Siliceous Deposits in the Pacific Region (Developments in Sedimentology 36)

DEVELOPMENTS IN SEDIMENTOLOGY 36

SILICEOUS DEPOSITS IN THE PACIFIC REGION Edited by

A. IIJIMA, J.R. HElN and R. SIEVER Geological Institute, University of Tokyo, T o k y o (Japan) United States Geological Survey, Menlo Park, Calif. (U.S.A.) Department of Geology, Harvard University, Cam bridge, Mass. (U.S.A.)

ELSEVIER SCIENTIFIC PUBLISHING COMPANY 1983 Amsterdam - Oxford - New Y ork

ELSEVIER SCIENTIFIC PUBLISHING COMPANY Molenwerf 1 P.O. Box 211,1000AE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 62,Vanderbilt Avenue New York, NY 10017

Library of Congress Cataloging in Publication Dala

Main entry under t i t l e : Siliceous deposits

i n the Pacific region.

(Developments i n sedimentology ; 36) "Pspers presented a t the Second International Conference an Siliceous Deposits in the Pacific Region held i n Two-Akita-Qoto, Japan between the 2 1 and 27 of August , 1981 The meeting in Japan was sponsored by UNESCO-NCG, Cet al. 3 "-Pref. Includes index. 1. Rocks, Siliceous--Congresses. 2, Petrol0 Pacific tmea--Congresses. I. Iijims, A. 11. Hein, J. R. (James R.) 111. Siever, Raymond. N. International Conference on Siliceous Deposits in the Pacific Region (2nd : 1981 : Tokyo, Japan, e t c . ) V. International Union of Geological Sciences. VI. Series: Developments in sedimentoiogy ; Y. 36. ~ ~ 4 9 5 . ~ 51982 4 552l.5 82-16453 Ism 0-444-42l29-7

......

-

ISBN 0-444-42129-7 (Vol. 36) ISBN 0-444-41238-7 (Series) 0 Elsevier Scientific Publishing Company, 1983 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permiasion of the publisher, Elsevier Scientific Publishing Company, P.O. Box 330,1000 AH Amsterdam, The Netherlands

Printed in The Netherlands

V

PREFACE This book contains twenty-five papers selected from t h i r t y - s e v e n papers presented a t t h e Second I n t e r n a t i o n a l Conference on S i l i c e o u s Deposits i n t h e P a c i f i c Region h e l d i n Tokyo-Akita-Kyoto, 1981.

C o r r e l a t i o n Program's (I.G.C.P.) years.

Japan between t h e 21 and 27 of August

The conference was t h e c u l m i n a t i n g event f o r t h e I n t e r n a t i o n a l Geological P r o j e c t 115, which had been a c t i v e f o r s i x

The meeting i n Japan was sponsored by UNESCO-IUGS,

t h e Japan Society

f o r t h e Promotion o f Science, the Geological Society o f Japan, and t h e Japanese Association o f Petroleum Techno1 o g i sts. Azuma I i j i m a , Professor o f Geology a t t h e U n i v e r s i t y o f Tokyo and chairman o f the Japanese Working Group o f P r o j e c t 115 coordinated the h i g h l y successful conference i n Japan.

I i j i m a a l s o i n i t i a l l y proposed and organized P r o j e c t 115.

James R. Hein, Geologist a t t h e Branch o f P a c i f i c - A r c t i c Marine Geology o f t h e U. S. Geological Survey was t h e i n t e r n a t i o n a l p r o j e c t leader and U . S. National Chairman f o r P r o j e c t 115.

Raymond Siever, Professor o f Geology a t Harvard

University, was coordinator along w i t h J. R. Hein and W. R. Danner, o f a Geological Society o f America sponsored Penrose Conference on S i l i c e o u s Rocks, which a l s o doubled as the F i r s t I n t e r n a t i o n a l Conference h e l d by members of P r o j e c t 115; t h e conference was h e l d i n Vancouver B.C.,

Canada i n 1978.

Twenty-three years have passed since t h e f i r s t compilation o f work concerning rrSiticu in

Sediments" ( e d i t e d by H. A. I r e l a n d ) was published as a Special

P u b l i c a t i o n o f t h e S.

E. P. M.

The g r e a t advances made i n t h e study o f f i n e -

grained s i l i c e o u s deposits since I r e l a n d ' s 1959 volume provided t h e impetus f o r t h i s volume and f o r I.G.C.P.

P r o j e c t 115 i n general.

The work o f I.G.C.P.

p r o j e c t 115 members w i l l be continued f o r an a d d i t i o n a l f i v e years as I.G.C.P. P r o j e c t 187, S i l i c e o u s Deposits o f t h e P a c i f i c and Tethys Regions. coordinates the work o f 110 s c i e n t i s t s from 25 countries.

P r o j e c t 187

Ms. Ayako Kamagata k i n d l y prepared t h e s u b j e c t index f o r t h i s volume and d i d much of t h e typing.

A l l t h e manuscripts i n t h i s volume were read c r i t i c a l l y by

the e d i t o r s and by one o r more o u t s i d e reviewers.

We thank t h e many s c i e n t i s t s

who helped t o improve t h e manuscripts contained i n t h e volume. provided continuous support and encouragement f o r our e f f o r t s . Azuma Iijima James R. Hein Raymond Siever

UNESCO-IUGS

1

CHAPTER 1 AN INTRODUCTION TO SILICEOUS DEPOSITS IN THE PACIFIC REGION

A. IIJIMA, J. R. HEIN, AND R. SIEVER

Though s i l i c a (Si02) has been used f o r c e n t u r i e s f o r many purposes, in recent years s i l i c o n as well as i t s oxide, s i l i c a , a r e becoming widely exploited. For example, s i l i c o n i s used in the semiconductor and coriiputer i n d u s t r i e s ; s i l i c a comprises the f i r e bricks f o r t h e U. S. space s h u t t l e , Columbia, and i s the raw material f o r glass and rilany other products. S i l i c o n and oxygen a r e the two most abundant elements of the e a r t h ' s c r u s t , 2877 and 462, respectively. S i l i c o n i s an iriiportant element of many minerals in rimt igneous, metamorphic, and sedimentary rocks of the e a r t h ' s c r u s t . In sediments and sediirientary rocks, s i l i c a comprises d e t r i t a l grains, s i l i c e o u s organic remains, and authi genic minerals. S i l i c e o u s deposits a r e fine-grained, s i l i c a - r i c h sediments and sedimentary rocks, f o r example c h e r t , s i l i c e o u s shale, diatomite, and so on. Amorphous s i l i c a (opal-A), c r i s t o b a l i t e (opal-CT and opal-C), tridyriiite, chalcedonic quartz, microquartz, and various combinations of these phases a r e the primary minerals of s i l i c e o u s deposits. The source of s i l i c a i s inost coimonly opaline biogenic debris, such as r a d i o l a r i a n s , s i l i c o f l a g e l l a t e s , diatoins, and sponge spicules. The t e s t s and f r u s t u l e s of these marine organisms accumulate in great q u a n t i t i e s over extensive areas of t h e sea f l o o r , e s p e c i a l l y around the polar regions (diatoms) and the equatorial b e l t of high biological productivity ( r a d i o l a r i a n s and diatoms). They a l s o accumulate in other areas where cold, n u t r i e n t rich waters upwell, such as offshore southern C a l i f o r n i a , north Africa, and Peru. These s i l i c a s e c r e t i n g organisms a r e the very basis of the food chain and they are in f a c t t h e cause of a l l the rmjor oceanic f i s h e r i e s world-wide. Phytoplankton, f o r example diatoms, produce much of the atmospheric oxygen. A f t e r hundreds of i e t e r s of burial t h e s i l i c e o u s biogenic debris i s transformed from opaline s i l i c a i n t o c r i s t o b a l i t e , and f i n a l l y i n t o quartz. These mineralogic transformations r e s u l t in t h e recycling of s i l i c a and t r a c e metals and the subsequent formation of s i l i c e o u s shale, porcelanite, and chert. Accompanying the transformation of the s i l i c a t e s t s and f r u s t u l e s i s a l t e r a t i o n of the carbon compounds of the plankton. This material (especially of diatoms) i s the source f o r vast accumulations of natural gas and hydrocarbons. For example, t h e Miocene Monterey and Onnagawa Formations a r e considered t o be t h e source rocks f o r some important petroleum deposits in California and Japan.

2

Siliceous

deposits

are

incorporated

into

oroyenic

belts

by

uplift

of

c o n t i n e n t a l m a r g i n and oceanic c r u s t o r by o b d u c t i o n and t h r u s t f a u l t i n y a t convergent p l a t e niaryins.

Such on l a n d s i l i c e o u s d e p o s i t s a r e found o f e v e r y

aye and a t many l o c a t i o n s i n t h e c i r c u i i i - P a c i f i c

region.

F r a n c i s c a n and Monterey Foriiiations o f C a l i f o r n i a , western

Canada

and Washington,

Onnagaua F o r m a t i o n o f Japan,

t h e Chichibu

Examples i n c l u d e t h e

t h e Cache Creek F o r i i i a t i o n o f and Sanbosan Groups

and t h e

,

t h e Vagoriipolkian S e r i e s o f E a s t e r n U.S.S.R.

Suiiiulony d i a t o i i i i t e o f t h e P h i l i p p i n e s ,

t h e P i s c o F o r m a t i o n o f Peru,

the

arid t h e

Nicoya Complex o f Costa Kica, t o naiiie o n l y a feu.

These s i l i c e o u s d e p o s i t s a r e

cormonly a s s o c i a t e d w i t h uraniuiii, i r o n , manganese,

barium, and phosphate ores.

In

suiluiiary

then,

siliceous

deposits

merit

detailed

study

1)

because:

S i l i c e o u s o r g a n i s m a r e t h e very base o f t h e marine f o o d c h a i n and produce much of

the

atiiiospheric

recycliny

of

oxyyen;

silica

therefore,

understanding

i n t h e marine environment

is

the

distribution

imperative;

2)

and

Siliceous

d e p o s i t s o c c u r i n g r e a t t h i c k n e s s and cover v a s t areas o f t h e sea f l o o r and geosynclinal deposits; occurrence

of

3) S i l i c e o u s d e p o s i t s a r e d i r e c t l y a c c o u n t a b l e f o r t h e

riiany hydrocarbon d e p o s i t s ;

i i i i p o r t a n t d e p o s i t s o f i r o n , manganese,

4)

Siliceous

deposits

occur w i t h

uranium, barium, and phosphate ores.

Because o f t h e b i o l o g i c o r i y i n o f many s i l i c e o u s d e p o s i t s , t h e i r yeocheiiiical transforination formations yeocheniists,

in

the

around

marine

the

environment,

Pacific,

it

and

is

their

essential

occurrence

that

in

many

paleontologists,

and g e o l o g i s t s work t o y e t h e r t o reach a h o l i s t i c u n d e r s t a n d i n y

about t h e o r i y i n and e v o l u t i o n o f these d e p o s i t s . d i s c i p l i n e s needed,

also,

c o o p e r a t e on comparative P r o j e c t 115 was

Not o n l y i s i n t e y r a t i o n o f

workers froiii a l l c o u n t r i e s around t h e P a c i f i c must studies.

formed.

W i t h t h e s e riiain p o i n t s i n iiiind I.G.C.P.

The aim o f I.G.C.P.

P r o j e c t 115 i s t o c o r r e l a t e

sediiiientary processes o f s i l i c e o u s d e p o s i t s i n t h e P a c i f i c b a s i n and i n t h e geosynclinal

areas o f

n e i g h b o r i n y o r o g e n i c b e l t s by means o f s t r a t i g r a p h y ,

p a l e o n t o l o g y , sediriientol ogy, sediiiientary p e t r o l o g y , and geochemistry. The

twenty-five

y e n e r a l overviews, diayenesis

and

southwest Japan.

chapters

i n this

book

are

organized

into

six

groups:

d i s t r i b u t i o n s o f s i l i c e o u s d e p o s i t s , cheiiiical sediinentology,

mineralogy,

diatomaceous

deposits,

and

bedded

cherts

in

Soiiie papers c o u l d e a s i l y be p l a c e d i n inore t h a n one cateyory.

Chapters i! and 3 a r e general overviews.

I n c h a p t e r 2, S i e v e r o u t l i n e s t h e

e v o l u t i o n o f t h e d e p o s i t i o n a l and c l i r l i a t i c environments,

and t h e sources o f

s i l i c a f o r t h e f o r m a t i o n o f c h e r t s as a c o n t i n e n t a l r i f t begins and widens i n t o an open ocean. time-temperature

The d i a g e n e t i c h i s t o r i e s o f t h e c h e r t s can b e s t be viebred i n plots,

subsidence h i s t o r y ,

where

temperature

is

dependent

on

heat

flow

b o t h of which are f u n c t i o n s o f p l a t e t e c t o n i c regimes.

and One

o f t h e iiiost i m p o r t a n t probleiils t o be s o l v e d c o n c e r n i n g t h e f o r m a t i o n o f c h e r t s i n the

Pacific

region

is

brhether o r

not

bedded c h e r t s

i n circuin-Pacific

3

orogenic b e l t s are e q u i v a l e n t t o deep-sea, open-ocean c h e r t s i n t h e P a c i f i c basin. I n Chapter 3 Hein and K a r l show t h a t t h e two groups o f c h e r t s are n o t the same based on l i t h o l o g i c associations,

sedimentation rates,

sedimentation,

sedimentary s t r u c t u r e s ,

Bedded c h e r t s

i n t h e orogenic b e l t s probably formed i n young ocean basins,

block f a u l t e d c o n t i n e n t a l margins,

modes o f formation,

iilechanisriis o f

and geochemistry.

back arc basins, o r adjacent t o i s l a n d arcs.

Chapters 4 through 9 deal w i t h t h e d i s t r i b u t i o n i n space and time o f c h e r t s I i j i m a and Utada sumnarize t h e occurrence and o r i g i n o f

i n t h e P a c i f i c region. siliceous

rocks

silicastone.

in

Japan

and

their

economic

importance

as

sources

T e r t i a r y s i l i c e o u s deposits, s i m i l a r t o t h e Monterey Formation o f

C a l i f o r n i a and composed mostly o f diatoms and t h e i r d i a g e n e t i c products, much o f n o r t h e r n Honshu and Hokkaido. occur

i n several

tectonic

s p i c u l e s and r a d i o l a r i a n s . cherts

formed

under

belts

Mesozoic and Paleozoic bedded c h e r t s

through Japan and are composed mostly o f

deep-sea

oceanic

conditions.

c h e r t s are most widespread. Peninsular Phillipines,

and

of

Koike

review The

The T r i a s s i c conodont-beariny

The d i s t r i b u t i o n o f c h e r t s i n Southeast Asia i s

Paleozoic c h e r t s occur mainly i n Indochina,

Malaysia,

Blocks

Iyo

b i o s t r a t i graphy o f c h e r t s from Japan.

o l d e s t conodont assemblage i s middle Ordovician. o u t l i n e d by Tan.

cover

I i j i m a and Utada f i n d no evidence t h a t these bedded

Paleozoic and Mesozoic conodont

Asia.

of

whereas Tertiary

Mesozoic cherts

and eastern Malaysia.

cherts

Thailand,

are widespread

i n melanges

occur

in

and

i n Southeast Indonesia,

the

Moore shows t h a t c h e r t s i n New Zealand

occur mainly i n the Permian-Jurassic arc-trench-basin

complex o f t h e Rangitata

Oroyen where c h e r t s are associated w i t h submarine volcanic rocks and f l y s c h , and i n t h e Late Cretaceous-Early

Tertiary

mrginal

basins o f western and

n o r t h e r n New Zealand where c h e r t s are associated w i t h s i l i c e o u s shale and limestone.

Widespread r a d i o l a r i a n c h e r t s i n allochthonous accreted t e r r a n e s

throughout t h e C o r d i l l e r a o f western North America,

which range i n age from

Ordovician

by

to

Holdsworth.

Middle

Cretaceous,

are

delineated

Murchey,

Jones,

and

These c h e r t sequences are d i v i d e d i n t o f o u r d i s t i n c t l i t h o l o g i c

associations:

(1) chert-carbonate

chert-argillite

basinal

water facies,

deposits,

deposited on a s u b s i d i n g platform,

(3) interbedded t u f f - c h e r t - a r g i l l i t e

and (4) t u r b i d i t e yraywacke-chert

c o n t i n e n t a l margin.

deposited i n deep water a t a

Gursky and Schmidt-Effing show t h a t t h e mostly Cretaceous

Nicoya Complex o f Costa Rica c o n s i s t s o f basalt, minor bedded chert, p l u t o n i c rocks.

(2) deep

and mafic

They suggest t h a t t h e r h y t h m i c a l l y bedded r a d i o l a r i a n c h e r t

accumulated i n an abyssal environment o f considerable r e l i e f i n t h e Mesozoic eastern P a c i f i c . Clear deposition studies.

evidence of

related

bedded

to

chert

the

depositional

sequences

has

not

settings been

Analyses o f the chemical coriiposition o f

and mechanism of

forthcoming

in

most

bedded c h e r t sequences,

4

however, are becoming very e f f e c t i v e f o r understandiny these basic sediilientary aspects o f c h e r t s as i s demonstrated i n papers o f Chapters 10, 11, and 12. Hein and co-workers suggest t h a t c h e r t s from western Costa Rica,

includiny the

Nicoya Complex, were deposited as t u r b i d i t e s o f mostly s i l i c e o u s d e b r i s i n deep water,

b u t near a c o n t i n e n t a l margin.

The d i s t i n c t coiliposition o f t h e Nicoya

Complex c h e r t s r e l a t i v e t o t h e open-ocean analyses o f bedded c h e r t s i n Japan, f o u r groups of elements: K,

Na, Mg,

Fe,

Cr,

c h e r t s from DSDP holes

Matsumoto and I i j i m a d i s c r i m i n a t e amony

SiO2 represents mostly biogenic s i l i c a ;

Rb occur i n d e t r i t a l components; Mn, Fe,

occur rnostly i n a u t h i g e n i c o r hydroyenous a d d i t i o n s ; carbonates.

By

using

t h e Mn/Al

sedimentation decreases,

i s well

Si0~-Al~0~-(Fe~0~+MnO)xlO. From cheiliical

i l l u s t r a t e d on a t e r n a r y p l o t of

ratio

that

Si, Al,

Ti,

V,

Zn,

Cu,

Ni,

and Ca and S r occur i n

increases

as

the

rate

of

they show t h a t r a d i o l a r i a n c h e r t beds i n a back-arc

marginal sea o f t h e Permian and T r i a s s i c Chichibu Terrane were deposited s l o w l y whereas

alternating

shale

layers

were

deposited

rapidly.

In

contrast,

radiolarian-diatomaceous bedded c h e r t s o f t h e T e r t i a r y Setogawa Terrane were deposited

i n an offshore,

arc-trench-gap

r a p i d l y than t h e Chichibu cherts. froin many p a r t s o f t h e world, mechanism o f

but

accumulated much more

Steinberg and co-workers

shou t h a t no s i n g l e

d e p o s i t i o n can describe a l l bedded c h e r t sequences,

siliceous turbidites, Al-Fe-Ti

basin

Based on the chemical composition o f c h e r t s

p e l a g i c deposition,

and t h a t

and diagenesis a l l p l a y a part.

Si-

and REE f l u c t u a t i o n s i n p e l a y i c r a d i o l a r i t e sequences correspond t o

changes i n paleogeography. Chapters 13 through 17 concern various aspects o f diayenesis: do s i l i c e o u s sediments l i t h i f y i n t o c h e r t s ? experinients

that

magnesium

hydroxide

t r a n s f o r m a t i o n o f opal-A t o opal-CT. of

early

diagenesis.

Dissolved

How and when

Kastner and Gieskes c o n f i r m from

compounds

foriii

as

nuclei

the

Nucleation i s slow under t h e c o n d i t i o n s silica

values

from

the

r a d i o l a r i a n s vary depending on temperature and a l k a l i n i t y .

dissolution

150°C t h e s o l u b i l i t y exceeds t h a t o f opal-A

of

Up t o 100°C t h e

s o l u b i 1it y o f t h e r a d i o l a r i ans approaches the s o l u b i 1it y o f opal -C, consumed.

for

whereas a t

a f t e r t h e a l k a l i n i t y has been

Based on p e t r o l o g i c a l study o f the Neogene s i l i c e o u s deposits o f

n o r t h e r n Japan, Tada and I i j i m a s h w t h a t various 4 A o p a l i n e phases form from different

materials.

Opal-CT

forms

from b i o g e n i c

opal;

o r i g i n a t e s from t h e a l t e r a t i o n o f s i l i c i c volcanic glass;

low c r i s t o b a l i t e

and t r i d y m i t e forms

by d i r e c t p r e c i p i t a t i o n as a l a t e stage weathering product.

They c o n f i r m t h e

i d e n t i f i c a t i o n o f mixtures o f t h e o p a l i n e phases by means o f X - r a y diffraction.

Garrison,

powder

Isaacs, and P i s c i o t t o describe t h e l i t h o f a c i e s o f t h e

Monterey Formation o f C a l i f o r n i a which i n c l u d e a lower calcareous-phosphatic f a c i e s and an upper s i l i c e o u s facies.

They show t h a t t h e s i l i c a diagenesis i s

c o n t r o l l e d by temperature and sediment composition such as t h e c l a y mineral

5

content.

Transformation o f

opal-A

and opal-CT

occurred by

rapid solution-

p r e c i p i t a t i o n accompanied by s i y n i f i c a n t compaction and by l i t t l e irioveirient o f M i z u t a n i and Shibata note t h a t t h e Rb-Sr and K - A r whole

s i l i c a between beds.

rock age o f s i l i c e o u s shales from t h e Mino D i s t r i c t , c e n t r a l Japan are about 18 my. less than t h e Middle Jurassic aye determined from r a d i o l a r i a n asseniblages, suygestiny t h a t 18 m.y.

were r e q u i r e d f o r diagenesis t o c l o s e t h e chemical

system w i t h respect t o t h e elements analyzed. chert

sections

cornpositions

i n Greece and I t a l y ,

that

the

temperatures

With samples from Cretaceous

Baltuck of

deduces from oxygen

diayenesis

were

greater

isotopic

for

cherts

associated w i t h inudstone than f o r those associated w i t h carbonate. Diatomaceous sediments are w i d e l y d i s t r i b u t e d both on t h e sea f l o o r i n h i g h l a t i t u d e areas and i n n o r t h e r n Japan and C a l i f o r n i a as discussed i n Chapters 19,

18,

and 20.

content

from

sediments.

B r e u s t e r proposes a method t o determine t h e biogenic opal the

bulk

chemical

composition

of

Antarctic

o f d e t r i t a l components from t h e t o t a l s i 1i c a content. that

diatoms

froiii B e r i n g Sea surface

stream,

productivity paleoclimatic assernblayes. activity,

sea-ice

conditions,

dichrotherrnal

water.

environments

of

deposits

Sancetta demonstrates

r e c o r d t h e hydrography and

D i f f e r e n t diatom assemblages d i s t i n g u i s h t h e

p r o d u c t i v i t y o f o v e r l y i n g waters. Alaskan

diatomaceous

The b i o y e n i c opal content can be obtained by s u b t r a c t i n g t h e s i l i c a

low-salinity Koizunii

shelf

discusses

Neoyene sections

as

water,

the

and

high-

sedimentary

determined frorii

and

diatom

Increased diatom p r o d u c t i o n d i d not always correspond t o v o l c a n i c

but t o c l i m a t i c c o o l i n g and t e c t o n i c d i f f e r e n t i a t i o n o f d e p o s i t i o n a l

basins which caused upwelling. Widespread Mesozoic and Late Paleozoic bedded c h e r t s o f southwest Japan are described and discussed r a d i o l a r i a n assemblages

i n Chapters 21 through 25. identified

Yao reviews

sixteen

from Mesozoic mudstone and chert.

New

assemblages d e f i n i n g p a r t s o f t h e T r i a s s i c and J u r a s s i c are described. describes t h e sedimentary s t r u c t u r e s i n Permian-Triassic District,

Imoto

c h e r t s o f t h e Tarnba

s t r u c t u r e s t h a t i n d i c a t e d e p o s i t i o n o f s i l i c e o u s d e b r i s by t u r b i d i t y

c u r r e n t s and/or bottom currents.

Nakazawa and co-workers describe bedded c h e r t

o f Cretaceous aye from the Shimanto B e l t , K i i Peninsula,

and suggest t h a t t h e

c h e r t formed i n a r e g i o n o f o f f - r i d g e volcanisin where inuch t e r r i g e n o u s d e b r i s accuniulated.

Ogawa.

Nakashirria,

and Sunouchi

propose t h a t many interbedded

sandstone-chert sections i n southwest Japan i n c l u d i n g t h e Shiilianto B e l t r e s u l t from t e c t o n i c i n t e r l a y e r i n g o f oceanic c h e r t and i s l a n d arc sandstone d u r i n g a c c r e t i o n associated w i t h mineralogic

composition

subduction. of

associated w i t h greenstones. chlorite,

and hematite

Sano describes

rhythmically

layered

the

occurrence

chert-shale

Q u a r t z v a r i e s i n v e r s e l y w i t h plagioclase,

i n c h e r t beds and mineral

and

sequences illite,

abundances a r e arranged

syrnmetrically about t h e center o f the beds, quartz being nnst abundant a t t h e

6

center. I n the

final

laminite,

to

laminae.

chapter,

describe

Yoshida

rocks

proposes

consisting

a new l i t h o l o g i c term, cherto f a l t e r n a t i n g quartz-claystone

Rocks belonging t o t h i s group are sedimentary but i n t h e past were

mistakenly

described

argillites.

The c h e r t - l a m i n i t e occurs i n many geosynclinal basins.

as

metamorphic

phyllites,

pelitic

schists,

or

7

CHAPTER 2

EVOLUTION OF CHERT AT ACTIVE AND PASSIVE CONTINENTAL MARGINS RAYMOND SIEVER

Department of Geological Sciences, Harvard University, Cambridge, Massachusetts 02138 (U.S.A.) ABSTRACT

Because paleo-oceanography i s largely determined by plate movements, the patterns of continental runoff, nutrients, and depth and distance from shore a f f e c t the occurrence of marine siliceous deposits; these sediments a r e gradua l l y l i t h i f i e d and typically end u p as overthrust continental margin chert sequences. In continental r i f t valleys the dominant modes of siliceous deposits are playa lake diatomites and alkaline-lake s i l i c a - z e o l i t e deposits. As a r i f t widens t o a narrow gulf o r sea, siliceous deposits will form i f continental t i l t s , paleoclimates, and previous geologic histories permit large r i v e r systems t o b r i n g abundant runoff and nutrients to the narrow sea. In t h i s milieu, the ultimate source of s i l i c a i s weathered continental d e t r i t u s , including earl i e r volcanics. Patterns of siliceous deposition as the narrow sea widens t o an open ocean depend on continental configurations and paleocl imates. Convergent margins involve continental margin and pelagic siliceous deposits inaccretionary prisms and, as continents converge, overthrust zones may involve both scraped-off pelagic sediments and continental apron sediments of the two continents. The diagenetic h i s t o r i e s of these different k i n d s o f siliceous sediments can best be diagrammed by plots o f time vs. temperature, where temperature i s dependent on heat flow and subsidence history, b o t h of which in t u r n are functions of plate tectonic history. Siliceous sediment i s deposited in many continental and oceanic sedimentary environments and evolves as a diagenetic rock-chert-in d i f f e r e n t kinds of burial environments. Plate tectonic environments, defined by plate boundaries of different kinds, control the occurrence of the sedimentary environments inwhich siliceous sediments form. They also control the course of burial history and therefore the course of diagenesis (Siever, 1979; Siever and Hager, 1981). In t h i s paper I propose a plate tectonic c l a s s i f i c a t i o n of siliceous sedimentary environments and follow the sediments deposited i n them t h r o u g h t h e i r burial histories. We may define plate tectonic environments as those associated withdivergent, convergent, and transform plate boundaries as well as i n t r a p l a t e areas of oceans and continents. For purposes of discussing siliceous rocks I r e s t r i c t the discussion t o s i x major environments: continental i n t r a p l a t e , continental r i f t valley, i n i t i a l spreading-narrow ocean, oceanic i n t r a p l a t e , passive continental

a margins, and active continental margins (Table I ) . TABLE I Plate tectonic categories of chert environments

Plate Category

Sedimentary environment

Detrital provenance

continental i ntrapl a t e

carbonate shelf

l i t t l e o r no continental terrigenous

continental r i f t valley

1acustrine

volcaniclastic; continental plutonic/ supracrustal

Si 1i ceous sediment t h i n , nodular, replacement

diatomite, magadiite, silica-zeolite, silica/non-marine evaporite

i n i t i a l spreadi ng-ocean

narrow shelf/embayment/strai t s

continental plutonic/ supracrustal

marine diatomite, siliceous clasti cs/evapori t e s

oceanic i n t r a plate

pelagic

terrigenous continent a l and oceanic volcanicl a s t i c

diatom/radiolarian ooze w i t h carbonate ooze and fine clastics

passive margins

active margins

subsiding continental continental plutonic/ s h e l f , shoreline, reef supracrustal

fore-arc/back-arc/ trench slope

a r c volcaniclastic/ arc massif and t e r rigenous

diatomite, si 1i ceous cl ast i c s , nodular rep1 acement diatomite/radiolarite, siliceous c l a s t i c s

SILICEOUS SEDIMENTS IN PLATE TECTONIC ENVIRONMENTS The siliceous sediments of continental areas f a r from plate boundaries a r e typically deposited i n carbonate depositional environments. These a r e found along the shorelines of shallow epicontinental seas t h a t may be dominated by carbonate buildups and reef belts of various kinds, depending on local structural and climatic conditions (Wilson, 1975). Terrigenous c l a s t i c s a r e of supracrustal rocks, typically preexisting sedimentary rocks. As a rule there are no i n i t i a l l y segregated siliceous sediments as such; the s i l i c a i s distributed in some homogeneous o r clumped fashion depending on the d i s t r i b u t i o n of the remains of silica-secreting organisms. Early diagenesis i n the f i r s t stages of shallow burial r e s u l t s i n the formation of nodular cherts replacing the carbona t e host (Siever, 1962).

9

Continental r i f t valley deposits containing abundant s i l i c a a r e deposited i n lacustrine environments (Eugster, 1969; Surdam and Eugster, 1976). These environments a r e dominated by a combination of volcaniclastic debris from nearby volcanoes and continental plutonic and supracrustal rock d e t r i t u s eroded from the r i f t borders. Volcaniclastics range from mafic t o s i l i c i c compositions. In humid regions, diatomaceous sediments may dominate, mixed w i t h clast i c s i n some lakes. Chemical precipitates such: as borates, s i l i c a t e s , and carbonates may be associated w i t h s i l i c a in lakes i n arid climates. In these environments more typically t h a n any others, the sources of the s i l i c a a r e not biogenic b u t derived from the s i l i c a released by the chemical a l t e r a t i o n of volcaniclastic material and of chemically.kprecipitated phases in non-marine evaporite lakes. The i n i t i a t i o n of a mid-ocean ridge and subsequent sea-floor spreading a t the s i t e of a former continental r i f t valley r e s u l t s i n narrow arms of t h e s e a , embayments, and s t r a i t s between the two r i f t e d borders of the formerly single continent. The change from a thinned continental t o an oceanic lithosphereand the change i n associated volcanism from mafic, intermediate, and s i l i c i c types t o t h o l e i i t i c basalts entrain the surface changes t h a t control sedimentary environments. Under arid conditions, because of the narrowness of the sea, evaporites may form, perhaps accompanied by some c l a s t i c s . The c l a s t i c materials a r e of continental plutonic and supracrustal s u i t e s and volcaniclastic material i s a t a m i n i m u m . The Gulf of California and the Persian Gulf o f f e r two d i f f e r ent examples o f t h i s kind of environment, the former providing an outstanding example of silica-deposition i n the presence of c l a s t i c s (Calvert, 1966). The distribution of runoff from the adjacent land masses and upwelling of deep waters i s important i n providing the essential nutrients f o r phytoplankton growth. In particular, large r i v e r systems bringing dissolved materials t o t h e narrow ocean may be the controlling influence on diatom abundance i n some areas. Here the ultimate source of the s i l i c a i s the weathering of terrigenous rocks. Upwelling may be an important mechanism even i n narrow seas, as analys i s of s i l i c a deposition i n the Gulf of California has shown (Calvert, 1966). As the opening sea widens, normal marine conditions evolve and the conditions f o r pelagic s i l i c a deposits are set. These are the well-known marine diatom and radiolarian oozes, mixed w i t h e i t h e r carbonate oozes o r f i n e clast i c s . The d e t r i t a l fraction of these sediments i s made u p of the general t e r rigenous component furnished t o the oceans by rivers and winds. In regions closer t o mid-ocean ridges, volcanic hot-spots, o r island arcs the volcaniclast i c component becomes important. The composition of t h a t volcaniclastic d e t r i tus i s a clue t o i t s derivation. The distribution of siliceous sediments i n the ocean f o r any plate-continent geography i s governed by the horizontal and

10 v e r t i c a l c i r c u l a t i o n and c l i m a t i c zones of t h e ocean, t h e r u n o f f from t h e cont i n e n t s and d i l u t i o n by c l a s t i c s , as was f i r s t o u t l i n e d i n d e t a i l by L i s i t s y n (1966).

Work o f t h e p a s t decade on t h e cores o f ' t h e Deep Sea D r i l l i n g Program

has g i v e n us much i n s i g h t i n t o d i s t r i b u t i o n s o f Mesozoic and C e n o z o i c s i l i c e o u s sediments and t h e i r diagenesis (Berger and von Rad, 1972; Heath and Moberly, 1971; Keene, 1975). The oceanic i n t r a p l a t e s e t t i n g grades i n t o t h e passive c o n t i n e n t a l margins o f t h e c o n t i n e n t edges formed by t h e o r i g i n a l r i f t i n g .

The slow subsidence o f

t h e c o n t i n e n t a l margins i s r e l a t e d t o t h e c o o l i n g and c o n t r a c t i o n o f t h e ocean i c l i t h o s p h e r e as i t moves away from t h e mid-ocean r i d g e where i t was formed (Parsons and S c l a t e r , 1977; Royden e t a l . ,

1980).

I s o s t a t i c adjustment t o t h e

increased sediment l o a d r e i n f o r c e s t h i s subsidence t o a c e r t a i n e x t e n t .

The

sedimentary environments o f t h e c o n t i n e n t a l shelves a r e w e l l known; t h e y a r e indeed some o f t h e environments b e s t explored by sedimentary g e o l o g i s t s .

How-

ever, they a r e n o t normally t h e s i t e o f e x t e n s i v e and t h i c k s i l i c a deposits. The formations we know o f i n t h e T e r t i a r y o f t h e U n i t e d States, such as those o f A t l a n t i c and G u l f Coastal P l a i n formations, a r e r e l a t i v e l y t h i n marine beds whose p r e c i s e environment o f d e p o s i t i o n remains u n c e r t a i n (Shattuck, 1907; Toulmin, 1955; Weaver and Wise, 1974).

They were deposited r e l a t i v e l y c l o s e t o

s h o r e l i n e s and i n t e r f i n g e r and i n t e r b e d w i t h f i n e c l a s t i c s .

Other s i l i c a f o r -

mations o f t h e c o n t i n e n t a l shelves a r e formed i n somewhat deeper waters o r f a r t h e r from shore.

The c o n d i t i o n s f o r diatomaceous deposits a r e determined by

h i g h phytoplankton p r o d u c t i o n i n r e l a t i o n t o u p w e l l i n g and oceanic c i r c u l a t i o n p a t t e r n s and t o near-shore n u t r i e n t supply from r i v e r s c a r r y i n g c o n t i n e n t a l runoff.

These deposits a r e associated w i t h i s o l a t i o n from abundant c l a s t i c s

t h a t would d i l u t e t h e s i l i c e o u s component.

Nodular c h e r t s formed by t h e r e -

placement o f carbonates a r e formed here as w e l l as i n shallow e p i c o n t i n e n t a l seas.

The Tampa (Miocene) f o r m a t i o n o f F l o r i d a i s one good example (Toulmin,

1955). Some o f t h e same sedimentary environments a r e found associated w i t h a c t i v e c o n t i n e n t a l margins, those r i m e d by subduction zones.

O f t h e many environ-

ments found i n a s s o c i a t i o n w i t h subduction zones, t h e trenches and a r c s them-

-

selves a r e r a r e l y t h e s i t e o f s i l i c a d e p o s i t i o n t h e sedimentation of t u r b i d i t e s i n t h e former and v o l c a n i c l a s t i c s i n t h e l a t t e r prevent t h a t . The s i t e s o f s i l i c a d e p o s i t i o n a r e p r i m a r i l y t h e f o r e - and back-arc basins, whose genera l c h a r a c t e r i s t i c s have been o u t l i n e d by Dickinson and Seely (1979).

Fore-arc

deposits may be o f e i t h e r nodular-replacement carbonate t y p e o r o f d i a t o m i t e s o r r a d i o l a r i t e s , depending on t h e c l i m a t e and t h e morphology o f t h e fore-arc region.

