Reefs and Carbonate Platforms in the Pacific and Indian Oceans (IAS Special Publication 25)

REEFS AND CARBONATE PLATFORMS IN THE PACIFIC AND INDIAN OCEANS

Reefs and Carbonate Platforms in the Pacific and Indian Oceans Edited by G. F. Camoin and P. J. Davies © 1998 International Association of Sedimentologists ISBN: 978-0-632-04778-9

SPECIAL PUBLICATION NUMBER 25 OF THE INTERNATIONAL ASSOCIATION OF SEDIMENTOLOGISTS

Reefs and Carbonate Platforms in the Pacific and Indian Oceans EDITED BY G. F. CAMOIN AND P. 1. DAVIES

b

Blackwell Science

© 1998 International Association of Sedimentologists published by Blackwell Science Ltd Editorial Offices: Osney Mead, Oxford OX2 OEL 25 John Street, London WC IN 2BL 23 Ainslie Place, Edinburgh EH3 6AJ 350 Main Street, Malden MA 02148 5018, USA 54 University Street, Carlton Victoria 3053, Australia 10, rue Casimir Delavigne 75006 Paris, France Other Editorial Offices: Blackwell Wissenschafts-Verlag GmbH Kurftirstendamm 57 I0707 Berlin, Germany Blackwell Science KK MG Kodenmacho Building 7-10 Kodenmacho Nihombashi Chuo-ku, Tokyo 104, Japan The rights of the Authors to be identified as the Authors of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the copyright owner. First published 1998 Set by Semantic Graphics, Singapore Printed and bound in Great Britain at the Alden Press Ltd, Oxford and Northampton The Blackwell Science logo is a trade mark of Blackwell Science Ltd, registered at the United Kingdom Trade Marks Registry

DISTRIBUTORS

Marston Book Services Ltd PO Box 269 Abingdon, Oxon OX14 4YN (Orders: Tel: 01235 465500 Fax: 01235 465555) USA Blackwell Science, Inc. Commerce Place 350 Main Street Malden, MA 02148 5018 (Orders: Tel: 800 759 6102 781 388 8250 Fax: 781 388 8255) Canada Login Brothers Book Company 324 Saulteaux Crescent Winnipeg, Manitoba R3J 3T2 (Orders: Tel: 204 224-4068) Australia Blackwell Science Pty Ltd 54 University Street Carlton, Victoria 3053 (Orders: Tel: 3 9347 0300 Fax: 3 9347 5001) A catalogue record for this title is available from the British Library ISBN 0-632-04778-X Library of Congress Cataloging-in-publication Data Reefs and carbonate platforms in the Pacific and Indian oceans I edited by G.F. Camoin and P.J. Davies. p. em. -(Special publication number 25 of the International Association of Sedimentologists) Includes bibliographical references and index. ISBN 0-632-04778-X l. Coral reefs and islands-Pacific Ocean. 2. Coral reefs and islands-Indian Ocean. 3. Rocks, Carbonate-Pacific Ocean. 4. Rocks, Carbonate-Indian Ocean. I. Camoin, G.F. (Gilbert F.) II. Davies, P.J, III. Series: Special publication ... of the International Association of Sedimentologists; no. 25. QE565.R426 1996 97-28586 551.42'4'09164-dc2l CIP

Contents

vm

Preface

G. F. Camoin & P. J Davies

Processes Operating 3

Exposure, drowning and sequence boundaries on carbonate platforms

W Schlager 23

The origin of the Great Barrier Reef-the impact of Leg 133 drilling

P. J. Davies & F. M. Peerdeman 39

Development and demise of mid-oceanic carbonate platforms, Wodejebato Guyot (NW Pacific)

G. F. Camoin, A. Arnaud- Vanneau, D. D. Bergersen, P. Enos & Ph. Ehren 69

Stable tropics not so stable: climatically driven extinctions of reef-associated molluscan assemblages (Red Sea and western Indian Ocean; last interglaciation to present)

M Taviani 77

Sedimentary cycles in carbonate platform facies: Fourier analysis of geophysical . logs from ODP Sites 865 and 866

P. Cooper

Platform Case Histories 95

Aptian-Albian eustatic sea-levels

U Rohl & J G. Ogg 137

Origin of white sucrosic dolomite within shallow-water limestones, ODP Hole 866A, Resolution Guyot, Mid-Pacific Mountains: strontium isotopic evidence for the role of sea water in dolomitization

P. G. Flood 145

Computer simulation of a Cainozoic carbonate platform, Marion Plateau, north-east Australia

K. Liu, C. J Pigram, L. Paterson & C. G. St C. Kendall v

Contents

VI

163

Quaternary and Tertiary subtropical carbonate platform development on the continental margin of southern Queensland, Australia

J F. Marshall, Y. Tsuji, H Matsuda, P. J. Davies, Y. lryu, N Honda & Y. Satoh 197

Pleistocene reef complex deposits in the Central Ryukyus, south-western Japan

Y. Iryu, T. Nakamori & T. Yamada

Oceanic Reef Case Histories Atolls and Volcanic Islands 219

Morphology and sediments of the fore-slopes of Mayotte, Comoro Islands: direct observations from a submersible

W -Ch. Dullo, G. F. Camoin, D. Blomeier, M. Colonna, A. Eisenhauer, G. Faure,

J Casanova & B. A. Thomassin 237

Tectonic and monsoonal controls on coral atolls in the South China Sea

Wang Guozhong 249

Steady-state interstitial circulations in an idealized atoll reef and tidal transients in a deep borehole by computer simulation

A.-M Leclerc, D. Broc, Ph. Jean-Baptiste & J Rancher

Active Margins 261

Environmental and tectonic influence on growth and internal structure of a fringing reef at Tasmaloum (SW Espiritu Santo, New Hebrides island arc, SW Pacific)

G. Cabioch, F. W Taylor, J Recy, R. Lawrence Edwards, S. C. Gray, G. Faure, G. S. Burr & T. Correge

Passive Margins 281

Lagoonal sedimentation and reef development on Heron Reef, southern Great Barrier Reef Province

B. T. Smith, E. Frankel & J S. Jell 295

Terrigenous sediment accumulation as a regional control on the distribution of reef carbonates

K J Woolfe & P. Larcombe

Contents 3 11

Comparison between subtropical and temperate carbonate elemental composition: examples from the Great Barrier Reef, Shark Bay, Tasmania (Australia) and the Persian Gulf (United Arab Emirates)

C. P. Rao, Z. Z. Amini & J. Ferguson 325

Index Colour plates facing p. 88, p. 160 and p. 304

VII

Preface

The Pacific and Indian Oceans, with their complex

journey and participated in what both promised

and diverse tectonic histories, are naturally fertile

and turned out to be a very lively meeting. To all

ground for the study of carbonate platforms, of­

those who participated in the Sydney meeting, we

fering different perspectives in age, scale, architec­

extend our thanks and appreciation. We appreci­

ture, global position and product from their better

ated your promptness, understanding and good

known Atlantic counterparts.

humour in an increasingly busy daily schedule.

Throughout the late 1980s and early 1990s, there­

This Special Publication has been a labour of

fore, platforms of many different types in these

love, in which many have played a part in the

oceans were the subject of important studies by the

consummation: first, the many authors who turned

Ocean Drilling Program and by national organiza­

presentations into publications, and second, but

tions such as the Australian Bureau of Mineral

almost as important, our many colleagues who have

Resources and the Japan National Oil Corporation.

acted as referees for the papers in this volume. An

It therefore seemed entirely appropriate to hold a

often thankless, but essential job,

meeting, not to compare the Pacific and Indian

would have been still-born without the help of so

this volume

Ocean platforms with others, but to define their

many, and particularly we thank the following: A.

diverse characteristics and growth response, with

Arnaud, H. Arnaud, M. Aurell, A. Bosellini, T.

the clear objective of expanding the spectrum of

Brachert, C. J. R. Braithwaite, A. Droxler, W.-Ch.

platform types useful as geological analogues. That

Dullo, A. Eisenhauer, D. Feary, R.N. Ginsburg, M.

meeting was held at the University of Sydney,

Grammer, D. Hopley, H. Kayanne, C. Kendall, I. G.

Australia, in July 199 5, and this Special Publication

Macintyre, J. Marshall, J.-P. Masse, L. F. Montag­

defines many of the deliberations presented at that

giani, T. M. Quinn, F. Rougerie, W. Schlager, P. K.

meeting. This book is therefore the first to examine

Swart, B. A. Thomassin and J. Wise. Finally, we

the carbonate platforms of two oceans which offer

wish to thank Michael Talbot, Andre Strasser and

such a wide diversity of tectonic and climatological

the staff at lAS for encouragement and help in the

variables, so relevant to platform development.

editing and publication of this volume.

The organization of the Sydney meeting would have been more difficult without the help and

G. F. CAMOIN

sponsorship provided by the Earth Resources Foun­

CEREGE, Aix-en-Provence, France

dation of the University of Sydney and the Petro­

P.J.

leum Exploration Society of Australia. However,

DAVIES

University of Sydney, Australia

that help and sponsorship would have been fruitless without the support of those who made the long

viii

Processes Operating

Reefs and Carbonate Platforms in the Pacific and Indian Oceans Edited by G. F. Camoin and P. J. Davies © 1998 International Association of Sedimentologists ISBN: 978-0-632-04778-9

Spec. Pubis int. Ass. Sediment. ( 1 998) 25, 3-21

Exposure, drowning and sequence boundaries on carbonate platforms W. S C H LA G E R Vrije Un iversiteit !Eart hScien c es, De Boelelaan 1085, 1081 HV Amsterdam, The Net herlands

ABSTRACT Events that reduce or terminate shoa1water carbonate production have a pronounced effect on the anatomy of reefs and carbonate platforms. Both exposure and drowning may cause such disturbances and thus generate bounding surfaces for sequence stratigraphy. The most common scenarios are: (i) exposure followed by shallow flooding, i.e. restoration of shallow-water conditions; (ii) exposure followed by drowning, and (iii) drowning without prior exposure. Sequence boundaries generated by scenarios (i) and (ii) fit into the standard systems-tract model of sequence stratigraphy; scenario (iii) does not, because it implies that a highstand systems tract is overlain by a transgressive tract withqut intervening exposure. As a rule, this contact is unconformable because it represents a profound change in sediment input and dispersal, and because drowning is often followed by extensive submarine erosion as the sharp topography of the drowned platform amplifies ocean currents. A growing number of case studies show that drowning without preceding exposure has produced distinct unconformities that can be used as regionally or even globally correlatable markers. They should be accepted as sequence boundaries.

INTRODUCTION platform top. This production is prolific but it is easily disturbed or terminated by environmental change such as sea-level fluctuations. Sea-level fall and exposure, for instance, will immediately termi­ nate carbonate production and convert the former carbonate factory into a site of carbonate destruc­ tion. Sea-level rise and deep flooding can submerge the platform top so deeply that production is re­ duced or terminated. Since Darwin ( 1842), the term 'drowning' is in use for deep flooding of reefs and carbonate platforms. Modern sedimentology distin­ guishes between 'incipient drowning', that is flood­ ing to less-than-optimal conditions yet still in the photic zone, and 'complete drowning' where the platform top becomes submerged below the photic zone and benthic carbonate production ceases for all practical purposes (Kendall & Schlager, 198 1; Read, 1982). Both exposure and drowning entail drastic changes in facies and lithology as well as in sediment input and dispersal. This paper assesses the relative importance of· exposure events and drowning events in controlling the anatomy of reefs and carbonate platforms. It

It is well known that either exposure or flooding with concomitant sediment starvation may generate signifi cant events and bounding surfaces in the stratigraphic record. Opinions differ on which of these contrasting processes dominates the record. Sequence stratigraphy assumes that exposure during the fall of sea-level produces the most pronounced events and consequently sequence boundaries were set at these junctures (Vail et a!., 1977; Posamentier & Vail, 1988;Sarg, 1988a). Flooding events such as transgressive surfaces and maximum flooding sur­ faces are assumed to be important but clearly less so than lowstand events. Genetic stratigraphy takes the opposite view. It assigns the dominant role to the flooding phase and considers lowstand exposure a less important intermission (Frazier, 1974; Gallo­ way, 1989a,b; Meckel & Galloway, 1996). The debate still continues and illustrates that the answer is not obvious and that the situation may vary from place to place. For carbonate platforms, this discussion takes on an added dimension because the sediment is pro­ duced within the depositional environment, at the

Reefs and Carbonate Platforms in the Pacific and Indian Oceans Edited by G. F. Camoin and P. J. Davies © 1998 International Association of Sedimentologists ISBN: 978-0-632-04778-9

3

4

W Schlager

relies on observations of seismic data, boreholes and large outcrops. The seismic expression of these events is an important part of this discussion. Furthermore, seismic images commonly show the entire platform edifi c e; outcrops rarely do. On the other hand, seismic images also contain pitfalls, such as the difficulty in distinguishing between gen­ uine unconformity and rapid lateral facies change (Schlager, 1992; Stafleu & Schlager, 1995) and the suppression of geologically important events if they are not endowed with sufficient impedance contrast. Outcrops and seismic models of outcrops can com­ pensate for these shortcomings of seismic data and serve as calibration points for seismic interpretation.

Production (%of maximum)

0 ' \

0

50

100

_ Inter-and supratidal

-!'--.;;:;� ;, ��--....---..:... - Meanlow water

E

� Zone of light .

0..

saturation

.. � b ;;..., �

� I

0

� E-o

2.6 0 0 '0:1' """

Time

(Z)

�Channcls1

Fig. 2. Migrated seismic profile across the elevated platform rims of Wodejebato Guyot and areal distribution of the carbonate platform.

shelves formed by the northern and north-eastern flank ridge, and along the ridge extending south-east towards Pikinni Atoll (see Bergersen, 1 993, 1 995; Camoin et a!. , 1 99 5). These elevated platform rims broaden across areas where the shelf margin widens (e.g. adjacent to the flank ridges) and are generally separated by a trough about 50 m deep (Fig. 2). As noted by Bergersen ( 1 993), the inner ridge appears at the edge of the summit plateau in all seismic profiles, with the exception of the south flank where faulting has removed portions of the original edifice. The maximum height of this ridge above the synchronous lagoonal deposits is 48 m, although the average height is 36 m (Bergersen, 1 995; Camoin et a!., 1 99 5); the top of the ridge varies in width from less than 1 00 m to a maximum of 800 m. The summit of the outer ridge is about 35 m lower than the inner ridge (Fig. 2). It is 200700 m wide and rises to about 50 m above the trough separating the two ridges. Slopes on the landward side of the inner ridge vary from less than 1 up to 1 0 whereas those on the seaward side are o

o,

slightly steeper (up to 1 5 ° ) and closely comparable with that of the outer ridge. The top of platform carbonates at Site 873 is 1 3 and 3 3 m lower than on the inner ridge (Sites 8 7 4 and 8 77) and 2 1 and 1 1 m higher than on the outer one (Sites 875 and 876) (Fig. 3). At the top of the guyot, a surface with perhaps a metre or more of relief with unconnected depres­ sions and bio-encrusted knobs was observed over an appreciable area of the sea-floor during video surveys on the perimeter of Wodejebato.

STRATIGRAPHY

Lithostratigraphy

The four major lithostratigraphical units drilled on Wodejebato Guyot are, from the top to the bottom (Premoli Silva et a!., 1 993) (Fig. 3): (i) manganese crust and manganese-phosphate coated limestone conglomerate; (ii) platform carbonates; (iii) ferrugi-

42

G. F.

Camoin et al.

Site 873 Site 877 mbsl

Site 874

1350

Site 876

Site 875

Algal-rudist

1400

II a

Bdst Rudst Fltst Gst 1450

Foraminiferlid

Wckest Pckst Gst

IV v

1550

Foraminifer-

Algal-coral Bdst

rudist-algal lllb

skeletal

1500

Algal-rudist Gst-Adst

VI

lid

Foraminifer

Gst Skeletal coral Pckst

skeletal

Ferrvglnous

Clay & claystone

lie

Gst

Basalt Volcan� breccia

[ill] D.S. 1

CJ D.S. 2

� D.S. 3

CJ D.S.4

..

Mn crust

..

Pelagic cap

Fig. 3. Lithostratigraphy and depositional sequences (DS, see the text) on Wodejebato Guyot (Sites 873-877). Numbers in the stratigraphical columns refer to lithostratigraphical units.

nous clay, claystone, and extremely altered vesicu­ lar basalt; and (iv) basalt and volcanic breccia. 40Ar/39 Ar ages obtained on the drilled basalts range from 78.4 to 85 Ma (Pringle & Duncan, 1 995). The basalts are reversely magnetized and attributed to Chron 3 3 R of early Campanian age (79 Ma) (Na­ kanishi & Gee, 1 995). Drilled volcaniclastic breccia at Site 869 (Sager et al., 1 993) and dredged material (Lincoln et al. , 1 993) suggest that Wodejebato expe­ rienced an earlier, probably Cenomanian, volcanic activity. Biostratigraphy

The oldest marine sediments underlying the plat­ form carbonates consist of black clays that contain a well-preserved late Campanian calcareous nano­ flora (CC22 biozone, around 76 Ma; see Erba et al., 1 995). Biostratigraphy of platform carbonates relies mainly on larger benthic foraminifers that constitute successive assemblages including Pseudorbitoides cf. trechmanni, Sulcoperculina sp. and Asterorbis sp., and finally Omphalocyclus macroporus (Fig. 4), rang-

ing in age from the late Campanian to the Maastrich­ tian (see Premoli Silva et al. , 1 995). The rudist assemblages recorded in these limestones include ra­ diolitids (Distefanella mooretownensis, Distefanella sp.) and caprinids (Mitrocaprina sp., Coralliochama orcutti, Coralliochama sp. , Plagioptychus aff.fragilis, P. aff. minor, Antillocaprina sp.), which have been reported in Campanian-Maastrichtian strata from Jamaica, Mexico, Cuba and California (see Camoin et al. , 1 995). Sr-isotope data indicated that the major part of the carbonate sequence on the inrter ridge may be attributed to the Maastrichtian, whereas Campanian values were measured for the lowermost part of the sequence (see Wilson et al., 1 995). Solution cavities occurring in platform carbon­ ates are partly filled by pelagic sediments that contain late Maastrichtian, early late Palaeocene, and early Eocene planktonic foraminifers (Erba et al., 1 995). The manganese crust and manganese­ phosphate coated conglomerate which cap the shallow-water limestones contain pelagic sediments ranging in age from the late Palaeocene to the middle Eocene (Erba et al., 1 995) (Fig. 4).

Site 873 Depth {mbsl)

Site 877

1350

Pal. Eo

Site 874 Site 876

Site 875

1400

.g ;:s

c

�

.!!! .E 0 ·;:::

�

�� � ;:: l'l...

11450

l} ;:s

:E

c;;· ""

� ;:s

1500

�

�;::

v

r;·

1550

G. gr gansseri +

Occurrence

1. -.]

..

48

G. F. Camoin et al.

environment, probably less than 1 0 or 20 m deep. Evidence of turbulent waters includes abrasion of shell fragments and the prevalence of grainstones and rudstones. The abundance of grainstones within the recovered cores and corresponding downhole logging signatures strongly suggests that the initial ridge of the atoll consisted of skeletal sand shoals with patchy organic frameworks. These frameworks were probably not wave-resistant structures because the organic binding was weak, but rigidity and sta­ bility may have been provided by early marine ce­ mentation (see 'Diagenesis' section). 3 The top of DS-2 is interpreted as a late HST and comprises very pale brown to white skeletal grain­ stones, packstones and floatstones, interlayered with sparse grey burrowed, dolomitic wackestones. The contact between these facies is typically an omission surface with borings, local erosion and reworking of intraclasts implying early lithification. Major components of skeletal packstones and floatstones consist of leached micritized fragments ofgastropods, rudists (radiolitids and scarce caprin­ ids) and other bivalves (inoceramids and probable pycnodonts); in situ radiolitid clusters occur locally. Benthic foraminifers include miliolids (Istrilocu­ lina), rotaliids, discorbids and, to a lesser extent, orbitoids and lituolids; placopsilinids encrust large bioclasts. Other grains include, in order of decreas­ ing abundance: fragments of green algae ( dasycla­ daceans: Terquemella), red algae (corallinaceans and peyssonneliaceans), corals (hexacorals and oc­ tocorals) and echinoderms. Calcisphaerulids and planktonic foraminifers (Rugoglobigerina sp.) occur in trace abundance. Minor dolomitic wackestone beds are character­ ized by the abundance of fenestrae that may form irregular networks of vertically elongated tiny tubes reminiscent of fluid escape structures. Skeletal content is low and limited to small gastropods (cerithids), benthic foraminifers (peneroplids, oph­ thalmiids, discorbids, textulariids, cf. Cuneolina, fragments of orbitoids), smooth-shelled ostracods, green algae (Terquemella) and few abraded frag­ ments of corals, red algae and rudists. A shallow-marine depositional environment, probably a few metres deep, may be inferred from the fossil content and the sedimentological criteria of the foraminifer-gastropod wackestones. The fine­ grained matrix and the lack of current indicators imply quiet-water conditions. The abundance of fenestrae is consistent with a shallow subtidal, inter­ tidal or supratidal environment. Periods of emer-

gence may be deduced from early dissolution of shells and the local reworking of caliche lithoclasts. The local prevalence of depauperate assemblages of very small foraminifers, smooth-shelled ostracods and gastropods (cerithids) may indicate temporary episodes of restriction and inimical ecological conditions (e.g. low illumination or oxygenation, temperature and salinity fluctuations, changes in nutrient content or turbidity). Skeletal packstones and floatstones interlayered with wackestone beds may have been redeposited from the outer debris shoals by currents.

Depositional sequence 3 (DS-3; Maastrichtian) DS-3 has been identified across the entire platform and varies in thickness from a minimum of 1 0 m on the inner ridge to a maximum of 9 5 m on the outer ridge; it is 30 m thick in the central part of the guyot (Fig. 3). The resolution of the seismic data pre­ cludes identification of the TST and HST stratal patterns in this depositional sequence, but carbon­ ate deposition was presumably aggradational after the flooding of the central volcanic high. The top of DS-3 corresponds to an unconformable sequence boundary interpreted as a pronounced surface of emergence associated with karstification, as sug­ gested by the hummocky upper surface of platform carbonates (Fig. 5) and isotopic data (see 'Diagene­ sis' section).

Central part ofthe guyot. Dominant facies consist of skeletal packstones and grainstones with wacke­ stones at the top of the carbonate sequence. Major skeletal constituents include fragments of rudists (radiolitids, up to 5 7% of the skeletal components), larger benthic foraminifers (abundant Sulcopercu­ iina and Asterorbis associated with Idalina and Vida/ina and then Omphalocyclus-respectively as­ semblages II and I in Fig. 4), echinoderms and gastropods. Red algae are locally abundant. Rela­ tive to the underlying DS-2, the prevalence of packstones and grainstones suggests a higher-energy environment and increased biotic diversity indi­ cates open marine conditions. Inner elevated platform rim. Dominant facies in­ clude skeletal grains tones-rudstones with few algal­ octocoral-rudist frameworks which are more abundant at Site 877 than at Site 874. Skeletal grainstones and rudstones are generally poorly sorted and medium, coarse or very coarse grained; a

Development and demise of mid-oceanic carbonate platforms few packstone beds occur at Site 877. Major com­ ponents include rudists (caprinids and radiolitids), corals (hexacorals and octocorals), calcareous sponges (chaetetids) and red algae (corallinaceans, peyssonneliaceans: Polystrata alba, solenopora­ ceans: Pycnoporidium). Large skeletal fragments are usually micritized and display millimetre-sized borings. Benthic foraminifers (orbitoids, rotaliids, nodosariids, miliolids and ophthalmiids) are mod­ erately abundant. Other grains are scarce and in­ clude echinoids, gastropods and green algae (Ter­ quemella). The local occurrence of calcisphaerulids and small ammonites may characterize a maximum flooding surface (Arnaud-Vanneau et al. , 1 995). Or­ ganic frameworks are formed by laminar coral colonies ( octocorals: Polytremacis; hexacorals), rud­ ists (radiolitids and caprinids) and red algal bushes (solenoporaceans) that are heavily encrusted by red algae (corallinaceans and peyssonneliaceans: Poly­ strata alba) and, to a lesser extent, by foraminifers and Bacinella. Probable Microcodium structures were noted at the top of DS-3 at Site 874. Fossils and sedimentological criteria suggest that the frameworks grew in a shallow-marine environ­ ment, probably less than 1 0-20 m deep, and characterized by high-energy conditions. Further evidence of turbulent waters includes abrasion of shell fragments and the prevalence of grainstone and rudstone textures.

Outer elevated platform rim. Dominant facies in­ clude coarse skeletal grainstones and packstones characterized by an extensive porosity (principally mouldic and intergranular). The major skeletal components include larger benthic foraminifers (abundant Sulcoperculina and Asterorbis associated with Omphalocyclus) and fragments of rudists, red algae and echinoderms; minor components are corals, ostracods and planktonic foraminifers. Large leached intraclasts of wackestone bearing moulds of small gastropods, ostracods and discor­ bid foraminifers probably result from the erosion and the reworking of the upper part of the DS-2 of the inner ridge (Enos et al., 1 995a). Depositional sequence 4 (DS-4; Maastrichtian) DS-4 is the last platform carbonate sequence and is only identified on the outer ridge, where a maxi­ mum thickness of 85 m is recorded at Site 876 (Fig. 3). Its deposition at the edge of the guyot and at a topographically lower level than the top of the

49

platform suggests that it was deposited during a sea-level lowstand, whereas the major part of the platform was subaerially exposed, thus correspond­ ing to a lowstand wedge. It is composed of poorly sorted white and highly porous coarse skeletal grainstones rich in benthic foraminifers (abundant Asterorbis assemblage VIa in Fig. 4 ); fragments of rudists and red algae represent 20% of the skeletal components, whereas echinoderm and coral frag­ ments are less abundant; minor constituents are calcareous sponges, stromatoporoids and green algae. -

Diagenesis

Sediments on guyots generally display a complex diagenetic history involving carbonate cementa­ tion, transformation of carbonate phases, alteration of morphology and chemistry of the particles, dis­ solution, cavity infillings, phosphatization, and sec­ ondary mobilization of Fe-Mn oxides.

Porosity and cementation In contrast to carbonates influenced by burial dia­ genesis, the sediments on guyots still exhibit a high percentage of porosity. Cementation includes a variety of cement morphologies that represent, presumably, a variety of diagenetic environments. Overburden was of minor importance in Wodeje­ bato sediments, as the stratigraphy indicates a maximum depth of burial less than 200 m, and pore-water was almost certainly at hydrostatic pres­ sure throughout the evolution of the guyot. Accord­ ingly, the definitive parameters in cementation were fluid chemistry and temperature. The Wodejebato platform carbonates may have been exposed to a wide range of environments including shallow marine, deep marine, meteoric phreatic, meteoric vadose and subaerial. No indication was found that hypersaline waters were ever present in Wodejebato (Enos et al. , 1 99 5b). The temperature end-members also spanned a limited range. These carbonates are today bathed in cold sea water, beneath the oceanic thermocline (temperatures about 1 0oC; Premoli Silva et al., 1 993), whereas at the time of deposition near sea-level in equatorial waters, ambient temper­ atures were probably over 2 5oC (Enos et al. , 1 99 5b ). There is no evidence of hydrothermal alteration within the clays that overlie the volcanic rocks at three sites (873, 874, and 877).

·

50

G. F. Camoin et a!.

Petrography. The major part of Wodej ebato plat­ form carbonates consists of skeletal sands that display intense leaching of the original grains result­ ing in high mouldic, vuggy and solution-enlarged interparticle porosity. These sediments are gener­ ally poorly lithified, except in the upper 70 m of the inner ridge, where sediments are strongly cemented by multiple generations of banded calcitic cements. The two major cement types recognized petro­ graphically within the platform carbonates of Wodej ebato Guyot include radiaxial (locally fi­ brous) calcites and prismatic limpid calcites. Other cement types are clearly less abundant and include columnar Mg calcite and syntaxial overgrowth ce­ ments (Enos et al., 1 99 5b ). 1 When present (upper 70 m of the inner ridge), the first cement phase typically corresponds to isopac­ hous fringes of amber to brownish, radiaxial calcites that exhibit a typical sweeping extinction and in­ clude bladed and fibrous morphologies (Fig. 7A-C); a botryoidal habit is evident in large intergranular cavities. These cements are largely confined to primary pore-space. Radiaxial calcites are generally interpreted as resulting from the lateral coalescence of radial fibrous cements (Kendall, 1 977), either aragonite (Bathurst, 1 977; Kendall, 1 985) or high-Mg calcite (Lohmann & Meyers, 1 977), or from direct marine precipitation (Sandberg, 1 98 5). Mag­ nesium contents and strontium values of Wodeje­ bato radiaxial calcites suggest a magnesian-calcite precursor (Enos et a!. , 1 99 5b ). 2 The second generation of cement consists of limpid, pyramidal, scalenohedral and equant (syn­ taxial overgrowths) calcite cements that locally cross-cut the isopachous fringes of radiaxial cal­ cites. They occur in all types of pores, including solution pores, but are more common in primary pores, where they post-date radiaxial calcites (Figs 7A-D). These cements also fill chambers of some foraminifers and cavities in the overlying Eocene pelagic cap. The rounding of tiny pyramidal crystals is attributed to dissolution, suggesting that the precipitation of these cements and dissolution over­ lapped or alternated in time. Blocky, pore-filling, low-Mg calcites can be the product of meteoric (Longman, 1 980) or of deep-marine (Schlager & James, 1 978) diagenesis. Stable isotope composition. As petrographical fea­ tures alone are not conclusive as diagenetic indica­ tors, depositional and diagenetic history may be tracked using the oxygen and carbon isotope com-

positiOns of depositional and late-stage cements. Stable carbon- and oxygen-isotopic compositions of the successive cement phases and matrix are plotted in Figs 8 and 9 (all values are given in PDB). When present, well-preserved rudist shell fragments were analysed to try to estimate the original shallow­ marine isotopic composition as a starting point for discriminating progressive diagenetic changes. Carbon and oxygen stable isotope composition of radiaxial calcites ranges from + 1 .22 to + 2. 77o/oo (average +2. 1 1 ± 0.44o/oo) and from -5.2 1 to -0. 1 2o/oo (average - 1 .26 ± 1 . 1 6o/oo) for o 13C and o180, respectively (Camoin et al., 1 99 5 ; Enos et a!. , 1 995b). o180 values are consistent with precipita­ tion in equilibrium with warm (20-3o·q, shallow­ marine waters, assuming an ice-free world (o180water 1 o/oo SMOW). These values are close to expectation for cements precipitated from late Cretaceous sea water (-2o/oo < o180 < - 1 . 8o/oo; o13C 3o/oo; Lohmann & Walker, 1 989). They are similar to those reported from Cretaceous radiaxial and fascicular-optic calcite cements in the Bahama Escarpment (Freeman-Lynde et al., 1 986) and from the Miocene of Enewetak Atoll (Saller, 1 986), and slightly higher in o180 than those measured on lower Cretaceous radiaxial calcites from the subsur­ face of East Texas and Mexico (Moldovanyi & Lohmann, 1 984) (Fig. 8). In contrast, these values are slightly lower than modern marine cements, including high-Mg calcite cements from Pikinni (Gonzalez & Lohmann, 1 98 5), but are within the theoretical field of o13C values for calcite precipi­ tated from modern Pacific sea water (i.e. +2.0 to +2.5o/oo; Kroopnick et a!., 1 977). Thus, petrograph­ ical observations and stable isotope data support precipitation in a shallow-marine environment. o13C and o180 values measured in clear, pyrami­ dal calcite cements fall into two distinct groups: 1 The first group display enriched isotopic ratios where o13C and o180 range respectively from + 1 .2 3 t o +3. 1 3o/oo (average +2. 1 9 ± 0.48o/oo) and from -2. 1 0 to + 1 . 1 1 o/oo (average: - 1 .05 ± 0.69o/oo) (Cam­ oin et al. , 1 995; Enos et a!., 1 995b) (Fig. 9). Similar isotopic values were obtained for deep marine equant spar cements formed below the thermocline in lower and mid-Cretaceous shallow-water lime­ stones exposed on the Bahama Escarpment (Freeman-Lynde et a!., 1 986), for Campanian gravity-flow deposits from the Bahamas (McClain et al. , 1 988) and for Campanian-Palaeocene hard-· grounds from the Caribbean (Anderson & Schnei­ dermann, 1 973) (Fig. 9). On the other hand, =

=

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0 �

;:s

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� 'l::l i:S" s, 0

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Fig. 7. Thin-section photomicrographs of rudist-coralgal frameworks and associated skeletal grainstone and rudstone of the inner elevated platform rim

(Sites 874 and 877). Early cements consist of thick isopachous fringes of radiaxial and fascicular-optic calcites with fibrous and bladed morphologies (2 in A and I in B). These calcites appear to be zoned by variations of inclusion density. The remaining pore-spaces are partly filled by internal micritic sediment (A) and/or by limpid, pyramidal and scalenohedral calcite cements (3 in A and 2 in B). In D, the primary pores are partly filled by limpid calcite cements. The local occurrence of clear granular calcite at the contact between grains and early calcite cements should be noted (I in A). m, mouldic pore; po, remaining porosity.

lJl

52

G. F. Camoin et a!.

o"c

Sites 873, 874 and 877 4 ,------,

o"c

Sites S73, 874 and 877 4 ,-------. NEP---

3

3 CE

AIC·C

2

2

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-2�--�----�-r�-.�� -8 -7 -6 -5 -4 -3 -2 -1 0 3 4 2

o'"o

o••o Fig. 9. Cross-plot of stable isotopic composition of late

Fig. 8. Cross-plot of stable isotopic composition of

syndepositional cements (shallow marine and meteoric) and matrix, Sites 873, 874 and 877. RC, Radiaxial cement; CC, columnar, bladed, magnesian calcite; EBS, equant blocky spars; BE, Bahama Escarpment shallow­ marine cements (Lower-Middle Cretaceous; Freeman-Lynde et a!. , 1 986); STC, shallow-marine cements in Stuart City trend strata (Lower-Middle Cretaceous; Prezbindowski ( 1 985) in McClain et a!. ( 1 988)); AIC-M, average isotopic composition of Maastrichtian calcites (Lohmann & Walker, 1 989); AIC-C, average isotopic composition of Coniacian calcites (Czemiakowski et a!. ( 1 984) in Moldovanyi & Lohmann ( 1 984)).

carbon-isotope values are slightly lower than for most modem shallow-marine cements (see Gonza­ lez & Lohmann, 1 985), suggesting a precipitation in relatively deep-marine waters where 813C of marine bicarbonate is 1 -2%o lighter than in surface marine waters (Kroopnick et al. , 1 977). This interpretation is strengthened by 8180 values that are consistent with precipitation in equilibrium with colder ( 1 020•C) waters of deep marine composition, assum­ ing an ice-free world (8180water 1 %o SMOW). 2 The second group of blocky spar cements has carbon- and oxygen-isotopic compositions more typical of meteoric diagenesis, especially depleted oxygen values with respect to radiaxial calcite ce­ ments and well-preserved rudist shells. 813C and 8180 values range respectively from -2. 3 7 to + l . 5 8%o (average +0. 1 4%o) and from -7.80 to -2.49%o (average: -5%o) (Fig. 9). All these samples come from throughout DS-3, from the top of DS-2 and from skeletal grainstone clasts embedded in the =

deep-marine cements, Sites 874 and 877. PLUC, Prismatic limpid cement with uniform extinction; BE, Lower-Middle Cretaceous from Bahama Escarpment (Freeman-Lynde et a!. 1 986); Leg 1 5, Campanian­ Palaeocene from Leg 1 5 DSDP (Anderson & Schneidermann, 1 973); NEP, Campanian from NE Providence Channel (McClain et a!. 1 98 8); CE, Aptian-Albian from Campeche Escarpment (Halley et a!. ( 1 984) in McClain et a!. ( 1 9 88)).

manganese crust. They indicate two episodes of subaerial exposure with meteoric-water and possi­ ble soil-gas influence, which may correspond to the termination of the platform.

Skeletal diagenesis Originally aragonitic skeletons exhibit differential preservation throughout the carbonate sequence, whereas dissolution of benthic foraminifers and red algae is peripheral and rare. Caprinid rudists dis­ play a gradational sequence of transformation, from pristine aragonitic shells to fragments totally re­ placed by limpid blocky calcites, through partial neomorphic replacement where relics of aragonitic skeletal structures are preserved in inclusion-rich coarse blocky calcite cements (Fig. 1 0). It is possible to relate the degree of stable isotope depletion to these specific fabrics. Stable carbon and oxygen compositions of well­ preserved aragonitic rudists ( 60- 1 00% aragonite;· Fig. lOA) range respectively from +0. 76 to +6. 3 5%o (average +2.95 ± l . 37%o) for 813C and

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CARBONATE PLATFORM DEVELOPMENT

600 m

- Phosphate-Mn crust

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Platform carbonates Volcanic basement

FORMATION OF PHOSPHATE-Mn CRUST

BOO m

1000 m

Fig. 14. Late Cretaceous and early Tertiary sedimentary evolution of Wodejebato. Subsidence path of volcanic

basement is calculated from the age-depth equation for oceanic crust proposed by Parsons & Sclater ( l 977) and modified by Crough ( 1 978) and Heestand & Crough ( 1 9 8 1 ); eustatic curves are from Haq et at. ( 1 987).

Vanneau et a!. , 1 993): late Campanian-Maastricht­ ian (Wodejebato) to latest Palaeocene-early middle Eocene (Limalok) in the Marshall Islands and Aptian-Albian (MIT, Takuyo-Daisan) in the Japa­ nese Seamounts. Consistent features in their devel­ opment include: (i) a prolonged initial period of subaerial exposure of the volcanic pedestal (gener­ ally of a few million years) characterized by the formation of soil horizons; (ii) the progressive flooding of the volcanic basement by organic-rich

clays, calcareous sandstones bearing plant remains and/or bioclastic limestones deposited in a reducing shallow-marine environment; and (iii) a burial of the carbonate platform by pelagic deposits, some­ times preceded by subaerial exposure. Subsidence and accumulation rates

The carbonate platforms that developed on these emergent seamounts kept pace with subsidence

Development and demise of mid-oceanic carbonate platforms rates, but none of the drilled platforms had a lifetime longer than 20 Myr. The MIT and Takuyo­ Daisan platforms developed for about 1 9 Myr and 1 5 Myr, respectively, whereas a lifetime of 1 0 Myr is estimated for the platform on Wodejebato and Limalok. Carbonate accumulation rates (not cor­ rected for compaction), range generally from 8.2 m Myr- ' (e.g. Takuyo-Daisan) to a maximum of 40 m Myc 1 (upper carbonate sequence of MIT). On Wodejebato, accumulation rates range from 1 3 . 7 m Myc ' in the central part of the guyot up to 25-45 m Myr- ' on the outer elevated platform rim; on the inner ridge, these rates are 1 7 m Myc 1 and 1 8. 3 m Myc1 at Sites 8 74 and 877, respectively. Accumulation rates of 30 m Myc ' are reported on Limalok. The lowest accumulation rates are related to the presence of hiatus(es) in the carbonate se­ quences (e.g. Takuyo-Daisan; central part and inner ridge of Wodejebato). Origin o f morphological features

A number of NW Pacific seamounts display atoll­ like features, including raised rims enclosing a central lagoon, flanked by steep talus leading down to volcanic slopes. Early-cemented organic frame­ works form thin intervals on the inner ridge of Wodej ebato but they cannot be compared to coral­ gal reefs that characterize modern Pacific atolls. None of the elevated platform rims drilled on early Cretaceous guyots (e.g. Takuyo-Daisan, Resolution) revealed the existence of organic frameworks. It seems likely that these systems kept pace with subsidence and sea-level changes not by construct­ ing a wave-resistant bulwark, but by producing vast quantities of loose carbonate sediment that formed intermittent skeletal and oolitic sand shoals. On the other guyots (e.g. MIT, Limalok), the perimeter rim was not drilled so its nature is unknown. The elevated platform rims appear to be depositional features rather than relict karst features related to the dissolution of low-energy limestones in the central part of the guyots and the concomitant lithification of high-energy rim deposits as sug­ gested by Van Waasbergen & Winterer ( 1 993). Another characteristic ofNW Pacific guyots is the occurrence of closed depressions or sinkholes, lo­ cally 100-200 m deep (e.g. MIT), stream channels and terraces on their summit. These features have been interpreted as the result of subaerial exposure, on the basis of their remarkable similarity to mod­ ern subaerial karst morphology (Van Waasbergen &

61

Winterer, 1 993). Other mechanisms for generating features that mimic the appearance of sinkholes on a submerged carbonate platform have been consid­ ered. They may include dissolution of carbonates related to the aragonite compensation depth and/or dissolution of carbonates associated with sulphur­ rich fluids (Haggerty & Van Waasbergen, 1 99 5), but none of them alone may account for the formation of such large-scale collapse features. It seems more likely that the surface topography of the guyots has been generated by a combination of several mecha­ nisms involving subaerial karstification and deep­ marine dissolution (Haggerty & Van Waasbergen, 1 99 5). It has been shown earlier that, on Wodeje­ bato, solution cavities affecting the upper 50 m of the carbonate cap were formed necessarily before the guyot sank below the aragonite compensation depth, thus implying early subaerial dissolution processes. Similar histories involving a subaerial exposure preceding drowning are recorded in the Mid-Pacific Mountains (Sager et al. , 1 993) and other dredged guyots along the Japanese Seamount chain (Grotsch & Flugel, 1 992). Subaerial karst sinkholes and troughs as much as 75 m deep are recorded in the upper part of Allison and Resolu­ tion guyots; furthermore, on Resolution Guyot, speleothems occur at least as deep as 60 m, imply­ ing that the guyot stood at least to this elevation above sea-level during karstification (Sager et al. , 1 993). The sedimentary evolution of many guyots is characterized by one or several substantial periods of exposure before rapid submergence. On Wodeje­ bato, the late development of the carbonate plat­ form is characterized by two successive periods of emergence: a first limited fall of sea-level and the water table may have induced juvenile karstifica­ tion, and then a deeper lowering of sea-level led to karstification and formation of a lowstand wedge, before a rapid rise of greater amplitude (Fig. 1 3). Meteoric diagenesis at the top of the carbonate sequence of Wodejebato is typified by sedimento­ logical evidence and stable isotope data (see 'Diagenesis' section). On Limalok Guyot, three intervals, including the top of the carbonate plat­ form, have been interpreted as indicative of mete­ oric diagenesis with low water/rock ratios (Wyatt et al., 1 99 5). Indications of subaerial exposure are also reported on MIT and Takuyo-Daisan guyots (Hag­ gerty & Van Waasbergen, 1 99 5 ; Jansa & Arnaud­ Vanneau, 1 99 5). With few exceptions, the depletion in 8 1 80 generally coincides with an increase in

62

G. F. Camoin et al.

porosity in the limestone, and meteoric cements are scarce in the Leg 1 44 platform limestones. A similar situation has been reported in modern atolls located in low rainfall areas, where meteoric cementation is reduced and the chief diagenetic processes consist of dissolution of the metastable carbonates and the development of karst terrane (see Wheeler & Aha­ ron, 1 99 1 ). The problem of the drowning

The drilling results of Leg 1 44 indicate that the formation of guyots was not synchronous. There were at least three major episodes of carbonate platform drowning within the tropical Pacific: Al­ bian, late Maastrichtian and middle Eocene (Pre­ moli Silva et a!., 1 993). The first episode of drowning was previously placed at 1 05- 1 1 6 Ma, after the early Aptian but before the middle Albian (Winterer & Metzler, 1 984), but it occurred during late Albian time (Sager et a!., 1 993; Premoli Silva et al. , 1 993; Erba et a!., 1 995). A number of mecha­ nisms, alone or in combination, may cause platform drowning. Among these are rapid relative sea-level changes, tectonics, volcanism and environmental deterioration causing reduced benthic growth rates or death (Schlager, 1 98 1 ; Hallock & Schlager, 1 986; Erlich et a!. , 1 990). Palaeolatitude data (Nakanishi & Gee, 1 99 5) do not support plate motion carrying reefs to latitudes beyond the Darwin Point as it was suggested by Winterer & Metzler ( 1 984). Because of restricted areal extent and low summit relief, small isolated platforms are probably more susceptible to drowning, which can place them out of the photic zone and effectively shut down the shallow-water 'carbonate factory'.

