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. SIEVE...

Author: A. Iijima | R. Siever | James R. Hein

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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.

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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.

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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<

0'

5'

10%

FIGURE I. Geographic Distribution of Chert-bearing sequences in Southeast Asia

_-._ THAILANO BILLITON

x SULAWESI

PHILIPPINES

CREmtEWS

Figwe 2. Stratigraphic Distributian of chert-bearing sequences in Scutheart A s k . Sequences without ossociotsd chertarerot shawn

82

Mesozoic c h e r t i s more widely d i s t r i b u t e d . T r i a s s i c c h e r t occurs i n northern Thailand, northern Peninsular Malaysia, Singapore, Indonesia, Sarawa k , and northern Pal awan I s 1and Jurassic -Cr etaceou s c h e r t occurs

.

i n Indonesia, Sarawak, Sabah, and t h e P h i l i p p i n e s b u t has n o t been reported i n Indochina, Thailand, and Peninsular Malaysia. Chert o f T e r t i a r y age or o l d e r occurs as blocks i n T e r t i a r y melange and imbricated canplexes i n Indonesia, t h e P h i l i p p i n e s , Sarawak, Sabah.

and

PA LAEOZOI C Lower Palaeozoic Thailand arid Malaysia The o l d e s t known c h e r t i n SE Asia, Ordovician-Silurian age, occurs i n micgeosynclinal s h e l f facies rocks i n NW Peninsular Malaysia and peninsular Thailand (Jones, 1973). These rocks are m a i n l y limestone w i t h sequences o f black carbonaceous s i l t s t o n e , shale, and chert. Micgeosynclinal basin f a c i e s rocks found i n NW Peninsular Malaysia and peninsular Thailand are S i l u r i a n t o Devonian i n age, and i n c l u d e carbonaceous shale, s i l i c e o u s mudstone, f l a g g y shale, s i l t s t o n e , a r g l l l i t e , s i l i c e o u s shale, and carbonaceous c h e r t w i t h minor amounts of q u a r t z i t e , metagraywacke, c a l c - s i l i c a t e rocks, and limestone. The Ordovician-Silurian c h e r t i n Kedah i n NW Peninsular Malaysia occurs i n a well-bedded sequence o f fine-grained, dark-gray t o black a r g i l l a c e o u s and s i l i c e o u s r o c k s w i t h i n d i v i d u a l beds varying from 2.5 t o 7.5 cm t h i c k . The c h e r t i s composed o f a l t e r e d remains o f Foraminifera and R a d i o l a r i a i n a f i n e - g r a i n e d mosaic o f i n t e r l o c k i n g quartz grains. Chemical analyses of 5 c h e r t samples g i v e an average composition o f 95.2% Si02, 1.77% A1203,

1.06% Fe203,

0.03% P205,

0.02% FeO,

0.07% MgO,

0.17% CaO,

0.07% Ti02,

and t r a c e t o 0.22% MnO (Courtier, 1974).

Eugeosynclinal rocks t h a t range i n age f r a n a t l e a s t t h e Lower Devonian i n t o t h e Carboniferous occur i n c e n t r a l Peninsular Malaysia. These r o c k s i n c l u d e conglanerate, q u a r t z i t e , graywacke, shale, p h y l l i t e , schist, s i l i c e o u s shale, r a d i o l a r i a n chert, and minor limestone. The c h e r t i s dark g r a y t o black, canmonly w e l l bedded, i n places i n t e n s e l y folded, and contains poorly-preserved R a d i o l a r i a moulds i n a c r y p t o c r y s t a l l i n e quartz m a t r i x c u t by v e i n l e t s o f m i c r o c r y s t a l l i n e quartz and f i n e p a r t i n g s o f carbonaceous m a t e r i a l (Jaafar, 1976).

a3

Upper Palaeozoic Indochina Upper Palaeozoic c h e r t i n Indochina i s of Devonian t o E a r l y I n eastern Laos and c e n t r a l Vietnam, geosynclinal Carboniferous age. rocks o f E i f e l i a n t o D i n a t i a n (Middle Devonian t o E a r l y Carboniferous) age i n c l u d e shale t h a t passes upward i n t o q u a r t z i t e , limestone, and r a d i o l a r i t e (Workman, 1977). Further north, black s i l i c e o u s shale, r a d i o l a r i t e , marl, and meta-limestone occur. I n northern Laos and west o f Vientiane (Fig. 1), c h e r t and chalcedony beds occur w i t h i n limestone. I n southern Vietnam, between Nha Trang and Ho Chi Minh bedded r a d i o l a r i t e occurs i n shale and sandy shale sequences. I n southern Kampuchea, r a d i o l a r i t e , limestone, marl, and jasper occur as minor p a r t s o f a succession o f shale, sandstone, and conglomerate. I n western Kampuchea, t h e Upper Palaeozoic rocks are subdivided i n t o a lower group of m a r l and r a d i o l a r i t e , and an upper group o f shale, marl,

City,

and sandy limestone. Thatland I n Thailand, Upper Palaeozoic c h e r t i s found i n two widelyd i s t r i b u t e d formations o f S i l u r i a n t o Permian age. The Kanchanaburi Series ( ? S i l u r i a n t o Carboniferous ? ) c o n s i s t s o f predaninantly shale, s i l t s t o n e , sandstone, graywacke, arkose, t u f f w i t h i n t e r c a l a t i o n s o f 1951). limestone, and beds o f banded c h e r t (Brown & aJ., The Rat Buri Limestone (Carboniferous? t o Permian) contains t h i n l a y e r s and beds o f c h e r t a t t h e base. I n peninsular Thailand, Garson g t d. (1975) noted t h a t c h e r t y limestone occurs as i r r e g u l a r lenses and undulating beds o f black c h e r t up t o 10 cm t h i c k , 1imestone.

interbedded w i t h dark-gray m i c r i t i c

M a1ays ia I n NW Peninsular Malaysia, Lower Carboniferous c h e r t occurs as minor lenses and nodules i n a sequence o f thick-bedded sandstone and subordinate mudstone, and a t t h e base o f a massive limestone sequence (Gobbett, 1973).

North o f Kuala Lunpur,

minor c h e r t beds occur i n

metasedimentary rocks o f Devonian age (Gan, 1974). I n c e n t r a l Peninsular Malaysia, c h e r t occurs as t h i n beds and nodules i n a sequence o f shale, mudstone, marl, and limestone o f Carbo-Permian t o T r i a s s i c age (Richardson, 1950). Chert occurs interbedded w i t h m a i n l y a r g i l l a c e o u s rocks o f Permian age i n NE Peninsular Malaysia (Rajah, 1973).

a4

In Sarawak, chert occurs i n a sequence made u p predaninantly of recrystallised limestone w i t h minor shale beds of Late Carboniferous t o Chert occurs i n the limestone as nodular lenses and Early Permian age. discontinuous beds t o 2 m thick, and i n the shale as bedded chert sequences. The chert i s usually pale gray i n colour, b u t sane are yellow, p i n k , or Nodular chert, formed by replacement of limestone, reddish-brown. contains r e l i c t f o s s i l s i n a microcrystalline quartz matrix. Chert, away from the limestone, i s coarsely c r y s t a l l i n e w i t h quartz-lined vugs (Wilford, 1965). MESOZOIC Triassic Thailand In northern Thailand, c h e r t occurs i n marine Triassic rocks consisting of a basal conglanerate, shale, sandy shale, arkosic sandstone, graywacke w i t h intercalations of limestone, cherty limestone, and chert. The sequence is Skythian t o Carnian/Norian i n age (Baum & aJ., 1970). Ma lays i a In NW Peninsular Malaysia, Triassic bedded chert occurs i n a predani nantly sandstone-shal e sequence containing minor intercalations o f mudstone, s i l t s t o n e , conglanerate, and locally, porcelanite and argillaceous chert. The chert i s generally well bedded, w i t h a wide range of colours fran white t o gray t o red, and contains poorly-preserved Radiolaria i n a matrix o f cryptocrystalline quartz and chalcedony which i s c m o n l y recrystallised to a mosaic of intergranular quartz (Burton, 1970). In western Sarawak, bedded chert occurs a s a minor part of a sequence of predominantly feldspathic sandstone, shale, arkose w i t h subordinate amounts of t u f f , tuffaceous sandstone, conglanerate, and limestone, a l l of Late Triassic age. Singapore Bedded c h e r t occurs in a sequence of mudstone, shale, sandstone, The sedimentary rocks and conglanerate, locally metamorphosed t o schist. a r e i n t e r s t r a t i f i e d w i t h s p i l i t i c lava, t u f f , and d o l e r i t e of middle Triassic t o Early Jurassic age (Anonynous, 1976).

85

I nd one s i a In west-central Sunatra, i n the Padang Highlands, Triassic chert occurs i n a sequence of bathyal marl, siliceous shale, and t h i n beds of limestone. On Bangka and Billiton Islands, radiolarian c h e r t is i n t e r calated w i t h shale, sandstone, s i l i c i c volcanic rocks, minor limestone, and t h e i r contactmetamorphic equivalents. These rocks a r e a t l e a s t of Triassic age, and probably extend back t o Permian or even Carboniferous ages ( B m e l e n , 1949; Hamilton, 1979). P h i 1i p p i nes

Middle Triassic bedded chert w i t h intercalations of black s l a t e and tuff occurs on northern Palawan and adjacent islands (Hashimoto and Sato, 1973). Jurassic-Cretaceous Ma1aysia In Sarawak, small amounts of chert occur as irregular nodules w i t h i n massive limestone of Late Jurassic t o Early Cretaceous age. Radiolarian chert occurs interbedded w i t h shale i n a marine succession t h a t includes shale, mudstone, sandstone, and s i l t s t o n e w i t h lesser amounts of conglanerate, limestone, t u f f , and tuffaceous sandstone of Late Jurassic t o Late Cretaceous age. The chert i s well bedded, dark gray t o black, and contains sponge spicules and well-preserved Radiolaria i n a microcrystalline The Radiolaria fauna indicates a Late Cretaceous age quartz matrix. (Wilford, 1965). On the western t i p of Sarawak and north of Kuching, bedded chert occurs a s blocks and lenses i n a chaotic assemblage o f sandstone, phyl 19t e , congl aneratic and bouldery s l a t e , conglanerate, calc-si 1i c a t e hornfels, marble, t u f f i t e , basalt, and gabbro i n a s l a t y p e l i t i c matrix. The chert varies i n colour f r a n pale- t o green-gray, p i n k , maroon, and brown t o pale yellowish. Individual c h e r t beds a r e 2 to 7 un thick, canmonly well bedded, i n places folded and distorted, forming blocks or lenses mostly 15 t o 25 an, b u t occasionally t o 100 m thick. Associated w i t h the c h e r t are veins of manganese minerals and goethite. Much of the chert contains Radiolaria indicating a Jurassic t o Cretaceous age (Tan, 1980). Chert and porcelanite occur i n a sequence t h a t includes sandstone, volcanic a r e n i t e , shale, calcareous s i l t s t o n e and shale, limestone, marl,

86

and conglanerate associated w i t h volcanic breccia, agglanerate, basalt, spilite,and diabase. In eastern Sabah and i n minor outcrops i n central Sabah and on Pulau Banggi, the chert is canmonly bedded, i n places folded, and varies i n colour fran l i g h t green, dark gray, orange, red, reddishbrown, t o dark brown. Sane chert beds yielded Radiolaria fauna indicating an Early Cretaceous or younger age (Leong, 1974, 1977). Indonesia In SE Kalimantan, subduction melange involving Cretaceous sedimentary rocks, including radiolarian chert, occurs in several areas. Tn the Meratus Mountains, widely varied lithologies including polymict breccia, glaucophane s c h i s t , greenschi s t , peridoti t e , serpenti n i t e , deep-ocean radiolarian chert, and c l a s t i c and carbonate rocks bearing middle Cretaceous pelagic foraminifers, are chaotically intercalated i n a steeply-dipping canplex. In NE Kalimantan, radiolarian chert occurs w i t h serpentinite, peridotite, diabase, s p i l i t e , basalt, greenstone, and scaly clay i n a probable subduction melange formed d u r i n g Cretaceous time (Hamilton, 1979). In southern Sumatra, red radiolarian chert occurs i n a bathyal sequence of black, thin-bedded siliceous shale, tuffaceous sandstone, and limestone associated w i t h a l i t t o r a l volcanic f a c i e s of andesite The Cretaceous bathyal rocks occur a t Gumai and Garba and basalt. Mountains and a t Ratai Bay. In west Sumatra, near Padang, Upper Jurassic t o Lower Cretaceous c h e r t occurs i n a sequence which includes limestone, shale, sandstone, and conglanerate. The c h e r t forms the upper part of the sequence and i s overlain unconfonably by Cenozoic volcanic rocks (Yancey and A l i f , 1977). In central Java, radiolarian c h e r t occurs i n a melange t h a t also involves Upper Cretaceous and Palaeocene sedimentary rocks. Rock types i n the melange include greenschist, amphibolite, eclogite, serpentinite, peridotite, gabbro, pillow-basalt, red radiolarian c h e r t , red pelagic limestone, scaly clay with’ sheared lenses of graywacke, s i l t s t o n e , conglanerate, quartzite, and quartz porphyry (Ketner & aJ., 1976). In Sulawesi, i n the NE part of the S o u t h Arm, chert occurs i n a melange consisting of chaotically intermixed and broken sandstone, s i l t s t o n e , sheared shale, red chert w i t h limestone, ultramafic rocks, amphibolite, glaucophane-lawsonite s c h i s t , and greenschist. Farther west along the Sadang Fault, a small mass of up-faulted melange consists of sandstone, In the s i l t s t o n e , red chert w i t h limestone, and ultramafic rocks. Bantimala area, red radiolarian c h e r t interbedded w i t h lenses of schist-

87

pebble cong lanerate i s overlain concordantly by siliceous shale. Radiolaria i n the chert and i n the lower parts of the shale a r e of l a t e Albian (Late Cretaceous) age (Hamilton, 1979). On Bunguran (Natuna) Islands, strongly folded c h e r t and metasedimentary rocks of probable Mesozoic (Jurassic-Cretaceous?) age a r e assoclated w i t h intermediate to ultrabasic igneous rocks including d i o r i t e , gabbro, The c h e r t i s bedded, serpentinite, t u f f , and anphibolite i n a melange. 1 t o 10 an thick beds, flexured and folded, and i s composed o f microc r y s t a l l i n e quartz and some chalcedony (Haile, 1970). Philimines Chert occurs i n a well-defined eugeosynclinal f a c i e s consisting of graywacke, shale, chert, and s p i l i t i c lava. These Cretaceous t o Palaeocene rocks a r e widely distributed i n the Philippines. Miranda (1977) described the rocks as a thick sequence of typically eugeosynclinal rocks consisting of chert, s p i l i t i c basalt and andesitic flows, graywacke, and cherty and o o l i t i c limestone. The chert and minor jasper range fran dark reddish t o chocolate-brown i n colour and a l t e r n a t e w i t h graywacke and shale. Sane o f the chert exhibits flexural chevron-like folding. In the southern Philippines, these eugeosynclinal rocks a r e considered t o be melanges (Hamilton, 1979). In eastern and NE Mindanao, chert and manganiferous jasper occur w i t h peridotite, serpentinite, gabbro, amphi bolite, diabase, basalt, red ferruginous shale, graywacke, and siltstone. On Panay, the subduction complex canprises polymict melange i n which a scaly clay matrix contains blocks of variegated chert, s c h i s t , serpentinite, gabbro, volcanic and metavolcanic rocks, and Tertiary c l a s t i c sedimentary rocks and llmestone (Hamilton, 1979). TERTIARY Ma 1ay sia Lower Cretaceous chert occurs as blocks i n melange of Tertiary age i n the Lupar Valley i n west Sarawak. The chert i s canmonly bedded, occasionally massive, comonly sheared, fractured, and i n places folded. The colour of the chert varies from white t o greenish-gray t o reddishbrown. Other rock types occurring as blocks i n the pervasively sheared pel i t i c matrix include mudstone, sand stone, shale, graywacke, hornf el s , conglomerate, limestone, basalt, s p i l i t e , gabbro, and serpentinite. This melange probably extends eastwards into Kalimantan (Tan, 1978, 1979).

88

In Sabah, Cretaceous t o Miocene chert occurs a s blocks i n chaotic These deposits a r e widespread i n eastern Sabah deposits of Miocene age. b u t minor i n western Sabah. The c h e r t i s mainly radiolarian c h e r t , canmonly bedded, i n places folded and sheared, and varies fran white, reddish-brown, t o red i n colour. Many of the c h e r t blocks were probably derived from the older Cretaceous-Eocene c h e r t - s p i l i t e sequence. Associated w i t h the chert i n the chaotic deposits a r e blocks of t u f f , tuffaceous sedimentary rocks, mudstone, sandstone, limestone, and volcanic rocks (Leong, 1974). Indonesia Melanges of Tertiary age a r e found on the Mentawai Islands west of Sumatra. On Nias Island, blocks i n the melange include c h e r t , red shale, 1imestone, sandstone, pebble cong lanerate, p i 1low-basal t, serpenti n i te, and garnet amphibolite. The matrix i s pervasively sheared s i l t s t o n e and mudstone. The chert is red, layered, and contains sparse, poorlypreserved, quartz-filled Radiolaria moulds i n a matrix of microcrystalline quartz (Moore and Karig , 1980). In the Banda Arc, Tertiary subduction melanges and imbricated canplexes a r e found on Timor, Seram, Buru, Roti, Savu, Kai, Tanimbar, and Babar. On Timor, rock types include deep- and shallow-water sedimentary rocks of ages from Permian t o Quaternary, including Cretaceous radiolarian chert, metamorphic rocks, ophiolitic rocks, continental crys t a l l i n e rocks, and others (Hamilton, 1979). Similar rock assemblages w i t h blocks of Jurassic to Cretaceous radiolarian chert and other rock types of various ages fran Permian t o Miocene a r e found on the other islands i n the arc. In east-central Sulawesi and the East and Southeast Arms, ophiolite, melange, and imbricated sedimentary and metamorphic rocks resulting f r m The metamorphic rocks subduction during Tertiary times a r e exposed. consist o f blueschist-facies rocks juxtaposed chaotically w i t h anphibolite, greenschist, phyllite, urinetanorphosed t o metamorphosed pelagic chert, and limestone. The sedimentary and metasedimentary rocks, varying i n ages fran Triassic and Jurassic t o Tertiary, include red and gray chert, limestone, shale, sandstone, s c h i s t , phyllite, s l a t e , quartzite, and pimontite-bearing metachert. Small masses o f Mesozoic and Tertiary limestone, marl, chert, and siliceous shale a r e tectonically intercalated w i t h the ophiolite (Hamilton, 1979). In the northern Moluccas, red radiolarian and brown c h e r t occur w i t h ophiolite and pelagic sedimentary rocks of Jurassic t o Palaeocene

89

age together w i t h shallow-water Oligocene t o Miocene s t r a t a i n subduction canplexes on Halmahera, Obi, Waigeo, and Karakelanj rslands (Hamllton,

1979).

P h i 1i D D i nes Chert beds, as much as 10 m t h i c k , occur i n a sequence o f i n t e r c a l a t e d b a s a l t i c flows, v o l c a n i c wackes, t u f f breccia, and limestone o f e a r l y Miocene age, i n the Caramines Norte province i n southern Luzon (Miranda and Caleon, 1979). Chert o f v o l c a n i c o r i g i n occurs w i t h keratophyre and andesite f l o w s o f Oligocene age i n the Nueva E c i j a province i n c e n t r a l Luzon (Antonia, 1976). On southern Palawan, ferruginous and manganiferous r a d i o l a r i t e and c h e r t occur together w i t h s e r p e n t i n i t e , p e r i d o t i t e and other u l t r a m a f i c rocks, gabbro, diabase, q u a r t z - d i o r i te, p i 1low-basal t, s p i l ite, and Palaeogene c l a s t i c sedimentary rocks i n a melange and imbricated canplex (Ham i1ton, 197 9). ACKNOWLEDGEMENTS The w r i t e r expresses h i s sincere thanks t o D r James R. Hein, U.S. Geological Survey, P r o j e c t Leader o f I.G.C.P. No. 115 and Prof. Azuna I i j i m a , U n i v e r s i t y o f Tokyo, f o r suggesting t h e o r i g i n a l idea f o r t h i s paper and p r o v i d i n g t h e o p p o r t u n i t y t o present it. G r a t e f u l thanks are due t o D r J.R. Hein, U.S. Geological Survey, Prof. P.H. S t a u f f e r , U n i v e r s i t y o f Malaya, and Prof. E.V.

Tamesis, U n i v e r s i t y o f P h i l i p p i n e s ,

f o r t h e i r c r i t i c a l reviews o f t h e manuscript. The data f o r t h i s paper a r e f r a n p r e v i o u s l y published sources and n o t f r a n unpublished personal observations i n the f i e l d . T h i s paper i s published w i t h t h e permission o f t h e Director-General , Geological Survey o f Malaysia. Research canpleted i n conjunction w i t h I.G.C.P. P r o j e c t 115, S i l i c e o u s Deposits o f t h e P a c i f i c Region. REFERENCES Anonymous, 1976. Geology o f the Republic o f Singapore. Public Dept., Singapore, 79p. Geology and mineral resources o f Nueva E c i j a Antonia, L.R., 1976. Bureau of Mines, Manila, Rept I n v e s t i g a t i o n No. 80, 17p. Baum, F., V. Braun, E., Hess, A., Koch, K.E., Kruse, G., Quarch, H., Siebenhuner, M., 1970. On the geology o f northern Thailand. Geol. J b. , 102: 23p. 1949. The geology o f Indonesia, 1A: Thnldague, Bemnelen, R.W.V., P r i n t i n g Office, 732p.

Works Province. and Beih. Govt.

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Brown, G.F., Buravas, S., Charaljavanaphet, J., Jalichandra, N., Johnston, W.D., Jr., Sresthaputra, V., and Taylor, G.C., Jr., 1951. Geologic reconnaissance o f t h e m i n e r a l deposits o f Thailand. U.S. Geol. Survey Bull., 9843183~. The geology and mineral resources o f the Baling area, Burton, C.K., 1970. Kedah and Perak. Geol. Survey Malaysia D i s t r i c t Mem., 12:150p. Courtier, D.C., 1974. The geology and mineral resources o f the neighbourhood o f Kulim, Kedah. Geol. Survey Malaysia Map B u l l . , 3:50p. Geol. Geology o f the Tanjong Malim area, Sheet 76. Gan, A.S., 1974. Survey Malaysia Ann. Rept 1973: 118-126. and others, 1975. The geology o f t h e t i n b e l t i n Peninsular Garson, M.S., Thailand around Phuket, Phangnga and Takua Pa. Overseas Mem. I n s t . Geol. Sci., No. 1:112p. Gobbett, D.J., 1973. Upper Palaeozoic. I n : D.J. Gobbett & C.S. Hutchison ( E d i t o r s ) , Geology o f t h e Malay Peninsula: West Malaysia and Singapore. John Wiley-Interscience, New York, 61-95. Notes on the geology o f t h e Tambelan, Anambas and Haile, N.S., 1970. Bunguran (Natuna) islands, Sunda Shelf, Indonesia, i n c l u d i n g r a d i o m e t r i c age determinations. United Nations ECAFE, CCOP. Tech. Bull., 3:55-89. Tectonics o f the Indonesian Region. U.S. Geol. Hamilton, W., 1979. Survey Prof. Paper 10783345~. Geological s t r u c t u r e o f North Palawan, Hashimoto, W., and Sato, T., 1973. Geol. and i t s bearing on the g e o l o g i c a l h i s t o r y o f t h e P h i l i p p i n e s . Palaeont. southeast Asia, 13: 145-161. The geology and m i n e r a l resources o f t h e Karak and Jaafar, A., 1976. Geol. Survey Malaysia D i s t r i c t Mem., ll:138p. Temerloh areas, Pahang. Jones, C.R., 1973. Lower Palaeozoic. In: D.J. Gobbett & C.S. Hutchison (Editors), Geology o f t h e Malay Peninsula: West Malaysia and Singapore. John Wiley-Interscience, New York, 25-60. Pre-Eocene rocks o f Java, Indonesia. Ketner, K.B., and others, 1976. U.S. Geol. Survey J. Res., 4:605-614. 1974. The geology and m i n e r a l resources o f t h e upper Segama Leong, K.M., V a l l e y and Darvel Bay area. Geol. Survey Malaysia Mem., 4 (revised): 354p. New ages from r a d i o l a r i a n cherts o f t h e c h e r t - s p i l i t e Leong, K.M., 1977. formation, Sabah. Geol. Soc. Malaysia Bull., 8:109-111. Mlranda, F.E., 1977. Geological-gecchemical survey o f Caramoan Peninsula, Caramines Sur. Bureau o f Mines, Manila, Rept I n v e s t i g a t i o n No. 86, 69p. Geology and m i n e r a l resources o f Miranda, F.E., and Caleon, P.C., 1979. Carmines Norte and p a r t o f Quezon Province. Bureau o f Mines, Manila, Rept I n v e s t i g a t i o n No. 94, 101p. S t r u c t u r a l geology o f Nias Island, 1980. Moore, G.F., and Karig, D.E., Indonesia: I m p l i c a t i o n s f o r subduction zone tectonics. h e r . J. Sci., 280:193-223. Geology and mineral resources o f the U l u L e b i r area o f 1973. Rajah, S.S., Trengganu, Kelantan and North Pahang. Geol. Survey Malaysia Ann. Rept 1972: 108-110. Ranneft, T.S.M., Hopkins, R.M., Jr., F r o e l i c h , A.J., and Gwinn, J.W., 1960. her. Reconnaissance geology and o i l p o s s i b i l i t i e s o f Mindanao. Assoc. P e t r o l . Geol. Bull., 44:529-568. Geology and mineral resources o f t h e neighbourhood Richardson, J.A., 1950. Geol. Survey Dept. Fed. Malaya o f Chegar Perah and Merapoh, Pahang, Mem., 43162~. Tan, D.N.K., 1978. Lower Cretaceous age f o r t h e c h e r t i n t h e Lupar Valley, west Sarawak. Geol. SOC. Malaysia Newsletter, 4:173-176. Tan, D.N.K., 1979. Lupar Valley, west Sarawak, Malaysia. Geol. Survey Malaysia Rept 13: 159p. 1980. S i l i c e o u s deposits ( c h e r t ) o f Malaysia. Geol. Survey Tan, D.N.K., Malaysia Geol. Papers 3: 100-113.

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Wilford, G.E. , 1965. Penrissen Area, west Sarawak, Malaysia. Geol. Survey Malaysia Rept 3: 195p. Workman, D.R., 1977. Geology o f Laos, Cambodia, South Vietnam and t h e eastern p a r t o f Thailand. Overseas Geol. Miner. Resour. No. 50, 33p. Yancey, T.E. , and A l i f , S.A. , 1977. Upper Mesozoic s t r a t a near Padang, west Sumatra. Geol. SOC. Malaysia B u l l . , 8:61-72.

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CHAPTER 7 CHERT-BEARING FORMATIONS OF NEW ZEALAND

P.R. N.Z.

MOORE Geological Survey, DSIR, Lower H u t t , New Zealand

ABSTRACT Chert occurs i n a v a r i e t y o f sedimentary formations, o f E a r l y Paleozoic t o E a r l y Cenozoic age. Cambrian and Ordovician c l a s t i c sequences i n c l u d e beds and lenses o f grey c h e r t and c h e r t y limestone. Bedded c h e r t s i n sequences o f t h e Permian-Jurassic Rangitata Orogen, and minor s i l i c e o u s rocks i n E a r l y Cretaceous formations, a r e c l o s e l y associated w i t h submarine ( s p i l i t i c ) v o l c a n i c rocks and red-green mudstone. S i l i c a may have been p a r t l y d e r i v e d from v o l c a n i c sources. L a t e Cretaceous mudstone and shale c o n t a i n r a r e c h e r t nodules. L e n t i c u l a r bedded, d o l o m i t i c c h e r t o f t h e Mead H i l l Formation (Maastrichtian-Danian), a basal member (up t o 300 m t h i c k ) o f t h e Amuri Limestone, shows evidence o f replacement o f b o t h quartzose sandstone and m i c r i t i c limestone. The s i l i c a i s most l i k e l y b i o g e n i c . Chert lenses and nodules a r e common i n p a r t s o f t h e Amuri, and l i t h o l o g i c a l l y e q u i v a l e n t E a r l y T e r t i a r y limestones. Permian-Jurassic c l a s t i c sequences a r e i n t e r p r e t e d , i n p a r t , as submarine f a n deposits, w h i l e Late Cretaceous-Early T e r t i a r y sediments probably accumulated i n an extensive, moderately shallow basin. INTRODUCTION New Zealand's g e o l o g i c a l r e c o r d i n c l u d e s c h e r t - b e a r i n g sedimentary sequences o f Cambrian, Ordovician, Permian-Jurassic, L a t e Cretaceous-Early T e r t i a r y ages ( F i g s 1, 2).

E a r l y Cretaceous , and

Recent s t u d i e s on t h e

Ordovician (Cooper, 1979), on Mesozoic r a d i o l a r i a (Feary and H i l l , 1978; Feary and Pessagno, 1980), and on m i n e r a l i z a t i o n associated w i t h bedded c h e r t s i n t h e Permian-Jurassic Rangitata Orogen (Stanaway e t a l . ,

1978) have

c o n t r i b u t e d s u b s t a n t i a l l y t o our knowledge o f s i l i c e o u s rocks i n these sequences.

L a t e Cretaceous-Early T e r t i a r y c h e r t s , however, have been

l a r g e l y overlooked s i n c e Thomson's (1916) e a r l y r e p o r t on t h e ' F l i n t beds' i n Marlborough, although some work i s i n progress.

Study o f these c h e r t s

should prove u s e f u l i n understanding t h e d i a g e n e t i c h i s t o r y o f economically i m p o r t a n t L a t e Cretaceous formations. This review o f c h e r t s and c h e r t - b e a r i n g formations i n New Zealand i s presented as a c o n t r i b u t i o n t o IGCP P r o j e c t 115 " S i l i c e o u s d e p o s i t s o f t h e P a c i f i c region".

P r e v i o u s l y unpublished data obtained by t h e w r i t e r and

o t h e r s has been i n c o r p o r a t e d where p o s s i b l e .

CAMBR IAN-ORDOV IC I AN The Early-Mid Cambrian B a l l o o n Formation i n northwest Nelson (Fig. 2 ) c o n s i s t s m a i n l y o f graded beds o f a r k o s i c sandstone and s i l t s t o n e w i t h conspicuous bands and lenses (up t o 50 m + t h i c k ) o f dark grey, laminated

94

F i g . 1. Chert-bearing formations i n North I s l a n d , New Zealand, and l o c a t i o n o f more important c h e r t occurrences ( s p e c i f i c l o c a l i t i e s mentioned i n t h e t e x t i n d i c a t e d by c i r c l e s ) . Compiled from various sources, i n c l u d i n g R i d d o l l s ( i n press) and S p o r l i (1978). For enlargement o f Hunua Range area see F i g . 3.

95

LATE CRETACEOUS-EARY TERTIARY

1m .PERMIAN- IURASSIC

.

I y h , M . l v H u I*rhrt*(plit.

CAMBRIAN

_ -- -

96

Black Shale; Cooper, 1979). The chert i s f i n e l y laminated and forms discrete layers 3-10 cm thick. SEM photographs reveal a "novacu1ite"type cryptotexture w i t h small organic ( ? ) microstructures composed of framboidal pyrite. Cherts contain 72-80% quartz, 5-12% i l l i t e , and up t o 8-10% pyrite. Cooper (1979) concluded t h a t the Anthill Shale was probably deposited i n a starved offshore basin, a t bathyal depths, and t h a t perhaps 25 t o 40% of the s i l i c a was contributed from biogenic sources. PERMIAN-JURASSIC (Rangi t a t a Orogen) The Rangitata Orogen consists of indurated and metamorphosed sedimentary rocks of the Late Paleozoic-Mesozoic New Zealand Geosyncline. A variety of stratigraphic, s t r u c t u r a l , and compositional subdivisions have been proposed (e.g. Andrews e t a l . , 1976; Carter e t a l . , 1978), although basically western sequences (Maitai , Murihiku terranes) a r e volcaniclastic and relatively simply deformed, while eastern parts of the Orogen (Torlesse Terrane) a r e predominantly quartzo-feldspathic, and strongly deformed. The CaplesPelorus, Waipapa, and Haast Schist terranes l i e between these two sequences (Figs 1, 2 ) . Bedded chert i s closely associated w i t h red and green a r g i l l i t e and s p i l i t i c volcanic rocks in the Waipapa, Caples and Torlesse terranes, and melanges contain blocks of these lithotypes. Chert i s extremely rare i n the Mai t a i and lluri h i ku terranes. Waipapa Terrane ( ? Permian-Jurassic) Bedded manganiferous chert i s comnon i n the northern part of the Waipapa Terrane (Fig. 1). In Northland, units of green, grey and red-brown chert a r e interbedded with massive sandstone, alternating sandstone and a r g i l l i t e , and concretionary mudstone. Chert i s thin-bedded and comnonly alternates with o r grades into siliceous a r g i l l i t e . On the Cavalli Islands chert i s stratigraphically overlain (and intruded) by pillow lava containing lumps of coral (Moore and Ramsay, 1979). Siliceous rocks a t Marble Bay contain radiolaria and a1 tered glass shards (Sporl i and Gregory, 1981). Chert a l s o occurs a s c l a s t s in "red m6lange" (sheared red mudstone with c l a s t s of various lithologies) along w i t h blocks o f f o s s i l i f e r o u s limestone and marble, a t Marble Bay and eastern Bay of Islands (Sporli and Gregory, 1981; Moore, i n press). In the east Auckland area units of l i g h t grey mudstone w i t h interbedded red mudstone, chert, and minor s p i l i t e form part of a c l a s t i c sequence more than 8.5 km thick (Schofield 1974, 1976, 1979). These units a r e separated

91

Fig. 3. Major sedimentary facies of the Waiheke Group (Waipapa Terrane) in the Hunua Range area, e a s t of Auckland (see Fig. 1 f o r location). Adapted from Schofield (1976, 1979). by thick sequences of alternating sandstone and mudstone beds and mudstone, and locally transgress across d i f f e r e n t c l a s t i c facies (Fig. 3). Chert constitutes < 10%of the "red bed" association in many areas, even though some chert lenses a r e u p t o 90 m thick.

98

Chert and interbedded s i l i c e o u s a r g i l l i t e c o n t a i n r a r e s i l t - s i z e d d e t r i t a l g r a i n s o f quartz, f e l d s p a r , e p i d o t e and mica; disseminated Mn oxides and s i l i c a t e s , hematite, g e o t h i t e , c h l o r i t e , c a l c i t e and sulphides; r e c r y s t a l l i s e d r a d i o l a r i a n t e s t s ; and a few o o l i t e s (0.01-0.3

mm) composed

o f b r a u n i t e and bemenite (Mayer, 1969; Stanaway e t a1 . , 1978).

Torlesse Terrane (Permian-Jurassic) I n eastern North I s l a n d ( F i g . 1) mdlanges c o n t a i n c h e r t blocks w i t h w e l l preserved e a r l y J u r a s s i c r a d i o l a r i a , e.g.

Waioeka Gorge (Feary and H i l l ,

1978; Feary and Pessagno, 1980), and l a r g e lenses o f r e d bedded c h e r t and v o l c a n i c rocks, e.g.

Ruahine Range ( S p o r l i and B e l l , 1976).

a 15 m-thick band o f thin-bedded,

Near Eketahuna

red-brown r a d i o l a r i a n c h e r t w i t h lenses o f

b i o c l a s t i c limestone and v e s i c u l a r t o amygdaloidal s p i l i t e i s i n d e p o s i t i o n a l c o n t a c t w i t h surrounding c l a s t i c rocks (Neef, 1974; pers. obs.). A t Red Rock P o i n t , Wellington, red, green, and grey a r g i l l i t e w i t h beds o f c h e r t i n t h e upper p a r t a r e conformably o v e r l a i n by p i l l o w l a v a (Wellman, 1949).

A l t e r n a t i n g sandstone and a r g i l l i t e a r e i n sedimentary contact.

Cherts c o n t a i n r a d i o l a r i a n casts, f i n e l y disseminated hematite, and minor z o i s i t e ; interbedded a r g i l l i t e s have a small v o l c a n i c component (Reed, 1957, 1958). I n t h e South I s l a n d ( F i g . 2 ) "volcanogenic a s s o c i a t i o n s " (Bradshaw, 1972) c o n s i s t i n g o f p i l l o w lava, p y r o c l a s t i c rocks, ferruginous-manganiferous c h e r t , and limestone a r e up t o 300 m t h i c k (e.g.

Cavendish H i l l s ) .

Silica

has l o c a l l y replaced lava, t u f f , and limestone, and s i l i c e o u s rocks c o n t a i n minor rhodonite, Mn oxides, and m a l a c h i t e (e.g. Malvern H i l l s , Speight, 1928).

A t Broken R i v e r a c l a s t i c sequence i n c l u d e s 30 m o f interbedded

c h e r t and shale (Andrews, 1974).

The c h e r t c o n t a i n s small e l l i p t i c a l patches, o f p o s s i b l e organic o r i g i n , and a l t e r e d glass shards. "Volcanogenic a s s o c i a t i o n s " a r e r a r e i n t h e western p a r t o f t h e Torlesse

Terrane, although beds o f r e d a r g i l l i t e ( " r e d beds") occur l o c a l l y (e.g. Lord Range, Andrews e t a l . ,

1974).

I n t h e L i e b i g and B u r n e t t Ranges, " r e d

beds" i n c l u d e minor w h i t e and green bedded c h e r t , a r e i n d e p o s i t i o n a l c o n t a c t w i t h surrounding c l a s t i c rocks, and c o n t a i n t h i n l a y e r s o f c u r r e n t bedded f i n e sand ( S p o r l i e t a l . ,

1974).

Metavolcanic rocks ( g r e e n s c h i s t s ) and meta-chert occur i n t h e Haast S c h i s t Terrane t o t h e west and south o f t h e Torlesse Terrane. Caples Terrane (Permian-Triassic) I n t h e Thomson Mountains p i l l o w l a v a and v o l c a n i c b r e c c i a u n i t s w i t h c h e r t , grey claystone, and r e d and green mudstone a r e 20 t o 100 m t h i c k

99

A t one locality bedded chert conformably overlies pillow lava and grades up into thin-bedded sandstone. Pods of limestone i n the chert contain fragments of the bivalve Atornodesrna. Volcanic units in the Humboldt Mountains include minor chert and manganiferous and ferruginous nodules (Bishop e t a l . , 1976). and can be traced l a t e r a l l y f o r u p t o 1 km (Turnbull, 1979).

Origin of the chert Stanaway e t a l . (1978) favoured a volcanic origin f o r chert, siliceous a r g i l l i t e and manganese deposits in the Waipapa Terrane, because of t h e i r localisation and proximity to submarine lavas. Siliceous sediments were, they suggest, precipitated out of sea-water enriched in S i , Al, K, Mn, and Fe from submarine volcanic exhalations; only minor amounts of clay were derived from in s i t u weathering of submarine t u f f s , d e v i t r i f i c a t i o n of volcanic glass and t e r r e s t r i a l sources. Reed (1957) reached a similar conclusion. Other workers have regarded the cherts as pelagic, deep-sea deposits (e.g. Sporli e t a l . , 1974; Sporli, 1975, 1978; Carter e t a l . , 1978; Sporli and Gregory, 1981), presumably because of the presence of radiolaria, and virtual absence of t e r r e s t r i a l d e t r i t u s . A biogenic origin f o r the s i l i c a i s implied. Andrews (1974) considered a tuffaceous origin was l i k e l y f o r the chert a t Broken River . Environment of deposition Some workers have suggested t h a t much of the Rangitata sequence was deposited in submarine fans in an arc-trench-ocean basin s e t t i n g (Carter e t a1 ., 1978; Turnbull, 1979; Howell, 1981). Andrews e t a l . (1976) considered that eastern parts of the Torlesse Terrane in the South Island, where c h e r t - s p i l i t e associations a r e relatively common, were deposited under shallow marine conditions. Apparently deeper water sediments t o the west contain only minor s p i l i t e and chert. The occurrence of bioclastic limestone and highly vesicular o r amygdaloidal pillow lava in "volcanogenic associations" suggests t h a t contemporaneous chert was deposited a t depths of l e s s than 4-5 kin, and probably much shallower (Moore, 1970). Volcanic rocks near Eketahuna could have been extruded into water l e s s than 500 m deep. Sporli (1978) and Sporli and Gregory (1981) argue t h a t the vesicular lavas and shallow-water limestones locally associated with them represent the upper parts of volcanic "highs", originally located a t mid-ocean ridges

100

or other oceanic sites, which were covered by deep-sea sediments (chert and red-green mudstone) before being tectonically emplaced into the clastic greywacke sequence along a subduction zone. This hypothesis assumes that the chert and red mudstone represent deep-sea oozes, for which there is little evidence. The main arguments for tectonic emplacement of volcanic associations are (e.g. Sporli, 1975): (a) occurrence of spilite, chert, and limestone blocks in m6langes; (b) ages different from the enclosing clastic sediments; and (c) lack of feeder dikes. These aspects could also be explained if the "m6langes" were originally olistostromes. Age differences between chertspilite associations (or bedded cherts) and enclosing strata have yet to be conclusively demonstrated (other than in melanges, e.g. Feary and Pessagno, 1980), and the lack of feeder dikes is somewhat tenuous evidence for tectonic emplacement. In the comparable Early Cretaceous sequence (Moore and Speden, 1979) dikes are rare, and very thin. EARLY CRETACEOUS (A1bian) Tupou Formation, in Northland, includes basic volcanic rocks with minor green to black chert (Hay, 1975; Fig. 1). At East Cape vitric tuff, argillaceous tuffite, and minor chert are interbedded with volcanic rocks of a spilite-keratophyre association in the Mokoiwi Formation (Speden, 1976; Pirajno, 1979). Tuffaceous sedimentary beds contain sparse radiolaria. The Pahaoa Group in Wairarapa includes common "red beds" (up to 8 m thick) composed of red-brown mudstone, argillite, and chert (Moore and Speden, 1979). Sills and flows of spilite, basalt, and dolerite are locally associated with the red beds, which are in depositional contact with enclosing flysch-like strata. In view of the close relationship between "red beds" and igneous rocks in many areas, it is tempting to assume a volcanic origin for the chert and associated sediments. However, the presence of sparse radiolaria in some cherts indicates that a biogenic origin for the silica is also possible. Environment o f deposition Moore and Speden (1979) considered that the Early Cretaceous sequence (Pahaoa Group) in Wairarapa was probably deposited in a fan-delta or submarine fan setting, in a rapidly subsiding basin, or fault-angle depression. The vesicle content of intercalated lavas indicates eruption at water depths of between 200 m and 700 m (C.P. Wood, pers. corn.), consistent with paleontological and other evidence. Speden (1976) concluded that Mokoiwi Formation was deposited in an outer shelf-upper slope environment.

101

Very rapid c l a s t i c sedimentation in a submarine fan could explain why "red beds" are rarely more than 2 t o 3 m thick, and why chert represents only a minor constituent of such units. LATE CRETACEOUS - EARLY TERTIARY North Island Early Tertiary formations i n Northland include green and chocolate shales w i t h chert lenses (Hay, 1960). In the Aponga Shale, lenses of mauve o r chocolate-coloured chert a r e u p to 30 cm thick. Siliceous shale and calcareous mudstone in the East Cape region (Rakauroa Formation and Mangatu Group) , and hard, non-calcareous t o calcareous mudstone (Whangai Formation) in southern Hawke's Bay and Wairarapa contain rare chert nodules and lenses. The Whangai Formation has been variously described as a siliceous a r g i l l i t e or shale, r h y o l i t i c t u f f (Kingma, 1971), and chert (Kingma, 1974). In the Whangai Range small black chert nodules w i t h d e t r i t a l quartz and recrystall ised radiolaria a r e r e s t r i c t e d t o the upper, calcareous part of the formation. Thin-bedded mudstone a t Mara contains lenses and large nodules of chert, dolomitic chert, and dolomite. Chert a l s o occurs in lithologic equivalents of the Amuri Limestone in southeast Wai rarapa.

South Island The Woolshed, Herring ( i n p a r t ) , and Mirza formations i n Marlborough are l a t e r a l equivalents of the Whangai Formation ( F i g . 4). Isolated lensoid chert nodules a r e present in the middle and upper parts of the Mirza Formation (Lensen, 1978). In the Coverham-Kekerengu area ( F i g . 2 ) , large siliceous concretions occur near the top of the Woolshed Formation, above a prominent lens of hard, dolomitic cherty s i l t s t o n e (Prebble, 1976). Chert i s absent from the Herring Formation to the southwest (Reay, 1980), and occurs only near the contact with overlying Mead Hill Formation i n the southeast, e.g. a t Mororimu Stream. Mead Hill Formation The Mead Hill chert has previously been regarded e i t h e r as a basal member of the Amuri Limestone ( F l i n t Beds member; Thomson, 1916; Hall , 1970; Prebble, 1976) o r a separate formation (Webb, 1966; Lensen, 1978). The lower boundary w i t h Herring, Woolshed, and B u t t formations (Fig. 4 ) i s well-defined, b u t the upper boundary has been placed e i t h e r where the chert occurs as isolated lenses and nodules, rather than continuous beds (Hall, 1970; Prebble, 1976) , o r where Amuri Limestone becomes dominant

102

-

Fig. 4. Late Cretaceous - Early Tertiary (Maastrichtian Danian) chertbear.ing formations in NE Marlborough, South Island. Data from Webb (1966), Hal 1 (1970), Prebble (1976) , Lensen (1978) and personal observations. CTB = Cretaceous - Tertiary boundary. (Webb, 1966). The formation reaches a maximum thickness o f 250 t o 300 m in the Coverham-Kekerengu area, b u t thins rapidly t o the NE and SW. A t Mororimu Stream (Fig. 2 ) i t i s 80 t o 100 m thick. Mead Hill Formation consists of lenticular beds of dark grey, black, and brown chert with greensand and limestone partings i n the lower part, and interbedded limestone in the upper part ( F i g s 4 , 5 ) . Chert lenses are mostly 0.3 t o 1 m long, and 5 to 15 cm thick. Calcite and dolomite rhombs a r e comnonly concentrated around the margins of chert lenses, dispersed through the chert, o r form separate carbonate lenses. Basal parts of the formation generally consist of a t h i n quartzose o r glauconitic sandstone (greensand) overlain by bedded, lenticular chert w i t h greensand pockets o r partings, and dolomitic lenses. The Cretaceous-Tertiary boundary l i e s within Mead Hill Formation, and a t Woodside Creek coincides w i t h a disconformable contact between thick-bedded

103

Fig. 5. Mead H i l l Formation, Clarence V a l l e y , Marlborough (S.N. Beatus photo). L e n t i c u l a r c h e r t beds ( w h i t e t o dark g r e y ) a r e separated by t h i n greensand p a r t i n g s . limestone w i t h c h e r t lenses and thin-bedded sandy o r s i l t y limestone (Strong, 1977).

Lensen (1978) b e l i e v e d t h e boundary l a y c l o s e t o t h e change from

greensand t o limestone p a r t i n g s ( c f . Fig. 4).

To t h e southwest, Mead H i l l c h e r t passes l a t e r a l l y i n t o greensand.. the northeast, r e l a t i o n s a r e more obscure.

In

The 60 m-thick greenish limestone

104

and interbedded chert a t Wharanui Point i s regarded as part of the Mead Hill Formation by Webb (1966), b u t as a separate (older) unit - Wharanui Point Limestone - by Prebble (1976). Part of the Wharanui Limestone consists of well-bedded red-brown, grey, and green chert with interbedded white limestone and dolomite beds; the chert contains abundant dolomite rhombs, and Maastrichtian foraminifera (C.P. Strong, pers. comn.). Further north, three separate lenses of white, pale green, and p i n k calcareous chert i n the B u t t Formation (Lensen, 1978; Fig. 4 ) a r e tentatively correlated w i t h the Mead Hill Formation. Composition of the Mead Hill chert i s not well known. Thomson (1916) noted t h a t basal chert beds a t Coverham contain abundant angular grains of d e t r i t a l quartz, comnon mica, and scattered glauconite. Small rectangular areas of f i n e l y c r y s t a l l i n e s i l i c a were thought t o represent possible replacement of d e t r i t a l feldspar. No radiolaria, sponge spicules, or diatom f r u s t u l e s were observed by Thomson. A t Mororimu Stream, where the basal 20 m of the formation consists of interbedded grey-black chert and quartz-mica sandstone, the chert contains common d e t r i t a l grains. Amuri Limestone Lower parts of the Amuri Limestone (Lower Limestone Member of Hall, 1970; Prebble, 1976) - typically a white, well-bedded c a l c i l u t i t e : foraminiferal biomicrite - contain scattered l e n t i c u l a r nodules and lenses of grey t o brown chert, and s i l i c i f i e d sponges (e.g. Woodside Creek). A t Chancet Rocks lenses of green to black chert a r e comnon i n the lower (Maastrichtian) part of the limestone, and well-preserved sponges occur i n the upper (Danian) part. Adjacent t o chert nodules, foraminiferal tests i n the limestone show progressive i n f i l l i n g and replacement by s i l i c a . Elsewhere, chert lenses and nodules have been reported from west of Cape Campbell, Awatere Valley, Kaikoura Peninsula, and Haumuri Bluff. Chert i s absent i n southwest Marlborough (Reay, 1980) and further south, i n Canterbury. Rare l e n t i c u l a r chert Rodules occur in the Middle Limestone Member a t Coverham (Hall , 1970). Outer Islands Alluvial chert pebbles and cobbles on the Chatham Islands ( F i g . 1) were derived from various Early Tertiary formations, particularly the Takatika Grit, T u t u i r i Greensand, and Te Whanga Limestone (Hay e t a l . , 1970; H.J. Campbell, pers. comn.). The Early Tertiary Tucker Cove Limestone (Eocene-01 igocene) on Campbell

105

Island contains common chert nodule horizons (Beggs, 1978) Origin of the chert The Mead Hill Formation, and chert lenses and nodules i n the Amuri Limestone show c l e a r evidence of replacement. S i l i c a was probably largely derived from dissolution of siliceous organisms, including sponge spicules and radiolaria, during diagenesis. Overburden pressure may have been an important factor in diagenesis, since the Amuri Limestone a t t a i n s i t s greatest thickness in the Coverham-Kekerengu area (c. 750 m, Lensen, 1978) where the Mead Hill chert i s also thickest. Chert lenses and nodules in the older c l a s t i c sequence contain similar evidence of replacement. Dissolution o f radiolaria, d e v i t r i f i c a t i o n of acidic t u f f , and perhaps localised i n t r a s t r a t a l solution o f the quartzrich sediments a r e possible sources of s i l i c a . Environment o f deposition Kingma (1974) envisaged that Late Cretaceous and Early Tertiary sedimentary rocks i n eastern New Zealand were deposited in a single, elongate trough - the Eastern Geosyncline. The source area has generally been regarded as a much denuded, deeply-weathered landmass, able t o supply only s i l t and clay t o the sedimentary trough. Recent work suggests t h a t the Whangai Formation may have been deposited in a large, r e l a t i v e l y shallow (neritic-upper bathyal) and perhaps partly enclosed basin. The Mead Hill Formation contains a rich and diverse planktonic microfauna, suggesting open-water, offshore conditions during i n i t i a l deposition of the Amuri Limestone sequence; depths o f 200 t o 600 m a r e inferred (Strong, 1977). Danian s t r a t a a t Woodside Creek were probably deposited i n shallower water, perhaps 100 t o 200 m. I t seems likely t h a t Mead Hill sediments accumulated in a local depression within the main Late Cretaceous basin, or i n the central part of a smaller, subsidiary basin. Either way, the depositional s i t e was almost completely isolated from c l a s t i c sediment sources. There i s l i t t l e consensus on the overall depth of deposition of the Amuri Limestone, b u t deep water conditions have often been inferred (e.g. Prebble, 1976). h u r i Limestone in SW Narlborough was probably deposited i n a shelf environment (Reay, 1980). CONCLUSIONS In Permian-Jurassic c l a s t i c sequences of the Rangitata Orogen chert i s commonly closely associated w i t h s p i l i t i c volcanic rocks (pillow lava), and

106

a t l e a s t some s i l i c a may have been c o n t r i b u t e d from submarine volcanism. D i r e c t evidence f o r a b i o g e n i c o r i g i n , f o r deep-water d e p o s i t i o n , and f o r t e c t o n i c emplacement o f t h e bedded c h e r t s , i s l a c k i n g i n many areas. L a t e Cretaceous-Early T e r t i a r y c h e r t s a r e c l e a r l y d i a g e n e t i c i n o r i g i n , and a b i o g e n i c source f o r t h e s i l i c a i s i n d i c a t e d . ACKNOWLEDGEMENTS I thank H.J.

Campbell, R.A.

Cooper, G.W.

Grindley, J.C.

Schofield,

C.P.

Strong, and C.P.

C.A.

Landis and F1.B. R e a y f o r r e v i e w i n g t h e paper; and P a t r i c i a White f o r

typing.

Wood f o r i n f o r m a t i o n and discussion; P.B.

Andrews,

Figures were d r a f t e d by Maureen Haronga.

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Andrews, P.B., 1974. D e l t a i c sediments, Upper T r i a s s i c Torlesse Supergroup, Broken R i v e r , n o r t h Canterbury. N.Z. J. Geol. Geophys. 17: 881-905. Andrews, P.B., Bishop, D.G., Bradshaw, J.D., and Warren, G. 1974. Geology o f t h e Lord Range, c e n t r a l Southern Alps, New Zealand. N.Z. J. Geol. Geophys. 17: 271-299. and Bradshaw, J.D. 1976. L i t h o l o g i c a l and Andrews, P.B., Speden, I.G., p a l e o n t o l o g i c a l c o n t e n t o f t h e Carboniferous-Jurassic Canterbury Suite, South I s l a n d , New Zealand. N.Z. J. Geol. Geophys. 19: 791-819. Beggs, J.H., 1978. Geology o f t h e metamorphic basement and L a t e Cretaceous t o Oligocene sedimentary sequence o f Campbell I s l a n d , southwest P a c i f i c Ocean. J. Roy. SOC. N.Z. 8: 161-177. Bishop, D.G., Bradshaw, J.D., Landis, C.A., and T u r n b u l l , I.H. 1976. L i t h o s t r a t i g r a p h y and s t r u c t u r e o f t h e Caples t e r r a n e o f t h e Humboldt Mountains, New Zealand. N.Z. J. Geol. Geophys. 19: 827-848. Bradshaw, J.D., 1972. S t r a t i g r a p h y and s t r u c t u r e o f t h e Torlesse Supergroup ( T r i a s s i c - J u r a s s i c ) i n t h e f o o t h i l l s of t h e Southern Alps near Hawarden (S60-61), Canterbury. N.Z. J. Geol. Geophys. 15: 71-87. Carter, R.M., Hicks, M.D., N o r r i s , R.J., and T u r n b u l l , I.M. 1978. Sedimentation p a t t e r n s i n an a n c i e n t arc-trench-ocean b a s i n complex: Carboniferous t o J u r a s s i c Rangitata Orogen, New Zealand. I n D.J. Stanley and G. K e l l i n g (Eds.) Sedimentation i n submarine canyons, fans, and trenches. Dowden, Hutchinson & Ross, Stroudsburg, Pennsylvania. pp. 340-361. Cooper, R.A., 1979. Ordovician geology and g r a p t o l i t e faunas o f t h e Aorangi Mine area, northwest Nelson, New Zealand. N.Z. Geol. Surv. Pal. B u l l . 47. Feary, D.A. and H i l l , P.H. 1978. Mesozoic R a d i o l a r i a from c h e r t s i n t h e Raukumara Peninsula,.New Zealand. N.Z. J. Geol. Geophys. 21: 363-373. Feary, D.A. and Pessagno, E.A. 1980. An E a r l y J u r a s s i c age f o r c h e r t w i t h i n t h e E a r l y Cretaceous Oponae Mglange (Torlesse Supergroup) , Raukumara Peninsula, New Zealand. N.Z. J. Geol. Geophys. 23: 623-628. G r i n d l e y , G.W., 1980. Sheet S13 Cobb ( 1 s t E d i t i o n ) Geological Map o f New Zealand 1:63 360. D S I R Wellington, N.Z. H a l l , W.H.M., 1970. A geology o f Coverham and t h e upper Waima Valley, Marlborough. Unpublished manuscript. Hay, R.F., 1960. The geology o f Nangakahia Subdivision. N.Z. Geol. Surv. B u l l . 61. Hay, R.F., 1975. Sheet N7 Doubtless Bay ( 1 s t E d i t i o n ) . Geological Map o f New Zealand 1:63 360. DSIR, Wellington, N.Z. Hay, R.F., Hutch, A.R., and Watters, W.A. 1970. Geology o f t h e Chatham I s l a n d s . N.Z. Geol. Surv. B u l l . 83.

107 Howell, D.G., 1981. Submarine f a n f a c i e s i n t h e Torlesse t e r r a n e , New Zealand. J. Roy. SOC. N.Z. 11: 113-122. Kingma, J.T., 1971. Geology o f Te Aute Subdivision. N.Z. Geol. Surv. B u l l . 70. Kingma, J.T.,1974. The g e o l o g i c a l s t r u c t u r e o f New Zealand. John Wiley & Sons. 407 pp. Lensen, G.J., 1978. S t r a t i g r a p h y - Marlborou h. pp. 383-390, 482-488 I n R.P. Suggate, G.R. Stevens, M.T. Te Punga 7Eds.). The geology o f New Zealand. Govt. P r i n t e r , Wellington. 820 pp. Mayer, W., 1969. P e t r o l o g y o f t h e Waipapa Group near Auckland, New Zealand. N.Z. J. Geol. Geophys. 12: 412-435. Moore, J.G., 1970. \.later c o n t e n t o f b a s a l t erupted on t h e ocean f l o o r . C o n t r i b u t i o n s Min. Pet. 28: 272-279. Moore, P.R. ( i n press). Geology o f t h e Urupukapuka Motuarohia i s l a n d group, eastern Bay o f I s l a n d s , Northland. Tane 27. Moore, P.R. and Ramsay, W.R.H. 1979. Geology o f t h e C a v a l l i I s l a n d s , n o r t h e r n New Zealand. Tane 25: 41-59. Moore, P.R. and Speden, I.G. 1979. S t r a t i g r a p h y , s t r u c t u r e , and i n f e r r e d environments o f d e p o s i t i o n o f t h e E a r l y Cretaceous sequence, e a s t e r n Wairarapa, New Zealand. N.Z. J. Geol. Geophys. 22: 417-433. Neef, G., 1974. Sheet N153 Eketahuna ( 1 s t E d i t i o n ) Geological Map o f New Zealand 1:63 360. DSIR, Wellington, N.Z. P i r a j n o , F., 1979. Geology, geochemistry, and m i n e r a l i s a t i o n o f a s p i l i t e - k e r a t o p h y r e a s s o c i a t i o n i n Cretaceous f l y s c h , East Cape area, New Zealand. N.Z. J. Geol. Geophys. 22: 307-328. Prebble, W.M., 1976. The geology o f t h e Kekerengu-Waima R i v e r d i s t r i c t , n o r t h e a s t Marlborough. Unpublished M.Sc. t h e s i s , V i c t o r i a U n i v e r s i t y o f We1 1ington. Reay, M.B., 1980. Cretaceous and T e r t i a r y s t r a t i g r a p h y o f p a r t o f t h e m i d d l e Clarence V a l l e y , Marlborough. Unpublished M.Sc. t h e s i s , U n i v e r s i t y o f Canterbury. Reed, J.J., 1957. Petrology o f t h e Lower Mesozoic rocks o f t h e W e l l i n g t o n d i s t r i c t . N.Z. Geol. Surv. B u l l . 57. Reed, J.J., 1958. A d d i t i o n a l data on t h e v o l c a n i c a r g i l l i t e s from Red Rock P o i n t , Wellington. N.Z. J. Geol. Geophys. 1: 635-640. R i d d o l l s , P.M. ( i n p r e s s ) . Geological Map o f New Zealand 1:2 000 000 (2nd E d i t i o n ) . DSIR, Wellington, N.Z. S c h o f i e l d , J.C., 1974. S t r a t i g r a p h y , f a c i e s , s t r u c t u r e , and s e t t i n g o f t h e Waiheke and Manaia H i l l Groups, e a s t Auckland. N.Z. J. Geol. Geophys. 17: 807-838. Schofield, J.C., 1976. Sheet N48 Mangatawhiri ( 1 s t E d i t i o n ) Geological Map o f New Zealand 1:63 360. DSIR, Wellington, N.Z. Schofield, J.C., 1979. P a r t Sheets N38, N39, N42, N43 Waiheke ( 1 s t E d i t i o n ) Geological Map o f New Zealand 1:63 360. DSIR, Wellington, N.Z. Speden, I.G., 1976. Geology o f M t T a i t a i , Tapuaeroa Valley, Raukumara Peninsula. N.Z. J. Geol. Geophys. 19: 71-119. Speight, R., 1928. The geology o f t h e Malvern H i l l s . N.Z. Geol. Surv. Memoir 1; S p o r l i , K.B., 1975. Waiheke and Manaia H i l l Groups, e a s t Auckland: comment. N.Z. J. Geol. Geophys. 18: 757-760. S p o r l i , K.B. , 1978. Mesozoic t e c t o n i c s , North I s l a n d , New Zealand. Geol. SOC. h e r . B u l l . 89: 415-425. S p o r l i , K.B. and B e l l , A.B. 1976. Torlesse mClange and coherent sequences, e a s t e r n Ruahine Range, North I s l a n d , New Zealand. N.Z. J. Geol. Geophys. 19: 427-447. S p o r l i , K.B. and Gregory, M.R. 1981. S i g n i f i c a n c e o f Tethyan f u s u l i n i d limestones o f New Zealand. I n M.M. Cresswell and P. V e l l a (Eds.) Gondwana F i v e - proceedings o f t h e f i f t h i n t e r n a t i o n a l Gondwana symposium. A.A. Bal kema , Rotterdam. pp. 223-229.

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Sporli, K.B., Stanaway, K.J., and Ramsay, W.R.H. 1974. Geology of the Torlesse Supergroup i n the southern Liebig and Burnett Ranges, Canterbury, New Zealand. J. Roy. SOC. N.Z. 4: 177-192. Stanaway, K.J., Kobe, H.W., and Sekula, J . 1978. Manganese deposits and the associated rocks of Northland and Auckland, New Zealand. N.Z. J . Geol. Geophys. 21: 21-32. Strong, C.P., 1977. Cretaceous-Tertiary boundary a t Woodside Creek, northeastern Marlborough. N.Z. J . Geol. Geophys. 20: 687-696. Thomson, J.A., 1916. The Flint-beds associated w i t h the Amuri Limestone of Marlborough. Trans. Roy. SOC. N.Z. 48: 48-58. Turnbull, I.M., 1979. Stratigraphy and sedimentology of the Caples terrane o f the Thomson Mountains, northern Southland, New Zealand. N.Z. J . Geol. Geophys. 22: 555-574. Webb, P.N., 1966. New Zealand Late Cretaceous foraminifera and stratigraphy. Unpublished Ph.D. t h e s i s , University of Utrecht, Netherlands. Wellman, H.W., 1949. Pillow lava a t Red Rock Point, Wellington. Trans. ROY. SOC. N.Z. 77: 306-312.

109

CHAPTER 8 DISTRIBUTION, AGE, AND DEPOSITIONAL ENVIRONMENTS OF RADIOLARIAN CHERT I N WESTERN NORTH AMERICA BENITA MURCHEY’,

DAVID 1. JONES1, and BRIAN

K. HOLDSWORTH2

p1. S .

Geological Survey, Menlo Park, C a l i f . Geological Survey, Menlo Park, C a l i f . The U n i v e r s i t y , Keele, England)

U.S.

(permanent address:

ABSTRACT Radiolarian c h e r t i s widespread i n allochthonous accreted terranes througho u t t h e C o r d i l l e r a o f western North America, where dated deposits range i n age from Ordovician t o middle Cretaceous. a r e described:

S i x p r i n c i p a l l i t h o l o g i c associations

(1) o p h i o l i t i c c h e r t association;

(2) alternating pillow-basalt/

c h e r t association; ( 3 ) s i l i c i c v o l c a n i c / c h e r t association; ( 4 ) subsidence association; ( 5 ) c l a s t i c / c h e r t association; and (6) melange c h e r t association. Each a s s o c i a t i o n may represent a d i s t i n c t environment. The ages and l i t h o l o g i c associations o f r a d i o l a r i a n c h e r t deposits i n 23 allochthonous C o r d i l l e r a n terranes a r e summarized. INTRODUCTION Paleozoic and Mesozoic r a d i o l a r i a n c h e r t i s abundant i n many o f t h e accreted terranes t h a t l i e west o f the c r a t o n a l margin o f western North America (Coney e t al., 1980). Chert occurs i n a s s o c i a t i o n w i t h various rock types; each a s s o c i a t i o n may i n d i c a t e a d i f f e r e n t d e p o s i t i o n a l environment. Here, we summarize t h e d i s t r i b u t i o n , age, l i t h o l o g i c associations, and d e p o s i t i o n a l environ ments o f r a d i o l a r i a n c h e r t w l t h i n t h e c o l l a g e o f separate accreted terranes i n western North America. AGE AND DISTRIBUTION

Recent advances i n . r a d i o l a r i a n b i o s t r a t i g r a p h y (Pessagno and Newport, 1972; Pessagno, 19’77a, b; Holdsworth e t a l . , 1978; Holdsworth and Jones, 1980a, b) have permitted p r e c i s e d a t i n g o f many c h e r t deposits. Thus f a r , dated r a d i o l a r i a n c h e r t i n western North America ranges i n age from Ordovician t o middle Cretaceous (Table 1). These r e c e n t l y obtained ages have fundamentally changed our ideas concerning s t r a t i g r a p h i c r e l a t i o n s throughout most o f t h e C o r d i l l e r a (e.g.,

Irwin e t al.,

1977, 1978; Whetten e t al.,

1978; Jones e t a l . ,

1980, 1981) and were instrumental i n the d e l i n e a t i o n o f numerous d i s c r e t e t e c t o n o s t r a t i g r a p h i c terranes (Fig. 1). Paleomagnetic determinations (Beck, 1976; Hillhouse, 1977; Stone and Packer, 1977), coupled w i t h paleobiogeographic

110 analyses (e.g. Monger and Ross, 1971), have demonstrated t h a t many o f these t e c t o n o s t r a t i g r a p h i c t e r r a n e s a r e a1 lochthonous and t h a t some a r e e x o t i c t o North America. LITHOLOGIC ASSOCIATIONS AND INFERRED DEPOSITIONAL ENVIRONMENTS D i s c r i m i n a t i o n o f v a r i o u s s i l i c e o u s f a c i e s i s fundamental f o r a comparison of t h e geologic h i s t o r i e s o f d i f f e r e n t c h e r t - b e a r i n g t e r r a n e s .

We recognize

s i x major c h e r t - b e a r i n g l i t h o l o g i c a s s o c i a t i o n s w i t h i n t h e C o r d i l l e r a o f western North America, each o f which may represent a d i s t i n c t d e p o s i t i o n a l environment (Table 2 ) . Ophiolitic chert association The c l a s s i c o p h i o l i t i c s u i t e o f s e r p e n t i n i t e , gabbro, and p i l l o w - b a s a l t i s capped by r a d i o l a r i a n c h e r t .

For convenience, we d e f i n e any bedded-chert

sequence capping a p i l l o w - b a s a l t sequence, w i t h o r w i t h o u t o t h e r o p h i o l i t i c igneous rocks, as an " o p h i o l i t i c c h e r t a s s o c i a t i o n . "

A t y p i c a l c h e r t sequence i s 50 t o 100 m t h i c k ; t h e basal p a r t i s , i n places, tuffaceous and may be i n t e r c a l a t e d w i t h minor p i l l o w b a s a l t , manganese ore, o r j a s p e r .

The basal

c h e r t i s commonly maroon o r dark red, whereas t h e upper c h e r t may vary i n c o l o r b u t i n most places i s r e d o r green.

The c h e r t i s c h a r a c t e r i z e d by

r e l a t i v e l y h i g h c o n c e n t r a t i o n s o f i r o n and ( o r ) manganese i n t h e lower p a r t o f the s e c t i o n and by low concentrations o f alumina throughout t h e s e c t i o n ( S t e i n b e r g and Mpodozis Marin, 1978). s p i c u l e s r a r e o r absent.

R a d i o l a r i a n s a r e abundant, and sponge

I n d i v i d u a l c h e r t beds c o n s i s t m a i n l y o f s i l i c a

c o n t a i n i n g l i t t l e carbonate, t e r r i g e n o u s c l a s t i c , o r v o l c a n i c components, as determined from f i e l d observations and t h i n sections.

Atypical o p h i o l i t i c

c h e r t a s s o c i a t i o n s may a l s o meet t h e d e f i n i t i o n o f o t h e r a s s o c i a t i o n s and are here designated by combining t h e two a s s o c i a t i o n names, e.g.9 silicic volcanic/ophiolitic chert association o r c l a s t i c / o p h i o l i t i c chert association. The c l a s s i c o p h i o l i t i c s u i t e represents d e p o s i t i o n of biogenic and p o s s i b l y hydrothermal s i l i c a on an oceanic c r u s t i n a marine environment removed from t h e i n f l u e n c e o f a r c volcanism and c o n t i n e n t a l - m a r g i n c l a s t i c sedimentation. A l l geologic f a c t o r s must be considered f o r a more s p e c i f i c environmental i n t e r p r e t a t i o n , e.g., abyssal p l a i n on a converging oceanic p l a t e , backarc spreading center, o r subsiding quiescent seamount. A1 t e r n a t i n g p i l l o w - b a s a l t / c h e r t a s s o c i a t i o n The d e p o s i t i o n a l i n t e r c a l a t i o n o f t h i c k (hundreds o f meters) p i l l o w and massive b a s a l t w i t h bedded c h e r t (1-30 m) c h a r a c t e r i z e s t h e a1 t e r n a t i n g pillow-basal t / c h e r t association.

The c h e r t commonly c o n t a i n s interbedded

b a s a l t b r e c c i a and b a s a l t i c t u f f , and may be manganiferous.

S i l i c i c volcanic

o r c o n t i n e n t a l d e t r i t u s , as determined by f i e l d observations and t h i n sections, are a p p a r e n t l y o n l y minor components of t h e c h e r t .

The c h e r t may grade i n t o

111

limestone. The biogenic silica was deposited during periods of volcanic quiescence on or near a volcanically active submarine edifice, such as a seamount, an aseismic ridge, or a spreading ridge. Silicic volcanic/chert association Varying admixtures of biogenic silica (radiolarian and (or) sponge bearing) and silicic volcanic detritus characterize the silicic volcanic/chert association. The lithologic spectrum ranges from (1) pillow basalt overlain by tuffaceous chert to ( 2 ) interbedded sil icic volcanic breccia, tuff, cherty tuff, and tuffaceous chert, including olistostromal blocks of fossiliferous limestone. The chert is commonly green to green-gray and may exhibit well-developed laminae or other sedimentary structures. Volcanic shard texture within cherty beds is well preserved in places (Jones et al., 1980). The chert may grade into or be interbedded with pelagic limestone. Deposition in a marine environment, with high productivity of siliceous plankton and nearby silicic volcanism (e.g., backarc basin, or oceanic plate adjacent to and downwind from a volcanic arc), created this association. Subsidence association A vertical succession from shallow-water fossiliferous carbonate or clastic rock to spicul itic and (or) radiolarian chert characterizes the subsidence association. Argillite or pelagic limestone commonly occurs in the transition zone between shallow-water sediment and overlying bedded chert. Rocks underlying the sequence may have continental, volcanic-arc, or oceanic affinities. The chert is typically gray or black, rarely red. The carbonate content of the chert and the ratio of sponge spicules to radiolarians commonly decrease upsection. Chert formed by partial replacement of limestone is locally common. Because chert alone is not diagnostic of the subsidence association, the chert unit may meet the definition of another association and can be designated by combining the two association names, e.g., silicic volcanic/subsidence association, clastic/subsidence association. Although we assume that subsidence is the most common cause for this lithologic succession, a rise in sea level could also create this sequence. A specific environmental interpretation must account for the origin of the shallow-water rocks at the base of the sequence as well as of the underlying platform. Clastic/chert association Silty chert and (or) chert interbedded with terrigenous detritus characterizes the clastic/chert association. Typically, chert deposits (1-30m) are interbedded with clastic or reworked carbonate rocks. The chert is commonly silty and limy, and exhibits such sedimentary structures as size grading, small scours, and laminae. The ratio o f sponge spicules to radiolarians may increase toward the continent.

112 This a s s o c i a t i o n represents a marine environment w e l l w i t h i n t h e deposit i o n a l sphere o f i n f l u e n c e o f a c o n t i n e n t a l margin, e.g.,

the continental

s i d e o f a backarc basin, a b l o c k - f a u l t e d marginal basin, o r an oceanic p l a t e adjacent t o a c o n t i n e n t a l margin.

S p e c i f i c environmental i n t e r p r e t a t i o n s

should i n c l u d e c o n s i d e r a t i o n o f t h e r e g i o n a l geology. Melange c h e r t a s s o c i a t i o n Interbedded s i l t y b l a c k a r g i l l i t e , minor graywacke, c h e r t and b a s a l t i c t u f f , w i t h o r w i t h o u t minor interbedded p i l l o w b a s a l t , l o c a l l y forms t h e d i s r u p t e d m a t r i x o f Mesozoic melange t e r r a n e s .

The presence o f interbedded b a s a l t i c t u f f ,

w i t h o r w i t h o u t interbedded b a s a l t , d i s t i n g u i s h e s t h i s a s s o c i a t i o n from t h e c l a s t i d c h e r t association.

The melange c h e r t a s s o c i a t i o n does n o t have t h e

t h i c k (hundreds o f meters) b a s a l t f l o w s c h a r a c t e r i s t i c o f t h e a l t e r n a t i n g pillow-basalt/chert association.

Although t h e melange c h e r t a s s o c i a t i o n

may be a t r a n s i t i o n a l f a c i e s between t h e a l t e r n a t i n g p i l l o w - b a s a l t / c h e r t a s s o c i a t i o n and t h e c l a s t i c / c h e r t a s s o c i a t i o n , we c l a s s i f y i t s e p a r a t e l y because (1) we have n o t seen t h i s t r a n s i t i o n i n t h e f i e l d and ( 2 ) t h e melange c h e r t a s s o c i a t i o n occurs w i t h i n a s p e c i f i c t e c t o n i c s e t t i n g . c h e r t u n i t s a r e commonly green, green-gray, o r red. r i c h and sponge s p i c u l e poor.

The t h i n (1-10 m)

The c h e r t i s r a d i o l a r i a n

The p e t r o g r a p h i c and geochemical p r o p e r t i e s of

t h e c h e r t a r e n o t known. The melange c h e r t a s s o c i a t i o n r e q u i r e s an environment o f d e p o s i t i o n adjacent t o a c t i v e b a s a l t i c volcanism as w e l l as t o a c o n t i n e n t a l margin.

