Calcretes (Reprint Series of the IAS 2)

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CALCRETES

Calcretes Edited by V. Paul Wright and Maurice E.Tucker © 1991 The International Association of Sedimentologists ISBN: 978-0-632-03187-0

CALCRETES ED ITED

BY

V. PAUL WRIGHT Postgraduate Research Institute of Sedimentology (PRIS), University of Reading AND

MAURICE E. TUCKER Department of Geological Sciences, University of Durham

REPRINT SERIES VOLUME 2 OF THE INTERNATIONAL ASSOCIATION OF SEDIMENTOLOGISTS PUBLISHED BY BLACKWELL SCIENTIFIC PUBLICATIONS OXFORD LONDON EDINBURGH BOSTON MELBOURNE PARIS BERLIN VIENNA

© 199I

The International Association

of Sedimentologists Published by Blackwell Scientific Publications Editorial officies:

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British Library Cataloguing in Publication Data Calcretes.

in a retrieval system, or transmitted,

1. Deserts. Sedimentation

in any form or by any means,

I. Wright, V. Paul

electronic, mechanical, photocopying,

II.

recording or otherwise without the

III. Series

prior permission of the copyright owner.

1953-

Tucker, Maurice E.

552.5 ISBN 0-632-03187-5

First published I991 Library of Congress Set by Setrite Typesetters, Hong Kong Printed and bound in Great Britain at the Alden Press, Oxford

Cataloging in Publication Data Calcretes/edited by V. Paul Wright and Maurice E. Tucker. p.

em. - (Reprint series v. 2

of the International Association of Sedimentologists) Includes index. ISBN 0-632-03187-5

l.

Calcretes.

l. Wright,

Il. Tucker, Maurice E. Reprint series

V.

Paul, 1953-

Ill. Series:

of the

International Association of Sedimentologists; v. 2. QE471.15.C27C25 552'.5-dc20

1991

Contents

vu

I77 A rendzina from the Lower Carboniferous of South Wales [pages 159-167 only plus

Preface

references]

I Calcretes: an introduction

V. P.

23

Quaternary Calcretes

WRIGHT

Sedimentology

25 Calcretes of Olduvai Gorge and the Ndolanya

1983

30159-179

I89 The role of fungal biomineralization in the

Beds of northern Tanzania

formation of Early Carboniferous soil fabrics

R. L. HAY & R.J. REEDER

V. P.

Sedimentology

Sedimentology

25

1978

649-673

51 Pellets, ooids, sepiolite and silica in three HAY &

Sedimentology

B.

33

831-838

caliche profiles in a Bahamian Pleistocene dune

WIGGINS

1980

1986

I97 Petrographic and geochemical analysis of

calcretes of the southwestern United States R.L.

WRIGHT

27 559-576

J.A. BEIER

Sedimentology 1987 34

991-998

69 Quaternary pedogenic calcretes from the Kalahari (southern Africa): mineralogy,

205

Biological Activity and Laminar Calcretes

genesis and diagenesis 207 Origin of subaerial Holocene calcareous

N.L. WATTS

Sedimentology

1980

27

crusts: role of algae, fungi and

661-686

sparmicritisation 95

Biological Activity and Calcrete Fabrics

C.F. KAHLE

Sedimentology

1977

24

413-435

97 Caliche profile formation, Saldanha Bay (South Africa)

23 I Calcification in a coccoid cyanobacterium associated with the formation of desert

G. F. KNOX

Sedimentology

1977

24

stromatolites

657-674

W. E. KRUMBEIN &

I15

C.F.

C.

GIELE

Sedimentology 1979 26 593-604

Biolithogenesis of Microcodium: elucidation KLAPPA


243

489-522

Biogenic laminar calcretes: evidence of calcified root-mat horizons in paleosols

I49 Rhizoliths in terrestrial carbonates:

V. P. WRIGHT,

classification, recognition, genesis and

WIMBLEDON

significance

Sedimentology

C.F. KLAPPA

Sedimentology

1980

27

261

613-629

PLATT &

N. H.

1988

35

W. A.

603-620

Aspects of Calcrete Petrography

263 Calcrete conglomerate, case-hardened

I67 Calcrete profiles in the Eyam Limestone (Carboniferous) of Derbyshire: petrology and

conglomerate and cornstone-a comparative

regional significance

account of pedogenic and non-pedogenic

A. E .

carbonates from the continental Siwalik

ADAMS

Sedimentology

1980

27

Group, Punjab, India

651-660

S. K. TANDON & D.


v

NARAYAN

353-367

Contents 321

279 Siliciclastic grain breakage and displacement

Calcretes and Palustrine Carbonates

due to carbonate crystal growth: an example 323

from the Lueders Formation (Permian) of

Lacustrine carbonates and pedogenesis:

north-central Texas, USA

sedimentology and origin of palustrine

C. BUCZYNSKI &

deposits from the Early Cretaceous Rupelo

Sedimentology

1987

H.S.

34

CHAFETZ

Formation, W Cameros Basin, N Spain

837-843

N.H. PLATT

Sedimentology

287 Near-surface shrinkage and carbonate replacement processes, Arran Cornstone 343

Formation, Scotland S.K. TANDON &

Sedimentology

1989

36

349 Index

1113-1126

301 The application of cathodoluminescence to interpreting the diagenesis of an ancient calcrete profile S.T. SOLOMON &

Sedimentology

1985

References

P.F. FRIEND

32

G.M. WALKDEN

877-896

VI

1989

36

665-684

Preface

Calcretes are an important component of many

calcretes from southern and eastern Africa and the

ancient fluvial, lacustrine and shallow-marine car

south-western United States. The examples are all

bonate sequences and they are widely developed in

of calcretes which show few biogenic features. The

many parts of the world at the present time. Cal

second section contains seven papers on modern and

cretes are useful to the earth scientist involved in

ancient calcretes which possess many biogenic fabrics,

reconstructing ancient environments, palaeoclimates

including the enigmatic Microcodium.

and palaeogeographies, and they may also reveal

section,

details of soil biota and chemistry. Papers on cal

laminar calcretes, some of which are the result of

cretes are published in journals of soil science,

calcification of root mats. Some specific textural

geomorphology, sedimentology and general geology,

features of calcretes are illustrated in the fourth

but in the last two decades the journal Sedimentology

section, with four papers describing examples from

with three

reprints,

The next

is concerned

with

has received many on this subject, so that a com

India, the USA, England and Scotland. The book is

pilation of them has been put together to make this

concluded with a case-history of lacustrine sedi

second reprint volume of the International Associa

mentation and pedogenesis, with a description of

tion of Sedimentologists.

palustrine limestones from Spain.

Calcretes have been studied by people from dif

This collection of reprints should illustrate the

ferent backgrounds and with different interests, so

range of calcrete occurrences and the great variety

that this book also provides a review of the work on

of textures and fabrics. It should serve as more than

calcretes as an introduction to the topic and the

an introduction to the subject and be of use to

papers that follow. Eighteen papers are reproduced

geologists, soil scientists and geographers.

here and they have been divided into five groups,

V. Paul Wright

each preceded by a short commentary. The first

Maurice E. Tucker

section has three papers which describe Quaternary

vii

CALCRETES: AN INTRODUCTION*

Calcrete is a near surface , terrestrial, accumulation of predominantly calcium carbonate, which occurs in a variety of forms from powdery to nodular to highly indurated. It results from the cementation and displacive and replacive introduction of calcium carbonate into soil profiles, bedrock and sediments, in areas where vadose and shallow phreatic ground waters become saturated with respect to calcium carbonate. This definition is modified from Goudie ( 1973) and Watts (1980, this volume). The term 'dolocrete' is used where the main carbonate phase is dolomite. Calcretes are not restricted to soil profiles (pedogenic calcretes) but can also occur, for example, below the zone of soil formation but within the vadose zone, or at the capillary fringe and below the water-table to form groundwater calcrete. A very general definition is preferred here be cause the term has been used very loosely in the past. It would serve no purpose to review the ter minological quagmire, but it is more important to identify the processes of formation and hydrological setting of terrestrial carbonate accumulations than to have a post-mortem on the misuse, or supposed misuse of the term. The term is not used to describe tufas, travertines, beachrock and lake carbonates. However, it is a moot point as to whether many types of simple carbonate cementation, such as that seen in aeolianites for example, are not classifiable as calcrete. Most calcretes are finely crystalline and in their more mature forms, consist of a more-or-less continuous secondary matrix of micrite or microspar grade carbonate. Thus the fabric differs from simple cementation which is typically more coarsely cry stalline in a grain/clast supported fabric. Goudie (1973) has provided a detailed review of the various terms used to describe calcrete materials. The term is virtually synonymous with 'caliche' in its current usage by English-speaking workers. Milnes (1991) provided an historical review of the development of ideas on calcrete formation. The most important and widespread calcretes are those which form in soil profiles (e .g. Fig. 1A). These *

Reading University, PRIS Contr. 115.


accumulations constitute calcic or petrocalcic hor izons (if continous and indurated) in the terminology of soil scientists. It has been estimated that such soils today cover an estimated 20 million km2 or about 13% of the total land surface (Yaalon, 1988), and it is little wonder that a huge literature exists on such soils. They are a prominent feature in climatic zones where a seasonal moisture deficit occurs, allowing CaC03 to accumulate (Goudie, 1973, 1983 ) . While calcretes are important in landscape development, providing a geomorphic 'threshold' for erosion, they are not as important in this respect as other duricrusts such as laterites and silcretes. They are not as re sistant to erosion as these other forms, since calcretes tend to form nearer the land surface and they are not part of thick saprolite profiles which typically form in more humid climates with deeper weather ing. Calcretes present problems for land use and are associated with serious soil erosion in many regions. Despite this fact, relatively little has been published on catenary relationships, calcrete thicknesses and soil erodability. Calcretes were just as widespread in the past as they are today. They have been widely recognized in ancient sedimentary sequences (e.g. Fig. 1 B , C), even from the Precambrian (Chown & Caty, 1983; Bertrand-Sarfati & Moussine-Pouchkine, 1983 ) . There are numerous records from Phanerozoic se quences where calcretes have been used as palaeo climatic indicators and to assess depositional rates in architectural models of alluvial sequences (e.g. Allen, 1974a; Steel, 1974; Hubert, 1978; Leeder, 1975; McPherson, 1979; Wright, 1982). Palaeo-calcretes have been described from two distinctive settings: alluvial/lacustrine and shallow water carbonate systems. In the latter case, as a result of the widespread development of exposure surfaces within Quaternary shallow-water carbon ates, and because of the realization that important stabilization and cementation take place during exposure to meteoric conditions, there was a surge of interest in identifying such exposure surfaces in ancient sequences. Descriptions of Quaternary limestone-hosted calcretes (Braithwaite, 1975, 1983; James, 1972; Read, 1974; Harrison, 1977) led to the

Calcretes: An Introduction

Fig. 1. (A) Stage 5 Quaternary calcrete, upper La Mesa surface, Rio Grande rift area, New Mexico. This profile is in excess of 400 ka and has a laminated crust capping the petrocalcic horizon. (B) Prismatic Stage 4 calcrete, Lower Devonian, Lydney, England. (C) Stage 3 calcrete, Upper Jurassic, Porto Novo, Portugal. Note overlying channel sandstone.

2


genic calcretes and was developed for geotechnical surveys. Table 1 is largely based on Netterberg's classification. The forms of calcrete recognized in this classi fication relate to stages seen in the development of calcrete profiles (Netterberg, 1980) . For example, scattered nodules with time pass to glaebular cal crete, to honeycomb calcrete , to hardpans, and may later weather to boulder calcrete if soil conditions change . This type of maturity-related classification (a chronosequence) has been offered by a number of workers for both Quaternary and pre-Quaternary calcretes (Gile et al., 1966; Allen, 1974a,b; Steel, 1974; Machette, 1985 ) . Machette ( 1985) has pro vided the most comprehensive sequence and has

discovery of numerous ancient examples (e.g. Walkden, 1974; Walls et al . , 1975; Harrison & Steinen, 1978; Adams, 1980; Adams & Cossey, 1981; Riding & Wright, 198 1 ; Wright, 1983) , and a suite of characteristic and diagnostic petrographic criteria quickly developed, thoroughly reviewed by Esteban & Klappa ( 1983). It is reasonable to state that while most sediment ologists working on calcretes in alluvial/lacustrine sequences (e.g. Allen, 1974a ,b; Freytet & Plaziat, 1982; Wright, 1982) interpreted the calcretes using the terminology and concepts of soil science , many carbonate sedimentologists treated calcretes as diagenetic features. This almost led to a dual ap proach , made worse because many soil scientists were unaware of the extensive literature on soil carbonates to be found in sedimentological journals. Recently these divisions have become blurred with much more 'cross-pollination' of ideas. In order to review calcretes severafbasic questions will be tackled: how can they be classified? where do they form? how do they form? what are the sources of the carbonate? what are the processes responsible for the profile form? what are the microstructures of calcretes? what are the geochemical controls and what isotopic criteria can be used to interpret palaeo-calcretes?

Table 1. Morphological classification of calcretes based on Netterberg (1967, 1980) and Goudie (1983)

CLASSIFICATION

Calcareous soil

Very weakly cemented or uncemented soil with small carbonate accumulations as grain coatings, patches of powdery carbonate including needle-fibre calcite (pseudomycelia), carbonate-filled fractures and small nodules

Calcified soil

A firmly cemented soil, just friable; few nodules. 10-50% carbonate

Powder calcrete

A fine, usually loose powder of calcium carbonate as a continuous body with little or no nodule development

Pedotubule calcrete All, or nearly all, the secondary carbonate forms encrustations around roots or fills root or other tubes (tubules)

Before reviewing how calcretes are classified 1t 1s worth examining the way calcretes fit into soil class ifications. No widely used soil classification includes calcrete (or some synonym) as a soil type (soil order). There are no soils called calcretes. Pedogenic calcretes occur within soil profiles, where they typically constitute several discrete horizons (e.g. calcic horizons or petrocalcic horizons) forming a sub-profile within the main soil profile. The use of the term 'calcrete profile' in this paper refers to a set of related calcic/petrocalcic horizons within a thicker soil profile. Calcretes typically occur within Aridisols, Vertisols and Mollisols (Soil Survey Staff, 1975). Five main types of classification are widely used; all are basically generic, reflecting our still relatively poor understanding of calcrete formation. At the simplest level calcretes can be classified on their morphology, with the system devised by Netterberg ( 1 967, 1980) , which supersedes that of Durand ( 1963 ) , being the most useful. This refers to pedo3

Nodular calcrete

(syn. glaebular calcrete of Netterberg, 1980.) Discrete soft to very hard concretions of carbonate-cemented and/ or replaced soil. Concentrations may occur as laminated coatings to form pisoids

Honeycomb calcrete

Partly coalesced nodules with interstitial areas of less indurated material between

Hardpan calcrete

(syn. petrocalcic horizon.) An indurated horizon, sheet-like. Typically with a complex internal fabric, with sharp upper surface, gradational lower surface

Laminar calcrete

Indurated sheets of carbonate, typically undulose. Usually, but not always, over hardpans or indurated rock substrates

Boulder/cobble calcrete

Disrupted hardpans due to fracturing, dissolution and rhizobrecciation (including tree-heave). Not always boulder grade. (Clasts are rounded due to dissolution)


primarily non-pedogenic carbonates modified by pedogenic processes. Shallow lake , pond or marsh (palustrine) carbonates are easily modified by ex posure and can resemble mature profiles , especially where extensive desiccation-related grainification has occurred (Wright, 1990a) (Fig. 2). This problem is discussed by Platt ( 1989 , this volume) and Esteban & Klappa ( 1983 ) . Very simply, the dense, fine grained carbonate which develops in low energy Jakes and marshes can be confused easily with con tinuous calcrete fabrics of hardpan (Stage 3) profiles, especially when desiccation has overprinted the original micrite . Calcretes developed on unconsolidated carbonate substrates also develop in a series of stages (Arakel, 1982) broadly similar to those on other host materials. Calcretes can also be classified according to their hydrological setting (Carlisle, 1980, 1983) (Fig. 3). A common misconception is that calcretes only form

recognized six stages of development (Table 2; Fig. 2). In this classification the role of parent (host) material is important and the distinction between calcretes developed in gravel-rich substrates , as against those in gravel-poor ones, is critical because the profiles develop much more rapidly in gravel rich substrates (Gile et a! ., 1966; Machette, 1985 ) . The most mature (Stage 4) profiles contain evi dence of polycyclic brecciation and cementation. The Ogallala cap rock of New Mexico and Texas is such an example and contains huge numbers of well rounded peloids and pisoids formed by the fractur ing, including circum-granular fracturing, and trans portation of these grains into fracture and cavity systems in the calcrete. This process of secondary grainification is discussed below, and by Hay & Wiggins ( 1 980, this volume) . A major problem arises in classifying some continental carbonates, which, while resembling mature calcretes (Stages 4-6) are

Table 2. Classification of pedogenic calcretes based on stages of development. From Machette (1985). High gravel content refers to >50% gravel. Low is less than 20% gravel. The per cent CaC03 refers to 60% in low gravel content

5

Any

Thick laminae (>10 mm); small to large pisoids above. Laminated carbonate may coat fracture surfaces

Indurated, dense, strong, platy to tabular. K,. horizon is 1-2 m thick

>50% in high gravel content. >75% in low gravel content

6

Any

Complex fabric of multiple generations of laminae, brecciated and recemented, pisolitic. Typically with abundant peloids and pisoliths in fractures

Indurated, dense, thick, strong tubular structure. K,. horizon is commonly >2 m thick

>75% in all gravel contents

Stage

2

3

4


,,�. .. } 11 � -r.;· . . . 2 secondary lime mudstone

2. (Upper part) Stages in calcrete development in fine-grained sediment. By Stage 4 the calcrete is dense and impermeable (petrocalcic horizon) leading to the pending of soil water and the formation of laminar calcrete. Stage 6 calcretes show evidence of extensive brecciation. Fractures become filled by peloids and cemented and refractured. The calcrete first develops a secondary lime mudstone fabric but later grainification takes place to form peloids. (Lower part) Calcretes can be difficult to distinguish from pedogenically modified shallow lake margin (palustrine) lime mudstones. These can also be affected by grainification (see text).

Fig.

w (f) 0 0

�

u

�w

a: I [l_

Fig.

Soil moisture zone

:-:�

Gravitational wa ter zone

Ca p i l lar y fringe �

�

... ' ' Groundwater

--- G R AVITATIONAL

ZONE NON- PEDOGENIC CALCRETE

local role of phreatophytic plants

• -

•---

- -

,�� .. ... .... _ : _..

.......-----!

I

CAPILL A R Y FRINGE NON-PEDOGENIC CALCRETE

'-¥

., _...:;.. ��--- PHREATIC (VALLEY) OR

J=

c a rbonate movement

3. Classification of calcretes by hydrologic setting (based on Carlisle, 1980, 1983).

5

G R OUNDWATER NON- PEDOGENIC CALCRETE


in very near-surface settings; extensive and _thick calcretes do occur in arid areas due to precipitation within or j ust below the capillary fringe (Arakel & McConchie, 1982), with or without the influence of phreatophytic plants (Semeniuk & Meagher, 198 1), at a depth down to many metres, or tens of metres, below the surface. It should be stressed that such calcretes can be very difficult to differentiate from the more common pedogenic forms, and it is likely that at least some calcretes, interpreted as pedogenic forms in the geological record, are of the phreatic or capillary fringe type (see below, p . 8). A mineralogical division of carbonate duricrusts has been proposed by Netterberg (1980) (Table 3 ) . I t would b e possible t o devise classifications for gypsiferous and siliceous calcretes/dolocretes but there seems little need for this. Dolocretes can also grade into ankeritic forms and these are common in the early Carboniferous marginal marine limestones of Europe (Muchez & Viane, 1987; Searl, 1988) . These ferroan dolocretes form in coastal paludal settings, typically associated with coals (Wright & Robinson, 1988). Another means of classification is microstructure (Fig. 4) and two end-member types have been re cognized (Wright, 1990c) . Alpha calcretes, which correspond to K-fabrics of Gile et al. (1965) and Bal

Table 3. Classification of calcretes and dolocretes based on dolomite content. After Netterberg ( 1980).

Name

o/o dolomite by mass of total carbonates

Approx. equivalent o/o MgC03*

Calcrete Magnesian calcrete Dolomitic calcrete Calcitic dolocrete Dolocrete

90

40

* o/o MgCO3

MgC03 MgC03

+

CaC03

X

100.

( 1975 ) , consist of dense, continuous masses of mic ritic to microsparitic groundmasses, typically with such features as crystallaria (including circum granular types); floating, etched or exploded skeleton grains; large euhedral crystals (commonly rhombic); crystal size mottling and displacive growth features. Beta calcretes exhibit microfabrics dominated by biogenic features such as rhizocretions, needle-fibre calcites (lublinite), microbial tubes, alveolar septal fabric, and Microcodium. In such fabrics much of the carbonate was precipitated in association with fungi or other soil micro-organisms. The implications

BETA

ALPHA

1 Dense m i crofabric

1

2 Nodules

2 N eedle fibre calcite

3

3 Calcified tubules

Complex cra cks a nd c r y s tallaria

=

Microbial coati ngs

Microcodium

4 Circum-granular cra cks

4

5 Rhombic cal cite c r y s tals

5 Alveolar s e pt a l fabric

6 Floating s e d i m ent grains

6 Calcified pellets

6

Fig. 4. Micromorphological classification of calcretes (see text). Based on Wright ( 1990c).


of these two types of microfabric are discussed below.

(Atkinson, 1977) while in semi-arid and arid soils it ranges from 0.6-4% to less than 0. 1% respectively (Brook et al., 1983). The relatively low Pco2 in arid and semi-arid soils is a contributory factor leading to carbonate precipitation (Marion et al. , 1985 ) . Pre late Palaeozoic soils would have had lower bio masses than later ones and lacked roots. They would have had lower Pco2 than modern soils. Changes in Pco2 in the atmosphere through time may also have been important in influencing carbonate mobility and precipitation in palaeo-calcretes. Pco2 is now regarded as an important control on carbonate mineralogy in marine settings (see review in Tucker & Wright, 1990) and it is possible that similar in fluences on mineralogy operate or operated in calcretes. The common ion effect is another factor which is important in the precipitation of groundwater cal cretes, for example near playas. It is a contributory factor in some pedogenic calcretes (Reheis, 1987 ) . The role o f organic processes, other than i n in fluencing Pco2 , has been underestimated. Cyano bacteria in soils may induce carbonate precipitation by the uptake of C02 (Krumbein & Giele, 1979, this volume), while bacteria are widely suspected of having the potential to cause extensive precipitation in soils (Krumbein, 1968, 1979; Boquet et al. , 1973 ; Pentecost & Terry, 1989), as a result of chemolitho tropic removal of C02, the production of extra cellular bases such as ammonia, sulphate or nitrate reduction and by the provision of low-energy surface sites for crystal nucleation. Fungi are particularly important in triggering carbonate precipitation (Callot et al. , 1985 ; Phillips & Self, 1987), and this may reflect the dumping of excess Ca2 + by the micro-organism (Phillips et al., 1 987) . I n summary, there is a range of mechanisms causing carbonate precipitation. Evaporation and evapotranspiration, and to a lesser extent , degassing, will be climatically controlled, and, of course, climate will influence the degree of biological ac tivity. Although the use of C and 0 stable isotopes has gone some of the way in assessing the rates of these processes (see below, p. 20) , their effects, and those of microbial activity on profile and micro structure, are not well understood. Microbial mech anisms are clearly important in the formation of beta calcretes but much more work is needed before the links between calcrete morphology/micromorpho logy and climate/biology are clear enough for their use in palaeoenvironmental interpretation.

MECHANISMS OF CARBONATE PRECIPITATION

Relatively little detailed work has been carried out on the mechanisms of carbonate precipitation, and an understanding of these processes and their prod ucts would provide a very powerful tool for inter preting ancient calcretes in the geological record. While some authors have stressed evaporation/ evapotranspiration and degassing as the main mech anisms of precipitation (Salomons & Mook, 1986 ) , other types are also important (Fig. 5 ) . Whatever the source o f the dissolved carbonate (as bicarbonate) in the calcrete (see below, p. 8 ) , its solubility will b e decreased by the removal of H20, C02 and by the addition of Ca2 + (common ion effect) . Water can be removed by direct evaporation or by evapotranspiration (Cerling, 1984; Salomons & Mook, 1986). Evapotranspiration is regarded as a major process in many semi-arid calcretes (Cerling, 1984) and is probably a major cause of rhizocretion formation. C02 loss is another major process (Salomons & Mook, 1986). The partial pressure of C02 in soils is typically much higher than in the atmosphere. Atmospheric Pco2 averages about 0.03%o; in tem perate soils it can reach 1 1 .5%, but averages 0.9%

M EC H A N I S M S OF CaC03 P R ECIPITATION

Co mm on ion effect

Evapotranspiration Microbial activity cyanobacteria, bacteria, fungi etc.

Fig. 5. Mechanisms of precipitation in calcretes (see text for details).

7

Calcretes: An Introduction GROUNDWATER CALCRETES

SOURCES AND MOVEMENT OF CaC03 IN

Calcretes, dolocretes and gypcretes of non-pedogenic origin are common in present-day arid alluvial basins but have not been recognized in the stratigraphic record . Groundwater (syn. phreatic, valley, channel calcrete) calcrete can cement and replace/displace very large volumes of sediment; for example , there are cemented Plio-Pleistocene alluvial fan gravels in Oman (the Wahiba Sands area) which have been diagenetically altered (replaced) to dolomite clays, apparently to depths of over 200 m (Maizels, 1987) . In Australia, groundwater calcretes and dolocretes are commonly kilometres wide (maximum of 10 km) , tens of kilometres long (maximum of 100 km) and have an average thickness of 10 m (Mann & Deutscher, 1978; Mann & Horwitz, 1979; Arakel & McConchie, 1982; Carlisle, 1983 ; Arakel, 1986) . On a local scale these carbonates may be lensoid and locally thickened zones occur as mounds or domes which break surface . They form from carbonate rich, mobile groundwaters which become progres sively concentrated during down-dip flow (Fig. 6). The carbonate is precipitated mainly in the capillary fringe zone , directly above laterally moving subsur face water, but it can also be precipitated below the water-table. The precipitation of carbonate is trig gered by several factors: C02 degassing, evapor ation/evapotranspiration and the common ion effect. Cementation preferentially occurs at 'highs' where basement irregularities bring groundwaters near to the surface, facilitating degassing and ev aporation/evapotranspiration. Where Ca or Mg bi carbonate-bearing waters mix with Ca or Mg sulphate or chlorite-rich playa groundwaters, precipitation due to the common ion effect occurs. Preferential formation of groundwater calcrete/dolocrete also occurs where drainages converge , where flow gradi ents decrease, where saline waters mix, or where permeabilities are low. Groundwater carbonates (calcrete or dolocrete) are typically micritic and densely crystalline although the pore size range of the host sediment is import ant. The carbonate may contain authigenic silica, clays (sepiolite , palygorskite) and gypsum . Figure 6A shows an idealized profile through a groundwater calcrete (see also Jacobson et at., 1988; Arakel et al., 1989). The growth of the carbonate is both dis placive and replacive and, as a result, nodular to massive forms develop. Shrinkage cracks and dis solution features may be abundant. During pro gressive cementation the profile becomes plugged

CALCRETE FORMATION

One still commonly held misconception about cal cretes is that the bicarbonate is sourced from groundwater ('per ascensum' model of Goudie, 1973, 1983) . This certainly applies to groundwater calcretes but in many areas where calcretes are forming near the surface , the water-table may be many tens of metres below the land surface (e.g. Gile & Grossman, 1979 ) . Carbonate cementation related to the capillary fringe is strongly controlled by the grain size of the host material which affects the amount of capillary rise - only a few metres in clays and much less in sands. The sources of CaC03 are varied (Goudie, 1973, 1983) and include rainfall (and seaspray) , surface runoff, groundwater, dust , bioclasts (e .g. terrestrial gastropods) , vegetation litter and rock. The main source of CaC0 3 in pedogenic calcretes is wind blown dust (Machette, 1985 ) . Ca-rich dust accumu lates on the soil surface and is dissolved by rainwater. The carbonate is translocated down into the soil and precipitates, typically at the depth of seasonal wett ing. In the Las Cruces area of New Mexico recent 3 dust fall contains 0.2 g of CaC0 3 per cm2 per 10 yr (Gile & Grossman, 1979), although much higher rates existed in the same area in the Pleistocene (Machette, 1985 ) . In the same area the concentration of Ca2 + in rainfall is also high, perhaps exceeding 5 mg of Ca2 + per litre of water (Machette, 1985 ) . The mechanism of downward movement of dis solved CaC0 3 is referred to as the 'per descensum' model (Goudie, 1983) and most readily explains calcretes developed well above the water-table and on non-calcareous substrates. However, where abundant CaC03 is available from the substrate, calcrete may form from redistribution of carbonate , especially if it contains 'metastable' carbonate (e.g. aragonite) . Rabenhorst & Wilding (1986) have shown that calcretes of the Edwards Plateau, Texas, formed by the in situ dissolution and reprecipitation of host limestones, resulting in stages of develop ment like those of other calcretes . It is likely that such 'redistribution' calcretes form much more rapidly than the dust-dependent type. Palustrine associated calcretes represent a special type of 're distribution' calcrete (Platt, 1989, this volume).

8

A %

POROSITY

CARBONATE

w (/) 0 0

%

TOP SOIL

.S?

c Q) CJ) 0 "0 Q)

'?-

<J)

� a;

pisolitic on slopes

��- LAMINAR

BRECCIATED

E

MASSIVE

'1

BRECCIATED

71-

12-

78%

15

t--- --

z

0 f=

() f= "'

c. E

� 0 ci z

5

Lam1nor calcrete Mass1ve calcrete

4

3 2

40

50

60

70

80

90

100

Weight per cenl CaC03

Fig. 8. Histogram showing weight per cent calcium c arbonate in c alcretes of the Olduvai and Ndo l anya Beds. The percentage of calcium carbonate is taken as the acid-so luble fractio n of samples dissolved in lOYo HCl (six samples) and in acetic acid buffered to a pH of 4·5 (ten samples). No s 1, 2, 12, and 13 are from the Ndolanya Beds. These data show that laminar c alcretes contain more calcium c ar bonate th an massive calcretes, and the massive c alcretes vary greatly in content o f calcium carbo nate.

Massive calcrete Massive calcrete in all but the lower of the Ndolanya calcretes consists of variably cemented and replaced aeolian tuff. Clay and zeolites are extensively replaced, and oolitic textures have resulted from micrite replacement of clay coatings. Micrite pellets have been formed by replacement of clay pellets. Pellets, ooids, and pseudo oolites of accretionary origin are both common and widespread in the lower Ndolanya calcrete. The calcium carbonate content ranges from 47-74"/o in the seven analysed oolitic samples (Fig. 8). The oolitic texture is grain-supported, hence these per centages represent substantial replacement in the Olduvai samples which were cemented by zeolites before being replaced by calcrete. Assuming a 25"/o porosity for the Olduvai samples, which were cemented by zeolites prior to replacement, then 25"/o CaC03 could represent simply cementation, 50"/o would represent cementation plus 33"/o replacement and 70"/o would represent 60"/o replacement. Thus, replacement is an 38

Calcretes of northern Tanzania

Fig. 9. Oo litic textures i n the massive calcretes of Fig . 3: (a) is a polished surface i n reflected light, and centres of ooids appear dark. The fabric is grain suppo rted by the ooids ; (b) is a n acet ate peel i n transmitted light , and a n ooid wit h co ncentric layers of micrite i s near centre of photo .

39

R. L. Hay and R. J. Reeder

Fig. 10. Pseudo-oo litic and oolitic textures illus trated by acetate peels of the massive calcrete at the base of the Ndutu Beds : (a) depic ts an early stage in the formatio n of pseudo-ooids in which micrite (light) is rep l acing clay (dark) at margins (m) and in concentric zo nes (c) with in pelleto id clay co ats; (b) depicts a late s tage of replacement. Oo id with concentric layers at lower left is also shown in Fig. 9 . Original grain outlines are indicated b y inked l ines.

40


Fig. 11. Pel letal m icrite of accretionary origin in ve ins of the upper calcre te of the Ndola nya Beds .

Cement is largely m icrospar, and fabric appears to be gra in s upporte d . This fabric is identical to the structure grumeleuse of Cayeaux (1935). Ph otographs are of ace tate peels .

41


important process in forming the massive calcretes. Swineford et al. (1958) are among the few workers who have demonstrated the major role of replacement in forming calcrete. Mineral grains, such as quartz, nepheline and augite, are etched and replaced to varying extents by calcite. These minerals are etched to about the same extent in the underlying tuffs, and most of the replacement in the calcrete represents the precipit ation of calcite around previously etched grains. Ooids, pseudo-ooids and pellets of replacement origin

These ooids and pseudo-ooids range in size from 70-500 J.!m in mean dimension, but generally fall between 90 and 300 J.!m (Figs 9 and 10). Pellets are usually smaller, most commonly ranging from 60-125 J.!m in mean dimension (see Fig. 11). Clay coatings of pelletoid grains have distinctive features which can be traced through various stages of replacement. Clay particles of the unaltered cortexes are often tangentially oriented, giving the cortex a radial extinction under crossed polars. Two distinct generations of clay coatings are commonly present, the older of which is typically a densely packed, dark clay, exhibiting a prominent radial extinction pattern. The outer, second generation is much lighter in colour, less dense, and somewhat silty, exhibiting a diffuse radial extinction. Remnant clay in ooids may preserve both the tangential orientation and evidence of two generations of coatings. Normally, the inner coating is much less replaced than the outer coating (Fig. lOa). Varying degrees of micrite replacement occur in coatings of the ooids and in pellets, and slightly replaced coatings or pellets lie adjacent to extensively replaced ones. These features clearly indicate a primary control, such as porosity, on micrite distribution. Ooids, pseudo-ooids, and pellets of accretionary origin

Accretion was important in forming pellets, ooids, and pseudo-ooids in the calcretes of the Ndolanya Beds, where they form layers and fill fractures. Pellets are more common than ooids and pseudo-ooids, and they form a grain-supported fabric. There is markedly less clay in the accretionary pellets and ooid coatings than in pellets and ooids of replacement origin, and its distribution is uniform, suggesting that accretion of micrite is the dominant process. These pellets, ooids, and pseudo ooids are similar to those of calcretes in coastal regions described by Multer & Hoffmeister, 1968; James, 1972; Siesser, 1973; Scholle & Kinsman, 1974; Ward, 1975. An accretionary origin is accepted by all of these but Ward (1975, p. 563), who suggests that the spherical shapes originate by desiccation fracturing of micrite and its sub sequent solution, deposition, and slight transportation. Calcite cement fabrics

The massive calcretes are cemented to varying degrees by micrite, microspar and, less commonly, spar. Both X-ray diffraction and electron microprobe analysis indicate the calcite is low in Mg ( 1 mol per cent MgC03). Microspar is most common and typically occurs as a pore-filling, interlocking mosaic of crystals. Grain sizes commonly increase toward Ihe centre of a pore (Fig. 11b), as often observed in cement fabrics. In many of the massive calcretes, microspar cementation of micrite pellets produces a 'clotted texture' or 'structure grumeleuse' identical to those described for many ancient, massive limestones (Cayeux, 1935; Carozzi, 1960; Bathurst, 1971). Micrite �

42


of the pellets is randomly embayed by microspar cement in this delicate interplay of calcite grains. This is clearly not a neomorphic fabric in these calcretes, but rather a primary pedogenic fabric. Non-calcite components The non-calcite fraction of the calcretes is a mixture of inherited and authigenic materials. Primary mineral grains, rock fragments, and vitroclasts generally form the largest amount of the acid-insoluble residue. Generally less common are clay minerals and zeolites, both primary and authigenic. Other minerals, including dolomite and opal, are additional authigenic constituents. Iron and manganese oxides occur widely, at least in the laminar calcretes, and silica-sesquioxide 'gel' is present in all of the analysed calcretes. Olduvai calcretes

Silt- and sand-size detritus in the laminar calcretes is similar in composition to that in the overlying sediments from which it has been incorporated. The clay fraction in two samples of laminar calcrete is composed of illite and dioctahedral chlorite with interlayered illite. The non-calcite fraction of the massive calcretes is composed largely of materials inherited from the parent aeolian tuff. Thus, it comprises mineral grains, rock fragments, and zeolites. Chabazite is the only zeolite represented in diffractograms of four of the five residues analysed, and phillipsite is the sole zeolite in the fifth. The dominance of chabazite in the calcretes contrasts with the over whelming predominance of phillipsite in the parent tuffs (Hay, 1976,table 25),suggest ing that phillipsite was selectively replaced by calcite. Pelletoid clay coats formerly unreplaced by calcite are waxy and appear unaltered, whereas partly replaced coats are porous and appear chalklike. The fraction finer than 2 f.tm in four of the five samples of massive calcrete dissolved in buffered acetic acid is wholly or largely dioctahedral chlorite with interlayered iiiite. Illite is the only clay mineral in one sample, and minor illite occurs in three others. A very weak 7·1 A peak, presumably of a kaolinite-type mineral, was noted in one sample. Dolomite is a common authigenic mineral which occurs principally as a late filling of fractures and other cavities in both laminar and massive calcrete,including pisoliths. Crystals are coarsest,10-50 f.tm in diameter,in the lower of the calcretes,including that at the base of the Ndutu Beds. The dolomite is disordered, as determined by lack of ordering reflections in Debye-Scherrer films. Dawsonite is found in all calcretes,but is common only in the calcrete at the base of the Ndutu Beds. Crystals of natrolite and phillipsite line some of the fractures in calcretes of the Masek Beds and at the base of the Ndutu Beds. Manganese and iron oxides have been deposited in some of the laminar calcretes. Both are concentrated in the darker layers,and probe analyses give maximum values of 50·5% MnO and 28·1% FeO,respectively. The manganese occurs largely in the form of dendrites, probably of hydrated Mn02• Silica-sesquioxide 'gel' was obtained from all eleven samples in which the acid solution of dissolved calcrete was neutralized with NH40H. The percentage of gel, weighed after drying overnight at about 90°C,ranges from 3·5 to 1 1·8,averaging 7·1% in samples dissolved in HCl; it averaged 1·3-3·0, averaging 2· 1 /';;,in samples dissolved in acetic acid buffered at a pH of 4·5. The gels were reduced to only 10-15% of their original weight on ignition, demonstrating that they are quite hydrous. One sample, 43


from an HCl solution, has the following composition, on a water-free basis, as analysed by X-ray fluorescence: Si02, 34·3% ; Al203, 38·9%; Ti02, 0·3%, Fe203, 4·9% ; MnO, 0·1%; MgO, 1·5% ; CaO, 18·0% ; Na20, 1·8%. Ndo/anya calcretes

Pumice, nepheline and other minerals in the upper Ndolanya calcrete are surpris ingly little altered, indicating that the calcrete was formed before the tuffs had been appreciably weathered. Partly replaced pelletoid clay coatings in the massive calcrete are white, porous and chalklike. Montmorillonite constitutes the fraction finer than 2 Jlm in the laminar calcrete. The same size fraction of one massive calcrete sample is principally dioctahedral chlorite with interstratified illite. Another sample exhibits only the 7·1 A peak of a kaolinite-type mineral. The dioctahedral chlorite and a 7·1 A clay mineral are considered authigenic, formed from montmorillonite, \-V hich is the clay mineral in beds below the calcrete. Opal coats root cavities in the upper calcrete. The lower Ndolanya calcrete contains partly altered mineral grains and montmoril lonite, and it lacks fresh glass. No authigenic minerals other than calcite have been identified in this lower calcrete. Thus, authigenic minerals are both less varied and less abundant in the Ndolanya calcretes than in those of Olduvai Gorge.