Sea-level changes Rapid drowning is in evidence on three guyots drilled during Leg 1 44: MIT, where late Albian pelagic deposits overlie mid- to late Albian platform carbonates; Wodejebato, where late Maastrichtian pelagic sediments fill cavities at the top of the Maastrichtian carbonate sequence; and Limalok, where mid-Eocene pelagic deposits overlie platform carbonates of nearly the same age (Erba et al. , 1 99 5). In all these situations, the time-span between the age of the youngest shallow-water limestones and the oldest pelagic sediments ranges from 1 to 3 Myr. This implies regression-transgression cycles of considerable amplitude and a duration of a few

million years. Such cycles are well known from continental margins and may have also been re­ corded in pelagic sequences (e.g. Albian; Sliter, 1 99 5). The drowning appears to be geologically instan­ taneous, as there is no transitional sequence be­ tween platform carbonates and pelagic deposits, as a result either of non-deposition or subsequent erosion during reflooding. The complete shut-down of platform carbonate production during reflooding cannot be explained solely by a rise in sea-level. Subsidence rates and long-term sea-level changes are at least one order of magnitude lower than the potential growth rates of carbonate platforms (Schlager, 1 9 8 1 ), even though the carbonate accu­ mulation rates reported on the Pacific guyots are generally lower than those reported on continent­ attached platforms �nd large isolated platforms. Accordingly, this suggest_�; that the cause of abrupt platform drowning may be related to deterioration of environmental conditions (e.g. changes in salin­ ity, temperature, ocean chemistry, circulation, or nutrient supply).

Nutrient excess Nutrient content is thought to control the balance between carbonate production and bioerosion on carbonate platforms (Hallock & Schlager, 1 986). Several major drowning events occurred during episodes of oxygen deficiency in the oceans, espe­ cially in the middle Cretaceous (Schlager, 1 99 1 ) The stratification in Cretaceous oceans was proba­ bly less stable and more prone to overturn than is temperature-dominated stratification in modern oceans, especially when overturn coincided with sea-level pulses (Hallock & Schlager, 1 986). Al­ though continent-attached platforms are more prone to nutrient excess than the mid-oceanic plat­ forms, several workers have attributed the demise of numerous mid-Cretaceous Pacific atolls to an excess of nutrient-rich waters. The mechanisms involved by these workers include volcanogenic upwelling (Vogt, 1 989), geothermal endo-upwelling (Rougerie & Fagerstrom, 1 994) or equatorial up­ welling (Larson et al. , 1 99 5). Because mid-plate submarine volcanism was widespread and intense in the middle Cretaceous Pacific, volcanogenic upwelling was considered by Vogt ( 1 989) as one possible mechanism for plat­ form demise. He pointed out an apparent temporal correlation between volcanism, anoxia and extinc.

Development and demise of mid-oceanic carbonate platforms tions during Aptian time. However, although the Aptian mid-plate volcanic episode overlaps, within dating errors, the 'Oceanic Anoxic Event 1 ' (Jen­ kyns, 1 980), new stratigraphical data show that most of the NW Pacific guyots drowned much later, during late Albian time (Winterer, 1 9 9 1 ; Grotsch & Hugel, 1 992; Premoli Siva et a!., 1 993; Sager et a!., 1 993) when volcanic activity had measurably de­ creased (see Fig. 3 of Vogt, 1 989). As pointed out by Rougerie & Fagerstrom ( 1 994), there is a scale problem for generalizing volcanogenic upwelling to the Pacific Ocean as a whole. Volcanogenic up­ welling cannot account for such a widespread, possibly global, demise of carbonate platforms (Grotsch & Flugel, 1 992) and the subsequent lack of recognized reef development in the Pacific between Cainomanian and Santonian time. The apparent temporal and spatial coincidence between the timing of guyot drowning and when these seamounts were carried across a palaeolati­ tude range of 0' to 1 2 ' S led some researchers to suggest that the equatorial zone may have played a role in the drowning equation. Larson et a!. ( 1 995) suggested that as these guyots entered this zone of higher nutrient availability and higher temperature, populations ofbioeroders increased and net carbon­ ate accumulation dropped. However, the strict ap­ plication of this model to Cretaceous carbonate platform communities may be questionable, as many, if not most, of the major Cretaceous shallow­ water carbonate producers (e.g. rudists) may have had ecological requirements different from those of modern building organisms (e.g. corals) (Camoin, 1 989). Furthermore, the equatorial upwelling is driven by trade winds and characterizes icehouse oceans that are governed by a thermohaline oceanic circulation (Hay, 1 98 8). The combination of high sea-surface temperatures, possible ice-free poles, and high sea-levels during a greenhouse climatic supercycle resulted in oceanic halothermal circula­ tion distinctly different from the thermohaline cir­ culation that characterizes icehouse periods. As a consequence, it has been suggested that no equato­ rial upwelling acted in the 'greenhouse' oceans, during Cretaceous and early Tertiary time (Hay, 1 988). Furthermore, extensive carbonate platforms are reported in many areas of the Maastrichtian palaeo-equatorial zone (e.g. Indian plate, East Afri­ can margin; Camoin et a!. , 1 993), suggesting that this zone was not so inimical to carbonate platform development.

63

Climatic and palaeoceanographic changes Short but strong perturbations of climate may explain both rapid sea-level changes and environ­ mental deterioration which could have removed the possibility of reef colonization and caused the complete drowning of carbonate platforms. Our drilling results, coupled with dredging data from other guyots, indicate that Pacific guyots preferen­ tially drowned during specific periods: late Albian, latest Maastrichtian and middle Eocene. Strong regression-transgression cycles connected to short­ term cooling events have been postulated for the late Albian (Grotsch & Fhigel, 1 992; Sager et a!. , 1 993), the late Maastrichtian (Camoin et a!. , 1 993) and the early middle Eocene (Haq et a!. , 1 987; Butterlin et a!., 1 993), and seemingly coincide with a drastic collapse of carbonate platforms through­ out the Tethyan realm. Widespread hiatuses and/or changes in calcare­ ous planktonic communities in the Pacific Ocean support the occurrence of strong palaeoceano­ graphic changes during late Albian time at shallow and deeper sites (Sliter, 1 99 5). Drowning of several carbonate platforms in the Tethys and Atlantic oceans supports a time-dependent event, as op­ posed to a geographical cause (i.e. palaeoequatorial zone), for the late Albian crisis (Larson et a!., 1 99 5). A plausible explanation is a cooling event, charac­ terized by a sharp shift of oxygen isotopes in upper Albian pelagic sediments (see Grotsch & Flugel, 1 992), that resulted in more vigorous circulation and deposition of more oxygenated sediments dur­ ing the Cenomanian (Larson et a!. , 1 99 5 ; Sliter, 1 99 5). During Maastrichtian time, a general cooling trend has been documented in many areas (Saltz­ man & Barron, 1 982; Frakes & Francis, 1 990; Huber & Watkins, 1 992; Camoin et a!., 1 993). A sharp decrease in sea-surface temperatures to about 2 1 ' C in the Pacific Ocean has been documented through oxygen-isotope fluctuations by Barrera et a!. ( 1 987). This time of moderately cool conditions began in the Maastrichtian and is thought to have continued, with fluctuations, into the late Palae­ ocene, when considerable warming occurred in bottom or polar water masses {see Camoin et a!. , 1 993). Maastrichtian time corresponds to a transi­ tional period between two modes of deep oceanic circulation (i.e. from a Cretaceous to a more Tertiary-type pattern), inducing a sharp decrease in organic productivity (see Camoin et a!., 1 993).

64

G. F. Camoin et a!.

Shifts of planktonic foraminiferal assemblages from high to lower latitudes and oxygen-isotopic data also suggest a cooling event in the early middle Eocene (see Shackleton, 1 986; Butterlin et a/. , 1 993; Diester-Haass & Zahn, 1 996). As a conclusion, the three episodes of drowning (i.e. late Albian, late Maastrichtian and mid Eocene) recorded in the Pacific coincided with rapid and high-amplitude sea-level fluctuations that acted in combination with environmental stress on the carbonate-platform ecosystems through climatic and palaeoceanographic changes in circulation and nutrient cycling.

SUMMARY AND CONCLUS IONS

The sedimentary evolution of NW Pacific guyots apparently yields a consistent scenario, irrespective of their age (Aptian-Albian, Campanian-Maastri­ chtian or Palaeocene-Eocene). A model for the development and the demise of mid-oceanic carbonate platforms is based on a detailed sedimen­ tological, seismic and geochemical study of Wode­ jebato Guyot (Marshall Islands, NW Pacific): 1 After a prolonged period of subaerial exposure (a few million years), the volcanic basement was pro­ gressively flooded and covered by organic-rich clays and pyrite-rich limestones deposited in a quiet and reducing shallow-marine environment. 2 The flooding of the volcanic platform resulted in the accumulation of carbonate sand shoals near the shelf margin, rimming an emerged volcanic island. The overlying sequence is a series of retrograda­ tional and then progradational sand shoals as the central part of the guyot was progressively flooded. 3 A number of Pacific guyots display atoll-like features including well-differentiated elevated plat­ form rims that enclose a central lagoon and are flanked by steep talus leading down to volcanic slopes. Early cemented organic frameworks were reported only as thin intervals on the inner ridge of Wodejebato. None of the drilled elevated platform rims on early Cretaceous guyots revealed the exist­ ence of reefal masses; rather they are composed of skeletal and oolitic sands. On the other guyots (e.g. MIT, Limalok}, the perimeter rim was not drilled, and its nature is unknown. 4 Later development of the carbonate platform is characterized by two successive periods of emer­ gence: a first, limited fall in sea-level, and a second, deeper fall in sea-level before the final drowning of

the carbonate platform during Maastrichtian time. The last platform carbonate sequence is a wedge deposited during a sea-level lowstand on the outer ridge, whereas the inner part of the platform was subaerially exposed and probably karstified. 5 After its demise, the carbonate platform sank rapidly into deep water, between the aragonite and the deeper calcite saturation depths, resulting in the dissolution of aragonite and concomitant precipita­ tion of calcite cements, possibly throughout the Cainozoic. The post-drowning succession includes a phosphate-manganese crust that formed from the late Palaeocene to the middle Eocene. 6 The drilling results of Leg 1 44 indicate that there were at least three major episodes of carbonate platform drowning within the tropical Pacific (late Albian, late Maastrichtian and middle Eocene) that apparently occurred during short-term regression­ transgression cycles connected to short-term cli­ matic and palaeoceanographic changes.

ACKNOWLEDGEMENTS

The authors wish to warmly thank the ODP Leg 1 44 Scientific Party and the shipboard personnel of the JOIDES Resolution. An early version of this manuscript benefited greatly from substantial re­ views by Wolfgang Schlager and Bruce Fouke.

REFERENCES AL AASM, I. & VEIZER, J. ( 1 986) Diagenetic stabilization of

aragonite and low-Mg calcite, II. Stable isotopes in rudists. J. sediment Petrol. , 56, 763-770. ALLAN, J.R. & MATTHEWS, R.K. ( 1 977) Carbon and oxygen isotopes as diagenetic and stratigraphic tools: surface and subsurface data, Barbados, West Indies. Geology, 5, 1 6-20. ANDERSON, T. & SCHNEIDERMANN, N. ( 1 973) Stable isotope relationships in pelagic limestones from the central Caribbean: Leg 1 5, Deep Sea Drilling Project. Initial Reports of the Deep Sea Drilling Project, 1 5 (Eds Edgar, N.T. & Saunders, J.B.), pp. 795-803. US Government Printing Office, Washington, DC. ARNAUD-VANNEAU, A., BERGERSEN, D., CAMOIN, G. et a/. ( 1 99 5) A model for the depositional sequences and systems tracts on small mid-ocean carbonate platforms: examples of Wodejebato Guyot (Sites 873-877) and Limalok Guyot (Site 8 7 1 ) . In: Proceedings of the Ocean Drilling Program, Scientific Results, 1 44 (Eds Haggerty, J.A., Premoli Silva, 1., Rack, F. & McNutt, M.K.), pp. 8 1 9-840. Ocean Drilling Program, College Station, TX. ARNAUD-VANNEAU, A., CAMOIN, G. & SHIPBOARD SCIEN­ TIFIC PARTY ( 1 993) Les edifices carbonates des atolls et

Development and demise of mid-oceanic carbonate platforms guyots d u Pacifique nord-occidental: resultats prelim­ inaires du Leg ODP 1 44. C.R. Acad. Sci. Paris, 317, 947-954. BARRERA, E., HUBERT, B.T., SAVIN, S.M. & WEBB, P.N. ( 1 987) Antarctic marine temperatures: late Campanian through early Palaeocene. Paleoceanography, 2, 2 1 -4 7. BATHURST, R. ( 1 977) Ordovician Meiklejohn bioherm, Nevada. Geol. Mag., 1 14, 308-3 1 1 . BERGERSEN, D. ( 1 993) Geology and geomorphology of Wodejebato (Sylvania) Guyot, Marshall Islands. In: The Mesozoic Pacific (Eds Pringle, M., Sager, W., Sliter, W. & Stein, S.), pp. 367-38 5 . American Geophysical Union, Wasington, DC. BERGERSEN, D. ( 1 99 5) Morphology of Marshall Islands guyots. In: Proceedings of the Ocean Drilling Program, Scientific Results, 1 44 (Eds Haggerty, J.A., Premoli Silva, 1., Rack, F. & McNutt, M.), pp. 5 6 1 -584. Ocean Drilling Program, College Station, TX. BUTTERLIN, J., VRIELYNCK, B., BIGNOT, G. et a/. ( 1 993) Lutetian (46 to 40 Ma). Explanatory notes. In: Atlas Tethys Paleoenvironmental Maps (Eds Dercourt, J., Ricou, L.E. & Vrielynck, B. ) , pp. 1 97-209. Gauthier Villars, Paris. CAMOIN, G. ( 1 989) Les plates-formes carbonatees du Tur­ onien et du Senonien de Mediterranee centrale (Algerie, Tunisie, Sicile). These Doct. es-Sci., Universite de Prov­ ence, Marseille. CAMOIN, G., ARNAUD-VANNEAU, A., BERGERSEN, D. et a/. ( 1 995) Anatomy and evolution of the inner perimeter ridge (sites 874 and 877) of a Campanian-Maastricht­ ian atoll-like structure (Wodejebato Guyot; Marshall Islands). In: Proceedings of the Ocean Drilling Program, Scientific Results, 1 44 (Eds Haggerty, J.A., Premoli Silva, 1., Rack, F. & McNutt, M.), pp. 27 1 -294. Ocean Drilling Program, College Station, TX. CAMOIN, G., BELLION, Y.J.C., DERCOURT, J.G. et a/. ( 1 993) Late Maastrichtian (69.5-65 Ma). Explanatory notes. In: Atlas Tethys Paleoenvironmental Maps (Eds Der­ court, J., Ricou, L.E. & Vrielynck, B. ) , pp. 1 79- 1 96. Gauthier Villars, Paris. CAMOIN, G., BERNET-ROLLANDE, M.C. & PHILIP, J. ( 1 988) Rudist-coral frameworks associated with submarine volcanism in the Maastrichtian of the Pachino area (Sicily). Sedimentology, 35, 1 23- 1 28 . COLLOT, J.Y., GREENE, H.G., STOKKING, L.B. et a/. , ( 1 992) Proceedings of the Ocean Drilling Program, Initial Re­ ports, 1 34. Ocean Drilling Program, College Station, TX. CROUGH, S.T. ( 1 978) Thermal origin of mid-plate hot-spot swells. Geophys. J.R. astron. Soc., 55, 4 5 1 -469. DIESTER-HAASS, L. & ZAHN, R. ( 1 996) Eocene-Oligocene transition in the Southern Ocean: history of water mass circulation and biological productivity. Geology, 24, 1 63 - 1 66. DUENNEBIER, F.K. & PETERSEN, L.K. ( 1 982) Summary report, !POD Site surveys in the western Pacific, R/V Kana Keoki cruise KK81 0626, Leg 4. Hawaii Institute of Geophysics, Honolulu. EMERY, K.O., TRACEY, J.l., Jr & LADD, H.S. ( 1 954) Geology of Bikini and nearby atolls. US geol. Surv. Prof Pap. , 260-A, 265 pp. ENOS, P., CAMOIN, G.F. & EBREN, PH. ( 1 995a) Sedimentary sequence from Sites 875 and 876, outer perimeter ridge,

65

Wodejebato Guyot. In: Proceedings of the Ocean Drill­ ing Program, Scientific Results, 1 44 (Eds Haggerty, J.A., Premoli Silva, 1., Rack, F. & McNutt, M.), pp. 295-3 1 0. Ocean Drilling Program, College Station, TX. ENOS, P., CAMOIN, G.F., WILSON, P.A. et a/. ( 1 995b) Diagenesis of platform carbonates, Wodejebato Guyot, Marshall Islands. In: Proceedings of the Ocean Drilling Program, Scientific Results, 1 44 (Eds Haggerty, J.A., Premoli Silva, 1., Rack, F. & McNutt, M.), pp. 789-8 1 8. Ocean Drilling Program, College Station, TX. ERBA, E., PREMOLI SILVA, 1., WILSON, P.A. et a/. ( 1 995) Synthesis of stratigraphies from shallow-water se­ quences at Sites 8 7 1 through 879 in the Western Pacific Ocean (Leg 1 44). In: Proceedings of the Ocean Drilling Program, Scientific Results, 1 44 (Eds Haggerty, J.A., Premoli Silva, I. Rack, F. & McNutt, M.), pp. 873886.0cean Drilling Program, College Station, TX. ERLICH, R.N., BARRETT, S.F. & Guo, B.J. ( 1 990) Seismic and geologic characteristics of drowning events on carbonate platforms. Bull. Am. Assoc. petrol. Geol. , 74, 1 523- 1 537. FRAKES, L.A. & FRANCIS, J.E. ( 1 990) Cretaceous paleocli­ mates. In: Cretaceous Resources, Events and Rhythms (Eds Ginsburg, R.N. & Beaudoin, B.), pp. 273-287. Kluwer Academic, Dordrecht. FREEMAN-LYNDE, R., FULKER WHITLEY, K. & LOHMANN, K.C. ( 1 986) Deep-marine origin of equant spar cements in Bahama Escarpment limestones. J. sediment Petrol., 56, 799-8 1 1 . GONZALEZ, L.A. & LOHMANN, K.C. ( 1 985) Carbon and oxygen isotopic composition of Holocene reef carbon­ ates. Geology, 13, 8 1 1-8 1 4. GROTSCH, J. & FLOGEL, E. ( 1 992) Facies of sunken early Cretaceous atoll reefs and their capping late Albian drowning succession (Northwestern Pacific). Facies, 27, 1 5 3- 1 74. HAGGERTY, J.A. &. VAN WAASBERGEN, R.J. ( 1 995) Geochemistry of MIT and Takuyo-Daisan carbonate platforms compared with other northwest Pacific guyots drilled during Leg 1 44: implications for their diagenetic history. In: Proceedings of the Ocean Drilling Program, Scientific Results, 1 44 (Eds Haggerty, J.A., Premoli Silva, 1., Rack, F. & McNutt, M.), pp. 84 1 -872. Ocean Drilling Program, College Station, TX. HALLOCK, P. & SCHLAGER, W. ( 1 986) Nutrient excess and the demise of coral reefs and carbonate platforms. Palaios, 1, 389-398. HAMILTON, E.L. ( 1 956) Sunken islands of the Mid-Pacific Mountains. Geol. Soc. Am. Mem. , 64, 1 -9 1 . HAMILTON, E.L. & REx , R.W. ( 1 959) Lower Eocene phos­ phatized Globigerina ooze from Sylvania Guyot. US geol. Surv. Prof Pap. , 260-W, 785-797. HAQ, B.U., HARDENBOL, J. & VAIL, P.R. ( 1 987) Chronology of fluctuating sea levels since the Triassic. Science, 235, 1 1 56- 1 1 67. HAY, W.W. ( 1 988) Paleoceanography: a review for the GSA centennial. Geol. Soc. Am. Bull., 100, 1 934- 1 956. HEESTAND, R.L. & CROUGH, S.T. ( 1 9 8 1 ) The effect of hot spots on the oceanic age-depth relation. J. geophys. Res., 86, 6 1 07-6 1 1 4 . HoLMES, M.A. ( 1 995) Pedogenic alteration o f basalts recovered during Leg 1 44. In: Proceedings of the Ocean Drilling Program, Scientific Results, 1 44 (Eds Haggerty,

66

G. F. Camoin et a!.

J.A., Premoli Silva, I., Rack, F. & McNutt, M.), pp. 3 8 1 -400. Ocean Drilling Program, College Station, TX. HUBER, B.T. & WATKINS, D.K. (1 992) Biogeography of Campanian-Maastrichtian calcareous plankton in the region of the Southern Ocean: paleogeographic and paleoclimatic implications. In: The Antarctic Paleoenvi­ ronment: a Perspective on Global Change (Eds Kennett, J.P. & Warnke, D.A.). Am. geophys. Union, Antarct. Res. Ser., Washington, DC, 56, 3 1 -60. JANSA, L.F. & ARNAUD-VANNEAU, A. (1 995) Carbonate buildup and sea-level changes at MIT Guyot, western Pacific. In: Proceedings of the Ocean Drilling Program, Scientific Results, 1 44 (Eds Haggerty, J.A., Premoli Silva, I., Rack, F. & McNutt, M.), pp. 3 1 1 -336. Ocean Drilling Program, College Station, TX. JENKYNS, H. C. (I 980) Cretaceous anoxic events: conti­ nents to ocean. J. geol. Soc. London, 137, 1 7 1 - 1 88 . KENDALL, A . ( 1 977) Fascicular-optic calcite. A replace­ ment of bundled acicular carbonate cements. J. sedi­ ment Petrol. , 47, 1 056-1 062. KENDALL, A. (1 985) Radiaxal fibrous calcite: a reappraisal. In: Carbonate Cements (Eds Schneidermann, N. & Harris, P.M.), Spec. Pub!. Soc. econ. Paleont. Miner., Tulsa, 36, 59-77. KONISHI, K. ( 1 989) Limestone of the Daiichi Kashima Seamount and the fate of a subducting guyot: fact and speculation from the Kaiko-"Nautile" dives. Tectono­ physics, 160, 249-265. KROOPNICK, P.M., MARGOLIS, S.V. & WONG, C.S. (I 977) 1i 1 3 C variations in marine carbonate sediments as indi­ cators of the C02 balance between the atmosphere and oceans. In: The Fate of Fossil Fuel C02 in the Oceans (Eds Anderson, N.R. & Malahoff, A.), pp. 295-32 1 . Plenum, New York. LARSON, R., ERBA, E., NAKANISHI, M., BERGERSEN, D.D. & LINCOLN, J.M. (1 995) Stratigraphic, vertical subsidence and paleolatitude histories of Leg 1 44 guyots. In: Pro­ ceedings of the Ocean Drilling Program, Scientific Re­ sults, 1 44. (Eds Haggerty, J.A., Premoli Silva, I., Rack, F. & McNutt, M.), pp. 9 1 5-934. Ocean Drilling Pro­ gram, College Station, TX. LINCOLN, J., PRINGLE, M. & PREMOLI SILVA, I. (1 993) Early and late Cretaceous volcanism and reef-building in the Marshall Islands. In: The Mesozoic Pacific (Eds Pringle, M., Sager, W., Sliter, W. & Stein, S.), Geophys. Monogr. Ser., Am. geophys. Union, Washington, DC, 77, 279305. LoHMANN, K.C. & MEYERS, W. ( 1 977) Microdolomite inclusions in cloudy prismatic calcites. J. sediment Petrol. , 47, 1 07 8- 1 088. LOHMANN, K.C. & WALKER, J. ( 1 989) The 0 1 8 0 record of Phanerozoic abiotic marine calcite cements. Geophys. Res. Lett. , 16, 3 1 9-322. LONGMAN, M.W. (1 980) Carbonate diagenetic textures from nearsurface diagenetic environments. Bull. Am. Assoc. petrol. Geol., 64, 46 1 -487. MATTHEWS, J.L., HEEZEN, B.Z., CATALANO, R. et a/. (1 974) Cretaceous drowning of reefs on Mid-Pacific and Japa­ nese guyots. Science, 184, 462-464. McCLAIN, W.R., FREEMAN-LYNDE, R.P. & LOHMANN, K.C. ( 1 988) Petrography and stable oxygen and carbon iso­ tope compositions of Campanian grainstones and rud­ stones, Northeast Providence Channel, Bahamas: ODP

Leg 1 0 1 , Hole 634A. In: Proceedings of the Ocean Drilling Program, Scientific Results, I 0 I (Eds Austin, J.A., Jr & Schlager, W.), 245-25 3 . Ocean Drilling Program, College Station, TX. MELIM, L.A., SWART, P.K. & MALIVA, R.G. (1 995) Meteoric-like fabrics forming in marine waters: impli­ cations for the use of petrography to identify diagenetic environments. Geology, 23, 755-758. MOLDOVANYI, E. & LOHMANN, K. (1 984) Isotopic and petrographic record of phreatic diagenesis: Lower Cre­ taceous Sligo and Cupido formations. J. sediment Petrol. , 54, 972-985. NAKANISHI, M. & GEE, J.S. (I 995) Paleomagnetic investi­ gations of volcanic rocks: paleolatitudes of the north­ western Pacific guyots. In: Proceedings of the Ocean Drilling Program, Scientific Results, 1 44 (Eds Haggerty, J.A., Premoli Silva, I., Rack, F. & McNutt, M.), pp. 5 85-604. Ocean Drilling Program, College Station, TX. ODP LEG 1 44 SHIPBOARD SCIENTIFIC PARTY (I 993) Insight on the formation of Pacific guyots from Leg 1 44. EOS, 74, 3 5 8 . PARSONS, B. & SCLATER, J.G. ( I 9 7 7 ) Analysis o f the variation of the ocean floor bathymetry and heat flow with age. J. geophys. Res. , 93, 1 1 753- 1 1 7 7 1 . PREMOLI SILVA, I., HAGGERTY, J., RACK, F. et a/. (Eds) (1 993) Proceedings of the Ocean Drilling Program, Ini­ tial Reports, 1 44. Ocean Drilling Program, College Station, TX. PREMOLI SILVA, I., NICORA, A., ARNAND-VANNEARU, A. et a!. (I 995) Paleobiogeographic evolution of shallow­ water organisms from Aptian to Eocene in western Pacific (Leg 1 44). In: Proceedings of the Ocean Drilling Program, Scientific Results, 1 44 (Eds Haggerty, J.A. Premoli Silva, I., Rack, F. & McNutt, M.), pp. 887-894. Ocean Drilling Program, College Station, TX. PRINGLE, M.S. & DuNCAN, R.A. (1 995) Radiometric ages of basement lavas recovered at Loen, Wodejebato, MIT and Takuyo-Daisan guyots, Leg 1 44, northwestern Pa­ cific Ocean. In: Proceedings of the Ocean Drilling Pro­ gram, Scientific Results, 1 44 (Eds Haggerty, J.A., Premoli Silva, I., Rack, F. & McNutt, M.), pp. 547-560. Ocean Drilling Program, College Station, TX. ROUGERIE, F. & FAGERSTROM, J.A. (1 994) Cretaceous history of Pacific basin guyot reefs: a reappraisal based on geothermal endo-upwelling. Palaeogeogr. Palaeocli­ matol. Palaeoecol., 112, 239-260. SAGER, W.W., WINTERER, E.L., FIRTH, J.V. et a/. (Eds) (1 993) Proceedings of the Ocean Drilling Program, Ini­ tial Reports, 1 43. Ocean Drilling Program, College Station, TX. SALLER, A. H. ( I 986) Radiaxial calcite in Lower Miocene strata, subsurface Enewetak atoll. J. sediment Petrol., 56, 743-762. SALTZMAN, E. & BARRON, E.J. (I 982) Deep circulation in the Late Cretaceous: oxygen isotope paleotemperatures from Inoceramus remains in DSDP cores. Palaeogeogr. Palaeoclimatol. Palaeoecol., 40, 1 6 7- 1 8 1 . SANDBERG, P.A. (I 985) Aragonite cements and their occur­ rence in ancient limestones . . In: Carbonate Cements (Eds Schneidermann, N. & Harris, P.N.). Spec. Puql. Soc. econ. Paleont. Miner., Tulsa, 36, 33-57. ScHLAGER, W. (I 9 8 1 ) The paradox of drowned reefs and carbonate platforms. Geol. Soc. Am. Bull. , 92, 1 97-2 1 1 .

Development and demise of mid-oceanic carbonate platforms ScHLAGER, W. ( 1 99 1 ) Depositional bias and environmen­ tal change-important factors in sequence stratigraphy. Sediment Geol. , 70, 1 09- 1 30. SCHLAGER, W. & JAMES, N.P. ( 1 978) Low-magnesium calcite limestones forming at the deep-sea floor, Tongue of the Ocean, Bahamas. Sedimentology, 25, 675-702. SCHLANGER, S., CAMPBELL, J. & JACKSON, M. ( 1 987) Post­ Eocene subsidence of the Marshall Islands recorded by drowned atolls on Harrie and Sylvania guyots. In: Seamounts, Islands and Atolls (Eds Keating, B., Fryer, P., Batiza, R. & Boehlert, G.), Geophys. Monogr. Ser., Am. geophys. Union, Washington, DC, 43, 1 65 - 1 74. SCHOLLE, P.A., ARTHUR, M.A. & EKDALE, A.A. ( 1 983) Pe­ lagic environment. In: Carbonate Depositional Environ­ ments (Eds Scholle, P.A., Bebout, D.G. & Moore, C. H.). Mem. Am. Assoc. petrol. Geol., Tulsa, 33, 620-69 1 . SHACKLETON, N.J. ( 1 986) Paleogene stable isotope events. Palaeogeogr. Palaeoclimatol. Palaeoecol. 57, 9 1 - 1 02. SLITER, W.V. ( 1 995) Cretaceous planktonic foraminifers from Sites 865, 866 and 869. In: Proceedings of the Ocean Drilling Program, Scientific Results, 1 43 (Eds Winterer, E.L., Sager, W.W., Firth, J.V. et a/.), pp. 1 5-30. Ocean Drilling Program, College Station, TX. VAN WAASBERGEN, R.J. & WINTERER, E.L. ( 1 993) Summit geomorphology of western Pacific guyots. In: The Meso­ zoic Pacific: Geology, Tectonics and Volcanism (Eds Pringle, M.S., Sager, W.W., Sliter, W.V. & Stein. S.). Geophys. Monogr., Am. geophys. Union, Washington, DC, 77, 3 3 5-366.

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VoGT, P.R. ( 1 989) Volcanogenic upwelling of anoxic, nutrient-rich water: a possible factor in carbonate-bank/ reef demise and benthic faunal extinctions? Geol. Soc. Am. Bull., 101, 1 225- 1 245. WHEELER, C.W. & AHARON, P. ( 1 99 1 ) Mid-oceanic carbon­ ate platforms as oceanic dipsticks: examples from the Pacific. Coral Reefs, 10, 1 0 1 - 1 1 4. WILSON, P.A., OPDYKE, B.N. & ELDERFIELD, H. ( 1 995) Strontium isotope geochemistry of carbonates from Pacific guyots. In: Proceedings of the Ocean Drilling Program, Scientific Results, 1 44 (Eds Haggerty, J.A., Premoli Silva, 1., Rack, F. & McNutt, M.), pp. 447-458. Ocean Drilling Program, College Station, TX. WINTERER, E.L. ( 1 99 1 ) The Tethyan Pacific during late Jurassic and Cretaceous time. Palaeogeogr. Palaeocli­ matol. Palaeoecol. 87, 2 53-265. WINTERER, E.L. & METZLER, C.V. ( 1 984) Origin and subsidence of guyots in Mid-Pacific Mountains. J. geo­ phys. Res., 89, 9969-9979. WYATT, J.L., QuiNN, T.M. & DAVIES, G.R. ( 1 995) Prelim­ inary investigation of the petrography and geochemistry of limestones at Limalok and Wodejebato guyots (Sites 87 1 and 874), Republic of the Marshall Islands. In: Proceedings of the Ocean Drilling Program, Scientific Results, 1 44 (Eds Haggerty, J.A. Premoli Silva, 1., Rack, F. & McNutt, M.), pp. 429-437. Ocean Drilling Pro­ gram, College Station, TX.

Spec. Pubis int. Ass. Sediment. ( 1998) 25, 69-76

Stable tropics not so stable: climatically driven extinctions of reef-associated molluscan assemblages (Red Sea and western Indian Ocean; last interglaciation to present) M. TA V I AN I Instituto di Geologia Marina del CNR, via Gobetti 101, 40129 Bologna, Italy

ABSTRACT

The Indo-West Pacific coral reefs have experienced dramatic faunal turnovers since the last interglacial period, as documented by local extinctions and contractions of associated faunas in the western Indian and Pacific oceans. Although refrigeration of sectors of the Indo-Pacific region is a possible concomitant limiting factor, rate of sea-level change is considered the most important constraint in controlling the ultimate fate of Quaternary coral reefs. Disruption of internal organization at times of rapid sea-level fluctuations may lead to progressive depauperation of coral reef biota through local extinctions. In the case of the shallow-silled Red Sea basin, effects of sea-level changes were dramatically amplified and the entire basin underwent massive destruction of its stenoecious biota as a result of the onset of high-salinity conditions. Similar disturbances punctuated the entire Quaternary ice age, possibly causing many, high-frequency faunal rearrangements of variable intensity in coral reef ecosystems. Habitat fragmentation within the Indo-West Pacific region is seen as a major mechanism to account for the high level of species-level biodiversity witnessed throughout the Cainozoic. Speciation is apparently promoted through gene-flow disruption among populations within the Indo-West Pacific during times of relative lowstands. Endemics seem to be preserved rather than lost, when populations reconnect at the re-establishment of highstand conditions. Thus, the Indo-West Pacific tropical region acts as a vast refuge at peaks of glacial difficulties. However, this is not the general rule for the tropics, as indicated by a decrease in coral diversity in the Caribbean. During the last glaciation, the capacity of the Red Sea and Persian Gulf to sequester C02 through calcification in both the pelagic and neritic domains practically reached zero. This fact, and the concomitant reduction of coral reefs in other areas of the Indo-Pacific region, should be taken into account in the evaluation of the global C02 cycle.

INTRODUCTION

short-term (101-102 yr: e.g. Loya, 1976, 1990; Mc­ Glade, 1990; Smith & Buddemeier, 1992) or very long-term disturbances (105-106 yr: e.g. Fager­ strom, 1987; Kauffman & Fagerstrom, 1993). Re­ sponse of reefs to stress at a time-scale ranging from 103-104 yr has only seldom been investigated, al­ though this is the scale of Pleistocene glacial cycles, which would allow testing of coral reef reaction to both temperature and sea-level fluctuations. The main factors controlling the fate of coral reefs during ice ages have been the topic of many debates, with suggestions ranging from sea-level fluctuations to changes in temperature regime (e.g. Stoddart, 1976; Taylor, 1978; Stanley, 1984; Crame, 1986; Paulay, 1990, 1991, 1996b).

The highly diversified coral reefs, corresponding in the oceans to what tropical rain-forests represent on the continents (Jackson, 1991), have long embodied the common notion of tropical stability (Newell, 1971). This assumption, however, is being chal­ lenged as our understanding of Pleistocene glacial age dynamics and its impact on marine biota grows. Significant faunal turnovers have punctuated the Cainozoic history, greatly affecting marine benthic communities, including tropical ones (see review by Stanley & Ruddiman, 199 5). The vulnerability of coral reef ecosystems to climatic disturbances should be explored at dif­ ferent time-scales. Most previous studies have fo­ cused on the response of coral reefs to either very

Reefs and Carbonate Platforms in the Pacific and Indian Oceans Edited by G. F. Camoin and P. J. Davies © 1998 International Association of Sedimentologists ISBN: 978-0-632-04778-9

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M. Taviani

Much concern exists about the recovery of coral reefs from habitat destruction. Proposed scenarios for reefs in the future are grim (McGlade, 1990; Stone, 1995), but coral reef resilience is still largely unknown (e.g. Loya, 1990; Kaufman, 1993). There­ fore, a better understanding of how these relatively fragile ecosystems respond to climatic disturbances and issues such as extinction and evolution rates of their faunal associates may ultimately provide guidelines for conservation policies (e.g. Wilkinson, 1991; Huston, 1994). The goal of this paper is to review the available evidence of reef-associated molluscan turnovers during the last glacial cycle in the western Indian Ocean, particularly the Red Sea, and to discuss their evolutionary and global-climatic meaning.

LAST GLACIAL EXTINCTIONS IN THE INDIAN OCEAN CORAL REEFS

General

Stoddart (1973, 1976) presented the concept of massive local extinctions in Indo-West Pacific reef communities caused by Pleistocene sea-level fluctu­ ations. The comparative work on Quaternary mol­ luscan assemblages from Aldabra atoll by Taylor (1978) provided documentation of significant changes in the structure of pre- and post-glacial Indian Ocean coral reefs. The notion of Pleistocene to Recent coral reef faunal turnovers was reinforced by the study of outcrops in Kenya, on the East African mainland (Crame, 1986). Emblematic of these two studies is the discovery that giant clams, i.e. Tridacna gigas (Linnaeus, 1758), T crocea Lamarck 1819 and Hippopus hippopus (Linnaeus, 1758) inhabited the reefs of the westernmost Indo­ West Pacific region during the last interglaciation. Today, the geographical range of these clams is considerably restricted (Rosewater, 1965) as a con­ sequence of climatically driven extinctions which affected the western Indian Ocean during the last glacial period (Crame, 1986). Whether these extinc­ tions were selective or more widespread is not known for sure. However, it is clear that the local extinction ofT gigas, other giant clams and gastro­ pods (Crame, 1986) should not be considered iso­ lated phenomena. In fact, the same fate was shared by other reef-associated molluscs in the Red Sea and Gulf of Aden. Moreover, similar trends have

been reported for Pacific Ocean coral reefs (Kohn, 1980; Vermeij, 1986; Paulay, 1996b). Extinctions of Red Sea marine biota during the last glacial age

By integrating deep-sea and coral reef data, Taviani (1996a,b) concluded that the Red Sea stenoecious fauna and flora were almost completely annihilated during the last glacial period by a combination of high salinity (c. 50o/oo at least) and sea-level changes. Present-day biota merely represent a recolonization which began c. 10 000 yr ago. This eradication of stenoecious biota from the Red Sea was previously suggested by Sewell (1948) and Gvirtzman et a!. (1977) among others. Taviani (1982, 1997b) has ob­ served that last interglacial (Eemian, isotopic sub­ stage 5e) coral reefs of the Red Sea-Gulf of Aden region house molluscan faunas which are qualita­ tively different from their modern counterparts. This author listed examples of molluscan extinctions, i.e. Diodora impedimentum (Cooke, 1885); geographi­ cal contractions, i.e. Rhinoclavis vertagus (Linnaeus, 1767), Cerithium madreporicolum (Jousseaume, 1930), Columbella turturina (Lamarck, 1822), Co­ nus litteratus (Linnaeus, 1758), Cucullaea cucullata (Roeding, 1798); and rarefactions, i.e. Cypraea mon­ eta (Linnaeus, 17 58), Oliva bulbosa (Roeding, 1798), Corbula taitensis (Lamarck, 1818). Analogous con­ tractions are observed among the corals Turbinaria peltata (Esper, 1794), Cycloseris vaughani (Bosch­ rna, 1923) and Pavona minuta (Wells, 1954): (see Dullo, 1987). Por (1989) has offered a different view on this last glacial basinal stress of the Red Sea. First, he argued that some metahaline-tolerant organisms (e.g. molluscs, echinoderms, corals) could stand salinities even greater than 48o/oo, as is the case today in the Persian Gulf, thus making a point against the complete destruction of the Red Sea fauna during the last glacial. He also reports that claimed endemism among certain fish species implies permanent viable conditions in the Red Sea, possibly since the Pliocene (Klausewitz, 1983). In practice, the hypothesis that diverse coral reefs and their associated stenoecious faunas could toler­ ate protracted periods (thousands of years) of salin­ ities in excess of 50o/oo is difficult to accept. Moreover, the significance of Red Sea fish ende­ mism is tempered by the admission of Klausewitz (1983), who reported these endemics in the Gulf of Aden, and therefore present in the Indian Ocean.

Stable tropics not so stable

Some Red Sea-Gulf of Aden gastropod endemics have sibling species in the Indo-West Pacific region, as exemplified by the pairs Cypraea tigris (Linnaeus, 1758) (Indo-Pacific)-C. pantherina (Lightfoot, 1786) (Red Sea-Gulf of Aden), Cypraea vitellus (Linnaeus, 1758) (Indo-Pacific)-C. camelopardalis (Perry, 1811) (Red Sea-Gulf of Aden), Cypraea talpa (Linnaeus, 1758) (Indo-Pacific)-C. exusta (Sowerby, 1832) (Red Sea-Gulf of Aden). It is im­ portant to note that these endemics have been ob­ served from last interglacial (5e) coral reefs of the Red Sea (Borri et a!., 1982, and personal data from Zabargad island, Egypt). Therefore, the origination of these endemics predates the proposed biological sterilization of the Red Sea during the last glacial period. In fact, these Red Sea endemics had to survive outside the basin. The Gulf of Aden has been proposed as a glacial refuge by Por (197 5). New evidence seems to indicate that Gulf of Aden coral reefs were seriously affected by the last glacial period. The extinction of the limpet Diodora imped­ imentum, and the contraction of the geographical range of the cerithiid Rhinoclavis vertagus, known from last interglacial coral reefs of both the Red Sea proper and the Gulf of Aden, and now absent from this region, support this view (Taviani, 1997b ). Studies by Taylor (1978) and Crame (1986) show that significant disturbances of coral reef biota of both western Indian Ocean islands and mainland at equatorial latitudes have occurred. To account for these pre-5e endemics, it seems probable that one or more geographical refuges did exist in the western Indian Ocean. Crame (1986) suggested that a pos­ sible refuge was Oman. It has always been tempting to look at the Red Sea as an ideal place where new species originate, given its relative isolation and 'abnormal' hydrolog­ ical attributes (e.g. Head, 1987; Por, 1989). The presence of a relatively high number of endemics seemed to support this view. However, the scenario of their eradication from the basin and their succes­ sive reintroduction from the Indian Ocean after the last glaciation seems to falsify this hypothesis. If these endemics were only confined to the Red Sea, how did they survive outside the basin, under conditions that did not mimic their Red Sea ones? The frequently emphasized biological and hydro­ logical uniqueness of the Red Sea (e.g. Head, 1987; Por, 1989) is possibly misleading for evaluation of its true role in the economics of the Indo-Pacific species-rich region. In fact, the Red Sea is simply a

71

satellite basin of the Indo-West Pacific, under the recurrent threat of major hydrological disturbances, linked to the very short-term glacial cycles. The Red Sea: a small-scale abrupt mass extinction

During the last glaciation, it is very likely that the Red Sea experienced the extinction events affecting vast areas of the Indian Ocean (Taylor, 1978; Crame, 1986). The basin, however, deviated from equilibrium as predicted by the island biogeography hypothesis, by the unexpected mass destruction, a result of intolerable hydrological conditions. It should be noted that the last glacial hydrological turmoil affecting the Red Sea, with its profound repercussions, may be described as a disturbance regime in island ecology (Whittaker, 1995). Unfor­ tunately, the critical steps of progressive reactions of Red Sea biota to increasingly harsh environmen­ tal conditions (disturbance) are virtually unknown. The signature of step-by-step disruption of the 'last interglacial equilibrium conditions' by increasing extinction rates not balanced by immigrations until its final collapse, is probably sealed within drowned relative lowstand reefs, at present unavailable to detailed palaeobiological analyses. The peculiar morphology of the basin amplified to apocalyptic levels what was a general disturbance of the tropical marine biota during the last glaciation because of progressive area reduction as a result of sea-level lowering. The final results were disproportionate only for the Red Sea, because there it ended in a true mass extinction. The Indo-West Pacific as a whole escaped massive glacial extinctions almost unharmed, thus helping to replenish the Red Sea as well as other peripheral areas where coral reef biota had been more or less completely stripped off. An extreme case is the shallow Persian Gulf, which becomes completely exposed and its biota des­ troyed at times of glacial maxima (Lambeck, 1996). Were it not for its location adjacent to a large marine tropical reservoir, almost all of the Red Sea stenoecious fauna and flora would have disap­ peared forever. For this reason, the Red Sea may serve as a scenario to understand better the pro­ cesses of mass extinction and successive recovery (Jablonski, 1986; McLaren, 1986). In my view the Red Sea faunal extinctions and reinvasions offer a clue to understanding speciation in tropical marine environments. Before describing

M. Taviani

72

the possible patterns of the origination and perpet­ uation of species in the Indo-West Pacific coral reef ecosystems, we must first analyse the two main factors thought to control the evolution of coral reefs, i.e. temperature and sea-level.