The presence

o f igneous rocks and t h e apparent syndepositional d i s r u p t i o n o f t h e sediment suggest s e a - f l o o r volcanism and t e c t o n i c a c t i v i t y , such as t h a t w i t h i n a rift zone o r a complex t r a n s f o r m f a u l t . LITHOLOGIC ASSOCIATIONS I N MAJOR CHERT-BEARING TERRANES AMERICA

OF WESTERN NORTH

Where possible, we have described and c l a s s i f i e d t h e c h e r t a s s o c i a t i o n s o f Some o f t h e c h e r t

t h e major c h e r t - b e a r i n g t e r r a n e s o f western N o r t h America.

deposits described below cannot y e t be c l a s s i f i e d because i n t e n s e t e c t o n i c d i s r u p t i o n has destroyed t h e o r i g i n a l s t r a t i g r a p h i c r e l a t i o n s ; o t h e r c h e r t The numbers r e f e r deposits a r e t o o i m p e r f e c t l y known t o a l l o w c l a s s i f i c a t i o n . t o t h e index map ( F i g . 1). Kagvik-Brooks Range (composite) t e r r a n e (1) R a d i o l a r i a n - r i c h s i l i c e o u s sedimentary rocks were deposited i n t h e n o r t h western Brooks Range i n t e r m i t t e n t l y from Late M i s s i s s i p p i a n t o E a r l y J u r a s s i c time.

These deposits a r e i n t e r l a y e r e d w i t h a r g i l l i t e , mudstone, b l a c k shale,

and limestone.

Paleozoic bedded c h e r t appears t o be t h e o r i g i n a l r o c k type,

whereas t h e Mesozoic bedded c h e r t commonly i s a s i l i c i f i e d r a d i o l a r i a n l i m e stone.

I n some t h r u s t sequences, t h i s s i l i c e o u s assemblage o v e r l i e s f o s s i l i -

113 ferous shallow-water carbonate rocks and i s , therefore, a subsidence association.

Although s i l t - o r sand-size c l a s t i c d e t r i t u s i s n o t c h a r a c t e r i s t i c o f

these rocks, f u t u r e geochemical studies may reveal a c o n t i n e n t a l d e r i v a t i o n f o r t h e a r g i l l i t e o r black shale ( c l a s t i d c h e r t association). The t e c t o n i c s e t t i n g o f some c h e r t w i t h i n t h e s t r u c t u r a l l y complex stack o f t h r u s t sheets o f the western Brooks Range i s c o n t r o v e r s i a l .

These rocks,

c a l l e d t h e I p n a v i k sequence (Martin, 1970) o r t h e Kagvik sequence (Churkin e t al., 1979), c o n s i s t s of basal b l a c k s i l i c e o u s shale and r i b b o n c h e r t aSSOCiated w i t h a n d e s i t i c flows, t u f f , and t u r b i d i t e s ( s i l i c i c v o l c a n i c / c h e r t assoc i a t i o n ) ; the u n i t contains important strata-bound lead-zinc deposits of a VOlCanOgeniC o r i g i n (Nokleberg and Winkler, 1979). The type o f t h e contacts ( d e p o s i t i o n a l versus t e c t o n i c ) o f the basal p a r t o f t h e Kagvik t e r r a n e on o l d e r s h e l f carbonate and c l a s t i c rocks i s disputed. Churkin e t a l . (1979) i n t e r preted t h e t e r r a n e as an oceanic f a c i e s near t h e c o n t i n e n t a l margin. Dutro (1980), M a y f i e l d (1980), Metz (1980), and M u l l (1980) asserted t h a t d e p o s i t i o n occurred i n an i n t r a c r a t o n a l basin o r an aulocogen.

The s i g n i f i c a n c e and

f a c i e s r e l a t i o n s o f t h e m e t a l l i f e r o u s a n d e s i t i c and keratophyric t u f f and flows t h a t c h a r a c t e r i z e t h e M i s s i s s i p p i a n p a r t o f t h e Kagvik sequence have n o t been adequately established.

The strong disagreements t h a t have a r i s e n concerning

t h e s e t t i n g o f t h i s c h e r t - r i c h assemblaae c l e a r l y p o i n t o u t the importance o f f a c i e s a n a l y s i s t o t e c t o n i c i n t e r p r e t a t i o n s , and t h e necessity f o r more r e f i n e d basin a n a l y s i s o f chert-bearing domains. Angayucham (2), Mystic (4), Red P a i n t (5), and McKinley ( 6 ) terranes Alaska has many small chert-bearing terranes i n f a u l t contact w i t h l a r g e r terranes (Jones e t a1 , 1981 ). I n t h e McKinley and Angayucham terranes, c h e r t i s i n t e r c a l a t e d w i t h t h i c k p i l l o w - b a s a l t sequences ( a l t e r n a t i n g p i l l o w -

.

b a s a l t / c h e r t association).

Chert i n the Red P a i n t t e r r a n e i s detached from the

underlying rocks b u t may have been p a r t o f an o p h i o l i t i c c h e r t association o r a subsidence association. The Mystic t e r r a n e contains a subsidence a s s o c i a t i o n o f Upper Devonian, Mississippian, and Pennsylvanian c h e r t o v e r l y i n g shallowwater Devonian carbonate rocks. Innoko ( 3 ) , West Fork (8), and Broad Pass ( 9 ) terranes I n the Broad Pass terrane, t h e dominant rock types a r e interbedded M i s s i s s i p p i a n c h e r t y t u f f , chert, and a r g i l l i t e ( s i l i c i c v o l c a n i c / c h e r t association) (Jones e t a1 1980). The West Fork t e r r a n e contains two separate c h e r t assoc i a t i o n s : (1) Lower J u r a s s i c massive t u f f w i t h r a d i o l a r i a n ghosts ( s i l i c i c

.,

v o l c a n i c / c h e r t a s s o c i a t i o n ) and (2) Upper Jurassic chert, a r g i l l i t e , and sandstone ( c l a s t i c / c h e r t a s s o c i a t i o n ) (Jones e t a l . , 1980). I n t h e s t r u c t u r a l l y deformed Innoko terrane, c h e r t y v o l c a n i c l a s t i c rocks a r e associated w i t h upper Paleozoic and lower Mesozoic r a d i o l a r i a n c h e r t ( s i l i c i c v o l c a n i c / c h e r t associat i o n ) although some c h e r t appears t o be associated w i t h a r g i l l i t e and coarse

114

clastic rocks (clastic/chert association?). Chul i tna terrane (7) The Chulitna terrane contains three distinct chert assemblages (Jones et al., 1980): (1) an ophioligic chert association--a thick sequence of red to graygreen Upper Devonian and Lower Mississippian radiolarian chert overlying pillow-basalt; (2) a silicic volcanic/subsidence association--Permian volcanic and chert conglomerate that fines upward to gray radiolarian chert, interpreted as a subsiding volcanic-arc seouence; and ( 3 ) a clastic/subsidence association-sandstone, argillite, and Upper Jurassic and Lower Cretaceous chert overlying Upper Triassic nonmarine red beds and minor shallow-water limestone; the environment of deposition was a subsiding basin adjacent to a continental margin. Wrangellia (10) and Cache Creek (12) terranes Subsidence associations in Wrangellia and the Cache Creek terrane represent subsiding-platform environments. The bedded-chert deposits overlie fossiliferous shallow-water limestone that caps Pennsylvanian andesitic volcanic and volcaniclastic rocks in Wrangellia and that overlies ophiolitic rocks in Cache Creek. In the Cache Creek terrane, shallow-water carbonate-debris blocks are mixed with coeval deeper water siliceous deposits and are capped by Triassic chert and arpillite. These terranes, which contain little continental detritus, may be accreted oceanic plateaus (Ben-Avraham et al., 1981; Coney et al., 1980). Chugach (ll), Bridge River (13), San Juan (14), and Blue Mountains (15) terranes The Chugach, Bridge River, San Juan, and Blue Mountains terranes are melange-bearing terranes with complex tectonic histories (Coney et al., 1980). Tectonic processes have displaced most of the chert deposits from the r original sedimentary associations. Both the Chugach and the San Juan terranes, however, contain ophiolitic chert and melange chert associations; the San Juan terrane also contains an alternating pillow-basalt/chert association. Depositional chert associations in the Bridge River and Blue Mountains terranes are unknown, although the presence of ophiolitic igneous rocks in both terranes suggests that an ophiolitic chert association existed before tectonic disruption. Triassic and Paleozoic of Klamath Mountains terrane (16) The Paleozoic and Triassic belt of the Klamath Mountains (Irwin, 1972) contains Permian, Triassic, and Jurassic radiolarians in widespread chert and cherty tuff (Irwin et al., 1977, 1978). The complex tectonic history of this area has destroyed the paleogeographic relations between individual chert bodies. At least two chert associations occur: (1) an ophiolitic association composed of red bedded-chert associated with pillow basalt, gabbro, diabase,

115

u l t r a m a f i c rocks, i n t e r p r e t e d as a dismembered o p h i o l i t e ( I r w i n , 1972); and ( 2 ) a s i l i c i c v o l c a n i c / c h e r t a s s o c i a t i o n composed o f c h e r t y r a d i o l a r i a n t u f f and t u f f a c e o u s c h e r t .

The s t r a t i g r a p h i c r e l a t i o n s between these two associa-

t i o n s i s n o t known; s t r u c t u r a l l y , they a r e mixed i n melange l i k e u n i t s accreted d u r i n g Late J u r a s s i c time ( I r w i n e t a l . ,

1982).

Eastern Klamath Mountains (17) and Northern S i e r r a (18) t e r r a n e s Both t h e Eastern Klamath Mountains and Northern S i e r r a t e r r a n e s c o n t a i n s i l i c i c volcanic/subsidence a s s o c i a t i o n s .

I n t h e Eastern Klamath Mountains

t e r r a n e , Lower and Middle T r i a s s i c r a d i o l a r i a n c h e r t associated w i t h b l a c k shale, t u f f and f l o w s i n t h e P i t Formation o v e r l i e s t h e Lower T r i a s s i c B u l l y

H i l l R h y o l i t e (Albers and Robertson, 1961).

I n t h e Northern S i e r r a terrane,

Paleozoic c h e r t i s interbedded w i t h o r o v e r l i e s a n d e s i t i c , d a c i t i c , r h y o l i t i c , o r l a t i t i c v o l c a n i c rocks i n t h e S i e r r a Buttes Formation and t h e T a y l o r Format i o n o f McMath (1966) ( D ' A l l u r a e t a l . ,

1977).

Moderately w e l l preserved

Late Devonian and Carboniferous r a d i o l a r i a n s occur i n t h e E l w e l l Formation o f D u r r e l l and D ' A l l u r a (1977) (DeVay and Stanley, 1979) and Peale Formation of D i l l e r (1908).

Bedded-chert occurrences i n b o t h t e r r a n e s were formed from

s i l i c e o u s sediment deposited on a subsiding-arc volcano adjacent t o a c t i v e arc volcanism. Golconda t e r r a n e (19) The s t r u c t u r a l l y complex Golconda a l l o c h t h o n o f Nevada may c o n t a i n several upper Paleozoic c h e r t a s s o c i a t i o n s .

Two c h e r t a s s o c i a t i o n s a r e t e c t o n i c a l l y

juxtaposed i n t h e Hoffman Canyon area o f t h e n o r t h e r n Tobin Range. s t r u c t u r a l l y h i g h e r assemblage ( U n i t D o f Stewart e t a l . , chert association.

The

1977) i s an o p h i o l i t i c

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

b a s a l t , manganese ore, and r a d i o l a r i a n - b e a r i n g jasper, and i s d e p o s i t i o n a l l y o r s t r u c t u r a l l y associated w i t h b l a c k f i n e l y laminated c h e r t (Murchey, 1982). I n t e r l a y e r i n g o f t h e c h e r t w i t h b a s a l t i s probably t e c t o n i c r a t h e r than depositional.

Both t h e r e d and b l a c k c h e r t s c o n t a i n Late M i s s i s s i p p i a n r a d i o l a r i a n

faunas. The s t r u c t u r a l l y lower assemblage ( U n i t C o f Stewart e t a l . , c l a s t i c / c h e r t association.

1977) i s a

Lower Permian carbonate t u r b i d i t e s o v e r l i e Upper

M i s s i s s i p p i a n , Pennsylvanian, and Lower Permian c h e r t ; i m b r i c a t e t h r u s t s repeat t h i s sequence.

The dark-gray and gray-green bedded c h e r t c o n t a i n s abundant

sponge spicules, abundant r a d i o l a r i a n s , and a few conodonts; t h e c h e r t a l s o c o n t a i n s d e t r i t a l c l a y and q u a r t z s i l t . The thick-bedded carbonate t u r b i d i t e s c o n t a i n sponge spicules, q u a r t z s i l t and sand, and r a r e E a r l y Permian f u s i l i nids.

Unit C ( c l a s t i c / c h e r t a s s o c i a t i o n ) may r e p r e s e n t an upslope, more

continentward environment r e l a t i v e t o U n i t D ( o p h i o l i t i c c h e r t a s s o c i a t i o n ) i n a l a r g e b a s i n (e.g., c o n t i n e n t a l margin.

open marine, backarc, o r g u l f ) adjacent t o a

116

Roberts Mountains terrane (20) Ordovician, S i l u r i a n , Devonian, and Lower Mississippian siliceous rocks of the Roberts Mountains allochthon i n Nevada contain interbedded terrigenous c l a s t i c and calcarenite rocks ( c l a s t i d c h e r t association). Most of the beddedchert deposits a r e Ordovician or Devonian. The Ordovician siliceous assemblages, the V i n i n i and Valmy Formations, a r e c l a s t i d c h e r t associations. Relative t o each other, the V i n i n i has a more continental a f f i n i t y , and the Valmy a more marine a f f i n i t y . Interbedded shale, calcarenite, quartz a r e n i t e , and s i l t y gray and green chert characterize the Vinini Formation. Most shale of the V i n i n i is bioturbated (Nereites tracef o s s i l assemblage) and probably represents deposition i n a deep-water normal marine (oxic) environment (Stanley e t a l . , 1977). The dominant rock types in the Valmy are black chert, basalt, quartz a r e n i t e , and g r a p t o l i t i c shale. The black laminated chert and g r a p t o l i t i c shale of the Valmy and parts of the V i n i n i represent deposition i n an anoxic environment (Churkin, 1974; Stanley e t a l . , 1977; Wrucke e t a l . , 1978). Vesicles and varioles i n pillow basalt of the Valmy indicate a possible submarine depth of emplacement of 4 km o r more (Wrucke e t a l . , 1978). Most paleogeographic reconstructions place the two facies in depositional continuity with each other (Stewart and Poole, 1974; Wrucke e t a l . , 1978). An eastern source f o r both the calcarenite and quartz a r e n i t e was postulated by many workers (Kay, 1960; Stewart and Poole, 1974; Churkin, 1974; Wrucke e t a l . , 1978). A few workers indicate a possible western source f o r the quartz arenite (Gilluly and Gates, 1965; Ketner, 1977), perhaps from a western microcontinent (Stewart and Poole, 1974, p. 51). If so, these two facies may be depositionally unrelated t o each other o r to the North American continent (Stanley e t a l . , 1977). Wrucke e t a l . (1978) suggested a backarc-basin environment to account f o r the m i x i n g of graptolite faunal provinces as well as the reducing environment; however, a partial offshore barrier i s not required to generate an expanded oxygen-minimum zone (Fischer and A r t h u r , 1977; Jenkyns, 1980). Devonian siliceous deposits in the Roberts Mountains allochthon a r e also predominantly a c l a s t i c / c h e r t association, characterized by mudstone, s i l t stone, chert, greenstone, b a r i t e and minor limestone, conglomerate, and sandstone. The mudstone i s the most common rock type in the eastern facies, the Woodruff and Cockalorum Wash Formations; the Woodruff i s so highly organic that i t can sometimes be recognized by i t s smell. S i l t y radiolarian chert occurs as t h i n stringers w i t h i n mudstone of the Woodruff. The western f a c i e s , the Slaven Chert and the Scott Canyon Formation, contains thick deposits of black bedded chert, commonly s i l t y , and greenstone ( c l a s t i c / o p h i o l i t i c chert

117 association).

Poole e t a l . (1977) postulated an inner-arc-basin depositional

s i t e for t h e Slaven and Woodruff, apparently f a r enough from an i s l a n d - a r c system t h a t the sediment contained l i t t l e s i l i c i c v o l c a n i c d e t r i t u s . Franciscan t e r r a n e (21) The Mesozoic rocks o f t h e Franciscan t e r r a n e o f C a l i f o r n i a comprise a t l e a s t f o u r depositional c h e r t associations.

The d i v e r s i t y and j u x t a p o s i t i o n

o f these f a c i e s r e f l e c t t h e complex t e c t o n i c h i s t o r y o f t h i s terrane; s p e c i f i c exampl es a r e described be1ow. O p h i o l i t i c chert a s s o c i a t i o n I n t h e Marin Headlands area immediately n o r t h o f San Francisco, 85 m o f Lower Jurassic, Upper Jurassic, Lower Cretaceous, and Upper Cretaceous (Cenomanian) c h e r t o v e r l i e s p i l l o w b a s a l t and u n d e r l i e s l i t h i c arkose (Murchey, 1980). I n t h e c h e r t sequence, volcanic d e t r i t u s i s rare, and s i l t o r sand-size terrigenous d e t r i t u s ( c l a s t i c / c h e r t association) occurs o n l y i n t h e upper few meters o f t h e c h e r t sequence.

I n a fault-bounded area o f t h e

western Marin Headlands, diabase dikes and s i l l s i n t r u d e t h e b a s a l t . Apparent removal o f t h e d e p o s i t i o n a l s i t e from c o n t i n e n t a l i n f l u e n c e ( e o l i a n excluded) f o r m i l l i o n s o f years suggests an open marine environment; S. K a r l ' s ( w r i t t e n communication, 1982) recent geochemical analyses o f t h e c h e r t substantiates t h i s i n t e r p r e t a t i o n . The succession from an o p h i o l i t i c c h e r t a s s o c i a t i o n t o a c l a s t i c / c h e r t a s s o c i a t i o n probably r e f l e c t s convergence o f t h e d e p o s i t i o n a l s i t e toward a c o n t i n e n t a l margin. A1 t e r n a t i ng p i 11ow-basal t / c h e r t a s s o c i a t i o n I n t h e Franciscan Central B e l t near Nicasio, a sequence o f tuffaceous manganese-stained maroon c h e r t (10-15 m) containing E a r l y Cretaceous r a d i o l a r i a n s and b a s a l t b r e c c i a and ash (15 m) i s sandwiched between hundreds of meters o f p i l l o w b a s a l t . The o r i g i n a l s i l i c e o u s sediment may have been deposited on t h e f l a n k s o f a submarine seamount d u r i n g a period o f v o l c a n i c quiescence. Melange c h e r t a s s o c i a t i o n

A good example o f a melange c h e r t a s s o c i a t i o n crops o u t i n a quarry near San Quentin. Green c h e r t and green b a s a l t i c t u f f are interbedded w i t h black a r g i l l i t e ; t h e e n t i r e assemblage i s f o l d e d and disrupted. C l a s t i d c h e r t association The Diablo Range contains Upper Jurassic o r Lower Cretaceous s i l t y c h e r t interbedded w i t h t h i c k sequences o f sandstone. Great Valley t e r r a n e (22) The Jurassic Coast Range o p h i o l i t e forms t h e basement o f t h e C a l i f o r n i a Chert i s associated w i t h these igneous rocks.

Great V a l l e y terrane.

118

A t P o i n t Sal, t u f f a c e o u s bedded c h e r t (20-25 m t h i c k ) caps an igneous I n the

o p h i o l i t e sequence ( s i l i c i c v o l c a n i c / o p h i o l i t i c c h e r t a s s o c i a t i o n ) . i n t e r p r e t a t i o n o f Hopson e t a1

.

(1981), t h e t u f f a c e o u s c h e r t represents sub-

carbonate-compensation-depth d e p o s i t i o n as t h e o p h i o l i t e approached an a c t i v e volcanic arc. I n t h e Stonyford area, a t h i c k ( 2 - 3 km) Ti02-enriched p i l l o w b a s a l t u n i t i s l o c a l l y i n t e r s t r a t i f i e d w i t h r e d manganiferous c h e r t , as much as 30 m t h i c k (Brown, 1364) ( a l t e r n a t i n g p i l l o w - b a s a l t / c h e r t a s s o c i a t i o n ) .

Hopson e t a l .

(1981) i n t e r p r e t t h e S t o n y f o r d v o l c a n i c rocks ( i n c l u d i n g t h e S t . John Mountain-Snow Mountain o u t l i e r s ) as remnants o f a T i t h o n i a n seamount, formed o f f t h e a x i s o f a spreading center. Cedros-Vizcaino (composite) t e r r a n e ( 2 3 ) The Cedros-Vizcaino t e r r a n e o f Baja C a l i f o r n i a i s a composite t e r r a n e c o n t a i n i n g two groups o f c h e r t a s s o c i a t i o n s :

(1) a s i l i c i c volcanic/ophiolitic

c h e r t association--Upper T r i a s s i c carbonate and s m e c t i t e - r i c h c h e r t o v e r l y i n g p i l l o w b a s a l t , interbedded w i t h t u f f a c e o u s c h e r t and v o l c a n i c l a s t i c sandstone, and g r a d i n g upward i n t o tuffaceous limestone (Pessagno e t a l . , e t al.,

1979, Rangin

1981), and ( 2 ) a c l a s t i c / o p h i o l i t i c c h e r t a s s o c i a t i o n - - L a t e J u r a s s i c

and E a r l y Cretaceous i l l i t i c c h e r t on Cedros I s l a n d o v e r l y i n g p i l l o w b a s a l t , grading i n t o o r interbedded w i t h graywacke i n places (Rangin, 1978; Rangin e t al.,

1981).

CONCLUSION O p h i o l i t i c c h e r t a s s o c i a t i o n s occur i n a l l s t r a t i g r a p h i c i n t e r v a l s , b u t t e c t o n i c events l i m i t e d t h e s t r a t i g r a p h i c d i s t r i b u t i o n o f t h e o t h e r f i v e l i t h o l o g i c associations.

Alternating pillow-basalt/chert associations are

predominantly Mesozoic.

S i l i c i c v o l c a n i c / c h e r t a s s o c i a t i o n s and subsidence

a s s o c i a t i o n s a r e predominantly T r i a s s i c o r o l d e r .

Clastic/chert associations

a r e common i n Paleozoic and upper Mesozoic terranes, uncommon i n lower Mesozoic t e r r a n e s .

Melange c h e r t a s s o c i a t i o n s a r e Mesozoic and a r e r e l a t e d

t o l a t e Mesozoic t e c t o n i c events. As more p a l e o n t o l o g i c . and s t r a t i g r a p h i c data accumulate, we expect more s i l i c e o u s f a c i e s t o be recognized.

Future s t u d i e s should r e f i n e t h e p e t r o -

graphic, geochemical, sedimentary, and faunal s i g n a t u r e s o f these a s s o c i a t i o n s . The a b i l i t y t o d i s c r i m i n a t e these f a c i e s and more c l e a r l y t o d e f i n e t h e e n v i ronments o f d e p o s i t i o n w i l l improve b a s i n analyses of t h e s i l i c e o u s d e p o s i t s i n t h e accreted t e r r a n e s o f western North America.

119

ACKNOWLEDGEMENTS W e acknowledge Bruce Wardlaw and Anita Harris, both of t h e U. S. Geological Survey, f o r conodont i d e n t i f i c a t i o n , and a l l the g e o l o g i s t s whose c h e r t samples and f i e l d d e s c r i p t i o n s helped us t o compile the data i n t h i s r e p o r t . David Howell and James R. Hein, both of t h e U. S. Geological Survey, and Azuma Iijima of the University of Tokyo, reviewed the manuscript. REFERENCES Albers, J . P. and Robertson, J . P . , 1961. Geology and o r e d e p o s i t s of e a s t Shasta copper-zinc d i s t r i c t , Shasta County, California. U.S. Geol. Surv. Prof. Pap. 338, 107 pp. Beck, M. E . , J r . , 1976. Discordant paleomagnetic pole p o s i t i o n s a s evidence of regional shear in the western Cordillera of North America: Am. J . S c i . , 276: 694-712. Ben-Avraham, Z., Nur, A., Jones, D. L. and Cox, A., 1981. Continental accretion: From oceanic plateaus t o a1 lochthonous t e r r a n e s : Science, 213: 47-54. Brown, R. D., J r . , 1964. Geologic map of t h e Stonyford quadrangle, Glenn, Colusa, and Lake Counties, C a l i f o r n i a : U.S. Geol. Surv. Min. Invest. Field Studies Map MF-279, 3 p p . , s c a l e 1 : 48,000. C h u r k i n , M . , J r . , 1974. Paleozoic marginal ocean basin-volcanic a r c systems in the Cordilleran Fold Belt. In: R. H. Dott, J r . , and R. H. Shaver ( E d i t o r s ) , Modern and Ancient Geosynclinal Sedimentation. SOC. Econ. Paleontol. Mineral, Spec. Publ. 19, pp. 174-192. Churkin, M . , J r . , Nokleberg, W . J . and Huie, C., 1979. Collision-deformed Paleozoic continental margin, western Brooks Range, Alaska: Geology, 7: 379-383. Coney, P. J . , Jones, D. L. and Monger, J . W. H . , 1980. Cordilleran suspect t e r r a n e s : Nature (London), 288: 329-333. D'Allura, J . A . , Moores, E. M. and Robinson, L . , 1977. Paleozoic rocks of the northern S i e r r a Nevada: Their s t r u c t u r a l and paleogeographic implications. In: J . H. Stewart, C. H. Stevens, and A. E. F r i t s c h e ( E d i t o r s ) , Paleozoic Paleogeography of the Western United S t a t e s : P a c i f i c Coast Paleogeography, Symposium 1 . Los Angeles, Society of Economic Paleontologists and Mineralogists, P a c i f i c Section, pp. 395-408. Devay, J . C. and Stanley, E . , 1979. Radiolaria from the Devonian Elwell Formation, northern S i e r r a Nevada, C a l i f o r n i a . Geol. SOC. Am. Abstr. Prog., 11- 412 ( a b s t r . ) . D i l l e r , J . S . , 1908. Geology of the T a y l o r s v i l l e region, California: U.S. Geol. Surv. B u l l . 353: 128 pp. Durrell, C. and D'Allura, J . , 1977. Upper Paleozoic s e c t i o n i n e a s t e r n Plumas and S i e r r a Counties, northern S i e r r a Nevada, California- Geol. SOC. Am. Bull., 88: 844-852. Dutro, J . T., 1980. Comment on "Collision-deformed Paleozoic continental margin, western Brooks Range, Alaska." Geology, 8: 355-356. Fischer, A. G., and Arthur, M. A., 1977. Secular v a r i a t i o n s in t h e pelagic realm. In: H. E. Cook and P. Enos ( E d i t o r s ) , Deep-Water Carbonate EnvironmGts. SOC. Econ. Paleontol. Mineral. Spec. Publ. 25, pp. 19-50. G i l l u l y , 3 . and Gates, O., 1965. Tectonic and igneous geology of the northern Shoshone Range, Nevada. U.S. Geol. Surv. Prof. Pap. 465, 153 p. Hillhouse, J . , 1977. Paleomagnetism of t h e T r i a s s i c Nikolai Greenstone, McCarthy quadrangle, Alaska. Can. 3. Earth Scie. , 14: 2578-2592. Holdsworth, B. K . , and Jones, D. L., 1980a. A provisional Radiolaria b i o s t r a t i g r a p h y , Late Devonian through Late Permian, U . S . Geological Survey Open-Fil e Report 80-876.

120

Holdsworth, B. K. and Jones, D. L., 1980b. Preliminary radiolarian zonation for Late Devonian through Permian time. Geology, 8: 281-285. Holdsworth, B. K., Jones, D. L., and Allison, C., 1978. Upper Devonian radiolarians separated from chert of the Ford Lake Shale, Alaska. U.S. Geol. Sur. J. Res., 6: 775-778. Hopson, C. A., Mattinson, J. M., and Pessagno, E. A., Jr., 1981. Coast Range Ophiolite, Western California. In: W. G. Ernst (Editor), The Geotectonic Development of California. EnglGood Cliffs, N.J., Prentice-Hall, pp. 418 510.

Irwin, W . P., 1972. Terranes of the western Paleozoic and Triassic belt in the southern Klamath Mountains, California. In: Geological Survey Research 1972. U.S. Geol. Sur. Prof. Pap. 8 0 F C , pp. C103-Clll. Irwin, W. P., 1981. Tectonic accretion of the Klamath Mountains. In: W. G. Ernst (Editor). The Geotectonic Development of CaliforniaFEnglewood Cliffs, N.J., Prentice-Hall , pp. 29-49. Irwin, W. P., Blome, C. D. and Jones, D. L., 1982. Age and tectonic implication of radiolarian chert in the North Fork terrane, Klamath Mountains, California: Geol. SOC. Am. Abstr. Prog., 14 (in press). Irwin, W. P., Jones, D. L. and Kaplan, T., 1978. Radiolarians from preNevadan rocks of the Klamath Mountains, California and Oregon. In: 0. G. Howel 1 and K. A. McDougall (Editors), Mesozoic Paleogeography o f T h e Western United States: Pacific Coast Paleogeography Symposium 2. Los Angeles, Society of Economic Paleontologists and Mineralogists, Pacific Section, pp. 303-310. Irwin, W. P., Jones, 0. L., and Pessagno, E. A., Jr., 1977. Significance of Mesozoic radiolarians from the pre-Nevadan rocks o f the southern Klamath Mountains, California: Geology, 5: 557-562. Jenkyns, H. C., 1980. Cretaceous anoxic events: From continents to oceans. J. Geol. SOC. London, 137: 171-188. Jones, D. L., Silberling, N. J., Berg, H. C. and Plafker, G. , 1981. Map showing tectonostratigraphic terranes of Alaska, columnar sections, and sumnary description of terranes: U.S. Geol, Surv. Open-File Rep. 81-792. Jones, D. L., Silberling, N. J., Csejtey, B., Jr., Nelson, W. H. and Blome, C. D., 1980. Age and structural significance of ophiolite and adjoining rocks in the Upper Chulitna District, South-central Alaska: U.S. Geol. Surv. Prof. Pap. 1121-A. Kay, M., 1960. Paleozoic continental margin in central Nevada, western United States: International Geological Congress, Zlst, Copenhagen, 1960, Proceedings, pt. 12, pp. 93-103. Ketner, K. B., 1977. Deposition and deformation of lower Paleozoic western facies rocks, northern Nevada. In: 3. H. Stewart, C. H. Stevens, and A. E. Fritsche, (Editors), PaleozoirPaleogeography of the Western United States. Pacific Section, Symposium 1. Los Angeles, Society of Economic Paleontologists and Mineralogists, Pacific Section, pp. 251-258. McMath, V . E., 1966. Geology of the Taylorsville area, northern Sierra Nevada, In: Geology o f Northern California: Calif. Div. Mines Geol. Bull. 19Oypp. 173-183. Martin, A. J., 1970.. Structure and tectonic history of the western Brooks Range, DeLong Mountains and Lisburne Hills, northern Alaska: Geol. SOC. Am. Bull. , 81 : 3605-3622. Mayfield, C. F., 1980. Comment on "Coll ision-deformed Paleozoic continental margin, western Brooks Range, Alaska." Geology, 8: 357-359. Metz, P. A. , 1980. Comment on "Collision-deformed Paleozoic continental margin, western Brooks Range, Alaska." Geology, 8: 360. Monger, J. W. H. and ROSS, C. A . , 1971. Distribution of fusulinaceans in the western Cordillera. Can. 3 . Earth Sci., 8: 259-278. Mull, C. G. , 1980. Comment on "Collision-deformed Paleozoic continental margin, western Brooks Range, Alaska." Geology, 8: 361-362. Murchey, B., 1980. Significance of chert age determinations in the Marin Headlands, California. Geol. SOC. Am., Abstr. Prog., 12(3): 144 (abstr.).

121 Murchey, B., 1982. Chert f a c i e s i n t h e Havallah sequence near B a t t l e Mountain, Nevada. Geol. SOC. Am. Abstr. Prog., 14 ( i n p r e s s ) . Nokleberg, W. J. and Winkler, G. R., 1979. Geologic s e t t i n g o f t h e l e a d and z i n c deposits, Drenchwater Creek area, Howard Pass quadrangle, western Brooks Range, Alaska. U.S. Geological Survey Open - F i l e 78-70-C, 16 pp. Pessagno, E. A., J r . , 1977a. Lower Cretaceous r a d i o l a r i a n b i o s t r a t i g r a p h y o f t h e Great V a l l e y sequence and Franciscan complex, C a l i f o r n i a Coast Ranges: Cushman Found. Foraminifera1 Res., Spec. Publ. 15, 87 pp. Pessagno, E. A., J r . , 1977b. Upper J u r a s s i c R a d i o l a r i a and r a d i o l a r i a n b i o s t r a t i g r a p h y of t h e C a l i f o r n i a Coast Ranges. Micropaleontology, 23(1): 56-113. Pessagno, E. A., Jr., Finch, J. W. and Abbott, 1979. Upper T r i a s s i c R a d i o l a r i a from t h e San H i p o l i t o Formation, Baja C a l i f o r n i a : MicropaleontOlOgy, 25: 160-197. Pessagno, E. A., Jr., and Newport, R. L., 1972. A technique f o r e x t r a c t i n g R a d i o l a r i a from r a d i o l a r i a n c h e r t : Micropaleontology, 18: 231-234. Poole, F. G., Sandberg, C. A. and Boucot, A. J., 1977. S i l u r i a n and Devonian paleogeography o f t h e western U n i t e d States. I n : J. H. Stewart, C. H. Stevens, and A. E. F r i t s c h e ( E d i t o r s ) , P a l e o z o E Paleogeography o f t h e Western U n i t e d States: P a c i f i c Coast Paleogeography Symposium 1. Los Angeles, S o c i e t y o f Economic 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 Section, pp. 39-66. Rangin, C., 1978. Speculative model o f Mesozoic geodynamics, c e n t r a l Baja C a l i f o r n i a t o n o r t h e a s t e r n Sonora (Mexico). I n : D. G. Howell, and K. A. McDougall , ( E d i t o r s ) , Mesozoic Paleogeographyyf t h e Western U n i t e d States: P a c i f i c Coast Paleogeography Symposium 2. Los Angeles, S o c i e t y o f Economic 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 Section, pp.85-106. Rangin, C. , Steinberg, M. and Bonnot-Courtois, C. , 1981. Geochemistry o f t h e Mesozoic bedded c h e r t s o f Central Baja C a l i f o r n i a (Vizcaino-Cedros-San B e n i t o ) : Imp1i c a t i o n s f o r paleogeographic r e c o n s t r u c t i o n o f an o l d ocean basin: E a r t h Planet. S c i . L e t t . , 54: 313-322. Stanley, K. O., Chamberlain, C. K. and Stewart, 3. H., 1977. D e p o s i t i o n a l s e t t i n g o f some eugeosynclinal Ordovician rocks and s t r u c t u r a l l y i n t e r leaved Devonian rocks i n t h e C o r d i l l e r a n m o b i l e b e l t , Nevada. I n : J. H. Stewart, C. H. Stevens, and A. E. F r i t s c h e ( E d i t o r s ) , Paleozoic Paleogeography o f t h e Western U n i t e d States: P a c i f i c Coast Paleogeography Symposium 1. Los Angeles, S o c i e t y o f Economic 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 Section, pp. 259-275. Steinberg, M. and Mpodozis Marin, 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 e t des sediments s i l i c i e u x oceaniques, s i g n i f i c a t i o n paleooceanographique: Oceanol. Acta, 1: 359-367. Stewart, J. H., MacMillan, J. R., Nichols, K. M. and Stevens, C. H., 1977. Deep-water Upper Paleozoic rocks i n n o r t h - c e n t r a l Nevada--a study o f t h e t y p e area o f t h e Havallah Formation. I n : J. H. Stewart, C. H. Stevens, and A. E. F r i t s c h e ( E d i t o r s ) , PaleozoiFPaleogeography o f t h e Western U n i t e d States: P a c i f i c Coast Paleogeography Symposium 1. Los Angeles, S o c i e t y o f Economic 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 Section, pp. 337-347. Stewart, J. H. and Poole, F. G., 1974. Lower Paleozoic and uppermost Precamb r i a n C o r d i l l e r a n miogeocline, Great Basin, western U n i t e d States: SOC. Econ. Paleontol. Mineral. Spec. Publ. 22, pp. 28-57. Stone, D. 6. and Packer, D. R., 1977. Tectonic i m p l i c a t i o n s o f Alaska Peninsula paleomagnetic data. Tectonophysics, 37: 183-201. Whetten, J. T., Jones, D. L., Cowan, D. S. and Zartman, R. E., 1978. Ages o f Mesozoic t e r r a n e s i n t h e San Juan Islands, Washington. I n : D. G. Howell and K. A. McDougall ( E d i t o r s ) , Mesozoic Paleogeography o F t h e Western U n i t e d States: P a c i f i c Coast Paleogeography Symposium 2. Los Angeles, S o c i e t y of Economic 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 Section, pp. 117-132.

122

Wrucke, C. T . , Churkin, J . , Jr. and Heropoulos, C . , 1978. Deep-sea origin o f Ordovician pillow basalt and associated sedimentary rocks, northern Nevada. Geol. SOC.Am. Bull., 89: 1272-1280.

123

CHERT- BEARING TERRANES 1. K a g v i k - B r o o k s R a n g e

2. A n g a y u c h a m 3. l n n o k o

4. M y s t i c 5. R e d P a i n t

6. M c K i n l e y 7. Chulitna 8.West

Fork

9. B r o a d P a s s

1O.Wrangellia 11. C h u g a c h 12. C a c h e C r e e k 13. B r i d g e River 14. San Juan 15. Blue Mountains 16. T r i a s s i c & P a l e o z o i c of Klarnafh Mountain 17. E a s t e r n Klarnath Mountains 18. Northern Sierra 19. G o l c o n d a 20. R o b e r t s Mountains 2 1. F r a n c i s c a n

.

22. G r e a t Valley 23. C e d r o s - V i z c a i n o

Fig. 1. D i s t r i b u t i o n o f major c h e r t - b e a r i n g t e r r a n e s i n western North America. Dashed p a t t e r n , North America autochthonous c r a t o n i c basement. Barbed 1ine, eastern l i m i t o f C o r d i l l e r a n Mesozoic-Cenozoic deformation.

124 TABLE 1. Ages o f d i a g n o s t i c r a d i o l a r i a n and ( o r ) conodont faunas from bedded c h e r t i n t e r r a n e s o f western North America.

\

z

z

z

zw

TERRANE

z

w -

~

(

Y

w

0

> 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 ?

B r ~ c ~ l a t e df l:a t t e n e d and aliqned r a d l o l a r i a n i :

hmtite

Ireccldted.

laminate<.

mllitP9

8RO-21-IC 11 880-?1-10

RR t o RRR*f

R

879-10-31

RR pY7OIene

1.8 (1

hmtlte hmtite zeolite hmtite lllitC Chlorite clay' chlorite

-1

R e d chert

RR

(I

880-22-18 I 1

Green chert

RRR

1.7

880-22-IC

*n-stained

RRR

(1

88o-z2-2

Gray and h r o n c h e r t Red chert R m n Chert Chert Vn-replaced Chert hed

880-22.11

I

879-30-LA 879-33-18 879-30-ZE 880-22-31

Pldya, Real

chert

I

R RR

'

6.3

RRR

1.1

RRR. F

2.5
880-22-3 I

*n-ltllnCd chert

PRR

-1

R00-22-38 I 1

RRR

2.5

880-22-3C

@range Chert lanlnde * i t h i n 880-22-3R 1 Rlwd-red chelt

RRR

880-22-N I

Green chert

RRR

880-22-30 11 880-22-3i

Red Chert Red shale

R

(I
RRO-22-4A

M"

Gray Chert

RR

6.3

l h i t e Chert

R

8.0

None t o RRR RRR

6.7

quartz. clay. z e o l l f e M" oxides

ore

V i o l e t - h r o r n chert Yn-itaincd c h c n

Laminated; replaced d e t r l t a l m i n c n l s : f o l i a t i o n defined hy i l l i t c ; a l t w d volcanic glass.

1.2

1.4

919-2-28

Mn-stained shale

R

qmrtz

979-2-2(:

Rrm-md chen

RR

quart2

919-3-1

B r o n Chert

RR

q"art2

2E"l l t C 3

hedding.

m g n e t i t r or p y r i t e stllhitr. wart2 s t l l h i t e ' and Other zcol i t e r . Chalcedony. q m n z ZC"l1 t e w a r t i . h a r i t e . clays. nn oxides

D e t r i t d l minerals; hurrowed i n p a e ; w Brccciated. a d e d heddlnq; l m l m t e d . Brcccldted: d e t r i t a l minerals. R a d i o l a r i m packstow lamime: laminated: gilded?

-1

hmtlte

1.2 Calcite henatlte

1.1

quartz. Chalcedony. c l a y Irmctltc') quaptz, Hn oiides. ~ d l ~ l t Itllhlte. e. 1 a u m n t it e

Lamlwted: g m d d hedding; muditone

grader t o r a d l o l a r i a n packitone.

s o l e mrks7 layers Of d e t r l t a l q Y d l t 2 anO feldspar: laminated: In part burrowed.

979-2-11 979-2-18 979-3-2 880-23-11 880-23-2

Mn-itaiMd h r o m chert Reddish-brow c h e r t Red c h e r t Red Chert Red Chert

880-2341

R c I Chert

RR R RRR RRR None t o RRR

quart* q"art2 quartz quartz quartz

RRR

quartz

henatlte

-1

hemtiti

-1 2.5 1.1 1.8

tl

880-23-38

B r m Chert

RR

quartz

(1

880-23-3C

Red and green c h e r t

RRR

qUdrtZ

880-234

Green c h e r t

RRR

qmrtz

880-23-3f

Mn-stalned c h e r t

RRR

quart*

880-23-4

Red Chert Red c h e r t

RRR

880-23-5

RR

quartz qmrtz

880-23.61

Gray t o lavender c h e r t

R t o RRR

q"lrtl

880-23-68 880-23-71

nmit E l F~..MLI

880-23-78 880-23-7C 880-24-31 880-24-38

880-24-3c

880-2441

5-23-2-78-1 5-23-2-78-2

HatlllO

'

Bdn4en red and unItdined chert W" "Ddulel Y e l l o l a n d red jasper

RRR

4.3

lahmiorite quartz

aug1te

bvaunl t e qmrtz

pyr01urite

R

clay ( m m t 1 t c or p a l y g o r s k l t r ) har1te. 9YdltZ. c l a y ( r m t i t e or p a l y g o r r k l t e ) h a d t L . quartz. W t l t C or p a l y g o m k l t e quartz, chalcedony. harlte W" OlidCI, qYart2

henatite henatlte

2.3

plagloclale smt1tc hmtlte

4.6

harite. h m a t l t e . quartz b a r i t e . Wn oxides. q u a r t z hemtiti q u a r t z . harlte

pym1"lltC

-1

nn ori4er. 9Ydrt2,

2.2

Microfauitr. Squashed and aligned radlolarlanr; k t r i t a l q u a r t z and feldspar; l i m l n i t e d . graded heddlrq. M l c m f a u l t s i graded bedding; laminated. M i c r o - b r e c c i a t e d pipes; laminated; graded bedding. hurrar7 8"rrOt.ed'

O e t r l t l l nlneralr; I l d M t e d i graded beddlng. Graded beddlng; l a n l n a t e d ; burrobed?

quartz. Wn oildcs. harite? q"1rtz. h a r i t e 1 01 zeol i t e ? EIIClte, apatite' M l e l I" q u a r t z quartz. r t l l b l t r . goethltc. c a l c i t e quartz. m t 1 t e . heulandlte

hmtlte

1.4

qmrtz

h m t l te? EalClte?

6.5

Y C l l W jasper tens frm 880-2441 Gray and r e d r l l l c l f i e d I1ltltD"F

R!

qmrtz

rtllblte

6.0

R

quart*

hrulandite

t1

S i l I C e w I henatlte lens I " blllt Red and y e l l w jasper Yhlte chert

None

qwthlte hmatlte quartz qllaptz

tl

goethlte. quartz

Hmatlte-gUirt2-gOCthlte layered mck.

hmtlte heulandlte

1.8 -2

c l a y ( s m t 1 t e ) . qYartZ quartz. chalcedony. hmatlte. harite? q u a r t z . chalcedony. Wn oxides q u a r t z , M n oxlder CalClte. q m r t f l . hmatlte. chlorite calcite. q u a r t z

B r e c d a t e d areas; l l l l t e aggregate e x t l m t l o n . l l l i t e throuqh m t r l x .

RR R

4-21-1-78

None t o RRR

880-28.1

880-28-11

9"drtZ

6.9

R'

qYdTt2 Calcite

qYlrtZ

R

C d l C lt

quart*

tl

O"dlt2

tl

e

880-28-18

RRR. F

ca 1CI t e

880-28-IC

RRR, F

Calcite

880-28-10

880-28-1H

harite. P"dTt2.

NOW

B r i g h t nd c h e r t l e n s I n 1arqcr yc110* jasper lens

R'

880-28-16

r t l l h i t e ? barite chalcedony M" axlder l e s l i t e . hdrlte? h a r 1 t e . ZeOlltC

IllItC

5-23-2-18-3

880-28-lE 880-28-1F

quartz. quartz, q"a?tz. qmrtz. quartz. I 11 l t e ?

VL110* jarper Bdnded red and y e l l o w jasper Red Jasper Red c h e r t

R

qIJartz

none RR t o RRR

quartz qvartz

none

quartz

none

quartz

hmtite hmtlte

6.3

Calcite

hytmltr hmtltr?

ca 1c1t e siderite wcthltr

2.3

CdlCltC

henatitd plag1oc1aIc~ Iectite?

c o l l o f o m 9"lltZ.

I n p a r t St02-replaced i n t e q l l l o w or i n t e r f l o u debris. quartz c m n t e d and m p l a c e d v o l c m l c I l l t i t o n e ; much pYF0Xe"e.

Brecciated; FOlIOflnn Wart& 8reCEiated; c o l l o f o m q " a r t l ; Brecciated.

laminated i n

C a l c i t e - r e p l a c e d r a d l o l a r l a n i ; feldspar. and q Y l l t Z I" c a l d t e w i n s .

Calcite. qyartz quartz. h m t l t c . CalClte

Lwki l l k e H a t l l l o mcks; w h p y r l t e .

5.4

v a r t 2 . CalClte.

M1CrObreCCla;

anphibole. h m t l t c qYaTtl. ZCOlltC~

o c t r i t a 1 mi"era1S; aggregate c r t l n c t l o n

3.1

wrt.

chlorlte.

l n i o l u h l e resldue = quartz. smctlte. and h e v l a n d l t c ; f l s h debris; laminated; b u r m r ? o r g a n i c - r l c h lamime; p y r i t e . F i s h d e h r t i ; p y r l t e : organic m t t e r ; laminated; hwvws? Replaced v o l c a n i c redlmnt? q u a r t z - c l a y Opaques.

1.0

4.6 CdlCltC henatitel k-feldspar

C d l c i te. chalcedony. qmrt2 quartz. Chlorite.

V o l ~ a n im~ k fragments: feldspar;

F r a g m n t e d hy v e l n l m .

"OlC1"1C ml"em1s. Of

Clays.

880-29-11 880-29-1C I 880-29-IC I 1 880-29-10

HI G o l f l t o

5.4

RR

Red chert

880-29-3A

Brm I I l l C e o u s shale lens I n h a s a l t

880-29-38

Red c h e n frm

lens

Black chert frm same lens

880-29-3C

4.2 5.7

NOW

880.29- 12

ran

5.3

RR None

Yell.%-red jasper Y h l t c warti Red < h e n Red chert

RR

4.7

R t o RRR

3.0

Now I 0 RRR

1.9

1.4

RRR None

-1

980-1-18 1

Chalcedony nodule f r a hasalt I n ~ 0 n 9 l v r a t c Y c l l n jasper I" 980-1-18 I I

RR

1.2

980-1-18 I 1 980-I-lC

Green chert R e d chert

RRR RRR

980-1 -10

Y e l l n . m d and r h l t e

RRR

980- 1-11

QWDOS

Illltc

1.8 1.1

-1

qodrtz. h m t l t e .

goethltc quartz. chalcedony

JaSpW

980-1-12 980-2-11

Jaco

Chalcedony nodule fim basalt G n e n c h c n lens i n haialt

3.1

None hlrneiiite

none

6.7

Inectlte r. r 1. r i .tr . clay?

980- 2- 18 980-2-lC I 980-2-1C la

Red c h e n Red Chert Green chert

980-2-1C I 1

Ouanz-replaced h a s a l t ?

980-2-10

Red a n d green chert

R t o RRR. F RRR R

qmnz q"llt2 PYIrtZ

NOW

qudrtz

R

quartz

Irctlte

hmtlte m"g.".-C.lCItC~ DldglaldSe CalClte m.l"CtltL? *&t1te calcite IrctlfL

980-2-IE a

Red 1 l m l f O M

980-2-12 b

MCtllllC C r Y I t

980-2-lE c

8aialt

RRR. F

chlorlte DYWXeW' Imtltc gc+fhlte

calcltc

ralclte. m r c t l t e . heulandlte s t l l h l t c . ~ a l c l t e . quartz. snectlte. h m t l t c . lpODhY1I f l e qUart2. C.1CltC. Itllh l t c . aDoDhYlllle

qYlnZ.

0.8 CdlCltC

5'' 5.2

'"

Benthic f O r d m i n l f L n : S p l c U l c S . O l C l t e lamime; laminated; graded heddlng; mqwtlte: r a d i o l a r l a n i a l l q m d and squashed.

Wdgnet1te rtr1n9ers.

apophyllltc. s t i l h l t e . m r c t l t e . c h l o r l t c . PULpellylte. ouanz. qoethlte

8 r e c ~ l a t e d : rand l a y e r ; w e t h l t e and hmtlte.

iaumntite. s t m t i t r

Lmimted; qraded'

IaUmntlte. c a l d t e . quartz. h m t l t i . InectltC 1amnt1te

Large mgnetltc p ~ a l n s ; lamlnated.

calclte.

Dyrlte.

k-feldspar

hemtlre

goeethlte mect1te

llllte

980-2-1F

Red Ilnrrtom lenses I " basalt

P8O-Z-IG

Bdlalt WIth a t t e n d 91111

Dld9lOCldlC

snect1te RRR. F None

PYP0Xe"e mgnct It C calclte ldhrddorlte

rlndr 980-2-IH

*

8 i m h a s a l t rlnd on 11neEItm

pyroxene CllClte

RR I F

ndevalte warse

foramlnlfcrr D n I C n t I l m l c s w l t h rove than me I n d l c a f l o n m n l t h a t r a d i a l a r l a n FOntCnt fmn place t o place or fmn l d m to l a m 1 1 1 w l t h l n t h e IamDlc.

mmt1te

d~hlbolc

RRR ahundant

**

hmtlte Celddonlte epldole?

YlrlCI

zco1 i t e ?

pumellyltr

Ccladonltc. h m t l t e . l l l l t e . goethltc; nagnetltc Idmime a l t e r n a t e w l t h h a i d l t .

153 mean 1.5 cm and median 1.5 cm. The age o f these c h e r t s i s not known. One outcrop near S a r d i n a l i s d i f f e r e n t and c o n s i s t s o f b l o c k s o f bedded, laminated,

and graded gray,

brown,

and w h i t e c h e r t i n t e r c a l a t e d i n basalt.

Chert b l o c k s a r e several meters i n length. from r a d i o l a r i a n s (Hein e t al.,

A Cenomanian age was determined

i n preparation).

Manganese i n t h e form o f nodules and lenses i n c h e r t was mined near Frances.

a

Blocks o f rird and black bedded c h e r t and jasper, surrounded by s o i l ,

crop o u t on t h e s i d e o f a steep h i l l . these outcrops i n d e t a i l .

K u i j p e r s and Oenyer (1979) describe

The age o f these c h e r t s i s not known.

Near H a t i l l o , r e d and y e l l o w jasper, and hematite and magnetite ores occur as lenses ( t o m n y meters i n l e n g t h ) between b a s a l t flows.

These deposits

belong t o t h e upper Nicoya Complex and were dated as Turonian (P.

Baumgartner,

personal communication, 1980). Red and y e l l o w j a s p e r and r e d and w h i t e c h e r t o v e r l i e b a s a l t and occur w i t h manganese ores near Judas.

These deposits a r e described by K u i j p e r s and

Oenyer (1979), b u t t h e age o f t h e c h e r t s i s not known.

We

s t u d i e d outcrops

Cartagena.

of

the

Sabana

Grande u n i t

near

Santa

Cruz

and

Near Cartagena, a 200 m t h i c k s e c t i o n o f r h y t h m i c a l l y bedded w h i t e

t o grey c h e r t and shale i s e a r l y t o middle Eocene i n age a t b o t h t h e t o p and bottom o f t h e s e c t i o n (Hein e t al.,

i n preparation).

s i l t s t o n e beds t h a t a r e comnonly graded a l s o occur.

T h i n sandstone and

Three k i l o m e t e r s t o t h e

east i s another c h e r t - s h a l e sequence t h a t grades upsection i n t o s i l i c i f i e d limestone then i n t o limestone.

Bedding t h i c k n e s s and s t y l e do not change from

t h e c h e r t t o t h e limestone and a t l e a s t some o f t h e c h e r t beds a r e v i r t u a l l y completely s i l i c i f i e d limestone beds. Eocene i n age (Hein e t al.,

These c h e r t s a r e Paleocene t o e a r l y

i n preparation).

J u s t n o r t h o f Santa Cruz (Fig.

1) t h e c h e r t - s h a l e s e c t i o n i s middle t o l a t e Eocene i n age (Hein e t al., i n p r e p a r a t i o n ) ; t h e r h y t h m i c a l l y bedded w h i t e t o gray c h e r t s and shale commonly a r e laminated. Chert beds range i n t h i c k n e s s from 3.5 t o 30 cm, mean 9.6 cm and median 8.8 cm, w h i l e shale beds range from 0.4 t o 5.3 cm, mean o f 2.35 cm and median o f 2.4 cm. Four areas o u t s i d e t h e Nicoya Peninsula were a l s o studied. surrounding

Jaco

places c o n t a i n 1imestone.

(Fig.

large 3 m X

100 m lenses o f chert,

c h e r t breccia,

I n t e r p i 1l o w c h e r t and 1imestone occur together.

limestones a r e red,

Costal areas

1) c o n s i s t o f p i l l o w e d and massive b a s a l t s t h a t i n and

Most c h e r t s and

b u t green and brown rocks a l s o occur i n t h e basalt.

Veining by quartz i s extensive, z e o l i t e and a p o p h y l l i t e veins a r e l e s s common (Table 1). suggesting

Long c h e r t deposition o f

lenses separate m s s i v e and p i l l o w e d lava flows, siliceous

sediments

during

periods o f

volcanic

quiescence.

R a d i o l a r i a n s e x t r a c t e d from a c h e r t lens g i v e a Campanian age

(Hein e t al.,

i n preparation).

P o o r l y preserved n a n n o f o s s i l s from a limestone

154

lens i n t h e b a s a l t can only be i d e n t i f i e d as Cretaceous o r Late Jurassic. Thus, these rocks a r e e i t h e r age c o r r e l a t i v e w i t h t h e upper p a r t o f t h e upper Nicoya t h r u s t sheet o r w i t h t h e lower p a r t o f t h e Sabana Grande u n i t . A conglomerate o f probable Paleogene age occurs near Quepos a t Punta

Catedral (Fig.

1).

Basalt, chert, and jasper o f t h e Nicoya Complex comprise

t h e only c l a s t s i n t h e conglomerate. sizes.

Chalcedony

nodules a r e common w i t h i n t h e b a s a l t clasts.

v a r i e t i e s i n c l u d e dark green, tan,

yellow,

red,

r a d i o l a r i a n chert.

Clasts a r e mostly o f pebble t o cobble

and

Chert

c h l o r i t e - r i c h c h e r t w i t h r a d i o l a r i a n ghosts,

white

jasper

containing

Quartz veins a r e ubiquitous,

radiolarians,

and

red

b r e c c i a t i o n i s common.

R e l a t i v e l y deep as w e l l as shallow water limestones o f Paleogene age, basalt, and t h i c k agglomerates occur several hundred meters n o r t h o f t h e conglomerate (Schmidt-Effing, 1979; Henningsen and Weyl, 1967). At

the

southern

tip

of

Osa

the

i n t e r c a l a t e d w i t h massive basalt.

Peninsula,

chert

overlies

and

is

Bedded and laminated limestone blocks occur

i n other beach outcrops b u t t h e s t r a t i g r a p h i c r e l a t i o n s a r e indeterminable. Sedimentary rocks include brecciated r e d and y e l l o w jasper, laminated gray and l i g h t brown t o green limestone, p a r t l y s i l i c i f i e d black limestone, red, qua r t z - r e p 1 aced hya 1o c l a s t it e w i t h r a r e Limestone commonly contains p y r i t e and

.

r a d i o l a r ia ns , and r e d c h e r t s t r i n g e r s o f organic matter.

Coccoliths from t h e limestone are abundant b u t t h i c k l y overgrown and i n d i c a t e an age o f l a t e Paleocene t o middle Pliocene, probably middle Eocene t o middle Miocene (Hein e t al.,

i n preparation).

The limestones were a c o c c o l i t h -

f o r a m i n i f e r a 1 ooze t h a t was probably cooked by t h e basalts.

Radiolarians from

t h e c h e r t are not age diagnostic. Outcrops along t h e coast o f Golfo Dulce, northwest o f G o l f i t o , c o n s i s t o f bedded gray t o l i g h t brown limestone o f l a t e Campanian t o e a r l y M a a s t r i c h t i a n ages as described by Dengo (1962),

Henningsen and Weyl (1967),

and Schmidt-

E f f i n g (1979). and f u r t h e r t o t h e northwest massive and p i l l o w e d b a s a l t s w i t h sparse meter- t o tens-of-meter-sized lenses o f c h e r t and 1imestone. Sedimentary rocks form lenses o f r e d and y e l l o w jasper c o n t a i n i n g r a d i o l a r i a n s , c h e r t breccia, red-brown chert, red r a d i o l a r i a n chert, black r a d i o l a r i a n chert, and brown s i l i c e o u s shale w i t h r a r e r a d i o l a r i a n s . Quartz, c a l c i t e , and pumpellyite veins a r e common, b a r i t e veins occur i n places. We were unable t o age date these rocks.

Bedding C h a r a c t e r i s t i c s Important t o t h e i n t e r p r e t a t i o n o f t h e Nicoya Complex sedimentary rocks i s an understanding o f t h e o r i g i n o f t h e rhythmic bedding i n t h e chert-shale sequences.

Sedimentary s t r u c t u r e s other than p a r a l l e l laminations,

graded

155 bedding, and t h e rhythmic bedding per B e , a r e few. Several beds c o n t a i n c o n t o r t e d sets o f laminae t h a t may be t h e Bouw "C" i n t e r v a l . Laminated beds alone c o u l d be i n t e r p r e t e d as products o f oceanic c u r r e n t s o r o f t u r b i d i t y c u r r e n t s , s p e c i f i c a l l y t h e p a r a l l e l - l a m i n a t e d i n t e r v a l o f Bouw sequences. general,

sedimentary

structures

support

a turbidity-current

In

mechanism o f

d e p o s i t i o n f o r t h e c h e r t beds and perhaps a l s o f o r p a r t o f t h e shale l a y e r s (see a l s o Gursky and Schmidt-Effing,

t h i s volume).

We attempted t o g a i n a b e t t e r understanding o f t h e nature o f t h e bedding by employing s t a t i s t i c a l analyses o f bed t h i c k n e s s as proposed by M i z u t a n i and Hattori

(1972), and Folk and McBride (1977).

however,

do n o t p r o v i d e c l e a r answers.

Results o f these analyses,

Some s t r a t i g r a p h i c

s e c t i o n s show

t h i c k n e s s d i s t r i b u t i o n s t h a t would be expected from t u r b i d i t e s , not.

others do

O f f o u r measured sections o f c h e r t s from t h e Nicoya Complex two show t h e

c h e r t s t o be t u r b i d i t e s ,

two do not, whereas only one s e c t i o n shows t h e shale

p a r t i n g s t o be t u r b i d i t e - l i k e . Sabana

Grande

section

N e i t h e r shale nor c h e r t beds from t h e one

measured

showed

a

turbidite

bedding-thickness

distribution. We f u r t h e r c h a r a c t e r i z e d beds by determining t h e mineralogy i n d e t a i l (up t o 20 samples per bed) across s i x beds. quartz c r y s t a l l i n i t i e s f o r

chert

parameters p l u s quartz/opal-CT

beds

We p l o t t e d q u a r t z peak areas and of

t h e Nicoya Complex,

and these

r a t i o s f o r rocks o f t h e Sabana Grande u n i t .

wide v a r i e t y o f p a t t e r n s emerged,

A

b u t most beds showed f l u c t u a t i o n s t h a t

corresponded w i t h t h e laminations o f t h e chert.

No general conclusions can be

drawn from such measurements on c h e r t s from t h e Nicoya Complex o r Sabana Grande u n i t . From t h e data a v a i l a b l e , t h e o r i g i n o f t h e bedding i s equivocal,

b u t our

best guess i s t h a t t u r b i d i t y c u r r e n t s deposited most o f t h e c h e r t s i n t h e Nicoya Complex and some o f t h e c h e r t s i n t h e Sabana Grande u n i t , interbedded

shales

represent

both

tails

of

turbidites

and

whereas

hemipelagic

deposition.

PETROGRAPHY AND MINERALOGY

X-ray mineralogy Cherts from 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 some rocks (Table 1).

I n contrast

rocks from t h e Sabana Grande u n i t c o n t a i n m c h opal-CT and some c a l c i t e .

Some

rocks from areas o u t s i d e t h e Nicoya Peninsula c o n t a i n much c a l c i t e (Table

1). The c r y s t a l l i n i t y o f q u a r t z i s low ( l e s s than 2 on Murata and Norman's (1976) s c a l e o f 10) i n most rocks from t h e Nicoya Complex (Table l), which i s i n d i c a t i v e o f low-temperature d i a g e n e t i c q u a r t z (Murata and Norman, 1976). I n

156

some areas on t h e Nicoya Peninsula quartz c r y s t a l l i n i t i e s are high, greater than 6; high c r y s t a l l i n i t i e s mark areas o f l o c a l i z e d hydrothermal a c t i v i t y t h a t r e c r y s t a l l i z e d t h e quartz. I n t h e Sabana Grande u n i t , rocks have low values f o r quartz c r y s t a l l i n i t y but c o n t a i n w e l l - t o moderately well-ordered opal-CT w i t h an average d(101) spacing o f 4.079

I n other sections from

A.

circum-Pacific areas, d(101) spacings vary between 4.040 and 4.131 A (Murata and Nakata, 1974; Hein e t al., 1978, 1982). The mineralogy o f c h e r t s from t h e Sabana

Grande

unit

indicates

that

they

probably

were

not

heated

temperatures greater than 60" t o 70°C during t h e i r h i s t o r y (Hein e t al., Hein and Yeh,

1982).

to

1978;

Large amounts o f opal-CT i n t h e c h e r t s from t h e Sabana

Grande u n i t d e f i n i t e l y sets them a p a r t from c h e r t s i n t h e Nicoya Complex i n terms o f mineralogic maturity. Rocks from areas outside t h e Nicoya Peninsula g e n e r a l l y have higher quartz c r y s t a l l i n i t i e s ,

b u t c r y s t a l l i n i t i e s vary over a

wide range. The most common v e i n mineral i s quartz and i t Occurs i n v i r t u a l l y every rock studied (Table 1). Peninsula a r e b a r i t e , oxides.

Other comnon v e i n minerals i n cherts on t h e Nicoya

stilbite,

laumontite,

c l a y minerals, hematite, and Mn

I n t h e Sabana Grande u n i t , quartz and c a l c i t e a r e t h e main minerals

i n veins.

Near G o l f i t o ,

Quepos, Jaco,

and t h e Osa Peninsula,

quartz and

c a l c i t e veins a r e comnon; other veins a r e made o f c l a y minerals, hematite, pumpellyite, laumontite, heulandite, a p o p h y l l i t e , and s t i l b i t e . No secondary minerals occur

that

would

indicate a

prehnite-pumpellyite facies. Thus, Nicoya Complex rocks a r e very

grade

of

metamorphism beyond t h e

low-grade

low-pressure metamorphic

rocks t h a t were probably not heated t o temperatures beyond 150" t o 25OOC. Local hydrothermal temperatures may have been greater. a t t e s t t o t h e great m o b i l i t y o f S i , A l ,

Secondary minerals

Ca, Mg, Fe, Mn, and perhaps K and Na

w i t h i n t h e Nicoya Complex and r e l a t e d rocks.

The Occurrence o f a p o p h y l l i t e

suggests t h a t some o f t h e hydrothermal f l u i d s were r i c h i n f l u o r i n e .

Petrography Petrographic d e s c r i p t i o n s o f Nicoya Complex c h e r t s a r e a p t l y provided by Gursky and Schmidt-Effing ( t h i s volume).

We b r i e f l y r e s t a t e a few p o i n t s t h a t

a r e o f special i n t e r e s t t o us and add a few observations. from t h e

Cherts and jaspers

Nicoya Peninsula a r e composed o f well-preserved

t o ghosts

of

r a d i o l a r i a n s (0 t o 90%) set i n a fine-grained m a t r i x o f microgranular quartz. Radiolarians a r e recognizable up t o a m a t r i x g r a i n - s i z e o f 12 t o 1511m. V i r t u a l l y a l l cherts c o n t a i n a t l e a s t t r a c e amounts o f r a d i o l a r i a n s Varying amounts o f hematite, Mn oxides, i l l i t e , c h l o r i t e , (Table 1). smectite,

and t r a c e s

of

detrital

quartz,

feldspar,

pyroxene,

and other

157

minerals a r e scattered through t h e rocks. Rarely, laminae o r lenses o f d e t r i t a l minerals occur i n t h e chert, e s p e c i a l l y a t Huacas and Playa Pedregosa. Jaspers generally a r e composed o f masses o f c o l l o f o r m t o o o l i t i c spherules o f c l e a r quartz imbedded i n masses o f g o e t h i t e - r i c h hematite-rich

(red) quartz.

(yellow) and

Cherts and jaspers a r e c u t by many veins as

l i s t e d i n t h e previous s e c t i o n and a r e c o m o n l y h i g h l y f r a c t u r e d g i v i n g t h e appearance o f microbreccias.

Microbreccia pipes are common and were probably

formed by f i l l i n g o f an open f r a c t u r e by fragments o f adjacent and o v e r l y i n g rocks t h a t were subsequently cemented by quartz. Most cherts a r e laminated and many a r e graded (Table 1).

Many c o n t a i n f l a t t e n e d and a l i g n e d r a d i o l a r i a n s ,

whereas more c l a y - r i c h c h e r t s show aggregate e x t i n c t i o n o f the c l a y minerals, mostly i l l i t e .

Ghosts o f d e t r i t a l minerals suggest t h a t quartz replaced some

primary minerals; DSDP (Deep

t h i s replacement process i s common i n cherts recovered on

Sea D r i l l i n g P r o j e c t ) Leg 62 (Hein e t al.,

1981).

Some c h e r t s were

i n i t i a l l y glassy volcanic debris t h a t was replaced and cemented by quartz, f o r example 880-24-313 (Table 1). Cherts from t h e Sabana Grande u n i t are generally more extensively burrowed and c o n t a i n more f o r a m i n i f e r s and sandstone beds than do c h e r t s from t h e Nicoya Complex. contain

I n t e r c a l a t e d d e t r i t a l sandstones i n Sabana Grande sections

radiolarians,

plagioclase,

quartz,

foraminifers,

hornblende,

and other minerals;

much

biotite,

pyroxene,

rocks a r e cemented by opal-CT f

quartz f c a l c i t e . Framework g r a i n s a r e g e n e r a l l y very angular but moderately well-sorted. Some c h e r t beds a r e s i l i c i f i e d limestones. These s i 1i c a replaced limestones look i d e n t i c a l i n t h i n s e c t i o n t o P a c i f i c DSDP c h e r t s which a l s o formed by replacement o f host carbonate rocks,

b u t i n a very

d i f f e r e n t environment. Rocks outside t h e Nicoya Peninsula a r e generally more fractured, more c a l c i t e (as veins and f o r a m i n i f e r s ) ,

volcanic d e t r i t u s ,

pyrite,

cherts

and fewer

radiolarians

than

do

i n the

contain

chlorite,

Nicoya

and

Complex.

Limestones a r e generally harder and more r e c r y s t a l l i z e d than those i n t h e Sabana Grande u n i t even though both groups a r e o f s i m i l a r ages.

GEOCHEMISTRY

The chemical data are presented as averages and r a t i o s f o r several rocks from a s i n g l e outcrop o r from groups o f outcrops, c o l o r e d rocks from a l l outcrops (Table 3).

and as averages o f same-

A d d i t i o n a l averages a r e given f o r

two shale beds, two basalts, t h r e e manganese ores, and a hematite ore from t h e Nicoya Complex f o r purposes o f comparison w i t h end-member suites.

Data w i l l

not be presented here f o r rocks outside o f t h e Nicoya Peninsula, most o f which c o n t a i n CaC03, b u t w i l l be presented elsewhere along w i t h rare-earth element

I -I

Wl!-Cl WI1.CT

wal-CT

snectite

-I

4.m7

0.16

157

1.057

0.a5

I10

I

4.095

0.1'

84

7.5

I

880.24.~

.

ranc

h e ~

I

RRR. F

waI-LT

4.088

0.54

86

I

658'0

982'0

110'0 151'0

110'0 i51'18

F

I

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5'Oi FT52 1'29

1'9b6 1'961

'0011

1'90D .

'9) -

1'9Fb

'18 '60 '0091

2

'I2 '98 '81

1'15 10'1 C'F2

2

12'1

10.0)

10'0)

I'll 65'1

6C'O

01'0

02'0) 11'0

02'0) 51'0

52'0

19'0

20.0)

01'0)

6'29 21'0

11'0

11'1

52'2

lO't6

VC.'W1

69'66 56'1 (2'0

P9'2

9 95'0

20'1 81'0

00'0

Lb'O

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'sic '5'I1 'OD '111 'Pi1 'S '1) '19 '02)

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95'66 (1'2

10'0

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168'0

2 '6 E'60 0'82 ('101 1:15

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1:oz 082 1 :5

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126'0 181'0

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60'0 80'0 10'0 M'O 9F'O

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8'15

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52'66 91'2

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29'0 01'0

85'2

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IC.2

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165'0 2E6'0

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12'86 bI'F

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98'1

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10'0

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02'0)

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81'0 20'0

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92'66 10'2 FO'O OI'OI 6F'O 10'0

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ow

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20u ~ O ~ I V

FF'IL

160

data f o r a l l t h e c h e r t s from Costa Rica (Hein e t al., i n preparation). Chemically, Nicoya Complex c h e r t s can be c l a s s i f i e d as low Fe low Mn v a r i e t y we c a l l chert, a high Mn v a r i e t y (more than 2% elemental Mn) we c a l l Mn chert, and a high Fe v a r i e t y (greater than 2% elemental Fe) we c a l l Fe c h e r t (Table 2).

Most b u t not a l l Fe c h e r t s t e x t u r a l l y a r e jaspers. A l l types

o f c h e r t and jaspers together average about 90% Si02 and 2.5% A1203 w i t h very

.

l i t t l e Ti02 i n most rocks Fee03 and MnO a r e h i g h l y v a r i a b l e and FeO i s low i n a l l s i l i c e o u s rocks. Most other elements occur i n small amounts w i t h t h e exception o f barium.

The high barium contents,

up t o 4.8% Ba,

were not

expected from our knowledge o f t h e chemistry o f other cherts, and average j u s t over 0.3% f o r a l l t h e c h e r t s and jaspers together, j u s t over 0.5% f o r Mn chert, and 2.3% f o r Mn ore. Other than h i g h Mn contents, Mn c h e r t i s d i s t i n g u i s h e d by very low FeO and high Ba, Co, Cu, and N i values, whereas Fe c h e r t i s d i s t i n g u i s h e d by l o w Al2O3, Ti02, K20, and Y, and h i g h CaO. Fe c h e r t i s a l s o d i s t i n g u i s h e d by a low value f o r Si/Si+Al+Fe and Al/Al+Fe+Mn and a h i g h value f o r Fe/A1. Compared t o chert, shale from t h e Nicoya Complex i s enriched i n Al203, Ti02, MgO, K20, B, Be, C r , and Sc, and depleted i n Si02, MnO, and Ba. Shale has t h e lowest value f o r Si/Si+Al+Fe. The various c o l o r s o f c h e r t i n t h e Nicoya Complex r e f l e c t some d i f f e r e n c e s i n t h e i r chemical compositions (Table 3). MnO and N i and high Zn and B.

Brown c h e r t i s characterized by low

Red chert,

being t h e dominant type i n t h e

Nicoya Complex, i s s i m i l a r i n chemical composition t o t h e average f o r c h e r t s i n t h i s study.

Black c h e r t s are characterized by very low FeO and h i g h MnO,

Co, Cu, and N i , whereas gray c h e r t s a r e high i n A1203 and l o w i n MnO. Green c h e r t contains rmch Al2O3, Ti02, MgO, K20, N i , V, and Zn and l i t t l e FepO3, and thus may be more contaminated by c l a y minerals than t h e other types o f chert; t h i s i s a l s o evident from t h e low Fe/A1 r a t i o and high Si/Al+Fe+Mn r a t i o . White c h e r t s a r e high i n Si02, Y, and Cu, and lower i n most other elements r e l a t i v e t o other types o f cherts. These d i f f e r e n c e s a r e t h e r e s u l t o f varying

amounts

of

terrigenous,

volcanic,

hydrothermal,

and authigenic

components i n t h e rocks. Cherts a t each outcrop o r set o f outcrops a r e a l s o d i s t i n c t i n t h e i r chemical compositions (Table 3).

Chert from B r a s i l i t o i s high i n B, Cu. and

Zn, and low i n V. Punta Salinas c h e r t i s characterized by low MnO, Ba, Sr, and Y. whereas Playa Real c h e r t s a r e high i n these elements p l u s N i and Co, but

low

i n Cr.

Playa Pedregosa c h e r t

i s h i g h i n those elements t h a t

c h a r a c t e r i z e contamination by d e t r i t a l o r volcanic debris, Al2O3, CaO, Na20, Be, and Cu. Near Sardinal, c h e r t s are characterized by high Ba and N i , and

low B.

Cherts from Huacas a r e e s p e c i a l l y l o w i n C r and Y, b u t h i g h i n V.

The

Mn-rich c h e r t s a t E l Frances c o n t a i n r e l a t i v e l y minor Al2O3, TiO2, MgO, C r ,

161 S r , and Y, whereas t h e Fe-rich c h e r t s a t H a t i l l o and Judas c o n t a i n rmch CaO, MgO, B, Sr, and V b u t l i t t l e l3a and Zr. These d i f f e r e n c e s can be seen i n t h e r a t i o s o f S i , A l , Fe, and Mn t h a t c h a r a c t e r i z e these Mn- and Fe-rich rocks.