The l a t t e r depends p a r t l y on t h e l o c a l sediment l o a d i n t r o d u c e d t o t h e

subduction zone a t t h e a c c r e t i o n a r y prism, and p a r t l y by t h e amount and age of

11

the pelagic sediment being accreted. In highly productive zones, diatom and radiolarian populations and hence oozes may be abundant. The c l a s t i c material associated with these environments i s , as may be expected, dominated by thevolcaniclastics of the arc. The distribution of these materials i s determined partly by the prevailing winds carrying the tephra t o e i t h e r fore- o r back-arc regions o r both. Older arc-massif plutonic and metamorphic rocks may be exposed t o erosion and so contribute terrigenous material t o the front and rear of the arc. A continent on the f a r side of a back-arc basin may c o n t r i b u t e i t s share of heterogeneous terrigenous material t o the back-arc. Diatomites, radio l a r i t e s , and mixed biogenic-clastic deposits a r e the most comnon in these environments. SEQUENTIAL DIAGENETIC EVOLUTION Plate movements t r a n s l a t e the original o r very early diagenetic siliceous sediments into other plate tectonic environments. The l a t t e r then determine the burial histories (Table 11). TABLE I1 Sequential diagenetic evolution of cherts

Burial , heat flow, structural evolution Shallow b u r i a l , low heat flow, long burial periods, l i t t l e deformation RIFT VALLEY Deep burial beneath passive continental shelves, i n i t i a l l y h i g h heat flow decreasing to low Failed r i f t : aulacogen type: Deep b u r i a l , h i g h heat flow decreasing to low Failed r i f t : parallel type: Shallow t o moderate burial , moderate t o low heat f 1ow NARROW OCEAN Deep burial beneath passive continental shelves, 1ow heat flow Shallow burial , decreasing heat flow on ocean PELAGIC OOZE f 1oor 1eadi ng t o Subduction o r accretion; i f l a t t e r ultimately to T h r u s t b e l t a t continental collision ACTIVE MARGIN Moderate t o deep burial, low heat flow, u l t i mately t o Thrust b e l t a t continental collision PASSIVE MARGIN Moderate t o deep burial, low heat flow, u l t i mately over-ridden by Thrust sheets a t continental collision

Primary deposition CONTINENTAL INTRARLATE

Continental i n t r a p l a t e deposits a r e normally subject t o r e l a t i v e l y shallowburi a l on stable continental platforms t h a t subside slowly, rarely t o depths great e r than 2 kilometers. Heat flow i s normally a t the low end of continental

12

heat f l o w values, those more t y p i c a l o f Precambrian t e r r a i n s (Roy e t a l . , 1968). Geothermal g r a d i e n t s tend t o be o f t h e o r d e r o f 30"C./km.

Deformation i s

s l i g h t and t h e o u t s t a n d i n g c h a r a c t e r i s t i c s o f t h e rocks a r e t h a t they have had l o n g b u r i a l h i s t o r i e s , i n t e r r u p t e d , when near t h e surface, by f r e q u e n t unconf o r m i t i e s (Sloss, 1963).

These may induce changes i n groundwater c i r c u l a t i o n

and chemical composition.

The k i n e t i c s o f t r a n s f o r m a t i o n s o f these s i l i c e o u s

rocks a r e governed by t h e l o n g times a v a i l a b l e , n o t h i g h temperatures.

These

c o n d i t i o n s operate on nodular replacement c h e r t s , which may have k i n e t i c s o f t r a n s f o r m a t i o n f a s t e r than those t y p i c a l o f marine deposits. S i l i c a d e p o s i t s o f rift v a l l e y s may have any o f t h r e e major outcomes: ( 1 ) deep b u r i a l beneath passive margins as t h e r i f t opens t o a spreading ocean; ( 2 ) deep b u r i a l beneath an aulacogen, formed as a f a i l e d r i f t o f a t r i p l e j u n c -

t i o n (Burk and Dewey, 1973; Hoffman e t a l . ,

1974); o r ( 3 ) shallow t o moderate

b u r i a l i n a f a i l e d rift p a r a l l e l t o t h e successful r i f t and i n b o a r d on t h e continent.

I n each o f these types, t h e h i g h heat f l o w o f t h e rift g i v e s way t o

t h e lower heat f l o w o f t h e c o o l i n g and c o n t r a c t i n g l i t h o s p h e r e . g r a d i e n t s , which r u n as h i g h as 60"C./km

The geothermal

i n the r i f t s , gradually attenuate t o

values c l o s e r t o t h e 30" norm f o r c o n t i n e n t s .

The wide v a r i e t y o f t e c t o n i c h i s -

t o r i e s o f these f a i l e d r i f t s prevents making any simple g e n e r a l i z a t i o n s . The s i l i c e o u s sediments o f a narrow ocean, formed by i n i t i a t i o n o f s e a - f l o o r spreading, q u i c k l y e n t e r t h e oceanic i n t r a p l a t e environment.

Depending on t h e

w i d t h o f t h e ocean a t t h e t i m e o f sedimentation, t h e sediments a r e more o r l e s s q u i c k l y b u r i e d under t h e advancing and t h i c k e n i n g c o n t i n e n t a l r i s e sediments o f There t h e sediments a r e s u b j e c t t o deep b u r i a l beneath

t h e former r i f t border.

passive c o n t i n e n t a l shelves, w i t h s t e a d i l y decreasing heat f l o w and r e l a t i v e l y low geothermal g r a d i e n t s .

The temperatures and times o f b u r i a l depend on t h e

abundance o f sedimentary supply t o t h e c o n t i n e n t a l margin and t h e d u r a t i o n o f t h e passive c o n d i t i o n .

I f t h e sedimentation i s abundant and t h e d u r a t i o n i s

over 100 Ma, as f o r t h e c o n t i n e n t a l shelves o f t h e A t l a n t i c Ocean, these sediments may be b u r i e d t o depths g r e a t e r than 10 km and reach temperatures c l o s e t o those o f t h e anchi-metamorphic o r t r u e g r e e n s c h i s t metamorphic f a c i e s . The s i l i c e o u s oozes o f t h e oceanic i n t r a p l a t e regions a r e u l t i m a t e l y en r o u t e t o a subduction zone, f o r t h a t i s where a l l o f t h e sea f l o o r ends up, sooner o r l a t e r .

While t h e sediment i s on t h e sea f l o o r i t becomes b u r i e d t o

t h e t y p i c a l shallow depths c h a r a c t e r i s t i c o f p e l a g i c environments.

As i t moves

away from t h e mid-ocean r i d g e , t h e oceanic 1i t h o s p h e r e s t e a d i l y cools , c o n t r a c t s and subsides, t h e heat f l o w decreasing w i t h age, t h i c k n e s s of sediment, and thickness o f l i t h o s p h e r e .

Because p e l a g i c sediments a r e r a r e l y more than a

k i l o m e t e r t h i c k , and because o f t h e thinness o f t h e c r u s t , a t l e a s t i n i t i a l l y , t h e geothermal g r a d i e n t s a r e high, up t o and exceeding 60"C./km

i n placesclose

13 t o ridges. spot.

The h e a t f l o w w i l l l o c a l l y i n c r e a s e i f t h e p l a t e o v e r r i d e s a h o t

T h i s t r a n s i e n t may s i g n i f i c a n t l y a f f e c t t h e course o f diagenesis.

Before t h e sediment moves i n t o t h e subduction zone i t may f i r s t become more deeply b u r i e d i f i t comes i n t o t h e r e g i o n o f an abyssal p l a i n covered by t u r b i d i t e s , whose sedimentation r a t e s a r e g r e a t e r than those o f p e l a g i c oozes.

Then

the sediment moves i n t o t h e t r e n c h i t s e l f , w i t h f u r t h e r t u r b i d i t e sedimentation.

AS i t moves f u r t h e r i t e i t h e r e n t e r s t h e a c c r e t i o n a r y p r i s m and i s scraped o f f t h e t o p o f t h e descending l i t h o s p h e r e o r i s subducted w i t h t h e oceanic l i t h o sphere.

One open q u e s t i o n concerns t h e s t a t e o f diagenesis o f c h e r t s i f and

when t h e y e n t e r t h e a c c r e t i o n a r y prism.

I f they have had a l o n g and deep b u r i -

a l h i s t o r y under t h e r e l a t i v e l y h i g h geothermal g r a d i e n t s o f some p a r t s o f t h e sea f l o o r , they w i l l have a l r e a d y become opal-CT, and/or m i c r o c r y s t a l l i n e q u a r t z .

perhaps p a r t l y chalcedony

This t r a n s f o r m a t i o n induces e m b r i t t l e m e n t t h a t

may be i n g r e a t c o n t r a s t t o t h a t o f i n t e r v e n i n g f i n e c l a s t i c s .

S i l i c a oozes

t h a t have had s h o r t e r and shallower b u r i a l h i s t o r i e s may s t i l l be r e l a t i v e l y unl i t h i f i e d and more e a s i l y deformed.

The d i f f e r e n c e i n e m b r i t t l e m e n t has s t r o n g

i m p l i c a t i o n s f o r t h e e x t e n t o f d u c t i l e behavior o f c h e r t s i n m6langes formed from a c c r e t i o n a r y prisms.

For, u l t i m a t e l y , c o l l i s i o n w i t h an approaching con-

t i n e n t w i l l r e s u l t i n p i l i n g up o f t h r u s t sheets as t h e c o n t i n e n t a l apron of t h e advancing c o n t i n e n t c o l l i d e s w i t h t h e a c c r e t i o n a r y prism. The s i l i c e o u s sediments o f t h e a c t i v e margins are, i n a way, t h e obverse o f t h e sediments o f t h e approaching c o n t i n e n t t h a t had a passive margin h i s t o r y . The e a r l y h i s t o r y o f a c t i v e margin sediments, b o t h f o r e - and back-arc,

i s one

o f moderate t o deep b u r i a l under t h e low heat f l o w c o n d i t i o n s c h a r a c t e r i s t i c o f t h e subduction zone, where t h e cool oceanic l i t h o s p h e r e extends t o g r e a t d e p t h s .

A r a p i d r a t e o f sedimentation w i l l r e s u l t i f v o l c a n i c l a s t i c , arc-massif and cont i n e n t a l margin t e r r i g e n o u s d e t r i t u s a r e w e l l supplied.

This high rate, to-

gether w i t h t h e low mantle heat f l o w , produces geothermal g r a d i e n t s t h a t may be s u f f i c i e n t l y low t h a t t h e k i n e t i c t r a n s f o r m a t i o n s o f s i l i c a may n o t have enough time t o become w e l l advanced.

When these a c t i v e margin sediments a r e caught up

i n t h e deformation a t t e n d a n t on c o n t i n e n t a l c o l l i s i o n , t h e s i l i c e o u s sediments may be o n l y p a r t i a l l y l i t h i f i e d and thus more p l a s t i c a l l y deformable. The sediments o f t h e passive margins have a l r e a d y been discussed by i m p l i c a tion.

As they accumulate t o g r e a t thicknesses, i f sediment supply i s abundant

and l i t h o s p h e r i c c o n t r a c t i o n subsidence l o n g continued, t h e s i l i c e o u s sediments a r e subjected t o h i g h temperatures a t t h e bottom o f t h e p i l e . Sediments depos i t e d mid-way i n t h e c o n t i n e n t a l s h e l f ' s e v o l u t i o n w i l l reach v a r i o u s diagenet i c stages dependent on t h e i r b u r i a l depth and t h e stage o f heat f l o w attenuation.

As noted above, t h e u l t i m a t e f a t e o f these passive margins i s continen-

t a l collision.

14 C a l c u l a t i o n o f geothermal g r a d i e n t s as v a r i o u s tine-dependent f u n c t i o n s f o r a l l o f these d i f f e r e n t t e c t o n i c environments i s n o t easy.

A t present, m o s t c a l - ,

c u l a t i o n s , such as those o f Royden e t a l . (1980), r e l y on r e l a t i v e l y simple models w i t h many necessary s i m p l i f y i n g assumptions.

They do n o t t a k e i n t o ac-

count s h i f t s i n thermal regimes o f any complexity, i m p o r t a n t d i f f e r e n c e s i n thermal c o n d u c t i v i t y r e l a t e d t o compaction and cementation, and, perhaps more i m p o r t a n t i n sedimentary basins, convective motions o f groundwaters t h a t may t r a n s f e r a good deal o f heat, thus g i v i n g much l o w e r geothermal g r a d i e n t s than would be p r e d i c t e d from conduction alone.

Yet i t i s - r e n a r k a b l e , c o n s i d e r i n g

a l l o f these u n c e r t a i n t i e s , how w e l l some heat f l o w m o d e l l i n g has worked i n predicting the diagenetic states o f thermally a l t e r e d minerals.

I n the follow-

i n g s e c t i o n I discuss on a most s i m p l i f i e d b a s i s t h e thermal regimes p r e d i c t e d from these c o n s i d e r a t i o n s . BURIAL AND DIAGENETIC HISTORIES Figures 1-4 show diagrammatically t h e e v o l u t i o n o f a c o n t i n e n t a l rift v a l l e y t o narrow ocean, widening ocean, passive and a c t i v e margins, and c o n t i n e n t a l c o l 1is i on.

WATER SEDIMENT

Fig. 1. Schematic c r o s s - s e c t i o n o f a c o n t i n e n t a l r i f t v a l l e y f i l l e d w i t h a l l u v i a l and l a c u s t r i n e sediments. T r i a n g l e i d e n t i f i e s marker bed #1, a s i l i c e o u s l a k e bed whose b u r i a l e v o l u t i o n i s shown i n t h e f o l l o w i n g f i g u r e s . A t t h i s stage o f t e c t o n i c e v o l u t i o n , heat f l o w i s h i g h and l o c a l magmatic a c t i v i t y may produce hydrothermal veins. S t a r t i n g a t t h e time o f f o r m a t i o n o f a r i f t v a l l e y s i l i c e o u s l a k e d e p o s i t , t h e s e r i e s t r a c e s t h e e v o l u t i o n through a t y p i c a l 100 Ma c y c l e .

I n the series are

shown two marker beds, one deposited i n t h e r i f t v a l l e y l a k e , t h e o t h e r

15

deposited as a p e l a g i c ooze on t h e r i d g e f l a n k s o f t h e widening ocean t h a t developed from t h e rift.

The s p e c i f i c geometries o f these s i t u a t i o n s a r e d e t e r -

mined by t h e c o n f i g u r a t i o n of p l a t e boundaries, t h e previous h i s t o r i e s o f t h e

Fig. 2. The rift v a l l e y begins spreading from an e a r l y d i v e r g e n t zone and a narrow marine embayment invades t h e v a l l e y . Marker bed #1 i s now b u r i e d beneath l a t e r r i f t v a l l e y sediments and e a r l y c o n t i n e n t a l margin sediments o f t h e nascent sea. Heat f l o w continues h i g h a t t h e spreading c e n t e r b u t begins t o a t t e n u a t e a t t h e c o n t i n e n t edges where t h i c k marginal sediments a r e accumul a t i ng

.

Fig. 3. Continued spreading has produced a wide ocean w i t h an a c t i v e mid-ocean r i d g e . On one s i d e a subduction zone has formed and convergence begins t o c l o s e the ocean w i t h accumulation o f an a c c r e t i o n a r y prism, f o r e - a r c basin, v o l canic arc, and back-arc basin. Marker bed # 2 , a p e l a g i c s i l i c e o u s ooze, i s i d e n t i f i e d by a square. As convergence continues t h i s bed i s b u r i e d by p e l a g i c sediment and then i s accreted t o t h e a r c complex. Heat f l o w i s h i g h a t b o t h mid-ocean r i d g e and a t t h e a c t i v e v o l c a n i c arc; i t i s g e n e r a l l y low a t t h e t r e n c h and a c c r e t i o n a r y prism. Back-arc basins may a l s o have h i g h heat flow. c o n t i n e n t s r i f t e d , t h e spreading r a t e s , and t h e changing oceanic c i r c u l a t i o n and c l i m a t e .

The l a t t e r determine p a t t e r n s o f u p w e l l i n g and phytoplankton pro-

d u c t i v i t y t h a t govern n o t o n l y t h e diatom d i s t r i b u t i o n b u t i n d i r e c t l y those of the r a d i o l a r i a as w e l l .

These c o n d i t i o n s a r e b e s t s p e l l e d o u t by u s e o f t w o types

16 o f diagrams.

Fig. 4. Continued convergence r e s u l t s i n c o n t i n e n t - c o n t i n e n t c o l l i s i o n w i t h e a r l i e r subduction o f t h e mid-ocean r i d g e . A complex t h r u s t b e l t forms h i g h mountains i n which marker beds #1 and 2 a r e deformed and u p l i f t e d , e v e n t u a l l y t o be exposed t o meteoric waters and erosion.

POST-DEPOSITIONAL TIME, Ma

m

I

Fig. 5 . Sedimentation d i gram howing depth o f b u r i a l vs. p o s t - d e p o s i t i o n a l time f o r a s i l i c e o u s l a c u s t r i n e sediment, marker bed #1, deposited on t h e f l o o r o f a rift v a l l e y . L a c u s t r i n e sedimentation a t a r a t e o f 0.05 0.10 km/Ma i s shown as c o n t i n u i n g f o r about 2 Ma, f o l l o w e d by f l u v i a l sedimentation f o r t h e p e r i o d from about 2 t o 3 Ma, f o l l o w e d by another episode o f l a c u s t r i n e s e d i mentation, then another f l u v i a l i n t e r v a l , a l l o f these l a t e r sediments overl y i n g t h e o r i g i n a l sediment.

-

17

Figure 5 shows such a diagram f o r the r i f t valley case. I assume a history of r i f t floor alluvial fan and alluvial plain sedimentation mixed w i t h volcanics and volcaniclastics. Intermittent lakes form by the damming of r i v e r systems by volcanics o r tectonic depressions. The history shown here shows the r a t e of accumulation, and therefore b u r i a l , in a lake f o r a l i t t l e over 2 Ma, followed by an overlay of fluvial deposits f o r another 1 Ma, returning t o fluvial conditions a f t e r t h a t . The r a t e s of sedimentation are averages of high sedimentat i o n rates f o r these conditions. The second type of diagram used t o calculate the e f f e c t s of burial on diagenesis i s the temperature-time diagram, a p l o t of the temperature i n the formation of i n t e r e s t as a function of time elapsed since sedimentation. I t i s derived from the depth-time curve by inference of the geothermal gradient derived by deduction of heat flow from tectonic position and average thermal conductiv i t i e s . Such gradients could be calculated as equilibrium curves, i n which case

POST-DEPOSITIONAL TIME, Ma I

2

3

4

5

20 0

30

0

0 L

40

-

W

a 50 2 k

a 60 a W

a 70 2 W

k

80 90

Fig. 6. Temperature-time diagram f o r the sedimentation diagram shown i n F i g . 5. The i n i t i a l temperature of marker bed #1 i s the surface mean annual temperature. Later temperatures a r e estimated using elevated heat flow values c h a r a c t e r i s t i c of continental r i f t valleys (about 4 heat flow u n i t s ) and average thermal conductivity f o r near-surface sedimentary rocks. Regions shown f o r conversion t o opal-CT and quartz are rough estimates f o r 80%yields i n the following temperature-time diagrams ( F i g s . 8 and 10).

18 t h e s i m p l i f y i n g assumption i s made t h a t a t every t i m e t h e t o t a l c r u s t a l t h i c k ness i s i n e q u i l i b r i u m w i t h deep-seated and indigenous heat sources on t h e one hand, and t h e mean annual surface temperature on t h e o t h e r .

Changes i n t h e

slope o f such a curve would be responses e i t h e r t o changes i n t h e e q u i l i b r i u m g r a d i e n t s when mantle heat sources change, o r a r e responses t o changing r a t e s o f sedimentation.

O r both.

The more a p p r o p r i a t e temperature-time curve shown i n

F i g u r e 6 comes from c o n s i d e r a t i o n o f t h e conductive transmission o f heat through a c r u s t a l s e c t i o n , s t a r t i n g from an e q u i l i b r i u m s t a t e and perturbed by t h e a d d i t i o n o f sediment o r removal by e r o s i o n ( B i r c h e t a l . , 1968).

Using t h e heat

flow equation takes i n t o account t h e heat l a g as c o l d sediment i s r a p i d l y l a i d down and g r a d u a l l y heats up by conduction from below.

I n a l l these cases we

assume no indigenous sources o f heat such as t h i c k sediments w i t h h i g h uranium contents.

This dynamic temperature-time curve shows l o w e r temperatures than

would an e q u i l i b r i u m curve.

Even a t t h i s r a t e , however, t h e sediment heats up

enough a f t e r 5 Ma t o approach 100°C.

Consideration o f t h e k i n e t i c s o f t h e t r a n -

s i t i o n from opal-A t o opal-CT as given by Kastner e t a l . (1977) suggests t h a t e a r l y i n t h e sediment's h i s t o r y , w h i l e i t i s s t i l l being a c t i v e l y b u r i e d , t h e h i g h geothermal g r a d i e n t s a r e s u f f i c i e n t t o t r a n s f o r m t h e b u l k o f t h e sediment i n t o opal-CT.

A f t e r several m i l i i o n years t h e opal-CT i s transformed t o quartz.

On t h i s t y p e o f curve, t h e curves f o r t r a n s f o r m a t i o n s a r e n e g a t i v e exponentials. More p r e c i s e experimental i n f o r m a t i o n i s needed t o c a l c u l a t e t h e exact p o s i t i o n s o f d i f f e r e n t y i e l d s as f u n c t i o n s o f t i m e and temperature b u t t h e general topol o g y must be as shown. The same two types o f curve, depth o f b u r i a l vs. time and temperature vs. time, a r e shown i n Figures 7 and 8 f o r t h e second example, a p e l a g i c ooze dep o s i t e d on a spreading sea f l o o r .

I n t h i s case, t h e r a t e o f sedimentation i s

much lower and depends n o t o n l y on l o c a l s i l i c a - s e d i m e n t producing sources b u t on spreading r a t e s , f a s t e r r a t e s corresponding t o lower sedimentation r a t e s . have shown t h r e e stages i n t h i s accumulation p a t t e r n .

The f i r s t i s sedimenta-

t i o n o f s i l i c e o u s ooze above t h e carbonate compensation depth (CCD) and companied by carbonate oozes.

I

SO

ac-

The second stage begins a t about 15 Ma, when t h e

sea f l o o r has subsided below t h e CCD and i s deposited w i t h no carbonate, thus l o w e r i n g sedimentation r a t e s ( i n t h e absence o f any i m p o r t a n t change i n t e r r i The t h i r d stage s t a r t s a t about 25 Ma, when t h e sea f l o o r

genous supply).

passes i n t o t h e r e g i o n o f an abyssal p l a i n r e c e i v i n g t u r b i d i t e deposits, which increases t h e sedimentation r a t e s i g n i f i c a n t l y .

Given t h i s curve we can e s t i -

mate an e q u i l i b r i u m p r o f i l e t h a t comes c l o s e t o a c t u a l i t y because o f t h e low sedimentation r a t e s . As w i l l be seen, t h e temperatures a r e e l e v a t e d s u f f i c i e n t l y over l o n g p e r i o d s o f t i m e t h a t t h e t r a n s i t i o n s from opal-A t o opal-CT t o q u a r t z occur w i t h i n 30-35 Ma.

This e s t i m a t e cannot be considered t o be a s i m p l e

19

POST-DEPOSITIONAL TIME, Ma

0

1000 DISTANCE FROM MOR AT 5 cm/yr SPREADING RATE

500

1500

Fig. 7. Sedimentation diagram f o r a p e l a g i c s i l i c e o u s ooze, marker bed #2, dep o s i t e d on a spreading sea f l o o r . S i l i c e o u s and carbonate sediments a r e depos i t e d w h i l e t h e sea f l o o r i s above t h e Calcium Carbonate Compensation Depth (CCD) from 0-15 Ma. A t 15 Ma t h e sea f l o o r has subsided below t h e CCD a n d s i l i ceous ooze alone i s deposited a t a lower t o t a l sedimentation r a t e . A t about25 Ma t h e sea f l o o r passes t o an abyssal p l a i n w i t h t u r b i d i t e s deposited a t amore s i g n i f i c a n t i n c r e a s e i n sedimentation r a t e . Distance from a mid-ocean r i d g e (MOR) i s shown assuming a u n i f o r m 5 cm/yr spreading r a t e . e x t r a p o l a t i o n from experimental data.

Rather i t i s i n f e r r e d from t h e sedimen-

t a r y p e t r o l o g i c work on t h e deep sea sediments r e p o r t e d i n t h e v a r i o u s volumes o f the Deep Sea D r i l l i n g Program, i n which c h e r t petrography can be matchedwith p l a t e age, depth o f b u r i a l , and approximate geothermal g r a d i e n t .

It i s c l e a r

t h a t any s i g n i f i c a n t l y l o n g t r a v e l t i m e o f a p l a t e w i l l r e s u l t i n c h e r t i f i c a t i o n under t h e monotonous b u r i a l regime o f t h e sea f l o o r . The f i n a l case i s t h e most d i f f i c u l t , t h e f u r t h e r h i s t o r y o f marker bed #2 deposited as a p e l a g i c ooze when i t approaches a subduction zone, perhaps t o be scraped o f f and i n c o r p o r a t e d i n t o an a c c r e t i o n a r y prism (Figures 9 and 10).

Now

shown on a d i f f e r e n t s c a l e t o a l l o w f o r much t h i c k e r depths o f b u r i a l , t h e f i r s t p a r t o f t h e curve f o r b u r i a l - t i m e i s t h e same as t h a t o f t h e e a r l i e r f i g u r e s . This curve then c a r r i e s on t h e h i s t o r y t o t h e t i m e when i t moves i n t o t h e t r e n c h and then a c c r e t i o n a r y prism.

The b u r i a l i n t h e a c c r e t i o n a r y p r i s m i s estimated.

We have no r e a l f i e l d data on geologic h i s t o r y i n r e l a t i o n t o b u r i a l o t h e r than

20

POST-DEPOSITIONAL TIME, Ma 5 10 15 20 25 30 35 0 0

0 L

W

a

3 + a

oz

0

5 10 15

20

W

n 25 2 30 W

t-

35

F i g . 8. Temperature-time diagram f o r the sedimentation diagram shown i n Figure 7. The i n i t i a l temperature of marker bed #2 i s the bottom water temperature. Later temperatures a r e estimated f o r gradually decreasing heat flow according to the model of Parsons and Sclater (1977).

the stratigraphic work of Audley-Charles (1972) and coworkers and of Karig and Moore (1975), Moore (1979) and others on fore-arc islands such as Nias, where the prisms a r e t h r u s t up above sea level. In any case, i t can be seen fromFigure 10 t h a t even a t the low heat flows c h a r a c t e r i s t i c of subduction zones, the depths and times a r e great enough t h a t continued diagenesis takes place, involving deformation accompanied by grain growth of the early microcrystalline q u a r t z . Any opal-CT l e f t unconverted i n e a r l i e r stages disappears here. A curve f o r a pelagic sediment deposited only a few million years before subduction would, of course, show a different history. Such a deposit may have neither s u f f i c i e n t time nor high enough temperatures a t the low heat flows of the accretionary prism to be converted completely and residues of unconverted opalCT m i g h t remain. From this consideration i t becomes c l e a r why cherts have already become completely l i t h i f i e d and mechanically b r i t t l e by the time they are-incorporated into m6langes. A t t h i s time they are a l s o f a r lower in temperature than the b r i t t l e - d u c t i l e ,transition t h a t takes place a t h i g h temperatures and confining pressures.

21

POST-DEPOSITIONAL TIME, Ma

20

40

I

I

100 1

\

3000

80

60

\

1

1000 0 DISTANCE FROM SUBDUCTION ZONE AT 5 c m / y r CONVERGENCE RATE

2000

Fig. 9. Sedimentation diagram f o r marker bed #2 from i n i t i a l deposition t o i n corporation i n an accretionary prism. The f i r s t part of t h i s curve i s the same as that i n Fig. 7 . After 60 Ma i t moves into the accretionary prism where i t is buried more deeply. After 100 Ma the sediment i s l i k e l y t o be involved i n a collisional orogeny and be incorporated i n t o a complex thrust b e l t as i n F i g . 4. This results i n surface erosion and decreasing depth of burial. CONCLUSION The p o i n t o f t h i s examination o f burial-temperature conditions i n relation t o s i l i c a diagenesis is t o serve as a guide to future work. If we a r e t o deduce the history of s i l t c a deposits best, we should be able t o analyze the interactions between the sedimentary environments, tectonics, and diagenetic transformations. If we were to know the kinetics of chert transformations in great det a i l as a function of time, temperature and chemical surroundings, we couldpossibly reconstruct the geothermal regimes responsible f o r those transformations. This requires good stratigraphic knowledge t o construct the depth o f burial

22

POST-DEPOSITIONAL TIME, Ma 20 40 60 80 100

Fig. 10. Temperature-time diagram f o r t h e sedimentation diagram shown i n F i g . 9 . Temperatures i n t h e a c c r e t i o n a r y p r i s m a r e estimated u s i n g t h e low heat f l o w values associated w i t h subduction zones. As a r e s u l t o f t h e u p l i f t and e r o s i o n a f t e r 100 Ma, t h e temperature s t a r t s t o decrease. The prolonged p e r i o d o f deep b u r i a l a t h i g h temperatures r e s u l t s i n c r y s t a l growth, pressure s o l u t i o n , and deformation o f q u a r t z g r a i n s d e r i v e d by conversion o f opal-CT. curve.

On t h e o t h e r hand, knowing something o f t h e p l a t e t e c t o n i c h i s t o r y we

m i g h t b e t t e r be a b l e t o r e c o n s t r u c t t h e o r i g i n a l times and environments ofdeposition.

I t i s l i k e l y t h a t t h e n e x t t a s k i s t o c o n s t r u c t these k i n d s o f curve

f o r r e a l deposits, those f o r which we have much knowledge, so t h a t we c a n a r r i v e a t t h e k i n e t i c s by i n f e r e n c e from t h e geology r a t h e r than e x t r a p o l a t i n g from n e c e s s a r i l y s h o r t - t e r m l a b o r a t o r y experiments.

Those experiments a r e needed t o

f u r t h e r s p e c i f y r a t e constants as f u n c t i o n s o f t h e p h y s i c a l s t a t e o f t h e a l t e r i n g s i l i c a and t h e chemical surroundings t h a t may so g r e a t l y a f f e c t t h e k i n e tics.

23 ACKNOWLEDGEMENTS This work was done under t h e auspices o f NSF Grant EAR-7904364. on the manuscript and many s t i m u l a t i n g discussions I thank A. Kas t ner

.

Forcomments I i j i m a and M.

REFERENCES Audley-Charles, M.G., 1972. Cretaceous deep-sea manganese nodules on Timor; i m p l i c a t i o n s f o r t e c t o n i c s and o l i s t o s t r o m e development. Nature, Phys. Sci., 240: 137-139. Berger, W.H. and von Rad, U . , 1972. Cretaceous and Cenozoic sediments from t h e A t l a n t i c Ocean. I n : D.E. Davies, A.C. Pimn e t a l . ( E d i t o r s ) , I n i t i a l Reports of the Deep Sea D r i l l i n g P r o j e c t . U.S. Government P r i n t i n g O f f i c e , X I V : 787954. Birch, F., Roy, R.F. and Decker, E.R., 1968. Heat f l o w and thermal h i s t o r y i n New England and New York. I n : E-An Zen, W.S. White, J.B. Hadley and J.B. Thompson, Jr. ( E d i t o r s ) , Studies o f Appalachian Geology, Northern and Maritimes. I n t e r s c i e n c e , New York, pp. 437-451. Burke, K. and Dewey, F.F., 1973. Plume-generated t r i p l e j u n c t i o n s : Key i n d i c a t o r s i n a p p l y i n g p l a t e t e c t o n i c s t o o l d rocks. J. Geol., 81: 406-433. C a l v e r t , S.W., 1966. Accumulation o f diatomaceous s i l i c a i n t h e sediments o f t h e Gulf of C a l i f o r n i a . Geol. SOC. Am. B u l l . , 77: 569-596. Dickinson, W.R. and Seely, D.R., 1979. S t r a t i g r a p h y and s t r u c t u r e o f f o r e a r c regions. Am. Assn. P e t r . Geol. B u l l . , 63: 2-31. Eugster, H.P., 1969. I n o r g a n i c bedded c h e r t s from t h e Magadi area, Kenya. Contri. M i n e r a l . and Petrology, 22: 1-31. Heath, G.R. and Moberly, R., Jr., 1971. Cherts from t h e western P a c i f i c , Leg 7 Deep Sea D r i l l i n g P r o j e c t . I n : E.L. Winterer e t a l . ( E d i t o r s ) , I n i t i a l Rep o r t s of t h e Deep Sea D r i l l i n g P r o j e c t . U.S. Government P r i n t i n g O f f i c e , V I I : 991-1007. Hoffman, P., Dewey, J.F., and Burke, K., 1974. Aulacogen and t h e i r g e n e t i c r e l a t i o n t o geosynclines, w i t h a P r o t e r o z o i c example from Great Slave Lake, Canada. I n : R.H. D o t t , J r . and R.H.Shaver ( E d i t o r s ) , Modern and Ancient Geosynclinal Sedimentation. SOC. Econ. Paleont. M i n e r a l . Spec. Publ. 19, pp. 38-55. Karig, D.E. and Moore, G.F. , 1975. T e c t o n i c a l l y c o n t r o l l e d sedimentation i n marg i n a l basins. E a r t h Planet. Sci. L e t t . , 26: 233-238. Kastner, M., Keene, J.B. and Gieskes, J.M., 1977. Diagenesis o f s i l i c e o u s o o z e s I. Chemical c o n t r o l s on t h e r a t e o f opal-A t o opal-CT t r a n s f o r m a t i o n an experimental study. Geochim. Cosmochim. Acta, 41: 1041-1059. Keene, J.B., 1975. Cherts and p o r c e l a n i t e s from t h e N o r t h P a c i f i c . I n : R.L. Larson, R. Moberly e t a l . ( E d i t o r s ) , I n i t i a l Reports o f t h e Deep Sea D r i l l i n g P r o j e c t . U.S. Government P r i n t i n g O f f i c e , X X X I I : 429-507. L i s i t s y n , A.P., 1966. Basic r e l a t i o n s h i p s i n d i s t r i b u t i o n o f modern s i l i c e o u s sediments and t h e i r connection w i t h c l i m a t i c zonation ( t r a n s l a t i o n ) . I n t . Geol. Rev., 9: 631-652, 842-865, 980-1004, 1114-1130. Moore, G.F., 1979. Petrography o f subduction zone sandstones from Nias I s l a n d , Indonesia. J. Sed. Petrology, 47: 71-84. Parsons, B. and S c l a t e r , J.G., 1977. An a n a l y s i s o f t h e v a r i a t i o n o f ocean f l o o r heat f l o w and bathymetry w i t h age. J. Geophys. Res., 82: 803-827. Roy, R.F., Decker, E.R., B l a c k w e l l , D.D., and B i r c h , F., 1968. Heat f l o w i n t h e U n i t e d States. J. Geophys. Res., 73: 5207-5221. 1980. Continental margin subRoyden, L., S c l a t e r , J.G., and von Herzen, R.P., sidence and heat f l o w : Important parameters i n f o r m a t i o n o f petroleum hydrocarbons. Amer. Assn. P e t r . Geol. B u l l . , 64: 173-187.

-

24

Shattuck, G.B., 1907. The Geolosv __ of C a l v e r t C0unt.y. Maryland Geol. Survev: C a l v e r t County, pp. 67-121. Siever, R., 1962. S i l i c a s o l u b i l i t y , 0"-2OO"C, and t h e diagenesis o f s i l i c e o u s sediments. J. Geol., 70: 127-149 Siever, R., 1979. P l a t e t e c t o n i c c o n t r o l s on diagenesis. J. Geol., 87: 127-155. Siever, R. and Hager, J.L., 1981. Paleogeography, t e c t o n i c s and thermal h i s t o r y of some A t l a n t i c margin sediments. I n : J.W. Kerr ( E d i t o r ) , North A t l a n t i c Borderlands, Canad. SOC. P e t r o l . Geol. Memoir 11, pp. 95-117. Siever, R., 1982. Geological problems i n t h e geochemistry o f sediments. 1n:J.A. Simon and R. Ber strom ( E d i t o r s ) , Perspectives i n Geology, I l l i n o i s Geol. Survey C i r c u l a r ? i n press). Sloss, L.L., 1963. Sequences i n t h e c r a t o n i c i n t e r i o r o f North America. Geol. SOC. Amer. B u l l . , 74: 93-114. Surdam, R.C. and Eugster, H.P., 1976. Mineral r e a c t i o n s i n t h e sedimentary dep o s i t s o f t h e Lake Magadi r e g i o n , Kenya. Geol. SOC. Amer. B u l l . , 87: 1739-

1752. Toulmin, L.D., 1955. Cenozoic geology o f southeastern Alabama, F l o r i d a , and Georgia. Amer. Assn. P e t r o l . Geol. B u l l . , 39: 207-235. Weaver, F.M. and Wise, S.W., 1974. Opaline sediments o f t h e southeastern coast a l p l a i n and Horizon A: b i o g e n i c o r i g i n . Science, 184: 899-901. Wilson, J.L., 1975. Carbonate Facies i n Geologic H i s t o r y . Springer-Verlag, Berl i n , Heidelberg, New York, 471 pp.

25

CHAPTER 3 COMPARISONS BETWEEN OPEN-OCEAN AND CONTINEIdTAL MARGIN CHERT SEQUENCES

JAMES R.

U.

HEIN,

Geological Survey,

S.

345 M i d d l e f i e l d Road, Menlo Park,

C a l i f o r n i a 94025

M.

SUSAN

KARL,

Department

of

Geological

Sciences,

Stanford

University,

Stanford, C a l i f o r n i a 94305

ABSTRACT I n order t o determine t h e d e p o s i t i o n a l environments o f orogenic b e l t cherts, we compared c h e r t sequences t h a t we s t u d i e d i n Costa Rica,

California,

and

Alaska w i t h c h e r t s recovered by t h e Deep Sea D r i l l i n g P r o j e c t d u r i n g Legs 62 and 69.

Leg 62 recovered cores i n open-ocean environments on Hess Rise and t h e

M i d - P a c i f i c Mountains, and Leg 69 recovered deposits from t h e south f l a n k o f t h e Costa Rica R i f t .

Comparisons o f

1 i t h o l o g i c associations,

sedimentary

s t r u c t u r e s , chemistry, sedimentation rates, and modes o f f o r m a t i o n c l e a r l y show t h a t c h e r t sequences o c c u r r i n g i n orogenic b e l t s a r e not analogous t o c h e r t s t h a t have been d r i l l e d i n open-ocean environments. sections we s t u d i e d apparently continental mrgins.

The orogenic b e l t c h e r t

formed i n t e c t o n i c a l l y produced basins near

No known c h e r t s o f any age d r i l l e d i n t h e P a c i f i c Ocean

basin resemble t h e r i b b o n c h e r t s observed on land. Cherts from t h e deep sea occur m i n l y as nodules o r lenses i n ,

o r above,

limestone and chalk, and compose a small percentage o f t h e o v e r a l l sedimentary sequences. 69,

T y p i c a l deep-sea open-ocean c h e r t s such as those from Legs 62 and

formed by replacement o f calcareous o r clayey sediments, whereas,

ribbon cherts California,

i n orogenic b e l t

and Alaska,

such as those from Costa Rica,

formed by d i a g e n e t i c r e c r y s t a l l i z a t i o n o f a l t e r n a t i n g

c o n t r a s t i n g sediment types, o r sandstone.

sequences,

bedded

such as s i l i c e o u s ooze and hemipelagic clay, t u f f ,

Many o f t h e c h e r t beds were l a i d down by t u r b i d i t y currents.