TEMPERATURE AND SEA-LEVEL CHANGES AS CORAL REEF LIMITING FACTORS

negatively affecting coral reef ecosystems of the Red Sea, which are located in the north-westernmost comer of the Indian Ocean. It is, however, difficult to believe that temperature was equally significant in causing important faunal rearrangements and extinctions in Aldabra and Kenya (Taylor, 1978; Crame, 1986) as well as in the Pacific (Kohn, 1980; Vermeji, 1986; Paulay, 1990, 1996b). Salinity, thought to have been responsible for mass extinction in the Red Sea proper, is intimately linked to the second disturbance, sea-level change.

Role of temperature

Modem coral reefs are constrained by winter min­ imum temperatures and are generally restricted within the 18·c isotherm (Newell, 1971; Belasky, 1996). Coral reefs can tolerate much lower winter temperatures, as documented in the Arabian Gulf where reefs routinely survive exposures to winter minima around Ire (Coles & Fadlallah, 1991). However, long-term temperature depression below the 18 • C isotherm will ultimately cause coral reef regression and extinction (e.g. Belasky, 1996). We do not have precise data about significant temperature changes of surficial waters in the trop­ ical region under scrutiny to be tied to the fate of individual coral reefs during the last glacial period. Stoddart (1973) drew a map showing a significant contraction of the 2o·c isotherm throughout the tropical belt during the glacial Pleistocene. This reconstruction, however, is biased by the absence of ground evidence. Most palaeotemperature data from the region are indirectly derived from stable oxygen isotope data on benthic and plankton Fora­ minifera and transfer functions using microfossils. Re-evaluation of the latter is now showing that temperature depression of the tropical Atlantic and eastern Pacific oceans during the ice ages was larger than previously thought by CLIMAP reconstruc­ tions (Mix, 1996). Micropalaeontological and isotopic data from offshore Red Sea indicate glacial temperatures 5 • C lower than at present in the central Red Sea, and 3.5·c lower than at present in the Gulf of Aden (Ivanova, 1985). Fluctuations of such amplitude are considered to be influential on tropical organisms (Barron, 1995). Long-term low temperatures dra­ matically affect tropical biota diversity in the Car­ ibbean region (Edinger & Risk, 1994). Before conditions of high salinity became the leading cause of their annihilation, lower temperatures during the last glacial period, may have been indeed a co-factor

Role of sea-level

Sea-level changes control coral reef growth and have been described as the dominant force influencing faunal diversity by affecting area-size (e.g. Wise & Schopf, 1981). Conversely, sea-level changes may also induce extinctions (Hallam, 1989), although probably only at the species level during the Cain­ ozoic (Jablonski, 1985). Paulay (1990, 1996b) examined the role of sea­ level fluctuations and concluded that it is the prin­ cipal mechanism driving oceanic island extinctions in the tropics. Fundamentally I agree with this view, although I believe that the rate of sea-level change is the most important factor. My contention is that fast rates of sea-level fluctuations (both rise and fall) impede the setting of reasonably diverse coral reefs, as they give no time for a fully topographical expansion and, therefore, formation of exploitable habitats where biotic interactions can be fostered at incremental rates. Back-reef (lagoonal) environ­ ments are more exposed to disruption and require more time for their full re-establishment, and this would explain why Pleistocene local extinctions affected mostly inner-reef specialists (Paulay, 1996b). By using the present-day situation as our stan­ dard, we can observe that recolonization of entire basins (Red Sea and Persian Gulf ), and wide sectors of continental and oceanic islands was achieved in the last few thousands year. The Red Sea is a particularly striking example, as it re-acquired its pre-glacial morphological and biological complexity from Bab-el-Mandab at 12 •N up to the northern reaches of Aqaba, at 29.27'N, in less than 10 kyr. Stable (still-stand) sea-level conditions were achieved in the last 5000-6000 yr, so setting a maximum age for the present-day reef complexes.

Stable tropics not so stable DISCUSSION

Tropical speciation by sea-level habitat-area fragmentation

Trends of local extinctions between the last and present interglacial periods are commonplace throughout the Indo-West Pacific region, as shown by their occurrence also in Pacific islands (Kohn, 1980; Vermeij 1986; Paulay, 1996b). Although geographically distant, such faunal turnovers basi­ cally represent similar responses of the Indo-West Pacific region to disturbances triggered by glacial ages. As discussed by Vermeij (1986) and Crame ( 1986), the tropical Indo-West Pacific served as a refuge throughout the Cainozoic. Briggs (199 5) proposed that the East Indies operates as a radia­ tion centre but also accumulates over time older genera and species generated elsewhere in the re­ gion. The tremendous advantage of having a refuge of such a size is dramatically shown by the modest rate of extinctions with respect to originations. According to available evidence, only one taxon (Diodora impedimentum) was apparently lost be­ cause of the severe disturbances to coral reefs in the western Indian Ocean during the last glacial, and it belonged to the most affected area, i.e. the Red Sea-Gulf of Aden. Many other species contracted their geographical range but did survive through the crisis. It is very important to observe that even 'endemics', whose area is by definition relatively small and whose survival is easily jeopardized by area-reduction, survived the biotic crisis. On the other hand, overall biodiversity of this region seems to have increased steadily throughout the Caino­ zoic. Is it that Neogene ice ages are beneficial to biodiversity in the tropical marine realm? It seems that the answer is yes, but why? As discussed, temperature drops have a negative effect on coral reefs, therefore there must be another link. Sea-level glacio-eustatic fluctuations are the most logical ex­ planation, as they caused habitat fragmentation within the continuum of the Indo-West Pacific region (e.g. Paulay, 1996b). In this scenario of enhanced 'provincialism', such disruptions elimi­ nated gene flow between mother and some periph­ eral populations, ultimately promoting speciation (e.g. Mayr, 1982; Fagerstrom, 1983; Stanley, 1986; Briggs, 1995; Johnson et al., 1995). From a single faunal core (the Indo-West Pacific), a number of original situations may arise at each significant sea-level fluctuation, when some viable, relict pop-

73

ulations may find the conditions suitable for genetic drift, so that a net gain in biodiversity is achieved by accumulation through time (Crame, 1986; Hus­ ton, 1994; Briggs, 1995). Furthermore, investiga­ tion of the Red Sea shows that, once originated, new species are not easily lost, and thus, biodiversity continuously increases. This increase probably di­ rectly correlates with the size of the refuge. The function played by refugia in the Indo-West Pacific, not only to preserve species but also to enhance speciation during the Pleistocene, appears, therefore, conceptually similar to the role embodied by refugia of the Amazonian rain-forests (Haffer, 1969; Lynch, 1988). The Pleistocene alone is punctuated by some 20 major ice cycles; such conditions accommodate multiple separations and re-unifications of marine biota at the scale of thousands or tens of thousands of years. Advances in taxonomy show that Indo­ West Pacific tropical ecosystems support a large number of sibling species, indicating a level of provinciality among reef organisms unknown until recently, (Knowlton, 1993; Knowlton & Jackson, 1994; Paulay, 1996a, and references therein). Sib­ ling species may be the product of isolation at times of relative sea-level lowerings, originating in some glacial refuges. Although not necessarily alternative, this model does not require changes in ocean circu­ lation to explain some patterns of coral reef evolu­ tion and biogeography (Veron, 199 5). The sea-level hypothesis can be tested by examining the parallel story of Caribbean tropical ecosystems. Caribbean coral reefs show a trend of reduction in biodiversity during the Cainozoic (e.g. Edinger & Risk, 1994). The difference in behaviour between the two tropical regions may be due largely to area differences. The lack of a sizeable area (and related faunal-reservoir) in the Caribbean makes the region's coral reef biota more vulnerable to unfavourable environmental changes (Edinger & Risk, 1994), and may be related to possible extinctions at the time of regressions (Boecklen & Simberloff, 1986). Sea-level changes are not necessarily the only explanation for extinction and origination rates of past tropical reef ecosystems. In fact, before the onset of Quaternary ice ages, with their paroxysmal sea-level fluctuations, temperature may have been the most important single factor in controlling the fate of tropical marine biodiversity (e.g. Stanley, 1986). It is also important to point out that this· model may possibly explain some patterns of short time-scale speciation, and is largely related to the

M. Taviani

74

nentic domain. For instance, global catastrophic events, such as those linked to bolid-impacts or anoxic oceans (McLaren, 1986), offer totally dif­ ferent avenues to extinctions and originations also at supraspecific level. Extinction and contraction of coral reefs during the last glaciation: global carbon implications

The hypothesis that coral reefs are important C02 reservoirs and sinks, by means of carbonate precip­ itation and dissolution, is supported by investiga­ tors concerned with 'greenhouse' scenarios (Berger, 1982; Kinsey & Hopley, 1991; Opdyke & Walker, 1992, Smith & Buddemeier, 1992; Milliman, 1993). Opdyke & Walker (1992) have recently suggested that the locus of global carbonate produc­ tion is modulated by sea-level changes and shifts from the deep sea during glacials to the shelves, which include coral reefs, during interglacials. Mil­ liman & Droxler (1996) have estimated that, during sea-level highstands, the neritic environments (which include coral reefs) produce an amount of carbonate comparable with carbonate sequestered by pelagic calcification at times of lower sea-level. Within this frame, the Red Sea is only incom­ pletely fulfilling what was expected of the simple CaCOrpump proposed by Opdyke & Walker (1992). In fact, calcification on the shelf is active during interglacial highstands (Kinsey & Hopley, 1991; Milliman, 1993; Milliman & Droxler, 1996) and the Red Sea contributes to reducing the oceans' capacity to hold C02• However, basinal high­ salinity conditions during the last glacial period prevented the establishment of biogenic carbonate­ factories in both the Red Sea deep-sea (pelagic) and shelf domains (Taviani, 1997a). Thus, carbonate production of the Red Sea did not contribute to sequester any significant amount of carbonate dur­ ing a part at least of the last glacial period, and this has been the case also for the Persian Gulf and other reefs of the Indo-West Pacific. The total number of missing calcification-reef factories is possibly signif­ icant enough to be taken into consideration in modelling global carbonate fluxes during the last glacial cycle.

CONCLUSIONS 1 During the last glaciation, the Indo-West Pacific coral reef belt suffered significant faunal turnovers

as documented by comparative studies of last inter­ glacial and modern reef-associated molluscan fau­ nas in the Red Sea, Gulf of Aden, Persian Gulf, western Indian Ocean and Pacific islands. 2 The rate of sea-level change is singled out as the most important factor in controlling the fate of Quaternary coral reefs. As these disturbances oc­ curred regularly throughout the late Cainozoic, one may hypothesize that they are responsible for mul­ tiple high-frequency faunal rearrangements in coral reef ecosystems through time (Paulay, 1991). 3 The habitat fragmentation within the Indo-West Pacific region during the Quaternary ice ages may promote speciation by severing gene flow during times of relative lowstands. The core of the Caino­ zoic Indo-West Pacific is probably acting as a refuge preserving older species during glacial times, thus enhancing an increase in biodiversity. 4 The significant reduction of coral reefs during the last glaciation should be considered when modelling the global C02 cycle.

ACKNOWLEDGEMENTS

CNR (Italian National Research Council) and EC grants SCI *CT91-0719, ERB SCI*CT92-0814 (RED SED) and EV 5V-CT94-0447 (TESTREEF) provided funding to investigate the Red Sea and the Western Indian Ocean. A. Crame, W.C. Dullo, L. Montaggioni, G. Paulay and J. Taylor are gratefully acknowledged for useful discussions on this topic and for providing references. P. Bart, G. Camoin, A. Droxler, B. Thomassin and J. Wise critically re­ viewed the paper and offered useful comments. Drawings were produced by G. Zini. This is IGM Scientific Contribution 1058 and TESTREEF Con­ tribution 18.

REFERENCES

BARRON, E.J. ( 1995) Tropical climate stability and impli­ cations for the distribution of life. In: Effects of Past Global Change on Life. Stud. Geophys., National Acad­ emy Press, Washington, DC, 108- 117. BELASKY, P. ( 1996) Biogeography of Indo-Pacific larger foraminifera and scleractinian corals: a probabilistic approach to estimating taxonomic diversity, faunal similarity, and sampling bias. Palaeogeogr. Palaeocli­ matol. Palaeoecol., 122, 119- 141. BERGER, W.H. (1982) Increase in carbon dioxide in the atmosphere during deglaciation: the coral reef hypothe­ sis. Naturwissenschaften, 69, 87-88.

Stable tropics not so stable BOECKLEN, W.J. & SIMBERLOFF, D. (1986) Area-based

extinction models in conservation. In: Dynamics of Extinction (Ed. Elliott, D.K.), pp. 247-276. John Wiley, New York. BORRI, M., SABELLI, B. & TAVIANI, M. (1982) Cypraea (Talparia) exusta Sowerby, 1832, from the central Red Sea: new data. Lav. Soc. Malacol. It., 103-107. BRIGGS, J.C. (1995) Global Biogeography. Developments in Paleontology and Stratigraphy, 14. Elsevier, Amster­ dam. COLES, S.L. & FADLALLAH, Y.H. ( 1991) Reef coral survival and mortality at low temperatures in the Arabian Gulf: new species-specific lower temperature limits. Coral Reefs, 9, 231-237. CRAME, J.A. (1986) Late Pleistocene molluscan assem­ blages from the coral reefs of the Kenya coast. Coral Reefs, 4, 183-196. DULLO, W.C. (1987) Fazies und Fossile Ueberlieferung der

Pleistozaen Riffterrassen and der Ostkueste des Roten Meeres. Habilitation Thesis, University of Erlangen, Germany.

EDINGER, E.N. & RISK, M.J. (1994) Oligocene-Miocene

extinction and geographic restriction of Caribbean cor­ als: roles of turbidity, temperature, and nutrients. Palaios, 9, 576-598. FAGERSTROM, J.A. ( 1983) Diversity, speciation, endemism and extinction in Devonian reef and level-bottom com­ munities, eastern North America. Coral Reefs, 2, 65-70. FAGERSTROM, J.A. (!987) The Evolution of Reef Commu­ nities. John Wiley, New York. GVIRTZMAN, G., BUCHBINDER, B., SNEH, A., NIR, Y. & FRIEDMAN, G.M. (1977) Morphology of the Red Sea fringing reefs: a result of erosional pattern of the last-glacial low-stand sea level and the following Ho­ locene recolonization. Mem. BRGM, 89, 480-491. HAFFER, J. (1969) Speciation in Amazonian forest birds. Science, 165, 131-137. HALLAM, A. (1989) The case for sea-level change as a dominant causal factor in mass extinction of marine invertebrates. Phil. Trans. R. Soc. London, Ser. B, 325, 437-455. HEAD, S.M. (1987) Introduction. In: Key Environments: the Red Sea (Eds Edwards, A.J. & Head, S.M.), pp. 121. Pergamon Press, Oxford. HusTON, M.A. (1994) Biological Diversity. Cambridge University Press, Cambridge. IVANOVA, E.V. (!985) Late Quaternary biostratigraphy and paleotemperatures of the Red Sea and the Gulf of Aden based on planktonic foraminifera and pteropods. Mar. Micropaleontol., 9, 335-364. JABLONSKI, D. (1985) Marine regressions and mass extinc­ tions: a test using the modern biota. In: Phanerozoic Diversity Patterns (Ed. Valentine, J.W.}, pp. 335-354. Princeton University Press, Princeton, NJ. JABLONSKI, D. ( 1986) Causes and consequences of mass extinctions: a comparative approach. In: Dynamics of Extinction (Ed. Elliott, D.K.), pp. 183-229. John Wiley, New York. JACKSON, J.B.C. (1991) Adaptation and diversity of reef corals. BioScience, 41, 475-482. JOHNSON, K.G., BUDD, A.F. & STEMANN, T.A. (1995) Extinction selectivity and ecology of Neogene Carib­ bean reef corals. Paleobiology, 21(1), 52-73.

75

KAUFFMANN, E.G. & FAGERSTROM, J.A. (1993) The Phan­

erozoic evolution of reef diversity. In: Biological Diver­ & Schluter, D.), pp. 315-329. University of Chicago Press, Chicago. KAUFMAN, L. (1993) Why the Ark is sinking. In: The Last Extinction (Eds Kaufman, L. & Malory, K.), pp. 1-46. MIT Press, Cambridge, MA. KINSEY, D.W. & HOPLEY, D. (1991) The significance of coral reefs as global carbon sinks. Response to Green­ house. Palaeogeogr. Palaeoclimatol. Palaeoecol., 89, 363-377. KLAUSEWITZ, W. (1983) Die Entwicklung des Roten Meeres und seiner Kustenfische. I. Evolutionszentrum. Natur. u. Museum, 113 (4), 103-111; II. Palaeogeogra­ phie, Palaeoekologie und Endemitenentwicklung. Natur u. Museum, 113(12}, 349-368. KNOWLTON, N. (1993) Sibling species in the sea? Ann. Rev. ecol. Syst., 24, 189-216. KNOWLTON, N. & JACKSON, J.B.C. (1994) New taxonomy and niche partitioning on coral reefs: jack of all trades or master of some? TREE, 9, 7-9. KOHN, A.J. (1980) Conus kahiko, a new Pleistocene gastropod from Oahu, Hawaii. J. Paleontol., 54, 534541. LAMBECK, K. (1996) Shoreline reconstructions for the Persian Gulf since the last glacial maximum. Earth planet. Sci. Lett., 142, 43-57. LOYA, Y. (1976) Recolonization of the Red Sea corals affected by natural catastrophes and man-made pertur­ bations. Ecology, 57, 278-289. LOYA, Y. (1990) Changes in a Red Sea coral community structure: a long-term case history study. In: The Earth in Transition (Ed. Woodwell, G.M.), pp. 369-384. Cam­ bridge University Press, Cambridge. LYNCH, J.D. (1988) Refugia. In: Analytical Biogeography (Eds Myers, A.A. & Giller, P.S.), pp. 311-342. McGLADE, J.M. (1990) The impact of global climate change on marine ecosystems. In: Global Climate and Ecosystem Change (Eds MacDonald, G.J. & Sertorio, L.}, pp. 165-184. Plenum, New York. McLAREN, D.J. (1986) Abrupt extinctions. In: Dynamics of Extinction (Ed. Elliott, O.K.}, pp. 37-46. John Wiley, New York. MAYR, E. (1982) Processes of speciation in animals. In: Mechanisms of Speciation (Ed. Barigozzi, C.), pp. 1-19. A.R. Liss, New York. MILLIMAN, J.D. (1993). Production and accumulation of calcium carbonate in the ocean: budget of a nonsteady state. Global biogeochem. Cycles, 7, 927-957. MILLIMAN, J.D. & DROXLER, A. (!996) Neritic and pelagic carbonate sedimentation in the marine environment: ignorance is not bliss. Geol. Rundsch., 85, 495-504. Mix, A.C. ( 1996) CLIMAP-style faunal transfer-function paleotemperatures re-assessed: cooler Ice-Age Tropics? Am. geophys. Union 1996 Spring Meeting, Program with abstract, 31A-3, S!57. NEWELL, N.D. (1971) An outline history of tropical or­ ganic reefs. Am. Mus. Novit., 2465, 1-37. OPDYKE, B.N. & WALKER, J.C.G. ( 1992) Return to the coral reef hypothesis: basin to shelf partitioning of CaC03 and its effects on atmospheric C02. Geology, 20, 733-736. PAULAY, G. (1990) Effects of late Cenozoic sea-level

sity (Eds Ricklefs, R.E.

76

M. Taviani

fluctuations on the bivalve faunas of tropical oceanic islands. Paleobiology, 16, 415-434. PAULAY, G. (1991) Late Cenozoic sea level fluctuations and the diversity and species composition of insular shallow water marine faunas. In: The Unity of Evolu­ tionary Biology (Ed. Dudley, E.C.), pp. 184-193. Dio­ scoroides Press, Portland. PAULAY, G. (1996a) Circulating theories of coral biogeog­ raphy. J. Biogeogr., 23, 279-282. PAULAY, G. (1996b) Dynamic clams: changes in the bivalve fauna of Pacific islands as a result of sea level fluctuations. Am. malacol. Bull., 12, 45-57. PoR, F.D. (1975) Pleistocene pulsation and preadaptation of biotas in mediterranean seas: consequences for Lessepsian migration. System. Zoo!., 24, 72-78. PoR, F.D. (1989) The Legacy of Tethys. Kluwer Academic, Dordrecht. RosEWATER, J. (1965) The family Tridacnidae in the Indo-Pacific. Indo-Pacific Mollusca, 1, pp. 347-396. SEWELL, S.R.B. ( 1948) The free-swimming planktonic Copepoda: geographical distribution. John Murray Exp. sci. Reps., 8, 317-595. SMITH, S.V. & BUDDEMEIER, R.W. (1992) Global change and coral reef ecosystems. Ann. Rev. ecol. Syst., 23, 89-118. STANLEY, S.M. (1984) Temperature and biotic crises in the marine realm. Geology, 12, 205-208. STANLEY, S.M. (1986) Population size, extinction, and speciation: the fission effect in Neogene bivalves. Paleo­ biology, 12, 89-110. STANLEY, S.M. & RuDDIMAN, W.F. (1995) Neogene ice age in the North Atlantic region: climatic changes, biotic effects, and forcing factors. In: Effects of Past Global Change on Life. Stud. Geophys., National Academy Press, Washington, DC, 118-133. STODDART, D.R. (1973) Coral reefs: the last two million years. Geography, 58, 313-323. STODDART, D.R. (1976) Continuity and crisis in the reef

community. Micronesica, 12, 1-9.

STONE, L. (1995) Biodiversity and habitat destruction: a

comparative study of model forest and coral reef eco­ systems. Proc. R. Soc. London, Ser. B, 261, 38 1-388. TAVIANI, M. (1982) Paleontological markers in late Pleis­ tocene raised coral reefs in the Red Sea. XI INQUA Congress, Moscow, Abstracts, 1, 307. TAVIANI, M (1997a) Axial sedimentation of the Red Sea transitional region (22 -25 "N): pelagic, gravity flow and sapropel deposition during the late Quaternary. In: •

Sedimentation and Tectonics of Rift Basins: Red Sea­ Gulf of Aden (Eds Purser, B.H. & Bosence, D.W.J.), pp. 47 1-482. Chapman and Hall, London.

TAVIANI, M. ( 1997b) Post-Miocene coral reefs of the Red

Sea: Glacio-eustatic controls. In: Sedimentation and Tectonics of Rift Basins: Red Sea-Gulf of Aden (Eds

Purser, B.H. & Bosence, D.W.J.), pp. 580-588. Chap­ man and Hall, London. TAYLOR, J.D. (1978) Faunal response to the instability of reef habitats: Pleistocene molluscan assemblages of Aldabra atoll. Palaeontology, 21, 1-30. VERMEIJ, G.J. ( 1986) Survival during biotic crises: the properties and evolutionary significance of refuges. In: Dynamics of Extinction (Ed. Elliott, D.K.), pp. 231246. John Wiley, New York. VERON, J.E.N. (1995) Corals in Space and Time. The Biogeography and Evolution of the Scleractinia. Univer­ sity of New South Wales Press, Sydney. WHITTAKER, R.J. ( 1995) Disturbed island ecology. TREE, 10, 42 1-425. WILKINSON, B.H. (1991) Coral reefs of the world are facing widespread devastation: can we prevent this through sustainable management practices? Proceedings of the 7th International Coral Reef Symposium 1, 11-21. WISE, K.P. & SCHOPF, T.J.M. ( 1981) Was marine faunal diversity in the Pleistocene affected by changes in sea level? Paleobiology, 7, 394-399.

Spec. Pubis int. Ass. Sediment. (1998) 25, 77-92

Sedimentary cycles in carbonate platform facies: Fourier analysis of geophysical logs from ODP Sites

865 and 866

P. COOPER

D epartment ofG eology andG eophysics, Univ ersity of Hawaii at Manoa, Bachman Hall lOS, 2444 Dol e Str e et, Honolulu, HI 96822, USA

ABSTRACT Shipboard analysis of recovered core materials at Ocean Drilling Program Sites 8 6 5 (Allison Guyot) and 866 (Resolution Guyot) suggested that rhythmic repetition of shallowing-upwards facies on several scales occurred within certain depth intervals. Poor recovery at these sites (averaging less than 16%) prevented visual confirmation of what were suspected to be Milankovitch cycles. In a previous study, Formation MicroScanner images from the sites were integrated with conventional log data and core descriptions to provide detailed stratigraphical columns. These detailed data confirmed shipboard conclusions regarding the presence of sedimentary cycles and provided details of facies changes that could be related to small-scale fluctuations in sea-level. Spectral analysis of the geophysical logs, which provide a continuous downhole record of lithological changes, was performed in an attempt to detect the presence of Milankovitch periodicities. For the initial analysis, a sample window was moved down the data at 3-m depth increments resulting in a spectrogram-like image. Imaging the entire length of the logged interval in this way revealed the presence of periodicities in the log data for the depth interval 250 mbsf (metres below sea-floor) to 490 mbsf in Hole 865 and for the intervals from 430 to 670 and from 9 3 5 to 1135 mbsf in Hole 866. Individual spectra within these selected intervals were then used to identify Milankovitch periodicities. At Site 8 6 5 , age controls were insufficient to provide time constraints for the logged interval ( 1 02 . 5-867.0 mbsf). However, the entire depth interval is probably of late Albian age. The strong cyclicity in the depth interval from 250 to 490 mbsf is mainly controlled by variations in porosity. Sedimentation rate and clay content increase slightly downhole. Fourier analysis of the gamma-ray and resistivity logs revealed high-amplitude spectral peaks corresponding to Milankovitch periodicities attributed to eccentricity and obliquity. The 4 1 3-kyr peak dominates all spectra; the spectral peak corresponding to the 123-95 kyr Milankovitch period has high amplitude for the interval from 330 to 490 mbsf. Vertical resolution was insufficient to resolve Milankovitch periodicities related to preces­ sion. At Site 866, the logged interval (78 .0-16 79.4 mbsf) corresponds to a time interval of about 3 3 Myr. Geophysical logs from two depth intervals showed strong cyclicity: 430-670 and 9 3 5-11 3 5 mbsf. Sedimentation rate is highly variable and increases downhole from about 30 m Myr-1 in the middle-to-late Albian section to about 80 m Myc1 in the Barremian section. Vertical resolution was insufficient to resolve frequencies corresponding to precession; the 41-kyr obliquity peaks are resolvable only in the lower portion of Hole 866A. The 413-kyr eccentricity cycle dominates all spectra. At both sites, the 123-9 5-kyr cycles could be related to alternations between dense wackestones and more porous packstones.

BACKGROUND

Early analyses of time series of climate-sensitive indicators in sediments and sedimentary rocks (e.g. Hays et a!., 1976) provided evidence that a major portion of climate variation is driven by changes in

insolation in response to perturbations in the Earth's orbital variables (Milankovitch, 1941) . Al­ ternative explanations for observed quasi-periodic climate changes have been presented in the litera-

Reefs and Carbonate Platforms in the Pacific and Indian Oceans Edited by G. F. Camoin and P. J. Davies © 1998 International Association of Sedimentologists ISBN: 978-0-632-04778-9

77

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P. Cooper

ture (e.g. Muller & MacDonald, 1995). Neverthe­ less, the quasi-periodicities of variations in orbital eccentricity (413 and 123-95 kyr), variations in obliquity or tilt ( 41 kyr), and variations in preces­ sion (23-19 kyr) are commonly observed in analy­ ses of cyclical sedimentation in a variety of sediment types and in many stratigraphical inter­ vals. Exactly how changes in the insolation patterns are translated into climatic changes remains the subject of extensive research. Because the magnitude of the change in insolation caused by the precession-obliquity-eccentricity cycles is so small (Berger, 1977), models that seek to explain climate forcing in terms of variations in in­ solation include atmospheric-oceanic feedback mechanisms that amplify the magnitude of the re­ sultant climate changes (Berger et a/., 1984; Barron et al., 1985; Sundquist & Broecker, 1985; Prell & Kutzbach, 1987; Sarnthein et al., 1988). For exam­ ple, Berger et al. ( 1984), Start & Prell ( 1984), & Ruddiman and Mcintyre ( 1984) have placed great emphasis on ice-sheet amplification of the relatively weak Milankovitch signals. Although the volume of polar ice caps during the Cretaceous-the time frame of this study-is not known accurately (Kemper, 1987), estimates sug­ gest that the growth patterns of ice caps with even greatly reduced volume would cause eustatic fluctu­ ations of the order of several metres, sufficient to cause emergence cycles at carbonate platforms (Hardie et al., 1986). The persistence of Milanko­ vitch periodicities through the Cretaceous, a time of warm seas and possibly minimal ice cover (Barron & Washington, 1985), implies that, whereas the presence of ice sheets may be an important factor in the modulation of global climate, their presence is not required for such modulation to occur (Fisher et al., 1985; Herbert et al., 1986). A variety of indicators is used to detect the presence of Milankovitch periodicities. Early stud­ ies of Quaternary pelagic sediments utilized varia­ tions in the percentage of calcium carbonate content and oxygen isotope ratios (e.g. Briskin & Harrell, 1980) for detecting Milankovitch periodic­ ities of the order of I 00 kyr. Oxygen isotope ratios for benthic marine foraminifers, as well, reflect global ice volume; numerous published studies have shown that Quaternary glacial regimes are related to orbital periodicities (Imbrie et al., 1984). Confirmation of the effects of climatic forcing in sediments of pre-Quaternary age extends over a range of sedimentary systems and depositional en-

vironments. Mean periodicities of 20, 40 and I 00 kyr have been detected for carbonate-marl deposits in the Cretaceous and lower Tertiary of the Apennines (e.g. Arthur & Fischer, 1977), in pelagic sediments drilled in the open ocean (e.g. Dean et a/. , 1977; Arthur, 1979; McCave, 1979), in Triassic shallow marine carbonates in the northern Alps (e.g. Fischer, 1964; Schwarzacher, 1964) and in Eocene (Bradley 1929), and Triassic lacustrine sediments (e.g. Van Houten, 1964; Olsen et al., 1978) in the USA. Climate indicators most useful for these sediment types include variations in porosity or grain size, the relative abundance and mineralogy of clays, and variations in one or more of the bioge­ nous components. Previous studies of geological time series from Cretaceous deep-ocean sediments (e.g. McCave, 1979; Arthur et al., 1984; Cotilion & Rio, 1984; de Graciansky & Gillot, 1985; Herbert & Fischer, 1986) revealed similar strong power in the eccentricity and precession peaks, regardless of the ocean of origin, indicating that this is truly a global signature that persists through geological time. Spectral analysis of logging data

Spectral analysis has proved to be a powerful method for extracting the dominant periodicities from climatic signals preserved in the sediments. The spectrum is calculated by taking the Fourier transform of a geological time series-some mea­ sure of the amplitude of climate change such as porosity plotted versus depth, which is used as a proxy for time. Such a spectral analysis can separate the dominant frequencies present in the data, giving them as cycles per unit depth. If sedimentation rates are well known, the frequencies can be converted to cycle periods. Spectral analysis works best in geo­ logical settings that contain a relatively continuous record of sedimentation. Jarrard & Arthur ( 1989) first explored the feasi­ bility of using spectral analysis of downhole geo­ physical logs to detect cyclic changes in mineralogy and porosity in Pleistocene sediments from Ocean Drilling Program (ODP) Sites 645 in the Labrador Sea and 646 in Baffin Bay. Their analysis revealed periodicities of roughly 20, 40 and 100 kyr in the sonic, resistivity and U/Th logs. They reasoned that fluctuations in bottom-water currents in response to Milankovitch forcing caused the observed varia­ tions in clay content and porosity. Amplitude spec­ tra of gamma-ray, sonic, and resistivity logs in upper Tertiary sediments at Site 693 on the Antarc-

S edimentary cycl es in carbonat e platformfaci es tic continental margin yielded obliquity and eccen­ tricity cycles (Golovchenko et a!., 1990). Obliquity and possibly eccentricity cycles also were observed in amplitude spectra of calcium and silica from Site 704 on Meteor Rise (Mwenifumbo & Blangy, 1991; Nobes et a!., 199 1). The results of an analysis of the natural gamma-ray log from Site 798 on the Oki Ridge suggested obliquity modulation of the Pliocene-Pleistocene aeolian dust influx to the Sea of Japan (DeMenocal et a!., 1992). Molinie & Ogg ( 1992) reported cycles of variable concentrations of radiolarians and clay, and a degree of silicification in upper Middle Jurassic to Lower Cretaceous radiolarites from spectral analysis of gamma-ray logs from Site 801. They used the wavelength of the eccentricity-modulated signals to determine sedi­ mentation rates. Glenn et a!. ( 1993) investigated the translation of climate modulation by Milankovitch­ like forcing into sedimentary cycles in mixed car­ bonate and siliciclastic sediments using sonic logs from Site 82 1 off the Great Barrier Reef, Austra­ lia. Climate modulation at that site incorporates such factors as lags in ocean-climate response to changes in insolation related to variations in orbital parameters, as well as independent changes in sediment supply (e.g. turbidites) and tectonic sub­ sidence. ODP site settings

This study uses two sets of geophysical logs obtained during Leg 143 at Allison Guyot (Site 865) in the central Mid-Pacific Mountains and at Reso­ lution Guyot (Site 866) in the western Mid-Pacific Mountains. Deep holes were drilled into the Creta­ ceous lagoonal facies of Allison and Resolution guyots to address fundamental problems concern­ ing guyot development (Sager et a!., 1993a). Hole 865A (Fig. 1), atop Allison Guyot, was drilled between the summit of the pelagic cap and the south rim of the guyot. The hole penetrated 140 m of pelagic cap and 698 m of late Albian shallow-water limestone, and bottomed at 870.9 mbsf (metres below sea-floor) in basaltic sills in­ truded into limestone. Clay, organic matter and the presence of pyrite in sediments from the lower part of the hole indicate a marsh-like setting with restricted-lagoonal environments in the upper part (Sager et a!., 1993b). Hole 866A (Fig. 1), atop Resolution Guyot, 7 16 km to the north-west of Site 865, was located about 1 km inboard, within a trough behind the perimeter mound. This deep hole

79

penetrated 1743.6 mbsf, through 25 m of pelagic sediments and 1620 m of Albian to Hauterivian shallow-water limestones overlying about 124 m of basalt. Dolomitized oolitic and oncolitic grain­ stones cover the volcanic basement and give way at 1400 mbsf to peritidal facies containing minor coral and rudist reef debris and beach sediments. Clusters of calcrete horizons are present in the lagoonal carbonates of the upper part of the platform (from about 680 mbsf ) (Sager et al., 1993c). Although average recovery was very low ( 15. 1o/o for Hole 865A, 15.4% for Hole 866A), cores and downhole logs suggested cyclic sedimentation on a metre-scale, particularly in the lagoonal facies of the upper part of Hole 866A (430-670 mbsf). Within this depth range, recurring metre-scale cycles typi­ cally began as laminated organic-rich mudstones and graded upward into bioturbated, less organic­ rich packstones and grainstones, and finally into wackestone with mouldic porosity. Facies from the lower part of Hole 866A (935-1 165 mbsf ) sug­ gested cyclical changes of the depositional environ­ ment from subtidal to intertidal-supratidal and returning to subtidal. Desiccation cracks and calci­ fied algal mat at the base of a typical sequence indicated emergence of the platform during low relative sea-level. A relative rise in sea-level resulted in reworking of the mudstone and algal mat as a flat-pebble conglomerate followed by deposition of a shallowing-upwards facies as the carbonate accu­ mulation rate outpaced the relative rise in sea-level. Many sequences are incomplete. In general, few peritidal sequences contain a complete record of climatic fluctuations because only high-amplitude sea-level changes will flood the peritidal platforms. Similar multimetre-scale peritidal sequences were identified by James ( 1977) and Shinn ( 1983) in both ancient and recent carbonate deposits. The analysis by Goldhammer et al. ( 1987) of peritidal facies from the Italian Triassic revealed a very complete record of the c. 20-kyr signal in the form of metre-scale peritidal sequences with vadose dia­ genetic, dolomitic caps. Many investigators (e.g. Gretzinger, 1986; Hardie et a!. 1986; Strasser, 199 1) have attributed such sequences to sea-level fluctuations related to climatic forcing as described by Milankovitch ( 1941). Much longer cycles (50-100 m) were revealed in laboratory analyses of MnO, Zn and Cu in core samples from Site 866 in the interval from 680 to 1400 mbsf (Rohl et al., 1995). Although these geochemical cycles are based on from four to six

80

P. Cooper

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samples per core, the trends are clear, and addi­ tional samples would not change the cycle lengths significantly. They are thought to correlate with packets of cycles seen in the lithology. A strong cyclicity was noted in the openhole log responses for Hole 865A from 250 to 490 mbsf (Fig. 2) and for Hole 866A from 430 to 664 and from 935 to 1 165 mbsf (Fig. 3). Further, the por­ tions of the geophysical logs displaying this cyclical character at both sites corresponded almost exactly to cored intervals displaying cyclic variations in lithologies. The high degree of correlation between the sonic and resistivity logs (Fig. 4) suggested that porosity variations were the dominant effect; how­ ever, it was difficult to confirm porosity variations and determine cycle length in the core material because of the poor recovery rate. A post-cruise study of the core, conventional log and Formation MicroScanner (FMS) log yielded a detailed litholog-

ical column that clearly revealed the nature of the cyclical sedimentation (Cooper et a!., 1995). Spectral analysis of the resistivity and gamma-ray logs from Sites 865 and 866 and the sonic log from Site 865 was undertaken (i) to investigate a possible relationship between the observed sedimentary rhythms and climatic forcing and (ii) to speculate on a possible cause.

DATA A standard suite of logging runs was made within the open borehole in Hole 865A between 100.5 and 867.0 mbsf and in Hole 866A between 74.5 andl 1679.4 mbsf or less (at Hole 866A the bottom of the: hole was not logged because of cave-ins at or above: 1680 mbsf; Sager et a!., 1993a). The sonic, medium-induction resistivity and natural gamma-

Sedimentary cycles in carbonate platform facies Hole 865A Gamma (API units) 100

0

Log resis.

Sonic (km/s)

% Recov.

70

200

81

the natural radioactivity of the formation and pro­ vides a qualitative evaluation of the clay or shale content, as radioactive elements tend to be concen­ trated in the crystal lattice of clay minerals. Cycle skipping and other noise caused by lack of centring of the tool compromised the quality of the sonic logs at both holes. The sonic log for Hole 866A was not used at all; the reprocessed sonic log for Hole 865A was used for the 4 10-490-m interval because no resistivity data were available for that interval. Resolution

300

400 :;:"' .0

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500

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900L------L--�--�� Fig. 2. Gamma-ray, resistivity and sonic logs for Hole 865A. Fourier analysis was performed on the stippled portions of the data. The resistivity and sonic logs suffered data losses from mechanical failure and noise contamination, respectively. (Note resistivity log scale is logarithmic.)

ray logs were used in this study. The sonic log responds to lithology and porosity; formation veloc­ ity is calculated by taking the inverse of the sonic log. The resistivity log responds primarily to poros­ ity variations. The natural gamma-ray log measures

Vertical resolution is the major limitation to using the downhole logging data for this purpose (Jarrard & Arthur, 1989; deMenocal et al., 1992); that is, the resolution and sampling interval of the tool, com­ bined with a particular sedimentation rate, deter­ mines the minimum detectable cycle. For example, the resistivity log has a 1-m vertical resolution (Schlumberger, 1987) and requires a minimum sedimentation rate of 20 m Myc 1 to detect the 95-, 123-, and 413-kyr eccentricity signals (because Fou­ rier analysis requires at least two points per cycle). Similarly, for 1-m resolution a sedimentation rate of 100 m Myc1 is required to detect precessional signals (23 and 19 kyr). Thus, the sedimentation rate and vertical tool resolution combine to act as a high-cut filter. The natural gamma-ray intensity logging tool has a resolution of 0.3 m and the sonic log has a resolution of 0.6 m (Schlumberger, 1987). Slight spectral contamination could exist near the Nyquist frequency because the sampling interval (0. 15 m) is smaller than the vertical resolution of the logs; however, that is outside the frequency range of interest here. Although the gamma-ray spectra possess more power at higher frequencies, the gamma-ray tool also is affected by residual noise that degrades the low-frequency resolution (de­ Menocal et al., 1992). As mentioned above, the response of the deposi­ tional environment to astronomical climate forcing is quasi-periodic in time but the data recorded by the logging tool are periodic in depth. As long as the sediment accumulation rate is constant over long time periods, then the cyclic variation of physical properties with depth (depth series) will approxi­ mate a variation with time (time series). Spectral analysis of a depth series at constant accumulation rate yields temporal cycle frequencies measured in cycles per Fourier window length. If the sedimenta­ tion rate and, therefore, the spatial wavelength, of

·

82

P. Cooper (a)

(b)

Hole 866A

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rhythmic sedimentation have Milankovitch peri· ods. Application of alternative methods of identifi­ cation of Milankovitch periodicities in the absence of suitable biostratigraphical control provided con· vincing evidence that the peaks observed in spectra of log data from Hole 865 were Milankovitch periodicities. Several questions arise as to what is the mechanism or origin of the cycles, how are forced climate changes translated into lithologies, and what is the significance of the sedimentary cycles of various thicknesses, great and small. Evidence of subaerial exposure strongly indicates sea-level oscillations of either eustatic or tectonic origin or some combination of the two as the mechanism for these Cretaceous carbonate plat· form cycles (Fischer, 1964; Grotzinger, 1986; Har­ die et a!., 1986; Strasser, 1991). Deposition of carbonates in a platform environment is primarily a function of space. The sedimentation rate is highly dependent on eustatic sea-level and the rate of tectonic subsidence. Variation in productivity may be a small factor, but is probably not an important cause of the observed cyclicity. In the lower part (> 600 m) of Hole 866A weath­ ering of the volcanic edifice was the main source of clay for the surrounding lagoons (Sager et a!., 1993c). Variations in clay supply may have been climatically controlled and a quiet, shallow­ lagoonal environment would have encouraged the near-source deposition of the clays derived from weathering of the volcanic edifice. With increasing water depth, circulation within the lagoonal envi·

1200

Fig. 10. Plot of spatial frequency of 123-kyr spectral peaks versus depth for a 40-m sliding window. The data window for Fourier analysis was centred first on 9 5 0 mbsf and moved downwards in increments of I 0 m. The shaded portion indicates a range in sedimentation rates of 758 7 m Myr-1 that brackets most of the time-depth period covered by the calculations. Excursions to lower sedimentation rates are seen at I 060-1070 mbsf.

ronment is more energetic, removing the fine sedi­ ment fraction and concentrating the coarse-grained components. A probable cause of the cyclic fluctu· ations in resistivity (porosity) and gamma-ray val· ues observed at these sites is increased deposition of clays during lowstands, and winnowing or transport of clays during highstands. The porosity-sensitive resistivity logs usually an­ ticorrelate with the gamma-ray logs. Clay content increases downhole and, in general, increased clay content, indicated by elevated gamma-ray intensi· ties, is associated with lower velocity and resistivity. Because porosity appears to be the main effect controlling the resistivity and sonic log responses, a likely explanation of these cycles is that they reflect climatic controls on sea-level, which, in turn, con· trois clay supply and sorting. Deposition in shallow lagoons is affected by (i) storms, (ii) changes in current patterns and migra· tion of tidal channels and (iii) short-term eustatic and tectonic sea-level fluctuations. Thus, superim­ posed on the weak Milankovitch 'signals' is some 'noise' related to depositional conditions, and both signal and noise may be affected by differential compaction and diagenetic overprinting. Small, multimetre-scale shallowing-upwards cycles are es· pecially well developed in the protected platform interior, lagoonal-peritidal settings, where typical cycles consist of mudstone grading up to grainstone or packstone-wackestone. Boundaries of small cy· cles from Hole 866A are commonly marked by calcified algal mat, bird's eye vugs and desiccation

Sedimentary cycl es in carbonate pl atform facies features indicating repeated exposure to subaerial diagenesis. Integration of core, FMS and conven­ tional logging data (Cooper et a!., 1995) indicate thicknesses for these shallowing-upwards cycles that range from 0.8 to 28.4 m at Hole 865 and from 0.3 to 1 1.2 m at Hole 866A (upper portion). Not all cycles are complete; migrating tidal channels, cur­ rents and storms inevitably erode the previously deposited sediments. Core materials recovered from nearby Sites 867 and 868 show evidence of storm deposition. Mean thicknesses of 5.6 and 3.4 m were deter­ mined for the shallowing-upwards sequences of Hole 865A and the upper portion of Hole 866A, respectively, from an analysis of the FMS logs (Cooper et a!.; 1995). These thicknesses are compa­ rable with the wavelengths of the E2-E3 cycles for Hole 865A and upper portions of Hole 866A-7.9 and 6.2 m, and 4. 1 and 3. 1 m, respectively. The mean thickness of shallowing-upwards sequences, as determined from the FMS log in the lower portion of Hole 866A (Cooper et a!., 1995), is 3.4 m, comparable with the observed 3.2-m spatial wavelength of the 4 1-kyr obliquity cycle. This sug­ gests that the 123-95-kyr eccentricity cycle is rep­ resented lithologically by packets of rhythmic packstone-wackestone alternations, whereas the 4 1-kyr obliquity cycle consists of some smaller portion of the packet. Three possible reasons for the absence of the 4 1-kyr signal in data from Hole 865A and from the upper portion of Hole 866A are (i) lack of spatial resolution, (ii) highly variable sedimentation rate, or (iii) very low amplitude. Sedimentation rates at Hole 865A (c. 65 m Mye1) and in the lower portion of Hole 866A (c. 77 m Myr-1 ) are high, so spatial resolution should be comparable at both sites. Deviations from the mean apparent sedimentation rate are also comparable at both sites. The major difference between the lower portion of Hole 866A and the upper portion of Hole 866A, and Hole 865A, is the greater abundance of clay below 600 mbsf. It is speculative, but the additional clay and organic matter from the eroding Resolution volcanic edifice may have amplified the obliquity signal. The distribution of clay seams and packets of clay seams throughout the core in the bottom portion of Hole 866A provides evidence of higher­ frequency climatic fluctuations; however, the sedi­ mentation rate is not high enough at this site to allow for resolution of spectral peaks shorter than the 4 1-kyr cycle.