CHEMICAL ASPECTS OF CALCRETE Chemical analyses for six laminar calcretes from Olduvai Gorge (Table 3) are similar to the mean composition for world calcretes computed by Goudie (1972). The Olduvai calcretes average 79·8% CaC03 and 7·2% of Si02, compared to Goudie's (1972) averages of 79·28% of CaC03 and 12·30% Si02• The Olduvai calcretes also compare rather closely with dense laminar calcretes from the High Plains, New Mexico (Aristarain, 1970). Bulk compositions of massive calcrete were not determined because of the highly variable nature of the replacement. Small-scale texturally related chemical variation within five calcite samples was studied using the electron microprobe. The elements analysed were Si, AI, Fe, Mg, Ca, Na, K and either Ti or Mn. We studied both laminar and massive calcretes of the Olduvai Beds, but only massive calcrete of the Ndolanya Beds. Within the laminar calcrete, the light laminae are essentially pure micrite with about 1 mole % MgC03 in solid solution. Dark laminae are potassic, presumably iilitic clay, diluted to varying extents with CaC03 (Fig. 12b) and with varying amounts of Mn and Fe, at least partly in the form of oxides. A few laminae are essentially pure clay, similar in com position to unaltered pelletoid clay coatings in the aeolian tuffs (Table 2). The composition of the clay in pseudo-ooids and pellets in the Olduvai calcretes varies considerably as a function of calcite replacement, as measured by increasing values of CaO (Fig. 13). The unaltered clay coatings and pellets, as noted earlier, have the composition of a phengitic illite with. an atomic Si/Al ratio averaging 2·5 and Al/K ratios generally between 2 and 4 (Table 2 : Fig. 13). In the most thoroughly studied sample, increasing replacement is accompanied by decreasing Si/AI ratios (Fig. 12) and increasing Al/K ratios (Fig. 13b). The Si/Al ratio drops to about I in highly replaced coatings and pellets. We interpret these data to indicate that increasing replacement of clay by calcite is accompanied by dissolution or leaching of phengitic illite clay and formation of clay approaching the composition of kaolinite or halloysite. 44


Table 3. Chemical analyses in percentage of laminar calcretes fro m Olduvai Gorge*

(I) Per cent s io . Al203 Ti02 Fe203 M nO MgO CaO NazO K .o Cl

s

ppm Nb Zr y Sr Rb Ba �C02 for MgO for CaO

7·14 2 ·85 0·43 2 ·62 0·09 I ·71 44·15 0·66 0·69 0·001 0 · 065

70 160 20 4120 40 400

(2)

6 28 2·42 0 - 41 2 · 04 0 · 06 1·84 4 3 · 54 1 ·95 0·70 0· 552 0 · 072

40 10 20 60 50 1350

(3)

6 · 30 2·15 0 · 37 1 . 88 0·08 1·94 45 ·12 0·89 0·88 0·006 0·029

40 170 30 5200 50 680

(4)

9·50 3 · 45 0 · 50 2 ·91 0·52 1 .1 1 44 ·12 0 · 45 0·80 0·039

90 1 30 20 2500 50 790

(5)

8·53 2 · 54 0·48 2 ·68 0 ·19 1 · 12 44 · 37 0· 58 0·95 0 · 018 0·022

60 120 20 2060 50 990

(6)

5 · 33 1 · 41 0·47 1 ·90 0 ·11 1 ·81 47 ·00 1 · 02 0 ·25 0 · 005 0 ·110

30 80 20 1040 30 220

I ·87 34·65

2·01 34· 17

2 · 12 35 ·41

1 ·21 34 ·63

1 · 22 34 ·82

1 · 98 36 · 89

To tal

97 · 39

95 · 58

97·83

99 · 59

97·83

98 · 31

% acidinsoluble

10·1

15 ·0

n. d .

14·2

22·7

11 · 1

* Analyses are b y R . N . Jack using X-ray fluorescence. A l l F e is given as Fe203. (1) Holocene calcrete ; U.C. M useum No . 478-2 34; (2 ) Calcrete at the base of the Naisiusiu Beds ; U.C. No . 478-249; (3) Calcrete at the base of the Naisiusiu Beds ; U.C. No . 478-254; (4) Calcrete at the base o f th e upper unit o f the Ndutu Beds ; U.C. No . 4 78-275; (5) Calcrete o f th e lower u n i t o f the M asek Beds ; U.C. No . 478-266 ; (6) Pisolithic coating of th e tuff clast in a co nglomerate of th e lower unit o f the M asek Beds ; U.S. No . 478-272 .

This inference is supported by the presence of a 7 · I A clay-mineral peak in a con centrate of altered clay coatings. The diffraction pattern is weak, however, and non crystalline aluminosilicate materials such as allophane may have resulted from the replacement process. The silica-sesquioxide gel that flocculated from solutions of dissolved calcrete may represent material formed in this way. The few analyses in two other samples scatter widely and show no clear trend. Reaction of illite to kaolinite and allophane(?) would yield Si, Fe, K and Mg. Aluminium (or aluminate) ions could also be released in the solution of illite. Phillipsite and perhaps other zeolites were dissolved in replacement by calcite. Thus, silicate alterations in the massive calcrete could have supplied components for the authigenic dolomite, dawsonite and zeolites. Although X-ray data indicate that the clay-mineral fraction in the upper Ndolanya calcrete has been changed mineralogically, microprobe analyses show no clear evidence 45

-

3 2

R . L. Hay and R. J. Reeder

(a)

---- �-

-

-

-

_

:_

KAOL INITE

.

.

-· _

20

40 60 80 Percent CoO of Tota l Analysis

0

3

��

•.

..: _

_

_

100

( b)

2

.

.

· .

.� (f)

..

·.

.

.

KAOLINITE

20

0

4

60

40

80

100

80

100

Percent CoO of Total Analysis

.

.

(c)

.

.. .

.

.

.

2o

20

40

.

60

Percent CoO of Total Analysis

Fig. 12. Atomic Si/Al ratio plotted against perce nt CaO in (a) pelletoid coatings re place d to varying e xte nts by calcite in Olduvai m assive calcre te at the base of the Ndutu Beds, (b) Olduvai lam inar cal cre te overlying the m assive calcrete of (a), and (c) pe lletoid coatings re place d to varying e xtents i n Ndolanya massive calcrete at the t o p of the Ndolanya Beds . Chemical data were obtained b y e lectron m icroprobe .

of chemical change in the replacement process. The Si/Al ratio, for example, scatters rather widely, but shows no trend as a function oheplacement (Fig. 12c). Mineralogic data, however, suggest that montmorillonite in the calcrete has been altered to diocta hedral chlorite and to a kaolinite-type mineral. Reaction of montmorillonite to form kaolinite or dioctahedral (aluminous) chlorite releases silica, but the microprobe results suggest that any silica released in clay-mineral reactions remained in the pelletoid coatings. The scatter of Si/Al ratios in analysed coatings (Fig. 12c) may indeed reflect small-scale movement of silica in the coatings. Finally, we wish to emphasize that this is only an introductory mineralogical and chemical study of silicate reactions, based on few samples. Despite their limitations, our data clearly show that zeolites and clay minerals have been altered and dissolved, and not simply displaced, in the development of these East African calcretes. 46


2 D- -

MUSCOVITE

2

0

- - - - - - - - - - - - - - - -7 KAOLINITE

10

4

12

� (b) __-Unaltered Clayclasts and Coats D

2

0-

.

- - - - - :...· - .!.. ....: ·_: - :- � - - . - - :;...

MUSCOVITE

2

0

100

•

0

....

z

12

10

4

14

(c)

0 80 0

14

'

.

60

11.1 40

� �

20 0

2

10

4

12

14

Fig. 13. Atomic Al/K ratio plotte d a gainst (a) Si/Al ratio in unreplace d clay c lasts and pelle toid c oa ts

of the Olduvai Beds (e, in calcre te ; _.. , in aeolian tuff), (b) Si/Al ratio in c lay coats re placed to varying extents ·in a sample of Olduvai mass ive calcre te at the base of the Ndutu Beds, and (c) Ca O in the ana lyses of (b). Chemical data were obtained by e lectron microprobe .

CON CL U SION S (I) Pedogenic calcretes of the Olduvai and Ndolanya Beds are closely associated with wind-worked volcanic ash of nephelinite or closely related composition. (2) A complete profile with calcrete typically consists of an unconsolidated sedi ment overlying laminar calcrete, which coats and overlies a massive calcrete. These profiles correspond closely to the mature, Stage 4 pedogonic calcretes of southern New Mexico (Gile et a!., 1966). Laminar calcretes of one Olduvai stratigraphic unit were deposited by vadose water below the level of pedogenesis. (3) An Olduvai calcrete profile can develop to a mature stage in only a few thousand years, probably reflecting the periodic fall of large quantitites of carbonatite ash, which is a source of readily available calcium carbonate. ·

47


(4) Replacement was a major process in the formation of massive calcrete. Oolitic textures were formed chiefly by micrite replacing pelletoid clay coatings around sand-sized grains. (5) Increasing replacement of clay by micrite in the Olduvai massive calcretes was accompanied by decreasing Si/Al and K/Al ratios, reflecting dissolution or leach ing of phengitic illite and formation of clay approaching the composition of halloysite or kaolinite. Si, AI, K and Mg were lost in the replacement and transformation of clay and zeolites and were probably precipitated in the form of dolomite, zeolites and dawsonite. In the upper Ndolanya calcrete, montmorillonite was altered to a kaolinite type mineral and to dioctahedral chlorite.

ACKNO WLEDGMENT S This study was supported by the National Science Foundation (grants EAR 72-0 1523 and EAR 76-84583). The electron microprobe used in this study was pur chased with NSF grant GA 38086. We are indebted to L. H. Gile and J. W. Hawley for showing us the calcrete sequence near Las Cruces, New Mexico, which gave us a standard of comparison for the East African calcretes. We profited from discussion with T. E. Cerling on clay-mineral analysis and calcrete pedogenesis. We thank J. Hampel for the photomicrographs and for assistance in X-ray diffraction analysis. Microprobe mounts were made by Len Leudke and Sharon Hudson, and thin sections are by S. J. Chebul. X-ray fluorescence analyses were made by R. N. Jack. The manu script was reviewed by L. H. Gile, N. P. James, and A. Goudie.

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ARISTARAIN,

BATHURST, R . G . C . ( 1 9 7 1 ) Carbonate Sediments and Their Diagenesis. Developments in Sedimentology,

12, pp. 505-5 1 3 . Elsevier Publishing Co . , Amsterdam. BRINDLEY, G .W. (1961)

The chlorite minerals. The X-ray Identification and Crystal Structures o.f Clay

Minerals (Ed. by G . Brown), pp. 242-296. Mineralogical Society, London.

A . (1 960) Microscopic Sedimentary Petrography. Wiley and Sons, New York. CAYEUX, L . ( 1 935) Les Roches Sedimentaires de France ; Roches Carbonatees. Masson, Paris. CERLING, T.E., HAY, R.L. & O NEIL , J. R. (1 977) Isotopic evidence for dramatic climatic changes in East Africa during the Pleistocene. Nature, 267, 1 37-1 3 8 . D AWSON, J . B . ( 1 962) The geology o f Oldoinyo Lengai. Bull. volcan. 24, 349-3 87. DAWSON, J.B. (1 964) Carbonatitic volcanic ashes i n northern Tanganyika. Bull. volcan. 27, 1-J l . FOLK, R.L. (1 965) Some aspects of recrystallization i n ancient limestones. In : Dolomite and Limestone Diagenesis : a Symposium (Ed. by L. C. Pray and R. C. M urray), Spec. Pubis Soc. econ.

CAROZZI,

'

Paleont. Miner. , Tulsa, 13, 14-48 . GARDNER,

L . R . ( 1 972) Origin of the Mormon· Mesa caliche, Clark County, Nevada. Bull. geol. Soc.

Am. 83, 1 43 - 1 5 6 .

L.H. & GROSSMAN, R.B. ( 1 967) Morphology of the argillic horizon in desert soils of southern New Mexico. Soil Sci. 106, 6-1 5 . GILE, L . H . , PETERSON, F . F . & GROSSMAN, R . B . (1 966) Morphological a n d genetic sequences of carbonate accumulation in desert soils. Soil Sci. 101, 347-360. GouDIE, A. (1 972) The chemistry of world calcrete deposits. J. Ceo!. 80, 449--463 . GouDIE, A. ( 1 973) Duricrusts in Tropical Landscapes. Clarendon Press, Oxford.

GILE,

48


HAWLEY, J.H., BACHMAN, G.O. & MANLEY, K. ( 1 976) Quaternary stratigraphy in the Basin and Range and Great Plains provinces, New Mexico and western Texas. Quaternary Stratigraphy of North America (Ed. by W. C. Mahaney), pp. 23 5-274. Dowden, Hutchinson, and Rose, Stroudsburg, Pa . , U.S.A. HAY, R.L. ( 1 963) Zeolitic weathering i n Olduvai Gorge, Tanganyika. Bull. geol. Soc. Am. 74, 1 28 1 1 286.

HAY, R.L. ( 1 976) Geology of the Olduvai Gorge. University of California Press, Berkeley. JAMES, N.P. ( 1 972) Holocene and Pleistocene calcareous crust (caliche) profiles : criteria for subaerial exposure. J. sedim. Petrol. 42, 8 1 7-83 6 . KAHLE, C. F. ( 1 977) Origin of subaerial Holocene calcareous crusts : Role of algae, fungi, and sparmi critization . Sedimentology, 24, 4 1 3-43 5 . LACROIX, A . ( 1 904) L a Montagne Petee et ses Eruptions. Masson, Paris. LATTMAN, L.H. ( 1 973) Calcium carbonate cementation of alluvial fans in southern Nevada. Bull. geol. Soc. Am. 84, 301 3-3028.

LEAKEY, M . D . , HAY, R.L., CuRTIS, G . H . , DRAKE, R.E., JACKES, M . K . & WHITE, T.D. (1 976) Fossil hominids i n the Laetolil Beds. Nature, 262, 460-466. MuLTER, H.G. & HOFFMEISTER, J.E. (1 968) Subaerial laminated crusts of the Florida Keys. Bull. geol. Soc. Am. 79, 1 83-192.

F . (1 969) Ages of calcretes in southern Africa. S. Afr. archaeol. Bull. 24, 347-374. S.R. ( 1 9 54) Average chemical compositions of some igneous rocks . Bull. geol. Soc. A m .

NETTERBERG, NocKOLDS,

65, 1007-1032.

M., HERLOCKER, D . & PENNYEVICK, L. ( 1 975) The patterns of rainfall i n the Serengeti ecosystem. E. Afr. wild!. J. 13, 347-374. READ, J.F. (1 974) Calcrete deposits and Quaternary sediments, Edel Province, Shark Bay, Western Australia. Mem. Am. Ass. Petrol. Ceo!. 12, 250-282. REEVES, C.C. (1 976) Caliche; Origin, Classification, Morphology and Uses. Estacado Books, Lubbock, Texas. ScHOLLE, P.A. & KINSMAN, D.J.J. ( 1 974) Aragonitic and high-Mg calcite caliche from the Persian Gulf-A modern analog for the Permian of Texas and New Mexico. J. sedim. Petrol. 44, 904NORTON-GRIFFITHS,

916. SIESSER,

W.G. ( 1 973) Diagenetically formed ooids a n d intraclasts i n South African calcretes. Sedi

mentology, 20, 539-55 1 .

A . , LEONARD, A.B. & FRYE, J . C. (1 958) Petrology o f the Pliocene pisolitic l imestone i n t h e Great Plains. Bull. Kans. geol. Surv. 130, 97-1 1 6 . WARD, W.C. (1 975) Petrology a n d diagenesis of carbonate eolianites o f northeastern Yucatan Peninsula, Mexico. I n : Belize Shelf: Carbonate Sediments, Clastic Sediments and Ecology (Ed. by K . F. Wantland and W. C. Pusey III). A m . Ass. Petrol. Ceo!. Stud. Ceo!. 2, 500-571 . WEAVER, C.E. & POLLARD, L.D. ( 1 973) The Chemistry of Clay Minerals. Developments in Sedi mentology, 15, Elsevier Publishing Co . , Amsterdam.

SwiNEFORD,

(Manuscript received

4

November 1 977 ; revision received 7 February 1 978)

49

Reprinted from Sedimentology ( 1980) 27 559-576

Pellets, ooids, sepiolite and silica in three calcretes of the southwestern United States

R. L. H AY & B R I A N W IGGI N S Department o f Geology and Geophysics, University o f California, Berkeley, Cal((ornia 94720, U.S.A.

ABS T R A C T Pellets and ooids are widespread and locally abundant i n mature calcrete profiles i n the Argus Range, California ; near Wickieup, Arizona ; and in Kyle Canyon, Nevada. Most concentrations of pellets and ooids either overlie laminar calcrete at various levels in the calcrete profile or fill subhorizontal fractures in the petrocalcic horizon. In all three profiles the petrocalcic horizon has been thickened by the pelletal, chemically deposited fracture fillings. Pellets range from 0·02 to 8·0 mm in diameter and consist principally of micritic calcite and sepiolite. Ooid coatings are chiefly calcite and opal or calcite and sepiolite. The pellets represent small concretions, some of which grew by accretion, either in void space or by displacing adjacent sediment, and the others of which were formed by cementation of pellet-shaped bodies of porous micrite. Ooid coatings with opal or sepiolite may have been deposited as a gel with sufficient strength for surface tension to thin the coatings over angular corners of nuclei so as to increase the roundness and sphericity of the particles. Major problems in calcrete genesis are (1) the cause of subhorizontal fractures and the mechanism for widening a fracture as sediment accumulates in it and (2) what determines the deposition of calcite, sepiolite, and opal as pellets and ooid coatings or as laminar layers.

INTRODUCTION

coatings over edges and corners of angular nuclei. Some is known, but much remains to be learned, about the authigenic clay minerals in calcretes of inland regions. Palygorskite or sepiolite in calcretes has been documented by Vanden Heuvel ( 1 966); G ardner ( 1972); Frye et a!. ( 1 974); Goolsby ( 1 975); Bachman & Machette ( 1 977); and Millot et a!. ( 1 977). Most of these writers attributed the formation of palygorskite and sepiolite to alteration of detrital montmorillonite and mixed-layer montmorillonite illite because the latter clays are commonly present in sediments below the calcrete but are generally absent or rare in the calcrete horizon with paly gorskite or sepiolite (see Gardner, 1 972). Bachman& Machette consider sepiolite a late-stage product formed in soils where palygorskite is dominant, and they did not find sepiolite in soils younger than middle-Pleistocene.

Micritic pellets and oolites o f pedogenic origin are widespread in the calcretes developed on carbonate sediments in coastal regions. Examples have been described by Multer & Hoffmeister ( 1 968), James ( 1 972), Siesser ( 1 973), Braithwaite ( 1 975) and Ward ( 1 975). The origin of most pellets is as yet poorly understood and no satisfactory explanation has been offered for pedogenic ooid coatings which thin over corners and edges so as to develop spherical particles . Calcretes o f inland semi-arid and arid regions have been studied much less, but oolites were noted and pellets were figured by Swineford, Frye & Leonard ( 1 958) in their study of the Ogallala calcrete of the High Plains. Bachman & Machette ( 1 977) report the occurrence of ooids and pellets in calcretes of the southwestern United States . These reports add little to the understanding of the origin of pellets and do not offer a hypothesis for the thinning of ooid Calcretes Edited by V. Paul Wright and Maurice E.Tucker © 1991 The International Association of Sedimentologists ISBN: 978-0-632-03187-0

51

R. L.

Hay and B. Wiggins horizon of Gile, Peterson & Grossman ( 1 965). Pellet refers to a spherical or ovoid particle of authigenic origin Jacking a nucleus and generally consisting of micritic calcite, with or without clay. Ooid refers to a spherical or ovoid particle Jess than 2 mm in diameter comprising a nucleus enclosed by one or more laminae. The majority of ooid coatings are concentrically banded. Pisolites are structurally similar to ooids but greater than 2 mm in diameter. Crumb aggregates are irregular micritic aggregates separated by voids or by cement (Braith waite, 1975). Clotted texture refers to a flocculent structure similar to that produced by coalescing pellets (Bathurst, 1 975, p. 86). Micrite, microspar, and spar are used to denote calcite grain sizes of 1 -5, -7- 1 5, and 15 fLm, respectively. An aqueous solution of methyl orange was used to aid in identifying sepiolite, both in the field and the laboratory. In at least nine-tenths of the samples, sepiolite stains pink to red either if pure or mixed with micritic calcite. Some of the rarer varieties of trioctahedral smectite (e.g., hectorite) can give a similar stain. Thin sections were used for studying textures, and sawed and smoothed slabs etched with 1 0% HCI were particularly useful in showing distribu tions of carbonate and non-carbonate minerals . The distribution of sepiolite on the slab is readily shown by methyl orange stain. The non-carbonate fraction of most samples was obtained by crushing a sample to pass through a 1 20-mesh sieve and dissolving the carbonate in a 1 0% solution of acetic

The present study is primarily a petrographic and mineralogical analysis emphasizing the distri bution, nature, and origin of pellets, ooids, sepiolite, and silica in three calcrete profiles of the south western United States . Structure of the calcretes was studied in the field, the main result being to show that the petrocalcic horizon of calcrete profiles can be substantially thickened by chemical precipitation in subhorizontal fractures . This study began in 1977 as a two-week recon naissance search for pelletal and oolitic calcretes . Pellets and ooids proved to be widespread and locally abundant in mature calcretes, and laboratory study was concentrated on samples from three calcretes developed in deposits of different mineral composition : ( 1 ) basalt and basaltic detritus in the Argus Range of southwestern California (Fig. 1 ) ; (2) silicic volcanic alluvial deposits near Wickieup, in west-central Arizona ; and (3) carbonate alluvial fan deposits in Kyle Canyon of the Spring Mountains of southwestern Nevada. Additional field study was made of the Argus Range and Kyle Canyon cal cretes .

T E R M I N O L O GY A N D M ETHO D S Calcrete is used here to designate a carbonate-rich, dominantly indurated profile or horizon of pedogenic origin. A petrocalcic horizon is a continuously cemented or indurated calcic horizon (Soil Survey Staff, 1975), usually synonymous with the K2m

·,·

Argus

�

Range

i I

::o\

�� Z l>

Mojave Desert • Barstow

Needles

N

..._Kingman -.

\ I Wickieup-_

35°

�...... 0

50

WK:

100 km 115°

Fig. 1. Index map showing the calcrete localities studied. AR indicates the Argus Range calcrete, WK indicates the

calcrete near Wickieup, and KC indicates the Kyle Canyon calcretes. 52

Three calcretes of SW U.S.A. 1 979). The general geology of this arn is shown on the Death Valley Sheet of the Geologic Map of California (I :250,000) published by the California Division of Mines and Geology ( 1 958) . The calcrete profi le exposed in the roadcuts com prises three units totalling about 2 m in t hickness (Fig. 3) . Unit 1, at the top, is 30-60 em t hick and is

acid buffered to a pH of 4·5-5· 0 with Li2C03• An hour's treatment in a 5% solution of acet ic acid was used in the later stages of the study, as this solution dissolves calcite much more rapidly, and comparative X-ray analyses showed no differ ence in clay minerals obtained by t he two methods. Diffractometer analyses were made of oriented and unoriented samples of most clay concentrates, and glycolation and heating were used to aid in some identifications. An A.R.L. e lectron microprobe was used to determine fabric-re lated chemical variations. Beam diameter was about 5-10 [Lm, and the sample current was 0·018 [Lamps.

LITTER OF BASALT BOULDERS DOMINANTLY SEMICONSOLIDATED CALCAREOUS CLASTIC SEDIMENT

LAYERS AND TABULAR BLOCKS OF DENSE LAMINAR AND PELLETAL LIMESTONE

1--'-"- 0 · 1 p.p.m., with increasing Mg 2 +. They drew atten tion, however, to the kinetic difficulties involved in transforming a sheet to a chain silicate, since energy requirements are high in the solid state. To overcome this problem they suggested that montmorillonite with the structure proposed by Edelman & Favejee (1 940), with alternate tetrahedra inverted, would be more easily transformed, particularly as the activity of Mg is one of the controlling factors in mont morillonite dissolution (Yaalon, 1 974). Under hypersaline conditions (high pH) Mg hydroxide apparently pre::: i pitates in the interlayer position forming chloritic minerals, but where there is less tendency to form hydroxides (lower pH) the Mg ions migrate to the octahedral sheet, increasing layer strain and forcing the tetrahedra to invert and form palygorskite (Weaver & Beck, 1 977, p. 2 1 0).

The occurrence of palygorskite in matted form discounts any possibilities of detrital origin (Singer & Norrish, 1 974). The mineral is considered authi genic, formed either from alteration of montmoril lonite (e.g. Yaalon & Wieder, 1 976) or secondly, by neoformation (e.g. Millot, Paquet & Ruellan, 1 969 ; Singer & Norrish, 1 974). Singer & Norrish (1 974) and Weaver & Beck (1 977) considered palygorskite for mation to be favoured by water-logged or brackish conditions, respectively, the primary requirement being alkaline Mg-rich conditions. Although Nahon & Ruellan (1 975) considered decalcification impor tant, Yaalon et al. ( 1 966), Mathiew, Thorez & Ek (1 975) and Yaalon & Wieder ( 1 976) thought the mineral was related to a calcification stage, the last authors showing that palygorskite disappeared on decalcification. Millot et al. ( 1977), in a study of calcrete developed on schist, identified palygorskite as an early stage mineral (their 'argillization' stage) suggesting that epigenesis of carbonates occurs at the expense of authigenic days. In the present study, however, palygorskite neoformation is demonstrably after or during calcite precipitation. 83

N L. Watts

- 52 4 7 8 1

•

. 524780

47·6·

. 524779

f--T'-'-+4.�-Y 49·1

• 00 CJOO . 46· 2 () 0 Q

(f) w a:

....

w :::
morphic low-Mg calcites, originally high-Mg calcites have been identified in three distinct settings : ( I ) In calcretes developed on Mg-rich host materials including dolerite, kimberlite and some pan sediments. The coarse crystal size of these high-Mg calcites suggest fairly slow precipitation from vadose waters locally enriched in magnesium. (2) In the lower levels of calcretes developed in and around saline depressions (e.g. Fig. 8). At the locality of the profile shown in Fig. 8 a groundwater level was seen at a depth of 3 m below the topo graphic surface. These groundwaters are probably preferentially enriched in magnesium compared to vadose waters and may account for the high-Mg calcites, precipitation being stimulated by C02 loss during capillary rise from the shallow ground waters. (3) The third occurrence of high-Mg calcites is somewhat problematic. They are found in the upp�r levels of immature calcretes developed on, or in, Mg-poor Kalahari sands (e.g. at Nata, Fig. 1 ; see

Origin of carbonates Source of calcium and carbonate ions

Aeolian dust is considered to be the major source of ions for calcrete formation (see Goudie, 1 973 ; Reeves, 1 976), but contributions from rainwater may be locally important (e.g. Gardner, 1 972). Biogenic 86

Kalahari calcretes: origin and diagenesis rapid precipitation accounting for the anhedral nature of the micritic crystals.

Watts, 1 977b). The carbonate is totally micritic and the crystals, as seen in SEM, are anhedral and re semble miniature rice-grains (Fig. 6c). It is difficult, at first, to account for the precipitation of these high-Mg calcites from vadose solutions with low Mg/Ca ratios. Glover & Sippel ( 1967) and Marschner (1 968) have shown that the Mg content of a given calcite is proportional to the Mg/Ca ratio of the solution fro m which it precipitates. Why then should high-Mg calcite precipitate from waters of low Mg/Ca ratios? It is known that magnesium (and other) ions affect and sometimes inhibit calcite nucleation (e.g. Bischoff 1 968 ; Bischoff & Fyfe, 1 968). De Boer (1 977) has stated that low-Mg calcite is the most stable poly morph at Mg/Ca ratios of 0-100 (or more), but in evaporitic lakes and in seawater high-Mg calcite forms, but never in true equilibrium with the pre cipitating water. Calcite precipitation may be induced by either C02 loss (e.g. Matthews, 1 969 ; Land, 1 970), increased temperature or evaporation (e.g. Thortensen, McKenzie & Ristvet, 1 972 ; Lippman, 1 973). In order to understand the origin of this micritic high-Mg calcite let us consider water of a low Mg/ Ca ratio (say around 1 ·0) within a vadose pore in calcrete. To produce rapid precipitation, strong evaporation and perhaps corresponding rapid Joss of C02 are needed, conditions to be expected within semi-arid calcretes. If evaporation is more rapid than calcite nucleation, the Mg/Ca ratio of the solution would remain constant but the salinity would increase. Such a solution, as shown by Folk & Land (1 975, Fig. 2), would thus tend to precipitate high-Mg calcite (and/or aragonite) and not the expected low-Mg variety. A similar situation would be expected within a kettle on almost complete evaporation of the water. Butler (1 975) studied the carbonate crust of a tea kettle into which water of a low Mg/Ca ratio had been introduced. Although the precipitated carbonates were to some extent affected by the fairly high temperatures within the kettle Butler found that from water with a Mg/Ca ratio of only 0· 15 high-Mg calcite (up to 12 mol% Mg C03) and ar< gonite precipitated. He attributed this to 'literally forced crystallization' and speculated that such a process should occur where fresh-water is trapped and subjected to evaporation. Butler's kettle is considered analogous to the conditions within a calcrete. The high-Mg calcite of the calcretes, precipitated by 'forced crystallization', will thus not be in equilibrium with the precipitating fluids, such

overall

The implications of such 'non-equilibrium' high Mg calcite precipitation are thus considerable, as it is possible that rapid evaporation (as opposed to C02 Joss) deviates from predicted thermodynamic con straints. The precipitation of high-Mg calcite micrite in calcretes from low Mg/Ca ratio waters leads one to conclude that thermodynamic equilibrium has not been reached. It is therefore possible that some pro cess similar to the 'stoichiometric saturation' of Thortensen & Plummer (1 977) occurs during rapid evaporation of pore water within the Kalahari calcretes in calcites above 4·0 mol% Mg C03• The results of Thortensen & Plummer (1 977) not only suggest that metastable thermodynamic equilibrium can never be achieved between high-Mg calcites and natural waters, but that with a constant Mg/Ca ratio of water the Mg content of calcite> (at 25°C) should be solely determined by the degree of supersaturation of the water with respect to calcite. High supersatura tion would occur with rapid evaporation and has already been shown to be a requirement for dis placive calcite crystallization within calcretes (Watts, 1 978). While it may be argued that Mg-enrichment in calcites can take place at elevated temperatures (e.g. Fuchtbauer & Hardie, 1 980), the concentration of magnesium in calcites within some Kalahari profiles cannot be explained by this factor alone. It is suggested here that rapid precipitation of calcite, from highly supersaturated solutions, is essentially kinetically controlled, and precipitation is too rapid for equilibrium to be attained. This implies that nucleation rate, in addition to temperature, should be considered when evaluating the partitioning of magnesium during calcite precipitation. Barnes & O'Neil ( 1 97 1 ) have described high-Mg calcite cements in a Holocene conglomerate from California forming in thermodynamic disequilibrium but isotopic equilibrium. Because such high-Mg calcite> are metastable, and rapidly transform to Jow-Mg calcite (see later), isotopic studies of calcretes should be undertaken with some care. Moreover, Usdowski , Menschel & Hoefs (1 980) have recently described isotopic disequilibrium in calcites resulting from rapid precipitation from highly supersaturated solutions. In view of the possible kinetic controls on calcite precipitation in calcretes outlined above, the work of Usdowski et a!. may have important impli cations in future isotopic studies of calcretes. 87

N. L. Watts and schizohaline (Folk & Siedlecka, I 974) dolomiti·· zation models. An intriguing aspect of Kalahari calcretes is the presence of features suggestive of a schizohalim� environment (see criteria outlined by Folk & Sied·· lecka, 1 974) ; for example, length-slow chalcedony, finely crystalline dolomite, euhedral dolomite, microspar and sparry calcite. This is not entirely unexpected in an environment with high evaporation and periodic flushing by rain water. It is, therefore, possible to draw certain analogies between the schizohaline and Kalahari calcrete diagenetic: environments. Finely crystalline, often cloudy, dolomite crystals are found in many pedogenic calcrete profiles. Their close association with pedogenic sepiolite (see above} suggests an origin from waters of similar composition to those required for sepiolite neoformation. It is proposed, therefore, that this finely crystalline: dolomite formed from solutions with high Mg/Ca ratios induced by evaporation ; as discussed below,. Mg-enrichment of the pore fluids is attributed predominantly to concomitant precipitation of low-Mg calcites within the profile. The moderately coarse, limpid dolomite, however, presents a different problem. It is predominant in older, thicker calcrete profiles and often occurs in solution channels. In places, high concentrations of limpid dolomite (and clear rims on finely crystalline: dolomite) are associated with the lower portions of calcrete profiles and appear to be related to (past?) groundwater influences. This dolomite is attributed to precipitation from low Mg/Ca ratio waters. Such waters may originate during periodic flushing of the profiles by rainwater which explains the association of limpid rhombs with solution channels. Higher concentrations of dolomite in older cal-· cretes are tentatively ascribed to mixing of vadose: and phreatic waters (possibly at a capillary front) at, and around, the groundwater table. The 'mixing zone' within Kalahari calcretes differs, however,. from the classical Dorag model in that, due t o evaporation, the overlying vadose waters are the more: saline. If such a model is universally applicable i n arid-zone continental environments, i t would pro-· vide an interesting alternative to the various dolo-· mitization models listed above.

Origin of aragonite One of the major problems encountered throughout this study has been the lack of data on carbonate minerals in 'fresh water' or continental environments. Although the major occurrences of aragonite cements are in marine or peritidal environments (see Bathurst, I 975), they have been described in vadose conditions, such as calcretes (e.g. Panos & Stele!, I 968 ; Scholle & Kinsman, I 974), in cave deposits (e.g. Fishbeck & Muller, I 97 1 ; Thraikill, I 97 1 ; Reams, I 974), and in fresh-water sediments (e.g. Konishi & Sakai, I 972) and soils (e.g. Veen & Arndt, I 973) where precipitation seems to be controlled by high Mg/Ca ratios. Preferential crystallization of aragonite is also favoured by elevated temperatures (e.g. Kinsman, I 965) and rapid loss of C02• Reams ( I 974) has d emonstrated experimentally that water with a low Mg content will precipitate aragonite, if stirred to induce C02 loss. Calcite nucleation has been shown to be slower than its rate of crystal growth (Matthews I 969) and kinetic factors usually favour aragonite crystallization over calcite (Reams, I 974). Further more, Butler ( 1 975) observed 25% aragonite, asso ciated with high-Mg calcite, in the upper crust of his kettle. It is suggested here that aragonite precipi tation in calcretes is possible, and could take place by rapid C02 loss (associated with evaporation) of low Mg/Ca ratio vadose waters. Origin of dolomite Because of the obvious importance of dolomites as major hydrocarbon reservoirs throughout the world, great efforts have been made to understand the origin of dolomite. This has resulted in the development of a number of dolomitization models which include evaporative reflux, evaporative pumping, compaction and dewatering of shales and the mixed-water ('Dorag') and schizohaline models (see Davies, I 979, for review). Opinions are generally divided over the need for low Mg/Ca ratios of the precipitating pore fluids (Morrow, I 978). Folk & Land (1 975) reviewed the problem and suggested that dolomite may pre cipitate from solutions of high Mg/Ca ratios if sufficient time for their ordering is available. Because of this ordering problem, they believed that at high salinities the Mg/Ca ratio must exceed 5-10 : I , but with reduced salinities (and thus slower crystalliza tion) Mg/Ca ratios of as low as I : I are sufficient. Such low Mg/Ca ratios can be attained by mixing of saline (marine) and fresh waters which is an inherent process in the Dorag (Badiozamani, 1 973)

Sources of magnesium for silicate authigenesis

�mll

dolomitization

In the above account details of silicate and carbonate� 88

Kalahari calcretes: origin and diagenesis loss and/or evaporation within the vadose pore fluids and subsequent inversion to low-Mg calcite. The primary calcite precipitation mechanisms are also thought to involve either rapid or slow evapora tion and/or C02 loss. Slow evaporation predomin antly gives low-Mg calcite with a consequent gradual i ncrease of magnesium concentration in the resulting solutions. Rapid evaporation may precipitate high Mg calcites which are in thermodynamic disequilib rium with the low Mg/Ca ratio vadose waters. Both passive (void-fill) and replacive (of detrital silicates) calcite occurs, silica being released in the latter. Under 'saline' conditions (in the broad sense) length-slow chalcedony and/or clinoptilolite precipitates, where as length-fast chalcedony and/or megaquartz are formed in 'non-saline' micro-environments. The silica is derived fro m replacive calcitisation and migrates down-profile to accumulate in the lower calcrete horizons. Finally, rapid evaporation may result in solutions highly supersaturated with respect to calcite, and displacive growth of calcite may occur (Watts, 1 978). Neomorphism of high-Mg to low-Mg calcite takes place fairly rapidly and magnesium is released. This, combined with the increased Mg/Ca ratio due to low-Mg calcite precipitation, increases the mag nesium concentration of the pore fluids to such a level that authigenic Mg-rich silicates may precipi tate. Clay authigenesis is an integral part of calcrete formation in the Kalahari, most profiles containing appreciable amounts of palygorskite and sepiolite. Palygorski te may form by reaction of Mg with mont morillonite or may be precipitated direct from solu tion in association with neoformed sepiolite and dolomite. Magnesium may also 'react' with (cement or neomorphic) low-Mg calcite to form replacive dolomite. The above pedogenic and diagenetic scheme com plies with most of the observations and results of the Kalahari calcretes. The model implies a dominantly 'closed' system but episodes of extremely high rainfall may obscure this and flush the system. Local precipitation of high-Mg calcite is thought to occur by capillary rise of groundwaters in, and around, saline depressions (e.g. Makgadikgadi). Microcrystalline dolomite, often associated with neoformed sepiolite, precipitates under evaporitic conditions from Mg-enriched pore fluids. Coarse, limpid dolomite is attributed to crystallization fro m low Mg/Ca ratio waters. These result either fro m flushing o f the calcrete profile during periods o f

authigenesis have been presented without detailed discussion of magnesium sources. The major cause of magnesium enrichment is believed to be the selective removal of calcium by low-Mg calcite precipitation raising the Mg/Ca ratio of the vadose solutions to conditions favourable for palygorskite, sepiolite and dolomite formation. In the description of mineral distributions in profiles, however, the inverse relationships between the mol% MgC03 of the calcites and palygorskite, sepiolite and dolomite content was shown. If some low-Mg calcite now present in the samples is a transformation product of original high-Mg calcites, magnesium released during alteration could have locally enriched the vadose waters and aided clay and dolomite formation. Land & Epstein (1 970) have suggested that loss of Mg from calcites may cause an increase in the Mg/Ca ratio of the fluids and induce dolomite precipitation. Folk & Land ( 1 975), whilst agreeing that this process is possible, thought that only minor amounts of dolomite would result. It is thought here, however, that such quantities of dolomite as seen in simple Quaternary Kalahari calcrete profiles may be explained by this mechanism. The removal of Mg from high-Mg calcites cannot take place rapidly by solid-state diffusion, certainly not at low temperatures, and it is probable that some dissolution-reprecipitation process is active. Such a process would be fairly rapid and preserve in detail original high-Mg calcite morphologies and textures.