The c h e r t s we s t u d i e d i n orogenic b e l t s a r e i n d e p o s i t i o n a l c o n t i n u i t y w i t h greenstone as w e l l as graywacke t u r b i d i t e s .

Sections r a r e l y ,

i f a t a l l , have

limestones associated w i t h them.

INTRODUCTION Comparison o f c h e r t s recovered from t h e P a c i f i c Ocean b a s i n by t h e DSDP (Deep Sea D r i l l i n g P r o j e c t ) w i t h c h e r t s i n several a c c r e t i o n a r y t e r r a n e s along

t h e western North and Central American continental margins has been frustrating because of the lack of analogues f o r the ribbon cherts from the accretionary terranes among the DSDP cores. Consequently, we have attempted t o i s o l a t e the c r i t i c a l features that distinguish various types of chert in order t o decipher t h e i r environments of deposition. The tools we have t o work with include sedimentary structures, bedding types, accumulation rates, micropal eontol ogy, chemistry, mineralogy, and lithologic associations. The variety of cherts defined by different combinations of these factors imply a corresponding variety of environments of chert format ion. The cherts considered in t h i s paper include samples from DSDP Legs 62 (Hess Rise and Mid-Pacific Mountains) and 69 (Costa Rica R i f t ) , a s well as samples from the Nicoya Complex of Costa Rica, the Franciscan Complex of California, and the Kelp Bay Group in southeastern Alaska (Fig. 1).

Other workers have interpreted cherts from the accretionary b e l t s as deepsea sediments deposited on seamounts or a t spreading centers because of the cherts common association with pillow basalts. Environmentally, Legs 62 and 69 recovered open-ocean, deep-sea deposits resting on basalts and should therefore be analogues of the orogenic belt chert sections, b u t , in f a c t , they a r e not analogous. Our observations of cores from other DSDP legs, a s well as of d r i l l i n g results in DSDP Initial Reports suggest that no cherts of any age d r i l l e d in the Pacific Ocean basin rese&le the ribbon cherts observed on land. The main differences we observed between the Leg 62 and 69 cherts, characteristic of cherts recovered from most DSDP legs, and the ribbon cherts from Costa Rica, California, and Alaska include: 1) Leg 62 and 69 cherts comprise l e s s than 5% of predominantly chalk/limestone sections, whereas ribbon cherts comprise a t least 50% of sequences consisting a l s o of basalt, shale, sandstone, and rarely limestone; 2 ) Leg 62 and 69 cherts occur as lenses, nodules, and stringers on a millimeter or centimeter scale, whereas ribbon cherts consist of beds up t o tens of centimeters thick that extend l a t e r a l l y for meters t o tens of meters; 3) Leg 62 and 69 cherts are characterized by extensive burrowing and burrow mottling, whereas ribbon cherts show l i t t l e i f any evidence of burrowing and commonly retain a laminated character; and 4 ) Leg 62 and 69 cherts show definite replacement textures as the result of volume-for-volume s i l i c i f i c a t i o n of the host chalks and limestones, whereas the ribbon cherts do not; radiolarians and radiolarian debris display original compositions and textures, and occasionally compose graded or convolute beds suggestive of

F i g u r e 1 . L o c a t i o n of L e g s 6 2 and 69 DSDP s i t e s c o n t a i n i n g o p e n - o c e a n c h e r t 6 and s i l i c e o u s o o z e s and of P a c i f i c r i m c o n t i n e n t a l m a r g i n c h e r t s e q u e n c e s .

28 t u r b i d i t e deposition. On t h e basis o f these d i f f e r e n c e s we conclude t h a t

other environments

besides mid-oceanic spreading centers and seamounts must be considered as t h e places o f o r i g i n f o r ribbon cherts.

PREVIOUS WORK At

t h e t u r n o f t h e century geologists recognized an a s s o c i a t i o n between

ribbon cherts and g r a p t o l i t i c shales i n Newfoundland and Great B r i t a i n (Peach and Horn, 1899; Dewey and F l e t t , 1911; Sampson, 1923; a l l in D i e t z and Holden, 1966) and concluded t h a t t h e c h e r t s must be deep marine deposits because t h e graptolitic

shales

were.

In

reviewing

the

literature with

respect

to

occurrences o f ribbon chert, D i e t z and Holden (1966) found c o n s i s t e n t r e l a t i o n s between black shale and c h e r t and some associated graywacke, t u f f , basalt, o r limestone. must have

They concluded from these l i t h o l o g i c associations t h a t t h e c h e r t s been deposited i n deep water, i n a so-called eugeosynclinal

environment, a t t h e t o e o f t h e slope o r on t h e c o n t i n e n t a l rise. Kimura (1973) found s i a l i c geosynclinal m t e r i a l oceanward o f greenstoneand chert-bearing eugeosynclinal m a t e r i a l o f t h e Shimanto, Nakamura, and Neogene b e l t s i n Japan.

He observed t h a t t h i s eugeosynclinal m t e r i a l had not

been deformed i n a subduction zone and concluded t h a t t h i s m a t e r i a l was not deposited on an oceanic plate. Cherts

in

Upper

Devonian

to

Middle

Permian

sequences

i n Japan

are

incorporated w i t h a l k a l i n e and t h o l e i i t i c rocks o v e r l y i n g S i l u r i a n t o Middle Devonian r e e f o i d 1imestone,

s h a l l ow r m r i ne v o l c a n i c l a s t i c sedimentary rocks,

and r h y o l i t i c t o a n d e s i t i c volcanic rocks. Kanmera (1974) i n t e r p r e t e d t h i s assemblage as being deposited i n an i n t e r a r c o r marginal basin generated during l a t e s t Devonian t i m e and which g r a d u a l l y widened through Permian time. R a d i o l a r i a n c h e r t s o f t h e Cretaceous Shimantogawa Group a r e i n t e r c a l a t e d w i t h tuffaceous l a y e r s which f i n e upward i n g r a i n s i z e (Kanmera, 1974). I i j i m a e t al. (1978) considered t h a t t h e T r i a s s i c c h e r t sections i n Japan were o r i g i n a l l y deposited i n low energy environments with i n t e r m i t t e n t i n f l u x e s o f terrigenous clay,

which c o n s i s t s predominantly o f i l l i t e ,

chlorite,

and

c r y p t o c r y s t a l l i n e quartz. They i n t e r p r e t e d t h i s sequence as being deposited during T r i a s s i c time i n a marginal sea i n t h e area o f c e n t r a l and southwestern Japan, between t h e Hida metamorphic rocks and t h e Kurosegawa-Ofunato t e r r a n e i n t h e outer p a r t o f t h e Japanese i s l a n d arc. A comparison by Shimizu and Masuda (1977) o f REE ( r a r e e a r t h elements) signatures o f Permian t o Jurassic cherts i n Japan w i t h c h e r t s and s i l i c e o u s

m i c r o f o s s i l s from DSDP samples showed d i f f e r e n t p a t t e r n s f o r t h e two groups o f

29

cherts.

As

the

c h a r a c t e r i s t i c of

Japanese

cherts

lacked

the

seawater and marine cherts,

negative

cerium

anomaly

they p o s t u l a t e d a t e r r i g e n o u s

i n f l u e n c e i n t h e f o r m a t i o n o f t h e Japanese cherts,

and favored marginal o r

land-enclosed sea environments. The Upper-Mesozoic

Ligurian cherts

i n northern

I t a l y consist

o f graded

r a d i o l a r i t e and s i l i c e o u s mudstone beds deposited on p i l l o w b a s a l t and o v e r l a i n by limestone and u l t i m a t e l y f l y s c h ( B a r r e t t , of

this

s e c t i o n show a

1981).

f l a t t e n e d Ce-anomaly

Because t h e lower c h e r t s

i n REE analyses

m a t e r i a l may have a f f e c t e d t h e e n t i r e s e c t i o n ( B a r r e t t , found t h a t t h e o v e r a l l major element composition, predominate

aluminosilicate

fraction

of

the

i n c r e a s i n g t e r r i g e o u s e f f e c t up section.

1981).

terrigenous Barrett also

as w e l l as t h e muscovite-

upper

cherts,

also

reflected

B a r r e t t concluded t h a t t h e c h e r t s

were deposited i n a rugged, slow-spreading oceanic r i d g e environment, which he a t t r i b u t e d t o e a r l y stages o f c o n t i n e n t a l r i f t i n g .

R i f t i n g ceased by M i d d l e

i n B a r r e t t ' s model, and was succeeded by b a s i n c l o s u r e along

Cretaceous time,

an e a s t e r l y d i p p i n g subduction zone. Differences i n t h e 6l80 values o f orogenic b e l t c h e r t s and DSDP c h e r t s l e d Kolodny and E p s t e i n

(1976)

t o propose a

shallower

water

environment

for

f o r m a t i o n o f c h e r t s found i n convergent margin sequences. Steinberg and M a r i n (1978) recognized a geochemical p a t t e r n i n c h e r t s they s t u d i e d from Greece. affinities,

Cherts

low i n t h e sequences had mid-oceanic

t h a t i s h i g h Mn and Fe values r e l a t i v e t o c h e r t s h i g h e r i n t h e

sequences which had c o n t i n e n t a l i m r y i n a f f i n i t i e s , h i g h e r A1 values. i t was

ridge

therefore

possible t o

through time and space,

track,

geochemically,

changing

They f e l t

environments

as w e l l as being a b l e t o d i s t i n g u i s h c h e r t s deposited

i n d i f f e r e n t environments. Blake e t al. California,

(1981) favored a r e s t r i c t e d rift basin, s i m i l a r t o t h e G u l f o f

f o r t h e Y o l l a B o l l y t e r r a n e o f t h e Franciscan Complex, which i s

composed predominantly o f nudstone, and thin-bedded and m s s i v e metagraywacke sandstone w i t h v o l c a n i c rocks and r a d i o l a r i a n c h e r t i n places. One o f t h e d i f f i c u l t i e s w i t h t h e common a s s o c i a t i o n o f r i b b o n c h e r t s and p i l l o w b a s a l t s i s t h a t t h e sequence observed on mid-ocean spreading c e n t e r s c o n s i s t s o f calcareous deposits on t h e basalt,

f o l l o w e d by s i l i c e o u s deposits

and p e l a g i c c l a y as t h e oceanic c r u s t moves away from t h e spreading a x i s and f a l l s because o f depth).

thermal

subsidence below t h e CCD

(carbonate compensation

W i n t e r e r and Jenkyns (1979) suggested t h a t i n small ocean basins and

along c o n t i n e n t a l margins t h e combination o f u p w e l l i n g w i t h attendant increased biogenic p r o d u c t i v i t y and an e l e v a t e d CCD w i t h p e r i o d i c i n f l u x e s o f c l a y would g i v e r i s e t o a l t e r n a t i o n s o f c l a y and s i l i c e o u s ooze.

I n a d d i t i o n they noted

t h a t t h e CCD was shallower i n t h e L a t e Mesozoic than i t i s today, and t h i s my have been a f a c t o r i n t h e d e p o s i t i o n o f r i b b o n c h e r t s i n orogenic b e l t s .

30

SILICEOUS DEPOSITS I t i s well established that biogenic s i l i c a (opal-A), including radiolarians, diatoms, silicoflagel lates, and sponge spicules, i s the primary source of s i l i c a f o r a l l the cherts we studied (Hein e t al., 1981a, 1982a; Kuijpers, 1980; unpublished d a t a ) . Some workers consider submarine volcanism t o be a primary alternative source of s i l i c a , whereas other workers believe t h a t volcanogenic s i l i c a plays e i t h e r a n indirect role by triggering blooms of silica-secreting organisms or a secondary role through a l t e r a t i o n of volcanic glass, b u t does not form primary deposits of chert (Bailey e t al., 1964; Mattson and Pessagno, 1971). Because siliceous organisms are always present, we i n f e r that biogenic s i l i c a i s the main source of s i l i c a in most cherts (Hein e t al., 1978; Iijima e t al., 1978). Siliceous deposits my accumulate through pelagic deposition under areas of high productivity (Van Andel e t al., 1975; Leinen, 1979) and by redeposition as siliceous turbidites.

Siliceous turbidites consist mainly of graded beds with

very rare cross-laminations and convolute layers, as well as varying amounts of radiolarian fragments and siliceous debris; t h i s evidence suggests that they are redeposited material from local topographic highs, as described by Nisbet a n d Price (1974) for Mesozoic ribbon cherts on Cyprus, by Barrett (1981) f o r Upper Mesozoic ribbon cherts in northern Italy, by Hein et al. (1981a, 1982a), f o r Mesozoic cherts in Costa Rica, by Torrini and Mattioli (in Speed and Larue, 1982) for Eocene radiolarites on Barbados, by Karl (unpublished data) f o r Upper Mesozoic ribbon cherts from the central belt of the Franciscan Complex, and by Tucholke, Vogt, et al. (1979) for alternations of graded calcareous siliceous ooze and clay from DSDP Sites 386 and 387, although these have a l s o been interpreted as radiolarian productivity cycles (McCave, 1979). S i t e s 386 and 387, located southeast and west of the island of Bermuda, respectively are the only two DSDP s i t e s of the f i r s t 500 s i t e s d r i l l e d from which rhythmically bedded r a d i o l a r i t e and clay t h a t could ultimately form ribbon cherts have been recognized. Of the Pacific DSDP s i t e s t h a t penetrated continental margin deposits, none contained rocks older t h a n Middle Tertiary, and most of the rocks that were recovered a r e poorly 1i t h i f i ed. However, 1i tho1 ogic associations distinguish sections from these continental margin s i t e s from those d r i l l e d into deep, open-ocean pelagic sequences. A t S i t e s 184 and 185 in the southern Bering Sea, laminated diatom ooze i s intercalated with clay, ash, terrigenous s i l t , and minor amounts of limestone. A t S i t e s 33 and 34, located off northern California, dark greenish-gray siliceous-fossil cherty mudstone was drilled. A t S i t e 173 offshore Oregon, muddy and, in places, sandy diatomaceous chert was associated with rare thin sand layers and common ash beds, overlying an ashbearing clayey dolomite t h a t was deposited on andesite. S i t e s 301 and 302,

31

drilled in the Sea of Japan, penetrated undiluted diatomaceous sediment of early through l a t e Pliocene age that contains increasing amounts of sand, ash The f i r s t layers, and terrigenous d e t r i t u s up section into the Pleistocene. appearance of calcareous microfossils in the Pleistocene a t S i t e s 301 and 302 i s thought t o mark depositional crossing of the CCD by Karig, Ingle, e t al. (1975). The layering in these continental margin and marginal basin sequences d r i l l e d by the DSDP i s on a scale of millimeters, and i s thus d i s t i n c t from the layering in ribbon cherts in orogenic belts which i s on a scale of a centimeter t o tens of centimeters. However, the continental margin sediments a r e unlithified, and diagenetic and l i t h i f i c a t i o n processes that will convert them t o rocks a r e poorly understood. I t should be noted that the types of sediment a r e comparable t o sequences recognized in orogenic belts. A t s i t e s d r i l l e d a t open-ocean spreading centers, such as S i t e 157 near t h e Galapagos Rift, the chert is calcareous and has well-preserved, mottled bedding and burrows; these features indicate pervasive bioturbation. Chert layers a l t e r n a t e with chalk and limestone, and biogenic s i l i c a decreases with depth in holes. Deeper in the Galapagos section chert occurs only as nodules and thin stringers (Heath, 1973). A t s i t e 504 of DSDP Leg 69 which was d r i l l e d into the Costa Rica r i f t , siliceous calcareous ooze i s mixed with ash and terrigenous material. Chert constitutes less than 2% of the sequence (Cann, Langseth, e t al., 1981). DSDP Leg 62 S i t e s 464, 465, and 466, d r i l l e d into Hess Rise and 463 into t h e Mid-Pacific Mountains, recovered chert only as isolated nodules and stringers in calcareous rock (Thiede, Vallier, e t al., 1981). S i t e s d r i l l e d into the deep, open-Pacific abyssal environment, such a s S i t e s 164, 393, 394, and 396, penetrated mottled t o finely laminated z e o l i t i c clays and z e o l i t i c cherty nudstones. A few cherty horizons occur, b u t for the most part, even sediments of Early Cretaceous age remain unlithified (Lancelot, 1973; Keene, 1975, 1976). Alternation of layers of chert and shale likely t o produce ribbon cherts have never been found in the open-ocean d r i l l cores.

FORMATION OF CHERT Cherts from DSDP Legs 62 and 69 a r e s i l i c i f i e d chalk and pelagic clay. They formed by cementation and volume-for-volume replacement of carbonate and clay (Hein e t al., 1981b. 1982b; Hein and Yeh, 1981, 1982). Excellent preservation of sedimentary structures, such as burrows, inherited from the host rock, and gradational relations with calcareous host rocks s u p p o r t the conclusion t h a t the chert originated by replacement. The replacement mechanism f o r the formation of open-ocean cherts i s well documented from other DSDP legs (Heath

32

and Moberly, nodules and

1971; Heath, 1973; Lancelot, 1973; Keene, 1975, 1976). Chert lenses may nucleate among l o c a l concentrations o f magnesium

compounds (Kastner e t al.,

1977) and grow u n t i l a l l t h e l o c a l biogenic s i l i c a

i n t h e host rock i s dissolved.

The biogenic s i l i c a , opal-A,

i n i t i a l l y p r e c i p i t a t e d as opal-CT,

i s dissolved and

comnonly i n burrows o r w i t h i n the chambers

o f t e s t s o f m i c r o f o s s i l s such as foraminifers. The metastable opal-CT u l t i m a t e l y becomes more ordered c r y s t a l l o g r a p h i c a l l y and then i s converted t o quartz w i t h increasing t i m e and temperature. I n contrast, ribbon c h e r t sequences apparently were i n i t i a l l y deposited as s i l i c e o u s oozes o r as t u r b i d i t e s composed mostly o f s i l i c e o u s d e b r i s w i t h v a r y i n g amounts o f admixed clay. Modern c o n t i n e n t a l margin deposits s i m i l a r l y may

consist

of

laminated

or

massive

diatomites

devoid

of

calcareous

microfossils.

Modern c o n t i n e n t a l margin s i l i c e o u s deposits may form e i t h e r

below t h e CCD,

o r i n very c o l d water (Leg 31, Sea o f Japan; Leg 19, Bering

Sea), o r i n anoxic s i l l e d basins, o r where t h e oxygen minimum zone o f t h e Ocean i n t e r s e c t s the c o n t i n e n t a l slope (Ingle, 1973, 1981). Topographic highs may c o n t r i b u t e s i l i c e o u s t u r b i d i t e s l o c a l l y ,

i n addition

s i l i c e o u s oozes may accumulate under zones o f high p r o d u c t i v i t y o f s i l i c e o u s plankton i n surface waters, such as i n high l a t i t u d e s o r along western margins o f continents (Keene, 1976; Fischer, 1977; Garrison and Fischer, 1969; W i n t e r e r and Jenkyns,

1979).

Episodes o f plankton p r o d u c t i v i t y may be c l i m a t i c a l l y

c o n t r o l l e d , e i t h e r on short o r long t e n cycles (such as periods o f increased glaciation).

Times

hemipelagic clays, Subsequently,

of

low

productivity

would be marked by

pelagic

or

t h e d e f a u l t sedimentary regime i n t h e deep ocean basins.

diagenetic

transformation

of

siliceous

ooze t o

opal-CT

and

u l t i m a t e l y t o quartz by s o l u t i o n - r e d e p o s i t i o n processes occurred w i t h i n t h e beds (Keene, 1976; Isaacs, 1980; B a r r e t t , 1981). I n s u m r y , t y p i c a l deep-sea open-ocean cherts t h a t were d r i l l e d by t h e DSDP formed by replacement o f calcareous o r clayey sediments whereas bedded cherts in

orogenic

belt

sequences

formed

by

diagenetic

recrystallization

of

a l t e r n a t i n g c o n t r a s t i n g sediment types, such as s i l i c e o u s ooze and hemipelagic clay, t u f f , o r sandstone; S t r a t i g r a p h i c Sequences:

Leg 62 and 69 Cherts

During Leg 62 o f the DSDP, c h e r t and p o r c e l a n i t e were recovered a t S i t e 463 on t h e Mid-Pacific Mountains and a t S i t e s 464, 465, and 466 on Hess Rise (Fig. 1). A t these s i t e s c h e r t m k e s up l e s s than 5% o f t h e section; t h e remainder c o n s i s t s o f chalk o r limestone (Fig. 2; Thiede, V a l l i e r , e t al., 1981; Hein e t al., 1981b). Chert l a y e r s and nodules a r e randomly d i s t r i b u t e d i n t h e calcareous host rock. The sediment a t S i t e 463 c o n s i s t s o f 822.5 m o f ,Quaternary through upper Barremian m n n o f o s s i l ooze, chalk, limestone, and

33

FRANCISCAN COMPLEX NICOYA COMPLEX LADD-BUCKEYE AREA COSTA RlCA

350 DSDP SITE 463

300

250

200

DSDP SITE 4 8 5 150

W

1

100

DSDP SITE 464 7

P 4.

50

Y

i Figure 2 .

DSDP SITE 488

DSDP SITE 504

T

Y

r

1P

W

I

P I

W

IY

0

P

1

Statigmphic cotwmzs of Leg 62 and 69 open-ocean chert sequences and

continental margin chert sequences. DSDP cherts: Sites 463, 464, 465, and 466 are typical of others drilled elsewhere i n the deep Pacific basin. The sections contain less than 5% chert, the remainder consists of chalk and limestone. Chert kzyers and nodules are mndomly distributed throughout the host rock and are a f w millimeters t o a f w centimeters i n siae. In S i t e 504, drilled on the Costa R i a R i f t , chert mkes up less than 2% of the total section and i s admixed with predomhzntly l e s s e r amounts o f c h e r t and volcanic ash.

S i t e 464 c o n s i s t s o f 310 m o f

Quaternary t o upper Miocene s i l i c e o u s clay,

lower Miocene t o Upper Cretaceous

brown clay, and Cenomanian t o lower A l b i a n c h e r t and nannofossil limestone o v e r l y i n g t h o l k i i t i c basalt. A t S i t e 465, 420 m o f Quaternary t o A l b i a n calcareous ooze,

limestone,

and minor chert over1 i e h i g h l y a l t e r e d trachyte.

A t S i t e 466, 312 m o f Quaternary t o upper A l b i a n calcareous sediment w i t h minor c h e r t and p y r i t i c c l a y were cored (Fig. 2).

These f o u r sections a r e t y p i c a l o f

those t h a t have been d r i l l e d elsewhere i n t h e deep P a c i f i c basin (Fig. 1). On DSDP Leg 69, S i t e 504 was d r i l l e d i n t o t h e south f l a n k o f t h e Costa Rica R i f t (Cann, Langseth, e t al.,

of

siliceous

1981). The cores from S i t e 504 c o n s i s t o f 270 m calcareous ooze and chalk w i t h v a r i a b l e amounts o f admixed

volcanic ash, terrigenous debris, and p y r i t e (Fig.

2).

The lowermost 30 m o f

34 EXPLANATION CHERT 100

3

RANCISCAN COMPLEX UARIN HEADLANDS

RHYTHMICALLY BEDDED CHERTISHALE MANOANIFEROUSIHEMATlTlC CHERT LENSES

90

SANDSTONE FRANCISCAN COMPLEX CLEAR L A K E AREA

SILICEOUS OOZE

80

SANDSTONEISHALE CONGLOMERATE

3

70

4

LIMESTONE

a

Y

a 80

50

I

I

CALCAREOUS OOZE

4

CALCAREOUS CHALK RHYTHMICALLY BEDDED LIMESTONEISHALE

40

COMPLETELY SILICIFIED RHYTHMICALLY BEDDED LIMESTONELSHALE CLAY .CLAYSTONE.ARGILLITE. PELAGIC CLAY

KELP OAY GROUP KELP BAY, ALASKA

BASALT

30 KELP OAY GROUP KELP BAY. ALASKA

[7 DIA0ASE.GABBRO.SERPENTlNE TUFFACEOUS SANDSTONE

20

[3 METATUFF 10

TUFFACEOUS ARGILLITE W

0

ANGULAR UNCONFORMITY TECTONIC CONTACT SCALE IN METERS

Figure 2. lcont .I siliceous calcareous ooze, chalk, and i n the lowermost 30 m of the section the chert i s admixed with predominantly siliceous limestone. In Pacific r i m cherts, chert comprises up t o 50% of the sequences. Chert is typically rhythmically interbedded oith terrigenous shales and sandstones that uere deposited by turbidity currents. Chert occurs i n a centimeter t o tens of centimeters-thick beds, and meter- t o hundreds of meters-thick sections. Sections mrety, if a t a l l , have limestones associated with them. s e c t i o n i s composed o f upper Miocene s i l i c e o u s limestone and l e s s e r amounts o f c h e r t and p o r c e l a n i t e t h a t o v e r l i e 6.2 than 2% o f t h e t o t a l section, s e c t i o n (Hein e t al.,

m.y.

o l d basalt.

Chert makes up l e s s

and approximately 15% o f t h e lowermost 30 m o f

1982b)..

I n summary, c h e r t s from t h e deep sea occur mainly as nodules o r lenses i n o r above limestone and chalk, o v e r a l l sedimentary sequence.

and they compose a very small percentage o f t h e The c h e r t s found w i t h z e o l i t i c c l a y s a l s o occur

as small l o c a l concentrations o f s i l i c e o u s material.

S t r a t i g r a p h i c Sequences:

Nicoya Complex, Franciscan Complex, and Kelp

Bay Group Cherts

O u r study has i n v o l v e d a comparison o f c h e r t s deposited i n t h e P a c i f i c Ocean

36 basin and c h e r t s exposed i n oroyenic b e l t s around t h e P a c i f i c rim. The o l d e s t sediments cored from t h e P a c i f i c Ocean f l o o r a r e Jurassic, so we have focused our a t t e n t i o n on a few Mesozoic a c c r e t i o n a r y t e r r a n e s along t h e western m r g i n

o f North and Central America.

I n p a r t i c u l a r we examined r i b b o n c h e r t s i n t h e

Nicoya Complex o f Costa Rica,

t h e Franciscan Complex o f C a l i f o r n i a , and t h e

Kelp Bay Group o f Alaska. The Nicoya Complex i s considered t o be an obducted o r trapped o p h i o l i t e sequence (Kui j p e r s ,

1980;

Nicoya Complex i s o f

Schmidt-Effing,

1979).

Radiolarian chert i n the

E a r l y through M i d d l e Cretaceous age and i s o v e r l a i n

unconformably by s i l i c e o u s ,

calcareous,

volcanic,

and t e r r i g e n o u s deposits,

which c o n s t i t u t e t h e Sabana Grande and o t h e r formations o f L a t e Cretaceous through Eocene age.

Most o f t h e Nicoya Complex c o n s i s t s o f basalt.

The

r a d i o l a r i t e s which compose l e s s than 2% o f t h e Nicoya Complex, occur w i t h shale

2;

i n r h y t h m i c a l l y bedded sequences and as lenses between b a s a l t f l o w s (Fig. Hein e t al.,

1981a, 1982a).

i s about 40 m.

The g r e a t e s t thickness o f t h e Nicoya r a d i o l a r i t e s

I n t e r p r e t a t i o n o f t h e bedding i n t h e Nicoya r a d i o l a r i t e s as

being t u r b i d i t y c u r r e n t deposits Effing,

1982),

(Hein e t al.,

1981a;

Gursky and Schmidt-

suggests t o us t h a t t h e c h e r t s were deposited e i t h e r near a

c o n t i n e n t a l m r y i n on oceanic c r u s t o r on a r i f t behind a v o l c a n i c a r c such as i s found behind t h e Marianas a r c ( K a r i g e t al., b a s i n (Kanmera, 1974; I i j i m a e t al., Radiolarian chert

1973) o r t h e Japanese back-arc

1978).

i n t h e Franciscan Complex occurs i n small amounts as

b l o c k s i n t h e westernmost Coastal B e l t , commonly as blocks and i n continuous stratigraphic

sequences

in

the

Central

Belt,

and

in

places

as

thin

intercalations

i n e x t r u s i v e v o l c a n i c rocks and sandstone i n t h e easternmost

Y o l l a B o l l y B e l t (Blake and Jones, 1980). Alternating chert-shale

sequences

--

ribbon cherts

C e n t r a l - B e l t c h e r t s i n t h e Franciscan Complex. 80 m i n thickness,

--

are t y p i c a l o f the

These s e c t i o n s range from 40 t o

were deposited on p i l l o w e d greenstone,

and a r e o v e r l a i n

conformably by a r g i l l i t e and graywacke beds i n t h e more complete sections, such as t h e one m a s u r e d a t Alexander Avenue (Murchey, 1980). o v e r l a i n by t u f f a c e o u s sandstone,

Also c h e r t s my be

such as a t t h e Geysers

(McLaughlin and

Pessagno, 1978). The Franciscan Complex s e c t i o n i n t h e Ladd-Buckeye area (Fig. 2 ) represents approximately 45 my. o f L a t e J u r a s s i c t o M i d d l e Cretaceous d e p o s i t i o n o f s i l i c e o u s d e b r i s t h a t i s uncontaminated by t e r r i g e n o u s m t e r i a l b u t conformably o v e r l a i n by M i d d l e and Upper Cretaceous t e r r i g e n o u s deposits. T h i c k sections o f c h e r t and graywacke a r e interbedded i n t h e Pacheco Pass area

(Blake and Jones,

Francisco (Trask,

1974),

i n t h e Ladd-Buckeye

area

southeast

1950) and i n t h e Y o l l a B o l l y t e r r a n e (Blake e t al.,

Chert sequences i n t h e Ladd-Buckeye area (Fig.

o f San 1981).

2) which a r e as t h i c k as 300 my

a r e i n t e r c a l a t e d w i t h bedded sandstone u n i t s o f s i m i l a r t h i c k n e s s and my

36

compose as nuch as 50% o f t h e section, although o v e r a l l , c h e r t makes up less than 0.Y6 o f t h e Franciscan Complex (Bailey, e t al., 1964). R a d i o l a r i a n ribbon cherts i n the Kelp Bay Group o f southeastern Alaska occur as lenses several t o a few tens o f meters t h i c k i n p i l l o w e d greenstone o r a r e interbedded

with

shale,

tuff,

and

medium

turbidites.

A t y p i c a l mode o f occurrence i s lenses i n greenstone o r a r g i l l i t e

d e p o s i t i o n a l l y o v e r l a i n by graywacke (Fig. several centimeters thick.

to

2).

coarse-grained

graywacke

Interbedded c h e r t s average

i n thickness and graywacke l a y e r s a r e as much as 2 m

Cherts i n t h e Kelp Bay Group m k e up l e s s than 0.5% o f t h e rocks, a

percentage s i m i l a r t o t h a t o f t h e c h e r t s o f t h e Franciscan Complex. I n summary, t h e c h e r t we observed i n orogenic b e l t s i s c h a r a c t e r i s t i c a l l y i n depositional individual

continuity

melange units. rarely,

with

greenstone

and graywacke

turbidites,

though

sections my have been subsequently t e c t o n i z e d and f a u l t e d i n t o Sections i n t h e accreted terranes along western North America

i f ever,

are associated w i t h limestone,

and i t i s t h i s f e a t u r e t h a t

d i s t i n g u i s h e s t h e c h e r t i n orogenic b e l t s from t h e c h e r t i n open oceans t h a t a r e associated predominantly w i t h limestone and chalk.

Sedimentary Structures Sedimentary s t r u c t u r e s p e r t i n e n t t o c h e r t deposits g e n e r a l l y include l a y e r thicknesses, l a t e r a l extent, c y c l i c i t y , and l o c a l deformations such as burrows, convolute layers, and s o f t sediment folds. As mentioned previously, c h e r t occurs only as s t r i n g e r s , lenses, o r nodules a few m i l l i m e t e r s t o a few centimeters i n s i z e w i t h i n calcareous sections i n

DSDP Legs 62 and 69 cores.

I n z e o l i t i c c l a y sections,

c h e r t forms f i n e l y

laminated l a y e r s whose thicknesses a r e on t h e scale o f millimeters. Chert i n t h e orogenic sequences studied occurs i n l a y e r s ranging from a centimeter t o several tens o f centimeters t h i c k and l a t e r a l l y extending from a

few meters t o tens o f meters.

The cherts a r e c h a r a c t e r i s t i c a l l y r h y t h m i c a l l y

i n t e r c a l a t e d w i t h nudstone o r shale i n t h e sections i n Costa Rica, C a l i f o r n i a , and Alaska.

Burrow s t r u c t u r e s a r e s t r i k i n g l y well-preserved i n t h e c h e r t s from

Legs 62 and 69,' and conspicuously l a c k i n g i n t h e c h e r t s from t h e c o n t i n e n t a l margins.

The lack o f burrowing and the preservation o f laminations i n t h e

c o n t i n e n t a l margin rocks suggest formation i n a low oxygen environment i n i m i c a l t o organisms,

such as described by I n g l e (1973),

and ( o r ) r a p i d deposition,

such as by t u r b i d i t y currents. Such anoxic environments are most l i k e l y t o occur along c o n t i n e n t a l margins r a t h e r than i n t h e open ocean where they probably occur only on t h e f l a n k s o f topographic highs. DSDP d r i l l data has shown t h a t burrowing i s extensive even a t great water depths on t h e ocean floor.

Other bedded c h e r t sections do c o n t a i n burrowing, b u t mostly on bedding

37 surfaces (cf. Folk and McBride, 1978). Sedimentary s t r u c t u r e s such as graded bedding, which i n v o l v e s grading mostly i n numbers and more r a r e l y i n s i z e s o f t h e more robust r a d i o l a r i a n s , marks,

t u r b i d i t y currents. (Hein, e t al.,

Such s t r u c t u r e s have been recognized i n t h e Nicoya Complex

1981a) and i n t h e c h e r t s o f t h e Franciscan Complex b u t not i n

c h e r t s from t h e Legs 62 and 69 sections. section

(Fig.

2)

Karl,

Chert beds i n t h e Alexander Avenue

c o n t a i n redeposited s i l i c e o u s d e b r i s

o v e r l a i n by claystones; (S.

sole

soft-sediment f o l d i n g , and c y c l i c bedding schemes suggest d e p o s i t i o n by

that

i s graded and

t h i s sequence suggests p e r i o d i c t u r b i d i t e d e p o s i t i o n

unpublished data).

Chert-shale

ratios

decrease

up

section a t

Alexander Avenue and grade i n t o t e r r i g e n o u s t u r b i d i t e s .

Rates o f Sedimentation Comparisons o f r a t e s o f accumulation o r sedimentation may be i n a p p r o p r i a t e because

deep-sea

open-ocean

cherts

are

replacement

products.

Rates

of

sedimentation o f t h e host p e l a g i c deposits, mostly calcareous oozes, a t Leg 62 s i t e s vary from 0.5 shallow-water debris,

The h i g h e r r a t e s i n c l u d e some redeposited

t o 58 m/m.y.

and t h e lower r a t e s a r e f o r p e l a g i c c l a y and s i l i c e o u s

c l a y (Thiede and V a l l i e r ,

e t al.,

1981).

S i t e s 504 and 505 o f Leg 69 show

r a t e s o f d e p o s i t i o n o f s i l i c e o u s calcareous ooze o f 50 t o 60 mln1.y. 1981).

(Sancetta,

These d e p o s i t i o n a l r a t e s a r e c o n s i s t e n t w i t h t h e range found f o r

calcareous p e l a g i c deposits o c c u r r i n g elsewhere i n t h e P a c i f i c (van Andel e t al.,

1975).

10 m/m.y.

Rates o f d e p o s i t i o n f o r s i l i c e o u s ooze a r e approximately 1-

and f o r p e l a g i c c l a y t y p i c a l l y 1 m/m.y.

Reports

of

rates .of

many bedded c h e r t sequences i n

sedimentation f o r

orogenic b e l t s a r e not s a t i s f a c t o r y .

o r less.

Sedimentation r a t e s a r e determined when

rocks l y i n g below and above t h e bedded c h e r t s can be dated and thus an average r a t e o f sedimentation can be calculated.

The problem i s t h a t bedded c h e r t

sequences a r e

sedimentary

composed o f

two k i n d s

of

probably accumulated a t v a s t l y d i f f e r e n t especially

true

i f . the

cherts

turbidites,

because

This i s turbidites

t h e i n t e r v e n i n g shales represent

most o f t h e d u r a t i o n o f d e p o s i t i o n f o r t h e s e c t i o n as a whole. true,

and each one

r a t e s o f sedimentation.

represent

e s s e n t i a l l y ' a r e deposited instantaneously,

rocks,

This i s a l s o

b u t t o a l e s s e r degree i f t h e c h e r t l a y e r s represent c l i m a t i c cycles.

Paleontologic d a t i n g o f t h e Nicoya Complex o f Costa Rica has not been e i t h e r extensive enough nor f i n e enough t o p e r m i t even average r a t e s o f sedimentation t o be c a l c u l a t e d yet.

The average r a t e o f sedimentation f o r Franciscan c h e r t s

a t Alexander Avenue (Fig.

2) i s 2 m/m.y.

( S . K a r l , unpublished data).

Other

average r a t e s o f sedimentation r e p o r t e d i n t h e l i t e r a t u r e f o r bedded c h e r t s e c t i o n s a r e 0.7

t o 1.0 m/m.y.

(Garrison and Fischer, 1969), 1.0

t o 5.3

m/m.y.

38 (McBride and Thompson, 19701, 3 t o 9 m/m.y. t o 34 m/m.y. ( I i j i m e t al., 1978). Rates of

accumulation (g/cm2/103yr)

(Schlager and Schlager,

1973), 27

a r e b e t t e r s u i t e d f o r comparisons o f

rocks o f d i f f e r e n t ages and l i t h o l o g i c c h a r a c t e r i s t i c s because t h e r a t e s a r e not a f f e c t e d by v a r i a t i o n s i n ages o r by amounts o f overburden (Van Andel e t al.,

1975).