89

The small cycles are grouped into larger sequences that indicate a more long-term cyclic deepening and shallowing of the depositional environment (Arnaud et a!., 1995). The distribution of MnO, Zn and Cu (Rohl & Strasser, 1995) displays cycles of the order of 50-100 m in length in the lower portion of Hole 866A. Peaks in the MnO content appear to be linked with facies deposited under the most restricted conditions and probably correspond to periods of maximum flooding (e.g. sequence boundaries 5, 7, 10, and 15 of Arnaud et a!. ( 1995)). The spatial wave­ lengths of these cycles are longer than the predicted wavelength for the 4 13-kyr eccentricity cycles (about 3 1 m). Cycles having periods much longer than 413 kyr are not resolved in the spectra because of the dominance of the 4 13-kyr peak, the low-cut filtering effect of the windowing process, and standardiza­ tion of the signal. Very long, irregularly spaced cycles of lowstand to highstand with distinctive dia­ genetic horizons are obvious in the core and logging data (Arnaud et a!., 1995; Cooper et a!., 1995), and, although their lengths are highly variable, they represent a likely explanation of the geochemical cycles observed by Rohl & Strasser ( 1995). A good deal of variation in the thickness of cycles exists between Site 865 and the Albian-age portion of Site 866, so there remains the possibility that non-periodic processes influenced cycle develop­ ment at both the local and regional scales. Spectral maxima that are not near the predicted locations of peaks may have a non-orbital source for the cyclic­ ity or may reflect a peak shift owing to change in sedimentation rate. Most likely, occasional collapse of sections of beach mound or fringing reef that normally protect the lagoon from wave energy can expose the lagoon to open-ocean conditions. The cycling of such events is not entirely random, because they involve lateral transport and build-up of carbonate debris, slope destabilization and col­ lapse. Variations in tectonic subsidence rate probably represent a major non-periodic contribution to cycle development. Rapid sediment accumulation rates of about 80 m Myr-1 during the Barremian slowed to about 30 m Myr-1 during the Albian­ Aptian at Resolution Guyot (Site 866). Accumula­ tion rates at the much younger Allison Guyot (Site 865) were about 65 m Mye1 during the Albian. We may infer from this that regional differences in tectonic subsidence rates exist that depend on the age of the volcanic edifice, and that the local

90

P. Cooper

subsidence rate may not be uniform throughout the growth of an individual platform and contains pulses of uplift as well. The relatively high­ frequency Milankovitch climate signal is, therefore, superimposed on a non-periodic low-frequency tec­ tonic signal. Because the carbonate platforms of the Mid-Pacific guyots are far removed from terrige­ nous influences, the Milankovitch cycling repre­ sents as 'pure' a eustatic signal as can be obtained. Therefore, it may be possible to extract the tectonic signal given excellent biostratigraphical or radio­ metric age control.

sediments as simultaneous vanat10ns in porosity and clay content. Comparison of the resistivity and gamma-ray spectra shows that all the dominant frequencies are common to the two logs. Clay content increases downhole and, in general, in­ creased clay content, indicated by elevated gamma­ ray intensities, is associated with low resistivity. Because porosity appears to be the main effect controlling the resistivity response, an explanation of these cycles is that they reflect climatic controls on sea-level that, in turn, control clay content and sorting.

CONCLUSIONS

ACKNOWLEDGEMENTS

Geophysical logs in the Cretaceous lagoonal carbon­ ate facies of Holes 865A and 866A show pro­ nounced cyclic variations in porosity and clay content. Spectral analysis of the logs revealed dominant peaks having spatial wavelength ratios that matched the ratios of Milankovitch eccentri­ city cycles. Mean sedimentation rates calculated from recon­ naissance spectral analysis of long (200-m) data windows range from 65 m Myc1 for Hole 865A to 33 m Myc ' for the upper portion of Hole 866A, and to 77 m Myc1 for the lower portion of Hole 866A. No sedimentation rates were estimated from shipboard stratigraphy, but these values are consis­ tent with the notion that Allison (Site 865) was a younger volcanic edifice subsiding at roughly twice the rate of Resolution (Site 866) throughout the late Albian. A sliding window spectral analysis of the lower portion of Hole 866A reveals details of fluctuations in sedimentation rates from 75 to 83 m Myc ' over the depth interval from 950 to 1 150 mbsf. A short episode of very low sedimentation rates from 1050 to 1070 mbsf corresponds to a cored interval con­ taining minor grainstones and is interpreted as a short-lived highstand stage. For both Hole 865A and the upper portion of Hole 866A, the average thickness of the mudstone­ wackestone-packstone shallowing-upwards cycles as measured from the interpreted Formation Micro­ Scanner images is approximately equal to the pre­ dicted wavelength of the E2-E3 cycles. No similar match was found for the El cycle. The 0 signal has high amplitude only in the lower portion of Hole 866A, perhaps because of the higher clay content. The Milankovitch periodicities are evident in the

This work was funded by the US Science Advisory Committee and the Joint Oceanographic Institu­ tions. This paper benefited from discussions with Hubert Arnaud, Peter Flood, Ursula R6hl, Will Sager, Annie Vanneau, and Jerry Winterer. SOEST Contribution no. 4532.

REFERENCES ARNAUD, H.M., FLOOD, P.G. & STRASSER, A. ( 1 995) Reso­ lution Guyot (Hole 866A, Mid-Pacific Mountains): facies evolution and sequence stratigraphy. In: Proceed­

ings of the Ocean Drilling Scientific Program, Results, 1 4 3 (Eds Winterer, E.L., Sager, W.W., Firth, J.V. & Sinton, J.M.), pp. 1 3 3- 1 60. Ocean Drilling Program, College Station, TX. ARTHUR, M.A. ( 1 979) North Atlantic Cretaceous black shales: the record at Site 3 9 8 and a brief comparison with some other occurrences. In: Initial Reports of the Deep Sea Drilling Project, 47-2 (Eds Sibuet, J.C. & Ryan, W.B. et a/.), pp. 7 1 9- 7 5 1 . US Goverment Print­ ing Office, Washington, DC. ARTHUR, M.A. & FISCHER, A. G. ( 1 977) Upper Cretaceous­ Paleocene magnetic stratigraphy at Gubbio, Italy: 1 . Lithostratigraphy and sedimentology. Geol. Soc. Am. Bull. , 88, 367-389. ARTHUR, M.A., DEAN, W.E., BOTTJER, D. & SCHOLLE, P.A. ( 1 984) Rhythmic bedding in Mesozoic-Cenozoic pe­ lagic carbonate sequences: the primary and diagenetic origin of Milankovitch-like cycles. In: Milankovitch and Climate (Eds Berger, A., Imbrie, J., Hays, J., Kukla, G. & Salzman, B.), pp. 1 9 1 -222. D. Reidel, Dordrecht. BARRON, E.J. & WASHINGTON, W.M. ( 1 98 5 ) Warm Creta­ ceous climates: high atmospheric C02 as a plausible mechanism. In: The Carbon Cycle and Atmospheric C02: Natural Variations Archean to Present (Eds Sundquist, E.T. & Broecker, W.S.), Geophys. Monogr. Am. geophys. Union, Washington, DC, 32, 546-5 5 3 . BARRON, E.J., ARTHUR, M.A. & KAUFFMAN, E.G. ( 1 98 5 ) Cretaceous rhythmic bedding sequences: a plausible

Sedimentary cycles in carbonate platform facies link between orbital variations and climate. Earth

planet. Sci. Lett., 72, 327-340. BERGER, A.L. ( 1 9 77) Power and limitations of energy balance climate model as applied to the astronomical theory of paleoclimates. Paleogeogr. Paleoclimatol. Pa­ leoecol. 2 1 , 227-2 3 5 . BERGER, A., IMBRIE, J., HAYS, J., KUKLA, G. & SALTZMAN, B. (Eds) ( 1 984) Milankovitch and Climate, Parts l and 2. D. Reidel, Dordrecht. BRADLEY, W.H. ( 1 929) The varves and climate of the Green River. US geol. Surv. Prof Pap. 158-E, 8 7- 1 1 0. BRISKIN, M. & HARRELL, J. ( 1 980) Time-series analysis of the Pleistocene deep-sea paleoclimate record. Mar. Geol. , 36, l -22. COOPER, P., ARNAUD, H.M. & FLOOD, P.C. ( 1 995) Forma­ tion MicroScanner log responses to lithology in guyot carbonate platforms and their implications: Sites 865 and 866. In: Proceedings of The Ocean Drilling Pro­ gram, Scientific Results, 1 43 (Eds Winterer, E.L., Sager, W.W., Firth, J.V. & Sinton, J.M.), pp. 329-372. Ocean Drilling Program, College Station, TX. COTILLON, P.H. & RIO, M. ( 1 984) Cyclic sedimentation in the Cretaceous at DSDP Sites 5 3 5 and 540 (Gulf of Mexico), 534 (central Atlantic), and in the Vicontian Basin (France). In: Initial Reports of the Deep Sea Drilling Project, 7 7 (Eds Buffer, R.T., Schlager, W. et al. ), pp. 3 3 9-376. US Government Printing Office, Washington, DC. DEAN, W.E., GARDNER, J.V., JANSA, L.F., CEPEK, P. & SEIBOLD, E. ( 1 9 77) Cyclic sedimentation along the con­ tinental margin of northwest Africa. In: Initial Reports of The Deep Sea Drilling Project, 4 1 (Eds Lancelot, Y., Seibold, E. et a/. ), pp. 965-989. US Government Print­ ing Office, Washington, DC. DE GRACIANSKY, P.C. & GILLOT, E. ( 1 9 85) Sedimentologic study of mid-Cretaceous carbonaceous limestones at Sites 549 and 5 50, northeast Atlantic. In: Initial Reports of The Deep Sea Drilling Project, 80 (Eds Graciansky, P.C. de & Poag, C.W. et al.), pp. 8 8 5-897. US Govern­ ment Printing Office, Washington, DC. DEMENOCAL, P., BRISTOW, J. & STEIN, R. ( 1 992) Paleocli­ mate applications of downhole logs: Pliocene­ Pleistocene results from Hole 798, Sea of Japan. In:

Proceedings of The Ocean Drilling Program, Scientific Results, 1 2 7/ 1 2 8 (Eds Pisciotta, K.A., Ingle, J.C., Jr, von Breyman, M.T., Barron, J. et al.), pp. 39 3-407. Ocean Drilling Program, College Station, TX. FISCHER, A.G. ( 1 964) The Lofer cyclothems of the Alpine Triassic. Kansas geol. Surv. Bull. , 169, 1 0 7- 1 49. FISCHER, A.G., H ERBERT, T.D. & PREMOU-SILVA, l. ( 1 9 85) Carbonate bedding cycles in Cretaceous pelagic and hemipelagic sediments. In: Fine-grained Deposits and

Biofacies of the Cretaceous Western Interior Seaway: Evidence of Cyclic Sedimentary Processes (Eds Pratt, L.M., Kauffman, E.G. & Zeit, F.B.), Soc. econ. Paleont. Miner., Tulsa, SEPM Guidebook, 9: 1 - 1 0 . GLENN, C.R., KROON, D. & WUCHANG, W . ( 1 993) Sedi­ mentary rhythms and climate forcing of Pleistocene­ Holocene mixed carbonate/siliciclastic sediments off the Great Barrier Reef. In: Proceedings of The Ocean Drilling Program, Scientific Results, 1 3 3 (Eds McKenzie, J.A., Davies, P.J. & Palmer-Julson, A., et al. ), pp. 1 89202. Ocean Drilling Program, College Station, TX.

91

GOLDHAMMER, R.K., DuNN, D.A. & HARDIE, L.A. ( 1 98 7 ) High-frequency glacio-eustatic sea-level oscillations with Milankovitch characteristics recorded in Middle Triassic platform carbonates in northern Italy. Am. J. Sci. , 287, 8 5 3-892. GOLOVCHENKO, X., O'CONNELL, S.B. & JARRARD, R. ( 1 990) Sedimentary response to paleoclimate from downhole logs at Site 6 9 3 , Antarctic continental margin. In: Pro­

ceedings of The Ocean Drilling Program, Scientific Results, 1 1 3 (Eds Barker, P.F., Kennett, J.P. et al.), pp. 2 3 9-25 1 . Ocean Drilling Program, College Station, TX. GROTZINGER, J. P. ( 1 986) Upward shallowing platform cycles: a response to 2.2 Billion years of low-amplitude, high-frequency (Milankovitch band) sea level oscilla­ tions. Paleoceanography, 1, 403-4 1 6. HARDIE, L.A., BOSELUNI, A. & GOLDHAMMER, R.H. ( 1 986) Repeated subaerial exposure of subtidal carbonate plat­ forms, Triassic, northern Italy: evidence for high fre­ quency sea level oscillations on a 1 04 year scale. Paleoceanography, 1, 447-457. HARLAND, W.B., ARMSTRONG, R.L., Cox, A.V., CRAIG, L.E., SMITH, A. G. & SMITH, D.G. ( 1 990) A Geologic Time Scale 1 989. Cambridge University Press, Cambridge. HAYS, J.D., IMBRIE, J. & SHACKLETON, N.J. ( 1 976) Varia­ tions in the Earth's orbit: pacemaker of the Ice Ages. Science, 1 94, 1 1 2 1 - 1 1 32 . HERBERT, T.D. & FISCHER, A.G. ( 1 986) Milankovitch climatic origin of mid-Cretaceous black shale rhythms in central Italy. Nature, 321, 7 3 9-793. HERBERT, T.D., STALLARD, R.F. & FISCHER, A. C. ( 1 986) Anoxic events, productivity rhythms and the orbital signature in a mid-Cretaceous deep-sea sequence from central Italy. Paleoceanography, 1, 495-506. IMBRIE, J.M., HAYS, J., MARTINSON, D. G., et al. ( 1 984). The orbital theory of Pleistocene climate: support from a revised chronology of the marine 1 80 record. In: Mi­ lankovitch and Climate (Eds Berger, A., Imbrie, J., Hays, J., Kukla, G. & Salzman, B.), pp. 269-3 05. D. Reidel, Dordrecht. JAMES, P. N. ( 1 97 7 ) Shallowing upward sequences in carbonates. Geosci. Can. , 4, 1 26- 1 36. JARRARD, R. & ARTHUR, M.A. ( 1 989) Milankovitch paleo­ ceanographic cycles in geophysical logs from ODP Leg 1 05 , Labrador Sea and Baffin Bay. In: Proceedings of The Ocean Drilling Program, Scientific Results, I 05 (Eds Srivastava, S.P, Arthur, M., Clement, B. et al.), pp. 7 5 77 72. Ocean Drilling Program, College Station, TX. JENKINS, G.M. & WATTS, D.C. ( 1 9 88) Spectra/Analysis and Its Applications. Holden-Day, San Francisco, CA. JENKYNS, H.C. ( 1 995) Carbon-isotope stratigraphy and paleoceanographic significance of the Lower Cretaceous shallow-water carbonates of Resolution Guyot, Mid­ Pacific Mountains. In: Proceedings of the Ocean Drilling Program, Scientific Results, 1 4 3 (Eds Winterer, E.L., Sager, W.W., Firth, J.V. & Sinton, J.M.), pp. 77-88. Ocean Drilling Program, College Station, TX. JENKYNS, H.C., PAULL, C.K., CUMMINS, D.l. & FULLAGAR, P.D. ( 1 995) Strontium isotope stratigraphy of Lower Cretaceous atoll carbonates in the Mid-Pacific Moun­ tains. In: Proceedings of The Ocean Drilling Program, Scientific Results, 1 4 3 (Eds Winterer, E.L., Sager, W.W., Firth, J.V. & Sinton, J.M.), pp. 89-97 . Ocean Drilling Program, College Station, TX.

92

P. Cooper

KEMPER, E. ( 1 9 8 7) Das Klimat der Kreide-Zeit. Geol. Jb. Reihe A, 96, 402 pp. MAYER, L.A., JANSEN, E., BACKMAN, J. & TAKAGAMA, T. ( 1 993) Climate cyclicity at Site 806: the GRAPE record. In: Proceedings of The Ocean Drilling Program, Scien­ tific Results, 1 30 (Eds Berger, W.H., Kroenke, L.A., Mayer, L.A., et a!.), pp. 623-639. Ocean Drilling Pro­ gram, College Station, TX. McCAVE, I.N. ( 1 9 7 9) Depositional features of organic­ carbon-rich black and green mudstones at Sites 3 8 6 and 3 8 7 , western North Atlantic. In: Initial Reports of The Deep Sea Drilling Project, 43 (Eds Tucholke, B., Vogt, P. et a!. ), pp. 4 1 1 -4 1 6. US Government Printing Office, Washington, DC. MILANKOVITCH, M. ( 1 94 1 ) Konon der Erdbestrah!ung und seine A nwendung aufdas Eiszeitprob!em. Royal Serbian Academy, Spec. Pub!. 133, Section of Mathematical and Natural Sciences, 33 (published in English by the Israel Program for Scientific Translation, for the US Depart­ ment of Commerce and the National Science Founda­ tion, Washington, DC, 1 969). MOLINIE, A.J. & 0GG, J.G. ( 1 992) Milankovitch cycles in Upper Jurassic and Lower Cretaceous radiolarites of the equatorial Pacific: spectral analysis and sedimentation rate curves. In: Proceedings of The Deep Sea Drilling Program, Scientific Results, 1 29 (Eds Larson, R., Lance­ lot, Y. et a!. ), pp. 529-547. Ocean Drilling Program, College Station, TX. MULLER, R.A. & MACDONALD, G.J. ( 1 995) Glacial cycles and orbital inclination. Nature, 377, 1 07. MWENIFUMBO, C.J. & BLANGY, J.P. ( 1 99 1 ) Short-term spectral analysis of downhole logging measurements from Site 704. In: Proceedings of The Ocean Drilling Program, Scientific Results, 1 1 4 (Eds Ciesielski, P.F., Kristoffersen, Y. , et a!. ), pp. 5 7 7 - 5 8 5 . Ocean Drilling Program, College Station, TX. NOBES, D.C., BLOOMER, S.F., MIENERT, J. & WESTALL, F. ( 1 99 1 ) Milankovitch cycles and nonlinear response in the Quaternary record in the Atlantic sector of the Southern Oceans. In: Proceedings of The Ocean Drilling Program, Scientific Results, 1 1 4 (Eds Ciesielski, P.F., Kristoffersen, Y. et a!. ), pp. 5 5 1 -5 7 6 . Ocean Drilling Program, College Station, TX. OLSEN, P.E., REMINGTON, C.L., CORNET, B. & THOMPSON, K.S. ( 1 978) Cyclic change in Late Triassic lacustrine communities. Science, 201 , 729-7 3 3 . PARK, J. & HERBERT, T.D. ( 1 987) Hunting for paleoclimate periodicities in a geologic time series with an uncertain time scale. J. geophys. Res., 92, 1 402 7- 1 4040. PRELL, W.L. & KUTZBACH, J.E. ( 1 9 87) Monsoon variability over the past 1 50,000 years. J. geophys. Res. , 92, 84 1 1 -8425. RbHL, U. & STRASSER, A. ( 1 995) Diagenetic alterations and geochemical trends in Early Cretaceous shallow-water limestones of Allison and Resolution guyots (Sites 865 to 868). In: Proceedings of The Ocean Drilling Program, Scientific Results, 1 4 3 (Eds Winterer, E.L., Sager, W.W., Firth, J.V. & Sinton, J.M.), pp. 1 97-230. Ocean Drilling Program, College Station, TX.

RUDDIMAN, W.F. & MciNTYRE, A. ( 1 984) Ice-age thermal response and climatic role of the surface Atlantic Ocean, 46 ' N to 63 'N. Geo!. Soc. Am. Bull. , 95, 3 8 1 -396. SAGER, W.W., WINTERER, E.L., FIRTH, J.V. et a/. (Eds) ( 1 993a). Proceedings of The Ocean Drilling Program, Initial Reports, 1 43 . Ocean Drilling Program, College Station, TX. SAGER, W.W., WINTERER, E.L., FIRTH, J.V., et a/. (Eds) ( 1 993b). Site 8 6 5 . Proceedings of The Ocean Drilling Program Initial Reports, 1 4 3 . Ocean Drilling Program, College Station, TX. SAGER, W.W., WINTERER, E.L., FIRTH, J.V., et al. (Eds) ( 1 993c). Site 866. Proceedings of The Ocean Drilling Program, Reports, 1 43 . Ocean Drilling Program, Col­ lege Station, TX. SARNTHEIN, M., WINN, K, DUPLESSY, J.C. & FONTUGNE, M.R. ( 1 9 88) Global variations of surface ocean produc­ tivity in low and mid latitudes: influences on C02 reservoirs of the deep ocean and atmosphere during the last 2 1 ,000 years. Paleoceanography, 3, 3 6 1 -399. SCHICKENDANZ, P.T. & BOWEN, E.C. ( 1 977) The computa­ tion of climatological power spectra. J. appl. Meteorol., 16, 3 5 9-367. ScHLUMBERGER ( 1 987) Log Interpretation Principles! Applications. Schlumberger Education Services, Hous­ ton, TX. SCHWARZACHER, W. ( 1 964) An application of statistical time series analysis to a limestone-shale sequence. J. Geol. , 72 , 1 9 5-2 1 3 . SHINN, E.A. ( 1 9 83) Tidal flat environment. In: Carbonate Depositional Environments (Eds Scholle, P.A., Bebout, D.C. & Moore, C.H.), Mem. Am. Assoc. petrol. Geol., Tulsa, 33, 1 7 1 -2 1 0. SLITER, W.V. ( 1 995) Cretaceous planktonic foraminifers from Sites 8 6 5 , 866, and 869: a synthesis of Cretaceous pelagic sedimentation in the Central Pacific Ocean basin. In: Proceedings of The Ocean Drilling Program, Scientific Results, 1 43 (Eds Winterer, E.L, Sager, W.W., Firth, J.V. & Sinton, J.M.), pp. 1 5-30. Ocean Drilling Program, College Station, TX. START, G.G. & PRELL, W.L. ( 1 984) Evidence for two Pleistocene climatic modes: data from DSDP Site 502. In: New Perspectives in Climatic Modeling, Berger, A. & Nicolis, C.), Developments Atmosphere Sciences, 1 6, pp. 3-22. Elsevier, Amsterdam. STRASSER, A. ( 1 99 1 ) Lagoonal-peritidal sequences in car­ bonate environments: autocyclic and allocyclic pro­ cesses. In: Cycles and Events in Stratigraphy (Eds Einsele, G., Rieken, W. & Seilacher, A.) Springer-Verlag, Berlin. SUNDQUIST, E.J. & BROECKER, W.S. (Eds) ( 1 9 85). The

Carbon Cycle and Atmospheric C02: Natural Variations Archean to Present, Geophys. Monogr., Am. geophys. Union, Washington, DC, 32. VAN HouTEN, F. B. ( 1 964) Cyclic lacustrine sedimentation, Upper Triassic Lockatong formation, central New Jer­ sey and adjacent Pennsylvania. Kansas geol. Surv. Bull. , 169, 497-5 3 1 .

Platform Case Histories

Reefs and Carbonate Platforms in the Pacific and Indian Oceans Edited by G. F. Camoin and P. J. Davies © 1998 International Association of Sedimentologists ISBN: 978-0-632-04778-9

Spec. Pubis int. Ass. Sediment. (1998) 25, 95-136

Aptian-Albian eustatic sea-levels

U. ROHL* and J . G. OGGt *Geosciences Department, Bremen University, PO Box 33 04 40, D-28334 Bremen, Germany; and tDepartment ofE arth and Atmospheric Sciences, Purdue University, West Lafayette, IN 4 7907, USA

ABSTRACT Carbonate banks record rapid falls of relative sea-level. as emergent surfaces and rapid rises as deepening or drowning events. During the combined Ocean Drilling Program Legs -143 and 144, four . carbonate banks of Aptian-Albian age were drilled on top, of seamounts ('guyots') that span a large region of the north-western Pacific. Simultaneous episodes of emergence ('sequence boundary') or deepening at these guyots must be the result of major eustatic sea-level events. From a combination of cored lithologies and downhole geophysical and geochemical logs we identified depositional sequences. A general geological age framework was assigned from biostratigraphical datum levels and chemo­ stratigraphical (carbon and strontium isotope) curves. Compensation for thermal subsidence rates allowed assignment of relative durations of the array of sequences within each stage. The number of upward-shallowing cycles or parasequences was also used 'to compare relative durations of sequences among sites. These Pacific carbonate banks record 12 Aptian and 1 2 Albian significant shallowing events, of which a third were associated with major episodes of emergence. The major events; on the guyots can be correlated easily with Aptian-Albian relative sea-level changes observed in European shelf successions, and both regions display the same number of minor events. Therefore, we can apply the relative timing of these events from the thermal subsidence compensation and parasequence counts within the Pacific banks to construct an improved scaling of the associated ammonite zones and biostratigraphical datum levels within the Aptian-Albian interval.

PACIFIC GUYOTS AS RECORDERS OF CRETACEOUS EUSTATIC SEA-LEVEL CHANGES

applicability was not documented. In addition, the derivation of a eustatic sea-level curve from the se­ quence stratigraphy of a continental margin is con­ troversial because shifts of deposition patterns can also be caused by irregular tectonic subsidence or uplift (e.g. Cloetingh, 1 9 88) or changes in sediment influx. Frequent high-amplitude swings of eustatic sea­ level during the late Cainozoic are caused by fluc­ tuations in continental ice sheets, but the proposed rapid major sequences in the Mesozoic (e.g. Haq et al., 1 98 7) create a perplexing dilemna-where to store massive amounts of water during a presumed globally warm climate. In particular, the mid­ Cretaceous is generally considered to be a period of elevated carbon dioxide levels and associated green-

Sequence stratigraphy has the basic premise that marginal-marine successions can be subdivided into distinct depositional packets bounded by unconfor­ mities. These sequence boundaries are generally in­ terpreted to be the product of rapid drops or regres­ sions of relative sea-level. If these sea-level falls are caused by eustatic falls of water levels in the world ocean, then the pattern of depositional sequences provides a means to achieve high-resolution corre­ lations among all continental margins. The assump­ tion of a master eustatic signal was implicit in the sequence stratigraphical charts and associated sea­ level curves published by the Exxon research group (e.g. Vail et a/., 1 977; Haq et a/., 1 987, 1 988; Vail, 1 987). These curves were mainly derived from out­ crops of European shelf sediments, and their global

Reefs and Carbonate Platforms in the Pacific and Indian Oceans Edited by G. F. Camoin and P. J. Davies © 1998 International Association of Sedimentologists ISBN: 978-0-632-04778-9

95

96

U Rohl & J. G. Ogg

house warming caused by accelerated submarine volcanism (e.g. Larson, 1 99 1 ; Berner, 1 994 ); there­ fore, a demonstrated existence of major short-term eustatic fluctuations would present a challenge to our models of the hydrological and climate systems of a 'hothouse' world. A test of eustatic sea-level excursions during the mid-Cretaceous requires depositional settings that are sensitive recorders of sequence boundaries and are isolated from postulated flexural oscillations of continental margins. A suite of shallow-water car­ bonate platforms on mid-ocean seamounts distant from the tectonic complications of ridges or trenches provides the desired traits. During the Aptian and Albian stages of the mid-Cretaceous, the tropical Pacific contained a thriving population of carbonate banks and atolls. Over millions of years, these isolated carbonate platforms had maintained a depositional environ­ ment close to sea-level as the underlying volcanic edifices underwent progressive thermal subsidence at average rates of about 30-50 m Myc1• However, rapid rises or falls in eustatic sea-level produced changes in accommodation space and associated deepening of facies or created episodes of tempo­ rary emergence. At the end of the Albian, these long-lived carbon­ ate banks were mysteriously terminated simulta­ neously with the demise of carbonate platforms from Albania to Venezuela (Arnaud et a!., 1 995). The Aptian-Albian carbonate caps drowned and

became part of the population of enigmatic flat­ topped seamounts, 'guyots', that punctuate the submarine bathymetry of the western Pacific. (The submerged flat-topped seamounts were named 'guyots' after the nineteenth-century geographer Arnold Guyot by Hess ( 1 946).) Not until tens of millions of years later, during the Campanian and Maastrichtian stages of latest Cretaceous, were car­ bonate platforms again a component of the tropical Pacific (Winterer et a!., 1 993). The terminal drown­ ing of these guyots is examined in a later section. Four Aptian-Albian carbonate platforms, span­ ning a region 40' in longitude and 20' in latitude (Fig. I ), were a main focus of deep drilling during Ocean Drilling Program (ODP) Legs 1 43 and 1 44 (Premoli-Silva et a!., 1 993; Sager et a!., 1 993). Leg 1 43 drilled two guyots in the area of the Mid-Pacific Mountains (Sites 865-868, Allison and Resolution guyots). Of the five guyots drilled during Leg 1 44, MIT and Takuyo-Daisan (Seiko) guyots contain Aptian-Albian shallow-water carbonate caps. The successions of up to 1 000 m thickness also recorded short- and long-term oscillations of relative sea­ level. A prime objective of these ODP legs was to decipher the sea-level history recorded by each guyot, then extract the major events common to all guyots. Any major relative sea-level fluctuation recorded simultaneously by the four dispersed guyots would require a tectonic uplift over an area equivalent to the entire continental USA; in which case, the

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ical to temperate environments, and to determine the effects of changes in climate and sea-level on their distribution. The focus of this paper is the transitional zone between tropical and temperate carbonates that occurs on the continental shelf offsouthern Queensland between Fraser Island and Noosa Heads (Fig. 2), directly to the south of the Great Barrier Reef. Regional geology

The continental margin of eastern Australia was formed by the opening of the Tasman Basin in the south between 80 and 60 Myr ago (Hayes & Ringis, 1973) and the Coral Sea Basin in the north between

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65 and 55 Myr ago (Weisse! & Watts, 1979). In the Tasman Basin, sea-floor spreading occurred along a NNW-trending ridge axis, offset by right-lateral transform faults, which separated the Lord Howe Rise from the Australian continent. The south-east Australian margin is unusually steep. The continen­ tal slope has gradients of 8 -12 and locally may be as steep as 20°. Jongsma & Mutter (1978) attributed this steep slope to the absence of rift­ stage tectonic elements typical of most Atlantic­ type margins. They suggested that the entire pre­ breakup rift basin remained attached to the Lord Howe Rise during the formation of the Tasman Basin. Etheridge et a!. (1990) applied the detach­ ment model of Wernicke (1981, 1985) to recon-

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c:: 300 m, and consists of fine to medium sands composed of biogenic carbonate and subordinate quartz sand, with some mud (less than 5o/o). The carbonate grains are mainly benthic Foraminifera, molluscs, bryozoans and unidentified bioclasts. Abundant planktonic Foraminifera and pteropod fragments are characteristic constituents of the deeper shelf terrace. The sediments also include granule-size brown carbonate lithic grains, which suggest either reworking of sediments from the bank and/or limestones outcropping on the outer shelf. THE Q U ATERNARY CARBONATE PLATFORM

This platform (Platform 3 in Fig. 4) lies directly beneath the present-day platform and is stratigraph­ ically continuous with it. Nine dredge sites were occupied on this mid-shelf carbonate platform, in water depths of 28-107 m. The dredge material consists of a variety of carbonate rocks, in particu­ lar, coral-algal boundstones and rhodolith float­ stones to rudstones. These limestones are characterized by the dominance of crustose coral­ line algae and hermatypic corals as constituents. At sites on the steep cliff east of the Gardner Banks, a plentiful supply of in situ coral-algal boundstone samples was obtained.

Petrographic description On the basis of rock texture, grain components, di­ agenetic features and carbonate mineral composi­ tion, dredge samples from the mid-shelf were di­ vided into the following rock types: (i) rhodolith floatstone-rudstone (RH-type limestone); (ii) coral­ algal boundstone (CA-type limestone); and (iii) bio­ clastic grainstone-packstone (BC-type limestone). Rhodolith floatstone-rudstone (RH-type limestone). These limestones are composed mainly of rhodoliths in a grainstone to packstone matrix made up of var­ ious bioclastic grain types (Fig. 13a & b). The lime­ stones are generally porous and consolidated. Most samples in this type consist predominantly of Mg­ calcite (average 74o/o), and subordinate low-Mg­ calcite and aragonite (average 16 and 1Oo/o, respectively). The RH-type limeston�s were recovered from the Gardner Banks in water depths of 28-60 m. The rhodoliths are 1-4 em in diameter, and have a typical concentric structure of crustose coralline algae (Fig. 13a & b). Most have been bored to some extent. The nuclei consist of corals, shell fragments and carbonate rock fragments. Most have smooth surfaces, although some are pinnacled (maerl rhod­ oliths). In places, the rhodoliths are closely com­ pacted, and little grainstone to packstone matrix is present. Where it occurs, the matrix consists of medium sand-size to granule-size bioclasts, plus fine calcareous material (Fig. 13c). The bioclasts include both encrusting and articulated coralline algae, benthic Foraminifera, echinoids, Halimeda, mol­ luscs and coral with minor amounts of angular to sub-angular quartz grains. The fine calcareous ma­ terial in the grainstone-packstone is mainly micrite and often contains peloids (Fig. 13c). There is little evidence of cementation in the RH-type limestones. Most cement appears to be Mg-calcite micrite in the matrix, plus some filling of pores by peloids. Some endolithic borings in the rhodoliths are filled by peloids and quartz silt. Geopetal fabrics are also present. The distribution of the RH-type limestones coin­ cides with that of present-day rhodoliths. The lime­ stones are coarse, poorly sorted, with a relatively low mud content, and resemble the present rhodo­ lith facies. ·

Coral-algal boundstone (CA-type limestone). These limestones are characterized by the dominance of

Subtropical carbonate platform development

183

(a)

Fig. 13. Rhodolith floatstone-rudstones (RH-type limestones). (a) Rhodolith rudstone showing rounded rhodoliths, of 2-3-cm d iameter, in an open matrix (52 m). (b) Rhodolith floatstone showing scattered rhodoliths of l -2-cm diameter in a packstone matrix (52 m). (c) Photom icrograph of the packstone matrix in the RH-type limestones. I ntergranular pore spaces are partly filled with finer calcareous materials containing peloidal grains. Plane-polarized light.

crustose coralline algae and hermatypic corals. They are widely distributed across the mid-shelf, but are most abundant on the steep cliff at the seaward edge of the platform. The CA-type limestones are brownish yellow to brown boundstones (e.g. Fig. 1 4a), composed mainly of abundant crustose coralline algae, herma­ typic corals, and encrusting Foraminifera, with minor amounts of benthic forams, gastropods and Halimeda. The coralline algae form thinly layered, thick crusts, and bind other bioclastic grains and finer calcareous material, along with minor quartz grains (Fig. 14b). A matrix, consisting chiefly of the tests of planktonic and benthic Foraminifera, mol­ luscan shell fragments and quartz grains with finer calcareous materials and terrigenous clay, is present between the algal layers and is bounded by them. The matrix often contains fine peloidal grains. The angular to subangular quartz grains are fine sand­ size. The carbonate content of this type of limestone is 77-93%. Like the RH-type limestones, the presence of unaltered coralline algae results in a dominantly

-

0.5 m m Mg-calcite mineralogy (average 78. 5%), whereas aragonite and low-Mg-calcite are relatively minor. The abundant boundstones from the cliff to the east of the Gardner Banks in water depths of 90110 m are generally pale yellow to yellowish brown, and are cobble- to boulder-size (Fig. 14a). Most are encrusted by living coralline algae. The layering of the coralline algae is distinct; each layer is less than I mm in thickness. In some of them, two genera­ tions of growth can be observed (e.g. Fig. 14c), with a very pale brown to yellow older layer overlain by later whitish coralline algae. The upper surface of the recent layer is encrusted by living coralline algae. In the CA-type limestones, cementation is not readily apparent, and most consolidation appears to reflect the binding effect of the coralline algae. Poorly developed fibrous aragonite cements are present in some intraskeletal pore spaces within coral fragments. Porosity is mainly framework po­ rosity, vuggy porosity and borings. Secondary po­ rosity, such as vuggy porosity and borings, is often filled with internal sediment consisting of fine cal-

184

JF

Marshall et al. (b)

(a)

-

1 .0 m m (c)

Fig. 14. Coral-algal wackestone-packstone-boundstone type limestones. (a) Algal boundstone showing overlapping sheets of coralline algae with high porosity between the layers (93 m). (b) Photomicrogra ph of an algal boundstone . The encrusting corall ine algae show multiple layering and bind fine calcareous materials, planktonic forams and quartz grains. (c) Algal boundstone. White newer generation of corall ine algae grows on a yellowish brown older ge neration. Note the distinct boundary between the two .

careous material, which sometimes has a peloidal texture, and angular quartz grains. Geopetal struc­ tures are observed in primary and secondary pores. A few poorly preserved calcareous nanofossils were found in the matrix of one specimen of coral­ algal boundstone. Most specimens are strongly over­ grown and difficult to identify. The presence of Ge­ phyrocapsa caribbeanica indicates that part of the matrix is no older than 1.66 Ma (Biodatum I I of Sato & Takayama, 1990). The absence of Pseudoe­ miliania lacunosa cannot be confirmed because of the rarity and poor preservation of the nanofossils. Bioclastic grainstone-packstone (BC-type limestone). BC-type rocks are composed mainly of a variety of bioclastic grains, with a minor amount of quartz grains, and show no binding by encrusting organisms (Fig. 15a & b). This type of limestone is not as com­ mon as the others, and was mainly recovered from the Gardner Banks. The carbonate content varies from 50 to 93%, but average values are around 83%. The major constituents are coralline algae, benthic

Foraminifera, corals, molluscs, Halimeda, bryozo­ ans and echinoderms. Grain size is generally me­ dium to very coarse sand- and granule-size, and sort­ ing is poor. The matrix consists of a lime mud that often contains peloids. This type of rock is poorly cemented and friable. The carbonate mineral com­ position is dominated by Mg-calcite (64-89%) with subordinate aragonite (2-28%) and low-Mg-calcite (8-22%). THE TERTIARY CARBONATE PLATFORM

Five dredge sites located on the outer shelf to mid-slope, in water depths of 270-600 m, recov­ ered well-lithified limestones of presumed shallow water origin from Platform 3. Petrographical description On the basis of grain components, diagenetic fea­ tures, and carbonate mineral composition, the dredge samples from Platform 3 were divided into

Subtropical carbonate platform development

185 (b)

-

1 . 0 mm Fig. 15. Bioclastic grainstone-packstone type limestones. (a) Bioclastic grainstone (60 m). (b) Photomicrograph of the

bioclastic packstone . I ntergranular pore-spaces are partly filled with micr itic matrix containing peloids. Plane-polarized light.

the following rock types: (i) well-cemented lime­ stone (WR-type limestone); and (ii) dolomitic lime­ stone (DL-type limestone). The well-lithified limestone and dolomitic lime­ stone samples are dense and have diagenetic features suggesting both marine and meteoric environments. Calcareous nanofossils suggest Oligocene to early Middle Miocene ages, and benthic forams indicate Early to Middle Miocene ages. The well-lithified limestones contain rhodoliths and large forams (e.g. Lepidocyclina). Well-cemented limestone (WR-type limestone). These limestones are white to pale brown, and com­ positionally and texturally are mainly rhodolith­ large foram-molluscan packstones, grainstones and floatstones. The rhodolith-large foram packstones are composed mainly of rhodoliths, encrusting and articulated coralline algal fragments, and large benthic forams with minor amounts of planktonic forams and sometimes quartz grains. The rhodoliths are white, 5-40 mm in diameter, and show typical concentric structure. Large forams (e.g. Lepidocy­ clina sp.) can be 5-40 mm in diameter. Some lime­ stones also contain larger coral fragments. One dis­ tinctive limestone is a yellowish brown to yellow floatstone that is fairly well cemented. This consists of mollusc shells, coralline algae, solitary corals, bry­ ozoans and forams in a wackestone matrix. Total carbonate content of the WR-type lime­ stones is very high, from 86 to 96%, and is pre­ dominantly low-Mg-calcite, with some Mg-calcite but no aragonite.