CONCLUSIONS

The Kalahari calcretes described in this paper result from dominantly pedogenic processes occurring episodically throughout the Pliocene to Recent in a semi-arid climate. Varying degrees of calcrete maturity are related to a number of interdependent factors : time, climate, host materials, carbonate source, geomorphological position, organic in fluences, sedimentation (or erosion) rate and various localized conditions. The interplay of such a number of parameters over an area as large as the Kalahari obviously results in a highly diverse suite of calcrete types, but such variations are frequently observed even on a single outcrop. Consequently, broad con clusions must be circumspect. Fig. 1 1 is a schematic flow diagram of the main processes operating i n the Quaternary calcretes. For simplicity, aragonite has been separated into a minor category, its precipi tation resulting from rapid C02 89

N. L. Watts

�

PRIMARY CARBONAT E PRECIPITAT ION

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SALINE

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REP ACI V E

(

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low-Mg c!c

original cement or c l inoptilol i te

megaquartz

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+ M g --+----.,

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_ _ _ _ _ _ neoformed

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pa l y gorsk i t e

Fig. 1 1 . Schematic flow diagram summarizing the major pedogenic a n d diagenetic processes within the Kalahari calcretes. Note aragonite is not shown in detail. See text for discussion.

concentration of magnesium (and the bulk (Mg/Ca ratios) of the Kalahari calcretes is similar to that of calcretes elsewhere (Watts, 1 977b). The model presented above and in Fig. 1 1 applies to the pedogenic calcretes of the Kalahari. It remains to be seen whether it can be applied to calcretes elsewhere. If not, then we must explain the domin ance of palygorskite and sepiolite in many calcrete profiles throughout the world and further research on this problem is essential. Detailed measurements of pore water chemistry, calcrete microclimate, and trace element and isotopic analyses of individual cement generations within calcretes would be of considerable use, and would greatly aid our under standing of this obviously major process in semi arid, continental environments.

rainfall or, in older, thicker calcretes, by mixing of saline vadose and fresh phreatic waters. The evidence presented indicates that high-Mg calcite is a significant component of the Kalahari calcretes contrasting sharply with calcretes from most other areas. It may be that original high-Mg calcite textures are present in other calcretes, but have not been identified. Textures suggestive of high-Mg calcite or aragonite precursor cements have been seen in thin sections of calcretes from Australia, North Africa (courte3y of M. G . Talbot) and central Africa (courtesy of J. Beauchamp) suggesting that, perhaps, the Kalahari material is fairly representative as far as carbonate cementation is concerned. In addition, the number of authentic Recent calcretes is remark able low (Goudie, 1 973), and thus if high- to low-Mg calcite transformation is rapid (it is too slow to be observed in the laboratory, R.B. de Boer, personal communication, 1 977) all of the calcite in relic calcretes should now be low-Mg calcite. Finally, it could be that the Kalahari material is exceptional. While it is true that some of the rocks surrounding the Kalahari are dolomites, and that some of the calcrete host-materials are quite Mg-rich, the overall

A CKNOWLEDGME N T S

This work was performed whilst the author was in receipt of a NERC postgraduate studentship at the Sedimentology Research Laboratory, University of Reading. I should like to thank all my friends and 90

Kalahari calcretes: origin and diagenesis colleagues at Reading for their help, and in particular my supervisors, Prof. J. R. L. Allen, Dr A. Parker and Dr R. Till for their efforts and invaluable advice. Prof. P. Allen, Mr G. Brown and Dr J. A. D. Dickson contributed greatly to the refinement of ideas and numerous colleagues and co-workers gave stimulat ing discussions. I am deeply indebted to Dr C. J. R . Braithwaite for his constructive comments on an earlier version of this paper. I am, however, wholly responsible for the interpretations and any errors included within this paper. I thank Shell I nter nationale Research Mij B.V. for their permission to publish this work.

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nodules in a vertisol near Katherine, North Territory, Australia. Nature Phys. Sci. 241, 37-40. VELDE, B. & HQWER, J. ( 1 963) Petrological significance of illite polymorphism in Palaeozoic sedimentary rocks.

2, 1 3 9-144. YAALON, D . H. & WIEDER, M. (1 976) Pedogenic paly

gorskite in some arid brown (calciorthid) soils i n Israel. Clay Miner. 1 1 , 73-80.

Ain. Miner. 48, 1 239-1254.

(Manuscript received I October I 978 ; revision received I July 1 980)

94

BIOLOGICAL ACTIVITY AND CALCRETE FABRICS

These papers illustrate the range of biogenic features

(1983) describes another early Carboniferous cal

found in calcretes (mainly beta calcretes). Knox

crete which contains horizons composed of small,

describes calcretes from South Africa containing

well-sorted,

abundant evidence of microbial carbonates in tubules

faecal pellets.

well-rounded

peloids

interpreted

as

and needle-fibre calcite. The formation of pedogenic

Carboniferous alveolar septal fabric is also de

packstones and micritization by boring is also de

scribed by Wright (1986a) and this material also

scribed. K.lappa, in two papers, describes root-related

exhibits needle-fibre calcite. Wright speculates that

carbonates in Spanish calcretes. The Microcodium

some ASF represents calcification within mycelial

study favours an endomycorrhizal origin with the

strands forming symbiotic ectomycorrhizal sheaths.

intracellular calcification of root cells. In the second

The role of fungi in forming coated grains in these

paper a wide range of rhizolith types is carefully

palaeo-calcretes is also described. Beier (1987) provides descriptions of microbial

documented.

fabrics in Bahamian Pleistocene calcretes. The stable

Adams describes 'alveolar texture' from early Carboniferous limestones.

Alveolar septal fabric

isotopic compositions are also reviewed.

may occur in any type of pore in a calcrete, either

All of these biogenic (beta-type) calcretes de

within root moulds or in intergranular pores. They

veloped on carbonate substrates, and in the case of

represent the sites of calcification within mycelial

Quaternary forms, in areas which have or had a

strands.

semi-arid to sub-humid climate. The alpha-type cal cretes described by Hay & Reeder, Hay & Wiggins

Peloids not only form by physical processes (Hay & Wiggins, 1980, this volume) but are commonly

and Watts in the previous section developed on

calcified faecal pellets. The extract from Wright

mainly silicate-rich substrates in more arid settings.

Fig. 13. Needle-fibre calcite from an Eocene calcrete near Montserrat, Barcelona, Spain. Note the mainly random distribution pattern but with a bundle of tightly-arranged needles crossing the field of view. From studies of Quaternary calcretes (see text) it appears that all these needles of low-Mg calcite formed within mycelial bundles, as parallel masses. Surprisingly these needles show no evidence of extensive overgrowths.


95

Reprinted from

Sedimentology (1977) 24 657-674

Caliche profile formation, Saldanha Bay (South Africa)

GORDONJ. KNOX KoninklijkejShell Exploratie en Produktie Laboratorium, Rijswijk (ZH), The Netherlands

ABSTRACT A sequence of gradational lithification events can be observed in caliche profiles, in the Saldanha Bay area (South Africa), from friable lightly cemented aeolian calcarenites or littoral shelly deposits through an intermediate semi-indurated zone to an upper strongly indurated zone (calcrete). Lightly cemented sediment fabrics exhibit bridge and meniscus cements, micritic druses and vadose compaction phenomena. The middle semi-indurated zones exhibit coated grains in which irregular borings and/or tubules with tangential acicular fibres contribute to coated grains. Random networks of acicular fibres also occur in void spaces. In fully indurated upper layers of the caliche profiles, fabrics of micrite and microspar (in voids) occur in complex brecciated macro-fabrics. The features represent changes in a sequence from the friable primary sedi ments to the calcretes. Fresh-water vadose flushing leaches grains and causes for mation of meniscus and bridge cements and uneven druses. In the middle zone, inorganic processes are aided by the action of micro-organisms; fungi, bacteria or algae which produce tubules and irregular borings; the overall effect of which is to break down original detrital carbonate particles and enclose them in a crypto crystalline micrite. The acicular fibres probably result from evaporation of super saturated solution. Mechanical processes cause fracturing, which repeated many times gives complicated brecciated fabrics within the upper indurated zone.

INTRODUCTION

Forming outcrops around Saldanha Bay, South Africa, are dominantly cal careous coastal deposits (Fig. 1), known as the Dorcasia Limestone (Du Toit, 1917), Langebaan Limestone (Visser & Schoch, 1973) or 'Coastal Limestone' (Siesser, 1972). These deposits consist of mixed aeolian sand and littoral shelly deposits. Depositional age is probably Middle to Late Pleistocene. Commonly, the deposits have developed a caliche profile, which were best developed as a hard, strongly indurated surface of calcrete. Similar profiles, more or less developed are present within sections of the coastal deposits (Fig. 2). Siesser (1973), has carried out studies on the diagenesis of calcretes from South Africa, including samples from Saldanha Bay. He described diagenetically-formed ooids and intraclasts which he interpreted as being the result of carbonate-mud Calcretes Edited by V. Paul Wright and Maurice E.Tucker © 1991 The International Association of Sedimentologists ISBN: 978-0-632-03187-0

97

G. J. Knox

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G. J. Knox

precipitation around detrital particles associated with slight expansion of the sedi ment. James (1972) in a study of surface calcareous crust profiles on Barbados has observed similar textures, but on a small scale also recognized tubule and needle-like structures. Recent examination of Saldanha Bay caliche samples collected on land and from beneath the bay shows that similar tubule structures to those described by James (1972) occur in these sediment fabrics. A sequence of fabric changes can be recognized from unlithified sediment to indurated calcrete caused by both inorganic and organic activities.

GEOLOGICAL SETTING

Saldanha Bay lies about 160 km NW of Cape Town and is set in an area of gentle relief without prominent drainage lines. Low hills may be topped by granite basement while lower areas consist of dunes. Flat-lying scrub-covered areas a few metres above sea-level may be covered by a caliche or calcrete. Within the granite basement are localized pockets of Miocene/Pliocene phosphatic sands. The coastal deposits are described by Siesser (1972), who relates them to various 'Coastal Sandstones' occurring in South Africa. The maximum thickness of the deposits recorded by Visser & Schoch (1973) in the Saldanha Bay area is 88 m. The average thickness is probably approximately 30 m. The aeolianites can be subdivided into fine-grained homogenous and coarser cross-bedded calcarenites. Fine-grained sands contain 'dikaba' root structures (Fig. 5b) similar to those defined by Glennie & Evamy (1968) and appear to have been deposited rather slowly, allowing plants to establish themselves from time to time. Coarser cross-bedded fossil dunes may have been established more rapidly under stronger wind conditions and may only rarely have sustained plant growth. The littoral shelly deposits consist of varying thicknesses of calcareous sands and shelly grainstones, which cover planation surfaces within dune sequences and then in turn may also be covered by aeolian sands. Interfingering of these deposits was related to their close association near to a Pleistocene fluctuating sea-level (Siesser, 1972). Equivalent environments occur today with development of coastal-associated recent dune and beach deposits. (Fig. 1). Comparison of thin sections of both aeolianites and shelly littoral sediments shows that the detrital constituents are essentially the same in both consolidated and present-day sediments. They consist dominantly of carbonate allochems and silici clastic detritals. The latter consist mainly of quartz and feldspar with lesser amounts of granite/gneiss lithoclasts, glauconite and rare heavy minerals. Carbonate allochems consist mainly of recognizable bioclastic fragments of echinoderms, foraminifera, gastropods and lamellibranchs. Some fragments show evidence of boring, which in the aeolian sands occurred before incorporation into a wind-blown environment. These bored cavities within allochems are of 50-100 l.l. dimensions and are not to be confused with the much smaller irregular borings described below. Siesser (1972) 1973) has determined by X-ray diffraction and atomic adsorption spectrophotometry that mineralogically the carbonate consists of low Mg-calcite. Cathode luminescence properties of the carbonate allochem fragments showed them to be non-luminescent, as were also the caliches. Stalder (1975) suggest this to be characteristic of fresh water influence. Disappearance of aragonite and Mg-calcite as original mineralogical 100

Caliche profile formation

constituents of shells of certain marine organisms is in agreement with this. Some aragonite, however, is present in land gastropod shells (Siesser, 1972).

CALICHE PROFILES

Caliche profiles have commonly developed in S Africa both at the present day and in the past (Netterberg, 1969a). They are not restricted to formation on calcareous substrates but are also formed on igneous basement or on soils developing from the latter. Usually, the top surface is indurated and therefore termed 'calcrete' (Lamplugh, 1907) or the 'K master soil horizon' of pedologists (Gile, Peterson & Grossmann, 1965). In South Africa Netterberg (1967, 1969b) has called the upper surface 'hardpan calcrete'. Morphologically, the Saldanha Bay examples vary (Fig. 3). Continuous pavements of indurated caliche are developed on calcareous sediments and cover widespread low-lying areas with considerable lateral variation in thickness. The indurated upper surface passes down into a less indurated caliche, which eventually grades into the substrate in the case of the coastal calcareous deposits. In addition continuous pavement ( ± 2 m thickness) may grade laterally into lenses, nodules, bifurcating sheets (10 em thickness) or irregular aggregates of indurated nodules (10-15 em diameter). Funnel-shaped structures 1-2 m high have filled solution hollows and cut through fossil dune profiles, which rest on the calcrete surface of a fossil caliche pro file. The downward development of the funnel has been arrested by the hard indu rated caliche layer (Fig. 5b). Thicknesses of the upper indurated caliche or calcrete on the present-day surface or interstratified within the coastal deposits may vary from a few centimetres to greater than a metre. In a few outcrops, caliche were observed lying directly over a non-calcareous substratum of fresh or weathered basement. Those resting on fresh basement are very similar in composition to examples developed upon aeolian calcareous deposits. Weathered basement becomes totally altered, as can be seen in a good quarry outcrop by Saldanha Municipality Workshop. Kaolinized basement passes upwards into clay residue with quartz wash bands. Above these is an 80 em caliche profile consisting of two parts,. (1) a semi-indurated zone containing irregular broken a-quartz crystals (derived directly from weathered basement) and some rounded quartz grains, cemented by micritic calcite with meniscus druse and (2) an upper part consisting of a calcrete rind which is extremely hard. Locally rinds may form directly on weathered basement. The macroscopic internal structure is rather complex (Fig. 4a, b) and similar to calcretes from New Mexico (Bretz & Horberg, 1949, Fig. 4; Ruhe, 1967, Fig. 3). They have an overall brecciated appearance in which variously sized fragments of calcrete, rare detrital quartz and feldspar, and terrestrial gastropods are enclosed in concentric crusts in which slight differences in texture and fabric are visible owing to colour and etching. Hollows are present which have been filled by blown sand or fragmented material from the same layer. Some infills have become lithified by recent cementation processes. Numerous fractured surfaces are present which penetrate into the calcrete; some have become closed by precipitation of carbonate cement. 10 1

G. J. Knox POSITION

IN SECTION S

THICKNESS

TYPICAL FORM (Indurated caliche)

aSb, Fig.2.

a) 30,20 and 18.2 metres

I

b) 2.0 qnd 0.9metres

0

lm

2m

al

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0

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.

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l m Note : Different combinations of J,n,m,.&,and 'Z may commonly occur.

Fig. 3. Different morphologies of indurated caliche. The calcareous sediment surrounding these indurated forms is usually partially indurated.

After formation, some profiles have been destroyed by weathering along slopes. Others have been covered by aeolian littoral deposits. Excellent examples of buried caliche (Fig. Sa, b) occur in a recent road and railway cutting on the approaches to Hoedjies point, where all types are present. At Blauwaterbaai a caliche profile has been partly eroded and covered by a littoral deposit. The caliche has formed on aeolian sand and is strongly indurated on its upper surface. The upper few centimetres 102


Fig.4a

Surface of a caliche layer near Olifantskop. Concretionary surfaces are

visi b l e within which brecciated fragments of caliche occur. A hollow

(h)

has been filled

by fragments and blown sand and is showing cementation. The complexity of this surface is shown by polished s a mples in Fig.4b

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Aeolianite sand cemented by calcite

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Calcrete formed from shelly deposit

IB

Original sand recrystalled so that original texture no longer recognisable

II

Concretionary calcrete plus sand grains

ill

Concentric calcite deposits,sometimes showing irregular patterns

II

Sand-filled hollow cemented by calcite

ill

Concretionary calcite layer with sand grains

l'il

Concretionary calcite-filled fractures and surfaces

I , II ,ill .... etc represent calcrete generations

FIG.4b INTERNAL STRUCTURE OF TWO CALCRETE SAMPLES 103

G. J. Knox

Fig. Sa. A buried indurated part of a caliche profile on Hoedjies point. The layer consists of a calcrete breccia over which a concretionary rind has formed. Underneath, semi-indurated caliche contains the remains of terrestrial gastropods, Above is a later aeolian sand.

Fig. Sb. Funnel-shaped indurated caliche (f). Notice that the structure widens towards the base, where it rests on a fossil brecciated calcrete (c). This calcrete passes downward into friable, loosely cemented aeolianite (a). Both above and below the calcrete are 'dikaba' root structures (d) preserved by differential cementation. Location railway cutting between Saldanha and Hoedjies Point.

104


are pitted by solution channels and possible borings which are truncated by the contact with the shelly deposit. The shelly deposit passes upwards into fine-grained sand and a caliche profile at the present surface, which is covered by recent dunes or a sparse loamy soil. In all cases primary structures such as bedding, roots, and most fossils are eradicated in the caliche profile development. Petrography of the caliche profiles

Within the profiles the caliches can be traced downwards from an indurated upper surface through a softer zone in which the primary rock becomes recognizable. The degree of alteration and cementation increases upward such that the original fabric and sedimentary structures are eventually no longer recognizable. The sequence of alterations described below are broadly in order of increasing calichification. Figs 6, 7 and 8 also represent the change from original sediments to hard indurated caliche. Fig. 9 summarizes these changes in relation to a generalized caliche profile. Textural features of the caliche profiles

Examination of caliche profile samples with the optical petrographic microscope and the scanning electron microscope (SEM) shows the following textural features: Leached particles

Numerous bioclastic fragments exhibit internal voids (Fig. 6b) lined by replacive microspar crystals ± 20 1-1m length. These voids are chiefly observable in the primary sediment grainstones which are extremely lightly cemented (Fig. 6a). Bridge and point-contact cement

Localized at grain contacts or as fine bridges with maximum lengths of 300 )lm (Fig. 6c) are patches of cement. Point-contact cement consists of a fine microspar; while bridges appear to have a micritic appearance. Examination with the SEM showed that some of the bridges consist of regular to irregular build-ups of sym metrical bundles of rods or fibres (Fig. 6d), between which considerable interparticle porosity occurs. Associated with these cementation features are the presence of long and slightly concavo-convex contacts between grains, indicating vadose compaction. This is a process by which fresh-water films around grains dissolve carbonate to allow original point contacts between grains to become flatter. Uneven druses

Numerous fine-bladed crystal druses (Fig. 6c) have developed on detrital grains and allochems. They point outwards into interparticle pore space and typically form broad crystals approximately 10-20 )lm length. Thickness varies considerably with local formation of microstalactitic dripstone features. Tubules

Semi-indurated zones exhibit meandering tubular structures (Fig. 6e, f ). Typically they may follow interparticle pore space or form coatings on· detrital grains. Branching structures are present, but often the tubules occur as broken aggregates. The tubules are generally hollow with walls consisting of normally orientated calcite crystals, 105

Fig. 6. (a) Calcarenite in which contacts owing to vadose compaction occur (c). Micritic bridge (b) and meniscus structures (m) are present between grains. Considerable primary pore space (0) is visible. (b) Aeolian calcarenite in which the centres of some bioclastic allochems are leached leaving ap1icritic margin. A drusy calcite (c) lines these intraparticle voids (v). A thin micritic coating (m) covers most of the grains. Primary pore space is considerable (0). (c) SEM photograph of a bridge structure (b) extending between grains (g). The latter are partly covered by a drusy calcite (c). (d) Close-up SEM photograph of bridge structure (see square 6c), which consists of regular bundles of acicular fibres orientated in two planes. The size of individual fibres is approximately 10 J..lm. (e) SEM photograph showing the outline of coated grains (g) in which the coatings consist of aggre gates of tubules (t) and some fibres (f ) and micritic calcite. (f) SEM photograph exhibiting coated grains in which the coatings consist mainly of tubules and micrite. Some of the tubules may continue for some distance, following a winding course between coated grains.

Fig. 7. (a) Composite-coated grain with other coated grains. The composite grain contains quartz (q) and a bioclastic allochem (a). The latter with the coating (c) shows an irregular fibre riddled with borings. A second coated grain also exhibits borings (b) in the micritic coating. Micrite has filled the intraparticle space and this too has a few borings. (b) Micrite from within a funnel-shaped body riddled with irregular borings (b) or build up of tubules. Some pore space occurs (0). (c) SEM photograph showing a void filled with an interlocking network of acicular fibres. A quartz grain (q) with an irregular micritic coating (c) occurs in the foreground. Tubules (t) are present to the right. (d) SEM photograph exhibiting coated grains (g) in which the coatings are made up of tubules (t) and tangential networks of acicular fibres. (e) SEM view of interlocking acicular fibres bound together· by membraneous coatings. Note the furrow running down the fibres-. (f) SEM view of the surface of an indurated caliche (calcrete) which consists of a microcrystalline aggregate of micritic calcite. A few voids (v) occur which may be lined by microspar calcite.

107

G. J. Knox

Fig. 8. SEM photograph of microspar calcite (c) filling a void within a microcrystalline micrite containing detrital quartz grains (q) and carbonate allochems (a).

TERMINOLOGY

MICROSCOPIC TEXTURES

CALICHE PROFILE

Netterberg This paper

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/

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DRUSE

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. ·

.

>. . ·

.

· :- .

.

.

Fig. 9. Schematic relationship of the position of observed textural features in the caliche profile. 108


which give the tubules a drusy exterior appearance. The central cavity is approx. 1 f.Lm in diameter. Irregular borings

Associated with the tubules and of similar dimensions are intensely bored allo chems and coating (Fig. 7a). Within the allochems they may be true borings, but within the coatings they may, in fact, be build-ups of tubules (Fig. 7b) around detrital grains or borings formed subsequent to the formation of the coatings. Acicular networks

Randomly patterned networks of calcite fibres (Fig. 7c) fill some intergranular spaces. Individual fibres average lengths of 30 f.Lm and thicknesses of slightly < 1 f.Lm. Contacts between 'fibres' appear to be marked by membraneous sheaths which give the networks rigidity (Fig. 7e). Networks may also form tangentially (Fig. 7d) t"o detrital particle surfaces to form coatings. Acicular networks are often associated with tubules and, like the latter, are present in semi-indurated caliches. Micritic fabrics

Strongly indurated caliches have an overall cryptocrystalline calcite fabric (Fig. 7f ) in which it is sometimes difficult to recognize original carbonate allochems which have faded out. Where allochems are still recognizable or where quartz grains are common the texture is not grain supported though some grain contacts occur. Some times, laminated micrite textures are present which lack any siliciclastic or carbonate grains. These represent surface rinds which are similar to rind-like fabrics, which Multer & Hoffmeister (1968) described from the Florida Keys. Completely indurated caliche has a mottled earthy appearance in thin section, in which a few quartz grains and ghosts of original allochems and chitinous exoskeletons of insects are visible. Calcite is totally cryptocrystalline. Tubules, borings and acicular networks are no longer recognizable and the rock is extremely hard and has low permeabilities. Similar fabrics are known in calcretes from Shark Bay, Australia from equivalent outcrop locations (Logan, Read & Davis, 1970, Figs 5-3, 9 and 8-6). Sparry fabrics

(1) Partially or completely filled vugs with equant blocky calcite (individual crystals approx. 50 �tm) occur in consolidated caliche (Fig. 8). (2) Concentric layers of blocky calcite occur within fractures in the indurated caliches. Relationship of the textures to caliche profiles

The textural features described above can be related in a general way to zones in the caliche profile (Fig. 9). Intraparticle voids, vadose compaction phenomena, bridge and meniscus structures and uneven druses are found in the lowest part of the profile and occur in the recognizable primary sediment. Tubules, ramifying borings and acicular fibres are present in the middle semi-indurated zone, while micritic fabrics and sparry calcite fillings of vugs and fractures occur in the uppermost indurated zone of calcrete. 109

G. J. Knox

In the middle and upper zones some overlap and combination of textural types occurs. Micritic rinds and fabrics may grade into coated grains in which the coating consists of acicular fibres and tubules. In other cases the coating is purely micritic or may be a combination of borings and micrite.

Origin of the fabrics

Progressive textural changes eventually result in the upper hard indurated surface . Some expansion of the original fabric appears to have taken place during these changes which are the complex result of mechanical, inorganic and organic processes in a vadose environment. Mechanical processes are represented by expansion in the surface layer of calcrete with the formation of fractures: creep along slopes, settlement of underlying sediment and the wedging action of plants have all led to the formation of fractures. These have subsequently been infilled by overlying unconsolidated wind-blown detritus or encrusting rinds of calcite. Repeated fracturing and infill eventually give complex brecciated fabrics (Fig. 4b). At lower levels, intraparticle voids, vadose compaction phenomena, bridge and meniscus cements and uneven druses are characteristic of freshwater vadose diagenesis. Similar fractures have been described by Land, MacKenzie & Gould, 1967), who devised a textural scheme for diagenetic evolution in Bermudan Pleistocene limestones. The tubules and irregular borings are sometimes part of composite textural structures, which Siesser (1973) has called diagenetically formed ooids and intra clasts. He describes their origin as being due to precipitation of carbonate mud around incipient ooids and intraclasts, which at the same time involved some expansion of the sediment and dissolution of original grains. Similar features described by Siesser (1973) mainly occur in the semi-indurated zone. Thin section examination shows that coated grains may commonly show point or tangential contacts which interrupt the coating. These contacts are probably original and alleviate the need for expansion of the whole sediment to incorporate the coatings. Although some expansion may occur, original pore space is high and allows coatings of micrite to build-up. On the SEM photographs some of these coatings are seen to consist of tubules and tangential needle fibres. The tubules record the presence of micro-organisms. In addition irregular borings, present in the semi indurated zones, were not present in the original sediments. As they also occur in coatings, they have formed during development of the caliche profile. They may have been caused by subaerial equivalents of algal or fungal boring organisms, which have caused gradual centripetal replacement of allochems by micrite (Bathurst, 1971, p. 383). The irregular borings were probably caused by the same organisms which produced the tubules.James (1972) has observed similar tubules from weathered zones. The tubules described byJames penetrate grains and thus may be similar in part to the ramifying borings from SaJdanha Bay caliches. In fact, James (1972) comments that such tubules are similar to blue-green algae tubes. In a microl biological investigation of two samples from the Nari-lime-crust, Israel, Krumbein (1968) found that a well developed microfl.ora, dominated by algae including bacteria, fungi and actinomycetae, was present. In addition he observed, in culture experi ments, that this microfl.ora could produce a large number of calcite crystals. Thus, the tubules may be the remains of fungal, algal or even bacterial activity, which 110


enclosed and coated allochems. It is not clear how the ramifying borings were made but it is likely that similar organisms were able to produce them. Later recrystal lization destroys these fabrics and gives the micritic coatings of the indurated caliche. James (1972) suggests that such needle fibres are the result of crystallization from highly supersaturated solutions caused by strong near-surface evaporation of void solutions. Salt linings of fractures/voids indicate that locally concentrated solu tions have precipitated halite. Salt spray blown inland by prevailing SSW winds, after incorporation and evaporation in caliche zones, would become very saline. Such spray could also carry Mg2+ ions, which would locally augment the Mg/Ca ratio. In non-marine speleothem environments, Miiller, Irion & Forstner (1972) observed that increased evaporation tends to increase the Mg/Ca ratio. According to Folk (1974), increasing the latter ratio will tend to poison the sideward growth of CaC03, so that fibrous crystals or elongated rhombs develop. A second possibility of origin is through the activities of micro-organisms. The needle fibres are of similar size to crystals produced in cultured solutions by micro flora from the Nari-lime-crust (Krumbein, 1 968). Gleason & SpackmaQ. (1973) have observed that blue-green algae produce fresh-water lime mats in the southern Ever glades. Different species can produce equant, acicular and encrusting structures, which become lithified over short distances. Ward (1970, inJames, 1972) observing similar needles in fossil weathering surfaces in calcarenites in Mexico, suggested that they are related in origin to fungi. The upper indurated zone has largely lost the above textural features. Two dominant fabric types remain: an overall cryptocrystalline texture, which tends to enmesh original grains, and destroy caliche textures of the semi-indurated zone and pockets of sparry calcite, which fill vugs and fractures. The micritization appears to have been a constant process which eventually changes original allochems and earlier cement structures. Chilingar, Bissell and Wolf (1967, p. 194) report the findings of Wolf(l963) that algae can cause the precipitation of crusts on detrital particles and cavity walls in a beach-rock environment. In addition, deposits and crusts of algal origin were found changed to a dense cryptocrystalline micrite. Thus, the absence of tubule structures and fibres in the upper caliche zone of Saldanha Bay samples does not exclude their previous existence, because overall micrite formation has destroyed them. As permeability is low within the indurated caliche, micritization can take place both within the sediment and on the surface to form rinds. In addition, pockets of solution may be held for longer periods in vugs and fractures where a slower crystal lization rate can take place to develop the sparry calcite. The final fabric of the indurated surface zones is very complex and is a result of the processes above. However, this is not a static situation; fractures, breccias and solution hollows are continuously forming. They may be infilled by rubble from the calcrete, wind-blown material or calcite rinds. Eventually, they become as tightly cemented as the rest of the horizon.

Source of the calcium carbonate

Detrital carbonate allochems are an obvious source for the calcium carbonate. Siesser (1 973) has suggested that solution and reprecipitation of the latter takes place in similar coastal sediments. Saldanha Bay samples show clear examples of solution 111

G. J. Knox

at grain contacts and within particles to produce voids. Bridge and meniscus cements indicate the beginning of calcium carbonate dispersal into pore spaces. Dispersal is augmented at a later stage by the activities of micro-organisms. Centripetal replacement of allochems was accompanied by the development of needle fibres and tubule structures which further clogged pore spaces. At the surface of these caliche profiles aggradation was dominant. A continuous supply of aeolian carbonate detritals is blown onshore by prevailing south to south westerly winds. Fractures, hollows and sinks in the indurated caliche profiles become filled with mixed siliciclastic-carbonate detritals. These younger infills exhibit varying degrees of calicification. Repeatedly, as the section on Hoedjies Point illustrates (Fig. 2a) individual caliche profiles (Fig. 5a, c) have been overwhelmed and covered by dunes. After stabilization these deposits have in turn developed caliche profiles. The area has a semi-arid climate with an average rainfall of 25 em per year (Mountjoy et al., 1970, p. 471). Thus, airborne dust may carry considerable amounts of calcium carbonate. The profiles developed on basement rocks probably derived some calcite by this means. In Texas and New Mexico, Brown (1956) and Ruhe (1967) consider that carbonate dust contributes to caliche formation especially in areas where a carbonate substrate is lacking. James (1972) notes that salt spray adds calcium carbonate to profile surfaces. The existence of halite on the surfaces of fractures indicates that evaporation of seawater trapped in the profiles has been common. The Saldanha Bay profiles clearly exhibit aggradation and association with coastal sediments. Solution, alteration and reprecipitation of original carbonate grains, par ticularly in pore spaces would ultimately reduce the thickness of the caliche profile compared to the original coastal deposit. This tendency is counteracted by the con tinual supply of material on aggrading profiles. Calcium carbonate is carried within the profiles in solution. At different levels in the profile solution has taken place. Periodic rainwater and seawater spray will per colate downwards. Aristarain (1970) from a study of the geo-chemistry of New Mexico caliches concluded that chemical elements were transported by water moving from top to bottom, thereforer educing the thickness of the profile. Multer & Hoff meister (1968) also invoke movement of calcium carbonate downwards by rainwater. Around Saldanha Bay, downward percolation clearly occurs freely in newly stabilized coastal deposits with high permeabilities. During this flushing bridge and meniscus cements develop concurrently with vadose compaction. During extended drought, ascending capillary water, supplied from a brackish water table, and salt water spray evaporate and precipitate micrite. Micro-organisms aid this process by developing characteristic textural types. Eventually, permeability is effectively destroyed so that capillary suction ceases and rinds develop on the surface. Mechanical fracturing allows restarting of processes. In some cases caliche profile development may end when the caliche profile becomes impermeable (Netterberg, 1969b). Around Saldanha Bay, it is more likely that the developing profiles are overwhelmed by aggrading coastal deposits.

CONCLUSIONS

Widespread caliche profiles occur in other parts of South Africa (Netterberg, 112

·


1969a, b; Viljoen et a!., 1975, Fig. 6). Morphologically, the examples described by Netterberg (1967, 1969b) are similar to the Saldanha Bay types and others described in the United States (Bretz & Horberg, 1949; Brown, 1956; Gile, 1961; Gile et a/., 1965; Ruhe, 1967, and Lattman, 1973), Barbados J ( ames, 1972) and Western Australia (Logan et a/., 1970). Caliche profiles have developed on both calcareous and non-calcareous substrates. A generalized profile consists of an upper indurated zone of calcrete passing down wards into a zone of lesser induration, which in turn passes into the original substrate. The indurated caliche can exhibit many forms such as laterally continuous pavements, nodules, lenses, bifurcating sheets, funnels and rinds. In all cases, the process. alters and eradicates original substrate fabrics and structures by dissolution, mobilization and precipitation of calcium carbonate. The internal fabrics and textures are very similar to those described byJames (1972) on Barbados. Siesser (1973) observed the basic fabrics but did not recognize needle fibres or tubule structures. These and centripetally micritized allochems which show irregular borings indicate the activities of micro-organisms during caliche profile development. Multer & Hoffmeister (1968) and Lattman (1973, Fig. 4b) also indicate that specifically algae may have been present, while Krumbein (1968) recognized a well developed microflora in the Nari-lime-crust, Israel. Conversely, many caliche profile developments are related solely to inorganic and mechanical processes (Aristarain, 1970; Bretz & Horberg, 1949; Netterberg, 1969b, and Siesser 1972). A simple answer to this may be that biogenic structures are destroyed as calichification proceeds. The Saldanha Bay profiles show that early bridge and meniscus cements, intraparticle voids and clearly discernible biogenic structures, in coastal calcareous deposits, are later destroyed by (a) aerobic decay of the organic matter of micro-organisms and (b) secondary destruction of biogenic . carbonate structures and fabrics by recrystallization. The biogenic structures may have been preserved by smothering of the profiles by later coastal deposits. On alluvial plain or pediment surfaces the tendency will also be for biogenic structures to be destroyed. Nevertheless, careful study of semi-indurated parts of profiles may bring biogenic structures to light in areas where caliche formation is considered only due to mechanical and inorganic processes. The Saldanha Bay examples illustrate mechanical, inorganic processes and also an influence owing to micro-organisms. ACKNOWLEDGMENTS

Permission to publish this paper was given by Salcon (a joint venture of the Amsterdam Ballast Dredging and the Royal Netherlands Harbour Works Company) and Shell Research B.V., The Hague. Logistical support for field work was given by the staff of Salcon and C. Hartman. Early drafts of the manuscripts were read by A.J. Keij, M. Epting and W. Schollnberger, who made constructive suggestions for its improvement. The SEM photographs were taken at the Central Laboratory, T. N. 0. Delft, The Netherlands. REFERENCES ARISTARAIN, L.F. (1970) Chemical analyses of caliche profiles from the high planes, New Mexico.

J. Geol. 78, 201-212.

113

G. J. Knox BATHURST, R.G.C. (1971) Carbonate Sediments and their Diagenesis. Developments in Sedimentology,

12, Elsevier Publishing Co., Amsterdam. BROWN, C.N. (1956) The origin of caliche on the north-eastern Llano Estacado, Texas. J. Geol. 64,

1-15. BRETZ, J.H. & HoRBERG, L. (1949) Caliche in south-eastern New Mexico. J. Geol. 57, 491-511. CHILINGAR, G.V., BISSELL, H.J. & WOLF, K.H. (1967) Diagenesis of carbonate rocks. In: Diagenesis

in Sediments (Ed. by G. Larsen and G.V. Chilingar), Developments in Sedimentology, 8, pp. 179-322. Elsevier Publishing Co., Amsterdam. Du ToiT, A.L. (1917), Report on the phosphates of Saldanha Bay. Mem. Geol. Soc. S Afr. 10, 38 p. FOLK, R.L. (1974) The natural history of crystalline calcium carbonate: effect of magnesium content and salinity. J. sedim. Petrol. 44, 40-53. GILE, L.H. (1961) A classification of Ca horizons in soils of a desert region, Dona Ana County, New Mexico. Proc. Soil Sci. Soc. Am. 25, 52-61. GILE, L.H., PETERSON, F.F. & GROSSMAN, R.B. (1965) The K Horizon: A master soil horizon of carbonate accumulation. Soil Sci. 99, 74-82. GLEASON, P.J. & SPACKMAN, W. (1973) The algal origin of a fresh-water lime mud associated with peats in the Southern Everglades. Abs. Prog. Geol. Soc. Am., Ann. Meetings SE Sect. 5, 5, 398. GLENNIE, K.W. & EvAMY, B.D. (1968) Dikaka: Plants and plant-root structures associated with aeolian sand. Palaeogeogr. Palaeoclim. Palaeoecol. 4, 77-87. JAMES, N.P. (1972) Holocene and Pleistocene calcareous crust (caliche) profiles: criteria for sub aerial exposure. J. sedim. Petrol. 42, 817-836. KRUMBEIN, W.E. (1968) Geomicrobiology and geochemistry of the 'Nari-Lime-Crust' (Israel). In: Recent Developments in Carbonate Sedimentology in Central Europe (Ed. by G. Muller and G.M. Friedman), pp. 138-147. Springer-Verlag, Berlin. LAND, L.S., MAcKENZIE, F.T. & GouLD, S.J. (1967) Pleistocene history of Bermuda. Bull. geol. Soc. Am. 78, 993-1006. LAMPLUGH, G.H., (1907) Geology of the Zambezi Basin around Batoka Gorge. Q. J. geol. Soc. Land. 63, 162-216. LATTMAN, L.H. (1973) Calcium carbonate cementation of alluvial fans in southern Nevada. Bull. geol. Soc. Am. 84, 3013-3028. LoGAN, B.W., READ, J.F. & DAVIES, G.R. (1970) History of carbonate sedimentation, Quaternary Epoch, Shark Bay, Western Australia. Am. Ass. Petrol. Geol. Mem. 13, 38-84. MOUNTJOY, A.B., EMBLETON, C. & MORGAN, W.B. (1970) Africa a geographical study. Hutchison, London (3rd impression). MOLLER, G., IRION, G. & FoRSTNER, U. (1972) Formation and diagenesis of inorganic Ca-Mg car bonates in the lacustrine environment. Natunvissenschaften, 59 Jg. 4, 158-164. MuLTER, H.G. & HoFFMEISTER, J.E. (1968) Subaerial laminated crusts of the Florida Keys, Bull. geol. Soc. Am. 79, 183-192. NETTERBERG, F. (1967) Some roadmaking properties of South African calcretes. Reg. Co11f Afr. Soil Mech. Foundation Eng. 4, 77-81. NETTERBERG, F. (1969a) Ages of calcretes in southern Africa. Bull. S. Afr. Archaeol. Soc. 24, 88-92. NETTERBERG, F. (1969b) The interpretation of some basic calcrete types. Bull. S. Afr. Archaeol. Soc. 24, 117-122. RUHE, R.V. (1967) Geomorphic surfaces and surficial deposits in southern New Mexico. State Bur. Mines Miner. Res. N. Mexico, Inst. Min. Techn. Campus Station, Mem. 18, 66 pp. SIESSER, W.G. (1972) Petrology of the Cainozoic coastal limestones of the Cape Province, South Africa. Trans. geol. Soc. S. Afr. 75, 177-185. SIESSER, W.G. (1973) Diagenetically formed ooids and intraclasts in South African calcretes. Sedi mentology 20, 539-551. STALDER, P.J. (1975) Cementation of Pliocene-Quaternary fluviatile clastic deposits in and along the Oman Mountains. Geologie Mijnb. , 54, 148-156. VILJOEN, R.P., VILJOEN, M., GROOTENBOER, J. & LoNGSHAW, T.G. (1975) ERTS Imagery: An apraisal of applications in geology and mineral exploration. Minerals Sci. Engng, 7, 132-168. VISSER, H.N. & SCHOCH , A.E. (1973) The geology and mineral resources of the Saldanha Bay area. Rep. S. Afr. Dept Mines Geol. Surv. Mem. 63, 150 pp. WoLF, K.H. (1963) Syngenetic to epigenetic processes, paleoecology and classification of limestones in particular reference to Devonian algal limestones of central New South Wales. Thesis, University of Sydney (unpublished).