The younger c h e r t s o f t h e Sabana Grande u n i t , known t o have been deposited near an i s l a n d arc (Stibane,

e t al.,

1977; Kuijpers, 1980; Lundberg,

1981),

c o n t a i n r e l a t i v e l y l o w amounts o f Fe2O3, K20, MnO, Y, Zn, and Zr, b u t a r e not e s p e c i a l l y enriched i n those elements c h a r a c t e r i s t i c o f d e t r i t a l minerals such as Al2O3, Ti02, and others. Cherts o f t h e Sabana Grande u n i t a r e s l i g h t l y enriched i n B and d e f i c i e n t i n Cu, N i , Y, Zn, K20, and MnO r e l a t i v e t o t h e average c h e r t from t h e Nicoya Complex; a l l other elements a r e comparable. F o r comparison,

Mn ore i s r i c h i n most o f t h e t r a c e elements,

Cu, N i , and Ba, b u t i s l o w i n Zr.

especially

I n contrast, Fe ore i s low i n most t r a c e

elements, e s p e c i a l l y Ba, b u t i s r i c h i n Cr, and e s p e c i a l l y r i c h i n Cu and V. A r e v e a l i n g p a t t e r n emerges when FezO3, MgO, and K20 values f o r c h e r t s a r e p l o t t e d on a t e r n a r y diagram (Fig. 3). Both t h e d i f f e r e n t c o l o r c h e r t s and

t h e c h e r t s from various outcrops p l o t along a s t r a i g h t l i n e t h a t o r i g i n a t e s a t t h e Fez03 apex and b i s e c t s t h e t r i a n g l e . This p a t t e r n means t h a t t h e r a t i o o f K20 and MgO remains constant i n a l l rocks and t h a t K20 and MgO occur i n one o r more mineral phases t h a t a r e d i f f e r e n t from t h e phase c o n t a i n i n g FezO3.

The

absolute amounts o f Fez03 versus K20+Mg0 vary according t o t h e distance t h a t t h e sediment was deposited from t h e Fez03 o r KZO+MgO sources.

Also note t h a t

t h e average c h e r t and shale from t h e Nicoya Complex p l o t close together suggesting t h a t t h e d e t r i t a l p l u s volcanic components (KzO+MgO) found i n t h e shale a r e t h e same as those t h a t occur as t r a c e components i n t h e chert. In o t h e r words, t h e m t e r i a l s t h a t contaminate t h e c h e r t a r e t h e same m a t e r i a l s t h a t make up t h e interbedded shales. This can a l s o be seen by normalizing t h e major elements of average c h e r t t o t h e SiO2 content o f t h e average shale from t h e Nicoya Complex; doing t h i s produces comparable compositions f o r c h e r t and shale o f t h e Nicoya Complex,

b u t w i t h s l i g h t l y more A1203 and Na20 and

s l i g h t l y l e s s TiO2, FezO3, MgO, and K20 i n t h e chert. Also p l o t t e d on t h e Fe203-~gO-K20 t e r n a r y diagram are p o i n t s from Costa Rica ores and basalt, from P a c i f i c Ocean c h e r t s and pelagic clays, and from average terrigenous shale.The average shale and c h e r t from t h e Nicoya Complex p l o t c l o s e t o values f o r both P a c i f i c pelagic c l a y s and f o r hemipelagic muds from o f f s h o r e Japan, b u t t h e i n d i v i d u a l Costa Rica c h e r t values span t h e range from average terrigenous shale t o East P a c i f i c Rise m e t a l l i f e r o u s sediment.

A constant r a t i o o f MgO

and K20 can be seen a l s o f o r c h e r t s and jaspers o f t h e Mid-Pacific Mountains ( 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 Hess Rise (central, n o r t h P a c i f i c ; DSDP Leg 62).

b u t f a v o r i n g more potassic compositions than Costa Rica rocks (Fig.

3)

due t o potassium metasomatism o f b a s a l t i c d e b r i s i n t h e Hess Rise and MidP a c i f i c Mountains (Hein and Vanek, 1981).

162 EXPLANATION

A

1 Nicoya Complex Mn Ore

A

2 Nicoya Complex Fe Ore

0

3 Average Nicoya Complex Cherls and Jaspers

0

4 Average Nicoya Complex Chert

0

5 Average Nicoya Complex Mn Chert 6 Average Nicoya Complex Fe Chert

0

. . 0

7 Average Nicoya Complex Brown Chert 8 Average Nicoya Complex Red Chert 9

Average Nicoya Complex Black Chert

10 Average Nicoya Complex Green Chert 1 1 Average Nicoya Complex Gray Chert 12 Average Nicoya Complex White Chert 13 Brasililo Cherls and Jaspers 14 Punla Saltnas Cherts and Jaspers 15 Playa Real Cherts and Jaspers 16 Playa Pedregosa Cherts and Jaspers

K,O

0

17 Sardinal Cherts and Jaspers

0

19 El Frances Cherls and Jaspers

0

2 0 Halillo Cherls and Jaspers

18 Huacas Cherts and Jaspers

50

2 1 Judas Cherls and Jaspers 23 Sabana Grande Cherts

*

24

A v e r a g e Nicoya Complex Shale

25

A v e r a g e Nicoya Complex Basalt

0 28

DSDP L e g 6 9 C h e r t . C o s t a Rita R i f t

0 27 Average Terrigenous Shale

/

I

0 28 Northwest Pacific Pelagic Clay

0

29 Easl-central Paclllc Siliceous Pelagic

163

Mn0)xlO

Alp 0,x 10

VXlO

F%Ure colored

4-

T e m P Y d%rms: cherts,

for

( A ) S i 0 2 - A l 2 0 3 - ( ~ e 2 0 3 ~ 0 ) ~ for 1 0 different

c h e r t s from d i f f e r e n t

outcrops

on

the

Nicoya

Peninsula, and f o r other rocks used f o r comparison. ( B ) K-Ea-VxlO f o r c h e r t s from d i f f e r e n t outcrops a the Nicoya Peninsula and f o r other rocks used f o r comparison. Symbols and sources of’ data are the m e QB f o r Figure 3 .

164

On t h e same t e r n a r y diagram green c h e r t s are t h e most MgO- and K20-rich, and Fe-cherts (jaspers) a r e r i c h e s t i n Fez03 as might be expected. A l l o t h e r c o l o r s p l o t almost on a s t r a i g h t l i n e connecting these two end members. Green c h e r t as w e l l as Mn ore p l o t t o t h e K20 side o f t h i s connecting l i n e , whereas w h i t e and gray c h e r t s c o n t a i n r e l a t i v e l y more MgO. On a t e r n a r y diagram of Si02-A1203-(Fe203~nO)x10 (Fig. 4A) Costa Rica c h e r t s and jaspers p l o t toward t h e A1 and Fe+Mn apices r e l a t i v e t o open-ocean DSDP c h e r t s (Legs 62 and 69).

The v a r i a t i o n i n A1 i n c h e r t s from Costa Rica

i s small compared t o v a r i a t i o n s i n t h e Si02 and Fe203MnO contents.

Costa

Rica Fe and Mn ores, and East P a c i f i c Rise m e t a l l i f e r o u s sediment p l o t near t h e (Fe203+MnO)x10 apex; Fe c h e r t and samples from H a t i l l o on t h e Nicoya Peninsula p l o t nearer t o these m e t a l - r i c h rocks and sediments than any o t h e r c h e r t s from Costa Rica. Cherts from t h e Sabana Grande u n i t a r e t h e most Si02r i c h and Fe203+MnO-poor, but a r e not t h e most A1203-rich as might be expected from t h e i r l o c a t i o n o f deposition near a volcanic arc. The r e l a t i v e c o n t r i b u t i o n s t o t h e c h e r t s o f basic

volcanic

debris,

d e t r i t a l - v o l c a n i c l a s t i c debris, and hydrothermal i n p u t can best be seen on a t e r n a r y p l o t o f K-Ba-VxlO (Fig. 48).

I r o n ore p l o t s c l o s e s t t o t h e VxlO apex

and probably represents t h e r e s i d u a l d e b r i s from intense weathering o f b a s a l t i c rocks. I r o n i n cherts, along w i t h Ba and Mn, i s o f mostly hydrothermal origin.

Mn ores p l o t c l o s e t o t h e Ba (hydrothermal) apex as does

East P a c i f i c Rise m e t a l l i f e r o u s sediment.

The average c h e r t from t h e Nicoya

Complex p l o t s midway bewteen t h e Ba-K j o i n , whereas t h e average shale from t h e Nicoya Complex p l o t s near t h e K apex.

Cherts from each outcrop d i s p l a y a

unique m i x t u r e o f t h e t h r e e end-member components.

DISCUSSION

Environment o f Deposition A prime purpose o f t h i s study i s t o determine t h e depositional environment

o f c h e r t s from t h e Nicoya Complex.

Deposition o f c h e r t s from t h e Sabana

Grande u n i t i n basins adjacent t o an i s l a n d arc,

e i t h e r i n t h e area o f t h e

arc-trench gap o r i n an i n t e r - a r c basin i s w e l l documented and we agree w i t h t h i s i n t e r p r e t a t i o n (de Boer,

1979;

Kuijpers,

sandstone and s i l t s t o n e beds

rich

i n amphibole and b i o t i t e w i t h i n t h e

1980;

Lundberg,

1982).

The

limestone and c h e r t from t h e Sabana Grande u n i t a t t e s t t o a nearby and emergent volcanic arc. The depositional s e t t i n g o f c h e r t s from t h e Nicoya Complex however, i s nuch more d i f f i c u l t t o e s t a b l i s h and marly environments have been proposed ranging from deep-sea open-ocean ( f a r away from a c o n t i n e n t o r i s l a n d a r c ) t o areas near t o t h e c o n t i n e n t a l slope (see references i n our I n t r o d u c t i o n section).

165

The b a s a l t s o f t h e lower and upper t h r u s t sheets o f t h e Nicoya Complex were thought t o be t y p i c a l o f mid-oceanic r i d g e b a s a l t s (MORB) and t h e Nicoya Complex was proposed t o be a piece o f oceanic c r u s t (Dengo, 1962; Henningsen and Weyl,

1967).

Wildberg e t al.

On t h e b a s i s o f major and minor element compositions, (1981) showed t h a t i n f a c t t h e b a s a l t o f t h e lower t h r u s t

sheet i s t y p i c a l o f MORB, t h a t i s , t y p i c a l o f oceanic o r back-arc basin crust, whereas b a s a l t s from t h e upper t h r u s t sheet include both p r i m i t i v e i s l a n d arc rocks and MORB.

The general b e l i e f t h a t t h e Nicoya Complex represented

oceanic c r u s t and recovery o f c h e r t s from many oceanic sequences by t h e Deep Sea D r i l l i n g P r o j e c t l e d many workers t o propose t h a t DSDP and Nicoya Complex c h e r t s a r e analogous.

However,

c l o s e comparison shows few s i m i l a r i t i e s

between DSDP open-ocean cherts recovered from t h e P a c i f i c and cherts t h a t occur i n orogenic b e l t s a t c o n t i n e n t a l margins (Hein and Karl, t h i s volume). Hein and K a r l argued t h a t many,

i f not most,

o f t h e orogenic b e l t c h e r t s

formed near c o n t i n e n t a l margins. The mineralogic and chemical data, and various arguments we present here, support a r e l a t i v e l y near-shore depositional environment f o r t h e shales and c h e r t s o f t h e Nicoya Complex: (1) The shale beds r h y t h m i c a l l y interbedded w i t h t h e c h e r t s o f t h e Nicoya Complex a r e composed o f i l l i t e - r i c h terrigenous c l a y minerals t h a t nust have been derived from a nearby c o n t i n e n t a l mss o r i s l a n d arc.

I n t h e South P a c i f i c a l l pelagic c l a y s a r e smectite-rich,

as a r e many i n

t h e North P a c i f i c , although i n t h i s l a t t e r area some a r e i l l i t e - r i c h ,

b u t they

always c o n t a i n other minerals not found i n t h e shale beds o f t h e Nicoya Complex.

Metamorphism o f t h e Nicoya Complex i s not great enough t o have

transformed o r i g i n a l smectite-rich beds t o i l l i t e . This conclusion i s a l s o supported by t h e occurrence o f smectite t h a t represents a l t e r e d b a s a l t i c (2) The Nicoya c h e r t beds were probably deposited by debris i n some rocks. t u r b i d i t y currents. Thus, slopes and basins f o r t h e generation and deposition

o f t u r b i d i t e s a r e necessary and occur most comnonly a t c o n t i n e n t a l margins, although such slopes and basins can occur a l s o on slow-spreading oceanic ridges, some aseismic ridges, and l o c a l l y around seamounts. (3) The chemical composition o f c h e r t s f r o m t h e Nicoya Complex i s s i m i l a r t o t h e composition o f c h e r t s f r o m , t h e Sabana Grande u n i t b u t d i f f e r s from c h e r t s from t h e deep P a c i f i c basin recovered during DSDP Legs 62 and 69; Leg 62 and 69 c h e r t s are t y p i c a l o f o t h e r P a c i f i c basin c h e r t s p e t r o g r a p h i c a l l y , t e x t u r a l l y , and i n l i t h o l o g i c associations.

These chemical d i f f e r e n c e s a r e evident i n t h e higher

A1203 contents i n most c h e r t s from t h e Nicoya Complex suggesting a g r e a t e r component o f e i t h e r terrigenous o r volcanic d e t r i t u s than i n t h e DSDP cherts.

Because c h e r t s from t h e Costa Rica R i f t recovered during DSDP Leg 69

o f t h e b a s a l t i c oceanic c r u s t we b e l i e v e i t i s terrigenous d e t r i t u s r a t h e r than t h e associated b a s a l t s causing t h e greater formed w i t h i n

meters

166 A1203 values i n t h e c h e r t s o f t h e Nicoya Complex. minerals

and

Complex.

volcanic

debris

were

found

Small amounts of d e t r i t a l

i n many

cherts

of

the

Nicoya

Some d e t r i t a l minerals were probably replaced by s i l i c a d u r i n g

diagenesis,

but s t i l l ,

t h e c h e r t s from t h e Nicoya Complex formed i n basins

(4) The

t h a t were g e n e r a l l y i s o l a t e d from i n p u t o f most d e t r i t a l debris.

constant r a t i o of K20 ( i l l i t e ) t o MgO ( s m e c t i t e ) i n c h e r t samples suggests t h a t both t e r r i g e n o u s and v o l c a n i c d e b r i s were c o n t i n u o u s l y s u p p l i e d t o t h e d e p o s i t i o n a l basins i n constant p r o p o r t i o n s but i n d i f f e r e n t absolute amounts depending

probably

on

the

distance

of

the

basin

from

the

terrigenous

source(s).

Hydrothermal i n p u t (Ba+Fe+Mn) v a r i e d g r e a t l y on a l o c a l s c a l e and

marks

spots

hot

i n the original

oceanic

settings.

(5)

The

lithologic

a s s o c i a t i o n and modes of formation o f c h e r t i n t h e Nicoya Complex a r e u n l i k e those

i n any o f

t h e holes d r i l l e d by t h e DSDP i n adjacent

Caribbean areas (Hein and K a r l , t h i s volume).

P a c i f i c and

(6) An argument used i n t h e past

i n f a v o r o f d e p o s i t i o n of t h e Nicoya c h e r t s i n very deep water i s t h e general absence o f carbonates and, t h e r e f o r e , accumulation below t h e CCD, even though t h e l e v e l of t h e CCD i s not w e l l known f o r t h e e a r l y Cretaceous o f t h e East Pacific.

However, two o t h e r ways e x i s t i n t h e present oceans ( t h a t may a l s o

have e x i s t e d i n t h e Cretaceous oceans) t o d e p o s i t s i l i c a - r i c h ,

carbonate-poor

sediments: (a) I n areas of i n t e n s e u p w e l l i n g c a l c i t e - s e c r e t i n g plankton cannot compete w i t h s i l i c a - s e c r e t i n g plankton.

Examples are t h e s i l i c e o u s oozes on (b) A t h i g h l a t i t u d e s ,

t h e s h e l f and slope o f f Peru and NW A f r i c a .

such as

o f f A n t a r c t i c a and t h e f a r North P a c i f i c , B e r i n g Sea, and A r c t i c Ocean regions s i l i c e o u s oozes p r e v a i l .

Intense u p w e l l i n g i n a s h e l f - s l o p e environment i s

c o n s i s t e n t w i t h o t h e r sedimentary c h a r a c t e r i s t i c s o f t h e Nicoya Complex. The combined evidence suggests t h a t sedimentary rocks o f t h e Nicoya Complex formed on oceanic c r u s t w i t h l o c a l i z e d ,

i n t e n s e hydrothermal a c t i v i t y ,

a l s o near t h e margin o f a c o n t i n e n t o r i s l a n d arc. anoxic as shown by t h e abundant laminated cherts. i n p u t s o f much t e r r i g e n o u s debris,

I n t h e modern ocean basins, areas

these c h a r a c t e r i s t i c s o f d e p o s i t i o n a l

basins such as t h e Gulf basins.

Basins were i s o l a t e d from

presumably e i t h e r by topographic highs,

i n t e r v e n i n g basins, o r an oceanic trench. that f u l f i l l

Azema

and

of

California,

Tournon

but

Most o f t h e basins were

(1982;

back-arc DSDP

Leg

basins are young ocean

basins, 67)

o r a r c - t r e n c h gap

described

Campanian-

M a a s t r i c h t i a n deposits t h a t o v e r l i e b a s a l t i c basement on t h e lower c o n t i n e n t a l slope o f f Guatemala s i t u a t e d not f a r t o t h e north. accreted t o t h e margin and are very d i f f e r e n t

These rocks were n o t

from oceanic rocks recovered 3

km t o t h e west on t h e COCOS p l a t e . Rocks o f t h e Nicoya Complex were probably deposited i n small basins a t a t e c t o n i c a l l y a c t i v e margin. We cannot define t h e environment o f d e p o s i t i o n any m r e p r e c i s e l y from t h e data a v a i l a b l e . Two unknown aspects o f t h e

167 geology o f t h e Nicoya Complex t h a t a r e needed t o h e l p decipher t h e geologic h i s t o r y i n c l u d e t h e age o f t h e b a s a l t a t each c h e r t outcrop and t h e age range o f c h e r t s w i t h i n each outcrop.

Only two r a d i o m e t r i c ages f o r b a s a l t s a r e

a v a i l a b l e , and probably o n l y one o f those (72.5 ( B a r r and Escalante,

1969).

my.)

represents a c o o l i n g age

Cherts from d i f f e r e n t outcrops i n t h e Nicoya

Complex were apparently deposited over a p e r i o d o f about 70 my., amount o f t i m e represented by any s i n g l e outcrop i s n o t known.

yet the

The o n l y data

we have i s f o r a s i n g l e 200 m s e c t i o n o f rocks from t h e Sabana Grande u n i t where b o t h t h e top and base o f t h e s e c t i o n date as e a r l y t o middle Eocene; we about 30 t o 40 m t h i c k ,

speculate t h a t each Nicoya outcrop,

consists o f

r e l a t i v e l y r a p i d l y deposited t u r b i d i t e s composed o f mostly s i l i c e o u s d e b r i s and i n t e r v e n i n g hemipelagic shales t h a t t o g e t h e r represent b r i e f periods o f time, perhaps a maximum o f 10 m.y. L i t t l e evidence e x i s t s t h a t can be used t o r e c o n s t r u c t t h e paleogeography o f t h e M i d d l e America area f o r E a r l y t o M i d d l e Cretaceous times.

Galli-

O l i v i e r (1979) and Schmidt-Effing

(1979) suggested t h a t t h e Middle American

arc

latest

came

i n t o existence i n the

Santonian o r

earliest

Campanian.

However, what t h e i r evidence r e a l l y i n d i c a t e s i s t h a t t h e e a r l i e s t Campanian i s t h e t i m e when t h e v o l c a n i c a r c became predominantly an emergent f e a t u r e ; a submarine r i d g e may have e x i s t e d much e a r l i e r as i s a l s o suggested by Wildberg e t al.'s

(1981) chemical data f o r t h e basalts.

Many scenarios f o r t h e o r i g i n

and e v o l u t i o n o f t h e Nicoya Complex have been proposed,

most d e a l i n g w i t h

c l a s s i c p l a t e t e c t o n i c models o f r a f t i n g o f sediment on an oceanic p l a t e as t h e r e s u l t o f s e a f l o o r spreading.

A d d i t i o n a l scenarios can a l s o be proposed,

f o r example, s t r i k e - s l i p movement o f t h e complex from a more s o u t h e r l y s i t e o f deposition,

adjacent

to

South America,

to

its

present

location,

or

by

"docking" o f small fragments o f c o n t i n e n t a l margin rocks t h a t formed elsewhere i n t h e P a c i f i c onto nuclear Central America.

L i t t l e evidence e x i s t s a t t h i s

t i m e f o r any o f t h e proposed scenarios.

Distance from Source Cherts o f t h e Nicoya Complex a r e chemically q u i t e v a r i a b l e i n Fe and Mn contents but

similar

represent about 70 m.y.

i n contents

of

o t h e r elements.

Cherts a p p a r e n t l y

o f h i s t o r y a t d i f f e r e n t outcrops w i t h i n t h e small area

o f t h e Nicoya Peninsula, which i s one o f t h e most enigmatic aspects o f these deposits, e s p e c i a l l y w i t h regards t o t h e s p a c i a l r e l a t i o n s o f t h e source rocks t o t h e d e p o s i t i o n a l basins. Galli-Olivier

(1979)

explained t h i s g r e a t v a r i e t y o f ages a t d i f f e r e n t

outcrops as t h e r e s u l t o f juxtaposed blocks i n a melange, although from our f i e l d work

a melange i n t h e c l a s s i c

sense t h a t

is,

b l o c k s i n a sheared

168

mudstone o r serpentine matrix, does not exist. Rather t h e Nicoya Complex c o u l d be described more as a megabreccia, l a r g e (several t o tens o f k i l o m e t e r sized) juxtaposed blocks. Schmidt-Effing (1979) and Schmidt-Effing e t al. (1980) explained t h i s vast age range as due t o s l i c e s o f an oceanic plateau accreted t o

the

arc.

However,

no

systematic

pattern

i n the

spacial

d i s t r i b u t i o n o f ages i s seen on t h e Nicoya Peninsula, which would be expected f o r e i t h e r o f these hypotheses.

Patterns may y e t be found when more of t h e

c h e r t bodies a r e dated. Despite t h i s great age range f o r t h e cherts.

the ratios of d e t r i t a l t o

volcanic (K$+MgO) debris remained constant, whereas absolute amounts d i d not, which b r i n g s up the question as t o whether o r not c h e r t s from some outcrops formed

closer

to

the

source

of

terrigenous

detritus

than

did

others. We

Systematic v a r i a t i o n s i n K20tMg0 w i t h ages o f t h e rocks are not apparent.

propose t h a t t h e c l o s e r a c h e r t p l o t s t o t h e MgO-KzO apices on t h e Fe203-MgOK20 t e r n a r y diagram (Fig. 3) t h e c l o s e r i t formed t o t h e terrigenous source(s). Other methods used t o measure r e l a t i v e p o s i t i o n s i n r e l a t i o n t o source terranes w i t h i n a s i n g l e depositional basin a r e provided by Steinberg e t al.

(1977),

Steinberg and Marin (1978),

include t h e r a t i o s o f Si/Si+Al+Fe,

and Rangin e t al.

Al/Al+Fe+Mn,

regression l i n e s on Fe/Al p l o t s (Tables 4 and 5).

Table 4. Localities

Fe/Al,

(1981),

and

and t h e slope o f t h e

Assuming t h a t t h e Nicoya

Data f o r A1 (x) and Fe (y) l i n e a r regressions. y-Intercept

B r a s i 1it o Punta Sa 1inas Playa Real Playa Pedregosa Huacas Sa r d i na 1 Judas Sabana Grande

0.585 5.170 -0.379 1.201 -0.063 0.537 3.528 -0.120

S1ope 0.782 -2.068 0.798 -0.107 1.746 0.410 0.816 0.676

Correlation Coefficient 0.353 -0.824 0.991 -0.103 0.831 0.381 0.128 0.989

c h e r t s formed i n t h e same basin o f deposition ( t h a t i s , a l l c h e r t s experienced similar

detrital-volcanic

sources),

we

ranked,

according

to

these

five

methods, t h e p o s i t i o n o f each outcrop p l u s t h e average shale o f t h e Nicoya Complex r e l a t i v e t o a terrigenous source (Table 5). The ranking o f any p a r t i c u l a r outcrop i s h i g h l y v a r i a b l e depending on t h e method used. The r a t i o Si/SitAl+Fe probably should not be used because o f the dominance o f biogenic s i l i c a and s i l i c a diagenesis.

On the basis o f t h e other f o u r methods, an

average o f t h e r e l a t i v e p o s i t i o n s o f each outcrop places them on a f i r s t

Table 5.

Relative distances from a terrigenous source t e r r a n e where c h e r t s from various outcrops were deposited, as determined by f i v e geochemical techniques. Outcrops c l o s e s t t o t h e source t e r r a n e a r e a t t h e top of l i s t s , those f a r t h e s t away a t t h e bottom. Average Nicoya Complex shale and Sabana Grande c h e r t s a r e included.

Si

Si+A1+Fe 1. Shale 2. H a t i l l o 3. Judas 4. Huacas 5. Playa Pedregosa 6. Punta Salinas 7. B r a s i l i t o 8. Sardinal 9. El Frances 10. Playa Real 11. Sabana Grande

A1

At+Fe+Mn Sardi nal Shale P1 aya Pedregosa Playa Real Sabana Grande Brasi 1 i t o Punta Sal inas Huacas E l Frances Hatil l o Judas

Fe

AT

Playa Real Sabana Grande Sardinal Shale P1 aya Pedregosa Rrasilito Punta Sal i nas Huacas E l Frances Hatillo Judas

Fe203-Mg0-K20 Average of Average of Last Slope of Fe/A1 Regression Lines Ternary Diagram Five Columns Four Columns

Punta Sal i nas* P1 aya Pedregosa Sardinal Sabana Grande Rrasil i t o Playa Real Judas Huacas

Playa Real Shale Sabana Grande Sardinal Rrasil i t o Playa Pedregosa Punta Salinas Huacas El Frances Hatillo Judas

Shale Sardi nal Playa Pedregosa Playa Real Sabana Grande Punta Salinas Brasilito Huacas Hati 1l o E l Frances Judas

Shale Sardinal Playa Real Sabana Grande P1 aya Pedregosa Punta Salinas Brasil i t o Huacas El Frances Hatillo Judas

* Average shale, El Frances, and H a t i l l o averages a r e based on two samples each, so meaningful regression l i n e s could not be calculated.

170 approximation i n t o two groups: Huacas, E l Frances, H a t i l l o , and Judas, and t h e o t h e r outcrops. The former group i s composed o f c h e r t s t h a t formed as lenses i n basalt,

o r c h e r t t h a t has a l a r g e hydrothermal component.

composition o f

rocks f r o m these outcrops probably

The chemical

i n d i c a t e s hydrothermal

a c t i v i t y r a t h e r than a g r e a t e r d i s t a n c e from a t e r r i g e n o u s source as suggested Shale places a t t h e t o p o f t h e l i s t (nearest t o t h e

by t h e r a t i o o f elements. source).

What i s s u r p r i s i n g i s rocks from t h e Sabana Grande u n i t , known t o

have formed near an i s l a n d arc, a r e placed near t h e middle o f t h e grouping o f outcrops t h a t show s m a l l e r hydrothermal signatures.

Whether o r not rocks from

t h e i n d i v i d u a l outcrops formed i n t h e o r d e r l i s t e d i n Table 4 r e l a t i v e t o a t e r r i g e n o u s source i s not known, b u t we b e l i e v e t h a t these l i n e s o f research warrant more e x t e n s i v e studies.

This t y p e o f evidence should l e a d t o more

r e f i n e d hypotheses regarding d i s t a n c e t o source areas.

The chemical evidence

suggests t h a t rocks from most outcrops were n o t formed a t g r e a t l y d i f f e r e n t distances from t h e source o f t e r r i g e n o u s debris.,

SUMMARY AND CONCLUSIONS Cherts i n t h e Nicoya Complex on t h e Nicoya Peninsula i n Costa Rica occur as 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 - s h a l e ,sequences t h a t r e s t on b a s a l t , and as lenses up t o 100 m long and 3 m t h i c k between b a s a l t flows.

Chert lenses

between b a s a l t f l o w s occur w i t h limestones near Jaco,

and t h e Osa

Peninsula.

Golfito,

Few t o abundant r a d i o l a r i a n s a r e set i n a f i n e - g r a i n e d m t r i x o f

m i c r o g r a n u l a r quartz.

Hematite, Mn oxides, c l a y minerals, and o t h e r d e t r i t a l

m i n e r a l s occur s c a t t e r e d through t h e cherts.

Cherts i n t h e o v e r l y i n g 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 . indicate that

Secondary m i n e r a l s

rocks o f t h e Sabana Grande u n i t were subjected t o maximum

temperatures o f about 60" t o 7OoC, whereas,

rocks o f t h e Nicoya Complex were

subjected t o temperatures ranging from about 150" t o 250"C, pressures.

Locally

intense

hydrothermal

activity

b o t h a t low

produced

greater

temperatures and degrees o f a l t e r a t i o n . Sedimentary s t r u c t u r e s suggest t h a t t h e r h y t h m i c a l l y bedded c h e r t s were deposited by

t u r b i d i t y c u r r e n t s and t h a t

the

interbedded i l l i t i c

shales

probably i n c l u d e b o t h t a i l s o f t u r b i d i t e s and hemipelagic deposits d e r i v e d from a nearby c o n t i n e n t o r i s l a n d arc. Costa Rica c h e r t s average 90% Si02 and 2.5% Al2O3, w i t h very low contents of T i 0 2 and h i g h Ba. c h e r t (red,

brown,

Mn and Fe a r e h i g h l y v a r i a b l e . green,

black,

grey,

The v a r i o u s c o l o r s o f

and w h i t e ) and c h e r t s from v a r i o u s

outcrops on t h e Nicoya Peninsula have d i s t i n c t chemical compositions.

Ternary

p l o t s o f Fe203-MgO-K20 show t h a t t h e r a t i o o f K20 and MgO remained constant f o r a l l t h e c h e r t s o f t h e Nicoya Complex, which a p p a r e n t l y represent about 70

171 my. o f deposition. K20 and MgO occur i n one o r more mineral phases t h a t d i f f e r from t h e phase where Fez03 occurs. The a b s o l u t e amounts o f Fez03 versus MgO and K20 vary 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

the

Fez03 and K20 p l u s MgO sources.

The m i n e r a l s t h a t

contaminate t h e c h e r t s a r e t h e same minerals t h a t make up t h e interbedded

i11it i c shale beds. The d i s t i n c t composition o f c h e r t s from t h e Nicoya Complex r e l a t i v e t o c h e r t s recovered by t h e DSDP from t h e open-ocean ternary

Plot

of

i s w e l l i l l u s t r a t e d on a The

Si02-A1203-(Fe203+MnO)xlO.

relative

amounts

of

contamination from d i f f e r e n t sources o f c h e r t s i n t h e Nicoya Complex and Sabana Grande u n i t can best be seen on a K-Ba-VxlO t e r n a r y diagram. The chemical

and m i n e r a l o g i c compositions,

sedimentary

structures,

and

comparisons w i t h rocks from t h e P a c i f i c Ocean b a s i n suggest t h a t c h e r t s from t h e Nicoya Complex formed i n small i n t e n s e hydrothermal a c t i v i t y ,

basins on oceanic c r u s t w i t h l o c a l l y

b u t near a c o n t i n e n t a l margin o r i s l a n d arc.

Modern environments w i t h 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

the

Gulf

of

California,

environments i n c l u d i n g summit basins.

back-arc

basins,

arc-trench

gap

Cherts from t h e Nicoya Complex probably

formed i n one o r more o f these types o f environments. Sabana Grande u n i t formed i n an a r c - t r e n c h (Stibane e t al.,

and

gap o r

The c h e r t s from t h e inter-arc

environment

1977; Kui j p e r s , 1980; Lundberg, 1982).

ACKNOWLEDGMENTS We thank Sara Monteith, L i s a Moryenson, Enomwoyi Kunde, and Marla Wilson, a l l o f t h e U.S.

Geological Survey,

f o r t e c h n i c a l assistance.

U n i v e r s i t y o f C a l i f o r n i a , Santa Cruz, David W. Geological manuscript.

Survey,

and

Susan

Stanford

N e i l Lundberg,

A.

University

Koski, U. reviewed

S.

the

Escuela Centroamericana, Ciudad U n i v e r s i t a r i a San Jose, provided We a p p r e c i a t e h e l p f u l discussions w i t h Hans

l o g i s t i c support f o r t h i s study. Gursky,

Karl,

S c h o l l and R.

W e s t f a l i s c h e Wilhelms U n i v e r s i t y .

Drs.

Joyce Blueford,

David Bukry

S. Geological Survey p r o v i d e d age dates. Rocks were analyzed for t h e i r chemical composition i n t h e U. S. Geological Survey's

and Charles Blome,

U.

a n a l y t i c a l labs by H.

Neiman,

E.

Campbell,

P. Arvscavage, P. Briggs, J. S .

Wahlberg, J. Taggert, J. Baker, and T. Fries.

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1954. X-ray d i f f r a c t i o n procedures.

Wiley,

Lundberg, N e i l , 1982. E v o l u t i o n o f t h e slope landward o f t h e M i d d l e America Trench, Nicoya Peninsula, Costa Rica. I n : Leggett, J.K. ( E d i t o r ) , Forearc Geology, Geol. SOC. London, pp. 431-447. Matsumoto, R. and I i j i m a , A., 1982. Chemical sedimentology o f some PermoJ u r a s s i c and T e r t i a r y bedded c h e r t s i n C e n t r a l Honshu, Japan. (This volume). S t o c h a s t i c a n a l y s i s o f bed-thickness M i z u t a n i , S. and H a t t o r i , I., 1972. d i s t r i b u t i o n o f sediments. Math. Geol., 4: 123-146. 1974. C r i s t o b a l i t e stage i n t h e diagenesis o f Murata, K.J. and Nakata, J.K., diatomaceous shale. Science, 184: 567-568. Murata, K.J. and Norman, M.B., Am. J. Sci., 276: 1120-1130. P e t t i j o h n , F.J., 1975. York, 628 pp. .

1976.

An index o f c r y s t a l l i n i t y o f quartz.

Sedimentary Rocks.

3rd. ed.,

Harper and Row,

New

Pimm, A.C., 1973. Trace element aeterminations compared w i t h X-ray d i f f r a c t i o n r e s u l t s o f brown c l a y i n t h e Central P a c i f i c . I n : Winterer, E.L., Ewing, J.L., 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. 511-513. P i s c i o t t o , K.A., 1978. Basinal sedimentary f a c i e s and d i a g e n e t i c aspects o f t h e Monterey Shale, C a l i f o r n i a . Ph.D. dissertation, University o f C a l i f o r n i a , Santa Cruz. 448 pp.

174 Rangin, Claude, Steinberg, Michel, and Bonnot-Courtois Chantal 1981. Geochemistry o f t h e Mesozoic bedded c h e r t s o f Centra! Baja C a l i f o r n i a (Vizcaino-Cedros-San Benito): imp1 i c a t i o n s for paleogeographic r e c o n s t s t r u c t i o n o f an o l d oceanic basin. E a r t h Planet. Sci. Letts., 54: 313-322. Schmidt-Effing, R., 1979. A l t e r und Genese des Nicoya-Complexes, e i n e r ozeanischen Palaokruste (Oberjura bis Eozan) im sudlichen Zentralamerika. Geol. Rundschau, 68: 457-494. Schmidt-Effing, R., Gursky, H.-J., Strebin, M., and Wildberg, H., 1980. The o p h i o l i t e s o f southern Central America w i t h s p e c i a l reference t o t h e Nicoya Peninsula (Costa Rica). Trans. 9 t h Caribbean Geol. Conf., Santo Domingo, Dominican Republic. Steinberg, M., Desprairies, A,, Fogelgesang, J.F., M a r t i n , A,, Caron, D., and Blanchet, R., 1977. R a d i o l a r i t e s e t sediments h y p e r s i l i c e u x oceaniques: une comparaison. Sedimentology, 24: 547-563. and Marin, C.M., Steinberg, 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 e t des sediments s i 1iceux oceaniques, s i g n i f i c a t i o n pa leooceani graphique. Oceanol ogica Acta, 1: 359-367. Stibane,

F.R.,

Schmidt-Effing,

R.,

and

Madrigal,

R.,

1977.

Zur

stratigraphisch-tektonischen Entwicklung der H a l b i n s e l Nicoya (Costa Rica) i n der Z e i t von oberkreide b i s u n t e r - T e r t i a r . 315-358.

Giessener Geol.

Schr.,

12:

Wildberg, H., Gursky, H.-J., Schmidt-Effing, R., and Strebin, M., 1981. Development o f t h e P a c i f i c O p h i o l i t e Sequence ("Nicoya Complex") i n northwestern Costa Rica (Central America). (Abs.), In: O p h i o l i t e s and Actualism. O f i o l i t i , 6: 48.

175

CHAPTER 11 CHEMICAL SEDIMENTOLOGY OF I N CENTRAL HONSHU, JAPAN R. MATSUMOTO and A.

Geological

1

SOME

AND

PERMO-JURASSIC

TERTIARY

BEDDED

CHERTS

IIJIMA

Institute,

F a c u l t y o f Science,

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

INTRODUCTION During t h e past

decade,

fine-grained

s i l i c e o u s rocks o f various

ages i n

Japan have been s t u d i e d e x t e n s i v e l y from t h e s t r a t i g r a p h i c and sedimentologic p o i n t s o f view.

I t has been w e l l

e s t a b l i s h e d t h a t most bedded c h e r t s are

composed p r i n c i p a l l y o f s i l i c e o u s skeletons such as r a d i o l a r i a n s h e l l s , spines, sponge

spicules,

Utada, 1982).

and diatom f r u s t u l e s

(Imoto and Saito,

o f bedded c h e r t s i n Japan. t h e major elements

from c e n t r a l

I i j i m a and

Shimizu and Masuda (1977) s t u d i e d t h e REE p a t t e r n s

o f Permo-Jurassic bedded c h e r t s o f c e n t r a l Honshu. analysed

1973;

There has been, however, few i n v e s t i g a t i o n s o f t h e geochemistry Suyari and Tiba (1977)

i n some T r i a s s i c and Cretaceous bedded c h e r t s

Shi koku and suggested t h e p o s s i b i l i t y o f chemical p r e c i p i t a t i o n

o f s i l i c a i n the matrix.

Yamazaki (1979) analysed t h e major and minor elements

i n t h e Chichibu c h e r t s o f t h e Inuyama d i s t r i c t . In this some bedded

paper,

we

compare t h e major

and massive c h e r t s ,

and minor element compositions of

s i l i c e o u s shale,

and r e l a t e d rocks i n t h e

Setogawa T e r r a i n and t h e Chichibu Geosyncline w i t h modern marine sediments. We a l s o discuss t h e d e p o s i t i o n a l environments and t h e mechanism o f sedimentation o f bedded c h e r t s i n comparison w i t h modern marine sediments. 2 2.1

SAMPLES AND EXPERIMENTAL PROCEDURES Samples analysed Sampling

samples

l o c a l i t i e s , are

shown

i n F i g u r e 1.

were c o l l e c t e d from t h e

Mariko-Okabe turbidites,

district,

where bedded c h e r t s

mudstones,

lavas ( I i j i m a e t al.,

Eocene t o

slump

beds,

1979, 1981).

micritic

I n the

Setogawa Terrain,

Middle Miocene are

section

i n the

associated w i t h t e r r i g e n o u s

limestones,

and b a s a l t p i l l o w

I n t h e Chichibu Geosyncline, samples were

c o l l e c t e d from t h e T r i a s s i c t o Jurassic s e c t i o n along t h e Kiso R i v e r i n t h e Inuyama d i s t r i c t ,

where

folded

Middle T r i a s s i c t o Middle Jurassic

bedded

cherts, Middle t o Upper Jurassic r e d s i l i c e o u s shale w i t h sporadic Mn-carbonate lenses,

and Upper Jurassic a r k o s i c sandstone and slump beds occur as t h r u s t

176

Figure 1. Sampling l o c a l i t i e s i n the Tertiary Setogawa Terrain (Mariko-Okabe) and the Permo-Jurassic Chichibu Geosyncline (Inuyama, Neo, Ashio, Kuzuu). Locations of modern sediment samples used i n this study are: KH-sites of the R/V Hakuho Maru, o DSDP-sites o f ' Leg 56/57, data from Sugisaki (1980) and Murdmaa e t a1.(1980), and A data from Aoki (1977). s l i c e s (Yao e t a l . , 1980; Mizutani e t a l . , 1981). We also analysed Permian t o Triassic bedded cherts i n the Neo and Kuzuu d i s t r i c t s and T r i a s s i c ( ? ) massive cherts and strata-bound Mn ore deposits from the Nogami and Manako Mines i n the Ashio d i s t r i c t .

or triple-layered small

amounts of

Chert beds in these sections are the single-

type of I i jima e t a l . (1978). Cherts commonly contain terrigenous quartz, feldspar, clay minerals, and organic

matter. Volcanogenic grains were not found in the Chichibu bedded chert studied, b u t trace amounts of altered v i t r i c shards were seen rarely i n the Setogawa bedded chert. Three offshore mud samples from the Nankai Trough and the continental slope off Northeast Japan, and nine pelagic clay samples from the Philippine Sea and the Pacific and Indian Oceans were taken from cores recovered by the R/V Hakuho Maru o f the Ocean Research I n s t i t u t e of the University of Tokyo.

2.2

Chemical analyses Rock samples were crushed and pulverized in tungsten carbide containers with a motor-driven mill. Sediment samples were powdered in an agate mortar a f t e r desalinization. Selected samples of bedded chert were leached w i t h an acid-reducing solution (hydroxylamine hydrochloride + acetic acid) following the method of Chester and Hughes (1967) t o obtain lithogenous and n o n - l i t h o -

177

genous contributions of minor elements in bedded cherts. Contents of nine major elements, Si, Ti, Al, Fe, Mg, Ca, Na, K, and P, were determined by X-ray fluorescence spectroscopy (XRF) using a Rigakudenki IKF-3064 with a chromium tube and fused disc samples. Ten minor elements, Ba, Cry Cu, Mn, Ni, Rb, Sr, V , Zn, and Zr, were also analysed by XRF using a tungsten tube and pressed powder samples. The precision of the determinations is estimated at about 3% for the major elements and 10-30% for the minor elements (Matsumoto and Urabe, 1980). 3 RESULTS OF ANALYSES Table 1 shows average chemical compositions of some bedded cherts and related rocks in the Setogawa Terrain and the Chichibu Geosyncline as well as of some modern marine sediments. For comparison, averages of published analyses are given in Table 1 for forty-one clays and silts from the continental shelf and slope off Northeast Japan, two clay-rich sediments from off the Izu Peninsula, forty-one pelagic clays from the Pacific and Atlantic Oceans, and six metalliferous sediments from the East Pacific Rise. 3.1

Multivariate analysis Interelemental relations among nineteen elements in the bedded cherts from the Setogawa Terrain and the Chichibu Geosyncline were investigated by means of the principal component analysis (PCA). PCA is a statistical method to obtain meaningful grouping of large numbers of variables. Calculations were performedat the Educational Computer Center of the University of Tokyo, using the PCA program originally prepared by Haga and Hashimoto (1980).The contributions of the first three principal components and factor loading of each element are shown in Figure 2. In the Tertiary Setogawa Terrain, the elements in bedded cherts are classified into three groups based on the first principal component: Si is positively loaded, probably reflecting the dilution of terrigenous, volcanogenic and some chemical materials by the influx of biogenic silica; Ca, P, Ba, and Sr show nearly zero or weakly negative loadings; and the other elements are negatively loaded. Ca, P, Ba, and Sr also show strongly negative loadings on the second component. Six major elements, Ti, Al, Fe, Mg, Na, and K, and four minor elements, Cry Mn, Rb, and Zr, behave similarly as indicated by strong negative loadings on the first component and nearly zero or weakly negative loadings on the second and third components. This association represents the terrigenous and volcanogenic suites characterized by aluminosilicates. The base metal elements such as Cu, Ni, V , and Zn are related to each other considering the loadings on the second and third components. In the Permo-Jurassic Chichibu Geosyncline, the behaviors of most elements

TABLE 1. The average chemical c o m p o s i t i o n o f c h e r t s and r e l a t e d r o c k s , and modern marine sediments.

Tertiary Setogawa Terrain Bedded Shale Number of chert part. samples 37 3 Si02 X Ti02

A1203 Fe2O3* MgO CaO

Na20 K20 p20 Ba PPm Cr cu Mn (MnO%) Ni Rb Sr

v

Zn Zr Ti02/A1203 Fe203/A1203 MnO/A1203 Cr/A1203 Cu/A1203 NiIA1203 FezOjlMnO

Permo-Jurassic Chichibu Geosyncline

Silic. Mass. Turbid.Basalt shale mudst. mudst. 2 2 6 17

Modern marine sediments (Present study) (Previous studies***) Bedded Shale Mass. Mass. Mn-ore Basalt**OffshorePelagic Nanno. Offshore Pelagic EPR chert part. mudst. chert deposits muds clay ooze muds clay sediments 31 3 3 6 9 1 6 3 6 3 43 41 6

77.31 .64.68 71.91 0.59 0.39 0.67 10.40 17.66 13.95 5.81 3.80 3.52 1.21 1.17 1.96 0.41 0.22 0.40 1.42 1.59 2.91 3.27 2.47 3.43 0.03 0.02 0

48.74 1.58 14.20 8.86 5.86 8.49 3.38 1.28

93.30 0.13 2.53 1.26 0.59 0.13 0.27 0.66 0.06

66.92 0.86 15.52 5.48 2.21 0.04 1.96 4.90 0.07

68.45 0.63 15.12 4.87 1.84 0.60 1.63 3.30 0.18

289 5 34 370 20 30 23 18 29 20

674 90 66 709 78 191 88 125 108 169

616 50 32 857 37 169 108 80 87 164

.055 .536 .0203 .0002 .0014

.055 .353 .0059 .0006 .0004 .0005 60

87.92 0.21 4.81 1.70 0.55 0.29 0.91 1.21 0.01

74.37 0.36 12.33 3.69 1.57 0.46 1.12 2.41 0.02

747 23 29 70 17 51 95 36 50 62

988 52 72 159 38 109 127 77 98 121

580 53 27 165 20 99 99 55 75 137

399 68 21 538 48 145 89 94 85 179

570 63 31 132

450 740 110 LO60

22

180 20

.044 .353 .0019 .0005 .0006 .0004 188

.030 .299 .0017 .0004 .0006 .0003 180

.038 .338 .0020 .0005 .0003

.038 .329 .0039 .0004 .0001 .0003 84

.042 .272 .0012 .0005 .0002

.0002

165

131 95 95 61 179

.0002

223

0.21

330 280 75 120 .I11 .624 .0092 .0052 .0008 .0013 68

.0008

26

94.90 0.06 1.13 1.29 0.54 0.31 0.06 0.34 0.02

37.52 0.43 7.58 5.49 3.87 4.23 0.31 2.10 1.33

569 1036 8 37 67 105 (1.38)(24.71) 20 134 14 53 13 53 18 113 28 117 9 75

.042 .053 .322 1.142 .0073 1.221 .0003 .0007 .0002 .0060 .0002 .0018 44 0.935

.057 .724 3.26 .0005 .0014 .0018 0.22

45.33 3.32 15.66 11.44 5.59 4.51 3.13 1.46

-

-

-

1320

-

-

.212 .732 .011

-

67

67.43 0.59 13.05 5.39 2.25 2.04 2.09 1.86 0.04

58.89 0.74 15.57 7.14 2.93 1.80 2.29 2.97 0.22

19.30 0.23 5.79 2.13 1.24 37.41 0.94 1.25 0.04

66.54 0.59 13.20 5.65 2.65 2.68 3.57 2.21 0.10

57.10 0.80 14.01 7.64 3.63 1.89 2.80 2.96 0.25

529 60 92 630 35 70 157 87 135 123

692 70 237 6504 115 99 152 120 111 141

1181 17 60 2038 41 30 7 70 44 55 120

702 44 63 541 35

1406 76 379 5098 207

6300 31 733 (3.82) 467

160 86 126

316 234 156 139

1006 192 320 130

.048 .039 .045 .459 .368 .413 .0062 .0539 .0454 .0005 .0004 .0003 .0007 .0015 .0010 .0003 .0007 .0007 66 8.5 8.1

-

-

.045 .428 .0053 .0003 .0005 .0003 135

-

24.30 0.35 4.72 14.36 1.96 2.60 3.10

-

-

-

.058 .074 .545 3.042 .0469 .807 .0005 .0007 .0027 .0155 .0015 .0099 12 3.8

Fe203*:total iron as FepO3, Mn(Mn0X):concentration of manganese is expressed in per cent when the MnO content exceeds 1%. EPR means Fast Pacific Rise. Basalt**:data from Tanaka(l970), Previous studies***:data from Goldberg and Arrhenius(l958). El Wakeel and Riley(l961). Turekian and Wedepohl(l961), Cressman(l962). BostrGm and Peterson(l969), Horowitz and Cronan(1976). Aoki!1977), Murdmaa et a1.(1980), Sugisaki(l980), and Minai et a1.(1981).

179 Tertiary bedded cherts in the Setogawa Terrain ( N: 37 ) Component Contribution(%) Factor loading

1

2

3

1

45.5

13.1

11.6

46.5

1 -.3 0

0

-1 l

'

Permo-Jurassic bedded cherts in the Chichibu Geosyncline ( N: 3 1)

"

4

-

-

i

-.3 0

0

.6 -1

I

I

I

2

3

11.3

10.7

--

1 -.3 0 I

I

Si Ti Al Fe

.8 -.6

0

.6

Ms Ca Na K P Ba Cr

cu Mn Ni Rb Sr V Zn

Zr

F i q u r e 2. P r i n c i p a l component a n a l y s i s T e r r a i n and t h e C h i c h i b u Geosyncline.

of

bedded c h e r t s

i n the

except f o r Fe and Mn a r e s i m i l a r t o t h o s e i n t h e Setogawa c h e r t . behave s i m i l a r l y t o Cu, weakly

negative

loadings

Ni, on

and Zn; the

Setogawa

Fe and Mn

t h e s e f i v e elements have n e a r l y z e r o o r

second component and p o s i t i v e l o a d i n g s on

t h e t h i r d component. 3.2

D e s c r i p t i o n o f s e l e c t e d elements Three m a j o r elements,

Cu,

and C r ,

Si,

Ti,

and Fe,

and f o u r m i n o r elements,

Mn,

Ni,

were s e l e c t e d f o r t h e d e s c r i p t i o n o f t h e geochemistry o f t h e s e

bedded c h e r t s and r e l a t e d r o c k s .

The c o n t e n t s o f T i 0 2 ,

Fe2O3,

MnO,

Ni,

Cu, and C r a r e p l o t t e d a g a i n s t A1203 i n F i g u r e 3 t o 8, because A1 i s r e p r e s e n t a t i v e o f t h e t e r r i g e n o u s and volcanogenic

materials of

known as one o f t h e most immobile elements

is

ratios o f Ti02,

FepO3,

MnO,

Ni,

Cu,

a l u m i n o s i l i c a t e s and

i n sediments.

The average

and C r t o A12 0 3 a r e shown i n Table 1.

Permo-Jurassic c h e r t s and mudstones f r o m t h e Inuyama,

Neo, and Kuzuu d i s t r i c t s

i n t h e C h i c h i b u Geosyncline a r e d e s c r i b e d c o l l e c t i v e l y as t h e C h i c h i b u c h e r t and t h e C h i c h i b u mudstone r e s p e c t i v e l y ,

because t h e y a r e s i m i l a r i n chemical

c o m p o s i t i o n among t h e s e d i s t r i c t s . 3.2.1

Silica

The s i l i c a c o n t e n t o f t h e Setogawa bedded c h e r t ranges f r o m 78 t o 98%,

180 -1.

1.58 332

0.8

/

Ti02

Fe

0.6

0

0.4

0.2

0

0

8

4

Ah03

12

-

2 'I.

16

F i g u r e 3. ( l e f t ) R e l a t i o n between t h e contents o f T i 0 2 and A1203 i n c h e r t s and r e l a t e d rocks i n t h e Setogawa T e r r a i n and Chichibu Geosyncline. Averages o f t h e modern marine sediments are a l s o shown. S and T i n d i c a t e shale p a r t i n g i n bedded c h e r t and mudstone i n t u r b i d i t e sequence, r e s p e c t i v e l y . The diagonal l i n e i n d i c a t e s t h e regression f o r a l l c h e r t s and mudstones. Symbols: Setogawa bedded Terrain; 0 bedded c h e r t o mudstone * b a s a l t , Chichibu Geosyncline; c h e r t A massive c h e r t mudstone b a s a l t , Modern marine sediments; x average o f o f f s h o r e muds +average o f p e l a g i c c l a y Aaverage o f East P a c i f i c Rise sediments. F i g u r e 4. ( r i g h t ) R e l a t i o n between t h e contents o f Fee03 and Al203. Line 1 i n d i c a t e s t h e r e g r e s s i o n f o r t h e Setogawa c h e r t and mudstone, and t h e Chichibu mudstone; l i n e 2, f o r t h e Chichibu c h e r t . The symbols are r e f e r r e d t o F i g u r e 3. averaging 88%, w h i l e t h a t o f t h e Chichibu bedded c h e r t f a l l s mostly w i t h i n t h e 90-98% range, averaging 93%. the

shale p a r t i n g s

Though t h e Chichibu c h e r t i s more s i l i c e o u s ,

are more s i l i c e o u s i n t h e Setogawa T e r r a i n (74% Si02

average) than those i n t h e Chichibu Geosyncline (67% Si02 average). Massive

cherts

associated

with

Mn

o r e deposits

i n t h e Ashio

district

are s l i g h t l y r i c h e r i n s i l i c a (95%) than bedded c h e r t s i n t h e Chichibu Geosync1 ine. 3.2.2

Titanium

Titanium

is

associated w i t h t e r r i g e n o u s and volcanogenic components. The

average T i 0 2 / A 1 2 0 3

r a t i o s o f r e s p e c t i v e c h e r t s and mudstones are 0.044 and

0.037 i n t h e Setogawa T e r r a i n and 0.055 and 0.049 i n t h e Chichibu Geosyncline. Both t h e younger and o l d e r c h e r t s and mudstones can be represented by t h e same r e g r e s s i o n l i n e ( F i g . 3),

suggesting t h a t t h e c h e r t s formed by a d d i t i o n

o f biogenic s i l i c a t o t e r r i g e n o u s c l a y . F o r t y - s i x modern o f f s h o r e muds have t h e average Ti02/A1203 r a t i o o f 0.045 and are represented by a s i m i l a r regression l i n e as t h e c h e r t s and mudstones from

-1.

181 Japan.

However,

f o r t y - s e v e n modern abyssal p e l a g i c c l a y s are r i c h e r i n Ti02

than t h e Japan mudstones and modern o f f s h o r e muds, and p l o t above t h e regression line. 3.2.3

T o t a l i r o n as Fe902

The average Fe2O3/Al2 0 3 r a t i o s o f t h e Setogawa c h e r t and mudstone are 0.353 and

and 0.327

respectively.

Although t h e y show a wide range,

mudstone are represented by t h e

t h e F e 2 0 3 / A 1 2 03 0.338

the chert

By c o n t r a s t ,

same r e g r e s s i o n l i n e .

r a t i o s o f t h e Chichibu c h e r t and mudstone are 0.536

respectively.

and

The r e g r e s s i o n l i n e o f t h e Chichibu c h e r t has a much

steeper slope than t h a t o f t h e r e l a t e d mudstone ( F i g . 4 ) .

I n o t h e r words,

t h e Chichibu c h e r t i s c h a r a c t e r i z e d by an anomalously h i g h c o n c e n t r a t i o n o f t o t a l i r o n compared w i t h t h e Chichibu mudstone.

Massive c h e r t s i n t h e Ashio

d i s t r i c t has higher Fe203/A1203 r a t i o s (1.142 average) than does bedded c h e r t . I n modern marine sediments,

Fez03

i s more concentrated i n p e l a g i c clays;

t h e average Fe203/A1203 r a t i o o f f o r t y - s e v e n p e l a g i c c l a y s i s 0.543,

whereas

t h a t o f f o r t y - s i x o f f s h o r e muds i s 0.427. 3.2.4

Manganese

Manganese belongs t o both base metal

and t e r r i g e n o u s a s s o c i a t i o n s .

The

MnO content v a r i e s between 0.017 and 0.045% i n t h e Setogawa mudstone and 0.05 and 0.13% i n t h e Chichibu mudstone.

The MnO content changes s y m p a t h e t i c a l l y

w i t h A1203 i n most Setogawa c h e r t and mudstone; MnO/A1203 r a t i o s between 0.001 and 0.003, curve I shale

shown i n F i g u r e 5.

parting closely

a l l b u t seven analyses have

and p l o t on t h e t h e o r e t i c a l d i l u t i o n

The t i e l i n e j o i n i n g a c h e r t bed and adjacent

I.

p a r a l l e l s t h e d i l u t i o n curve

Chichibu c h e r t has an average MnO/A1203

r a t i o o f 0.0203,

times

Hence,

that

of

t h e Chichibu mudstone.

By c o n t r a s t ,

the

approximately t h r e e

t h e Chichibu c h e r t deviates

g r e a t l y from t h e d i l u t i o n curve 11; t h e t i e l i n e s j o i n i n g t h e Chichibu c h e r t t o adjacent shale p a r t i n g are o b l i q u e t o t h i s d i l u t i o n curve. c l o s e l y associated w i t h strata-bound

Massive c h e r t ,

Mn o r e deposits i n t h e Ashio d i s t r i c t ,

has h i g h MnO contents (0.39-3.7%) and MnO/A1203 r a t i o s (0.22-3.5). I n modern, marine sediments, i n forty-seven Calcareous similar

to

t h e MnO contents

p e l a g i c c l a y s and 0.03

oozes those

t o 0.12% i n f o r t y - s i x

from Shatsky Rise show MnO/A12 03 of

average p e l a g i c

ranges from 0.21

clays

ratios

(0.0478).

o f f s h o r e muds.

(0.0454

This

t o 1.5% average)

suggests

that

t h e Mn-rich p e l a g i c c l a y was d i l u t e d by Mn-free carbonates.

Modern hydrothermal

deposits from t h e East P a c i f i c Rise have e x t r a o r d i n a r i l y

h i g h MnO contents

and MnO/A1203 r a t i o s and occupy t h e u p p e r - l e f t p a r t o f t h e diagram i n F i g u r e 5 .

182

A

PPM 1000 AA

+ * + + ++ + +

+

MnO

A

+

Ni lO(1

10

0.oot.l 0

1

5

AliOs

10

15 *I.

Figure 5. ( l e f t ) Relation between the contents of MnO and Al2O3. Logarithmic curve I and I 1 represent the theoretical trend of dilution of mudstone by s i l i c a in the Setogawa Terrain and the Chichibu Geosyncline, respectively. Broken lines join a set of chert bed and shale parting in bedded chert. Symbols are referred t o Figure 3. Figure 6. ( r i g h t ) Relation between the contents o f Ni and A12O3. The solid curves, broken lines, from Figure 5, and symbols are referred t o Figure 3. 3.2.5 Nickel Nickel i s typically a non-detrital component. The N i content in the Setogawa chert and mudstone i s , as a whole, lower than that of the Chichibu chert and mudstone (Fig. 6 ) . The Ni/A1203 r a t i o of chert beds of bedded chert i s generally higher than that of related mudstone, particularly in the Chichibu Geosyncline. The average r a t i o s are 0.0004 f o r chert and 0.0003 f o r mudstone in the Setogawa Terrain, and 0.0009 f o r chert and 0.0004 f o r mudstone in the Chichibu Geosyncline. Therefore, most of the Chichibu chert values p l o t above the theoretical dilution curve I1 in Figure 6. I n modern marine sediments, N i concentrates most in pelagic clays. The Ni contents range from 57 t o 380 ppm i n pelagic clays and from 24 t o 48 ppm i n offshore muds.

183

F i g u r e 7. ( l e f t ) R e l a t i o n between t h e contents o f Cu and Al2O3. curves, broken l i n e s , and symbols are r e f e r r e d t o Figures 3-5.

The s o l i d

F i g u r e 8 . ( r i g h t ) R e l a t i o n between t h e contents o f C r and A1203. The s o l i d curve i s t h e t h e o r e t i c a l d i l u t i o n t r e n d o f t h e Setogawa and Chichibu mudstones by s i l i c a . Broken l i n e s and symbols are r e f e r r e d t o F i g u r e 3.

3.2.6

Copper

Copper i s a l s o a n o n - d e t r i t a l t o that

of

nickel.

element and t h e behavior i s q u i t e s i m i l a r

The Cu content i s d i s t i n c t l y higher i n p e l a g i c c l a y

than i n o f f s h o r e mud and o l d e r mudstone ( F i g . 7 ) .

The Cu/A1203 r a t i o o f t h e

Chichibu c h e r t i s approximately f o u r times t h a t o f r e l a t e d mudstone,

while

t h e r a t i o s o f t h e Setogawa c h e r t and mudstone are s i m i l a r . 3.2.7

Chromium

Chromium i s presumably d e r i v e d mainly from t e r r i g e n o u s c l a y because t h e content o f C r changes i n harmony w i t h A l 2 O 3 . older

mudstone

The C r contents o f most o f

and modern marine mud are s i m i l a r t o

each other,

ranging

from 40 t o 90 ppm r e g a r d l e s s o f l i t h o l o g i c types, geologic ages, and environments o f d e p o s i t i o n ( F i g . 8 ) .

3.3

P a r t i t i o n i n g o f minor elements i n c h e r t and shale p a r t i n g Table 2 shows t h e abundances o f minor elements i n b u l k samples and l i t h o -

genous

residues,

and t h e

lithogenous

and non-lithogenous

contributions t o

each s e t o f c h e r t and adjacent shale p a r t i n g o f t h e Setogawa and Chichibu bedded c h e r t s .

184 TABLE 2. P a r t i t i o n i n g o f minor elements i n bedded c h e r t s .

Element

Miocene bedded c h e r t in Setogawa C h e r t bed Shale parting A B C D A B C D ppm

Ba

ppm

839 829 <3 <3 17 14 25 32 <3 <3 14 12 79 66 23 22 16 19 21 20

Cr

cu

Mn Ni Rb Sr

v Zn

Zr

%

-

-

82 78

18 22

86 84 96 84 95

14 16 4 16 5

1

-

ppm

%

99

-

1056 57 80 147 33 97 209 85 104 122

ppm 982 55 59 120 24 90 195 79

88 117

%

93 96 74 82 73 93 93 93 85 96

T r i a s s i c bedded c h e r t in Inuyama c h e r t bed Shale parting A

ppm

%

7 4 26 18 27 7 7 7 15 4

167 <3 31 433 23 36 21 16 32 24

B

C

D

ppm

%

154 <3 13 173 6 33 18 15 20 23

A

ppm

%

92

a

39 40 26 92 86 94 62 96

61 60 74 8 14 6 38 4

-

B

349 72 70 927 100 193 40 90 140 125

-

C

ppm 320 63 27 826 85 172 36 90 135 122

D

%

%

92 88 90 89 85 89 90 100 96 98

8 12 10 11 15 11 10 0 4 2

A : T o t a l amount of minor e l e m e n t i n b u l k s a m p l e s ; B:Amount i n i n s o l u b l e r e s i d u e s ; C:% of l i t h o g e n o u s c o n t r i b u t i o n ; D:% of n o n - l i t h o g e n o u s c o n t r i b u t i o n .

I n t h e Setogawa gray bedded c h e r t ,

t h e base metal contents i n most l i t h o -

genous r e s i d u e s exceed 80% o f t h e t o t a l

78% o f t h e Mn and 82% I n the Triassic

abundances;

o f t h e Cu are i n t h e lithogenous f r a c t i o n o f t h e c h e r t bed. r e d bedded chert*

i n t h e Inuyama d i s t r i c t ,

however,

Cu, Mn,

and N i i n t h e

lithogenous

residues

o f t h e c h e r t bed c o n t r i b u t e o n l y 26-40% o f t h e t o t a l

abundances;

that is,

60-74% of t h e base metals are contained i n t h e leachable

phases,

adsorbed

components,

oxides,

hydroxides,

carbonates,

and

so

on.

In t h e T r i a s s i c shale p a r t i n g s , on t h e c o n t r a r y , g r e a t e r than 85% o f t h e Cu, Mn, and N i are i n t h e lithogenous f r a c t i o n . 4 4.1

DISCUSSION

Hydrogenous accumulation o f base metal elements The r a t i o s o f Fe, Mn, N i ,

and Cu t o A1203 i n c h e r t beds o f t h e Chichibu

bedded c h e r t are much higher than those i n shale p a r t i n g s .

I f t h e base metals

migrated e f f e c t i v e l y from t h e shale p a r t i n g s t o t h e c h e r t ,

t h e base metal

contents i n the shale p a r t i n g s would be much s m a l l e r than those contents f o r average mudstone o r s l a t e .

I n fact,

however,

t h e shale p a r t i n g s g e n e r a l l y

have t h e same range o f base metal contents as do average mudstone and s l a t e i n t h e Chichibu Geosyncline

(Miyashiro and Haramnura,

1962):

Consequently,

t h e d i f f e r e n c e s i n t h e r a t i o s between c h e r t and shale p a r t i n g seem n o t t o be caused by t h e m i g r a t i o n o f o r i g i n a t e d from d i f f e r e n c e s

base metals d u r i n g diagenesis b u t probably

i n the

amounts o f elements deposited w i t h t h e

o r i g i n a l sediments. ~~

-

*Gray bedded c h e r t i n t h e Inuyama d i s t r i c t has 50-65% o f t h e base metals i n t h e non-lithogenous f r a c t i o n (Yamazaki, personal communication).

186

The base metals in the bedded chert did not precipitate from hydrothermal solutions. In the Ashio district, Mn and Cu are highly concentrated in massive cherts that are closely associated with strata-bound Mn ore deposits; the ore precipitated syngenetically on the sea floor by hot springs or other hydrothermal processes (Watanabe, 1957). Imoto et al. (1971 documented the high MnO content in the hanging and foot wall cherts of the MnO ore deposits in the Tanba district, about 100 km west of Inuyama. From these observations alone, most of the Mn and Cu in the massive cherts conceivably could have precipitated from hydrothermal solutions. However, the ratio in chert beds of bedded chert sections is large (26-188) Fe203/MnO compared with massive chert (0.941, Mn ore deposits (0.221, and hydrothermal deposits of the East Pacific Rise (3.8): The ratios of modern diatom and radiolarian oozes are 22.3-24.5 (Cressman, 1962). These differences in the ratios were caused by variations of MnO rather than FezO3. The leachable, non-lithogenous contribution of Cu, Mn, and Ni in the Triassic red chert in the Inuyama district is 60-74%; in modern pelagic clays these element contributions are 56% for Cu, 68% for Mn, and 45% for Ni (Chester and Messina-Hanna, 1970). The leachable, non-lithogenous fractions of these base metals in marine sediments probably formed by slow and steady hydrogenous accumulation from normal sea water (e.g. Bonatti et al., 1976; Krishnaswami, 1976). Therefore, the incorporation of these base metals into the Chichibu chert is considered to have occurred mainly through hydrogenous accumulation. The similarity of the base metals/A12 0 3 ratios of chert beds and shale partings in the Setogawa bedded chert sections is consistent with the low contribution of non-lithogenous hydrogenous elements to both chert and shale. The distinction in the ratios between the Chichibu and Setogawa cherts is probably due to difference in the rates of sedimentation as discussed below. 4.2

Rate of sedimentation of bedded cherts Figure 9 illustrates the relation between MnO/A1203 ratio and the rate of sedimentation of deep sea muds and calcareous and siliceous oozes. The ratio of lithogenous MnO to A 1 2 0 3 was calculated at about 0.003 from the data given in Table 2, and this value is considered to be the ratio of the 'background' terrigenous component. The MnO/A1 2 03 ratio increases with the increase of hydrogenous accumulation of non-lithogenous Mn in slowly deposited sediments. Using the diagram in Figure 9, we can estimate the rate of sedimentation of bedded chert, provided that the chemical compositions of sea water in the Setogawa Terrain and the Chichibu Geosyncline were the same as that of normal sea water, and that the MnO content in bedded chert did not change during diagenesis.

186

MnO -

-Chichibu

-

A1203 0.01

-

0.005

-

chert

0.0203

shale part ing

-'

0.0059

Setogawa chert

0.001

4

.

0.5

1

2

0.0022

4'1

1:5 16

4

Rate of sedimentation

3i

4

s i 120 mmllO3yrs

shale part ing o*oola

F i g u r e 9. R e l a t i o n between MnO/A1203 r a t i o and t h e r a t e o f sedimentation o f deep sea muds. The s o l i d diagonal l i n e i s t h e r e g r e s s i o n l i n e f o r t h i r t y - t h r e e marine muds. Broken l i n e i n d i c a t e s t h e average MnO/A1203 r a t i o i n t h e l i t h o genous f r a c t i o n . Average r a t i o s o f c h e r t beds and shale p a r t i n g s i n t h e Setogawa and Chichibu bedded c h e r t s are a l s o i n d i c a t e d . Symbols: A Kobayashi (1980), Minai e t a1.(1981) and t h i s study; 0 Sugisaki (19801, Scientific p a r t y (19801, and t h i s study; Krishnaswami (1976). The MnO/A1203 r a t i o o f the Krishnaswami ' s data were c a l c u l a t e d using t h e average contents o f A1203 o f the pelagic clay. The average r a t e o f sedimentation o f t h e Permo-Jurassic bedded c h e r t i n t h e Chichibu Geosyncline i s estimated t o be 7.5 m / 1 0 3 y r s f o r s i n g l e - and t r i p l e l a y e r e d c h e r t beds and 41 m / l O 3 y r s f o r shale p a r t i n g s .

Matsuda e t a1.(1980)

estimated t h e average r a t e o f sedimentation o f t h e bedded c h e r t i n t h e Inuyama d i s t r i c t a t about 2.8 m / 1 0 3 y r s on t h e b a s i s o f conodont and r a d i o l a r i a n b i o stratigraphy.

The d i f f e r e n c e i n t h e two e s t i m a t i o n s i s probably t h e r e s u l t

o f compaction which was n o t considered by Matsuda e t a l . In

the

T e r t i a r y , Setogawa

Terrain,

t h e estimated r a t e o f

o f bedded c h e r t i s f a s t f o r b o t h c h e r t and shale p a r t i n g s , 100 m / 1 0 3

y r s (Fig.

.

Iij i m a e t a1 (1981

9);

sedimentation

being more than

i t i s u n c e r t a i n which one was deposited f a s t e r .

estimated t h e average r a t e a t about 18-69 mn/103 y r s based

on nannoplankton and r a d i o l a r i a n b i o s t r a t i g r a p h y . 4.3

Sedimentation o f bedded c h e r t The

Permo-Jurassic

the central

part of

Chichibu

spicule-radiolarian

t h e marginal

t h a t was a few hundred k i l o m e t e r s

sea o f

bedded c h e r t

formed

t h e Chichibu Geosyncline,

in

a sea

wide and s i t u a t e d between t h e Hida Continent

187 and

Kurosegawa-Ofunato

I s 1ands.

Terrigenous m a t e r i a l s were m o s t l y trapped

i n r a t h e r narrow depressions created Iijima

and Utada,

1982).

by t h e adjacent

lands

The p o s t u l a t e d d e p o s i t i o n a l

supported by t h e present geochemical study.

(Kimura,

1977;

environment i s w e l l

The contents o f base metals such

as Fe, Mn, Cu, and N i and t h e r a t i o s o f each metal t o A 1 2 q i n shale p a r t i n g s of

t h e bedded c h e r t are s i m i l a r t o those i n t h e modern o f f s h o r e mud;

the

r a t i o s i n c h e r t are h i g h e r and somewhat s i m i l a r t o those i n modern oceanic sediments due t o t h e h i g h c o n t r i b u t i o n o f hydrogenous elements caused by slow r a t e s o f sedimentation o f c h e r t ; t h e s i m i l a r Ti02/A1203 r a t i o b o t h i n c h e r t and shale p a r t i n g s ,

which i s lower than t h a t i n t h e modern brown clay,

a t e r r i g e n o u s o r i g i n f o r t h e c l a y i n t h e bedded c h e r t .

suggests

Moreover,

a small

p o s i t i v e Ce anomaly o f t h e bedded c h e r t i n t h e Kamiaso d i s t r i c t , about 50 k n east o f Neo, suggests a nearshore environment (Shimizu and Masuda, 1977). The rhythmic

layering o f

bedded c h e r t

has been explained

as

resulting

from p e r i o d i c r a p i d d e p o s i t i o n o f s i l i c e o u s skeletons by t u r b i d i t y c u r r e n t s (McBride and Folk, of

siliceous

1979;

organisms

Kalin e t al., (Garrison

1979) o r by f l u c t u a t i n g p r o d u c t i v i t y

and Fischer,

1969;

Fischer,

1977) along

w i t h continuous and slow d e p o s i t i o n o f p e l a g i c o r hemipelagic c l a y s .

However,

these hypotheses are i n c o n s i s t e n t w i t h t h e r e s u l t s o f t h e present geochemical study;

that is,

deposited a t

t h e c h e r t beds o f t h e Chichibu bedded c h e r t were probably

a slower

r a t e than t h e shale p a r t i n g s .

Most o f c h e r t beds

i n t h e Chichibu bedded c h e r t show t h e t r i p l e - l a y e r e d t y p e composed o f t h e middle h i g h - s i l i c e o u s c h e r t l a y e r and t h e upper and lower a r g i l l a c e o u s c h e r t l a y e r s which change c o n t i n u o u s l y t o t h e adjacent shale p a r t i n g s :

Radiolarian

s h e l l s are r a t h e r u n i f o r m l y and randomly packed i n t h e m a t r i x c o n s i s t i n g o f f i n e r fragments o f s i l i c e o u s skeletons ( I i j i m a e t al.,

1978).

The symmetrical

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

a chert

Hence,

the

bed and adjacent

Permo-Jurassic

shale

p a r t i n g s ( I i j i m a and Utada,

radiolarian

bedded

chert

probably

1982).

accumulated

dominantly by s e t t l i n g o f s i l i c e o u s skeletons through t h e sea water column in

a

low

energy

environment,

though

occasional

winnowing

of

sedimentary

p a r t i c l e s by weak bottom c u r r e n t s i s i n d i c a t e d by i n t e r c a l a t i o n s o f laminated s p i c u l e - r i c h c h e r t beds. Periodic,

rapid influxes

of

terrigenous

o r i g i n a t e d e i t h e r from t u r b i d i t y currents, suspension o f

fine-grained

sediments

c l a y s as shale p a r t i n g s may be bottom c u r r e n t s ,

by major storms.

o r from p e r i o d i c

The continuous b u t

sharp c o n t a c t s between c h e r t beds and shale p a r t i n g s seem t o i n d i c a t e abrupt changes i n d e p o s i t i o n a l process; t h e y seem c o n t r a r y t o r e l a t i v e l y slow deposition.