Diagenetic fabrics include isopachous and equant sparry calcite cements. Isopachous rim cements grow radially from the surfaces of bioclastic grains, and are dominant in grainstones from the upper slope (Fig. 16a). They consist of fibrous or bladed crystals, about ! 50 J.l.m in length. The outer zone of the rim cement is sometimes brown and dusty in appearance. In some examples, two generations of isopachous cement could be observed (Fig. 16b & c). Sparry calcite cement is common as a late-stage pore fill in these limestones. The sparry calcite cement overlies the isopachous rim cement and fills intergranular and intraskeletal pores (Fig. 16a). It occurs as a drusy mosaic of equant, subhedral to anhedral crystals, 15-400 J.l.m in diameter. Dolomitic limestone (DL-type limestone). Dredging of the middle and upper slope off Fraser Island recovered some dolomitic limestone samples. All samples containing dolomite were classified as DL­ type regardless of other rock components and rock textures. They consist of rhodolith-large foraminif­ eral floatstone and bioclastic packstone, grainstone and mudstone, and their dolomite content ranges from 15 to 37%. White dolomitic rhodolith-large-foram float­ stone is well consolidated, with a packstone to wackestone matrix (Fig. 17a). The main bioconstit­ uents are crustose coralline algae and large benthic Foraminifera (Lepidocyclina sp.). The dolomite oc­ curs mainly as a cement, lining pores created by dissolution or boring and growing on micritic inter­ nal sediments which occupy the lower parts of

186

J. F. Marshall et al. (b)

-

0.2 m m

0 .5 m m

Fig. 16. Well-consolidated limestone type (438 m). (a) Photomicrograph of a bioclastic grainstone showing articulated coralline algal fragments and other skeletons lined by isopachous cement. Remaining pore-space has been filled with sparry calcite cement. Plane-polarized light. (b) Photomicrograph of two generations of isopachous rim cements (F 1 and F2 ). The two cement layers are intercalated with micrite (I). Plane-polarized light. (c) Photomicrograph showing the second-stage isopachous rim cement growing directly on the first-stage isopachous rim cement, and followed by internal sediment. Plane-polarized light.

(b)

-

0.2 m m

0.2 mm

Fig. 17. Dolomitic limestone type ( 5 9 2 m). (a) Photomicrograph o f dolomitic limestone. The dolomite occurs as

equant, subhedral to euhedral pore-lining cement. Micritic internal sediment below the dolomite cement is often dolomitized. The dolomite cements are covered by the next stage of internal sediment, equant mosaic sparry calcite . cement and isopachous bladed calcite rim cements. Plane-polarized light. (b) Photomicrograph of euhedral dolomite crystals within a vug or boring. The dolomite crystals are covered by equant mosaic calcite cements. Plane-polarized light.

187

Subtropical carbonate platform development dissolution pores (Fig. 17b). The dolomite cement consists of equant subhedral to euhedral crystals, about 50 Jlm in length. The micritic sediments beneath are also often dolomitized. The dolomite cement is commonly also covered by internal sedi­ ments and/or fibrous to bladed isopachous calcite cement, which in turn are succeeded by equant mosaic calcite cement (Fig. 17a). The dolomites have an isotopic compositional range of 0 1 80p08 from + 1.58 to +4.27o/oo and 0 1 3CPDB from + 1.69 to + 3.45o/oo (Table 2). The c) 1 3C and c) 1 80 values of the dolomites provide a regular pattern of variation. Dolomites with more 1 80-enriched oxygen tend to have more 1 3C­ enriched carbon. Dolomitic limestones with a higher dolomite content are also more 1 80- and 1 3C­ enriched.

DISCUSSION

The environmental significance of subtropical shelf carbonates

The separation of shelf carbonates into chlorozoan and foramol assemblages by Lees & Buller (1972) and their attribution to temperature-salinity changes (Lees, 1975) presents a definite division between tropical and temperate carbonates. How­ ever, the boundary between the two has not been

investigated in any detail, and the nature of this boundary remains largely unknown. Along the east coast of Australia, Marshall & Davies (1978) deter­ mined the boundary to be at 24 • S, based on the southernmost occurrence of surface reefs. It is apparent that such a boundary cannot be so clear­ cut, and that some overlap of assemblages should occur. The skeletal composition of low-relief bio­ herms on the mid-shelf off Fraser Island supports this contention. The Gardner Banks, although dom­ inated by coralline algae, contain hermatypic corals and Halimeda, an association that is definitely tropical. However, warm temperate shelf assem­ blages, some 150 km to the south, contain no trace of Halimeda, and, apart from the presence of coralline algae and corals, have a greater abundance of bryozoans and barnacles, an association that is distinctly temperate (Tsuji et a!., 1994). Conversely, bioherms the same distance to the north form surface reefs, which, although areally restricted, are definitely tropical. This suggests that the boundary between tropical and temperate assemblages con­ sists of a zone, some 300 km wide, that contains within it a distinctly subtropical assemblage. The narrowness of this boundary (less than 8% of the total length of the east Australian shelf) reflects the sensitivity of these assemblages to subtle changes, presumably in water temperature. How­ ever, there is no perceptible change in surface water temperature over this distance, other than slight

Table 2. Carbon and oxygen isotopic composition (in PDB) of the Tertiary platform samples

Sample number 1 05/DR/005 - 1 -2 -4 -7 -9 1 05/DR/006 -4 1 05/DR/009 -3 -5 -7 -8 1 05/DR/0 1 0 - 1 -2 -3 -4 -6

Water depth (m) 592

420 27 1

438

Calcite

Dolomite

Rock type

o 1 3C

WR WR DL

1 . 399 1 .693 2.690 2.695 2 . 5 76 1 . 993 2.729

1 .064 1 .059 0.784 0.728 2 . 36 5 1 . 559 2.422

3 .086 3.080 1 .944 1 .285 1 .946 1 .900 2.027 0.822 2. 1 88 2.238

2.835 2.808 0.820 1 . 1 42 -0.06 1 1 .330 1 .420 -0.745 1 .308 1 .992

DL DL DL DL DL DL DL DL WR WR WR WR WR

a t so

o 1 3C

a t so

3.463

3.644

2.76 8 2.444 2.840 1 .689 3.449 3.430

3.259 2.686 3.292 1 .586 4.272 4.257

1 .9 1 2

2.247

1 88

J. F

Marshall et al.

differences in summer and winter ranges. It may be that other, as yet unknown, factors are involved. Possibly seasonal changes in oceanic circulation, involving the EAC and the gyres immediately to the north, are important. Whereas the subtropical and warm temperate assemblages are similar, the ab­ sence of Halimeda in the south being the most apparent difference, both zones are distinctly dif­ ferent from coral-Halimeda-dominated tropical assemblages to the north and bryozoan-mollusc­ foram-dominated cool temperate assemblages further south. The most obvious difference is the dominance of crustose coralline algae, along with subordinate and distinctly species-limited herma­ typic corals, in both the subtropical and warm tem­ perate assemblages. This change from coral-Halimeda to coralline algae-coral-Foraminifera to bryozoan-Foramini­ fera-mollusc is very similar to that identified on the Brazilian shelf off the east coast of South America (Carannante et a!., 1 988). There, four major types of carbonate lithofacies were distinguished on the shelf: 1 Chlorozoan-characterized by hermatypic corals and Halimeda, associated with molluscs, benthic foraminifers, echinoids, bryozoans, sponges and coralline algae. This lithofacies is typical of tropical areas containing well-developed coral reefs. 2 Chloralgal-contains large amounts of calcareous green algae but no hermatypic corals. This litho­ facies is present in tropical-subtropical zones where coral reefs have not developed. No equivalent lithofacies has been identified in subtropical assem­ blages from eastern Australia, but it is particularly abundant in the Great Barrier Reef as Halimeda bioherms and biostromes. 3 Rhoda/gal-dominated by abundant encrusting coralline algae, bryozoans and variable amounts of large benthic foraminifers and barnacles. This litho­ facies is well developed in transitional warm temperate-subtropical zones. Locally, bryozoans can be abundant, especially towards cooler areas. 4 Molechfor-characterized by abundant mollusc fragments, benthic foraminifers (many of them arenaceous) and echinoids. Barnacles can be the main constituents; serpulids and bryozoans may be present. Encrusting coralline algae are generally absent. This lithofacies characterizes cold-temper­ ate carbonate shelves. Carannante et a!. ( 1 988) also pointed out that the distribution of these lithofacies is obviously subject to complex environmental factors related primarily to latitude and depth, both of which control water

temperature. However, factors affecting salinity and temperature, nutrient levels, light penetration, etc., may also play a fundamental role. This apparent sensitivity to temperature changes within the transitional zone should be reflected in the sedimentary record, particularly during glacial­ interglacial cycles. We have no evidence at this stage to verify this; presumably the frequency of Pleis­ tocene glacial cycles is too high to preserve a legible record of varying carbonate assemblages. On the other hand, more gradual changes do appear to be preserved. The Early to Middle Miocene carbonate platform off Fraser Island contains similar skeletal elements to the modern shelf. From the results of our present study, we would interpret this assem­ blage as being deposited in a subtropical environ­ ment. This is more appropriate than a cool temperate or tropical interpretation because of the lack of both abundant bryozoans on one hand and corals-Halimeda on the other. The rhodolith-large foram assemblage is more in keeping with the transitional environment of the present study area, where coralline algae, both as crusts and, more significantly, rhodoliths, predominate over all other bioconstituents. Subtropical carbonate platform development

Recent studies (e.g. Davies et a!., 1 987, 1 989; Pigram et a!., 1 993) have detailed substantial work aimed at understanding the evolution of the Great Barrier Reef and its relation to other large carbonate platforms off tropical north-east Australia, namely the Marion and Queensland plateaux. These studies have postulated that the development of carbonate platforms off eastern Australia is underpinned by the northward movement of the Australian plate throughout the Cainozoic. This northward drift of the Australian continent has progressively moved north-east Australia from a temperate to a tropical environment. From an analysis of the rate of drift, in conjunction with palaeoclimate reconstructions for the Tertiary, Davies et al. ( 1 987) postulated that the present tropical carbonate platforms of north-east Australia would be underlain by temperate carbon­ ate accumulations. An essential consequence of the drift hypothesis is that subtropical and temperate facies should not only underlie the tropical facies of the marginal plateaux, but also that subtropical car­ bonate platforms would be contemporaneous with their tropical counterparts, but further to the south. Therefore, off eastern Australia subtropical facies

Subtropical carbonate platform development could have occurred south of the Marion Plateau in the Early to Middle Miocene, and have occurred south of the Great Barrier Reef from the Pleistocene to the present day. Miocene subtropical carbonate build-ups Results from ODP Leg 133 off north-east Australia have confirmed that large tropical reef platforms developed beneath the Marion and Queensland plateaux in the Early and Middle Miocene (Davies et al., 1991). It also appears that a very large subtropical carbonate build-up occupied the outer continental shelf to the east of Fraser Island at the same time. We have named this feature the Fraser Platform. On seismic profiles (Figs 4 & 6) the Fraser Platform can be seen as a 10-km-wide feature with poor reflection characteristics and truncated sea­ wards by the steep upper continental slope. Changes in the reflection character allow the differentiation of an outer (Platform 1) and inner zone (Platform 2) of poor reflections, with dipping reflectors in front of Platform 2, and the progradation of a strongly reflecting bedded upper section over Platform 1. The interpretation of the seismic sequences sug­ gests: 1 Build-up of the Fraser Platform on the shelf edge with a steep seaward facing slope (Platform I) part of which may be erosional. West of the shelf edge platform, lagoonal or leeside sediments were depos­ ited, and further west, Platform 2 was sporadically developed. If the platform (non-bedded) and la­ goonal (bedded) sediments are stratigraphically re­ lated then they define deposition during a slowly rising sea-level, demonstrated by the upward and outward progradation of the bedded and lagoonal facies over the shelf edge platform. Sediment thick­ ness is of the order of 450 m, similar to that beneath the Marion Plateau (Pigram et al. , 1993). 2 Exposure of the Fraser Platform with subaerial erosion and the probable development of karst. The steep slope of the margin is probably partly related to erosion at this time. 3 Flooding of the Fraser Platform, with backstep­ ping and growth of a new platform (Platform 3) on the mid- to outer shelf. Quaternary subtropical carbonate build-ups Seismic, bathymetric, side-scan sonar and sediment data indicate the development of a large Quaternary carbonate platform beneath and seaward of the

189

Gardner Banks. These results indicate that subtrop­ ical carbonates, as compared with temperate car­ bonates in general, are capable of forming bioherms of some magnitude. The present-day bioherms of the Gardner Banks stand as much as 20 m above the surrounding sea-floor. They also cover a wide area of the mid-shelf. Seismic reflection data indi­ cate that they are at least 60 m thick. There appears to have been backstepping of Platform 3 during the Quaternary from the I 05 m cliff at the edge of the mid-shelf to the present location of the Gardner Banks. Nanofossil and planktonic foraminiferal bio­ stratigraphy shows that limestone samples dredged from this build-up were deposited during the Qua­ ternary, and nanofossil assemblages indicate an age possibly younger than 0.39 Ma (Biodatum 3). From a consideration of their age and sea-level history, the timing and environment of the formation of these limestones can be considered as follows: 1 Precursor sediments, similar to present-day sedi­ ments of the area, were deposited during the Qua­ ternary in an environment basically similar to that which exists today. 2 The limestones have not suffered marked mete­ oric diagenesis, although the dredge sites are situ­ ated between supposed highstand and lowstand levels of eustatic oscillations during the Pleistocene. It is possible that the limestones have been lithified since the last postglacial transgression. The lime­ stones have only been weakly cemented, possibly under relatively quiet water conditions. Diagenetic features of the subtropical platforms

The diagenetic features of the Tertiary limestones of the Fraser Platform suggest that diagenesis occurred in both marine and non-marine environments. Isopachous fibrous to bladed rim cements indicate a submarine environment, whereas bladed to equant sparry calcite cement is considered to have formed in fresh to brackish water. Dissolution features within coralline algal fragments also indicate mete­ oric diagenesis. The dolomitic limestones have undergone a vari­ ety of diagenetic processes, such as dolomite cemen­ tation, dolomitization, internal sediment deposition and cementation, and cementation by isopachous bladed calcite and equant mosaic calcite. The first stage of diagenesis was the formation of vuggy po­ rosity and/or boring porosity, because no diagenetic fabric is cut by vuggy porosity. Porosity created by

190

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Marshall et al.

the first stage of dissolution and/or boring was locally filled with micrite. Subsequently, euhedral pore­ lining dolomite cement deposition and dolomitiza­ tion of pore-filling sediments occurred. In the re­ maining pores, deposition of micritic (peloidal) sediments, pore-filling and isopachous bladed calcite cements followed dolomitization, and these were succeeded in turn by equant blocky calcite cement. The oxygen isotopic composition of the dolomite ranges from + 1.58 to +4.27%o. Calcites equili­ brated with these dolomites are predicted to have 8 1 80 values of +0. 13 to +2.82%o. The isotopic composition of the Fraser Platform dolomites sug­ gests that they were precipitated from sea-water slightly more 1 80-enriched than normal sea-water at 25 • c. Enrichment in 1 80 is generally considered to correspond to a decrease in water temperature and/or evaporation. There is no evidence that an evaporitic environment existed, whereas the oxygen isotopic value of calcites equilibrated with the dolomites is equal to that of limestones formed in water depths of 0- 150 m. This suggests that the dolomite formed from slightly cool normal sea­ water. Unfortunately, there are few clues to the origin of the dolomite, so it is very difficult to clarify the mechanism and timing of dolomitization of this subtropical carbonate platform. However, some inferences can be made by comparing the Fraser Platform dolomites with results from ODP Leg 133 off north-eastern Australia (Davies et a!., 1991). D,olomitic intervals exist throughout the Tertiary to Quaternary section of the Marion Plateau (Pigram et al., 1993). From the petrographical and micro­ palaeontological evidence, the diagenetic sequence observed in the Marion Plateau cores can be di­ vided into three stages: 1 late Early to early Middle Miocene: the plateau formed a shallow marine environment, with mic­ rite, isopachous fibrous and botryoidal aragonite cements forming on grains; 2 middle Middle to middle Late Miocene: the pla­ teau was emergent and underwent meteoric diagen­ esis, resulting in the formation of syntaidal overgrowths, bladed to granular pore lining ce­ ments, and mouldic and vuggy porosity; 3 late Late Miocene: the plateau was again flooded and became a deep marine environment, as evi­ denced by borings, apatite crusts, coarse equant sparry calcite and pore filling with hemipelagic clay. Dolomitization is considered to have occurrecl on at least three occasions: the middle Middle Mi-

ocene, the late Late Miocene and the post-Late Pliocene. Some of the diagenetic textures observed in the carbonates of the Marion Plateau are absent in the dredged limestones off Fraser Island, but there is no significant difference between the order of diage­ netic events for the two areas. The first diagenetic event was dissolution and/or boring. Both micrite and isopachous fibrous to bladed calcite cements can be recognized forming before and after the dolomite cement and dolomitization. These tex­ tures are generally considered to have formed under shallow marine conditions. Equant to blocky calcite cements follow the isopachous calcite cement, and are similar to coarse equant spar in the dolomites of the Marion Plateau, which are considered to have formed in a deep marine environment as a late­ stage cement (Pigram et a!., 1993). The presence of hemipelagic planktonic foraminiferal clay that post­ dates the equant calcite cement supports this view. Precipitation of dolomite cement and dolomiti­ zation are considered to have occurred at a stage when the environment was changing from shallow marine to somewhat deeper water. This is sup­ ported to some extent by the oxygen isotopic values, which suggest that the dolomite formed from cool normal marine waters. This dolomitization appears to correspond to the second (Late Miocene) stage of dolomitization within the Marion Plateau carbon­ ates, which also occurred in a marine environment changing rapidly from shallow to deep marine. Comparison between the Ryukyus and the southern Queensland shelf

As noted, one of the few areas where subtropical carbonates have been studied in any detail is the Ryukyu Islands (Tsuji, 1993). The Ryukyu Island Arc (Fig. 18), which extends c. 1200 km from Ky­ ushu south-westward to Taiwan, rims the north­ western Pacific Ocean, and is bounded by the Okinawa Trough to the north-west and the Ryukyu Trench to the south-east. The Kuroshio boundary current flows to the north-east through the Okinawa Trough and extends the influence of tropical water into the Ryukyus. To the west of Miyako Island, Tsuji (1993) recognized five sedimentary facies on the shelf: 1 reef facies (0-60 m), made mainly of autochtho-· nous hermatypic corals and encrusting algae; 2 near reef sand facies (0-90 m), characterized by tests of shallow benthic Foraminifera such as Cal..

19 1

Subtropical carbonate platform development

.�/1 �1( cc;!HIu§YJ/1 �/E/1

fJIJ rCU�UrC OrC!EI1 fNJ

il

8 124· 0

,.•.

136.

500 km

Fig. 18. Location map of the Ryukyu Islands showing the area studied by Tsuji ( 1 993).

carina and Marginopora, plus Halimeda segments; 3 muddy sand-sandy mud facies (20-60 m), con­ sisting of a very fine sand-size carbonate fraction and lime mud; 4 rhodolith and large foraminiferal gravelly sand facies (60-200 m), containing pebble- to cobble-size rhodoliths and/or the large benthic Foraminifera Cycloclypeus; 5 bryozoan sand facies (80-200 m), rich in bryo­ zoan fragments. Tsuji et al. ( 1989) and Tsuji ( 1993) showed that tidal currents dominate the shelf area, and that

current speeds are high in the open sea where the reef, rhodolith and large Foraminifera, and bryo­ zoan facies are distributed, and low in the restricted area where the muddy sand facies is distributed. The most obvious differences between offshore Miyako Island and offshore Fraser Island lie in their relative tectonic setting and oceanography. The differences in tectonic settings create differences in their respective large- and small-scale topography, and in the amount of terrigenous influx. The large topographical setting created by island-arc develop­ ment has strongly influenced the deposition of

192

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Marshall et a!.

carbonates in the Ryukyus. The Okinawa Trough acts as a barrier to siliciclastic input from the continent, and consequently, the Ryukyu arc is a high carbonate area. On the other hand, the conti­ nental shelf of southern Queensland exists in a passive margin setting where terrigenous grains are constantly supplied from the continent, although Fraser Island acts as a barrier to siliciclastic input. In both areas the influence of strong currents is apparent, particularly with respect to rhodolith formation, but the measurement of near-bottom currents on the shelf and upper slope has demon­ strated a significant difference in the hydrological regime between the areas. Off Fraser Island, current measurements confirm that the southward-flowing East Australian Current dominates the hydrological regime, whereas near-sea-bed current measure­ ments off Miyako Island indicate a predominantly tidal influence amplified by the topography and the opposing flow of the East China Sea at high tide and the Pacific Ocean at low tide. However, despite different tectonic and oceano­ graphic settings, the carbonate sediments in the southern Ryukyus and southern Queensland are very similar. Coralline algae are one of the most important carbonate constituents in both areas. They have created a rhodolith-large Foraminifera facies in the southern Ryukyus and a rhodolith­ dominated facies in southern Queensland. The rhodoliths in both areas can be classified as deep­ water rhodoliths (Bosence, 1983) from their occur­ rence. Rhodoliths are distributed on the outer shelf in the Ryukyus at depths of between 60-150 m, and on the mid-shelf off southern Queensland at depths of between 30 and 140 m. Corals, bryozoans and Halimeda are associated with coralline algae in both areas. It appears that similar settings, in terms of their subtropical latitudinal position and range of water temperatures, have determined their com­ mon biofacies. The formation of rhodoliths rather than crusts can be attributed to the high-energy oceanographic regime in both areas. The in-situ direct measure­ ment of near-bottom currents on the shelf and slope has proven the existence of high-velocity currents, up to 130 em s-1 at depth, controlling the distribu­ tion of coarse sediments both in the Ryukyus and southern Queensland. In the study area off Fraser Island, the sediments on the shelf and upper slope are mud free. The EAC impinges on the continental shelf and the distribution of shelf sediments is strongly influenced by the current (Harris et a!.,

1996). Tsuji ( 1993) described coarse, mud-free sediments off Miyako Island, and indicated that tidal currents in the Ryukyus provide the high­ energy deeper shelf environment essential to win­ now the mud fraction and move rhodoliths; a process important to their concentric growth, par­ ticularly when it is superimposed by processes such as swells or typhoons. The high-energy environ­ ments in both areas provide the necessary boundary conditions for coarse sediment, specifically rhode­ lith, formation and accumulation.

CONCLUSIONS

For the past 3 0 yr, substantial emphasis has been placed on depositional models of carbonate rocks, particularly from an exploration viewpoint. The knowledge of sedimentary facies helps to predict the distribution and the heterogeneity of reservoirs, and carbonate sedimentary facies models have aided the exploration for hydrocarbons in carbonate reser­ voirs throughout the world (e.g. Scholle et al., 1983; Roehl & Choquette, 1985). However, the great majority of these models have been developed on the basis of studies of tropical shelf carbonates. Models based on temperate and subtropical carbon­ ates are virtually non-existent. This is despite the fact that the sedimentary facies identified in the present study area are very similar to carbonate facies in many parts of the world, some of which are oil-producing. In this paper we make the case for the recognition of extensive subtropical build-ups composed largely of coralline algae, corals, bryozoans and Foramin­ ifera. The combination of relatively shallow-water hermatypic corals and relatively deep-water rhodo­ liths would seem to be a contradiction, but it is exactly this association that typifies the subtropical shelf environment. Although coralgal associations are more often tropical, it is the relative abundance of coralline algae, whether as crusts or rhodoliths, together with the restricted distribution and diver­ sity of the corals, that makes this environment unique. Another attribute that can be associated with subtropical shelf carbonates is the variation between Halimeda towards the tropical boundary and bryozoans towards the temperate boundary.. They can also be separated from temperate shelf carbonates in that they can develop bioherms that produce significant vertical relief. In this sense, subtropical carbonates are a true watershed be-

Subtropical carbonate platform development tween majqr shelf carbonate depositional environ­ ments. Such build-ups, growing at the same time as their better understood tropical counterparts, occurred in the Miocene and Quat�rnary of north-eastern Aus­ tralia. This indicates that these assemblages will shift latitudinally in response to climate change, and this is supported by the existence of warm temperate assemblages beneath tropical carbonate platforms further north. We believe that recognition of these environmentally sensitive assemblages in ancient carbonates will prove to be invaluable in palaeogeographical and palaeoclimate reconstruc­ tions.

ACKNOWLED GEMENTS

We wish to thank the captain and crew of R.V. Rig Seismic for the successful completion of Cruise 105 off southern Queensland. We also thank the AGSO seismic and geological technicians for their usual high standard of support. David Feary and Gary Bickford (both AGSO) contributed their time and expertise to the cruise. We thank Peter Harris (previously Ocean Sciences Institute, Sydney Uni­ versity, but at present at the Antarctic CRC, Hobart) for his expertise and dedication with the current meter deployments and interpretation. We acknowledge our appreciation to Hiroshi Nakagawa (Japan Oil Engineering Co. Ltd) for his effort in identification of the foraminifers, and Keiko Ha­ tano and Manami Aikawa (JNOC) for X-ray dif­ fraction and stable isotope analyses. We thank Colin Braithwaite and Ian Macintyre for their reviews of the manuscript, and Gilbert Camoin for his editorial assistance. This paper is published with the permission of the Executive Directors of the Australian Geological Survey Organisation and the Technology Research Center of the Japan National Oil Corporation.

REFERENCES

BOLAND, F.M. & HAMON, B.V. ( 1 970) The East Australian Current, 1 96 5- 1 96 8 . Deep-Sea Res., 17, 777-794. BONE, Y. & JAMES, N.P. ( 1 993) Bryozoans as carbonate sediment producers on the cool-water Lacepede Shelf, southern Australia. Sediment. Geol., 86, 247-27 1 . BOREEN, T.D. & JAMES, N.P. ( 1 993) Holocene sediment dynamics on a cool-water carbonate shelf: Otway, south­ eastern Australia. J. sediment. Petrol., 63, 574-588.

1 93

BOREEN, T., JAMES, N., WILSON, C. & HEGGIE, D. ( 1 993) Surficial cool-water carbonate sediments on the Otway continental margin, southeastern Australia. Mar. Geol., 1 1 2, 3 5-56. BOSENCE, D.W.J. ( 1 983) Description and classification of rhodoliths (rhodoids, rhodolites). In: Coated Grains (Ed. Peryt, T.M.), pp. 225-242. Springer-Verlag, Berlin. CARANNANTE, G., ESTEBAN, M., MILLIMAN, J.D. & SIMONE ( 1 988) Carbonate lithofacies as paleolatitude indicators: problems and limitations. Sediment. Geol., 60, 333346. CHURCH, J.A. ( 1 987) East Australian Current adjacent to the Great Barrier Reef. Aust. J. mar. freshw. Res., 38, 67 1 -6 8 3 . COLLINS, L.B. ( 1 988) Sediments and history o f the Rott­ nest Shelf, southwest Australia: a swell-dominated, non­ tropical carbonate margin. Sediment. Geol., 60, 1 5-50. CONOLLY, J. R. & VON DER BORCH. C.C. ( 1 967) Sedimenta­ tion and physiography of the sea floor south of Austra­ lia. Sediment. Geol., 1, 1 8 1 -220. DAVIES, P .J., SYMONDS, P.A., FEARY, D.A. & P IGRAM, C.J. ( 1 987) Horizontal plate motion: a key allocyclic factor in the evolution of the Great Barrier Reef. Science, 238, 1697-1700. DAVIES, P.J., SYMONDS, P.A., FEARY, D.A. & P IGRAM, C.J. ( 1 989) The evolution of the carbonate platforms of northeast Australia. In: Controls on Carbonate Platform to Basin Development (Eds Crevello, P., Sarg, J.F., Read, J.F. & Wilson, J.L.), Spec. Pub!. Soc. econ. Paleont. Miner., Tulsa, 44, 233-258. DAVIES, P.J., McKENZIE, J.A., JuLSON, A.P. et a!. ( 1 99 1 ) Preliminary Results of the Ocean Drilling Program, Leg 133-Northeast Australia. US Government Printing Of­ fice, Washington, DC. DAVIES, P.J. & TsuJI, Y. et a!. ( 1 992) Tropical and temper­ ate carbonate environments: the effects of sea level, climate and tectonics on facies development-post cruise report. Bur. Min. Res. Aust. Rec. 1 9921 17, 85 pp. ELLIS, P.L. ( 1 966) The Maryborough Basin. APEAJ. 3036. ETHERIDGE, M.A., SYMONDS, P.A. & LISTER. G.S. ( 1 990) Application of the detachment model to reconstruction of conjugate passive margins. In: Extensional Tectonics and Stratigraphy of the North Atlantic Margins (Eds Tankard, A.J. & Balkwill, H.), Mem. Am. Assoc. petrol. Geol, Tulsa, 46 , 23-40. GILLESPIE, R. & POLACH, H.A. ( 1 979) The suitability of marine shells for radiocarbon dating of Australian prehistory. In: Radiocarbon Dating (Eds Berger, R. & Suess, H.), pp. 404-42 1 , University of California Press, Berkeley. GRIMES, K.G. ( 1 982) Stratigraphic drilling report-GSQ Sandy Cape 1 -3R. Queensland Gov. Mining J., 83, 224-233. HAMON, B.V. ( 1 965) The East Australian Current 1 9601 964. Deep-Sea Res. , 12, 899-92 1 . HARRIS, P.T., TSUJI, Y., MARSHALL, J.F., DAVIES P.J. HoNDA, N. & MATSUDA, H. ( 1 996) Sand and rhodolith­ gravel entrainment on the mid- to outer-shelf under a western boundary current: Fraser Island continental shelf, eastern Australia. Mar. Geol. , 1 29, 3 1 3-330. HAYES, D.E. & RINGIS, J. ( 1 973) Seafloor spreading in the Tasman Sea. Nature, 243, 454-459.

194

J. F

Marshall et al.

IRYU, Y. ( 1 985) Study on the recent rhodoliths around the Ryukyu Islands. In: Prompt Reports of the Comprehen­ sive and Scientific Survey in the Ryuku Archipelago, 2, 1 23-1 3 3 , Kagoshima University (in Japanese with Eng­ lish abstract). JAMES, N.P. ( 1 990) Cool water carbonates: viable ana­ logues for Paleozoic limestones. International Sedimen­ tological Congress, Nottingham, p. 275 (abstract). JAMES, N.P. & BONE, Y. ( 1 99 1 ) Origin of a cool water, Oligo-Miocene deep shelf limestone, Eucla Platform, southern Australia. Sedimentology, 38, 323-34 1 . JAMES, N.P. & VON DER BORCH, C.C. ( 1 99 1 ) Carbonate shelf edge off southern Australian: a prograding open­ platform margin. Geology, 19, 1 005- 1 008. JAMES, N.P., BONE, Y., VON DER BORCH, C.C. & GOSTIN, V.A. ( 1 992) Modern carbonate and terrigenous clastic sediments on a cool-water, high-energy, mid-latitude shelf; Lacepede Shelf, southern Australia. Sedimentol­ ogy, 39, 877-904. JONES, H.A. ( 1 973) Submerged shorelines and channels on the east Australian continental shelfbetween Sandy Cape and Cape Moreton. Bur. Min. Res. Aust. Rec., 1 97 3/46, 7 pp. JoNES, H.A., DAVIES, P.J. & MARSHALL, J.F. ( 1 975) Origin of the shelf break off southeast Australia. J. geol. Soc. Aust., 22 , 7 1 -78. JONGSMA, D. & MUTTER, J.C. ( 1 978) Non-axial breaching of a rift valley: evidence from the Lord Howe Rise and the southeastern Australian margin. Earth planet. Sci. Lett. , 39, 226-234. KONISHI, K., SCHLANGER, S.O. & OMURA, A. ( 1 970) Neo­ tectonic rates in the Central Ryukyu Islands derived from 230Th coral ages. Mar. Geol., 9, 225-240. LEES, A. ( 1 975) Possible influence of salinity and temper­ ature on modern shelf carbonate sedimentation. Mar­ . Geol. , 19, 1 59-1 98. LEES, A. & BULLER, A.T. ( 1 972) Modern temperate-water and warm-water shelf carbonate sediments contrasted. Mar. Geol. , 13, M67-M73. LITTLER, M.M., LITTLER, D.S. & HANISAK, M.D. ( 1 99 1 ) Deep-water rhodolith distribution, productivity, and growth history at sites of formation and subsequent degradation. J. exp. Mar. Bioi. Ecol. , 150, 1 63- 1 80. MARSHALL, J.F. ( 1 97 1 ) Phosphatic sediments on the eastern Australian continental slope. Bur. Min. Res. Aust. Rec., 1 97 1 /59, 9 pp. MARSHALL, J.F. ( 1 972) Morphology of the east Australian continental margin between 2l"S and 33"S. Bur. Min. Res. Aust. Rec., 1 972170, 1 8 pp. MARSHALL, J. F. ( 1 977) Marine geology of the Capricorn Channel area. Bur. Min. Res. Aust. Bull. , 163, 8 1 pp. MARSHALL, J.F. ( 1 978) Morphology and shallow structure of the continental shelf of southern Queensland and northern New South Wales. Bur. Min. Res. Aust. Rec., 1 978/ 1 00, 25 pp. MARSHALL, J.F. ( 1 979) The development of the continen­ tal shelf of northern New South Wales. Bur. Min. Res. J. Aust. Geol. Geophys., 4, 28 1 -288. MARSHALL, J. F. ( 1 9 80) Continental shelf sediments: southern Queensland and northern New South Wales. Bur. Min. Res. Aust. Bull., 207, 33 pp. MARSHALL, J. F. & DAVIES, P.J. ( 1 978) Skeletal carbonate variation on the continental shelf of eastern Australia.

Bur. Min. Res. J. Aust. Geol. Geophys. , 3, 85-92. MATSUDA, S. ( 1 987) Non-articulated coralline algae in the Ryukyus. Earth Monthly (Chikyu), 9, 1 62- 1 68 (in Jap­ anese). MATSUDA, S. ( 1 989) Succession and growth rates of en­ crusting crustose coralline algae (Rhodophyta, Cryp­ tonemiales) in upper fore-reef environment off Ishigaki Island, Ryukyu islands. Coral ReefS, 7, 1 85- 1 95. MILLIMAN, J.D. ( 1 974) Marine Carbonates. Springer­ Verlag, Berlin. MINNERY, G.A. ( 1 990) Crustose coralline algae from the Flower Garden Banks, Northwestern Gulf of Mexico: controls on distribution and growth morphology. J. sediment. Petrol. , 60, 992- 1 007. MINOURA, K. & NAKAMORI, T. ( 1 982) Depositional envi­ ronment of algal balls in the Ryukyu Group, Ryukyu Islands, southwestern Japan. J. Geol., 90, 602-609. MONTAGGIONI, L.F. ( 1 979) Environmental significance of rhodolites from the Mascarene reef province, western Indian Ocean. Bull. Cent. Rech. Explor. Prod. Elf Aquitaine, 3, 7 1 2-722. NELSON, C.S. ( 1 988) Non-tropical shelf carbonates­ modern and ancient. Sediment. Geol., 60, 5 1 -70. NELSON, C.S., KEANE, S.L. & HEAD, P.S. ( 1 988) Non­ tropical carbonate deposits on the modern New Zealand shelf. Sediment. Geol., 60, 7 1 -94. NODA, M. ( 1 9 84a) Ryukyu Limestone ofOkinoerabu-jima (Part I )-Stratigraphy. J. geol. Soc. Japan, 90, 26 1 -270 (in Japanese with English abstract). NODA, M. ( 1 984b) Ryukyu Limestone of Okinoerabujima (Part 2)-Sedimentary facies. J. geol. Soc. Japan, 90, 3 1 9-328 (in Japanese with English abstract). PALMIERI, V. ( 1 984) Neogene Foraminiferida from GSQ Sandy Cape l -3R bore, Queensland: a biostratigraphic appraisal. Palaeogeogr. Palaeoclimatol. Palaeoecol., 46, 1 65 - 1 8 3 . P HIPPS, C.V.G. ( 1 970) Dating o f eustatic events from core taken in the Gulf of Carpentaria, and samples from the New South Wales continental shelf. Aust. J. Sci. , 32, 329-330. PIGRAM, C.J., DAVIES, P.J. & CHAPRONIERE, G.C.H. ( 1 993) Cement stratigraphy and the demise of the Early­ Middle Miocene carbonate platform on the Marion Plateau. In: McKenzie, J.A., Davies, P.J., Palmer-Jut son, A. et al., 499-5 1 2. Ocean Drilling Program, College Station, TX. REID, R.P. & MACINTYRE, l.G. ( 1 988) Foraminiferal-algal nodules from the eastern Caribbean: growth history and implications on the value of nodules as paleoenviron­ mental indicators. Palaios, 3, 424-43 5 . ROEHL, P.O. & CHOQUETTE, P.W. ( 1 98 5) Perspectives on world-class carbonate petroleum reservoirs. Am. Assoc. petrol. Geol. Bull., 69, 1 48 (abstract). SATO, T. & TAKAYAMA, T. ( 1 990) Age determination based on calcareous nannofossils. J. lap. Assoc. petrol. Tech­ no!., 55, 1 2 1 - 1 28. SCHOLLE, P.A., BEBOUT, D.G. & MOORE, C.H. (Eds) ( 1 983) Carbonate Depositional Environments. Mem. Am. As­ soc. petrol. Geol., 33, 708 pp. SHEPARD, F.P. ( 1 963) Submarine Geology, 2nd edn. Harper and Row, New York. STUIVER, M. & POLACH, H.A. ( 1 977) Discussion: reporting on C- 1 4 data. Radiocarbon, 19, 3 5 5-363.

Subtropical carbonate plat form development SYMONDS, P.A., DAVIES, P.J. & PARISI, A. ( 1 983) Structure and stratigraphy of the Central Great Barrier Reef. Bur. Min. Res. J. Aust. Geol. Geophys., 8, 277-29 1 . TsuJI, Y. ( 1 993) Tide influenced high energy environ­ ments and rhodo1ith-associated carbonate deposition on the outer shelf and slope off the Miyako Islands, southern Ryukyu Arc, Japan. Mar. Geol. , 1 13, 25 5-27 1 . TSUJI, Y., SUNOUCHI, H., YAMAMURA, T., KODATO, T., YUKI, T. & IWAMOTO, H. ( 1 989) Present-day carbonate sediments and their depositional environments, western offshore Miyako Island, Ryukyus. Earth Monthly (Chikyu), 11, 6 1 2-6 1 7 (in Japanese). TsuJI, Y., HONDA, N., MATSUDA, H. et a!. ( 1 994) Sedimen­ tology of subtropical to temperate carbonates and the East Australian Current off eastern Australia. Earth Monthly (Chikyu), 16, 407-4 1 1 (in Japanese). VEEH, H.H. & VEEVERS, J.J. ( 1 970) Sea-level at - 1 75 m off the Great Barrier Reef 1 3600 to 1 7000 years ago. Nature, 226, 5 36-537. WASS, R.E., CONOLLY, J.R. & MACINTYRE, R.J. ( 1 970) Bryozoan carbonate sand continuous along southern

195

Australia. Mar. Geol., 9, 63-73. WEISSEL, J.K. & WATTS, A.B. ( 1 979) Tectonic evolution of the Coral Sea Basin. J. geophys. Res., 84, 4572-4582. WERNICKE, B. ( 1 98 1 ) Low-angle normal faults in the Basin and Range province: nappe tectonics in an extending orogen. Nature, 291, 645-648. WERNICKE, B. ( 1 985) Uniform-sense normal simple shear of the continental lithosphere. Can. J. Earth Sci., 22, 1 05 - 1 2 5 . WYRTKI, K . ( 1 960) The surface circulation in the Coral and Tasman Seas. CSIRO Div. Fish. Ocean. Tech. Pap., 8, 1 -44. WYRTK, K. ( 1 966) East Australia Current. In: Encyclopae­ dia of Oceanogtophy (Ed. Fairbridge, R.). Reinhold, New York. YuKI, T., IWAMOTO, H., TsuJI, Y. et a!. ( 1 988) Cyclic sedimentary sequences and their lateral variation in the Ryukyu Limestone (Pleistocene), lrabu Island, Oki­ nawa, Japan. Soc. econ. Paleont. Miner. Annual Midyear Meeting. Abstracts, p. 59.

Spec. Pubis int. Ass. Sediment. (1998) 25, 197-213

Pleistocene reef complex deposits in the Central Ryukyus, south-western Japan Y. I R Y U , T. NAKAMO RI and T. YAMADA Institute of Geology and Paleontology, Graduate Sch ool ofScience, Tohoku University, Aobayama, Sendai 980-77, Japan

ABSTRACT The Ryukyu Group, composed of Pleistocene reef complex deposits that locally pass laterally into terrestrial sediments, is extensively distributed over the Ryukyu Islands. The carbonate rocks are divided into four facies: coral, rhodolith, Cycloclypeus-Opercu/ina, and poorly sorted detrital limestones. Their depositional environments are specified based on the distribution and depth range of the present-day reef biota and associated sediments around the Ryukyu Islands. The stratigraphical succession of the Ryukyu Group is investigated at Toku-no-shima, Okierabu-jima and Yoron-jima, Central Ryukyus. Here reef complex deposits are associated with terrestrial sediments formed at relatively high and low sea-level stands. The highstand deposits are thick, occur extensively, and consist of terrestrial and marine conglomerates, and coral, rhodolith and poorly sorted detrital limestones that are arranged from inland proximal to coastal distal parts. Lowstand deposits are thin, composed mainly of coral limestone, and distributed in very limited areas at elevations less than the highstand deposits. Abundant rhodoliths occur in the deep fore-reef to insular shelf areas in the Pleistocene to present-day Ryukyus, which perhaps indicates that nutrient-rich marine environments observed in Halimeda banks have never prevailed over the shelves on the Ryukyus.

INTRODUCTION The Ryukyu Islands (Ryukyus) are located to the south-west of mainland Japan and consist of several tens of islands and islets, extending from Tanega­ shima (30° 44'N, 131oO'E) in the north-east to Yonaguni-jima (24o27'N, 123oO'E) in the south­ west (Fig. 1). These islands are an active island arc (Ryukyu Arc) generated by subduction of the Phil­ ippine Sea Plate beneath the Eurasian Plate and bounded by the East China Sea on the north-west and by the Pacific Ocean on the south-east. Pleistocene reef complex (coral reef and associ­ ated fore-reef and moat) deposits accompanied by terrestrial sediments are distributed over most of the islands of the Central and Southern Ryukyus and reach up to c. 200 m in elevation. These deposits have been called the Riukiu Limestone (Yabe & Hanzawa, 1930) or the Ryukyu Group (MacNeil, 1960). The name 'Ryukyu Group' is adopted in this study, because the unit contains both carbonate and siliciclastic rocks.

Fig. 1. Map showing localities of the study area.