(Manuscript received 9 May 1975; revision received 21 October 1976)

Reprinted from Sedimentology (1978) 25 489-522

Biolithogenesis of Microcodium: elucidation

COLIN F . KLAPPA* Jane Herdman Laboratories of Geology, University of Liverpool, Liverpool, U.K.

AB STRACT

Petrographic studies of Tertiary and Pleistocene caliche from the western Mediterranean show some unusual calcite structures. These structures were desig nated Microcodium elegans GlUck 1912. New data are presented which question earlier interpretations with regard to the origin of this structure. The new discovery of Microcodium in Recent soils extends its stratigraphic range into the Holocene. Retention of fine detail in Recent samples, revealed by light microscopy and SEM, has suggested an origin hitherto unconsidered, calcification of mycorrhizal asso ciations. Ancient and Recent Microcodium fabrics are compared; sufficient preservation of ultrastructure in the Ancient indicates a homologous origin. Environmental, stratigraphic and palaeoecological significance of Microcodium is discussed; correct recognition indicates existence of a palaeoso( and hence is a valuable criterion for recognition of continental conditions, cessation of sedimenta tion, subaerial exposure, and time-equivalent horizons. In particular, Microcodium is a characteristic component of caliche in the western Mediterranean. A review of the literature suggests that its presence may have been overlooked ot misinterpreted in other parts of the world and, thus, may be more widespr�ad than' hitherto suspected. This study, in its embryonic stage of development, illumines the potential importance of biolithogenesis within terrestial carbonates.

INTRODUCTION

The term Microcodium was used by GlUck (1912) to describe unusual calcite crystals from the marine Miocene of southern Germany (Baden). From their shape and arrangement, evoking 'cells' in palisades around small nuclei, GlUck created the genus Microcodium elegans to designate these calcite crystals, which he considered to be organic in origin. He attributed them to siphonaceous algae and, hence, placed Microcodium in the Codiaceae of the Chlorophyta. Later workers (e.g. Jodot, 1935; Moret, 1952a), doubted their organic origin, considering them to be purely physicochemical precipitates. Interest in Microcodium was directed mostly towards an organic versus inorganic debate until Johnston (1953) argued convincingly in favour of an organic origin. * Present address : Memorial University of Newfoundland, Department of Geology, St John's, Newfoundland, Canada AlB 3X7.


115

Colin F. Klappa

Curved faces and presence of certain internal structures are criteria commonly put forward to argue against a purely inorganic crystallization. In addition, Bodergat (1974) has demonstrated, using isotopic ratios, that the carbon within the calcite is organic in origin, having been metabolised by Microcodium in the meteoric environ ment. Thus, the analytical results of Bodergat (1974) corroborate the morphological evidence. Further details on ultrastructure, presented in this paper (p.499), establishes the organic nature of Microcodium beyond doubt. Apart from the mere noting of its presence, and recording its facies associations, our understanding of Microcodium was not advanced until Lucas & Montenat (1967) described and interpreted its internal structure. More recently, Bodergat (1974) has given a detailed petrographic and geochemical analysis of Microcodium. This work, together with that of Esteban (1972, 1974), also provides an excellent literature review on this hitherto enigmatic organism. Esteban states that the genus Microcodium refers to elongate, petal-like calcite prisms about 1 mm long and hexagonal in basal section. The prisms are grouped in spherical, elliptical, sheet or bell-like clusters (Esteban, 1974). This description refers to his Microcodium (a) form and appears to embrace types 1 (epis de mai's) and 2 (colonies en laminae) described by Bodergat (1974). Esteban (1972) also defines a new form of Microcodium (b), which is distin guished by its smaller size and quadrangular section of its prisms. Nevertheless, because a complete range of sizes exists between (a) and (b) forms, Esteban (1972, 1974) considered it unnecessary to define another genus. Furthermore, he gives additional ground for not establishing taxonomic refinement (and/or confusion!) as follows . . .'Another reason is that we do not know what Microcodium is.' Microcodium has been discussed in several studies concerned with caliche (Esteban, 1972, 1974; True, 1975a, b). Similarly, my interest in Microcodium was initiated from an examination of caliche samples of Pleistocene age collected in the western Mediter ranean (mainland Spain and the Balearic Islands). In addition, the new discovery of Microcodium within soil samples collected from the province of Murcia (south eastern Spain) extends its stratigraphic range into the Holocene. True (1975b, p. 51) states'... Or les microcodiums, qui jouent un role primordial dans ces systemes, n'ont jamais ete signales encore dans les encrofitements actifs actuels.' The discovery of Microcodium in the Recent necessitates a revision of this quotation. Because Microcodium forms a significant component of many of these samples it was considered unscientific to pass off Microcodium as a 'whim of nature' (an attitude mentioned, although not necessarily followed, by Cuvillier & Sacal, 1961). As a consequence, a detailed examination of this material has led to new findings on Microcodium which are presented herein. Purpose

The intention of this paper is: (1) to document, for the first time, the presence of Microcodium in samples collected from the Pleistocene of Ibiza (Balearic Islands); (2) to demonstrate the existence of Microcodium in the Recent; (3) to provide further details on Hie inicrofabrics of Microcodium ; (4) to demonstrate that Ancient and Recent Microcodium structures are the product of the same phenomenon. Sufficient details are retained in Ancient Microcodium sampled from the Eocene of northeast Spain and southern France to allow such direct comparisons; (5) to point out the environmental significance of Microcodium ; (6) to stimulate a search for Microcodium

116

Biolithogenesis of Microcodium

whose presence may have been overlooked or misinterpreted; and (7) to suggest an hitherto unconsidered origin for Microcodium. Previous investigations

Well documented specimens of Microcodium have been recorded by French geologists in lacustrine, paludal (swamp), alluvial, and fluvial deposits, at palaeokarstic horizons, and as 'contaminants' in marine facies. Microcodium occurs dominantly in Tertiary rocks, being particularly conspicuous at the Cretaceous-Eocene boundary. The stratigraphic levels at which Microcodium occurs share several common charac teristics: a carbonate-rich substratum, indications of subaerial exposure and fre quently the presence of an overlying palaeosol. Microcodium appears to be associated with continental conditions during periods of negligible sedimentation, i.e. with sedi mentation rates being insufficient to preclude pedogenetic processes. The Garumnian and Vitrollian of French stratigraphy are examples of such facies. Most recorded in situ colonies of Microcodium are from rocks of Eocene age. In younger rocks they may be detrital but evidence for in situ growth within Pleistocene rocks has been documented (Bodergat, 1974; Ward, 1975). Visual evidence that Microcodium has grown in situ is indicated by grain truncations within the host rock. This relationship with the enclosing substrate has led to the suggestion that Micro codium actively dissolves carbonate in search of trapped organic matter (Lucas & Montenat, 1967; Esteban, 1972, 1974; Bodergat, 1974; Bodergat, Triat & True, 1975; True, 1975a, 1975b). Insoluble residues (e.g. layer lattice silicates, quartz grains) are pushed aside and the dissolved carbonate is reprecipitated to form the calcite prisms of Microcodium. This scheme of development, however, does not explain fully the observed micro fabrics and raises questions of actual processes. For instance, Bodergat (1974) has pointed out that although there may be many micro-organisms capable of dissolving carbonate (e.g. certain algae, fungi, lichens, actinomycetes, bacteria), the same organ ism cannot also bring about a reprecipitation. This line of reasoning led Bodergat (1974) to the hypothesis that two organisms (at least) have played a role in producing the structure of Microcodium. She suggested that actinomycetes were responsible for the destructive component of her 'biocorrosion-biosynthesis' system but the mechan isms of biosynthesis and the type of organism responsible for such a process are not discussed. Although the study of Microcodium has been largely confined to France, its presence has not been overlooked completely in other parts of the world. For example, Microcodium has been reported from the Miocene in Germany (Gllick, 1912), the Eocene and Pleistocene in Spain (Esteban, 1972, 1974), the Permian in Russia (Maslov, 1956), the Eocene in Switzerland (Kamptner, 1960), the Devonian and Carboniferous in North America (Wood & Basson, 1972), the Eocene of Turkey (Richard, 1967), the Pleistocene in Mexico (Ward, 1975) and in the Miocene from the Pacific Islands of Bikini and Saipan (Johnson, 1953, 1957). The single reference found (from an extensive, though not exhaustive literature survey) of Microcodium occurring in the Recent is by Marie (1957).According to this worker, Microcodium is present in littoral deposits from the Bay of Along, Indochina, although its presence is explained as being due to reworking rather than in situ growth. Table 1 summarizes pertinent citations on Microcodium, giving the geographical

1 17

Table 1. Summary of published citations and undocumented reports on Microcodium, giving geographical location, geological age of Microcodium and/or age of

substrate(s), environment and interpretation with respect of origin Author(s) and year

...... ......

00

Sacco (I 886)* Capeder (1904)* Gli.ick (19 1 2) Edwards (1932) Jodot (1935) Rech-Frollo (1948) Moret (1952a, b) Johnson (1953, 1957, 1961a, b) Rutte ( 1954) Faure-Muret & Fallot (1954) Cuvillier (1955) Gubler (1955) Maslov (1956) Demangeon (1956) Cuvillier & Sacal (1961)

Geological age

Geographical location

Interpretation

Envi; onment (lithofacies)

Lithothamnian alga Coral Codiacean alga

Marine Marine Marine Freshwater limestones Marine Lacustrine Marine Marine Marine Marine Marine; continental

Inorganic origin Complex diagenetic (inorganic) I norganic origin Alga Green alga Alga Alga

Conglomerates Epicontinental

Blue-green alga (Demorarpales) Vegetation Alga (?)

Maritime Alps, France

Eocene Miocene L. Eocene Miocene Danian U. Cretaceous Miocene Miocene Maastrichtian-Lutetian Cretaceous-Lutetian Eocene Permian; Palaeogene Montain Maastrichtian-Danian

Fran9ois & Sigal (1957) Marie (1957) Boulanger & Cros (1957) Allard et a!. (I 959) Kamptner (1960, 1962) Guillaume (1961) Paquet (1961) Moret & Flandrin (1961) Durand (1962)

Baden, Germany Trieste, Italy Baden, Germany N. Pyrenees, France French Alps & Pyrenees Pacific Islands Baden, Germany Maritime Alps, France Mouries, France Maritime Alps Russia Languedoc Meaux (Seine-et-Marne, France) Landes, France France; Indochina Limoux (Aude), France Bresse, France Jura, Switzerland Doubs, France Ardeche, France Alps, France Provence, France

Landenian Cretaceous; Tertiary Montain Tertiary Eocene Turonian-Coniacian Sannoisian U. Cretaceous U. Cretaceous; Eocene

Marine Lacustrine Marine; brackish; lacustrine Fluvio-lacustrine

Gottis (1963) Bourrouilh & Magne (1963) Lapparent (1966)

Mouries, France Minorca, Balearics Alps, France

Neocomian(s); Eocene Pliocene(s); Pleistocene Eocene

Karst Marine Continental

Lacustrine; Fluviatile Lacustrine; Continental Limestones; sandstones Lacustrine

Red alga; liverworts Organic

Organic (recrystallized) Analogous to stromatolites Dasycladacean algae CMizzia; Macroporella)

Obligatory heterotrophs

Table 1 continued o n page 493

� � �

(S � � �

Table 1 continued

Author(s) and year

...... ...... \0

Lucas & Montenat ( 1967) Richard ( 1967) Misik ( 1968) Bodelle & Campredon ( 1968) Freytet ( 1969) Roux ( 1970) Nury, Rey & Roux ( 1970) Freytet ( 1971a, 1971b, 1973) Plaziat ( 1971) Masse, Triat & True ( 1972) Esteban ( 1972, 1974) Wood & Basson ( 1972)t Bodergat ( 1974)

Geographical location

Drome, France Gocek, Turkey Brezova, Yugoslavia Alps, France and Italy Languedoc, France Castellane, Basal Alps Rouet, France Languedoc, France Languedoc, France S.E. France N.E. Spain Missouri, U.S.A. France; Spain, incl. Minorca (Balearics) Digne-Valensole, France Gigot ( 1974) Paris, Basin, France Freytet ( 1975) Yucatan, Mexico Ward ( 1975) Bodergat, Triat & True ( 1975) S.E. France S.E. France True ( 1975a, 1975b) Mallorca (Balearics) Calvet et al. ( 1975) Montenat & Echallier ( 1977) S.E. Spain Spain, incl. Ibiza (Balearics) Klappa (this study)

Geological age Eocene Cretaceous-Tertiary Maastrichtian Eocene U. Jurassic(s); Maastrichtian Eocene-Oligocene Oligocene-Miocene Eocene Thanetian-Sparnacian Eocene Eocene; Pleistocene Devonian-Carboniferous(s) Mesozoic-Pleistocene M iocene Eocene Pleistocene Paleogene Pleistocene Pleistocene Eocene-Recent

Environment (lithofacies)

Interpretation

Continental

Filamentous bacteria

Marine Brackish-laguna] · Karst

Alga to (3'

:::-:

So c

Fluvio-lacustrine, palustrine Fluviatile conglomerates Caliche Non-carbonate shales Various Conglomerates Fluvio-laucustrine Aeolianites Continental Calcareous crusts; palaeosols Aeolianites Caliche Caliche

� ;:s "' "'

Bacteria; blue-green alga Fungus Micro-organisms, actinomyctees ( ?)

Roots ( ?) Micro-organisms Roots ( ?) Bacteria ( ?) Mycorrhiza (root+ fungus)

* Cited in Sturani ( 1963); t probably not Microcodium (Klappa, personal observations; Dr J. M. Wood, written communication).

1:;•

� �

;:;·

.... c (") c >:>...

$2' ;::!

Colin F. Klappa

location, geological age and stratigraphic relationships, together with interpretations (if attempted) on origin. Problems of terminology

Early attempts to describe Microcodium directed me to problems of terminology. The term 'Microcodium' has been used in various ways. Some workers refer to Micro codium as the actual (hypothetical) organism, while others have used it to describe the observed calcite structure. Two important considerations may have contributed to this semantic problem: firstly, whether Microcodium is organic or inorganic; and secondly, whether the calcite is part of the skeleton (as in calcareous algae) or a later precipitate within a vacated or original chamber. Terms frequently used such as 'thallus', 'cell', 'vacuole', 'filament' have genetic overtones. Because of potential confusion in terminology, the morphological term 'Microcodium grain' is used in this paper to describe individual prisms or units. In instances where a number of Micro codium grains form an organized arrangement the term 'aggregate' is employed.

GEOGRAPHIC LOCATION AND GEOLOGICAL SETTING Ibiza-PI eistocene

The island of Ibiza, situated between the latitudes of 39° 6' N and 38° 5' N and longitudes 4° 45' E and 5° 1' E, is an emergent part of the Balearic Platform in the western Mediterranean (Fig. I b and b). Carbonates (limestones, dolomites, marls) are the dominant lithologies, ranging in age from Muschelkalk (Trias) to Recent. Except for the more elevated areas, the solid geology is covered by Quaternary sedi ments and calcareous crusts (including caliche sense stricto, i.e. having a pedogenetic origin). Microcodium has been recorded in calichified bedrock (dominantly carbonates but also in profiles with igneous substrates) of Tertiary and Mesozoic age, and in aeolianites and colluvial silts of Pleistocene age (Klappa, unpublished data). Southeastern Spain - occurrence of Recent Microcodium

Recent Microcodium was discovered in southeastern Spain, 8 km south of Cieza (30 km NNW of Murcia). The site occurs along an unmetalled forest track (38° 11' N, 2° 18' W) on the east side of the road Mula-Cieza (Fig. 1c). The bedrock of upland areas is composed of Upper Triassic dolomites, whereas the lower slopes and valleys consist of Lower Eocene lime muds and unconsolidated or poorly consolidated marls. Conglomeratic slope deposits, which overlie the solid geology, also lack consolidation apart from their uppermost layers. At or near the surface, subaerial vadose pedogenetic and diagenetic processes have led to the form ation of caliche profiles in various stages of development, from nodular to thin laminated crusts. Present-day pedogenetic processes, however, are causing modi fication and/or destruction of these indurated layers, mainly by the mechanical penetration of root systems. Samples containing Microcodium were located 25 em below the present-day surface in a rubbly calcareous soil. Anastomosing channels, 1·0 mm wide, were noticed on the surfaces of many pebble-sized clasts (Fig. 3a). The arrangement of the 120


c

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Fig.

• Pleistocene

+ Recent

1. Recorded localities of caliche profiles containing Microcodium sampled in this study. Age of

is denoted by symbol. (a) General map of western Mediterranean ; (b) Ibiza, Balearics, (c) location of recorded Recent Microcodium, southeast Spain.

Microcodium

channel networks suggests that roots have caused peripheral dissolution of these pebbles. Evenari, Shanan & Tadmor (1971) have noted that the surfaces of pebble sized grains, in otherwise fine-grained soils, tend to be sites of greater moisture content and, therefore, provide readily available water for the indigenous flora. While many channels were devoid of any material, some contained rows of white to translucent, ellipsoidal grains (Fig. 3b). Their morphology and ordered arrangement initiated the idea that they may be the same as Microcodium grains recorded from the Pleistocene of lbiza. Subsequent laboratory examination substantiated this preliminary field observation (Figs 2a-e, 3b-d, 4a and b, 6a-d). The possibility that these Microcodium grains are detrital, having been reworked from older geological successions, can be discounted for several reasons. Firstly, organized aggregates occur within the present-day soil profile. Secondly, the soil matrix surrounding and supporting undisturbed aggregates is friable; the aggregates

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Fig. 2. SEM photomicrographs of Recent Microcodium aggregates and grains. Loose sediment mounts, Cieza, S.E. Spain. (a) Ellipsoidal Microcodium aggregate with grains showing interference· growth boundaries; (b) enlargement of (a), showing surface detail. Note pore pattern and presence of subsurface channels (arrow); (c) elongate Microcodium aggregate with grains in concentric layers; (d) enlargement of (c), showing grain surface concavities and naturally etched Microcodium grains; (e) single Microcodium grain with concave faces. Note subsurface tubular networks with tube dia meters of 1·0 �tm or less (arrow); (f) detail of (e), showing protuberances on surface of naturally etched Microcodium grain. Scale in ).lm.

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Fig. 3. SEM photomicrographs of Recent Microcodium gra ins. Untreated surface samples, Cieza, S.E. Spain. (a) Surface of dolomite lithoclast fragment with pitted channels due to the corroding action of Microcodium; (b) linear aggregate of Microcodium grains within surface channel of dolomite lithoclast; (c) Enlargement of (b), showing intragranular protuberances (arrow) within a completely dissolved (natural) Microcodium grain. (d) detail of (c). Lower surface is partially etched wall. Aerial fungal hypha is probably post-cavity formation . Residual intragranular structures are arrowed; (e) surface of 'bored' wall showing 1·0 J.!m diameter cylindrical pores surrounded by fibrillar mat (arrow). The latter is interpreted as disaggregated plant cell wall material; (f) naturally etched Micro· codium grain showing anastomosing subsurface (originally) tubes, 1· 0 J.!m diameter. Scale in J.!m . 123

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Fig.

4. Loose sediment mounts of Recent Microcodium grains, Cieza, S.E. Spain. (a) Microcodium grains surrounded and penetrated by fungal hyphae. Grains have been mounted i n Polyric immersion oil and viewed under the petrographic microscope, P . P.L.; (b) 'floating' elongate Microcodium grains supported by fungal hyphae. Note rhizomorph on right-hand side (arrow) . SEM natural (untreated) surface; (c) detail of Microcodium grain with pitted surface. Note that several pits are surrounded by raised borders (arrow). SEM photomicrograph . Scale in J.lm.

could not maintain form if mechanically churned or transported. Thirdly, aggregates unaffected by chemical corrosion, are extremely delicate; even the slightest pressure of a steel needle is sufficient to cause disaggregation. Fourthly, moribund fungal mycelia (masses of hyphae) surround and penetrate Microcodium grains (Fig. 4a and b). Fifthly, partially decayed vascular plant material surrounds Microcodium aggre gates. Similar plant debris, and also fungal hyphae, are present throughout the soil matrix. Finally, some of the fine details of Microcodium grain ultrastructure, as revealed by SEM (Figs 2e, 3e and f ), are considered unlikely to be preserved completely in the Ancient.

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Fig. 5. SEM photomicrographs of polished and etched rock samples from a calcified terra-rossa soi l .

Pleistocene, Ibiza. (a) Isodiametric Microcodium grains with- convex a n d concave boundaries. Indi vidual grains have been outlined in ink; (b) detail of (a), showing linear pore pattern (accentuated by etching in dilute HCl acid) indicated by arrow; (c) single Microcodium grain showing cracks and tubular pores; (d) detail of (c), indicating presence of filamentous structures (tips just visible) within tubular pores.- Scale in 1-1m.

LABORATORY

ANALY SI S

OF

MICROCODIUM

Sample preparation for petrographic analysis

The following procedures were adopted for samples containing Microcodium from lbiza, mainland Spain and southern France. (A) Standard petrographic thin sections. (i) Unstained; (ii) etched (1·5% hydro chloric acid for I 0 sec) and stained: (a) combined Alizarin red S and potassium ferricyanide (Dickson, 1966), (b) Feigl's solution (Feigl, 1943), (c) Clayton yellow (Winland, 1971); (iii) decalcified (10% hydrochloric acid until all carbonate was removed); and (iv) as (iii) plus staining with Gentian Violet dissolved in 90% methanol (Gurr, 1965). 125

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Fig. 6. Petrographic details of internal structures of Recent Microcodium grains from Cieza, south eastern Spain. Grains have been mounted in Polyric oil and viewed under the petrographic micro scope. (a) Elongate Microcodium grain displaying a radial-fibrous calcite fabric. Note discrete peripheral nucleus for part of radial-fibrous calcite (arrow); (b) Microcodium grain consisting of bundles of radial-fibrous calcite. (c) central tubular structure (10 J.!m diameter) and smaller tubular pores . ( 1·0 J.!m diameter) within single Microcodium grain; (d) enlargement of (c). Scale in J.lm.

(B) Loose sediment mounts. (i) Binocular examination; and (ii) temporary slide mounts using polyric immersion oil to coat Microcodium grains. (C) Scanning electron microscopy. (i) Freshly fractured rock surfaces; (ii) polished and etched (1·5/o hydrochloric acid for 30 sec) chips of Microcodium ; and (iii) Micro codium grains, hand-picked under a binocular microscope from friable, calcareous sediments. Prepared samples for SEM were mounted on a 1·0 em diameter specimen stub using Durofix adhesive mixed with acetone (50:50). Acetone was used to ensure an even spread of adhesive to minimize charging effects. Prior to coating, the specimens were oriented and features recorded under a 40 x magnification Nikon zoom binocular microscope. Using a MGN SG-2 12" coating unit, the mounted samples were coated under vacuum with 60/o gold-palladium. Oriented specimens were viewed employing a Cambridge Stereoscan, Mark IIA, operated at an accelerating voltage of 20 kV, with a beam angle of 45° and a working distance of 9-11 mm. Petrography

Abundance. Point-count analysis of fifty thin sections contammg Microcodium of Pleistocene age indicated that this component ranges from less than 0·5/o to 43/o of the total rock by volume, with an average of 17/o. 126


Size.The sizes of individual Microcodium grains were measured for loose sediment mounts and thin sections which contained them. Apparent dimensions (measurement on grains with various orientations in thin section) showed a range from 100 J..lm to 200 J..lm, averaging 120 J..lm, for long axes, and from 30 J..lm to 100 J..lm, averaging 70 J..l m for grain widths (diameters of isodiametric grains and/or transverse sections). The maximum observed length for loose grain mounts was 375 J..lm, considerably less than the typical 1·0 mm prisms of Eocene Microcodium described by Esteban (1974). Shape. The shape of individual Microcodium grains varies from well-defined prisms with length:width ratios of 2:1 to 3:1 (Fig. 7a), to vague ellipsoidal or subspherical outlines. Transverse sections show hexagonal, quadrangular or subspherical outlines (Fig. 8c). Curved faces, both convex and concave,lI tend to be commoner than straight (Fig. 2a). Many Microcodium grains display re-entrants or embayments (Fig. 7b) giving shapes that cannot be attributed simply to mechanical abrasion or fracture during transport.

Fig. 7. Photomicrographs of petrographic thin sections from the Pleistocene of Ibiza. P.P.L. (a) Microcodium aggregate composed of apparently overlapping prismatic grains (due to interference growth) with linear pore patterns (dark areas); (b) single detrital Microcodium grain with irregular ('wiggly') tubular pores radiating from periphery. Note : (i) re-entrant at top; and (ii) partial micri tization at base. Grain to lower left is a partially micritized coralline algal fragment; (c) part of Microcodium aggregate stained with Alizarin red S and potassium ferricyanide. Radial-fibrous fabric has taken up stain (red, indicating non-ferroan calcite), whereas grains or parts of grains with u niform extinction have remained unstained; (d) detail of (c), showing stained radial-fibrous calcite (lower half) and u nstained monocrystalline calcite (upper half). Scale in 11m .

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Arrangement. The arrangement of individual grains with each other and the enclosing substrate varies from single, isolated crystals, to organized aggregates The latter may occur as rosettes ('epis de ma:is' of Franr;:ois & Sigal, 1957) (Fig. lOa), with prisms radiating from a central nucleus (Fig. 9a), or as groups of prismatic or isodiametric grains lacking any discernible radial or concentric pattern (Fig. 8a). Another type of arrangement, although rare in Pleistocene samples from Ibiza but common in the Eocene of northeastern Spain and southern France, is a laminar or sheet-like layering of prismatic grains (long axes normal to layers). In general, how ever, because of reworking isolated detrital grains of Microcodium or fragmented Microcodium aggregates are far more numerous than in situ growth forms. Optical properties. Optical properties suggest a calcite mineralogy for the Micro codium grains. In plane-polarized light, grains are colourless or pale brown, but contain dark radial-fibrous inclusions. The filamentous inclusions appear white using reflected light. SEM examination of Microcodium grains reveals that non carbonate filaments may be present (Figs 5d and lOb) but in many cases, selective leaching has produced pits which give an optical effect analogous to the porous'shell residue micrite' described by Alexandersson (1972). Some Microcodium grains have a rhombohedral cleavage but many show only an irregular pattern of cracks (Figs 5a, c, 7c and d). Extinction patterns vary between grains and within the same grain. Uniform (relatively uncommon) to aggregate (suggesting a number of sub-crystals) extinction may be observed in one part, while a sweeping extinction may be present in the remainder of an individual grain. Some grains show a complete radial-fibrous structure with fibres radiating from a point on the perimeter of an individual grain and not from the centre as in typical spherulites (Figs 6a, 7c and d). Adjacent grains show also a radial-fibrous structure radiating from the same point (Fig. 8c). As a result, a pseudo-uniaxial cross is formed when the juxtaposed grains are seen between crossed nicols. Because of this extinction pattern, combined with overlapping sub-crystals, optical interference figures were not readily obtained. In cases where interference figures could be recognized a uniaxial-negative figure corroborated the evidence in favour of calcite. Ultrastructure. In several Microcodium aggregates the calcite was removed by etching in dilute hydrochloric acid (1·5/';;). Total dissolution revealed the presence of a network of branching filaments 1·0-2·0 !liD in diameter. The filaments may have been originally transparent (common in fungi, for example) but the presence of iron and/or natural organic staining rendered them visible in reflected and transmitted light. SEM examination of Microcodium from the Pleistocene of Ibiza shows several interesting features that have not, to my knowledge, been documented elsewhere. Linear patterns of tubular cavities 0·5-2·0 !liD cross-section can be seen in grains (Fig. 5b). These may radiate from a larger tubular cavity 5·0 !liD in diameter (Fig. 5a) or from a point on the grain perimeter. Similar tubular patterns can be seen within Recent Microcodium grains mounted in immersion oil and viewed under the petro graphic microscope (Fig. 6d). Some tubes have a prismatic cross-section which per haps suggest that these cavities are moulds of aragonite needles but two points of observation do not favour such an interpretation. Firstly, the tubes are not necessarily straight but show curved or wiggly shapes (Figs 2e, 6d and 7b), whereas aragonite needles have planar crystal faces. Secondly, gentle etching of the calcite prior to SEM coating, revealed the presence of residual structures less than 1·0 !liD diameter within

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Fig. 8. Photomicrographs of petrographic thin sections from Pleistocene deposits, Ibiza, P . P . L .

(a) Fragmented, detrital aggregate consisting o f isodiametric Microcodium grains 'floating' in a calcisiltite matrix; (b) in situ Microcodium grains with dark inclusions or pores (cf. 'shell-residue micrite' of Alexandersson, 1972); (c) detail of (b), showing dark inclusions radiating from discrete nuclei (arrows). Scale in J.Lm.

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Fig. 9. SEM photomicrographs of fractured samples from the Eocene of northeastern Spain and

southern France. (a) Transverse section of Microcodium cylinder composed of radiating petal-like prisms; (b) oblique section of cylinder with constrictions or disc-like structures. Cylinder axis lies NE-SW in photomicrograph; (c) contact between Microcodium grains (right) and calcisiltite matrix (left). Concentric bands within Microcodium grains may be trapped insoluble residues at successive growth fronts; (d) detail of (c) at junction. Note micro-honeycombed structure (arrow) at periphery of Microcodium grain; (e) walls between and within Microcodium grains consisting of porous clay sized aggregates; (f) enlargement of (a) . Scale in J.!m.

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Fig. 10. SEM photomicrographs of P leistocene Microcodium, Ibiza. (a) Partially collapsed rod of Microcodium, resembling a 'corn on the cob'. Isodiametric grains which constitute the rod similarly show signs of deflation; (b) detail of surface features in a, revealing presence of filamentous structures within, and traversing across, Microcodium grains. Scale in J.tm.

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some of the tubes (Figs 5c and d). These structures seem to have been unaffected by acid etching, a point which further argues against them being aragonite.Because of the fineness of scale it has not been possible to isolate these filaments for further tests. Viewed with SEM, the tubular pores can be equated in petrographic thin sections with the so-called dark, filamentous inclusions that show a radial pattern (Figs 7b and 8c). It is significant that these filaments radiate from a point on the periphery of the Microcodium grain and not from the centre. Furthermore, the calcite within the Micocodium grain also shows a radial-fibrous fabric (Figs 6a, b, 7c and d). The crystallites that make up this radial-fibrous fabric similarly radiate from the grain perimeter, unlike spherulites which radiate from a central point. This indicates growth from a 'wall' into a_cavity. Moreover, the radial arrangement for the vacated or tubular pores, likewise suggests penetration from the walls of Microcodium grains. The question now arises as to what constitutes this 'wall' and its relationship, if any, to the tubular pores. In thin section, Microcodium grains from the Eocene of northern Spain show walls between, and cross-walls within, petal-shaped grains. The walls consist of dusty, pale to dark brown microcrystalline calcite (staining and etching in dilute hydrochloric acid indicates that at least part of wall has a calcite mineralogy). SEM observations of fractured surfaces show that these walls are made up of equant, subhedral to anhedral clay-sized grains with a high intergranular porosity (Figs 9e and f ). The fine grain size and high porosity is probably responsible for the dusty appearance of the walls. No organized fabric was recognized within these walls; the micrite has an apparently random fabric. SEM studies of Recent Microcodium grains from southern Spain provide further surface and internal features that have relevance to the above outlined details, and perhaps more fundamentally, provide evidence for overall interpretation (p. 5 10). The surfaces of Recent Microcodium grains show a granular appearance with bordered pits and cylindrical pores 0·4-1·0 J.lm in diameter (Figs 2b, f and 4c). The pores appear to be connected to a subsurface anastomosing network of tubes (Figs 2b and c). Indeed, some Microcodium grains with the surface layer removed by fracturing or natural flaking, reveal the presence of a fine network of tightly bound, branching or coiled tubes 0·4-0·8 J.lm in diameter (Fig. 3f). This network does not appear to be continuous throughout the grain but restricted to a thin superficial layer less than 5·0 J.lm thick. This layer corresponds to the 'wall' of Ancient Microcodium grains. The ultrastructure of the remainder of the grain, both in the Recent and Ancient, consists of foliate calcite (Figs 2c, d, 9e and f). Bodergat ( 1974) stated that the 'platy' calcite was oriented perpendicular to the long axes of elongate Microcodium grains. Although this orientation was recorded in many grains examined in this study, calcite plates were observed also with a sub-parallel alignment with respect to grain long axes. The complex ultrastructure of Microcodium grains, as revealed by SEM examin ation, perhaps helps to explain the reasons for anomalous optical properties and staining patterns of the same grains when viewed under a petrographic microscope. Mineralogy

In order to substantiate the petrographic data several microchemical tests were carried out to evaluate the mineralogy of Microcodium grains. Staining thin sections with a filtered solution containing Alizarin red S and potassium ferricyanide combined 132


(Dickson, 1966), revealed certain anomalous features. Staining colours were weak or absent in many Microcodium grains, or present within irregular cracks (possibly be cause of the difficulty of thorough washing within micro-pores). In grains with com posite extinction patterns a red stain (non-ferroan calcite or aragonite) was commonly taken up only by the fibrous part of the grain (Figs 7c and d) whereas the clear spar, with uniform extinction, remained unstained. According to Dickson (1966) the intensity of the combined stain depends on the amount of iron present, the orientation of the c axis with respect to the plane of the thin section, and the concentration of the acid in solution. Additional factors considered in the course of this work that may have affected the staining pattern and colour intensity include: the presence of organic matter, particularly mucilaginous films; the presence of non-carbonate clay-sized grains; the micro-porosity of individual crystals (p. 502 and Figs 2b and Sa-d); and the presence of other foreign ions (in addition to iron). Because of the atypical staining patterns as outlined above, some Microcodium grains were suspected of having a mineralogical composition other than pure calcite. Further microchemical tests were carried out for the presence of aragonite (Feigl's solution, Feigl, 1943), high magnesium calcite (Clayton yellow, Winland, 1971) and dolomite (Alizarine cyanide green, Davies & Till, 1968). The presence of these minerals was not detected using these methods. Because Microcodium is considered to be organic in origin, several microbiological tests were carried out. It is well known that certain plants secrete crystals (cystoliths) within their cells (Cutter, 1969). Most contain calcium; calcium oxalate is the commonest organic compound found within plant tissues, although calcium carbonate also occurs.Such crystalline deposits are generally considered to constitute deposits of waste products. Examples of oxalate crystals-whewellite (CaC204• H20), and weddellite (CaC204• 2H20)-are organic salts of inorganic cations (calcium) and organic acids. Following the procedure of Gurr ( 1965), a test was carried out to detect calcium oxalate. Observation of the chemical reactions, both under the binocular and petro graphic light microscopes, suggested the presence of calcium oxalate within part of the Microcodium grains tested, but not throughout. The reliability of the method is unknown, however, and further geochemical tests were undertaken. Hand-picked Microcodium grains were ground to a fine powder and prepared for X-ray diffraction analysis. A Guinier camera was employed, using CuKa radiation, for mineral identification. The results showed that only calcite was present in these samples. If other compounds were present, as earlier tests seemed to indicate, then their trace amounts were masked by calcite.

CO M PARATIVE A P PROACH- MICROCODIU M AND TERRESTIAL VEGETATION Preliminary observations

This section examines cumulative data gained from field and laboratory studies on Eocene to Recent Microcodium, giving due-and in my opinion, long overdue consideration to present-day soil systems. Until now, Microcodium has disguised itself so well that it has not been recognized

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in the Recent. After studying Pleistocene samples from Ibiza containing abundant Microcodium grains, this discrepancy seemed somewhat puzzling. Did Microcodium really become extinct at the end of Pleistocene, or has the environment of its formation not been studied by geologists? The presence of Microcodium near or within rhizocretions has been recorded by Calvet et al. (1975) in Pleistocene aeolianites from Mallorca. Dr W.C.Ward (written communication) suggests that Microcodium from the Pleistocene of Yucatan may, in some way, be related to roots. Moreover, the conviction that roots are of paramount importance in determining many textures in caliche, and are related to the formation of Microcodium, has been amplified in valuable discussions with Dr M. Esteban, of the University of Barcelona. From my field observations, cumulative evidence indicated that roots play an important role in determining macromorphological features of caliche profiles (author's unpublished data). This realization led to a search for rhizospheres (Gk. rhiza root) on a microscopical scale. =

Biogenic aspects

In earlier sections (Petrography, Mineralogy), several features were described but left unexplained, namely: the occurrence of walls, radial-fibrous calcite and filaments radiating from discrete points on the grain perimeter; preservation of grain mor phology; and possible presence of calcium oxalate. In order to account for these observed features a new model is proposed in this paper (p.510) which contends that Microcodium is the result of calcification of mycorrhizae. Before the morphological observations can be interpreted in the light of this proposal, it is deemed necessary to clarify terminology and introduce basic concepts of plant anatomy and physiology. Great variability exists in the shape and structure of roots (Fahn, 1974). This is related mainly to root function, i.e. whether they are storage roots, succulent roots, aerial roots, pneumatophores, prop roots, or whether they contain symbiotic fungi (to form mycorrhizae). Nevertheless, the general anatomy of young roots shows several common characteristics which can be conveniently divided into the following zones: (I) the root cap (situated at the tip of the root); (2) the epidermis (outermost layer of cells), including root hairs which are projections or tubular outgrowths of the epi dermal cells; (3) the root cortex (parenchyma tissue surrounding the vascular cylinder and bounded on the outskirts by the epidermis); and (4) the vascular or central cylinder (consisting of xylem and phloem). It is well known that roots provide habitats for many soil micro-organisms (Burges, 1958). Fungi may form a union with roots to make composite structures known as mycorrhizae. It should be made clear, however, that not all fungal attacks on roots are necessarily mycorrhizal; many fungi are parasitic or saprophytic, whereas a mycor rhiza is defined as 'a symbiotic association between a non-pathogenic (or weakly pathogenic) fungus and living, primary cortical cells of a root' (Marks & Kozlowski, 1973). Mycorrhizae can be divided into two main categories (ecto- and endo-), although a transitional stage has also been recognized (ectendomycorrhizae). Marks & Kozlowski (1973) define these as follows: Ectomycorrhiza fungus is confined exclusively to the intercellular spaces of cortical cells of the host root; Endomycorrhiza . fungus is confined exclusively to the intracellular spaces of cortical cells of the .