Few,

i f any,

r a d i o l a r i a n t e s t s i n t h e shale p a r t i n g s supports r a p i d

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

The p e r i o d i c i t y and abrupt a l t e r n a t i o n

o f l i t h o l o g i e s w i t h i n t h e bedded c h e r t sections suggest t h a t shale p a r t i n g s

188

are d i s t a l t u r b i d i t e s deposited from d i l u t e turbidity currents. The Chichibu bedded chert formed in the center of a marginal sea, where the production of siliceous organisms was moderate and steady and there was l i t t l e supply of terrigenous clay except f o r occasional and periodic influxes. The Tertiary Setogawa bedded chert was deposited as aggregates of siliceous skeletons of mainly diatoms on and around offshore banks that were about 10 km wide: Water depths were above the oxygen m i n i m u m zone and the banks were about 50 km off the paleocoast of Japan. Terrigenous t u r b i d i t e s and slump beds f i l l e d in areas adjacent t o the banks on the continental slope (Iijima e t a1 1981 1. These postulated depositional environments are well supported by the present geochemical study. The contents of base metals such as Fe, Mn, Cu, and Ni i n the Setogawa mudstones including the shale partings of bedded chert sections are low, and comparable t o those in modern offshore muds; the base metals/Al203 r a t i o s i n chert are somewhat similar t o those in the mudstone. The contribution o f hydrogenous elements was low and the average r a t e o f sedimentation h i g h f o r b o t h chert and shale partings, being more than 100 mn/103 yrs. Rapid accumulation of biogenic s i l i c a on the flat-topped banks on the continental slope indicate a h i g h productivity of s i l i c a organisms, mainly diatoms, i n the water column over the banks. The marked difference in the r a t e of sedimentation between the Setogawa chert and the Chichibu chert was presumably caused by the difference in the assemblage of s i l i c a organisms. Diatomaceous siliceous shale of Miocene s t r a t a in Hokkaido, Northern Japan, was deposited rapidly, 90-235 mn/lO3 yrs (Iijima e t a l . , 1982). Rapid deposition of shale partings of the Setogawa bedded chert sections was probably the r e s u l t of d i l u t e turbidity currents as suggested by the rather common association of bedded chert w i t h terrigenous turbidites. The Setogawa chert i s generally more argillaceous while the shale partings are more siliceous than those i n the Chichibu Geosyncline. Microsedimentary structures such as obscure lamination, cross bedding, and cut-and-fill structure are sporadically observed i n the Setogawa chert. These f a c t s may indicate a sporadic mixing and winnowing of diatom f r u s t u l e s and fine-grained terrigenous materi a1 s by bottom currents.

.,

5 SUMMARY Concentrations and s t a t i s t i c a l analyses of mdjor and minor elements i n bedded cherts and related rocks of the Permo-Jurassic Chichibu Geosyncline and the Tertiary Setogawa Terrain i n central Honshu compared w i t h the same data f o r modern marine sediments lead t o several r e s u l t s : Fe, Mn, Ni, and Cu are generally more concentrated in pelagic clays than i n modern offshore muds. The contents of these metals i n shale partings from bedded chert sections,

189

siliceous shale, and mudstone in terrigenous t u r b i d i t e sections are similar t o or lower t h a n those in modern offshore muds. Chemical evidence suggests that bedded cherts formed by mixing of siliceous skeletons w i t h offshore muds. I n the Chichibu Geosyncline, the r a t i o s of Fe, Mn, N i , and C u t o A1 in chert beds of bedded chert sections are d i s t i n c t l y larger than those in the associated shale partings. In the Setogawa Terrain, the r a t i o s of chert beds and shale partings are similar. The r a t e of sedimentation of the Chichibu bedded chert as estimated from MnO/A1203 r a t i o was 7.5 m/103 yrs f o r chert beds and 41 m/103 yrs f o r shale partings. The r a t e of sedimentation of the Setogawa bedded chert was f a r greater than t h a t of the Chichibu chert. ACKNOWLEDGMENTS T h i s research was supported partly by Grant-in-Aid f o r Fundamental Scientific Research from the Ministry of Education of Japan (No. 254254, A. Iijima).

Professor R. Siever (Harvard University) and Dr. J.R. Hein ( U . S . Geological Survey) read the manuscript c r i t i c a l l y . Dr. A. Kato (National Science Museum), Professor K . Kobayashi (Ocean Research I n s t i t u t e of the University of Tokyo), Mr. Y . Kakuwa (College of General Educatioin of the same University), and Mr. K. Yamazaki (Mitsui Mining Company) kindly offered samples of manganese ore deposits and massive chert from the Ashio d i s t r i c t , modern marine sediment, chert and mudstone from the Kuzuu d i s t r i c t , and chert from the Neo d i s t r i c t , respectively. REFERENCES Aoki, S., 1977. Physical, chemical and clay mineralogical properties of the two sediments from Nankai Trough and i t s environs. La mer ( B u l l . SOC. franco-japonaise d'oceanographie), 15:116-120. Bonatti, E . , Zerbi, M . , Kay, R . , and Rydell, H., 1976. Metalliferous deposits from the Appennine ophiolites: Mesozoic equivalents of modern deposits from oceanic spreading centers. Geol. SOC. Amer. Bull., 87:83-94. Bostrom, K. and Peterson, M.N.A., 1969. The origin o f a l u m i n i u m poor ferromanganoan sediments i n areas of high heat flow on the East Pacific Rise. Mar. Geol., 7:427-447. Chester, R. and Hughes, M.J., 1967. A chemical technique f o r the separation of ferro-manganese minerals, carbonate minerals and adsorbed trace elements from pelagic sedimerks. Chem. Geol., 2:249-262. Chester, R. and Messiha-Hanna, R.G., 1970. Trace element partition patterns in North Atlantic deep-sea sediments. Geochim. Cosmochim. Acta, 34:11211128. Cressman, E.R., 1962. Non d e t r i t a l siliceous sediments. In: M. F$leischer ( E d i t o r ) , Data of Geochemistry, sixth edition. U.S. Geol. Surv. Prof. Paper, 440T:Zp. Fischer, A.G., 1977. Pelagic sediments as clues t o earth behavior. I n : Giampaolo, P i a l l i ( E d i t o r ) , Paleomagnetic stratigraphy of pelagic carbonate sediments: Mem. SOC. Geol. I t a l i a , 15:l-18. Garrison, R.E. and Fischer, A.G., 1969. Deep-water limestones of the Alpine Jurassic. I n : Friedman, G.M. (Editor), Depositional environments in carbonate rocks. SOC. Econ. Paleontologists Mineralogists Spec. Pub., 14:20-54.

190 Goldberg, E.D. and Arrhenius, G.O.S., 1958. Geochemistry o f P a c i f i c p e l a g i c sediments. Geochim. Cosmochim. Acta, 13:153-212. Haga, T. and Hashimoto, S., 1980. Regression a n a l y s i s and p r i n c i p a l component a n a l y s i s . Nikkagiren-shuppansha, Tokyo, 228 pp. 1976. The geochemistry o f basal sediments Horowitz, A. and Cronan, D.S., from t h e n o r t h A t l a n t i c Ocean. Mar. Geol., 20:205-228. jima, A. ,Kakuwa, Y., Yamazaki, K., and Yanagimoto, Y., 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 c e n t r a l Japan. J. Fac. Sci. Univ. Tokyo, Sec.11, 19:369-400. jima, A., Inagaki, H., and Kakuwa, Y., 1979. Nature and o r i g i n o f t h e Paleogene c h e r t s i n t h e Setogawa T e r r a i n , Shizuoka, c e n t r a l Japan. J. Fac. Sci. Univ. Tokyo, Sec.11, 2O:l-30. jima, A., Matsumoto, R., and Watanabe, Y., 1981. Geology and s i l i c e o u s deposits i n T e r t i a r y Setogawa T e r r a i n o f Shizuoka, c e n t r a l Honshu. J. Fac. Sci. Univ. Tokyo, Sec.11, 20:241-276. I i j i m a , A. and Utada, M., 1982. Recent developments i n sedimentology o f s i l i c e o u s deposits i n Japan. I n : I i j i m a , A. e t a l . ( E d i t o r s ) , S i l i c e o u s deposits i n t h e P a c i f i c region. E l s e v i e r , Amsterdam ( i n p r e p a r a t i o n ) . I i j i m a , A., Tada, R., and Matsumoto, R.. 1982. Sedimentary petrographic study o f t h e Neogene System i n Hokkaido. I n : Tanai, T. ( E d i t o r ) , Bios t r a t i g r a p h y o f t h e Neogene System i n Hokkaido. Hokkaido Univ. Press, Sapporo, 67-74. Imoto, N., Honjo, H., Ohmae, M., and Nakata, S . , 1971. D i s t r i b u t i o n o f manganese and i r o n i n bedded manganese deposits i n t h e Paleozoic system o f t h e Tamba D i s t r i c t , Japan. Geol. SOC. Japan, Memoir, 6:165-172. Imoto, N. and Saito, Y., 1973. Scanning e l e c t r o n microscopy o f c h e r t . B u l l . Natn. Sci. Mus., 19:35-42. K a l i n , O., Patacca, E., and Renz, O., 1979. J u r a s s i c p e l a g i c deposits from Southeastern Tuscany; aspects o f sedimentation and new b i o s t r a t i g r a p h i c data. Ecolog. Geol. Helv., 72/3:715-762. Kimura, T., 1977. Japan I s l a n d s ( I ) , Age o f Chichibu Geosyncline. KokinShoin, Tokyo, 243 pp. Kobayashi, K., Tonouchi, S . , Furuta, T., and Watanabe, M., 1980. Paleomagnetic r e s u l t s o f deep-sea sediment cores c o l l e c t e d by R.V. Hakuho Maru i n a p e r i o d 1968-1977 compiled w i t h associated i n f o r m a t i o n . B u l l . Ocean Res. I n s t . Univ. Tokyo, 13:l-148. Krishnaswami , S . , 1976. Authigenic t r a n s i t i o n elements i n P a c i f i c p e l a g i c clays. Geochim.Cosmochim. Acta, 40:425-434. Matsuda, T., I s o z a k i , Y., and Yao, A., 1980. S t r a t i g r a p h i c r e l a t i o n o f T r i a s s i c Jurassic system i n Inuyama d i s t r i c t , Mino B e l t . Abs. Paper, 87th Ann. Meet. Geol. SOC. Japan, 107. 1980. An automatic a n a l y s i s o f major elements Matsumoto, R. and Urabe, T., i n s i l i c a t e rocks w i t h X-ray fluorescence spectrometer u s i n g fused d i s c samples. J. Japan. Assoc. Min. Petr. Econ. Geol., 7 6 : l l l - 1 2 1 . McBride, E.F. and Folk, R.L., 1979. Features and o r i g i n o f I t a l i a n Jurassic r a d i o l a r i t e s deposited on c o n t i n e n t a l c r u s t . J. Sedim. Petrol., 49:837-868. Minai, Y . , Tominaga, T., Nakamura, Y., and Wakita, H., 1981. Geochemistry of oceanic sediments c o l l e c t e d from KH80-3. I n : Kobayashi, K. ( E d i t o r ) , P r e l i m i n a r y r e p o r t on t h e Hakuho Maru Cruise KH80-3. Ocean Res. I n s t . Univ. Tokyo, 188-191. Miyashiro, A. and Haramura, H., 1962. Chemical composition o f Paleozoic s l a t e s I V . Zonal d i s t r i b u t i o n o f geosynclinal sediments and t h e p o s i t i o n of r e g i o n a l metamorphic b e l t s . J. Geol. SOC. Japan, 68:75-82. Mizutani, S., Imoto, N., Yao, A., Ichikuni,K., I s h i d a , K., Nakazawa, K., Otsuka, T . , Shimizu, D., and Suyari, K., 1981. T r i a s s i c bedded c h e r t and associated rocks i n t h e Inuyama area, c e n t r a l Japan. 2nd I n t . Conf. S i l i c e o u s Deposits i n t h e P a c i f i c Region. IGCP #115, Japan. 156-210. Murdmaa, I., Gordeev, V . , Kuzmina, T., Turanskaya, N., and Mikhailov, M., 1980. Geochemistry o f t h e Japan Trench sediments recovered on Deep Sea D r i l l i n g P r o j e c t Legs 56 and 57. I n : S c i e n t i f i c p a r t y , I n i t i a l Reports

191

of the Deep Sea D r i l l i n g P r o j e c t , Vol. 56, 57, P t . 2 , Washington (U.S. Government Printing O f f i c e ) 1213-1232. Shimizu, H. and Masuda, A., 1977. Cerium i n c h e r t as an i n d i c a t o r of marine environment of i t s formation. Nature, 266:346-348. S c i e n t i f i c Party, 1980. Introduction and s i t e r e p o r t s . In: S c i e n t i f i c party 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 , Vol. 56, 57, Pt.1, (U.S. Government P r i n t i n g Office) 1-446. Sugisaki, R., 1980. Major element geochemistry of the Japan Trench sediments, Legs 56 and 57, Deep Sea D r i l l i n q Project. In: S c i e n t i f i c party, 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 , Vol. 56, 57, P t . 2 , Washington (U.S. Government Printing O f f i c e ) 1233-1249. Suyari, K. and Tiba, T . , 1977. Chemical composition of t h e c h e r t from Shikoku d i s t r i c t . J . S c i . U n i v . Tokushima, 10:59-66. Tanaka, T., 1970. Chemical composition of geosynclinal volcanic rocks from t h e Paleozoic Chichibu Group i n c e n t r a l Japan. J . Geol. SOC. Japan, 76:323335. Turekian, K . K . and Wedepohl, K.H., 1961. Distribution of the elements i n some major u n i t s of the e a r t h ' s c r u s t . Geol. SOC. Amer. Bull., 72:175192. Yamazaki, K . , 1979. Petrological and geochemical study of bedded c h e r t along Nippon Rhein, Kiso River, c e n t r a l Japan. Master thesis a t the Geological I n s t i t u t e , University of Tokyo (MS). Yao, A., Matsuda, T., and Isozaki, Y . , 1980. T r i a s s i c and J u r a s s i c Radiolarians from t h e Inuyama a r e a , c e n t r a l Japan. J. Geosci. Osaka City Univ., 23:135154. Wakeel, S. K . El and Riley, J . P . , 1961. Chemical and mineralogical s t u d i e s of deep-sea sediments. Geochim. Cosmochim. Acta, 25:llO-146. Watanabe, T . , 1957. Genesis of bedded manganese deposits and cupriferous p y r i t e d e p o s i t s i n Japan. Min. Geol. (Japan), 1:87-97.

193

CHAPTER 12 GEOCHEMICAL CONTRIBUTION TO THE UNDERSTANDING OF BEDDED CHERT M.STEINBERG

, C.BONNOT-COURTOIS , S.TLIG.

L a b o r a t o i r e de Geochimie des Roches Sedimentaires.E.R.A.765. Paris-Sud.Bat.504. 91405 0RSAY.Cedex. France.

U n i v e r s i t e de

INTRODUCTION I n bedded c h e r t , i n

s p i t e o f t h e f a c t t h a t a l l elements,except s i l i c a , a r e i n

minor o r t r a c e quantities,geochemical

s t u d i e s can p r o v i d e good i n d i c a t i o n s r e -

l a t i v e t o c o n d i t i o n s o f r a d i o l a r i t e sedimentation.Three

aspects o f t h i s p a r t i -

c u l a r geochemistry w i l l be considered : (1) i n c e r t a i n b a s i n s , i t i s p o s s i b l e t o demonstrate t h e e x i s t e n c e o f geochemical g r a d i e n t s which have a palaeogeog r a p h i c s i g n i f i c a t i o n ; ( 2 ) systematic a n a l y s i s o f rhythmic i n t e r b e d s o f c h e r t and shale g i v e i n t e r e s t i n g i n d i c a t i o n s concerning t h e o r i g i n o f t h i s character i s t i c f e a t u r e o f bedded c h e r t ; ( 3 ) A1,Fe and r a r e e a r t h elements (REE) p e r m i t a s i g n i f i c a n t comparison between r a d i o l a r i t e s and oceanic sediments. For o u r purpose,we shal use m i n e r a l o g i c a l data o b t a i n e d by X.R.D.,major element a n a l y s i s o b t a i n e d by X r a y fluorescence and REE a n a l y s i s performed by neutron a c t i v a t i o n u s i n g a chemical s e p a r a t i o n a f t e r i r r a d i a t i o n ( T r e u i l e t a1.,1973;

C o u r t o i s and Jaffre~ic-Renault~l977).

1. PALAEOGEOGRAPHIC ZONATION I N A CRETACEOUS RADIOLARITE FORMATION OF TURKEY I n a study concerning Mesozoic bedded c h e r t s o f Pindos (Greece) and Cretaceous c h e r t s from NW P a c i f i c ocean (Leg.32 DSDP),we p r e v i o u s l y proposed ( S t e i n berg and Mpodozis,l978)

t o use t h e Al/Fe r a t i o o f c h e r t s i n o r d e r t o ( 1 ) de-

monstrate palaeogeographic z o n a t i o n i n r a d i o l a r i t e basins d u r i n g a given geolog i c a l period,(2)

f o l l o w t h e e v o l u t i o n o f t h e sedimentary environment w i t h i n a

b a s i n through time.Subsequently,this and REE. Thus,recently,we

method was completed u s i n g t h e A l / T i r a t i o

e s t a b l i s h e d t h e r e l a t i v e palaeogeographic p o s i t i o n o f

several Upper J u r a s s i c r a d i o l a r i t e formations i n v o l v e d i n r a t h e r complicated t e c t o n i c u n i t s o f t h e Cedros and San B e n i t o i s l a n d s (Baja California,Mexico) (Rangin e t a l . ,1981). Another example,presented here,concerns Cretaceous bedded c h e r t s i n Turkey ( M a r t i n e t al.,l981).These

r a d i o l a r i t e s belong t o t h e Catal tepe u n i t (Poisson

1977; Gutnic e t a1.,1979)

which c o n s t i t u t e s p a r t o f t h e complex zone o f Antalya

i n southern Turkey (Fig.1A) .This u n i t comprises t h r e e terms : (1) y e l l o w

194

IFe

Fig.1.

C

A : S i m p l i f i e d s t r u c t u r a l map showing t h e l o c a t i o n o f t h e s t u d i e d s e c t i o n s . 1 = Bey D a g l a r i calcareous p l a t f o r m ; 2 = Antalya t h r u s t sheets (Catal tepe u n i t except); 3 = Catal tepe u n i t . B : Bey D a g l a r i p l a t f o r m ; 1 a = n e r i t i c limestones, 1 b = p e l a g i c limestones. Catal tepe u n i t : 2 a = T r i a s s i c marls w i t h blocks , 2 b = J u r a s s i c t o Turonian b r e c c i a s , 2 c = Upper Cretaceous r e d marls and c l a y s , 2 d = Upper Cretaceous bedded c h e r t s . C : Regression l i n e s between Fe and A l . A= Derekijy s e c t i o n ; B= Elmali b e l i s e c t i o n ; C= B i l e l y e r i s e c t i o n D : REE p a t t e r n s f o r E l m a l i b e l i samples (1,2 and 3) and Derekoy samples (4,5 and 6 ) .

195

Triassic marls containing reefal limestone and sandstone blocks,(2) Early Jurassic to Turonian calcarenites and coarse breccias,(3) Upper Cretaceous calcareous breccias,red claystones,radiolarites and some tuffs. Important changes of facies occur from North to South within the Catal tepe unit.Thus,in Bilelyeri area (Fig.1A and lB),the thickness of the Jurassic-Turonian breccia is reduced and their constituents are finer than those within the Elmali beli area;furthermore,radiolarites are more frequent.Southward,in the Derekoy area,breccia thickness is further reduced and the entire unit is formed almost entirely of radiolarites and red clays.This lateral evolution indicates a palaeogeographic zonation:breccias are coarser and thicker near the source of detrital material which was constituted by a calcareous platform (Bey Dag1ari)located northward (in the present structural situation) .Since this platform is composed only of limestones of Triassic to Late Cretaceous age,it cannot be a major souce of clays associated with radiolarites. Three sections of Upper Cretaceous bedded cherts exposed at Elmali beli, Bilelyeri and Derekoy were studied in order to establish whether a paleogegra phic zonation,similar or different from that of the underlying formations,could be demonstrated. 1.1 Mineralogical results In addition to quartz,the main constituent,two clay minerals are present in all samples : smectite generally prevails over illite.Moreover,chlorite exists only at Elmali beli where feldspars are also present.If feldspars and chlorite are detrita1,this could mean that the Elmali beli area was closer to the source than other sections. 1.2 Geochemistry The ratio Si/(SitAltFe) has been computed from the results of chemical ana1ysis.It gives information concerning the biogenic silica content of the rock in comparison with alumino-silicates and ferruginous minerals (see Table 1). It should be noted that these radiolarites are devoid of carbonate. Furthermore,many authors studying radiolarites have suggested that a strong statistical correlation exists between iron and aluminium (Cressman,l962; Audley-Charles,l965;'Saito,1972 ; Leoni,1974; Steinberg and Mpodozis,l978; Baltuck,l980 ).Our chemical results were converted into ppm. following which the equation of regression lines between Fe and A1 and coefficients of determination were computed (Table l).Regression lines have been drawn on Fig.1C. REE patterns of certain representative samples are represented on Fig.lD., according to the usual method (Coryell et al.,l963).Analytical data have been normalized to the average of shales (Piper,1974).

196

1

TABLE 1

Ratios Si/(Si+Al+Fe) and c o r r e l a t i o n between Fe and A1 i n samples from Turkey Location

N

-

Si/(Si+Al+Fe)

Derekoy

6

0.90

Elmal i be1 i

9

0.74

9 -

0.94

Bilelyeri

C o rar e l a t i o n Fe-A1 y = a x + b

I

0.92 0.91

I

0.94 0.57

'

I

-

3100

+

4600

+

2260

1

N: numbers o f samples analyzed; r: c o e f f i c i e n t o f d e t e r m i n a t i o n o f r e g r e s s i o n l i n e s . y = ax + b : r e g r e s s i o n l i n e equation where a = slope o f l i n e s drawn on F i g 1 C and b expressed i n ppm. The value o f Si/(Si+Al+Fe) corresponding t o samples o f E l m a l i b e l i i s lower than t h a t o f t h e two o t h e r sections.This suggests a g a i n t h a t t h e E l m a l i b e l i area was c l o s e r t o t h e sources o f d e t r i t a l m a t e r i a l . I t may be noted t h a t t h e slope o f t h e r e g r e s s i o n l i n e Fe-A1

( F i g . lC,parameter a i n Table 1) f o r t h e

E l m a l i b e l i samples although l o w e r than t h a t o f t h e Derekoy one,is

similar to

t h a t f o r the B i l e l y e r i material.

It has been noted p r e v i o u s l y t h a t , w i t h i n a g i v e n b a s i n a t h i s slope increases w i t h d i s t a n c e from d e t r i t a l sources ( S t e i n b e r g and Mpodozis,l978 ; Rangin e t al.,l981).This

e v o l u t i o n i s r e l a t e d b o t h t o t h e d i f f e r e n t geochemical behaviour

o f A l , and t o Fe (and Mn) (Bostrom and Peterson,l969).Aluminium

may be conside-

r e d as being t y p i c a l l y d e t r i t a l w h i l e i r o n and manganese may have a deep-seated oceanic origin.Thus,with

i n c r e a s i n g d i s t a n c e from d e t r i t a l sources,the r a t i o

Fe/A1 should increase a n d , i f these elements a r e c o r r e l a t e d , t h e r e g r e s s i o n l i n e slope increases.Present results,therefore,suggest t h a t t h e E l m a l i b e l i s e c t i o n was probably s i t u a t e d c l o s e r t o d e t r i t a l sources than DerekiSy. REE p a t t e r n s o f E l m a l i b e l i samples ( F i g . 1D) a r e f l a t o r present a minor cerium anomaly;moreover,they

present a minor p o s i t i v e europium anomaly w h i l e

samples o f t h e Derekoy area a r e more d e p l e t e d i n Ce and do n o t p r e s e n t any enrichment i n Eu. The Eu anomaly expressed by t h e E l m a l i b e l i samples i s probab l y r e l a t e d t o f e l d s p a r s which a r e n e a r l y t h e o n l y m i n e r a l s capable o f concent r a t i n g Eu i n magmatic processes ( P h i l p o t t s , 1970) .This e n r i c h m e n t a r e l a t e d t o t h e r e d u c t i o n o f Eu3+ i n t o Eu2+,is n o t thought t o occur i n t h e sedimentary environments.This r e s u l t t h e r e f o r e suggests t h a t f e l d s p a r s i n t h e E l m a l i b e l i samples a r e a i n r e a l i t y , d e t r i t a l and n o t r e l a t e d t o diagenesis. Furthermore,a n e g a t i v e cerium anomaly i s u s u a l l y r e l a t e d t o a u t h i g e n i c m i n e r a l s which tend t o i n h e r i t t h e seawater REE p a t t e r n (Goldberg e t a1.,1963 Piper, 1974). I n f a c t , i t

a l s o i s r e l a t e d t o t h e more o r l e s s open c h a r a c t e r

;

197 o f the environment w i t h i n which a u t h i g e n e s i s takes p l a c e and a l s o t o the k i n e t i c s o f t h i s process ( D e s p r a i r i e s and Bonnot-Courtois,l980).In any case,the strong Ce anomaly f o r t h e Derekoy samples suggests t h a t these bedded c h e r t s were deposited i n a more open d i s t a l environment r e l a t i v e t o those o f E l m a l i b e l i , t h u s c o n f i r m i n g previous results.The palaeogeographic z o n a t i o n o f Upper Cretaceous r a d i o l a r i t e s o f t h e Catal tepe u n i t t h e r e f o r e i s s i m i l a r t o t h a t o f u n d e r l y i n g formation. Thus,it i s p o s s i b l e t o use major elements and REE t o demonstrate palaeogeographic z o n a t i o n w i t h i n c e r t a i n r a d i o l a r i t e formations.However,in

order t o

achieve this,a major c o n d i t i o n must be f u l f i l l e d : i n each s t u d i e d section, sedimentation should be n e a r l y homogeneous ; i m p o r t a n t reworking must be absent a t l e a s t i n t h e c h e r t beds ( s h a l e i n t e r b e d s were n o t considered i n t h e previous study).This i s n o t always t h e case and,in c e r t a i n s e c t i o n s examined i n t h e f o l l o w i n g paragraphs,we

s h a l l see t h a t sedimentation can be v e r y heterogeneous

even when o n l y c h e r t beds a r e considered.Furthermore,we may note t h a t , i n t h i s case,no c o r r e l a t i o n e x i s t s between A1 and Fe. 2. COMPARISON OF CHERT AND SHALE BEDS COMPOSITION Most r a d i o l a r i t e formations a r e c h a r a c t e r i z e d by rhythmic i n t e r b e d s o f c h e r t and shale.Although,many

sedimentological and p e t r o g r a p h i c a l observations have

been made concerning t h i s t y p i c a l f e a t u r e (see I i j i m a e t a1.,1978,1979;

Mcbride

and Folk,1979 and references herein),we a r e n o t aware o f any study devoted t o a systematic comparison o f t h e chemical composition o f c h e r t and shale.Because such a comparison c o u l d p o s s i b l y g i v e some i n d i c a t i o n s concerning t h e o r i g i n o f t h e rhythmic bedding o f r a d i o l a r i t e s , w e

have s t u d i e d t h r e e s e c t i o n s c o r r e s -

ponding t o v a r i o u s sedimentological environment. 2.1

Analytical result s The f i r s t s e c t i o n crops o u t i n t h e Karaman-Ermenek area,another

t h e Antalya r e g i o n i n southern Turkey (Gokdeniz,l981).

portion o f

The age o f these r a d i o -

l a r i t e s i s s t i l l u n c e r t a i n ( T r i a s s i c ? ) b u t they conformably o v e r l i e T r i a s s i c nodular limestones and.are f o l l o w e d by s i l i c e o u s limestones whose t o p i s Upper Cretaceous (Senonian).Bedded c h e r t s occur i n r e g u l a r continuous beds separated by shale i n t e r b e d s . I n d i v i d u a 1 beds o f c h e r t vary i n t h i c k n e s s from 7 t o 10 cm w h i l e shales a r e 2 t o 4 cm t h i c k . Twenty beds have been analyzed

and t h e i r s i l i c a contents and values o f the

Al/Fe r a t i o r e p o r t e d on Fig.2. The f o l l o w i n g remarks can be made : ( 1 ) n o t s u r p r i s i n g l y , t h e s i l i c a c o n t e n t o f shale i n t e r b e d s i s lower than i n c h e r t beds; however d i f f e r e n c e s a r e r e l a t i v e l y low (10 t o 15% Si02 o r l e s s ) ; ( 2 ) Al/Fe values fluctuate,these

being more i m p o r t a n t i n c h e r t beds (0.8 t o 1.4) than i n

198

s

I

I

80

90

L

Fig.2. Bedded c h e r t s from Karaman-Ermenek ( T u r k e y ) . V a r i a t i o n s o f S i O and Al/Fe i n a continuous s e c t i o n o f c h e r t beds ( f i l l e d squares) and s i a l e i n t e r beds ( c i r c l e s ) . shales (0.98 t o 1.23) ; ( 3 ) f l u c t u a t i o n s i n s i l i c a c o n t e n t and Al/Fe values a r e n o t r e l a t e d , c h e r t beds a r e n o t s y s t e m a t i c a l l y r i c h e r i n Fe o r A1 than shale

interbeds. The second example concerns a s e c t i o n w i t h i n t h e Carboniferous l y d i t e s o f t h e Montagne N o i r e (South o f France).In t h i s area,Upper Devonian nodular limestones ( c a l l e d : g r i o t t e s ) a r e s t r a t i g r a p h i c a l l y o v e r l a i n by Tournaisian

( = E a r l y M i s s i s s i p p i a n ) r a d i o l a r i t e , t h e n by o t h e r nodular limestones,a ceous recurrence of E a r l y Visean age,followed Visean f l y s c h (Geze,1949;

Boyer e t a1.,1968;

t h i c k n e s s i s approximately 25 m.,rocks

sili-

by t u r b i d i t i c limestones and a Michel,l981).

The r a d i o l a r i t e

being dark grey t o black,due t o o r g a n i c

m a t t e r ( 1 t o 6% o r g a n i c carbon) and l o c a l l y a l s o c o n t a i n phosphate nodules. Three main f a c i e s can be recognized : (1) l y d i t e s where c h e r t s occur i n regu l a r continuous beds,whose mean t h i c k n e s s i s about 4 cm w h i l e shale i n t e r b e d s a r e 1.5 cm t h i c k . C h e r t beds have sharp upper and lower c o n t a c t s w i t h shale. ( 2 ) l y d i t e s where c h e r t beds a r e g e n e r a l l y continuous b u t have i r r e g u l a r surfaces;the c o n t a c t between c h e r t and shale can be g r a d a t i o n a l ; ( 3 ) l y d i t e s where c h e r t beds a r e uneven,discontinuous,gradational

c o n t a c t s w i t h shale a r e f r e -

quent.Radiolaria can be concentrated i n laminae and g r a d i n g due t o s i z e v a r i a t i o n o f t h e r a d i o l a r i a n s h e l l s were sometimes observed.Al1 these rocks a r e devoid o f carbonate,except 3) *

a t t h e very t o p o f t h e r a d i o l a r i t e f o r m a t i o n ( f a c i e s

199

u 70

60

80

..;t I

0.5

1

1.5

.

1

I

I

I

I

b

2

25

3

3.5

4

All

Fig.3. Variations of SiO and Al/Fe in a section of carboniferous bedded chert from the Montagne Noire fFrance).Chert beds = f i l l e d squares ;shale interbeds = circles.

200 F i f t y samples were analyzed and r e s u l t s are reported on Fig.3,which

indicates

t h a t :(1) shale interbeds are poorer i n s i l i c a than c h e r t beds,differences being notably more important than i n the previous Turkish s e c t i o n (20 t o 30% Si02); (2) f l u c t u a t i o n s o f Si02 and Al/Fe values a r e s y s t e m a t i c a l l y r e l a t e d : shale interbeds are always r i c h e r i n A1 than t h e i r adjacent c h e r t beds; (3) compared t o shale interbeds,the composition o f c h e r t beds change up the s e c t i o n and t h r e e p o r t i o n s can be d i s t i n g u i s h e d corresponding approximatively t o t h e t h r e e f a c i e s described above.In the f i r s t (zone AYFig.3),fluctuations o f Al/Fe r a t i o from one c h e r t bed t o t h e f o l l o w i n g a r e small; shale interbeds are char a c t e r i z e d by a l i m i t e d increase o f Al/Fe values. I n t h e second (Zone B,Fig.3), although f l u c t u a t i o n s o f Al/Fe r a t i o i n c h e r t beds increase,the main modificat i o n concerns shale interbeds i n which t h e Al/Fe r a t i o becomes very d i f f e r e n t from t h a t o f the c h e r t bed.Finally,in

t h e upper p a r t o f the s e c t i o n (zone C,

F i g . 3 ) , f l u c t u a t i o n s o f Al/Fe values a r e large,both i n c h e r t and shale beds. The t h i r d example (Fig.4) concerns o n l y c h e r t beds o f the r a d i o l a r i t e format i o n o v e r l y i n g the o p h i o l i t e s u i t e o f North Appenines ( I t a l y ) . I t crops o u t i n the area o f S t a t a l e (Mitr6,1978),where

f a c i e s a r e s i m i l a r t o those described

by B a r r e t t (1980) .Nineteen c h e r t beds,directly o v e r l y i n g lavas,were analyzed and r e s u l t s reported on Fig.4.

%Si02 4 '

0.5

70

80

90

I

1

Fig.4. V a r i a t i o n s o f Si02 and Al/Fe i n c h e r t beds o v e r l y i n g o p h i o l i t e s ( S t a t a l e Appen ines ,I t a l y )

.

One may note t h a t : (1) d i f f e r e n c e s i n s i l i c a content between these c h e r t beds are more important than those between c h e r t and shale i n o t h e r sections studied; ( 2 ) f l u c t u a t i o n s o f Al/Fe values a r e wide and t h e i r r e l a t i o n w i t h s i l i c a v a r i a t i o n s i s n o t clear.However,Fe

g e n e r a l l y i s i n c r e a s i n g when S i

201 decreases n o t a b l y i n t h e lower p a r t o f t h e s e c t i o n . I n t h e upper part,Al/Fe f l u c t u a t i o n s weaken and t h i s was observed i n o t h e r s e c t i o n s i n the Appenines. Our r e s u l t s can be sumnarized as f o l l o w s :

-

i n some cases (Karaman-Ermenek s e c t i o n ) ,shale bed composition d i f f e r s o n l y

slightly

from t h a t o f c h e r t beds;differences

between them having no systematic

trend;

-

i n o t h e r cases (Montagne N o i r e section),shale

partings are very d i f f e r e n t

from c h e r t beds and i n t h e s e c t i o n studied,they a r e s y s t e m a t i c a l l y r i c h e r i n A1

-

the composition o f c h e r t beds can be r e l a t i v e l y c o n s t a n t (Karaman-Ermenek,

zones A and B o f Montagne N o i r e ) o r f l u c t u a t e s c o n s i d e r a b l y (Zone C Montagne Noire,Appenines). 2.2. Discussion. According t o McBride and F o l k (1979),four processes may e x p l a i n t h e r h y t h m i c i t y o f r a d i o l a r i t e s : "(1) d i a g e n e t i c segregation o f s i l i c a from i n i t i a l l y sub-homogeneous s i l i c e o u s mud; ( 2 ) episodes o f r a p i d and slow p r o d u c t i o n o f r a d i o l a r i a i n s u r f a c e waters d u r i n g a c o n s t a n t r a t e o f mud d e p o s i t i o n ; ( 3 ) episodes o f c u r r e n t d e p o s i t i o n o f r a d i o l a r i a n s i l t d u r i n g constant mud deposit i o n ; ( 4 ) episodes o f c u r r e n t d e p o s i t i o n o f mud d u r i n g constant r a d i o l a r i a sedi m e n t a t i on". ( a ) Diagenesis. I t i s c e r t a i n l y i m p o r t a n t i n bedded c h e r t f o r m a t i o n because t h e o r i g i n a l t h i c k n e s s o f beds ,whatever t h e y were,must derably.Furthermore,a

have been reduced consi

-

l a r g e amount o f s i l i c a i s necessary t o c o n v e r t s i l i c e o u s

ooze,with an i n i t i a l p o r o s i t y o f a t l e a s t 70 t o 80% i n t o dense c h e r t . I f diagen e s i s i m p l i e s o n l y m i g r a t i o n o f s i l i c a , t h e n c h e r t and shale beds should d i f f e r o n l y i n t h e i r S i content. T h i s i s i n f a c t observed i n most p a r t s o f t h e Karaman-Ermenek s e c t i o n where,in a d d i t i o n , t h e s i l i c a c o n t e n t o f t h e shale beds does n o t d i f f e r much from t h a t o f t h e c h e r t ( F i g . 2 ) .

On t h e contrary,shale

interbeds

o f t h e Montagne N o i r e a r e s y s t e m a t i c a l l y r i c h e r i n A1 than c h e r t s . I n t h i s case one may assume e i t h e r a d i a g e n e t i c m i g r a t i o n o f A1 and/or Fe,or mechanical p o s t - d e p o s i t i o n segregation o f s i l i c e o u s s h e l l s and o t h e r m i n e r a l s . Aluminium i s known as an i n e r t element,and w h i l e i r o n i s more mobile,we do n o t however have any evidence o f such a migration.As f o r segregation,we agree w i t h I i j i m a e t a1.(1978) and McBride and F o l k (1979),who consider t h a t t h i s process i s very questionable.Davis

(1918) i n f e r r e d t h a t segregation takes p l a c e

w h i l e t h e sediment was a ge1,a c o n d i t i o n t h a t has never been observed i n a marine environment.Moreover,it

i s pointed out t h a t the content o f non-siliceous

m a t e r i a l i n Appenine c h e r t s ( F i g . 4 ) can a t t a i n 35 t o 45% w i t h o u t l e a d i n g t o any e v i d e n t segregation

.

202

Thus,we consider generally that beds and interbeds have been deposited separately.However,chemical characteristics of the Karaman-Ermenek bedded cherts can be explained by diagenesis alone. (b) Fluctuating radiolarian production combined with a constaht rate of clay deposition. According to this mode1,favored by Garrison and Fisher (1969), chert beds and shale interbeds should differ only by their silica content,a nearly constant flux of clay being diluted by variable quantities of biogenic sil.ica.Although this process could explain the composition of Karaman-Ermenek cherts,it does not satisfactorily explain that of the Montagne Noire and Appenines; for these latter,variations in the composition of the non-siliceous material must be assumed.Nevertheless,we may emphasize that these variations do not necessarily conflict with fluctuations of radiolarian production. (c) Fluctuating current deposition of radiolarian silt associated with a constant rate of mud deposition. Theoretically,two possibilities should be considered .Firstly,before reworking ,radio1 arian ooze may be contaminated by muds similar in composition to the "background clayey sediment",and thus,the composition of chert and shale beds,with the exception of silica,could therefore be rather similar.Thus;the chemical composition of Karaman-Ermenek bedded cherts could also be justified by this mechanism.0n the contrary,if radiolarian silt is not contaminated or are mixed with materials different in composition from that of the "background sediment",then,chert and shale beds composition will differ.Furthermore,the composition of the "background sediment" and that of the shale interbeds should be nearly constant through time while chert bed composition could be either constant or variable,depending on whether source areas of radiolarian silt change or not. In the Montagne Noire,chert beds and shale partings are chemically different although shale bed composition is nearly constant only in the lowest part of the section (Zone A,Fig.3) which could thus correspond to this third model. In the Appenines,we have no data concerning shale beds but chert bed composition fluctuates widely (Fig.4) and therefore could be related to fluctuating current deposition of radiolarian silt reworking various materials.In the same area,results concerning Pb isotopes led Barrett (1980) to a similar conclusion. He established that the amount of Fe and the volcanic contribution of Pb are greatest in the basal and lower cherts (near the ophiolite suite) while in upper cherts,poorer in Fe,volcanogenic Pb decreases and is mixed with increasing amounts of Pb of terrigenous and radiolarian silica origin.As mentionned later a similar conclusion may be inferred from REE,Fe and A1 analysis.

203

(d) Fluctuating current deposition of mud associated with a constant rate of radiolaria supply. According to this mode1,favored by Iijima et a1.(1978) in their study of Triassic radiolarite of Japan,chert and shale interbed composition generally will differ. Moreover,in this model ,the "background sediment" is radiolarian ooze,thus chert bed composition should be nearly constant while shale bed composition could be either constant or variable,depending on whether source areas of muds change or not.Zones A and B of the Montagne Noire section (Fig.3) are characterized by a nearly constant composition of cherts while,in shale interbeds,the Al/Fe ratio fluctuates widely.Thus,shale partings could correspond to current deposits. 2.3. Conclusions The rhythmic bedding of radiolarites can not be related to a single process. Furthermore,geochemical results sometimes can be justified by several mechanisms.In the Karaman-Ermenek section,the composition of chert and interbeds may be related to : (1) fluctuations o f the radiolarian productivity, (2) episodic radiolarian silt deposition,and ( 3 ) diagenesis. In the Montagne Noire,the lowest part of the section may correspond to nearly constant mud deposition interrupted by radiolarian silt arrival or,more probably,to a continuous radiolarian ooze sedimentation and fluctuating current deposition of mud;this mechanism being highly probable in the middle part of this section (Zone BYFig.3).As for the upper part (Zone C,Fig.3) where composition of cherts as well as that of shales fluctuates widely,this rhythmicity could be related to reworking of siliceous ooze and clay on a continental slope thus leading to a pre-flysch facies. In the Appenines,currents bringing radiolarian silt have reworked various materia1s:sub-marine weathering products o f lavas and,subsequently,terrigenous clays. Finally,it is noted that the two processes implying currents do not necessarily conflict with fluctuations of radiolarian productivity;although these variations can not be detected as they are masked by current effects.Moreover, diagenesis and/or superficial weathering can enhance initial differences between beds and interbeds. 3. COMPARISON BETWEEN RADIOLARITE AND OCEANIC SEDIMENTS. Most sedimentologists consider that radiolarites not only are ancient equivalent of deep oceanic radiolarian ooze but may also correspond to almost any type of marine sediment diluted by biogenic silica.Thus,it is of interest to find certain geochemical feature permitting one to precise the nature of these sediments.

204

Robertson and Hudson (1974) have compared t r a c e element c o n t e n t (Ni,Co, Cu ...) o f c h e r t s from Cyprus w i t h deep sea clays.Using REE,we have demonstrated t h e t e r r i g e n o u s o r i g i n o f c l a y s i n r a d i o l a r i t e s from Greece ( S t e i n b e r g e t a l . , 1977).More r e c e n t l y , B a r r e t t (1980) has u t i l i z e d Pb-isotopes t o d e t e c t volcanogenic c o n t r i b u t i o n t o bedded c h e r t sedimentation from t h e Appenines. 3.1. A1-Fe- (

1 REE

-

Ce ) t r i a n g u l a r diagrams

Our p a r t i c u l a r goal i s t o compare r a d i o l a r i t e s w i t h oceanic sediments as v a r i e d as p o s s i b l e . F o r t h i s purpose,we u t i l i z e t h r e e parameters :amounts o f A1 ,Fe and

I REE-Ce.

Aluminium and i r o n were s e l e c t e d because they occur o n l y i n t r a c e amount i n R a d i o l a r i a ( M a r t i n and Knauer,1973) and,thus

a r e r e p r e s e n t a t i v e o f t h e non

-

b i o s i l i c e o u s f r a c t i o n o f cherts.Moreover,aluminium may be considered as being a t y p i c a l l y t e r r i g e n o u s element w h i l e i r o n may have a deep seated oceanic o r i g i n , b e i n g more p a r t i c u l a r l y abundant i n t h e products o f submarine weathering o f b a s a l t s and i n hydrothermal d e p o s i t s (Bostrom and Peterson,l969;

Bonatti e t

a1 . ,1972). For REE concentrations,cerium was excluded because,due t o i t s well-known a b i l i t y t o change o x i d a t i o n s t a t e , i t can be 'depleted o r e n r i c h e d r e l a t i v e l y t o o t h e r r a r e e a r t h s . REE a l s o a r e i n t r a c e amounts i n r a d i o l a r i a ( I REE being c l o s e t o 10 ppm, Spirn,1965;

Bonnot-Courtois,unpublished data ) F u r t h e r -

more,REE contents a r e more o r l e s s c h a r a c t e r i s t i c o f c e r t a i n types o f sediments Thus,in s u p e r f i c i a l sediments from t h e S.E.

P a c i f i c ocean,collected along two

t r a n s e c t s between T a h i t i and t h e Peru trench,a n e a r l y r e g u l a r decreasing o f REE c o n t e n t was observed ( C o u r t o i s and H o f f e r t Y l 9 7 7 ) . T y p i c a 1 p e l a g i c r e d c l a y s

c o n t a i n several hundred ppm o f REE near t h e f r e n c h Polynesia,as w e l l as t h e sediments from t h e Bauer Deep (Dymond e t a l . ,1977) w h i l e t h e sediments c l o s e t o South America have REE c o n t e n t l e s s than 100 ppm. I n submarine weathering products o f basalts,REE c o n t e n t i s c l o s e t o 20 t o 50 ppm (Frey e t a1.,1974; Hoffert,l980;

Bonnot-Courtois.1980)

while i n "sensu-stricto"

d e p o s i t s , i t i s lower ( 5 t o 15 ppm) ( C o r l i s s e t a1.,1978;

hydrothermal

Bonnot-Courtois,l981

a and b ) . D i f f e r e n t types o f marine sediments can be d i s t i n g u i s h e d on a t r i a n g u l a r diagram wher A1,Fe and ( IREE-Ce) values a r e p l o t t e d ( Fig.5)

.

F i e l d s corresponding t o (1) p e l a g i c r e d clays, ( 2 ) submarine weathering products o f basalts,(3)

hydrothermal d e p o s i t s a r e c l e a r l y d e l i m i t e d w h i l e

f i e l d s ( 4 ) and ( 5 ) a r e more d i f f i c u l t t o characterize.Both c o n t a i n t e r r i g e n o u s and volcanogenic m a t e r i a l s , t h e former p r e v a i l i n g i n f i e l d ( 4 ) .

205

K

E

-

C

e

x103

J

T\m

m

\ \

J Al

/

O

Fe

Fig.5.Recent oceanic sediments.Circles : SE Pacific ocean transect ; black dots : pelagic red clays; empty circles :sediments closer to South America ; black squares : submarine weathering products o f basalts ; empty squares : pelagic sediments from SW Indian ocean ; black triangles :hydrothermal deposits (FAMOUS and Galapagos area) ; empty triangles : diatomaceous oozes (Sea of Japan,Leg.Sl,DSDP). Samples of bedded chert collected in areas where they are overlying ophiolite suites are reported on Fig.6.0ne sample i s in the field of hydrothermal deposits.It cropped out in the Cedros island (Mexico) at the contact between sediments and pillow-lavas and contain more than 40% Fe203 . At least one sample from each section studied falls within field (2).They correspond to chert collected near the contact with ophiolites.Thus,as one progresses up the chert formation,the influence of volcanogenic products decreases;samples fall in the field (5),certain samples from Mexico being in the field of pelagic clay . Finally,samples of bedded chert overlying sedimentary formations are illustrated on Fig.7.Most fall in the field (5),only three being in the field of pelagic red clays. 3.2.Discussion Thus,radiolarites overlying ophiolites are contaminated by sediments rich in iron.According to their REE content they are closer to basaltic submarine weathering products than to "sensu-stricto" hydrothermal deposits.This

206

Al

Fe

Fig.6.Bedded cherts overlying ophiolites.Black squares:San Francisco (California,USA);black dots:Cedros island (Mexico);black triang1es:Nicoya complex (Costa Rica);open circ1es:Appenines ' ( 1taly);empty squares:Ballantrae (England).

Al

Fe

Fig.7.Bedded cherts overlying sedimentary formations.Squares:Rif (Morocco); circ1es:Montagne Noire (France);triangles:Lombardy (Italy); inverted triangles Pindos (Greece).

207

contamination,previously demonstrated in the Appenines (Barrett,l980);seems to be generally present in other Mesozoic formations (California,Mexico,Costa Rica) as well as in Paleozoic rocks (Ballantrae,England). Most radiolarites being red coloured are often compared to pelagic red clays.This comparison i s not correct if one considers pelagic red clays as sediments almost devoid o f terrigenous material ,presenting a very low sedimentation rate and generally rich in ferruginous smectites and zeolites.Most radiolarites,if not al1,contain terrigenous components but the parameters considered (A1,Fe and REE)do not provide enough information permitting a more precise characterization. Thus ,for instance ,radio1ari tes from the Montagne Noire were certainly deposited close to the continent.Nevertheless,their A1 content is not very high (Fig.7) probably because the continent was covered by iron-rich lateritic weathering formations.Moreover,REE content of most continental crust materials fluctuate within a narrow field. 4.CONCLUSIONS These studies permit the following conclusions. It is often possible to determine palaeogeographic zonation within radiolarite basins when sedimentation is sufficiently homogeneous and does not include marked variations related to a predominent current deposition. The rhythmic feature of bedded cherts is not related to a single process. Productivity fluctuations of radiolaria may often be obliterated by the action of currents.Shale as well as chert beds may be related to current deposition. Finally,contamination of basal radiolarites overlying ophiolite suites by volcanogenic material is confirmed. Howeverymost bedded cherts contain terrigenous phases and the exact equivalents of recent pelagic red clays seem very rare. Acknowledgements We are grateful to Dr P.Azema,J.Marcoux,A.Poisson and Pr.L Leclaire who provided samples of bedded cherts and Indian oceanic sediments.We also thanck Pr.B.H.Purser for improvement of the English.This research has been supported partly by the.Centre National de la Recherche Scientifique (ERA 765,ATP.IPOD). REFERENCES Audley-Charles,M.G.,1965.Some aspects of the chemistry of Cretaceous siliceous sedimentary rocks from Eastern Timor.Geochim.Cosmochim.Acta,29:1175-1197 Baltuck,M.,1980.Chemistry and provenance of Mesozoic pelagic sediments,Pindos 429. mountains,Greece.Z6th Intern.Geol.Cong.Paris,Abstract,II:

208

BarrettYT.J.,1980.The Pb-isotopic composition of Jurassic chert overlying ophiolites i n the North Appenines,Italy.Earth and Planet.Sc.Lett.,49 : 193-204. Bonatti,E.,Kraemer,T.,and Rydel1,H.1972.Classification and genesis of submarine iron manganese deposits.in Ferromanganese deposits on the ocean floor.D.Horn (Editor).I.D.O.E. National Science Foundation.149-165. Bonnot-Courtois,C.198O.Le comportement des terres rares au cours de l'alteration et ses cons~quences.Chem.Geo1.,30 : 119-131. Bonnot-Courtois,C.1981 a.Distribution des terres rares dans les depots hydrothermaux de la zone Famous et des Ga1apagos.Comparaison avec les sediments metalliferes.Marine Geol.,39 : 1-14. Bonnot-Courtois,C. 1981 b.Geochimie des terres rares dans les principaux milieux de formation et de sedimentation des argiles.These Dr.Sc.Orsay 215p. Bostrom,K. and Peterson,M.N.A. 1969.The origin of aluminium poor ferromanganoan sediments in areas of high heat flow on the East Pacific Rise.Marine Geo1.,7,5 / 427-447. Boyer,F.,Krylatov,S.,Lefevre,J. and Stoppe1,D. 1968. Le devonien superieur et la limite devono-carbonifere en Montagne Noire (France).Lithostratigraphie. Biostratigraphie (Conodontes).Bull.Centre Rec.Oau,S.N.P.A.2 : 5-33. Corliss,J.B.,Lyle,M.,Dymond,J. and Crane,K. 1978. The chemistry of hydrothermal mounds near the Galapagos rift.Earth and Planet.Sc.Lett.,40 : 12-24. Coryell,C.D.,Chase,J.W. and Winchester,J.W. 1963. A procedure for geochemical interpretation of terrestrial rare earths abundance patterns.J.Geophys.Res., 68 : 559-566. Courtois,C.and Jaffrezic-Renaul t,N. 1977. Utilisation des proprietes echangeuses d'ions du bioxyde d'etain pour l'analyse des lanthanides par activation neutronique.Comptes Rendus Acad.Sc.ParisyS6r.D,284: 1139-1142. Courtois,C. and Hoffert,M. 1977. Distribution des terres rares dans les sediments superficiels du Pacifique Sud-Est.Bull.Soc.Geol.Fr.,XIX : 1245-1251. Cressman,E.R. 1962. Data of geochemistry : non detrital siliceous sediments. U.S.Geol.Surv.Prof.Pap.,440 T,1-22. Davis,E.F. 1918. The radiolarian cherts of the Franciscan Group.Univ.Ca1ifornia Pub.Geo1 .,11,3 : 235-432. Desprairies,A. and Bonnot-Courtois,C. 1980. Relation entre la composition des smectites d'alteration sous-marine et leur cortege de terres rares. Earth and Planet. Sc.Lett.,48 : 124-130. Dymond,J. ,Corliss,J.B. and Heath,G.R. 1977. History of metalliferous sedimentation at Deep Sea Drilling Site 319 in the South Eastern Pacific.Geochim. Cosmochim.Acta,41,6 : 741-753. Frey,F.A. ,Bryan,W.B. and Thompson,G. 1974. Atlantic ocean floor : geochemistry and petrology of basalts from Legs 2 and 3 of the Deep Sea Drilling Project. J.Geophys.Res.,79 : 5507-5527. Garrison,R.E.and Fischer,A.G. 1969. Deep water limestones of the Alpine Jurassic.in G.M.Friedman (Editor).Depositional environments in carbonate rocks. Soc.Econ.Paleont.Mineral.Spec.Publ.,l4 : 20-54. Geze,B. 1949. Etude geologique de la Montagne Noire et des Cevennes meridionales.Mem.Soc.Geo1 .Fr. ,no 5,XX. Gbkdeniz,S. 1981. Recherches g6ologiques dans les Taurides occidentales entre Karaman et Ermenek,Turquie. These 3eme Cycle,0rsay,no3006.

209

Goldberg,E.D.,Koide,M.,Schmidt,R. and Smith,R. 1963. Rare earth distributions in the marine environment.J.Geophys.Res.,68 : 4029-4217. Gutnic ,M. ,Monod,O. Poisson,A. and Dumont ,J .F. 1979. Geol ogie des Taurides occidentales (Turquie).Mem.Soc.Geol.Fr.,LVIIIyno 137,112~. Hoffert,M. 1980. Les "argiles rouges des grands fonds" dans le Pacifique Dr.Sc.Strasbourg.231p. Centre Est.Authigen&se,transport,diagenese.These and Sciences g@ologiques,n" 61. Iijima,A.,Kakuwa,Y.,Yamazaki,K. and Yamagimoto,Y. 1978. Shallow sea,organic origin of the Triassic bedded chert in central Japan.Fac.Sci.Univ.Tokyo, Sec.I1,19 : 369-400. Iijima,A., Inagaki,H. and Kakuwa,Y. 1979. Nature and origin o f the Paleogene cherts in the Setagowa terrain,Shizuoka,central Japan. Fac.Sci.Univ.Tokyo Sec.I1,20 : 1-30 Le0ni~R.1974. La rocce silicce non detriche dell Apenino centro-settentrionale Att.Soc.Tosc.Sci.Nat.Pisa,Mem.81 : 187-221. Martin,A.,Marcoux,J.,Poisson,A., and Steinberg,M. 1981. Mise en evidence de gradients geochimiques dans une serie radiolaritique mesozoique des Taurides occidentales (Turquie),consequences paleogeographiques .Comptes Rendus Acad.Sci.Paris,Ser.D, 292 : 525-530. Martin,J.H. and Knauer,G.A. 1973. The elemental composition of plankton.Geoch. Cosmochim.Acta.,37 : 1639-1653. McBride,E.F. and Folk,R.L. 1979. Features and origin of Italian Jurassic radiolarites deposited on continental crust.J.Sedim.Petro1. ,49 : 837-868. Miche1,D. 1981. Paleoenvironnement des calcaires noduleux et lydiennes en Montagne Noire (Devonien superieur-Dinantien).Th@se 3eme Cycle,Orsay,385 p. Mitre-Salazar,L.M. 1978. Contribution a 1 'etude de 1 'Apennin septentrional : la region de Bracco (Province de Genes et de la Spezia,Italie).These Dr. Ing. Universite Paris VI. Philpotts,J.A. 1970. Redox estimation from calcaulation of Eu2+ and Eu3+ concentration in natural phases. Earth and Planet.Sc.Lett.,g : 257-268. Piper,D.Z. 1974. Rare earth elements in the sedimentary cycle : a summary. Chem.Geol.,l4 : 285-304. Poisson,A. 1977. Recherches geologiques dans les Taurides occidenta1es.These Dr.Sc .Orsay,n) 1902. Rangin,C., Steinberg,M.and Bonnot-Courtois,C. 1981. Geochemistry of the Mesozoic bedded cherts of Central Baja California (Vizcaino-Cedros-San Benito). Implications for palaeogeographic reconstructions of an old oceanic basin. Earth Planet. Sc.Lett. ,54 : 313-322. Robertson,A.H. and Hudson,J.D. 1974. Pelagic sediments in the Cretaceous and the Tertiary history of the Troodos massif,Cyprus. in J.HsU and H.C.Jenkyns (Editors),Pelagic sediments on land and under the sea.Int.Ass.Sedim.,Spec. Publ.n)l : 403-436. Saito,Y. 1972. Some aspects o f chemical composition of cherts.Bul1.Nat.Sci.Mus. Japan. ,19 : 403-414. Spirn,R.V. 1965. Rare earth distributions in the marine environment.Ph.D.Thesis. 165p. Steinberg,M. ,Fogel gesang ,J. F. ,Courtoi s ,C. ,Mpodozi s ,C. ,Desprairies,A. , Martin, A., Caron,D. and BlanchetBR.1977.D@termination de l'origine des feldspaths et des phyllites presents dans des radiolarites mesogeennes et des sediments hypersiliceux oceaniques par l'analyse des terres rares.Bull.Soc.Geo1. Fr., 19,4 : 735-740.

210

Steinberg,M. and Mpodozis-Marin,C. 1978.Classification geochimique des radiolarites et des sediments siliceux oceaniques : signification pal6o-oceanographique.0ceanologica Acta,l : 359-367. Treui 1 ,M. ,Jaffrezic,H. ,Deschamps ,N. ,Derre,C. ,Guichard,F. ,Joron,J .L . ,Pel letier B.,Novotny,S. and Courtois,C. 1973. Analyse des lanthanides,du hafnium,du scandium,du chrome,du mangan@se,de cobalt,du cuivre et du zinc dans les mineraux et les roches par activation neutronique. J.Radioanal.Chem.,l8 : 55-68.

211

CHAPTER 13

OPAL-A TO OPAL-CT TRANSFORMATION: A KINETIC STUDY M. KASTNER and J. M. GIESKES

Scripps I n s t i t u t i o n o f Oceanography, La J o l l a , Ca i f o r n a 92093 (U.S.A.) ABSTRACT K i n e t i c experiments on t h e opal-A t o opal-CT t r a n s f o r m a t i o n were conducted a t 50", 75", loo", 125", and 150"C, i n 0.03 M MgC1, and 0.03 M NaHCO, s o l u t i o n s , w i t h r a d i o l a r i a n s ( s p e c i f i c s u r f a c e area, SSA, 5.3 m2/g) and Ludox s i l i ca (SSA 61.5 m2/g). For b o t h s t a r t i n g m a t e r i a l s , t h e changes i n d i s s o l v e d magnesium and i n a l k a l i n i t y a r e e q u i v a l e n t a t a l l temperatures studied. This confirms t h a t t h e prev i o u s l y p o s t u l a t e d magnesium hydroxide compound (MHC) (Kastner e t a l . , 1977) serves as a nucleus f o r opal-CT c r y s t a l l i z a t i o n a t a l l temperatures. The changes i n d i s s o l v e d magnesium and i n a l k a l i n i t y decrease as t h e MHC continues t o p r e c i p i t a t e . I n t h e experiments w i t h r a d i o l a r i a n s , d i s s o l v e d s i l i c a values a t 50", 75", and 100°C reach values approaching t h e s o l u b i l i t y v a l u e o f B - c r i s t o b a l i t e ( a "disordered" opal-CT); t h e d i s s o l u t i o n r a t e o f opal-A i s t h e r a t e c o n t r o l l i n g step. A t 125" and 150"C, a f t e r t h e a l k a l i n i t y has been consumed, t h i s value i s exceeded, and a t 150"C, i t reaches t h e s o l u b i l i t y value of opalA. Thus, under these c o n d i t i o n s , t h e c r y s t a l l i z a t i o n o f opal-CT i s t h e r a t e c o n t r o l l i n g step. I n t h e experiments w i t h Ludox s i l i c a , d i s s o l v e d s i l i c a v a l ues a t a l l temperatures reached c o n c e n t r a t i o n s i n excess o f B - c r i s t o b a l i t e s o l u b i l i t y . An apparent steady-state between r a t e s o f MHC n u c l e i formation, opal-CT c r y s t a l l i z a t i o n , and opal-A d i s s o l u t i o n has been achieved. The 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 decreases c o n s i d e r a b l y w i t h decreasing temperatures. E x t r a p o l a t i o n t o 0°C o f t h e k i n e t i c experimental r e s u l t s w i t h r a d i o l a r i a n s i n d i c a t e s t h a t under e a r l y d i a g e n e t i c c o n d i t i o n s , t h e r a t e o f MHC n u c l e i f o r m a t i o n i s very slow b u t s t i l l s i g n i f i c a n t , i n p a r t i c u l a r i n d i a g e n e t i c environments w i t h adequate s u p p l i e s o f d i s s o l v e d Mgt2 and (OH)-, f o r example, from sea water and carbonate d i s s o l u t i o n . INTRODUCTION Opaline s i l i c a from b i o g e n i c sources, once deposited i n t h e sediment, undergoes e v o l u t i o n a r y changes w i t h time.

The pathways and r a t e s o f t h i s diagene-

t i c e v o l u t i o n depend on t h e n a t u r e o f t h e i n i t i a l s i l i c a phase and v a r i o u s physico-chemical c o n t r o l s , t h e most i m p o r t a n t o f which a r e temperature, pressure, pH, pore f l u i d chemistry, and s p e c i f i c s u r f a c e area. I n t h e marine environment, t h e i n i t i a l s i l i c a phase i s p r i m a r i l y b i o g e n i c opal-A (Jones and Segnit, 1971).

I n t h e m a j o r i t y o f cases, i t transforms t o

q u a r t z c h e r t through i n t e r m e d i a t e metastable opal-CT (Jones and Segnit, 1971) by a s o l u t i o n p r e c i p i t a t i o n mechanism (Carr and Fyfe, 1958; Mizutani, 1966; S t e i n and K i r k p a t r i c k , 1976; Kastner e t a l . , 1977). I n open spaces, such as

212

c a v i t i e s , opal-CT often occurs in the form o f lepispheres ranging i n s i z e from 4 t o 30 vm (e.g. Wise and Kelts, 1972; Berger and von Rad, 1972; Kastner e t a l . , 1977; Kastner, 1979, 1981). The most common form, however, i s massive opal-CT (Keene, 1975, 1976), some of which r e s u l t s from the coalescence of lepispheres. Earlier experimental work (e.g. Campbell and Fyfe, 1960; M i z u t a n i , 1966, 1977) led t o the conclusion t h a t temperature and pressure are the important physico-chemical factors which control the r a t e of opal-A t o opal-CT transformation. Numerous f i e l d observations (e.g. Bramlette, 1946; Millot, 1970; Greenwood, 1973; Lancelot, 1973; Keene, 1976; Pisciotto, 1978), however, suggested t h a t "lithological" factors such as the clay/carbonate r a t i o a l s o strongly influence the r a t e of t h i s transformation. An experimental study by Kastner e t a l . (1977) explained the observed general relationship between " l i thology" and r a t e of transformation independent of burial depth (temperature and pressure) and sediment age. These authors demonstrated from hydrothermal experiments t h a t a magnesium hydroxide compound (henceforth referred t o as MHC) containing Mg+' and (OH)- in a 1 : 2 r a t i o , and as yet unidentified, serves as a nucleus which i s chiefly responsible f o r the observed enhanced transformation rates of opal-A t o opal-CT in carbonate rocks. This MHC nucleus appears t o serve as a template f o r opal-CT c r y s t a l l i z a t i o n . The experiments were carried o u t mainly a t 150°C. A l t h o u g h short-term experiments, a t room temperature f o r a maximum of 3 weeks, suggested that s i l i c a precipitation occurred on c a l c i t e surfaces, no significant changes in solution chemistry were observed, with the exception of an increase i n dissolved s i l i c a . This can be understood in terms of extremely low rates o f MHC formation a t room temperature and of minor amounts of s i l i c a involved. Observations of the growth of opal-CT lepispheres i n marine sediments (Wise and Kelts, 1972; Wise and Weaver, 1974), which show much the same h a b i t a s those i n the 150°C experiments by Kastner e t a l . (19771, seem t o indicate t h a t the above mechanism i s indeed valid a t the temperatures of ocean f l o o r diagenesis. The main objectives of the present experimental study were t o extend our previous work t o lower temperatures t o t e s t the above hypothesis, and t o gain information on the rate-controlling step of this reaction as influenced bytemperature, solution chemistry, and by the specific surface area of the opal-A s t a r t i n g material. As no r e l i a b l e methods e x i s t t o determine accurately the weight percent of opal-A remaining in these experiments and of the very small quantities of opal-CT crystallized in many of the experiments, no attempts could be made t o calculate the activation energy of this reaction.

213

EXPERIMENTAL Numerous solution compositions, r a n g i n g from sea water t o fresh water and various a r t i f i c i a l solutions were tested i n our previous experimental study on the mechanism of opal-A to opal-CT transformation (Kastner e t a1 , 1977). One of the major conclusions was t h a t Mg+2 and (OH)- enhance the r a t e of t h i s transformation. Accordingly, f o r the present experiments solutions of 0.03 M MgC1, and 0.03 M NaHCO, were prepared w i t h i n i t i a l pH (25°C) values of 8.0 adjusted by NaOH addition. No attempt was made t o maintain constant pH, and a small decrease i n pH was generally observed d u r i n g the experiments. One hundred milligrams of opal-A were transferred into prewashed pyrex glass tubes containing 15 ml of the above solution (about 2/3 of the tube volume). The pyrex tubes were then sealed under vacuum w i t h about 1/3 of the volume available f o r vapor space. The sealed tubes were then transferred into heating blocks which were maintained a t 50°, 75", looo, 125O, and 150°C w i t h an accuracy of 1°C f o r periods of u p t o 30 days. Most experiments were r u n i n d u p l i cates, and several three t o four times. The experiments were stopped a t regular intervals (1, 3, 5, 7 , 14, and 30 days), the solutions were immediately separated from the solids by f i l t e r i n g through a 0.45 pm millipore f i l t e r . Thequenching time f o r the 150°C was about 5 minutes. Because of the high dissolved s i l i c a values in many of the experiments, i t was imperative t o analyze thesolutions almost immediately a t the end of each experiment. The solutions wereanalyzed f o r s i l i c a , Mg'2, and a l k a l i n i t y ; reproducibility f o r duplicate experiments was f 1.5% f o r s i l i c a and f 1%Mg+2 and a l k a l i n i t y . The methods usedwere essentially the same as those described by Gieskes (1974). Starting materials were low surface-area Eocene radiolarians (5.3 m2/g) and purified Ludox s i l i c a (61.5 m2/g). Specific surface areas were measured by means of a "Quantasorb" (Quantachrome Corporation) continuous-flow adsorption apparatus. Multipoint isotherms were determined, and accuracies are 3% of the measured values. The solids were analyzed microscopically, by X-ray d i f fraction and scanning electron microscope with an X-ray energy dispersive attachment. The solution chemical data are presented i n Tables I and I 1 and plotted in Figures 1 through 4. The specific surface areas a r e given in Table I 1 1 and plotted i n Figure 5.

.

*

*

RESULTS AND DISCUSSION

Changes i n a l k a l i n i t y and dissolved Chemical compositions of solutions kinetic experiments w i t h radiolarians plotted against time i n Figures 1 and

magnesium a r e given i n Tables I and I 1 f o r the and Ludox s i l i c a , respectively, and are 2. As in our e a r l i e r experiments a t

214 150°C (Kastner e t a l . ,

1977), a t a l l temperatures used i n t h i s i n v e s t i g a t i o n

t h e changes i n d i s s o l v e d magnesium and a l k a l i n i t y a r e e q u i v a l e n t , as shown i n Figures 3 and 4, i n which t h e changes ( A ) i n magnesium and a l k a l i n i t y b o t h i n meq/l a r e p l o t t e d a g a i n s t time.

For t h e experiments i n which t h e A Mg and A

a l k a l i n i t y values a r e i d e n t i c a l , o n l y A a l k a l i n i t y i s p l o t t e d . i n d i c a t e t h a t n u c l e i o f MHC p r e c i p i t a t e d a t a l l temperatures.

2 0 " .v)

These r e s u l t s

215

I

*

I

CnY

-A

za

aa

3000

10

-fl

2000

20 20

1000

10 10

0 4000

5 0

I b 30 30

20

4

Cb

-A

20 20

2000

10 10 10

0

0

6

6000

30 15

4000

20 10

2000

10

5

4 30 20

2

10 0

10

20

30

0

0

tc

-A

216

rrY

CnY

za

za

aa 10

2

5

I'

-A

aa

--B

-A

30 5OoC

0

-B

20 10

4

0

0

125OC

20

30 20

10

10

0

0

DAYS A OpaI-A ,& 8-Cristobalite

30 20

H4Si04(mM)

10

0

+

DAYS

Q

AMg ( m e q N ) AAlkalinity (meq//)

Fig. 3. Changes in dissolved s i l i c a and in A magnesium and A a l k a l i n i t y as a function of time and temperature i n experiments w i t h Eocene radiolarians. Missing A magnesium data points indicate an exact overlap with a l k a l i n i t y . The arrow next t o 8 represents the s o l u b i l i t y value o f 13-cristobalite~and the arrow next t o A represents the s o l u b i l i t y value of opal-A. The r a t e of change in magnesium and a l k a l i n i t y does not vary considerably w i t h time a t 50" and 75"C, while i t decreases rapidly as a function of time a t lOO", 125", and 150°C. The rapid decrease i s in part a r e s u l t of s l i g h t decreases i n pH values, b u t i s due primarily t o decreasing rates of MHC nuclei formation and decreasing a1 kalinity and magnesium concentrations. In the 15OOC experiments with radiolarians, f o r example, a l l the MHC nuclei formed within the f i r s t one t o three days (Table I and Figs. 1 and 3 ) . A t 100°C, however, the compound was s t i l l forming a f t e r 30 days, i n both

75'c

5OoC Time (days)

H SiO4 TIIM)

Mg+2*

(mM)

A1 ka-** linity (meq/l)

1 3 7 14 30

206 456 768 1140 1400

29.6 29.4 29.1 28.9 28.4

33.3 32.8 31.6 31.3 30.9

*

**

l0O0C

A1 kaH Si04 Mg+2 l i n i t y (mM) (meq/l) ?uM)

863 1700 2400 2740 2920

28.0 27.2 25.9 24.6 22.5

27.5 26.3 24.4 21.7 19.0

15OoC

125'C

A1 kaH Si04 Mgt2 l i n i t y ~ I J M ) (mM) (meq/l)

23.8 22.9 20.4 19.6 18.1

2110 2550 3860 4080 4360

(meq/l)

3155 5920 6080 6620 6760

20.5 18.8 12.2 10.4 8.2

uM)

(mM)

Alkalinity (meq/l)

6480 8310 8900 10250

14.9 14.5 14.8 14.0

2.3 1.7 1.4 1.4 1.4

18.8

16.2 15.6 15.9 15.8

3.0 2.9

Concentration o f dissolved Mg+2 o f a l l s t a r t i n g solutions was 29.8 mM. A l k a l i n i t y o f s t a r t i n g s o l u t i o n o f 5OoC experiments was 33.6 meq/l and o f 75'.

H SiOr ?vM)

H SiOr MS+2

Mg+2* A l k a l i n i t y * * (M)

(nw/l)

2073 2566

31.7 31.3

Ei : 28.9

2480 2364

31.0 29.7

28.3 26.4 24.5

I I

?VM)

3052 3399 3449 3510 3505 3570

28.3 26.2 26.2 25.5 24.6 23.8

Alkalinity (meq/l)

and 150°C was 31.3 meq/l.

ii:; 19.1 18.0 16.4 14.9

1 I

H SiO,

Mgt2

(m)

Alkalinity (meq/l)

3906 4681 4740 4940 4917 5042

23.6 21.0 20.5 20.2 19.7 19.4

14.8 9.2 8.3 7.7 6.7 5.9

Concentration o f dissolved Mg+2 o f a l l s t a r t i n g s o l u t i o n s was 31.6

**

125',

l0O0C

75OC

5OoC Time (days)

looo,

A l k a l i n i t y o f a l l s t a r t i n g s o l u t i o n s was 31.0 meq/l.

mM.

218 r a d i o l a r i a n s and Ludox s i l i c a experiments, although a t a v e r y slow r a t e (Tables I and I 1 and Figs. 1 through 4 ) .

A remarkably s i m i l a r t r e n d i s seen i n t h e s p e c i f i c s u r f a c e area (SSA) data (Table I11 and F i g . 5).

The data suggest an i n c r e a s e i n SSA t h a t c o i n c i d e s

w i t h t h e changes i n a l k a l i n i t y and d i s s o l v e d magnesium.

The c o r r e l a t i o n canbe

understood i n terms o f MHC-nuclei f o r m a t i o n and embryonic opal-CT l e p i s p h e r e c r y s t a l l i z a t i o n w i t h r e l a t i v e l y l a r g e SSA ( c . f . ,

a l s o Figs. 6c, d, and 7a, b ) .

TABLE I I I a S p e c i f i c surface areas (SSA) o f k i n e t i c experiments w i t h r a d i o l a r i a n s T"C

Time (days)

SSA (mz/g)

---

s t a r t i n g material 14 7 14

5.3 24.2 30.4 56.1

75 100 100

TABLE I I I b S p e c i f i c s u r f a c e areas (SSA) o f k i n e t i c experiments w i t h Ludox s i l i c a

ToC

Time (days)

---

s t a r t i n g material 30 14 30 1 3 7 14 30

50 75 75 100 100 100 100 100

SSA

(m2/s)

61.5 88.7 133.6 141.0 118.6 148.5 161.2 164.0 172.6

I n i t i a l slopes o f t h e curves r e p r e s e n t i n g changes i n d i s s o l v e d magnesiumand a l k a l i n i t y f o r t h e experiments w i t h r a d i o l a r i a n s ( F i g . 1) a t 50°, 75", and 100°C y i e l d an approximately l i n e a r r e l a t i o n between t h e l o g a r i t h m o f t h e s l o p e E x t r a p o l a t i o n t o zero degrees and t h e i n v e r s e o f t h e absolute temperature. c e n t i g r a d e i n d i c a t e s an i n i t i a l slope o f 3 X lO-3mM Mg+2/day o r approximately of 1 mM Mg+Z/year, which i s a maximum estimate. Thus, under e a r l y d i a g e n e t i c c o n d i t i o n s i n t h e r e l a t i v e l y c o l d p e l a g i c environment, t h i s r e a c t i o n w i l l be v e r y slow and most probably q u a n t i t a t i v e l y i n s i g n i f i c a n t w i t h i n a p e r i o d o f a

219

m9q

-

2001

t OO

20

10

30

DAYS

Fig. 5. Changes i n s p e c i f i c s u r f a c e area as a f u n c t i o n o f t i m e and temperature i n experiments w i t h Ludox s i l i c a . few thousands of years, b u t may become s i g n i f i c a n t i n a few tens o f thousands o f years, and c e r t a i n l y i n l e s s than one m i l l i o n years. We do n o t know whether i n our experiments t h e f o r m a t i o n o f t h e MHC nucleus involves the coprecipitation o f s i l i c a o r s i l i c a t e ( s ) .