L i t t l e data e x i s t s ,

however, on r a t e s o f accumulation.

According

t o W i n t e r e r and Jenkyns (1979) c h e r t s i n orogenic b e l t s accumulated a t about 1g/cd/1O3yrs,

whereas Cenozoic open-ocean

about 0.1g/cd/103yrs.

Deep-sea

r a d i o l a r i a n oozes accumulated a t

carbonate deposits accumulate a t r a t e s o f

g / c d / 1 0 3 y r s (Van Andel e t al.,

about 0.2 t o 0.5

1975).

I n summary, t h e data a r e sparse, t h e r a t e s d i f f i c u l t t o c a l c u l a t e , and t h e determined average r a t e s f o r turbidity

current

c h e r t sequences d i f f i c u l t t o i n t e r p r e t .

deposition

for

chert

beds

in

orogenic

belts

When can

be

e s t a b l i s h e d w i t h confidence, then perhaps i t might be more meaningful t o r e p o r t number o f events p e r

lon

U n i v e r s i t y o f Toronto,

years,

t h e frequency o f d e p o s i t i o n (T.

personal communication,

1981).

J.

Barrett,

B a r r e t t estimated t h a t

an average o f 1 t o 4 t u r b i d i t e c h e r t beds occurred p e r

lo4

y e a r s i n sequences

from n o r t h e r n I t a l y .

SUMMARY

I n terms o f l i t h o l o g i c association,

mineralogy,

and sedimentology,

it i s

apparent t h a t t h e DSDP Legs 62 and 69 c h e r t s a r e q u i t e d i s t i n c t from t h e r i b b o n c h e r t s o f t h e Nicoya Complex, t h e Franciscan Complex, and t h e Kelp Bay Group. Open-ocean

biogenic

d e p o s i t s accumulated

under

zones

of

high biologic

They accumulated most r a p i d l y near t h e equator i n t h e e q u a t o r i a l

productivity.

zones o f convergence.

The predominance o f s i 1iceous over calcareous b i o g e n i c

m a t e r i a l i n such deposits depended on t h e p o s i t i o n o f t h e CCD, which i s known to

change through

Winterer,

1975).

time

geologic I n addition,

(Van Andel

et

al.,

1975;

Bosellini

and

biogenic oozes t h a t s e t t l e d through t h e water

column may have been reworked l o c a l l y by bottom currents.

B i o l o g i c a c t i v i t y on

t h e sea f l o o r commonly homogenizes these deposits. Continental

margin

biogenic

deposits

accumulated

under

zones

of

high

p r o d u c t i v i t y r e s u l t i n g from u p w e l l i n g o f n u t r i e n t - r i c h water along c o n t i n e n t a l margins ( I n g l e ,

1973, 1981).

High p r o d u c t i v i t y promoted r a p i d accumulation o f

b i o g e n i c d e b r i s and organic decay;

they depleted t h e water column o f oxygen,

r e s u l t i n g i n an anoxic environment i n i m i c a l t o t h e p r e s e r v a t i o n o f calcareous tests

and

preservation

adverse of

t o the

fine

existence

laminations.

of Also,

infaunal

activity,

i n these

zones

thus of

allowing

very

high

p r o d u c t i v i t y calcareous organisms c o u l d not compete w i t h t h e s i l i c e o u s p l a n k t o n which t a k e up n u t r i e n t s more r a p i d l y than they do.

Terrigenous sediment was

39 i n e v i t a b l y i n t e r c a l a t e d w i t h o r deposited on these b i o g e n i c deposits a t t h e c o n t i n e n t a l margin. The main d i f f e r e n c e s between t h e Leg 62 and 69 c h e r t s and t h e orogenic b e l t ribbon c h e r t s appear t o be r e l a t e d t o t h e p r o x i m i t y o f t h e s i t e o f d e p o s i t i o n t o l a n d and t o t h e CCD.

The c h e r t s o f t h e Nicoya Complex, Franciscan Complex,

and Kelp Bay Group a l l

l a c k calcareous m a t e r i a l and a r e associated w i t h

conformable t e r r i g e n o u s deposits,

suggesting d e p o s i t i o n b o t h i n p r o x i m i t y t o

land and below an e l e v a t e d CCD.

Modern environments

that

s a t i s f y these

c o n d i t i o n s i n c l u d e r i f t e d c o n t i n e n t a l margins, such as t h e G u l f o f C a l i f o r n i a , and s i l l e d basins l o c a l l y

i s o l a t e d from t e r r i g e n o u s m a t e r i a l ,

such as t h e

wrench basins o f t h e C a l i f o r n i a c o n t i n e n t a l borderland (Crouch,

1981).

It

seems u n l i k e l y t h a t t h i c k s e c t i o n s r e p r e s e n t i n g many tens o f m i l l i o n s o f y e a r s o f uncontaminated s i l i c e o u s ooze would accumulate i n an a c t i v e t r e n c h - i s l a n d a r c s e t t i n g although t h i n s e c t i o n s o f r i b b o n c h e r t s r e p r e s e n t i n g m i l l i o n s o f years o f d e p o s i t i o n have been r e p o r t e d (Hein and McLean, 1980). environments o f d e p o s i t i o n i n c l u d e r i f t e d i n t e r a r c basins, Marianas arc, More work

Other p o s s i b l e

such as west o f t h e

o r marginal seas o r back a r c basins such as t h e Sea of Japan.

needs t o be done,

a t present we do not have enough evidence t o However, t h e evidence we have imp1 i e s

d i s c r i m i n a t e between these environments.

t h a t orogenic b e l t c h e r t s deposited i n t e c t o n i c a l l y c r e a t e d basins w i t h i n t h e i n f l u e n c e o f a c o n t i n e n t a l margin d i f f e r from sections d r i l l e d i n t h e openocean P a c i f i c basins.

ACKNOWLEDGMENTS We thank Drs. Survey, Mary

M.

C.

Blake and D.

G.

Howell,

both a t t h e U.S.

f o r many h e l p f u l suggestions f o r improving t h i s paper. Ann

McCall,

Lisa

Moryenson,

and

Marla

Wilson

Geological

Sara Monteith,

provided

technical

assistance.

REFERENCES Bailey, E. H., I r w i n , W. P., and Jones, D. L., 1964. Franciscan and r e l a t e d rocks and t h e i r s i g n i f i c a n c e i n t h e geology o f western C a l i f o r n i a . C a l i f o r n i a Div. Mines and Geol. Bull., 183: 177 pp. B a r r e t t , T. J., 1981. S t r a t i g r a p h y and sedimentology o f J u r a s s i c bedded c h e r t o v e r l y i n g o p h i o l i t e s i n t h e n o r t h Apennines, I t a l y . Sedimentology ( I n press).

40

Blake, M. C., Jayko, A. S:, metamorphism and t e c t o n i c northern C a l i f o r n i a . In: London ( I n press).

and Howell, D. G., 1981. Sedimentation, a c c r e t i o n o f t h e Franciscan assemblage o f Forearc Geology, The Geological Society o f

Blake, M. C. and Jones, D. L., 1974. O r i g i n o f Franciscan melanges i n northern California. SOC. Econ. Paleontologists and Mineralogists, Spec. Pub., 19:

345-355. Blake, M. C., Jr., and Jones, D. L., 1980. The Franciscan assemblage and r e l a t e d rocks i n northern C a l i f o r n i a : A r e i n t e r p r e t a t i o n . In: Ernst, W. G., ( E d i t o r ) , The Geotectonic Development o f C a l i f o r n i a , Ruby, v. 1, N.J. Prentice H a l l , Inc., pp. 307-328. B o s e l l i n i , A. and Winterer, E. L., 1975. Pelagic limestone and r a d i o l a r i t e o f t h e tethyan Mesozoic: A genetic model. Geology, 3: 279-282. Cann, J. R., Langseth, M. G., e t al., D r i l l i n g Project, Leg 69, v. 69: ( I n press).

1982. I n i t i a l r e p o r t s o f t h e Deep Sea Washington (U.S. Govt. P r i n t i n g O f f i c e )

Crouch, J. K., 1981. Northwest margin o f C a l i f o r n i a Continental Borderland: Marine Geology and Tectonic Evolution. Am. Assoc. Petrol. Geol. Bull.,

65: 191-218. Dewey, H. and F l e t t , J. S., 1911. On some B r i t i s h p i l l o w - l a v a s and t h e rocks associated w i t h them. Geol. Mag., 8: 241-248. Dietz, R. S. and Holden, J. C., 1966. Deep-sea deposits i n b u t not on t h e continents. Bull. Am. Assoc. Petroleum Geologists, 50: 351-362. Fischer, A. G., 1977. Pelagic sediments as clues t o e a r t h behavior. In: Giampaolo, Pialli, (Editor), Paleomagnetic s t r a t i g r a p h y o f pelagic carbonate sediments. Mem. SOC. Geol. I t a l i a , 15, pp. 1-18. Folk,

R. L. and McBride, E.,,F., 1978. R a d i o l a r i t e s and t h e i r r e l a t i o n t o subjacent "oceanic c r u s t i n L i g u r i a , I t a l y . J. Sed. Petrol., 48: 1069-

1102. Garrison, R. E. and Fischer, A. G., 1969. Deep-water limestones o f t h e Alpine Jurassic. In: Friedman, G. M., (Editor), Depositional environments i n carbonate rocks. SOC. Econ. P a l e o n t o l o g i s t s and Mineralogists, Spec. Pub.

14: 20-54. Gursky, H.-J. and Schmidt-Effing, R., 1982. Sedimentology o f t h e r a d i o l a r i t e s w i t h i n t h e Nicoya o p h i o l i t e complex, Costa Rica, Central America. (This volume). Heath, G. R., 1973. Cherts from t h e eastern P a c i f i c , Leg 16, Deep Sea D r i l l i n g Project. In: Van Andel, T. H., Heath, G. R., e t al., I n i t . Repts. DSDP, 16: Washington (U.S. Govt. P r i n t i n g O f f i c e ) , pp. 609-613. Heath, G. R. and Moberly, R., 1971. Cherts from t h e western P a c i f i c , Leg 7, Deep Sea D r i l l i n g Project. I n : Winterer, E. L., Riedel, W. R., e t al., Init. Repts. DSDP, 7: Washington, (U.S. Govt. P r i n t i n g O f f i c e ) , pp. 991-1007. Hein,

Scholl, D. W., Barron, J. A., Jones, M. G., and M i l l e r , J., Diagenesis o f Late Cenozoic diatomaceous deposits and formation o f t h e bottom s i m u l a t i n g r e f l e c t o r i n the southern Bering Sea. Sedimentology, 25: 155-181. J.

1978.

R.,

41 Hein, J. R. and McLean, Hugh, 1980. Pajeogene sedimentary and volcanogenic rocks from Adak Island, Central A l e u t i a n Islands, Alaska. U.S. Geol. Sur. Prof. Pap. 1126-E: pp. El-E16. Hein, J. R. and Yeh, H.-W., 1981. Oxygen i s o t o p e composition o f c h e r t from t h e M i d - P a c i f i c Mountains and Hess Rise, Deep Sea D r i l l i n g P r o j e c t Leg 62: I n : Thiede, Jdrn, V a l l i e r , T. L., e t al., I n i t . Repts. DSDP, 62: Washington, (U.S. Govt. P r i n t i n g O f f i c e ) , pp. 749-758. Hein,

J. R., K u i j p e r s , E. P., and Denyer, Percy, 1981a. Paleogene and Cretaceous c h e r t s o f western Costa Rica. Abstrs., Second I n t e r n a t i o n a l Conference on S i l i c e o u s Deposits o f t h e P a c i f i c Region, Tokyo, Japan, 2127 August 1981, pp. 59-61.

Hein, J. R., V a l l i e r , T. L., and A l l a n , M. A., 1981b. Chert p e t r o l o g y and geochemistry, M i d - P a c i f i c Mountains and Hess Rise, Deep Sea D r i l l i n g P r o j e c t Leg 62. I n : Thiede, Jdrn, V a l l i e r , T. L., e t al., I n i t . Repts. DSDP, 62: Washington (U.S. Govt. P r i n t i n g O f f i c e ) , pp. 711-748. Hein,

J. R. and Yeh, H.-W., 1982. Oxygen-isotope composition o f secondary s i l i c a phases, Costa Rica R i f t , Deep Sea D r i l l i n g P r o j e c t Leg 69. In: Langseth, M. G., e t al., I n i t . Repts. DSDP, 69: Washington, Cann, J. R., (U.S. Govt. P r i n t i n g O f f i c e ) ( I n press).

Hein,

J. R., K u i j p e r s , E. P., Denyer, Percy, and Sliney, R. E., 1982a. Petrology and geochemistry o f Cretaceous and Paleogene c h e r t s from Western Costa Rica, (This volume).

Hein, J. R., Sancetta, Constance, and Morgenson, L. A. 1982b. Petrology and geochemistry o f s i l i c i f i e d upper Miocene chalk, Costa Rica R i f t , Deep Sea D r i l l i n g P r o j e c t Leg 69. In: Cann, J. R., Langseth, M. G., e t al., I n i t . Repts. DSDP, 69: Washington (U.S. Govt. P r i n t i n g O f f i c e ) ( I n press).

Iij i m a , h u m , Kakuwa, Yoshitaka, Yamazaki, Kenichi, and Yanagimoto, Yutaka, 1978. Shallow-sea, organic o r i g i n o f t h e T r i a s s i c bedded c h e r t i n Central Japan. J. F a c u l t y Sci., U n i v e r s i t y o f Tokyo, Sec. 11, 19: 369-400. I n g l e , J. C., Jr., 1973. Summary comnents on Neogene b i o s t r a t i g r a p h y , p h y s i c a l s t r a t i g r a p h y , and pa 1eo-ocea nog rap hy in t h e m r g ina 1 n o r t h e a s t e r n Pac if ic von Huene, R., e t al., I n i t . Repts. DSDP, 18: Ocean. In: Kulm, L. D., Washington (U.S. Govt. P r i n t i n g O f f i c e ) , pp. 949-960. Ingle, J. C., Jr., 1981. O r i g i n o f Neogene d i a t o m i t e s around n o r t h P a c i f i c rim. In: The Monterey Formation and Related S i l i c e o u s Rocks o f California. SOC. Econ. P a l e o n t o l o g i s t s and M i n e r a l o g i s t s , P a c i f i c Sec., pp. 159-179. 1980. Diagenesis i n t h e Monterey Formation examined l a t e r a l l y Isaacs, C. M., along t h e coast near Santa Barbara, C a l i f o r n i a . Ph.D. Thesis, S t a n f o r d U n i v e r s i t y , 329 pp. Kanmera, Kametoshi, 1974. Paleozoic and Mesozoic geosynclinal volcanism i n t h e Japanese I s l a n d s and associated c h e r t sedimentation. SOC. Econ. P a l e o n t o l o g i s t s and M i n e r a l o g i s t s Spec. Pub. 19: 161-173. Karig, D. E., Ingle, J. C., e t al., 1975. I n i t i a l Report o f t h e Deep Sea D r i l l i n g P r o j e c t , 31: Washington (U.S. Govt. P r i n t i n g O f f i c e ) , 927 pp.

42

Kastner, M., Keene, J. B., and Gieskes, J. M., oozes I. Chemical c o n t r o l s on t h e transformations an experimental study. 41: 1041-1051.

-

1977. Diagenesis o f s i l i c e o u s r a t e o f opal-A t o opal-CT Geochim. e t Cosmochim. Acta,

Keene, J. B., 1975. Cherts and p o r c e l a n i t e s from t h e n o r t h P a c i f i c , DSDP Leg 32. I n : Larson, R. L., Moberly, R., e t al., I n i t . Repts. DSDP, 32: Washington (U.S. Govt. P r i n t i n g O f f i c e ) , pp. 429-507. Keene, J. B., 1976. The d i s t r i b u t i o n , mineralogy, and petrography o f b i o g e n i c and a u t h i g e n i c s i l i c a from t h e P a c i f i c Basin. Ph.D. Thesis, U n i v e r s i t y o f C a l i f o r n i a , San Diego, 264 pp. Kimura, Toshio, 1973. The o l d ' i n n e r ' a r c and i t s deformation i n Japan. In Coleman, P. J., ( E d i t o r ) , The Western P a c i f i c I s l a n d Arcs, Marginal Seas, Geochemistry. Crane, Russak and Co., Inc., N.Y., and U n i v e r s i t y o f Western A u s t r a l i a Press, pp. 255-273. 1976. S t a b l e i s o t o p e geochemistry o f deep sea Kolodny, Y. and Epstein, S.. cherts. Geochim. e t Cosmochim. Acta, 40: 1195-1209. K u i j p e r s , E. P., Costa Rica, 255.

1980. The geologic h i s t o r y o f t h e Nicoya o p h i o l i t e complex, and i t s geotectonic s i g n i f i c a n c e . Tectonophysics, 68: 233-

Lancelot, Y., 1973. Chert and s i l i c a diagenesis i n sediments from t h e c e n t r a l Pacific. I n : Winterer, E. L., Ewing, J. I., e t al., I n i t . Repts. DSDP, 17: Washington (U.S. Govt. P r i n t i n g O f f i c e ) , pp. 377-405. Leinen, M., 1979. Biogenic s i l i c a accumulation i n t h e c e n t r a l e q u a t o r i a l P a c i f i c and i t s imp1 i c a t i o n s f o r Cenozoic paleoceanography. Geol. SOC. Am. Bull., 90: 1310-1376. 1971. Caribbean Eocene volcanism and t h e Mattson, P. H. and Pessagno, E. A., e x t e n t o f Horizon A. Science, 174: 138-139. McBride, E. F. and Thompson, A., Region, Texas. Geol. SOC. Am.

1970. The Caballos Novaculite, Spec. Pap. 122, 129 pp.

Marathon

McCave, I. N., 1979. Diagnosis o f t u r b i d i t e s a t S i t e s 386 and 387 by p a r t i c l e counter s i z e a n a l y s i s o f t h e s i l t (2-40 vm) f r a c t i o n . I n Tucholke, B. E., Vogt, P. R., e t al., I n i t . Repts. DSDP, 43: Washington, (U.S. Govt. P r i n t i n g O f f i c e ) , pp. 395-416. McLaughlin, R. J. and Pessagno, E. A., Jr., 1978. S i g n i f i c a n c e o f aye r e l a t i o n s above and below Upper J u r a s s i c o p h i o l i t e i n t h e Geyser-Clear Lake Region, C a l i f o r n i a . J. Res. U.S. Geol. Surv., 6: 715-726. Murchey, Benita, 1980. S i g n i f i c a n c e o f c h e r t age determinations i n t h e M a r i n C o r d i l l e r a n Section, Headlands. Geol. SOC. Am. Absts. w i t h Progs., C o r v a l l i s , Oregon, pp. 144. Nisbet, E. G. and P r i c e , I., 1974. Siliceous turbidites: bedded c h e r t s as redeposited ocean r i d g e - d e r i v e d sediments. In: Pelagic Sediments: on l a n d and under t h e sea. HSU, K. J. and Jenkyns, H. C. ( E d i t o r s ) , I n t e r . Assoc. Sediment. Spec. Pub. 1: 351-366. Peach, 8. N. and Horn, J., Survey, U.K. v. 1.

1899.

The S i l u r i a n rocks o f B r i t a i n .

Mem.

Geol.

43

Sampson, Edward, 1923. The f e r r u inous chert formations of Notre Dame Bay , Newfoundland. J. Geol. 31: 571-598. Sancetta, Constance, 1981. Bi o s t r a t i g r a p h i c and paleoceanographic events in the eastern equatorial Pacific: Results of DSDP Leg 69. In: Cann, J. R., Langseth, M. G., et a l . , I n i t . Repts. DSDP, 69: Washington (U.S. Govt. Printing O f f i c e ) , ( I n press). Schlager, W. and Schlager, M., 1973. radiolarites (Tauglboden-Schichten, Sedimentology, 20: 65-89.

C l a s t i c sediments associated with Upper J u r a s s i c , Eastern Alps).

Schmidt-Effing, R., 1979. A l t e r und Genese des Nicoya-Komplexes, e i n e r ozeanischen Palaokrusta (Oberjura b i s Eozan) im sudlichen Zentralamerika. Geol. Rundschau, 68: 457-494. Shimizu, Hiroshi and Masuda, Akimasa, 1977. of marine environment of i t s formation.

Cerium in c h e r t a s an indication Nature, 266: 346-348.

Steinberg, M. and Marin, C. M., 1978. C l a s s i f i c a t i o n geochimique des r a d i o l a r i t e s et des sediments s i l i c e u x oceaniques, s i g n i f i c a t i o n paleooceanographique. Oceanologica Acta, 1: 359-367. Thiede, Jbrn, V a l l i e r , T. L., et a l . , 1981. I n i t i a l Reports of the Deep Sea D r i l l i n g P r o j e c t , Leg 62, v. 62: Washington (U.S. Govt. P r i n t i n g Office), 1113 pp. T o r r i n i , R. and M a t t i o l i , G. (In prep.). In: Speed and Larue, 1982, Barbados a r c h i t e c t u r e and implications f o r a c c r e t i o n , Jour. Geophys. Res. (In press). Trask, P. D., 1950. Geologic descriptions of the mnganese deposits of California. California Div. of Mines Bull., 152: 378 pp. Tucholke, B. E., Vogt, P. R., et al., 1979. I n i t i a l Reports of the Deep Sea D r i l l i n g P r o j e c t , Leg 43, v. 43: Washington (U.S. Govt. P r i n t i n g Office), 1115 pp. Van Andel, T. H., Heath, G. R., and Moore, T. C., Jr., 1975. Cenozoic history and paleoceanography of the c e n t r a l equatorial P a c i f i c Ocean. Geol. SOC. Am. Mem. 143: 134 p. Winterer, E. L. and Jenkyns, H. C., o r i g i n of Mesozoic ribbon chert. with Progrs., 7: 542-543.

1979. Radiolarians, diatoms, and the Geol. SOC. Am. Annual Meeting Abstrs.

45

CHAPTER 4 RECENT DEVELOPMENTS IN THE SEDIMENTOLOGY OF SILICEOUS DEPOSITS IN JAPAN Azuma IIJIMA and Minoru UTADA Geological Institute, the University of Tokyo, 7-3-1 Hongo, Tokyo 113 (Japan)

INTRODUCTION Fine-grained siliceous deposits are involved to varying extents in all Phanerozoic strata of Japan except the Cambrian, which is absent. They occur in marine sections with the exceptions of a few continental diatomites of Neogene and Quaternary age. The diversity of occurrences and lithologic types as well as the complex geologic history of Japan as a compound orogenic belt in the Circum-Pacific region provide one of the most favorable areas in which siliceous deposits are investigated. Economic as well as scientific interests have stimulated the investigation of siliceous rocks; some are used as silicastone ores, some contain metallic ores such as copper and manganese, and some are oil source rocks and reservoirs. Extensive work has been done on various aspects of the siliceous deposits of Japan, as listed in the bibliography by Hein (1980). In this paper we summarize recent developments in the sedimentology of the siliceous deposits of Japan. The last section on "Fine-grained silicastone ore deposits" has been written by Utada; the other sections, by Iijima. 1

2

CLASSIFICATION AND TERMINOLOGY OF FINE-GRAINED SILICEOUS ROCKS IN JAPAN Consolidated fine-grained siliceous deposits of Japan have various appearances and are called different names. Their classification i s mainly based on the degree of consolidation, the rate of mixing with clay, and the dominant kind of siliceous skeletons. Chert [chito in Japanese] is a dense and very hard siliceous sedimentary rock composed principally' of cryptocrystall ine and microcrystal 1 ine opal ine silica, quartz or a mixture of the two. The S i O n content of quartzose chert is generally ranges from 88% to 98%. The primary porosity is usually less than 10%. Chert occurs in the form of nodules, lenses, and layers. Bedded chert [sojo chito] is a thick, widespread chert body which shows specific rhythmic interlayering with shale partings, usually with even stratification. The classification of chert in Japan is based on two different criteria which are independent of each other. According to the first criterion, chert is divided into opaline or quartzose based on the crystalline state of the main

46

s i l i c a constituent.

Opaline c h e r t i n c l u d e s amorphous s i l i c a , opal-CT,

c r i s t o b a l i t e , and t r i d y m i t e .

low

By t h e second c r i t e r i o n , c h e r t i s c l a s s i f i e d i n t o

primary and secondary ( o r replacement) based on t h e o r i g i n o f t h e main s i l i c a constituent.

Primary c h e r t o r i g i n a t e s as an aggregate o f s i l i c e o u s skeletons

o r as a chemical p r e c i p i t a t e : chemical c h e r t .

The former i s c a l l e d b i o g e n i c c h e r t ; t h e l a t t e r ,

The biogenic c h e r t i s subdivided i n t o e a r l y and

on t h e stage o f c h e r t i f i c a t i o n .

later based

Early chert originates during progressive

b u r i a l ; l a t e r c h e r t i s formed by s i l i c a cementation o f porous s i l i c e o u s s e d i ments d u r i n g u p l i f t ( I i j i m a and Tada, 1981).

Secondary c h e r t i s produced by

s u b s t i t u t i o n o f o p a l i n e s i l i c a o r q u a r t z f o r n o n - s i l i c e o u s sediments; e.g., carbonate-replacement c h e r t , t u f f - r e p l a c e m e n t c h e r t , clay-replacement c h e r t , and so on.

Thus we can describe c h e r t s i m p l y by means o f t h e above c l a s s i f i -

c a t i o n , l i k e primary r a d i o l a r i a n quartzose bedded c h e r t , biogenic l a t e r o p a l i n e c h e r t , carbonate-replacement nodular c h e r t , and so on. P o r c e l a n i t e [ t o k i g a n ] has been r a t h e r u n f a m i l i a r i n Japan.

I t i s used as t h e

f i e l d name r e p r e s e n t i n g t h e i n t e r m e d i a t e r o c k type between porous d i a t o m i t e and dense c h e r t i n Neogene s e c t i o n s ( I i j i m a and Tada, 1981) a f t e r Bramlette (1946). S i l i c e o u s shale o r mudstone [ k e i s h i t s u ketsugan o r deigan] i s commonly used f o r t h e r o c k t y p e i n t e r m e d i a t e between c h e r t o r p o r c e l a n i t e and claystone. w i t h Hard shale [ k o s h i t s u ketsugan],

Together

i t i s o f t e n vaguely used f o r Neogene

s i l i c e o u s rocks i n n o r t h e r n Japan w i t h o u t regard t o s i l i c a content, and i s sometimes synonymous w i t h p o r c e l a n i t e . S i l i c a s t o n e [ k e i s e k i ] i s used as a general t e r m f o r s i l i c a ores composed e s s e n t i a l l y o f q u a r t z aggregates, n o t w i t h s t a n d i n g t h e i r o r i g i n . 3

DISTRIBUTION

OF SILICEOUS DEPOSITS I N SPACE AND TIME I N THE JAPANESE ISLANDS

Fine-grained s i l i c e o u s rocks i n t h e Japanese I s l a n d s occur l a r g e l y i n marine s e c t i o n s from Ordovician t o T e r t i a r y age.

Continental diatomaceous sediments

a r e n o t t r e a t e d here because o f t h e i r r a r i t y .

I n t h e l a s t decade t h e age o f

t h e marine s i l i c e o u s rocks has been a c c u r a t e l y determined by means o f diatom, r a d i o l a r i a n , and conodont b i o s t r a t i g r a p h y , as reviewed by Koizumi (1982); Yao (1982); and I g o and Koike (1982) i n t h i s volume.

Regional s t u d i e s a r e s t i l l i n

progress, and t h i s i s a b r i e f overview o f t h e i r d i s t r i b u t i o n i n space and time. The t e c t o n i c i n t e r p r e t a t i o n i s p r i n c i p a l l y based on "The Japanese Islands", w r i t t e n by Kimura (1977/80).

Fig. 1 shows t h e g e o t e c t o n i c d i v i s i o n s o f t h e

Japanese I s l a n d s and t h e d i s t r i b u t i o n o f Neogene s i l i c e o u s rocks i n n o r t h e r n Japan. 3.1

The s t r a t i g r a p h y o f t h e s i l i c e o u s rocks i s summarized i n Fig. 2. Ordovician and Siluro-Devonian

S i l i c e o u s s h a l e c o n t a i n i n g Ordovician conodonts and r a d i o l a r i a n s was r e c e n t l y

41

discovered a t F u k u j i i n t h e G i f u Prefecture (Igo e t a l . , Nishiyama, 1980). Japan.

1980; Furutani and

As f a r as we know, t h i s i s t h e o l d e s t s i l i c e o u s rocks i n

I t i s associated w i t h Siluro-Devonian and Carboniferous r e e f limestones

a t t h e margin o f t h e Hida Terrane, which i s a segment o f t h e o l d Asian c o n t i nent. Siluro-Devonian s i l i c e o u s shale associated w i t h Fuvosites-HuZisites r e e f limestone and s i l i c i c t u f f occurs s p o r a d i c a l l y i n t h e Kurosegawa-Ofunato B e l t , which i s considered as a detached remnant o f t h e o l d continent. 3.2

Upper Paleozoic and Hesozoic S i l i c e o u s Rocks i n t h e Chichibu Geosyncline

A s u i t e o f geosynclinal deposits o f Late Paleozoic and E a r l y Mesozoic age are widespread i n t h e Chichibu Terrane, where the Chichibu Geosyncl i n e e x i s t e d between t h e o l d Hida c o n t i n e n t and t h e Kurosegawa-Ofunato B e l t forming a t r a c t o f shallow sea and i s l a n d s .

An eugeosyncline p e r s i s t e d d u r i n g E a r l y Carbonifer-

ous t o E a r l y Permian times i n a s s o c i a t i o n w i t h vast submarine mafic volcanics. I t s shallow sea environment i s r e f l e c t e d by t h e common a s s o c i a t i o n o f f u s u l i n i d and r e e f limestones w i t h t h e volcanics. bedded c h e r t and s i l i c e o u s shale.

It contains a r a t h e r small amount o f

During t h e Late Permian, T r i a s s i c and

Jurassic, i t changed t o a miogeosyncline, i n which submarine mafic volcanism d i d n o t occur, excepting a few r e s t r i c t e d areas.

Terrigenous c l a s t i c sediments predominate i n t h e Upper Permian and Jurassic formations, w h i l e spiculer a d i o l a r i a n bedded c h e r t and s i l i c e o u s shale a r e abundant i n t h e T r i a s s i c .

The

Upper T r i a s s i c c h e r t i s t h e most widespread s i l i c e o u s rocks i n Japan. The bedded c h e r t together w i t h t u r b i d i t e s , slump beds and occasional m i c r i t i c limestone represents t h e o f f s h o r e f a c i e s i n t h e c e n t r a l p a r t o f t h e mioThe metamorphosed equivalents o f such

geosyncline, which was a marginal sea.

T r i a s s i c sedimentary rocks are i d e n t i f i e d i n t h e Ryoke and Sanbagawa C r y s t a l l i n e Schists on t h e both sides o f t h e Median Tectonic L i n e i n Southwest Japan (Toyohara, 1977; Kano, 1979; Suyari e t a l . ,

1980).

A t t h e both i n n e r and o u t e r margins o f t h e miogeosyncline, t h e o f f s h o r e c h e r t f a c i e s changes t o a shallow s h e l f f a c i e s w i t h Monotis and even t o p a r a l i c coal measures.

Jurassic

r a d i o l a r i a n s i l i c e o u s shale and bedded c h e r t occur a t some places

- e.g.,

Inuyama, KUZUU, and Yamizo - i n a s s o c i a t i o n w i t h amnonite-bearing shales, and massive sandstone c o n t a i n i n g d r i f t wood and a l i t t l e conglomerate.

3.3

Upper Paleozoic and T r i a s s i c S i l i c e o u s Rocks i n t h e Sanbosan Geosyncline

A s u i t e o f geosynclinal deposits ranging from Late Carboniferous t o E a r l y Jurassic age a r e d i s t r i b u t e d i n t h e Sanbosan Terrane between t h e KurosegawaOfunato B e l t and t h e Butsuzo Tectonic Line. They f i l l e d t h e Sanbosan Geosyncl i n e t h a t formed outboard, south o f t h e Chichibu Geosyncl ine. Radiolarian bedded c h e r t i s sporadic i n t h e Permo-Carboniferous s t r a t a , b u t occurs commonly

48

I'.+1

HlDA OLD CONTINENT

0 .. 0

INNER C H l C H l B U T E R R A N E

MTL MEDIAN TECTONIC LINE OUTER C H l C H l B U T E R R A N E

++ KUROSEGAWA-OFUNATO

m -

BELT

SANBOSAN TERRANE

BTL BUTSUZO TECTONIC LINE SHIMANTO TERRANE

(IIIID S E T O G A W A

TERRANE

'i2 E

Fig. 1. Geotectonic map o f the Japanese Islands ( s l i g h t l y m o d i f i e d Kimura, 1977/1980). Hatched area shows t h e d i s t r i b u t i o n o f Miocene diatomaceous and s i l i c e o u s rocks i n northern and west coast o f Japan: I = Japan Sea coast, I 1 = c e n t r a l Hokkaido, and I11 = e a s t Hokkaido. 1 = Tenpoku, 2 = Kitami, 3 = Akita, 4 = N i i g a t a , 5 = Yamizo, 6 = KUZUU, 7 = Chichibu, 8 = Inuyama, 9 = Tanba, 302, 438, 439 = DSDP s i t e s .

I I , I s~F~lJ~~ I

CHICHIBU TERRANE

INNER

PL I

OUTER

KUROSEGAWA-

SANBOSAN TERRANE

SHIMANTO

SETOGAWA TERRANE

SORACHI TERRANE

JAPAN SEA COAST

CENTRAL HOKKAIDO

EASTERN HOKKAIDO

R i WAE :

MI0 T OLI EOC PAL

L KE

L J-FJ

E

R P-

L E

L CE D S

A

A A S I L I C I C AND INTERMEDIATE VOLCANICS

' vvv

SUBMARINE MAFIC VOLCANICS

0 OPHIOLITE

Fig. 2. Stratigraphic distribution o f the primary bedded chert and siliceous shale facies in each of the tectonic units o f the Japanese Islands. P (D

50 i n the Triassic.

Flafic t o i n t e r m e d i a t e v o l c a n i c s and shallow-marine l i m e s t o n e

a r e associated w i t h t h e T r i a s s i c c h e r t a t t h e southern margin o f t h e Sanbosan Geosyncline.

It i s noteworthy t h a t c o n t i n e n t a l o r shallow-marine environments

p e r s i s t e d on b o t h margins o f t h e geosyncline through L a t e Paleozoic a n d T r i a s s i c age (Kimura e t al.,

3.4

1975; Murata, 1981).

Mesozoic S i l i c e o u s Rocks i n t h e Sorachi Geosyncline The Sorachi Geosyncline was widespread i n c e n t r a l Hokkaido, extending t o

Sakhalin I s l a n d d u r i n g Permian t o E a r l y Cretaceous times i n a s s o c i a t i o n w i t h an immense volume o f s e r p e n t i n i t e , gabbro, and b a s a l t p i l l o w l a v a , which a r e i n t e r p r e t e d as a segment o f t h e o l d oceanic c r u s t .

R a d i o l a r i a n bedded c h e r t

and s i l i c e o u s shale o f T r i a s s i c t o E a r l y Cretaceous age a r e widespread and c l o s e l y associated w i t h t h e p i l l o w l a v a and m a f i c t u f f .

The s i l i c e o u s rocks

extend southward t o t h e c o n t i n e n t a l slope area o f f Sanriku ( I i j i m a e t a l . , 1980).

The d e t a i l e d s t r a t i g r a p h y and geologic s t r u c t u r e have n o t been f u l l y

investigated.

3.5

Cretaceous and T e r t i a r y S i l i c e o u s Rocks i n Shimanto and Setogawa Terranes Thick s t r a t a o f L a t e Jurassic, Cretaceous, and T e r t i a r y ages, c a l l e d t h e

Shimanto Supergroup, a r e widespread i n t h e Shimanto Terrane, where t h e Cretaceous Shimanto and T e r t i a r y Setogawa Geosynclines formed on t h e southeast o f t h e Sanbosan Geosyncline along t h e P a c i f i c Ocean coast from t h e Boso Peninsula t o Okinawa Island.

Thus, t h e Chichibu, Sanbosan, Shimanto, and

Setogawa geosynclinal basins s h i f t e d toward t h e P a c i f i c Ocean through geologic time. Bedded c h e r t and s i l i c e o u s shale, sometimes interbedded w i t h t u r b i d i t e sandstones, slump beds, s i l i c i c t u f f s , and m a f i c volcanics, comprise t h e o f f s h o r e facies.

I n western Shikoku, i n which t h e t y p e l o c a l i t y o f t h e Shimanto

Supergroup i s located, t h e Upper Cretaceous o f f s h o r e f a c i e s changes l a t e r a l l y t o a contemporaneous shallow s h e l f f a c i e s a t t h e northwest margin o f t h e Shimanto geosyncline (Yanai, 1981 ).

I n t h e Setogawa Terrane on t h e southeast

s i d e o f t h e Shimanto Terrane i n c e n t r a l Honshu, r a d i o l a r i a n and diatomaceous bedded c h e r t s occur a t some horizons i n t h e Eocene t o M i d d l e Miocene section. The Miocene o f f s h o r e c h e r t f a c i e s changes l a t e r a l l y t o a shallow s h e l f f a c i e s w i t h l a r g e molluscs and such l a r g e r f o r a m i n i f e r s as LepidocycZinu and

Miogypsina. This t r a n s i t i o n takes p l a c e through t h e o f f s h o r e slope f a c i e s over a d i s t a n c e o f 25 km along t h e s t r i k e o f t h e Setogawa Geosyncline ( I i j i m a e t a l . , 1981).

51

3.6

Neogene S i l i c e o u s Rocks i n Northern Japan Diatomaceous s i l i c e o u s rocks i n c l u d i n g d i a t o m i t e , p o r c e l a n i t e , c h e r t , and

s i l i c e o u s shale a r e widespread i n t h i c k marine Miocene s e c t i o n s o f n o r t h e r n Japan.

The s i l i c e o u s rocks accumulated i n t h r e e major sedimentary basins.