Reefs and Carbonate Platforms in the Pacific and Indian Oceans Edited by G. F. Camoin and P. J. Davies © 1998 International Association of Sedimentologists ISBN: 978-0-632-04778-9

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Stratigraphical studies on the Ryukyu Group were carried out from the morphostratigraphical viewpoint in the 1960s (Nakagawa, 1967, 1969). In this paradigm, the Group was regarded as terrace­ forming deposits: coral reefs were developed at high sea-level stands and the islands have risen to form a flight of terraces from inland to the coast. Uranium­ series dating was introduced to the field of studying the Group by Konishi and his co-workers (Konishi, 1967; Konishi et a!., 1970, 1974). They revised the stratigraphy of the Group based on the resulting ages, conducted a correlation of the Group with reef deposits in other regions such as Barbados and New Guinea, and estimated rates of vertical displace­ ment of the Ryukyu Islands. The Okinawa Quater­ nary Research Group (1976) and Takayasu (1976, 1978) expressed a quite different opinion from the morphostratigraphers. They divided the Ryukyu Group into two basic stratigraphical units: the older, main constituent unit with extensive distri­ bution; and the younger terrace-forming deposits with limited distribution. The older unit was thought not to be coral reef deposits but to be basinal sediments. In the stratigraphical works of these decades, sedimentological and palaeontologi­ cal features were not fully analysed, so that neither an anatomy of reef complex deposits nor a palaeo­ bathymetric interpretation was provided. Stratigraphical and sedimentological studies of the Ryukyu Group since the middle of the 1970s have shown that the Group is composed principally of plural reef complex deposits which are compara­ ble with those of the present-day Ryukyus (Noda, 1976; Nakamori, 1986; Iryu et a!., 1992). The extensive investigations of the distribution of mod­ ern biota and sediments around the Ryukyus (Na­ kamori, 1986; Iryu & Matsuda, 1988; Matsuda et a!., 1992; Tsuji, 1993, Iryu et a!., 1995) have allowed precise palaeoenvironmental determina­ tions, especially of palaeobathymetry, of the bio­ and lithofacies of the Group(Nakamori, 1986; Iryu, 1992; Iryu et a!., 1992; Nakamori et a!., 1995a). This paper aims to provide a basic framework for establishing a reef stratigraphy for the Ryukyu Group. We here present a classification of carbonate and siliciclastic rocks of the Group and specify dep­ ositional environments of bio- and lithofacies based on the known distribution of marine biota and sed­ iments around the Ryukyus. Using such a frame­ work, the Group is shown to be divisible into plural reef complex deposits by referring to stratigraphical examples of the Ryukyu Group in the islands of

Toku-no-shima, Okierabu-jima and Yoron-jima, Central Ryukyus (Fig. 1). The Pleistocene reef for­ mations in the three islands are discussed.

LITHOLOGY OF THE RYUKYU GROUP AND DEPOSITIONAL ENVIRONMENTS The Ryukyu Group consists of both carbonate and siliciclastic rocks (Table 1). Six major facies have been identified in the carbonate rocks: coral, rhod­ olith, Cycloclypeus-Operc ulina, Halimeda, poorly sorted detrital, and well-sorted detrital limestones (Nakamori et a!., 1995a). Of these, two facies (Halimeda and well-sorted detrital facies) do not occur in the study area. Carbonate rocks Cora/limestone

·

Coral limestone is defined as a limestone in which autochthonous hermatypic corals are contained. The volume of hermatypic corals can be as high as 40%. This facies is up to 50 m thick and occurs at the proximal part of a single reef complex deposit, overlying conglomerate and sandstone or the base­ ment rocks. It grades laterally into distal rhodolith limestone. This facies can be subdivided into two subfacies. One is characterized by the occurrence of autochthonous corals embedded with bioclasts of corals, coralline algae, foraminifers and molluscs, forming a floatstone-like structure (Fig. 2A). The other comprises massive to encrusting forms of hermatypic corals and nongeniculate coralline algae that accumulate to form a framestone structure (Fig. 2B). Dendritic (i.e. branching) corals act as baffles in places. The former sub facies is much more abundant than the latter. Hermatypic corals are distributed from reef flat to fore-reef slope down to depths of 100 m in the present-day Ryukyu Islands. Iryu et a!.(1995) noted that the corals are common to abundant to depths of 50 m and that they are extremely reduced in number and diversity at greater depths. It follows that the coral limestone largely accumulated at depths up to 50 m. Depositional depth of the coral limestone can be determined more precisely by examining fossil coral communities and non­ geniculate coralline-algal assemblages. Nakamori (1986) and Iryu (1992), respectively, discriminated

Table 1.

Classification of carbonate and siliciclastic rocks of the Ryukyu Group

Carbonate rocks Coral limestone

Rhodolith limestone

Cycloc/ypeus-Operculina limestone

Poorly sorted detrital limestone Siliciclastic rocks Conglomerate and sandstone

Expected depositional environment

Definition

Sedimentological features

Palaeontological features

Limestone (framestone) with autochthonous hermatypic corals

Massive or bounded by corals and coralline algae to show biological framework structures

Associated with coralline algae, echinoids, molluscs and larger foraminifers

Reef flat and fore-reef slope (0-50 m deep)

Limestone (rudstone) consisting of more than 20% by volume rhodoliths

Massive but exhibiting parallel- to cross-bedding in places

Associated with C. carpenteri and solitary corals

Insular shelf (50-150 m deep)

Limestone (rudstone) with abundant C. carpenteri and/or 0. ammonoides

Massive with micrite

Associated with rhodoliths

Depths from 50 to !50 m where C. carpenteri dominates and I 0-200 m where 0. ammonoides dominates

Poorly sorted limestone (packstone) composed of organic skeletons

Parallel- or cross-bedding conspicuous

Siliciclastic conglomerate with intercalated beds of sandstone

Massive with muddy matrix Parallel- or cross-bedding conspicuous

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Y lryu, T. Nakamori & T. Yamada

Fig. 2. Carbonate rocks of the Ryukyu Group. (A) Coral limestone with massive hermatypic corals, Porites sp., on an outcrop at Kunigami-misaki (Cape Kunigami), Okierabu-jima. Note that the corals are 'floating' in surrounding bioclasts. (B) Coral limestone with encrusting form of corals and non-geniculate coralline algae, showing a biological framework structure, exposed at Kametsu, Toku-no-shima. (C) Rhodolith limestone that crops out north of Masana, Okierabu-jima. Note concentrations of rhodoliths. The rhodoliths comprise multiple species of nongeniculate coralline algae and/or contain several individuals of an encrusting foraminifer Acervulina inhaerens. (D) Cycloc/ypeus­ Operculina limestone dominated by C. carpenteri (arrowed) from Uchijiro, Okierabu-jima. (E) Cycloclypeus-Operculina limestone dominated by 0. ammonoides (small lenticular forams) from Okidomari, Okierabu-jima. (F) Well-bedded, poorly sorted detrital limestone exposed to the west of Tamina, Okierabu-jima.

Pleistocene reef complex deposits, Ryukyus five coral commumties and four coralline-algal assemblages, each of which represents a particular depth range (Table 2). Rh odolith limestone The rhodolith limestone, characterized by rocks with more than 20% of the total volume made up of rhod­ oliths, is widespread, encloses areas of coral lime­ stone, and grades laterally into poorly sorted detrital limestone(Fig. 2C). It is mostly less than 20 m thick. This limestone is generally massive, although parallel- to cross-bedding is occasionally observed. The rhodoliths are well rounded, range in mean diameter from 0.5 to 8 em, and consist mainly of multiple species of thin, encrusting nongeniculate coralline algae and an encrusting foraminifer Acer­ vulina inhaerens, which together form an irregularly concentric internal structure. These rhodoliths occur associated with poorly sorted bioclasts of coralline algae, larger foraminifers ( Cycloc lypeus carpenteri and Operculina ammonoides), molluscs and bryozo­ ans. Solitary corals and brachiopods are also found. It is well documented that the modern rhodoliths analogous to fossil forms in the Ryukyu Group are extensively distributed on the insular shelves rang­ ing in depth from 50 to 150 m around Miyako-jima (Tsuji, 1993) and Okinawa-jima (Iryu et a!., 1995). Thus, the rhodolith limestone is thought to have been deposited in such deep fore-reef to insular shelf areas. Cycloclypeus-Operculina limestone This limestone is distinguished from the other facies by the abundance of larger foraminifers, C. carpenteri and/or 0. ammonoides. In most cases, this limestone is found associated with the rhoda­ lith limestone and exposed sporadically, forming a relatively thin massive bed (less than 5 m). It consists of up to pebble-sized bioclasts of coralline algae, bryozoans, foraminifers and molluscs as well as the tests of C. carpenteri and/or 0. ammonoides, and displays a grain-supported texture. Both fora­ minifers can occur together but in general one or the other predominates in this facies (Fig. 2D & E). It was confirmed that C. carpenteri inhabits the deep fore-reef to island shelf zones corresponding to the range of rhodoliths off Miyako-jima (Tsuji, 1993) and Okinawa-jima (Iryu et a!., 1995). Kodato & Nakagawa (1993) reported that 0. ammonoides occurs at depths from 30 to 60 m off Miyako-jima,

201

whereas Reiss & Hottinger (1984) found this spe­ cies in a much broader depth range ( 10-140 m) in the Gulf of Aqaba. It can be concluded that the limestones dominated by C. carpenteri and by 0. ammonoides were deposited at depths of 50-150 m and 10-200 m, respectively. Poorly sorted detrital limestone This limestone consists mainly of poorly sorted, coarse-grained sand- to pebble-sized bioclasts of foraminifers, bryozoans, corals and coralline algae, with less abundant molluscs and brachiopods (Fig. 2F). Autochthonous hermatypic corals are en­ tirely lacking. Rhodoliths, C. carpenteri and 0. ammonoides are found in places. Intergranular space is filled with micrite. It occurs at the most distal part of a single reef complex deposit of the Ryukyu Group and reaches more than 50 m in thickness. Rhythmic stratification (5-20 em thick) is common, with occasional large-scale (up to 1 m thick and 5-20 m across) cross-bedding. Common to abundant occurrences of bryozoan skeletons in this facies indicate that it was deposited on the insular shelf at depths greater than 50 m, because Iryu et a!. (1995) showed that the skeletons are comparably more abundant on the shelf at depths from 40 to 160 m off Miyako-jima. Siliciclastic sediments The siliciclastic sediments of the Ryukyu Group con­ sist of terrestrial and marine conglomerate accom­ panied by sandstone, and their thickness reaches up to 90 m. The terrestrial conglomerate consists of poorly sorted, angular to subangular, up to boulder-sized gravel with massive, muddy to sandy matrix (Fig. 3A). It occurs, overlying the basement rock, at the most proximal part of a single reef complex deposit and grades laterally into the marine con­ glomerate. The clasts are of metamorphic slate, sandstone and basalt derived from the basement rock. Generally, a clast-supported texture is evident in this facies. The marine conglomerate is characterized by poorly sorted, subangular to subrounded granules and pebbles, and by a matrix of fine- to coarse­ grained sand(Fig. 3B & C). It overlies the terrestrial conglomerate or covers directly the basement rocks. It is more or less bedded, commonly showing well­ defined cross-bedding. Fossil marine organisms

N 0 N

Fossil coral communities and nongeniculate coralline-algal assemblages of the Ryukyu Group and their depositional environments; the comparable modem coral communities and algal assemblages were defined by Nakamori (1986) and lryu & Matsuda (1988), respectively

Table 2.

Community or assemblage

Characteristic species or taxonomic groups

Other features

Hermatypic corals Acropora spp. (Branching) Community A Porites spp.

Expected depositional environment

Comparable modem community or assemblage

Moat to reef crest of fringing reef or protected shallow water of patch reefs

Porites cylindrica Com. Porites nigrescens Com. Heliopora coerulea Com.

Community B

Acropora spp. (Tabular)

Reef edge

Acropora hyacinthus Com.

Community C

Acropora spp. (Tabular) Porites spp. Favia spp. Platygyra spp.

Fore-reef slope (0-15 m deep)

Favia stelligera Com.

Community D

Pectiniidae Favia spp. Platygyra spp.

Fore-reef slope (10-30 m deep)

Oxypora lacera Com.

Community E

Leptoseris spp. Pachyseris spp.

Fore-reef slope to insular shelf (>30 m deep)

Leptoseris scabra Com.

� ......

�

F �

�

iS �

Occasionally associated with Cycloclypeus carpenteri, Operculina ammonoides and rhodoliths

0

:::!. � �

;;:: �

Nongeniculate coralline algae Hydrolithon onkodes Assemblage A Lithophyllum insipidum

0-20 m deep

Assemblage I

Assemblage B

Hydrolithon murakoshii Neogoniolithon fosliei Neogoniolithon sp. A sensu lryu & Matsuda (1994) Lithophyllum insipidum

20-35 m deep

Assemblage II

Assemblage C

Mesophyllum purpurascens

35-50 m deep

Assemblage III

Assemblage D

Lithothamnion australe Lithothamnion sp.

Associated with C. carpenteri and " rhodoliths

>50 m deep

�

�

Pleistocene reef complex deposits, Ryukyus

203

Fig. 3. Siliciclastic rocks of the Ryukyu Group. (A) Terrestrial conglomerate with subangular to subrounded clasts of the Mesozoic volcanic or volcaniclastic rocks on an outcrop near Yakomo, Okierabu-jima. (B) Thick, well-bedded marine conglomerate of unit I overlain by the coral limestone (arrowed) of unit 2, exposed at the mouth of Okuna-gawa, Toku-no-shima. Note an arrowhead indicating their boundary. (C) Marine conglomerate with thin layers rich in Operculina ammonoides, at Wanjouhama, Okierabu-jima.

such as corals, molluscs, larger foraminifers (0. am­ monoides), bryozoans and echinoids are abundant, which makes this conglomerate calcareous(Fig. 3C). Gravel with thin coralline-algal crusts (algal-coated gravel) occasionally occurs, forming concentrations. It is impossible to determine the depositional envi­ ronment of the marine conglomerate precisely. A broad depth range from 0 to 200 m appears to be the best estimate, and is based on the occurrence of var­ ious marine fossils including 0. ammono ides (10200 m deep).

STRATIGRAPHY Toku-no-shima In Toku-no-shima, basement rocks of the Ryukyu Group are Mesozoic metamorphic slate, sandstone and basalt, and Tertiary granitic rocks, forming hilly areas(up to 600 m in elevation) at the centre of the island. The Group consists of conglomerate and coral, rhodolith and poorly sorted detrital lime­ stones, and crops out on the hillside to the coastal plain at less than 200 m elevation (Figs 4 and 5). The following four stratigraphical units (units 1-4) are discriminated.

204

Y.

Iryu, T. Nakamori & T. Yamada

B'

Toku-no-shima

2km

'======---

Legend

AD B-1� C-16 C-2· C-3§ C-40 D-1� . . F --- --- E-2 Do D-2 D E-1 � ...,..'-'"• ,'.•;' . ,..•.. . . . E-3 !§§! E-4·Qlill] � G >-;.,..,_ • "•

·

Fig. 4. Geological map of the southern part of Toku-no-shima. (A) Recent beach and alluvial deposits; (B-E), the

Ryukyu Group: (B) coral limestone of unit 4; (C) unit 3 (C-1, coral limestone; C-2, rhodolith limestone; C-3, detrital limestone; C-4, conglomerate and sandstone); (D) unit 2 (D-1, coral limestone; D-2, conglomerate and sandstone); (E) unit I (E-1, coral limestone; E-2, rhodolith limestone; E-3, detrital limestone; E-4, conglomerate and sandstone). (F) basement rocks; (G) fault.

Unit 1, the lowest unit of the Group, is widely exposed in the southern part of Toku-no-shima and is distributed from 0 to 120 m elevation. The thickness is up to 90 m. It consists of conglomerate and coral, rhodolith and poorly sorted detrital limestones. The terrestrial conglomerate accumu­ lates on the basement rocks and grades laterally into marine conglomerate with intercalated beds of sandstone. The coral limestone overlies the con­ glomerate or unconformably covers the basement rocks. It occurs continuously from 0 to 120 m elevation. The rhodolith and detrital limestones are laterally equivalent to the coral limestone. Unit 2 (less than 30 m in thickness) consists of thin coral limestone and conglomerate and crops out at elevations of 20-70 m with very limited distribution. It lies unconformably on the rhodolith limestone and conglomerate of unit 1 (Fig. 3B). Unit 3 is another reef complex deposit, consisting

of conglomerate and coral, rhodolith and poorly sorted limestones, which overlies units 1 and 2. These four facies are distributed from inland prox­ imal to coastal distal parts in this order, and are arranged more or less parallel to the coast. Unit 3 overlies unit 1 unconformably and covers unit 2 conformably. It is distributed up to 200 m elevation and its thickness reaches 70 m. Unit 3 forms ter­ races at two levels: a higher terrace at about 160200 m elevation (the Itokina Terrace of Nakagawa (1967)), consisting of coral limestone and corre­ sponding to the former reef flat; and a lower terrace at elevations from 30 to 70 m(the Kametsu Terrace of Nakagawa (1967 }}, composed of the rhodolith and detrital limestones which accumulated on the insular shelf. Unit 4 crops out in the southern part of Toku-rio­ shima with limited, sporadic distribution. It con­ sists mainly of coral limestone associated with

205

Pleistocene reef complex deposits, Ryukyus m 200

100

A'

Fig. 5. Geological cross-sections of

the southern part of Toku-no­ shima across the lines A-A' and B-B' indicated in Fig. 4. Legend as in Fig. 4.

B' 2km ==--===---

rhodolith limestone and conglomerate containing pebble- to boulder-sized clasts of the limestones assignable to the lower units. It is less than 5 m in thickness. Coral limestone of this unit unconform­ ably overlies unit 3 at several localities less than 50 m in elevation. Okierabu-jima In Okierabu-jima, basement rocks of the Ryukyu Group are Mesozoic sedimentary rocks (turbidite and volcaniclastic-volcanic rocks), and Tertiary granodiorite and porphyrite. They form a topo­ graphic high and are unconformably overlain by the Ryukyu Group, which covers most of the island except for O-yama (246 m in elevation) and crops out at 0-200 m elevation. Two sedimentary units are discriminated in the Group (Figs 6 & 7). The lower unit consists of conglomerate with intercalated beds of sandstone, and reef complex limestones. This unit occurs extensively, rising to 130 m elevation, and is up to 90 m thick. Thick terrestrial conglomerate covers the basement rocks in the most proximal part of the unit. Marine conglomerate lies on the basement rocks on the northern coast or on the terrestrial conglomerate to the south-west of O-yama(Fig. 7). The reef complex limestones overlie the terrestrial and marine con­ glomerates or directly cover the basement rocks.

They consist mainly of coral limestone associated with poorly sorted detrital limestone. The coral limestone occurs continuously from 0 to 130 m elevation. The detrital limestone is laterally equiv­ alent to the coral limestone. The upper unit ( < 50 m thick) unconformably overlies the lower unit (Fig. 8). It is also composed of underlying conglomerate-sandstone and reef complex limestones. The lowest strata of this unit are proximal marine conglomerate and distal sand­ stone, which are distributed from 30 to 150 m elevation. The marine conglomerate is rather thin ( < 1.5 m thick) and occurs along a road encircling O-yama at 110-130 m. It passes laterally into the distal sandstone (thickening to 30 m) which is exposed to the north of O-yama and along the northern coast of the island. The sandstone is calcareous in places with abundant 0. ammonoides. Coral limestone occurs at elevations of 120-200 m, encircling O-yama. Rhodolith limestone and poorly sorted detrital limestone with abundant rhodoliths, Cycloclypeus and Operculina extend, surrounding the coral limestone. These strata are laterally equiv­ alent to the coral limestone. The upper unit forms terraces at two levels: the higher one, from 150 to 200 m elevation (the Shimoshiro Terrace of Naka­ gawa (1967)), is considered to have been a reef flat of the Pleistocene fringing reef; the lower one, from 30 to 100 m elevation (the Shinjo and Serikaku

206

Y lryu, T Nakamori & T Yamada

Okierabu-jima

n

n

lc

N Legend

-t-

AD B-1 m B-2 1!1 8 B-4 � B-5 G_] C-1 iji(;§ C-2 § B-3

B-

C-3

f}}t��

Em

0

2km

Fig. 6. Geological map of the western half of Okierabu-jima. A, Recent beach and alluvial deposits; B & C, the Ryukyu Group: B, upper unit (B-1, coral limestone; B-2, rhodolith limestone; B-3, Cycloclypeus-Operculina limestone; B-4, detrita1limestone; B-5, conglomerate and sandstone); C, lower unit (C-1, coral limestone; C-2, detrital limestone; C-3, conglomerate and sandstone); E, basement rocks.

Terraces of Nakagawa (1967)), is composed of insular shelf deposits. By stratigraphical position, reef geometry and topographical features (distributional elevations of the Ryukyu Group and terraces), the lower and upper units of Okierabu-jima are correlated with unit 1 and unit 3 in Toku-no-shima, respectively. Yoron-jima The basement of the Ryukyu Group is composed mainly of Mesozoic turbidites and volcaniclastic rocks with limestone olistholiths. This limestone is probably Permian in age. The Group unconform­ ably overlies the basement rocks and covers most of the island. It is less than 50 m thick and composed of siliciclastic rocks (conglomerate and sandstone)

and reef complex limestones (Fig. 9). There is a difference in stratigraphical succession of the Group between eastern and western parts of the island. In the eastern part, conglomerate and sandstone inter·· calating coral limestone overlie the basement rocks. These strata crop out in very limited areas at the surface, so their nature is uncertain, although litho.. logical data were given by some previous workers (e.g. Noda, 1976). They are overlain by coral, rhodolith and poorly sorted detrital limestones. The coral limestone occurs continuously from 0 to 97 m elevation and laterally grades into the rhodolith and poorly sorted detrital limestones exposed in the northern and western peripheries of the island. In the western part, sandstone and conglomerate ·are lacking and coral limestone overlies the basement rocks.

207

Pleistocene reef complex deposits, Ryukyus 0

0 A

0

250m 200 150 100

Fig. 7. Geological cross-sections of

the western half of Okierabu-jima across the lines 0-A, 0-B and 0-C indicated in Fig. 6. Legend as in Fig. 6.

The Ryukyu Group in Yoron-jima correlates with unit 1 in Toku-no-shima by stratigraphical position and topographical features. Age assignment Geological ages of the Ryukyu Group in three islands are determined by radiometric methods (uranium­ series and electron spin resonance (ESR) methods) and calcareous nanofossil biostratigraphy. Omura (1982) dated corals from the lower part of unit 1 exposed at the southern tip of Toku-no-shima by the U-series method. Although his resulting coral ages were close to or beyond the limitation of the 230Th/ 234U method(> 300 ka), he estimated the ages to be 387-709 ka from the mean 234UF38U activity ratio (Table 3). Nakamori et al. (199 Sa) measured ESR age at the point where Omura (1982) collected his samples and reported an age of 481 ± 17 ka. This agrees well with the U-series ages. Calcareous nano­ fossils detected from a lower horizon of unit 1 and lower and middle horizons of unit 3 by Nakamori et al. (1995a) indicate a CN14a zone(Okada & Bukry, 1980), the age of which ranges from 890 to 390 ka (Takayama & Sato, 1987). After considering all the data available, it is concluded that the ages of the Group in Toku-no-shima range widely from c. 400 to 900 ka.

50

f===--""'!1 km

Ikeda et al. (1991) reported ESR ages ranging from 730 to 840 ka for the lowest horizons of the lower unit in Okierabu-jima (Table 3). Calcareous nanofossils were recovered from the lowest horizon of the upper unit, indicating a CN14a zone (Okada & Bukry, 1980). Consequently, the age of the Ryukyu Group in Okierabu-jima is considered to be in a range from c. 400 to 900 ka. The same is true of the Group in Yoron-jima, where calcareous nano­ fossils detected from the uppermost horizon of the sandstone and conglomerate in the western part of the island indicate a CN14a zone and no radiomet­ ric age has been reported. As a consequence, the geological ages of the Ryukyu Group in the three islands are not well constrained, and range widely from c. 400 to 900 ka.

DISCUSSION Pleistocene reef formation in the Central Ryukyus Our stratigraphical and sedimentological study re­ veals that there are two different types of reef complex deposits on the three islands of the Central Ryukyus. One is represented by units 1 and 3 on Toku-no-shima and their correlative deposits on Okierabu-jima and Yoron-jima. The single unit is

208

Y Iryu, T Nakamori & T Yamada

A

Fig. 8. Field photographs

showing stratigraphical relationship between the lower and upper units of the Ryukyu Group in Okierabu-jima. (A) Coral limestone (c) overlain by Cycloclypeus-Operculina limestone dominated by 0. ammonoides (o) with thin, granule- to pebble-sized gravel layer at its base, exposed to the north of Serikaku. (B) Cycloclypeus­ Operculina limestone dominated by C. capenteri (cc) overlying coral limestone (c) on an outcrop near Ashikyora.

thick (up to 90 m) and extensively distributed on the islan·ds, and shows a wide range of elevations exceeding 100 m. It consists of a complete set of reef complex sediments: showing, from inland to the coast, terrestrial and marine conglomerates and coral, rhodolith and poorly sorted detrital lime­ stones. This arrangement is conformable with the distribution of biota and sediments in the present­ day reef complex around the Ryukyu Islands (lryu et a!., 1995). The other type includes units 2 and 4 on Toku-no-shima. Each unit is thin (less than 30 m), sporadically exposed, and composed mainly of conglomerate and coral limestone associated with occasional thin beds of rhodolith limestone. It does not form a complete set of reef deposits. These two types of reef complex deposits accumulated alternately: the former type of deposits lies at

greater elevations above sea-level (elevations of units 1 and 3 are 150 and 2.00 m, respectively) than the latter (units 2 and 4 are at 70 and 50 m, respectively). Taking account of these stratigraphi­ cal successions and positions, it is supposed that the: reef deposits constitute a record of glacio-eustatic sea-level changes, superimposed on a record of tectonic movement. The reef deposits at greater elevations(units 1 and 3 and their correlatives) may have accumulated during the rises of sea-level or at: highstands during interglacial episodes. Units 2 andl 4 may have been formed at lower stands of sea-level than units 1 and 3. These lowstands happened! twice, following the highstands that resulted in units 1 and 3. It is uncertain whether these lower stands correspond to other highstands in interglacial-· interstadial episodes or lowstands in glacial epi-·

209

Pleistocene reef complex deposits, Ryukyus

Yoron-jima N

t A'

A

Fig. 9. Geological map and

cross-section of Yoron-jima. A, Recent beach and alluvial deposits; B, the Ryukyu Group (B-1, coral limestone; B-2, rhodolith and detrital limestones; B-3, conglomerate and sandstone intercalating coral limestone); C, basement rocks; D, fault.

�:f

t:�m

� � m

A'

o�==-91km Legend

AD

sodes. At present, precise chronological data for the Ryukyu Group are apparently lacking, and thus it is impossible to specify the timing of reef formation in the Ryukyus and to correlate reef deposits with deep-sea oxygen isotope records. Two different types of reef deposits also occur in the Huon Peninsula, Papua New Guinea. It is known that a well-developed flight of terraces along the north-east coast of the Huon Peninsula consists of reef complex deposits with subordinate deltaic gravel formations (Chappell, 1 97 4, 1980), and thus this area is regarded as one of the best fields for studying Quaternary sea-level changes, similar to Barbados (Broecker et al., 1 968) and the Kabola Peninsula on Alor Island, Indonesia (Hantoro et al., 1 994). These terraces are thought to be an off­ lapping sequence of coral reefs, each comparable with modern reefs on the coast. In reality, the reef

B-1

a

B-2

�

B-3

0

c�

D�

deposits did not accumulate in such a simple manner as previously thought and their deposi­ tional history is more complex. Nakamori et at. ( 1 995b) showed that the deposits are divisible into several facies comparable with those of the Ryukyu Group, and undertook palaeoenvironmental inter­ pretation based on investigations of biota in the modern reefs on the Huon Peninsula, referring to the stratigraphical results of the Ryukyu Group. They found that both interglacial and interstadial reefs occur. The interglacial reefs are volumetrically much larger than the interstadial ones, with shallow facies characterized by abundant occurrences of shallow-water corals such as A. h yacinthus and A. palifera composing the interior bulk from inception to terrace surface (Nakamori et at., 1995b, Fig. 6). Consequently, the interglacial reefs maintained the shallow, frame-building communities throughout

2 10 Table 3.

Y.

Iryu, T. Nakamori & T. Yamada

Recently reported, radiometric and ESR ages of the Ryukyu Group in Okierabu-jim and Toku-no-shima

Locality

Sample number

Elevation (m)

Method of dating

Age (ka)

Reference

Toku-no-shima Isen-zaki

80-2-9-5

.

Table 2. U-Th geochemistry and ages based on TIMS

activity ratio (%o) decay corrected

23oTh/z3su activity ratio

Ages (ka) TIMS

Ages (ka) TIMS (corr) 2cr

234u;zJsu

232 Th (p.p.b.)

230 Th (p.p.t.)

23Bu (p.p.m.)

234u;z3su activity ratio (%o)

1 4 . 18 (23)

8. 73 (23)

3 . 3 7 {I)

144 ± 5

151 ± 5

0. 1 586 (4 1 )

16.2 (5)

1 6. 1 (5)

Reworked corals of the LGM cemented to the wall or slope 5 .46 (9) J95I 8.69 ( 1 9) Unidentified -205 9 . 70 { I I ) -220 62.89 (25) Porites J36

3 . 3 79 (5) 3 .04 { I )

1 43 ± 4 1 47 ± 6

1 50 ± 4 1 56 ± 7

0. 1 577 (36) 0. 1 95 9 (22)

1 6 . 1 (4) 20.3 (4)

1 6.0 (4) 1 9.7 (4)

Drowned reef J99/1 Porites

- 1 05

1 5.02 { I I)

6.8 ( 1 2)

3.35 {I)

1 39 ± 4

1 45 ± 4

0. 1 350 (3 1)

1 3.8 (4)

1 3.6 (4)

Recent talus J88 Acropora

- 1 00

1 3.30 ( 1 3)

0.93 1 (49)

3 .484 (9)

153 ± 6

153 ± 6

0.0 1 639 (86)

1 .5 6 (8)

1.46 (8)

Sample

Taxon

Depth (m)

In situ shallow-water cora/gal veneer J6 1 I Acropora - 1 60

�

� tl

:;:::

� (I)

�

Errors are given in 2cr values in parentheses referring to the last digits. Our 232Th concentrations are much higher than those found in corals from oceanic islands, which typically have less than 0.5 p.p.b. (Edwards et a/. , 1 987; Chen et a/. , 1 99 1 ). We therefore attribute the high 232Th content to the possible entrapment of detrital or marine materials (clays) from the adjacent areas. To account for this entrapment, our values are corrected for inherited U and Th assuming a U/Th ratio of 3.8. As can be seen in the last column, the corrections to the original ages are minor to negligible.

�

225

Mayotte for e-slope mor phology and sediments ·50-

platform edge

JAGO Dive 187, Mayotte North of Grand Recif du Nord

drowned reel

·100 sand slope 2.'i"

JAGO Dive 190, Mayotte Barrier Reef West of Pte. Mohila

50

100

·150

·200

11lQ

caves, cliffs, ledges,

-150

wall

..

vertk;.r���-� 9uuie� "

·200

1l

�.'i"

cemented slope

JAGO Dive 201, Mayotte -so-

Grand Recif du Nord-Est

platform edge

-250 . 100

1

.

'"

300 (

talus and large cipit boulders

. 150 -350 �u·

cemented slope . 200

begin of

. 250

sediment slope . 250

. so ·150

.!..

vertical chutes and gullies '

cemented slope

JAGO Dive 203, Mayotte Recif Bandele

platform edge

J.'i" caves, cli�s ledges,

drowned reel

. 100 wall

·200 - 150 talus and large cipit boulders

. 250

cemented slope - 200

erosional clifl with internal bedding

.1.�·

U

.'iU

IIMl

I�U

2UJ

L... L...LLI�L...J . __J_ � __L_l_.l

C .w - 250

latus and cipit boulders

sediment stope

Fig. 2. The morphology of the dive sites exhibits an overall pattern which consists of a sediment slope, a cemented slope and a steep wall. Note the slight dominance of cipit boulders on the windward margins. Drowned reefs occur on top of the wall and in some places even shallower, as at dive site 1 87. The five selected profiles are representative of all sites investigated (site 1 87 is representative of sites 1 88 and 1 89 plus a tectonic escarpment as at site 1 93 below 250 m; site 1 90 is similar to site 1 91; site 1 93 represents sites 1 92, 194 and 1 97; site 20I represents sites 1 86, 206, 1 98 and 207; site 203 is representative of sites 202, 204 and 205). They all are drawn to the same scale with no vertical exaggeration.

Upslope, grain size changes abruptly from silt to sand and even gravel around 250-m depth. The sedi­ ment consists of a mixture of recent unconsolidated material and reworked carbonates (Fig. 3b) charac­ terized by corroded surfaces and distinctive colours (i.e. white versus grey, respectively). The recent particles are mostly derived from the present-day shallow-water environment and include bioclasts of scleractinians, molluscs and echinoderms. The pre­ dominant portion of Halimeda plates among these skeletal grains, however, is derived from the living crops on top of the terraces between 70 and 100 m deep. Fossil material that is grey represents eroded

carbonates and reworked corals from the cemented slope and wall above (Fig. 3b). Accumulations of huge blocks occur on top of the sediment slope, up to several cubic metres in size (Fig. 3c). We have seen them in nine dives concen­ trated along the eastern and south-western margin of the barrier reef (Figs 1 & 2), which is the site most hit by cyclones. These blocks do not display any distinctive arrangement according to depth. They provide a hard substrate for various benthic organisms on a sediment slope where unlithified material prevails. At two sites, 192 and 193 (Fig. 1 ), tectonic es-

·

226

W-Ch. Dullo et a!.

Mayotte for e-slope mor phology and sediments carpments (Fig. 2, profile 193) provide exposures of the underlying cemented limestones. These natural outcrops exhibit well-bedded carbonates gently in­ clined downslope, which support horizontal ledges (Fig. 3d). The well-bedded carbonates are predom­ inantly composed of skeletal grainstones with mi­ nor packstones. The prime skeletal grains consist of two types of Halimeda, which can be differentiated according to the size of their cortical tubes (Fig. 3e). Other skeletal grains include fragments of coralline algae, and shallow- and deeper-water benthic fora­ minifers dominated by amphisteginids; however, bryozoans, gastropods and echinoids are rare. Cemented slope and wall

Upslope around 200 m water depth, the sediment slope grades into the cemented slope (Fig. 3t) com­ posed of well-cemented grainstones with a shallow­ water derived biota; reworked shallow-water corals include Acr opora, Por ites and Goniopora. The incli­ nation of the cemented slope is predominantly around 60', with a minimum inclination of 40'. The surface of the cemented slope corresponds to a submarine hardground, as indicated by the occur­ rence of multiple generations of borings. The rock surface becomes increasingly flaky upslope until the occurrence of the typical ledge rocks (Fig. 4a) on the steep wall generally between 190 and 90-m water depth. The ledges have a dense cover of living benthic organisms among which sponges prevail macroscopically. Unlithified sediment may accu­ mulate on top of the ledges or in small depressions,

3. (Opposite) (a) Contour ripples seen from the submersible; width of field of view is 1 m. Dive 188, 290-m water depth. (b) Sediment slope, exhibiting present skeletal grains represented by bioclasts of scleractinians, molluscs and echinoids as well as ancient clasts exhibiting a corroded surface (differentiated by colour) seen from the submersible; width of field of view is 1 m. Dive 1 90, 250-m water depth. (c) Cipit boulders seen from the submersible; the block is about 1 .5 m across. Dive 197, 3 1 0-m water depth. (d) Ledges on the deeper cliff from the older cemented slope system seen from the submersible; note the linear arrangement indicating bedding geometries. Dive 1 93, 260-m water depth. (e) Ha/imeda-packstone. Sample J76, dive 193, 250-m water depth, thin-section. (f) Cemented slope seen from the submersible. The cemented beds are steeply inclined (around 40') showing less than 5 em of loose sedimentary veneer. Dive 1 97, 240-m water depth; the inclination is predominantly around 60' with a minimum inclination of 40'.

Fig.

227

suggesting that most sediment is usually bypassing these steep slopes. A sharp increase in inclination occurs between 190 and 160 m depth, where the cemented slope steepens and forms an almost vertical wall (75 90') as a prominent part of the cemented slope. This wall is a typical feature of most of the investi­ gated slope sites of the island (Fig. 2). The surface of the wall is covered by irregularly arranged ledges, which may protrude up to half a metre from the wall. Along the SW side of the island, the cemented slope may continue up to 120-m water depth (Fig. 2, profile 193). Here, the local absence of ledges is probably related to a strong bypass of sediments, which suppresses their development be­ cause of episodic erosion. There are two karst systems within the bathy­ metric range of the cemented slope and the wall (Fig. 2). Their occurrence is limited to distinct levels. The first level is located between 150- and 155-m water depth and consists of small solution caves smaller than 3 m (Fig. 4b). These caves may con­ tinue up to 2 m horizontally into the wall or the cemented slope. Furthermore, the surfac� of the wall exhibits small-scale solution features, such as karren and kamenitza morphologies. However, no petro­ graphic evidence of subaerial exposure was ob­ served. A thin coralgal veneer has started to grow over the karst features in a few sites. In situ shallow­ water scleractinians (Acr opora, probably A. danai, and Por ites) derived from the initial framework of this coralgal facies at 152-m water depth were dated at 18.4 ± 0.5 ka (Table 1), and at -160 m at 16.1 ± 0.5 ka by TIMS (Table 2). The corals were partly encrusted by the Foraminifera Acer vulina in­ haer ens and by the coralline algae Lithophyllum sp. and Por olithon onkodes. Because of the very limited space for coralgal growth during this time of low­ stand of sea-level, most of the biota broke off and were transported downslope. Therefore, reworked corals and associated skeletal material derived from this ancient shallow-water reef environment were trapped below 180-m present-day water depth on top of the ledge rocks or on the cemented slope. These corals gave ages ranging from 19.1 ± 1.1 to 18.0 ± 0.5 ka. (Table 1), and from 19.7 ± 0.4 to 16.0 ± 0.4 ka by TIMS (Table 2). The second karst horizon occurs between 120and 125-m water depth, where we found caves more than 3 m deep and wide. Furthermore, karst chan­ nels and solution pipes may connect different caves, which we could prove by chasing fish from one to ·-

228

W-Ch. Dullo et a!.

Mayotte for e-slope mor phology and sediments another. The surface of the carbonates exhibits karren and kamenitza features as well. Two major lithofacies types were sampled from the wall. One is composed of Halimeda grainstones and packstones or skeletal grainstones and rud­ stones. They represent a reef talus facies forming the rocks of the wall or of the cemented slope. Therefore, corals occur only as fragments. Ages obtained from these cemented reef talus sediments range between 37.4 ± 0.8 and 27.6 ± 0.9 ka (Table 1). Although we did not see unequivocal internal bedding during all our dives, we could observe a clear indication of an inclined internal bedding in dive 197 (Fig. 3f). The other lithofacies comprises a thin veneer of a coralgal fabric which grows on the surface of the cemented rocks of the steep slope and the wall. This lithofacies type is made up of a scleractinian frame­ work forming the nucleus of the ledge rocks. Por ites and Acr opora are common; however, additional platy-growing species of Pavona were also recovered (Fig. 4c). This primary biogenic fabric, still in situ, acted as a trap for skeletal sands produced in the nearby shallow-water reef environment. These cor­ als, together with encrusting organisms represented by red algae (corallinaceans and peyssonneliaceans), foraminifers and especially vermetid gastropods, bathymetrically indicate a reef environment less than 30 m deep and probably within the upper 10 m. Ages obtained on these in situ corals sampled from the coralgal veneer of the cemented slope

Fig. 4. (Opposite) (a) Ledge rock seen from submersible. Note the tiny threads at the margin of the ledge, which may contribute to the stabilization of the sediment. Width of field of view is 0.6 m. Dive 20 1 , 1 80-m water depth. (b) Karst features seen from submersible at 1 5 0-m water depth. Note the solution caves and karren features. Some parts of the corroded surfaces are covered with a thin sedimentary veneer, less than I em. Width of field of view is 0.9 m, dive 1 98. (c) Ledge rock showing Pavona sp. ( 1 ) as a major constituent encrusted by coralline algae (2). Note the open sponge boring in the upper right-hand corner (3). Sample 1 1 0 1 , dive 203, 1 7 5 m water depth, thin-section. (d) Drowned reefs seen from the submersible. Note the typical plate-like growth form of shallow-water Acropora colonies, now completely overgrown by coralline algae. The fish are c. 1 5 em long for scale. Dive 1 97, 90-m water depth. (e) Porites ( 1 ) with encrusting algae (2). Sample J 1 02, dive 203, 95-m water depth, thin-section. (f) Leptoseris fragilis ( 1 ) framework composed of this special scleractinian and several crusts of Acervulina inhaerens (2) coral line algae (3), and peyssonneliaceans (4). Sample J71A, dive 292, 90-m water depth, thin-section.

229

and the wall range from 33.5 ± l .O to 18.8±0.4 ka (Table 1). From two sites (19 1 and 199) we were able to ram off samples from the upper part of the wall at around ll0-m and l05-m present water depth. These sam­ ples can be characterized as framestones represent­ ing a real reef facies. The state of preservation of the corals within these rocks is good to moderate. There­ fore, we obtained only two ages of 55.6 ± 2.1 and 33.6 ± l.l ka (Table 1), which provide only a hint for the time of reef growth. As these reef rocks form the uppermost part of the wall, we assume that they correspond to the talus facies described above, which occurs bathymetrically deeper and forms the lower part of the wall and the cemented slope. Another group of samples, which is not yet ce­ mented, consists of loose coral debris deriving from the Holocene reef. As most of the reefs produce more carbonate than they can accommodate within their shallow-water environment, carbonate material is transported off the reef downslope and occurs as debris at various depths. Hence, the ages obtained from these samples are young, indicating the still continuing formation of the Holocene talus (Tables l & 2). Drowned reefs

On top of the wall, drowned reefs are found concen­ trated bathymetrically between l00 m and 90 m. In contrast to the coralgal veneers recovered from the cemented slope and the wall, they form small mounds, elevated by up to 3 m in comparison with the surrounding sediment. They are at present covered by living zooxanthellate scleractinians (Leptoser is fragilis and L. explanata) especially adapted to this twilight zone (Fricke et a/., 1987), Halimeda, sponges and calcareous red algae, as well as gorgonians and antipatharians. Some convincing examples exist where the ancient growth morphol­ ogy of the constituent shallow-water scleractinians is still seen below the intense encrustation of mainly coralline algae (Fig. 4d). These drowned reef mounds are fragile and it is relatively easy to ram off huge parts with the chisel mounted to the keel of the submersible. Therefore, we obtained samples from the internal parts. The lower part, up to 2 m in height, is composed of shallow-water corals (e.g. Por ites, Pocillopora) with associated encrusting organisms (calcareous red al­ gae (Fig. 4e), vermetid gastropods and foramini­ fers). Skeletal sand is trapped within this fabric. It is

230

W-Ch. Dullo et a!.

composed of coated grains and foraminifers among which miliolids are common, indicating a shallow­ water origin. This coral community is gradationally replaced upward by deeper-water platy growth forms at between 2 and 3 m of mound height. The uppermost part of the mounds, 20 em thick, com­ prises a lithified L eptoser is framework (Fig. 4f), representing the present-day calcareous commu­ nity. One in situ shallow-water coral (Por ites) was dated at 13.6 ± 0.4 ka by TlMS (Table 2). Two other corals were dated at 10.1 ± 0.2 (Cyphastr ea) and at 2.9 ± 0.3 ka (Leptoser is) (Table 1). They probably represent the transition from shallower to deeper environments, as the latter is still living at this depth. In a few dives (189, 192, 193 and 204, Fig. 1; 193, Fig. 2), we recognized a second level of drowned reefs between 65 and 55 m, already covered by a veneer of living platy scleractinians. The base of these coralgal associations is composed of shallow­ water corals such as branching Pocillopora and Acr o­ pora, although these were recorded as small pieces. Bathymetrically shallower terrace steps have not been included in this survey because of severe swell conditions.

parable ages (Table 1). According to published sea­ level curves for the last 130 kyr (e.g. Bard et a!., 1990), there is a prominent sea-level lowstand oscillating around 80 m deep during late isotope. stage 3 (Fig. 5). During that time (Table 1 ), shallow­ water reefs developed together with their associated talus facies on the present-day deeper fore-reef and may have contributed to the overall morphology of this prominent terrace (Fig. 2). This implies that the: cemented slope and parts of the wall record pro-· cesses during sea-level lowstand, a condition which was responsible fo� the equivalent formation on the: fore-slopes of the Bahamas (Grammer & Ginsburg, 1992). The overall morphology of the wall, mainly re-· lated to intense erosion with the formation of reef outrunners (Land & Moore, 1977; James & Gins-· burg, 1979; Grammvr, 199 1), even known in the: fossil record as cipjt p�:mlders (Bosellini, 1989;; Biddle et a!., 1992), and subsequent karstification,, was created during a rapid lowering of sea-level starting around 26 ka (Fig. 5) and approaching the: last glacial maximum (LGM). As sea-level fall stopped around 22 ka, the karstification processes bathymetrically moved down to 150-m present

DISCUSSION

The oldest structure seen during the dives is repre­ sented by the submarine outcrops of the volcanic basement at site 197, 320 m deep. On top of these volcanic rocks, a series of well-bedded, Halimeda­ dominated packstones and grainstones (James & Ginsburg, 1979; Ginsburg eta!., 1991) were depos­ ited and are exposed in two tectonically formed cliffs at sites 193 and 198, between 250 and 300 m deep. Comparable inclined packstones have been reported from Bahamian fore-slopes by Grammer eta! . (1993). As we did not obtain any absolute ages from these rocks, we may only assume a late Pliocene to early Pleistocene age; this is the most likely age, consistent with the subsidence history of the island and the age of the volcanic rocks (Nougier et a!., 1986). The oldest samples we could date to be derived from the uppermost part of the wall have ages of 55.6 ± 2. 1 and 33.6 ± 1.1 ka, respectively. These carbonates represent framestones that belong to a reef facies. Associated with this reef facies is a reef talus forming the deeper part of the wall and the cemented slope. Samples from this part show com-

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Age (ky BP) 5. Sea-level curve based on normalized 8180 curve determined by Shackleton ( 1987) (continuous line) and determined by Labeyrie et a!. ( 1987) (dotted line) after Bard et a!. ( 1990). The isotope stages are indicated by numbers.