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.

.


host root; and Ectendomycorrhiza . . fungus occupies the intercellular spaces of the root and penetrates some (or all) of the adjacent cortical cells. As a result of fungal infection, the plant cell may be structurally modified in a characteristic way. A particular type of mycorrhiza, known as vesicular-arbuscular, usually applies to an endomycorrhiza where the fungal hyphae inside the cortical cells are either coiled or divided into haustoria! branches (Figs 3c and d). Haustoria (Latin haustor drinker) are generally regarded as specialized absorbing organs. They may be knob-like in shape, elongate, or branched like a miniature root system (Alexopoulous, 1 962). Although the role fungi play in mycorrhizal associations is unclear, the general consensus of opinion is that mycorrhizal infections assist in the absorption of mineral salts, particularly in soils where the levels of available minerals are low (Burges, 1958). This point may be of particular relevance to caliche profiles which are poor in many essential mineral salts for normal plant growth. The actual mechanisms for uptake of nutrients into mycorrhizae are imperfectly understood as the following statement by Kelley (1950, p. 12) indicates '. . . whether materials get from the soil into the plant by mechanical means or black magic is left to the imagination of the reader'. Recent work on mechanisms subsequent to Kelley's cynicism is reviewed by Bowen (1973). His discussion may have some relevance to the problem of calcification of Microcodium, but much remains to be learned before actual processes are understood. For the time being, the above quotation from the work of Kelley (1950) can also be applied to this problem, even though several possible mechanisms for calcification have been outlined (p. 512). The possibility that Microcodium is related to a fungus-root association is con sistent with previously noted associations recorded on a macroscale. For example, Ancient Microcodium has been found within palaeosols, particularly calcareous soils (Bodergat, 1974), at unconformities indicating subaerial exposure of marine succes sions (Lapparent, 1966; Esteban, 1972), and at palaeokarstic horizons (Freytet, 1969). With respect to the latter, the presence of Microcodium within deep fractures and solution hollows has raised doubt as regards an algal origin for Microcodium ; algae generally require light for their vital life processes. Because of this factor, Gottis (1963) suggested that Microcodium was an obligatory heterotroph. Lucas & Montenat (1967) overcame this problem by considering Microcodium to be the result of the activities of colonial bacteria. Wood & Basson (1972) state that the occurrence of their specimens and presence of chitin suggest that the organism could be a fungus. They note (p. 212) that '. . . if this organism is a fungus, the question as to how M. elegans was able to live in the absence of light . . . would be answered. ' Similarly, mycorrhizae (fungus-root symbiosis) occur generally in a subterranean environment and, thus, do not require direct light. Several pertinent general comments regarding plant roots may help to convey the reasons for emphasizing their importance with respect to the occurrence of Micro codium. Roots are responsible for acid reactions that may stimulate rock decom position. Roots add C02 to soil-air and soil-water, thus increasing the production of carbonic acid which lowers the pH of circulating waters. This may lead to dissolution of carbonate minerals. Roots provide channels which allow easier circulation of water and air. Roots penetrate joints and cracks, causing mechanical disintegration. Roots are surrounded by a concentration of micro-organisms within the rhizosphere, or may provide habitats for micro-organisms on the root surface (rhizoplane), or actually .

=

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within root tissues (intracellular infection). Such micro-organisms contribute to changes in the chemical micro-environment by respiration, secretion of acids, organic decomposition and other complex interacting processes. Because of the biogeochemical complexity of soil formation and modification, it is not always possible to substantiate such generalities as outlined above. For example, the assumption that roots increase acidity in soil around them by excretion of carbon dioxide, and possibly H+ ions, may not be valid. It has been suggested that roots take up on average more anions than cations and would therefore tend to pass out HC03ions, rather than H + ions to preserve electrical neutrality (Gray & Williams, 1971). This would increase the pH of the soil around the roots and possibly counteract the effect of carbon dioxide which would diffuse away from the root region more rapidly than the carbonate ions. Nevertheless, whatever the complex processes may be, field observations indicate that roots can both dissolve solid rock and act as nuclei for cementation. Cementation around roots leads to the formation of rhizocretions (Kindle, 1925). Dissolution by roots, on the other hand, provides the biochemical-corrosion component for Micro codium, i.e. the attack by Microcodium on the substrate may be a function of rhizo solution. The preservation of plant form (biosynthesis component) still requires explanation; it is suggested herein that penetrative fungal hyphae within plant cells and walls provide a template for such preservation. Synthesis of model

Following the above rudimentary introduction on plant physiology and anatomy, it is now possible to present evidence to substantiate the claim that observed morpho logical features of Pleistocene and Recent Microcodium are the result of mycorrhizal associations. This will be presented in two sections, (i) evidence for the presence of roots, and (ii) evidence for fungal presence and modification. Unfortunately, because the biological systems are no longer living it is impossible to demonstrate that the root-fungal association is definitely a symbiotic one. Neverthe less, the morphological similarity between the material studied here and actual mycorrhizal associations is considered sufficient to suggest that this was the case. (i ) Evidence for the presence of plant roots

(I) Size of Microcodium grains. Grains (Microcodium) from lbiza and southeastern Spain have similar dimensions to plant cells. (2) Shape of Microcodium grains. Grain shapes are similar to parenchymatous and collenchymatous plant cells. (3) Non-planar grain boundaries (Figs 2d, e, 5a, b, and 7a-c). Convex, concave and re-entrant faces are common for Microcodium grains; curved faces are not typical of inorganically precipitated calcite. (4) Arrangement of Microcodium aggregates. The cylindrical arrangement (Fig. 2c) in many aggregates is like that of the cortical layer of a plant root. (5) Occurrence of in situ Microcodium aggregates within channels (Fig. 3b). Channels in caliche (millimetre scale) are common. Many owe their origin to root channels. (6) Insoluble residues from indurated caliche profiles. Xylem vessel members (specialized vascular plant cells used for transporting water) and other plant remains 136


have been extracted from Pleistocene caliche. Microchemical tests indicate that these residues contain lignin. (7) Presence of calcium oxalate. Plants may secrete crystals within their cells. According to Kelley (1950), however, a general opinion held maintains that fungal hyphae do not penetrate raphide cells (cells containing bundles of acicular needles). On the other hand, Kelley (1950) also mentioned the work of Busich (1913) who said that a fungus is not warded off by calcium oxalate but on the contrary forms it.Thus the minor amounts of calcium oxalate detected in Microcodium grains may be the product of fungal activity rather than of vascular plant secretion. (8) Surface features of Microcodium grains. Bordered pits on grain surfaces (Fig. 2f) correspond to positions of connection between cells of thin cytoplasmic strands (plasmodesmata). (9) Presence offine strands or fibrils (Fig. 3e). Fungal (?) bores (1 0 J.lm diameter) which penetrate Microcodium walls are surrounded by fine fibres (less than 0·05 J.lm diameter). The latter are interpreted as macrofibrils that constitute plant cells walls. ( 1 0) Subterranean habitat of Microcodium. Microcodium is encountered dominantly within a subterranean environment that shows pedological features, i.e. within a soil which by definition supports a biological ecosystem. ·

(ii) Evidence for the presence offungi (1) Filaments (1·0 J.lm diameter) radiating from the grain perimeter (Figs 7b and 8c). In thin section, radiating filaments show a radial arrangement with respect to Micro codium walls. These are interpreted as intracellular fungal hyphae, possibly haustoria (special absorptive hyphae that invade living cells). In endo- and ectendomycorrhizae these hyphae radiate out from the cell wall into the cell lumen. (2) Presence of filamentous networks. Following acid etching of Microcodium grains, rod- or needle-shaped structures (0·4--1·0 J.lm diameter) were revealed by examination under the SEM (Fig. 5d). Their size and shape are compatible with them being of fungal origin. In some Microcodium aggregates the arrangement of filaments forms an organized pattern. The total structure resembles a sclerotium which is a firm, rounded, often hard, mass of hyphae devoid of spores that forms a resting stage (Marks & Kozlowski, 1973).Trappe (1971) shows a photomicrograph (his fig.5, p.25) of a microtomed section of the surface of a sclerotium of Cenococcum graniforme. The hypha! arrangement in Microcodium resembles that of the mycorrhizal mantles formed by this fungus. Trappe (1971) illustrates such a mycorrhizal mantle (his fig. 4, p. 24) which is similar in shape and size to Microcodium aggregates sampled from the Eocene of northern Spain. (3) Coiled filaments or finely divided branches within Microcodium walls (Fig. 3f). These are considered to be fine networks of closely packed fungal hyphae within cell walls. Fungi are composed dominantly of chitin, whereas cellulose constitutes most of the cell wall in higher plants with minor amounts of lignin, tannins and pectic sub stances.Cellulose is rapidly broken down by microbial decomposition but chitin, when associated with polyphenols contained within the hypha! wall, resists decay for much longer (Potgieter & Alexander, 1966).Therefore, it is possible that the presence of fungi within cell walls preserves the cell form.A tentative proposal made here is that such a template is the reason for preservation of plant morphology. (4) Bores within Microcodium grains (Fig.3e).Tubular pores (0·4-0·1 J.lm diameter)

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within Microcodium grains are thought to be moulds of intracellular fungal hyphae. The hyphae have been later embedded in calcite so that the 'bores' are, strictly speaking, embedment structures (Bromley, 1970). (5) Protuberances in naturally etched Microcodium grains (Figs 2f and 3c). Rod shaped protuberances (0·5 J..Lm diameter) within naturally etched grains have been observed by SEM. Such intragranular (intracellular) structures, whose dimensions and forms are similar to haustoria, are interpreted as the fungal component of vesi cular-arbuscular mycorrhizae. (6) Hypertrophied Microcodium grains (Figs 2a and c). Hypertrophy (Gk. hyper= over + trophe food) is an excessive enlargment of cell size without an increase in cell number (Alexopoulous, 1962). Hypha! infection (intracellular) exerts a physical pressure on adjacent cortical cells (fungi forming mycorrhizal associations do not generally penetrate beyond the cortical layer). Enlargement of cells, because of this fungal infection, leads to interference during growth between adjacent cells. Con cavities and bizarre cell shapes (Figs 2e, 6a and b) are considered to be the result of this phenomenon. (7) Presence ofperitrophic (surrounding surfaces of roots) mycelia. Viewed under a binocular or petrographic microscope, Recent Microcodium grains from southeastern Spain were observed to be surrounded by networks of fungal hyphae (Fig. 4a). These form a mycelium (mass of fungal hyphae) or a rhizomorph (densely packed mass of fungal hyphae that resembles a tree root). SEM examination shows that these hyphae are slightly larger than the previously mentioned filaments. Hence, a dimorphism exists in the diameter of relatively thick aerial hyphae (2·0-5·0 J..Lm diameter) around Recent Microcodium grains, and finer hyphae ( 1·0 J..Lm or less in diameter) on and within these grains (Fig. 4b). A similar example of hypha! dimorphism has been recorded by Nicolson (1967). Thus, by documenting the fabric (size, shape and arrangement) of Microcodium grains by light and scanning electron microscopical observations and comparing the results with known biological features, Microcodium is interpreted as a product of mycorrhizal activity. =

MECHANI S M S FOR CALCITE PRECIPITATION WITHIN PLANT CELL S-CALCI FICATION

This section is an attempt to indicate possible mechanisms that may lead to the accumulation of calcite within plant cells, and conclusions reached are to be regarded as highly tentative. The term 'calcification' is used here to include preservation of plant form by calcite precipitation within vacated or original pore spaces (vacuoles) and metasomatic replacements of organic compounds by calcium carbonate. Treat ment of this subject can be conventionally divided into several categories as follows. I. Phenomena associated with plant growth

(i) Direct biochemical (a) Metabolic products of plants during normal growth, (b) metabolic products of symbiotic or parasitic micro-organisms, (c) secretion of substances in an attempt to flush out foreign intruders, and (d) selective uptake or rejection of ions by sorption (Lovering, 1959), ion exchange, or contact exchange (Keller & Frederickson, 1952).

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(ii) Indirect biochemical (a) Change in partial pressure of C02 because of synthesis and respiration of plant and associated microflora, (b) change in solute concentration because of evapo transpiration, (c) change in pH because of plant and microbial activity, and (d) exudation of organic acids and the sloughing off of plant debris.leading to changes in the chemical micro-environment of the rhizosphere. (iii) Physico-chemical (a) Accumulation of H 20 in the proximity of the root because of surface tension effects, and (b) diurnal and seasonal changes in temperature which affect the solubility of calcium carbonate. II. Phenomena associated with plant decay

(i) Microbial decomposition (a) Decay of plant organic material, and (b) decay of micro-organisms associated with plants during life (symbiotic or parasitic) or after death (saprophytic). (ii) Metabolic products of saprophytic organisms Documented isotopic analyses on Ancient (Eocene) Microcodium are given by Bodergat ( 1974).With respect of 1 3C/12C ratios, the conclusion reached by J. C. Fontes (who carried out the isotopic work) is quoted by Bodergat ( 1974, p. 209) as follows: '. . . pratiquement tout le carbone qui s'integre au carbonate a ete prealablement metabolise. ' Bodergat points out that this analytical evidence confirms the organic nature of Microcodium and that the carbon is photosynthetic in origin. By tacitly assuming that a 13C/12C isotopic analysis on Recent Microcodium would give similar results, only the above mentioned mechanisms for inducing calcification that involve metabolic processes will be considered in greater detail. Metabolic products ofplants during normal growth. As a result of metabolic activity in the cell, some plants form ergastic substances as cell inclusions (Cutter, 1969). Such substances include proteins, starch, fats, oils and crystals. Some of these may be waste products, other are stored food material. Crystalline deposits in various forms occur in the cells of certain plants and are generally considered to be waste products (Cutter, 1969). The size, shape and arrangement of isodiametric Microcodium grains are consistent with those of certain types of plant cells (particularly the parenchymatous cells of vascular plants). At first, this led to the idea that Microcodium grains were secretory crystals within plant cells. The observation, however, that Microcodium has destroyed or at least modified the substrate (Fig.3a) by dissolution and reprecipitation indicates that the product cannot be attributed simply to passive cavity filling of cell lumina. Metabolic products of symbiotic, parasitic and saprophytic soil-plant micro organisms. Little is known regarding the ability of microbes to precipitate calcium carbonate (Alexandersson, 1974) but the culture experiments of Krumbein (1968) may have some relevance in this context. He demonstrated that the microflora from a nari lime-crust (caliche) could produce large quantities of calcite. Likewise, Adolphe & Billy ( 1974) have observed the precipitation of calcite by bacteria in vitro, but it is unclear whether the phenomenon is the result of a direct or indirect biochemical control.

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Metabolic secretion in response to foreign bodies. This idea may be treated with the greatest scepticism as I am unable to substantiate whether such mechanisms occur in nature for this given situation. Reasoning by analogy with pearl formation in oysters, the suggestion is put forward here that the penetration of plant cells by micro organisms may induce the secretion of substances in order to flush out intruders. Although mycorrhizae involve symbiotic rather than parasitic associations, the physical presence of fungal hyphae within the cytoplasm, or even restricted to the cell wall, may cause the release of certain substances. Alternatively, there may be a chemotactic response, stimulated by substances diffusing from the fungal hyphae. Whatever the exact mechanisms are, it is evident from consideration of the above possibilities that higher and lower plants have the ability to exert direct or indirect biochemical controls which may culminate in calcification of plant tissues. It is not denied that the above comments are speculative and should be treated with caution. The chemical complexities of the soil micro-environment are beyond the scope of this study. Nevertheless, as a concluding remark in this section, it can be stated unequivocally from direct observation that calcification has taken place.

ANCIENT MICROCODIU M

It is not the intention of this paper to give a complete petrographic analysis of Ancient Microcodium. This has been covered by the study of Bodergat (1974). After reviewing this work, together with the studies of Esteban (1972, 1974), Freytet (1969, 197 l a, 1971b, 1973), Lucas & Montenat (1967) and True (1975a, 1975b), and using my own findings on the same material, it has been possible to make comparisons between Ancient and Recent Microcodium. Description

The illustrations and descriptions of Bodergat (1974) and Esteban (1972, 1974), generally agree with observations made in the course of this work. Viewed with the light and scanning electron microscopes, Microcodium grains include the following features. The calcite that makes up the grains is non-limpid. Grains are dominantly prismatic in shape. Inclusions are commonly aligned parallel or sub-parallel to the length of the prisms. Grain boundaries typically display curved faces, both concave and convex, and show re-entrant outlines. Lines of insoluble residues occur between prisms and at the boundary between the Microcodium aggregate and the attacked rock. Some transverse sections perpendicular to the long axes of prisms have a central tubular hollow. Fractured grains show that the ultrastructure of the calcite that makes up the solid part of the prism is composed of a pile of thin plates, commonly although not invariably, oriented perpendicularly to the long axes of the prism. Some differences and additional features have been noted in the material examined in this study. For example, some samples show that the calcite plates that constitute the individual prisms are aligned sub-parallel to the long axes of elongate Microcodium grains. Another point at variance with previous studies concerns the insoluble residues. The presence of insoluble clay residues at the contact between the Microcodium grain and the enclosing substrate was not recognized in all samples (Figs 9a and c).Instead, in some cases, a micro-honeycombed structure (hitherto undocumented) with pore

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diameters of 1·0 Jlm was observed (Fig. 9d). An additional feature, not previously recorded in Ancient (Eocene) Microcodium, is the presence of disc-shaped constrictions along rod-shaped aggregates (Fig. 9b). Interpretation of new features recorded in Ancient Microcodium

A micro-honeycombed structure within Microcodium grains has apparently been found in Eocene samples studied by Dr L. Pomar (Esteban, written communication). Esteban thinks the units may be bacterial precipitates. In line with observations of Recent and Ancient Microcodium made during this study, however, it is considered that the walls forming the honeycombed structure are the result of calcite precipitation on intracellular fungal hyphae. Following decay of the hyphae, the resulting pores mark their former positions. The origin of the constrictions is not clearly understood, but as a tentative proposal it is suggested that they may represent the nodes of a plant (parts of a plant stem where one or more leaves arise). If this proves correct, then calcification of plant cells penetrated by fungal hyphae may not be restricted only to the root zone as emphasized earlier. Thus, preservation may be simply a function of environment; structures already enclosed within a sediment (roots) have a better chance of survival than aerial parts of a plant which tend to be rapidly oxidized (Barghoorn, 1952). The sample illustrated in Fig. 9b may reflect a case of unusual preservation.

COMPARI SONS BETWEEN ANCIENT AND RECENT

MICR O C O D I UM

On a megascopic scale, the occurrence of Microcodium at specific lithostratigraphic levels, namely at surfaces indicating subaerial exposure and within dominantly continental facies, is perpetuated throughout the geological record. The presence of Microcodium within calcareous sediments affected by pedogenesis suggests that a genetic relationship exists between Microcodium and the rhizosphere of lime-rich soils. Observation at microscopic level indicates the presence of radiating filamentous structures within single Microcodium grains. Pitted surfaces, and tubular pores of 1·0, Jlm diameter or less, are characteristic of Ancient and Recent Microcodium (Figs 2f, 4c 9c and d). Shapes of grains typically show curved faces (Figs 2a, e, 3b, 6a, b, 9a and b) a feature which militates against a purely inorganic origin. Some Ancient Microcodium aggregates, however, contain prisms with straight faces. Monocrystalline calcite with uniform extinction is characteristic of Ancient Microcodium, whereas Pleistocene and Recent Microcodium grains tend to be composed of a number of sub-crystals, commonly displaying a radial-fibrous fabric. The latter gives aggregate or sweeping extinction patterns.These differences are thought to result from subsequent diagenetic modification of Ancient Microcodium. Following the classification of Bodergat (1974), type I ('epis de mals' = corn on the cob) and type 2 ('colonies en laminae' = laminar colonies) are common habits shown by Ancient Microcodium. Type 3 (isodiametric grains forming a cortical layer that surrounds a central canal, Fig. 8b) is apparently rare in the Ancient, whereas samples of Pleistocene and Recent age are dominantly of this form. The size-range for type 3 grains is similar for Ancient and Recent Microcodium but types 1 and 2 tend to be 141

Colin F. Klappa

composed of somewhat larger, elongate prisms up to 1·0 mm in length. Such mono crystalline, elongate grains of Ancient Microcodium, commonly have cross-walls normal to the length of the prism (Figs 9e and f). Cross-walls within single Micro codium grains have not been recorded in Quaternary samples. The widths of Ancient and Recent Microcodium grains are similar; the greater lengths of the former may be due simply to recrystallization of a number of isodiametric grains. Thus, the 'cross walls' may, in reality, mark the sites of former outer walls of juxtaposed, more or less isodiametric, single grains. With regard to the smaller Microcodium (b) forms of Esteban (1972) which are possibly equivalent to the 'seed-plots' of Lucas & Montenat (1967) and Microcodium described by Montenat & Echallier (1977), their occurrence at the perimeter of rhizocretions or subjacent to root channel walls, displaying either a concentric arrangement in transverse sections or palisade rows in longitudinal sections, suggests an intimate relationship with plant root systems. They occur both in the Ancient and Recent. Their origin may be the result of calcite encrustation on fungal hyphae. Fungi commonly form a peritrophic mantle around root surfaces, utilizing sloughed-off debris and exudates of the root as sources of food. Both calcite encrusted fungal hyphae and detached calcified root hairs have been recorded by SEM around rhizo cretions and root moulds (Klappa, 1978). Microcodium (b) form grains typically have a central tube (unfilled) or channel, or a central rod (filled), which is surrounded by a layer of calcite 5-20 J.tm thick. This arrangement gives Microcodium (b) grains an overall diameter of between 10-40 J.lffi. The diameter and shape of the central tube or rod allows distinction between the two suggested origins; root hairs have diameters of 5-17 J.tm according to Dittmer ( 1949) and tend to be straight unless penetrated by fungi, whereas fungal hyphae tend to be somewhat narrower (0·5- 10 J.tm) depending on species (personal observations) and are commonly sinuous. This morphology, combined with the cut-effect gives a spaghetti-like appearance in thin section. Microcodium is considered to be, on the basis of this study, the product of calci fication of plant cells whose forms have been maintained by fungi which show mycor rhizal associations. Since mycorrhizae are not restricted to a particular species of plant, variations in form may exist between different plant species. Moreover, given a time-span from the Eocene to the present-day, morphological differences between Ancient and Recent Microcodium are to be expected. As well as the involvement of completely different species, sufficient time is also availabl� for variation to result from evolutionary change within a single plant species. As a concluding remark in this section, it is pointed out that variations between Ancient and Recent Microcodium may also be explained (away), in part, by the 'cinderella' of carbonate sedimentology, that is, by diagenesis.

CO N SE QUENCE S AND I MPLICATIO N S Misinterpretations, oversights

( ?)

In reviewing the literature, in an attempt to elucidate the origin of Microcodium, several petrographic descriptions were found that show affinities with those of Microcodium. For example Seghal & Stoops (1972, pp. 67-68) state that '. . . a puzzling form of calcite accumulation is the occasional occurrence of sand-sized, single,

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rounded calcite grains with wavy extinction.' They suggest that abrasion during trans port was responsible for the rounded shapes, and the wavy extinction is attributed to their derivation from metamorphic rocks. They also note that similar kinds of calcite crystals have been observed by other authors (AI Rawi, Sys & Laruelle, I 968 ; Altaie, Sys & Stoops, I 969) who could not categorically state their origin. Schlanger ( I 964, p. D I I ) states '. . . small radiating and sheaf-like clusters of stubby, acicular and rhombohedral crystals have formed in some mosaics. These crystals are coffee coloured and show weak pleochroism or absorption in shades of light yellow brown. Absorption is greater paraliel to the long axis of the crystals. These crystals show high birefringence and indices of refraction greater than calcite; they have not been identified.' Unfortunately, neither of the above descriptions are furnished with any illus trations, thus, direct comparisons are difficult to make. None the less, some points in their descriptions may be applied to Microcodium and perhaps should be interpreted with this consideration in mind. Folk ( 1971), in discussing unusual neomorphic fabrics illustrates an example of 'neomorphic bladed calcite forming a very crudely oriented (N .B4.C) crust on an intraclast' (his fig. 84B, p. I 65). He mentions a '. . . microspar matrix of blades L/W 2: I to 4: 1, often circular in cross-section. Some of these particles taper at their end, others splay out like a worri toothbrush. They have slightly undulose extinction, and appear to be made of poorly defined fibres.' Folk's illustration (his fig. 84B) shows a remarkable similarity with Plate V, figs 2 1, 22 and 23 of Calvet, Pomar & Esteban ( 1975). The latter figure illustrates rhizocretions surrounded by structures considered analogous to Microcodium (b). Harbaugh ( I96 I ), in a discussion on calcite fabrics in late Paleozoic limestones from Kansas, Texas and New Mexico, established four specific types of visibly crystalline calcite. He suggests that his 'blade calcite', characterized by tapered, blade shaped crystals bunched in flower-like aggregates, probably formed by recrystal lization under mild shearing stresses. The photomicrograph illustrating blade calcite (his Plate I B, p. 99), is alarmingly similar to Ancient Microcodium. No plausible explanation is given as to how mild shearing stresses could produce this form ; perhaps a closer examination at the stratigraphic horizon from which this sample was taken might lead to the discovery of a former Microcodium-attacked subaerial exposure surface. The observation that Harbaugh's ( I 96 I ) blade calcite shows a close resem blance to Microcodium was noted independently by Misik ( 1968). With such limited data it is not the intention here to reinterpret the above citations. They are pointed out only to give Microcodium a hearing before sentence is passed or deferred regarding unusual calcite fabrics. Geological importance

The chronological and stratigraphical importance of Microcodium is well estab lished in the French geological literature. Correct recognition allows precision of correlation and is equally effective in application to sedimentological studies. As already indicated, Microcodium is intimately related to a land surface, which, by definition is a disconformity and represents a time-equivalent horizon marker. Thus, the presence of in situ Microcodium indicates terrestrial conditions, and may provide evidence for subaerial exposure in otherwise marine lithofacies. 143

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An origin associated with root systems necessitates the presence of a soil cover. Therefore, Microcodium may be used as a criterion for recognition of palaeosols. Soil formation, itself, requires a cessation or pause in sedimentation sufficiently long to allow pedogenetic processes to act on a given substrate. A prerequisite for the development of Microcodium appears to be a lime-rich soil. Petrographic analysis demonstrates that Microcodium not only modifies or obliter ates pre-existing textures but provides a potential source for sand-sized detrital calcite grains in reworked sediments. Thus, the cumulative effect of Microcodium on sub strate may provide valuable clues that assist in palaeoenvironmental reconstructions.

SUM MARY AND CONCLUSIONS

From samples of caliche collected in the western Mediterranean, calcite grains showing atypical fabrics have been recorded. A review of the literature indicates that such grains have been designated Microcodium, a hypothetical organism considered by early investigators to be possibly algal in origin. More recently, several workers have presented models which, although considering Microcodium to be the result of microbial activity, favour fungal, actinomycete, or bacterial interference. This study presents a new model based on petrographic examination of Eocene to Recent samples of Microcodium. Cumulative evidence, interpreted in the light of modern plant-soil ecosystems, suggests a mycorrhizal origin for Microcodium. The significant points resulting from this study are summarized as follows. ( 1 ) Occurrence. Caliche samples collected from mainland Spain and the island of lbiza, Balearics, reveal the presence of Microcodium grains constituting up to 43% of the rock by volume. The occurrence of Microcodium on Ibiza has not been pre viously documented. (2) Age. Stratigraphic, palaeontological and lithological relationships indicate the presence of significant quantities of Microcodium in the Eocene and Pleistocene. The existence of Microcodium in the Recent is reported here for the first time. (3) Unusual fabrics. Preservation of fine detail in Pleistocene and Recent samples, as revealed by SEM, portrays an exceedingly complex ultrastructure. The presence and subsequent calcification of microtubules, filamentous structures, radiating pore systems within grains, and protuberances, pits and raised borders on grain surfaces give a somewhat bewildering array of calcite fabrics when viewed in thin section. (4) Ancient versus Recent Microcodium. Sufficient details are retained in Ancient Microcodium to allow fruitful comparisons with Recent samples. Their origins are considered to be homologous. (5) Origin. Previous investigations regarding the origin of Microcodium are outlined. New field and petrographic data are at variance with earlier studies and have led to the formulation of a new model of formation. Microcodium is reinterpreted as being the result of calcification of mycorrhizae, a symbiotic association between fungi and cortical cells of roots. (6) Geological importance. Correct recognition of Microcodium has wide appli cation in terms of environmental, stratigraphic and palaeoecological studies. Emphasis is placed on Microcodium being a pedological feature and, thus, a valuable criterion for the recognition of the existence of a palaeosol.

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(7) Scope and limitations. It is hoped that this paper will at least provide an in troduction to those unfamiliar with this potentially useful diagenetic 'whim of nature' and lead to a search for a better understanding of distribution, environmental par ameters and diagenetic processes. Refinement is required to clarify many poorly understood but fundamental details resulting from this study. Such progress can be achieved only by an interdisciplinary approach.

AC K N 0 WLED G MENTS

It is a pleasure to acknowledge my debt to Prof. R.G.C.Bathurst for his continual guidance, astute criticism and intangible encouragement during the course of this work.To Dr M.Esteban, I am sincerely grateful for his advice, teaching and stimulat ing discussion during field studies in Spain and the friendship and genial hospitality shown by him and his 'Grup d'Estudi de Calcaries' at the University of Barcelona. SEM instrument time was provided by the Department of Botany, University of Liverpool, and operation was aided by the superlative technical assistance of C. J. Veltkamp. Special thanks are due also to many members of the Botany Department at Liverpool, especially to Dr J. C. Collins, Dr H. A. McAllister, Dr G. Russell and Dr S. T. Williams whose fruitful discussions helped to clarify biogenic aspects presented in this study. I greatly appreciate the comments of Prof. R. G. C. Bathurst, Dr P. J. Brenchley and Dr S.T.Williams which gave constructive criticism to an earlier draft of this work. Dr P. Enos provided useful suggestions during preparation. I am greatly indebted to J. Lynch for his cartographical assistance. Dr J. W. Wood kindly provided specimens from Palaeozoic shales of Missouri. I thank also the referees, Dr P. Freytet and Dr W. E. Krumbein, for their helpful suggestions on the original manuscript. Financial support provided by a NERC Research Studentship (Grant No. GT4/ 75/GS/131) is gratefully acknowledged.

REFERENCE S ADOLPHE, J.P . . & BILLY,

C. ( 1974) Biosynthese de calcite par une association bacterienne aerobe. 2873-2875. ALEXANDERSSON, E.T. (1972) Micritization of carbonate particles : processes of precipitation and dissolution in modern shallow-marine sediments. Bull. geol. Inst. Univ. Uppsala, 3, 201-236 . ALEXANDERSSON, E.T. ( 1974) Carbonate cementation i n coralline algal nodules i n the Skagerrak, North Sea : biochemical precipitation in undersaturated waters. J. sedim. Petrol. 44, 7-26. ALEXOPOULOl)S, C .J. ( 1962) Introductory Mycology, 2nd Ed. Wiley and Sons, New York. ALLARD, P., GANNAT, E . , LAPAICHE, N . , LEFAVRAIS-RAYMoi-m, A. & MARIE, P. ( 1959) Sur un niveau a Microcodium a Ia base du Tertiaire de Bresse. C. r. somm. Seanc. Soc. geol. Fr. 6, 150-151. AL RAWI, G .J., Svs, C. & LARUELLE, J. ( 1968) Pedogenetic evolution of the soils of the Mesopotamian Flood Plain. Pidologie, Gent, 18, 63-109. ALTAIE, F.H., Svs, C. & STOOPS, G. ( 1969) Soil groups of Iraq-their classification and characteriza tion. Pidologie, Gent, 19, 65-148. BARGHOORN, E.S. (1952) Degradation of plant tissues in organic sediments. J. sedim. Petrol. 22, 34--41. BoDELLE, J. & CAMPREDON, R. (1968) Les formations a Microcodium dans les Alpes-Maritimes franco-italiennes et Jes Basses-Alpes. Leur importance paleogeographique. Mem. Bur. Rech. Geol. Min. 58, 453-471 . C. r. hebd. Seanc. Acad. Sci. , Paris, 278,

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A . M . ( 1974) Les microcodiums, milieux et modes de developpement. Docum. Lab. Geol. 137-235 . BODERGAT, A . M . , TRIAT, J . M . & TRuc, G. ( 1 975) L'origine organique des microcodiums : exemple du role des microorganismes dans Ia biocorrosion des roches carbonatees et Ia biosynthese de Ia calcite en milieu continental. Int. Sediment. Congr. 9th, Nice, 1 975. Theme, 2, 7-10. BOULANGER, D. & CROS, P. ( 1 957) Presence de Microcodium dans Ia region de Limoux (Aude). Bull. Soc. geol. Fr. Ser . 6, 7, 353-354. BouRROUILH, R. & MAGNE, J. ( 1 963) A propos de depots du Pliocene superieur et du Quaternaire sur Ia cote nord de l'ile de Minorque (Baleares). Bull. Soc. geol. Fr. Sir. 7, 5, 298-302. BowEN, G. D . ( 1 973) Mineral nutrition of Ectomycorrhizae. I n : Ectomycorrhizae, Their Ecology and Physiology ( Ed . by G . C. Marks & T. T. Kozlowski), pp. 1 5 1-205 . Academic Press, New York. BROMLEY, R.G. ( 1970) Borings as trace fossils and Entobia cretacea Portlock, as an example. In : Trace Fossils (Ed. by T. P. Crimes & J. C. Harper), pp. 49-90. Geol. J. Special Issue, 3. BuRGES, A. ( 1 958) Micro-Organisms in the Soil. Hutchinson, London. BusiCH, E. ( 1913) Die endotrophen Mykorhiza der Asclepidaceae. Verh . zoo!. bot. Ges. Wien, 63, 240264. CALVET, F., PoMAR, L. & EsTEBAN, M. ( 1 975) Las Rizocreciones del Pleistoceno de Mallorca. lnst. Invest. Geol. Univ. de Barcelona, 30, 3 5-60. CUTTER, E.G. ( 1 969) Plant Anatomy : Experiment and Interpretation Part I. Cells and Tissues. Edward Arnold Ltd, London. CuviLLIER, J. ( 1955) Sur l'origine de Microcodium . Bull. Soc. geol. Fr. Ser. 6, 5, 295-297. CuviLLIER, J. & SACAL, V. ( 1 961) Stratigraphic correlations by microfacies in Western Acquitaine. Int. sedim. petrogr. Ser. 2, E. J. Brill, Leiden . DAVIES, P.J. & TILL, R. ( 1 968) Stained dry cellulose peels of ancient and Recent impregnated sedi ments. J. sedim . Petrol. 38, 234-237. DEMANGEON, P. ( 1956) Importantes formations dues a des algues calcaires dans le Montien rouge (Vitrollien) du Midi de Ia France. C. r. hebd. Seanc. Acad. Sci. , Paris, 242, 1905-1 907. DICKSON, J.A.D. ( 1 966) Carbonate identification and genesis as revealed by staining. J. sedim. Petrol. 36, 491-505. DITTMER, H.J. ( 1 949) Root hair variations in plant species. Am. J. Bot. 36, 1 52-1 55 . DURAND, J.P. ( 1962) Role e t repartition des Microcodium dans les formations fiuviolacustres pro venc;ales du Cretace superieur et de !'Eocene. C. r. somm. Seanc . Soc. geol. Fr. 9, 263-265. EDWARDS, W.N. ( 1 932) Lower Eocene plants from !stria. Ann. Mag. nat. Hist. 56, 21 3-216. EsTEBAN, M . ( 1 972) Una nueva forma de prismas de Microcodium elegans Gluck 1912 y su relaci6n con el caliche del Eoceno Inferior, Marmella, provincia de Tarragona (Espana). Rev. Inst. inv. Geol. Dipt. Prov. Barcelona, 27, 65-81. ESTEBAN. M. ( 1 974) Caliche textures and Microcodium. Boll. Soc. geol. ita!. 92, Suppl. 1 973, 105-1 25. EVENARI, M., SHANAN, L. & TADMOR, N. ( 1 971) The Negev: the Challenge of a Desert, Harvard University Press, Cambridge, Mass. FAHN, A. ( 1974) Plant Anatomy, 2nd Ed. Pergamon, Oxford. FAURE-MURET, A. & FALLOT, P. ( 1954) La formation a Microcodium au pourtour de I'Argentera Mercantour. Bull. Soc. geol. Fr . Sir. 6, 4, 1 1 1-138. FEIGL, F. ( 1 943) Laboratory Manual of Spot Tests, Academic Press, New York. FOLK, R.L. ( 1971) Unusual neomorphism of micrite. In : Carbonate Cements (Ed . by 0. P. Bricker), pp. 163-168. Johns Hopkins Press, Baltimore. FRANCOIS, S. & SIGAL, J. ( 1957) Quelques donnees nouvelles sur Ia morphologie et Ia repartition stratigraphique des Microcodium Gluck 1 9 1 2 . C. r. somm. Seanc. Soc. geol. Fr. 10, 1 68-171 . Fac. Sci. Lyons, 62,

FREYTET, P.

( 1969) Un nouveau gisement de Microcodium cavernicoles : le paleokarst de St Beauzille (Herault). C. r. somm. sianc . Soc. geol. Fr. 5, 1 66.

FREYTET,

P. ( 1971 a) Les depots continentaux et marins du Cretace superieur et des couches de passage a l'E ocene en Languedoc. Bull. Bur. Rech. Geol. Min. Sir. 2 , Sect. 1, 4, 1-54.