The

f i r s t b a s i n on t h e Japan Sea s i d e extends from t h e Oshima Peninsula i n southwest Hokkaido, through t h e Oga and Noto Peninsulas, t o Oki I s l a n d , where a l t e r e d s i l i c i c t o i n t e r m e d i a t e green v o l c a n i c s o f E a r l y t o Middle Miocene age u n d e r l i e o r even s u b s t i t u t e

t h e s i l i c e o u s rocks.

The second b a s i n e x i s t s i n c e n t r a l

Hokkaido from Tenpoku t o Hidaka, extending southward t o t h e c o n t i n e n t a l s l o p e o i f t h e Kitakami M a s s i f ( I i j i m a e t a l . ,

1980) and f a r t h e r probably t o t h e

Kamenoo Formation i n t h e Joban d i s t r i c t .

The t h i r d b a s i n i s i n e a s t e r n

Hokkaido, where t h e r e a r e many t h i n i n t e r c a l a t i o n s o f s i l i c i c and i n t e r m e d i a t e v o l c a n i c l a s t i c rock.

Each o f t h e t h r e e basins i s separated by landmass

composed o f p r e - T e r t i a r y basement rocks.

Phosphatic r o c k i s l i t t l e i f any i n

t h e Miocene sections, though t h e geologic age and l i t h o l o g i e s o f t h e s i l i c e o u s rocks a r e very s i m i l a r t o those o f t h e Monterey Formation o f C a l i f o r n i a .

4

SEDIMENTATION AND EMPLACEMENT OF PRIMARY BEDDED CHERT The o r i g i n and genesis o f bedded c h e r t have l o n g been debated i n Japan and

research continues.

The most prominent f e a t u r e o f Japanese bedded cherts,

i r r e s p e c t i v e o f g e o l o g i c age from Carboniferous t o Miocene, i s t h e rhythmic i n t e r - l a y e r i n g w i t h shale p a r t i n g s .

Most i f n o t a l l c h e r t beds were o r i g i n a l l y

deposited as an aggregate o f s i l i c e o u s skeletons (Imoto and Saito, 1973; I i j i m a e t a1

. , 1978;

Iij i m a and Tada, 1981 ; Tada, 1981 ).

There i s a tendency f o r

r a d i o l a r i a n s t o predominate i n t h e p r e - T e r t i a r y c h e r t and diatoms, i n t h e T e r t i a r y , e s p e c i a l l y i n Miocene c h e r t and p o r c e l a n i t e : t r a t e s p o r a d i c a l l y w i t h o u t r e g a r d t o g e o l o g i c age.

Sponge s p i c u l e s concen-

Terrigenous c l a y s c o n s t i t u t e

most o f t h e shale p a r t i n g s , which a r e composed l a r g e l y o f expandable and nonexpandable sheet s i l i c a t e s as w e l l as amorphous m a t e r i a l i n t h e younger rocks. These a r e w h o l l y transformed i n t o i l l i t e , c h l o r i t e and q u a r t z i n t h e o l d e r bedded c h e r t s (Iwao, 1961; I i j i m a e t a l . ,

1978, 1979; I i j i m a and Tada 1981).

Fine-grained v o l c a n i c l a s t i c m a t e r i a l s , p a r t i c u l a r l y s i l i c i c t o m a f i c v i t r i c fragments, a r e sometimes mixed t o v a r y i n g degrees w i t h t h e t e r r i g e n o u s c l a y s ( I i j i m a e t al.,

1979, 1981; I i j i m a and Tada, 1981; Sano e t a l . ,

1979).

Red

bedded c h e r t s c o n t a i n a small amount o f h e m a t i t e p a r t i c l e s as r e d pigment. These a r e a p t t o change t o green f e r r u g i n o u s s i l i c a t e l i k e c h l o r i t e under some burial conditions ( I i j i m a e t al.,

1978).

It i s noteworthy t h a t t h e r e were few,

i f any, s i l i c e o u s skeletons o r i g i n a l l y i n t h e shale p a r t i n g s , and t h a t t h e nons i l i c a mineral assemblage o f t h e shale p a r t i n g i s g e n e r a l l y t h e same as t h e associated bedded c h e r t .

52

4.1 Mechanism of Formation of Bedded Chert The mechanism of formation of the rhythmic layering in bedded chert is an important problem remaining to be solved. Many models have been proposed, all of which can be grouped into the following three: (1) Diagenetic differentiation of a homogeneous sil ica-clay-water mixture into chert beds and shale partings. (2) Sedimentary differentiation of a homoaeneous siliceous skeleton-clay-water mixture into chert beds and shale partings -siliceous turbidite. (3) Double accumulation model. a) Sedimentation of clay at a constant and slow rate interrupted by intermittent and rapid precipitation of silica, b) Sedimentation of siliceous skeletons at a constant and slow rate interrupted by intermittent and rapid accumulation of clay. The diagenetic differentiation model long ago was proposed by Davis (1918) based on laboratory experiments and field observations of the Franciscan Chert and the Ilonterey Shale. In Japan, Shoji (1967) performed similar experiments using synthetic silica gel and pulverized clay slate and insisted that the "Upper Paleozoic" bedded chert in the Ashio Mountains formed in the process of diagenesis. We believe it is impossible for such diagenetic differentiation to occur in an aggregate of dominant siliceous skeletons and little clay. The siliceous turbidite model is not unrealistic when the chert-shale beds are compared with the flysch-type alternation of terrigenous turbidites. In fact, cyclic sedimentation of siliceous turbidite is sometimes found in bedded diatomaceous porcelanite in the Neogene sections of northern Japan. Kano (1979) reported siliceous turbidite in the lower part of the Onnapawa Formation from the south coast of the Oga Peninsula. Siliceous turbidite occurs in bedded porcelanite of the Wakkanai Formation in the Tenpoku district o f northern Hokkaido (Fukusawa, 1982). Also, it is seen in thick-bedded porcelanite in the lower part of the Tatsukobu Formation in the Kitami district of eastern Hokkaido. In northern and eastern Hokkaido, the turbidite unit has a thickness of 10-30 cm: It begins with a sharp contact - frequently an erosional surface - at the base, which is followed by a 1-3 cm-thick sandy layer composed of terrigenous and volcaniclastic grains, larger forms of siliceous skeletons, reworked glauconite pellets, and reworked clasts of diatomaceous shale in the siliceous matrix; the sandy layer grades into massive diatomaceous porcelanite; and microcross-lamination sometimes occurs in the gradational zone. The top shale parting is not always clear. Some chert beds in the Miocene Okabe Formation of the Setogawa Terrane are probably siliceous turbidites which are interbedded with turbidite sandstones and slump beds (Iijima et al., 1981). Undoubted siliceous turbidites characterized by asymmetric layering have not

53

been reported in pre-Tertiary bedded chert. Iijima et al. (1978) reported four layering types in Triassic bedded cherts: single-layered, triple-layered, laminar and striped types as shown in Fig. 3. These types are also recognized in Tertiary bedded chert of the Setogawa Terrane (Iijima et al., 1979). The triple-layered type is usually predominant, and is characterized by symmetric, rather obscure layering composed o f the middle clay-poor, purer chert layer and the upper and lower argillaceous chert layers; the latter layers grade into both the top and bottom shale partings unless microstylolites develop at the boundaries. The distribution o f siliceous skeletons is correspondingly symnetric, and larger radiolarian tests tend to concentrate at the top and bottom margins of the radiolarian chert bed (Iijima et al., 1978). There are no evidences o f the selective dissolution of the tests in the shale partings: In other beds, the tests are usually well preserved in mudstone. Furthermore, the chemical composition of the triple-layered chert bed and neighboring shale partings shows a symnetric pattern (Fig. 4): The Si02 content, which islargely biogenic silica, is highest in the middle chert layer, decreasing gradually toward the shale partings on the both sides of the chert bed: By contrast, the content of TiOz, A1203,total Fey MgO, K20, Ni, Zn, Cr and Zr, which are principally derived from clay particles, are lowest in the middle layer, increasing gradually toward the top and bottom shale partings (Yamazaki, 1979). The symmetric pattern cannot be explained by the siliceous turbidite model but by the double accumulation model. Iwao (1976) investigated Permo-Triassic bedded chert of the Chichibu Terrane and concluded that silica precipitated rapidly and intermittently while clay accumulated at a slow but constant rate in the geosynclinal basin. He seems to invoke an inorganic chemical precipitate rather than siliceous skeletons as the main source of the silica. Actually, most bedded cherts in Japan consisted originally of siliceous skeletons. Consequently, Iwao's model implies a difficulty in postulating a long period of non-existence of siliceous plankton, represented by the shale parting, alternating with a short period of explosive productivity. By contrast, Iijima et al. (1978, 1979, 1981) consider that clay was flushed rapidly and intermittently to form shale partings while siliceous skeletons were deposited at a slow but constant rate. Chert beds in the bedded chert show significantly greater values of the !lnO/Al203 ratio compared with shale partings, this suggesting rapid and slow accumulation of the shale partings and chert beds, respectively (Matsumoto and Iijima, 1982). The flushing of the shale partings might result from either distal turbidity currents or bottom currents at intervals of a few thousands to a few tens of thousands of years. This estimate is derived from the average rate of sedimentation of some Japanese bedded cherts; 2-34 mm/lOOO yrs (Iijima et al., 1978, 1981; Igo

54

Laminar

Striped

Graded

Fig. 3. Layering types 0 . s i n g l e c h e r t beds i n bedded c h e r t formations.

0.05 0.1

1

0.5

5

T i

10

60

100

80

MINOR ELEMENTS (ppm)

-3 t 1

f

f 100

0

100

t

“ i . t t I

200

0

50

f, 100

0

500

1000

Fig. 4. Symnetrical d i s t r i b u t i o n o f major and minor elements w i t h i n a t r i p l e l a y e r e d c h e r t bed and t h e adjacent shale p a r t i n g s i n t h e Late T r i a s s i c bedded c h e r t a t Inuyama i n t h e Mino d i s t r i c t , c e n t r a l Honshu.

55

and Koike, 1982; Yao, 1981), and from t h e average thickness o f a c h e r t bed o f 25-64 mn ( I i j i m a e t al.,

1978, 1979). The mechanism o f sedimentation o f t h e single-layered type i s l i k e l y s i m i l a r

t o t h a t o f t h e t r i p l e - l a y e r e d one.

That o f t h e laminar and s t r i p e d types i s

n o t f u l l y understood. S i t e o f Sedimentation o f Bedded Chert

4.2

There are two c o n t r a s t i n g t h e o r i e s on t h e s i t e o f sedimentation o f bedded c h e r t i n Japan.

One, based on t r a d i t i o n a l geosyncline theory, concludes t h a t

the bedded c h e r t accumulated i n an eu- o r miogeosynclinal basin together w i t h associated t u r b i d i t e s , shale, slump beds, sandstones, limestones, and/or mafic volcanics (Kimura, 1974, 1977/80; Kimura and Tokuyama, 1971; Kimura e t al., 1975). The other, t h e melange theory, based on modern p l a t e tectonics, assumes t h a t t h e bedded c h e r t accumulated o r i g i n a l l y on an oceanic p l a t e under an open ocean environment, being brought w i t h t h e p l a t e t o a subduction zone.

A t the

associated trench, which was t h e s i t e o f sedimentation o f terrigenous slope deposits such as t u r b i d i t e s and slump beds, t h e c h e r t was t e c t o n i c a l l y mingled w i t h terrigenous deposits t o form an a c c r e t i o n a r y prism (Kanmera, 1976; Suzuki and Hada, 1979).

This problem i s being a c t i v e l y debated even f o r rocks i n the

same province, e.g.,

i n the Shimanto and Sanbosan Terranes. So f a r as we know, no bedded c h e r t equivalent t o modern s i l i c e o u s ooze i s

found i n Japan, except f o r t h e r e d r a d i o l a r i a n c h e r t o f t h e Horokanai o p h i o l i t e s u i t e i n c e n t r a l Hokkaido, on which we reserve judgement. Bedded c h e r t d i f f e r s from s i l i c e o u s ooze i n many respects:

(1) The rhythmic l a y e r i n g i n bedded c h e r t never appears i n t h e s i l i c e o u s oozes encountered i n many D.S.D.P. cores. (2) The chemical composition and abundances o f minor and t r a c e elements o f bedded c h e r t s are s i g n i f i c a n t l y d i f f e r e n t from those o f open-ocean p e l a g i c clays and r a d i o l a r i a n oozes.

I n p a r t i c u l a r bedded cherts, even r e d v a r i e t i e s , a r e

d e f i c i e n t i n such base metals as Fey Mn, Cu, and N i compared w i t h t h e openocean sediments.

Bedded cherts a r e e s s e n t i a l l y a m i x t u r e o f biopenic s i l i c a

and o f f s h o r e terrigenous mud (Hatsumoto and I i j i m a , 1982). (3) S i g n i f i c a n t e f f e c t s o f f r e s h water on t h e sedimentation o f some bedded cherts a r e r e f l e c t e d by a small p o s i t i v e Ce anomaly i n t h e d i s t r i b u t i o n p a t t e r n o f the r a r e e a r t h elements i n t h e T r i a s s i c c h e r t o f t h e Mino d i s t r i c t i n t h e Chichibu miogeosyncline (Shimizu and Masuda, 1977); t h e i n t e r c a l a t i o n o f a coal seam i n t h e same d i s t r i c t ( I i j i m a e t al., 1978); and t h e existence o f brackish diatom Stephmropyxis i n t h e Oligocene c h e r t of t h e Setoaawa Terrane i n c e n t r a l Honshu ( I i j i m a e t al., 1979, 1981). (4)

The average r a t e o f sedimentation o f bedded c h e r t i s much l a r g e r than t h a t

56

o f modern s i l i c e o u s ooze.

It i s estimated t o be 2.1

mm per 1000 y r s on an

average f o r t h e Upper T r i a s s i c c h e r t i n t h e Tanba and Mino d i s t r i c t s o f t h e Chichibu Terrane (Yao, 1981); 6 mn and 23 mn per 1000 y r s f o r t h e Oligocene and Miocene chert, r e s p e c t i v e l y , i n t h e Setogawa Terrane ( I i j i m a e t al.,

1981); and

27-34 tnm per 1000 y r s f o r some T r i a s s i c c h e r t i n c e n t r a l Japan ( I i j i m a e t a l . , 1978).

Even t h e smallest value, 2.1

tnm/lOOO

y r s , i s f a r greater, because i t

corresponds t o a sedimentation r a t e o f a t l e a s t 10 mm/lOOO y r s f o r modern sediments when t h e compaction r a t e o f t h e dense T r i a s s i c c h e r t w i t h many m i c r o s t y l o l i tes p a r a l l e l t o t h e bedding plane i s compensated f o r according t o methods used by Tada (1981). What i s t h e d e p o s i t i o n a l environment o f bedded c h e r t o f Japan?

Thenecessary

c o n d i t i o n s f o r t h e formation o f bedded c h e r t a r e high p r o d u c t i v i t y o f s i l i c e o u s organisms, t h e segregation o f coarser c l a s t i c m a t e r i a l , t h e p e r i o d i c i n f l u x o f clay, and moreover, t h e persistence o f such c o n d i t i o n s f o r a l o n g time - o n t h e order o f a m i l l i o n t o t e n m i l l i o n years.

There a r e two favorable s i t e s f o r

such c o n d i t i o n s - o f f s h o r e banks on a c o n t i n e n t a l slope and o f f s h o r e basins (Fig. 5). Offshore bank topography i s sometimes b u i l t on a foundation o f a f l a t - t o p p e d b a s a l t i c sea mount, as i n t h e case o f t h e Paleogene c h e r t o f t h e Setogawa Terrane ( I i j i m a e t a l . ,

1981) and t h e T r i a s s i c c h e r t o f t h e Sanbosan

Terrane i n c e n t r a l Kyushu (Kanmera, 1968).

Banks a r e sometimes constructed

t e c t o n i c a l l y , as i n t h e case o f t h e Miocene c h e r t o f t h e Setogawa Terrane ( I i j i m a e t al.,

(1)

1981).

Offshore basins are found i n a marginal sea, as i n t h e

OFFSHORE BANK ON CONTINENTAL SLOPE

BANK CONSTRUCTED BY SUBMARINE MAFIC VOLCANISM OR TECTONICALLY (2)

OFFSHORE BASIN IN MARGINAL SEA

-100-200

km

SWELLS PROTECTING INFLUX OF COARSER CLASTICS INTO BASIN Fig. 5. Favorable s i t e s o f sedimentation o f bedded c h e r t shown i n black. ( 1 ) S l i g h t l y modified Fig. 13 o f I i j i m a e t a l . (1981). ( 2 ) Modified Fig. IV-4B o f Kimura (1977).

case of the Triassic chert i n the Chichibu miogeosyncline (Iijima e t a l . , 1978; Kimura e t a l . , 1975) and i n the Sanbosan Geosyncline (blurata, 1981). The existence of a swell between the basin and the mainland i s most favorable because i t prevents the influx of coarser terrigenous material into the basin (Kimura, 1977). The water depth of the offshore banks o r basins is not necessarily deep, only deep enough t h a t siliceous skeletons are not swept away by surface waves and t i d a l currents. Actually, many of the bedded cherts of Japan areconsidered t o have accumulated under relatively shallow environments (Iijima e t a l . , 1978, 1981 ; Kanmera, 1968; Kimura, 1977/80). The forementioned Miocene bedded porcelanite of a siliceous t u r b i d i t e origin of eastern Hokkaido was also deposi t e d under r e l a t i v e l y shallow environments, considering the conformable superposition on non-marine s t r a t a . Emplacement of Bedded Chert Bedded chert frequently underwent the deformation which indicates the mode of emplacement from the s i t e of sedimentation. Intraformational folds due to penecontemporaneous submarine sliding sometimes indicate t h a t they accumulated originally a t a shallower depth. Chert beds frequently are involved i n a gigantic nappe structure l i k e the Setogawa Nappe in the Setoaawa Terrane ( l i jima e t a l . , 1981). Olistostromes and complicated deckenpakets composed o f many t h r u s t s l i c e s o f variable geologic ages a r e known from many l o c a l i t i e s i n the Chichibu Geosyncline; f o r example, Kuzuu (Yanagimoto, 1973), eastern tlino (Kano, 1979), Inuyama (Iijima e t a l . , 1978; Yao e t a l . , 1980), west Chugoku (Toyohara, 1977), and so on. These phenomena might be interpreted a s suggesting a melange which formed i n a past subduction zone. However, geologic evidence based on detailed f i e l d and laboratory work leads t o the conclusion t h a t the bedded cherts contained i n such structures was emplaced by large-scale gravity slidings which occurred inside geosynclinal basins (Iijima e t a l . , 1978, 1981; Kimura, 1977/80). Paleomagnetic methods a r e sometimes used t o estimate the emplacement of bedded chert and associated greenstone, although they seem t o be unreliable with respect to the restoration of such complicated structures. 4.3

5

DIAGENESIS AND LITHIFICATION OF FINE-GRAINED SILICEOUS DEPOSITS Northern Japan, i n which marine siliceous rocks of Neogene age arewidespread and thick, provides an excellent f i e l d f o r study of the diagenesis and l i t h i f i cation of siliceous deposits, particularly because there a r e some o i l f i e l d s from which we can obtain subsurface data. L i t t l e work has been done on this k i n d of investigation i n older bedded chert a s compared w i t h extensive work on the Neogene siliceous rocks.

58

TABLE 1 Silica and zeolite zones in biogenic siliceous rocks and interbedded silicic volcaniclastic rocks of Neogene marine sections. Biogenic siliceous rocks

Si 1 icic vol canicl astic rocks

Silica zones

Silica zones Silicic glass (Opal-A)

Opal -A Opal -CT Quartz

5.1

I

i

Tridymite cementation from percolating groundwater

Zeolite zones I

'

Low cri stobali te

I1

Quartz

111

IV

Silica Diagenesis Amorphous silica and various opaline minerals appear in the process of sedimentation and diagenesis o f the siliceous deposits and interbedded volcaniclastic sediments; they are eventually transformed into quartz, which is thermo-dynamically stable in sedimentary rock regimes. Recently, Iijima and Tada (1981), Tada (1981), and Tada and Iijima (1982) clarified the ways in which different types of opaline minerals form from different initial materials in Neogene siliceous deposits, as sumnarized in Table 1. Biogenic opal, constituted of diatom frustules, radiolarian tests and sponge spicules, changes to opal-CT, which crystallizes into quartz during burial diagenesis. The crystallinity of opal-CT, as represented by the decrease of d(101) spacing, increases with burial temperature. As a result, biogenic opal , opal-CT and quartz are distributed in a vertically zonal arrangement, which i s recognized in surface and subsurface sections in northern Japan (Hitsui and Taguchi, 1977; Honda, 1978; Kano, 1979; Aoyagi and Kazama, 1980; Iijima and Tada, 1981; Tada, 1981). The silica mineral zoning in biogenic siliceous deposits corresponds .nearly to the zeolite zoning formed by burial diagenesis o f silicic vitric volcaniclastic sediments (Iijima, 1980); that is, the biogenic opal zone corresponds to Zone I (fresh glass, lacking in zeolites); the opal-CT zone, Zone I1 (clinoptilolite and/or mordenite, and low-cristobalite); and the quartz zone, Zone III (analcime and quartz) and Zone IV (albite and quartz). Silica burial diagenesis tends to proceed a little earlier, that is to say, occurs at a little lower temperatures than zeolite diagenesis. The analyses of core of deep holes in the Neogene sections indicate that silica diagenesis as well as zeolite diagenesis is dependent primarily on burial temperature.

59

S i l i c a zoning due t o b u r i a l diagenesis i s o f t e n m o d i f i e d by l a t e r a d d i t i o n a l cementation w i t h l o w - t r i d y m i t e and perhaps quartz, which p r e c i p i t a t e from p e r c o l a t i n g groundwaters d u r i n g u p l i f t ( I i j i m a and Tada, 1981).

Low-tridymite

cementation occurs n o t o n l y i n t h e opal-CT zone b u t a l s o i n t h e biogenic opal zone; e.g.,

l o w - t r i d y m i t e nodules i n t h e Shinzan d i a t o m i t e d e p o s i t o f t h e Oga

Peninsula i n A k i t a .

I t i s , moreover, very d i f f i c u l t t o d i s c r i m i n a t e an opal-CT

and low t r i d y m i t e m i x t u r e from opal-CT on t h e X-ray powder d i f f r a c t o g r a m :

The

m i x i n g s e r i o u s l y i n f l u e n c e s t h e r e a d i n g o f t h e d(101) spacing o f opal-CT (Tada and I i j i m a , 1982).

Consequently, t h e analyses o f diagenesis u s i n g surface

s e c t i o n s should be i n t e r p r e t e d very c a r e f u l l y . S i l i c a diagenesis i s a l s o a f f e c t e d by l o c a l hydrothermal a l t e r a t i o n .

The

a l t e r a t i o n p a t t e r n o f s i l i c e o u s shale i n t h e Onnagawa Formation o f t h e Oga

POROSITY 0

0

20

40

60

80 %

1

I

I-

w n A

0 POROSITY

a -

ESTIMATED

[I

3

m

3

4 km

Fig. 6. P o r o s i t y - b u r i a l depth diagram i n Neogene subsurface s e c t i o n s o f marine diatomaceous and s i l i c e o u s rocks ( d o t t e d ) and mudrocks (hatched) i n n o r t h e r n Japan. P o r o s i t y estimated from bent laminae due t o d i f f e r e n t i a l compaction o f carbonate c o n c r e t i o n s i s a l s o p l o t t e d .

60

Peninsula i s a case in point (Hosoyamada e t a l . , 1981). 5.2

Lithification of Biogenic Siliceous Deposits How and when do biogenic siliceous deposits consolidate t o chert? How much do s i l i c a transformations contribute t o the consolidation? Recently, Tada (1981) investigated these problems using many core samples from deep holes penetrating i n t o the Neogene siliceous deposits of northern Japan. He concluded t h a t the diatomaceous siliceous deposits a r e not indurated t o dense chert b u t t o porcelanite d u r i n g progressive burial, even a t a depth of 4500 m a t 13OOC i n the quartz zone. The quartzose porcelanite s t i l l retains 15-20% porosity a t this d e p t h . The i n i t i a l porosity of diatomaceous earth i s around 80%, which decreases gradually t o 15-20% a t a depth of 4500 m y irrespective of the clay content. The porosity-burial depth relation can be shown t o be a smooth curve which almost matches t h a t of Neogene mudstones, excepting f o r very porous diatomites a t depths l e s s than about 1000 m (Fig. 6 ) . Any significantly abrupt porosity changes do not occur a t the biogenic opal t o opal-CT and the opal-CT t o quartz transformations, although the textures and the pore structures of the siliceous rocks change d r a s t i c a l l y . Otherwise, the compaction r a t e of porcelanite can be estimated from the d i f f e r e n t i a l compaction of the carbonate concretions formed a t an early stage of burial, and the values a r e not inconsistent w i t h the porosity-burial depth relation. I t can be deduced from these f a c t s t h a t the siliceous rocks a r e not cemented w i t h additional s i l i c a during progressive burial i n s p i t e o f the dissolution of siliceous skeletons, the reprecipitation of opal-CT in the neighboring intergranular micropores, and the c r y s t a l l i z a t i o n of microquartz. Consequently, the decrease of porosity of opaline andquartzose porcelanites i s considered t o be caused by physical compaction. About a 10 kmdeep burial m i g h t be needed t o form dense c h e r t , extrapolating from the porosity-burial d e p t h diagram. However, t h i s seems t o be too much, estimating from the terranes of older bedded chert. Pressure solution t h a t causes the comnon microstylolitic structure found i n the older chert may play an important role i n the formation o f . e a r l y quartzose chert. Later opaline c h e r t i n the Neogene siliceous sections were caused by additional low-tridymite cementation during u p l i f t (Iijima and Tada, 1981). I t i s uncertain whether l a t e r quartzose chert e x i s t s . The chertification of calcareous siliceous sediments, l i k e most deep-sea chert collected by Glomar Challenger cruises during the Deep Sea Drilllng Project, seems t o be quite different from t h a t of the aforementioned non-calcareous siliceous deposits. Preservation of siliceous skeletons i n bedded chert i s discussed by Saito and Imoto (1978), Iijima e t a l . (1978) and Kakuwa (1982).

61

6 FINE-GRAINED SILICASTONE ORE DEPOSITS 6.1 Diatomite Ore Deposits Diatomite i s one of the most important siliceous rocks f o r industrial raw materials (Ichikawa, 1953). Since 1662, diatomite from the Noto Peninsula has been used as a undercoat of lacquered japan ware. Recently more than l o 4 tons per year have been used f o r admixtures of Portland cement, absorbents, f i l t e r s and other uses. The main diatomite deposits are i n Neogene o r Quaternary s t r a t a . The Shinzan, Tsuzuriko, and Katsurane diatomite deposits in northern Akita and the Wajima, Wakura and Iizuka diatomite deposits in the Noto Peninsula, which have been actively worked, a r e i n l a t e Miocene marine sections. On the other hand, Quaternary diatomite deposits a r e mainly found i n lacustrine sediments, most of them distributed i n western Japan. Diatomite from Yatsuka in Okayama Prefecture, i s composed only of Stephanodiscus, QcZateZZa and other non-marine diatoms and is of the best quality. 6.2

S i l i c i f i e d Chert Ore Deposits S i l i c i f i e d radiolarian chert had been actively mined f o r important refractory raw materials f o r the s t e e l industry. Radiolarian cherts of Japan usually contain a small amount of impurities, while some s i l i c i f i e d ores are highly pure, the s i l i c a content more than 95% in weight. A typical silicastone ore, which i s customarily called "Akashiro-Keiseki" [Red-and-white silicastone], i s very d i s t i n c t i v e i n appearance. I t i s composed of red o r green cherty breccia w i t h white quartz veins, very hard and beautiful. Very many l o c a l i t i e s of the deposits are known from the Chichibu Terrane. Among them, productive ones are mostly situated i n the Tanba d i s t r i c t and the Kochi Prefecture. About 260 l e n t i c u l a r o r massive small deposits a r e scattered throughout the Tanba d i s t r i c t . The total ore reserves i n this d i s t r i c t were calculated to be about 460,000 metric tons or more in 1951 (Iwao e t a l . , 1951). Most of the deposits a r e sporadically formed i n Permian chert beds which overlie basaltic lava flows o r pyroclastics and underlie thick bedded chert. The ores are l e n t i c u l a r o r massive. From geological occurrences and petrographic features, Iwao (1976) concluded t h a t the ores. were the products of submarine hydrothermal a c t i v i t i e s which s t a r t e d subsequent t o the outflow of the basalt lavas. Several s i l i c a stone deposits are known in the Sanbosan Terrane. They are l e s s productive and low quality. The color of most ores i s white. However, "Kaize s i l i c a sand", which i s of very h i g h quality, i s a l s o derived from hydrothermally s i l i c i f i e d chert associated w i t h a small amount of pyrophyllite, kaolinite, s e r i c i t e , montmorillonite, diaspore and b a r i t e (Katayama e t a l . , 1955). This hydrothermal a1 t e r a t i on i s s imi 1a r t o t h a t of " Roseki -type" pyrophyl 1i t e mineral i z a t i on related t o solutions w i t h low alkali-ion activity/hydrogen ion a c t i v i t y r a t i o s .

62

S i l i c a s t o n e o r e deposits o f limestone o r i g i n a r e a l s o known i n t h e Chichibu Terrane i n t h e A t e t s u and Taishaku d i s t r i c t o f Okayama and Hiroshima Prefectures.

They a r e c a l l e d "Nan-keiseki"

[ s o f t silicastone].

The ores a r e found

i n limestone as s t r a t i f o r m bodies o r i n many small massive bodies, and composed o f c r y p t o c r y s t a l l i n e chalcedonic q u a r t z and s u i t a b l e f o r ceramic use, e s p e c i a l l y f o r t h e cement i n d u s t r y (Ueno, 1956).

Though some researchers have i n s i s t e d

t h a t t h e s i l i c i f i c a t i o n i s r e l a t e d t o a weathering process, a p a r t o f t h e s i l i c a may have been i n t r o d u c e d from hydrothermal s o l u t i o n s .

One evidence f o r t h i s i s

t h e k a o l i n i t i z a t i o n t h a t i s recognized a t some places along d i k e rocks. 6.3

Hydrothermal S i l i c a s t o n e Ore Deposits The Ugusu s i l i c a and a l u n i t e d e p o s i t i n t h e I z u Peninsula i s t h e most impor-

t a n t s i l i c a d e p o s i t f o r t h e Japanese b o t t l e - g l a s s i n d u s t r y a t present.

The

d e p o s i t was formed by hydrothermal a l t e r a t i o n o f P l e i s t o c e n e and Miocene a n d e s i t i c l a v a f l o w s and p y r o c l a s t i c s ( I i j i m a and Iwao, 1970).

Iwao (1962,

1963) s t u d i e d t h e geologic occurrence, petrography and geochemistry o f t h e deposit.

The main a l t e r e d mass has l a t e r a l dimensions o f about 10 km2 and a

depth o f about 600 meters.

V e r t i c a l zoning i n t h e a l t e r a t i o n envelope i s recog-

n i z e d from t h e s i l i c a - a l u n i t e zone a t t h e t o p t o t h e c l a y zone a t t h e bottom. Judging from t h e manner o f l e a c h i n g o f a l a r g e amount o f chemical components, t h e a l t e r a t i o n may have progressed i n a n e a r l y open system and t h e r e a c t i n g s o l u t i o n s may have had a v e r y h i g h hydrogen-ion a c t i v i t y .

The Japanese I s l a n d s

a r e crowded w i t h a c t i v e and e x t i n c t geothermal areas forming s i m i l a r s i l i c a o r e deposits by t h e emission o f h o t - s p r i n g waters, f u m a r o l i c o r s u l f a t i c gasses. The worked mines, however, a r e few.

The Beppu mine and Satsuma I o j i m a mine i n

Kyushu produce o p a l i n e s i l i c a rock f o r cement i n d u s t r y . ACKNOWLEDGMENTS T h i s research i s i n p a r t f i n a n c i a l l y supported by t h e Grant-in-Aid f o r S c i e n t i f i c Research (Nos. 254254, 534035) from t h e M i n i s t r y o f Education o f Japan.

We a r e g r a t e f u l t o t h e Japanese Working Group o f t h e I.G. C. P.

P r o j e c t 115 f o r t h e i r cooperation. critically.

Professor Raymond Siever read t h e m a n u s c r i p t

Miss Ayako Kamgata made t h e t y p e s c r i p t .

REFERENCES Aoyagi, K. and Kazama, T., 1980. Transformational changes o f c l a y minerals, z e o l i t e s and s i l i c a m i n e r a l s d u r i n g diagenesis. Sedimentology, 27: 179-188. Bramlette, M.N., 1946. The Monterey Formation o f C a l i f o r n i a and t h e o r i g i n o f i t s s i l i c e o u s rocks. U.S. Geol. Surv. P r o f . Paper, 212: 1-57. Davis, E.F., 1918. The r a d i o l a r i a n c h e r t s o f t h e Franciscan group. B u l l . Dept. Geol. Univ. C a l i f . Publs., 11: 235-432. Fukusawa, H., 1982. S i l i c e o u s t u r b i d i t e i n t h e Wakkanai Formation a t

63

Toi kanbetsu, Tenpoku, northern Hokkaido. ( i n preparation) Furutani, H. and Nishiyama, H., 1980. Paleozoic radiolarian f o s s i l s of F u k u j i , G i f u Prefecture. Abstract Paper 87th Ann. Meet. Geol. SOC. Japan, p . 135. Hein, J.R., 1980. Bibliography o f fine-grained siliceous deposits. U.S. Geol. Surv. Open-file Report 80-391, 122 pp. Honda, S., 1978. Composition of the so-called hard shale of the Onnagawa formation of Miocene age. Geol. SOC. Japan Memoir, 15: 103-118. Hosoyamada, K., Tada, R. and Iijima, A., 1981. Alteration of siliceous shale and volcaniclastic sediments i n the Neogene section o f the Oga Peninsula, Akita. Abstract Paper 8 8 t h Ann. Meet. Geol. SOC. Japan, p. 226. Igo, .H., Adachi, S. , Furutani , H. and Nishiyama, H. , 1980. Ordovician f o s s i l s f i r s t discovered i n Japan. Proc. Japan Acad., 56: 499-503. -and Koike, T., 1982. Conodont biostratigraphy of cherts in the Japanese Islands. In: A. Iijima e t a l . (Editors), Siliceous Deposits in the Pacific Region. Elsevier, Amsterdam ( i n preparation). Iijima, A., 1980. Geology of natural z e o l i t e s and z e o l i t i c rocks. Pure and Appl . Chem. , 52: 21 15-21 30. -and Iwao, S., 1970. Geology of the Ugusu d i s t r i c t , western Izu. J . Geol. SOC. Japan, 76: 591-604. , Kakuwa, Y., Yamazaki, K. and Yanagimoto, Y . , 1978. Shallow-sea, organic origin of the Triassic bedded chert i n central Japan. J . Fac. Sci. Univ. Tokyo, Sec. 11, 19: 369-400. , Inagaki, H. and Kakuwa, Y . , 1979. Nature and origin of the Paleogene cherts i n the Setogawa Terrane, Shizuoka, central Japan. J . Fac. Sci. Univ. Tokyo, Sec. 11, 20: 1-30. -, Matsumoto, R. and Tada, R . , 1980. Zeolitic and s i l i c a diagenesis and sandstone petrography a t s i t e s 438 and 439, DSDP/IPOD Leg 57 off Sanriku, northwest Pacific. In: M. Lee and L.N. Stout (Editors), I n i t i a l Reports o f the Deep Sea Drilling Project. Washington D.C., 56/57: 1143-1158. -, Matsumoto, R. and Watanabe, Y . , 1981. Geology and siliceous deposits i n Tertiary Setogawa Terrane of Shizuoka, central Honshu. J . Fac. Sci. Univ. Tokyo, Sec. 11, 20: 241-276. -and Tada, R . , 1981. S i l i c a diagenesis o f Neogene diatomaceous and volcaniclastic sediments i n northern Japan. Sedimentology, 28: 185-200. Imoto, N. and Saito, Y . , 1973. Scanning electron microscopy o f chert. B u l l . Natn. Sci. Mus. Tokyo, 16: 397-400. Iwao, S., 1961. Clay petrography o f some s l a t y intercalations i n layeredcherts - a preliminary note. Clay Sci., 1 : 1-7. -, 1962. Geology of the s i l i c a deposits i n the Tamba d i s t r i c t , Japan. Mining Geol., 12: 334-345. -, 1963. Further consideration on the rock a l t e r a t i o n i n Ugusu, an extinct geothermal area. Japan J . Geol. Geogr. , 34: 81-91. -, 1976. A tentative estimation of the r a t e of growth of the "akashiro" silicastone deposits i n Japan. Mining Geol., 26: 1-12. , Ansai, T. and Okano, T., 1951. Brick silicastone deposits i n Tanba d i s t r i c t , Kyoto and Hyogo Prefectures. B u l l . Geol. Surv. Japan, 6: 133-157. Kakuwa, Y . , 1982. Preservation of radiolarians i n bedded cherts. ( i n preparation) Kanmera, K., 1968. On some sedimentary rocks associated w i t h geosynclinal volcanic rocks. Geol. SOC. Japan Memoir, 1 : 23-32. -, 1976. Comparison between past and present geosynclinal sedimentary bodies. Kagaku (Science) , Iwanamishoten, Tokyo, 46: 284-291 , 371-378. Kano, K., 1979. Deposition and diagenesis of siliceous sediments of the Onnagawa Formation. Sci. Rep. Tohoku Univ., Ser. 3, 14: 135-189. Kano, K., 1979. Giant deckenpaket and olistostrome in the Eastern Mino d i s t r i c t , central Japan. J . Fac. Sci. Univ. Tokyo, Sec. 11, 20: 31-59. Katayama, N., Takano, Y. and Sato, Y., 1955. Kaize s i l i c a sand deposit, Nagano Prefecture. Mining Geol., 5: 64. Kimura, T . , 1974. The ancient continental margins of Japan. In: C.A. Burk and C.L. Drake (Editors), The geology of continental margins, pp. 817-829.