Fig.

231

Mayotte for e-slope mor phology and sediments water depth, creating small caves, karren and ka­ menitza features. After most parts of the karst had been formed and sea-level may have started to rise slowly at the end of the LGM, scleractinians started to grow on the cemented slope. The overall slope inclination around 150-m water depth is very steep except between sites 192 and 197 (Figs 1 & 2). There is almost no space to accrete a real rock onto these steeply inclined walls. Therefore, the coral veneer growing in this ancient shallow-water envi­ ronment during the end of the LGM produced more bioclastic debris than was recorded as in situ fabric. Accordingly, we collected reworked corals as frag­ ments already cemented to the deeper parts of the fore-slopes exhibiting an age corresponding to the LGM (Table 1) (Colonna et al., 1996). This is in good agreement with the dates of 17 ± 1 ka (by U/Th a counting) obtained by Veeh & Veevers ( 1970) on corals (Galaxea clavus) 17 5 m deep in the Middle Great Barrier Reef. Similar dates, 17.595 ± 0.07 ka and 15.58 ± 0.05 ka (by U/Th TlMS) have been published by Bard etal. (1992) from the flanks of Mururoa, and Fairbanks (1989) has presented an age of 18 200 ya (by 14 C, which corresponds to 2 1.93 ± 0.15 ka by U/Th) for Porites aster oides 124 m deep from Barbados.

Sea-level rise recorded during the last deglacia­ tion, averaging around 8 mm yr-1 (Fairbanks, 1989), interrupted sediment transport off the wall and created new space for accommodation as soon as the wall was flooded. During this interruption, hard­ grounds and laminar micritic crusts were formed and lithified. The results of Brachert & Dullo (1991) demonstrate that this type of hardground and lami­ nar micrite was formed under conditions of rapidly rising sea-level. We also assume similar conditions and timing for the formation of laminated micrites occurring on Belize fore-slopes (James & Ginsburg, 1979). During this rapid sea-level rise, the surface of the starved cemented slope would correspond to a diastem, thus recording a typical drowning uncon­ formity as reported by Grammer & Ginsburg (1992) and Grammer et al. (1993). This is verified by tex­ tural evidence (i.e. boring) indicating that the surface of the slope is a submarine hardground. The build-ups located on top of the deeper forereef terrace at 90 m-100-m water depth represent a cor­ algal succession, frequently beginning with shallow­ water stony corals and ending in an encrustation by a L eptoser is framework. This indicates a classical drowning event. According to our present knowl­ edge, the process of deglaciation is marked by pulses

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are indicated at the 2cr level. Note the two pulses of rising sea-level, which may have caused the observed drowning events of the reef mounds at I 00-90 m and 65-55 m on Mayotte fore-slopes. Modified after Bard et al. ( 1 9 90).

232

W-Ch. Dullo et a!. -50

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7. Summarizing sketch of the recorded sea-level changes on the fore-slopes of Mayotte. ( I ) Reefs grew during isotope stage 3. The dated samples indicate a reef growth between 55 and 2 7 ka for a sea-level around -80 to -90 m. The shallower part might have formed during the early part of stage 3. (2) Maximum drop in sea-level occurred between 22 and 18 ka. The emerged parts were karstified and the steep wall was formed because of erosion, which led to the accretion of coarse material and cipit boulders. (3) Rise of sea-level and formation of ledges on the wall. The new creation of space for accommodation of sediment reduced sedimentation in the 'deep'. This favoured the formation of a hiatus and the coeval cementation of the talus (cemented slope). Reefs started to grow on top of the wall, and were drowned as a result of the B0lling meltwater pulse (meltwater pulse l A). (4) Continuous rise in sea-level. Reefs started to grow on the shallower cliff around 60 m below the present-day sea-level, and were drowned as a result of the Post Younger Dryas meltwater pulse (meltwater pulse I B). (5) Onset of Holocene reef growth on top of the karstified Pleistocene limestones. (6) From 3 ka, sea-level has remained in the present position. As a consequence, new sediment has started to accumulate downslope, forming the sedimentary slope. Selected locations of samples are shown by sample number and age obtained.

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_.r_..-:h abstract). LUDWIG, W.J., KUMAR, N. & HOUTZ, R.E. (1979) Profi es on buoy measurements in the South China Sea Basin. J. geophys . Res ., B7, 3505-3518. MAXWELL, J.D.H. (1968) Atlas of the Great Barrier Reqf Elsevier, Amsterdam. MILLIMAN, J.D. (1974) Marine Carbonates . Springer­ Verlag, Berlin. PuRDY, E.G. (1974) Reef configurations: cause and effeet. In: Reefs in Time and Space (Ed. Laporte, LF.), Sp 250

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,.,,_ 0), or from erosion of existing terrigenous substrate (T < 0), and 'turbid reefs' result. With an increasing absolute magnitude of terrigenous sedimentation, reef death occurs through a combination of light attenuation, burial or erosion. Continued divergence from the x-axis leads to the formation of siliciclastic shelf sediments or carbonate lags (ITI> R). (Modified after Woolfe & Larcombe, 1997 .) =

307

This understanding is likely to improve studies of the physiological responses of corals to various aspects of sedimentation.

SEA-LEVEL CYCLES

The evidence from Halifax Bay indicates that the availability of a suitable substrate is a greater control on reef initiation and development than is water clarity, at least for initial colonization. Substrate character is controlled by sediment accumulation rather than by turbidity. Cycles of transgression and regression imposed by eustatic and tectonic factors will thus act in combination with local oceano­ graphic and sediment transport regimes, and will be of far greater importance for the establishment of reefs than sediment supply per se. During transgressive phases, shelf sediments are commonly transported landward, increasing the volume of sediment in the coastal zone. Within the central GBR lagoon, the isobaths in the 40-20-m depth range are relatively linear and evenly spaced, indicating that during lower (and rising) post-glacial and Holocene sea-levels, the coastline would have been relatively linear (Harris et a/., 1990;Larcombe & Woolfe, 1997). With the likely presence of SE trade winds, substantial northward movement of coastal sediment would have occurred, limiting the availability of stable, clean substrates. In concert with a rapidly rising sea-level, few coastal or inner­ shelf reefs would have been initiated. However, as sea-level approached the level of the modern 101 5-m isobath, a highly indented coastline evolved (Belperio, 1978). The embayments provided loci for sedimentation and attenuated the coastal boundary layer, and coral reefs were able to initiate in areas where sediment accumulation was mini­ mal, including exposed headlands (Fig. 8).

CONCLUSIONS

Coral-dominated carbonate provinces are not re­ stricted to clear-water conditions. 'Coastal turbid­ zone reefs' occur throughout the central GBR, and in the Gulf of Papua. Although near-bed turbidity is necessary for fine-grained terrigenous sediment ac­ cumulation, the presence of highly turbid water does not necessarily lead to net sediment accumu­ lation. Indeed, reef establishment on the GBR shelf is currently occurring in some of the most turbid

308

K. J. Woolfe & P. Larcombe

"Coastal turbid-zone reefs" on gravel patches of Pleistocene substrate Shore-detached sediment wedge Patch reef

Depth >20m Inner-shelf sediment wedge " Turbid inner-shelf waters

""'

Mid-shelf lag surface

"

Clear mid-shelf waters

Prevailing SE trade winds

20-40 km

Fig. 8. Perspective view of a sedimentary model of the central GBR coastline, showing terrigenous sediment bodies and geomorphical features (partly after Belperio. 1978), sites of coral reef growth and simplified oceanographic features. The terrigenous sediment wedges are attached to the shoreline near the major sediment sources and in the depositional embayments, but lie offshore further downdrift (and) on eroding (linear) coastlines. Where the sediment wedge lies offshore, coarse substrates are exposed in the nearshore zone, forming sites of coral growth and potential reef formation (from Larcombe & Woolfe, 1997).

parts of the inner shelf, where resuspension and erosion are exposing new hard substrate. In noting the relationships of some corals to terrigenous sediment bodies, we stress the distinc­ tion between sediment supply to the coast, sedi­ ment flux on the shelf, and sediment accumulation. The sediment accumulation rate is a more funda­ mental control on reef development than is sedi­ ment supply. However, the occurrence of terrigenous sediment within developing reefs is probably not, by itself, an indication of increased sediment supply. Increasing terrigenous compo­ nents within the upper portion of coral cores may reflect increased sediment trapping rather than increased sediment accumulation, supply or water turbidity. Substrate availability is more important as a

control upon reef initiation and growth than is water clarity. Substrate availability is controlled by local oceanography and sediment transport, super .. imposed upon regional sedimentation patterns, and in the longer term, tectonic and global eustatic: change. High sediment fluxes through an area do not necessarily limit substrate availability.

ACKNOWLEDGE MENTS

This paper draws on data collected for a variety of projects. Financial and logistical support was pro.. vided by the Cooperative Research Centre for Sustainable Development of the Great Barrier Reef, the Australian Institute of Marine Science, the Australian Research Council, James Cook Univer..

Sediment accumulation and reef distribution

sity, Ok Tedi Mining Ltd and the Townsville Port Authority. We thank our colleagues Bob Carter and Tim Naish for their editorial comments, and Alan Orpin for helpful advice. Colin Braithwaite pro­ vided inspired suggestions upon the manuscript, and comments were also received from an anony­ mous reviewer. Arnstein Prytz processed the turbid­ ity datasets, and, finally, we thank Richard Purdon for his skilful and patient drafting assistance with Figs 1 , 2, 4 & 8. REFERENCES

W.H. (1978) Coral reef morphogenesis: a multidi­ mensional model. Science, 202, 831-857. AIDAB (1994) Papua New Guinea and Gulf Coastal Zone Management Plan, pre-feasibility study, Final Report. Australian International Development Assistance Bu­ reau, Canberra, ACT. ALONGI, D.M. & ROBERTSON, A.l. (1995) Factors regulat­ ing benthic food chains in tropical river deltas and adjacent shelf areas. Geomar. Lett., 15, 145-152. ANONYMOUS ( 199 5) Old photos chart destruction of Aus­ tralia's reef. New Sci. , 4, 7. BELPERIO, A.P. (1978) An inner shelfsedimentation model for the Townsville region, Great Barrier Reef province. PhD thesis, James Cook University, Townsville, Qld. BELPERIO, A.P. (1979) The combined use of wash load and the bed material load rating curves for the calculation of total load: an example from the Burdekin River, Aus­ tralia. Catena, 6, 317-329. BELPERIO, A.P. (1983) Late Quaternary terrigenous sedi­ mentation in the Great Barrier Reef lagoon. In: Proceed­ ings of the Great Barrier Reef Conference (Eds Baker, J.T., Carter, R.M., Sammarco, P.W. & Stark, K.P.), pp. 71-76. James Cook University, Townsville, Qld. BELPERIO, A.P. (1988) Terrigenous and carbonate sedimen­ tation in the Great Barrier Reef province. In: Carbonate­ Clastic Transitions (Eds Doyle, L.J. & Roberts, H.H.), Developments in Sedimentology, 42, pp. 143-174. Elsevier, Amsterdam. BIRD, M.l., BRUNSKILL, G.J. & CHIVAS, A.R. (1995) Carbon-isotope composition of sediments from the Gulf of Papua. Geomar. Lett. , 15, 153-159. BRUNSKILL, G., WOOLFE. K. & ZAGORSKJS, I. (1995) Dis­ tribution of riverine sediment chemistry on the shelf, slope and rise of the Gulf of Papua. Geomar. Lett. , 15, 160-165. CARTER, R.M., JOHNSON, D.P. & HOOPER, K.G. (1993) Episodic post-glacial sea-level rise and the sedimentary evolution of a tropical embayment (Cleveland Bay, Great Barrier Reef shelf, Australia). Aust. J. Earth Sci. , 40, 229-255. CoRTES, J. & RISK, M.J. (1985) A reef under siltation stress: Cahuita, Costa Rica. Bull. mar. Sci. , 36, 339-356. DODGE, R.E. & VAISNYS, J.R. (1977) Coral populations and growth patterns: responses to dredging and turbidity associated with dredging. J. mar. Res. , 35, 715-730. DoNE, T. ( 1982) Patterns in the distribution of corals ADEY,

309

commumties across the central Great Barrier Reef. Coral Reefs, 1, 95-107. HARRIS, P.T., DAVIES, P.J. & MARSHALL, J.F. (1990) Late Quaternary sedimentation on the Great Barrier Reef continental shelf and slope east of Townsville, Australia. Mar. Geol. , 94, 55-77. HARRIS, P.T., BAKER, E.K., CoLE, A.R. & SHORT, S.A. (1993). A preliminary study of sedimentation in the tidally dominated Fly River Delta, Gulf of Papua. Continent. ShelfRes., 13, 441-472. HAYWARD, A.B. (1982) Coral reefs in a clastic sedimentary environment: fossil (Miocene, SW Turkey) and modern (Recent, Red Sea) analogues. Coral Reefs, 1, 109-114. HoPLEY, D. ( 1982) The Geomorphology of the Great Barrier Reef Quaternary Development of Coral Reefs. Wiley-Interscience, New York. HOPLEY, D. (1983) Evidence of 15000 years of sea-level change in tropical Queensland. In: Australian Sea-levels in the Last 15000 Years: a Review (Ed. Hopley, D.), De­ partment of Geogaphy, James Cook University, Towns­ ville, Qld, Monogr. Ser, Occas. Pap., 3, pp. 93-104. HoPLEY, D. (1995) Continental shelf reef systems. In: Coastal Evolution: Late Quaternary Shoreline Morpho­ dynamics (Eds Carter, R.W.G. & Woodrolfe, C.D.), pp. 303-340. Cambridge University Press, Cambridge. HOPLEY, D. & MURTHA, G.G. (1975) The Quaternary Deposits of the Townsville Coastal Plain. Geography Department, James Cook University, Townsville, Qld, Monogr. Ser, 8. JAMES, N.P. & BouRQUE, P.A. (1992) Reefs and mounds. In: Facies Models: Response to Sea-level Change (Eds Walker, R.G. & James, N.P.), pp. 323-347. Geological Association of Canada. JOHNSON, D.P. & CARTER, R.M. (1987) Sedimentary framework of mainland fringing reefdevelopment, Cape Tribulation area. Great Barrier Reef Marine Park Au­ thority, Townsville, Qld, Technical Memorandum, 14. JoHNSON, D.P. & SEARLE, D.E (1984) Post-glacial seismic stratigraphy, central Great Barrier Reef, Australia. Sedi­ mentology, 31, 335-352. LARCOMBE, P. & Rmo, P.V. (1994)Data interpretation. In: Townsville Port Authority Capital Dredging Works 1 993: Environment Monitoring Program (Eds Benson, L.J., Goldsworthy, P.M., Butler, I.R. & Oliver, J.), pp. 165194. Townsville Port Authority. LARCOMBE, P. & WOOLFE, K. (Eds) (1996) Great Barrier Reef Terrigenous Sediment Flux and Human Impacts, 2nd edn. CRC Reef Research Centre, Research Sympo­ sium Proceedings, Townsville, Qld. LARCOMBE, P. & WOOLFE, K. (1997) sedimentary and sea-level controls on the distribution of Holocene inner­ shelf coral reefs, Great Barrier Reef, Australia. Mar. Geol. (submitted). LARCOMBE, P., RIDD, P.V., WILSON, B. & PRYTZ, A. (1994) Sediment data collection. In: Townsville Port Authority Capital Dredging Works 1 993: Environment Monitoring Program (Eds Benson, L.J., Goldsworthy, P.M., Butler, I.R. & Oliver, J.), pp. 149-164. Townsville Port Author­ ity. LARCOMBE, P., CARTER, R.M., DYE, J., GAGAN, M.K. & JOHNSON, D.P. (1995a) New evidence for episodic post­ glacial sea-level rise, central Great Barrier Reef, Austra­ lia. Mar. Geol. , 1 27, 1-44.

310

K J Woolfe & P. Larcombe

P., RIDD, P.V., WILSON, B. & PRYTZ, A. (1995b) Factors controlling suspended sediment on inner-shelf coral reefs, Townsville, Australia. Coral Reefs, 14, 163171. MILLIMAN, J.D. (1995) Sediment discharge to the ocean from small mountain rivers: the New Guinea example. Geomar. Lett., 15, 127-133. MILLIMAN, J.D. & MEADE, R.H. (1983) World-wide deliv­ ery of river sediments to the oceans. J. Geol. , 91, 1-21. Moss, A.J., RAYMENT, G.E., REILLY, N. & BEST, E.K. (1993) A preliminary assessment of sediment and nutrient exports from Queensland coastal catchments. QueenslandDepartment of Environment and Heritage, Brisbane, Environment Technical Report 5. MURRAY-WALLACE, C.V. & BELPERIO, A.P. (1991) The last interglacial shoreline in Australia-a review. Quat. Sci. Rev., 10, 441-461. NEIL, D.T. & Yu, B. (1995) Simple climate-driven models for estimating sediment input to the Great Barrier Reef lagoon. In: Great Barrier Reef Terrigenous Sediment Flux and Human Impacts (Eds Larcombe, P. & Woolfe, K.), pp. 67-73. CRC Reef Research Centre, Research Symposium Proceedings, Townsville, Qld. 0HLENBUSCH, R. (1991) Post-glacial sequence stratigraphy and sedimentary development of the continental shelf off Townsville, central Great Barrier Reefprovince. Honours thesis, Geology Dept., James CookUniversity,Towns­ ville, Qld. OK TED! MINING LIMITED (1988) Sixth supplemental agree­ ment environmental study, 1 986-1 988, final draft report, Vols I, II and III, unpublished. PASTORAK, R.A. & BILYARD, G.R. (1985) Effects of sewage pollution on coral reef communities. Mar. Ecol. Progr. Ser. , 21, 175-189. QuEENSLAND GovERNMENT (1993) The Condition of River Catchments in Queensland. Department of Primary Industries, Brisbane. RoGERS, C.S. (1982) The marine environments ofBrewers Bay, Perseverance Bay, Flat Cay and Saba Island, St. Thomas, U.S VI., with emphasis on coral reefs and seagrass beds (November 1978-July 1981). Department of Conservation and Cultural Affairs, Government of the Virgin Islands. RoGERS, C.S. (1983) Sublethal and lethal effects of sedi­ ments applied to common Caribbean reef corals in the LARCOMBE,

·

field. Mar. Pollut. Bull., 14, 378-382. C.S. (1990) Responses of coral reefs and reef organisms to sedimentation. Mar. Ecol. Progr. Ser. , 62, 185-202. SHULMEISTER, J. (1995) Holocene climate change in Queen­ sland: implications for sea-level change and coastal sedimentation. In: Great Barrier Reef Terrigenous Sed­ iment Flux and Human Impacts (Eds Larcombe, P. & Woolfe, K.), p. 91. CRC Reef Research Centre, Research Symposium Proceedings, Townsville, Qld. SMITH, A. (1978) Case study: Magnetic Island and its fringing reels. In: Geographical Studies of the Townsville Area (Ed. Hopley, D.), Department of Geography, James Cook University, Townsville, Monogr. Ser. Oc­ cas. Pap., 2, pp. 59-64. STAFFORD-SMITH, M.G. , KALY, U.L. & CHOAT, J.H. (1994) Reactive monitoring (short-term responses) of coral species. In: Townsville Port Authority Capital Dredging Works 1 993: Environmental Monitoring Program (Eds Benson, L.J., Goldsworthy, P.M., Butler, I.R. & Oliver, J.), pp. 23-53. Townsville Port Authority. THOM, B. G. & WRIGHT, L.D. (1983) Geomorphology of the PurariDelta. In: The Purari-Tropical Environment ofa High Rainfall Basin (Ed. Petr, T.), pp. 47-65. Dr W. Junk, The Hague. VERON, J.E.N. (1995) Corals in Space and Time: the Biogeography and Evolution of the Scleractinia. Univer­ sity of New South Wales Press, Sydney. WAY, A.J. (1987) Post-glacial stratigraphy of Upstart Bay off the Burdekin River, north Queensland. MSc thesis, James Cook University, Townsville, Qld. WOLANSKJ, E. & ALONGI,D.M. (1995) A hypothesis for the formation of a mud bank in the Gulf of Papua. Geomar. Lett. , 15, 166-171. WOLANSKJ, E., KING, B. & GALLOWAY, D. (1995) Water circulation in the Gulf of Papua. Continent. Shelf Res., 15, 185-212. WOOLFE, K. & LARCOMBE, P. (1997) Terrigenous sedimen­ tation and coral reef growth: a conceptual framework. Mar. Geol. (submitted). WOOLFE, K.J., DALE, P.J. & BRUNSKJLL, G.l. (1995) Sedi­ mentary CIS relationships in a large tropical estuary: evidence for refractory carbon inputs from mangroves. Geomar. Lett. , 15, 140-144.

RoGERS,

Spec. Pubis int. Ass. Sediment. (1998) 25, 311-323

Comparison between subtropical and temperate carbonate elemental composition: examples from the Great Barrier Reef, Shark Bay, Tasmania (Australia) and the Persian Gulf (United Arab Emirates) C. P. RA O*, Z. Z. A M I N I* and J. F E R G U SONt *Department of Geology, University of Tasmania, GPO Box 252C, Hobart, Tas. 7001, Australia; and tAustralian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia

ABSTRACT

The Persian Gulf (United Arab Emirates) is renowned for subtropical carbonates. Extensive subtropical (Great Barrier Reef and Shark Bay) to temperate (Tasmania) carbonates are also forming in shallow seas around Australia. These carbonates differ in the types and proportions of skeletal to non-skeletal grains and cements, and are forming in normal to hypersaline shallow- marine environments. The elemental composition of subtropical carbonates differs from that of their temperate counter­ parts mainly because of differences in seawater temperature, carbonate mineralogy, salinity, rate of precipitation and the proportion of skeletal to non- skeletal grain composition. Differences in the Mg concentrations of the bulk carbonates result from variations in the temperature of sea water and carbonate mineralogy. Sr concentrations are higher in subtropical carbonates relative to temperate ones because of a higher proportion of aragonite in the tropical carbonate and calcite mineralogy. Na values increase with increases in salinity and rate of precipitation. Under reducing conditions appreciably higher Mn and Fe concentrations enter the calcite lattice compared with aragonite. The results from this study demonstrate that modern subtropical carbonate elemental composition differs distinctly from that of temperate carbonates. Thus, these differences can be used in the recognition of the ancient spectrum of subtropical to temperate carbonates based on the relative concentrations of elements and their ratios.

INTRODUCTION

Extensive subtropical to temperate carbonates are forming in shallow seas in many areas, particularly around Australia (Rao, 1996a). A similar spectrum of ancient subtropical to temperate carbonates may exist in the stratigraphical record but only a few ancient non-tropical carbonates have been identi­ fied (e.g. Nelson, 1978; Rao, 1981a, 1988; Brook­ field, 1988; Draper, 1988; Boreen & James, 1995). To fill this gap in our understanding, we present here a comparison of the elemental composition of subtropical Great Barrier Reef and Shark Bay car­ bonates, Australia, and subtropical Persian Gulf carbonates, United Arab Emirates, with temperate Tasmanian carbonates, Australia. Temperate carbonates differ from tropical carbon­ ates in the proportions of skeletal and nonskeletal-

grains (Lees, 1975), the mineralogy and products of diagenesis (e.g. Rao, 1981b; Reeckman, 1988; Rao & Adabi, 1992), oxygen and carbon isotopic compo­ sition (Rao & Green, 1983; Rao & Nelson, 1992), and in the concentrations of major and minor ele­ ments (Rao, 1990a; 1996b; Rao & Jayawardane, 1994). Elements that are mainly used in understand­ ing the origin of carbonates are Ca, Mg, Sr, Na, Mn and Fe (Veizer, 1983a,b). The Mg contents are sen­ sitive to variations in seawater temperature (e.g. Bur­ ton & Walter, 1991), and aragonite and calcite min­ eralogy (Milliman, 1974). The Sr contents vary with the proportion of aragonite and calcite, seawater temperature (Morse & Mackenzie, 1990) and rate of precipitation (Carpenter & Lohmann, 1992). The Na contents depend on salinity (Land & Hoops,

Reefs and Carbonate Platforms in the Pacific and Indian Oceans Edited by G. F. Camoin and P. J. Davies © 1998 International Association of Sedimentologists ISBN: 978-0-632-04778-9

311

312

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Rao, Z. Z. Amini & J. Ferguson

197 3), biochemical fractionation and rate of precip­ itation (Busenberg & Plummer, 1985). The Mn and Fe concentrations are sensititive to changes in oxi­ dizing and reducing conditions (Veizer, 1983a,b; Morrison & Brand, 1987). The results from this study demonstrate that tropical carbonates differ distinctively from temperate carbonates. These re­ sults can be used to recognize ancient subtropical and temperate carbonates.

METHODS OF STUDY

Bulk sample powders were dissolved in l N HCI and analysed for Ca, Mg, Sr, Na, Fe and Mn by atomic absorption spectrophotometry (AAS). These values were normalized for 37% Ca to facilitate comparison of elemental composition in pure carbonate frac­ tions. The detection limits are ± 1o/o for Ca and Mg and ± 5 p.p.m. for Sr, Na, Mn and Fe (Robinson, 1980). Data of subtropical carbonates are listed in Table 1. The Tasmanian temperate carbonate ele­ mental data are from previous publications (Rao, 198 l b; Rao & Adabi, 1992; Rao & Jayawardane, 1993, 1994; Rao & Amini, 1995) and unpublished data.

SEDIMENTOLOGICAL FEATURES

Petrographical examination of samples analysed in this study showed that the bulk samples vary con­ siderably in skeletal and non-skeletal grain compo­ sition, micrite, spar and terrigenous constituents.

high carbonate facies range from reef to reworked reef debris and algae. Most of the reefs are shelf, lagoonal or elongate platform reefs (Maxwell, 1968). The core samples studied from the Great Barrier Reef are from a transect off north-east Australia (Fig. 1). Petrographical examination of these samples revealed abundant biotic constitu­ ents, particularly algae (Halimeda), foraminifers, molluscs, echinoderms and rare bryozoans, corals, worm tubes and crustaceans, and mud, debris and terrigenous material. Non-skeletal grains are rare in the Great Barrier Reef carbonates. Shark Bay

Modern carbonates at Shark Bay, Western Australia (Fig. 2), are forming in an arid to semi-arid climate, at a subtropical latitude of 26 S. The samples stud­ ied here are from Hamelin Pool (Fig. 2), which is a landlocked embayment in Shark Bay that contains carbonates. Seawater temperatures range from 15 to 29oC. The salinity of seawater varies considerably from oceanic (35-40o/oo), to metahaline (40-53o/oo) and hypersaline (56-70o/oo). The salinity variation is due to strong evaporation and restriction of seawater inflow by the Faure sill (Fig. 2). Organic communi­ ties change with salinity. In the metahaline phase, seagrass communities dominate. In the hypersaline phase, the biota is restricted to bivalves and algae with non-skeletal grains, such as ooids, intraclasts and pellets. The Shark Bay samples are stromatolitic or microbial limestones with foraminifers, molluscs o

144 143 142

176

Great Barrier Reef

The Great Barrier Reef is the largest modern reef province in the world. It is about 2000 km long, 23-290 km wide and is composed of about 2500 reefs. It is situated off the north-east of Australia in latitudes ranging from 9 to 24oS. Open seawater temperatures range from 21 to 29 C. The salinity variation is small and is between 35 and 36.6o/oo because of strong ocean water circulation. Three major sediment facies were recognized from the modern northern Great Barrier Reef (Flood & Orme, 1988). These are coastal terrigenous sand facies, transitional facies and impure carbonate algal facies. The impure carbonate facies are ex­ tremely heterogeneous mixtures of terrigenous mud and debris of corals, algae and other skeletons. The

"-., "-.. 122cm "-., "' 100cm""-....._ ""20km

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o

137 Scm

139 22cm 63cm

1 03cm

176cm

63cm /

230cm

242cm "-..

140 212

/

/

/

320cm

Fig. 1. Core samples from Great Barrier Reef, north-east

Australia.

Subtropical and temperate carbonate elemental composition

313

Table 1. Elemental composition of subtropical bulk carbonates from the Great Barrier Reef, Shark Bay and Persian

Gulf Sample no.

Mg(%)

Sr(p.p.m.)

Na(p.p.m.)

Mn(p.p.m.)

Fe(p.p.m.)

Great Barrier Reef Top 83/16 30 em top 83/13 330 cm 83/13 200 em 83/13 200 em 83/20 200 em 83/20 100 em 83/2 137 em Core 140 down 22 em Core 140 down 85 em Core 140 down 175 em Core 140 down 230 em Core 140 down 320 em Core 137 top 5 em Core 137 down 103 em Core 137 down 242 em Core 144 top Core 144 down 122 em Core 212 top Core 212 down 63 em Williamson No. 4 Mynidon3-186(27.1-28.61) Boulder! 7A(19.65-20.75) RibbonS No. 37A OTI No. 5(15.05-16.73) Coral No. I Coral No. 2

1.71 2.35 1.72 1.58 1.93 1.51 2.25 2.21 0.73 0.87 0.81 1.11 0.74 1.47 1.10 1.15 2.66 3.17 2.22 2.27 0.25 0.36 0.11 0.31 0.11 0.24 0.28

3592 3651 5687 5925 4874 6188 3239 3390 7049 7184 7247 6790 7499 5757 7409 7097 2542 2484 3177 3051 860 1775 813 1543 7135 1474 3695

9853 21 661 12 439 12 141 12 355 12 241 14 189 14 478 5968 7442 7718 8269 6885 8459 II 510 9059 17 790 33 036 4933 4877 444 522 534 1085 4219 896 1962

155 436 94 85 95 95 333 452 12 16 15 21 13 47 56 51 299 159 13 14 29 30 17 12 7 13 8

3871 10 735 3818 3221 3178 3953 6414 7507 261 556 455 680 355 2123 2832 2595 17 595 26 772 226 167 245 67 175 132 46 168 95

0.30 0.34 0.29 0.40 0.46 0.50 0.66 0.65 0.35 0.29 0.32 0.33 0.37 0.38 0.40 0.58 0.44 0.42 0.43 0.43 0.37 0.34 0.46 0.49 0.38 0.40 0.39

9039 8935 8654 8176 7369 7390 7099 6944 8127 8404 8228 8048 7996 7989 8664 8291 8211 8160 8194 7986 8488 8144 8617 8454 8365 8238 7789

5656 5518 5037 4884 4658 4766 5078 4562 4791 4799 4819 4703 4662 4457 5504 5037 4359 4278 4343 4428 5021 4464 6568 5807 5593 6273 7432

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5

333 315 269 330 263 261 269 228 262 254 312 282 298 298 734 829 568 527 543 526 422 593 1880 1813 1294 1330 1104

Shark Bay lA IB IC ID IE IF 2A 2B 2C 2D 2E 2F 2G 2H 3A 3B 3C 3D 3E 3F 3G 3H 4A 4B 4C 4D 4E

Continued on p. 314.

314

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Rao, Z. Z. Amini & J. Ferguson

Table 1. Continued.

Sample no.

Mg(%)

Sr(p.p.m.)

Shark Bay (Continued) SA 58 5C 50 SE

0.40 0.31 0.30 0.37 0.43

8781 8615 8398 8235 8289

5688 4639 5136 4583 6067

5 5 5 5 5

861 1193 1412 1456 1249

0.66 1.16 0.54 0.45 0.73 0.99

5955 6925 7191 6989 6584 3740

4029 14 295 4436 3176 3347 2811

21 75 18 18 32

219 1626 232 285 363 679

Na(p.p.m.)

Mn(p.p.m.)

Fe(p.p.m.)

Persian Gulf I

2 3 4 5 6

and micrite (Logan et al., 1974; Burne & Moore, 1987). Persian Gulf

The Persian Gulf is a landlocked sea that covers a 2 large area (c. 226 000 km ). The average water depth is about 35 m, and it is virtually cut off from open

Fig. 2. Samples from Hamelin Pool, Shark Bay, Western Australia.

Ill

ocean circulation. The average Gulf water tempera­ tures fluctuate mostly between 4o·c in summer and 15·c in winter. The salinity is in the range 40-50%o in open shallow waters, and 60-70%o in remote la­ goons, rising to 60 to > 1 OO%o in coastal embay­ ments. The facies patterns along the Trucial coast of the United Arab Emirates of the Persian Gulf de­ pend on three major factors (Purser & Evans, 1973): orientation of the shoreline with respect to 'shamal' winds, proximity to the Qatar Peninsula (an up-wind barrier) and the presence of the Great Pearl Bank coastal barrier. The western regiori. is protected lat­ erally by the Qatar Peninsula from 'shamal' winds. The subtidal facies are mainly carbonate muds with molluscs. The intertidal flat facies are pelletal sands with imperforate foraminifers. The protection de­ creases rapidly to the east, where the sabkha embay­ ments contain fringing reefs, and oolitic and mollus­ can sands. The central region is protected from 'shamal' winds by the Great Pearl Bank, which acts as a coastal barrier with tidal deltas and channels and passes laterally to coastal lagoons, intertidal flats and wide coastal sabkha (supratidal flat). On the seaward side of the barrier, spectacular coral reefs and oolitic tidal deltas have formed. The barrier is covered by molluscan (bivalve) sand; whereas lagoons contain mud with imperforate foraminifers, gastropods, pel­ lets and gastropods. Intertidal flats have abundant algal mats. Supratidal flats contain algae, mud, evaporites and dolomite. The north-eastern region is unprotected from 'shamal' winds, and a linear coast­ line has developed. The sediments here are muddy sands and clean sands (with oolites, pellets and skel­ etons); these suffer the effects of maximum wave fetch, which has led to the development of major

Subtropical and temperate carbonate elemental composition longshore spit systems. The Persian Gulf samples studied are from beaches on the Trucial coast. These contain abundant ooids (up to 95%) with intraclasts and various amounts of biota, such as molluscs, for­ aminifers, echinoderms, skeletal debris and micrite. Tasmania

Carbonate is the predominant sediment on the tem­ perate shelf off Tasmania (Fig. 3). Siliciclastics occur in water shallower than about 30 m and carbonates in deeper water (greater than 30 m). The modern carbonates are mixed with, and grade into, pre­ Holocene glacial carbonates on the outer continental shelf between 130 and 200 m. The average winter and summer surface-water temperatures on the shelf around Tasmania are about 11 C and 16 C, respec­ tively. During the maximum of the last glacial period (c. 18 000 yr BP), sea-level dropped about 130 m and shallow-marine carbonates formed around Tasma­ nia; surface-water temperatures were about 4 C lower than they are now. The salinity of the deep to shallow Tasman Sea ranges from 34.4o/oo to 35. 7o/oo because of mixing of water masses (Rao & Huston, 1995). In summer, seawater temperatures and salino

315

ity are higher than winter ones, because of the influx of the Eastern Australian Current. In winter, seawa­ ter temperatures and salinity decrease as a result of introduction of subantarctic water. Through the year, the Tasman Sea water is mixed with low­ salinity and -temperature deep Antarctic intermedi­ ate water. The temperate Tasmanian carbonates comprise mainly skeletal grains, intragranular ce­ ments and rare non-skeletal grains (Rao, 1981b, 1990b). In Tasmanian carbonates bryozoans are the dominant fauna in all samples, along with some mol­ luscs, foraminifers, echinoderms, brachiopods, algae and coccoliths. The samples studied here are from eastern and western Tasmania, collected by dredging on a grid of 10 nautical miles.

o

o

ELEMENTAL COMPOSITION

Subtropical bulk carbonates

Elemental composition of subtropical bulk carbon­ ates presented here is from the Great Barrier Reef, Shark Bay and Persian Gulf samples. Magnesium

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In these samples, the concentration of Mg ranges from 0.11 to 3.2%. Aragonite contains < 1o/o Mg whereas high-Mg calcite contains 1-3.2% Mg (Fig. 4). These ranges are similar to equilibrium Mg concentrations observed in other tropical world­ wide carbonates (Veizer, 1983a,b) .

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l8l Mud jgj Carbonate 44°8 Fig. 3. Distribution of shelf carbonates and siliciclastics

off Tasmania.

Strontium The Sr concentrations range from 813 to 9039 p.p.m. in examples presented here (Fig. 4) and are similar to those in other tropical carbonates (Morse & Mackenzie, 1990). The variation of Sr in the sam­ ples studied here is mainly related to relative propor­ tions of aragonite containing around 9000 p.p.m. Sr and calcite with Sr values;;;. 813 p.p.m.. Sodium The Na content in the bulk carbonates studied here ranges from 444 to 33 036 p.p.m. with a mean value of 6780 p.p.m.. These Na values are posi­ tively correlated with Mg concentrations, with a significant r2 value of 0.67 (Fig. 5). The increase in Na with Mg values results from the increase in the

316

C. P.

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Aragonite

Rao, Z. Z. Amini & J. Ferguson 500

+ ppmSr: P.G. • ppmSr: GBR.

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2.5

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3.5

Mg%

Fig. 4. Variation of Sr and Mg in bulk carbonates from the Great Barrier Reef(GBR), Persian Gulf (PG) and Shark Bay (SB). In most samples Sr concentrations decrease with increasing Mg values because of increasing amounts of high- Mg calcite and decreasing amounts of aragonite. The biotic calcite line is after Carpenter & Lohmann(1992).

proportions of calcite in the samples. These Na values are much higher than those in other world­ wide carbonates (Veizer, 1983a,b). As explained below, these anomalously high concentrations of Na are related t' 1 o/o Mg. Sim­ ilarly, Fe concentrations (Fig. 7; 46-26 772 p.p.m.; mean 1,907 p.p.m.) are small in aragonite samples with < 1o/o Mg, whereas Fe concentrations are high in calcite samples with > 1o/o Mg. These Mn and Fe concentrations in the bulk samples studied here (Figs. 3, 6 & 7) are higher than those in other world­ wide calcitic and aragonitic carbonates (Morrison &

• ppmNa GBR. o ppmNa SB

High-Mg calcite

30000

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.,20000 z E 15000 Aragonite 0.. 0..10000

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25000

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y 5992.987x 1917.828, r2 .667 -5000,+--��---+���-�---+ .5 1.5 0 3.5 2 2.5 3 =

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Mg%

Fig. 5. Variation of Na and Mg in bulk carbonates from

the Great Barrier Reef(GBR), Persian Gulf(PG) and Shark Bay(SB). It should be noted that Na values increase with increasing Mg contents because of the abundance of high- Mg calcite biota in the Great Barrier Reef samples. The Shark Bay and Persian Gulf samples have Na values higher than 2700 p.p.m. because of higher salinity of sea water.(See text for details.)

8:woo0

•

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Aragonite

•

�

5000 0

•

A ppmFe: GBR.

mil1

.5

' :··. 1.5

2

2.5

3

3.5

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Fig. 7. Variation of Fe and Mg in bulk carbonates from

the Great Barrier Reef(GBR), Persian Gulf(PG) and. Shark Bay(SB). It should be noted that Fe concentrations are low in aragonitic samples and high in high- Mg calcitic samples.

317

Subtropical and temperate carbonate elemental composition 30000

ppmFe: P.G. • ppmFe: GBR. o ppmFe: SB

+

25000 20000 �15000 E

2::10000 5000 0

=.79

-5000 -5000

0

5000 10000 15000 20000 25000 30000 35000 ppmNa

aragonite (Veizer, 1983a,b; Rao, 1996a). Therefore, low Mg values in the Shark Bay and Persian Gulf samples are due to high aragonite content, very low to high Mg values in the Great Barrier Reef samples are related to calcite to aragonite mixed mineralogy, and mainly intermediate Mg values in temperate Tasmanian carbonates result from the occurrence of low- to intermediate-Mg calcite. As Mg content in calcite increases with rising temperature (Mucci, 1987; Burton & Walter, 1991), subtropical Great Barrier Reef samples contain higher Mg values compared with temperate Tasmanian carbonates (Fig. 9).

Fig. 8. Variation of Fe and Na in bulk carbonates from

the Great Barrier Reef(GBR), Persian Gulf (PG) and Shark Bay(SB). It should be noted that Fe and Na concentrations are positively correlated.

Brand, 1987). The Fe and Na values are positively correlated, with a significant? value of 0.8 (Fig. 8). Comparison with temperate carbonates

Magnesium The Mg contents in carbonates are low in Shark Bay, moderate in the Persian Gulf, high in Tasma­ nia, and range from lowest to highest in the Great Barrier Reef (Fig. 9). The Mg concentrations are high and range up to a few per cent in calcite, whereas Mg contents are small and are < 1o/o in

8

0.. 0.. bO

�

3.5 3 2.5 2 1.5

+

The Sr concentrations in bulk carbonates are low in the temperate Tasmanian carbonates, moderate in the Great Barrier Reef samples, high in the Persian Gulf samples and highest in the Shark Bay samples (Fig. 10). The amount of Sr in aragonite is between 8000 and 10 000 p.p.m., whereas in calcite Sr ranges from 900 to 1800 p.p.m. The highest Sr concentrations of around 8000 p.p.m. in Shark Bay are related to the predominance of aragonite in a mainly aragonite-calcite mixture. Low to moderate Sr concentrations in the Great Barrier Reef result from the mixture of calcite and aragonite, whereas the lowest Sr concentrations in temperate Tasma­ nian carbonates are due to the predominance of intermediate-Mg calcite and some aragonite (Rao & Adabi, 1992).

14000r'-�-�--���--�-�......

MgppmGBR

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eMgppm TAS

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SrppmSB

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1

-.5-'--r---�--�----.-�--,--L 0

20

40

60

80

100

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0

20

40

60

80

100

Percentile

Fig. 9. Percentile distribution of Mg in bulk carbonates

Fig. 10. Percentile distribution of Sr in bulk carbonates

from the tropical Great Barrier Reef (GBR), Persian Gulf(PG) and Shark Bay(SB), and temperate carbonates from Tasmania(TAS).(See text for details.)

from the tropical Great Barrier Reef(GBR), Persian Gulf(PG) and Shark Bay(SB), and temperate carbonates from Tasmania(TAS).(See text for details.)

318

C. P.

+

25000

Rao, Z. Z. Amini & J. Ferguson

300�----�----��--y,

NappmGBR

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100 50 0

20

40

60

80

100

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20

40

60

80

100

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Fig. 11. Percentile distribution of Na in bulk carbonates from the tropical Great Barrier Reef (GBR), Persian Gulf(PG) and Shark Bay(SB), and temperate carbonates from Tasmania(TAS).(See text details.)