FREYTET,

P. ( 1 971 b) Paleosols residuels et paleosols alluviaux hydromorphes associes aux depots fiuviatiles dan� le Cretace superieur et l' Eocene basal du Languedoc. Revue Geogr. phys. geol. dyn. Ser. 2, 13, 245-268. FREYTET, P. ( 1 973) Petrography and paleo-environment of continental carbonate deposits with particular reference to the Upper Cretaceous and Lower Eocene of Languedoc (Southern France). Sedim. Geol. 10, 25-60. 146

Biolithogenesis of Microcodium FREYTET,

P. ( 1 975) Quelques observations petrographiques sur les calcaires continentaux rencontres

a !'excursion de mai 1 974 de I' A.G .B.P. : facies lacustres, modifications pedologiques, Micro

1 5-23. P. ( 1974) Presence de couches inframiocenes a Microcodium dans Ia marge orientale du Bassin Tertiaire de Digne-Valensole. C. r. hebd. Seanc. Acad. Sci. , Paris, Ser D, 278, 2087-2090. GLUCK, H. ( 1 9 1 2) Eine neue gesteinbildende Siphonee (Codiacee) aus dem marinen Tertiiir von SUddeutschland. Mitt. bad. geol. Landesanst. Bd. 7, 3-24. GoTns, M . ( 1 963) Sur un cas d'heterotrophie de Microcodium. Bull. Soc. geol. Fr. 5, 838-843. GRAY, T.R .G . & WILLIAMS, S .T. (1971) Soil micro-organisms. Oliver & Boyd, Edinburgh. GUBLER, Y. (1 955) L' E ocene subbrian has not been preserved (Figs 5e, f, 6e and f). Petrifaction of cortical cells by calcite is a common, but by no means the commone>t, form of preservation. Petrifaction of cell walls is of greater importance in samples studied here (Figs 7 and 8). In Fig. 7 the rhizolith consists of a petrified epi dermis. Root hairs radiate outwards from the root (Fig. 7a). Calcified root hairs occur as tube> in the surrounding micritic matrix, giving a vermicular or spaghetti-like texture when viewed in thin se::tion (Fig. 5f). Closer inspection of the rhizolith shown in Fig. 7a reveals that micron-sized plates of calcite are present in the position of the middle lamella between adjacent cells (Fig. 7c and d). Similar fabrics have been found in other rhizoliths (Fig. 8). The rhizolith of Fig. 8a is composed of calcified parenchymatous cells and xylem vessels which have been preserved in plate-shaped and rhombic micrite (Fig. 8b-f). The protoplast or cell contents have not been calcified in this sample. The cells are now voids (Fig. 8d and e) or partially filled with needle fibres of calcite (Fig. 8c). Thus, it appears that the middle lamella, originally composed of or containing calcium pectate, is a preferential site for calcification. Thus, cell morphology of the roots is maintained. In thin section, calcite impregnated or replaced middle lamellae replicate the cellular pattern of root cells (Fig. 5d). In Fig. 5d root anatomy has not been preserved in detail but the concentric layering of cells can be discerned. This form of preservation is especially characteristic of rhizoliths which occur within sheet calcrete horizons.

GENESIS OF RHIZOLITHS

The role of plants in biological weathering is well known (Keller & Frederickson, I952). Plant roots accelerate weathering of rocks by exchanging H+ ions from the roots for Ca2+, Mg2+, K+, etc., ions in the rocks. Keller & Frederickson (1952) suggested that the surface chemistry of a plant root could be explained by the Debye-Hiickel double layer system. The root and adhering water film is surrounded by an ionic double layer of which the root has a strong negative charge which is balanced by a surrounding area of positive ions (typically H +) . Thus, the high concentration of H + ions in the diffuse ionic double layer around roots will accelerate weathering of surrounding minerals if the released metal cations are removed from the system. The above outlined mechanism of biological weathering may account for root borings and the breakdown of host materials, but the production of cemented cylinders of calcium carbonate around roots to form rhizocretions and the calcification of root tissues to form root petrifactions require further explanation. Ca2 + ions are not removed from the rhizosphere in environments conducive to the formation of rhizoliths within calcrete profiles. On the contrary, calcium carbonate is the stable mineral phase in such environments. The problem is to explain why this should be so. Gray & Williams (197I) have questioned the assumption that roots increase acidity in the sur rounding soil as a result of excretion of C02 and H+ ions. Some roots take up more anions than cations. Such roots maintain electrical neutrality by passing out HC0-3 ions rather than H + ions. In so doing, the pH of the surrounding soil is raised, rathe.r than lowered. This may trigger precipitation of calcium carbonate around roots, thus leading to the formation of rhizocretions. Johnson (I 967) listed further possible ways of forming rhizocretions. He stated (p. I 54): 'Root sheaths apparently form in one or more of five biochemical ways, dependent upon (I) the presence of organic acids exuded by living plant roots; (2) symbiotic relations between roots and certain soil bacteria; (3) symbiotic relations between roots and certain soil fungi; (4) the presence of some blue green soil algae which have calcium carbonate precipitating bacteria housed in their slime sheaths; (5) calcium exclusion properties of some plants which promote the precipitation of calcium carbon ate outside the root'. Although Johnson admitted 161

162

Rhizoliths in terrestrial carbonates

the possibility that the formation of rhizocretions may result from a combination of these processes, he favoured the first one. Carozzi (1967) described calcite-cemented sand stone around roots of Iroko trees from the Ivary Coast and attributed the cementation to calcareous secretions related to wounds kept unhealed by insect activity. Calvet et al. (1975) suggested that rhizocretions, occurring in Pleistocene aeolianites from Mallorca formed by: (1) progressive root penetration, pro ducing a closer packing of sand grains around the roots; (2) formation of a calcareous envelope (sheath), resulting mainly from the activity of micro organisms, the effects of organic acids and evapo transpiration; and (3) centripetal filling of chalky material following death and decay of the root. Kindle ( 1925, p. 744) suggested that the presence of certain bacteria, or of fungi, on. living roots may constitute the initial factor in the development of root concretions. That micro-organisms are present in and around roots has been noted in this study and elsewhere (Burges, 1958; Alexander, 1961; Gray & Williams, 1971; Russell, 1973; Klappa, 1979a, b). Whether they play an active role in, or are incidental to, the formation of rhizoliths is another question. The suggestions of Kindle (1925), Johnson (1967) and Calvet et al. (1975) are reasonable but not readily proved (Klappa, 1978a, p. 5 14). With respect to root petrifactions, it seems to be more than coincidental that sites for calcification in these structures correspond to naturally occurring calcium-rich layers within plant tissues, notably the middle lamella (organic cement of calcium pectate between cell walls). Thus, a substrate or template control appears to govern the form of preservation in petrified samples examined in this study (Figs 7 and 8). A similar control has been found for the formation of some calcified filamentous micro organisms (algae, fungi, actinomycetes; see Klappa, 1979a) which, together with rhizoliths, are common

and characteristic biogenetic carbonate structures of pedodiagenetic calcretes (Klappa, 1978c). The formation of tubules and rhizocretions, on the other hand, involves dissolution of mineral components within the rhizosphere and reprecipitation of some or all of the dissolved minerals around the root (Figs 3, 4, 5a-c) and/or introduction of CaC03-rich solutions from elsewhere. This process may take place during the life of the root (Fig. 3d) (Kindle, 1925) or during its decay. Hoffmeister & Multer (1965), in their description of an inferred sequence of events which led to the formation of a 'fossil mangrove reef rock' from Florida, suggested that the slow decomposition of buried root material released C02 which combined with available water, forming H2C03• This action dissolved calcite and produced carbonate-bearing solutions which percolated through pore spaces of the calcareous-quartzitic sand substrate. Reprecipi tation of the CaC03 in the sand immediately adjacent to the rotting root cemented quartz grains together, forming a hard cylindrical rim around the root (Hoffmeister & Multer, 1965, p. 851). They envisaged that the hard cylindrical rim (equivalent to root tubule of this study)' . . . slowly grew outward as the action continued and resulted in a coating con siderably thicker than the original periderm. At the same time continued decay of the organic material, surrounded by the hard but still porous rim, pro vided an environment for calcification within the woody structure and for replacement of the tissue itself by CaC03'. In some root tubules examined in this study the tubular wall is composed of cryptocrystalline calcite with virtually no porosity. Because the outer wall of the tubule forms an effective barrier between the decaying root within the tubule and the outer surface of the tubule itself, a change in the chemical micro environment as a result of decaying root organic matter would be unable to cause further buildup of CaC03 on the outer wall.

Fig. 9. (a) Vertical section through calcrete hardpan. Tubular voids are root moulds. Some moulds contain cylinders composed of microcrystalline calcite to give an alveolar texture. Polished slab. Cala Bassa, Ibiza; calcrete hardpan of Pleistocene age. (b) Alveolar texture consisting of ramifying micritic walls. White areas are root moulds. Black peloids (arrows), composed of cryptocrystalline calcite, are interpreted as calcified faecal pellets. Thin section, PPL. Same sample as a. (c) Detail of b showing transverse sections (T) and longitudinal sections (L) through micritic cylinders. Thin section, PPL. (d) Detail of micritic wall shown in c. Wall is composed of micron-sized calcite needles (cf tangential needle fibres; James, 1972). Arrow points to columnar calcite crystals which have their long axes perpendicular to needle calcite walls. Thin section, polarizers at 45°. (e) Cylinders composed of calcite needles oriented tangentially with respect to the surface of the cylinders but random with respect to the long axes of the cylinders. Tubular voids (arrows) are root moulds. SEM. Location: Tarragona, NE Spain. (f) Detail of e. Needle calcite wall has a banded fibrous fabric when viewed in transverse section and a hyphantic fibrous fabric when viewed in longitudinal section. Equant microcrystalline calcite precipitated on needle wall (arrow) leads to thickening of wall. SEM.

163

Colin F. Klappa

At many field outcrops in the study area, root tubules were observed around living roots (Fig. 3d). It was noted that root tubules started at some distance (mm) from the root-sediment interface on the outer periphery of the rhizosphere. Laboratory examination of these samples indicated that the outer diameter of the tubule is slightly greater than the maximum extent of root hair penetration into the sediment. In other words, the outer diameter of the tubule is approximately at, or just beyond, the rhizosphere. With decay of the root hairs the rhizo sphere decreases and precipitation of CaC03 can occur near the root surface since C02 evolution from root hairs is terminated. New root hairs will grow lower down the root as the growing root tip pene trates further into the sediment. Thus, formation of the tubules is viewed as a centripetal process, similar to that described by Calve! et al. (1975). However, the significant difference between the process envisaged here and that of Calve! et at. (1975) is that centripetal tubule formation can occur around living roots as well as around decaying roots. The root tubule, once formed, provides a conduit for downward percolating solutions. When the root within the tubule can no longer maintain viability (perhaps because of the tubule itself), C02 levels are reduced. The root, following its death, begins to decay and releases proteins and sugars which in crease alkalinity of the ambient rhizosphere. If precipitation of calcite takes place within or on the decaying root, some anatomical features of the root may be preserved; the end result being a petrified root (root petrifaction) surrounded by a root tubule.

ASSOCIATED FEATURES

Field and petrographic observations indicate that roots of higher plants are partly or totally respon sible for numerous and characteristic features of calcretes and calcretized aeolianites (Kiappa, 1978b). Apart from rhizoliths themselves, roots are pri marily responsible for the formation of vertically elongate glaebules or concretionary soil structures (Fig. 3b), sheet calcrete layers, brecciation textures and the formation of some tepee structures (Klappa, 1980), brittle fracture, channel and mouldic poro sity, and alveolar textures (cf. Esteban, 1974). Roots, together with symbiotic fungi, also are responsible for the enigmatic structure Micro codium as demonstrated by Klappa (1978a).

Calcretization involves modification or oblitera tion of precursor fabrics, textures and structures in a given host material and the production of new fabrics, textures and structures. Roots modify -and destroy rocks (e.g. 'rhizomicritization', results from dissolution of silt-sized or larger carbonate grains and/or cement and reprecipitation of released CaCOa as microcrystalline calcite). Roots also become calcified. Thus, roots are fundamental contributors to pedodia-genetic processes and resulting products of calcretization, the products being rhizoliths and related features as outlined above.

A CKNOWLEDGMENTS

This study evolved from part of a Ph. D. dissertation on calcretes from coastal regions of the western Mediterranean, completed at the University of Liverpool under the advisorship of Robin G. C. Bathurst. I wish to express my gratitude to him, and to Mateu Esteban, Francese Calvet and Lluis Pomar for their encouragement and stimulating dis cussion during the course of this work. I also thank s.- T. Williams for providing SEM facilities in the: Department of Botany, University of Liverpool andl C. J. Veltkamp for technical assistance. The manuscript was reviewed critically by J. A. D. Dickson, M. Esteban, J. D. Hudson and N. P. James; I am grateful for their comments and sug gestions which have improved this contribution considerably. Financial support for field and laboratory studies was provided by the Natural Environment Research Council (Research Studentship Grant No. GT4/75/ GS/131) which is gratefully acknowledged.

REFERENCES

M. (1961) Introduction to Soil Microbiology. Wiley & Sons, New York. 472 pp. AMERICAN GEOLOGICAL INSTITUTE t)972) Glossary of Geology. Washington, D.C. 805 pp. AMIEL, A.J. (1975) Progressive pedogenesis of eolianite sandstone. J. sedim. Petrol. 45, 513-519. B AL, L. (1975) Carbonate in soil: a theoretical con-· sideration on, and proposal for its fabric analysis. II. Crystal tubes, intercalary crystals, K fabric. Neth. J. Agric. Sci. 23, 163-176. BoYD, D.W. (1975) False or misleading traces. In: -The Study of Trace Fossils (Ed. by R. W. Frey), pp. 65-83. Springer-Verlag, New York. ALEXANDER,

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C.J.R. (1975) Petrology of palaeosols and other terrestrial sediments on Aldabra, Western Indian Ocean. Phil. Trans. R. Soc. Land. 273, 1-32. BREWER, R. (1964) Fabric and Mineral Analysis of Soils. Wiley & Sons, New York. 470 pp. BROMLEY, R.G., CURRAN, H.A., FREY, R.W., GUTSCHICK, R.C. & SuTTER, L.J. (1975) Problems in interpreting unusually large burrows. In: The Study of Trace Fossils (Ed. by R. W. Frey), pp. 351-376. Springer Verlag, New York. BuRGES, A. (1958) Micro-Organisms in the Soil. Hutchin son, London. 188 pp. CALVET, F., PoMAR, L. & EsTEBAN, M. (1975) Las Rizocreciones del Pleistoceno de Mallorca. lnst. Invest. Ceo!. Univ. Barcelona, 30, 35-60. CAROZZI, A.V. (1967) Recent calcite-cemented sandstone generated by the Equatorial tree Iroko (Chiorophora cxcelsa), Daloa, Ivory Coast. J. sedim. Petrol. 37, 597-600. DURAND, J.H. (1949) Essai de nomenclature des croutes. Bull. Soc. Sci. Nautrelles Tunisie, 3 4, 141-142. EsTEBAN, M. (1974) Caliche textures and Microcodium. Boll. Soc. geol. !tal. 92, Suppl., 1973, 105-125. ESTEBAN, M. (1976) Vadose pisolite and caliche. Bull. Am. Ass. Petrol. Ceo!. 60,2048-2057. FAIRBRIDGE, R.W. & TEICHERT, C. (1953) Soil horizons and marine bands in the coastal limestones of Western Australia. J. Proc. R. Soc. New South Wales, 86, 68-87. GLENNIE, K.W. & EvAMY, B.B. (1968) Dikaka: plants and plant-root structures associated with aeolian sand. Palaeogeog. Palaeoclimat. Palaeoecol. 4, 78-87. GRAY, T.R.G. & WILLIAMS, S.T. (1971) Soil Micro organisms. Oliver & Boyd, Edinburgh. 240 pp. HARRISON, R.S. (1977) Caliche profiles: indicators of near-surface subaerial diagenesis, Barbados, West Indies. Bull. Can. Petrol. Ceo!. 25, 123-173. HOFFMEISTER, J.E. & MULTER, H.G. (1965) Fossil mangrove reef of Key Biscayne, Florida. Bull. geol. Soc. Am. 16,845-852. JAMES, N.P. (1972) Holocene and Pleistocene calcareous crust (caliche) profiles: criteria for subaerial exposure. J. sedim. Petrol. 42, 817-836. JOHNSON, D.L. (1967) Caliche on the Channel Islands. Miner. Jnf . Calif Div. Mines Ceo!. 20, 151-158. KELLER, W.D. & FREDERICKSON, A.F. (1952) Role of plants and colloidal acids in the mechanism of weathering. Am. J. Sci. 250, 594-608. KINDLE, E.M. (1923) Range and distribution of certain types of Canadian Pleistocene concretions. Bull. geol. Soc. Am. 34,609-648. KrNDLE, E.M. (1925) A note on Rhizocretions. J. Ceo!. 33,744-746. KLAPPA, C.F. (1978a) Biolithogenesis of Microcodium: elucidation. Sedimentology, 25, 489-522. KLAPPA, C.F. (1978b) Morphology, composition and genesis of Quaternary calcretes from the western BRAITHWAITE,

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Mediterranean: a petrographic approach. Unpublished Ph.D. Thesis, University of Liverpool, 446 pp. KLAPPA, C.F. (1978c) Biogenetic carbonate structures in Quaternary calcretes, western Mediterranean. lOth Int. Sediment. Congr., Jerusalem, 365 (abstract). KLAPPA, C.F. (1979a) Calcified filaments in Quaternary calcretes: organa-mineral interactions in the subaerial vadose environment. J. sedim. Petrol. 49,955-968. KLAPPA, C.F. (1979b) Calcification and significance of soil filamentous micro-organisms in Quaternary calcretes, eastern Spain. Bull. Am. Ass. Petrol. Ceo!. 63,480. KLAPPA, C.F. (1980) Brecciation textures and tepee structures in Quaternary calcrete (caliche) profiles from eastern Spain: the plant factor in their forma tion. Ceo!. J. IS, 81-89. NoRTHROP, J.I. (1890) Notes on the geology of the Bahamas. Trans. N.Y. Acad. Sci. 10, 4-22. PERKINS, R.D. (1977) Depositional Framework of Pleisto cene Rocks in south Florida. Mem. geol. Soc. Am. 147, 131-198. PLAZIAT, J.C. (1971) Racines ou terriers? Criteres de distinction a partir de quelques exemples du Tertiaire continental et littoral du Bassin de Paris et du Midi de Ia France. Consequences paleographiques. Bull. Soc. geol. Fr. ser 7, 13, 195-203. READ, J.F. (1974) Calcrete deposits and Quaternary sediments, Edel Province, Western Australia. Mem. Am. Ass. Petrol. Ceo!. 22, 250-282. RussELL, E.W. (1973) Soil Conditions and Plant Growth, lOth ed. Longman, London. 849 pp. RuTH, N. ST J. (1927) Replacement vs impregnation in petrified wood. Econ. Ceo!. 22, 729-739. SARJEANT, W.A.S. (I 975) Plant trace fossils. In: The Study of Trace Fossils (Ed. by R. W. Frey), pp. 163179. Springer-Verlag, New York. SHERMAN, G.D. & IKAWA, H. (1958) Calcareous con cretions and sheets in soils near South Point, Hawaii. Pacific Sci. 12, 255-257. STEINEN, R.P. (1974) Phreatic and vadose diagenetic modification of Pleistocene limestone: Petrographic observations from sub-surface of Barbados, West Indies. Bull. Am. Ass. Petrol. Ceo!. 58, 1008-1024. STRAKHOV, N.H. (1970) Principles of Lithogenesis, vol. 3. Oliver & Boyd, Edinburgh. 577 pp. TEICHERT, C. (1950) Late Quaternary sea-level changes at Rottnest Island, Western Australia. Proc. R. Soc. Victoria, 59, 63-79. VALETON, I. (1971) Tubular fossils in the bauxites and the underlying sediments of Surinam and Guyana. Geologie Mijnb. 50, 733-741. WARD, W.C. (1975) Petrology and diagenesis of carbon ate Eolianites of northeastern Yucatan, Mexico. In: Studies in Geology, 2. Belize Shelf: Carbonate Sedi ments, Clastic Sediments and Ecology, pp. 500-571. Am. Ass. Petrol. Geol.

(Manuscript received 28 September 1979; revision received 18 February 1980)

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Sedimentology

(1980) 27 651-660

Calcrete profiles in the Eyam Limestone (Carboniferous) of Derbyshire: petrology and regional significance

A. E. A D A M S Department of Geology, University of Manchester, Manchester M13 9PL

ABSTRACT Calcrete profiles (caliche) have been recognized in the Eyam Limestone from the Lower Carboniferous in the area around Monyash, Derbyshire. They occur at the top of the flank facies surrounding carbonate-mud buildups ('knoll reefs'). Four units make up the complete profile. These are from base to top: (I) grain-supported sediment with rhizocretions, (2) matrix-supported sediment with alveolar texture, (3) pelleted calcrete, (4) laminar calcrete. Commonly one or more units are missing from the profile. Calcretes indicate subaerial exposure. The carbonate buildups of the Eyam Limestone were completely exposed soon after deposition, requiring a fall in sea-level probably in excess of 10m. This discovery demands a review of previous regional palaeoenvironmental studies.

INTRODUCTION

Calcretes (Caliche) form through the accumulation and re-distribution of carbonate in soil-profiles and indicate subaerial weathering and unconformity in otherwise marine limestone sequences (Reeves, 1970; Read, 1976). During the last few years there has been an increasing volume of literature on the subject of Pleistocene and Recent calcrete deposits. Authors such as Multer & Hoffmeister (1968), James (1972) and Read (1976) have emphasized the importance of recognizing calcretes, and in particu lar distinguishing them from superficially similar algal stromatolite deposits which indicate tidal or shallow subtidal environments. Read (1976) noted that there are few published descriptions of ancient calcrete profiles. In particu lar, there is a shortage of information on details of calcrete microtexture which may enable calcretes to be identified even where the more obvious large-scale features are absent. Palaeozoic examples described in the literature include those by Harrison & Steinen (1978) and Walls, Harris & Nunan (1975). The aim of this contribution, there fore, is to describe an example of an ancient calcrete profile with particular emphasis on microtextures, Calcretes Edited by V. Paul Wright and Maurice E.Tucker © 1991 The International Association of Sedimentologists ISBN: 978-0-632-03187-0

and to discuss its implications for the regional geology. GEO L O G IC A L SETTIN G

The calcrete profiles discussed here occur in the Eyam Limestone of the area around Monyash, Derbyshire (Fig. 1). The Eyam Limestone occurs in the topmost part of the Carboniferous Limestone and has been assigned to the P2 zone on the basis of bivalve-goniatite faunas. Previous work on the Eyam Limestone includes stratigraphical studies (e.g. Shirley, 1959) and broad scale palaeoenvironmental studies (Brown, 1973). Biggins (1969) briefly des cribed the geology of the P2 zone in Lathkill Dale while making a comparison with coeval Carboni ferous Limestone at High Tor near Matlock 15 km to the southwest. In the Monyash area the Eyam Limestone conformably overlies pale-coloured, fossiliferous shelf limestones of the Monsal Dale Beds. The Eyam Limestone comprises up to 30 m of well-bedded, dark grey, fine-grained limestones. Included in the sequence are a number of lenticular bodies of pale-coloured, massive, fine-grained 167

A. E. Adams

KEY

lllllllJ

Black mudstone (Upper Carboniferous) Unconformity Basin fill facies Buildup facies

]

Eyam Limestone (0 to 30m), p2 zone

D

Monsal Dale Beds (base not seen in Monyash area), D2 zone

""'

Fault

�

Old quarry

Fig. 1.

Map showing distribution of principal rock-types and location of quarry exposures, Lathkill Dale. Grid line numbers refer to 1 km squares in National Grid Reference square SK.

limestone each surrounded by coarse crinoidal lime stones which exhibit depositional dips away from the massive core. These features are usually called 'knoll-reefs' although the author prefers the more general term carbonate-mud buildup since many of the attributes of reefs as reviewed by Braithwaite (1973) and Heckel (1974), such as wave resistance and ecological zonation, have not been demonstrated. The terminology of such structures and their origin has caused much controversy and the subject is reviewed by Wilson (1975). Following general usage, the massive fine-grained limestones of the buildup are here called the buildup-core facies, the coarse crinoidal limestones, the buildup-flank facies and the bedded strata above the buildups, the basin fill facies. The buildups generally occur at the base of the Eyam Limestone and rest directly on the Monsal Dale Beds, but a few occur higher in the sequence. In the area studied, higher formations have generally

been removed by erosion although in Monyash village there is a small patch of shale attributed to the: Upper Carboniferous (Fig. 1). PETRO LO GY OF THE CA LCRETES General description

The calcretes occur at the junction between the: coarse, crinoidal buildup-flank facies and the dark-· coloured fine-grained limestones of the basin-fill facies. This contact is well exposed in and around Ricklow Quarry near the head of Lathkill Dale (Fig. 1). The sediments on which the calcretes have developed are bioclastic grainstones and packstones (classification of Dunham, 1962) in which the bio .. clasts are crinoid plates and ossicles, together with fragments of brachiopods and bryozoans. The calcretes are characterized by a series of 168

Calcrete profiles in Eyam Limestone

structures which always occur in the same strati graphical order although not all profiles show the complete range of structures. The four units identi fied are, from base to top: (I) grain-supported sediment with rhizocretions, (2) matrix-supported sediment with alveolar texture, (3) pelleted calcrete, (4) laminar calcrete. These units are described in detail and the terminology explained below. The thickest calcrete profile observed is 50 em, but more usually they are 10-20 em thick. They are much thinner than many of the Quaternary profiles des cribed in the literature (see, for example, Read, 1974, 1976). A typical complete profile is illustrated in Fig. 2. ;;: '

c

� "

carbonate mudstone

]]

. :I

em

20 30

laminar calc rete

pelleted calcrete matrix·- supported sediment with alveolar texture

�

"' ·;:;

grain- supported sediment with rhizocretions

.E -" c

0 :;:: ' a.

0

�

3! "3 ..c

�

0"

'--0

"-0

0

o,--,

�

�

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0

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alveolar texture

·.•

calcrete pellets

# 0

host packstone or grainstone

By analogy with Quaternary examples, the tube structures are interpreted as coated plant roots. Perkins ( 1977) noted that the precipitation of fine grained calcite around small root tubules in Ple.isto cene calcretes from Florida may be so extensive as to produce a 'root-rock'. He called such structures root tube calcifications, while noting that similar structures described by other workers had been given different names. Harrison & Steinen ( 1978) figure 'root voids' from dense pelleted micrites in Recent and ancient calcretes which are similar structures to those described here. The nomenclature of plant root structures in sediments has been reviewed by Klappa ( 1978). He used the term 'rhizoliths' to include all organo sedimentary structures produced by roots. This would include simple root moulds and root casts as well as root petrifications. Klappa used the term 'rhizo cretion' for concretionary cemented coatings around living or decaying roots and described examples from Quaternary calcretes of the western Mediterranean. They comprise sheaths of micritized and micrite cemented carbonate grains around roots. Although the Carboniferous structures described here have no remaining traces of organic matter, they are very similar to the Quaternary structures described by Klappa and are thus interpreted as rhizocretions.

rhizocretions crinoid plates

Matrix-supported sediment with alveolar texture

'-f-.. shell fragmeuts

The second unit of the calcrete profile contains a higher proportion of fine-grained matrix than does the unit beneath. It is matrix-supported and has a transitional contact with the grain supported sedi ments below. This unit displays a characteristic texture comprising irregular-shaped but approxi mately equidimensional pores filled with sparry calcite separated by a network of interconnecting walls of micrite (Figs 4 and 5). The diameter of the pores varies from 0· 1 to 0·5 mm and the width of the walls from 0·01 to 0·1 mm. A similar structure was figured by Esteban ( 1974) in Pleistocene calcretes from Spain and named 'alveolar texture'. Esteban noted that the texture is apparently exclusive to calcrete and suggested that it might be formed by the disso lution of the interiors of pisoliths. In a later publica tion, however, Esteban stated that alveolar texture is 'probably related to rhizocretion fabrics' (Esteban, 1976). A comparable texture is described and illustrated by Braithwaite ( 1975) from Quaternary terrestrial sediments on the Indian Ocean Island of Aldabra. He

Fig. 2. Generalized complete calcrete profile· on top of buildup-flank facies� Eyam Limestone, Ricklow Quatry.

Grain-supported sediment with rhizocretions

The lowermost unit of the calcrete profile has a gradational contact with the host packstone or grainstone. The principal features of this unit, as seen in thin section, are thin micritic coatings on bioclasts and circular to irregular spar-filled cavities, having diameters of 0·25-o:s mm, surrounded by walls of fine-grained carbonate up to 0·2 mm thick (Fig. 3). The walls often.show an irregular concentric lamin ation and may include small aggregates of micrite (pellets). In this case they have a clotted appearance. These spar-filled cavities are sections through irregu lar tubes. In hand specimen these tubes impart a brown stain to an otherwise pale grey limestone. The micritic coatings are often continuous between individual sediment grains and have the appearance of binding the sediment together. Thus they formed after final deposition of the sediment grains. 169

A. E. Adams

Photomicrograph showing irregular spar-filled voids separated by a network of micrite walls. Interpreted as alveolar texture formed by the deposition of fine-grained carbonate around decaying rootlets. Scale bar represents 0· 5 mm.

Fig. 3.

Fig. 4. Photomicrograph showing irregular spar-filled voids surrounded by coatings of vaguely laminated micrite. Interpreted as rhizocretions (see text). Scale bar represents 0· 5 mm.

170


Fig. 5. Photomicrograph showing well-developed alveolar texture from immediately below laminar unit. Scale bar represents 0·5 mm.

called it 'vesicular structure' and suggested that it was caused by the formation of large numbers of gas bubbles during alternate wetting and drying of the sediments. Harrison & Steinen (1978) described arcuate sheaths of micrite surrounding a network of voids in Recent and Carboniferous calcretes. In the latter case voids are cement filled. These sheaths were interpreted as cement precipitates on the surfaces of rootlets and root hairs which originally occupied the voids. These were thus called 'arcuate branching root sheaths'. They are similar to the structures present in the Eyam Limestone, although the arcuate form of the micrite sheaths is not so evident. The most detailed description of alveolar texture is that of Klappa (1978) from Quaternary calcretes from the western Mediterranean. He states, 'alveolar texture consists basically of a number of cylindrical to irregular pores, which may or may not be filled with calcite cement, separated by a network of anastomosing micritic walls'. The pores have similar dimensions to those in the Carboniferous structures described here. In the Quaternary examples the walls consist of calcite needle fibres. Klappa ob served that whereas decaying roots in Quaternary calcretes might become coated with needle micrite, Jiving roots would always lack such a coating. He

therefore suggested that calcite precipitation was caused by a change in micro-environment produced by root decomposition. Partial collapse of decom posing roots followed by calcite precipitation would lead to the irregular structure seen in alveolar texture. Alveolar texture is thus a special type of rhizocretion fabric. Simple rhizocretions as described from the under lying unit, also occur occasionally in this unit. In some cases a trace of alveolar texture can be seen within such rhizocretions. Pelleted calcrete

The pelleted calcrete has a transitional contact with underlying units. In hand specimen it appears as pale grey or cream, structureless fine-grained sedi ment. In thin section a clotted texture is visible resulting from the aggregation of irregular micritic pellets up to 0·3 mm in diameter. Micritized shell fragments and unfilled or spar-filled voids are also present. Calcrete pellets are common components of many Quaternary profiles (Read, 1976). They may have formed by the alteration of skeletal grains during calcrete formation (James, 1972) or as cement 171

A. E. Adams

aggregates (Kiappa, 1978). The pelleted calcrete in the Eyam Limestone is the least common unit of the calcrete profile and where present forms a layer up to 1·5 em thick. Laminar calcrete

The laminar unit of calcrete profiles may not be volumetrically the most important part of the profile but it has generally drawn the most discussion, partly because it is readily visible in field studies. and because of its similarity to stromatolites. In Quater nary calcretes the laminar horizon commonly under lies loose soils (James, 1972; Multer & Hoffmeister, 1968; Read 1974, 1976). In the Eyam Limestone described here, the laminar unit rests sharply on the underlying units. This sharp contact is in contrast to the transitional contacts between other units of the profile. The laminar unit is up to 3 em thick and is a dense pale to dark grey micritic limestone. In quarry exposures the laminar unit can be seen. to be discontinuous, apparently filling slight depressions in the underlying sediment surface. The deposit is finely laminated and the laminae are undulating. As the unit thins laminae are successively cut out giving an impression of uncon formity with overlap (Fig. 6). The laminae average about 0·4 mm in thickness and have slightly un dulating boundaries. As with many Quaternary

calcretes, the laminae .in the calcretes described here are texturally similar and thus much less obvious in thin section than in hand specimen. They are differen tiated by variations in pigment content (Read, 1976). Few workers have considered the origin of laminar calcretes although Klappa (1979) has shown that some laminar calcretes in the Quaternary of the western Mediterranean may form through the activities of successive growths of lichens. In many places alveolar texture is well developed in the laminar unit. Flattened rhizocretions and other spar-filled voids are also present. Klappa (1978) records the presence of alveolar textures within laminar horizons in Quaternary calcretes from the western Mediterranean. Other vadose diagenetic features

Since calcrete profiles form above the water table, diagenetic features such as vadose internal sediments (Dunham, 1969) and dripstone and meniscus ce ments might be expected. Such features are recorded from Quaternary calcretes by Perkins (1977) and Klappa (1978) and from a Jurassic example by Bernoulli & Wagner (1971). It is difficult to demonstrate how much of the cement in the profiles from the Eyam Limestone was precipitated in the vadose zone. Firstly, much of the matrix is micritic sediment and sparry calcite is

Fig. 6. Polished sample of laminated unit showing irregular nature of laminae and microunconformities with overlap. Interpreted as laminar calcrete. Scale bar in em. 172


restricted to sheltered areas such as the undersides of shell fragments. This does not necessarily indicate vadose cementation. Secondly, in grainstones much of the cement is in the form of syntaxial overgrowths on crinoid fragments. Any preferential downwards growth of overgrowths during cementation in the vadose zone may have been obscured by earlier or later periods of cementation in the phreatic zone. In the Eyam Limestone downwards-thickening over growths, if present, are indistinguishable using a petrographic microscope even with stained sections. However, in a few cases there are examples of coarse prismatic calcite cements to be seen on the under surfaces of shell fragments, even where they are overlain by sparry calcite (Fig. 7). Such cements may have formed in the meteoric vadose zone. A possible example of vadose sediment is also illustrated by Fig. 7. Sediment in the cavity was formed after the sparry calcite described above (and so post-dates vadose cementation) and is plastered on the roof of the cavity as well as at the base. Perkins (1977) illustrates a similar feature in vadose-altered Quaternary limestones from Florida. He suggested that the centre space of the void, now occupied by blocky spar, was occupied by an air bubble formed

during desiccation and this caused sediment to be plastered on the roof of the cavity. Discussion

As previously mentioned, many workers have emphasized the importance of recognizing calcretes and distinguishing them from algal stromatolites. It is relevant therefore, to list briefly the criteria which aid identification of the calcrete in this particular example. (1) The presence of features exclusive to calcretes such as rhizocretions and alveolar texture. (2) The presence of distinctive sediment types occurring in a particular order, characteristic of Quaternary calcrete profiles. (3) The presence of 'clotted' textures in the pelle ted horizon, formed by the alteration of pre-existing sediment or by the aggregation of small cement crystals. (4) The nature of the laminar horizon-it possesses many of the characteristics of laminar calcretes as distinct from algal stromatolites as noted by Read (1976), for example, (i) laminations caused by differential pigmentation rather than by

Fig. 7. Photomicrograph showing possible examples of vadose cement and sediment (see text for discussion). Shell fragments are partly silicified. Scale bar represents 0·5 mm.

173

A. E. Adams

significant textural variations, (ii) laminar horizons discontinuous, tending to fill depressions in under lying surface rather than thickening over highs, (iii) presence of microunconformities within laminar zone and (iv) absence of bioturbation. It is also relevant to discuss differences between the profile in the Eyam Limestone and other calcrete profiles described in the literature. The smaller thickness of the profile has already been mentioned. Profile thickness is related, in part, to the length of time available for development, but the ,Processes occurring within calcretes are complex and as yet poorly understood (Kiappa, I978). Climate may also be an important factor. Robbin & Stipp (1979) used radiocarbon dating to determine the age of laminated crusts from Florida Keys. Calculated rates of accumulation varied from I cm/2000 years to I em/ 4000 years. The thickest-known development of the laminar zone in the Eyam Limestone is 3 em perhaps suggesting that subaerial exposure lasted at least 6000 years. Many Quaternary and ancient profiles show inter digitation of sediment types suggesting repeated profile development (Read, I976). The Carbonifer ous profile described here shows no such features and this is perhaps an indication that there was a single fairly short -lived episode of subaerial exposure. Although the distinction made between vadose pisoliths and calcrete pisoliths by Esteban (I976), has meant that pisoliths might not be as diagnostic of calcretes as was once thought, they are nevertheless present in many profiles (James, I972; Read, I974). No such features have been recognized here. In some Quaternary profiles (e.g. Shark Bay, Read, I974) pisoliths occur in loose soils overlying indurated laminar calcretes. Had these been present in the Carboniferous examples they might have been removed by erosion before or during the ensuing marine incursion. The exact sequence of sediment types in both Recent and ancient calcretes is highly varied (Read, I976). The sequence in the Eyam Limestone shows an increasing intensity of alteration towards the top of the profile as would be expected, but the reasons for the order in which the units occur is not clear. Further work on the processes occuring in Recent calcretes is needed.

elusive evidence that theEyamLimestone experienced at least one episode of subaerial exposure during the Lower Carboniferous. Because of the nature of the exposure around Ricklow Quarry it is not Clear whether the original crest of the buildup is now exposed. Thus it cannot be shown whether calcretes formed over the whole buildup or only where they can be seen today, at the base and on the lower flanks of the buildup. However, since the sides of the buildup dip fairly steeply (at up to 30°) it is likely that downslope movement of soils would at least cause thickening of the calcretes on the lower flanks. It is this interpretation which is included on the summary diagram (Fig. 8). At Shark Bay, Western Australia, Read (I974) has described Quaternary soils which are thickest on the flanks of dunes and thin towards dune crests and inter-dune depressions as a result of downslope movements. Since the calcretes in theEyamLimestone occur on the lower flanks of the buildups, and the buildups had a positive relief on the sea-floor during depo sition, the buildups must have stood well above sea A.

B.

Deposition of buildup core and flank facies

Relative drop in sea level, exposure of buildups and development of calcretes

_-

runoff

'""

,,,

C.

Gradual submergence of buildups, deposition of dark, lagoonal

D.

limestones around buildups

Further submergence, re -establishment of normal marine conditions, accumulation of coral and beari limestones

REGIONAL SI GNIFICANCE OF CA LCRETES

Summary diagram illustrating depositional and early diagenetic history of carbonate mud buildups, Eyam Limestone, Lathkill Dale.

Fig. 8.