W o l l a s t e t a l . (1968)

and Couture (1977) found t h a t homogeneous s o l u t i o n s o f chemical compositions s i m i l a r t o ours m i g h t p r e c i p i t a t e p o o r l y c r y s t a l l i n e s e p i o l i t e a t 25°C.

In

t h i s connection, t h e observations by Donnelly and M e r r i l l (1977) a r e o f i n t e r e s t ; they found a good c o r r e l a t i o n between t h e S i / A 1 and Mg/A1 r a t i o s i n b u l k sediment samples o f p e l a g i c s i l i c e o u s carbonate sediments.

The authors ex-

p l a i n e d t h i s o b s e r v a t i o n by a d s o r p t i o n o f magnesium on b i o g e n i c opal-A s u r f a ces.

They, however, d i d n o t observe such a c o r r e l a t i o n i n samples o f p e l a g i c

s i l i c e o u s non-carbonate o r s i l i c e o u s c l a y - r i c h sediments.

Furthermore, e x p e r i -

mental work by W i r t h and Gieskes (1979) and as y e t unpublished r e s u l t s by Kent (personal communication) i n d i c a t e t h a t a d s o r p t i o n o f magnesium on amorphous s i l i c a surfaces i s i n s i g n i f i c a n t below pH values o f 8.7 t o 9.0 a t 25°C.

We

p r e f e r , t h e r e f o r e , an e x p l a n a t i o n i n terms o f t h e above mechanism o f MHC n u c l e i formation, perhaps c o n t a i n i n g s i l i c a , which then serves as a template f o r opalCT c r y s t a l l i z a t i o n and growth. Changes i n d i s s o l v e d s i l i c a Dissolved s i l i c a values i n t h e experiments w i t h Eocene r a d i o l a r i a n s a t 50°, 75", lity

and 100°C ( F i g . l a , b, c ) appear t o reach values approaching t h e s o l u b i v a l u e o f F o u r n i e r ' s (1973) " 6 - c r i s t o b a l i t e " ,

b i l i t y values f o r opal-CT a r e a v a i l a b l e .

b u t n o t o f opal-A.

No s o l u -

I t s solubility i s inferred to l i e

220

between chalcedony and opal-A. This " B - c r i s t o b a l i t e " i s t h e o n l y s i l i c a phase which l i e s w i t h i n t h i s r e g i o n ( c l o s e t o t h e upper boundary) and f o r which s o l u b i l i t y data a r e a v a i l a b l e .

I t i s , t h e r e f o r e , assumed t h a t t h e s o l u b i l i t y o f F o u r n i e r ' s " B - c r i s t o b a l i t e " represents t h e s o l u b i l i t y o f a considerably " d i s -

ordered" opal-CT.

A t 125" a f t e r 44 days and a t 150°C a f t e r 3 days, t h e solu-

b i l i t y value of " 8 - c r i s t o b a l i t e " i s exceeded, as shown by t h e dashed l i n e s i n Figure I d , e.

A t 150"C, t h e s o l u b i l i t y value o f opal-A i s reached a f t e r 30

days. The r e s u l t s of experiments w i t h Ludox s i l i c a ( F i g . 2) d i f f e r from those presented i n F i g u r e 1.

Although r a t e s o f p r e c i p i t a t i o n o f MHC n u c l e i a r e somewhat

A t a l l temperat u r e s (50", 75", and 100°C), c o n c e n t r a t i o n s i n excess o f t h e " B - c r i s t o b a l i t e "

g r e a t e r , d i s s o l v e d s i l i c a values show v e r y r a p i d increases.

s o l u b i l i t y value i s a t t a i n e d , b u t do n o t reach t h e s o l u b i l i t y value o f opal-A. Opal-CT i n t h e h a b i t o f lepispheres o r embryonic l e p i s p h e r e s formed i n a l l t h e experiments ( F i g s . 6 and 7). The r a t e s o f opal-A d i s s o l u t i o n , o f MHC n u c l e i formation, and o f opal-CT l e p i s p h e r e s ' growth a r e very d i f f e r e n t i n t h e two s e t s o f experiments.

Rates

o f opal-A d i s s o l u t i o n have p r e v i o u s l y been shown t o be s t r o n g l y dependent on temperature (Lawson e t a l . ,

1978), t o be p r o p o r t i o n a l t o t h e square o f t h e spe-

c i f i c s u r f a c e area (Goto, 1958), and a l s o t o be dependent on pH o r on s u r f a c e charge ( W i r t h and Gieskes, 1979).

The SSA o f t h e Ludox s i l i c a s t a r t i n g m a t e r i -

a l was almost twelve times l a r g e r than t h a t o f t h e Eocene r a d i o l a r i a n s (Table

111).

I n t h e experiments w i t h r a d i o l a r i a n s a t 50", 75", and 1OO"C,

the r a t e o f

opal-A d i s s o l u t i o n was t h e r a t e - c o n t r o l l i n g step; t h e r a t e s o f MHC n u c l e i prec i p i t a t i o n and opal-CT c r y s t a l l i z a t i o n were r a p i d enough t o prevent s i l i c a conc e n t r a t i o n s from exceeding t h e s o l u b i l i t y of " 8 - c r i s t o b a l i t e . "

The opal-CT

A t 125" and 150°C a f t e r 44 and 1 t o 3 days, r e s p e c t i v e l y , as t h e a l k a l i n i t y was conc r y s t a l l i n i t y , w i t h i n t h i s range, can thus n o t be t o o "disordered."

sumed (Table I)no a d d i t i o n a l MHC n u c l e i c o u l d form.

From t h i s t i m e on, t h e

r a t e o f growth o f t h e e x i s t i n g embryonic opal-CT lepispheres was t h e new r a t e c o n t r o l l i n g step; t h e d i s s o l u t i o n r a t e o f t h e remaining opal-A was r a p i d enough r e l a t i v e t o opal-CT growth r a t e t o exceed t h e s o l u b i l i t y o f " ~ - c r i s t o b a l i t e " and a t 150°C even t o reach t h e s o l u b i l i t y value o f opal-A. The increased SSA o f Ludox s i l i c a caused t h e s i l i c a c o n c e n t r a t i o n s t o r i s e above t h e s o l u b i l i t y value o f " 8 - c r i s t o b a l i t e " even a t 50", 75", and 100°C (Table I 1 and Figs. 2 and 4).

A steady s t a t e between r a t e s o f opal-A d i s s o l u -

t i o n , MHC n u c l e i formation, and opal-CT c r y s t a l l i z a t i o n was a p p a r e n t l y reached a t 75" and 100°C.

A comparative a n a l y s i s between t h e two s e t s o f experiments w i t h Ludox s i l i c a and r a d i o l a r i a n s s t a r t i n g m a t e r i a l s i s presented i n F i g u r e 8.

Differences i n

221

Fig. 6 .

(See caption a t top of following page.)

222

Fig. 6. Scanning electron microscope photographs of experiments w i t h radiolarians. ( a ) Opal-CT lepispheres t h a t formed a t 50°C a f t e r 7 days. ( b ) OpalCT lepispheres t h a t formed a t 100°C a f t e r 1 day. ( c ) Embryonic opal-CT lepispheres s t i l l forming a t 50°C a f t e r 30 days. ( d ) Embryonic opal-CT lepispheres s t i l l forming a t 100°C a f t e r 7 days. White bar i n lower r i g h t of ( a ) represents 75 pm f o r ( a ) , 15 pm f o r ( b ) , 10 pm f o r ( c ) , and 2 pm f o r ( d ) .

a

b

Fig. 7. Scanning electron microscope photographs of experiments w i t h Ludox s i l i c a . ( a ) Embryonic opal-CT lepispheres a t 50°C a f t e r 14 days. ( b ) Small opal-CT lepispheres and coalescing embryonic lepispheres a t 100°C a f t e r 14days. White b a r in lower l e f t of ( a ) represents 1 pm f o r both photographs. dissolved s i l i c a and i n A a l k a l i n i t y (and therefore also i n A magnesium) a t 50", 75", and 100°C a r e plotted against each other. An increase i n the d i f f e r ence i n A a l k a l i n i t y represents an increase in the difference i n yield of MHC nuclei formation, whereas a decrease in the difference i n dissolved s i l i c a indicates a decrease in the d i f f e r e n t i a l r a t e o f opal-CT c r y s t a l l i z a t i o n and growth. As discussed above, the dissolution r a t e of opal-A was the rate-controlling step in the experiments w i t h radiolarians a t these three temperatures, and dissolved s i l i c a values were always higher in the Ludox experiments than i n the experiments w i t h radiolhrians. A t 50"C, d u r i n g the f i r s t seven days the r a t e of MHC nuclei formation i n the Ludox experiments was only s l i g h t l y higher than i n the radiolarian experiments. The difference i n dissolved s i l i c a was immediately very large; i t even i n creased during the f i r s t three days, b u t then s t a r t e d t o decline rapidly. Thus, during t h i s i n i t i a l period the r a t e of dissolution o f Ludox s i l i c a , with a very high SSA, was so rapid t h a t the r a t e of MHC nuclei formation was the rate-cont r o l l i n g step f o r the c r y s t a l l i z a t i o n of opal-CT. Between 7 and 30 days, the differential yield of MHC nuclei formation increased, and the d i f f e r e n t i a l r a t e

223

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A Alk. Ludox - A Alk. Radiolarians (meq/l) Fig. 8. Differences in dissolved s i l i c a and A a l k a l i n i t y between Ludox s i l i c a and radiolarian experiments a t 50", 75", and 100°C as a function of t i n e . of opal-CT c r y s t a l l i z a t i o n and growth decreased. The difference between the two s e t s of experiments affected the nature of the newly-formed opal-CT. In the radiolarian experiments, i n addition t o embryonic opal-CT lepispheres, several larger well-developed ones grew (Fig. 6c and a , respectively); in the Ludox experiments, only numerous embryonic lepispheres were observed (Fig. 7a). A t 75"C, most of the MHC nuclei formed during the f i r s t three t o seven days; thereafter c r y s t a l l i z a t i o n and growth of opal-CT became more important than duri n g the f i r s b t h r e e t o seven days. Figure 8 indicates t h a t while the different i a l yield of MHC nuclei formation approaches zero, the difference in dissolved s i l i c a values approaches a constant value of approximately 650 urn. T h i s value most probably represents an apparent steady s t a t e between the rates of MHC nucl e i formation, opal-A dissolution, and opal-CT c r y s t a l l i z a t i o n and growth. A t lOO"C, a very large number of MHC nuclei formed during the f i r s t three days, b u t the opal-CT c r y s t a l l i z a t i o n r a t e was s t i l l significantly slower than the dissolution r a t e o f Ludox s i l i c a a t t h i s temperature. After the f i r s t t h r e e

224

days, the d i f f e r e n t i a l yield of MHC nuclei formation decreased s t e a d i l y , and the c r y s t a l l i z a t i o n and growth of opal-CT became increasingly important. Interestingly, a similar constant value of A dissolved s i l i c a i s being approached, however, more rapidly than a t 75°C. Figure 7 clearly shows t h a t the embryonic "disordered" opal-CT lepispheres which formed a f t e r 14 days a t 100°C are much b e t t e r developed and larger than those formed a f t e r the same amount of time a t 50°C (Fig. 7b and a , respectively). No large well-developed opal-CT lepispheres, which formed i n the radiolarian experiments a t a l l temperatures, were observed i n the Ludox experiments even a f t e r 30 days a t 100°C (forexample, compare Figs. 6a, b w i t h 7 b ) . In about two t o three months, a l l the dissolved s i l i c a values i n the Ludox s i l i c a experiments a r e expected t o approach the s o l u b i l i t y values of "B-cristobal i t e I' I t was shown by Kastner e t a l . (1977) t h a t a t higher dissolved s i l i c a concentrations a more "disordered" opal-CT c r y s t a l l i z e s than a t lower dissolved s i l i c a concentrations. S i l i c a studies i n the Monterey Formation, California (e.g. Murata and Nakata, 1974; Murata and Larson, 1975; Pisciotto, 1978; Isaacs, 1981) have shown t h a t the degree of "disorder" of opal-CT influences i t s potent i a l t o transform diagenetically t o quartz. Hurd and Theyer (1975) have shown t h a t the SSA and s o l u b i l i t y of radiolarians decrease significantly w i t h geologic age. Similar observations were reported by Willey (1980) f o r synthetic opal-A which was aged f o r various time intervals. No simple relationship between the s o l u b i l i t y (as affected by SSA o r age) of biogenic opal-A and the degree of "disorder" of opal-CT should, however, be expected. W e have shown t h a t the nature of the opal-CT which crystallized depends on a complex interaction primarily between the dissolution r a t e of opalA (which t o a large degree i s influenced by i t s SSA), the solution chemistry, and the temperature.

.

CONCLUSIONS Kinetic experiments a t 50°, 75O, 100°, 125", and 15OoC, w i t h two d i f f e r e n t opal-A s t a r t i n g materials in solutions containing magnesium and a l k a l i n i t y indicate: 1. That a t a l l temperatures of this investigation a magnesium hydroxide compound (MHC) serves as a nucleus f o r the c r y s t a l l i z a t i o n of opal-CT. 2. Rates of this reaction are strongly temperature-dependent and slow considerably w i t h decreasing temperatures. Nevertheless, even a t early diagenetic temperatures, reaction rates should lead t o significant MHC nuclei formation w i t h i n a period of a few tens of thousands of years, t h u s making the formation

226

of MHC nuclei formation an early diagenetic process in environments in which a supply of Mg+2 and (OH)- is available, for example, Mg+2 from sea water and (OH)- from carbonate dissolution or bacterial reduction of dissolved sulfate. 3. In the transformation of opal-A t o opal-CT, the rate-controlling step depends on the solution chemistry, the nature of the opal-A starting material, and temperature. (a) When Mg+2 and (OH)- are available for MHC nuclei formation and the specific surface area of the opal-A is relatively low, the dissolution rate of opal-A will be the rate-controlling step. (b) When either Mgt2 or (OH)- or both were or are no more available for MHC nuclei formation, nucleation or crystallization and growth of opal-CT, respectively, will be the ratecontrolling step, regardless of the specific surface area of opal-A. (c) In the presence o f opal-A of high specific surface area, even if Mg+2 and (OH)are available, initially crystallization of opal-CT will be the rate-controlling step. With time, the dissolution rate of opal-A will become the ratecontrolling step. ACKNOWLEDGMENTS We are grateful to Drs. R. Siever, Harvard University, Y. K. Bentor, Scripps Institution of Oceanography and Hebrew University, and A. Iijima, University of Tokyo for their critical reviews of the manuscript and for their constructive comments. We also thank Mr. D. B. Kent for conducting several of the specific surface area analyses. This research was supported by National Science Foundation Grants 0CE78-09652 (M.K.) and 0CE80-23966 (J.M.G.). Additional support from Chevron Oil Research Co., La Habra, California is greatly appreciated. REFERENCES Berger, W.H. and von Rad, U., 1972. Cretaceous and Cenozoic sediments from the Atlantic Ocean. In: D.E. Hayes et al. (Editors), Initial Reports of the Deep Sea Drilling Project, Val. 14. U.S. Government Printing Office, Washington, D.C., pp. 787-954. Bramlette, M.N., 1946. The Monterey Formation of California and the origin of its siliceous rocks. U.S. Geol. Surv. Prof. Pap. 212, 55 pp. Campbell, A.S. and Fyfe, W.S., 1960. Hydroxyl ion catalysis of the hydrothermal crystallization of amorphous silica: a possible high temperature pH indicator. Amer. Mineral. , 45: 464-468. Carr, R.M. and Fyfe, W.S., 1958. Some observations on the crystallization of amorphous silica. Amer. Mineral., 43: 908-916. Couture, R.A., 1977. Synthesis of some clay minerals at 25°C; palygorskite and sepiolite in the ocean. Ph.D. Thesis, Scripps Institution of Oceanography, 237 pp. Donnelly, T.W. and Merrill> L., 1977. The scavenging of magnesium and other chemical species by biogenic opal in deep-sea sediments. Chem. Geol., 19: 167-186. Fournier. R.O.. 1973. Silica in thermal waters: laboratorv and field investigation. Sympos. Hydrogeochem. Biogeochem., Tokyo, Japan, 1970 Proc. , pp. 122-139.

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.,

221

Wirth, G.S. and Gieskes, J.M., 1979. The i n i t i a l k i n e t i c s o f t h e d i s s o l u t i o n o f v i t r e o u s s i l i c a i n aqueous media. J. C o l l . I n t e r f . Sci., 68: 492-500. Wise, S.W., J r . and K e l t s , K.R., 1972. I n f e r r e d d i a g e n e t i c h i s t o r y o f a weakly s i l i c i f i e d deep-sea chalk. Trans. G u l f Coast Ass. Geol. SOC., 22: 177-203. Wise, S.W., Jr. and Weaver, F.M., 1974. C h e r t i f i c a t i o n o f oceanic sediments. I n t e r n t l . Assoc. Sedimentol. Spec. Publ. 1, pp. 301-326. Wollast, R., Mackenzie, F.T., and B r i c k e r , O.P., 1968. Experimental p r e c i p i t a t i o n and genesis o f s e p i o l i t e a t e a r t h - s u r f a c e c o n d i t i o n s . h e r . Mineral., 53: 1645-1662.

229

CHAPTER 14 IDENTIFICATION OF MIXTURES OF OPALINE SILICA PHASES AND ITS IMPLICATION FOR SILICA OIAGENESIS

R. TADA and A. IIJIMA Geological Institute, the University of Tokyo, 7-3-1 Hongo, Tokyo 113 (Japan)

ABSTRACT Three -4 A silica phases - tridymite, opal-CT and cristobalite - are identified in Neogene siliceous sediments in northern Japan, in which two or three of them may coexist. X-ray diffraction analyses of artificial mixtures of tridymite - opalCT, opal-CT - cristobalite and tridymite - cristobalite were performed to observe the effect of mixing on the strongest -4 A peak of each phase. Though it never splits, the peak of the tridymite - opal-CT mixtures may be shouldered, acute or rounded. The peaks of tridymite - cristobalite and opal-CT - cristobalite mixtures are usually tailed toward the low angle side and even split in one case. In addition to these visual distinctions, peak shape analyses o f artificial mixtures were performed and peak width, skewness and acuteness were determined. We can distinguish mixtures of two opaline silica phases with specific ratios by these visual distinctions and peak shape characteristics. 1.

INTRODUCTION It is generally considered that natural opaline silica is composed of two or more phases. Jones and Segnit (1971) classified them into three structural groups, opal-A ,opal-CT and opal-C. Opal-A and opal-CT are widely known from biogenic siliceous sediments (Ernst and Calvert, 1969; Rex, 1969, 1970; Calvert, 1971a, b; Wise et al., 1972; among others), well-ordered low cristobalite (probably similar to opal-C) has been found in altered silicic vitric tuff. In addition, a fourth silica phase, disordered tridymite, occurs as pore cement and veins in opaline chert, porcelanite and silicic tuff (Iijima and Tada, 1981). In a thick column of marine siliceous sediments, biogenic opal (opalA) converts eventually to the stable phase, quartz, through an intermediate phase of opal-CT with an increase of burial depth (Bramlette, 1946; Ernst and Calvert, 1969; Murata and Nakata, 1974; Murata and Larson, 1975; von Rad et al., 1978; Hein et al., 1978; Kano, 1979; Iijima and Tada, 1981; Tada and Ii jima, 1982). Hydrothermal experiments also confirm this transformation sequence (Mizutani, 1966; Ernst and Calvert, 1969; Oehler, 1973; Florke et al., 1975; Kastner et al., 1977; Kano, 1979; Kano and Taguchi, 1980). Opal -CT is interpreted as disordered low cri stobal i te with varying degrees

230 of

tridymite

peak

stackings

increases

in

o r d e r i n g (Florke,

(Florke,

intensity 1955;

s i l i c e o u s sediments, 1977;

I i j i m a and Tada,

and

Jones and Segnit,

decreases

Jones and Segnit,

the

(101)

depth (Murata and Nakata, and Taguchi,

1955;

process

and Segnit,

d-spacing

I t s (101)

with

progressive

1977).

I n marine

spacing decreases w i t h an i n c r e a s e o f b u r i a l

1974; Murata and Larson, 1975; M i t s u i , 1975; M i t s u i

Hein e t

al.,

1978;

von Rad e t a l . ,

1978;

is

strongly

affected

1971; Mizutani,

1977).

, 1978;

In

1977; Kano,

siliceous

sediments,

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

cases,

(Jones

1979; Kano and Taguchi, 1980), which 1977; Hein

Iij i m a and Tada, 1981).

natural

such

1979;

I n hydrothermal experiments,

by temperature and elapsed t i m e

i s a l s o confirmed i n n a t u r a l s i l i c e o u s sediments (Murata e t al., e t al.

Kano,

1981); t h i s i s i n t e r p r e t e d as a s o l i d s t a t e r e a c t i o n based

on an oxygen i s o t o p e study (Murata e t a l . , the

in

1971; Mizutani,

1971).

however,

other

opaline

silica

o f t e n c o e x i s t w i t h opal-CT.

phases I n such

t h e (101) peak o f opal-CT seems t o be a f f e c t e d by t h e m i x i n g o f phases

( I i j i m a e t al.,

1980; I i j i m a and Tada, 1981).

I n t h i s a r t i c l e , t h e experimental r e s u l t s o f t h e X-ray d i f f r a c t i o n a n a l y s i s of

a r t i f i c i a l m i x t u r e s o f two o p a l i n e s i l i c a phases are described and t h e the - 4

modification o f

A peak o f each o p a l i n e s i l i c a phase by m i x i n g o f

o t h e r s i l i c a phases i s discussed.

Then t h e r e s u l t s a r e a p p l i e d t o t h e a n a l y s i s

o f t h e d i f f r a c t i o n p a t t e r n s o f n a t u r a l s i l i c e o u s sediments and t h e coexistence o f two o p a l i n e s i l i c a phases i n them i s confirmed. Finally,

2.

f o u r t r a n s f o r m a t i o n sequences o f s i 1i c a phases are established.

IDENTIFICATION OF MIXTURES OF TWO OPALINE PHASES I t i s very important t o i d e n t i f y m i x t u r e s o f two o p a l i n e phases,

because

such m i x t u r e s are considered t o occur commonly i n Neogene s i l i c e o u s sediments (Iijima

et

al.,

1980;

I i j i m a and Tada,

seems t o be most p r a c t i c a l Identification of von Rad e t a l .

opal-A

(1978).

as

a quick

1981).

X-ray

powder

diffraction

method t o i d e n t i f y such mixtures.

i n t h e presence o f opal-CT

has been described by

Opal-A i n t h e presence o f t r i d y m i t e o r c r i s t o b a l i t e

can be i d e n t i f i e d by t h e same method. We have examined a r t i f i c i a l m i x t u r e s o f opal-CT - t r i d y m i t e , opal-CT c r i s t o b a l i t e , and t r i d y m i t e c r i s t o b a l l t e i n t h i s study. As a r e s u l t of

-

c a r e f u l analysis,

-

i t i s p o s s i b l e t o d i s t i n g u i s h t h e m i x t u r e s and determine

s p e c i f i c r a t i o s i n some cases. 2.1.

Experimental

(i) Samples. Opaline s i l i c a samples used f o r t h e m i x i n g experiments were c o l l e c t e d from s i l i c e o u s sediments, a l t e r e d v i t r i c t u f f s and veins i n s i l i c e o u s sediments i n n o r t h e r n Japan,

as l i s t e d i n Table 1 and described

231 below: Tridymite

: The t r i d y m i t e sample was obtained from a v e i n

(disordered)

i n o p a l i n e c h e r t i n a surface s e c t i o n o f t h e Miocene Chokubetsu Formation, Kamiatsunai, of

southeast Hokkaido (Table 1A).

the vein

is

in

I i j i m a and Tada (1981).

The p e t r o g r a p h i c a l d e s c r i p t i o n The X-ray

d i f f r a c t i o n powder

p a t t e r n i s c h a r a c t e r i z e d by a p a i r o f s t r o n g peaks a t 4.11 w i t h a moderate peak a t 2.51

A and a t 4.32 A

A and a broad hump a t about 3.9 A ( F i g . 1F).

It c l o s e l y resembles an opal-CT p a t t e r n w i t h a l a r g e r d(101) spacing, though t h e peak a t 4.32 However,

t h e thermal p r o p e r t i e s o f t r i d y m i t e and opal-CT are d i f f e r e n t from

each other; "C

A

for

A i s a l i t t l e more i n t e n s e i n t r i d y m i t e than i n opal-CT.

that

i s t r i d y m i t e does

eleven days,

tridymite-like

wood opal,

w h i l e opal-CT

mineral

was

n o t change even a f t e r heated a t 1000 crystallizes t o cristobalite

reported

by M i t c h e l l

and T u f t s

(Fig.

1A).

(1973)

from

and i t s thermal p r o p e r t i e s are t h e same as t h a t o f t h e t r i d y m i t e

used i n t h i s study.

Recently,

some opals from bentonites,

deep-sea c h e r t s

and gem opals from v o l c a n i c f i e l d s were i d e n t i f i e d as disordered low t r i d y m i t e based on e l e c t r o n d i f f r a c t i o n a n a l y s i s

(Wilson e t a l . ,

Akizuki

also

and Shimada,

19791,

and t h e y

seem t o

1974; Sanders,

1975;

be i d e n t i c a l w i t h t h e

t r i d y m i t e used i n t h i s study. Opal-CT : Three opal-CT samples w i t h d i f f e r e n t d(101) spacings were obtained from o p a l i n e p o r c e l a n i t e s o f t h e Miocene Wakkanai Formation i n deep d r i l l h o l e s i n Tenpoku, as l i s t e d i n Table l B , C and D. and almost f r e e o f v o l c a n i c m a t e r i a l s ;

The p o r c e l a n i t e s are homogeneous

no a d d i t i o n a l s i l i c a cement i s recog-

TABLE 1. Opal-CT,

tridymite

and

cristobalite

d-spacing o f

No. Samples -4 A peak ( A )

samples

Occurrence

used

for

mixing

experiments.

Local it y

, southeast

A

tridymite

4.11

vein i n opaline chert

Kamiatsunai ( F i g . 1F).

B

opal-CT

4.09

muddy p o r c e l a n i t e

C

opal-CT

4.08

muddy p o r c e l a n i t e

Core from a depth o f 905 m o f MITI-Hamayuchi Hole i n Tenpoku, n o r t h Hokkaido ( F i g . 1E). C u t t i n g from a depth o f 1110 m o f t h e same h o l e ( F i g . 1D).

D

opal-CT

4.06

porcel a n i t e

E

c r i s t o b a l i t e 4.04

vitric fine t u f f

Hokkaido

C u t t i n g from a depth o f 1650 m o f t h e Wakkanai-oki #1, Tenpoku (Fig. 1C). C u t t i n g from a depth o f 910 m o f Futada Ak-1 Hole i n Futada, Akita (Fig. l a ) .

232

20

25

30

35

O20 CU KK

F i g . 1. X-ray d i f f r a c t i o n powder p a t t e r n s o f s i l i c a phases which a r e d i f f e r e n t i a t e d by b o t h spacing and shape. A ) Low c r i s t o b a l i t e c r y s t a l l i z e d from opal-CT by h e a t i n g a t 1000°C f o r several days. B ) Low c r i s t o b a l i t e i n v i t r i c t u f f . C) 4.06 A opal-CT i n p o r c e l a n i t e . D) 4.08 A opal-CT i n muddy p o r c e l a n i t e . E) 4.09 A opal-CT i n muddy p o r c e l a n i t e . F ) T r i d y m i t e i n a seggregation vein i n opaline chert. G) Opal-A i n t h e Shinzan Oiatomite o f t h e Onnagawa Formation, Oga, Akita. L o c a l i t i e s o f B F r e f e r t o Table 1.

-

233

nizable in them and the opal-CT contained in them was transformed from biogenic opal-A during burial diagenesis within a narrow stratigraphic interval of as much as 150 m (Iijima and Tada, 1981; Tada and Iijima, 1982). Thus the opal-CT in them seems to be nearly a single phase. The X-ray diffraction powder pattern of the three opal-CT samples are characterized by two peaks: (1) a strong peak at 4.09, 4.08 and 4.06 A, respectively, attributed to the (101) reflection of cristobalite and (2) a weak peak at about 4.3 A attributed to the (200) reflection of tridymite, the latter often being masked by the (100) reflection of quartz associated. A moderate peak also appears at 2.51 - 2.50 A (Fig. lC, D and E). The (101) peak is rather broad, with a width (at 60 % intensity of the peak) of 0.10 - 0.15 A even after treatment with H2SiF6. Opal-CT crystallizes to low cristobalite with evidence of minor tridymite (referable to opal-C) at 1000 OC for several days; that is, the (101) reflection becomes sharper, its d-spacing shifting to 4.08 - 4.06 A, with a moderate peak at 2.49 A and weak peaks at 4.30 A, 3.15 A and 2.86 A (Fig. 1A). Cristobalite (well-ordered) : The cristobalite sample was obtained from a silicic vitric tuff of the Miocene Manaitayama Volcanics in a drillhole in Akita (Table 1E). The X-ray diffraction powder pattern of the cristobalite i s characterized by a sharp and intense peak at 4.04 A and a moderate peak at 2.494 A. Weak peaks are also present at 2.851 A and 3.145 A (Fig. 1B). The (101) reflection is sharp with a width of 0.03 - 0.05 A. The pattern does not change by heating at 1000°C for several days. Opaline silica phases contained in pulverized porcelanite and tuff samples were concentrated by immersion in 20 % H2SiF6 solution for two days at room temperature; insoluble residues were centrifuged in heavy liquid (bromoform + acetone, S.G.= 2.33). Pulverized vein opal was centrifuged in the heavy liquid. Even after the separation, as much as 25 % of quartz remained as determined by ordinary X-ray methods (Fig. 1). Three pairs of tridymite - opal-CT and one pair each of tridymite - cristobalite and of opal-CT cristobalite samples under 300 mesh were separately mixed at given ratios in an agate motor. with acetone for 10 to 20 minutes. The mixing ratios were 4/1, 2/1, 1/1, 1/2 and 1/4 for tridymite - opal-CT mixtures. In addition, a ratio of 9/1 was added for the tridymite - cristobalite and opal-CT cristobalite mixtures. (ii) Determination of d-spacing of the -4 A peak. About 0.1 g of mixtures of the concentrated samples was placed in a non-reflecting quartz holder and analysed with a Rigaku-denki X-ray diffractometer at a scanning speed of 0.25'28/min. and fixed time of 10 sec. The d-spacing read at the top position of the peak was obtained by averaging three measurements whose precision is within f 0.005 A.

234 8

8

I

A

I

I

I

21

22

23

"28

I

I

I

21

22

23

"28 Cu Ka

F i g . 2. Some t y p i c a l X-ray d i f f r a c t i o n powder p a t t e r n s o f - 4 A peak o f m i x t u r e s o f two o p a l i n e s i l i c a phases. A) Shouldered ,peak o f 1/2 m i x t u r e o f t r i d y m i t e - 4.08 A opal-CT. B ) Rounded peak o f 1/1 m i x t u r e o f t r i d y m i t e - 4.06 A opal-CT. C) S p l i t t e d peak o f 4/1 m i x t u r e o f t r i d y m i t e - c r i s t o b a l i t e . D) T a i l e d peak o f 1/2 m i x t u r e o f 4.06 A opal-CT- c r i s t o b a l i t e .

TABLE 2. The v i s u a l opal-CT

distinctions

- cristobalite

T

- 4.09

;; CT

i n the

-4

and t r i d y m i t e

2 / 1

shouldered (acute)

1 1 1 1 / 2 1 / 4

acute (shouldered)

opal-CT,

mixtures.

0

T

- 4.08

CT

T

- 4.06

CT

9 1 1

4 / 1

-

A peak shapes o f t r i d y m i t e

- cristobalite

4.06 A CT - C r

T

-Cr

(tailed)

shouldered

. (shouldered)

shouldered

(tailed)

split

(shouldered)

shouldered

(tailed)

t a i 1ed

shouldered

(shouldered) rounded s houl dered

t a i 1ed

t a i 1ed

(tailed)

(tailed)

( t a i 1ed)

( t a i led)

shouldered (shouldered)

( ) means n o t s i g n i f i c a n t

235 Shape a n a l y s i s o f t h e - 4 A peak.

(iii) of

the - 4

mixtures,

t h e peak was obtained by step-scanning a t every 0.05'28

time of 40 sec. because

and f i x e d

The peak shape was expressed as numerals r e p r e s e n t i n g width,

skewness and acuteness. peak,

I n order t o compare t h e shape

A peak o f t h e o r i g i n a l o p a l i n e s i l i c a phases w i t h t h a t o f t h e i r

it

The w i d t h was determined a t 60 % i n t e n s i t y o f t h e

i s a f f e c t e d by t h e 4.255

A peak o f t r i d y m i t e below 60 % i n t e n s i t y .

A peak o f q u a r t z and t h e 4.32 The skewness was d e f i n e d as t h e

d i f f e r e n c e between t h e mean and median values o f t h e d-spacing o f t h e peak above 60 % i n t e n s i t y .

The acuteness was d e f i n e d by t h e equation;

(dZ5 - d75 / peak width, where d25 and d75 are spacings a t 25 and 75 % r e s p e c t i v e l y o f t h e cumulative i n t e n s i t y o f t h e peak above 60 % i n t e n s i t y . The width,

skewness and acuteness were c a l c u l a t e d as t h e mean o f measurements

w i t h a p r e c i s i o n o f 2 0.006 A, 2 0.002 A and L 0.01, 2.2.

respectively.

Results (i)

Tridymite

-

opal-CT

A A A A

TRI DY M I 1'E

4 1 1

1 / 4 4.09 OPAL-CT

fin

I

21

22

mixtures.

The - 4 A peaks o f t r i d y m i t e ,

TRIDYMITE

TRIDYMITE

4 / 1

1 / 4 0

4.08 OPAL-CT

4.06 A

opal-

AA

fln

- OPAL-CT

21

22

21

2220

cu

Kd

F i g . 3. Step scanning X-ray d i f f r a c t o g r a m s o f - 4 A peak above 60 % i n t e n s i t y o f t r i d y m i t e , opal-CT and t h e i r mixtures.

236 CT and t h e i r m i x t u r e s are shown as drawn from step-scanning

(Fig.

3).

diffractograms

The peaks o f t h e m i x t u r e s d i d n o t s p l i t a t any m i x i n g r a t i o s ,

b u t t h e d-spacings s h i f t g r a d u a l l y between two values o f t r i d y m i t e and opal-CT corresponding t o t h e m i x i n g r a t i o shown i n F i g . 4A. peak shape, all

such as

'shouldered',

'acute'

and

p a i r s o f m i x t u r e s a t c e r t a i n mixing r a t i o s

i n Table 2.

Visual d i s t i n c t i o n s i n t h e

'rounded'

are recognized f o r

( F i g s . 2,

3) and summerized

The peak w i d t h tends t o a maximum value a t a m i x i n g r a t i o o f 4/1;

a t t h a t r a t i o t h e w i d t h i s s i g n i f i c a n t l y l a r g e r than t h a t o f e i t h e r t r i d y m i t e o r opal-CT

(Fig.

48).

The peak skewness and acuteness change w i t h m i x i n g

r a t i o s and sometimes d e v i a t e from those o f both t r i d y m i t e and opal-CT

(Fig.

4C, D ) . The most d i s t i n c t c h a r a c t e r i s t i c s o f t h e - 4 A peak o f t r i d y m i t e

-

opal-

CT mixtures are a shoulder on t h e h i g h angle s i d e o f t h e peak and an acute o r rounded o f t h e maximum o f t h e peak ( F i g . 2 ) .

They appear commonly when

t h e d-spacings o f t h e t r i d y m i t e and opal-CT are s u f f i c i e n t l y d i s t a n t (~0.003A). TRIDYMITE

B

4 11

0,

211

"

111

El

iz

112

11

114

--

OPAL-CT

0.10

D-SPACING

(A)

WIDTH

TRIDYMITE

TRIDYMITE

4 11 211

4 11 211

=z " z

z

111

L

Y

zI:

112

0.14

z

-

0.18

(i)

111 112 114

114

OPAL-CT

OPAL-CT

SKEWNESS

(A)

ACUTENESS

F i g . 4. Diagrams showing t h e r e l a t i o n between m i x i n g r a t i o and d-spacing (A), peak w i d t h (B), peak skewness (C) and peak acuteness ( 0 ) o f - 4 A peak o f t r i d y m i t e - opal-CT mixtures. Tridymite, opal-CT, o tridymite 4.09 A opal-CT mixture, A t r i d y m i t e - 4.08 A opal-CT m i x t u r e , tridymite4.06 A opal-CT m i x t u r e . The bars i n d i c a t e p r e c i s i o n .

+

-

237 I

I

I

1

I

1

0.34

v)

0.32

v)

w

z

W

c,

:: 0.30

4.06

4.08 4.10 D-SPACING

4.12

(i)

F i g . 5. Diagram showing the relation between d-spacing and acuteness of - 4 A peak of tridymite - opal-CT mixtures. The hatched area represents tridymite and opal-CT. Symbols are same as in Fig. 4.

When the d-spacing are near each other ( ~ 0 . 0 0 2A ) , these characterisitcs are rarely observed. However, the peak shape analysis provides further information. The peak width of opal-CT decreases with decreasing d-spacing while that of tridymite i s consistent. I t i s obvious t h a t the 4/1, 2/1 and 1/1 tridymite 4.09 A opal-CT mixture and the 4/1 tridymite - 4.08 A opal-CT mixture show larger peak widths than those of opal-CT and tridymite. The peak acuteness o f tridymite and opal-CT gradually increases w i t h decreasing d-spacing. The 4/1 and 2/1 tridymite - 4.09 A opal-CT mixtures and the 1 / 2 and 1/4 tridymite - 4.08 A opal-CT mixtures have smaller values of acuteness, while the 2/1 tridymite - 4.06 A opal-CT mixture has larger value of acuteness than that o f opal-CT ( F i g . 5 ) . There i s no obvious relation between the peak skewness, and d-spacing of the - 4 A peaks of tridymite, opal-CT and t h e i r mixtures except f o r a larger skewness of 4/1 tridymite - 4.09 A opal-CT and 1/4 tridymite - 4.06 A opal-CT mixtures. Together w i t h the visual charact e r i s t i c s , the shape analysis enables us t o distinguish the opal-CT - tridymite mixtures from both tridymite and opal-CT i n some cases as i s sumerized i n Table 3. The (101) reflection o f (ii) 4.06 A opal-CT - c r i s t o b a l i t e mixture. 4.06 A opal-CT, c r i s t o b a l i t e and t h e i r mixtures are shown in Fig. 6, and t h e i r visual distinctions are summerized i n Table 2. The peak does not

238 s p l i t i n any m i x t u r e s b u t t a i l e d towards i t s low angle side.

A i n t h e 9/1 mixture,

o f t h e - 4 A peak i s 4.05

i s t h e same as c r i s t o b a l i t e , width

tends

to

The d-spacing

decreasing t o 4.04 A, which

i n 4/1 and o t h e r m i x t u r e s ( F i g .

7A).

The peak

decrease w i t h decreasing m i x i n g r a t i o and t h e values stay

w i t h i n t h e range o f opal-CT

and c r i s t o b a l i t e ( F i g .

78).

t h e range o f t r i d y m i t e ,

The peak skewness

b u t i t i s almost w i t h i n

shows a maximum value a t 4/1 and 2/1 m i x i n g r a t i o ,

opal-CT and c r i s t o b a l i t e ( F i g . 7C).

The peak acuteness

i s a maximum a t 4/1 and minimum a t 1/1 m i x i n g r a t i o s ( F i g . 7D), which being considerably l a r g e r and smaller,

r e s p e c t i v e l y than t h a t o f t r i d y m i t e , opal-CT

and c r i s t o b a l i t e . The

4.06

A

from t r i d y m i t e ,

opal-CT opal-CT

-

cristobalite

m i x t u r e s are e a s i l y d i s t i n g u i s h a b l e

and c r i s t o b a l i t e ,

except f o r

mixtures w i t h l i t t l e

4.06 OPAL-CT

CRISTOBALITE

-

21

22

-

21

22 028 CU Ka

F i g . 6. Step scanning X-ray d i f f r a c t o g r a m s o f - 4 A peak above 60 % i n t e n s i t y o f 4.06 A opal-CT, c r i s t o b a l i t e and t h e i r mixtures; and o f t r i d y m i t e , c r i s t o b a l it e and t h e i r mixtures.

239 opal-CT, Moreover,

by t h e s t r o n g t a i l

o f the - 4

A peak toward i t s low angle side.

t h e r e s u l t o f shape a n a l y s i s provides f u r t h e r i n f o r m a t i o n as suml?a-

r i z e d i n Table 3.

F i g . 8 shows t h a t t h e peak w i d t h o f t h e mixtures, e s p e c i a l l y

w i t h abundant opal-CT,

tends t o be l a r g e r than t h a t o f opal-CT and c r i s t o b a l i t e

w i t h t h e same d-spacing. cristobalite and 1/4.

The acuteness i s n o t i n t h e range o f opal-CT and

w i t h t h e same d-spacing

a t most m i x i n g r a t i o s except f o r 9/1

The peak skewness o f t h e 4/1 and 2/1 m i x t u r e s i s l a r g e r than t h a t

o f opal-CT and c r i s t o b a l i t e w i t h t h e same d-spacing. ( i i i ) T r i d y m i t e - c r i s t o b a l i t e mixture. The -4 A r e f l e c t i o n s o f t r i d y m i t e , c r i s t o b a l i t e and t h e i r m i x t u r e s are shown i n Fig.6 and t h e i r v i s u a l d i s t i n c t i o n s are

sunarized

of

-4

of

4.06

in

A peak of

Table

2.

Visual

-

tridymite

A opal-CT

-

and shape

cristobalite

cristobalite

are more obvious i n some cases.

mixture,

analytical

characteristics

m i x t u r e c l o s e l y resemble those though t h e v i s u a l

For example,

distinctions

t h e - 4 A peak o f 9/1 m i x t u r e

OPAL-CT o r

OPAL-CT or

) .

BALITE \

4.04

OPAL-CT or

4.08 D-SPACING

SKEWNESS

4.12-

(i)

(I)

ACUTENESS

F i g . 7. Diagrams showing t h e r e l a t i o n between m i x i n g r a t i o and d-spacing (A), peak w i d t h ( B ) , peak skewness (C) and peak acuteness (D) o f - 4 A peak o f tridymite c r i s t o b a l i t e and 4.06 A opal-CT c r i s t o b a l i t e mixtures. Tridymite, 0 opal-CT, A c r i s t o b a l i t e , o t r i d y m i t e c r i s t o b a l i t e , + 4.06 A opal-CT - c r i s t o b a l i t e . The bars i n d i c a t e p r e c i s i o n .

-

-

240

has a shoulder on t h e h i g h angle s i d e and t h e peak o f t h e 411 m i x t u r e i s s p l i t . 3.

IMPLICATION FOR SILICA DIAGENESIS

3.1.

Two o p a l i n e s i l i c a phases i n n a t u r a l s i l i c e o u s sediments Opal ine c h e r t and dense p o r c e l a n i t e f r e q u e n t l y occur i n t h e opal -CT zone

i n t h e surface sections o f t h e Neogene s i l i c e o u s sediments o f n o r t h e r n Japan. They are composed o f an opal-CT framework, which i s transformed from biogenic opal-A

during

groundwater

burial,

during

t r i d y m i t e t o opal-CT content

of

and

uplift

tridymite (Iijima

cement

precipitated

and Tada,

1981).

from

percolating

The m i x i n g r a t i o o f

i n them i s estimated t o be l e s s than 111, because t h e

t r i d y m i t e cement i s l e s s than 35 % and t h a t o f t r i d y m i t e p l u s

opal-CT i s more than 70 % (Tada and I i j i m a ,

1982).

I n t h e c h e r t and dense

p o r c e l a n i t e which appear i n t h e upper p a r t o f opal-CT zone, i n d-spacing

of

-4

A peaks between opal-CT

be l e s s than 0.002

A.

Consequently,

be t h e o n l y v i s u a l

characteristics

o f m i x i n g experiments.

an acute peak shape i s expected t o

of

c h e r t and dense p o r c e l a n i t e ( F i g . 9A). and lower p a r t o f t h e opal-CT zone, For example,

opaline

chert

this

mixture,

j u d g i n g from r e s u l t s

such a - 4 A peak i s recognized i n o p a l i n e

I n fact,

be d i s t i n g u i s h e d from opal-CT

the difference

and t r i d y m i t e i s considered t o

with

about

I n some o p a l i n e c h e r t s i n t h e middle a broad and shouldered - 4 A peak can

a small

d(101)

spacing

and t r i d y m i t e .

500 m below t h e opal-A/opal-CT

boundary

TABLE 3. Shape a n a l y t i c a l c h a r a c t e r i s t i c s o f - 4 A peak o f t r i d y m i t e

- cristobalite

opal-CT

T - 4.09 A CT

W

A

S

T - 4.08 A CT W

no d a t a t

-

+

(-)

(+I

0

0

0

M i Xing ratio

- opal-CT,

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

A

S

no data t

+

T - 4.06 A CT 6 ; 4

W

A

iRCT

T

Cr

S

no d a t a

t

(+I

+

+

+

+

+

+

+

+

+

-

+

t

-

+

(+I +

-

-

(+ 1

t

(+I

-

W : peak width, A : peak acuteness, S : peak skewness. + and mean l a r g e r and s m a l l e r values o f W , A o r S o f t h e m i x t u r e s than t r i d y m i t e , opal-CT o r c r i s t o balite, respectively.

241 0.20

0.15 h

O

S

=

I

I-

0.10

3;

0.05 precision

0.00 4.04

4.06

4.08

D-SPACING

4.10

(i)

Fig. 8. Diagram showing t h e r e l a t i o n between d-spacing and peak w i d t h o f -4 A peak o f t r i d y m i t e c r i s t o b a l i t e and 4.06 A opal-CT - c r i s t o b a l i t e mixtures. The hatched area represents t r i d y m i t e , opal-CT and c r i s t o b a l i t e . Symbols are same as i n F i g . 7.

-

I

1

I

I

0

= 4.098 A 0 skewness = -0.002 A acuteness = 0.299

d

/ Acute

\

21

22

21

22

21

22 "28 Cu Koc

Fig. 9. X-ray d i f f r a c t i o n powder p a t t e r n s o f - 4 A peaks o f n a t u r a l s i l i c e o u s sediments i n which two o p a l i n e s i l i c a phases c o e x i s t . A ) Opaline c h e r t o f t h e Miocene Chokubetsu Formation i n Shakubetsu, southeast Hokkaido, comprisi n g opal-CT and t r i d y m i t e ( A t 76-127). B) Opaline c h e r t o f t h e Miocene Okoppezawa Formation i n Atsunai , southeast Hokkaido, comprising opal -CT and t r i d y m i t e ( A t 76-23). C) V i t r i c p o r c e l a n i t e from t h e sub-bottom depth o f 935 m, S i t e 439, DSDP Leg 57, o f f Sanriku, comprising opal-CT and c r i s t o b a l i t e ( F i g . 7D, I i j i m a e t al., 1980).

242

collected from the surface section of the Miocene Okoppezawa Formation in Atsunai, southeast Hokkaido, shows this (Fig. 9B). The (101) spacing of opal-CT in the host porous porcelanite is 4.09 A. The mixing ratio of tridymite to opal-CT in this chert is estimated at about 1/2.5 from the 6 % and 26 % porosity of the opaline chert and host porcelanite, respectively, and from the opaline silica content of 70 ?: 10 % in the chert, assuming that the reduction in porosity in the chert is caused by the additional cementation of tridymite. The values of the d-spacing, width, skewness and acuteness, and a slightly shouldered peak shape are consistent with those of the 4.09 A opal-CT - tridymite mixtures with mixing ratios of 1/1 and 1/2. Opal-CT and cristobalite often coexist in vitric siliceous sediments. For example, in the vitric porcelanite of Miocene age from a subbottom depth of 935 m, Site 439, off Sanriku, DSDP Leg 57, opal-CT is transformed from radiolarian and diatom skeletons, while cristobalite occurs as linings of of voids of dissolved glass shards (Iijima et al., 1980). The -4 A peak of the mixture splits, as shown in Fig. 9C. Shouldered, split, or tailed -4 A peaks are recognized in many other vitric siliceous sediments (Iijima and Tada, 1981 1. Silica diaaenesis The close relationship between opaline silica phases and their occurrence has been pointed out by Iijima and Tada (1981) based on their study of silica di agenesi s in the Neogene diatomaceous sediments in northern Japan. Opal-A, which comprises siliceous skeletons in diatomite and diatomaceous mudstone, is transformed into opal -CT during buri a1 di agenesis , as recognized by many workers (e.g. Murata and Larson, 1975; Hein et al., 1978; Iijima and Tada, 1981). Opal-CT is the main constituent of opaline porcelanite and opaline chert in the opal-CT zone. Iijima and Tada (1981) distinguished two kinds of "opal-CT" in the opal-CT zone of surface sections; that is, one transformed from siliceous skeletons during progressive burial and the other precipitated from percolating groundwater in cement and veins during uplift. In the postscript, they stated that the latter is not opal-CT but disordered low tridymite. It is shown that opal-CT and tridymite mixtures in siliceous sediments can be identified by means o f the X-ray powder diffraction method, as described in this study. Tridymite also occurs as interstitial cement o f later opaline chert nodules in the Shinzan Diatomite o f the opal-A zone in Onnagawa Formation, Oga, Akita. It is noteworthy that opal-CT and tridymite together exhibit lepispheres growing in open spaces, and that the morphology is indistinguishable. In addition, tridymite makes up the main constituent o f precious opal precipitated from hydrothermal solution in vugs of altered glassy silicic volcanics at Hosaka, Fukushima (Tsutsumi 3.2.

243 and Sakamoto,

1973;

Akizuki

and Shimada,

1979).

The modes o f occurrence

s t r o n g l y suggest t h a t t r i d y m i t e s p r e c i p i t a t e d i r e c t l y from e i t h e r h i g h - s i l i c a groundwater

or

hydrothermal

and i n t e r s t i t i a l

pores.

solutions

The d(101)

i n open

spaces

such as

veins,

vugs

spacing o f opal-CT g r a d u a l l y decreases

w i t h an i n c r e a s e o f b u r i a l depth (Murata and Nakata, 1974; Murata and Larson, 1975;

Mitsui,

and Tada,

1975;

1981,

Mitsui

and Taguchi,

among o t h e r s ) .

causes t h e s h i f t of

d-spacing

1977;

However,

-4

of

von Rad e t al.,

1978;

Iijima

m i x i n g o f two o p a l i n e phases a l s o

A peak as i s r e v e a l e d by t h i s study.

It i s necessary t o s p e c i f y t h a t t h e - 4 A o p a l i n e s i l i c a t o be s t u d i e d should

be a s i n g l e opal-CT phase when u s i n g t h e d(101) spacing as d i a g e n e t i c scale. Silicic burial

volcanic

diagenesis

I n porous

glass

changes

to

in silicic vitric

silicic

vitric

tuffs of

at

early

stages

of

zeolitic

Zone

which

1980).

a l t e r t o aggregates o f

C r i s t o b a l i t e c o e x i s t s w i t h opal-CT i n v i t r i c s i l i c e o u s rocks

opal-CT

originates

i s d e r i v e d from s i l i c i c g l a s s eventually Shale been

-4

orders

to

A

silica

t o fibrous

in

are

chalcedonic

1980).

while

crsitobalite

Furthermore, opal-CT

diagenesis

i n t h e Monterey

Such we1 1-ordered c r i s t o b a l i t e s have n o t

siliceous

eventually

quartz

i s the exclusive

skeletons

( I i j i m a e t al.,

1975).

biogenic

phases

siliceous

c r i s t o b a l i t e during burial

(Murata and Larson, recognized

from

cristobalite

-4 A

mordenite

s i l i c a phase. which

smectite,

11,

clinoptilolite, in

and

opal-A

t u f f s o f Zone I ( I i j i m a e t al.,

with

sediments

transformed the

optical

i n northern

Japan.

All

t o quartz,

commonly f i r s t

length-fast

e l o n g a t i o n and

then t o aggregates o f m i c r o c r y s t a l l i n e q u a r t z . I n conclusion,

at

least four

s i l i c a phase t r a n s f o r m a t i o n

sequences

have

been e s t a b l i s h e d i n t h e Neogene s i l i c e o u s sediments o f n o r t h e r n Japan: ( 1 ) Biogenic s i l i c e o u s t e s t s (opal-A) +opal-CT

+ (crstobalite) +quartz

( 2 ) S i l i c i c v o l c a n i c g l a s s +-opal-A + c r i s t o b a l i t e + q u a r t z ( 3 ) High s i l i c a s o l u t i o n + t r i d y m i t e + q u a r t z ( 4 ) Low s i l i c a s o l u t i o n + q u a r t z ACKNOWLEDGEMENTS This

study i s p a r t o f a d o c t r a l d i s s e r t a t i o n submitted t o t h e Geological

Institute, Special

t h e U n i v e r s i t y o f Tokyo by R. Tada under t h e guidance o f A. I i j i m a .

thanks go t o A s s i s t a n t Professor Minoru Utada and O r .

for

their

discussion

the

University

of

and

suggestions.

California

at

We wish

Berkeley

and

Ryo Matsumoto

t o thank Prof. Prof.

M.

R.L. Hay of

Kastner o f Scripps

I n s t i t u t i o n o f Oceanography f o r t h e i r r e a d i n g o f t h e t y p e s c r i p t .

244

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245

Rex, R.W., 1969. X-ray mineralogy studies - Leg 1. In: J.I. Ewing et al. (Editors), Initial Reports of the Deep Sea Drilling Project, Volume I. U.S. Government Printing Office, Washington, pp. 354-367. Rex, R.W., 1970. X-ray mineralogy studies - Leg 2. In: M.N.A. Peterson et al. (Editors), Initial Reports of the Deep Sea Drilling Project, Volume 11. U.S. Government Printing Office, Washington, pp. 329-346. Sanders, J.V., 1975. Microstructure and crystallinity of gem opals. Amer. Min., 60: 749-757. Tada, R. and Iijima, A., in preparation. Petrology and diagenetic changes o f Neogene siliceous sediments in northern Japan. Tsutsumi, S . and Sakarnoto, T., 1972. Opal from Hosaka, Fukushima Prefecture, Japan. Jour. Japan. Assoc. Min. Petr. Econ. Geol ., 68: 295-302 (in Japanese with English abs.). von Rad, U . , Riech, H. and Rosch, H., 1978. Silica diagenesis in continental margin sediments of northwest Africa. In: J. Gardner, J. Herring (Editors), Initial Reports of the Deep Sea Drilling Project, Volume XLI. U.S. Government Printing Office, Washington, pp. 879-897. Wilson, M.J., Russell, M.D. and Sait, J.M., 1974. A new interpretation of the structure of disordered-cristobalite. Contr. Mineral. Petrol., 47: 1-6. Wise, S.W.Jr, Buie, B.F. and Weaver, F.M., 1972. Chemically precipitated sedimentary cristobalite and the origin of chert. Ecolog. Geol. Helv. 65: 157-163.

247

CHAPTER 15 FACIES AND DIAGENESIS OF THE MIOCENE MONTEREY FORMATION, CALIFORNIA: CAROLINE M.

A SUMMARY

I S A A C S l , KENNETH A. PISCIOTT02, AND ROBERT E. GARRISON3

ABSTRACT The Miocene Monterey Formation records t h e deep marine phase o f a major l a t e T e r t i a r y c y c l e o f basin formation and f i l l i n g associated w i t h wrench-fault t e c tonism along t h e C a l i f o r n i a margin. Monterey sedimentation occurred i n basinf l o o r , slope, s h e l f , and r i d g e - t o p environments which were v a r i o u s l y a f f e c t e d by t h e oxygen-minimum zone. I n many places t h e Monterey Formation c o n s i s t s o f a lower calcareous f a c i e s , a middle t r a n s i t i o n a l phosphatic f a c i e s , and a t h i c k upper s i l i c e o u s f a c i e s composed o f diatomaceous rocks and t h e i r d i a g e n e t i c e q u i v a l e n t s ( c h e r t , p o r c e l a n i t e , etc.). Aquagene t u f f s and v o l c a n i c rocks u n d e r l i e t h e Monterey l o c a l l y . The f o r a m i n i f e r - c o c c o l i t h mudstones o f t h e calcareous f a c i e s represent an e a r l y Miocene phase o f equable c l i m a t e and r e l a t i v e l y l o w - n u t r i e n t water masses. The widespread s i l i c e o u s f a c i e s represents r a p i d l y deposited diatom ooze, r e c o r d i n g h i g h plankton p r o d u c t i v i t y caused by c l i m a t i c c o o l i n g and i n t e n s i f i e d u p w e l l i n g i n t h e l a t e Miocene. The middle phosphatic f a c i e s formed d u r i n g a middle Miocene t r a n s i t i o n from l o w - f e r t i l i t y calcareous d e p o s i t i o n t o h i g h - f e r t i l i t y diatomaceous d e p o s i t i o n . Diagenesis o f t h e Monterey Formation t y p i c a l l y i n v o l v e d t h e w i d e l y recognized sequence o f s i l i c a phases, b i o g e n i c opal-A + d i a g e n e t i c opal-CT + diagene t i c quartz. Accompanying t h i s sequence was a t y p i c a l d i a g e n e t i c succession o f rocks: d i a t o m i t e s and diatomaceous shales + opal-CT c h e r t s , p o r c e l a n i t e s , and mudrocks + q u a r t z cherts, p o r c e l a n i t e s , and mudrocks. The two phase transforma t i o n s occurred by s o l u t i o n and i n - s i t u p r e c i p i t a t i o n o f s i l i c a accompanied by abrupt r e d u c t i o n s o f 15 t o 30 p o r o s i t y percent r e s u l t i n g from compaction. Est i m a t e s i n d i c a t e t h a t opal-CT rocks became abundant a t about 40" t o 50" C o r a t b u r i a l depths o f 750 t o 1100 m y whereas q u a r t z rocks became abundant a t about 80" C o r a t b u r i a l depths o f 1500 t o 2400 m. The temperatures a t which t h e s i l i c a phases transformed v a r i e d somewhat w i t h t h e abundance o f d e t r i t a l c l a y , b u t t h e presence o f c a l c i t e had l i t t l e e f f e c t on t h e k i n e t i c s o f t h e transformations. Past geothermal g r a d i e n t s can be i n f e r r e d from t h e t h i c k n e s s o f t h e opal-CT zone and from t h e r a t e o f change o f opal-CT d-spacings w i t h depth i n porcel aneous rocks. Thermal diagenesis o f t h e abundant organic matter, l a r g e l y kerogen, produced a h i g h l y a s p h a l t i c hydrocarbon f r a c t i o n - - a n d probably generated o i l - - a t tempera t u r e s lower than 100" C. Secondary carbonate minerals, m a i n l y dolomite, exh i b i t complex p a t t e r n s o f carbon- and oxygen-isotope r a t i o s and probably formed a t a l l stages o f diagenesis. INTRODUCTION Miocene diatomaceous sediments occur throughout t h e world, most prominently Among these Pac f i c deposits,

i n a b e l t t h a t r i m s much o f t h e P a c i f i c Basin.

t h e Monterey Formation o f t h e C a l i f o r n i a Coast Ranges (Fig.

1) i s o f special

1 U.S. Geological Survey, Menlo Park, C a l i f o r n i a (USA) Sohio Petroleum Co., San Francisco, C a l i f o r n i a (USA) E a r t h Sciences Board, U n i v e r s i t y o f C a l i f o r n i a , Santa Cruz, Cal f o r n i a (USA)

248

i n t e r e s t because o f i t s tiidespread exposures and i t s r o l e as a major source and r e s e r v o i r rock f o r hydrocarbons. The Monterey Formation, l i k e i t s c o u n t e r p a r t s elsewhere i n t h e P a c i f i c r e gion, shows exceedingly complex p a t t e r n s o f sedimentary f a c i e s and diagenesis. Monterey sedimentary f a c i e s vary i n time and space because o f v a r i a b l e i n t e r a c t i o n s among Neogene t e c t o n i c , c l i m a t i c ,

and oceanographic events.

As d i s -

cussed belov, these events combined t o c r e a t e sediment-starved marginal basins i n California coincident w i t h c l i m a t i c cooling, phytoplankton p r o d u c t i v i t y , hemi p e l a g i c

sediments

i n t e n s i f i e d upwelling,

high

and r e l a t i v e l y r a p i d d e p o s i t i o n o f o r g a n i c - r i c h

beneath coastal

upwell i n g systems.

These sediments

commonly e x h i b i t unusual p a t t e r n s o f diagenesis because they contained l a r g e amounts o f two i n h e r e n t l y unstable components: fine-grained

organic matter.

Authigenic

b i o g e n i c s i l i c a (opal-A)

m i n e r a l s such as p y r i t e ,

and

apatite,

dolomite, and c a l c i t e began t o form very soon a f t e r d e p o s i t i o n , whereas s i l i c a phases were transformed

and

petroleum was generated

during l a t e r

burial.

S l i g h t v a r i a t i o n s i n t h e composition o f t h e o r i g i n a l sediment a p p a r e n t l y regul a t e d t h e t i m i n g and e x t e n t o f d i a g e n e t i c r e a c t i o n s ; determine,

commonly i n s u b t l e ways,

these v a r i a t i o n s also

t h e outcrop appearance o f d i a g e n e t i c a l l y

produced s i l i c e o u s rocks i n t h e Monterey. Our knowledge o f Monterey rocks i s l a r ' g e l y t h e r e s u l t o f f i e l d and microscopic s t u d i e s by M.

N. B r a m l e t t e (1946) and o f geochemical research by K. J.

Murata and h i s co-workers a t t h e U.S.

Geological survey (see References).

SOUTHERN CALIFORNIA BORDERLAND /

More

249

recent i n v e s t i g a t i o n s ,

i n c l u d i n g discussions o f modern environments p o s s i b l y

analogous t o those o f t h e Monterey, are r e p o r t e d i n a symposium volume e d i t e d by Garrison e t a l . (1981) and an accompanying f i e l d guide e d i t e d by Isaacs (see Isaacs , 1981a). I n t h e s e c t i o n s t h a t f o l l o w we o u t l i n e t h e geologic characteristics,

lithologic

and d e p o s i t i o n a l environments o f Monterey deposits, and we de-

t a i l aspects o f t h e i r d i a g e n e t i c m o d i f i c a t i o n . sedimentology i s summarized by R. i s summarized by C. w r i t t e n by

setting,

M.

The m a t e r i a l on l i t h o f a c i e s and

E. Garrison, and t h e m a t e r i a l on diagenesis

Isaacs, except f o r t h e s e c t i o n on secondary carbonates

K. A. P i s c i o t t o .

SEQUENCE OF NEOGENE LITHOFACIES, CALIFORNIA COAST RANGES

I n g l e (1973,

1975, 1980, 1981a, b) has demonstrated t h a t v e r t i c a l succes-

sions o f Cenozoic sedimentary r o c k s i n t h e c i r c u m - P a c i f i c r e g i o n have s t r i k i n g ly similar lithofacies.

T y p i c a l l y , t h i s succession i n c l u d e s : ( 1 ) Oligocene t o

lower Miocene volcanic,

continental

,

and n e r i t i c marine c l a s t i c rocks, which

represent t h e i n i t i a l stages o f b a s i n formation; shales,

mudstones,

f o l l o w e d by (2) deep marine

and superjacent diatomaceous s t r a t a ,

which correspond t o

r a p i d subsidence and t h e development o f marginal and borderland basins; capped by ( 3 ) c l a s t i c d e p o s i t s - - t u r b i d i t e s f i n a l stages o f basin f i l l i n g .

and marine sandstones--which represent t h e

I n t h e southern Coast Ranges o f C a l i f o r n i a t h i s

A t least five lithofacies

succession o f f a c i e s i s p a r t i c u l a r l y w e l l developed. can be d i s t i n g u i s h e d :

(1) a lower non-marine u n i t c o n s i s t i n g o f c o n t i n e n t a l

f l u v i a l and a l l u v i a l c l a s t i c rocks; ( 2 ) a shallow marine f a c i e s , m a i n l y a r k o s i c s t r a n d and nearshore sandstones; tuffs

and v o l c a n i c

rocks,

( 3 ) a deep marine phase t y p i f i e d by aquagene

foraminifer-coccolith

mudstones

and shales,

superjacent phosphatic mudrocks and s i l i c e o u s and diatomaceous s t r a t a ; second shallow marine f a c i e s ,

and

(4) a

dominantly n e r i t i c sandstones, f o l l o w e d by; ( 5 )

an upper non-marine u n i t u s u a l l y markedly d i s c o r d a n t w i t h t h e u n d e r l y i n g sandstones and conglomerates (Figs. 2 and 3). The complete v e r t i c a l sequence i s

not present

everywhere i n C a l i f o r n i a n o r

a r e s p e c i f i c l i t h o f a c i e s always p r e c i s e l y o f t h e same age (Fig. 3), two reasons f o r t h e complex s t r a t i g r a p h i c r e l a t i o n s o f middle and upper T e r t i a r y rocks i n t h e Coast Ranges.

D e p o s i t i o n o f n o n s i l j c e o u s marine muds b e f o r e and a f t e r

d e p o s i t i o n o f t h e Monterey r e s u l t e d i n a d d i t i o n a l f a c i e s i n some areas, such as t h e western Transverse Ranges;

i n other l o c a l i t i e s ,

nondeposition and e r o s i o n

account f o r missing f a c i e s , as do marked l a t e r a l f a c i e s changes (e.g. marine facies--see

discussions

i n Lagoe,

1981,

i n t o non-

and P i s c i o t t o and Garrison,

1981). Local v a r i a t i o n s aside, a general succession o f 1 i t h o f a c i e s , r e c o r d i n g a major Neogene c y c l e o f basin f o r m a t i o n and f i l l i n g , can be recognized through much o f t h e southern Coast Ranges (Fig. 2).

250

APPROXIMATE PALEOBATHYMETRY (METERS)

0

APPROXIMATE RATES OF SEDIMENTATION AND SUBSIDENCE ( M/MY)

GENERALIZED UPPER TERTIARY FACIES,COAST RANGES,CALIFORNI A

1000 2000

F

Ntmmorine Facies

5

Shallow Marine Facies 4

GENERALIZED FACIES OF THE MONTEREY FORMATION

t

Deep Marine Facies 3

Siliceous Facies

Phosphatic Facies

Shallow Marine Facies

Calcareous Facies

2

Nonmor ine Facies I

Fig. 2. Generalized upper T e r t i a r y sedimentary f a c i e s o f t h e Coast Ranges, C a l i f o r n i a , and f a c i e s o f t h e Monterey Formation ( f r o m P i s c i o t t o and Garrison, 1981). Bathymetry and r a t e s o f sedimentation and subsidence general i z e d from Graham (1976a) and I n g l e (1980). The deep marine f a c i e s o f t h i s succession ( f a c i e s 3 i n Fig. 2),

i n places

u n d e r l a i n by v o l c a n i c rocks, commonly c o n t a i n s a d i s t i n c t t h r e e - f o l d v e r t i c a l s u b d i v i s i o n c o n s i s t i n g o f a lower calcareous f a c i e s , a middle t r a n s i t i o n a l u n i t o f phosphatic rocks, and an upper s i l i c e o u s f a c i e s (Fig. 2). I n t h e past, some workers

i n t h e western p a r t o f t h e southern Coast Ranges a p p l i e d t h e term

"Nonterey Formation" o n l y t o t h e upper s i l i c e o u s f a c i e s , because o f t h e l a t t e r ' s d i s t i n c t i v e l i t h o l o g y , and assigned t h e lower f a c i e s t o separate formations.

Most r e c e n t workers have i n c l u d e d a l l t h r e e deep marine f a c i e s as mem-

bers o f t h e Monterey Formation.

A t some l o c a l i t i e s ,

the vertical facies i s

somewhat more complicated and b e t t e r represented by s u b d i v i s i o n s i n t o more than t h r e e members. For example, Isaacs (1980, 1981b) d i v i d e d t h e Monterey i n t h e Santa Barbara-Point Conception area o f t h e Transverse Ranges i n t o f i v e i n f o r m a l members, t h e lower and upper two o f which correspond b r o a d l y t o t h e calcareous and s i l i c e o u s f a c i e s r e s p e c t i v e l y , t h e middle trm t o t h e phosphatic f a c i e s . As w i t h t h e more general Neogene sequence, t h e complete v e r t i c a l succession o f deep marine l i t h o f a c i e s comprising t h e Monterey Formation and r e l a t e d rocks i s n o t present everywhere i n t h e Coast Ranges.

Where t h i s deep marine succes-

s i o n i s n o t developed, t h e r e most commonly i s a t h i c k s e c t i o n o f sandy t u r b i d i t e s o v e r l a i n by s i l i c e o u s s t r a t a , as, f o r example, i n t h e P i r u - F i l l m o r e area and i n t h e Santa Monica Mountains o f southern C a l i f o r n i a (Fig. 1). Below, we n o t e b r i e f l y some o f t h e major c h a r a c t e r i s t i c s o f t h e u n d e r l y i n g v o l c a n i c rocks and t h e t h r e e deep marine f a c i e s o f t h e Monterey.

251 Underlying v o l c a n i c rocks These are lower and middle Miocene, c h i e f l y v o l c a n i c l a s t i c and shallow i n t r u s i v e rocks o f b a s a l t i c , a n d e s i t i c , and r h y o l i t i c composition.

They occur i n

w i d e l y separated p a r t s o f t h e Coast Ranges, e i t h e r s t r a t i g r a p h i c a l l y below t h e Monterey Formation o r interbedded w i t h it. tons,

subaerial

flows,

air fall

tuffs,

They are present as shallow p l u -

and submarine ash flows.

The best

s t u d i e d example i s t h e lower and middle Miocene Obispo Formation o f t h e San L u i s Obispo area (Fig.

1).

Fisher

(1977),

Surdam (1980),

and Surdam and

Stanley (1981a, b ) have shown t h i s u n i t t o be l a r g e l y t h e product o f d e p o s i t i o n

from submarine ash f l o w s and v o l c a n i c l a s t i c t u r b i d i t y c u r r e n t s on t h e f l a n k o f a v o l c a n i c ridge. Some workers, n o t a b l y Tal i a f e r r o (1933) , c o r r e l a t e d h i g h diatom p r o d u c t i v i t y and diatomaceous sedimentation i n t h e Miocene w i t h t h e r e l e a s e o f s i l i c a from t h i s volcanism; phase o f

t h e main pulse o f volcanism, however, occurred b e f o r e t h e main

Miocene

siliceous

d e p o s i t i o n (Fig.

3).

More l i k e l y ,

this

late

Oligocene t o middle Miocene volcanism was r e l a t e d t o t e n s i o n a l f a u l t i n g t h a t accompanied b a s i n development and r a p i d subsidence,

and t h e l a t e r phases o f

s i l i c e o u s sedimentation r e f l e c t i n t e n s i f i e d upwelling,

comparable t o t h a t i n

t h e present G u l f o f C a l i f o r n i a ( C a l v e r t , 1966a). Calcareous f a c i e s The dominant rock types siltstone,

i n t h i s f a c i e s are f o r a m i n i f e r - c o c c o l i t h

shale,

and mudstone, as w e l l as limestone marking t h e beginning o f r a p i d

subsidence and deep-water sedimentation i n many p a r t s o f t h e Coast Ranges.

The

exact ages and formation names vary.

Some calcareous u n i t s o f t h i s k i n d a r e

assigned t o separate l o c a l formations,

whereas i n o t h e r l o c a l i t i e s such u n i t s

are i n c l u d e d as a lower member o f t h e Monterey Formation. from middle L u i s i a n t o l a t e s t Zemorrian (about 14-22 m.y. the timing o f local

subsidence and b a s i n development

T h e i r ages range

ago), depending upon along t h e C a l i f o r n i a

margin. These calcareous rocks vary from massive and apparently burrowed rocks t o unburrowed, laminated, o r g a n i c - r i c h rocks which must have been deposited i n anoxic settings.

Interbedded w i t h t h e calcareous rocks a r e s i l i c i c l a s t i c t u r b i -

d i t e s and f o r a m i n i f e r a l t u r b i d i t e s c o n t a i n i n g displaced faunas.