Sodium The Na concentrations in bulk carbonates are low in the temperate Tasmanian. carbonates, low to moderate in the Persian Gulf samples, high in the Shark Bay samples, and range from lowest to highest in the Great Barrier Reef samples (Fig. 1 1). Na contents in the samples studied are related to the combination of salinity, biochemical fractionation and growth rate. The salinity of sea water around Tasmania and the Great Barrier Reef is 35o/oo, whereas salinity ranges up to 300o/oo in tidal flats and lagoons in Shark Bay and the Persian Gulf (Rao, 1996a). The increase in Na from the temper­ ate Tasmanian to the Persian Gulf and Shark Bay bulk carbonates is due to increasing salinity. The lowest Na values observed in the Great Barrier Reef samples (Fig. 11) represent normal seawater salinity of 35o/oo, whereas the highest Na values in the Barrier Reef samples are due to a combination of biochemical fractionation and growth rate. The biotic calcites have higher Na values relative to abiotic calcites as a result of biochemical fraction­ ation (Morrison & Brand, 1987; Rao, 1996a). The Na values increase with increasing rates of crystal growth (Busenberg & Plummer, 1985). The rate of formation of tropical carbonates is greater than that of temperate ones (Rao, 1994, 1996a).

Fig. 12. Percentile distribution of Mn in bulk carbonates

from the tropical Great Barrier Reef (GBR), Persian Gulf(PG) and Shark Bay (SB), and temperate carbonates from Tasmania(TAS).(See text for details.)

samples, moderate in the Persian Gulf carbonates, high in the temperate Tasmanian carbonates, and lowest to highest in the Great Barrier Reef samples (Fig. 12). The Fe contents are similar in the Persian Gulf and Shark Bay samples, moderate in the temperate Tasmanian carbonates, and lowest to highest in the Great Barrier Reef samples (Fig. 13). The vari�tion of Mn and Fe in modern carbonates is related to carbonate mineralogy, oxidizing and reducing conditions, and availability of Mn and Fe released from terrigenous sediments. The concen-

10000 �----��-�--�-----'--t--r-'-T + FeppmGBR 9000 • FeppmPG 8000 o FeppmSB 7000 !!. Feppm TAS §_ 6000 �5000 p.. 4000 3000 2000 100 �G������==:_�_j

�1

0

20

40

60

80

100

Percentile

Fig. 13. Percentile distribution of Fe in bulk carbonates

Manganese and iron The Mn contents are lowest

m

the Shark Bay

from the tropical Great Barrier Reef (GBR), Persian Gulf(PG) and Shark Bay(SB), and temperate carbonates from Tasmania(TAS).(See text for details.)

Subtropical and temperate carbonate elemental composition trations of Mn and Fe are low in aragonite (< 20 p.p.m.), whereas calcite can take up to a few per cent (Mucci, 1987; Rao & Jayawardane, 1994). Thus, aragonitic carbonates from Shark Bay and the Persian Gulf contain low Mn and Fe concentrations relative to calcitic carbonates from temperate Tas­ mania. Low Mn and Fe concentrations enter calcite in oxidizing conditions, whereas high concentra­ tions of these elements can occur in calcite in reducing environments. Tasmanian carbonates studied are from water depths from 30 to 200 m, where conditions are predominantly reducing. In contrast, the Shark Bay, Persian Gulf and Great Barrier Reef carbonates are from water depths less than 50 m, where conditions are predominantly oxidizing. The availability of Mn and Fe released from terrigenous sediments is low in Shark Bay and the Persian Gulf, because in these areas mostly pure carbonates are forming. In contrast, the temperate Tasmanian carbonates and the Great Barrier Reef carbonates grade into coastal siliciclastics deposits that release Mn and Fe into shallow seas in these regions.

DISCUSSION

The vanatJon of major and minor elements in subtropical to temperate carbonates is mainly re­ lated to: (i) carbonate mineralogy; (ii) seawater temperatures; (iii) seawater composition; (iv) frac­ tionation of elements; (v) rate of precipitation; (vi) oxidizing and reducing conditions; (vii) pC02level, and (viii) salinity. Carbonate mineralogy

Carbonate mineralogy is a major control on Mg, Sr, Na, Mn and Fe values in modern carbonates. Abiotic aragonite contains low concentrations of Mg ( < 1%), Mn (mostly 5 p.p.m.) and Fe (> 50 p.p.m.), and variable amounts of Mg and Na relative to abiotic aragonite (Morrison & Brand, 1987). Abi­ otic calcite and biotic calcite have a similar range of Mg concentrations, because of the occurrence of similar types of low-Mg to high-Mg calcites. Mg and Sr (Fig. 4), and Mg and Na (Fig. 5) concentrations are positively correlated in bulk carbonates because

319

of variable amounts of aragonite and calcite com­ ponents, biotic content and salinity. Seawater temperature

Seawater temperature determines the carbonate mineralogy and thus affects the concentrations of elements in carbonates. Experimental studies dem­ onstrate that entirely abiotic aragonite forms at > 30·c, mixtures of abiotic aragonite and high-Mg calcite precipitate between 16 and 30·c, whereas low-Mg calcite forms at < 3·c (Kinsman & Hol­ land, 1969). Biotic carbonate mineralogy also varies with seawater temperature (Lowenstam, 1954; Morse & Mackenzie 1990). The mol o/o MgC03 in abiotic calcite decreases with lower seawater temperature because of changes in calcite mineralogy from high-Mg to low-Mg calcite (Mucci, 1987; Burton & Walter, 1991). Similarly, mol o/o MgC03 in many biotic calcites decreases with lower seawater temperature because of changes in calcite mineralogy from high-Mg to low-Mg calcite (Chave, 1954; Morse & Mackenzie, 1990). The biota that show the temperature depen­ dence in calcite mineralogy are benthic foramin­ ifera, bryozoans, echinoids and barnacles, and these are important constituents in temperate carbonates. Higher Mg contents in the Great Barrier Reef samples relative to temperate Tasmanian ones are due to higher seawater temperatures in tropical regions. Seawater composition

In sea water, the concentrations of Mg, Sr and Na increase with increasing salinity, and Fe and Mn contents depend on input of terrigenous material that contains high concentrations of Fe and Mn. The Mg concentrations in marine calcite are not related to the rate of precipitation or saturation of CaC03 (Mucci, 1987; Burton & Walter, 1991). Therefore, the relationships between Mg, Sr, Fe and Mn values observed in modern carbonates are due to seawater temperature, seawater composition, oxidizing and reducing conditions, and pC02 in seawater. Similar slopes of regression lines of Sr and Mg between biotic and abiotic calcites are due to uniform composition of Mg and Sr in normal sea water (Fig. 4; Carpenter & Lohmann, 1992). Sr and Na concentrations in sea water increase with in­ creasing salinity. High Na concentrations in carbon­ ate from Shark Bay and the Persian Gulf are due to

320

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Rao, Z. Z. Amini & J Ferguson

an increase in salinity. Sr concentrations in arago­ nite from Shark Bay and the Persian Gulf are similar despite the increase in salinity. This is because the Sr concentrations in carbonates are not significantly affected by increasing concentrations of Sr in sea water. Higher concentrations of Mn and Fe in bulk carbonates that contain > 1% Mg than in those samples that contain < 1% Mg indicate that Mn and Fe enter calcite preferentially compared with aragonite. Fractionation of elements

The incorporation of a trace element into a CaC03 lattice is governed by the distribution coefficient (D) in the following equation (Mcintire, 1963; Kins­ man, 1969): (mMelmCa)S

=

D (mMelmCa) W,

where m is molar concentration, Me is trace ele­ ment, Ca is calcium, and S and W indicate solid phase (i.e CaC03) and water, respectively. This equation is valid only when the system is at com­ plete equilibrium and the water and solid phase do not show any concentration gradients in Me during­ precipitation (homogeneous distribution law; Gor­ don et a!., 1959). The distribution coefficient D is related to the equilibrium constant K by several equations (Veizer, 1983b). These show that the dis­ tribution coefficient is a complex function of tem­ perature, pressure and the chemical compositions of the liquid and solid phases. Distribution coefficients involving various ele­ ments incorporated into CaC03 during the precipi­ tation of aragonite and calcite have been summa­ rized by Veizer (1983a,b). The distribution of the minor and trace elements is governed by the follow­ ing conditions of the distribution coefficient (Veizer, 1983a,b; Morrison & Brand, 1987): 1 When D 1, the precipitated carbonate contains similar amounts of Me relative to the carrier in both liquid and solid; 2 When D > 1, there is an enrichment of the Me concentration in the precipitated solid phase rela­ tive to its proportion in the liquid phase; 3 When D < 1, there is a proportional depletion of the minor and trace elements in the solid phase relative to their proportions in the liquid phase. In general, cations that are larger than Ca (e.g. Sr, Na) are preferentially incorporated into the open orthorhombic structure of aragonite. Cations that are smaller than Ca (e.g., Mg, Fe, Mn) are preferen=

tially incorporated into the tighter rhombohedral structure of calcite. The precise calculation of dis­ tribution coefficients is only necessary to determine the exact water chemistry. The magnitude and sign of distribution coefficient are sufficient to provide information on the seawater composition and the effect of diagenesis (Veizer, 1983a,b; Morrison & Brand, 1987). Rate of precipitation

The rate of precipitation affects the concentrations of Mg, Sr, Na and Mn in calcitic carbonate, partic­ ularly in biotic calcite. Kolesar ( 1978) observed that the Mg concentration in the coralline alga Calliar­ t hron are related to its growth rate. In summer months the growth of the alga is slow, which means that the demand for Mg during calcification is matched by the supply from sea water. In winter months the growth of the alga is so fast that the Mg in solution cannot keep pace with the demand. Thus, the growth rate of algae is also a function of water temperature. The Sr distribution coefficient is correlated with changes in calcite precipitation rate (Lorens, 1981; Mucci & Morse, 1983; Mucci, 1988). Slow rates correspond to equilibrium conditions and high rates to kinetic effects. Thus relatively high Sr concentrations in biotic calcite result from rapid precipitation associated with shell growth of marine organisms (Fig. 4; Carpenter & Lohmann, 1992). Experimental studies indicate that the amounts of Na and S04 in abiotic calcite increase linearly with increasing crystal growth rate (Busenberg & Plummer, 1985). The number of crystal defects increases with increasing crystal growth rate. The amount of Na that can be incorporated in the calcite depends on the number of crystal defects. The large variation of Na values in biotic calcite (Land & Hoops, 1973; Busenberg & Plummer, 1985) is related to salinity, crystal growth rates, ionic substitution and other causes. Experimental studies indicate that Mn concentra­ tions decrease with increasing crystal growth rate (Lorens, 1981; Mucci, 1988; Pingitore et a!. 1988), in contrast to Mg, Sr and Na values for which concentrations increase with increasing crystal growth rate. Lower concentrations of Mn relative to Mg, Sr and Na in bulk carbonates and biotic calcite are due to the fast rate of formation of tropical carbonates and biota.

Subtropical and temperate carbonate elemental composition Oxidizing and reducing conditions

Mn and Fe concentrations are sensitive to oxidizing and reducing conditions (Eh). In oxidizing waters Mn and Fe rapidly precipitate as highly insoluble ferric and manganoan oxyhydroxides, and the wa­ ter will contain very small concentrations of these elements. In reducing sea waters (anaerobic waters) Mn and Fe can enter the calcite lattice in apprecia­ ble amounts (Mucci, 1988). pC02 in sea water

The Mg concentrations in marine calcite vary with seawater temperature, pC02 levels and sulphate concentrations in sea water (Burton & Walter, 1991 ). Regardless of temperature, Mg concentra­ tions increase linearly with decreasing pC02 levels. Therefore, variations in Mg concentrations other than those attributable to a given temperature are due to changes in pC02 level. Salinity

Na concentrations in abiotic aragonite and calcite indicate the salinity of sea water (Land & Hoops, 1973). In biotic calcite and aragonite Na concentra­ tions are due to both salinity and rates of crystal growth (Busenberg & Plummer, 1985; Morrison & Brand, 1987). The positive correlation observed between Na and Mg concentrations in bulk carbon­ ates from the Great Barrier Reef, Persian Gulf and Shark Bay (Fig. 5) is due to a combination of salinity and growth rate of biota. Na values in the Great Barrier Reef bulk carbonate, which has formed in normal seawater salinity of 35o/oo, are much higher than those of the Persian Gulf and Shark Bay because of rapid growth of biota, which increases the amount of crystal defects into which Na is incorporated. Na values in marine abiotic aragonite forming at normal salinity of 35o/oo are around 2700 p.p.m. Na values in the Persian Gulf bulk carbonates (2500- 13 500 p.p.m.) and in the Shark Bay bulk carbonates (4000-7000 p.p.m.) are much higher than 2700 p.p.m. partly because of higher salinities of about 40-60o/oo. Preservation of original 'elemental' signal in ancient carbonates

Ancient carbonates are affected by meteoric and/or burial diagenesis. This normally leads to a decrease

32 1

in Mg, Sr and Na, and an increase in Mn and Fe concentrations relative to modern carbonates (Veizer, 1983a,b). Despite these changes, the rela­ tive concentrations and ratios of elements depict original mineralogy and elemental concentrations (Rao, 1981a, 1990b, 1991). For example, tropical Ordovician Tasmanian carbonates, interpreted to be originally aragonitic on the basis of petrograph­ ical features, contain high Sr, moderate Na and low Mn, similar to relative elemental concentrations in modern tropical aragonitic carbonates (Rao, 1981a, 1990b). In contrast, subpolar calcitic Permian polar carbonates, which contain glacial dropstones, con­ tain equal amounts of Sr and Na and higher amounts of Mn and Fe, similar to modern calcitic carbonates (Rao, 1991). Similarly, originally cal­ citic temperate Cainozoic limestones from New Zealand were found to contain lower concentra­ tions of Mg and Sr and higher concentrations of Na, Fe and Mn than tropical carbonates (Winefield et al., 1996). In these calcitic non-tropical carbonates, the ratio of Sr/Na is about unity or less, in contrast to higher Sr/Na ratios greater than three in origi­ nally aragonitic tropical carbonates (Rao, 1991; Winefield et al., 1996).

CONCLUSIONS

The major features of elemental composition in subtropical and temperate carbonates are as follows: 1 The Mg and Sr concentrations in the bulk carbonates from the Great Barrier Reef, Persian Gulf and Shark Bay are similar to those of other tropical carbonates, whereas Na, Mn and Fe con­ centrations in these carbonate localities are higher than in other subtropical carbonates because of high salinity in the Persian Gulf and Shark Bay areas and abundant biota in the Great Barrier Reef area. The Mg contents in temperate Tasmanian carbonates are higher than those of aragonitic subtropical carbonates. Sr concentrations in temperate carbon­ ates are lower than in subtropical counterparts because of calcitic mineralogy of temperate carbon­ ates. The Na contents are lower and Mn and Fe concentrations are higher in temperate carbonates relative to tropical aragonitic carbonates because of normal salinity of sea water, higher rate of precipi­ tation and the formation of temperate carbonates in a reducing (dysaerobic) environment. 2 Inorganic components have a similar range of Mg, higher Sr, and lower Na, Mn and Fe concentra-

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tions relative to biotic equivalents. Non-skeletal grains are absent in temperate carbonates. Cements occur in temperate carbonates but these were not chemically analysed because of the fine crystal size of cements. 3 Carbonate mineralogy is a major control on Mg, Sr, Na, Mn and Fe concentrations in tropical and temperate carbonates. Subtropical carbonates are entirely aragonite or mixtures of aragonite and calcite, whereas temperate carbonates are mainly calcite with some aragonite. 4 Seawater temperatures determine carbonate mineralogy and thus affect the concentrations of elements in carbonates. Mg and Sr concentrations decrease with lower seawater temperature. 5 In sea water, the concentrations of Mg, Sr and Na increase with increasing salinity, and Mn and Fe contents depend on input of terrigenous material. 6 The concentration of elements in carbonates is determined by distribution coefficients. Cations larger than Ca are preferentially incorporated into the open orthorhombic structure of aragonite, whereas cations that are smaller than Ca are prefer­ entially incorporated into the tighter rhombohedral structure of calcite. 7 Mg, Sr and Na values increase and Mn values decrease with increasing rate of crystal growth. 8 Appreciable Mn and Fe concentrations enter the calcite lattice in preference to aragonite in reducing conditions. 9 Mg concentrations other than those attributable to a given temperature are due to changes in pC02 levels. 10 Na values in abiotic and biotic carbonates in­ crease with increasing salinity and rate of precipita­ tion. 1 1 Relative cocentrations of elements and their ratios can be used to differentiate originally tropical aragonitic limestones from calcitic non-tropical limestones.

ACKNOWLEDGEMENTS

Financial assistance was provided by a grant from the University of Tasmania. We thank P. Robinson for supervision of chemical analysis, and Richard Orme and John Marshall for providing the Great Barrier Reef samples. We also thank Lucien Mon­ taggioni and Gilbert Camoin for their valuable comments, which improved an earlier version of the manuscript.

REFERENCES

T.D. & JAMES, N.P. (1995) Holocene sediment dynamics on a cool-water shelf: Otway, southeastern Australia. J. sediment. Petrol., 63, 574-588. BROOKFIELD, M.E. (1988) A mid-Ordovician temperate carbonate shelf-the Black River and Trenton Lime­ stone .Groups of southern Ontario, Canada. In: Non­ tropical Shelf Carbonates-Modern and Ancient (Ed. Nelson, C.S.). Sediment. Geol., 60, 137-153. BURNE, R.V. & MOORE, L.S. (1987) Microbialites: orga­ nosedimentary deposits of benthic microbial communi­ ties. Palaios, 2, 241-254. BURTON, E.A. & WALTER, L.M. (1991) The effects of pC02 and temperature on magnesium incorporation in calcite in seawater and MgC12-CaC12 solutions. Geochim. Cos­ mochim. Acta, 55, 777-785. BUSENBERG, E. & PLUMMER, N.L. (1985) Kinetic and thermodynamic factors controlling the distribution of S04 and Na in calcites and selected aragonites. Geochim. Cosmochim. Acta, 49, 713-725. CARPENTER, S.J. & LOHMANN, K.C. (1992) Sr/Mg ratios of modern marine calcite: empirical indicators of ocean chemistry and precipitation rate. Geochim. Cosmochim. Acta, 56, 1817-1849. CHAVE, K.E. (1954) Aspects of the biogeochemistry of magnesium 2: Calcareous sediments and rocks. J. Geol., 62, 587-599. DRAPER J.J. (1988) Permian limestone in the southeastern Bowen Basin, Queensland: an example of temperate car­ bonate deposition. In: Non-tropical Shelf Carbonates­ Modern and Ancient (Ed. Nelson, C.S.). Sediment. Geol., 60, 155-162. FLOOD, P.G. & 0RME, G.R. (1988) Mixed siliciclastic/ carbonate sediments of the northern Great Barrier Reef province, Australia. In: Carbonate-Clastic Transitions (Eds Doyle, L.J. & Roberts, H.H.), pp.175-205. Elsevier, Amsterdam. GORDON, L., SALUTSKY, M.L. & WILLARD, H.H. (1959) Precipitation from Homogeneous Solution. John Wiley, New York. 2 KINSMAN, D.J.J. (1969) Interpretation of Sr + concentra­ tion in carbonate minerals and rocks. J. sediment. Petrol., 49, 937-944. KINSMAN, D.J.J. & HOLLAND, H.D. (1969) The coprecipi­ tation of cations with CaC03• IV, The coprecipitation of 2 Sr + with aragonite between 16 to 96'C. Geochim. Cosmochim. Acta, 33, 1-17. KOLESAR, P.J. (1978) Magnesium in calcite from the coralline alga. J. sediment. Petrol., 48, 815-820. LAND, L.S & HooPs, G.K.(l973) Sodium in carbonate sediments and rocks: a possible index to salinity of diagenetic solutions. J. sediment. Petrol., 43, 614-617. LEES, A. (1975) Possible influence of salinity and temper­ ature on modern shelf carbonate sedimentation. Mar. Geol., 19, 159-198. LOGAN, B.W., HOFFMAN, P. & GEBELEIN, C.D. (1974) Algal mats, cryptalgal fabrics and structures, Hamelin Pool, Western Australia. In: Evolution and Diagenesis of Qua­ ternary Carbonate Sequences, Shark Bay, Western Aus­ tralia. (Eds Logan, B.W., Read, J.F., Hagan, G.M. et a!.), Mem. Am. Assoc. petrol. Geol., Tulsa, 22, 140-194. LORENS, R.B. (1981) Sr, Cd, Mn and Co distribution BOREEN,

Subtropical and temperate carbonate elemental composition coefficients in calcite as a function of calcite precipita­ tion rate. Geochim. Cosmochim. Acta, 45, 553-561. LowENSTAM, H.A. (1954) Factors affecting the aragonite: calcite ratios in carbonate-secreting marine organisms. J Geol. , 62, 284-322. MAXWELL, W.G.H. (1968) Atlas of the Great Barrier Reef Elsevier, Amsterdam. MciNTIRE, W.L. (1963) Trace element partition coeffi­ cients-a review of theory and applications in geology. Geochim. Cosmochim. Acta, 27, 1209-1264. MILLIMAN, J.D. (1974) Recent Sedimentary Carbonates I, Marine Carbonates. Springer-Verlag, Berlin. MORRISON, J.O. & BRAND, U. (1987) Geochemistry of Re­ cent marine invertebrates. Geosci. Can., 13, 237-254. MORSE, J.W. & MACKENZIE, F.T. (1990) Geochemistry of Sedimentary Carbonates, Developments in Sedimentol­ ogy, 48. Elsevier, Amsterdam. Muccr, A. (1987) Influence of temperature on composi­ tion of magnesian calcite overgrowths precipitated from seawater. Geochim. Cosmochim. Acta, 51, 19771984. Mucci, A. (1988) Manganese uptake during calcite pre­ cipitation from seawater: conditions leading to the formation of a pseudokutnahorite. Geochim. Cosmo­ chim. Acta, 52, 1859-1868. MuCCI, A. & MORSE, J.W. (1983) The incorporation of 2 Mg2 + and Sr + into calcite overgrowths: influences of growth rate and solution composition. Geochim. Cos­ mochim. Acta, 47, 217-233. NELSON, C.S. (1978) Temperate shelf carbonate sediments in the Cenozoic of New Zealand. Sedimentology, 25, 737-771. PINGITORE, N.R., EASTMAN, M.P., SANDIDGE, M., 0DEN, K. & FREIHA, B. (1988) The co-precipitation of manga­ nese(II) with calcite: an experimental study. Mar. Chern., 25, 107-120. PuRSER, B.H. & EVANS, G. (1973) Regional sedimentation along the Trucial Coast, SE Persian Gulf. In: The Persian G ulf (Ed. Purser, B.H.), pp. 211-231. Springer­ Verlag, Berlin. RAo, C.P. (198la) Geochemical differences between trop­ ical (Ordovician) and subpolar (Permian) carbonates, Tasmania, Australia. Geology, 9, 205-209. RAo, C.P. (1981b) Cementation in cold-water bryozoan sand, Tasmania, Australia. Mar. Geol., 40, M23-M33. RAo, C.P. (1990a) Geochemical characteristics of cool­ temperate carbonates, Tasmania, Australia. Carbonates and Evaporites, 5, 209-221. RAo, C.P. (1990b) Petrography, trace elements and oxygen and carbon isotopes of Gordon Group carbonates (Ordovician), Florentine Valley, Tasmania, Australia. Sediment. Geol., 66, 83-97. RAo, C.P. (1991) Geochemical differences between sub­ tropical (Ordovician), cool temperate (Recent and Pleis­ tocene) and subpolar (Permian) carbonates, Tasmania, Australia. Carbonates and Evaporites, 6, 83-106. RAo, C.P. (1994) Implications of isotopic fractionation and temperature on rate of formation of temperate

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shelf carbonates, eastern Tasmania, Australia. Carbon­ ates and Evaporites, 9, 33-41. RAo, C.P. (l996a) Modern Carbonates: Tropical, Tem­ perate and Polar. Carbonates, Hobart, Tasmania, Aus­ tralia. RAo, CP. (1996b) Elemental composition of marine cal­ cite from modern temperate shelf brachiopods, bryozo­ ans and bulk carbonates, Eastern Tasmania, Australia. Carbonates and Evaporites, 11, 1-18. RAo, C.P. & ADABI, M.H. (1992) Carbonate minerals, major and minor elements and oxygen and carbon isotopes and their variation with water depth in cool, temperate carbonates, western Tasmania, Australia. Mar. Geol., 103, 249-272. RAo, C.P. & AMINI, Z.Z. (1995) Faunal relationship to grain-size, mineralogy and geochemistry in Recent tem­ perate shelf carbonates, Western Tasmania, Australia. Carbonates and Evaporites, 10, 114-123. RAo, C.P. & GREEN, D.C. (1983) Oxygen- and carbon­ isotope composition of cold shallow-marine carbonates of Tasmania, Australia. Mar. Geol. , 53, 117-129. RAo, C.P. & HusTON (1995) Temperate shelf carbonates reflect mixing of distinct water masses, eastern Tasma­ nia, Australia. Carbonates and Evaporites, 10, 105-113. RAo, C.P. & JAYAWARDANE, M.P.J. (1993) Mineralogy and geochemistry of modern temperate carbonates from King Island, Tasmania, Australia. Carbonates and Evaporites, 8, 170-180. RAo, C.P. & JAYAWARDANE, M.P.J. (1994) Major minerals, elemental and isotopic composition in modern temper­ ate shelf carbonates, Eastern Tasmania, Australia: im­ plication for the occurrence of extensive ancient non­ tropical carbonates. Palaeogeogr. Palaeoclimatol. Palaeoecol. , 107, 49-63. RAo, C.P. & NELSON, C.S. (1992) Oxygen and carbon isotope fields for temperate shelf carbonates from Tas­ mania and New Zealand. Mar. Geol., 103, 273-286. REECKMAN, S.A. (1988) Diagenetic alterations in temper­ ate shelf carbonates from southeastern Australia. Sedi­ ment. Geol. , 60, 209-219. RoBINSON, P. (1980) Determination of calcium, magne­ sium, manganese, strontium, sodium and iron in the carbonate fraction of limestones and dolomites. Chern. Geol., 28, 135-146. VEIZER, J. (1983a) Chemical diagenesis of carbonates: theory and application of trace element technique. In: Stable Isotopes in Sedimentary Geology (Ed. Arthur, M.A.). Soc. econ. Paleont. Miner., Short Course, 10, 3/1-3/100. VEIZER, J. (1983b) Trace element and stable isotopes in sedimentary carbonates. In: Carbonates: Mineralogy and Chemistry (Ed. Reeder, R.J.), Rev. Mineral., Min­ eral. Soc. Am. Washington, DC, 1 1, 265-299. WINEFIELD, P.R., NELSON, C.S. & HODDER, A.P.W. (1996) Discriminating temperate carbonates and their diage­ netic environments using bulk elemental geochemistry: a reconnaissance study based on New Zealand Cenozoic limestones. Carbonates and Evaporites, 11, 19-31.

Index

Page numbers in italics refer to figures; those in bold refer to tables.

active margins, fringing reef growth and structure, New Hebrides Island Arc 261-77 Allison Guyot, Mid-Pacific Mountains 79-90 age-depth model l l 0-12 Albian sequences 130-3 Aptian sequences 129-30 comparisons 109 location maps 80, 96, 138 summary 107 Alps, French, Cretaceous, Vercors Platform 13, 14 Alps, Southern, Triassic Schlern Platform 7 Alps, Swiss, Cretaceous, Helvetic Domain l l , 13 ammonites, time-scale calibration

113

Aptian-Albian, Pacific Albian sequence boundaries

115-16 Aptian sequence boundaries

117

and European sequences 112-16 eustatic sea-levels 116-18 Japanese seamounts 60 time-scale calibration 113 Arkansas, Mississippian, Burlington platform, exposure and drowning unconformities, close superposition 18 atolls see coral atolls Aube, Folkestone, correlation with Aptian-Albian, Pacific 114 Australia development of subtropical shelf carbonates 188-9 see also Great Barrier Reef Australia, E climate and oceanography 169 distribution of carbonate sediments 165 Fraser Island and Platform

166-89 Australia, NE, Marion Plateau

11

Boussinesq's approximation of incompressibility 250 Bowling Green Bay, Queensland, sediment accumulation

296-302 bryozoan-foraminifera-mollusc assemblage, and coral­ Halimeda assemblage 187 Burdekin River delta, Queensland, sediment accumulation

8,

145-61 Australia, W, Shark Bay, geographical and geological setting 312-14

61-2 porous media equations 250-l post-drowning evolution 59 sedimentology 4-5 seismic expression of exposure and drowning events 15-16 sequence stratigraphy 7-9 subtropical vs temperate, four examples 311-23 subtropical development, Quaternary and Tertiary

296-302 Burlington platform, exposure and drowning unconformities, close superposition 18

Cainozoic carbonate platform, Marion Plateau, NE Australia, computer simulation 145-61 Campanian, initiation of guyots 57 Campanian-Maastrichtian, development of platforms 57,

58

112-15 composite sequence­ biostratigraphy-isotope scale

without exposure 9-15 as global markers 17 nutrient excess 62-4 sea-level changes 62 as sequence boundaries 16-17 topography 6 erosion during drowning 4 exposure before drowning, seismic evidence 17-18 groundwater flow, porous media equations 250-l insights into demise 59-61 origin of morphological features

B0lling meltwater pulse 231-3 borehole response, tidal transients, computer simulation 256-58 Borneo, Oligocene of Kalimantan 9,

Canada, Western Canada Basin, Devonian reefs and platforms

15

carbon dioxide levels, sea-water 321 carbon reservoir function of coral reefs 74 carbonate ancient, preservation of elemental signal 321 composition, subtropical and temperate 311-23 distribution control by terrigenous sediment

295-310 terrigenous sediment supply

295-310 fractionation of elements 320 lithofacies, Brazil 188 production vs water depth 4 and rate of precipitation 320 carbonate platforms computer simulation 145-61 correlation with European continental margins and Haq curve 110, 112-16 development 57-9 drowning events 62-4 defined 3, 59 with exposure 7-9

163-95 unconformities, scenarios

interglaciation to present 70 Cleveland Bay, Queensland, sediment accumulation 296-302 climate change origin of Great Barrier Reef 32-6 see also sea-level changes coastal turbid-zone reefs, central GBR and Gulf of Papua

295-310 Comanchean platforms, southern USA, Cretaceous 12 Comoro Islands, Mayotte, morphology and sediments 219-36 computer simulation borehole response, tidal transients

256-58 groundwater flow, porous media equations 250-l Marion Plateau, Cainozoic carbonate platform, N E Australia 145-61 steady-state interstitial water circulation, coral atolls 249-56 coral atolls cross section 252 homogeneous atoll platform 253-4 porous media equations 250-l

Reefs and Carbonate Platforms in the Pacific and Indian Oceans Edited by G. F. Camoin and P. J. Davies © 1998 International Association of Sedimentologists ISBN: 978-0-632-04778-9 325

5

see also named locations chloralgal lithofacies 188 chlorozoan lithofacies 188 clam, Tridacna spp., last

Index

326 coral atolls (cont.) South China Sea, tectonic and monsoonal controls 237-48 steady-state interstitial water circulation, computer simulation 249-56 transition zone 254-6 coral reefs carbon reservoir function 74 coastal turbid-zone reefs, central GBR and Gulf of Papua

295-310 conceptual existence fields, carbonate production and terrigenous sedimentation 306 development and lagoonal sedimentation, Heron Reef, S Great Barrier Reef Province

281-94 extinction and contraction during Last Glaciation 74 internal structure, Tasmaloum, SW Pacific 261-277 limiting factors 72 coral-Halimeda assembly, and bryozoan-foraminifera-mollusc assembly 187 Cretaceous Comanchean platforms, southern USA 12 Helvetic Domain, Swiss Alps 11,

13

Maracaibo platform, Venezuela Pacific guyots as recorders of

12

sea-level 95-100 polar ice caps 78 Pyrenees, S 9, 12 Shuaiba platform, Persian Gulf 12 Vercors Platform, French Alps 13,

14 Cycloclypeus-Operculina limestone, features 20 l

Darcy's law 250 d'Entrecasteaux Zone, New Hebrides Island Arc, location map 262 Devonian reefs and platforms, Western Canada Basin 15 dolomite, Barremian 140 dolomite, white sucrosic, origin in shallow water 137-44 dolomitization Queensland shelf 190 role of sea water, strontium isotopes

137-44 . drowned reefs 223 drowning events, carbonate platforms 3

Espiritu Santo, SW Pacific, fringing reef biofacies sequences 265-7, 272-3 o180 and o13C, laminated stromatolitic crusts 269 geomorphology and lithology 264-5

lithification 267-70, 273 location map 262 schematic cross-sections 263, 271 tectonic and environmental growth influences 261-77 European margin, sea-level curve, correlation with Aptian-Albian, Pacific 114 EXXON global sea-level chart comparison with Marion Plateau data 159 correlation with carbonate platform sequences 110, 112-16 as a working model 147-8, 156,

157, 158-60

Fieberling Guyot, oscillatory currents 5 Fly River, Gulf of Papua, sediment accumulation 302-5 foraminifera, time-scale calibration

113

Fourier analysis, sedimentary cycles at ODP Sites Leg 143, geophysical logs 77-92 Fraser Island and Platform 166-9 dolomite, comparison with Marion Plateau 190 location maps 166-8 Miocene subtropical carbonate buildups 189 Quaternary subtropical carbonate buildups 189 sedimentology 172-87 rhodoliths, radiocarbon age 178 seismic line 170 seismic stratigraphy 171-2 sub-reef environment, comparison with Great Barrier Reef

34-6 fringing reef, Tasmaloum, SW Pacific, tectonic and environmental growth influences 261-77

Gardner Banks 167, 187 location maps 168, 174-5 seismic data 170 Great Bahama Bank, platform history 5-6 Great Barrier Reef central coastline inner shelf, sediment accumulation 296-302 sedimentary model 308 turbid-zone reefs 295-310 geographical and geological setting

312 location map 24-5 origins 32-6 reconciliation of seismic, sedimentological and isotopic data 30-2 Great Barrier Reef Province, Heron Reef, lagoonal sedimentation

281-94

Gulf of Papua, turbid-zone reefs, sediment accumulation 302-5 guyots age-depth models l l 0-12 correlation of sequences 107-110 defined 39 drowning events 116-18 typical sectional view 98

Halifax Bay, Queensland, sediment accumulation 296-302 Haq curve see EXXON Haynesville platform, Jurassic, Texas 14 Helvetic Domain, Cretaceous, Swiss Alps 11, 13 Heron Reef, Great Barrier Reef Province lagoonal sedimentation 281-94 location map 282 radiocarbon dating 289 ryyf growth history 289-91 seisq�ic profiling and sediment si'mpling 282-9 Huevo Guyot see Resolution Guyot

Indian Ocean, W, last glacial extinctions 70-2 Indo-West Pacific habitat fragmentation 73 molluscan assemblage extinctions

69-76 see also Pacific, NW

JOIDES Resolution see ODP, route location maps Jurassic, Haynesville platform, Texas 14

karst processes

4-5

Lady Elliot Reef, Bunker Group, reef growth history 291 lagoonal sedimentation and reef development, Heron Reef, S Great Barrier Reef Province

281-94 last glacial maximum 231 logging data, spectral analysis

78-9

Maastrichtian cooling of sea-surface 63 drowning of platforms 59 magnesium, subtropical vs temperate shelf carbonates 315, 317 manganese and iron, subtropical vs temperate shelf carbonates

316, 318 Maracaibo platform, Cretaceous, Venezuela 12 Marion Plateau, NE Australia

Index chronostratigraphical diagram 150 comparison with EXXON global sea-level chart 153, 159 computer simulation 145-61 carbonate parameters 154 diagenetic sequence, 3 stages 190 dolomite, comparison with Fraser shelf 190 mega-seismic sequences and events in cover sequence 149 Marshall Islands, Wodejebato Guyot, NW Pacific, development and demise model 39-68 Maryborough Basin 167 Mayotte, Comoro Islands geological setting 220-1 location map 220 morphology and sediments 219-36 U-Th geochemistry and ages

223-4

Mid Pacific Mountains see Allison and Resolution Guyots mid-oceanic carbonate platforms, NW Pacific, development and demise model 57-64 Milankovitch periodicities, ODP Leg

143

83-4, 86

Miocene Luahua platform, South China Sea 9, 10 Mississippian, Arkansas, Burlington platform, exposure and drowning unconformities, close superposition 18 MIT Guyot, NW Pacific 96, 107 age-depth model 110-12 Albian sequences 127-9 Aptian sequences 126-7 comparisons 109 molechfor lithofacies 188 molluscan assemblage extinciions, Indo-West Pacific 69-76 monsoonal controls, coral atolls, South China Sea 237-48

327

8180, laminated stromatolitic crusts

269

normalized 8180 curve

230

71 Pacific, NW guyots as recorders of Cretaceous eustatic sea-level changes

95-136 seamount morphology 61-4 Wodejebato Guyot, development and demise model 39-68 see also Indo-West Pacific Pacific, SW, Tasmaloum, fringing reef tectonic and environmental growth influences 261-77 Paluma Shoals, Halifax Bay, Queensland, corals and sedimentation 298-300, 302 Papua, Gulf of, sediment accumulation 302-5 passive margins Heron Reef, Great Barrier Reef Province 281-94 terrigenous sediment, control on reef carbonate distribution

295-310 patch reefs, origins 291 Persian Gulf geographical setting 314-15 Shuaiba platform, Cretaceous 12 Pleistocene reef complex deposits, Central Ryukyus, SW Japan

197-213 porous media equations 250-1 Purari River, Gulf of Papua, sediment accumulation 302-4 Pyrenees, S, Cretaceous 9, 12

Quaternary subtropical carbonate platform development, S Queensland

163-95 Navier-Stokes domains 252 New Hebrides Island Arc, active margins, fringing reef growth and structure 261-77

ODP Leg 133 Great Barrier Reef 23-38 route location map 25 ODP Leg 143 route location maps 80, 96, 138 see also Allison Guyot; Resolution Guyot ODP Leg 144 route location map 40 see also MIT, Takuyo-Daisan and Wodejebato Guyots . Oligocene of Kalimantan, Borneo 9,

11

One Tree Reef, reef growth history

291 oxygen isotopes

Red Sea last glacial extinctions 70-2 small-scale mass extinction 71-2 sibling species in Indo-West Pacific

Tasmaloum, SW Pacific, fringing reef, tectonic and environmental growth influences 261-77 Queensland climate and oceanography 169 East Australia Current 169 Plateau 24 shelf, comparison with Ryukyus, SW Japan 190-2 subtropical carbonate platform development, Quaternary and Tertiary 163-95

radiocarbon dating Heron Reef, Great Barrier Reef Province 289 rhodoliths, Fraser Island and Platform 176, 178 Tasmaloum, SW Pacific, fringing reef, laminated stromatolitic crusts 269 Rayleigh numbers 251, 252

tropic stability, last interglaciation to present 69-76 Resolution Guyot, Mid-Pacific Mountains age-depth model 110-12 Albian sequences 122-4 Aptian sequences 119-22 comparisons 109 location maps 80, 96, 138 origin of white sucrosic dolomite

137-44 reconstruction of emersion horizons 101 strontium isotopes, evidence for role of sea water in dolomitization

137-44 summary 103-5, 104-5 rhodalgal lithofacies 188 rhodolith limestone, features 20 I rhodoliths, fore-reef, Ryukyu Group

210-11 rhodoliths, radiocarbon age, Fraser Platform 176, 178 Ryukyu Group, SW Japan carbonate and siliciclastic rocks

199

comparison with S Queensland shelf

190-2 corals and corallines 202 location maps 191, 197 Pleistocene reef complex deposits

197-213 sedimentary facies 190-1 stratigraphy 203-7

sea-level changes Aptian-Albian 95-136 carbonate platforms drowning events 62 theory 97-100 and practice I 00-3 European margin, correlation with Aptian-Albian, Pacific 114 limiting factor of coral reefs 72 meltwater pulses 231-3 normalized 8'80 curve 230 schematic eustatic curve, Aptian-Albian, Pacific 116-18 and sediment accumulation 307-8 sea-water carbon dioxide levels 321 composition 319-20 density equations 250 salinity, tolerances 70 temperature carbonate mineralogy 319 limiting factor of coral reefs 72 sediment accumulation control on reef carbonate distribution 295-310 coral reefs, potential problems 305

Index

328 sediment accumulation (cont.) methods of measurement 302 sedimentary cycles, carbonate platform facies, ODP Sites 865 and 866 logs 77-92 SEDPAK platform modelling 156 setup and input variables used in modelling 155 Seiko Guyot see Takuyo-Daisan Guyot seismic data Gardner Banks 170 Great Barrier Reef 30-2 Heron Reef, Queensland 282-3 mega-seismic sequences, Marion Plateau, NE Australia 8, 149 Wodejebato Guyot, NW Pacific 40-1 sequence boundaries 3 Shark Bay, W Australia geographical and geological setting 312-14 location map 314 Shuaiba platform, Cretaceous, Persian Gulf 12 sodium subtropical shelf carbonates 315 temperate shelf carbonates 318 South China Sea coral atolls characteristics and location

242-3

classification 247 location map 239 tectonic and monsoonal controls 237-48 types 240 Miocene Luahua platform 9, IO tectonic map 238 spectral analysis, logging data 78-9 strontium subtropical shelf carbonates 315 temperate shelf carbonates 317 strontium isotopes, evidence for role of sea water in dolomitization 137-44

subtropical shelf carbonates vs temperate, composition, four examples 311-23 composition, comparisons 313-14 development, Australia 188-9 environmental significance 187-8 Quaternary and Tertiary, S Queensland 163-95 subtropical and temperate carbonate platforms, diagenetic features 189-90

Tahiti, P7 borehole 256-8 Takuyo-Daisan Guyot, NW Pacific 96, 105-6 age-depth model 110-12 Albian sequences 126 Aptian sequences 124-6

comparisons I09 Tasmaloum see Espiritu Santo Tasmania, shelf carbonates and siliciclastics 3 I 5 tectonic controls, coral atolls, South China Sea 237-48 tectonic growth influences, Tasmaloum, Espiritu Santo, SW Pacific 261-77 temperate shelf carbonates, vs subtropical 311-23 terrigenous sediment control on reef carbonate distribution 295-310 world total annual 29 5 Tertiary subtropical carbonate platform development, S Queensland 163-95 Texas, Haynesville platform, Jurassic 14 thermal convection, inducing steady­ state hydraulic water circulation 249-56 tidal forcing, borehole response, computer simulation 256-8 tidal transients, borehole response, computer simulation 256-58 time-scale calibration 113

Triassic Schlern Platform, Alps, Southern 7 TRIO/CASTEM, simulation of thermal convection systems 251 tropic stability, Red Sea and Western Indian Ocean 69-76 turbidites, threshold condition 305

U-Th geochemistry and ages, Mayotte 223-4 Upstart Bay, Queensland, sedimP.nt accumulation 296-302 USA, Comanchean platforms, Cretaceous 12

Vanuatu (New Hebrides Island Arc), active margins, fringing reef growth and structure 261-77 Venezuela, Maracaibo platform, Cretaceous 12 Vocontian Basin, correlation with Aptian-Albian, Pacific II4

Walther's Law of Succession 23-38 water depth, vs carbonate production 4 Wodejebato Guyot, NW Pacific depositional history 57-9, 60 development and demise model 39-68 diagenesis 49-57 morphology and seismic data 40-1 paragenetic sequence and interpreted relative sea-level changes 56 post-drowning facies 57 shallow-water platform carbonates 44-56 stratigraphy 41-3 weathering profiles 44

Younger Dryas meltwater pulse 231-3

Reefs and Carbonate Platforms in the Pacific and Indian Oceans Edited by G. F. Camoin and P. J. Davies © 1998 International Association of Sedimentologists ISBN: 978-0-632-04778-9

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