64

-,

1977/80. The Japanese Islands. Kokin-shoin, Tokyo, Vol. 1 , 243 pp., Vol. 2, 916 pp. -and Tokuyama, A., 1971. Geosynclinal prisms and tectonics i n Japan. Geol. SOC. Japan Memoir, 6: 9-20. , Yoshida, S. and Toyohara, F . , 1975. Paleogeography and earth movements of Japan i n the Late Permian t o Early J u r a s s i c Sambosan stage. J . Fac. Sci. Univ. Tokyo, Sec. 11, 19: 149-177. Koizumi , I. , 1982. Diatoms and sedimentary environments of Neogene s i l i c e o u s deposits. In: A. Iijima e t a l . (Editors), Siliceous Deposits i n the Pacific Region. Elsevier, Amsterdam ( i n preparation). Hatsumoto, R. and Iijima, A., 1982. Chemical sedimentology of some bedded cherts in Japan. Ibid. ( i n preparation). Mitsui , K. and Taguchi , K. , 1977. S i l i c a mineral diagenesis i n Neogene Tertiary shales i n the Tempoku d i s t r i c t , Hokkaido, Japan. J . Sedim. Petrol. , 47: 158-167. Murata, A., 1981. A large overthrust and the paleogeography of the Kurosegawa and Sambosan Terranes. J . Geol. SOC. Japan, 87: 353-367. Saito, Y. and Imoto, N., 1978. Chertification of s i l i c e o u s sponge spicule deposit. Geol. SOC. Japan Memoir, 15: 91-102. Sano, H. , Kanmera, K. and Sakai, T. , 1979. Sediments associated w i t h greenstones of the Shimanto terrane. J. Geol. SOC. Japan, 85: 435-444. Shimizu, H. and Masuda, A . , 1977. Cerium i n c h e r t s a s an indication of marine environment of i t s formation. Nature, 266: 346-348. Shoji, R., 1967. Occurrence and petrographical studies of Paleozoic c h e r t of the western Ashio Mountains, Japan. Jubilee P u b l . Comnem. Prof. Sasa, 60th Birthday, Sapporo, pp. 171-183. Suyari, K., Kuwano, Y. and Ishida, K., 1980. Discovery of the Late T r i a s s i c conodonts from the Sambagawa Metamorphic Belt proper i n western Shikoku. J. Geol. SOC. Japan, 86: 827-828. S u z u k i , T. and Hada, H., 1979. Cretaceous tectonic m6lange of the Shimanto b e l t i n Shikoku, Japan. J . Geol. SOC. Japan, 85: 467-479. Tada, R., 1981. Diagenesis of Neogene diatomaceous sediments i n northern Japan. Doctoral Dissertation, Geological I n s t i t u t e , University of Tokyo (MS). -and I i jima, A. , 1982. X-ray powder d i f f r a c t i o n of mixtures of % 4 1 s i l i c a minerals - implication t o s i l i c a diagenesis. In: A. Iijima e t a l . (Editors), Siliceous Deposits i n the Pacific Region. Elsevier, Amsterdam ( i n preparation). Toyohara, F., 1977. Early Mesozoic tectonic development of the northwestern Chichibu geosyncline i n west Chugoku, Japan. J . Fac. Sci. Univ. Tokyo, Sec. 11, 19: 253-334. Ueno, M., 1956. On the Nankeiseki (Ganister) deposits i n the area including the Atetsu and Taishaku d i s t r i c t , Okayama and Hiroshima Prefecture. Bull. Geol. Surv. Japan, 7: 111. Yamazaki, K., 1979. Petrological and geochemical study of bedded c h e r t along the Nippon Rhein, Kiso River, central Japan. Master t h e s i s a t theGeologica1 I n s t i t u t e , University of Tokyo (MS). Yanagimoto, H. , 1973. Stratigraphy and geological s t r u c t u r e of the Paleozoic and Mesozoic formations i n the v i c i n i t y of KUZUU, Tochigi Prefecture. J . Geol. SOC. Japan, 79: 441-451. Yanai, S. , 1981. The s t r a t i g r a p h i c a l and paleogeographical s i t u a t i o n s of the Upper Cretaceous Uwajima Group of the shelf-facies within the Shimanto SuperGroup, western Shikoku, Japan. J . Geol. SOC. Japan, 79: 441-451. Yao, A. , 1981. Mesozoic and Paleozoic radiolarian c h e r t s , their d i s t r i b u t i o n i n space and time and t h e i r depositional environments. Abstract Paper 88th Ann. Meet. Geol. SOC. Japan, pp. 55-56. , 1982. Late Paleozoic and Mesozoic radiolarians from Southwest Japan. In: A. Iijima e t a l . (Editors), Siliceous Deposits i n the Pacific Region. Elsevier, Amsterdam ( i n preparation). , Masuda, T. and Isozaki , T. , 1980. T r i a s s i c and Jurassic radiolarians from t h e Inuyama area. J . Geosci. Osaka City Univ., 23: 135-154.

66

CHAPTER 5

CONODONT BIOSTRATIGRAPHY OF CHERTS I N THE JAPANESE ISLANDS Hisayoshi I G O

1

and Toshio KOIKE

2

' I n s t i t u t e o f Geoscience, The U n i v e r s i t y o f Tsukuba, I b a r a k i (Japan) 2Geological I n s t i t u t e , Yokohma N a t i o n a l U n i v e r s i t y , Yokohama (Japan)

ABSTRACT Previously, most c h e r t d i s t r i b u t e d i n t h e Japanese I s l a n d s was regarded as Permo-Carboniferous t h a t i s belonging t o t h e Chichibu System. We s t u d i e d conodonts t o e s t a b l i s h t h e geologic age o f these c h e r t s . Our conodont study proved t h a t T r i a s s i c c h e r t s a r e e x t e n s i v e l y d i s t r i b u t e d throughout t h e Japanese I s l a n d s and bought t o l i g h t many i n t e r e s t i n g problems concerning s t r a t i g r a p h y , sedimentary environment, geologic s t r u c t u r e , and geologic e v o l u t i o n . We summarized t h e s t r a t i g r a p h i c and geographic d i s t r i b u t i o n o f c h e r t s t h a t were dated by conodont b i o s t r a t i g r a p h y . INTRODUCTION Bedded c h e r t s a r e one o f t h e most common sedimentary rocks i n t h e Paleozoic t o Mesozoic geosynclinal deposits i n t h e Japanese Islands.

Geomorphically

these c h e r t s c o n s t i t u t e h i g h mountain peaks, canyons, water f a l l s , and o t h e r scenic landscapes i n Japan.

The progress i n b i o s t r a t i g r a p h y o f these c h e r t s

a c c e l e r a t e d g r e a t l y d u r i n g t h i s decade. Previous t o o u r study, most c h e r t s were thought t o be t y p i c a l Carboniferous and Permian eugeosyncl i n a l (Chichibu Geosyncl i n e ) deposits because t h e c h e r t s a r e associated c l o s e l y w i t h fusul inacean limestones and submarine v o l c a n i c and v o l c a n o c l a s t i c rocks. Ehara (1927) was t h e f i r s t who t r i e d t o s e t t l e t h e geologic age of c h e r t s by s t u d y i n g r a d i o l a r i a n fauna.

He described several species o f r a d i o l a r i a n s

c o l l e c t e d from t h e T r i a s s i c Kochigatani and Zohoin Series and some Paleozoic rocks.

Subsequently, Fujimoto (1933, and o t h e r s ) succeeded i n determining

t h e geologic age o f c h e r t s i n v a r i o u s r e g i o n s by study o f r a d i o l a r i a n s . Fujimoto used t h e s o - c a l l e d percentage method t o study r a d i o l a r i a n fauna and he i n s i s t e d t h a t most r a d i o l a r i a n c h e r t s a r e Upper Paleozoic and J u r a s s i c i n age.

Kobayashi and Kimura (1944) c r i t i c i z e d F u j i m o t o ' s view and p o i n t e d

o u t i n a c c u r a c i e s o f t h e percentage method.

T h i s d i s c u s s i o n was one of t h e

most e x c i t i n g i n t h e Geological S o c i e t y o f Japan b e f o r e t h e War time.

66 Since 1963 we have s t u d i e d conodonts i n Japan.

As a r e s u l t , we found

t h a t most o f t h e s o - c a l l e d Upper Paleozoic c h e r t s are a p p a r e n t l y T r i a s s i c . T r i a s s i c conodonts a r e a l s o e x t r a c t e d from some c h e r t s b e l i e v e d t o be J u r a s s i c . Our study, and r e c e n t l y several o t h e r a u t h o r s ' c o n t r i b u t i o n s t o conodont b i o s t r a t i g r a p h y , have provided many new data concerning geologic and sedimentary environment of cherts, and t h e geologic e v o l u t i o n o f t h e Japanese I s l a n d s d u r i n g Late Paleozoic t o E a r l y Mesozoic times. Here, we summarized t h e s t r a t i g r a p h i c and geographic d i s t r i b u t i o n o f c h e r t s based on conodont b i o s t r a t i g r a p h y . PRE-CARBONIFEROUS S i l u r i a n , Devonian, and newly found Ordovician rocks f r e q u e n t l y i n c l u d e i n t e r c a l a t e d s i l i c e o u s rocks, however, most o f them a r e s i l i c e o u s a c i d i c t u f f s o r nodular c h e r t s i n limestone.

Conodonts a r e r a r e i n these sedimentary rocks.

Several fragments o f Lower Devonian conodonts were found from s i l i c e o u s t u f f s o f t h e Ono Formation exposed i n southern Kitakami Massif, n o r t h e a s t e r n Honshu. CARBON IFEROUS B i o s t r a t i g r a p h y o f Carboniferous conodonts was e s t a b l i s h e d f o r t h e lower p a r t s o f t h e Akiyoshi, Atetsu, Omi, and o t h e r limestones which a r e l a t e E a r l y t o Middle Carboniferous i n age (Igo, Hy. and Koike, 1975). The f o l l o w i n g assemblage zones were established.

I

Idiognathodus delicatus-Diplognathodus atetsuensis

Moscovian Atokan

zone

LNeogondolella clarki-Idiognathoides corrugatus Zone

Bashkirian rrdiognathodus sinuosus-Streptognathodus expansus Zone

Namurian

Morrowan

i

Neognathodus bassleri symmetricus zone Idiognathoides noduliferus-Paraganthodus nagatoensis

Visean

Chesterian

zone

c

Gnathodus bilineatus-Paragnathodus nodosus Zone Gnathodus bilineatus-Gnathodus a f f . texanus Zone

Compared w i t h t h e Permian and T r i a s s i c , l i t t l e i s known about t h e Carboni f e r o u s conodont-bearing c h e r t s i n Japan.

Among Carboniferous s e c t i o n s i s

t h e Mamba Formation d i s t r i b u t e d i n t h e Kwanto Massif which c o n t a i n s age n o s t i c conodonts i n c h e r t s (Igo, Hy., 1972; Takizawa, 1979).

diag-

These c h e r t s

y i e l d Idiognathodus sinuosus and o t h e r Lower Pennsylvanian conodonts. Bedded c h e r t s of t h e Mamba exposed a t Okunoiri, Kamiyoshida c o n t a i n r e d d i s h dolomite l a y e r s and o v e r l i e a l e n t i c u l a r limestone mass. s t e e p l y j n c l i n e d c h e r t and d o l o m i t e a l t e r n a t e .

On t h e f l a n k o f t h i s l e n s Lower Pennsylvanian conodonts

a r e abundant i n b o t h l e n t i c u l a r limestone and bedded c h e r t .

Lower Pennsylvanian

conodont-bearing c h e r t i s a l s o exposed i n t h e I t u k a i c h i d i s t r i c t , west o f

67

Tokyo, i n t h e southern Kwanto Massif ( I g o , Hy. and Kobayashi, 1974). The Daigo Group d i s t r i b u t e d i n Naka County, Tokushima P r e f e c t u r e c o n t a i n s conodont-bearing c h e r t .

I s h i d a (1977) r e p o r t e d t h e occurrence o f Idiognath-

oides sulcatus sulcatus from t h e lower p a r t and Neogondolella clarki and

o t h e r s from t h e upper p a r t o f t h i s group.

The former species was a l s o r e p o r t e d

from t h e Profusulinella zone o f t h e Omi Limestone, N i i g a t a P r e f e c t u r e (Igo, Hy. and Koike, 1964).

The l a t t e r species ranges from t h e Profusulinella t o

Fusulinella zones i n Japan (Koike,

1967) and European Russia.

Sporadic occurrence o f Carboniferous conodonts are a l s o r e p o r t e d f o r c h e r t s d i s t r i b u t e d i n v a r i o u s r e g i o n s o f t h e Chichibu T e r r a i n o f t h e Outer Zone of Southwestern Japan.

However, d e t a i l e d i n v e s t i g a t i o n o f these conodonts has

n o t been completed.

PERM IAN Recently, Hh. I g o (1979a) attempted t h e zonation o f Permian conodonts He e s t a b l i s h e d t h e f o l l o w i n g conodont assemblage zones and faunas.

i n Japan. Upper Middle

Lower

Anchignathodus typicalis-Diplognathodus Sp. fauna

c c

Diplognathodus lanceolatus-Diplognathodus nodosus Zone Diplognathodus oertlii-Neogondolella pequopensis Zone Neogondolella bisselli-Sweetognathus whitei Zone Streptognathodus elongatus fauna

These zones and faunal d i v i s i o n s were found i n limestones t h a t were dated by fusulinaceans.

Supplemental data were obtained from c h e r t s .

Occurrence o f t h e Lowest Permian conodont faunas a r e o n l y known from l i m e stone.

The n e x t h i g h e r Lower Permian conodont zone, Neogondolella bisselli-

Sweetognathus whitei zone i s recognized i n green bedded c h e r t cropping Out

a t Minokuchi and Tamanouchi, I t s u k a i c h i Town, west o f Tokyo. These c h e r t s a r e interbedded w i t h tuffaceous limestone, d o l o m i t i c limestone, and t u f f . Pale green, gray and p a r t l y r e d bedded c h e r t exposed a t Mitsuzawa, I t s u k a i c h i a l s o y i e l d s Neogondolella bisselli and Sweetognathus whitei fauna and Overl i e s t h e Lower Carboniferous rocks.

A t t h e base o f t h i s c h e r t l i e s d o l o m i t i c

limestone b r e c c i a formed by submarine s l i d i n g (Igo, Hh., cation).

personal communi-

Cherts o f Permian age seem t o be widespread n o t o n l y i n t h e Kwanto

Massif b u t a l s o i n o t h e r regions. The Tamba Research Group (1979) and I s h i g a and Imoto (1980) r e p o r t e d t h e occurrence o f Anchignathodus and fragments o f o t h e r conodonts from t h e Tamba Group, west o f Kyoto.

These c h e r t s are o v e r l a i n by d o l o m i t i c sandstone

c o n t a i n i n g Sweetognathus whitei and o t h e r s were a l s o o b t a i n e d from c h e r t i n the Sasayama area, Hyogo P r e f e c t u r e . Bedded c h e r t i s most prominent i n t h e uppermost Lower t o lower p a r t of

68 t h e Middle Permian i n t h e Chichibu eugeosynclinal d e p o s i t s o f Japan. Limestone c o n t a i n i n g Pseudofusulina vulqaris and several o t h e r c h a r a c t e r i s t i c species o f Pseudofusulina and Parafusulina a r e exposed i n various regions. These limestones a r e interbedded w i t h v o l c a n i c and v o l c a n o c l a s t i c rocks and chert. rence

Conodonts from these c h e r t s a r e few except f o r t h e sporadic occurO f

Neostreptoqnathodus pequopensis, Diploqnathodus nodosus, and o t h e r s

i n t h e Kwanto and Mino-Hida Massifs.

Red bedded c h e r t o f t h e Shimadani

Formation i n Hachiman, Mino Massif c o n t a i n s abundant conodonts. Neogondolella idahoensis, Diploqnathodus oertlii, Anchignathodus minutus, and o t h e r s are

y i e l d e d from t h e lower p a r t o f t h i s formation.

The Shimadani Formation

over1 i e s t h e Kuchibora Formation which y i e l d s c h a r a c t e r i s t i c f u s u l inaceans, such as, Maklaya pamirica and Cancellina nipponica (Igo, Hh.,

1979a).

Chert beds o f t h e Wakasugi Group i n Tokushima Prefecture, Shikoku y i e l d a Middle Permian conodont assemblage c o n s i s t i n g o f Neogondolella serrata serrata, Neoqondolella idahoensis, and o t h e r s ( I s h i d a , 1977).

Although t h e study o f conodont i s n o t complete, t h e s i m i l a r s t r a t i g r a p h i c l e v e l s o f conodont-bearing c h e r t s a r e known from H i r o s a k i , Asanai, and Orikabe i n n o r t h e r n Kitakami and Iwaizumi B e l t s , northwestern Honshu (Toyohara e t al.

, 1980).

Upper Permian conodonts a r e p o o r l y known even i n calcareous f a c i e s . Recently, Hy. I g o and h i s c o l l a b o r a t o r s (1980) found Uppermost Permian f o r a m i n i f e r a 1 limestone l e n s a t Menashidomari, n o r t h e a s t Hokkaido. limestone l e n s i s embedded i n v o l c a n i c rock and c h e r t .

The

The c h e r t beds y i e l d

r a d i o l a r i a n s and fragmental Anchignathodus minutus and o t h e r Upper Permian conodonts. TRIASSIC Previous t o our conodont study, pelecypods and ammonites were t h e most r e l i a b l e s t r a t i g r a p h i c i n d i c a t o r s o f T r i a s s i c rocks o f Japan although t h e r d i s t r i b u t i o n was t h e most r e s t r i c t e d compared t o o t h e r geologic periods. However, we now know t h a t T r i a s s i c rocks a r e more e x t e n s i v e than rocks o f t h e Permian and Carboniferous periods.

T r i a s s i c c h e r t s associated w i t h

v o l c a n i c and v o l c a n o c l a s t i c rocks, c l a s t i c sedimentary rocks, and some limestones make up considerable p a r t s o f t h e Chichibu and Sambosan geosynclinal t e r r a i n s i n t h e Japanese I s l a n d s .

A d e t a i l e d T r i a s s i c conodont zonation was e s t a b l i s h e d m a i n l y from small limestone masses i n southwestern Japan, such as Tao Formation i n Ehime P r e f e c t u r e and Kamura Formation i n Miyazaki P r e f e c t u r e (Koike, 1979b, Watanabe e t a l . ,

1979).

S i m i l a r s t r a t i g r a p h i c occurrence o f conodonts i n

c h e r t s a r e recorded i n v a r i o u s regions.

The f o l l o w i n g i s an up-to-date

T r i a s s i c conodont zonation f o r s e c t i o n s i n Japan.

69

Rhaet ian

Misikella posthernsteini Zone Misikella hernsteini ZOne Epigondolella bidentata Zone

Norian

Epiqondolella mu1 tidentata Zone

c

Carnian

zone

Epigondolella spatulata Epigondolella nodosa zone

Neogondolella plygnathiformis Zone

Ladinian-Carnian

Neogondolella foliata

Ladinian

Carinella mungoensis Zone

Anisian-Ladinian

Neogondolella excelsa-Neogondolella constricta zone

Anisian

Neogondolella bulgarica zone

Spathian-Anisian

Neospathodus timorensis zone

Spathian

{

Smi t h i a n

zone

Neospathodus homeri Zone Neospathodus triangularis-Neospathodus ? collinsoni Zone Neospathodus conservativus-Neospathodus dieneri Zone

T r i a s s i c conodont-bearing c h e r t i n t h e I n n e r Zone T r i a s s i c c h e r t s occur e x t e n s i v e l y i n t h e Chichibu Miogeosynclinal T e r r a i n

o f t h e I n n e r Zone o f Japan i n c l u d i n g t h e Okukinu-Okutadami, Toriashi-Torinoko,

Ashio, Yamizo-

Kiso-Hida, Mino-Neo, Tamba, and western Chugoku regions.

The basements of these mountainous r e g i o n s c o n s i s t o f monotonous, t h i c k s t r a t a , and they have l o n g been c a l l e d t h e "Yamaguchi Facies" o f t h e Upper Paleozoic Chichibu System (Kobayashi, 1941).

Kimura e t a l . (1975) grouped these s e d i -

mentary rocks as t h e Chichibu Miogeosyncl i n a l sedimentary rocks.

This mio-

geosynclinal area was an eugeosyncline u n t i l t h e Middle Permian, except f o r southern Kitakami where t h e eugeosynclinal stage ended d u r i n g Middle Carboniferous time (Kimura e t a l . ,

1975).

Recently, J u r a s s i c r a d i o l a r i a n - b e a r i n g

shale associated w i t h T r i a s s i c c h e r t has been found i n t h e above mentioned regions (Yao, 1972, 1970; Yao e t a l . ,

1980; M i z u t a n i e t a l . ,

1981).

As a

r e s u l t , some T r i a s s i c c h e r t formations seem t o be l a r g e e x o t i c sheets o r masses i n t h e J u r a s s i c s t r a t a (Aono e t a l . ,

1981).

Cherts d i s t r i b u t e d i n t h e above mentioned r e g i o n s a r e gray, green, and subordinate r e d bedded c h e r t s and a r e tens o f meters t o more than several hundreds o f meters t h i c k . common.

I n t r a f o r m a t i o n a l t e c t o n i c and slump f o l d s a r e

Small o r e bodies o f manganese a r e common, b u t most have been complete-

l y mined.

The lowest p a r t o f t h e T r i a s s i c sequences o f t h e I n n e r Zone represented by p a l e green t o b l a c k s i l i c e o u s shale and t h i n c h e r t l a y e r s .

The s i l i c e o u s

shale i s e x t e n s i v e and y i e l d s a Spathian t o A n i s i a n conodont assemblage. The famous whetstone, N a r u t a k i i s h i , exposed i n t h e h i l l s west o f Kyoto i s one o f t h e t y p i c a l examples.

70

The n e x t h i g h e r zones, t h e L a d i n i a n Carinella mungoensis and Neogondolella foliata zones a r e m i s s i n g o r p o o r l y represented i n t h e n o r t h Kwanto and

southernmost Tohoku regions.

However, these zones a r e w e l l d e f i n e d i n t h e

Kiso and Mino Massifs, c e n t r a l Japan. The Carnian t o Norian conodont zones are extensive throughout t h e I n n e r Zone. The Neogondolella polyqnathiformis zone o f t h e Carnian and t h e Epiqondolella spatulata and

Epigondolella bidentata zones o f t h e Norian a r e almost

completely represented i n these regions, b u t t h e Epiqondolella multidentata zone seems t o be m i s s i n g i n t h e c h e r t f a c i e s .

The uppermost Norian conodont

zone, t h e Misikella hernsteini zone and t h e Rhaetian Misikella posthernsteini zone a r e recognized i n places, b u t may be more w i d e l y d i s t r i b u t e d than suggested from known occurrences.

The Norian t o Rhaetian conodont-bearing

c h e r t s i n t h i s province are m o s t l y o v e r l a i n by t h i c k s e r i e s o f c l a s t i c sedimentary rocks o r r a d i o l a r i a n c h e r t s .

As a l r e a d y p o i n t e d out, J u r a s s i c

r a d i o l a r i a n s were found i n t h e shale and c h e r t i n t h e Ashio, Kiso, Mino, and Hida Massifs. Recently,

I s o z a k i and Matsuda (1980) found almost complete Upper T r i a s s i c

conodont zones i n t h e Tamba Group, western h i l l s o f Kyoto. 30 meter t h i c k c h e r t beds y i e l d successively

Approximately

Neogondolella polygnathiformis,

Epigondolella abneptis, Epigondolella postera, Epigondolella bidentata, Misikella hernsteini, and Misikella posthernsteini. Misikella posthernsteini

w i t h o u t t h e a s s o c i a t i o n o f Misikella hernsteini i n d i c a t e s a Rhaetian age i n Europe.

T h i s i s t h e f i r s t c o n f i r m a t i o n o f t h e Rhaetian conodont zone as w e l l

as t h e e x i s t e n c e o f t h e marine Rhaetian i n Japan. The Adoyama Formation exposed i n t h e Kuzu area, Ashio Massif contains abundant N o r i a n conodonts (Conodont Research Group, 1974; Koike e t a l . , 1974).

The Adoyama Formation c o n s i s t s mainly o f bedded c h e r t and green o r

r e d shale and limestone conglomerate occupy i n i t s basal p a r t .

The l i m e -

stone conglomerate c o n t a i n s pebbles o f b o t h Permian and T r i a s s i c limestone, c h e r t , and o t h e r sedimentary rocks.

T r i a s s i c conodonts r a n g i n g from t h e

Spathian t o Norian were recovered from limestone pebbles and limesand cement. The conglomerate over1 i e s t h e c h e m i c a l l y weathered s u r f a c e o f t h e Permian limestone.

Green o r r e d shale o v e r l i e s t h i s conglomerate w i t h o u t any break.

Recently, Hy. I g o and Hh. I g o found J u r a s s i c r a d i o l a r i a n s from t h e green and r e d shales.

Therefore, we must change our previous conclusion (Koike e t a l . ,

1974) concerning geologic age o f t h i s limestone conglomerate and shale. The unconformity between t h e Permian Nabeyama and T r i a s s i c Adoyama c h e r t Formation was n o t T r i a s s i c b u t r a t h e r J u r a s s i c .

The r e l a t i o n between limestone conglo-

merate and shale sequences and t h e T r i a s s i c Adoyama c h e r t i s t e c t o n i c and t h e c h e r t was t h r u s t over t h e former sequences.

Aono (1980) s t u d i e d "Decken-

paket" s t r u c t u r e i n autochthonous o f f - s h o r e deeper f a c i e s and allochthonous

71

shallow water f a c i e s in t h i s d i s t r i c t . The Yabuhara Formation d i s t r i b u t e d i n t h e Kiso Massif c o n s i s t s of wellbedded c h e r t t h a t yielded numerous conodonts. Conodonts a r e r a r e i n the lower p a r t . Neogondolella bulgarica, Carinella mungoensis, Neogondolella foliata, Epigondolella spatulata, Epigondolella abneptis and o t h e r s a r e successively found from the upper p a r t s (Kazama, 1980). The t o t a l thickness of t h i s formation i s about 600 m. Based on the occurrence of conodonts, t h e Anisian c h e r t s , including t h i n a l t e r n a t i n g s h a l e beds, a r e t h i c k e r than those of o t h e r ones. The estimated r a t e of deposition of c h e r t s t o g e t h e r with i n t e r c a l a t e d t h i n shale i s a s follow; lower Anisian 185 mm/lO’y; upper Anisian t o lower Ladinian 12 mm/lO’y; upper Ladinian t o lower Carnian 7.5 m / 1 0 3 y ; upper Carnian t o lower Norian 3 . 5 mm/103y; middle Norian 3.0 mm/103y. The r a t e of deposition of the Carnian and Norian s i l i c e o u s sequence i s slow in c o n t r a s t t o the r a t e determined by Iijima e t a l . (1978). The mentioned T r i a s s i c c h e r t contains well-preserved r a d i o l a r i a n s . Interbedded t h i n s i l i ceous shale contains occasionally abundant r a d i o l a r i a n s and i s r a d i o l a r i t e . Chert i s extensive in the Mino Massif and i s s t r u c t u r a l l y complicated (Mizutani, 1964; Yoshida, 1972; Igo, Hh., 1979b; Kano, 1979). Hh. Igo found almost complete Upper Spathian t o Norian conodont zones i n t h i s d i s t r i c t . The c h e r t s e c t i o n s a r e s t r a t i g r a p h i c a l l y repeated a s t h e r e s u l t of g r a v i t y sliding. Similar s t r u c t u r e s were a l s o reported from western Chugoku Massif by Toyohara (1977). Spathian t o Norian conodont-bearing c h e r t i s reported from the Nichihara a r e a , Shimane Prefecture. These c h e r t s a r e a l s o repeated s t r a t i g r a p h i c a l l y as t h e r e s u l t of g r a v i t y s l i d i n g . The T r i a s s i c conodont zones, ranging from the Upper Spathian t o Norian a r e almost continuously represented i n c h e r t t h a t i s about 40 m t h i c k (Tanaka 1980). T r i a s s i c conodont-bearing c h e r t i n the Outer Zone T r i a s s i c c h e r t a l s o occurs extensively i n t h e Chichibu and Sambosan Terrains of the Outer Zone of Japan. These c h e r t s were a l s o regarded a s Upper Paleozoic except the Togano and Sambosan Groups i n Shikoku. In the Kwanto Massif and t h e medial zone of Hokkaido, t h e Upper J u r a s s i c Torinosu Limestone i s c l o s e l y associated w i t h c h e r t . Therefore, t h e c h e r t i s considered t o be J u r a s s i c . Chert exposed i n the Outer Zone i s associated with eugeosync1 inal sedimentary rocks. The c h e r t i s s t r u c t u r a l l y complicated. Conodont age d a t e s have shown the existence of T r i a s s i c c h e r t s . The Spathian t o Anisian conodont zones a r e recognized in s i l i c e o u s shale and c h e r t t h a t a r e comon i n the Inner Zone. This c h a r a c t e r i s t i c l i t h o f a c i e s i s known from southern Hokkaido t o Amamioshima o f f of southern Kyushu, a d i s t a n c e o f about 1800 km. The Anisian conodont zones a r e widespread, b u t

72

the Ladinian chert represented by the C a r i n e l l a mungoensis zone is r e s t r i c t e d . The Carnian Neogondolella polygnathiformis zone i s well represented i n cherts distributed elsewhere i n the Outer Zone. The lower Norian Epigondolella s p a t u l a t a , Epigondolella bidentata and the uppermost Norian Misikella herns t e i n i zones are recognized extensively i n cherts of the Outer Zone. The Sorachi and Hidaka Groups exposed in the medial zone of Hokkaido cons t i t u t e the basement of the Cretaceous Ezo-Sakhalin Geosyncline. The former group i s characterized by thick volcanic and volcanoclastic rocks and subordinate chert and limestone. The Hidaka Group consists of a thick c l a s t i c sequence with t h i n intercalations of chert and limestone. Although the Sorachi and Hidaka Groups were almost barren of age diagnostic f o s s i l s , the geologic age of these units was considered as Jurassic and Late Paleozoic. Hy. Igo e t a l . (1974, 1978), Hashimoto e t a l . (1975) and some others reported the occurrence of Triassic conodonts and Upper Permian microfossils from limestone and chert of both groups. Most chert of these groups yields Carnian and Norian conodonts, b u t the middle Carnian Neogondolella polygnathiformis zone seems t o be most extensive. Upper Paleozoic t o Triassic s t r a t a dated by conodonts occur i n northern Kitakami and the southwestern part of Hokkaido (Sakagami e t a l . , 1969; and others). Recently, Toyohara e t a l . (1980) summarized the preliminarly r e s u l t s of the Upper Paleozoic t o Triassic stratigraphy f o r t h i s region. The Spathian t o Norian conodont zones are found b u t several other zones are missing. This comprehensive study together with the research of the medial zone of Hokkaido made clear the geotectonic relation o f the basement rocks o f the islands of Honshu and Hokkaido. The Kamiyoshida Formation distributed in Chichibu, Kwanto Massif was the type section of the uppermost part of the Chichibu System. This formation consists of chert and terrigenous c l a s t i c rocks. Chert yields rich Triassic conodont faunas ranging from the Spathian t o Norian, b u t the Ladinian fauna i s missing as the r e s u l t of an unconformity. The Norian Epigondolella spatuiata-bearing cherts are extensive and thick in the Kamiyoshida Formation (Takizawa, 1979). According t o recent preliminary investigation of Hy. Igo and Sashida, shale intercalated w i t h the Triassic chert i n the Kaiyoshida yields Jurassic radiolarians. Therefore, stratigraphy of t h i s formation established by Takizawa will be revised i n the near future. Chert associated with terrigenous c l a s t i c rocks and volcanic and volcanoc l a s t i c rocks, slump beds, and limestone i s distributed widely i n the southern part of the Kwanto Massif. These sedimentary rocks are characteristic of the Chichibu and Sambosan Eugeosyncl ines. Recently, the authors have found many Triassic conodonts from chert, shale, and 1 imestone. These conodonts indicate Spathian t o Norian age. Hisada (1979, 1981) mapped t h i s mountainous

73

r e g i o n and produced much new data concerning t h e s t r a t i g r a p h i c p o s i t i o n o f chert.

He a l s o found J u r a s s i c r a d i o l a r i a n s from shale interbedded w i t h t h e

T r i a s s i c c h e r t and concluded t h a t most T r i a s s i c c h e r t i s e x o t i c sheet o r mass formed by submarine g r a v i t y s l i d i n g d u r i n g J u r a s s i c o r l a t e r p e r i o d . Chert i s e x t e n s i v e i n t h e Outer Zone o f Southwest Japan, i. e. K i i , southe r n Shikoku, and Kyushu.

T r i a s s i c conodont zones a r e almost completely

represented i n t h e Kashiwagi d i s t r i c t , c e n t r a l p a r t o f t h e K i i Peninsula (Makino, 1976).

Maejima and Matsuda (1977) found abundant T r i a s s i c conodonts

i n c h e r t i n t h e Yuasa d i s t r i c t , Wakayama Prefecture. I s h i d a (1981) recognized t h e Spathian t o Norian conodont zones w i t h i n a 26 meter t h i c k c h e r t s e c t i o n i n t h e Nakagawa Group i n Tokushima Prefecture, Shikoku.

The zones a r e repeated by i n t r a f o r m a t i o n a l f o l d i n g and s l i p planes

caused by submarine s l i d i n g .

The Togano Basin o f Kochi Prefecture, Shikoku

i s a c l a s s i c area of T r i a s s i c s t r a t i g r a p h y i n Japan.

Recently, Koike and

Kishimoto (1979) s t u d i e d conodonts from c h e r t s o f t h e Sambosan Group and other units.

Conodont assemblages i n d i c a t i v e o f t h e Spathian t o N o r i a n

were found i n c h e r t o f t h e Shien Formation exposed i n t h i s basin.

The Togano

Formation i s a t y p i c a l sequence o f t h e Sambosan Geosyncline and c o n s i s t s o f c h e r t , sandstone, shale, and limestone.

Chert o f t h i s f o r m a t i o n y i e l d e d

conodonts i n d i c a t i n g Carnian and Norian ages.

Chert i s e x t e n s i v e i n t h e

Gokase-Shiiba d i s t r i c t , Miyazaki Prefecture, Kyushu. d i r e c t e x t e n s i o n o f t h e Sambosan B e l t o f Shikoku.

This d i s t r i c t i s the

Murata (1981) c o l l e c t e d

many conodont-bearing c h e r t s i d e n t i f i e d as Spathian t o Norian i n ages (Koike and Murata, 1979). The Konose Group i n Kumamoto P r e f e c t u r e i s a l s o t y p i c a l o f t h e Sambosan Geosyncline (Kanmera,l969,

and o t h e r s ) .

Carnian and Norian conodonts were

found i n c h e r t and limestone o f t h e Konose Group (Koike, 1979a). CONCLUSION Bedded c h e r t s t y p i f y t h e Japanese Upper Paleozoic t o Mesozoic geosyncline. Recent i n v e s t i g a t i o n o f conodonts c o n t r i b u t e d much i n d e c i p h e r i n g t h e s t r a t i g r a p h i c p o s i t i o n and geochronology o f t h e c h e r t s .

Our conodont study

proved t h a t T r i a s s i c c h e r t i s e x t e n s i v e l y d i s t r i b u t e d i n Japan.