This f a c i e s

a l s o c o n t a i n s common c o n c r e t i o n s and l e n t i c u l a r beds composed o f a u t h i g e n i c d o l o m i t e or, l e s s commonly, o f c a l c i t e . These calcareous

rocks

are

coccoliths, foraminiferal tests,

i n t e r p r e t e d as

hemi p e l a g i c muds composed o f

and s i l t - s i z e t o fine-sand-size

siliciclastic

grains; t h e presence i n most rocks o f diatom f r u s t u l e s , d i a g e n e t i c opal-CT, d i a g e n e t i c q u a r t z shows t h a t diatoms were a p a r t o f t h e o r i g i n a l b i o t a .

or

252

Phosphatic facies The widely distributed middle Miocene phosphatic shales and mudstones (Oickert, 1966; Pisciotto, 1978) are commonly calcareous, b u t are also siliceous i n places, particularly in the gradational zone between phosphatic and overlying siliceous facies. The phosphate occurs mainly as small (<1-8 cm) discrete nodules and sandsize peloids of cryptocrystalline a p a t i t e (collophane) and also as diffuse masses. I n places, a p a t i t e i n f i l l e d and replaced foraminifera1 t e s t s ; in other places, no c a l c i t e precursor i s obvious. These types of phosphate occur mainly i n laminated organic-rich layers. This occurrence suggests t h a t they formed i n low-oxygen waters, perhaps similar t o the waters of the present day Peruvian continental margin where phosphatization of diatomaceous sediments i s occurring today a t the edges of the oxygenminimum zone, in the region where the zone intercepts the upper-slope/outershelf (Veeh e t a l . , 1973; Burnett, 1977, 1980; Burnett e t a l . , 1980). This modern phosphatization results from dissolution o f organic material i n anoxic pore waters which release organic phosphorus, leading i n turn t o a p a t i t e prec i p i t a t i o n and replacement near the sea floor d u r i n g early diagenesis. An early diagenetic origin for the Monterey phosphate nodules a1 so seems 1 i kely because many are bounded by compactionally bent laminae in the host sediment and a few are reworked into intraformational conglomeratic layers (Pisciotto, 1978). Siliceous facies The siliceous facies of the Monterey Formation i s the most widespread of the three deep marine facies. Though as old as Saucesian in a few places, i n most areas of the Coast Ranges siliceous rocks f i r s t appear i n lower Mohnian s t r a t a (about 11 t o 12 m.y. ago). They persist through the Mohnian stage and locally into the Pliocene; thus, t h i s major pulse of diatomaceous sedimentation lasted about 5 t o 7 my. (Fig. 3). Diatomite and diatomaceous mudrock and their diagenetic equivalents--chert , porcelanite, and siliceous mudrock--are the principal rock types o f the s i l iceous facies.. Intercalated i n the dominantly siliceous rocks are beds of sandstone, mudstone, shale, breccia, pelletal and nodular phosphorite, volcanic a s h , and diagenetic carbonate (both c a l c i t i c and dolomitic). Bramlette (1946) has provided superb descriptions of these rocks, which are also discussed by Pisciotto and Garrison (1981). Rhythmic bedding and millimeter- t o meter-thick cycles are conspicuous chara c t e r i s t i c s of the siliceous facies. Pisciotto (1978) recognized three orders of cycles w i t h time scales varying from years t o thousands of years (see also

253 AGE

1.l.W

FACIESOF MONTEREY :ORMATION AND ADJACENT UNITS

CALIFORNIA BENTHIC JRAMINIFERAL STAGE

EPOCH

-

CLIMATIC EVENTS

TECTONIC EVENTS

t

PLIDCENE

5

1ASIN DEVELOPMENT IN OFFSHORE NORTHERNAND ENTRAL CALIFORNIP

1

ANTARCTIC GLACIATION 10

CHANGEIN PLATE MOTION

A 'OLCANISM SOUTHERN CALIFORNlr

15

-_

__

WIDESPREAD IASlN DEVELOPMENT AND SUBSIDENCE

20 "ON-GLACIAI WORLD

VOLCANlSh ENTRAL Ah SOUTHERN COAST RANGES

25 OLIGOCENE

._

-_ _

v

,RIDGECONTINENT COLLISION

30

-

Fig. 3. Summary o f major Miocene events i n t h e C a l i f o r n i a Coast Ranges and t h e i r c o r r e l a t i o n w i t h Monterey f a c i e s ( f r o m P i s c i o t t o and Garrison, 1981). Time alignment o f Miocene p r o v i n c i a l stages i s a f t e r I n g l e (1980). P i s c i o t t o and Garrison,

1981).

Laminated s i l i c e o u s rocks are common i n t h i s

f a c i e s and appear t o mark e i t h e r basin f l o o r d e p o s i t i o n i n anoxic basins or outer-shelf-slope Soutar e t al., logs).

environments

i n t e r s e c t e d by t h e oxygen-minimum

zone

(cf.

1981, and Donegan and Schrader, 1981, f o r p o s s i b l e modern ana-

One cmmon k i n d of

c y c l e c o n s i s t s o f m e t e r - t h i c k massive,

burroved

l a y e r s interbedded w i t h m e t e r - t h i c k laminated, unburrowed l a y e r s (see s i l i c e o u s f a c i e s i n Fig.

2;

Govean,

1980;

Isaacs,

1980,

1981b; Govean and Garrison,

1981); t h i s c y c l e may record f l u c t u a t i o n s i n t h e i n t e n s i t y and/or p o s i t i o n o f t h e oxygen-minimum zone.

Another very common c y c l e i s rhythmic i n t e r b e d d i n g o f

5- t o 10-cm-thick s i l i c e o u s beds, adjacent beds being separated e i t h e r by bedding-plane p a r t i n g s o r by p a p e r - t h i n shale l a y e r s . On t h e b a s i s o f sedimentary s t r u c t u r e s , p a t t e r n as

the

Berg and Kersey (1981) i n t e r p r e t e d t h i s k i n d o f bedding

product

t u r b i d i t y currents.

of

redeposition

o f diatomaceous muds from d i l u t e

254

TECTONIC AND PALEOCEANOGRAPHIC SETTING OF MONTEREY DEPOSITION I n g l e (1980, 1981a, b ) has suggested t h a t a combination o f t e c t o n i c , c l i m atic,

and oceanographic

events c o i n c i d e d t o c r e a t e t h e c o n d i t i o n s

accumulation o f Monterey b a s i n a l sediments (Fig. Garrison,

1981).

3;

f o r the

see a l s o P i s c i o t t o and

Tectonism, which created a s e r i e s o f extensional basins i n

t h e Coast and Transverse Ranges,

r e s u l t e d from t h e c o l l i s i o n o f an oceanic

spreading r i d g e w i t h t h e western margin o f t h e American p l a t e , beginning i n l a t e Oligocene time, about 29 m.y.

ago (Blake e t al.,

1978).

T h i s was f o l l o w e d

somewhat l a t e r by development o f t h e San Andreas t r a n s f o r m system, along which several small basins developed a t d i f f e r e n t times between about 10 and 20 m.y. ago, e i t h e r (1) i n areas o f t e n s i o n where l a t e r a l f a u l t s curved o r s t r i k e - s l i p f a u l t s i n t e r s e c t e d , o r ( 2 ) where syndepositional f o l d i n g created small en echel o n basins o r l a r g e marginal troughs such as t h e San Joaquin basin. i n p l a t e motion about 10 m.y. basins.

A change

ago r e s u l t e d i n t h e c r e a t i o n o f e n t i r e l y new

Diachronous b a s i n development thus e x p l a i n s many o f t h e Neogene f a c i e s

v a r i a t i o n s p r e v i o u s l y noted. The basins created along t h e western margin o f t h e American p l a t e i n e a r l y Miocene time and i n middle t o l a t e Miocene time underwent r a p i d subsidence ( a t r a t e s o f 200 t o 850 m/my according t o Graham, 1976a, and I n g l e , 1980, 1981a), which was n o t matched by t h e supply o f t e r r i g e n o u s sediment.

They thus sub-

sided as starved basins which c o l l e c t e d m a i n l y p e l a g i c and hemipelagic biogenous sediments.

Coincident w i t h these t e c t o n i c events was a g l o b a l c o o l i n g t h a t accompanied t h e onset o f A n t a r c t i c g l a c i a t i o n s t a r t i n g a t about 15 t o 16 m.y. e t al.,

1981).

The high-standing,

ago (Woodruff

r e l a t i v e l y warm oceans o f e a r l y and e a r l y

middle Miocene t i m e became replaced i n t h e middle and l a t e Miocene by g l a c i a l l y i n f l u e n c e d c o l d e r oceans w i t h f l u c t u a t i n g b u t g e n e r a l l y low sea-level ( V a i l and Hardenbol, 1979).

stands

Probable consequences o f t h i s c l i m a t i c change were

compression o f g l o b a l l a t i t u d i n a l c l i m a t i c b e l t s toward t h e equator and i n t e n s i f i e d c o a s t a l upwelling, e s p e c i a l l y i n regions o f eastern boundary currents. F o l l o w i n g I n g l e (1980, 1981a, b ) , we thus speculate t h a t t h e t h r e e - f o l d d i v i s i o n o f t h e Monterey deep marine f a c i e s (Figs. 2 and 3) i s a general r e c o r d o f these c l imatic-oceanographic events.

Calcareous plankton (mainly coccol itho-

p h o r i d s ) predominated i n t h e warm, l o w - f e r t i l i t y seas o f e a r l y t o e a r l y m i d d l e Miocene time; consequently, t h e r e s u l t i n g sediments were carbonate-rich and now c o n s t i t u t e t h e lower calcareous f a c i e s .

The h i g h e r n u t r i e n t l e v e l s engendered

by l a t e middle t o l a t e Miocene c o o l i n g and i n t e n s i f i e d u p w e l l i n g l e d t o phytoplankton populations dominated by diatoms and t h e consequent predominantly s i l i c e o u s sedimentation o f t h e upper s i l i c e o u s f a c i e s . Phosphatic rocks are present i n a t l e a s t small amounts i n many p a r t s o f t h e Monterey succession,

b u t they a r e p a r t i c u l a r l y prominent and widespread i n

255

p a r t s o f m i d d l e Miocene age, s p e c i f i c a l l y o f t h e R e l i z i a n and L u i s i a n stages (Fig.

3).

The reason f o r t h i s i s not y e t obvious,

(about 13-16 m.y.

although t h i s t i m e p e r i o d

ago) c o n t a i n s b o t h t h e maximum Miocene sea-level stand ( V a i l

and Hardenbol, 1979) and t h e onset o f A n t a r c t i c g l a c i a t i o n .

Perhaps t h e com-

bination o f

and l o w - l a t i t u d e

high-standing,

still

r e l a t i v e l y warm,

middle-

oceans and increased v e r t i c a l c i r c u l a t i o n may be e s s e n t i a l f o r phosphogenesis.

Sheldon (1980) r e l a t e d t h i s type o f phosphogenesis t o phosphorus w i t h -

drawal from t h e deep-ocean phosphorus s i n k by u p w e l l i n g i n coastal t r a d e wind b e l t s a t times o f t r a n s i t i o n from n o n g l a c i a l t o g l a c i a l c o n d i t i o n s . tively,

J.

C.

I n g l e , Jr.

( o r a l communication,

Alterna-

1981) has suggested t h a t such

t r a n s i t i o n a l i n t e r v a l s may be t h e t i m e o f g r e a t e s t v e r t i c a l mixing and upwelling; t h i s would l e a d t o very h i g h p r o d u c t i v i t y i n c o a s t a l zones, deposition o f organic-rich

sediments beneath these zones,

prodigious

and p o s s i b l y t h e

generation o f abundant o r g a n i c a l l y l i n k e d a u t h i g e n i c phosphatic m a t e r i a l i n anoxic sediments ( B u r n e t t , 1977, 1980). Basinward encroachment o f c l a s t i c t u r b i d i t e fans o r prograding shal low-water c l a s t i c wedges ( t h e Santa M a r g a r i t a Formation and analogous u n i t s ) brought t h i s i n t e r v a l o f p e l a g i c and hemipelagic sedimentation t o a close, a t l e a s t i n t h e nearshore basins, i n l a t e Miocene t o Pliocene time.

These c l a s t i c u n i t s record

increased t e r r i g e n o u s sediment supply and t e c t o n i sm. The pronounced g l o b a l drops i n sea l e v e l o f l a t e Miocene t i m e ( V a i l and Hardenbol, 1979) may have a l so c o n t r i b u t e d t o these c l a s t i c b l a n k e t s by exposing new sources on s h e l f areas and by i n c r e a s i n g stream g r a d i e n t s (see a l s o Surdam and Stanley, 1981a, b). DEPOSITIONAL ENVIRONMENTS F i g u r e 4 p o r t r a y s s c h e m a t i c a l l y our i n t e r p r e t a t i o n o f t h e c h i e f d e p o s i t i o n a l environments o f t h e Monterey Formation.

CONTINENTAL SLOPE

STARVED BORDERLAND BASIN

Major c o n t r o l s were t h e topography o f

STARVED BORDERLAND RIDGE

PROXIMAL BASIN

OUTER SHELF

256 the fractured

borderland r e g i o n and t h e p o s i t i o n

minimum zone.

Nearshore proximal basins commonly were s i t e s o f mixed c l a s t i c -

hemipelagic d e p o s i t i o n , i n c l u d i n g t u r b i d i t e fans,

and i n t e n s i t y o f t h e oxygenwhereas o f f s h o r e d i s t a l ba-

sins, starved o f c l a s t i c sediment, r e c e i v e d predominantly hemipelagic sediment a t i o n (Blake,

1981).

Hemipelagic d e p o s i t i o n a l s o dominated on c o n t i n e n t a l

slope f a c i e s (Summerhayes, 1981). Basins whose s i l l depths were i n t e r s e c t e d by t h e oxygen-minimum zone were anoxic and c h a r a c t e r i z e d by predominantly laminated, o r g a n i c - r i c h ,

unburrowed

sediments; more aerated o x i c basins, i n c o n t r a s t , had burrowing i n f a u n a l popu1 a t i o n s and massive burrowed sediments ( C a l v e r t , 1964, 1966a, b). Offshore fault-bounded

r i d g e s and a n t i c l i n a l

highs were sediment-starved

s i t e s o f predominantly hemi p e l a g i c sedimentation.

I n some places, a p p a r e n t l y

when these r i d g e s were w i t h i n t h e oxygen-minimum zone o r near i t s edges, t h e y were a l s o s i t e s o f p h o s p h a t i z a t i o n and g l a u c o n i t i z a t i o n . tion

affected

waters,

., 1980). Most

and o u t e r - s h e l f

sediments

S i m i l a r phosphatizabathed

by

low-oxygen

i n a s e t t i n g somewhat analogous t o t h a t o f modern p h o s p h a t i z a t i o n o f

slope-shelf a1

upper-slope

sediments west o f South America ( B u r n e t t , 1977, 1980; B u r n e t t e t

prior

interpretations

of

Monterey d e p o s i t i o n a l

emphasized t h e importance o f b a s i n - f l o o r deposition.

s i t e s have t a c i t l y

But extensive slope and

s h e l f d e p o s i t i o n i s i m p l i c i t i n t h e paleogeographic r e c o n s t r u c t i o n s o f Graham (1976a, b),

Isaacs (1980), Lagoe (1981),

and others.

A l l o f these environments

may c o n t a i n laminated, anoxic f a c i e s (Fig. 4) and s i m i l a r b e n t h i c f o r a m i n i f e r a 1 assemblages dominated by low-oxygen

forms such as t h e b o l i v i n i d s .

Upper-

s l ope/outer-she1 f sedimentation may be more i n f l u e n c e d by f l u c t u a t i o n s i n t h e p o s i t i o n and i n t e n s i t y o f t h e oxygen-minimum zone,

producing a l t e r n a t i o n s o f

laminated unburrowed and massive burrowed beds (see s i l i c e o u s f a c i e s i n Fig. 2; Govean, 1980; Govean and G a r r i son, 1981).

But s i m i 1a r a1t e r n a t i ons may occur

i n b a s i n f l o o r sequences due t o o v e r t u r n o f b a s i n a l waters (Huslemann and Emery, 1961).

Thus d i f f e r e n t i a t i o n among basin, slope, and s h e l f environments

i s d i f f i c u l t and remains an important challenge f o r f u t u r e work. DIAGENESIS' A f t e r d e p o s i t i o n , Monterey s t r a t a were g e n e r a l l y b u r i e d r a p i d l y , a t t a i n i n g several hundred meters o f b u r i a l w i t h i n a few m i l l i o n years. f l uence on t h e i r d i a g e n e t i c h i s t o r y was b u r i a l temperature;

The primary i n sediment canposi-

t i o n , b u r i a l depth, and p o s s i b l y h e a t i n g r a t e had important secondary e f f e c t s . I n t h e f o l l o w i n g sections, we summarize c u r r e n t knowledge o f diagenesis i n terms o f s i l i c a diagenesis and t h e s t r u c t u r e o f o p a l i n e s i l i c a s , b u r i a l tempera t u r e and depth o f d i a g e n e t i c changes, l i t h o l o g i c changes, r e d u c t i o n o f poro s i t y , organic m a t t e r m a t u r a t i o n and o i l generation, and f o r m a t i o n o f secondary

258 ANGSTROMS

259 and Norman, 1976). Other important c h a r a c t e r i s t i c s o f t h i s sequence are: ( 1 ) a decrease i n t h e p r i n c i p a l d-spacing o f opal-CT (described by Murata and Larson, 1975, as t h e d ( 1 0 1 ) spacing o f c r i s t o b a l i t e ) from about 4.12 t o about 4.04

1 with

i n c r e a s i n g diagenesis;

( 2 ) t h e presence o f a broad peak character0 A i n opal-CT having h i g h p r i n c i p a l d-spacing values; and ( 3 ) t h e presence o f two minor X-ray d i f f r a c t i o n peaks c h a r a c t e r i s 0 t i c o f a - c r i s t o b a l i t e near 3.1 A and 2.8 A i n opal-CT w i t h low (4.04-4.05 i s t i c o f a-tridymite

near 4.3

i)

p r i n c i p a l d-spacing values.

A general model f o r s i l i c a diagenesis i n t h e Monterey Formation was devel-

J.

oped by K.

Murata and co-workers.

M a r t i n e z Creek (Fig.

Study o f s i l i c e o u s rocks a t Chico

1 ) i n t h e Temblor Range showed t h a t t h e depth p r o f i l e o f

oxygen i s o t o p e r a t i o s i s c h a r a c t e r i z e d by two abrupt step r e d u c t i o n s t h a t c o r respond t o t h e two s i l i c a phase transformations, t o q u a r t z (Murata and Larson, 1975).

opal-A t o opal-CT and opal-CT

R e - e q u i l i b r a t i o n o f t h e oxygen i n s i l i c a

w i t h pore water, as i n d i c a t e d by t h e step reductions, t o g e t h e r w i t h changes i n b u l k d e n s i t y values (see p o r o s i t y s e c t i o n below) and petrographic and SEM feat u r e s showed t h a t t h e two s i l i c a phase t r a n s f o r m a t i o n s occurred by a process o f t o t a l solution-precipitation of silica.

The same l i n e s o f evidence i n d i c a t e d

t h a t t h e " o r d e r i n g " o f opal-CT proceeded by a s o l i d - s t a t e process.

On t h e ba-

s i s o f experimental r e c r y s t a l l i z a t i o n o f Monterey opal-CT rocks a t 300"-500"

C,

E r n s t and C a l v e r t (1969) had p r e v i o u s l y suggested t h a t t h e mechanism o f quartz f o r m a t i o n was a s o l i d - s o l i d mental

data

(Mizutani,

reaction,

( S t e i n and K i r k p a t r i c k ,

b u t r e i n t e r p r e t a t i o n o f these e x p e r i 1976)

and o t h e r

1977) have supported Murata and Larson's

experimental

(1975)

studies

conclusion t h a t

s o l u t i o n - p r e c i p i t a t i o n was t h e t r a n s f o r m a t i o n mechanism i n t h e Monterey. To e x p l a i n t h e metastable f o r m a t i o n o f opal-CT,

Murata and Larson (1975)

p o i n t e d o u t t h a t p r e c i p i t a t i o n o f a s i l i c a phase i s a p p a r e n t l y n o t favored unl e s s t h e c o n c e n t r a t i o n o f s i l i c a i n s o l u t i o n i s o n l y s l i g h t l y over-saturated w i t h respect t o t h a t p a r t i c u l a r phase (see a l s o Kastner e t a l . the

solubility

o f opal-CT

i s i n t e r m e d i a t e between t h a t

, 1977).

Because

o f amorphous opal

( h i g h e r ) and t h a t o f quartz (lower), opal-CT forms more e a s i l y from a s o l u t i o n i n e q u i l i b r i u m w i t h opal-A than does q u a r t z (see a l s o Fournier,

1973).

To ex-

p l a i n why q u a r t z forms so s l o w l y i n comparison w i t h r a t e s i n d i c a t e d by extrapol a t i o n o f E r n s t and C a l v e r t ' s (1969) experimental data t o low temperatures (14,000 years a t 110" C ) , Murata and Larson (1975) suggested t h a t " w e l l ordered" opal-CT i s a necessary precursor f o r formation of q u a r t z a t low temp e r a t u r e because s o l u t i o n s i n e q u i l i b r i u m w i t h h i g h l y "disordered" opal-CT a r e t o o supersaturated t o form quartz; r a t e s based on experimental data, measured a t temperatures over 200" C where t h e s o l u b i l i t i e s o f t h e v a r i o u s s i l i c a p o l y morphs are much a l i k e , do n o t t a k e i n t o account g e o l o g i c a l l y important d i f f e r ences among t h e s o l u b i l i t i e s a t lower temperatures (Murata and Larson, 1975).

260

The influence of sediment composition on s i l i c a diagenesis was investigated i n detail by Isaacs (1980, 1982) who demonstrated the typical diagenetic sequence of s i l i c a phases (opal-A opal-CT .+ quartz) in rocks of a remarkably wide range o f compositions--including rocks with over 70% c a l c i t e , rocks w i t h over 80% d e t r i t a l minerals, and rocks with as l i t t l e as 5% biogenic s i l i c a . Isaacs (1982) studied sample s e t s from the same lithostratigraphic and biostratigraphic level i n an uplifted homocline t h a t had been subjected t o increasing burial temperatures and depths in a l a t e r a l trend extending more than 55 km along the Santa Barbara coast ( F i g . 1). Results showed t h a t differences in the kinetics (or r e l a t i v e temperatures) of s i l i c a phase transformations were consistently related to differences in the proportion of s i l i c a and d e t r i t a l minerals (Fig. 6). Results also showed t h a t d-spacings of newly formed opal-CT 0 are significantly smaller (4.08 A ) in d e t r i t a l - r i c h rocks than in s i l i c a - r i c h rocks (4.12 and t h a t increased diagenesis produced an approximately constant -f

w)

RELATIVE DETRITAL MINERAL CONTENT, WEIGHT Yo

261

r a t e of decrease in d-spacings. These contrasts among s i l i c a phases in rocks of varying compositions can be used t o rank, with considerable precision, the diagenetic maturity of the Monterey a t different l o c a l i t i e s (Keller, 1982). The influence of d e t r i t a l minerals on the kinetics of s i l i c a phase transformations probably r e s u l t s from the smectite (Kastner e t a l . , 1977) in the mixed-layer illite-smectite clay t h a t is dominant in the d e t r i t a l mineral fract i o n (Isaacs, 1980). The pattern (Fig. 6 ) i s apparently determined mainly by the kinetics of formation and character of newly formed opal-CT (Isaacs, 1982). Because "ordering" of opal-CT i s apparently required before quartz can form, differences i n the temperature of q u a r t z formation are probably only indirectly related t o d e t r i t a l mineral content as a result of the pattern o f opal-CT formation and maturation. Relations between opal-CT and d e t r i t a l mineral abundance observed i n the Monterey Formation underscore the need for further investigation of the struct u r e of opal-CT. In f a c t , neither the structure of the mineral opal-CT nor the mineralogic significance of the d-spacing o f opal-CT i s a t a l l c l e a r , and longstanding doubts about the validity of tridymite as a t r u e s i l i c a polymorph pers i s t ( E i t e l , 1957; Wilson e t a l . , 1974; Jones and Segnit, 1975; von Rad e t a l . , 1977; Isaacs, 1982). Moreover, although regarded by Murata and h i s co-workers as an ordering process, changes i n the mineralogic structure of opal-CT are not ordering in the s t r i c t sense. X-ray diffractograms of opal-CT change with depth from patterns characteristic of a-tridymite to patterns d i s t i n c t l y chara c t e r i s t i c of a - c r i s t o b a l i t e ( F i g . 5 ) . Because the change evidently occurs in nature by a solid-state process, detailed mineralogic analysis of the sequence may c l a r i f y the mineralogic structure of opal-CT. Changes i n the refractive index (Murata and Larson, 1975) and i n the adsorptive c h a r a c t e r i s t i c s (Isaacs, 1980) of opal-CT during "ordering" suggest t h a t water o r hydroxyls in the s i l i ca structure may be of importance. The influence of carbonate on s i l i c a diagenesis was also examined, by comparing two s e r i e s of sample s e t s immediately above and below the horizon a t which disseminated carbonate disappears from the lithostratigraphic sequence i n the Santa Barbara coastal area (Isaacs, 1982). Surprisingly, almost no influence of carbonate could be detected when rocks w i t h equal proportions of s i l i c a and d e t r i t a l minerals were compared. The presence of carbonate influenced s i l i c a diagenesis in only one type of rock--rocks in which s i l i c a i s a t l e a s t 8 times more abundant t h a n clay. I n such rocks, minor amounts of diagenetic quartz commonly formed early in diagenesis, mainly within benthic foraminifera1 t e s t s . Also early i n diagenesis, diagenetic q u a r t z partly or completely replaced some clay-poor calcareous rocks to form rare beds o r nodules of dense black chert. Measurements of compaction around the nodular type of q u a r t z chert show t h a t the nodules formed when host rocks had 50 t o 60% porosity, a

262

p o r o s i t y i n d i c a t i n g t h a t h o s t rocks were diatomaceous (Isaacs, 1980). Bramlette (1946, p. 50-51) a l s o concluded t h a t such c h e r t s formed e a r l y , on t h e b a s i s o f t h e d i s t r i b u t i o n o f Monterey-derived cobbles i n i n t r a f o r m a t i o n a l and o v e r l y i n g Miocene and P1 iocene conglomerates.

A1 though v o l m e t r i c a l l y rare,

these dense quartz c h e r t s a r e o f p a r t i c u l a r importance because t h e y can be misl e a d i n g i n t h e d e t e r m i n a t i o n o f s i l i c a phase zones,

temperatures o f q u a r t z

f o r m a t i o n i n more common rocks, and rock paragenesis. TEMPERATURE AND BURIAL DEPTH OF SILICA PHASE TRANSFORMATIONS Most Monterey s e c t i o n s s t u d i e d have been p a r t l y o r e n t i r e l y u p l i f t e d since t h e i r deepest b u r i a l , and i n many places o v e r l y i n g sediment has been completely eroded.

Moreover, present-day

thermal

e q u i v a l e n t t o those o f t h e past. temperature o f

regimes a r e a p p a r e n t l y n o t everywhere

Values from t h e Monterey f o r t h e depth and

s i l i c a phase t r a n s f o r m a t i o n s a r e thus,

f o r t h e most

part,

r e f i n e d guesses. Estimates o f maximum p r i o r overburden suggest t h a t a t Chico Martinez Creek abundant opal-CT formed a t a depth o f 800 t o 1100 m, t h a t t h e opal-CT zone was about 1300 m t h i c k ,

and t h a t d i a g e n e t i c q u a r t z formed a t a depth o f 2100 t o

2400 m (Murata and Larson,

1975; Murata e t al.,

1977).

I n t h e Santa Maria

b a s i n (Fig. l), where present geothermal g r a d i e n t s are much h i g h e r (42" t o 67" C per km) than i n t h e Temblor Range, P i s c i o t t o (1981a) estimated t h e t o p o f t h e

opal-CT zone t o have l a i n a t b u r i a l depths o f 750 t o 900 m and i t s base a t 1300 t o 1600 m, w i t h a t o t a l t h i c k n e s s ranging from 550 t o 800 m. deep s t r a t i g r a p h i c t e s t w e l l OCS-Cal 78-164 No. 1 (Fig.

I),

I n the offshore where t h e present

geothermal g r a d i e n t i s 48" C per km, t h e base o f t h e opal-CT zone i s a t subsea-floor

depths o f about 1650 m (Beyer,

1979; McCulloh, 1979; McCulloh and

Beyer, 1979). Because o f r e - e q u i l i b r a t i o n w i t h pore water d u r i n g diagenesis, oxygen i s o tope r a t i o s o f s i l i c a can be used t o estimate temperatures o f s i l i c a phase f o r m a t i o n i f t h e i s o t o p e composition o f t h e pore water and an a p p r o p r i a t e expression f o r s i l ica-water f r a c t i o n a t i o n are known.

U n f o r t u n a t e l y , numerous

proposed expressions f o r i s o t o p e f r a c t i o n a t i o n i n t h e quartz-water system a r e n o t s t r i c t l y a p p l i c a b l e t o t h e opal-water system (see a l s o P i s c i o t t o , and 6l80 values o f pore water have n o t been measured. ever,

1981a)

As a best estimate, how-

and because o f i t s agreement w i t h L a b e y r i e ' s (1974) curve f o r diatom-

water f r a c t i o n a t i o n between 0" and 30" C, s i o n developed f o r Clayton e t al.

quartz-water

(1972).

Murata e t a l .

(1977) used an expres-

f r a c t i o n a t i o n between 200" and 500" C by

R e s u l t i n g estimates f o r t h e phase t r a n s f o r m a t i o n tem-

peratures a t t h e two sections i n t h e Temblor Range are c o n s i s t e n t , assuming t h a t t h e 6 l80 o f i n t e r s t i t i a l water was 0.0 r e l a t i v e t o standard mean ocean water (SMOW)

a t Chico Martinez Creek and was +3.7 permil SMOW a t T a f t - - b o t h

263

values w i t h i n the range ( 0 t o +6 permil SMOW) of o i l f i e l d formation waters i n C a l i f o r n i a (see Murata and o t h e r s , 1969, 1977). Estimated temperatures a r e then 48' f 8" C f o r formation of opal-CT and 79' 2' C f o r formation of q u a r t z , but comparably higher ( o r lower) 6 l80 values f o r both s e c t i o n s would s l i g h t l y increase ( o r decrease) these estimates. Calculations from i n f e r r e d overburden thickness, average thermal c o n d u c t i v i t i e s , and l i k e l y heat flow values suggest t h a t the estimated transformation values a r e reasonable f o r a heat flow r a t e of about 0.8 heat flow units (60% of present values i n the area) , whereas s i g n i f i c a n t l y lower transformation temperatures a r e improbable (Murata and o t h e r s , 1977). A temperature of about 85' C f o r the transformation of opal-CT t o quartz was more r e c e n t l y suggested from evaluation of the o f f shore deep s t r a t i g r a p h i c t e s t well OCS-Cal 78-164 No. 1 (McCulloh, 1979). Isotope values and thermal d a t a were extensively studied by P i s c i o t t o (1981a) i n t h e s u r f a c e and subsurface of t h e Santa Maria area. On t h e b a s i s of values commonly found i n DSDP h o l e s , P i s c i o t t o (1981a) assumed a pore water 6l80 value of 0.0 permil SMOW. Resulting temperatures f o r the transformation of opal-A t o opal-CT range from 18' t o 44" C using Knauth and E p s t e i n ' s (1976) expression o r from 28' t o 56" C using Clayton e t a l . ' s (1972) expression; temp e r a t u r e s f o r the transformation of opal-CT t o quartz range from 31" t o 68" C using Knauth and Epstein's (1976) expression o r from 44" t o 80" C using Clayton e t a1 I s (1972) expression. Temperatures based on estimated maximum overburden and on present thermal g r a d i e n t s c a l c u l a t e d from equilibrium borehole temperature values i n d i c a t e t h a t the temperature of the transformations f a l l s w i t h i n the range of 50' t o 54' C f o r the transformation of opal-A t o opal-CT and 77" t o 110" C f o r t h e transformation of opal-CT t o quartz ( P i s c i o t t o , 1981a). The r e l a t i o n between opal-CT d-spacings and burial depth was i n v e s t i g a t e d by Murata and Larson (1975), Murata and Randall (1975), and P i s c i o t t o (1981a). In porcelani tes a t Chico Martinez Creek, d-spacings decrease w i t h depth ( a c r o s s a 1300-111 i n t e r v a l ) r a p i d l y between 4.11 and 4.09 A , very slowly between 4.09 and 0 4.07 A, and a t an intermediate r a t e below 4.07 A (Murata and Larson, 1975); i n p o r c e l a n i t e s of the T a f t a r e a , by c o n t r a s t , the decrease w i t h depth ( a c r o s s an 800-111 i n t e r v a l ) is l i n e a r (Murata and Randall , 1975). The rapid decrease of dspacings w i t h depth i n 3 s e c t i o n s of the Santa Maria basin was produced by high geothermal g r a d i e n t s i n t h i s area ( P i s c i o t t o , 1981a). Murata and Randall (1975) regarded opal-CT d-spacings a s temperature i n d i c a t o r s , and they contoured d-spacing values a s isothermal s u r f a c e s i n o r d e r t o d e l i n e a t e the d e t a i l s of f o l d s i n the Monterey Formation of t h e Temblor Range. Because dspacings vary widely w i t h rock composition (see Fig. 6 ) , t h i s method is accura t e only when the rocks compared a r e s i m i l a r i n composition, and results must be adjusted somewhat f o r d i f f e r e n c e s i n heating r a t e s (Mizutani , 1977).

.

264

Estimates o f t h e t h i c k n e s s o f t h e opal-CT zone were used by P i s c i o t t o (1978, 1981a)

to

approximate

past geothermal

gradients

i n t h e Santa Maria area.

P i s c i o t t o (1981a) a l s o suggested, however, t h a t t h e thermal h i s t o r i e s o f specif i c well

s i t e s i n t h e Santa Maria b a s i n a f f e c t e d t h e temperature o f phase

transformations.

Diagenetic h i s t o r i e s r e c o n s t r u c t e d on t h e b a s i s o f M i z u t a n i ' s

(1967, 1970, 1977) k i n e t i c model i n d i c a t e t h a t opal-CT began t o form between 9" and 27" C whereas quartz began t o form between 35" and 61" C.

Most s i l i c a con-

versions i n t h e Santa Maria area thus occurred w i t h i n t h e past 3 t o 4 my.,

in

response t o accelerated r a t e s o f sedimentation d u r i n g t h e Pliocene. LITHOLOGIC CHANGES WITH OIAGENESIS Recent

diagenetic

s t u d i e s have l e d t o general

recognition that

i n the

Monterey Formation most rocks b e a r i n g d i a g e n e t i c q u a r t z are n o t c h e r t y - - t h a t i s , t h e y a r e n e i t h e r waxy nor glassy nor e s p e c i a l l y b r i t t l e - - a n d t h a t many glassy, b r i t t l e ,

hard rocks bear o n l y opal-CT as a d i a g e n e t i c s i l i c a mineral.

Although Bramlette (1946) d i d not emphasize these r e l a t i o n s , he s t a t e d c l e a r l y (p. 16) t h a t dense v i t r e o u s s i l i c e o u s rocks c o u l d c o n t a i n e i t h e r o p a l i n e s i l i c a o r quartz, and he i n d i c a t e d (p. 48) t h a t quartz was t h e d i a g e n e t i c s i l i c a mine r a l i n many porcelaneous rocks.

Because o f c o n f l i c t i n g usage d u r i n g t h e past

decade i n t h e deep-sea l i t e r a t u r e ,

where t h e term " c h e r t " denotes a q u a r t z -

bearing rock, t h e s t r i k i n g l y matte t e x t u r e o f most q u a r t z rocks i n t h e Monterey Formation has been s t r o n g l y reemphasized by r e c e n t workers (Murata and Nakata, 1974; Murata and Larson, 1975; P i s c i o t t o , 1978, 1981a; Isaacs, 1980, 1981a). Petrography,

X-ray d i f f r a c t i o n a n a l y s i s ,

and chemical a n a l y s i s have shown

t h a t d i f f e r e n c e s i n t h e t e x t u r e and p h y s i c a l p r o p e r t i e s o f rocks are c l o s e l y r e l a t e d t o t h e rocks' b u l k mineral c o m p o s i t i o n - - p a r t i c u l a r l y t o t h e p r o p o r t i o n o f s i l i c a and d e t r i t a l m i n e r a l s (Bramlette, Pisciotto,

1978;

Isaacs, 1981a).

1946;

Murata and Larson,

1975;

Moreover, n e a r l y a l l d i f f e r e n c e s i n t e x t u r e

d e r i v e from o r i g i n a l compositional d i f f e r e n c e s - - n o t s i l i c i f i c a t i o n (that is, s i l i c a i n f i l l i n g ) .

from d i f f e r e n t degrees o f

Isaacs (1980, 1981d) proposed t h a t

v a r i a t i o n s i n d i a g e n e t i c surface t e x t u r e s r e s u l t m a i n l y from v a r i a t i o n s i n t h e c r y s t a l l i z a t i o n c h a r a c t e r i s t i c s o f s i l i c a t h a t forms i n t h e v i c i n i t y o f v a r y i n g abundances o f c l a y minerals.

Because sand- and s i l t - s i z e d e t r i t a l g r a i n s a r e

l e s s l i k e l y than c l a y m i n e r a l s t o i n f l u e n c e t e x t u r e s s t r o n g l y and because d i f f e r e n t c l a y m i n e r a l s probably i n f l u e n c e c r y s t a l l i z a t i o n t e x t u r e s d i f f e r e n t l y , rock t e x t u r e s cannot be r e l a t e d t o s p e c i f i c d e t r i t a l contents by a scheme t h a t is

universally

applicable.

In t h e Monterey Formation,

i l l i t e - s m e c t i t e i s t h e dominant c l a y mineral

, ratios

where mixed-layer

o f s i l i c a t o clay minerals

range from about 6 t o 18 i n c h e r t s and c h e r t y p o r c e l a n i t e s , from about 1.5 t o 8 i n p o r c e l a n i t e s , and from about 0.5 t o 2 i n s i l i c e o u s mudrocks (Isaacs, 1981a). Detailed

study o f l a t e r a l

sample s e t s i n t h e Santa Barbara area shows t h a t

265

F i g . 7. Outcrop comparison showing increased diagenesis i n l a t e r a l l y e q u i v a l e n t rocks o f t h e Monterey Formation, from diatomaceous shale w i t h f l a k y surface ( t o p ) t o opal-CT p o r c e l a n i t e ( c e n t e r ) t o q u a r t z p o r c e l a n i t e (bottom). Note s i m i l a r i t i e s o f both p o r c e l a n i t e s i n bedding, f l a g g i n e s s , and m a t t e surface t e x t u r e . From Isaacs (1981a).

266 t h e paragenesis o f rocks i n t h e Monterey Formation g e n e r a l l y proceeds thus (Isaacs, 1980, 1 9 8 1 ~ ) :

*

d ia t om it e diatomaceous shale ( d i atom- r ic h ) diatomaceous shal e ( d i atom-poor)

+

+

opal-CT c h e r t opal-CT p o r c e l a n i t e opal-CT mudrock

* +

+

quartz chert quartz porcelanite quartz mudrock.

I n these sequences, most a t e r a t i o n o f physical p r o p e r t i e s occurs d u r i n g opal CT formation,

-

which i n v o l v e s a marked increase i n b u l k d e n s i t y , hardness, co-

hesion, and b r i t t l e n e s s and a marked decrease i n p o r o s i t y (Isaacs, 1981a, c). The formation o f d i a g e n e t i c q u a r t z i s accompanied by a moderate increase i n b u l k d e n s i t y and cohesion and a moderate decrease i n p o r o s i t y (Fig. 7). Textures i n c a l c i t e - b e a r i n g rocks w i t h as much as 50% c a l c i t e are a l s o r e l a t e d m a i n l y t o t h e p r o p o r t i o n o f s i l i c a t o d e t r i t a l minerals.

I n addition,

however, t h e presence o f c a l c i t e (which i s m a i n l y i n t h e form o f c o c c o l i t h s ) appears t o have i n f l u e n c e d t e x t u r e s s l i g h t l y by promoting graininess. calcareous and d o l o m i t i c rocks w i t h unusually sparse c l a y , porcel aneous r a t h e r than cherty--a c o c c o l i t h rocks (Hein e t al.,

1981).

Some

f o r example,

are

re1 a t i o n a1 so observed i n some r a d i o l a r i a n During t h e diagenesis o f calcareous rocks

i n t h e Santa Barbara coastal area, d o l o m i t e replaced c a l c i t e i n rocks o f n e a r l y a l l compositions (although r a r e l y i n diatomaceous rocks); t h e r e s u l t i s a comp l e t e a r r a y o f d o l o m i t i c c h e r t s , d o l o m i t i c p o r c e l a n i t e s , and d o l a n i t i c mudrocks (Isaacs,

1981a, c).

D o l o m i t i c rocks are somewhat more cohesive than composi-

t i o n a l l y e q u i v a l e n t calcareous rocks because extensive cementation accompanied pervasive dolomite c r y s t a l l i z a t i o n , whereas i n general l i t t l e o f t h e c a l c i t e i n coccol it h s is r e c r y s t a l 1ized. POROSITY REDUCTION

P o r o s i t y r e d u c t i o n i n diatomaceous and d i a g e n e t i c a l y d e r i v e d rocks i s ma nl y i n f l u e n c e d by t h e mechanical s t r e n g t h o f t h e rocks, t h e geochemical i n s ab i l i t y o f opal, and t h e k i n e t i c s o f s i l i c a phase transformations. The p a t t e r n o f p o r o s i t y r e d u c t i o n d u r i n g e a r l y b u r i a l o f diatomaceous sediment has been e s t a b l i s h e d by Hamilton (1976) from study o f sediments i n t h e Bering Sea.

Here, diatomaceous sediments show a general decrease i n i n - s i t u

values o f f l u i d - s a t u r a t e d b u l k d e n s i t y from 1.24 g/cm3 near t h e s u r f a c e t o 1.45 g/cm3 a t 500 meters and a general decrease i n i n - s i t u p o r o s i t y values from 86% near t h e s u r f a c e t o 71% a t 500 meters.

I n comparison w i t h chalk and deep-sea

mud, diatomaceous sediment shows a remarkably h i g h r e t e n t i o n o f p o r o s i t y d u r i n g b u r i a l (Hamilton, 1976), one o f t h e c h a r a c t e r i s t i c s t h a t has made i t o f unique economic importance throughout h i s t o r y (Eardl ey-Wilmot

, 1928;

Durham, 1973).

268 P O R O S I T Y ,%

POROSITY,%

0 0

20

40

60

I

I

I

80

100

0

20

40

60

:. '/

80

100

0

I

- 500 n u)

-

I

-

I b-

L

W W I-

IW

v)

-

W

sediment

-

s

-

It

-

W

n

- 1000

n

270

d i r e c t a s s o c i a t i o n between l o s s o f p o r o s i t y and f o r m a t i o n o f d i a g e n e t i c q u a r t z i s c l e a r from l a b o r a t o r y measurements o f i n d i v i d u a l beds i n t h e t h i c k t r a n s i t i o n a l zone i n which rocks w i t h c o n t r a s t i n g s i l i c a phases a r e i n t i m a t e l y i n t e r bedded (Fig. 9).

W i t h i n t h i s zone, opal-CT-bearing rocks have d r y b u l k densi-

t i e s as low as 1.4 g/cm3 and p o r o s i t i e s as h i g h as 39%, whereas quartz-bearing beds have d r y b u l k d e n s i t i e s as h i g h as 2.1 g/cm3 and p o r o s i t i e s as low as 10% (Isaacs, 1981d).

E v a l u a t i o n o f t e x t u r a l features,

o f b u l k chemical compositions, quartz

formed mainly

by

f i e l d r e l a t i o n s , t h e range

and p e r m e a b i l i t y values shows t h a t d i a g e n e t i c

in-situ

recrystallization of

s i l i c a and

strongly

suggests t h a t compaction was t h e p r i n c i p a l mechanism o f p o r o s i t y r e d u c t i o n (Isaacs, 1981d). Other p a t t e r n s o f p o r o s i t y l o s s , although l e s s canmon, a r e a l s o present i n t h e Monterey Formation.

Sparsely s i l i c e o u s (5-25% s i l i c a ) o r g a n i c - r i c h marl s,

f o r example, d i f f e r from more common rocks i n t h a t ( 1 ) l e s s p o r o s i t y was l o s t d u r i n g s i l i c a phase t r a n s f o r m a t i o n s , ( 2 ) s i l i c a phases transformed a t s l i g h t l y d i f f e r e n t temperatures,

and--most i m p o r t a n t - - ( 3 ) as much as 10 t o 15 p o r o s i t y %

was l o s t w i t h i n t h e opal-CT zone (see Fig. 8a). i n t e r m e d i a t e between t h a t o f chalks (Fig.

This p a t t e r n , which i s c l e a r l y

8b; Hamilton,

1976; Scholle,

1977)

and t h a t o f h i g h l y diatomaceous rocks, suggests t h a t t h e p o r o s i t y l o s s w i t h i n t h e opal-CT zone i s due t o t h e absence o f t h e r i g i d s i l i c a framework t h a t i s present i n more h i g h l y s i l i c e o u s rocks (Isaacs, 1981d).

Cementation by i n f i l -

l i n g and replacement a l s o occurs i n some rocks, p a r t i c u l a r l y i n pure dolomites and nodular c h e r t s (Murata and Larson, 1975; Isaacs, 1981d; P i s c i o t t o , 1981b). DIAGENESIS OF ORGANIC MATTER

The Monterey Formation

i s widely

recognized as t h e

principal

petroleum

source rock i n C a l i f o r n i a , and t h e formation i s a l s o a major petroleum r e s e r v o i r rock i n many l o c a l i t i e s (e.g.,

Taylor, 1976).

Reported abundances o f o r -

ganic m a t t e r range from 1 t o 25% by weight and average as h i g h as 12% i n t h e sparsely

siliceous

organic

shale member o f t h e Santa Barbara coastal

area

(Taylor, 1976; Isaacs, 1981b; Surdam and Stanley, 1981b). Several recent papers have addressed aspects o f thermal m a t u r a t i o n o f o r ganic m a t t e r i n t h e Monterey.

I n s h a l l o w l y b u r i e d middle Miocene sediments

t h e organic m a t t e r i s almost from t h e southern C a l i f o r n i a borderland (Fig. l), e n t i r e l y amorphous kerogen having hydrocarbon/organi c-carbon (HC/C) r a t i o s as l o w as 0.0016, expressed as 0.16% (Claypool i n Taylor, 1976); a small amount o f e x i n i t i c m a t e r i a l (humic kerogen) i s a l s o present (Surdam and Stanley, 1981b). Visual examination o f t h e kerogen i n t h e Pismo s y n c l i n e i n d i c a t e s t h a t i t i s s a p r o p e l i c and would be considered t y p e I kerogen (Surdam and Stanley, 1981b). However, t h e H/C and O/C r a t i o s o f kerogen i n both t h e Santa Barbara coastal area and t h e P i s m s y n c l i n e (Isaacs, 1980; Surdam and Stanley, 1981b) show t h a t

271

272

TABLE 1. Changes i n elemental compositiona and i n atomic r a t i o s o f elements i n kerogen, as diagenesis increases ( f r o m t o p t o bottom) i n l a t e r a l l y e q u i v a l e n t rocks from t h e Monterey Formation o f t h e Santa Barbara coastal area. Values a r e averages o f between 4 and 11 samples ( f r o m Isaacs, 1980, pp. 247-255).

A B C

D

Zone o f interbedded opal-A and opal-CT rocks Upper opal-CT zone Lower opal-CT zone and zone o f i n t e r bedded opal-CT and q u a r t z rocks Q u a r t z zone

C

H

0

N

H/C

O/C

C/N

67.9

7.8

20.5

3.7

1.37

0.229

21.3

74.0

8.7

14.1

3.3

1.40

0.143

26.5

77.4

9.0

10.6

3.2

1.37

0.103

31.2

78.7

9.0

8.9

2.5

1.36

0.094

37.0

a weight percent normalized s u l f u r - f r e e basis. same age from t h e Santa Barbara coastal area c o n t a i n 159 t o 40,000 ppm hydrocarbon and HC/C r a t i o s o f 1.5 t o 43.5% (Taylor, 1976, p. 41-42). Claypool

( i n Taylor,

immature as i n d i c a t e d by h i g h l y a s p h a l t i c e x t r a c t s and, fraction,

According t o

1976), b o t h groups o f samples are apparently t h e r m a l l y i n t h e hydrocarbon

by t h e predominance o f aromatic over s a t u r a t e d hydrocarbons and o f

branched-cyclic

saturates

over

straight-chain

paraffins.

In

the

Point

Conception deep s t r a t i g r a p h i c t e s t w e l l OCS-Cal 78-164 No. 1, analyses o f d r i l l c u t t i n g s from Monterey e q u i v a l e n t s a t sub-sea-floor y i e l d t h e f o l l o w i n g mean values:

depths o f 1700 t o 1800 m

hydrocarbon abundance, 4800 ppm; HC/C,

r a t i o o f s a t u r a t e d hydrocarbons t o aromatic hydrocarbons, r a t i o i n kerogen, 1.30; 1979).

0.17;

18.7%; atomic

H/C

and C/N atomic r a t i o i n kerogen, 34.5 (Claypool e t al.,

Gas chromatographic a n a l y s i s o f t h e s a t u r a t e d hydrocarbons shows t h a t

n - p a r a f f i n s have a pronounced even-carbon-number preference across t h e range o f "c16

t o 2-C24,

a preference a l s o c h a r a c t e r i s t i c

o f o i l s produced from t h e

Monterey Formation i n t h e Santa Maria b a s i n ( M i l n e r e t al., Claypool

et

unsaturated

al.,

1979).

steroid

Details o f the e a r l y diagenetic

hydrocarbons

have a1 so

1977,

p.

138;

sequence among

been e s t a b l ished by Giger

and

Schaffner (1981). The

exact

temperature a t which petroleum i s generated

i n t h e Monterey

Formation has n o t been s e t t l e d , b u t several recent s t u d i e s suggest t h a t s i g n i ficant

amounts o f o i l

a r e generated a t temperatures o f 85" t o 95" C from

marginal l y mature source rocks.

C1 aypool e t a1

.

(1979) c h a r a c t e r i z e d t h e

Organic m a t t e r i n t h e P o i n t Conception deep s t r a t i g r a p h i c t e s t w e l l between sub-sea-floor depths o f 1100 and 2600 m as t h e r m a l l y immature t o submature, b u t t h e y a l s o noted t h a t o i l shows were r e p o r t e d d u r i n g d r i l l i n g and t h a t t h e

274

t h i c k n e s s o f beds and l a y e r s passing through carbonate concretions, amounts t o a f a c t o r o f 3 o r 4 (Bramlette, 1946). P r e s e r v a t i o n o r r e s o l u t i o n o f sediment a r y s t r u c t u r e s and t e x t u r e s decreases on a microscopic scale. replaced f o r a m i n i f e r t e s t s are common; 1981b).

Coccoliths,

fecal

Laminations and

p e l l e t s ( ? ) are r a r e ( P i s c i o t t o

which a r e abundant i n undolomitized s t r a t a o f t h e phos-

p h a t i c and calcareous f a c i e s , are g e n e r a l l y absent o r unrecognizable i n dolom i t i z e d zones.

However, beds o f c o c c o l i t h mudstone and shale t h a t a r e l a t e r -

a l l y and s t r a t i g r a p h i c a l l y e q u i v a l e n t t o d o l o m i t i c i n t e r v a l s (Isaacs,

1980)

suggest t h a t some d o l o m i t e d e r i v e s from primary c o c c o l i t h debris. I s o t o p i c analyses o f Monterey dolomites show t h a t oxygen-isotope values g e n e r a l l y f a l l w i t h i n a narrow range o f 23 t o 38 permil SMOW, whereas carbon isotopes vary considerably,

from -16

t o +21 permil

belemnite standard (PDB) (Murata e t a1

., 1969;

r e l a t i v e t o t h e Peedee

P i s c i o t t o , 1981b; Roehl , 1981).

The oxygen-isotope values have been used f o r paleothermometry, experimental water.

assuming t h e

f r a c t i o n a t i o n expressions f o r e i t h e r c a l c i t e - w a t e r o r dolomite-

Exact temperatures o r ranges o f temperatures o f f o r m a t i o n a r e d i f f i c u l t

t o s p e c i f y because o f t h e u n c e r t a i n t y i n applying t h e c a l c i t e - w a t e r expression t o dolomite,

because t h e

o f t h e e q u i l i b r a t i n g f l u i d s i s unknown, and

6 l8O

because, once nucleated, d o l o m i t e may continue t o grow d u r i n g b u r i a l and w i t h age.

Formation temperatures may be comparable t o bottom water temperatures

(3'-16"C),

as i n d i c a t e d by t h e s i m i l a r i t i e s between trends i n t h e sI80-age pro-

files

Monterey dolomites

for

oxygen-isotope

and Murata, 1979). 78°C)

a t t h e Chico M a r t i n e z Creek s e c t i o n and t h e

composition o f deep-sea Miocene benthic f o r a m i n i f e r s (Friedman A l t e r n a t i v e l y , t h e temperature range may be expanded

t o r e f l e c t g r e a t e r and v a r y i n g depths o f formation,

straightforward

computations

using

the

dolomite-water

(loo-

as suggested by

and

calcite-water

The broad range o f carbon isotopes suggests a complex o r i g i n .

With some ex-

f r a c t i o n a t i o n expressions ( P i s c i o t t o , 1981b; Roehl , 1981). ceptions, t h e r e i s an apparent c l u s t e r i n g o f l i g h t - and heavy-carbon dolomites i n c e r t a i n t e c t o n i c d i s t r i c t s o f t h e C a l i f o r n i a Coast Ranges t h a t a r e bounded by major f a u l t s (Roehl, uncertain.

The cause o f t h i s apparent d i s t r i b u t i o n i s

1981).

Depth-related trends i n

613C values perhaps r e f l e c t f o r m a t i o n i n

v a r i o u s zones o f organic matter decay (Table 2; P i s c i o t t o , 1981b and references therein).

Negative

6 I 3 C values can d e r i v e from t h e abundant l i g h t - c a r b o n COP

i n shallow zones o f aerobic m i c r o b i a l o x i d a t i o n and anaerobic s u l f a t e reduction.

More p o s i t i v e values occur below, i n a zone where methane generation by

C02 r e d u c t i o n i n m i c r o b i a1 metabolic processes produces heavy-carbon b i c a r bonate

in

interstitial

waters

through

preferential

removal

of

H12C03-.

Alternatively,

heavy carbon values may be i n h e r i t e d from Cop produced t o g e t h e r

with

during

methane

acetate

fermentation.

If light

C02 i s added

a t rates

TABLE 2. Oiagenetic zones and o x i d a t i o n - r e d u c t i o n Mahoney, 1981).

r e a c t i o n s f o r subsurface m i c r o b i a l metabolic processes (from P i s c i o t t o and

Observed o r Estimated 6 C I 3 Range (permil POB) O i agenet i d Zones and

Approximate Depth Range

Oxidation-Reduction Reactions

o f Zone below Sea F l o o r (m)

Microbi a1 o x i d a t i o n (aerobic r e s p i r a t i o n ) : CH20 + 02

-+

0-0.01

c02a

CH4

-18 t o -28

--b

-15 t o -30

-- b

C02+ H20

M i c r o b i a l s u l f a t e r e d u c t i o n (anaerobic): CH20 + S04'2

+

0.01-10

Carbonate No P P t ( C u r t i s , 1978) 0 t o -25

S-* + 2C02 + 2H20

Methanogenesis:

methane production and

10-1000

-20 t o + 10

-47 t o -90

50-2500

-10 t o -20

-60 t o -80

-10 t o + 15

carbonate p r e c i p i t a t i o n (anaerobic): Me+*

+

2HC03-

+

8H+

-+

CH4 + MeC03 + 3H20

where Me = Cay Mg, Fe, etc. Thermocatalytic decarboxylation g e n e r a l l y : f a t t y acids + n-a1 kanes + f a t t y acids + C02

0 t o -10 (-25?)

Note: Reactions, depth i n t e r v a l s , and i s o t o p i c data compiled from Cooper and Bray (1963), Eisma and Jung (1969), Claypool e t a l . (1973), Claypool (1974), Claypool and Kaplan (1974), I r w i n e t a l . (1977), C u r t i s (1978), Hei-n e t a l . (1979). a Total dissolved CO2, mainly as bicarbonate. Methane n o t present.

276

g r e a t e r than i t s removal by reduction t o methane, I3c values may again decrease and light-carbon carbonates can form within the interval of thermoc a t a l y t i c decarboxylation. A t any one l o c a l i t y , dolomite may have formed c m p l e t e l y within one of these zones or p a r t i a l l y i n several zones, making the precise decoding of i s o t o p i c values a formidable task. The importance of more f u l l y understanding the o r i g i n of dolomite beds i n the Monterey Formation i s underscored by t h e i r s i g n i f i c a n c e a s f r a c t u r e d reservoirs of hydrocarbons. The best and most p r o l i f i c r e s e r v o i r s , found mainly i n the Santa Maria region, a r e f o r the most part i n phosphatic and calcareous mudstones and shales of the Monterey Formation t h a t have been embrittled by dolomitization. Textural r e l a t i o n s within dolomitic breccias a r e c m p l e x , and many samples contain several generations of coarse and i s o t o p i c a l l y d i s t i n c t i v e dolomite cement completely o r p a r t l y f i l l i n g f r a c t u r e s . Although the precise cause of brecciation i s uncertain, i t i s thought t h a t regional stresses induced repeated periods of rock d i l a t i o n followed by natural hydraulic f r a c t u r i n g (Roehl , 1981; Redwine, 1981). ACKNOWLEDGMENTS Our conversations with James C. Ingle Jr., Stephan A. Graham, Robert R. Cmpton, Larry A. Beyer, and K. J. Murata have contributed importantly t o the formulation of our ideas about the Monterey Formation. R. E. Garrison and K. A. P i s c i o t t o a r e grateful t o the National Science Foundation (Grant EAR7622131) and t o t h e Union Oil Company Foundation f o r support of t h e i r research. We a l s o thank L i l l i a n M. Wood and Suzanne Harris f o r typing t h e manuscript; P h y l l i s A. Swenson f o r d r a f t i n g and r e d r a f t i n g the f i g u r e s ; and Larry A. Beyer, James R. Hein, Azuma I i j i m a , and James C. Ingle, J r . f o r reviewing t h e manuscript. George E. Claypool and L e s l i e B. Magoon 2150 reviewed p a r t s of the manuscript. REFERENCES Alpha, T.R., 1970. Physiographic diagram of the southern C a l i f o r n i a borderland. U.S. Geol. Surv. Open-file Rept. , Dec. 3, 1970. Berg, R.R. and Kersey, U.G., 1981. Origin of t h i n , s i l i c e o u s beds i n Monterey Shale, E l k H i l l s Field, California (abs.). Am. Assoc. Pet. Geol. Bull., 65: 900. Beyer, L.A., 1979. Geological and geophysical implications of density and sonic logs. In: H.E. Cook ( E d i t o r ) , Geologic s t u d i e s of the Point Conception deep s t r a t i g r a p h i c test well OCS-CAL 78-164 no. 1, outer continental shelf, southern C a l i f o r n i a , United States. U.S. Geol. Surv. Open-File Rept., 791218: 59-70. Blake, G.H., 1981. Biostratigraphic r e l a t i o n s h i p of Neogene benthic foramini f e r a from the southern California o u t e r continental borderland t o the Monterey Formation. In: R.E. Garrison et a l . ( E d i t o r s ) , The Monterey Formation and Related S i l i c e o u s Rocks of California. P a c i f i c Sect. S.E.P.M. Spec. Publ., pp 1-14.

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Eisma, E. and Jung, J.W., 1969. Fundamental aspects o f t h e generation o f petroleum. I n : G. E g l i n t o n and M.T.J. Murphy ( E d i t o r s ) , Organic Geochemistry. Springer-Verlag, New York, N.Y., pp. 676-698. E i t e l , W., 1957. S t r u c t u r a l anomalies i n t r i d y m i t e and c r i s t o b a l i t e . Am. Ceram. SOC. B u l l . , 36: 142-148. Ernst, W.G., and C a l v e r t , S.E., 1969. An experimental study o f t h e r e c r y s t a l l i z a t i o n o f p o r c e l a n i t e and i t s b e a r i n g on t h e o r i g i n o f some bedded cherts. Am. J. Sci., 267-A: 114-133. F i s h e r , R.V., 1977. Geologic guide t o subaqueous v o l c a n i c rocks i n t h e Nipomo, Pismo Beach and A v i l a Beach areas. Geol. SOC. Am. Penrose Conference on Subaqueous Volcanic Rocks, Santa Barbara, Nov. 1977, F i e l d t r i p guidebook, 29 p. Fournier, R.O., 1973. S i l i c a i n thermal waters: l a b o r a t o r y and f i e l d i n v e s t i g a t i o n s . I n : I n t . Symp. Hydrogeochem. Biochem., Tokyo, 1970, Proc., 1: 122139. Friedman, I. and Murata, K.J., 1979. O r i g i n o f dolomite i n Miocene Monterey Shale and r e l a t e d formations i n t h e Temblor Range, C a l i f o r n i a . Geochim. Cosmochim. Acta, 43: 1357-1365. Garrison, R.E., Douglas, R.G., P i s c i o t t o , K.A., Isaacs, C.M. and I n g l e , J.C., Jr. ( E d i t o r s ) , 1981. The Monterey Formation and Related S i l i c e o u s Rocks o f C a l i f o r n i a . P a c i f i c Sect. S.E.P.M. Spec. Publ., 327 p. Giger, W., and Schaffner, C., 1981. Unsaturated s t e r o i d hydrocarbons as i n d i c a t o r s o f diagenesis i n immature Monterey shales. Naturwissenschaften, 68: 3739. Govean, F.M., 1980. Some paleoecologic aspects o f t h e Monterey Formation, C a l i f o r n i a . Ph. D. t h e s i s , Univ. C a l i f . , Santa Cruz, C a l i f o r n i a , 278 p. Govean, F.M. , and Garrison, R.E. , 1981. S i g n i f i c a n c e o f laminated and massive d i a t o m i t e s i n t h e upper p a r t o f t h e Monterey Formation, C a l i f o r n i a . I n : R.E. Garrison e t a l . ( E d i t o r s ) , The Monterey Formation and Related S i l i c e o u s Rocks o f C a l i f o r n i a . P a c i f i c Sect. S.E.P.M. Spec. Publ., pp. 181-198. Graham, S.A., 1976a. T e r t i a r y sedimentary t e c t o n i c s o f t h e c e n t r a l S a l i n i a n Block o f C a l i f o r n i a . Ph.0. t h e s i s , S t a n f o r d Univ., Stanford, C a l i f o r n i a , 510 P. Graham, S.A., 1976b. T e r t i a r y s t r a t i g r a p h y and d e p o s i t i o n a l environments near F r i t s c h e , H. TerBest, Indians Ranch, Monterey County, C a l i f o r n i a . I n : A.E. Jr. and W.W. Wornardt ( E d i t o r s ) , The Neogene Symposium. P a c i f i c Sect. S.E.P.M. Spec. Publ., pp. 125-136. Grechin, V.I., P i s c i o t t o , K.A. , Mahoney, J.J. and Gordeeva, S.N. , 1981. Neogene s i l i c e o u s sediments and rocks o f f southern C a l i f o r n i a and Baja C a l i f o r n i a , Deep Sea D r i l l i n g P r o j e c t Leg 63. I n : R.S. Yeats, B.U. Haq 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 , 63. U.S. Govt. P r i n t . O f f . , Washington, D.C., pp. 579-593. Hamilton, E.L., 1976. V a r i a t i o n s o f d e n s i t y and p o r o s i t y w i t h depth i n deep-sea sediments. J. Sediment. Petrol., 46: 280-300. Hein, J.R., O ' N e i l l , J.R. and Jones, M.G., 1979. O r i g i n o f a u t h i g e n i c carbonates i n sediments from t h e deep B e r i n g Sea. Sedimentology, 26: 681-705. Hein, J.R., S c h o l l , D.W., Barron, J.A., Jones, M.G. and M i l l e r , J., 1978. 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 t h e southern Bering Sea. Sedimentology, 25: 155-181. Hein, J.R., V a l l i e r , T.L., and A l l a n , M.A., 1981. Chert p e t r o l o g y and geochemi s t r y , 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 : J.Thiede, T.L. V a l l i e r 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 , 62. U.S. Govt. P r i n t . O f f . , Washington, D.C., pp. 711748. Hislemann, J. and Emery, K.O., 1961. S t r a t i f i c a t i o n i n recent sediments o f Santa Barbara basin as c o n t r o l l e d by organisms and water character. J. Geol., 69: 279-290. I n g l e , J.C., Jr., 1973. Summary canments on Neogene b i o s t r a t i g r a p h y , physical s t r a t i g r a p h y , and paleoceanography i n t h e marginal n o r t h e a s t e r n P a c i f i c Ocean. I n : L.D. Kulm, R. von Huene e t a l . ( E d i t o r s ) , I n i t i a l Reports o f t h e

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Deep Sea D r i l l i n g P r o j e c t , 18. U.S. Govt. P r i n t . O f f . , Washington, D.C., pp. 949-960. I n g l e , J.C., Jr., 1975. Summary o f Paleogene-Neogene i n s u l a r s t r a t i g r a p h y , paleobathymetry, and c o r r e l a t i o n s , P h i l i p p i n e Sea and Sea o f Japan region. I n : D.E. Karig, J.C. I n g l e , Jr. 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 , 31. U.S. Govt. P r i n t . O f f . , Washington, D. C., pp. 837-855. I n g l e , J.C., Jr., 1980. Cenozoic paleobathymetry and d e p o s i t i o n a l h i s t o r y o f selected sequences w i t h i n t h e southern C a l i f o r n i a c o n t i n e n t a l border1 and. I n : W.V. S l i t e r ( E d i t o r ) , Studies i n micropaleontology. Cushman Found. Foram. Res. Spec. Publ., 19: 163-195. Jr., 1981a. Cenozoic d e p o s i t i o n a l h i s t o r y o f t h e n o r t h e r n c o n t i n I n g l e , J.C., e n t a l borderland o f Southern C a l i f o r n i a and t h e o r i g i n o f associated Miocene diatomites. I n : C.M. Isaacs ( E d i t o r ) , Guide t o t h e Monterey Formation i n t h e C a l i f o r n i a c o a s t a l area, Ventura t o San L u i s Obispo. P a c i f i c Sect. Am. Assoc. Pet. Geol. Spec. Publ., 52: 1-8. I n g l e , J.C., Jr., 1981b. O r i g i n o f Neogene d i a t o m i t e s around t h e n o r t h P a c i f i c r i m . I n : R.E. Garrison e t a l . ( E d i t o r s ) The Monterey Formation and r e l a t e d s i l i c e o u s rocks o f C a l i f o r n i a . P a c i f i c Sect. S.E.P.M. Spec. Publ., pp. 159179. I r w i n , H., C u r t i s , C.D. and Coleman, M., 1977. I s o t o p i c evidence f o r source o f d i a g e n e t i c carbonates formed d u r i n g b u r i a l o f o r g a n i c - r i c h sediments. Nature, 269: 209-213. Isaacs, C.M., 1980. Diagenesis i n t h e Monterey Formation examined l a t e r a l l y along t h e coast near Santa Barbara, C a l i f o r n i a . Ph.D. t h e s i s , Stanford Univ., Stanford, C a l i f o r n i a , 329 p. Isaacs, C.M. 1981a. F i e l d c h a r a c t e r i z a t i o n o f rocks i n t h e Monterey Formation along t h e Coast near Santa Barbara, C a l i f o r n i a . I n : C.M. Isaacs ( E d i t o r ) , Guide t o Monterey Formation i n t h e C a l i f o r n i a coastal area, Ventura t o San L u i s Obispo. P a c i f i c Sect. Am. Assoc. Pet. Geol. Spec. Publ., 52: 39-53. Isaacs, C.M., 1981b. L i t h o s t r a t i g r a p h y o f t h e Monterey Formation, Goleta t o P o i n t Conception, Santa Barbara Coast, C a l i f o r n i a . I n : C.M. Isaacs ( E d i t o r ) , Guide t o t h e Monterey Formation i n t h e C a l i f o r n i a coastal area, Ventura t o San L u i s Obispo. P a c i f i c Sect. Am. Assoc. Pet. Geol. Spec. Publ., 52: 9-23. Isaacs, C.M., 1981c. O u t l i n e o f diagenesis i n t h e Monterey Formation examined l a t e r a l l y along t h e Santa Barbara Coast, C a l i f o r n i a . I n : C.M. Isaacs ( E d i t o r ) , Guide t o t h e Monterey Formation i n t h e C a l i f o r n i a coastal area, Ventura t o San L u i s Dbispo. P a c i f i c Sect. Am. Assoc. Pet. Geol. Spec. Publ., 52: 25-38. Isaacs, C.M. , 1981d. P o r o s i t y r e d u c t i o n d u r i n g diagenesis o f t h e Monterey Formation, Santa Barbara c o a s t a l area, C a l i f o r n i a . I n : R.E. Garrison e t a l . ( E d i t o r s ) , The Monterey Formation and Related S i l i c e o u s Rocks o f C a l i f o r n i a . P a c i f i c Sect. S.E.P.M. Spec. Publ., pp. 257-271. Isaacs, C.M., 1982. I n f l u e n c e o f rock composition on k i n e t i c s o f s i l i c a phase changes i n t h e Monterey Formation, Santa Barbara area, C a l i f o r n i a . Geology ( i n press). Jones, J.B. and Segnit, E.R., 1971. The n a t u r e o f opal I. Nomenclature and cons t i t u e n t phases. J, Geol. SOC. A u s t r a l i a , 18: 57-68. Jones, J.B. and Segnit, E.R., 1972. Genesis o f c r i s t o b a l i t e and t r i d y m i t e a t l o w temperatures. J. Geol. SOC. A u s t r a l i a , 18: 419-422. Jones, J.B. and Segnit, E.R., 1975. Nomenclature and t h e s t r u c t u r e o f n a t u r a l d i s o r d e r e d ( o p a l i n e ) s i l i c a . Contrib. Mineral. Petrol., 51: 231-234. Kastner, M., Keene, J.B. and Gieskes, J.M., 1977. Diagenesis o f s i l i c e o u s oozes. I. Chemical c o n t r o l s on t h e r a t e o f opal-A diagenesis an experimental study. Geochim. Cosmochim. Acta, 41: 1041-1059. Keene, J.B., 1975. Cherts and p o r c e l l a n i t e s from t h e N o r t h P a c i f i c , Deep Sea D r i l l i n g P r o j e c t Leg 32. I n : R.L. Larson e t a l . ( E d i t o r s ) , I n i t i a l Reports Govt. P r i n t . O f f . , Washington, o f t h e Deep Sea D r i l l i n g P r o j e c t , 32. U.S. D.C., pp. 429-507. K e l l e r , M.A., 1982. L i t h o s t r a t i g r a p h y and diagenesis o f t h e Monterey Formation near D j a i , C a l i f o r n i a . Am. Assoc. Pet. Geol. Programs P a c i f i c Sect. Meet.,

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281 ( E d i t o r s ) , I n i t i a l Reports o f t h e Dee Sea D r i l l i n g P r o j e c t , 63. U.S. Govt. P r i n t . O f f . , Washington, D.C., pp. 5 9 c 6 0 9 . Redwine, L.E., 1981. Hypothesis combining d i l a t i o n , n a t u r a l h y d r a u l i c f r a c t u r ing, and d o l o m i t i z a t i o n t o e x p l a i n petroleum r e s e r v o i r s i n Monterey Shale, Santa Maria area, C a l i f o r n i a . I n : R.E. G a r r i s o n e t a l . ( E d i t o r s ) , The Monterey Formation and R e l a t e d S i l i c e o u s Rocks o f C a l i f o r n i a . P a c i f i c Sect. S.E.P.M. Spec. Publ., pp. 221-248. Roehl , P.O., 1981. D i l a t i o n b r e c c i a t i o n - - a proposed mechanism o f f r a c t u r i n g , petroleum e x p u l s i o n and d o l o m i t i z a t i o n i n t h e Monterey Formation, C a l i f o r n i a . I n : R.E. G a r r i s o n e t a l . ( E d i t o r s ) , t h e Monterey Formation and r e l a t e d s i l i c e o u s rocks o f C a l i f o r n i a . P a c i f i c Sect. S.E.P.M. Spec. Publ., pp. 285-315. Scholle, P.A., 1977. Chalk diagenesis and i t s r e l a t i o n t o petroleum explorat i o n : o i l from chalks, a modern m i r a c l e ? Am. Assoc. Pet. Geol. Bull., 61: 982- 1009. Sheldon, R.P., 1980. E p i s o d i c i t y o f phosphate d e p o s i t i o n and deep ocean c i r c u l a t i o n - - a hypothesis. In: Y.K. Bentor ( E d i t o r ) , Marine Phosphorites-Geochemistry, Occurrence, Genesis. SEPM Spec. Publ., 29: 239-247. Soutar, A., Johnson, S.R. and Baumgartner, T.R., 1981. I n search o f modern Garrison e t al. d e p o s i t i o n a l analogs t o t h e Monterey Formation. I n : R.E. ( E d i t o r s ) , The Montcrey Formation and R e l a t e d S i l i c e o u s Rocks o f C a l i f o r n i a . P a c i f i c Sect. S.E.P.M. Spec. Publ., pp. 123-147. Spotts, J.H. and Silverman, S.R., 1966. Organic d o l o m i t e from P o i n t Fermin, C a l i f o r n i a . Am. Mineral., 51: 1144-1155. Stein, C.L. and K i r k p a t r i c k , R.J., 1976. Experimental p o r c e l a n i t e r e c r y s t a l l i z a t i o n k i n e t i c s : a n u c l e a t i o n and growth model. J. Sediment. P e t r o l 46: 430-435. Stosur, J.J. and David, A,, 1976. P e t r o p h y s i c a l e v a l u a t i o n o f t h e d i a t o m i t e f o r m a t i o n o f t h e L o s t H i l l s f i e l d , C a l i f o r n i a . J. Petroleum Tech., 28: 11381144. Summerhayes, C.P., 1981. Oceanographic c o n t r o l s on o r g a n i c m a t t e r i n t h e Miocene Monterey Formation, o f f s h o r e C a l i f o r n i a . In: R.E. G a r r i s o n e t a l . ( E d i t o r s ) , The Monterey Formation and Related S i l i c e o u s Rocks o f C a l i f o r n i a . P a c i f i c Sect. S.E.P.M. Spec. Publ., pp. 213-219. Surdam, R.C., 1980. Diagenesis o f t h e Obispo Formation. I n : Am. Assoc. Pet. Geol. Short Course Notes on Sandstone Diagenesis, Monterey, C a l i f . , June, 1980. Surdam, R.C. and Stanley, K.O. , 1981a. S t r a t i g r a p h i c and sedimentologic framework o f t h e Monterey Formation, Pismo s y n c l i n e , C a l i f o r n i a . I n : C.M. Isaacs ( E d i t o r ) , Guide t o t h e Monterey Formation i n t h e C a l i f o r n i a coastal area, Ventura t o San L u i s Obispo. P a c i f i c Sect. Am. Assoc. Pet. Geol. Spec. Publ., 52: 83-91. Surdam, R.C. and Stanley, K.O., 1981b. Diagenesis and m i g r a t i o n o f hydrocarbons i n t h e Monterey Formation, Pismo s y n c l i n e , C a l i f o r n i a . In: R.E. G a r r i s o n e t a l . ( E d i t o r s ) , The Monterey Formation and Related S i l i c e o u s Rocks o f C a l i f o r n i a . P a c i f i c Sect. S.E.P.M. Spec. Publ pp. 317-327. Tal i a f e r r o , N.L., 1933. The r e l a t i o n o f volcanism t o diatomaceous and associated s i l i c e o u s sedjments. Univ. C a l i f . Publ., Dept. Geol. Sci. Bull., 23: 156. T a y l o r , J.C., 1976. Geologic a p p r a i s a l o f t h e petroleum p o t e n t i a l o f o f f s h o r e southern C a l i f o r n i a : t h e b o r d e r l a n d compared t o onshore c o a s t a l basins. U.S. Geol Surv. Circ., 730: 43 p. T i s s o t , B.P. and Welte, D.H., 1978. Petroleum Formation and Occurrence. Springer-Verlag, New York, N.Y., 538 p. V a i l , P.R. and Hardenbol, J., 1979. Sea-level changes d u r i n g t h e T e r t i a r y . Oceanus, 22: 71-79. Veeh, H.H., B u r n e t t , W.C. and Soutar, A., 1973. Contemporary p h o s p h o r i t e on t h e c o n t i n e n t a l margin o f f Peru, Science, 181: 845-847. von Rad, U., Riech, V. and Rosch, H., 1977. S i l i c a diagenesis i n c o n t i n e n t a l margin sediments o f f northwest A f r i c a . In: Y. Lancelot, E. S e i b o l d 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 , 41. U.S. Govt.