The calcrete profiles described here present con174


level during formation of the calcretes. (Fig. 8). Today the buildups have a relief of 5-10 m above the base of the surrounding buildup-flank sediments. Even allowing for some differential compaction of buildup-flank and basin-fill facies over build-up-core facies, a fall of at least several metres in the level of the sea relative to the land would have been neces sary. Wilson (1975) in his summary on the origin of Carboniferous carbonate-mud buildups noted that many geologists believe such structures accumulated below wave-base, because of their fine-grained nature and Jack of any wave-washed or sorted talus. Furthermore since calcareous algae are generally absent from such structures, including the build-up core and flank facies of the Eyam Limestone, although abundant in many other Lower Carboni ferous limestones, it is likely that the buildups accumulated below the photic zone. Even making allowance for muddy waters reduc ing the depth of the photic zone, and a sheltered sea in which normal wave-base was high, it seems likely that a fall of sea-level much greater than the minimum of 5-10 m would have been required to bring the base of the buildups above sea-level. Laminated crusts have been described from the older D zone limestones of Derbyshire by Walkden (1974). These crusts, also interpreted as calcretes, are associated with palaeokarstic mammillated surfaces and are overlain by clay beds interpreted by Walkden as weathered volcanic ashes. Palaeokarstic surfaces associated with clay beds have not been recognized in the sections described here. Walkden estimated times of 30,000-100,000 years to be necessary for the development of the karstic surfaces and it is possible that the Eyam Limestone was not exposed sufficiently long for such features to develop. At Ricklow Quarry calcretes may be overlain directly by further lime stones without any noticeable parting, in other words they may occur completely within a unit which might be described as a single 'bed'. In sections where the laminar unit is absent, recognition of the calcrete is not possible in the field; rhizocretions and alveolar texture can only be distinguished by detailed sampling and the examination of acetate peels or thin sections under the microscope. Since such studies are lacking in the Carboniferous Limestone of many areas, it is probable that many similar examples exist elsewhere. Environmental syntheses of the Carboniferous Limestone in Derbyshire are few. An attempt has been made by Ford (1977) and he noted the probable transgression which resulted in the deposition of

progressively deeper-water sediment from the mass ive bioclastic Monsal Dale Beds, envisaged as shallow shelf deposits, through the dark well-bedded Eyam Limestone to the Longstone Mudstone which occurs in the area 5 km to the northeast of Monyash. This follows the typical Lower Carboniferous se quence of deepening water sediments envisaged by Ramsbottom (1973). Ford (1977) noted that the buildups may repre sent regressive phases which may be related to local tectonism contemporaneous with deposition rather than to the cyclic transgression concept for the Lower Carboniferous as a whole proposed by Ramsbottom (1973). Whether the period of subaerial exposure described here is only local or can be traced to other areas remains to be seen and will be difficult to show because of the paucity of exposure. Nevertheless a considerable change in sea-level apparently occurred around Monyash. If local changes of this magnitude are to superimposed on a regional cyclic pattern it will be very difficult to apply the new Lower Carbon iferous stratigraphy, demonstrated in other areas by Ramsbottom (1973), to the Peak District. The limestones immediately overlying the cal crete are thinly bedded dark-coloured limestones with a limited fauna. Such limestones have often been called 'basinal facies' in Derbyshire (see Ford, 1968). However, petrographic studies show that these limestones have a biota of gastropods, foraminifera and calcareous algae. It is suggested here that rather than being 'basinal', these limestones formed in restricted stagnant lagoons around the buildups as they became submerged. Fully marine fossils such as corals and brachiopods do not occur in any numbers until the level of the top of the buildups is reached (Fig. 8).

ACKN OWLE D GMENTS

The author would like to thank Mr G. S. Evans for assistance in the field and laboratory, Professor R. G. C. Bathurst for helpful discussions and Dr F. M. Broadhurst for his encouragement, and for critical comments on the manuscript.

REFERENCES

BERNOULLI, D. & WAGNER, C.W. (1971) Subaerial dia

genesis and fossil caliche deposits in the Calcare Massiccio Formation, Lower Jurassic, Central 175

A. E. Adams

Apennines, Italy. Neues Jb. Palaont. Abh. 138, 135149. BIGGINS, D. (1969) The structure, sedimentology and

KLAPPA, C. F. (1978) Morphology, composition and genesis of Quaternary calcretes from the western Mediter ranean: a petrographic approach. Unpublished Ph.D.

Thesis, University of Liverpool.

palaeoecology of a Carboniferous reef knoll at High Tor, Derbyshire. Unpublished Ph.D. thesis, Uni

KLAPPA, C.F. (1979) Lichen stromatolites: Criterion for

versity of London.

subaerial exposure and a mechanism for the formation of laminar calcretes (caliche). J. sedim. Petrol. 49,

BRAITHWAITE, C.J.R. (1973) Reefs: just a problem of semantics. Bull. Am. Ass. Petrol. Geol. 57,1100--1116. BRAITHWAITE, C.J.R. (1975) Petrology of palaeosols and

387-400. MULTER, H.G. & HOFFMEISTER, J.E. (1968) Subaerial

other terrestrial sediments on Aldabra, western Indian Ocean. Phil. Trans. R. Soc. Ser. B, 273, 1-32. BROWN, M.C. (1973) Limestones of the eastern side of the

laminated crusts of Florida Keys.

Bull. geol. Soc. Am.

79, 183-192.

PERKINS, R.D. (1977) Depositional framework of Pleisto

cene rocks in South Florida. In: Quaternary Sedi mentation in South Florida (Ed. by P. Enos and R. D. Perkins). Mem. geol. Soc. Am. 147, 131-198.

Derbyshire outcrop of the Carboniferous Limestone.

Unpublished Ph.D. Thesis, University of Reading. DuNHAM, R.J. (1962) Classification of carbonate rocks according to depositional texture. In: Classification of Carbonate Rocks (Ed. by W. E. Ham). Mem. Am.

RAMSBOTTOM, 'W.H.C. (1973) Transgressions and regres

sions in the Dinantian: A new synthesis of British Dinantian stratigraphy. Proc. Yorks. geol. Soc. 39,

Ass. Petrol. Geo/. 1, 108-121.

DuNHAM, R.J. (1969) Early vadose silt in Townsend Depositional En

567-607. READ, J.F. (1974) Calcrete deposits and Quaternary

vironments in Carbonate Rocks (Ed. by G. M. Friedman). Spec. Pub/. Soc. econ. Paleont. Miner.,

sediments. Edel Province, Shark Bay, Western Australia. In: Evolution and Diagenesis of Quaternary

Tulsa, 14,139-181.

Carbonate Sequences, Shark Bay, Western Australia. Mem. Am. Ass. Petrol. Geo/. 22, 250-282.

Mound (Reef), New Mexico. lri:

EsTEBAN, C.M. (1974) Caliche textures and Microcodium. Bull. Soc. Geol. It. (sup.) 92, 105-125. EsTEBAN, C.M. (1976) Vadose pisolite and caliche. Bull.

READ, J.F. (1976) Calcretes and their distinction from

Stromatolites. In: Stromatolites (Ed. by M. Walter), pp. 55-71. Elsevier Publishing Co., Amsterdam. REEVES, C.C., Jr (1970) Origin, classification and geologic history of caliche on the southern High Plains, Texas and eastern New Mexico. J. Geo/. 78,352-362. RoBBIN, D.M. & STIPP, J.J. U979) Depositional rate of laminated soilstone crusts, Florida Keys. J. sedim.

Am. Ass. Petrol. Geo/. 60, 2048-2057.

FORD, T.D. (1968) The Carboniferous Limestone. In: The Geology of the East Midlands (Ed. by P. C. Sylvester-Bradley and T. D. Ford), pp. 59-79. Leicester University Press. FoRD, T.D. (Ed.) (1977) Limestones and Caves of the Peak District. Geo. Abstracts, Norwich. HARRISON, R.S. & STEINEN, R.P. (1978) Subaerial crusts, caliche profiles, and breccia horizons. Comparison of some Holocene and Mississippian exposure surfaces, Barbados and Kentucky. Bull. geol. Soc. Am. 89,

385-395. HECKEL, P.H. (1974) Carbonate buildups in the geologic

record: a review. ln: L. F. Laporte). Spec.

Reefs in Time and Space (Ed. by Pub/. Soc. econ. Paleont. Miner.,

Tulsa, 18, 90-154. JAMES, N.P. (1972) Holocene and Pleistocene calcareous

crust (caliche) profiles: criteria for subaerial exposure. J. sedim. Petrol. 42, 817-836.

Petrol. 49, 175-180.

SHIRLEY, J. (1959) The Carboniferous Limestone of the Monyash-Wirksworth area, Derbyshire. Q. J. geol. Soc. Land. 114, 411-429.

WALKDEN, G.M. (1974) Paleokarstic surfaces in Upper Visean (Carboniferous) Limestones of the Derbyshire Block, England. J. sedim. Petrol. 44, 1232-1247. WALLS, R.A., HARRIS, W.B. & NUNAN, W.E. (1975) Calcareous crust (caliche) profiles and early subaerial exposure of Carboniferous carbonates, north eastern Kentucky. Sedimentology, 22, 417-440. WILSON, J.L. (1975) Carbonate Facies in Geologic History. Springer-Verlag, New York.

(Manuscript received 8 October 1979; revision received 20 February 1980)

176

Reprinted from Sedimentology (1983) 30159-179

A rendzina from the Lower Carboniferous of South Wales V. PA U L W R IG H T*

Department of Geology, University College, Cardiff

ABSTRACT

A thin calcrete-crust horizon from the Lower Carboniferous Llanelly Formation of South Wales consists of two parts. an upper laminated unit and a lower peloidal unit. The former is interpreted as a subaerial stromatolite and the latter as an A horizon of a palaeosol. Comparisons are made with the A horizons of rendzinas and it is concluded that the calcrete-crust represents a complete rendzina profile. This fossil rendzina contains abundant evidence of a soil fauna in the form of fecal pellets and small burrows.

GEOLOGICAL SETTING

INTRODUCTION

The calcrete-crust horizon has been named the Darrenfelen Pedoderm (Wright, 1981a). A pedoderm is a mappable palaeosol unit which has characteristics and stratigraphic relationships that permit its recog nition in the field (Brewer, Cook & Speight, 1970, p. I 06). It has been found at five localities in the outcrop area of the Llanelly Formation in South Wales. This formation comprises part of the atten uated Lower Carboniferous succession in the north east part of the South Wales coal field (Fig. lA), which consists in the main of shallow, subtidal and peritidal limestones deposited on the northern (land ward) edge of a carbonate shelf which covered much of South Wales (Wright, Raven & Burchette, 1981). The units comprising the sequence are shown in Fig. I (B). The Llanelly Formation is composed of four distinct members (Fig. l C); the Clydach Halt and Gilwern Clay members are floodplain deposits with sheet-flood, stream-flood and high-sinuosity channel sandstones and conglomerates, and claystones with calcrete profiles (Wright, 1982). The Penllwyn Oolite Member is a thin oolitic unit separated from the underlying Cheltenham Limestone Member by an oncolitic grainstone, the Uraloporella Bed, containing replaced aragonite cements and the problematical tubiform microfossil Uraloporella (Wright, 1981c). The Cheltenham Limestone Member consists of a . series of peloidal limestones of lagoonal to supratidal facies-type deposited as a facies mosaic (Wright,

There are now many descriptions from both recent and ancient carbonate sequences of so-called caliche or calcrete crusts. These horizons develop on subaeri ally exposed carbonate sediments and rocks, and studies of Recent and Pleistocene forms (e.g. Multer & Hoffmeister, 1968; James, 1972; Read, 1974; Harrison, 1977) have led to the recognition of many characteristic features which enable similar crusts to be recognized in ancient sequences (e.g. Walkden, 1974; Walls, Harris & Nunan, 1975; Harrison & Steinen, 1978; Somerville, 1979; Adams, 1980; Riding & Wright, 1981 ; Wright, 198l b). Subaerial crusts form in a variety of ways. Some are purely accretion ary, like a subaerial dripstone, others result from purely pedogenic processes, some from the activ ities of lichens (Kiappa, 1979) while others result from the calcification of algal mats (Krumbein & Giele, 1979). This paper aims to document a variety of features which occur in a 'calcrete crust'-like horizon in the Lower Carboniferous of South Wales, using infor mation from soil microscopy. This horizon contains abundant evidence that a soil fauna was active during its formation.

* Present address: Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, U.K.


177

V. P.

Wright

MILLSTONE GRIT (NAM URIAN)

DOW LAIS

� . .

[)

LIMESTONE

0

• CARBONIFEROUS LIMESTONE

LLANELLY FM.

A 0

/

OOLI TE 0

GROUP

0 LOWER LIMESTONE

GILWER N

SHALES CLAY MR. OLD RED SANDSTONE (DEVONIAN)

.

PENLLW Y N 0

B

OOLITE MR.

Sml

LIMESTONE MR. CLYDACH HALT MR.

c Fig. I. Geological setting of the Darrenfelen Pedoderm. (A) position of outcrop area. (B) major stratigraphic subdivisions in the outcrop area. (C) subdivisions of the Llanelly Formation.

1981a), although occasionally containing fining-up ward shoaling units (Wright, 198l b). The calcrete-crust described herein occurs 0.5-1 m below the Uraloporella Bed and has been found at five localities; at the Graig quarry on the eastern side of Gilwern Hill near Abergavenny (British Grid Refer ence S021, 2475, 1250); at the Clydach Halt Lime Works near Brynmawr (S021, 2342, 1261); at Llanelly Quarry near Brynmawr (S021, 2233, 1237); at Craig y Gaer near Brynmawr (S021, 2232, 1328), and Cwar yr Hendre near Tredegar (SOO I, 0995, 1492). Detailed descriptions of these localities have been given in Wright (198l a).

FIELD APPEARANCE

The calcrete crust appears as a thin (2-12 em) light grey to buff, fine-grained fenestral limestone. It is overlain at all localities by a thin green clay (Wright, 1981b, plate 1a) which at Craig y Gaer contains small ( l -3 em diameter) round and subspherical nodules of fine-grained spar identical to calcrete nodules de scribed from elsewhere in the Llanelly Formation (Wright, 1982). In the field two units can be recognized in the crust; first, there is a thin (under 4 em), finely laminated unit which may occur separately or overlie the second type, which consists of a thicker highly 178

Carboniferous rendzina (Fig. 2A). Some of the latter can be traced horizontally for distances of 30 em. Large spar-filled vertical cracks and subhorizontal clay-filled fractures also occur as well as irregular patches of the underlying grainstone (Fig. 2B). In thin section, the unit is seen to have a very variable fabric but is composed essentially of three components, grains, coatings of various types and fenestrae.

fenestral unit (Fig. 2) containing peloids and larger aggregates. The laminated unit has been figured and described in a earlier paper (Wright, 198lb). These horizons overly a peloidal grainstone, 10-20 em thick and the laminated unit has a sharp boundary where it overlies the peloidal grainstone but the peloidal fenestral unit has a gradational contact. These two horizons will be described separately.

Grains

PELOIDAL FENESTRAL UNIT

In vertically cut slabs, this unit is seen to contain farge numbers of irregular and horizontal tubular fenestrae

There are three distinct major grain types in this unit; large irregular peloids, small, well sorted peloids and peloid aggregates.

Fig. 2. Polished sections through the peloidal layer. (A) is from Llanelly Quarry, showing fenestrae; Note the trains of tubular fenestrae running from left to right across the section. (B) is from Craig y Gaer showing fenestrae and it incorporates darker patches of the associated peloidal grainstone. 179

Fig. 3. (A) Small pellets occurring between the larger peloids; scale bar is I mm long. (B) Concentration of pellets in the upper half of photograph overlying an area of poorly sorted peloids with irregular calcite coatings. Many of the pellets are compacted (welded) together. Scale bar is I mm long. (C) Area of compacted pellets traversed by tubular fenestrae; note the pellets in the fenestra at the top left, and the septa-like structures in some of the fenestrae; scale bar is 2 mm long. (D) Pellet coatings and bridges (arrowed) on larger sediment-grain peloids; scale bar is 0· 5 mm long. (E) Pellet coatings (arrowed) on peloids; scale bar is 0·5 mm long. (F) Tubular fenestra (burrow?) partially filled with pellets; note the irregular domed roof and chamber-like termination. The matrix consists of compacted pellets. Scale bar is I mm long.

Fig. 4. (A) Chamber-like structure within a complex tubular fenestra (see Fig. SF) showing pellets in the roof (arrowed). The

pellets are coated by a fine crust of fibrous calcite. Scale bar is 0·5 mm long. (B) Pellets in the roof (arrowed) of a large chamber like structure in a complex tubular fenestra (see Fig. SF). The pellets also have a fine fibrous crust. Note the sparry haloes around the peloids above the roof (see Fig. 7). Scale bar is 0·5 mm long. (C) Pellet septa (arrowed) in a tubular fenestra; note the sparry haloes around the overlying peloids. The small pellets are packed in between the large peloids. Scale bar is 0·5 mm long. (D) Irregular fenestra containing a pellet arch (arrowed); note geopetal crystal silt.overlying a pellet layer. The matrix consists of compacted pellets. Scale bar is l mm long. (E) Pellet bridge structure; is this a simple pellet bridge similar to those in Fig. 3D or a pellet tube-lining formed by some organism? Scale bar is l mm long. (F) A pellet tube structure. This is not simply a series of pellet bridges connecting peloids but a distinct pellet tube, but was it formed by a root simply pushing pellets aside or by some animal constructing a protective tube? Note the regular sparry halo around the peloid in the upper left quadrant. Scale bar is 0·5 mm long.

V. P.

Wright

Most of the peloidal fenestral unit is a poorly sorted , grainstone containing irregular, subrounded large peloids, 200-1200 11m in diameter, ostracode valves and quartz grains of medium sand size. This material is identical to the underlying peloidal grainstones interpreted as shallow, restricted subtidal deposits (Wright, 198la). The small peloids, which occur in huge numbers, are very well sorted, and consist of well rounded, spherical to ovoid peloids 20-50 11m in size but averaging 40 !J.m. They occur in a variety of distribu tions, e.g. they may occur as fillings between the larger peloids (Fig. 3A) or may make up the whole of the fabric (Fig. 3B) forming a finer, better sorted grain stone. These small peloids may also occur as com pacted masses (Fig. 3C), or may coat the larger peloids and form bridges between them (Fig. 3D, E). They may also occur inside fenestrae, as geopetal fills (Fig. 3F), or in the walls of fenestrae (Fig. 4A, B, C) and sometimes packed between the larger peloids in the walls (Fig. 4C). They also occur as septa-like structures within tubular fenestrae (Fig. 4C) or as arch-like structures within large fenestrae (Fig. 4D) or concentrically arranged in tubes in the intergranular spaces between very large peloids (Fig. 4E, F). These various distributions are shown schematically in Fig. 5. Small peloids are more abundant at the top of the peloidal unit, and sometimes form dense compacted masses (Fig. 3C) in the middle of the unit. These masses appear in hand specimen as grey bands, up to 1 em thick, transected by spar-filled tubular fenestrae. The pellets are less numerous towards the base of the unit where they often occur as coatings and bridges. The third grain type consists of compound grains, up to 5 mm in diameter made up of small peloids (Fig. 6A). These structures have smooth outlines and are not simply aggregates.

the peloidal limestones in the Cheltenham Limestone Member, no peloids have been encountered which show the same degree of sorting, or are the same shape and size as those in the peloidal fenestral unit. They are, therefore, considered to be of pedogenic or biological origin and not primary sedimentary grains. Similar peloids to these small forms have also been described as an important component of Recent calcrete crusts (James, 1972, p. 823 and Harrison, 1977, p. 133), and those described by Harrison are very similar indeed to those described here. Harrison noted the similarities between soil fecal pellets and such peloids but instead interpreted them as small nodules (glaebules of the soil terminology). It is surprising that few of the detailed descriptions of Recent and fossil calcrete crusts mentions fecal pellets, which are a very important component of many other soil types and especially those developed on carbonate parent materials such as rendzina soils (Bridges, 1978). The small peloids described here are identical in size, shape and degree of sorting to the fecal material of the smaller soil animals such as mites, collembolas and some enchytraeid worms, e.g. compare Fig. 3(A, B, F) with those in Babel (1975),

FABRIC TERM

skeletal grains

ooo 00

granular fabric

pellets

dropping fabric

-6 welded agglomeratic fabric

pellets between grains

pellet coats and bridges

coated and linked distribution

�

Interpretation

�0

The larger peloids, ostracode fragments and quartz grains are identical to those in the underlying peloidal grainstones and were presumably derived from them. Accepting that this unit, by virtue of its macro- and microscopic similarities to descriptions of calcrete crusts (see below), is a pedogenic deposit, then the sediment grains would be described as the skeleton grains of a soil (Brewer, 1964). The smaller peloids have no counterpart in the underlying peloidal grainstone, indeed during the examination of hundreds of peels and thin sections of

in tubular fenestrae or as pellet tubes

CD

tubulic distribution

QK;)

8:o Fig. 5. Fabrics and distributions of pellets. See text for

details.

182

Fig. 6. (A) Large compound pellets set in a matrix of smaller pellets. Many of these compound pellets resemble earthworm fecal material. Scale bar is 2 mm long. (B) Peloids (sediment grains) coated by a fine spar; note the meniscus-like thickenings (arrowed). Scale bar is 0·5 mm long. (C) Gravitational'cement' (arrowed) composed of very fine spar; scale bar is 0·5 mm long. (D) Irregular fenestra with a clay-lined bottom (white arrow) overlain by geopetal crystal silt (black arrow). Scale bar is 0·5 mm long. (E) The upper third of the photograph shows a silt-rich clay laminae thought to represent an argillan. This overlies peloids. Scale bar is l mm long.

V. P.

Wright

Bal (1970, 1973) and de Coninck eta/. (1974). Even though the peloids are now composed of micrite, they are interpreted as calcified fecal pellets because of their shape and size similarities to Recent soil fecal pellets, because of their high degree of sorting as compared with associated sedimentary grains and by their presence in the walls of burrow-like fenestrae (see below). These peloids are quite unlike calcrete glaebules which occur in other palaeosols in the Llanelly Formation (Wright, 1982). Fecal pellets are an important component in modern soils and Bal (1973) has provided a useful terminology for such pellets. Using this terminology these Carboniferous pellets would be described as spherical to ellipsoidal, medium fine excrements, and would be said to occur in a heaped distribution (in groups), from single (discrete pellets) to strongly welded (compacted) forms (see Fig. 5). The distribution of these fecal pellets provides additional information of processes which were at work in this soil. Those areas which lack fecal pellets, i.e. composed only of skeleton grains (large peloids), would be said to have a granular soil fabric (Brewer, 1964) (Fig. 5) and form grainstones in the petrographic sense similar to those beneath the peloidal unit. In descriptions of soil fabrics fecal pellets are not usually treated as skeleton grains since they form distinct fabrics, and a variety of specialized terms are available to describe them. The areas which consist only of fecal pellets (Fig. 3B) are said to form a dropping fabric (Babel, 1975) (Fig. 5) or a separated distribution in the sense of de Coninck eta!. (1974, p. 268). The areas where pellets fill the intergranular spaces between large peloids (skeleton grains) (Fig. 3A) are described as having an agglomeratic fabric (Kubiena, 1938, p. 146 and Brewer, 1974, p. 39) (Fig. 5). A coated distribution refers to the skeleton grains coated by pellets (Figs 3D, E and 5) and a linked distribution refers to skeleton grains linked by pellet bridges or braces (Figs 3D, E and 5) (de Coninck eta/., 1974, p. 268). The pellets associated with tubular fenestrae show a tubulic distribution in the sense of Bal (1973) and these are discussed at length below. The dropping fabric, the agglomeratic, coated and linked distributions and the areas of strongly welded pellets all probably owe their origin to concentrations caused by illuviation, the washing down of material in suspension, such that the pellets became mixed with the skeleton grains. Such concentrations in Recent soils have been called mecaconcentrations by Jongerius (1970, p. 320). Some mixing of pellets and

skeleton grains could also have been caused by faunal activity or by the churning of the horizon caused by shrink-swell cycles. By analogy with Recent soils the: pellets were probably produced in the upper, organic rich, part of the soil and were washed down. Such processes are well documented in Recent soils (e.g. Babel, 1975, p. 429; Bal, 1970, Jongerius & Schelling, 1960). The experiments of Wright & Foss (1968) have proved that silt-sized particles (and presumably pellets) up to 50 �m in diameter, are easily moved down through sand by flowing water. Evidence for the action of this process in the peloidal fenestral unit is clearly seen in the partial geopetal fills of many fenestrae (Fig. 3F) and by the overall decrease in the amount of pellets down through the unit. The high concentrations which occur locally in the middle of the horizon probably reflect areas where permeability was reduced resulting in a change in flow velocity and the deposition of the pellet load. Such a reduction in permeability in soils is usually caused by a change in grain size or packing or by the presence of grain coats. Bal (1970, p. 20) describes similar pellet concentrations in Recent soils. The aggregations of pellets to form dense welded (com pacted) masses is a common feature of fecal pellet rich soils today, and such compaction may result from pedoturbations (Jongerius, 1970) caused by faunal or root activity or by shrink-swell cycles. The most common cause, is however, the decay of the pellets, (Bal, 1970, p. 28; Bal, 1973; Bjorkhem & Jongerius, 1974; de Coninck eta/., 1974, p. 270; Jongerius & Schelling, 1960, p. 703). The pellets, as mentioned above, are similar in shape and size to those of the smaller soil arthropods such as the mites, collembolas and some enchytraeids (see also discussion on tubular fenestrae below). The: determination of the composition of soil faunas from their pellets is a difficult task and there are a number of pitfalls. First, pellets are not diagnostic of particular soil organisms for taxonomically different animal groups can produce very similar fecal pellets (Babel, 1975, p. 422); secondly, the size of a pellet is not always a reliable guide to the size of the animal which produced it because pellet size varies with growth stage of the animal (Bal, 1973, p. 64); thirdly the same: animal can produce markedly different pellets de-· pending on the type of vegetation it is feeding on (Bal, 1970; van der Drift, 1964, p. 79); lastly, pellets can shrink considerably on drying with a consequent change in shape (Bal, 1973, p. 65). Subsequent diagenetic changes such as calcification (see below) may also have affected both shape and size. Despite: 184

Carboniferous rendzina these problems, the pellets in the Darrenfelen Pedod erm are remarkable for the uniformity of shape and size which suggests a probable lack of diagenetic deformation, and that the fauna was probably of low diversity. The larger pellet aggregates which also occur in this unit are very similar to the larger fecal pellets in Recent soils (cf. Babel, 1975 and Bal, 1973). The smooth edges of these larger pellets argues against them being simple mechanical soil aggregates (crumbs). What inferences can be drawn about the productiv ity of the fauna from the pellet evidence? The huge numbers of pellets present might indicate that the fauna was very abundant or that it was active for a long period. In Recent soils, pellets can accumulate in considerable quantities where there is an absence of organisms ingesting them, e.g. earthworms normally consume the smaller fecal pellets to 'form larger aggregates rather like those described above (Babel, 1975, p. 428). Thus, pellets in a soil where secondary ingestors are absent have a higher preservation potential than those in other soils. Other factors can influence preservation potential, and the preservation potential of pellets is enhanced in dry conditions (Fitzpatrick, 1971, p. 223), the existence of which is proved by the occurrence of calcretes and evaporites in the Llanelly Formation (Wright, 1981a). The early calcification of these pellets would also have enhanced their chances of preservation (see below). Thin section examination of the pellets has only revealed a very fine micritic microstructure and no recognizable organic structures have been seen. The pellets would originally have been made largely or wholly of organic matter and their preservation suggests early calcification. This calcification is probably analogous to the calcification of fecal pellets in Recent carbonate environments, and although this calcification is common, it is not well understood (Bathurst, 1975, p. 364) but it may be related to bacterial decay. The early calcification of fecal pellets has been noted in Recent caliches in Spain by Klappa (1978, p. 189).

L. ( 1970) Morphological investigation and the role of the soil fauna in their genesis. Geoderma, 4, 5-36. BAL, L. (1973) Micromorphological analysis of soils. Lower levels in the organization of organic soil materials. Soil Surv. Pap. 6. Netherlands Soil Survey Institute, Wagen ingen, 174 pp. BATHURST, R.G.C. (1966) Boring algae, micrite envelopes and the Lithification of molluscan biosparites. Geol. J. 5, 15-32. BATHURST, R.G.C. (1975) Carbonate Sediments and their Diagenesis. Elsevier, Amsterdam. 658 pp. BJORKHEM, U. & JONGERJUS, A. (1974) Micromorphological observations in some podzolised soils from central Sweden. In: Soil Microscopy (Ed. by G. Rutherford), pp. 3 2Q-33 2. Limestone Press, Kingston, Ontario. BRAITHWAITE, C.J.R. (1975) Petrology of palaeosols and other terrestrial sediments on Aldabra, western Indian Ocean. Phil. Trans. R. Soc. B, 273, 1-3 2. BREWER, R. (1964) Fabric and Mineral Analysis of Soils. Wiley, New York. 470 pp. BREWER, R. (1974) Some considerations of micromorpho logical terminology. In: Soil Microscopy (Ed. by G. Rutherford), pp. 28-48. Limestone Press, Kingston, Ontario. BREWER, R.,COOK, K.A.W. &SPEIGHT,J.G. (1970) Proposal for soil stratigraphic units in the Australian Stratigraphic Code. J. geol. Soc. Aus. 17, 103-109. BREWER, R. & HALDANE, A.D. (1957) Preliminary experi ments in the development of clay orientation in soils. Soil Sci. 84, 301-308. BRIDGES, E.M. (1978) World Soils. Cambridge University Press. 128 pp. BULLOCK, P. & MACKNEY,.D. (1970) Micromorphology of strata in the Boyn Hill Terrace Deposits, Buckingham shire. In: Micromorphological Techniques and Applications (ED. by D. A. Osmond and P. Bullock). Soil Surv. Tech. Monogr. 2, 97-106. BUURMAN, P. (1980) Palaeosols in the Reading Beds (Paleocene) of Alum Bay, Isle of Wight, U.K. Sediment ology, 27, 593-606. DE CONINCK, F., RIGHI, D., MAUCORPS, J. & ROBIN, A.M. (1974) Origin and micromorphological nomenclature of organic matter in sandy spodosols. In: Soil Microscopy (Ed. by G. Rutherford), pp. 263-280. Limestone Press, Kingston, Ontario. DuNHAM, R.J. (1971) Meniscus cement. In: Carbonate Cements. John Hopkins Studies in Geology No. 19 (Ed. by 0. P. Bricker), pp. 297-300. Baltimore. ESTEBAN, M. (1974) Caliche textures and Microcodium. Suppl. Boll. Soc. geol. ita/. 92, 105-125. EsTEBAN, C.M. (1976) Vadose pisolite and caliche. Bull. Am. Ass. Petrol. Geol. 60, 2048-2057. FITZPATRICK, E. A. (1971) Pedology: a systematic approach to soil science. Oliver & Boyd, Edinburgh. 306 pp. FLETCHER, J.E. & MARTIN, P.W. (1948) Some effects of algae and moulds in the rain-crust of desert soils. Ecology, 29, 95-100. FRIEDMANN, l., LIPKIN, Y. &0CAMPO-PAUS, R. (1967) Desert algae of the Negev (Israel). Phyco/ogia, 6, 185-200. GOLUBIC, S. & CAMPBELL, S.E. (1979) Analogous microbial forms in Recent subaerial habitats and in Precambrian charts. Gloethece coerulea Geitler and Eosynechococcus moorei Hofmann. Precamb. Res. 8, 201-217. GROVER, G.M. & READ, J.F. (1978) Fenestral and associated

BAL,

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Wright (I 974) Calcrete deposits and Quaternary sedi ments. Edel Province, Shark Bay, Western Australia. In:

vadose diagenetic fabrics of tidal flat carbonates, Middle Ordovician New Market Limestone, south western Vir ginia. J. sedim. Petrol. 48, 453-473. HARRISON R.S. (1977) Caliche profiles: indicators of near surface subaerial diagenesis, Barbados, West Indies. Bull. Can. Petrol. Geol. 25, 123-173. HARRISON, R.S. & STEINEN, R.P. (I 978) Subaerial crusts, caliche profiles, and breccia horizons. Comparison of some Holocene and Mississippian exposure surfaces, Barbados and Kentucky. Bull. geol. Soc. Am. 89, 385-395. JAMES, N.P. (1972) Holocene and Pleistocene calcareous crust (caliche) profiles: criteria for subaerial exposure. J. sedim. Petrol. 42, 817-836. JONGERIUS, A. (1970) Some morphological aspects of regrouping phenomena in Dutch soils. Geoderma, 4, 311331. JONGERIUS, A. & SCHELLING, J. (1960) Micromorphology of organic matter formed under the influence of soil organ isms, especially soil fauna. 7th int. Congress of Soil Science, vol. 2. Madison, Wisconsin, U.S.A. KLAPPA, C.F. (I 978) Morphology, composition and genesis of

READ, J. F.

Evolution and Diagenesis of Quaternary Carbonate Se quences, Shark Bay, Western Australia. Mem. Am. Ass. Petrol. Geo/. 22, 250-282.

R & WRIGHT, V.P. (1981) Palaeosols and tidal flat/lagoon Sequences on a Carboniferous carbonate shelf: sedimentary associations of triple disconformities. J. sedim. Petrol. 51, 1323-1339. RIGHI, D. & DE CONINCK, F. (1974) Micromorphological aspects of Humods and Haplaquods of the Landes du Medoc, France. In: Soil Microscopy (Ed. by G. Ruther ford), pp. 567-588. Limestone Press, Kingston, Ontario. ROLFE, W. D.l. (1980) Early invertebrate terrestrial faunas. In: The Terrestrial Environment and the Origin of Land Vertebrates (Ed. by A. L. Panchen) Systematics Association (Special Volume), IS, 117-157. Academic Press, London. SIMONSON, R.R. (1978) A multiple-process model of soil genesis. In: Quaternary Soils (Ed. by W. C. Mahoney), pp. 2-25. Geo. Abstracts, Norwich. SOIL SURVEY STAFF (1975) Soil Taxonomy, Agricultural Handbook 436. USDA, Washington. SOMERVILLE, l.D. (1979) A cyclicity in the early Brigantian (D2) Limestones east of the Clwydian Range, North Wales and its use in correlation. Geol. J. 14, 69-86. TERUGGI, M.E. & ANDREIS, R.R. (I 971) Micromorphological recognition of paleosolic features in sediments and sedimentary rocks. In: Paleopedology (Ed. by D. H. Yaalon), pp. 161-171. International Society of Soil Sciences and Israel University Press. VAN DER DRIFT, J. (I 964) Soil fauna and soil profile in some inland-dune habitats. In: Soil Micromorphology (Ed. by A. Jongerius), pp. 69-81. Elsevier, Amsterdam. WALKDEN, G.M. (I 974) Paleokarstic surfaces in the Upper Visean (Carboniferous) Limestone of the Derbyshire Block, England. J. sedim. Petrol. 44, 1232-1247. WALLS, R.A. HARRIS, W.B. & NUNAN, W.E. (1975) Calcareous crust (caliche) profiles and early subaerial exposure of Carboniferous carbonates, north eastern Kentucky. Sedimentology, 22, 417-440. WALLWORK, J.A. (I 970) The Ecology of Soil Animals. McGraw-Hill, New York. RIDING,

Quaternary calcretes from the Western Mediterranean: a petrographic approach. Unpublished Ph.D. Thesis. Uni

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Carboniferous rendzina Lower Carboniferous of South Wales. Geol. Mag. ll8, 97100. WRIGHT, V.P. (198lc) Algal aragonite-encrusted pisoids from a Lower Carboniferous schizohaline lagoon. J. sedim. Petrol. 51, 479-489. WRIGHT, V.P. (1982) Calcrete palaeosols from the Lower Carboniferous, Llanelly Formation, South Wales, Sedim. Geol. 33, 1-33. (Manuscript received

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26 February 1982; revision received 28 June 1982)

187


The role of fungal biomineralization in the formation of Early Carboniferous soil fabrics V. PAUL WRIGHT Department of Geology, Wills Memorial Building, University of Bristol, Queen's Road, Bristol BS8 IRJ

ABSTRACT

Paleosols in the Lower Carboniferous limestones of South Wales commonly contain needle-fibre calcite which is an unusual form of calcite recently shown to form by the calcification of fungal hyphae in present day soils. The needle-fibre calcite occurs in two associations in the paleosols: as coatings on sediment grains and as rhizocretions. The former can be compared with the microbial grain coatings of Quaternary calcretes. The latter represent the sites of fungal coats on roots and are interpreted as probable ectomycorrhizae, a symbiotic fungal sheath-root association. These findings suggest that biomineralization was important in the formation of soil fabrics during the Carboniferous as it is in present day soils.

INTRODUCTION

pseudomycelia in the soil science literature (e.g.

Calcretes have now been widely documented in

regarded, but never proved until recently, to be of

ancient sedimentary sequences and have proved useful

fungal origin. The origins of this form of calcite have

Kubiena,

1938; Fitzpatrick, 1984). They have been

for a variety of interpretive purposes. Studies of

been discussed by many workers and were reviewed

Quaternary soil carbonates have shown the impor

in Wright

tance of biomineralization in the formation of many

these elongate crystals are analogous to 'whisker'

( 1984). Some authors have speculated that

crystals formed by growth during extremely high

calcretes (Callot, Guyon & Mousain, 1985; Calvet, 1982; Ca1vet & Julia, 1983; Calvet, Pomar & Esteban, 1975; Esteban, 1974; Kahle, 1977; Klappa, 1978, 1979, 1980; Krumbein & Giele, 1979; Ward 1975, 1978). During a study of subaerial exposure surfaces

However, the illustration provided by these authors

within the Lower Carboniferous (Mississippian) of

more closely resembles lublinite, a bizarre form of

South Wales, a variety of biogenic calcrete fabrics

calcite consisting of stacked, en echelon flattened

degrees of supersaturation caused by rapid degassing of C02 and/or evaporation. This view has most recently been repeated by Given & Wilkinson

(1985).

have been found, including abundant calcified fungal

rhombs (Stoops,

hyphae. The aim of this paper is to describe the

has stressed the strong association between needle

1976; Ward, 1978). Wright ( 1984)

distribution of these fungal fabrics and to compare

fibre calcite and micro-organisms, especially fungi.

them with their Quaternary counterparts.

This view was also reached by Harrison

( 1977) and (1978) during detailed studies of Quaternary calcretes. Recently Chafetz, Wilkinson & Love (1985) Ward

have also suggested a possible biogenic influence.

NEEDLE FIBRE CALCITE

The most significant contribution to this problem Needle-fibre calcite consists of extremely elongate

has come from Callot, Guyon & Mousain

needles of low-magnesium calcite. The needles are

carefully documented the formation of needle-fibre

typically a few microns wide and up to several hundred

calcite by the calcification of soil fungal hyphae of the

( 1985), who

microns long. It is an unusual habit of calcite yet is

Basidiomycetes. A number of authors have noted that

very common in soils. Aggregates of needles resemble

fungi have the ability to concentrate various ions,

fungal mycelia and have long been referred to as

including calcium (Sihanonth & Todd,


189

1977), which

V. Paul Wright

can result in either the formation of CaC03 in the outer walls of hyphae (Callot, Mousain & Plassard, 1985) or in the formation of calcium oxalate which is easily broken down by bacteria allowing calcium to form bicarbonate and carbonates (Cromack et a!., 1977). The evidence presented by Callot, Guyon & Mousain ( 1985) shows that at least some forms of needle-fibre calcite are organically precipitated. While it cannot be ruled out that needle-fibre calcite has more than one origin most inorganically precipitated calcite in soils has a polyhedral micritic or more rarely a rhombic habit (Folk, 1974). Claims that needle-fibre calcite forms in abiotic settings by inorganic precipi tation have never been substantiated by experimen tation. The origin of lublinite, however, remains uncertain. Based on the above, the presence of needle-fibre calcite is taken as reliable evidence that fungi were once present in a soil or paleosol. However, such observations can be taken a stage further when the actual distribution of the fungi (needle-fibre calcite) is studied.

by exposure surfaces. The material described here comes from the Heatherslade Geosol, a paleosol which occurs at the top of the Chadian Gully (or Caswell Bay) Oolite in South Wales (Fig. I) (Wright, 1984). The top of the oolite is an irregular surface veneered by a calcrete crust in the Gower (Wright, 1982, 1984) but is overlain by a thick calcretised regolith and petrocalcic horizon as at Miskin near Cardiff (Riding & Wright, 1981) (Fig. 2). The needle-fibre calcite occurs in these paleosols in two associations: (i) within irregular micritic grain coatings on ooids and (ii) composing the walls of 'alveolar textures' (in the sense of Esteban, 1974), that is, elongate tubules with irregularly curved partitions (septae).