The Spathian

t o Norian conodont zones a r e commonly recognized elsewhere, among which t h e Carnian and Norian conodont-bearing c h e r t s seem t o be more w i d e l y d i s t r i b u t e d than o t h e r T r i a s s i c d i v i s i o n s . REFERENCES CITED Aono, H., 1980. Geologic s t r u c t u r e and s t r a t i g r a p h y o f t h e Ashio Mountains. Msc. Thesis I n s t . Geosci. Univ. Tsukuba (MS). , e t a l . , 1981. G r a v i t y - s l i d i n g s observable i n t h e Mesozoic o f t h e Yamizo M o u n t a i n s i n n o r t h e a s t Japan. S c i . Rep. I n s t . Geosci. Univ. Tsukuba, Sec. B,

74

2: 17-44. Conodont Research Group, 1974. Conodonts a t t h e Permian-Triassic boundary i n Japan-Stratigraphy and faunas o f t h e Nabeyama and Adoyama formations i n Karasawa area, southeast Ashio Mountains-. E a r t h Sci., 28:86-98. Ehara, S., 1927. Faunal and s t r a t i g r a p h i c a l study o f t h e Sakawa Basin, Shikoku, Japan. J. Geol. Geogr., 5:l-40. Fujimoto, H., 1933. Geology o f t h e e a s t e r n p a r t o f t h e Kwanto mountainland. J. Geol. SOC. Tokyo, 4O:l-15. Hashimoto, W. e t a l . , 1975. F i r s t c o n f i r m a t i o n o f t h e Permian System i n t h e c e n t r a l p a r t o f Hokkaido. Proc. Japan Acad., 51:34-37. Hisada, K., 1979. S t r a t i g r a p h y and sedimentary petrography o f t h e Chichibu t e r r a i n i n t h e Urayama d i s t r i c t , Saitama Prefecture. Msc. Thesis, I n s t . Geosci. Univ. Tsukuba (MS). -, 1981. S t r a t i g r a p h y and geologic s t r u c t u r e o f t h e Chichibu and Shimanto t e r r a i n s of t h e Kanto mountains, Japan. Dsc. Thesis, I n s t . Geosci. Univ. Tsukuba (MS). Igo, Hh. , 1979a. B i o s t r a t i g r a p h y o f Permian conodonts. P r o f . Kanuma Mem. Vol , 5-20. -, 1979b. Conodont b i o s t r a t i g r a p h y and r e s t u d y o f g e o l o g i c s t r u c t u r e i n t h e eastern p a r t o f t h e Mino b e l t . I b i d . , 103-113. Igo, Hy., 1972. Conodonts, as a new index f o s s i l i n Japan. J. Geogr., 81: 143-151. -, and Kobayashi, F., 1974. Carboniferous conodonts from t h e I t s u k a i c h i d i s t r i c t , Tokyo, Japan. Trans. Proc. Pal. SOC. Japan, n.s. 96:411-426. -, and Koike, T., 1964. Carboniferous conodonts from t h e Omi Limestone, N i i g a t a P r e f e c t u r e , c e n t r a l Japan. I b i d . , 53:179-193. , and -, 1975. Carboniferous conodont zones i n Japan and Southeastern A s i a ( A b s t r a c t ) . V I I I I n t . Cong. Carb. S t r a t . Geol. A b s t r a c t : 108-109. -, e t a l . , 1974. On t h e occurrence o f T r i a s s i c conodonts from t h e Sorachi Group i n t h e Hidaka Mountains, Hokkaido. J. Geol. SOC. Japan, 80:135-136. -, e t a l . , 1980. On geologic age o f t h e Hidaka and Sorachi Groups. i n Kimura, T. ( E d i t o r ) , Restudy o f geosyncline and s t r u c t u r a l d i v i s i o n s i n t h e n o r t h e r n p a r t o f t h e Japanese Islands, 69-75. I i j i m a , A. e t a l . , 1978. Shallow-sea, o r g a n i c o r i g i n o f t h e T r i a s s i c bedded c h e r t i n c e n t r a l Japan. J. Fac. Sci. Univ. Tokyo, sec. 2 , 19:369-400. I s h i d a , K., 1977. Reexamination o f t h e Paleozoic and Mesozoic formations i n t h e southern zone o f t h e Chichibu b e l t i n e a s t e r n Shikoku by means o f conodonts and f u s u l i n i d s . J. Geol. SOC. Japan, 83:227-240. 1981. F i n e s t r a t i g r a p h y and conodont b i o s t r a t i g r a p h y o f a bedded-chert member o f t h e Nakagawa Group. J. Sci. Univ. Tokushima, 14:107-137. I s h i g a , H. and Imoto, N., 1980. Some Permian r a d i o l a r i a n s i n t h e Tamba d i s t r i c t , southwest Japan. E a r t h Sci., 34:333-345. I s o z a k i , Y. and Matsuda, T., 1980. Age o f t h e Tamba Group along t h e Hozugawa " A n t i c 1 i n e " , western h i 1 1s o f Kyoto, southwest Japan. J. Geosci. Osaka City Univ., 23:115-134. Kanmera, K., 1969. L i t h o - and b i o - f a c i e s o f Permo-Triassic geosynclinal l i m e stone of t h e Sambosan b e l t i n southern Kyushu. Pal. SOC. Japan, Spec. Paper, 14:13-39. Kano, K., 1979. Gigant Deckenpaket and o l i s t o s t r o m e i n t h e e a s t e r n Mino d i s t r i c t , c e n t r a l Japan. J. Fac. Sci., Univ. Tokyo, sec. 2, 20:31-59. Kazama, S., 1980. S t r a t i g r a p h y and conodont faunas i n Kisofukushima Town and i t s environs, Nagano Prefecture, c e n t r a l Japan. Graduation Thesis, I n s t . Geosci. Univ. Tsukuba (MS). Kimura, T. e t a l . , 1975. Paleoqeoqraphy and e a r t h movements o f Japan i n t h e l a t e r Permian t o e a r l y J u r a i s i c Sambosan stage. J. Fac. Sci. Univ. Tokyo, sec. 2 , 19:149-177. Kobayashi, T., 1941. The Sakawa orogenic c y c l e and i t s b e a r i n g on t h e o r i g i n of t h e Japanese I s l a n d s . J. Fac. S c i . Imp. Univ. Tokyo, sec. 2, 5:219-578. -, and Kimura, T., 1944. A study on t h e r a d i o l a r i a n rocks. I b i d . , 7:75-178. Koike, T., 1967. A Carboniferous succession o f conodont faunas from t h e A t e t s u

.

_.

75 limestone i n southwest Japan. S c i . Rep. Tokyo Kyoiku Daigaku, sec. C, 9: 279-31 8. , 1979a. B i o s t r a t i g r a p h y o f T r i a s s i c conodonts. P r o f . Kanuma Mem. vol., 21 -77. -, 1979b. Conodont b i o s t r a t i g r a p h y i n t h e Taho limestone ( T r i a s s i c ) , S h i r o kawa-cho, Higashiuwa-gun, Ehime P r e f e c t u r e . I b i d . , 115-126. -, and Kishimoto, M., 1979. S t r a t i g r a p h y and conodont faunas i n t h e Sambosan t e r r a i n a t t h e v i c i n i t y o f t h e Togano basin, Sakawa-cho, Kochi Prefecture. I b i d . , 139-153. -, and Murata, A., 1979. T r i a s s i c s t r a t i g r a p h y and conodont b i o s t r a t i g r a p h y i n t h e Sambosan t e r r a i n a t Gokase and S h i i b a areas, Nishiusuki-gun, Miyazaki P r e f e c t u r e . I b i d . , 147-153. -, e t a l . , 1974. Geological s i g n i f i c a n c e o f t h e unconformity between t h e Permian Nabeyama and T r i a s s i c Adoyama formations i n t h e v i c i n i t y o f KUZUU, T o c h i g i P r e f e c t u r e . J. Geol. SOC. Japan, 80:293-306. Maejima, W. and Matsuda, T., 1977. Discovery o f T r i a s s i c conodonts from "Paleoz o i c " s t r a t a i n t h e n o r t h e r n s u b b e l t o f t h e Chichibu b e l t i n t h e n o r t h o f Yuasa, Wakayama P r e f e c t u r e and i t s g e o l o g i c a l s i g n i f i c a n c e . J. Geol. SOC. Japan, 83:599-600. Makino, Y., 1976. On t h e s t r a t i g r a p h y o f t h e Chichibu System i n t h e Kashiwagi d i s t r i c t , c e n t r a l p a r t o f t h e K i i mountainland, c e n t r a l Japan. J. Geol. SOC. Japan, 82:297-310. Mizutani, S., 1964. S u p e r f i c i a l f o l d i n g o f t h e Paleozoic System o f c e n t r a l Japan. 3. E a r t h Sci. Nagoya Univ., 12:17-83. -, e t a l . , 1981. J u r a s s i c formations i n t h e Mino area, c e n t r a l Japan. Proc. Japan Acad., 57, ser. B:194-199. Murata, A., 1981. A l a r g e o v e r t h r u s t and t h e paleogeography o f t h e Kurosegawa and Sambosan t e r r a i n s - i n t h e case o f t h e Gokase area i n c e n t r a l Kyushu. J. Geol. SOC. Japan, 87:353-367. Sakagami, S. e t a l . , 1969. Conodonts from t h e Kamiiso limestone and considera t i o n o f i t s g e o l o g i c a l age:J. Geogr., 78:415-421. Takizawa, S., 1979. S t r a t i g r a p h y o f t h e Chichibu t e r r a i n i n t h e n o r t h e r n p a r t o f t h e Kwanto mountains. P r o f . Kanuma Mem. Vol., 89-102. Tamba Research Group, 1979. Paleozoic and Mesozoic Systems i n t h e Tamba b e l t ( P a r t 5)-Permian and T r i a s s i c conodont f o s s i l s i n t h e northwestern h i l l s o f Kyoto City. E a r t h Sci., 33:247-254. Tamura, M. e t a l . , 1978. A f i n d o f T r i a s s i c molluscs from t h e Buko limestone formation, Chichibu, Saitama P r e f e c t u r e . Proc. Japan Acad., 54, ser. B: 41 -44. Tanaka, K., 1980. Kanosashi Group, an o l i s t s t r o m e , i n t h e N i c h i h a r a area, Shimane P r e f e c t u r e . J. Geol. SOC. Japan, 86:613-628. Toyohara, F., 1977. E a r l y Mesozoic t e c t o n i c development o f t h e northwestern Chichibu geosyncline i n west Chugoku, Japan. J. Fac. S c i . Univ. Tokyo, sec. 2, 19:253-334. -, e t a l . , 1980. Geosyncline o f t h e n o r t h e r n Kitakami-Oshima Peninsula. i n Kimura, T. ( E d i t o r ) , Restudy o f geosyncline and s t r u c t u r a l d i v i s i o n s i n t h e n o r t h e r n p a r t o f t h e Japanese Islands, 27-36. Watanabe, K. e t a l . , 1.979. Conodont b i o s t r a t i g r a p h y i n t h e Kamura limestone ( T r i a s s i c ) , Takachiho-cho, Nishiusuki-gun, Miyazaki Prefecture. Prof. Kanuma Mem. Vol., 127-137. Yao, A., 1972. R a d i o l a r i a n fauna from t h e Mino b e l t i n t h e n o r t h e r n p a r t of t h e Inuyama area, c e n t r a l Japan. P a r t I:Spnogosaturnalids. J. Geosci. Osaka City Univ., 15:21-64. -, 1979. R a d i o l a r i a n fauna from t h e Mino b e l t i n t h e n o r t h e r n p a r t of t h e Inuyama area, c e n t r a l Japan, P a r t 11: N a s s e l l a r i a 1. I b i d . , 22:21-72. , e t a l . , 1980. T r i a s s i c and J u r a s s i c r a d i o l a r i a n s from t h e Inuyama area, c e n t r a l Japan. I b i d . , 23:135-154. Yoshida, S., 1972. C o n f i g u r a t i o n o f Yamaguchi zone-Analytical study on a f o l d zone. J. Fac. Sci, Univ. Tokyo, sec. 2, 18:371-429.

76

Fig. 1 Map showing l o c a t i o n s o f i m p o r t a n t Upper Paleozoic through T r i a s s i c c h e r t formations. 1 Menashidomari, 2 Sorachi d i s t r i c t , 3 Hidaka d i s t r i c t , 4 H i r o s a k i , 5 Orikabe, 6 Iwaizumi, 7 Asanai, 8 Okutadami, 9 Okukinu, 10 Yamizo-Toriashi-Torinoko Massifs, 11 Ashio Massif, 12 KUZU, 13 Mamba, 14 Kamiyoshida, 15 I t s u k a i c h i , 16 Omi, 17 Hida Massif, 18 Kiso Massif, 19 Mino Massif, 20 Gujiohachiman, 21 Neo, 22 Tamba d i s t r i c t , 23 Sasayama, 24 Kashiwagi, 25 Yuasa, 26 Atetsu, 27 Nichihara, 28 Akiyoshi, 29 Nakagawa, 30 Sambosan, 31 Togano, 32 Tao (Taho), 33 Kamura, 34 Gokase, 35 Shiiba, 36 Konose, 37 Amamioshima.

77

1 Sakawa

i

I I

1

s. Kitakami Gokase Togam 6. Tokushima. Nakagawa

'Pm

Okuchichibu-

Nichihara

Fig. 2. S t r a t i g r a p h i c and geographic d i s t r i b u t i o n o f i m p o r t a n t c h e r t formations of t h e Upper Paleozoic through T r i a s s i c i n t h e Japanese I s l a n d s . 1 s t r a t a w i t h o u t c h e r t , 2 c h e r t - b e a r i n g zone c o n t a i n i n g conodonts, 3 c h e r t s w i t h o u t conodonts, 4 " s h e l f f a c i e s " T r i a s s i c , 5 h i a t u s , e r o s i o n a l o r tectonic.

79

CHAPTER 6

CHERTS W SOUTHEAST ASIA

D.N.K. TAN Geological Survey of Malaysia, Kuching.

ABSTRACT Chert of Palaeozoic age i s found mainly i n Indochina, Thailand, and Lower Palaeozoic c h e r t i s confined t o peninsular Peninsular Malaysia. Thailand, and northwest and central Peninsular Malaysia. Upper Palaeozoic chert is widespread i n Thailand, Indochina, and Peninsular Malaysia, w i t h an isolated occurrence i n west Sarawak. Mesozoic chert i s widespread i n Southeast Asia. Triassic chert is found i n northern Thailand, northern Peninsular Malaysia, Singapore, Indonesia, Sarawak, and northern Palawan. Jurassic-Cretaceous chert occurs i n Indonesia, Sarawak, Sabah, and the Philippines b u t has not been reported i n Thailand, Indochina, and Peninsular Malaysia. Chert of Tertiary age or older occurs as blocks i n Tertiary melange and imbricated canplexes i n Indonesia, the Philippines, and the eastern Malaysian s t a t e s of Sarawak and Sabah. INTRODUCTION I will review the occurrences of cherts i n Southeast Asia, between approximately 96' 134'E longitudes and 23N ' - 12's l a t i t u d e s , which includes Indochina (Vietnam, Laos, Kampuchea), Thailand, Malaysia, The geographic distribution of chertIndonesia, and the P h i l i p p i n e s . bearing sequences i n Southeast Asia and their stratigraphic d i s t r i b u t i o n A larqe amount of are shown i n Fiqure 1 and Fiqure 2 respectively. q e o l w i c a l 1itera.ture concerninq SE Asia i s available b u t only a few studies a r e related t o c h e r t deposits. The c l a s s i c work on the resional More recent regional studies qeolow of the area is by Bemnelen (1949). include those of Workman (1977) and Hamilton (1979). The oldest c h e r t i n SE Asia, Early Palaeozoic aqe, occurs i n peninsular Thailand and Peninsular Malaysia. Upper Palaeozoic chert is found mainly i n Indochina, Thailand, and Peninsular Malaysia w i t h an isolated occurrence i n Sarawak (Fig. 1).

-

20%

15'

lo
w n -

w n

1 KAGV IK-BROOKS RANGE

2 ANGAYUCHAM

V

2 z

v)

I

1

3 INNOKO

4 MYSTIC 5 RED PAINT 6 McKINLEY

7 CHULITNA X

8 WEST FORK 9 BROAD PASS

10 WRANGELLIA

11 CHUGACH

12 CACHE CREEK X (XX

13 BRIDGE RIVER 14 SAN JUAN 15 BLUE MOUNTAINS ~~

16 T r & Pz KLAMATH MTNS

xx

xx

_ .

xx

17 EASTERN KLAMATH MTNS

I

X

-

~

18 NORTHERN SIERRA

19 GOLCONDA X

!O ROBERTS MOUNTAINS

!1 FRANCISCAN !2 GREAT VALLEY

!3 CEDROS-VIZCAINO

(XX

I x

B CL 0

Characteristics, distribution, and inferred depositional environments o f six radiolarian chert associations in western North America. Compound lithologic associations are designated by asterisks.

TABLE 2.

ASSOCIATION NAnE

LITHOLOGIC ASSOCIATIONS

STIMATEO THICKNESS F CHERT DEPOSITS

OPHIOLITIC CHERT ASSOC I AT1ON

Thick c h e r t sequence (low i n s i l i c i c volcanic o r terrigenous d e t r i t u s ) o v e r l i e s p i l l o w b a s a l t ( + serpentine, gabbro. Basal c h e r t may 6e i n t e r calated w i t h basalt, manganese ore, jasper. '

Thick (to 5 h o r more)

Chert and b a s a l t breccia i n t e r l e a v e d between t h i c k (100's of meters) p i l l o w basalt units.

Chert sequence has h i g h s i l i c i c v o l canic component. May i n c l u d e volcanic breccia, tuff, c h e r t y t u f f , tuffaceous.

TESRANE DISTRIBUTION (see Fig. 1)

:LASTfC

, 11, LOW

INFERRED OEPOSITIONAL ENVIRONWENT

14, 16, Marine basin

9, 20*, 21, '2'. 23'

Thin (1-2071)

Variable

Marine environmenl dominated by a c t ? ' b a s a l t i c volcanisl

2, 6, 14, 21, 22

LOW

SILICIC VOLCANIC/ CHERT ASSOCIATION

I

CHERT COMPONENTS

-

1

1

I

I

I

1

1

!

''

3'

7p

8s

Slope o r basin adjacent t o a c t ? arc volcanism

9, 16, 17'. 18*, 22*, 23*

LOW

-

~

SUBSIOENCE ASSOCIATION

Chert (+ interbedded hemipelagic rocks) o v e r l i e s shallow- marine o r subaerial deposits.

Variable

1, 4, 7*. 10. 12, 17'. 18'

Lowhigh

I

Chert sequence has h i g h c l a s t i c component. May include s i l t y chert, c l a s t i c rucks, c a l c a r e n i t e .

Thin-medium (1-307I)

High

L~~

FELANGE CHER1 ASSOCIATION

Interbedded black a r g i l l i t e , chert, and s i l i c i c tuff (+ graywacke o r p i l l o w basalt). Kn%n occurrences are t e c t o n i c a l l y disturbed.

Thin (1-lOn)

Jnknowr

-

Subsiding platform

~

RareIRareabundant abundant

I *_owl

Rarec m n

I

1I I I

I

Abundant

1

Rare

Lowhiqh

1, 3?, 7',

8,

19, 20*, 21,

Continental margin

23'

I

Rare

T e c t o n i c a l l y and Unknown marine basin neai c o n t i n e n t a l marg

127

CHAPTER 9 SEDIMENTOLOGY OF RADIOLARITES WITHIN THE N I C O Y A OPHIOLITE COMPLEX, COSTA RICA, CENTRAL AMERICA H.-J.

GURSKY and R. SCHMIDT-EFFING

Geologisch-Palaontologisches I n s t i t u t der U n i v e r s i t a t , Lahnberge, 0-3550 Marburg, Fed. Rep. o f Germany. ABSTRACT Gursky, H.-J. and Schmidt-Effing, R., 1982. Sedimentology o f r a d i o l a r i t e s w i t h i n t h e Nicoya O p h i o l i t e Complex, Costa Rica, Central America. Mesozoic through Paleogene o p h i o l i t e complexes form e x t e n s i v e p a r t s o f t h e southern Central American basement. I n Costa Rica t h e o p h i o l i t e i s c a l l e d t h e Nicoya Complex which can be d i v i d e d i n t o t h r e e u n i t s : The Lower Nicoya Complex i s composed o f b a s a l t s and mafic p l u t o n i c rocks f o l l o w e d by t h e Punta Conchal Formation, a r a d i o l a r i t e s e r i e s which separates t h e Lower from t h e Upper Nicoya Complex c o n s i s t i n g m a i n l y o f submarine b a s a l t f l o w s w i t h i n c l u s i o n s o f sedimentary rocks The Punta Conchal Formation, emphasized i n t h i s paper, has a t h i c k n e s s of some 50 m and c o n s i s t s o f r h y t h m i c a l l y t h i n - s t r a t i f i e d r a d i o l a r i a n c h e r t s . Homogeneous bedding i s much more comnon than graded l a m i n a t i o n and microcrosslamination. E a r l y d i a g e n e t i c deformation i s common such as bed d i s r u p t i o n s , slump f o l d s , and b r e c c i a t i o n . P e t r o g r a p h i c a l l y , t h e s i l i c e o u s rocks a r e q u a r t z c h e r t s w i t h micro- t o c r y p t o c r y s t a l l i n e t e x t u r e s w i t h a g r e a t v a r i e t y o f colours. I n places magmatogenic h e a t i n g caused r e c r y s t a l l i z a t i o n . Sedimentation was i n i t i a t e d a t l e a s t i n t h e lowermost Cretaceous and l a s t e d up t o t h e H a u t e r i v i a n o r t o t h e Barremian. We suggest t h a t c h e r t s were deposited i n a p e l a g i c abyssal environment i n t h e l a t e Mesozoic eastern P a c i f i c Ocean t h a t was c h a r a c t e r i z e d by a consider a b l e r e l i e f . Redepositional processes due t o d e n s i t y c u r r e n t s p a r t i c i p a t e d i n t h e sedimentation presumably f a r away from an emerged l a n d mass. Sporadic g r a v i t a t i o n a l s l i d e s a l s o occurred.

.

INTRODUCTION Southern Central America c o n s i s t s o f two fundamental g e o t e c t o n i c u n i t s . The most e x t e n s i v e u n i t c o n s i s t s o f igneous and sedimentary rocks o f t h e Central American i s l a n d arc, which has been s t r o n g l y u p l i f t e d forming a c l o s e d isthmus between North and South America. The rocks o f t h i s u n i t were formed from t h e Upper Cretaceous t o t h e Quarternary. O p h i o l i t e complexes, i n t e r p r e t e d as oceanic basement (e. g.,

Schmidt-Effing e t a l . ,

1981), crop o u t

i n a s e r i e s o f peninsulas along t h e P a c i f i c coast o f Costa Rica and Panama and may be comparable t o t h e Basic Igneous Complex o f Colombia and Ecuador.' Along t h e c o a s t l i n e , e s p e c i a l l y t h e Nicoya Peninsula i n Costa Rica, e x c e l l e n t s e c t i o n s o f o p h i o l i t e c r o p o u t ( F i g . 1).

128 The o p h i o l i t e sequence i n Costa Rica i s named Nicoya Complex a f t e r t h e Nicoya Peninsula (Dengo, 1962). I t c o n s i s t s o f presumably several k i l o m e t e r s o f massive b a s a l t , p i l l o w b a s a l t , v o l c a n i c l a s t i c b r e c c i a , and m a f i c , m o s t l y gabbroic, p l u t o n s and s i l l s (Wildberg e t a l . ,

1981a). Pelagic s i l i c e o u s rock

and limestone, repeatedly i n t e r c a l a t e d i n t h e b a s a l t and b r e c c i a , and overl y i n g t h e o p h i o l i t e , p e r m i t s t r a t i g r a p h i c s u b d i v i s i o n o f t h e Nicoya Complex (Stibane e t a l . ,

1977; Schmidt-Effing,

1979; Wildberg e t a l . ,

1981).

T h i s paper concerns Lower Cretaceous c h e r t s w i t h i n t h e Nicoya Complex. FUNDAMENTAL TYPES AND OCCURRENCES OF SILICEOUS ROCKS I N THE NICOYA COMPLEX Regional d i s t r i b u t i o n Diverse s i l i c e o u s rocks a r e about 1 o r 2 volume percent o f t h e Nicoya Complex and they occur i n most peninsulas o f t h e P a c i f i c coast o f Costa Rica and western Panama (Fig. 1).

Caribbean Sea

Fig. 1. Sketch map o f southern Central America. - Hatched areas: o p h i o l i t i c rocks. Black dots: main c h e r t outcrops w i t h i n t h e o p h i o l i t e r e f e r r i n g t o thickness, b i o s t r a t i g r a p h i c a l s i g n i f i c a n c e , o r i n t e r r e l a t i o n t o igneous rocks (see t h i s s e c t i o n ) .

129

Tectonically isolated Middle Cretaceous r a d i o l a r i t e s w i t h very well preserved radiolarians crop out i n central Santa Elena Peninsula (SchmidtEffing, 1980) and a t i t s southwestern coast recrystallized and tectonically considerably deformed bedded radiolarian cherts occur associated w i t h deformed basalt. The radiolari t e s of the Lower Cretaceous Punta Conchal Formation, which crops out i n northwestern Nicoya Peninsula, make u p most o f the sedimentary rocks in the ophiolite complex and separate i t into Lower and Upper parts (Wildberg e t a l . , 1981; Fig. 2 ) . I t represents a widespread , temporal 1y 1ong ,

not to scale overlying sedimentary rocks

1

upper Nicoya Complex

v v v v -v-v-

v

v

v

v

-I

deep and shallow water sandstone

v

4

Punta Conchal Formation

deep and shallow m M water limestone overlying siliceous rock, siliceous limestone intercalated radiolarite

Lower Nicoya

volcaniclastic breccia pillow and massive basalt

Complex

........

...x

...4' x

x

ultramafic x

x x x x x

1.: .................

rocks of Santa Elena ~

1++++4

intrusive gabbro, diorite, tonalite

KJ

peridotite, dunite, partly serpentinized

Fig. 2. Lithostratigraphic sketch of composite sections on the Nicoya Peninsula, Costa Rica.

130

autochthonous sedimentary i n t e r r u p t i o n of the o p h i o l i t e sequence; t h e subj a c e n t Lower Complex igneous rocks a r e geochemically d i s t i n c t from the superj a c e n t Upper Complex and have probably d i f f e r e n t o r i g i n s (Gursky e t a l . , 1982). In western, c e n t r a l , and southern Nicoya Peninsula, and i n the peninsulas of Herradura, Quepos, Osa (L. Lew, Pennsylv. S t a t e Univ.: w r i t t e n communicat i o n , 1981), Son’a, and Azuero, bedded r a d i o l a r i a n c h e r t and massive s i l i c e o u s rock of mostly unknown ages l o c a l l y occur i n a s s o c i a t i o n with massive and pillow b a s a l t ( F i g . 1 ) . Mode of occurrence

Lenses of r a d i o l a r i t e ( s i m i l a r l i t h o l o g y a s Punta Conchal Formation described below) w i t h maximum lengths of 20 m and u p t o 1.5 m in thickness crop out i n massive b a s a l t along t h e northwestern c o a s t of Nicoya Peninsula (e. g . , near B r a s i l i t o , Fig. 3). I n t h i n s e c t i o n s most of these s i l i c e o u s rocks appear

Fig. 3 . Index map of the Nicoya Peninsula showing l o c a l i t i e s c i t e d i n the t e x t . Dotted area: Nicoya Complex, white area: Cretaceous through Quarternary sedimentary and volcanic cover. - 1 Nancital, 2 Punta Gorda, 3 Punta S a l i n a s , 4 B r a s i l i t o , 5 Punta Conchal, 6 Punta Sabana, 7 Playa Pedregosa, 8 Tamarindo, 9 F l o r i d a , 10 Rio Morote, 11 Pochote.

131 t o have been deformed by p o s t d e p o s i t i o n a l magmatic events; e. g.,

radiolaria

and l a m i n a t i o n were r e c r y s t a l l i z e d , groundmass g r a i n s were coarsened, b u t bedding i s s t i l l preserved. Sedimentary x e n o l i t h s occur as s i n g l e b l o c k s o r groups o f b l o c k s i n t h e Upper Nicoya Complex, e i t h e r c l o s e t o o r f a r away from t h e i r o r i g i n a l s t r a t i graphic p o s i t i o n . They form i r r e g u l a r , roundish, o r l e n t i c u l a r bodies w i t h sizes o f up t o many cubic meters. When they occur as groups o f l o n g i s h blocks, they may be p a r a l l e l y o r i e n t a t e d (e. g., near Tamarindo). The x e n o l i t h s show unconformable c o n t a c t s due t o i n c o r p o r a t i o n by igneous flows. The t y p i c a l i n t e r n a l s t r u c t u r e s are: Bending o f t h e sedimentary bedding i n t h e blocks, breakage o f t h e beds a t t h e c o n t a c t margins, m o s t l y s t r a t i f o r m i n t r u s i o n s o f small igneous d i k e s and break o f f o f s m a l l e r blocks, c o n t a c t metamorphism i n c l u d i n g f o r m a t i o n o f c r u s t s , thermal r e c r y s t a l l i z a t i o n , d i s c o l o r a t i o n , and i n places complete o b l i t e r a t i o n o f d e p o s i t i o n a l s t r u c t u r e s . Rare, autochthonous, massive i n t e r p i l l o w c h e r t was t h e r m a l l y r e c r y s t a l l i z e d and, t h e r e f o r e , primary c h a r a c t e r i s t i c s were o b l i t e r a t e d (e. g.

, near

Florida).

I n some areas o f t h e Nicoya Complex r e d and yellow, massive j a s p e r rocks a r e a l s o found. Because they a r e s t r o n g l y r e c r y s t a l l i z e d , no d e p o s i t i o n a l s t r u c t u r e s a r e v i s i b l e . They occur i n massive b a s a l t s as i r r e g u l a r compact bodies o f up t o several cubic meters i n s i z e , as f i l l i n g s o f narrow f r a c t u r e s , and as small d i k e - l i k e i n t r u s i o n s i n r a d i o l a r i t e sequences. Dense groups o f q u a r t z - f i l l e d f i s s u r e s , o r i e n t a t e d i n places c o n c e n t r i c a l l y (cabbage head s t r u c t u r e s ; e. g.,

Punta S a l i n a s ) , a r e conspicuous. These c h e r t s p o s s i b l y

represent completely a l t e r e d r a d i o l a r i t e o r hydrothermal h e m a t i t e - r i c h s i l i c a mineralizations. Reworked fragments o f s i l i c e o u s rocks as products o f endogenous o r exogenous e r o s i o n a l processes on t h e sea f l o o r a r e commonly found i n v o l c a n i c l a s t i c b r e c c i a i n t h e upper p a r t o f t h e Upper Nicoya Complex. Angular t o subangular, v i v i d l y brownish r e d t o brownish y e l l o w g r a i n s o r blocks o f c h e r t occur i r r e g u l a r l y d i s t r i b u t e d w i t h i n t h e m o s t l y homogeneous b r e c c i a s (e. g., near Pochote). PUNTA CONCHAL FORMATION D e f i n i t i o n and general d e s c r i p t i o n Sections o f a r e l a t i v e l y t h i c k r a d i o l a r i t e s e r i e s , most probably forming a s i n g l e s t r a t i g r a p h i c u n i t , crop o u t i n many i n l a n d and c o a s t a l exposures o f t h e northwestern Nicoya Peninsula and mark t h e boundary between t h e Lower and t h e Upper Nicoya Complex i n wide areas. Due t o complicated polyphase t e c t o n i c deformation d u r i n g t h e L a t e Cretaceous and t h e T e r t i a r y , t h i s Formation was r e p e a t e d l y f o l d e d and f a u l t e d which r e s u l t e d i n an i r r e g u l a r p a t t e r n o f d i s -

132

continuous outcrops (Wildberg e t a l . , 1981). As the s e r i e s can be lithologic a l l y and stratigraphically defined i n several sections, however, i t i s given the name Punta Conchal Formation ( F i g . 4 ) . The type l o c a l i t y i s the small cape of Punta Conchal i n Northwest Nicoya Peninsula. Additional c h a r a c t e r i s t i c s can be observed in inland and other coastal sections (e. g., near Nancital, Punta Gorda, Punta Sal inas, northern Playa Pedregosa).

.. .... . . . . . .......... . . ... . . . . ... ........ f

m

v

v

v

v

v

v

v

v

v

v

v

massive basalt flow of the Upper Nicoya Complex with chilled bottom contact and chert xenoliths

v v v v v q ; ; ; v - v v v v v

40

zone of contact metamorphism thick-bedded or massive radiolarite alternation of very thin-bedded radiolarites and shales slump horizon with folds, lenticular bodies, and chaotic masses

30

v v v "=

v

v

\,

discolorations and concretions of chert

V

20

V V

massive basalt sill witti thermal contacts and chert xenoliths

V V

V

manganese nodules

V

10

typical thin-bedded radiolarite and pinch-and-swell structure

V V

V

radiolarite with arenitic-tuffaceous layers

V V V

0

basal very thin-bedded chert

V

v

~

v

l

v

L .................................... v v v v v v v v v

I . . . . .

v v

v v

v v

v v

v v

v

v v

v

v

v

v

I

v

massive basalt of the Lower Nicoya Complex

Fig. 4. Idealized synoptic section o f the Punta Conchal Formation.

133 The sequence i s g e n e r a l l y u n d e r l a i n by p o s t d e p o s i t i o n a l b a s a l t s i l l s , so t h a t t h e c o n t a c t between b o t h rock u n i t s i s unconformable (e. g.,

Punta S a l i -

nas). The lowermost sedimentary beds a b r u p t l y break a t t h e rough plane o f t h e contact. Some sedimentary b l o c k s a r e n e a r l y broken o f f by small i n t r u d i n g b a s a l t dikes and show s l i g h t bending o f t h e beds. Other completely separated blocks w i t h s t r o n g i n t e r n a l deformation " f l o a t " i n t h e b a s a l t below t h e contact. A t Punta Gorda t h e Formation o v e r l i e s a p p a r e n t l y conformably massive b a s a l t a t a sharp, r e l a t i v e l y even i n t e r f a c e w i t h a r e c r y s t a l l i z e d , dark grey, up t o 0.5 m t h i c k , m i l l i m e t e r - t o centimeter-bedded a l t e r n a t i o n o f manganiferous and pure c h e r t . During t h e n e x t s t r a t i g r a p h i c a l l y h i g h e r 30 cm t h e bed thickness of t h e pure c h e r t l a y e r s increases t o centimeter- and d e c i m e t e r - s t r a t i f i e d tuffaceous r a d i o l a r i t e s . Upsection, manganiferous and l a t e r c l a s t i c components decrease.

A t t h e t o p c o n t a c t o f t h e Formation ( i n c o n t r a s t t o what K u i j p e r s s t a t e s , 1980) massive b a s a l t f l o w s o f t h e Upper Nicoya Complex cover t h e r a d i o l a r i t e s (e. g.,

Punta Gorda, Fig. 6). S t r a t a a r e wrenched o f f , t h e uppermost sedimen-

t a r y rocks a r e baked ( c h i l l e d margin), and c r u s t s formed a t t h e i n t e r f a c e . The o r i g i n a l t h i c k n e s s o f t h e Punta Conchal Formation probably reached a t l e a s t 50 m a f t e r compaction, although i n t h e a c t u a l outcrops much l e s s i s preserved due t o p o s t d e p o s i t i o n a l igneous, t e c t o n i c , o r e r o s i o n a l processes. A t Punta Conchal i t s e l f , some 35 m was measured. Several l i t h o l o g i c a l types appear t h a t can be assigned t o d i f f e r e n t subfacies. Monotonous, non-calcareous centimeter- t o decimeter-thick massive r a d i o l a r i t e s t r a t a w i t h m i l l i m e t e r t h i c k shale i n t e r c a l a t i o n s comprise t h e t y p e s e c t i o n and dominate most exposures. A d d i t i o n a l l y , m a i n l y i n t h e upper p a r t o f t h e Formation, v e r y t h i n s t r a t i f i e d ( l e s s than 1 cm) r a d i o l a r i t e s occur w i t h e q u a l l y t h i c k shale l a y e r s o r thick-bedded r a d i o l a r i t e s (more than 10 cm) w i t h o r w i t h o u t s h a l y i n t e r c a l a t i o n s (e. g.,

near N a n c i t a l ) . A r e n i t i c i n t e r c a l a t i o n s o f igneous d e t r i t u s

a r e present near t h e base o f t h e Formation. The l i t h o f a c i e s a r e f u r t h e r v a r i e d by c o n t a i n i n g amounts o f hematite and manganese minerals, as w e l l as by thermal r e c r y s t a l l i z a t i o n . B a s a l t s i l l s and d i k e s i n t r u d e d some s e c t i o n s (e. g.,

Punta Gorda, Punta Sabana).

I n l o c a l i t i e s w i t h l i t t l e thermal metamorphism, many w e l l preserved r a d i o l a r i a n faunas occur (e. g.,

t y p e l o c a l i t y ) . Faunas show t h a t

Sphaerostylus lanceola zone

--

--

besides t h e

t h e Staurosphaera septemporata zone i n t h e

sense o f Riedel and S a n f i l i p p o (1974) i s present, thus t h e sedimentation, perhaps a l r e a d y i n i t i a t e d i n t h e T i t h o n i a n , l a s t e d a t l e a s t t o t h e H a u t e r i v i a n o r t h e Barremian. Whether s i m i l a r r a d i o l a r i t e outcrops i n c e n t r a l and southern Nicoya Peninsula and o t h e r P a c i f i c peninsulas a r e e q u i v a l e n t t o t h e

134

P u n t a Conchal Formation i s unknown, because comparable r a d i o l a r i t e s of Middle Cretaceous ages (Albian t o Turonian) a l s o occur in places (e. g . , Potrero Grande in Santa Elena Peninsula, see Schmidt-Effing, 1980; Rio Morote, see Azema e t a1 , 1979).

.

Macroscopic sedimentary structures I n a l l sections of the Punta Conchal Formation (Fig. 4 ) , thin-bedded radiol a r i t e s with very thin-bedded shale partings predominate (typical r i b b o n cherts) with a rhythmic s t r a t i f i c a t i o n of the type "ABAB...". This i s a l s o reflected by the generally regular colour rhythms of the reddish brown radiol a r i t e beds and the s l i g h t l y darker shaly intercalations. I n places greyish red, grey, brownish v i o l e t , brown, yellow, pale green, and black colours occur. Fresh, splintery rocks break with angular t o conchoidal smooth planes and show dull t o subvitreous l u s t r e . These features r e f l e c t the high degree of l i t h i f i c a t i o n caused by s i l i c i f i c a t i o n and recrystallization. These mature q u a r t z cherts a r e fine-grained and homogeneously compact. Macrofossils a r e absent. Beds show mostly internal parallel laminations, homogeneous bedding being l e s s common. Rhythmic gradations of the "ABCABC..." type occur only near the base of the Formation, where arenitic-tuffaceous layers a l t e r n a t e w i t h p e l i t i c and radiolarian-rich intervals. Apart from these gradations, the genetic cause of the macroscopic rhythmic bedding cannot be d i r e c t l y deduced. Well defined s t r a t a range in thickness from 0.5 t o 15 cm. I n every outcrop, however, a c l e a r maximum of a roughly constant thickness dominates, mostly of 4 t o 5 cm. Generally the bedding planes a r e not perfectly parallel b u t uneven and wavy and can even wedge o u t (pinch-and-swell s t r u c t u r e s ) . The s o f t e r shaly intercalations react in the same manner, and i n slump folds o r under tectonic s t r e s s , shales partly migrated into cores of folds. The surfaces of the bedding planes a r e seldom clearly seen, they show irregular wavy t o knobby surfaces without marks. Graded alternations between siliceous sedimentary rocks and stratiform manganese mineralizations represent a special lithotype. Pure ore layers of centimeter-thickness w i t h sharp soles slowly grade t o r a d i o l a r i t e layers until the next rhythm abruptly s t a r t s . In places dense manganese nodules a r e intercalated between the r a d i o l a r i t e beds (e. g . , Punta Conchal). They show smooth discoidal t o ellipsoidal shapes and diameters from 0.5 t o 1 0 cm. Horizons of early diagenetic p l a s t i c sediment deformation occur in several sections (e. g . , Punta Gorda, P u n t a Sabana, Nancital). They are developed as irregular commonly recumbent folds w i t h an isoclinal geometry. Only few beds a r e deformed, adjacent beds being uneffected.

135

Slump folds show generally transport directions towards the East or West (Fig. 5). One horizon contains a mass of l e n t i c u l a r s l i d e bodies t h a t reach lengths of more t h a n 1 m and thicknesses of several decimeters and represent intense disruption of b e d s ; chaotic intraformational s l i d e masses with thicknesses of u p t o several meters represent another, s t i l l more intense deformation of the original stratigraphic succession (both a t Punta Gorda).