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.,

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CHAPTER 16 DIAGENESIS OF JURASSIC SILICEOUS SHALE I N CENTRAL JAPAN S. MIZUTANI' and K. SHIBATAL

'Department o f E a r t h Sciences, Nagoya U n i v e r s i t y , Nagoya 464 (Japan) 2Geological Survey o f Japan, Yatabe, I b a r a k i 305 (Japan) S i l i c e o u s shales o f t h e J u r a s s i c Mazegawa Formation i n t e r c a l a t e d w i t h i n a t h i c k s e c t i o n o f b l a c k shales a r e described. Chemical compositions o f cons t i t u e n t m i n e r a l s determined w i t h t h e electronprobe microanalyser r e v e a l t h a t rhodochrosite, dolomite, and c a l c i t e c o e x i s t i n a s i n g l e sample. Dolomite A cryptocrystalline fillrhombohedra a r e commonly rimmed w i t h rhodochrosite. i n g o f a r a d i o l a r i a n s k e l e t o n i s composed o f s i l i c a , c h l o r i t e , and micaceous c l a y mineral. B i o s t r a t i g r a p h i c a l l y , t h e Mazegawa Formation i s assigned t o t h e lower T i t h o n i a n (approximately 145 Ma), whereas whole-rock samples g i v e a Rb-Sr isochron age o f 128 f 3 Ma. K - A r ages f o r f o u r o f t h e same group o f samples average The age o f 128 ?la i s i n t e r p r e t e d as 129 Ma, ranging from 125 Ma t o 133 Ma. i n d i c a t i n g t h e time when t h e whole rock became c h e m i c a l l y closed w i t h respect t o t h e Rb-Sr system. The t i m e d i f f e r e n c e (145 - 128 = 17 Ma) represents t h e d u r a t i o n o f chemical diagenesis b e f o r e t h e system was closed. 1

INTRODUCTION

Diagenesis, d u r i n g which s o f t sediments g r a d u a l l y become hard rocks, i s undoubtedly a continuous process.

We can e x a c t l y d e f i n e t h e beginning o f d i a -

genesis as t h e t i m e o f d e p o s i t i o n , b u t t h e end o f t h e process i s d i f f i c u l t t o recognize.

On t h e o t h e r hand, t h e i s o t o p i c age o f a sedimentary rock repre-

sents t h e t i m e when t h e r o c k became a closed system w i t h r e s p e c t t o t h e chemi c a l species measured.

T h e o r e t i c a l l y , t h e r e should be a d i f f e r e n c e i n t i m e

between a f o s s i l age and an i s o t o p i c age, because t h e former g i v e s us an age of d e p o s i t i o n , whereas t h e l a t t e r s i g n i f i e s t h e end o f chemical diagenesis.

If

r e l i a b l e age data a r e o b t a i n e d by b o t h methods, we can l i m i t t h e d u r a t i o n o r time-span o f chemical diagenesis.

Since diagenesis i s s t r o n g l y c o n t r o l l e d by

temperature, and t h e process continues w i t h time, t h e d u r a t i o n i s e s s e n t i a l l y r e l a t e d t o t h e thermal h i s t o r y t h a t t h e sediments experienced i n t h e course o f accumulation, b u r i a l and ensuing u p l i f t . I n t h i s paper, we d e s c r i b e t h e Upper J u r a s s i c Mazegawa Formation i n t h e Mino Terrane, c e n t r a l Japan, c o n c e n t r a t i n g on t h e m i c r o p a l e o n t o l o g i c , m i n e r a l o g i c , and geochronologic aspects o f t h e s i l i c e o u s shales, and discuss t h e diagenesis o f t h e s i l i c e o u s shales i n terms o f temperature and time. 2

GEOLOGIC SETTING I n t h e Mino Terrane ( F i g . 1 ), heterogeneous assemblages o f graywacke sand-

284

stone, shale, c h e r t , greenstone, and limestone occur i n places associated w i t h a small amount o f i n t r a f o r m a t i o n a l conglomerate. from Late Paleozoic t o J u r a s s i c (Mizutani e t a l . ,

The sediments range i n age 1981).

The Upper J u r a s s i c

Mazegawa Formation from which samples o f t h i s study were taken, i s w e l l exposed along t h e banks of Mazegawa (Maze R i v e r ) o f Hida-Kanayama i n t h e c e n t r a l p a r t of t h e Mino Terrane (Mizutani, 1981). The Mazegawa Formation i s composed of s i l i c e o u s shale, more than 30 m i n thickness, g r a d i n g upward t o b l a c k shale through a sequence o f f i n e l y bedded s i l i c e o u s shale a l t e r n a t i n g w i t h b l a c k shale ( F i g s . 1 and 5 ) .

Bedded c h e r t s e x h i b i t i n g i n t r a f o r m a t i o n a l f o l d s do n o t

i n t e r f i n g e r w i t h t h e b l a c k shale, nor w i t h t h e s i l i c e o u s shale, b u t a r e found as e x o t i c blocks.

Judging from t h e l i t h o l o g i c s i m i l a r i t i e s o f t h e c h e r t s t o

t h e T r i a s s i c c h e r t s i n t h e Mino Terrane, t h e y a r e b e l i e v e d t o be o f T r i a s s i c age.

The c h e r t s i n t h e Mazegawa area, t h e r e f o r e , occur as an o l i s t o l i t h o r

allochthonous sheet embedded i n t h e Upper J u r a s s i c sedimentary rocks. Sandstone blocks a r e a l s o jumbled i n t h e b l a c k shale.

A s m a l l - s c a l e quartz-porphyry dike, probably o f L a t e Mesozoic age, d i s c o r d a n t l y i n t r u d e s t h e sedimentary rocks.

The c o u n t r y r o c k , however, shows no

evidence o f a l t e r a t i o n o r thermal e f f e c t s . 3

PETROGRAPHY OF SILICEOUS SHALE Most s i l i c e o u s shales a r e dark gray o r dark greenish gray, i n places showing

s t r a t i f i c a t i o n p a r a l l e l t o which weak f i s s i l i t y i s sometimes developed ( F i g . 5). I n massive and more s i l i c e o u s p a r t s , conchoidal f r a c t u r e s a r e i n v a r i a b l y observed.

Some s i l i c e o u s shales a r e much darker i n c o l o r owing t o manganese

o x i d e which was converted from primary manganese carbonate contained i n t h e rock.

Manganese carbonate ( r h o d o c h r o s i t e ) nodules a1 so occur though r a r e l y ,

i n t h e Mazegawa Formation and i n t h e o v e r l y i n g b l a c k shale. The s i l i c e o u s shales c o n t a i n v a r y i n g amounts o f r a d i o l a r i a n remains enclosed i n a m a t r i x composed o f v e r y f i n e - g r a i n e d f l a k y m i n e r a l s and c r y p t o c r y s t a l l i n e material.

Subcyl i n d r i c a l o r subconical s h e l l s o f r a d i o l a r i a n f o s s i l s 1 i e sub-

p a r a l l e l t o laminae, and a r e s l i g h t l y deformed, though scarcely, as i f they were compressed from a d i r e c t i o n p e r p e n d i c u l a r t o t h e bedding plane.

The

s h e l l s a r e always made up o f anhedral aggregates o f m i c r o c r y s t a l l i n e q u a r t z o r chalcedonic q u a r t z ( l e n g t h - f a s t ) , whereas most o f t h e i n n e r chambers a r e f i l l e d w i t h c r y p t o c r y s t a l l i n e m i n e r a l s so small i n s i z e t h a t i n d i v i d u a l g r a i n s cannot be i d e n t i f i e d under t h e o p t i c a l microscope. An e l e c t r o n p r o b e m i c r o a n a l y s i s o f t h i s c r y p t o c r y s t a l l i n e m a t e r i a l i n a r a d i o l a r i a n chamber from manganiferous s i l i c e o u s shale (SM04) c o l l e c t e d from a m i d d l e p a r t of t h e Mazegawa Formation, suggests a composition r o u g h l y o f s i l i c a (1/4), c h l o r i t e (1/4), and mica c l a y (1/2) (see Table 1 ) .

A l l these

285

sandstone

bggg

siliceous shale (Late Jurassic)

bedded c h e r t ( T r i a s s i c ? )

F i g . 1.

Geologic map o f t h e Mazegawa a r e a , Hida-Kanayama, c e n t r a l Japan. The Mino Terrane i s shown i n t h e i n s e t by a s t i p p l e d p a t t e r n .

286

minerals are derived from clayey material and radiolarian tests in the primary sediments and crystallized supposedly during early stages of diagenesis.

A1

A

: dolomite rimmed with rhodochrosite

-

microcrystal 1 ine aggregate rhodochrosite

' of

microcrystal 1 ine lomite rimmed with

Fig. 2.

Sketch showing paragenesis and analytical spots of carbonate minerals and of radiolarian test filled with clay minerals in sample SM04.

The Mazegawa Formation is composed wholly of siliceous shale, which usually includes carbonate minerals showing some partial varieties in composition. Optically, two types of carbonate minerals are distinctly observed; one is euhedral and generally rhombohedral, approximately 0.1 - 0.2 mm in diameter as "A", "B", or "C" in Fig. 2, and the other is a cloudy aggregate (0.05 - 0.2 INII in diameter) of anhedral.microcrystall ine carbonate grains as "0" and "E" in Fig. 2. Curiously, almost all of the carbonate grains of the rhombohedral form are zoned, i.e. euhedral grains are rimmed with a syntaxial growth of a carbonate mineral of higher refractive index (Figs. 2 and 3 ) . Chemical compositions of these carbonate minerals, together with the calculated atomic percent ++ ++ ++ of Ca", Mg , Fe , and Mn , show that assemblages of microcrystalline grains ("0" and "E" in Fig. 2) are Mn-carbonate, whereas euhedral grains are Ca-carbonEuhedral grains of differate (Bl) and Ca-Mg carbonate (Al, A2, C1, and C2). ent compositions are commonly rimmed with Mn-carbonate - A1 and A2 rimmed with

287

Ax and Ay, C1 and C2 w i t h Cx, and B1 w i t h Bx ( F i g s . 2 and 4 ) . Chemical compositions and morphological p r o p e r t i e s o f these carbonate minera l s suggest t h a t m i c r o c r y s t a l l i n e aggregates, u b i q u i t o u s l y disseminated i n t h i s manganiferous s i l i c e o u s shale, a r e primary carbonates. t r i b u t i o n o f minerals i n t h i n sections

Judging from t h e d i s -

F i g . 2 ) , however, t h e m i g r a t i o n of chem-

i c a l species took p l a c e i n a r e s t r i c t e d small volume w i t h i n a domain o f t h e 3 o r d e r o f 0.1 mm i n s i z e . TABLE 1 E l e c t r o n microprobe a n a l y s i s o f c r y p t o c r y s t a l l i n e aggregate o f c l a y m i n e r a l s ( " R " i n F i g . 2 ) i n manganiferous s i l i c e o u s shale (SM04).

R ( i n SM04) ~

(i)x1/4 + (ii)xl/4 + ( i ii ) x l / 2

(i) quartz

(ii) chlorite Shirozu(l978)

(iii) mica c l a y Shimoda e t a l . (1 969)

100.00

~~

Si02

56.11

55.73

26.04

48.44

Ti02

0.02

tr

-

tr

21.95

21.91

19.96

33.84

Fe203 FeO

*

0.71

1.85

0.49

4.97

5.34

21.34

MnO

0.34

0.12

0.47

CaO

0.04

0.06

-

0.11

MgO Na20

3.56

5.12

18.56

0.95

0.01

0.25

0.50

7.70

4.70

-

4.51

6.00

99.21

99.94

2'3

K2°

H2° total

*

11 .78**

I

measured as FeO

100.00

100~.00

**

-

tr

9.40

6.02** 99.75

H20(+) + H20(-)

S i l i c e o u s shale o f t h e Mazegawa Formation c o n t a i n s s i l t - s i z e d c l a s t i c g r a i n s of q u a r t z and p l a g i o c l a s e i n small amount, g e n e r a l l y l e s s than 1 %. The presence of a v o l c a n i c r o c k fragment having a v i t r o c l a s t i c t e x t u r e i n d i c a t e s volcanism a t t h e t i m e o f d e p o s i t i o n .

X-ray examination r e v e a l s t h a t i n t h e s i l i c e o u s

shale, quartz, p l a g i o c l a s e , i l l i t e , and c h l o r i t e a r e common; i n a d d i t i o n , rhodoc h r o s i t e occurs i n manganiferous s i l i c e o u s shale.

I n sample SM02, a 5 mn t h i c k

l a y e r i s interbedded i n which s i l t and fine-sand s i z e d c l a s t i c g r a i n s o f quartz, fragments o f shale, s i l t s t o n e , c h e r t , v o l c a n i c rock, p o r p h y r i t i c rock, and q u a r t z o - f e l d s p a t h i c rock occur.

As w i l l be discussed l a t e r , some o f these

c l a s t i c g r a i n s a r e i n f e r r e d t o have been d e r i v e d from much o l d e r t e r r a n e s i n t h e provenance.

288

A1

A2

Ax

CaO

36.9

36.4

10.2

MgO FeO

20.9

19.5

MnO

Ay

B1

Bx

C1

C2

Cx

Dx

Dy

Ex 7.17

9.29 56.3

9.50 33.8

36.2

8.02

5.32

6.12

2.86

2.31

2.01

2.13 20.8

21.3

1.89

1.83

2.20

1.77

0.31

0.31

0.49

0.37

0.24

0.41

0.37

0.13

0.17

0.10

0.24

0.13 46.7

48.9

0.11 48.2

0.15

0.10

0.29

0.15 51.1

55.9

53.0

55.6

158.21 56.13 60.07 60.81 58.91 60.20 55.04 57.75 61.25 63.46 61.69 64.67 Ca++

55.7

57.2

19.9

18.1

Mg++

43.8

42.6

Fe++

0.2

0.1

0.5

0.5

Mn++

0.3

0.1

71.9

75.2

F i g . 3.

7.7

6.2

94.4 4.7

18.7

53.6

54.8 44.9

15.7

10.2

11.9

13.4

5.2

4.8

6.0

4.6

5.9

45.9

0.7

0.5

0.2

0.1

0.3

0.6

0.5

0.2

0.2

74.9

0.3

0.2

78.8

54.4

81.6

81.8

Photomicrograph showing d o l o m i t e rhombohedra r i m e d w i t h rhodoc h r o s i t e ( " A " i n F i g . 2) i n sample SM04.

289

Mg magnesi te

Ca calcite Fig. 4.

4

’I

Bx

\

Mn

CI Ex rhodochrosite

Compositions of carbonate minerals in manganiferous siliceous shale (SM04) plotted on Ca-Mg-Mn diagram (cf. Fig. 2 and Table 2).

BIOSTRATIGRAPHY Radiolarian remains are a major constituent of siliceous shale of the Mazegawa Formation. Mesozoic radiolarian biostratigraphy has advanced in recent years principally as the result of development of laboratory techniques for extracting fossils, the use of the scanning electron microscope, and the accumulation of data from deep-sea sections. Using these methods, Mizutani (1981) carried out a biostratigraphic study on the Hazegawa Formation and concluded that the radiolarian fossils are of Late Jurassic aqe (Table 3). An extensive survey of the Mino Terrane disclosed the widespread occurrence of Jurassic rocks in this region, the youngest Jurassic rocks being the Flazegawa Formation characterized by radiolarian fossil association tentatively called the !ti&fusus baileyi Assemblage (Mizutani et a1 ., 1981). Since no significant difference exists in radiolarian fossil associations between the upper and the lower parts of the rlazegawa Formation, the radiolarian species listed in Table 3 are interpreted to be of a single assemblage; the abundant or characteristic species are Mirifusus baileyi Pessagno, Parvicingula mashitaensis n. sp., and Eucyrtidium ptyctum Riedel and Sanfilippo. This assemblage is correlative with that of Zone 2 of the radiolarian zonation established by Pessagno (1977) for the California Coast Ranges. His zonation, biostratigraphically integrated with the Buchia zonation of Jones et al. (1969)

290

10 8

6

SCALE

4 2 0

dark gray(N3) & greenish-gray(5G6/1) finely bedded bk.sh. & sil. shale dark greenish gray(5GY4/1) sil. Sh. medium dark gray(N4) sil. shale dark greenish gray(5G4/1) siliceous tuffaceous shale dark greenish gray(5G4/1) siliceous tuffaceous shale medium dark gray(N4) sil. shale medium dark gray(N4) sil. shale medium dark gray(N4) sil. shale brownish gray(5YR4/1) sil. shale medium gray(N5) siliceous shale dark gray(N3) manganiferous siliceous shale dark greenish gray(5GY4/1) sil. sh. medium dark gray(N4) sil. shale dark greenish gray(5GY4/1) sil. sh. medium gray(N5) shale, very siliceous medium gray(N5) siliceous shale dark greenish gray(5GY4/1) sil. Sh. SMn6

Fig. 5.

---- A ---

dark greenish gray(5GY4/1) sil. Sh.

Stratigraphic section o f the Mazegawa Formation with description o f samples. Rock color is expressed by designations from the Geological Society o f America's Color Chart.

291

TABLE 3 Radiolarian species in the Mazegawa Formation after Mizutani (1981). SAMPLES

A B C D E F G H I J K L M N O P Q R

Archaeospongoprunum imlayi Archaeospongoprunum sp. A Tritrabs cfr. ewingi Paronaella sp. Orbiculiforma sakaii n. sp. Orbiculiforma kanayamaensis n. sp. Emiluvia sp. 8 Praeconocaryomma sp. 9 Pantanellium cfr. riedeli 10 Pantanellium sp. A 1 1 Tripocyclia blakei 12 Acaeniotyle diaphorogona 13 Will iriedel lum cfr. crystal 1 inum 14 Archaeodictyornitra sp. A 15 Hsuum maxwelli 16 Parvicingula mashitaensis n. sp. 1 7 Mi ri fu sus ba i 1 ey i 18 Pseudodictyomitra minoensis n. sp. 19 Pseudodictyomitra okamurai n. sp. 20 Pseudodictyomitra sp. A 21 Spongocapsula sp. A 22 Spongocapsula sp. B 23 Spongocapsula sp. C 24 Xitus gifuensis n. sp. 25 Protunuma fusiformis 26 Podobursa tr i acan tha 27 Eucyrtidium ptycturn 28 Pseudoeucyrtis sp. 29 Napora sp. 30 Saitoum sp.

r

SAMPLES for biostratigraphic study

A B C D E F G H I J K L M N O P Q R

1 2 3 4 5 6 7

r r C r C A r r r

c

c

r r r r r C r r C r r r C r r C

C r r

C r r r r r r

r

C r r C C C C r r r r C C r C C A r r r C r r

C r C r r A r r

r C C C C r r r r

r

A r A r r r r r r r r r r C C C

r C C r r r r r r r r r

C r r C A A r r r r r r

A C A r r r r r r r r r

1 2 3 4 5 6

r C

1

8 r 9 10 11 12 13 r 14 15 r 16

17 18 19 r 20 21 22 23 24 25 26 27 28 29 30

292

and w i t h ammonites described by Imlay and Jones (1970), demonstrates t h a t Zone 2 i s lower T i t h o n i a n . P a r v i c i n g u l a mashitaensis n. sp. bears a c l o s e resemblance

t o P. b o e s i i

(Parona) described by Baumgartner e t a l . (1980), who t r e a t e d t h i s species t o g e t h e r w i t h o t h e r ones c o l l e c t i v e l y as t h e P a r v i c i n g u l a b o e s i i (Parona) group. They a l s o described M i r i f u s u s b a i l e y i Pessagno as one o f t h e species o f t h e M i r i f u s u s m e d i o d i l a t a t u s (Rust) group.

R a d i o l a r i a n f o s s i l s o f these groups

a r e r e p o r t e d by Foreman (1978) from t h e A t l a n t i c deep-sea cores o f Leg 41, c o r r e l a t i v e w i t h t h e Kimmeridgian

-

Tithonian.

As l i s t e d i n Table 3, however,

M i r i f u s u s b a i l e y i Assemblage o f t h e Mazegawa Formation c a r r i e s r a d i o l a r i a n f o s s i l s o f t h e E a r l y Cretaceous type.

For example, A c a e n i o t y l e diaphorogona

Foreman i s found from deep-sea cores o f t h e E a r l y Cretaceous (Foreman, 1973, p.258,

pl.2,

figs.2-5)

p.613,

p1.2HY f i g . l l ) ,

and D i c t y o m i t r a b o e s i i Parona described by Foreman (1975, v e r y s i m i l a r t o P a r v i c i n g u l a mashitaensis n. sp.,

found i n deep-sea cores assigned t o t h e E a r l y Cretaceous. i n Table 3, though

is

R a d i o l a r i a n species

d i v e r s i f i e d , i n d i c a t e t h a t t h e Mazegawa Formation i s b i o -

s t r a t i g r a p h i c a l l y c o r r e l a t i v e w i t h t h e T i t h o n i a n (Mizutani, 1981). 5

ISOTOPIC AGE S i l i c e o u s shale o f t h e Mazegawa Formation was dated by K-Ar and by Rb-Sr

methods (Shibata and M i z u t a n i , 1980).

The Rb-Sr whole-rock i s o c h r o n has,

except f o r one sample (SM05). an apparent age o f 128 87Sr/86Sr

r a t i o o f 0.7155 f 0.0004 ( F i g . 6).

f

3 Ma w i t h an i n i t i a l

I n a d d i t i o n , t h e K-Ar whole r o c k

ages average 129 Ma f o r f o u r samples f a l l i n g on t h e Rb-Sr i s o c h r o n p l o t .

This

concordant r e s u l t i n d i c a t e s t h e t i m e when t h e r o c k became c l o s e d t o t h e migrat i o n o f elements, a t l e a s t w i t h r e s p e c t t o elements i n t h e Rb-Sr and K-Ar systems. Two samples, however, do n o t f i t w e l l w i t h t h e age r e s u l t mentioned above. Sample SM05, t h e Rb-Sr p l o t o f which f a l l s s i g n i f i c a n t l y o f f t h e isochron, has a K - A r age o f 157 f 5 Ma.

I n t e r e s t i n g l y , a Rb-Sr model age f o r t h i s sample

was c a l c u l a t e d t o be 158 Ma by assuming t h e i n i t i a l r a t i o o f 87Sr/86Sr

= 0.7155.

As discussed by Shibata and M i z u t a n i (1980), sample SM05 i s i n t e r p r e t e d t o cont a i n much more d e t r i t a l than d i a g e n e t i c m a t e r i a l .

On t h e o t h e r hand, sample

SM02 has a K - A r age o f 165 f 4 Ma, d e s p i t e t h e f a c t t h a t i t l i e s on t h e Rb-Sr i s o c h r o n p l o t o f 128 Ma.

Petrographic examination r e v e a l s t h a t t h i s sample

c o n t a i n s coarser c l a s t i c fragments and i s v e r y heterogeneous i n composition. The d i f f e r e n c e between t h e K-Ar age and t h e Rb-Sr isochron f o r t h i s sample i s , therefore,

a t t r i b u t e d t o d i f f e r e n c e s i n composition o f t h e samples used f o r age

determinations. The i n i t i a l 87Sr/

86

S r r a t i o f o r s i l i c e o u s shale o f t h e Mazegawa Formation

(Ro = 0.7155 f 0.0004) i s c o n s i d e r a b l y h i g h e r than t h a t o f contemporaneous

293

87s 0.77 0.76

/

K-Ar whole-rock age

0.75 SM10

0.74

/

0.73 0.72

-

6

z- T

1

,*'

S M O ~SMOl~ ~ ~ ,' 9402 - _ _ _ _- --'

/R,

=

128 Ma:SM09

T = 133 Ma:SM10 ___________ - - - - - ,----- T = 125 130 Ma:SM04 Ma:SM06 ,r--

T

=

129 Ma

= 0.7155 f 0.0004

0.71 0.70

0

Fig. 6.

5

10

15 20 87Rb/86Sr

25

30

Isotopic ages of the Mazegawa Formation after Shibata and Mizutani ( 1980).

marine Sr (Ro = 0.707, Peterman et al., 1970). Evidently, the ratio in sediments is controlled by the overall average of the 87Sr/ 86Sr ratio of the source material as well as of the Rb/Sr ratio and age. The high initial 87Sr/86Sr ratio for siliceous shale of the Mazegawa Formation shows that Sr had not equilibrated with marine Sr; rather the isotopic composition was controlled by the detrital material, suggesting the existence o f older rocks in the provenance. As a matter of fact, Precambrian metamorphic clasts occur in the Jurassic Kamiaso conglomerate exposed about 15 km south of Hida-Kanayama (Shibata et al., 1971). Precambrian rocks with a 2000 Ma age (Shibata and Adachi, 1974) must have supplied the depositional area with much detrital material.

DISCUSSION The Rb-Sr whole-rock isochron age, 128 Ma, and K-Ar ages, averaging 129 Ma of the siliceous shales of the Mazegawa Formation correspond to the Hauterivian. Generally, K-Ar ages are more readily affected by tectonic movements if the rock is raised in temperature than a Rb-Sr isochron age. In the present study, as 6, there exists a concordance between K-Ar and Rb-Sr ages; conseshown in Fig. 6

294

quently, tectonic movements had little, if any, influence on the chemical system. Geologically, the siliceous shales of the Mazegawa Formation had not been involved in any thermal event before 128 Ma except burial. These geological and geochemical data strongly indicate that the age of 128 Ma represents the time of diagenesis, more strictly, the time when the chemical system was closed during burial to migration of the chemical species concerned. The radiolarian assemblage identified in the Mazegawa Formation is Tithonian. This would be about 145 Ma according to the time scale of Armstrong and McDowall (1974) or about 140 Ma according to that of Van Hinte (1976). Since the fossil age denotes the age of deposition, the time difference (145 - 128 = 17 Ma, if Armstrong and McDowall's time scale is adopted) shows the time-span of extensive chemical diagenesis (Fig. 7). It is highly probable that by 128 Ma ago the system was closed to migration of most chemical species over distance of greater than centimeters. Carbonate minerals in the siliceous shales indicate, however, distinct compositional differences over small distances of the order of 0.1 mrn. In the last stage of chemical diagenesis, material transfer and reorganization in which the system Composias a whole equilibrated had to be restricted to such small domains. tional changes observed under the microscope from Mn-carbonate ("0" or "E" in Fig. 2) to dolomite ("A" or "C" in Fig. 2) on the one hand, and from Mn-carbonate to calcite ( " 6 " in Fig. 2) on the other, both having terminated with crystallization of rhodochrosite rims may be three-phase equilibrium at a low temperature (cf. Goldsmith and Graf, 1960). This peculiar trend of compositional change in carbonate minerals is ascribed to the composition of the starting carbonate material and the chemical environment, i .e. alteration of Mn-carbonate in a marine environment. Microcrystalline aggregates of anhedral rhodochrosite disseminated in manganiferous siliceous shale are presumed to have been formed as a primary precipitate together with the deposition of other fine-grained materials, but the origin of the manganese remains uncertain. All the radiolarian tests in the siliceous shales of the Mazegawa Formation are composed of microcrystalline quartz; the inner chamber of the skeletons were invariably filled with chalcedonic quartz or a cryptocrystalline aggregate of silica, chlorite, and/or micaceous clay mineral. The test consisted originally of opaline silica, the present quartz mineralogy resulting from diagenesis. The time required for formation of quartz under deep-sea conditions i s estimated at 40 - 100 Ma (Siever, 1979); Riech and von Rad (1979), for example, stated Chemical that quartz chert is found in deep-sea sediments older than 60 Ma. and mineralogic changes in sediments are essentially kinetic processes tending towards equilibrium. As discussed for silica minerals by Mizutani (1970, 1977), Utada (1974), Murata et al. (1979), and Pisciotto (1981), the rates of change are controlled by temperature, hence the whole process is dependent on thermal

295

history. formations i n Japan, where subsurface temperatures a r e thought t o have been much h i g h e r than those beneath t h e sea bottom; Utada

clay: carbonate:

illite

detrital clay Mn-carbonate

s Td (in nann )Tj 0.0aom;

calcite dolomite-rhodochrosite

296

To date, our knowledge o f t h e d u r a t i o n o f chemical diagenesis as t r e a t e d i n t h i s paper i s i n s u f f i c i e n t , and d i s c u s s i o n cannot be extended f u r t h e r .

I t can be

speculated, however, t h a t t h e d u r a t i o n o f chemical diagenesis i s governed by t e c t o n i c c o n d i t i o n s , and t h e r e s u l t o f t h i s study o f f e r s an example o f d i a genesis o f sediments i n t h e West P a c i f i c area. ACKNOWLEDGEMENTS We a r e much indebted t o D r . Kazuhiro Suzuki o f Nagoya U n i v e r s i t y f o r making e l e c t r o n microprobe analyses.

D r . E. A. Pessagno, J r . , U n i v e r s i t y o f Texas a t

Dallas, and D r . K. Nakaseko, Osaka U n i v e r s i t y , gave f r u i t f u l suggestions on t h e r a d i o l a r i a n b i o s t r a t i g r a p h y , f o r which we would l i k e t o r e c o r d our g r a t i t u d e . Thanks a r e extended a l s o t o D r . R. Siever o f Harvard U n i v e r s i t y and D r . J. R. Hein o f U. S. Geological Survey, who read an e a r l y d r a f t o f t h i s paper o f f e r i n g many i n s t r u c t i v e comments. REFERENCES Armstrong, R.L. and McDowall, W.G., 1974. Proposed refinement o f t h e Phanerozoic time scale. Abs. I n t . Mtg. Geochron. Cosmochron. I s o t o p e Geology ( P a r i s ) . Azuma, Y., 1979. Middle Miocene c h e r t from t h e Niu Mountains, Fukui Prefecture, c e n t r a l Japan ( i n Japanese w i t h E n g l i s h a b s t r a c t ) . J. Geol. SOC. Japan, 85: 59-66. Baumgartner, P.O., De Wever, P., and Kochert, R., 1980. C o r r e l a t i o n o f Tethyan l a t e J u r a s s i c - e a r l y Cretaceous r a d i o l a r i a n events. Cah. Micropal., 1980.2: 23-72. Eberl, D. and Hower, J., 1976. K i n e t i c s o f i l l i t e formation. Geol. SOC. Amer. B u l l , 87: 1326-1 330. Ernst, W:G. and C a l v e r t , S.E., 1969. An experimental study o f t h e r e c r y s t a l l i z a t i o n o f p o r c e l l a n i t e and i t s b e a r i n g on t h e o r i g i n o f some bedded c h e r t . Amer. J. Sci., 267-A:114-133. Foreman, H., 1973. R a d i o l a r i a from DSDP Leg 20. I n Heezen, B.C. e t a l . ( E d i t o r s ) , U. S. Govt., P r i n t . I n i t . Rept. Deep Sea D r i l l i n g P r o j e c t , Washington, D.C., O f f i c e , 20:249-305. Foreman, H . , 1975. R a d i o l a r i a from t h e N o r t h P a c i f i c , Deep Sea D r i l l i n g P r o j e c t , Leg 32. Larson, R.L. e t a l . ( E d i t o r s ) , I n i t . Rept. Deep Sea D r i l l i n g P r o j e c t , Washington, D.C., U. S . Govt. P r i n t . O f f i c e , 32: 579-676. Foreman, H . , 1978. Mesozoic R a d i o l a r i a i n t h e A t l a n t i c Ocean o f f t h e northwest coast o f A f r i c a , Deep Sea D r i l l i n g P r o j e c t , Leg 41. I n Lancelot, Y. e t a l . ( E d i t o r s ) , . I n i t . Rept. Deep Sea D r i l l i n g P r o j e c t , Washington, D.C., U. S . Govt. P r i n t . O f f i c e , 41 :739-761. Goldsmith, J.R. and Graf, D.L., 1960. Subsolidus r e l a t i o n s i n t h e system CaC03J. Geol., 68:324-335. MgCD -MnCO Imlay, 3R.W. 2nd Jones, D.L., 1970. Ammonites from t h e Buchia zones i n n o r t h western C a l i f o r n i a and southeastern Oregon. U. S. Geol. Surv., P r o f . Paper 647-B:Bl-859. Jones, D.L. , B a i l e y , E.H. and Imlay, R.W., 1969. S t r u c t u r a l and s t r a t i g r a p h i c Paskenta area, s i g n i f i c a n c e o f t h e Buchia zones i n t h e Colyear Springs C a l i f o r n i a . U. G. Geol. Surv., P r o f . Paper 647-A:Al-A24. Laudise, R.A., 1959. K i n e t i c s o f hydrothermal q u a r t z c r y s t a l l i z a t i o n . J. Amer. Chem. SOC., 81 :562-566.

.

.

-

291

Matsumaru, K., Mizuno, E., and Azuma, Y . , 1980. C o n s i d e r a t i o n concerning t o t h e Mioq sina-0 e r c u l i n a f o s s i l assemblages found from t h e Kaetsu r e g i o n , Fukui P r e f Z F t u r e *Japanese w i t h E n g l i s h a b s t r a c t ) . J. Saitama Univ. Fac. Educ., 29:51-58. M i z u t a n i , S., 1966. Transformation o f s i l i c a under hydrothermal c o n d i t i o n s . J. E a r t h Sci., Nagoya Univ., 14:56-88. M i z u t a n i , S., 1970. S i l i c a m i n e r a l s i n t h e e a r l y stage o f d i a g e n e s i s . SedimentOlOgy,l5:419-436. M i z u t a n i , S., 1977. P r o g r e s s i v e o r d e r i n g o f c r y s t o b a l i t i c s i l i c a i n t h e e a r l y stage o f diagenesis. Contr. M i n e r a l . P e t r o l . , 61:129-139. M i z u t a n i , S., 1981. A J u r a s s i c f o r m a t i o n i n t h e Hida-Kanayama area, c e n t r a l Japan ( i n Japanese w i t h E n g l i s h Appendix and d e s c r i p t i o n ) , B u l l . Mizunami Fossi 1 Museum, 8 : 147-1 90. M i z u t a n i , S., H a t t o r i , I.,Adachi, M., Wakita, K., Okamura, Y . , Kido, S., Kawaguchi, I . , and Kojima, S., 1981. J u r a s s i c f o r m a t i o n s i n t h e Mino area, c e n t r a l Japan. Proc. Japan Acad., 57:194-199. Murata, K.J., Dibblee, T.W., Jr. and Drinkwater, J.L., 1979. Thermal e f f e c t s o f l a r g e bodies o f i n t r u s i v e s e r p e n t i n i t e on o v e r l y i n g Monterey shale, southern D i a b l o Range, Cholame area, C a l i f o r n i a . U. S. Geol. Surv., P r o f . Paper, 1082: 1-8. Pessagno, E.A., J r . 1977. Upper J u r a s s i c R a d i o l a r i a and r a d i o l a r i a n b i o s t r a t i g raphy o f t h e C a l i f o r n i a Coast Ranges. Micropaleontology, 23:56-113. Peterman, Z.E., Hedge, C.E., and T o u r t e l o t , H . S . , 1970. I s o t o p i c composition o f s t r o n t i u m i n sea water throughout Phanerozoic time. Geochim. Cosmochim. Acta, 34: 105-1 20. P i s c i o t t o , K.A., 1981. D i a g e n e t i c t r e n d s i n t h e s i l i c e o u s f a c i e s o f t h e Monterey Shale i n t h e Santa Maria r e g i o n , C a l i f o r n i a . Sedimentology, 28:547-571. Riech, V. and Von Rad, U., 1979. S i l i c a diagenesis i n t h e A t l a n t i c Ocean: d i a g e n e t i c p o t e n t i a l and t r a n s f o r m a t i o n s . I n M. Talwani e t a l . ( E d i t o r s ) , Deep D r i l l i n g R e s u l t s i n t h e A t l a n t i c Ocean, M. Ewing Ser. 3, Amer. Geophys. Union, 315-340. Shibata, K., Adachi, M., and M i z u t a n i , S., 1971. Precambrian r o c k s i n Permian conglomerate from c e n t r a l Japan. J. Geol. SOC. Japan, 77:507-514. Shibata, K. and Adachi, M., 1974. Rb-Sr whole-rock ages o f Precambrian metamorphic r o c k s i n t h e Kamiaso conglomerate from c e n t r a l Japan. E a r t h Planet. S c i . L e t . , 21 :277-287. Shibata, K. and Wizutani, S., 1980. I s o t o p i c ages o f s i l i c e o u s s h a l e from HidaKanayama, c e n t r a l Japan. Geochem. J., 14:235-241. Shimoda, S., Sudo, T., and Oinuma, T., 1969. D i f f e r e n t i a l thermal a n a l y s i s curves o f mica c l a y m i n e r a l s . Proc. I n t e r n . Clay Conf., 1969, Tokyo, 1:197206. Shirozu, H., 1978. C h l o r i t e m i n e r a l s . I n T. Sudo and S. Shimoda ( E d i t o r s ) Clays and c l a y m i n e r a l s o f Japan, Development o f Sedimentology, 26:243-264. 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. Utada, M., 1965. Zonal d i s t r i b u t i o n o f a u t h i g e n i c z e o l i t e s i n t h e T e r t i a r y pyroc l a s t i c r o c k s i n Mogami d i s t r i c t , Yamagata P r e f e c t u r e . Sci. Pap. C o l l . Gen. Educ. Univ. Tokyo, 15:173-216. Utada, M., 1974. An i n t e r p r e t a t i o n o f t h e types o f a l t e r a t i o n i n t h e Neogene sediment,s r e l a t i n g t o t h e i n t r u s i o n o f v o l c a n o - p l u t o n i c complexes, c o r r e l a t i n g t o t h e c a l c u l a t e d temperature p a t t e r n s . S c i . Pap. C o l l . Gen. Educ. Univ. Tokyo, 24:65-77. Van H i n t e , J.E., 1976. A J u r a s s i c t i m e scale. Amer. Assoc. P e t r o l . Geol. B u l l . , 60~489-497.

299

CHAPTER 17 SOME SEDIMENTARY AND DIAGENETIC SIGNATURES I N THE FORMATION OF BEDDED RADIOLARITE

M I R I A M BALTUCK Scripps I n s t i t u t i o n o f Oceanography, La J o l l a , C a l i f o r n i a 92093 (U.S.A.)

ABSTRACT Widespread evidence o f primary sedimentary s t r u c t u r e s i n Pindos bedded r a d i o l a r i t e i n d i c a t e s t h a t some bedding planes and surfaces e x i s t e d on the Other evidence sea f l o o r subject to bi,oturbation and c u r r e n t winnowing. from the same l i t h o s t r a t i g r a p h i c u n i t i n d i c a t e s a diagenetic component i n t h e formation o f bedded chert. Pressure d i s s o l u t i o n o f m a t r i x material s i S one means (besides c u r r e n t winnowing o r c u r r e n t deposition) o f achieving a r a d i o l a r i a n "packstone" texture. U1 t i m a t e l y pressure d i s s o l u t i o n destroys even the r e s i s t a n t quartz r a d i o l a r i a n molds, r e s u l t i n g i n opaque horizons o f i n s o l u b l e residue r i c h i n Fe and A l . The existence o f s t y l o l i t e s i n t h i n sections c u t p a r a l l e l to bedding suggest a h o r i z o n t a l shear pressure a t some time, most l i k e l y during a l p i n e tectonism, which implies t h a t s t y l o l i t i z a t i o n continues very l a t e i n the h i s t o r y o f the sediment. S i l i c i f i c a t i o n o f r a d i o l a r i a n pack- and wackestones can obscure r a d i o l a r i a n s and other sedimentary features. I n an attempt to g w g e the i n f l u e n c e o f associated sediment on the r a t e o f s i l i c a diagenesis, authigenic quartz from a range o f r a d i o l a r i a n l i t h o types was analyzed f o r oxygen ,potope composition. Results show a strong c o r r e l a t i o n between l i g h t 61) o f the quartz and h i g h aluminum content o f t h e sediment, where aluninum i s an index o f c l a y content. INTRODUCTION The bedded r a d i o l a r i t e problem has been a subject o f debate f o r Over century.

During

the l a s t decades pal eoenvironmental analyses have he1 ped

c h a r a c t e r i z e the geologic s e t t i n g s required f o r accumulation sediments (e.g.

and 1974;

of

s i l iceous

to c l a r i f y the s i g n i f i c a n c e o f associated l i t h o l o g i c facies

Audley-Charles,

Garrison,

a

1965;

Iijima et

Grunau, a1

1965;

. 1978;

Garrison

and

Fischer,

1969;

F o l k and McBride, 1978; McBride and

Folk, 1979; Diersche, 1980; Ricou and Marcoux, 1980; Baltuck, 1982; Jenkyns and Winterer, i n preparation). Progress i n understanding the processes Of formatinn o f the bedded aspect o f r a d i o l a r i tes, t y p i c a l l y characterized as " t h i n and smooth-bedded, the beds being separated by w a f e r - t h i n p a r t i n g s of shale" (Hallam, 1975), has lagged somekhat.

I n p a r t t h i s i s because group-

i n g a l l r a d i o l a r i t e s under t h i s d e s c r i p t i o n o v e r s i m p l i f i e s the problem: l i t h o l o g y v a r i e s w i d e l y from Hallam's r a d i o l a r i t e . I n a1 pine terminology " r a d i o l a r i t e " i s appl i e d to bedded and knobby c h e r t , s i l iceous 1imestone, and s i l i c e o u s mudstone. The term has a broad range o f lithologic connotation

as

well

as the i m p l i c a t i o n t h a t r a d i o l a r i a n s are cmmon fos-

300

s i l s . Davis' (1918) i n t e r p r e t a t i o n o f the interbedded mudstone as simply a r e s u l t o f diagenetic unmixing o f c l a y from the c h e r t has been challenged by recent studies (e.g., Nisbet and P r i c e , 1974; Kal i n e t a1 1979; F o l k and

.,

McBride, 1978) which d i r e c t l y address the o r i g i n o f bedding and c l e a r l y show the presence o f primary sedimentary s t r u c t u r e s t h a t separate c l a y from chert. I n t h i s paper I emphasize the wide v a r i e t y o f bedding s t y l e s ,

sedimen-

t a r y and diagenetic textures, and c o l o r occurring i n r a d i o l a r i t e sections from the Pindos Zone o f Greece, s t r e s s i n g t h a t w h i l e bedding i n r a d i o l a r ites

is

of

primary sedimentary o r i g i n , a long and complex diagenetic and

weathering h i s t o r y s t r o n g l y o v e r p r i n t s some

original

features

and

exag-

gerates others. Data used i n t h i s argunent come from f i e l d observations o f t e x t u r a l and sedimentological re1 ations, petrographic studies, and e l e c t r o n microprobe

analyses.

I n a d d i t i o n , a range o f r a d i o l a r i t e l i t h o l o g i e s was

analyzed f o r &0l8 o f diagenetic quartz i n order to i d e n t i f y the

influence

o f sediment type on the r a t e o f s i l i c a diagenesis. Most

textural

and

sedimentological

features

described

here

were

observed i n r a d i o l a r i t e s o f the Pindos Zone, which i s a be1 t o f imbricate t h r u s t sheets i n Western Greece composed l a r g e l y o f Mesozoic pelagic sediments o r i g i n a l l y deposited i n the Pindos Trough. The r a d i o l a r i t e s o f the Pindos Zone range i n thickness from seventy to three hundred meters, range age from Middle Jurassic to E a r l y Cretaceous and o v e r l i e limestones o f E a r l y Jurassic age. The basal f i f t e e n o r so meters o f Pindos r a d i o l a r i t e s in is

composed o f

a very s i l i c e o u s f i s s i l e - w e a t h e r i n g mudstone which grades

upward over a few meters i n t o very c h e r t y thickness.

Silica

content

is

as

beds

thirty

to

eighty

meters

much as 95% i n these beds, decreasing

upwards s t r a t i g r a p h i c a l l y from t h i s u n i t .

The c h e r t beds are

overlain

by

s i l iceous pelagic limestone o f Tithonian-Berriasian age. Above t h i s , thin-bedded s i l iceous marl c o n s t i t u t e s the uppermost u n i t o f Pindos radio1 a r i t e s (F1eury, 1977 1. ANALYTICAL METHODS Most samples used i n t h i s study were c o l l e c t e d and described by Baltuck (1982). (e.g.

These

878-64).

are

designated

Other samples (W)

w i t h the i n i t i a l B and the year c o l l e c t e d were

contributed

by

E.

L.

Winterer.

Besides the a n a l y t i c a l methods described below, petrographic study was performed on some 200 t h i n sections o f c h e r t and s i l i c e o u s mudstone, marl, and 1 imestone. The chemical composition o f the samples from Karpenision t h a t were analyzed f o r i s o t o p i c composition was f i r s t measured on a P h i l i p s Automated X-ray wavelength spectrometer (AXS) using procedures described i n

Bal tuck,

301

(1982 1. Silica stable isotope -

analysis samples o f chert, siliceous marl, and siliceous mudstone were preAll treated to i s o l a t e the quartz phase using the procedure described by Syers e t a l . (1968) and modified by Pisciotto (1978). The isotopic composition was measured on a V. G. Micromass 602 isotope mass spectrometer. Dupli= cates were run on a third o f the samples, and a standard (NBS 28, 60l8 10.0

2 .14%1 was run w i t h each s u i t e o f analyses. Isotopic data are reported u s i n g the 6 notation:

60l8samp1e =

with M O W as standard.

Analytical error for oxygen i n quartz

is about 2

.2%.

Figure 1. Index map of Greece showing l o c a l i t i e s cited i n Figures 2,3, and 4. IZ = Ionian Zone, KL = Klinovon, KA = Karpenision, KR = Kremasta, PR = Proussos, DA = Dafni, KK = Kato Klitoria, AN = Anthusa. Pindos Zone i s outlined.

302

Figure 2. R a d i o l a r i t e bedding s t y l e s , showing every break o r p a r t i n g between beds i n one meter o f section. L o c a l i t i e s are shown i n Figure 1. S o l i d beds are chert; dashed l i n e s are marl o r mudstone where l e n g t h of dashes decreases w i t h f i s s i l i t y . Figure 2m i s an exception: s o l i d beds are m i c r i t i c limestone and slashed areas designate bands o r nodules of replacement s i l i c a .

E l e c t r o n microprobe a n a l y s i s Two k i n d s o f microprobe analyses were made (a)

quantitative

on

petrographic

sections:

analyses o f major element chemistry: a working curve was

derived from the Bureau des Recherches Geologiques e t Mineres (France) and Smithsonian mineral standards. I n d i v i d u a l spots were analyzed using a 4 w i d t h beam. ( b ) q u a l i t a t i v e analyses t h a t detected the r e l a t i v e number o f counts o f a chosen element were done i n t r a n s e c t across the t h i n section using a l o p w i d t h beam to average across l o c a l spot concentrations m a t e r i a l . Analyses were done on a Camebax E l e c t r o n Microprobe.

of

FIELD OBSERVATIONS The r a d i o l a r i t e s o f Greece c o n t a i n a wealth o f bedding styles, l i t h o l o gic

textures,

rep1 acement

features and sedimentary structures.

The wide

v a r i e t y o f bedding s t y l e s i n the r a d i o l a r i t e u n i t o f the Pindos Zone and o f the Ionian Zone i s i n d i c a t e d i n Figure 2, which i l l u s t r a t e s v e r t i c a l l i t h o l o g i c changes i n bed-by-bed d e t a i l over a meter thickness a t various l o c a l ities. Bedding d i f f e r s markedly i n s t y l e even i n one l o c a l i t y (e.g. Kato K l i t o r i a section, Figure 2a, g d j ) ranging from thin-bedded mudstone weathers

to

that

a f i s s i l e appearance and obscures bedding through the charac-

t e r i s t i c chert-mudstone a1 t e r n a t i o n to

chert

beds

sharply

separated

by

303 -

paper-thin

partings.

An

additional

s t y l e o f bedding i n r a d i o l a r i t e s i n

which t h i n beds o f s i l i c e o u s limestone c o n t a i n abundant bands and nodules o f c h e r t i s i l l u s t r a t e d i n Figure 2m. Most a l p i n e geologists would map these beds as r a d i o l a r i t e s .

The broad range o f t e x t u r e and l i t h o l o g y b e l i e

the n o t i o n o f a s i n g l e u n i f i e d process o f bedding formation. The beds i n Figure 2 (excepting Figure 2m) are arranged roughly i n order o f increasing c h e r t content and from 2a to 2 j t h e r e i s a correspondi n g increase i n thickness o f the more r e s i s t a n t (i.e. s i l i c e o u s ) beds. The increase suggests a r e l a t i o n s h i p between the p r o p o r t i o n o f mud i n a s t r a t i graphic section and the thickness o f the more r e s i s t a n t beds.

In

Figures

2k and 1, on the other hand, thin-bedded cherts have no shaley interbeds. Weathering o f outcrops commonly r e s u l t s i n increased

f i s s i l ity,

which

tends to obscure and i n a muddy section may even overvhelm signs o f bedding (e.g. F i g u r e 2a). The f i s s i l e nature o f c h e r t y interbeds may be i n p a r t enhanced by weathering o f the s l i g h t l y muddier beds. The e f f e c t i s stronge s t i n h e a v i l y tectonized areas where f r a c t u r e s provide many conduits f o r ground water c i r c u l a t i o n . Other sedimentary features Unmistakable primary sedimentary s t r u c t u r e s are abundant throughout the l i t h o l o g i e s of Pindos and other r a d i o l a r i t e s . These i n d i c a t e the existence o f bedding surfaces a t the s e a f l o o r and the e a r l y coherence o f beds s t i l l soft

enough

to

deform

during

slmping.

Cruziana-type tracks and round

r e s t i n g traces l i t t e r many exposed bedding surfaces o f the Pindos r a d i o l a r i t e s (Figure 3a), and wispy burrows can be observed i n bedding cross sect i o n ( F i g u r e 3b--sample p i c t u r e d i s from S i c i l y ) . F l u t e casts and starved c u r r e n t r i p p l e s docunent the existence o f bottom water c u r r e n t s i n the sedimentary environment as w e l l as syn-sedimentary existence o f bedding. Figu r e 5b i s a photomicrograph o f a small starved c u r r e n t r i p p l e o r p o s s i b l y a f i l l e d burrow showing the c h a r a c t e r i s t i c l e n s o f coarser rounded by s i l i c i f i e d mudstone.

grain

size

sur-

A sedimentary s l y p i s preserved i n an exposure i n c e n t r a l Greece. The slunped beds are t w i s t e d and broken b u t r e t a i n the thickness o f associated

undisturbed beds t h a t u n d e r l i e coherence

of

the

slunp.

This

demonstrates

an

early

beds and i m p l i e s t h a t e a r l y cementation may preserve bedding

thickness p r i o r to post-sl unping compaction. D i a g e n e t i c features

Some diagenetic features i n r a d i o l a r i t e beds are obvious upon

examina-

t i o n o f sediments a t the outcrop. Diagenetic processes may n o t d e f i n e o r i ginal bedding surfaces i n r a d i o l a r i t e s , b u t diagenesis can s i g n i f i c a n t l y

304

F i g u r e 3. Pr-imary sedimentary s t r u c t u r e s . a. Cruziana-type t r a c k s (arrows) along a beddi ng plane. Anthusa. b. Burrows ( a r r m - c r o s s s e c t i o n o f a c h e r t bed. S i c i l y (photo courtesy o f E. L. Winterer). enhance

or

obscure

original

sedimentary

features.

Extensive bands and

nodules o f rep1 acement c h e r t c u t across o r i g i n a l sedimentary s t r u c t u r e s host

limestone beds.

t h e bands and nodules these replacement f e a t u r e s t h i c k n e s s and s t y l e .

bedding

may

define

a

secondary

F i g u r e 4 i l l u s t r a t e s the e f f e c t s o f increas-

i n g degree o f s i l i c i f i c a t i o n o f limestone, s t a r t i n g w i t h nodules stone

beds

in

Where limestone beds are reduced i n thickness around

lime-

in

t h a t can r e s u l t i n e i t h e r wavy- o r flat-bedded c h e r t depending

on the p r o p o r t i o n s o f c h e r t i n the bed.

Coalesced

nodules

in

an

evenly

bedded c h e r t occur i n a number o f sections, commonly i n c l o s e s t r a t i g r a p h i c a s s o c i a t i o n w i t h wavy bedded c h e r t .

These c o u l d represent

an

extreme

of

complete nodular s i l i c i f i c a t i o n o f h o s t rock o r a merging through d i f f e r e n t i a l compaction o r d i s s o l u t i o n o f marl o r carbonate and d i a g e n e t i c cementat i o n o f i n d i v i d u a l nodules. Evidence f o r the t i m i n g o f nodular c h e r t development i s ambiguous. support

o r i g i n , some c h e r t bodies c u t across o r i g i -

o f a post-compactional

nal bedding f e a t u r e s and commonly do n o t a f f e c t the thickness o f beds.

On

the

other

hand,

In

limestone

shale and marl l a y e r s commonly deform around

nodules, i m p l y i n g e a r l y compaction o f s o f t sediment around hard nodules. As f u r t h e r c o m p l i c a t i o n , p r e l i m i n a r y s t u d i e s o f the s h i f t i n 6 0 l 8 o f q u a r t z across a c h e r t nodu’le suggest an age d i f f e r e n c e o f as m i l l i o n years

between

quartz

precipitation

nodules (Bal tuck, i n p r e p a r a t i o n ) . thing

much

as

4.5

a t c e n t e r and edge o f c h e r t

This does n o t

necessarily

imply

any-

about o r i g i n a l nodule formation b u t does show t h e p o t e n t i a l f o r v e r y

l o n g term diagenesis. O R I G I N OF SOME CHARACTERISTIC RADIOLARITE TEXTURES AND STRUCTURES

Some o f t h e c h a r a c t e r i s t i c t e x t u r e s o f r a d i o l a r i t e s have been mentioned earlier

and c i t e d as evidence o f p r i m a r y o r d i a g e n e t i c processes o f f o n a -

305

Figure 4. Limestone s i l i c i f i c a t i o n sequence r e s u l t i n g i n bedded chert. a. Dark s i l i c a nodules i n even bedded m i c r i t i c limestone. Anthusa. b. With reduced proportions of limestone, t h e beddinq follows thickness v a r i a t i o n of c h e r t nodules. Anthusa. c. Knobby chert. Limestone absent o r reduced t o r i n d s around c h e r t nodules. Anthusa. d. F l a t bedded chert. Nodules coalesce t o f a i r l y f l a t bedding planes. Some d i s s o l u t i o n o f nodules probably occurred t o r e s u l t i n good f i t between lenses. Karpenision. tion.

I n t h i s section r e s u l t s o f the petrographic and e l e c t r o n

observations

of

radiolarian

"sands"

, mu1 ti-banded

microprobe

c h e r t beds, and chert-

mudstone interbeds are presented. R a d i o l a r i a n packstone The concentration o f r a d i o l a r i a n s i n t o a packstone, o r grain-supported t e x t u r e has been i n t e r p r e t e d as evidence o f primary sedimentary processes, through t u r b i d i t e deposition o f graded beds where a r a d i o l a r i a n sand e.g., i s the coarse basal member o f a Bouma t u r b i d i t e sequence, o r through current

winnowing

radiolarian (e.g.,

resulting

packstone

Garrison and

in

between

Fischer,

starved beds

1969;

.,

of

ripples:

i.e.,

mud-supported

Nisbet

and

Price,

pockets

of

radiol arian chert 1974;

Folk

and

McBride, 1978; Kal i n e t a1 1979). Fie1 d evidence supports the l a t t e r mechanism M e r e pockets o f packstone a c t u a l l y occur (e.g. see Figure 5b), b u t petrographic and microprobe studies suggest diagenesis i s a1 so import a n t i n concentrating r a d i o l a r i a n s . Figure 5a i l l u s t r a t e s a t y p i c a l r a d i o l a r i a n packstone and spot chemical

306

L

Figure 5. Radiolarian packstones and s t y l o l i t i z a t i o n . a. Photo-mosaic micrograph of a radiolarian packstone upon which electron microprobe major element analyses were performed. Diagram on r i g h t i l l u s t r a t e s the A1 0 and Fe O3 content o f points i n photo on l e f t . Note very high increasz ?n aluminh and especially i n i r o n a t the s t y l o l i t i c lithofacies boundary between radiolarian packstone and radiolarian mudstone. W72-76, Scale Bar i s 2mn. Solid c i r c l e s a r e %Fe 0 ,open c i r c l e s are %A1 0 b. Clay seams coalescing l a t e r a l l y t o form d i s k opaque s t y l o l i t i c lay&- of insoluble

.

307

residue. Lens-shaped concentration o f r a d i o l a r i a n s (arrow) may be a primary sedimentary s t r u c t u r e , the remnant o f a starved c u r r e n t r i p p l e o r p o s s i b l y a f i l l e d burrow. 878-53, Scale Bar i s lmm. c. Vestige o r c l a s t o f radiol a r i a n mudstone i n r a d i o l a r i a n wackestone. V e s t i g i a l contents g e n e r a l l y preserve b e t t e r w a l l s t r u c t u r e and i n c l u d e smaller r a d i o l a r i a n s than are found i n surrounding packstone. Radiolarians t h a t a r e half-contained w i t h i n the v e s t i g e (arrows) a r e b e t t e r preserved i n t h e h a l f i n s i d e t h e mudstone. 879-2, Scale Bar i s .lm. compositions as determined by e l e c t r o n microprobe analysis. The border between r a d i o l a r i a n packstone (above) and r a d i o l a r i a n mudstone (be1 ow) i s accentuated by an opaque horizon o f unusually h i g h a1 uminum and i r o n cont e n t . Radiolarians are g e n e r a l l y f i l l e d and r e c r y s t a l l i z e d i n t o r a d i a l l y f i b r o u s chalcedony o r drusy quartz spheres from h i c h the spines and d e l i cate pore s t r u c t u r e have been l o s t through d i s s o l u t i o n . Radiolarian molds are surrounded by and truncated by dark opaque s t r i n g e r s o f h i g h Fey K,and A1 content, which represent the i n s o l u b l e residue o f c h e r t pressure solution (stylolitization). The photomicrograph i n Figure 5b shows a l a t e r a l t r a n s i t i o n from c l a y seams to s t y l o l i t e s . Radiolarians are n o t size-graded i n the b i o c l a s t i c layers. Figure 5c i l l u s t r a t e s a c l a s t o r v e s t i g e o f r a d i o l a r i a n mudstone contained i n a m a t r i x o f h i g h l y r e c r y s t a l l i z e d r a d i o l a r i a n packstone.

The

dark

material i n the c l a s t i s c l a y and i r o n

oxide. The r a d i o l a r i a n s i n the v e s t i g e are r a t h e r b e t t e r preserved and l e s s concentrated. A few r a d i o l a r i a n s on the v e s t i g e border (Figure 5c, arrows) are p a r t i c u l a r l y i n s t r u c t i v e since they e x h i b i t preservational features o f both the h e a v i l y s t y l o l i t i z e d packstone ( r e c r y s t a l l i z e d , missi n g w a l l s t r u c t u r e s ) and o f the l e s s digested v e s t i g e (some w a l l preservation, o r i g i n a l r a d i o l a r i a n diameter).

structure

Most seam o r i e n t a t i o n i n the s t y l o l i t e s i s roughly p a r a l l e l to bedding. F i g u r e 6 i s a photomicrograph o f a Pindos t h i n section c u t p a r a l l e l to sediment bedding. There i s a modest development o f pressure s o l u t i o n l i n e a t i o n s roughly perpendicular to bedding. This may have i m p l i c a t i o n s f o r the t i m i n g o f pressure s o l u t i o n i n r a d i o l a r i a n packstones: history

of

Greece

shows

regional

geologic

t h a t horizontal shear pressure was imposed upon

these rocks during a l p i n e t e c t o n i c t h r u s t i n g i n Cenozoic time, over 100 m i l l i o n years a f t e r deposition o f the sediment. I f t h i s t e c t o n i c horizont a l shear produced the s t y l o l i t e s observed perpendicular

to

the

bedding,

the sediment would have been subject to pressure s o l u t i o n and thus to

then

dissolution Ground

water

a1 t e r a t i o n

at

syn-

or

possibly

post-tectonic

conditions.

c i r c u l a t i o n would have become an important agent among post-

depositional sediment m o d i f i e r s . Mu1ti-banded c h e r t beds ---A s i n g l e r a d i o l a r i a n c h e r t bed commonly c o n s i s t s o f several

layers

of

'

308

Figure 6. L i n e a t i o n o f c l a y seams and weakly developed s t y l o l i t e s i n a t h i n s e c t i o n c u t p a r a l l e l t o bedding. Such o r i e n t a t i o n i s a r e s u l t of h o r i z o n t a l compression which most l i k e l y occurred d u r i n g a l p i n e tectonics. B78-624. d i f f e r e n t c o l o r s and/or texture. have sharp boundaries.

Layers may merge i n t o other l a y e r s o r may

Over a few meters

of

lateral

distance,

some o f

these l a y e r s reduce to p a r t i n g s t h a t p h y s i c a l l y separate the c h e r t layers, r e s u l t i n g i n two separate beds. Sharply defined boundaries most commonly occur a t t e x t u r a l breaks and the border may be sharpened by s t y l o l i t e s . V i s i b l e l a y e r i n g which does n o t form bedding i s i l l u s t r a t e d i n the photomicrograph i n Figure 7a showing r e c r y s t a l l i z a t i o n o f a p r e v i o u s l y s t y l o l i t i z e d r a d i o l a r i a n wackestone. The l i g h t l a y e r d i f f e r s from the dark by:

( 1 ) a change i n preservation:

r a d i o l a r i a n s are composed o f f i n e r grained

quartz and are l e s s d i s t i n c t from the surrounding

matrix

in

the

lighter

l a y e r , and ( 2 ) a change i n color. As shown e a r l i e r , the l i t h o l o g y i s darker mainly because o f the s t y l o l i t i c concentration o f i n s o l u b l e r e s i dues,

i.e.,

Fe-oxides and clays.

There i s no s t y l o l i t i c development along

t h e edges o f ,the r e c r y s t a l l i z e d area, and t h i s r e c r y s t a l l i z a t i o n event does n o t develop any f u r t h e r pressure s o l u t i o n features. Rather i t appears to homogenize the sample, removing o r d i s s i p a t i n g the i n s o l u b l e residue and recrystallizing

radiolarians

and

matrix

to

similar

texture

so t h a t

radio1 arians become ghost features. Sharper changes w i t h i n a bed are accompanied by changes i n composition, i n c o l o r , and i n r a d i o l a r i a n preservation, and some are commonly delineated by c l a y seams and/or s t y l o l i t i c i n s o l u b l e residues. a

striking

example;

Figure

7b shorn another.

Figure 5b

illustrates

I n Figure 5b, a c l a y - r i c h

309

Figure 7. a. S i l i c i f i c a t i o n and r e c r y s t a l l i z a t i o n o f a r a d i o l a r i a n packstone, r e s u l t i n g i n l i g h t e r c o l o r through d i s s i p a t i o n o r r e m o b i l i z a t i o n o f opaque minerals and g h o s t l i k e r a d i o l a r i a n textures. 878-21, Scale Bar i s .lm. b. Abrupt j u x t a p o s i t i o n o f d i f f e r e n t l i t h o l o g i e s through pressure s o l u t i o n . Upper u n i t i s s i l i c i f i e d mudstone, lower i s a s i l i c i f i e d r a d i o l a r i a n r i c h mudstone. R e c r y s t a l l i z a t i o n and d i s s o l u t i o n o f t h e lower u n i t i s apparent i n t h e increased concentration o f r a d i o l a r i a n s , t h e i r l o s s o f d e l i c a t e features such as spines and pores, t h e i r f l a t t e n e d e l l i p s o i d a l cross section, and t h e development o f s t y l o l i t e s . U l t i m a t e l y even the r e s i s t a n t r a d i o l a r i a n molds a r e dissolved l e a v i n g an opaque h o r i z o n of i n s o l u b l e residue. ( S t y l o l i t e boundary i s marked w i t h an a s t e r i x . ) R a d i o l a r i a n concentration as a r e s u l t o f pressure s o l u t i o n was c a l c u l a t e d by comparing t h e number o f r a d i o l a r i a n s i n t h e two areas o u t l i n e d . Volume r e d u c t i o n i s about two t h i r d s . 878-56, Scale Bar i s .lm.

seam grades l a t e r a l l y i n t o an opaque, more s t y l o l i t i c l a y e r separating green r a d i o l a r i a n packstone from green muddy chert. Preservation o f del icate r a d i o l a r i a n s i n the muddy p o r t i o n c o n t r a s t s sharply w i t h the f.ibrous and

drusy

t e x t u r e s o f the capsules o f r a d i o l a r i a n molds i n the packstone.

I n Figure 7b a r a d i o l a r i a n packstone i s juxtaposed against a r a d i o l a r i a n mudstone i n sharp, s t y l o l i t e - d e f i n e d contact. The higher concentration o f i n s o l u b l e residue i n the packstone i s

shown

by

the

heavier

opacity

of

matrix. Radio1arians are g e n e r a l l y more f l a t t e n e d and truncated i n the pac kstone. Mu1 t i - c o l o r e d and banded l a y e r i n g i n a c h e r t bed enhances l a y e r i n g by at

l e a s t two diagenetic mechanisms: ( 1 ) the s e l e c t i v e r e c r y s t a l l i z a t i o n o f

o r i g i n a l l y uniform rock r e s u l t i n g i n a change i n preservation and i n intensity

o f c o l o r and ( 2 ) pressure s o l u t i o n r e s u l t i n g i n enhanced sharpness

Of

310

the c o n t a c t between two d i f f e r e n t l i t h o l o g i e s .

Changes i n primary sedimen-

t a t i o n could a1 so achieve a m u l t i - l a y e r e d appearance through o r i g i n a l depos i t i o n a l j u x t a p o s i t i o n o f d i f f e r e n t sedimentary textures. C hert-mudstone interbeds

One of the most i n t r i g u i n g puzzles i n r a d i o l a r i t e sedimentology i s the o r i g i n of the chert-mudstone pairs. A c h e r t sample from the Lombard basin of I t a l y showing the c h a r a c t e r i s t i c symmetry around a very s i l i c e o u s center was selected f o r petrographic and chemical analysis.

Figure 8 shorn a pho-

tomicrograph, the r e s u l t s o f microprobe spot chemical analyses, and a quali t a t i v e broader-beam t r a n s e c t recording the change i n A1 concentration across the bed. I n t h i n section the sample appears more clayey and i r o n r i c h a t the top and bottom, w i t h the development o f c l a y seams and s t y l o l i t e s increasing i n frequency o u t from the center. The spot chemical anal y s e s (microprobe beam diameter o f 4 V) confirm t h i s observation a1 though i n d i v i d u a l p o i n t s can obscure the trend. The qua1 i t a t i v e t r a n s e c t measures Changes i n comconcentrations over a broader microprobe diameter o f 1%. p o s i t i o n must p e r s i s t over distances o f a beam

size)

Transects diagenetic

to

register

confirm

significantly

petrographic

boundary,

only

a

few m i l l i m e t e r s

(depending

on

above the background noise l e v e l .

interpretation:

There

is

no

sharp

gradual increase i n non-sil iceous component

outward from the center. The bed i s s t i l l very s i l i c e o u s a t i t s edges. I n general , mudstone interbeds are a1 so very s i l iceous. F i s s i l i t y of the i n t e r b e d s i s commonly n e g l i g i b l e i n outcrops, e s p e c i a l l y where exposures are c o n t i n u a l l y freshened by erosion, e.g., i n outcrops l i n i n g stream beds, a1 though a c l a y seam o r p a r t i n g may s t i l l del i n e a t e the interbed. The f i s s i l e interbeds r e s u l t from a combination o f an o r i g i n a l

concen-

t r a t i o n o f c l a y and other insolubles, change i n supply o f primary sediment, diagenetic pressure-sol u t i o n enhancement o f the primary concentration i n seams

and

stylol ites,

and recent weathering.

Weathering may be enhanced

along conduits created by s t y l o l i t e boundaries and thus accent the

fissil-

ity. DISCUSSION

Despite the bewildering v a r i e t y o f l i t h o t y p e s and bedding

styles

that

f a l l under the heading o f r a d i o l a r i t e s , sedimentary s t r u c t u r e s such as burrows, tracks, f l u t e casts, and slunps are widespread and common and

some

other

radiolarites,

in

Pindos

i n d i c a t i n g t h a t r a d i o l a r i t e bedding formed

e a r l y as a r e s u l t o f depositional processes on the sea

floor.

Microprobe

transects across fresh-looking beds show a gradational change i n composit i o n r a t h e r than sharp diagenetical ly-induced s h i f t s i n composition.

311

80

00 100

%SiO2

0

2

4

6

6

Abmhum

10

% A$09.% Fe203

Oualitative Transect

Figure 8. Photomicrograph mosaic o f a t y p i c a l c h e r t bed from t h e Lombardy Basin o f I t a l y . On t h e f a r r i g h t i s a q u a l i t a t i v e t r a n s e c t r e c o r d o f r e l a t i v e aluminum content across a t y p i c a l bedded chert. I n t h e center and Fe 03. The black are microprobe chemical analyses o f Si02’ A1 0 s t r i p e i n t h e photomicrograph i s s i l v e r pain? ?overing En organic glue t o prevent contamination; t h e sharp t o n a l d i f f e r e n c e a t center i s an a r t i f a c t o f the photomosaic. Aluminum shows a gradual increase i n content from bedding center t o edges. W79-880Cy Scale Bar i s 2mn. S o l i d c i r c l e s are %Fe203, open c i r c l e s are %A1203. Nevertheless

diagenetic

textures

leave

their

signature.

Chert

rep1 aces

limestone beds, and ubiquitous s t y l o l i t e s and c l a y seams t e s t i f y t o compaction and widespread pressure-sol ution. Even t h i n sections c u t parallel

to

bedding

exhibit

some l i n e a r i t y i n s t y l o l i t i c development, a

phenomenon which most l i k e l y r e l a t e s to h o r i z o n t a l shear during a l p i n e tectonics. I n t h i s case, r a d i o l a r i t e beds were f i r s t subject to diagenetic m o d i f i c a t i o n on the sea f l o o r , and l a t e r to subaerial and ground water m o d i f i c a t i o n during and a f t e r t e c t o n i c a c t i v i t y . Nagy (1970) c a l c u l a t e d t h a t modern ground water c i r c u l a t i o n i n Pre-Cambrian cherts

could

contam-

i n a t e c h e r t rocks w i t h alkanes as much as 30 carbons long. Dissolved s i l i c a species (H2Si04) are i n t r u e s o l u t i o n and should thus be unimpeded by c h e r t porosity

and

permeability

limitations.

Folk

and McBride (1978) r e p o r t

f i n d i n g young c h e r t nodules a b u t t i n g j o i n t s o f l a t e o r post-tectonic o r i g i n i n Jurassic r a d i o l a r i t e s o f I t a l y . Some Pindos r a d i o l a r i t e s show young

312 Remobil i-

c a l c i t e veins i n c h e r t truncated a g a i n s t r a d i o l a r i a n s t y l o l i t e s .

z a t i o n o f s i l i c a i n c h e r t thus does n o t cease w i t h t e c t o n i c emplacement. The extensive compaction and p o s s i b l y the c o n c e n t r a t i o n o f r a d i o l a r i a n s and

associated

s t y l o l i t e s suggests t h a t l a r g e amounts o f s i l i c a and o t h e r

sedimentary m a t e r i a l have been remobil i z e d o r even removed tion.

from

the

sec-

simple c a l c u l a t i o n can be a p p l i e d to estimate the amount o f sec-

A

t i o n l o s t by comparing the number o f r a d i o l a r i a n s i n a packstone

with

the

number i n associated muddy 1 i t h o l o g i e s and assuming o r i g i n a l l y s i m i l a r textures.

Some diagenesis of the packstone i s apparent i n the

truncated

radiol arians.

elongated

and

I n F i g u r e 7b t h e r e are twenty-three r a d i o l a r i a n s

i n the .5 X .7m box o u t l i n e d i n the muddy c h e r t and s i x t y - t h r e e i n the box outlined

i n r a d i o l a r i a n packstone.

The r a d i o l a r i a n s i n the packstone thus

have been concentrated i n t o a volume t h a t i s reduced to a t h i r d o f i t s o r i ginal

muddy

chert lithology.

To make t h i s c a l c u l a t i o n an assumption must

be made which may g r a v e l y o v e r s i m p l i f y the sediment l i t h o l o g y , f o r we seen

have

t h a t o r i g i n a l f e a t u r e s such as the c l a y c o n t e n t and r a d i o l a r i a n abun-

dance can v a r y w i t h i n a bed, and t h a t s t y l o l i t e s clay

content

is

highest.

begin

p o t e n t i a l l o c a l i z e d vol m e l o s s and c o n c e n t r a t i o n o f dissolution

development where

Nevertheless the c a l c u l a t i o n gives an idea o f radiol arians

through

Keene (1976) documents an order o f magni-

o f matrix material.

tude r e d u c t i o n i n sediment volune from r a d i o l a r i a n ooze to c h e r t i n P a c i f i c Ocean

sediments,

thus the c a l c u l a t i o n here may c o n s i d e r a b l y underestimate

v o l m e reduction. ISOTOPE ANALYSES OF OXYGEN I N S I L I C A I n an attempt to estimate the i n f l u e n c e o f h o s t l i t h o l o g y on of

silica

widely

diagenesis

varying

stratigraphic

(for

in

composition

was

selected

from

s e c t i o n o f Karpenision f o r oxygen i s o t o p e analyses.

submarine

authigenic

rate

r a d i o l a r i t e s , a s u i t e o f r a d i o l a r i t e samples

radiolarites)

t h e oxygen isotopes o f c l a y and f e l d s p a r are g e n e r a l l y l i g h t e r of

the

quartz,

precautions

were

taken

Of

the

Because

than

those

d u r i n g sample

p r e p a r a t i o n to remove a l l s i l i c a t e s o t h e r than q u a r t z from the sample p r i o r to

oxygen

isotope

analyses.

This

procedure leaves d e t r i t a l as w e l l as

biogenic q u a r t z i n the i s o l a t e d sample, b u t petrographic observation cates

that

detrital

quartz

is

a

indi-

t r i v i a l concern i n Pindos r a d i o l a r i a n

cherts. Table 1 shows the chemical and i s o t o p i c data f o r the and

Figure

9

samples

p l o t s t h e i s o t o p i c values a g a i n s t Si02,Ca0,

bonate index), and A1203 ( a c l a y index) composition f o r each to q u a r t z i s o l a t i o n .

analyzed

( a calcium carsample

prior

Isotope composition o f these s i l i c i c rocks appears to

be independent o f actual s i l i c a c o n t e n t o r o f calcium content, b u t w i t h t h e

313

Table 1. 0l8 Lithology Studies Sample Number Lithology

% A I ~ O%CaO ~

6i8-6 s i l i c e o u s mudstone 9.27 6.29 B78-16 s i l i c e o u s mudstone 6.76 .48 878-23 chert .65 2.12 678-37 chert 1.53 .15 678-40 chert 1.90 .21 678-106 s i l i c e o u s marl 5.23 17.49 *Assuming quartz formation i n presence of SMOW a. Knauth and Epstein, 1976 b. Clayton e t a l . , 1972

%sio2 60180/oo 69.49 80.34 88.46 97.06 96.82 55.45

29.16 32.41 34.55 30.10 32.83 32.29

TOC*

n

34.4 21.0 12.6 31.0 19.3 21.5

49.0 34.1 25.3 44.5 32.2 34.6

exception of one point there is a well-defined r e l a t i o n s h i p between 6018 and aluninum content. A l i n e a r regression to f i t the data was calculated:

60l8 = 34.63

-

.5(%A1203)

The c o r r e l a t i o n c o e f f i c i e n t r i s 0.90. DISCUSSION OF OXYGEN ISOTOPES

The temperatures of quartz formation calculated from the isotopic data reflect diagenetic conditions, assuming quartz formation is i n equil ibrium w i t h Standard Mean Ocean Water (SMOW) (Clayton et a1 1972; Knauth and Epstein, 1976). The c o r r e l a t i o n of %A124 w i t h l i g h t e r isotopic r a t i o s i n a sample strongly suggests the existence of a rough r e l a t i o n s h i p between A1203 (an index of c l a y content) and temperature of quartz p r e c i p i t a t i o n ,

.,

1

1

28.