GRAIN COATS

The calcrete crust in the Gower and the regolith lithoclasts at Miskin both contain abundant micritic coats on grains. The individual grains (ooids) are coated by a single layer of dark micrite up to 500 llm in thickness. These coats are typically irregular in thickness and commonly display irregular protuber ances including elongate branched filaments, which average 27!lm wide (range 10-60 !lm) and up to several hundred microns long, which may form bridges between grains (Figs. 3, 4). Under the SEM, poorly preserved needles of calcite occur in parallel sets (Fig. 4F). The needles are similar in size to those in the tubular structures described

GEOLOGICAL SETTING

Subaerial exposure surfaces represented by palaeo karsts and paleosols are common in the Lower Carboniferous of South Wales. The sequence repre sents deposition on a carbonate ramp (Wright, 1986) and contains a number of oolitic sandbodies capped

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limestones, occur both between them and also beneath the lower one. The upper, thicker, porous marine limestone bed, with its laminated calcret e, is overlain by a fragmented white micritic limestone with freshwater gastropods. Both porous beds and their laminar calcretes are laterally discontinuous and the upper bed, in particular, is reduced to a mass of large cobble to boulder-sized, subrounded to rounded, spherical to oblate clasts for tens of metres along strike. These relationships suggest phases of in situ reworking during deposition of the unit, reminiscent of regolith horizons. A similar series of deposits has been described by Perkins (1977) from the Pleistocene of Florida. These cla�ts, and the parent rock from which they were derived, are calcite cemented, bioclastic, peloi-

dal, medium to coarsely grained sandstones and

limestones. The matrix around the clasts is a calcareous

clay-rich sand. This study concentrates on the lower much thicker

laminar calcrete. Thin sections of the sandstone clasts show a variety of calcrete textures such as floating sediment grains set in a dense micritic matrix passing

down into a grain-supported fabric in which the grains

exhibit irregular, non-laminated micritic coatings up to a few hundred microns thick. Some small (average I mm diameter), subspherical nodules with sharply defined margins occur, containing several quartz grains surrounded by micrite. Shell moulds are abundant and are only partially occluded by finely crystalline bladed calcite cement. The micrite matrix also exhibits sub-millimetre wide irregular cracks, 248

Biogenic laminar calcretes

downward into the substrate as sub-horizontal to sub vertical, irregular (wavy) dark, micritic stringers. These are up to several centimetres in length and averaging 1 mm in width (Fig. 5). Internally these stringers have the same microstructure as the laminar calcrete. Thin sections of the calcrete are strikingly like those at Miskin in exhibiting small tubiform cylinders (here as open pores) with micritic and microspar micro laminar walls (Fig. 6a). The only differences with this horizon and that at Miskin are that alveolar-septal structures are more common, and that dense, micritic laminae occur. These latter occur both within the · main part of the horizon, separating irregular cylin drical-micro-laminar layers, and near its top where they are more common and several millimetres thick (Fig. 6b). These thicker, dense zones have very few pores, exhibit a very fine wavy lamination and include layers of finely crystalline inclusion-rich, brown bladed calcite (Fig. 6b).

including circumgranular cracks. Bifurcating tubular pores are also common, up to 500 f.!m in diameter, and locally showing alveolar-septal structure. This suite of fabrics shows that the unit underlying the laminar calcrete was modified by pedogenic processes. The laminar calcrete itself is up to 50 mm thick (Fig. 5) but it thins over projections on the substrate to 15 mm. It is light buff-beige in colour and contains fragments of the underlying lithology up to 10 mm in diameter. The laminae, which have relief of one to two millimetres, are of two types: dark laminae, 1 mm or less in thickness which are traceable for several centimetres, separated by lighter coloured, highly contorted laminae. The waviness and contorted appearance is due to an abundam:e of small tubiform pores (under 1 mm diameter) occurring within the lighter coloured layers. The pores themselves are traceable laterally for only one or two millimetres. The base of the horizon is sharp and shows no evidence of previous endolithic activity at the contact. However, from the base there are extensions passing

Laminar calcretes from the Upper Jurassic-Lower Cretaceous of the Cameros Basin, northern Spain

Similar calcretes (Fig. 9b) occur in the Upper Jurassic Lower Cretaceous Tierra de Lara Group (Group I of Salomon, 1982) of the western Cameros Basin of northern Spain (Platt, 1986) (Fig. 7). They occur in both siliciclastic (Sefiora de Brezales Formation) and carbonate-dominated sequences (Rupelo Formation) (Fig. 7), and four different facies associations have been recognized (Fig. 8). 1 Conglomerate/sandstone association (Fig. 8.1) Laminar calcretes occur in the alluvial Sefiora de Brezales Formation both within sandstones and capping channelized conglomerates. These clastic sediments have been interpreted as sandflat and streamflood deposits, respectively (Platt, 1986). The laminar calcretes which are 0·05-0·20 m thick, form laterally discontinuous, undulating horizons traceable for up to 5 m. They either cap the conglomerates, suggesting that their formation took place after the abandonment of the channels, or occur within the sandstone units where they are associated with cross cutting, 1-2 mm diameter, carbonate stringers inter preted as fine-scale rhizocretions (Platt, 1986) (Fig. 8).

Fig. S. Polished sample of the laminar calcrete from the Town Gardens Member. Note the biomoulds in the calcretized sandstone beneath the horizon. A small micrite stringer is arrowed. The mottled appearance of the sandstone is due to differential cementation by pedogenic micrite which occurs as grain coats.

2 Marl/paleosol association (Fig. 8.2) Laminar calcretes occur in both the upper part of the Sefiora de Brezales Formation and in the basal Rupelo 249

V. P. Wright, N. H. Platt and W. A. Wimbledon

Fig. 6. Photomicrogr.aphs of the Town Gardens Member laminar calcrete. (a) porous, micro-laminar microfabric showing fine

tubular and larger irregular pores (some with a crude septal structure resembling alveolar-septal structure) set in a micro laminar matrix). Scale bar represents 0·5 mm. (b) dense micritic laminae forming layers several millimetres thick. Scale bar represents 0·5 mm.

250

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Cathodoluminescence and calcrete diagenesis ((CS); Table I, column 2) may have been precipitated in the meteoric phreatic zone before subaerial expo sure, cannot be completely discounted. In contrast, the light brown-coloured spar (B; Table I, column 2) is demonstrably a subaerial vadose cement, having been precipitated concurrently with subaerial breccia tion (demonstrated below) and being absent below the I m deep calcrete profile. First (major) bright orange luminescent cement (2) (Table I, column I) This distinctive 'marker' cement within the cathodol uminescence cement stratigraphy coats earlier non luminescent cements as a thin layer (Fig. 5B). Its boundaries are sharply defined and pitted. This brightly luminescent cement is believed to have formed under the influence of subaerial diagenetic conditions because of its association with needle-fibre calcite (discussed later). Non-luminescent cements with brightly luminescent pitted subzones andfinal pore-filling medium luminescent cements with smooth subzones (3) and (4) (Table I, column I) I n transmitted light this sequence of pore-filling cements is characterized by clear inequant blocky crystals which increase in size dramatically towards pore centres (Fig. 5A and Table I, column 2). On the basis of transmitted light petrography and cathodoluminescence subzone morphology (after cri teria of Meyers, 1974), cements (3) and (4) (Fig. 5 and Table I, column I) are interpreted to have been precipitated during meteoric p hreatic diagenesis. This finding is in accordance with earlier petrographic and geochemical work of Walkden & Davies ( 1983) and Walkden & Berry ( 1984) who described a closely similar sequence of meteoric phreatic cements (their zones 1-3) from Asbian successions in Anglesey and Derbyshire ·respectively. Meteoric phreatic cements form an estimated 70-85% of pore-fill in lithologies of the calcrete profile. Relationship of cementation to calcrete fabric formation

By combining transmitted light and cathodolumines cence petrography, the timing of cement precipitation in the calcrete profile can be related to the evolution of fabrics which formed in response to other subaerial diagenetic processes.

Subaerial brecciation Cross-cutting relationships (Fig. 6A, B) show that clear, prismatic, non-luminescent spar cement pre dates brecciation. Brown-stained inclusion rich spar cements are both cross-cut by brecciation fractures and form the first cement to coat brecciation fracture walls. This shows that subaerial brecciation occurred during the precipitation of brown stained non luminescent cements (Table 1, column 3). Younger generations of cement are u naffected by subaerial brecciation. Calcification ofplant roots Both subaerial cements and micrites define calcified plant root tubules indicating that plant root growth/ decay occurred concurrently during early cementation and subaerial micritization (Table 1, column 3). Subaerial micritization The exact relationship between precipitation of non luminescent cements and the onset of subaerial micritization is unclear. I f ' floating' grain fabrics (Fig. 3) are formed by aggradational micritization (consid ered earlier), this may indicate that the non-lumines cent cements (I) (Table 1, column 1) were precipitated before or concurrently with the onset of subaerial micritization. It will be shown that subaerial micriti zation occurred both before and after a major dissolution event (Dissolution Hiatus; Table I) and possibly continued after the precipitation of needle fibre calcite. These observations and interpretations show that the non-luminescent cements (I) (Table 1, column 1) were precipitated concurrently with subaerial breccia tion, micritization and the formation of calcified plant root tubules. This implies that the non-luminescent cements formed during subaerial vadose diagenesis.

RECURRING DISSOLUTION

Textures visible only with cathodoluminescence pro vide evidence that dissolution occurred repeatedly during subaerial vadose diagenesis. Dissolution sur faces cut both cements and allochems and are coated by a bright orange luminescent phase which marks their presence. In Fig. 6, dissolution surfaces are visible in the coral wall structure (Fig. 6B, b) and along the inter-crystalline bou ndaries of non-lumines cent subaerial vadose cements (Fig. 6B, a). 309

S. T. Solomon and G. M. Walkden

Fig. 6. (A) Thin section, plane polarized light: a coral fragment in the calcrete profile cut by a subaerial brecciation fracture

(f) filled by spar cements. A: unbrecciated intra-particle pore, B: brecciated intra-particle pore, C: coral wall. (B) Under cathodoluminescence; dissolution textures formed during subaerial diagenesis. l , 3, 4: pore-filling cements (as Fig. 5B) a, b, c: locations referred to in the text. Field of view: l· 35 mm.

Only the non-luminescent subaerial vadose cements display dissolution textures of this type, demonstrating that dissolution occurred before the onset of meteoric phreatic cementation.

bright luminescent phase defining them. In brecciated pores (Fig. 6B, pore B) the bright orange luminescent phase not only 'invades' intercrystalline boundaries (Fig. 6B, a) but also forms a thin, even coating on the terminations of the non-luminescent subaerial vadose cements (Fig. 6B, c). In adjacent unbrecciated pores showing the same cement stratigraphy (Fig. 6B, pore A and Fig. 7); again, this first bright luminescent

Origin of the textures

The process responsible for forming the textures can be determined by examining the characteristics of the 310

Cathodoluminescence and calcrete diagenesis 1). This petrographic observation is of major impor tance since it establishes that the bright luminescent phase is a cement precipitate. This implies that the textures were produced by erosion/degradation of substrates which occurred before precipitation of the first bright orange luminescent cement. The brightly luminescent cement merely acts as a passive marker highlighting the morphology of the solution corroded substrate. We consider dissolution by calcium carbon ate undersaturated pore fluids to be the most likely cause of substrate erosion. The texture seen with cathodoluminescence varies with substrate type. Three major textures are visible within the calcrete profile defined by the first bright orange luminescent cement filling the following substrates:

(I) Dissolution ofnon-luminescent subaerial vadose cements

Fig. 7. Thin section, cathodoluminescence; an unbrecciated

intraparticle pore in the same coral as Fig. 6(B). The same sequence of spar cements fill the pore but dissolution textures are absent. Field of view: 0· 7 mm.

Comparison of transmitted light and cathodolumi nescence views of Fig. 6 shows that the bright orange luminescent cement has filled space opened along intercrystalline boundaries; the prismatic crystalline morphology of the non-luminescent cement control ling the appearance of the texture seen with cathodo luminescence. Laminar pores are believed to have been created

phase coats the non-luminescent subaerial vadose cements, but does not 'invade' intercrystalline bound aries. This first bright orange luminescent phase occurring in both pores is an identifiable part of the cathodoluminescence cement stratigraphy; being the first brightly luminescent cement (2) (Table I, column

Fig. 8. Thin section, cathodoluminescence, a dissolution texture developed in the wall structure of a cora]· to a limited depth from a circumgranular brecciation fracture (f). A: unbrecciated pore showing no associated dissolution textures. Field of view: 2· 3mm.

311

S. T. Solomon and G. M. Walkden along intercrystalline boundaries which represent more favourable sites for dissolution (higher free energy) than the non-luminescent calcite crystals. (2) Dissolution offibrous calcite walls ofmarine allochems The bright luminescent cement also fills rugose coral wall ultrastructure (Figs 6B and 8). This texture, seen with cathodoluminescence, is closely comparable with the rugose coral wall ultras�ructure figured by Sand berg (1975) (Fig. 9). It seems likely that this texture

Fig. 9. Sandberg (1975, plate 4-S.E.M; rugose coral wall ultrastructure). We thank Dr P. Sandberg for kindly permitting us to use this photomicrograph. Fig. 10. Thin section, cathodoluminescence; dissolution

was formed by dissolution along the inter-crystalline boundaries of calcite fibres which constitute the coral wall ultrastructure. The resulting laminar pores would have been filled by bright orange luminescent cement producing the texture shown in Fig. 8. It is expected that brachiopods and other fibrous walled allochems, similarly affected by dissolution and filled by brightly luminescent cement, will show a texture reflecting the arrangement of calcite fibres in their wall ultrastru c ture.

textures developed in matrix micrite and fibrous calcite walled allochems immediately adjacent to a (now) spar cement filled subaerial brecciation fracture. Field of view: l-4mm.

probably because of the microcrystalline size of the substrates. The examples cited above show that the morphology/structure of the partially dissolved sub strate controls the appearance of the texture seen with cathodoluminescence. The brightly luminescent ce ment defining the textures only penetrates a limited distance into the substrate from the palaeo-pore surface (e.g. Fig. 10). This is interpreted to indicate that dissolution acted at pore-substrate interfaces and selectively dissolved underlying substrates to a limited depth. Dissolution textures only occur adjacent to subaerial

(3) Dissolution of micrite substrates (including the products of both marine and subaerial micritization) I nfilling brightly luminescent cement defines a fine random anastomosing texture (Fig. 10). Even at high magnification the resolution of this texture is poor,

312

Cathodoluminescence and calcrete diagenesis brecciation fractures and in the larger inter-, intrapar ticle and mouldic pores within the calcrete profile. The genetic implications of these observations are considered below. A model for dissolution during subaerial diagenesis

It is proposed that extensive fabric dissolution and weakening occurred during subaerial vadose dia genesis. This was probably caused by strongly under saturated (with respect to CaC03) fluids travelling through the developing calcrete profile. These fluids (percolating rainwaters?) moved through larger pores and along subaerial brecciation fractures which may have acted as fluid pathways within the relatively impermeable calcrete (which was already partially cemented and micritized). Progressive dissolution of substrates occurred to a limited depth in the walls of pores through which the dissolving fluids travelled. The solution-etched surfaces of substrates were sub sequently filled by a bright orange luminescent cement producing the characteristic textures described. Pores containing non-luminescent cements or other sub strates which show no evidence of dissolution are believed to have been closed or simply by-passed during this stage of d iagenesis (e.g. Fig. 6B, pore A and Fig. 7). Fig. 11. Thin section, cathodoluminescence; cemented in Recurring dissolution

terparticle pore in calcrete profile. Brightly luminescent cement subzones (a and b) define two dissolution surfaces which pre-date the major dissolution event defined by textures associated with first major bright luminescent subzones. Field llf view: 0· 57 mm.

So far, only one major d issolution hiatus has been discussed. However, some lithologies within the calcrete profile show evidence of two earlier dissolu tion hiatuses which are recognizable by the same cathodoluminescence textures, but defined by rela tively dull orange luminescent cements (Fig. 11; subzones a and b). This proves that fabric dissolution was a recurring process during the precipitation of non-luminescent subaerial vadose cements.

ination of these micrite fabrics with cathodolumines cence has revealed that many have an acicular cathodoluminescence signature. In particular, the micritic pore-lining and pore-bridging fabrics appear as a dense mesh of luminescent calcite needles up to 50 11m long and 5J.!m wide (Fig. 12). Small calcite needle-fibres are occasionally visible in transmitted light, but their petrographic characteristics are best studied with cathodoluminescence.

SUBAERIAL MICRITIZATION AN D TH E FORMATION OF N E E DL E-FIBR E CALCIT E

Much of the original marine matrix of lithologies in the calcrete profile has been subaerially altered to micrite which displays a variety of characteristic fabrics (already described). Normally, the term micrite (Folk, 1959) is used to describe calcite crystals 1-4 Jlm in diameter which typically appear brown and are poorly resolved in transmitted light. However, exam-

Cathodoluminescence petrography of needle-fibre calcite

Needle-fibres are straight, unbranched and usually form steep sided rhombs or, more rarely, rods with blunt prismatic terminations. They have variable packing density and are commonly arranged in one of the following textures:

313

S. T. Solomon and G. M. Walkden

Fig. 12. (A) Thin section, plane polarized light; subaerial 'micrite' from the calcrete profile showing arcuate pore bridging structure. (B) Under cathodoluminescence; subaerial 'micrite' appears as dense mesh of brightly luminescent needle-fibre calcite. Note that needle-fibre meshes act as substrates for later pore-filling cements (non-luminescent and medium luminescent). Field of view: 1·65 mm.

(1) Random, or subparallel orientated acicular meshes. (2) Tangential surface coatings on pore walls. (3) Bifurcating arcuate pore bridging structures con sisting of subparallel orientated needles (this texture closely resembles alveolar texture of Esteban, 1974).

All three textures can occur within the same pore (Fig. 138). The timing of needle-fibre calcite growth relative to other stages of cementation can be determined by cement stratigraphy ; the needle-fibre calcite has acted as a substrate for later meteoric phreatic cements (Fig. 128), but overlies non-luminescent subaerial vadose 314

Cathodoluminescence and calcrete diagenesis

Fig. 13. (A) Thin section, plane polarized light; subaerially brecciated coral (C) in the calcrete profile. f: circumgranular brecciation fracture. Early non-luminescent prismatic cements (CS, B as Fig. 5A) line the intraparticle pore which is partly filled by pore bridging 'micrite' (PM). (B) Under cathodoluminescence; pore bridging 'micrite' appears as needle-fibre calcite lining the intraparticle pore and coating the dissolution-eroded surface of early non-lum'inescent cements. Dissolution textures also occur in association with the circumgranular brecciation fracture. Field of view: l·l mm.

The luminescence of needle-fibre calcite varies from bright orange to dull brown. Non-luminescent needle fibre calcite has been recorded by the authors in an Asbian calcrete from Cockermouth, Cumbria. This suggests that luminescence colour is not directly determined by the geochemical or physical conditions

cements (Fig. 13B). Needle-fibres always coat disso lution eroded surfaces and may show the same luminescence as the filling cement (Fig. 13B). Conse quently, we believe that needle-fibre calcite formed during the later stages of subaerial diagenesis (Table 1). 315

S. T. Solomon and G. M. Walkden that permit growth of acicular calcite. Needle-fibre luminescence also varies on a microscopic scale forming two common fabrics:

also may have formed by in situ overprinting of the original marine fabric.

( l ) Zoned needle-fibres : brightly luminescing needle fibres which occasionally show micrometre-sized non-luminescent cores. This fabric probably rep resents continued growth of needle-fibre calcite from pore water with changing geochemistry (e.g. Eh, pH, activator or quencher ion concentration). (2) Irregular patchy, or layered fabrics : such fabrics are caused by variations in luminescence which occur as irregular patches and layers on scales of ten to hundreds of micrometres (Fig. 14B). Contacts are typically irregular and gradational over a few tens of micrometres. These variations in luminescence could reflect activator ion com position changes of the inflowing cementing pore water. Alternatively, they may be caused by the presence of organic compounds in the calcrete profile which have locally influenced pore water geochemistry. Decaying plant roots could have created localized geochemical effects (Klappa, 1980) which may account for the brightly lumines cent zones often associated with calcified plant root tubules (Fig. 4B).

Origin of needle-fibre calcite morphology

Needle-fibre calcite has been recognized in transmit ted light by numerous authors and occurs in both Recent and ancient calcretes and also in soils. Explanations for the origin of needle-fibre habit fall into two categories; either organically or inorganically controlled growth. Workers preferring organic controls (Ward, 1970; Klappa, 1979, 1980 ; Calvet & Julia, 1983; Wright, 1984) have cited the relationship of needle-fibres to calcified plant root structures and have suggested that needle-fibre calcite formed as an indirect result of the activity of micro-organisms. In particular, some authors consider the biochemical reactions operating in the microenvironments around plant roots and fungal hyphae to be important. In this study, catho doluminescence has shown that needle-fibre calcite is abundant and widespread throughout the calcrete profile. This may imply a pervasive genetic process rather than one restricted to biological microenviron ments. Consequently, although biogenic and pedo genic processes may have contributed to the formation of needle-fibre calcite an inorganic process is favoured as the dominant crystal growth control. Buckley (1951) indicated that needle crystals may be produced by extreme supersaturation of the precipitating solution. James (1972) discussed inor ganic controls and suggested that 'needle-fibres' in a 'laminated crust profile' of Pliestocene reef limestones in North Barbados crystallized from rapidly evapora ting pore solutions, which may have quickly reached high degrees of supersaturation. Knox (1977) and Braithwaite (1983) also came to this conclusion. Harrison (1977) described similar needle-fibres which were mostly confined to root voids. Harrison (1977) suggested that supersaturation required for needle production might only have been satisfied in the plant root microenvironment. Needle-fibre calcite from the Quaternary succession of Barbados is particularly well illustrated by Esteban & Klappa (1983) and closely resembles the needle-fibre calcite seen in the Carbon iferous calcrete profile of this study. Tightly packed acicular meshes of calcite crystals (pseudomycelium) are a common nearsurface feature of recent soils (e.g. Fitzpatrick, 1971, fig. 83; Bal, 1975) usually occurring in carbonate-rich horizons approximately I m thick. Generally, pseudomycelium structures are believed to form rapidly (often within

Partially micritized marine allochems have been identified occurring within meshes of needle-fibre calcite (Fig. 14). This suggests that needle-fibre calcite

Fig. 14. Thin section, cathodoluminescence; needle-fibre calcite displaying irregular patchy luminescence. Partially subaerially micritized foraminifera occur within a mesh of needle-fibre calcite. Field of view: 1-4 mm.

316

Cathodoluminescence and calcrete diagenesis months) due to precipitation of leached calcium carbonate in the soil profile in response to evaporation. The needle-fibres of this Lower Carboniferous calcrete profile are closely similar to those cited above and, by comparison, are also believed to have been precipitated from pore solutions which became CaC03 supersaturated in response to near-surface evaporation. The relationship between needle-fibre calcite and subaerial micrite

Establishing the genetic relationship between needle fibre calcite and subaerial micrite has been aided by combining transmitted light and cathodolumines cence petrography. Transmitted light petrography has shown that needle-fibre calcite does not occur in optical continuity with the enclosing cement. This indicates that needle-fibres are not a relic or pseudo morph phase but were formed as we now see them. Needle-fibres always occur in close association with micrite, but because they form self-supporting pore bridging structures without the presence of micrite (Fig. 15), it seems likely that needle-fibre calcite cement and micrite formed independently. However, cathodoluminescence reveals that the micrite phase is not simply a fill of the acicular calcite meshes (Fig. 12B). Consequently, a more complex diagenetic relationship must be considered. Similar needle-fibre calcite/micrite associations have been described in transmitted light in studies of Quaternary calcretes by many authors (Harrison, 1977; H arrison & Steinen, 1978; Klappa, 1978, 1980). Both Knox (1977) and Calvet & Julia (1983) have

noted that in Recent calcretes needle-fibre calcite rapidly became unstable and recrystallized to micrite. Wright (1984) also considered this mode of degrada tion when describing needle-fibre calcite from a Dinantian calcrete in South Wales. Calvet & Julia (1983) noted that needle-fibre calcite 'reorganizes' (by degrading neomorphism) into crypto-microcrystalline anhedral crystals of low magnesium calcite. The needle-fibre calcite of this study may have undergone the same transformation. This is supported by slight crystal form differences from well-defined steep-sided needle-fibres (Fig. 12B) to relatively di ffuse rod-like crystals with blunt terminations (Fig. 13B). While crystal orientation may partly account for the apparent difference in form, it cannot explain the relatively diffuse appearance of many needle-fibres, particularly those associated with dense micritic fabrics. To conclude, the needle-fibre calcite cement may have partially neomorphosed to form micrite, the pore-bridging micritic fabrics reflecting the original arrangement of calcite needle-fibres.

Needle-fibre calcite, a common feature of ancient calcretes?

Additional examples of this needle-fibre calcite/ micrite relationship have been recorded by the authors in Asbian calcretes from both Derbyshire and Cum bria. It is predicted that the use of cathodolumines cence to examine calcretes will reveal that needle fibre calcite is a more common feature of ancient calcrete profiles than the geological literature currently indicates.

15. Thin section, plane polarized light; pore lining and bridging needle-fibre calcite occurring in close association with similar fabrics in the 'micrite' matrix of the calcrete profile. Field of view: 0· 72 mm.

Fig.

3 17

S. T. Solomon and G. M. Walkden distinguished primarily by its acicular cathodolumi nescence signature. Pore lining and bridging fabrics are characteristic, although smaller and more densely packed needle-fibres occur in association with micritic laminae and other subaerial micrite fabrics. This may indicate that second generation micrite contributed to the continued formation of earlier micrite fabrics ; possibly by the alteration of needle-fibre calcite (already discussed). Cathodoluminescence cement stratigraphy shows that second generation micrite overlies both non-luminescent subaerial vadose ce ments and the major dissolution hiatus. It also fills brecciation fractures demonstrating that it is the younger of the two micrite generations present in this calcrete (Table 1). Where acicular cathodolumines cence signature is poorly defined and dissolution textures and other identifiable cements are absent, the distinction of these two micrite generations is not possible. To conclude, during the formation of this ancient calcrete profile subaerial micritization appears to have been a continual process, initially altering the original marine sediment, but later also affecting needle-fibre calcite cements which partially occluded porosity.

Micrite stratigraphy

Two generations of non-marine micrite can be distinguished primarily on the basis of cathodolumi nescence signature. However, their different relative ages can be demonstrated only when dissolution textures and a full spar cement stratigraphy can be identified. Under cathodoluminescence first generation mi crite appears diffuse with no distinguishable crystal form and a dull to intermediate orange luminescence. In transmitted light it has a grey-brown colour and displays all the micrite fabrics described, except pore lining and bridging textures. Dissolution textures and brecciation fractures cross-cut this generation of micrite (Fig. 16 and Table 1). Second generation micrite (Table I , column 3) is

DIAG EN E TIC HISTOR Y

The combined use of transmitted light and cathodo luminescence microscopy has revealed the interrela tionships of the major diagenetic processes operative during subaerial emergence. This has enabled the diagenetic history of the calcrete profile to be reconstructed (Table 1 ). Subaerial alteration of the marine sediment was multistaged, involving the simultaneous action of many diagenetic processes during a single phase of emergence in the Late Asbian. Subaerial diagenesis was probably halted by the onset of the next phase of cyclic shelf sedimentation and the influx of marine pore water into the calcrete profile.

CONCLUSIONS

( ! ) A laterally extensive ancient calcrete profile has been identified in the Late Asbian shallow marine shelf limestones of the Llangollen area, North Wales. (2) In thin section, lithologies of the calcrete profile have a strongly altered and variable fabric, possessing micritic textures and secondary pore types of subaerial diagenetic origin. (3) Using cathodoluminescence to determine rela-

Fig. l6. Thin section, cathodoluminescence; two generations

of subaerial micrite distinguished in cathodoluminescence. First generation micrite (m l ) is offset by a subaerial brecciation fracture together with a brachiopod fragment (BR) and non-luminescent cements ( l -as Fig. 5). Second generation 'micrite'/needle-fibre calcite (m2) and meteoric phreatic cements infill the brecciation fracture. Field of view: 2·05 mm.

318

Cathodoluminescence and calcrete diagenesis precipitation of meteoric phreatic cements. (d) Needle-fibre calcite is considered to be a cement precipitate which may have almost completely recrystallized to micrite during the late stages of subaerial diagenesis. (e) In many parts of the calcrete profile two genera tions of non-marine micrite can be distinguished using cathodoluminescence. This two-part 'mi crite stratigraphy' requires the presence of either the full spar cement stratigraphy or dissolution textures and brecciation fractures to prove the different ages of subaerial micrite.

tive ages, the calcite cements of the calcrete profile have been divided into a three-part cement strati graphy : (I ) non-luminescent cements (subaerial vadose); (2) first (major) bright orange luminescent cement (subaerial vadose); (3) non-luminescent cements with brightly lumi nescent pitted subzones and final pore filling medium luminescent cements with smooth subzones (meteoric phreatic). (a) Precipitation of the non-luminescent subaerial vadose cements was concomitant with subaerial brecciation, micritization and the calcification of plant root tubules. (b) Subaerial vadose cements have a restricted verti cal stratigraphic range ; they occur both in the laminated calcareous crust of the calcrete profile and to depths of l m in the immediately underlying lithologies, but are absent from the rest of the Late Asbian succession.

(6) Combined transmitted light and cathodolumi nescence petrography show that the diagenetic history of the calcrete profile was multistaged, with many subaerial diagenetic processes acting simultaneously during a single phase of emergence.

AC KNO WLEDGMENTS

We would like to thank Drs V. P. Wright, C. J. R. Braithwaite and W. C. Ward for critically reading this manuscript and providing helpful suggestions.

(4) Textures, visible only with cathodolumines cence, are characteristic of recurring fabric dissolu tion. Dissolution took place during, and immediately after, precipitation of non-luminescent subaerial va dose cements.

REFERENCES

(a) The texture seen with cathodoluminescence is controlled by the microstructure/crystal morphol ogy of the partially dissolved substrate. (b) Dissolution textures are generally confined to the walls of larger pores and brecciation fractures which probably acted as fluid pathways in the calcrete during early subaerial diagenesis.

BAL, L. ( 1 975) Carbonate in soil : a theoretical consideration on, and proposal for its fabric analysis. Crystic, calcic and fibrous plasmic fabric. Neth. J. agric. Sci. 23, 1 8-35. BRAITHWAITE, C.J.R. ( 1 975) Petrology of palaeosols and other terrestrial sediments on Aldabra, western Indian Ocean. Phil. Trans. R. Soc. B, 273, 1-32. BRAITHWAITE, C . J . R . ( 1 983) Calcrete and other soils in Quaternary limestones : structures, processes and applica tions. J. geol. Soc. London, 140, 351-363. BuCKLEY, H.E. ( 1 9 5 1 ) Crystal Growth. Wiley, New York. CALVET, F. ( 1 982) Constructive micrite envelope developed in vadose continental environment in Pleistocene eoliantes of Mallorca (Spain). A cta geol. Hisp. 17, 1 69-1 78. CALVET, F. & JULIA, R. ( 1 983) Pisoids in the caliche profiles of Tarragona (N.E. Spain). In: Coated Grains (Ed. by T. M. Peryt), pp. 456-473. Springer-Verlag, Berlin. DICKSON, J . A. D . ( 1 965) A modified staining technique for carbonates in thin section. Nature, 205, 587. EsTEBAN, M . ( 1 974) Caliche textures and microcodium. Suppl. Bull. Soc. geol. ita/. 92, 1 05-1 25. EsTEBAN, M. & KLAPPA, C . F. ( 1 983) Subaerial exposure. In: Carbonate Depositional Environments (Ed. by P. A. Scholle, D. G. Bebout and C. H. Moore). Mem. Am. Ass. Petrol.

(5) Much of the original marine matrix of litholo gies in the calcrete profile has been subaerially altered to form micrite. Under cathodoluminescence this micrite often has an acicular appearance consisting of meshes of calcite needle-fibres. (a) The luminescence of needle-fibres is highly vari able suggesting that activator ion concentrations are not controlled by the geochemical or physical conditions required for the growth of acicular calcite. (b) Needle-fibre calcite was probably formed in response to localized supersaturation of meteoric pore fluids caused by periods of near-surface evaporation. (c) Cement stratigraphy demonstrates that needle fibre calcite formed after early subaerial vadose cementation and dissolution, but before the

Geo/. 33.

FITZPATRICK, E.A. ( 1 97 1 ) Pedology : a Systematic Approach to Soil Science. Oliver & Boyd, Edinburgh. FoLK, R.L. ( 1959) Practical petrographic classification of limestones. Bull. Am. Ass. Petrol. Geo/. 43, 1-8. GRAY, D.I. ( 1 98 1 ) Lower Carboniferous shelf pa/aeoenviron-

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S. T. Solomon and G. M. Walkden ments in North Wales. Unpublished Ph.D. Thesis. Univer sity of Newcastle on Tyne. HARRISON, R.S. ( 1 977) Caliche profiles : indicators of near surface subaerial diagenesis, Barbados, West Indies. Bull. Can. Petrol. Geol. 25, 1 23-1 73. HARRISON, R.S. & STEINEN, R.P. ( 1 978) Subaerial crusts, caliche profiles, and breccia horizons : comparison of some Holocene and Mississippian exposure surfaces, Barbados and Kentucky. Bull. geol. Soc. Am. 89, 385-396. JAMES, N . P. ( 1 972) Holocene and Pleistocene calcareous crust (caliche) profiles : criteria for subaerial exposure. J. sedim. Petrol. 42, 8 1 7-836. KLAPPA, C. F. ( 1978) Morphology, composition and genesis of Quaternary calcretes from the western Mediterranean : a petrographic approach. Unpublished Ph.D. Thesis, Univer

sity of Liverpool. KLAPPA, C.F. ( 1 979) Calcified filaments in Quaternary calcretes : organo-mineral interactions in the subaerial vadose environment. J. sedim. Petrol. 49, 955-968. KLAPPA, C.F. ( 1 980) Rhizoliths in terrestrial carbonates : classification, recognition, genesis and significance. Sedi mentology, 27, 6 1 3-629. KNOX, G.J. ( 1 977) Caliche profile formation, Saldanha Bay (South Africa). Sedimentology, 24, 657-674. MEYERS, W.J. (1974) Carbonate cement stratigraphy of the Lake Valley Formation (Mississippian) Sacramento Mountains, New Mexico. J. sedim. Petrol. 44, 837-86 1 . MULTER, H . G . & HOFFMEISTER, J .E. ( 1968) Subaerial laminated crusts of the Florida Keys. Bull. geol. Soc. Am. 79, 1 83-192. SANDBERG, P.A. ( 1 975) Bryozoan diagenesis : bearing on the

(Manuscript received 2 January

nature of the original skeleton of rugose corals. J. 587-606. SOMERVILLE, I.D. ( 1977) The sedimentology and stratigraphy Palaeontol. 49,

of the Dinantian limestones in the Llangollen area and East of the Clwydian range, North Wales. Unpublished Ph.D.

Thesis, Queen's University of Belfast. SOMERVILLE, I. D. ( 1 979) Minor sedimentary cyclicity in Late Asbian (Upper Dl) limestones in the Llangollen district of North Wales. Proc. Yorks. geol. Soc. 42, 3 1 7-34 1 . WALKDEN, G . M . ( 1974) Palaeokarstic surfaces i n Upper Visean (Carboniferous) limestones of The Derbyshire Block, England. J. sedim. Petrol. 44, 1 232-1 247. WALKDEN, G . M . & BERRY, J . R. ( 1 984) Syntaxial over growths in muddy crinoidal limestones : cathodolumines cence sheds new light on an old problem. Sedimentology, 31, 251 -268. WALKDEN, G . M . & DAVIES, J. ( 1 983) Polyphase erosion of subaerial omission surfaces in the Late Dinantian of Anglesey, North Wales. Sedimentology, 30, 861 -878. WALLS, R.A., BURLEIGH HARRIS, E. & N UNAN, W.E. ( 1 975) Calcareous crust (caliche) profiles and early subaerial exposure of Carboniferous carbonates, northeastern Ken tucky. Sedimentology, 22, 4 1 7-440. WARD, W.C. ( 1 970) Diagenesis of Quaternary eo/iantes of NE Quintana Roo, Mexico. Ph.D. Thesis, Rice University, Houston, Texas. WATTS, N .L. ( 1 978) Displacive calcite : evidence from recent and ancient calcretes. Geology, 6, 699-703. WRIGHT, V.P. (1 983) A rendzina from the Lower Carboni ferous of South Wales. Sedimentology, 30, 1 59-179. WRIGHT, V.P. (1984) The significance of needle-fibre calcite in a Lower Carboniferous paleosol. Geol. J. 19, 23-32.

1985 ; revision received 2 May 1985)

320

CALCRETES AND PALUSTRINE CARBONATES

Palustrine carbonates (marsh carbonates) are wide

nates.

spread in continental deposits, particularly the later

limestones have more negative values, reflecting

Stable

isotope

data from

the

palustrine

Mesozoic and Tertiary of Europe. Platt, in describing

extensive pedogenic modification and the input of

material from the Cameros Basin in Spain, discusses

light 'meteoric' oxygen and light organic carbon

the problems of distinguishing calcretes from pedo

isotopes.

genically modified lacustrine and palustrine carbo-

pebbles'.

These

limestones

also

contain

'black

Fig. 16. Palustrine limestone, Oligocene, Bembridge Limestone, Isle of Wight, England. The original sediment, a marsh/pond bioclastic wackestone, has been affected extensively by pedogenic processes. In (A) calcrete nodules occur within the sediment, which also contains peloids and coated grains. A large dissolution void formed and was filled with darkened intraclasts. In (B) a laminar calcrete is succeeded by sediment containing peloids, coated grains (some with gastropod nuclei, others lime mud), and intraclasts of laminated crust, perhaps derived from rhizobrecciation. An elongate cavity near the top is filled by internal sediment and then calcite spar. Original sediment, with moulds of gastropods and bivalves, occurs at the very bottom and top of the specimen.


321


Lacustrine carbonates and pedogenesis: sedimentology and origin of palustrine deposits from the Early Cretaceous Rupelo Formation, W Cameros Basin, N Spain NI G E L H. P L A T T Geologisches lnstitut, Universitiit Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland

ABSTRACT

The Berriasian Rupelo Formation of the W Cameros Basin consists of a 2-200 m thickness of marginal and open lacustrine carbonate and associated deposits. Open lacustrine facies contain a non-marine biota with abundant charophytes (both stems and gyrogonites), ostracods, gastropods and rare vertebrates. Carbonate production was mainly biogenic. The associated marginal lacustrine ('palustrine') facies show strong indications of subaerial exposure and exhibit a wide variety of pedogenic fabrics. Silicified evaporites found near to the top of the sequence reflect a short hypersaline phase in the lake history. The succession was laid down in a low gradient, shallow lake complex characterized by wide fluctuations of the shoreline. Carbon and o xygen stable isotope analyses from the carbonates show non-marine values with ranges of b13C from -7 to -l l%0 and 0 a: (!'

red marls

z 0

evaporitic;:

chert

i=
0 w

u

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