Mi c roscopi c structures Radiolarian skeletons make u p a small fraction of u p t o 85 volume percent (generally 20 t o 50 percent). Additionally, sponge spicules and very scarce foraminifera occur. Microfossils a r e in places obliterated by recrystallization. Radiolarians occur i n a micro- t o cryptogranular groundmass consisting of s i l i c a with hematitic pigment. Microfossils a r e composed of colourless mosaic quartz o r chalcedony and a r e f i l l e d w i t h fine-grained s p a r i t i c q u a r t z or chalcedony. Tests a r e well preserved and v i s i b l e as pale annular struct u r e s , where hematite- or manganese-rich f i l l i n g s and coatings occur.

F i g . 5. Combined representation of 115 slump fold directions in the P u n t a Concha1 Formation. - Equal area projection (lower hemisphere): rotated fold axes (concentration isolines 0 , 4 , and 8 %). Rose diagram: resulting directions of slump movements (10"-classes).

136 I n t h i n s e c t i o n s r a d i o l a r i a n s a r e r e g u l a r l y d i s t r i b u t e d o r concentrated i n homogeneously bedded, m i l l i m e t e r - t h i c k laminae. Redeposi t i o n a l f e a t u r e s a r e r a r e : m i l l i m e t e r - t o c e n t i m e t e r - t h i c k g r a d a t i o n s i n t e r p r e t e d as m o d i f i e d "Bouma sequences" w i t h s t r o n g l y reduced bases. They c o n s i s t o f o n l y one r a d i o l a r i a n - r i c h i n t e r v a l ( " r a d i o l a r i a n s i l t i t e " ) i n which t h e g r a i n s i z e o f t h e skeletons and t h e i r q u a n t i t y decrease upward on an average u n t i l n e a r l y f o s s i 1- f r e e , extreme1y f in e - g r a i ned , hemati t e - r i ch m a t e r i a1 domi nates ( F i g

. 7).

I n r a d i o l a r i a n - p o o r s i l i c e o u s rocks 0.5 mm, c l o s e l y spaced, p a r a l l e l lamin a t i o n s a r e caused by enrichment i n o r l a c k o f pigment and by c o n c e n t r a t i o n o f d e t r i t a l igneous minerals. Doubtful m i c r o c r o s s - s t r a t i f i c a t i o n i s scarce and r i p p l e marks were never observed. Only t h e uppermost two Bouma i n t e r v a l s were developed. Probably, extended low energy c u r r e n t s suspended and r e d i s t r i b u t e d f i n e - g r a i n e d mater i a l s . They were probably extremely weak g r a v i t y c u r r e n t s w i t h r e l a t i v e l y

Fig. 6. Top c o n t a c t o f Punta Concha1 Formation (Punta Gorda). Thin-bedded r a d i o l a r i t e o v e r l a i n by massive b a s a l t .

137

Fig. 7. P o o r l y graded lamina i n r a d i o l a r i t e bed (microphotograph, n i c o l s // , s c a l e b a r 1 m).

small suspension loads ( l o w v e l o c i t y , low d e n s i t y t u r b i d i t y c u r r e n t s ) , whose s t r e n g t h was t o o weak f o r erosion. Even graded beds were commonly n o t devel o p e d because o f t h i n laminae and s c a r c i t y o f coarser-grained p a r t i c l e s (see: B a r r e t t , 1979). B i o t u r b a t i o n i s r a r e : simple, small, w e l l - d e f i n e d burrow s t r u c t u r e s occur w i t h i n r a d i o l a r i a n - r i c h beds. R a d i o l a r i a n t e s t s have been g r e a t l y enriched i n these non-ramose, r e c t i l i n e a r o r curved, some m i l l i m e t e r l o n g tubes ( F i g . 8). R e c r y s t a l l i z a t i o n s t r u c t u r e s a r e due t o d i a g e n e t i c and thermometamorphic events. I r r e g u l a r d i s c o l o r a t i o n s and macroscopic d e s t r u c t i o n o f s t r a t i f i c a t i o n commonly accompany t h e microscopic d i s i n t e g r a t i o n o f r a d i o l a r i a n s . When t h e rocks became completely s i l i c i f i e d and chalcedony transformed t o quartz, extremely hard and s p l i n t e r y c h e r t s were formed, which c h a r a c t e r i z e t h e Nicoya Complex. The groundmass became coarser-grained,

t h e f o s s i l t e s t s were t r a n s -

138 formed t o "ghosts" ( F i g . 9), many completely disappearing. I n places i n t r a f o r m a t i o n a l b r e c c i a i s v i s i b l e i n t h i n s e c t i o n s as w e l l as i n p o l i s h e d s e c t i o n s etched by h y d r o f l u o r i c acid. Chert was broken t o cubic c e n t i m e t e r - s i z e d p a r t i c l e s w i t h o u t d i s i n t e g r a t i o n o f t h e rock coherence, o r completely crushed t o angular fragments connected by a quartz-hematite groundmass. I n d i v i d u a l components i n places show d i f f e r e n t stages o f r e c r y s t a l l i z a t i o n . These fragments may represent d e p o s i t i o n a l i n t r a - o r e x t r a c l a s t s o r shear b r e c c i a a t c o n t a c t s t o b a s a l t .

Fig. 8. Burrow i n r a d i o l a r i t e showing concent r a t i o n o f r a d l o l a r i a n s (microphotograph, n i c o l s // , s c a l e b a r 1 mm).

139

Fig. 9. Thermally r e c r y s t a l l i z e d r a d i o l a r i t e from baked c o n t a c t zone i n Fig. 6 (microphotograph, n i c o l s +, s c a l e b a r 1 mn.

SOME CONCLUSIONS The t h i c k r a d i o l a r i t e s o f t h e Punta Concha1 Formation i n t e r c a l a t e d i n t h e Nicoya Complex b a s a l t s , represent an extended i n t e r r u p t i o n o f t h e b a s a l t i c magmatism i n t h e Mesozoic presumably eastern P a c i f i c Ocean. A p e r i o d o f some

20 t o 30 m i l l i o n years (Tithonian/Lowermost Cretaceous t o H a u t e r i v i a n o r Barremian) shows l i t t l e evidence o f v o l c a n i c a c t i v i t y . R a d i o l a r i t e s were dep o s i t e d on t h e Lower Nicoya Complex which represents oceanic c r u s t (geochem i c a l evidence, Wildberg e t al.,

1982)

p o s s i b l y generated a t a P a c i f i c

spreading r i d g e . The Upper Nicoya Complex represents a second, geochemically d i s t i n c t , u n i t o f submarine b a s a l t s and p l u t o n i c rocks t h a t began t o form i n t h e l a t e Lower Cretaceous. During f o r m a t i o n o f t h e younger b a s a l t s , t h e r a d i o l a r i t e s e r i e s underwent considerable deformation i n c l u d i n g r e c r y s t a l l i z a t i o n and i n t r u s i o n s of d i k e s and s i l l s . Formation o f v o l c a n i c l a s t i c deposits probably occurred on steep slopes o r a t v o l c a n i c centers.

140 We i n t e r p r e t t h e data t o represent t h e f a c i e s model shown i n F i g u r e 10. I t i s p r e l i m i n a r y , as t h e evidence does n o t a l l o w f i n a l decisions. The environment o f d e p o s i t i o n was s i t u a t e d on oceanic sea f l o o r probably i n deep water below t h e CCD (Schmidt-Effing,

1979

calcareous components were found. High p u r i t y o f

, only

v e r y few o r i g i n a l l y

he b i o g e n i c r a d i o l a r i t e s ,

absence o f coarse-grained t e r r i g e n o u s c l a s t i c s i n t h e main p a r t s o f t h e sect i o n s , r e l a t i v e l y h i g h hematite content, and o n l y l o c a l evidence o f weak submarine c u r r e n t s suggest an open, 0,-rich,

b a s i n a l realm i n which a l s o manga-

nese nodules formed. R e l a t i v e l y s t a b l e and u n i f o r m environmental c o n d i t i o n s , l a t e r a l l y extended over more than 500 km,,

p e r s i s t e d d u r i n g e a r l y Lower

Cretaceous. Igneous d e t r i t u s o c c u r r i n g l o c a l l y a t t h e base o f t h e Formation may have been d e r i v e d from i n t r a o c e a n i c areas. We i n f e r t h a t besides normal p e l a g i c sedimentation, which p o s s i b l y created t h e s h a l y p a r t i n g s o f t h e r a d i o l a r i t e beds, low energy t u r b i d i t y c u r r e n t s presumably c o n t r i b u t e d t o t h e f o r m a t i o n o f t h e rhythmic s t r a t i f i c a t i o n t h a t was m o d i f i e d i n places by r e g i o n a l f a c t o r s . I n a d d i t i o n , slumping took p l a c e on presumably s l i g h t l y i n c l i n e d slopes, and a considerable p a l e o r e l i e f o r i e n t a t e d north-south t o northeast-southwest may be r e c o n s t r u c t e d from t h e slump directions.

F

n o t to scale

sea

- level

Fig. 10. Envir'onmental model o f t h e Punta Conchal Formation. - 1 Lower Nicoya Complex (oceanic c r u s t ) , 2 submarine r e l i e f ( P a c i f i c spreading r i d g e ? ) , 3 b a s a l t f l o w s o f t h e Upper Nicoya Complex c o n t a i n i n g c h e r t x e n o l i t h s d i r e c t l y above t o p c o n t a c t o f t h e Punta Conchal Formation, 4 b a s a l t s i l l s w i t h c h e r t x e n o l i t h s u n d e r l y i n g and i n t r u d i n g i n t o t h e r a d i o l a r i t e s , 5 c h e r t lenses, 6 l o c a l small sedimentary ponds, 7 r a d i o l a r i t e s o f t h e Punta Conchal Formation, 8 manganese nodules, 9 slump f o l d s , 10 l e n t i c u l a r s l i d e bodies.

141

ACKNOWLEDGMENTS Financial support was provided by the Deutsche Forschungsgemeinschaft (Bonn) and the Deutscher Akademischer Austauschdienst (Bonn). We g r a t e f u l l y acknowledge f i e l d excursions and discussions with E. Kuijpers, M. S t r e b i n , and H. Wildberg and c r i t i c a l revision of t h e manuscript by J . Hein. REFERENCES Azema, J . , Sornay, J . , and Tournon, J . , 1979. Decouverte d'Albien sup'erieur d ammonites dans l e materiel vol cano-s'edimentaire du "Complexe de Nicoya" (Province de Guanacaste, Costa Rica). C.R. somm. SOC. g'eol. France, 1979 ( 3 ) : 129-131. Azema, J. and Tournon, J . , 1980. La peninsule de Santa Elena, Costa Rica: u n massif u l t r a b a s i q u e c h a r r i e en marge pacifique de l ' k ' e r i q u e Centrale. C.R. Acad. S c i . , ,290: 9-12. B a r r e t t , T.J., 1979. Origin of bedded c h e r t s overlying o p h i o l i t i c rocks in the I t a l i a n North Appennines, and implications of the o p h i o l i t e - p e l a g i c sediment sequence f o r sea f l o o r processes. Ph. D. Thesis, Univ. Oxford: 419 p. (unplublished). Dengo, G., 1962. Estudio geolbgico de l a regibn de Guanacaste, Costa Rica. San Jose de C.R. ( I n s t . geogr. C.R.), 112 p. Gursky, H.-J., Schmidt-Effing, R . , S t r e b i n , M . , and Wildberg, H., 1982. The o p h i o l i t e sequence i n northwestern Costa Rica (Nicoya Complex): o u t l i n e s of s t r a t i g r a p h i c a l , geochemical, sedimentological , and t e c t o n i c a l data. Actas 5" Congr. latinoamer. Geol. Buenos Aires, 13 p. ( i n press). Hein, J.R., Kuijpers, E.P., and Denyer, P . , 1981. Paleogene and Cretaceous c h e r t s of western Costa Rica ( a b s t r a c t ) . In: I i j i m a , A. (ed.): The second i n t e r n a t i o n a l conference on s i l i c e o u s d e p o s i t s i n t h e P a c i f i c region. Abstract papers. Tokyo (Japan. Work. Group IGCP 1151, 59-61. Kuijpers, E.P., 1979. La geologia del Complejo O f i o l i t i c o de Nicoya, Costa Rica. Informe semestral (Inst. geogr. C.R.), j u l i o a diciembre 1979: 15-75. Kuijpers, E.P., 1980. The geologic h i s t o r y of the Nicoya o p h i o l i t e complex, Costa Rica, and i t s geotectonic s i g n i f i c a n c e . Tectonophysics, 68: 233-255. Lundberg, N . , 1980. Evolution of the Middle America Trench s l o p e , Nicoya Peninsula, Costa Rica. Santa Cruz (Univ. C a l i f . ) , 24 p. (manuscr.). Riedel, W.R. and Sanfilippo, A . , 1974. Radiolaria from the southern Indian Ocean, DSDP Leg 26. In: Davies, T., Luyendyk, B., e t a l . : I n i t i a l Rep. DSDP (Washington D.C.), 26: 771-813. Sano, H., 1982. Bedded c h e r t s associated with greenstones i n t h e Sawadani and Shimantogawa Groups, Southwest Japan. ( t h i s i s s u e ) . Schmidt-Effing, R., 1979. A l t e r und Genese des Nicoya-Komplexes, e i n e r ozeanischen Palaokruste (Oberjura bis Eozan) im sudl ichen Zentralamerika. Geol Rdsch., 68: 457-494. Schmidt-Effing, R. , 1980. Radiolarien d e r M i t t e l k r e i d e aus dem Santa ElenaMassiv von Costa Rica. N. J b . Geol. Palaont. Abh., 160 ( 2 ) : 241-257. Schmidt-Effing, R . , Gursky, H.-J., S t r e b i n , M . , and Wildberg, H., 1981. The o p h i o l i t e s of southern Central America with special reference t o the Nicoya Peninsula (Costa Rica). Trans. 9th Caribbean geol. Congr. Santo Dmingo, 17 p. ( i n p r e s s ) . Stibane, F.R., Schmidt-Effing, R., and Madrigal, R., 1977. Zur s t r a t i g r a p h i s c h tektonischen Entwicklung d e r Halbinsel Nicoya (Costa Rica) i n der Z e i t von Ober-Kreide bis Unter-Tertiar. GieRener geol. Schr. , 12: 315-358. Tournon, J . and Azkma, J., 1980. Sobre l a e s t r u c t u r a y l a petrologia del macizo u l t r a b a s i c o de Santa Elena (Provincia de Guanacaste. Costa Rica). Informe semestral ( I n s t . geogr. C.R.), enero a j u n i o 1980: 17-54.

.

142 Wildberg, H., Gursky, H.-J., Schmidt-Effing, R., and S t r e b i n , M., 1981. Der Ophiolith-Komplex d e r H a l b i n s e l Nicoya, Costa Rica, Zentralamerika. Zbl. Geol. Palaont., T e i l I , 1981 (3/4): 195-209.

143

CHAPTER 10 PETROLOGY AND GEOCHEMISTRY OF CRETACEOUS AND PALEOGENE CHERTS FROM WESTERN COSTA R I C A JAMES R. HEIN1, E R I C P. KUIJPERSZ, PERCY DENYERE, and ROSEMARY E. SLINEYI Geological

U.S.

94025,

Survey,

345 M i d d l e f i e l d Road,

Menlo

Park,

California

U.S.A.

Escuela

Centroamericana

de

Geologia,

Apartada

Postal,

35,

Ciudad

U n i v e r s i t a r i a , San Jose, Costa Rica.

ABSTRACT Rhythmically bedded and m s s i v e c h e r t s occur i n t h e Nicoya Complex o f Costa Rica,

which

includes

rocks

o f T i t h o n i a n through Santonian ages,

and i n

o v e r l y i n g pelagic, volcanic, and v o l c a n i c l a s t i c sequences o f Campanian through Eocene ages,

such as t h e Sabana Grande and Rivas Formations.

makes up about two percent o f t h e Nicoya Complex,

and g e n e r a l l y o v e r l i e s

b a s a l t i n a lower t h r u s t sheet on t h e Nicoya Peninsula. t h r u s t sheet on t h e Nicoya Peninsula, t h e Osa Peninsula, and 3 m t h i c k ,

Bedded c h e r t

W i t h i n t h e upper

and a t places near Jaco,

G o l f i t o , and

c h e r t occurs mainly as lenses, t o a maximum o f 100 m l o n g

between b a s a l t flows.

Near Quepos, Nicoya Complex b a s a l t and

c h e r t compose t h e c l a s t s i n a conglomerate o f probable Paleogene age. Rhythmic bedding, p a r a l l e l laminations, graded bedding, and r a r e Bouma "C" i n t e r v a l s suggest t h a t these c h e r t s were deposited by t u r b i d i t y currents,

and

t h a t interbedded shales a r e probably i n p a r t t a i l s o f t u r b i d i t e s and i n p a r t hemipelagic clays.

Rhythmic bedding i n t h e younger c h e r t s o f t h e Sabana

Grande u n i t my be due i n p a r t t o t u r b i d i t y c u r r e n t deposition. Cherts o f t h e Nicoya Complex a r e composed o f q u a r t z w i t h minor amounts o f hematite,

plagioclase, 'and c l a y minerals.

I n a d d i t i o n t o these minerals,

c h e r t s o f t h e Sabana Grande u n i t a l s o c o n t a i n much opal-CT and some c a l c i t e . The c r y s t a l l i n i t y o f t h e quartz i s low, t y p i c a l o f low-temperature d i a g e n e t i c quartz, except l o c a l l y where hydrothermal a c t i v i t y caused r e c r y s t a l l i z a t i o n o f t h e quartz. zeolites,

Secondary minerals i n veins barite,

apophyllite.

clay

minerals,

i n c l u d e quartz,

hematite,

Mn

oxides,

calcite,

calcium

pumpellyite,

and

M i n e r a l compositions i n d i c a t e t h a t t h e Sabana Grande u n i t and

t h e Nicoya Complex were subjected t o maximum temperatures o f 60' 150" t o 250°C r e s p e c t i v e l y ,

b o t h a t low pressures.

t o 70°C and

L o c a l l y temperatures my

144

have been g r e a t e r because o f hydrothermal a c t i v i t y . P e t r o g r a p h i c a l l y , t h e c h e r t s c o n s i s t o f few t o abundant r a d i o l a r i a n s set i n a f i n e - g r a i n e d m a t r i x o f microgranular quartz.

Varying amounts o f a u t h i g e n i c

and d e t r i t a l m i n e r a l s a r e s c a t t e r e d through t h e cherts.

Rarely,

laminae o r

lenses o f d e t r i t a l minerals occur. Chemically, t h e c h e r t s average about 90% SiO2 and 2.5% Al203. and T i low i n most o f t h e rocks studied;

Ba i s h i g h

Mn and Fe a r e h i g h l y variable.

Average chemical compositions a r e d i s t i n c t f o r c h e r t s a t d i f f e r e n t outcrops o f t h e Nicoya Complex and Sabana Grande u n i t on t h e Nicoya Peninsula because d i f f e r e n t amounts o f volcanic,

hydrothermal,

and d e t r i t a l i n p u t s occured on a

A t e r n a r y p l o t o f K-Ba-VxlO most c l e a r l y d i s t i n g u i s h e s between

l o c a l scale.

t h e v a r i o u s m a t e r i a l s t h a t contaminate t h e cherts, b a s i c v o l c a n i c d e b r i s (V), v o l c a n i c l a s t i c - d e t r it a l d e b r i s ( K ) , and hydrothermal input (Ba ).

A t e r n a r y P l o t of Fe203-MgO-K20 shows t h a t t h e r a t i o o f K20 and MgO constant i n a l l rocks t h a t represent more than 70 m.y. of

remained

deposition.

K20 and MgO occur i n one o r more mineral

d i f f e r e n t from t h e phase c o n t a i n i n g Fe2O3.

phases t h a t a r e

The r a t i o o f t h e a b s o l u t e amounts

of Fez03 versus MgO and K20 v a r i e s according t o t h e d i s t a n c e t h a t t h e sediment was deposited from Fez03 and K20 p l u s MgO sources. D e t r i t a l and v o l c a n i c m i n e r a l s contaminating t h e c h e r t a l s o make up t h e interbedded shales. The chemical and mineralogic compositions o f t h e cherts, oceanic deposits,

comparisons w i t h

and modes o f d e p o s i t i o n suggest t h a t Nicoya Complex c h e r t s

were deposited on oceanic c r u s t with l o c a l l y i n t e n s e hydrothermal a c t i v i t y , b u t i n small

basins near a c o n t i n e n t a l margin.

Modern environments t h a t

d i s p l a y these c h a r a c t e r i s t i c s i n c l u d e young ocean basins such as t h e G u l f of California, Nicoya

back-arc basins, and a r c - t r e n c h gap environments.

Complex

environments.

probably

formed

in

one

or

more

of

Cherts o f t h e

these

types

of

The c h e r t s from t h e Sabana Grande u n i t formed i n an a r c - t r e n c h

gap o r i n t e r - a r c environment.

INTRODUCTION Many aspects o f t h e Nicoya Complex, l o c a t e d i n western Costa Rica (Fig.

l),

have been discussed i n recent p u b l i c a t i o n s i n c l u d i n g r e g i o n a l geophysics (de Boer,

1979),

Olivier,

r e g i o n a l geology

1979),

(Dengo,

1962; Goossens e t al.,

1977;

Galli-

b i o s t r a t i g r a p h y o f t h e c h e r t and limestone (Schmidt-Effing,

1979), sedimentology o f c h e r t , limestone, and o v e r l y i n g t u r b i d i t e s (Gursky and Schmidt-Effing, al.,

1977;

t h i s volume;

Kuijpers,

1980),

Lundberg,

1982), s t r u c t u r a l geology (Stibane e t

and b a s a l t and u l t r a m a f i c rock chemistry and

p e t r o l o g y (Henningsen and Weyl,

1967; Wildberg e t al.,

1981), y e t t h e o r i g i n

o f t h e Nicoya Complex, e s p e c i a l l y t h e d e p o s i t i o n a l environment o f t h e

145

\,

CARIBBEAN SEA

\

C O S T A RlCA ‘\

I

7

\ 1 BRASlLlTO

PPUNTA SALINAS 3 PLAYA REAL 4 PLAYA PEDREGOSA

PACIFIC OCEAN

;i\i

SHUACAS BCARTEGENA 7SARDINAL 8 EL FRANCES

1 1 SANTA CRUZ

SCALE 1:700.000 0 , , , ,25KM

Figure 1 .

Index nnp and outcrop l o c a l i t i e s i n Western Costa &a.

1 Localities

studied on the Nicoya Peninsula are numbered, other outcrops studied occur near Jaco, Quepos, Cotfito, and on the Osa Peninsula. The Nkoya COnpleX i 8

shaded. sedimentary

rocks,

remains unclear.

Each model proposed t o e x p l a i n t h e

e v o l u t i o n o f these rocks f a i l s t o account f o r c e r t a i n aspects o f t h e geology. We studied rocks of t h e Nicoya Complex and rocks o f t h e o v e r l y i n g Sabana Grande u n i t , and age-equivalent

rocks,

from t h e northern h a l f o f t h e Nicoya

Peninsula, t h e Osa Peninsula, and near G o l f i t o , Quepos, and Jaco located along t h e west coast o f Costa Rica (Fig.

1).

The age o f t h e Nicoya Complex i s Late

Jurassic through Santonian, whereas t h e o v e r l y i n g rocks, here r e f e r r e d t o as t h e Sabana Grande u n i t , a r e Campanian through Eocene i n age. For various areas

146 on t h e Nicoya Peninsula d i f f e r e n t workers have used t h e Sabana Grande d e s i g n a t i o n t o represent rocks deposited d u r i n g d i f f e r e n t t i m e periods. A f i n a l d e f i n i t i o n has not been agreed upon.

We use t h e term i n an i n f o r m a l and

general way f o r t h e Late Cretaceous and Paleogene rocks. Here, we present data on t h e chemistry, petrology, and x-ray mineralogy o f c h e r t and limestone i n t h e Nicoya Complex and Sabana Grande u n i t .

We discuss

our r e s u l t s i n terms o f models presented f o r t h e Nicoya Complex by o t h e r workers

and

then

delineate

the

characteristics

of

the

environments

of

d e p o s i t i o n o f c h e r t s as constrained by our and o t h e r workers' data.

METHODS

Techniques 1981).

of

x-ray

d i f f r a c t i o n were described by Hein e t a l .

Norman (1976), (1974).

and opal-CT

Opal-CT

d-spacings

by t h e method o f Murata and Nakata

c r y s t a l l i t e s i z e s were measured p e r p e n d i c u l a r t o d(101),

u s i n g t h e Scherrer equation (Klug and Alexander, not

(1978,

Q u a r t z c r y s t a l l i n i t i e s were determined by t h e method o f Murata and

have a

1:l

correlation

coefficient;

the

1954).

Q u a r t z and opal-CT do

quartz/opal-CT

correlation

c o e f f i c i e n t depends on t h e c r y s t a l l i n i t y o f each mineral, and probably f a l l s between 1:6 and 1:9

(Cook e t al.,

r e l a t i v e q u a r t z and opal-CT c o e f f i c i e n t o f 1:8,

1975;

Pisciotto,

1978).

The values o f

l i s t e d i n Table 2 a r e based on a c o r r e l a t i o n

and thus may be somewhat more o r l e s s than t h e t r u e

r e l a t i v e amounts. The oxides o f S i , A l ,

Ti,

Fe. Mn, Mg, Ca, Na, K, and P were determined by

x-ray fluorescence spectroscopy. i s 1 t o 2%.

8, Co, C r , Cu, N i ,

Accuracy ranges from 0.4 t o 1%and p r e c i s i o n

V, Y, Zn, and Z r were measured by i n d u c t i v e l y

coupled emission p l a s m spectroscopy. Pb,

Sc,

Sn,

Ag, Ba, Be, Cd, Ce, Ga, La, L i , Mo, Nb,

and S r were determined by q u a n t i t a t i v e emission spectroscopy.

V o l a t i l e s were measured by l o s s on i g n i t i o n a t 900°C.

FeO was determined by

f u s i o n w i t h HF and H2SO4; b o r i c a c i d and phosphoric a c i d were added and t h e s o l u t i o n t i t r a t e d w i t n 0.1N potassium dichromate. E i g h t y t h i n sections and,polished and HF-etched slabs were studied.

RESULTS L i t h o l o g i c Associations and Ages The Nicoya Complex (Oengo, 1962) c o n s i s t s o f T i t h o n i a n t o Campanian c h e r t and l e s s e r amounts o f limestone b o t h r e s t i n g on and i n t e r c a l a t e d w i t h p i l l o w e d and massive b a s a l t f l o w s (Fig.

2).

Cherts make up about 2% o f t h e rocks,

b a s a l t s being t h e predominant l i t h o l o g y ; sedimentary and v o l c a n i c b r e c c i a s a r e common l o c a l l y .

On t h e Nicoya Peninsula, t h e Nicoya Complex i s d i v i d e d i n t o a

147 1000-

900-

EXPLANATION

aoo-

RHYTHMICALLY BEDDED CHERT a SHALE MANGANIFEROUS 8 HEMATITIC CHERT LENSES

700-

SANDSTONE

600--

CONGLOMERATE BASALT

500--

DIABASE, GABBRO, SERPENTINE RHYTHMICALLY BEDDED LIMESTONE 8 SHALE

400--

VVv

300.-

COMPLETELY SILICIFIED RHYTHMICALLY BEDDED LIMESTONE a SHALE ANGULAR UNCONFORMITY TECTONIC CONTACT

SCALE IN METERS

200-

loo--

0-

Figure 2 .

Composite generaliaed s t r a t i g r a p h i c s e c t i o n of

the Late JU?~.~66ic

and Cretaceous Nicoya Complex and overlying Campanian through Eocene rocks oj’ the Sabana Cmnde u n i t , from the northern Nicoya Peninsula. lower

unit

composed

of

basalt

and

meter-

to

40-m-thick

sections

of

r h y t h m i c a l l y bedded c h e r t t h a t ranges i n age from T i t h o n i a n t o middle A p t i a n (Schmidt-Effing, 1979; Kui jpers, 1980), a l t h o u g h we r e p o r t an a d d i t i o n a l r a d i o l a r i a n age date t h a t extends t h e age range i n t o t h e l a t e Albian.

This

lower u n i t o f t h e Nicoya Complex i s i n low-angle t h r u s t f a u l t c o n t a c t w i t h a s t r u c t u r a l l y o v e r l y i n g sequence o f b a s a l t t h a t c o n t a i n s i n t e r c a l a t e d lenses o f chert, jasper,

1imestone, and s i 1iceous i r o n - r i c h rocks of Cenomanian t o e a r l y

148

Campanian ages. Chert lenses range up t o 3 m thick and 100 m long and a r e commonly massive. In places, tabular blocks of bedded chert, t o tens of meters on a side and commonly laminated, a r e intercalated w i t h basalt, such as Cenomanian chert near Sardinal on the Nicoya Peninsula (Fig. 1). Other minor lithologies occurring in the Nicoya Complex include diabase, gabbro, plagiogranite, Mn ores, and others. Detailed descriptions of a l l these rock types found on the Nicoya Peninsula can be found in the papers l i s t e d in our Introduction. The upper and lower parts of the Nicoya Complex a r e overlain with angular unconformity by limestone, s i l i c i f i e d limestone, rhythmically bedded 1 imestone-shale sequences, chert, basalt, sandstone, and conglomerate of Campanian t o Oligocene ages. In the southern part of the Nicoya Peninsula these younger rocks may overlie the Nicoya Complex with local conformity, b u t t h e contact i s mrked by sedimentary breccias (Lundberg, 1982). Outcrops containing chert on the Nicoya Peninsula were described in some detail by Stibane e t a l . , (1977), Kuijpers and Denyer (1979). Schmidt-Effing (1979), Kuijpers (1980), and Gursky and Schmidt-Effing ( t h i s volume) so we will not repeat the general lithologic descriptions here. W e will, however, describe the cherts per 8e f r m twelve areas on the Nicoya Peninsula (Fig. 1). Each area, named a f t e r the nearest town, beach, or coastal peninsula (Table l ) , consists of from two (Huacas) t o six outcrops (Sardinal). Rocks near Santa Cruz and a t two of the three outcrops studied near Cartagena belong t o the Sabana Grande unit; the r e s t belong t o the Nicoya Complex. Near Sardinal, cherts from six outcrops a r e probably part of the lower Nicoya Complex whereas one section of chert and basalt belongs t o the upper thrust sheet. Cherts near Brasilito (type locality of the Punta Concha1 Formation o f Gursky and Schmidt-Effing, t h i s volume; Sardinal Formation of de Boer, 1979) a r e thin rhythmically bedded red, brown, and black radiolarian chert-shale sequences that r e s t on basalt and contain various admixtures of manganese and iron oxides and basaltic debris. In one measured section chert beds range in thickness from 1.0 t o 14.5 cm w i t h a mean of 7.8 cm and median of 9.2 cm, while interbedded shales range from 0.1 t o 4.0 cm with the mean a t 1.75 cm and median a t 2.1 cm. In another section chert beds range in thickness from 0.4 t o 14.5 cm with a mean of 8.5 cm and median of 8.8 cm, while shales range from 0.1 t o 7.5 cm, mean 3.3 cm and median 3.5 cm. Cherts a r e Tithonian t o Valanginian in age (Pessagno . i n Galli-Olivier, 1979; Gursky and SchmidtE f f i ng , t hi s vo 1ume ). Basalt and red and green chert on the flanks of Punta Salinas are Blocks of bedded and massive chert up t o extensively veined by quartz. several tens of meters on a side occur in basalt. Smeared-out remnants of shale layers characterize the massive chert. On top of Punta Salinas crops

149

out we1 1-bedded gray and greenish-gray laminated chert, very d i f f e r e n t i n appearance from t h e c h e r t s t h a t l i n e t h e beach; (880-22-2, Table 1); t h e contact i s not exposed. Shale interbeds o f t h e laminated c h e r t s are paper t h i n and c h e r t beds range from 4 t o 17 cm t h i c k . on r a d i o l a r i a n s i s A l b i a n (Hein e t al.,

The age o f t h i s c h e r t based

i n preparation),

the age o f the

underlying r e d c h e r t i s not known. A t Playa Real a r e sections

chert-shale

o f red-brown,

green,

and black thin-bedded

sequences i n a road c u t 1 km from t h e beach,

and massive t o

coarsely bedded red c h e r t t h a t contains l a r g e blocks and veins o f manganese ore l o c a t e d a t t h e n o r t h end o f t h e beach.

I n t h e thin-bedded c h e r t s occur

beds t h a t a r e f i n e l y laminated and t h a t are v i r t u a l l y completely replaced by manganese oxides. a r e present. and median 5.4

Pinch and swell s t r u c t u r e s and soft-sediment deformation

Chert beds range i n thickness from 3.2 t o 14.5 cm, mean 5.3 cm cm, w h i l e interbedded shales range from 0.1 t o 8.0

mean 1.9 cm and median 2.0 cm. Hauteriv i a n (Sc hmi d t - E f f ing, 1979).

cm t h i c k ,

Cherts range i n age from Tithonian t o

Outcrops a t Playa Pedrogosa c o n s i s t o f a complex o f brown, white, gray, and red m s s i v e and bedded c h e r t s t h a t r e s t on and a r e i n t e r c a l a t e d w i t h basalt. Massive c h e r t s commonly show remnants o f smeared-out shale interbeds. bedded c h e r t s a r e extensively f r a c t u r e d and r e c r y s t a l 1ized. o f c h e r t a r e set i n a m a t r i x o f sheared white, yellow, paste.

I n places, blocks and brown s i l i c a

A t one outcrop a block o f bedded w h i t e c h e r t occurs w i t h separate

blocks o f massive red, brown, and v i o l e t chert. diffuse.

White

Contacts between blocks are

Age o f these c h e r t s i s not known.

Outcrops around Huacas show small blocks ( t o -25 m) o f thin-bedded r e d and Mn-rich c h e r t i n t e r c a l a t e d i n massive and p i l l o w e d b a s a l t flows. Some i n t e r p i l l o w c h e r t occurs. Basalts a r e r e l a t i v e l y f r e s h (black) t o h i g h l y a l t e r e d (reddish-brown). An i s o l a t e d block o f c h e r t i n one outcrop i s s i m i l a r t o c h e r t s a t B r a s i l i t o , b u t more h i g h l y altered. Cherts a r e probably T i t h o n i a n t o H a u t e r i v i a n i n age (Schmidt-Effing, Near Cartagena a 40-m-thick

1979).

outcrop i s very

similar t o the B r a s i l i t o

s e c t i o n except t h a t bedding i s poorly expressed.

Also,

beds o f green clay’may represent a l t e r e d ash beds. known.

t h i n s t r i n g e r s and

Age o f t h e c h e r t i s not

A s e r i e s o f outcrops n o r t h o f Sardinal c o n s i s t s o f mostly red, r h y t h m i c a l l y

bedded chert-shale sections. Chevron f o l d s and shear planes are comnon. Many beds a r e laminated. I n a r e c e n t l y made road cut, a 40-m-section o f t h i n bedded c h e r t changes i n c o l o r from brown a t t h e top t o red, purple, yellow, green, gray, shale. 4.7

and f i n a l l y t o black a t t h e base.

Interbeds a r e a red f i s s i l e

A t t h i s outcrop c h e r t beds range i n thickness from 1.0 t o 8.5

cm and median 5.5,

cm, w h i l e shale interbeds range from 0.3

cm, mean

t o 5.8

cm,

ARO-21-11I

9RR

RRO-21-IA 11

RRR

1.3

PP

880-21-18

RRR

RPO-?l-IC 1

RadlolaTian packstone: i t y l o l l t e i or m i c m f a u l t i : w a d e d heddlng. laminated.

1.0

zm1rte7 h m t 1t e ?

ZOO m) i s c h a r a c t e r i z e d by h i g h e r summer temperature and s a l i n i t y o f t h e surface water (32.0°/,,,

7-8°C;

Takenouti and Ohtani, 1974; Coachman and Charnell

, 1979).

This water i s d e r i v e d from t h e North P a c i f i c Alaskan Stream, which e n t e r s t h e Bering Sea through A l e u t i a n i s l a n d passes ( F a v o r i t e , 1974). Waters o f t h e m i d - c o n t i n e n t a l s h e l f ( ~ 1 0 0m) a r e o f lower summer s a l i n i t y and temperature (31-31.5°/00,

0-10°C;

Coachman and Charnell

, 1979).

a horizontal s a l i n i t y gradient, w i t h s a l i n i t y increasing offshore. i s present d u r i n g a few months of t h e w i n t e r .

There i s Sea-ice

This s h e l f water i s i n f l u e n c e d

by l o w - s a l i n i t y water from Alaskan r i v e r s . The o u t e r c o n t i n e n t a l s h e l f (100-200 m) i s t h e t r a n s i t i o n zone where Alaskan Stream/Bering water and s h e l f water a r e mixed l a t e r a l l y by temperat u r e j s a l i n i t y f i n e s t r u c t u r e between 20 and 80 m i n t h e water column (Coachman and Charnell

,

1979).

Temperature and s a l i n i t y covary w i t h i n t h e f i n e s t r u c t u r e .

The seaward l i m i t of t h i s t r a n s i t i o n zone i s marked by a steep s a l i n i t y f r o n t a t t h e s h e l f - s l o p e break (Kinder and Coachman, 1978). Modern sediments u n d e r l y i n g these regions show dramatic differences

in

336 t h e diatom assemblages, r e f l e c t i n g t h e hydrographic f e a t u r e s (Sancetta, 1981a, b ) . Sediments o f t h e b a s i n show a s t r o n g dominance o f p e l a g i c species, p a r t i c u l a r l y D e n t i c u b p i s marina, which serves as a marker f o r Alaskan Stream/Bering 2 ).

water (Fig.

These sediments a r e diatomaceous oozes, w i t h a r e l a t i v e l y

F i g u r e 2 : Abundance o f D. marina i n s u r f a c e sediments. This species i s a t r a c e r f o r ' t h e Alaskan Stream. low d i v e r s i t y (H=1.66), due t o t h e overwhelming dominance o f one species. Sediments o f t h e m i d - c o n t i n e n t a l s h e l f (