.

0 0

4

i

b

b

4

8

11

18

20 Oc.0

60

00

70

00

80

100.uO1

lo

.Va

Figure 9. Oxy en isotope results from Greek cherts. 60l8 is p l o t t e d a g a i n s t %A1 OJsolid d o t s ) , %CaO ( c i r c l e s ) , and %SiO (squares) of samples before quar?z i s o l a t i o n . (Sample numbers a r e op8the f i g h t s i d e of the figure.) Aluminum increases w i t h decreasing 60 , indicating higher temperature a t quartz p r e c i p i t a t i o n .

314

implying t h a t quartz forms l a t e r ( g r e a t e r depth) i n the presence o f clay. This i n t e r p r e t a t i o n i s supported by the experimental data o f Kastner e t a l .

(19771, Wo

showed

that

the

presence o f c l a y slowed the r a t e o f s i l i c a

diagenesis through i t s uptake o f magnesium ions and hypothesized t h a t

mag-

nesium acts as a nucleation s i t e f o r opal CT i n s i l i c a diagenesis. CONCLUSIONS 1) Formation o f bedded r a d i o l a r i t e s as

observed

today

is

a

complex

process t h a t begins w i t h a change i n primary sedimentation. 2) Sharpness o f contacts o f c h e r t w i t h the f i s s i l e b u t s i l i c e o u s mudstone interbeds i s enhanced by the development o f s t y l o l i t e s . 3) F i s s i l i t y can be heightened by subaerial and ground water weathering, p a r t i c u l a r l y as t h i s may be conf i n e d to conduits defined by the i n s o l u b l e residues o f s t y l o l i t e s . 4 ) Results o f p r e l i m i n a r y isotope analyses on q u a r t z - i s o l a t e d samples from a v a r i e t y o f r a d i o l a r i t e l i t h o l o g i e s show l i g h t e r i s o t o p i c composition o f authigenic quartz i n samples o f o r i g i n a l l y h i g h aluninum (i.e., c l a y ) conI n Karpenision samples the p r e d i c t a b l e value o f ,0l8 i n quartz as a f u n c t i o n o f A1203 i n the o r i g i n a l specimen suggests the p o s s i b i l i t y o f the

tent. use

of

a1 uninum

content

manetry i n t h i s region.

as an i n d i c a t o r o f s i l i c a diagenetic paleother-

5) Weakly developed

stylolites

perpendicular

to

bedding and oxygen isotope composition o f quartz independently suggest the i n f l u e n c e o f diagenesis long a f t e r sediment deposition. ACKNOWLEDGEMENTS I thank G. Anderson and J .

operating

the

Killingley

for

help

and

instruction

in

f l u o r i n a t i o n l i n e , M. Orona, F. Matsumoto, and M. Beach for

I i j i m a , and R. t y p i n g the manuscript, and E. L. Winterer, M. Kastner, A. Siever f o r the improvements i n t h i s paper's content as a r e s u l t o f t h e i r critiques. The a u t h o r ' s f i e l d work i n 1978 was supported by the French C.N.R.S. g r a n t number g r a n t number 6510041. Subsequent work was supported by N.S.F. EAR 78-10786.

REFERENCES Audley-Charles, M. G., 1965. Some aspects o f the chemistry o f Cretaceous s i l iceous sedimentary rocks from eastern Timor. Geochim. Cosmochim. Acta., 29: 75-1192. Baltuck M., 1982. Provenance and d i s t r i b u t i o n o f Tethyan pelagic and hemipelagic s i l i c e o u s sediments, Pindos Mountains, Greece. Sed. Geol., 31:

63-88. Baltuck, M., i n preparation. I s o t o p i c evidence f o r the sequence o f quartz p r e c i p i t a t i o n i n c h e r t nodules. Clayton, R. N., O'Neil , J. R., and Mayeda, T. K., 1972. Oxygen isotope exchange between quartz and water. J. Geoph. Res., 77: 3057-3067.

315

Davis, E. F., 1918. The r a d i o l a r i a n cherts o f the Franciscan Group. Univ. C a l i f . Publ., Dept. Geol. Sci. Bull., 11: 235-432. Diersche, V., 1980. Die r a d i o l a r i t e des Oberjura i m M i t t e l a b s c h n i t t der NOrdl ichen Kal kal pen. Geotekt Forsch., 58: 1-217. Fleury, J. J., 1977. De Lamia a Messolonghi. La nappe du Pinde-Olonos e t 1 ' U n i t e de Megdhovas. I n : Dercourt e t a1 ( E d i t o r s ) Reunion extraord i n a i r e de l a societe geologique de France en Grece. B u l l . Soc. Geol. France, 1977, no. 1: 53-60, Huitieme Journee. 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 to subjacent "oceanic c r u s t " i n L i g u r i a , I t a l y . Jour. Sed. Petr., 48:

.,

Garrison, R. E. and A. G. Fischer, 1969. Deep water limestones and rad i o l a r i t e s o f the Alpine Jurassic. I n : Friedman, G. M. ( E d i t o r ) De os i t i o n a l environments i n carbonate rock, a sym osium. Tulsa, O k l a k Soc. Econ. Pal. and m e r a l Spec. Pub., T4* Garrison , R. E., 1974. Radio1 a r i a n cherts, pel agic 1imestones , and igneous rocks i n eugeosyncl i n a l assemblages. In:Hsa, K. J. , and Jenkyns, H. C. , ( E d i t o r s ) Pela i c sediments on l a n d and under t h e sea. I n t . Ass. Sed. Spec. Pub.-T-%f-400. Grunau, H. R., 1965. Radiolarian cherts and associated rocks i n space and time. Eclogae Geol. Helv., 58: 157-208. Hallam, A., 1975. J u r a s s i c Environments. Cambridge U n i v e r s i t y Press, Cambridge, 269 p. I i j i m a , A., Kakuwa, Y., Yamazaki, K., and Yanagimoto, H., 1978. Shallowsea, organic o r i g i n o f the T r i a s s i c bedded c h e r t i n Central Japan. J. Fac. Sci. Univ. Tokyo, Sec. 11, 19: 369-400. Jenkyns, H. C., and E. L. Winterer, i n preparation. Paleo-oceanography o f Mesozoic ribbon r a d i o l a r i t e s . Kal i n , O., Patacca , E. , and Renz, 0. ,1979. Jurassic pel agic deposits from southeastern Tuscany; aspects o f sedimentation and new b i o s t r a t i g r a p h i c data, Eclogae Geol Helv., 72: 715-762. Kastner, M., Keene, J. B., Gieskes, J. M., 1977. Diagenesis o f s i l i c e o u s oozes--1. chemical controls on the r a t e o f opal-A to opal-CT transformation--an experimental study. Geochim. Cosmochim. Acta., 41:

.

Keene, J. B., 1976. The d i s t r i b u t i o n , mineralogy, and petrography o f biogenic and authigenic s i l i c a from the P a c i f i c Basin. U n i v e r s i t y o f C a l i f . San Diego, Ph.D. Thesis, 264 p. Knauth, L. P., and Epstein, S., 1976. Hydrogen and oxygen isotope r a t i o s i n nodular and bedded cherts. Geochim. Cosmochim. Acta, 40: 1095-1108. McBride, E. F., and Folk, R. L., 1979. Features and o r i g i n o f I t a l i a n Jurassic r a d i o l a r i t e s deposited on continental c r u s t . Jour. Sed. Petrol., 49: 837-868. Nagy, B., 1970, P o r o s i t y and p e r m e a b i l i t y o f the e a r l y Pre-Cambrian Onverwacht Chert: O r i g i n o f the hydrocarbon f r a c t i o n . Geochim. Cosmochim. Acta., 34: 525-527. Nisbet, E. G., and Price., I.,1974. S i l i c e o u s t u r b i d i t e s : bedded cherts as redeposited ocean-ridge derived sediments. I n : Hsa, K. J., and Jenkyns, H. C., ( E d i t o r s ) , Pela i c sediments fl and the

under

311

CHAPTER 13 THE DETERMINATION OF B I O G E N I C OPAL I N HIGH LATITUDE DEEP SEA SEDIMENTS BREWSTER, NANCY ANN

Union O i l o f C a l i f o r n i a , P. 0. Box 6176, Ventura, C a l i f o r n i a

93006

ABSTRACT The biogenic opal c o n t e n t o f A n t a r c t i c deep sea sediments i s estimated by subt r a c t i n g i s o l a t e d non-biogenic s i l i c a f r a c t i o n s from t h e t o t a l s i l i c a c o n t e n t o f the samples.

Q u a n t i t a t i v e l y s i g n i f i c a n t sources o f non-biogenic s i l i c a i n

A n t a r c t i c Ocean sediments a r e c l a y - a s s o c i a t e d s i l i c a and d e t r i t a l q u a r t z .

In

t h i s way, t h e r e l a t i o n i s d e r i v e d : BIOGENIC OPAL = TOTAL SI02

-

CLAY-ASSOCIATED SILICA

-

DETRITAL QUARTZ.

Clay-associated s i l i c a i s estimated from aluminum and magnesium concentrations i n the sediment, u s i n g a normative c a l c u l a t i o n .

D e t r i t a l q u a r t z content i s de-

termi ned by X-ray d i f f r a c t i o n . INTRODUCTION V a r i a t i o n s i n t h e biogenic opal c o n t e n t o f A n t a r c t i c Ocean marine sediments r e f l e c t changes i n t h e p r o d u c t i v i t y o f marine p l a n k t o n i n surface waters. F l u c t u a t i n g p r o d u c t i v i t y i s r e l a t e d t o changing oceanographic c o n d i t i o n s as t h e A n t a r c t i c Ocean basin evolved throughout t h e Cenozoic.

Accurate measurement o f

biogenic opal i n sediments a l l o w s t h e r e c o n s t r u c t i o n o f t h e paleoceanographic c o n d i t i o n s i n t h e A n t a r c t i c Ocean i n terms o f c l i m a t e , upwelling, oceanographic c i r c u l a t i o n , and t e c t o n i c h i s t o r y (Brewster, 1980). Leinen (1977) developed a method t o determine t h e amount o f b i o g e n i c opal i n s m e c t i t e - r i c h P a c i f i c sediments.

This study describes a method t o determine t h e

content o f biogenic opal i n sediments h i g h e r i n terrigeneous c o n s t i t u e n t s and lower i n s m e c t i t e c o n t e n t than P a c i f i c deposits.

Leinen's method used a norma-

t i v e c a l c u l a t i o n t o p r e d i c t t h e t o t a l amount o f non-biogenic s i l i c a i n a b u l k sediment sample.

By s u b t r a c t i n g non-biogenic s i l i c a from t h e t o t a l s i l i c a con-

The

tent, t h e remaining s i l i c a f r a c t i o n was considered t o be b i o g e n i c opal.

equation used t o p r e d i c t non-biogenic opal was d e r i v e d from S i , A l , and Mg conc e n t r a t i o n s i n selected P a c i f i c sediments which represent t h e dominant m i n e r a l ogies encountered i n t h e P a c i f i c region. I n c o n t r a s t , sediments from t h e Southern Ocean a r e dominated by t h e c l a y mineral i l l i t e , which has a h i g h e r A1 and lower Mg c o n t e n t than smectite.

In

a d d i t i o n , i c e - r a f t e d d e t r i t a l q u a r t z i s present i n s i g n i f i c a n t amounts i n Southern Ocean sediments (Cook e t a l . ,

1974; 1975; Jacobs, 1974).

Because o f these

d i f f e r i n g mineral c o n s t i t u e n t s , an equation based on P a c i f i c sediment chemistry cannot be used t o estimate non-biogenic s i l i c a i n t h e Southern Ocean.

Instead

318 a d i f f e r e n t e q u a t i o n must be generated f r o m A1 and Mg c o n c e n t r a t i o n s d e t e r m i n e d f r o m Southern Ocean sediments. DETERMINATION OF BIOGENIC OPAL The r e l i a b l e d e t e r m i n a t i o n o f o p a l i n sediments i s c o m p l i c a t e d f o r modern sediments, and was, u n t i l r e c e n t l y , i m p o s s i b l e f o r sediments o l d e r t h a n one m i l -

li o n y e a r s .

Techniques used t o e v a l u a t e o p a l c o n t e n t i n c l u d e d X-ray d i f f r a c t i o n

( E l l i s , 1972; C a l v e r t , 1966; Goldberg, 1958), chemical d i s s o l u t i o n and l e a c h i n g (Kamatani and Oda, i n press; Hashimoto and Jackson, 1960), and i n f r a r e d s p e c t r o scopy ( C h e s t e r and E l d e r f i e l d , 1968). P r e v i o u s t e c h n i q u e s proved u n r e l i a b l e f o r sediments o l d e r t h a n one m i l l i o n years.

X-ray d i f f r a c t i o n d e t e r m i n a t i o n o f opal, i n v o l v i n g t h e thermal conver-

s i o n o f amorphous o p a l t o c r y s t a l l i n e opal-CT,

underestimates opal i n o l d e r

sediments because thermal c o n v e r s i o n o f aged opal t o opal-CT i s n o t complete. The X-ray t e c h n i q u e a l s o c a n n o t d i s t i n g u i s h o p a l w h i c h has d i s s o l v e d and r e p r e c i p i t a t e d as a s i l i c e o u s c o a t i n g on o t h e r sediment g r a i n s (Heath, 1974).

Dif-

f e r i n g s o l u b i l i t i e s o f o p a l o f d i f f e r e n t ages make d i s s o l u t i o n t e c h n i q u e s f o r t h e determination o f opal u n r e l i a b l e .

I n f r a r e d techniques a r e n o t accurate f o r

m a r i n e sediments w i t h an opa1:quartz r a t i o l e s s t h a n 3; i n a d d i t i o n , common m a r i n e c o n s t i t u e n t s such as p a l a g o n i t e i n t e r f e r e w i t h t h e d e t e r m i n a t i o n (Chester and E l d e r f i e l d , 1968). These l i m i t a t i o n s s t i m u l a t e d t h e development o f approaches t h a t make an i n i t i a l a n a l y t i c a l d e t e r m i n a t i o n o f t o t a l s i l i c a i n a b u l k sediment f r o m w h i c h n o n - b i o g e n i c s i l i c a f r a c t i o n s a r e i s o l a t e d and s u b t r a c t e d . f i r s t used t h i s approach f o r d e t e r m i n i n g b i o g e n i c o p a l .

A r r h e n i u s (1952)

Quantitatively signi-

f i c a n t sources o f n o n - b i o g e n i c s i l i c a i n t h e p e l a g i c r e a l m a r e d e t r i t a l q u a r t z and c l a y - a s s o c i a t e d s i l i c a . T h i s s t u d y uses t h e r e l a t i o n : BIOGENIC OPAL =

TOTAL Si02

-

CLAY-ASSOCIATED SILICA

-

DETRITAL QUARTZ.

C l a y - a s s o c i a t e d s i l i c a i s t h e most d i f f i c u l t s i l i c a f r a c t i o n t o d e t e r m i n e . L e i n e n (1977) suggested, u s i n g a c o n s t a n t Si02:A1203 r a t i o t o e s t i m a t e non-biogenic s i l i c a i n Central P a c i f i c clays. t o 4:1,

L e i n e n used a range o f r a t i o s f r o m 3 : l

depending on t h e age and l o c a t i o n o f t h e P a c i f i c samples.

L e i n e n ' s approach was improved w i t h a n o r m a t i v e c a l c u l a t i o n i n v o l v i n g b o t h A1 and Mg c o n t e n t o f samples, w h i c h p r o v i d e d f o r v a r i a t i o n s i n c l a y m i n e r a l o g y . The e q u a t i o n developed t o e s t i m a t e n o n - b i o g e n i c s i l i c a i n P a c i f i c sediments was:

si

(non-biogenic)

= 4.33 A1

+

2 1.36 Mg (Leinen, 1977).

L e i n e n ' s c a l c u l a t i o n i n c l u d e s t h e o p a l t h a t has d i s s o l v e d and r e p r e c i p i t a t e d o n t o sediment g r a i n s .

319

Fig. 1. L o c a t i o n f o r D.S.D.P. S i t e s used i n t h i s study. Biogenic s i l i c a cont e n t i s determined f o r S i t e s 266 and 277. Samples from S i t e s 264A, 266, 268, 277, and 278 were used t o study clay-associated s i l i c a . This study uses a normative c a l c u l a t i o n t h a t w i t h A1 and Mg p r e d i c t s t h e amount o f clay-associated s i l i c a .

I n a c l a y mineral, t h e t r i o c t a h e d r a l and

dioctahedral s i t e occupations o f A1 and Mg a r e r e l a t e d t o t h e s i t e occupation o f Si.

The amount o f A1 i n t h e octahedral and t e t r a h e d r a l s i t e s o f t h e c l a y

mineral s t r u c t u r e l i m i t s t h e amount o f S i t h a t w i l l f i t i n t h e c r y s t a l s t r u c t u r e . Mg s u b s t i t u t i o n f o r t r i v a l e n t c a t i o n s i n t h e octahedral s i t e s o f marine c l a y s c o r r e l a t e s w i t h t h e amount o f A1 s u b s t i t u t i o n f o r S i i n t e t r a h e d r a l s i t e s .

Fe

i s a s t r u c t u r a l c o n s t i t u e n t o f marine c l a y m i n e r a l s b u t i t s r e l a t i o n t o S i and A1 cannot be determined w i t h o u t i s o l a t i o n o f t h e c l a y m i n e r a l s from o t h e r i r o n bearing phases i n t h e sediments. Although t h e i n t e r r e l a t i o n s o f A l , Mg, and S i w i t h i n t h e c r y s t a l l a t t i c e a r e complex, they can be d e f i n e d s t a t i s t i c a l l y using m u l t i p l e r e g r e s s i o n a n a l y s i s . F i r s t , a r e g r e s s i o n equation d e f i n i n g t h e r e l a t i o n between clay-associated A1 and Mg, and clay-associated S i i s e s t a b l i s h e d .

Then, t h i s equation can be used

t o p r e d i c t clay-associated s i l i c a from chemical data determined from marine sed-

320

;i

266

0

DlAlMl WZE

MlXm MNIO WZE. lunmr CLAY. DlATCM CLAY.

DlATM WZE

0

[ W

z w

NAW CHALK. CLAVSTM

277

V

0

FOW

-

"Iy)

CHALK,

NANM WZE

u

CHERT IOWLfS

Fig. 2.

General l i t h o l o g i c column f o r S i t e s 266 and 277.

iments.

Once b u l k s i l i c a values a r e corrected f o r clay-associated s i l i c a , they

a r e corrected f o r d e t r i t a l quartz as determined by X-ray d i f f r a c t i o n . maining s i l i c a i s assumed t o be biogenic opal.

The re-

The accuracy o f t h i s technique

i s about 55% opal. To c h a r a c t e r i z e t h e r e l a t i o n between A1 and Mg, and clay-associated s i l i c a , 1 7 sediment samples t h a t showed no recognizable opal i n smear slides, were sel e c t e d from f i v e D.S.D.P.

s i t e s i n t h e A n t a r c t i c Ocean.

A regression equation

was generated from t h e 17 "opal-free" samples, and then used t o p r e d i c t c l a y associated s i l i c a f o r 45 A n t a r c t i c Ocean D.S.D.P. samples t h a t span t h e Cenozoic. S i t e s 266 and 277 i n the A n t a r c t i c The s e t o f 45 samples a r e from D.S.D.P. Ocean (Figure 1 ) .

Sediments from these two s i t e s form a n e a r l y continuous se-

321 quence through t h e Cenozoic ( 0 t o 53 m.y. B.P;

Figure 2 ) .

Each sample i s actu-

a l l y a composite sample c o n s i s t i n g o f from one t o e i g h t samples t h a t when combined represent a o n e - m i l l i o n year i n t e r v a l .

The paleoceanography o f t h e Antarc-

t i c Ocean was s t u d i e d from t h e d e t e r m i n a t i o n o f opal i n these samples (Brewster 1980). METHODS Sample p r e p a r a t i o n The 17 samples from O.S.D.P.

S i t e s 264A, 266, 268, 277, and 278 ( F i g u r e 1 )

were chosen on t h e basis o f n e g l i g i b l e opal c o n t e n t i n o r d e r t o determine a r e g r e s s i o n equation.

These samples c o n t a i n c l a y and d e t r i t a l quartz, o r carbonate,

c l a y , and d e t r i t a l quartz.

The A l , Mg, and S i c o n c e n t r a t i o n s were determined by

atomic a b s o r p t i o n spectroscopy and were processed by m u l t i p l e r e g r e s s i o n analys i s i n which S i i s t h e dependent v a r i a b l e , and A1 and Mg a r e t h e independent variables. The s e t o f 17 o p a l - f r e e samples as w e l l as t h e s e t o f 45 o p a l - c o n t a i n i n g samples were f r e e z e d r i e d , ground, and p u t through a sample s p l i t t e r t e n times t o homogenize t h e sediment.

S a l t c o n t e n t was determined on t h e b a s i s o f

water l o s s u s i n g t h e r e l a t i o n :

% salt =

% water l o s s X 0.035 - % water loss.

A l l weight percents were c a l c u l a t e d on a s a l t - f r e e b a s i s (van Andel e t a l , 1975). Weight percent c a l c i u m carbonate was determined u s i n g t h e Karbonat Bombe ( M i l l e r and Gastner, 1971) f o r samples w i t h h i g h CaC03 c o n t e n t and LECO Carbon Analyzer f o r samples c o n t a i n i n g l e s s than 25% CaC03 (Hays e t al., e t al.,

1970).

1972; Badger

Samples were r u n i n d u p l i c a t e , o r u n t i l values agreed by 2%.

Q u a r t z was determined by X-ray d i f f r a c t i o n ( E l l i s , 1972; T i l l and Spears, 1969) w i t h an accuracy o f 55% (carbonate-free).

I f sample volume was i n s u f f i -

c i e n t f o r X-ray a n a l y s i s , q u a r t z values were i n t e r p o l a t e d from a p l o t o f o t h e r q u a r t z values, and c r o s s checked w i t h published q u a r t z data from X-ray analyses i n t h e I n i t i a l Reports (Cook e t a l . ,

1974; 1975).

A rough e s t i m a t e o f opal c o n t e n t can be estimated from t h e same d i f f r a c t o g r a m s used f o r q u a r t z d e t e r m i n a t i o n by measuring t h e amount o f background s c a t t e r . For t h e 17 " o p a l - f r e e ' ' samples, an average o f about 1.4% opal by weight was measured. This opal c o n t e n t represents opal presumably t o o f i n e - g r a i n e d t o be seen i n smear s l i d e s .

The opal values estimated by X-ray d i f f r a c t i o n were sub-

t r a c t e d from t h e b u l k s i l i c a values t o g i v e more r e a l i s t i c o p a l - f r e e b u l k s i l i c a values. To prepare samples f o r chemical a n a l y s i s , 200 t o 500 mg o f each sample was d i g e s t e d i n T e f l o n - l i n e d s t e e l bombs, a t 110" C w i t h aqua r e g i a and h y d r o f l u o r i c acid.

A f t e r n e u t r a l i z a t i o n w i t h b o r i c a c i d (Bernas, 1968), samples were analyzed

f o r S i , A l , and Mg by atomic a b s o r p t i o n spectroscopy (Tables 1 and 3).

Samples

322 TABLE 1 . S a l t - c o r r e c t e d chemical data f o r 17 o p a l - f r e e samples.

264A

2-3

1.03

0.60

0.31

0.17

3-4 14-15

1.02

0.58

0.24

0.19

266

17.94

3.57

3.88

0.94

15-16

15.83

4.94

3.45

0.90

16-17

14.00

5.25

3.07

0.79

17-18

14.55

4.41

3.40

0.92

18-1 9

16.01

4.47

3.00

0.81

19-20

10.29

3.19

2.19

0.62

20- 21

15.46

3.44

3.03

0.84

4-1

32.79

12.31

5.59

1.01

9- 1

31.64

9.99

6.31

1.27

18-1

32.79

14.61

6.68

1.39

268

20- 1

32.46

12.84

6.51

1.50

277

29-30

1.22

0.49

0.29

0.22

37-38

2.70

0.70

0.55

0.25

52-53

3.02

0.24

0.78

0.33

8- 1

28.87

6.87

5.40

1.48

268

278

were r u n i n d u p l i c a t e , w i t h a p r e c i s i o n o f 1-2% f o r A l , Mg, and S i f o r samples low i n CaC03, and from 2-5% f o r samples h i g h i n CaC03.

Comparison w i t h the

Standard Rock and t h e North P a c i f i c Sediment Standard i n d i c a t e d no

U.S.G.S.

systematic e r r o r . RESULTS Equation generated f o r e s t i m a t i o n o f c l a y - a s s o c i a t e d s i l i c a I n o p a l - f r e e samples, t h e o n l y s i g n i f i c a n t s i l i c a f r a c t i o n s a r e clay-associat e d s i l i c a and q u a r t z .

The amount o f q u a r t z i s s u b t r a c t e d from t h e t o t a l s i l i c a

c o n t e n t (determined by atomic a b s o r p t i o n spectroscopy and c o r r e c t e d f o r t h e very s l i g h t amount o f o p a l ) , and t h e remaining s i l i c a i s assumed t o be associated w i t h clay.

Clay-associated s i l i c a i s regressed a g a i n s t A1 and Mg concentrations.

D i f f e r e n t combinations o f t h e f i v e parameters Al,

A12, Mg, Mg2, and AlxMg were

used i n t h e search f o r t h e best model ( o r equation) t o c h a r a c t e r i z e clay-associated s i l i c a .

The r e g r e s s i o n a n a l y s i s associates a c o e f f i c i e n t w i t h each parame-

323

n W

a

40

3 v)

a W

I

a

30

5 v)

n W

4

20

V

0 v) v)

7

4

10

V

I

I

10

I

30

20

I

40

CLAY-ASSOCIATED SILICA. PREDlCTE D

F i g . 3. Comparison o f c l a y - a s s o c i a t e d s i l i c a measured i n 17 o p a l - f r e e sediment samples w i t h t h a t p r e d i c t e d by a r e g r e s s i o n e q u a t i o n . Diagonal l i n e i n d i c a t e s a one t o one correspondence between p r e d i c t e d and measured c l a y - a s s o c i a t e d s i l i c a . t e r i n t h e model, and p r o v i d e s a y - i n t e r c e p t .

It a l s o furnishes s t a t i s t i c a l in-

f o r m a t i o n d e s c r i b i n g how c l o s e l y t h e e q u a t i o n - p r e d i c t e d c l a y - a s s o c i a t e d s i l i c a compares t o t h e measured c l a y - a s s o c i a t e d s i l i c a v a l u e . The e q u a t i o n p r o v i d i n g t h e b e s t f i t t o t h e chemical d a t a i s :

si

( c l ay-associated)

=

4.54 A1

-

1.02 AlxMg

-

1.88.

T h i s e q u a t i o n was s e l e c t e d f r o m 40 equat ons, on t h e b a s i s o f c o r r e l a t i o n c o e f f i c i e n t s , T-values,

and s t a n d a r d e r r o r s

Table 2 ) .

The r e s i d u a l s between t h e

p r e d i c t e d and measured c l a y - a s s o c i a t e d s l i c a a r e v e r y s m a l l ; p l o t t i n g t h e meas u r e d vs. t h e p r e d i c t e d v a l u e s d e f i n e a

i n e whose s l o p e approximates one ( F i g -

ure 3). Most o f t h e v a r i a n c e i n t h e e q u a t i o n

s due t o A1 .

i m p o r t a n t , a c c o u n t i n g f o r o n l y 1.6% o f t h e v a r i a n c e .

The AlxMg t e r m i s n o t as Thus, c l a y - a s s o c i a t e d s i l -

i c a i n Southern Ocean c l a y s can a l m o s t be p r e d i c t e d u s i n g a s i m p l e ratio.

U s i n g t h e a d d i t i o n a l AlxMg t e r m improves t h e model s l i g h t l y .

Si02:A1203 r a t i o suggested by t h e r e g r e s s i o n e q u a t i o n i s 2.26.

Si02:A1203 The

This r e s u l t i s

324

TABLE 2. S t a t i s t i c s o f r e g r e s s i o n equation

variab le

regression

std. e r r o r o f

computed

coefficient

reg. coef.

T-Val ues

A1

4.54100

0.53544

8.481

A1 X Mg

-1.01 633

0.36039

-2.820

intercept

-1.87990

mu1 t i p l e c o r r e l a t i o n c o e f f i c i e n t

0.986

standard e r r o r o f e s t i m a t e

1.313

c o n s i s t e n t w i t h t h e mineralogy o f t h e Southern Ocean region, which i s predomin a n t l y i l l i t e , having an average Si02:A1203 r a t i o o f 2.50 (Weaver and P o l l a r d , 1973).

The AlxMg term i n t h e model becomes i n c r e a s i n g l y i m p o r t a n t as o l d e r

samples a r e encountered a t g r e a t e r depth which c o n t a i n more smectite (Jacobs, 1974). Clay-associated s i l i c a estimates f o r t h e 45 A n t a r c t i c Ocean samples The r e g r e s s i o n equation i s a p p l i e d t o t h e A1 and Mg c o n c e n t r a t i o n s determined by atomic a b s o r p t i o n spectroscopy f o r t h e 45 composite samples (Table 3) t o pred i c t a clay-associated s i l i c a value f o r each sample ( F i g u r e 4).

Clay-associated

s i l i c a ranges from about 0 t o 5 w t . % throughout much o f S i t e 277, f o r Eocene and Oligocene deposits.

T h i s i s expected as most o f these sediments a r e nannofossil

oozes c o n t a i n i n g l i t t l e o r no c l a y .

The r e g r e s s i o n equation p r e d i c t s s l i g h t l y

n e g a t i v e clay-associated s i l i c a values f o r a few Paleogene samples.

This c o u l d

be due t o an o v e r - e s t i m a t i o n o f t h e d e t r i t a l q u a r t z content, o r i t c o u l d be an a r t i f a c t o f t h e equation f o r samples w i t h a combination o f low c l a y c o n t e n t and h i g h carbonate content.

The discrepancies a r e w e l l w i t h i n t h e l i m i t s o f standard

e r r o r and t h e s l i g h t l y n e g a t i v e values a r e taken t o be zero.

A l , Mg, and S i c o n t e a t a r e a l l h i g h e r i n t h e Neogene a t S i t e 266, than i n t h e Paleogene sediments a t S i t e 277 ( F i g u r e 4 ) .

This i s due t o t h e g r e a t e r amount

o f c l a y i n t h e Neogene sediments, compared t o t h e Paleogene, where c l a y c o n t e n t i s v e r y low.

Clay-associated s i l i c a i n t h e Neogene, p r e d i c t e d from t h e A1 and

Mg contents, ranges from 4 t o 35 w t . %, averaging about 24 w t . %. Determination o f b i o g e n i c opal i n t h e Southern Ocean Clay-associated s i l i c a and d e t r i t a l q u a r t z contents a r e s u b t r a c t e d from t h e b u l k s i l i c a c o n t e n t as t h e f i n a l s t e p i n c a l c u l a t i n g b i o g e n i c opal content. I n

from A1 and Mg concentrations according t o the regression equation.

10

5 IM I

M

f

Fig. 4. Concentrations o f A1 and Mg i n b u l k sediments from S i t e s 266 and 277 corrected f o r s a l t . Percent c l a y i s determined a f t e r the biogenic opal i s estimated (Figure 5 ) , by s u b t r a c t i n g percent quartz from percent non-biogenic content. The percent non-biogenic content i s determined by s u b t r a c t i n g percent opal + percent CaC03 from 100%. Percent S i (clay-associated) i s predicted

% SiOz CiQ-OSSOCbiOd

% Mg % Al %; Clay

a d d i t i o n , t h e percentage o f non-biogenic d e b r i s i s d e r i v e d by s u b t r a c t i n g percent opal + percent CaC03 from 100%. The non-biogenic " r e s i d u e " c o n s i s t s p r i m a r i l y o f c l a y and q u a r t z ; i t c l o s e l y resembles t h e curve r e p r e s e n t i n g percent c l a y and percent c l a y - a s s o c i a t e d s i l i c a i n Figure 4. Paleogene sediments a t S i t e 277 c o n t a i n considerable CaC03 w i t h a t r a c e o f o p a l , c l a y , and d e t r i t a l quartz.

Neogene sediments from S i t e 266 c o n t a i n more

w

N Q,

% OPAL

0

XI

%

SQ

50

% SiO,

bull

clay-aaloclatad

% DETRITAL QUARTZ

% NON-BIOGENIC

10

F i g . 5. C o n c e n t r a t i o n s o f some sedimentary components i n b u l k sediments a t S i t e s 266 and 277. and t h e s i l i c a f r a c t i o n s used t o compute p e r c e n t o p a l . Because t h e p e r c e n t n o n - b i o g e n i c m a t e r i a l i s p r i m a r i l y c l a y and d e t r i t a l q u a r t z , t h i s c u r v e i s v e r y s i m i l a r t o b o t h t h e p e r c e n t c l a y c o n t e n t c u r v e ( F i g u r e 4 ) and t h e p e r c e n t c l a y - a s s o c i a t e d s i l i c a curve.

327 opal, clay, and quartz, and l e s s CaC03.

Figure 5 shows t h a t d u r i n g t h e e a r l y

Miocene, t h e clay-associated s i l i c a accounts f o r most o f t h e s i l i c a , w i t h a d e t r i t a l q u a r t z c o n t e n t o f as h i g h as 10%. The curves o f opal and c l a y - a s s o c i a t e d s i l i c a contents increase from 13 m.y.B.P.

t o 10 m.y.B.P.,

and then b o t h curves

show l e s s e r amounts o f opal and c l a y , decreasing t o 7 m.y.B.P.

From 7 m.y.B.P.

t o present, biogenic opal becomes t h e dominant type o f s i l i c a i n Southern Ocean sediments.

D e t r i t a l q u a r t z and clay-associated s i l i c a decrease i n t h e Neogene

a t S i t e 266 and occur i n t r a c e amounts i n Recent sediments ( F i g u r e 5 ) . DISCUSSION AND CONCLUSIONS

A r e l i a b l e estimate o f biogenic opal can be c a l c u l a t e d w i t h o u t a d i r e c t measurement o f opal.

The method, l i k e t h a t described by Leinen (1977), i s an i m -

provement over previous ones because i t can measure opal c o n t e n t f o r p r e - P l e i s t o cene sediments and f o r d e p o s i t s t h a t have undergone v a r i o u s d i a g e n e t i c s i l i c a phase transformations.

I n p a r t i c u l a r , t h e technique i s an improvement because

i t d e t e c t s opal t h a t has d i s s o l v e d and r e p r e c i p i t a t e d as a cement i n t h e sediment

m a t r i x , and onto sediment g r a i n s as overgrowths.

The technique cannot d e t e c t

opal which has d i s s o l v e d and has been consumed by t h e enrichment o f degraded c l a y s (Heath and Dymond, 1977).

A Si02:A1203 r a t i o c l o s e l y estimates t h e s i l i c a associated w i t h c l a y i n SouthA Mg term improves t h e r e s u l t s s l i g h t l y . I n o l d e r , smec-

e r n Ocean sediments.

t i t e - r i c h sediments from t h i s r e g i o n which c o n t a i n h i g h e r Mg contents, t h e Mg term becomes necessary t o e s t i m a t e t h e amount o f s i l i c a associated w i t h c l a y . Determination o f opal depends on t h e accurate e s t i m a t i o n o f non-biogenic s i l i c a associated w i t h c l a y s from t h e Southern Ocean samples, and a p p l i e s o n l y t o Southern Ocean samples.

The d e r i v e d equation should n o t be used i n areas where

m i n e r a l s u i t e s i n t h e sediments a r e g r e a t l y d i f f e r e n t from those used i n t h i s study.

A normative d e t e r m i n a t i o n f o r biogenic opal must employ a r e g r e s s i o n

equation based on c l a y m i n e r a l s s p e c i f i c t o t h e r e g i o n being studied.

I f clay

m i n e r a l s change s i g n i f i c a n t l y i n o l d e r sediment deposits down core, more than one equation c o u l d be necessary f o r a s i n g l e l o c a l i t y . Sediments i n r e g i o n s o f h i g h d e t r i t a l i n f l u e n c e c o n t a i n o t h e r sources o f nonbiogenic s i l i c a , p r i m a r i l y q u a r t z and f e l d s p a r .

Where present i n s i g n i f i c a n t

abundance, these d e t r i t a l m i n e r a l s must be measured and considered i n t h e correct i o n s f o r non-biogenic s i l i c a .

A p o s s i b l e source o f e r r o r l i e s i n t h e d e t e r m i n a t i o n o f t h e r e g r e s s i o n equat i o n . The generation o f t h e p a r t i c u l a r equation i s based on chemical data from " c o n t r o l " samples considered t o be o p a l - f r e e . However, no sediment sample from t h e A n t a r c t i c Ocean was observed t h a t d i d n o t c o n t a i n a t l e a s t a small amount o f opal as detected by X-ray d i f f r a c t i o n a n a l y s i s .

Because o f t h i s , t h e regres-

328

TABLE 3. Sal t-corrected chemical data f o r 45 Antarctic samples.

Site

Age Interval (m.y.

266

- 1) (1 - 2 )

(0

(2 (3 (4 (5 (6

277

1

-

3) - 4) - 5) - 6) - 7) (7 - 8) (8 - 9) ( 9 -10) (1 1-12) (12-13) (13-14) (14-15) (15-16) (16-17) (17-18) (18-19) (19-20) (20-21) (21 -22) (22-23) (28-29) (29-30) (30-31 ) (31 -32) (32-33) (33-34) (34-35) (35-36) (36-37) (37-38) ( 38- 39)

A1 (average)

Mg (average)

x

x

Si (average) %

0.75 1.84 4.24 2.65 3.96 4.05 2.81 3.48 4.11 5.88 5.85 5.87 0.89 3.88 3.45 3.07 3.36 3.00 2.19 3.03 4.04 5.32 0.43 0.29 0.24 0.21 0.20 0.21 0.40 0.36 1.25 0.55 0.65

4.70 0.66 1.05 0.73 1.03 1.04 0.71 0.81 1 .oo 1.40 1.40 1.39 0.32 0.94 0.90 0.79 0.92 0.81 0.62 0.84 1.32 1.87 0.33 0.22 0.21 0.14 0.14 0.16 0.21 0.18 0.17 0.25 0.28

33.60 35.72 31.59 33.86 30.98 31.58 20.34 18.59 27.55 31.74 26.51 28.04 4.12 17.94 15.83 14.00 14.55 16.01 10.29 15.46 17.96 22.06 2.21 1.22 1.27 1.18 1.13 1.06 1.64 1.59 1.38 2.70 3.31

329 TABLE 3.

Site

(Continued).

Age

A1

Mg

Si

Interval

(average)

(average)

(average)

%

%

%

(m.y.

277

1

(39-40)

0.94

0.35

5.88

(40-41 )

1.07

0.38

6.41

(42-43)

0.70

0.26

4.30

(44-45)

0.51

0.24

2.63

(45-46)

0.40

0.21

3.98

(46-47)

0.24

0.18

1.49

(47-48)

0.22

0.15

1.12

(48-49)

0.15

0.16

0.78

(49-50)

0.26

0.22

1.09

(50-51 )

0.37

0.26

1.47

(51 -52)

0.49

0.28

2.98

(52-53)

0.78

0.33

3.02

s i o n e q u a t i o n may induce some systematic e r r o r , i n t h i s study, a biased over-est i m a t i o n o f clay-associated s i l i c a . I n t h e f i n a l c a l c u l a t i o n t h i s would r e s u l t i n a s l i g h t under-estimation o f biogenic opal.

I n samples w i t h n e g l i g i b l e opal,

and l i t t l e clay, a v e r y s l i g h t o v e r - e s t i m a t i o n o f clay-associated s i l i c a may r e s u l t i n a s l i g h t l y n e g a t i v e value f o r b i o g e n i c opal.

The magnitude o f such an

e r r o r should be small and w i t h i n t h e margin o f a n a l y t i c a l e r r o r . Other systematic e r r o r s c o u l d r e s u l t from s e l e c t i o n o f a p a r t i c u l a r r e g r e s s i o n equation.

For samples c o n s i s t i n g o f nannofossil ooze and n e g l i g i b l e c l a y , an

equation w i t h a n e g a t i v e y - i n t e r c e p t may p r e d i c t s l i g h t l y n e g a t i v e values f o r clay-associated s i l i c a .

T h i s i s observed i n some o f t h e nannofossil oozes a t

S i t e 277, t h a t c o n t a i n n e g l i g i b l e c l a y .

The n e g a t i v e values f o r clay-associated

s i l i c a a r e small, however and a r e taken t o be zero.

Again, t h e e r r o r i s w i t h i n

t h e 1i m i t s o f standard' e r r o r . Once t h e biogenic s i l i c a c o n t e n t o f sediments i s established, t h i s i n f o r m a t i o n can be a p p l i e d as a r e l i a b l e index o f t h e p r o d u c t i v i t y o f surface p l a n k t o n through t i m e (Brewster, 1980; Leinen, 1979).

V a r i a t i o n s i n surface p r o d u c t i v i t y a r e r e -

l a t e d t o p a r t i c u l a r paleoceanographic changes d u r i n g t h e Cenozoic.

Establishing

these f l u c t u a t i o n s a i d s i n t h e study o f c l i m a t i c v a r i a t i o n s , oceanographic c i r c u l a t i o n and water mass development, p o l a r g l a c i a t i o n , and t h e t e c t o n i c e v o l u t i o n o f t h e Southern Ocean basin.

Brewster (1980) determined these p a l e o c l ima-tic-and

330 oceanographic c o n d i t i o n s i n t h e A n t a r c t i c Ocean, based on t h e amounts o f opal as determined by t h e techniaue presented herein. ACKNOWLEDGMENTS This study b e n e f i t t e d from discussions w i t h T j . van Andel, Erwin Suess, and Margaret Leinen.

A p p r e c i a t i o n i s expressed t o Azuma I i j i m a and Jim Hein who

c r i t i c a l l y reviewed t h e manuscript. ing Project.

Samples were provided by t h e Deep Sea D r i l l -

F i n a n c i a l support was provided by a g r a n t from t h e National Science

Foundation No. 0CE75-21833.

Union O i l provided support f o r p a r t i c i p a t i o n and pre-

s e n t a t i o n o f the paper a t t h e I G C P conference i n Japan. REFERENCES

Arrhenius, G., 1952. P r o p e r t i e s o f t h e sediment and t h e i r d i s t r i b u t i o n . Reports o f t h e Swedish Deep Sea Expedition, 5:6-91. Badger, R. G., Gerard, R. D., Benson, W. E., B o l l i , H. M., Hay, W. W., Roghwell, W. T. J r . , Ruef, M. H., Riedel, W. R. and Sayles, F. L., 1970. 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 , 4. U. S. Govt. P r i n t i n g O f f i c e , Washington, D. C., 753 pp. Bernas, B., 1968. A new method f o r decomposition and comprehensive a n a l y s i s o f s i l i c a t e s by atomic a b s o r p t i o n spectrometry. Anal. Chem., 40:1682-1684. Brewster, N. A . , 1980. Cenozoic biogenic s i l i c a sedimentation i n t h e A n t a r c t i c Ocean. Geol. SOC. Am. B u l l . P a r t I,91:337-347. Calvert, S. E., 1966, Accumulation o f diatomaceous s i l i c a sediments i n t h e G u l f o f C a l i f o r n i a . Geol. SOC. Am. B u l l . , 77:569-596. Chester, R. and E l d e r f i e l d , H., 1968. The i n f r a r e d d e t e r m i n a t i o n o f opal i n s i l i ceous deep sea sediments. Geochim. Cosmochim. Acta, 32:1128-1140. Cook, H. E., Zimmels, I.and M a t t i , J. C., 1974. X-ray mineralogy data, Campbell Plateau and Southern Tasman Sea. I n Kennett, J. P., Houtz, R. J. and others, 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 , 29. U. S. Govt. P r i n t i n g O f f i c e , Washington, D. C., 1173-1186. Cook, H. E., Zimmels, I. and M a t t i , J. C., 1975. X-ray mineralogy data A u s t r a l A n t a r c t i c region, Leg. 28, D.S.D.P. I n Hayes, D. E., Frakes, L. A., and o t h e 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 , 28. U. S. Govt. P r i n t i n g O f f i c e , Washington, D. C., 981-998. E l l i s , D. B., 1972. Holocene sediment o f t h e South A t l a n t i c Ocean: t h e c a l c i t e compensation depth and c o n c e n t r a t i o n o f c a l c i t e , opal, and quartz, Master's t h e s i s , Oregon S t a t e U n i v e r s i t y , 77 pp. Goldberg, E. D., 1958. Determination o f opal i n marine sediments. J. Mar. Res., 17: 178-182. Hashimoto, I. and Jackson, M. L., 1960. Rapid d i s s o l u t i o n o f allophane and k a o l i n i t e - h a l l o y s i t e a f t e r dehydration. I n Proceedings o f t h e 7 t h National Conferenct on Clays and Clay Minerals, Washington, D. C., Pergamon Press, London, 102-113. Hays, J. D., Cook, H. E. 111, Jenkins, D. G., Cook, F. M., F u l l e r , J. T., G o l l , R. M., Milow, E. D. and Orr, W. N., 1972. 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 , 9. U. s. Govt. P r i n t i n g O f f i c e , Washington, 0. C., 1205 pp. Heath, G. R., 1974. Dissolved s i l i c a i n deep-sea sediments. I n Hay, W. W. ( E d i t o r ) , SOC. Econ. Paleont. Miner. Spec. Pub. 20:77-93. Heath, G. R. and Dymond, J. 1977. Genesis and t r a n s f o r m a t i o n o f m e t a l l i f e r o u s sediments from t h e East P a c i f i c Rise, Bauer Deep, and Central Basin, Northwest Nazca p l a t e . Geol. SOC. Am. B u l l . , 88:723-733. Jacobs, M. B., 1974. Clay mineral changes i n A n t a r c t i c deep sea sediments and Cenozoic c l i m a t i c events. J. Sed. P e t r o l . , 44:1079-1086. Kamatani, A. and Oda, H., i n press. Determination o f b i o g e n i c s i l i c a i n marine sediments by a l k a l i n e s o l u t i o n . Deep Sea Research.

331 Leinen, M., 1977. A normative c a l c u l a t i o n f o r d e t e r m i n i n g opal i n deep sea s e d i ments. Geochim. Cosmochim. Acta, 41 : 671-676. 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 i m p l i c a t i o n s f o r Cenozoic paleoceanography: Summary. Geol SOC. Am. B u l l . , 90:801-803. M u l l e r , G. and Gastner, G., 1971. The "Karbonat-Bombe," a simple d e v i c e f o r t h e d e t e r m i n a t i o n o f t h e carbonate c o n t e n t i n sediments, s o i l s , and o t h e r m a t e r i a l s . N. Jb. Miner. Mh. 1971: 466-469. T i l l , R. and Spears, D. A., 1969. The d e t e r m i n a t i o n o f q u a r t z i n sedimentary r o c k s u s i n g an X-ray d i f f r a c t i o n technique. Clays and Clay Minerals, 17:323-327. Van Andel. T j . H., Heath, G. R. and Moore, T. C., 1975. Cenozoic h i s t o r y and paleoceanography o f 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 Ocean, Geol. SOC. Am. Memoir 143, 134 pp. Weaver, C. E. and P o l l a r d , L. D., 1973. The c h e m i s t r y o f c l a y m i n e r a l s . E l s e v i e r Amsterdam, 213 pp.

.

333

CHAPTER 19 FOSSIL DIATOMS AND THE OCEANOGRAPHY OF THE BERING SEA DURING THE LAST GLACIAL EVENT Constance Sancetta, Lamont-Doherty Geological Observatory, Palisades, N.Y. 10964, U.S.A. ABSTRACT Previous a n a l y s i s o f over 200 c o r e t o p s from t h e Bering and Okhotsk Seas d e f i n e d several assemblages o f diatoms t h a t show s i g n i f i c a n t r e l a t i o n s t o hydrography and p r o d u c t i v i t y o f t h e seas.

These i n c l u d e markers f o r 1 ) t h e

3 ) o u t e r c o n t i n e n t a l s h e l f t r a n s i t i o n water, 4 ) subsurface T-minimum, 6 ) s p r i n g p r o d u c t i v i t y . Here I

Alaskan Stream, 2 ) sea-ice, m i d - s h e l f water, 5) a

r e p o r t on a n a l y s i s o f two cores from t h e e a s t e r n Bering Sea.

A core on t h e

o u t e r c o n t i n e n t a l s h e l f shows a sharp change downward i n which t h e modern Alaskan Stream and t r a n s i t i o n assemblages a r e replaced by t h e sea-ice assemblage.

14C and amino a c i d dates i n d i c a t e an age o f 17,000 years B.P.

change.

for this

A core a t t h e base o f t h e c o n t i n e n t a l s l o p e has f i v e l i t h o l o g i c

u n i t s t h a t may correspond t o oxygen i s o t o p i c Stages 1 through 4 (Holocene t o t h e beginning o f t h e l a s t g l a c i a l ) .

T h i s c o r e a l s o shows a downward d i s -

appearance o f t h e Alaskan Stream assemblage, which i s replaced by a d i v e r s e group i n d i c a t i n g sea-ice, today.

low p r o d u c t i v i t y , and somewhat lower s a l i n i t y than

The dominant elements i n t h e g l a c i a l i n t e r v a l a r e species t h a t a r e

abundant i n t h e modern Sea o f Okhotsk, which may be i n d i c a t o r s o f subs u r f a c e water formed by prolonged w i n t e r ice-cover.

Taken together, t h e

cores i n d i c a t e t h a t d u r i n g t h e Wisconsin t h e Bering b a s i n was covered by seai c e f o r several months o f t h e year, w i t h p r o d u c t i v i t y o c c u r r i n g i n t h e sumer.

On t h e o u t e r c o n t i n e n t a l s h e l f sea-ice would appear t o have p e r s i s t e d

f o r over 6 months, and p o s s i b l y as much as 9 months o f t h e year. a f t e r 17,000 years B.P.

Some t i m e

r e t r e a t o f t h e i c e caps and r e s u l t a n t r i s e i n sea-

l e v e l allowed h i g h e r s a l i n i t y waters o f t h e Alaskan Stream t o e n t e r t h e Bering Sea, and sea-ice formation decreased t o i t s present occurrence of o n l y 1 - 2 months of t h e w i n t e r on t h e c o n t i n e n t a l s h e l f . INTRODUCTION Diatoms a r e t h e o n l y common m i c r o f o s s i l s i n sediments o f t h e Bering Sea, where 1 i t h o l o g i e s range from diatomaceous s i l t y c l a y t o diatomaceous ooze. Furthermore, d i a t m s form t h e base o f t h e f o o d c h a i n i n t h e region, so t h a t the h i g h p r o d u c t i v i t y c h a r a c t e r i s t i c o f t h e Bering Sea i s dependent upon t h i s group o f algae.

334 The Bering Sea i s t h e most n o r t h e r n e x t e n s i o n o f t h e P a c i f i c (52-65"N) and t h e s o l e communication between t h e P a c i f i c and A r c t i c Oceans.

The

effects o f c l i m a t i c changes i n v o l v i n g p o l a r warming o r c o o l i n g should be p a r t i c u l a r l y w e l l recorded i n sediments from t h i s marginal sea. Jous6 (1962, 1967) examined diatoms i n p i s t o n cores from t h e western s i d e o f t h e Bering Sea, i d e n t i f y i n g f i v e s t r a t i g r a p h i c u n i t s ( " h o r i z o n s " ) , which she b e l i e v e d represent a l t e r n a t e g l a c i a l and i n t e r g l a c i a l episodes.

The " i n t e r -

g l a c i a l s " a r e c h a r a c t e r i z e d by h i g h e r abundances o f p e l a g i c and boreal diatoms, w h i l e " g l a c i a l s " show n e r i t i c and a r c t o b o r e a l species, i n a d d i t i o n t o e x t i n c t species, suggesting c o l d e r waters and lowered sea-level w i t h increased e r o s i o n and reworking.

Koizumi (1973) examined t h e abundance of cold-water species i n

several Deep Sea D r i l l i n g P r o j e c t cores taken near t h e A l e u t i a n arc; h i s a n a l y s i s i n d i c a t e s d e f i n i t e f l u c t u a t i o n s through t h e Pleistocene, b u t h i s sample i n t e r v a l s a r e t o o w i d e l y spaced i n t h e cores t o p r o v i d e much d e t a i l on oceanographic events. I n t h i s study I examined t h e diatoms i n two cores from t h e south-eastern Bering Sea.

Changes i n diatom abundance and d i v e r s i t y , combined w i t h l i t h o -

l o g i c a l d i f f e r e n c e s and abundance o f i n d i v i d u a l marker species a l l o w a f a i r l y d e t a i l e d i n t e r p r e t a t i o n of ocean c o n d i t i o n s d u r i n g t h e l a s t g l a c i a l sequence (Wisconsin). MATERIALS AND METHODS 6119 was taken from t h e o u t e r c o n t i n e n t a l s h e l f o f t h e eastern Bering Sea (54"59'N,

166'26'W;

145 m); RC14-121 i s from t h e base o f t h e c o n t i n e n t a l slope

(54"51'N,

170"41'W;

2532 m).

6119 was sampled a t 10 cm i n t e r v a l s , w h i l e RC14-

121, which has a much h i g h e r r a t e o f sedimentation, was sampled a t 50 cm intervals.

The diatom a n a l y s i s was based on smear s l i d e s , thus p r e s e r v i n g as

c l o s e l y as p o s s i b l e t h e q u a n t i t a t i v e r e l a t i o n s h i p s of t h e sediment components. I n each sample, t h e f i r s t 300-400 specimens encountered were i d e n t i f i e d , and t h e i r abundance converted t o percent.

L i t h o l o g i c d e s c r i p t i o n s a r e qua1 i t a -

t i v e , based on t h e smear s l i d e s . 14C d a t i n g was done on t h e b u l k sediment f o r RC14-121, using a gas prop o r t i o n a l counter (S. Robinson, USGS, personal communication, 1981).

I n G119

an amino a c i d d a t e was r u n on one sample composed o f mollusc fragments.

In

a d d i t i o n , 6119 i s c o r r e l a t e d s t r a t i g r a p h i c a l l y w i t h a nearby c o r e where t h e age of a gastropod s h e l l was determined by 1 4 C d a t i n g (J.V. Gardner, USGS, personal communication, 1980). MODERN HYDROGRAPHY AND DIATOM ACCUMULATION Mean c i r c u l a t i o n i n t h e area i s t o t h e northwest, p a r a l l e l i n g t h e bathymetric contours (Kinder and Schumacher, 1981a; Fig.

1 ).

Water on

335 t h e c o n t i n e n t a l s h e l f e v e n t u a l l y t u r n s t o t h e north, and passes through t h e Bering S t r a i t s i n t o t h e A r c t i c Ocean.

tConslant

Cunnt

Water on t h e c o n t i n e n t a l slope and

+--uncerldn

Currenl

F i g u r e 1 : Surface c i r c u l a t i o n i n t h e southeastern Bering Sea i n r e l a t i o n t o core l o c a t i o n s . Depth cont o u r s a r e i n meters. The c u r r e n t along t h e A l e u t i a n Peninsula i s i n t e r m i t t e n t . ( A f t e r Kinder and Schumacher, 1981). b a s i n f o l l o w s a c y c l o n i c path, f i n a l l y e x i t i n g from t h e Bering Sea i n t h e East Kamchatka Current. The Alaskan Stream/Bering water seaward of t h e s h e l f - s l o p e break (>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 (
o f water depth, s i n c e b e n t h i c algae cannot s u r v i v e i n deep waters where t h e p e n e t r a t i o n o f l i g h t i s low (Fig. m

40

--

-

* IM

~

l 1.0

l 150

'

3

).

A secondary group a r e species of

....

' IM

" 170

I#O

170

10

110

110

I,

130

F i g u r e 3 : Abundance of MeZosiru and ParaZia i n surface sediments. These species a r e i n d i c a t o r s of m i d - t o - i n n e r s h e l f waters.

337 Nitzschia which a r e common i n sea-ice b u t n o t i n t h e water column (Horner and Alexander, 1972). Bering s h e l f (Fig.

This group i s most common i n sediments o f t h e n o r t h e r n

4 ) , and a r e i n d i c a t o r s o f t h e presence o f w i n t e r sea-ice

on t h e s h e l f and of t h e h i g h s p r i n g p r o d u c t i v i t y f o l l o w i n g i c e m e l t i n g (Sancetta, 1981a,b).

The sediments a r e diatom-bearing sandy s i l t s , w i t h an

i n t e r m e d i a t e d i v e r s i t y o f diatoms (H=2.11).

F i g u r e 4 : Abundance o f N i t z s c h i a species i n s u r f a c e sediments. These species a r e i n d i c a t o r s o f prolonged sea-ice cover. I n t h e t r a n s i t i o n zone (100-200 in) d i v e r s i t y i s h i g h e s t (H=.2.52) and t h e sediment i s a diatomaceous s i l t y c l a y .

D. maraim and o t h e r p e l a g i c species

a r e present i n low numbers, r e f l e c t i n g t h e i n f l u e n c e o f t h e Alaskan Stream/ Bering water.

Benthics and N i t z s c h i a species a r e a l s o present, b u t i n lower

n m b e r s than on t h e m i d - s h e l f , r e f l e c t i n g t h e increased water depths and

A characteristic species o f t h i s o u t e r c o n t i n e n t a l s h e l f t r a n s i t i o n zone i s Thalassionerna r e l a t i v e l y i n f r e q u e n t presence of sea-ice i n t h e area.

nitzschioides,

a cosmopolitan diatom t h a t has a wide t o l e r a n c e f o r temperature

and s a l i n i t y v a r i a t i o n s (Smayda, 1958).

T h i s species appears t o be an i n d i c a -

t o r o f l a t e r a l m i x i n g o f s h e l f and Alaskan Stream/Bering waters, and o f t h e existence o f v e r t i c a l finestructure. Two o t h e r groups occur i n surface sediments, although t h e y a r e n o t major c o n t r i b u t o r s t o t h e assemblages.

One group (Thalassiosira trifulta and

Actinocyclus curvatulus) occurs i n low numbers i n sediments from t h e Bering

basin.

These species a r e abundant i n modern sediments o f t h e Sea o f Okhotsk,

and may be r e l a t e d t o t h e occurrence of t h e subsurface temperature minimum (Sancetta, 1981b).

Dichothermal water i s a pronounced temperature minimum

( u s u a l l y
338

Dichothermal water does occur i n t h e

c l i n e which prevents v e r t i c a l mixing.

Bering Sea today (Takenouti and Ohtani, 1974), b u t t h e minimun i s n o t as pronounced as i n t h e Sea o f Okhotsk. The o t h e r group occurs m o s t l y on t h e c o n t i n e n t a l s h e l f , and c o n s i s t s o f members of t h e genus Thalassiosira (T. nordenskioezdii, T. hyaZina, T.

decipiensl. These species a r e common i n t h e p l a n k t o n and sediments o f most boreal c o n t i n e n t a l margins, and seem t o be i n d i c a t o r s of low temperatures (<4"C) and s a l i n i t i e s i n t e r m e d i a t e between those o f t h e coasts and o f t h e open ocean.

MICROFOSSILS AND LITHOLOGIES I N 6119 AND RC14-121 G119 on t h e o u t e r c o n t i n e n t a l s h e l f (145 m) has two l i t h o l o g i c u n i t s . The upper u n i t (0-50 cm) i s a diatomaceous s i l t y c l a y .

D i v e r s i t y o f diatoms i s

5 ) , and t h e assemblage i s c h a r a c t e r i z e d

somewhat g r e a t e r i n t h i s u n i t (Fig.

DIVERSITY

- 20

10

I

L

GI19 (145 m)

22 ,000 IbAI

29.300f320 I'*CI

F i g u r e 5 : Cumulative species abundance and diatom d i v e r s i t y in G119. D i v e r s i t y i s measured by t h e Shannon index. AA = amino a c i d date. Shading shows t h e environment i n d i c a t e d by each species: D. marina = A1 askan Stream/Bering Water, T. nitzschioides = Outer S h e l f T r a n s i t i o n Water, Thalassiosira species = Spring P r o d u c t i v i t y , Melosira and ParaZia species = M i d - s h e l f Benthics, Nitzschia species = Sea-ice. by t h e presence of D. marina and T. nitzschioides, and low numbers o f Nitzschia species.

The lower u n i t (60-160 cm) i s a s i l t y c l a y w i t h r a r e diatoms.

D i v e r s i t y i s lower, and t h e assemblage i s c h a r a c t e r i z e d by t h e predominance

of Nitzschia species and v i r t u a l absence o f D. marina and T. nitzschioides.

339 Benthic diatoms and t h e ThaZucsiosira group show l i t t l e d i f f e r e n c e between t h e two u n i t s . An amino a c i d d a t e on a s h e l l hash a t 75 cm g i v e s an age o f about 22,000 ybp. I n a d d i t i o n , t h e i n t e r v a l 100-110 cm can be c o r r e l a t e d s t r a t i g r a p h i c a l l y w i t h a nearby core, i n which 1 4 C was r u n on a gastropod s h e l l , y i e l d i n g an age o f 29,300

f

320 ybp (J.V. Gardner, USGS, personal communication, 1980).

RC14-121 (2532 m) can be d i v i d e d i n t o f i v e l i t h o l o g i c u n i t s (Fig.

6 ).

RC14-121

(2532 m)

mA,o.,m S l r e m m l r l n l Woln

Eq s.0

Y,d-.",ll

.

Ice

0s p m p

PlOd"
8.",hCC.

OICMlh.,~! WOII, Relorlino

F i g u r e 6 : Cumulative species abundance and diatom d i v e r s i t y i n RC14-121. Codes and symbols as i n F i g u r e 6. T. trifuZta/A. curvutuZus = Dichothermal Water. U n i t I (0-1.5 in) i s a diatom ooze.

D i v e r s i t y i s v e r y low a t t h e t o p o f t h e

u n i t , due t o t h e overwhelming dominance o f one species (D. m a r i n a ) , and i n creases toward t h e bottom.

The assemblage i s c h a r a c t e r i z e d by h i g h numbers

o f D. marina and an absence o f b e n t h i c species and N i t z s c h i a . 3.0 m) i s a diatom-bearing clayey s i l t .

U n i t I 1 (1.5-

Although t h e number of diatoms i s

low, d i v e r s i t y i s moderately h i g h (Fig.

6 ).

l a g e d i f f e r s g r e a t l y from t h a t o f U n i t I .

D. marina i s r a r e and t h e assemb-

The composition o f t h e assemb-

l a g e i s dominated b y . N i t z s c h i a species and t h e T. trifuZta/A. w i t h r a r e b e n t h i c diatoms. y i e l d e d an age o f 23,340

f

c u r v a t u l u s group,

I 4 C a n a l y s i s on a b u l k sediment sample a t 3.3 m 260 ybp.

U n i t I 1 1 (3.0-10.0 m) i s a diatom ooze,

dominated by ThaZassiosira species and t h e T. trifuZta group.

The assemblage

d i f f e r s from t h a t o f U n i t I 1 by a moderate increase o f D. marina ( F i g . and a corresponding decrease o f t h e N i t z s c h i a group.

6 ) D i v e r s i t y i s very high

i n t h e m i d d l e p a r t o f t h i s u n i t , w i t h lower values a t t h e t o p and bottom. Midway through t h e u n i t (5.0-6.0 Pliocene diatoms.

m) t h e r e i s a minor amount of reworking of

U n i t I V (10.0-12.0

toms and moderately h i g h d i v e r s i t y .

m) i s a c l a y e y s i l t w i t h v e r y r a r e d i a The most d i s t i n c t i v e aspect of t h e assemb-

340 l a g e i s t h e increased abundance o f b e n t h i c diatoms, and an i n c r e a s e i n t h e number o f r e s t i n g spores.

U n i t V (12.0-14.5

m) i s a diatomaceous clayey s i l t

w i t h an increase i n Nitzschia species and t h e T. trifuZta group and c o n t i n u i n g D i v e r s i t y i s about t h e same

presence o f b e n t h i c diatoms and r e s t i n g spores.

A t 13.5-14.0 m reworked E a r l y Oligocene calcareous nanno-

as t h a t o f U n i t I V . f o s s i l s occur.

PALEOENVIRONMENTAL INTERPRETATIONS The 1 4 C dates, combined w i t h an assumption o f r e l a t i v e l y constant sediment a t i o n r a t e s , i m p l y t h a t 6119 spans about 45,000 years (sedimentation r a t e 3.6 cm/Kyr).

I f so, t h e l i t h o l o g i c u n i t s correspond t o well-known stages i n g l o b a l

climatic history.

U n i t I i n b o t h cores represents t h e Holocene p o s t - g l a c i a l i n -

t e r v a l (0-11,000 ybp), i d e n t i f i e d as oxygen-isotope Stage I i n numerous s t u d i e s (e.g.

Shackleton, 1977).

5 ) covers t h e l a t t e r p a r t

U n i t I 1 of 6119 (Fig.

o f t h e Wisconsin ( o r Weichselian) g l a c i a l (11,000 i s o t o p e Stage 2 and p a r t of Stage 3.

-

45,000 ybp), i n c l u d i n g

I n RC14-121 U n i t I 1 represents t h e l a s t

advance and r e t r e a t o f t h e l a t e Wisconsin i c e ( i s o t o p e Stage 2; 11,000

-

20,000

U n i t 111 of RC14-121 i s e q u i v a l e n t t o t h e Wisconsin i n t e r s t a d i a l ( i s o -

ybp). tope Stage 3; 20,000

-

60,000 ybp); U n i t I V r e f l e c t s t h e beginning o f t h e

g l a c i a l i n t e r v a l (Stage 4; 60,000

-

75,000 ybp).

The age of U n i t V i s specula-

t i v e , b u t t h e general s i m i l a r i t y t o U n i t I V suggests t h a t i t may a l s o represent t h e i n c e p t i o n o f t h e Wisconsin event.

The h i g h c o n c e n t r a t i o n o f d e t r i t u s i n

U n i t s I V and V i m p l i e s an increased sedimentation r a t e d u r i n g t h i s i n t e r v a l . I f these ages a r e accepted, t h e sequence o f hydrographic events i n t h e southeast Bering Sea can be reconstructed.

By t h e end o f t h e l a s t i n t e r g l a c i a l

i n t e r v a l ( U n i t V, RC14-121) e n t r y o f r e l a t i v e l y warm and h i g h - s a l i n i t y Alaskan Stream water t o t h e Bering Sea through t h e A l e u t i a n passes was a l r e a d y r e s t r i c ted, (low number of D. marina).

There was e r o s i o n o f t h e exposed c o n t i n e n t a l

s h e l f probably by t h e Kuskokwim R i v e r (as shown by t h e reworked nannofossils, abundant d e t r i t u s and common b e n t h i c diatoms).

The common r e s t i n g spores,

which a r e r e l a t i v e l y heavy and robust, suggest some s i z e - s o r t i n g by c u r r e n t action.

During t h e t i m e covered by U n i t V, annual formation and d u r a t i o n o f

sea-ice i n b a s i w w a t e r s increased ( i n c r e a s i n g abundance o f N i t z s c h i a ) ; t h e i c e melted d u r i n g t h e summer and surface waters warmed, w h i l e dichothermal water was created below t h e h a l o c l i n e (presence of T. trifulta). The moderately h i g h d i v e r s i t y suggests some seasonal c o n t r a s t , r e s u l t i n g i n v a r i a b l e p r o d u c t i v i t y . The i n i t i a t i o n o f t r u e g l a c i a l c o n d i t i o n s ( U n i t I V ) r e s u l t e d i n an i n t e n s i f i c a t i o n of t h e c o n d i t i o n s o u t l i n e d above.

The southeast Bering Sea was

almost completely i s o l a t e d from t h e N o r t h P a c i f i c , and s a l i n i t y values were consequently lowered ( v e r y low abundance o f D. marina).

Sea-ice f o r m a t i o n and

341 the presence o f dichothermal water continued (presence o f N i t z s c h i a and T.

trifulta). Dichothermal water may have been more common, due t o a s t r o n g e r h a l o c l i n e r e s u l t i n g from t h e lowered s u r f a c e s a l i n i t y . There was an i n c r e a s e i n e r o s i o n o f t h e c o n t i n e n t a l s h e l f ( h i g h abundance o f b e n t h i c s and c l a s t i c s ) , due t o lowered sea-level

o f the continental shelf.

--

t h e Kuskokwim R i v e r must have extended across most D i v e r s i t y and p r o d u c t i v i t y appear t o have been

s i m i l a r t o t h e previous i n t e r v a l . Conditions were c o n s i d e r a b l y m i l d e r d u r i n g t h e m i d d l e Wisconsin i n t e r s t a d i a 1 ( U n i t 111, RC14-121).

T h i s i n t e r v a l a c t u a l l y represents a s e r i e s of

stades and i n t e r s t a d e s , d u r i n g which c l i m a t e a1 t e r n a t e l y improved and d e t e r i orated.

Global sea-level was i n t e r m e d i a t e between t h e modern l e v e l and t h a t

o f t h e f u l l g l a c i a l i n t e r v a l s (Dreimanis and Raukas, 1975), so t h a t connect i o n w i t h t h e North P a c i f i c was a l t e r n a t e l y increased and decreased ( f l u c t u a t i n g abundance o f D. marina, i n moderate numbers).

The d i v e r s e diatom assemb-

lage, combining elements o f several water types, suggests a good deal of seasonal v a r i a b i l i t y i n t h e basin, w i t h d i f f e r e n t p l a n k t o n assemblages f l o u r i s h i n g a t d i f f e r e n t times o f year.

During t h e w i n t e r sea-ice may have been

present (low numbers o f Nitzschia), i n which case t h e upper water column would have been u n i f o r m l y c o l d (below O'C) w i t h v e r t i c a l homogenization.

I n spring,

as t h e i c e began t o m e l t , a p r o d u c t i v i t y bloom occurred, c h a r a c t e r i z e d by t h e

Thalassiosira group, which a r e cornon members o f t h e modern s p r i n g plankton.

As summer progressed t h e s u r f a c e waters warmed, w h i l e subsurface dichothermal water was formed below t h e h a l o c l i n e (presence o f T. trifulta).

Overall,

s a l i n i t y o f t h e b a s i n waters appears t o have been somewhat lower t h a n t h a t o f today (D. marina a secondary species; ThaZassiosira groups more common). 6119, on t h e o u t e r c o n t i n e n t a l s h e l f , c o n t a i n s a r e c o r d of t h e l a s t p a r t of t h i s i n t e r s t a d i a l i n t e r v a l (60-160 cm; Fig.

5 ).

Water depth a t t h i s s i t e ,

today 145 m, must have been considerably shallower, so t h a t t h e environment would be more t h a t o f an i n n e r s h e l f .

The s h e l f appears t o have been covered

by f l o a t i n g sea-ice f o r much o f t h e year (overwhelming dominance o f NLtzschia)

--

perhaps t h i n n i n g and m e l t i n g f o r o n l y a few months o f t h e smmer, a l l o w i n g

s u r v i v a l o f t h e b e n t h i c diatoms and t h e short-blooming T'halassiosira group. The complete absence o f D. marina and v i r t u a l absence o f T. nitzschioides suggests s h e l f waters o f d i s t i n c t l y lower s a l i n i t y t h a n a t t h e s i t e today s a l i n i t i e s , i n f a c t , more l i k e those o f t h e i n n e r s h e l f (about 31.0"/,,).

-The

t r a n s i t i o n a l zone o f t h e modern o u t e r c o n t i n e n t a l s h e l f must have been g r e a t l y compressed, perhaps s i t u a t e d o v e r t h e upper c o n t i n e n t a l slope, and t h e s a l i n i t y g r a d i e n t across t h e f r o n t may have been steeper than today.

A t t h e end o f t h e Wisconsin ( U n i t 11, RC14-121; t o p o f U n i t 11, G119), t h e r e was a major advance o f c o n t i n e n t a l g l a c i e r s and a corresponding drop i n

342

sea-level ( F l i n t , 1971).

T o t a l abundance o f diatoms i n b o t h cores i s v e r y low

due t o t h e g r e a t i n c r e a s e i n c l a s t i c s t r a n s p o r t e d by r i v e r s and p o s s i b l y a l s o ice-rafting. deep.

S i t e 6119 must have been b a r e l y submerged, perhaps o n l y 20-30 m

The diatom assemblage i n t h i s i n t e r v a l i s e s s e n t i a l l y t h e same as i n

t h e lower p a r t o f U n i t 11, i m p l y i n g c o n t i n u a t i o n o f t h e c o n d i t i o n s o f prolonged w i n t e r sea-ice cover and o n l y s h o r t s p r i n g ( o r summer) blooms. The t e r m i n a l Wisconsin a t RC14-121 i s represented by an increase o f c l a s t i c s , probably r e f l e c t i n g advance o f t h e Kuskokwim R i v e r t o t h e edge of the continental shelf.

D u r a t i o n ( o r a r e a l e x t e n t ) o f sea-ice o v e r t h e b a s i n

increased ( h i g h numbers o f U i t z s c h i a ) and summer dichothermal water was present (2'. trifulta). The Alaskan Stream was a g a i n excluded from t h e Bering Sea

( v i r t u a l absence of D. m a r i n a ) , and s a l i n i t y o f b a s i n water may have been lowered as a r e s u l t . U n i t I i n b o t h cores represents t h e Holocene and t h e end o f t h e P l e i s t o cene.

Northern hemisphere i c e r e t r e a t e d r a p i d l y ( F l i n t , 1971) w i t h c o i n c i d e n t

r i s e i n sea l e v e l .

I n b o t h cores t h e disappearance o f sea-ice and i n c r e a s i n g

communication w i t h t h e Alaskan Stream i s w e l l recorded (decrease o f t h e U i t z s c h i a species and increase o f D. marina).

I n b a s i n waters (RC14-121)

disappearance o f sea-ice was abrupt, and occurred a t t h e same t i m e as a cess a t i o n o f o f f - s h e l f t r a n s p o r t by t h e Kuskokwim (absence o f benthics and c l a s tics).

During t h e Pleistocene-Holocene t r a n s i t i o n , s a l i n i t y and temperature

g r a d u a l l y increased and dichothermal water, a product o f severe w i n t e r c o o l i n g , became l e s s common (decrease o f !?. trifulta). The Pleistocene-Holocene t r a n s i t i o n i s p a r t i c u l a r l y well-documented i n 6119 (Fig.

5 ).

The l a t e s t

Wisconsin environment (50-60 cm) was t h a t o f an ice-covered i n n e r - t o - m i d d l e s h e l f (Nitzsckia dominant, w i t h b e n t h i c s and TkaZassiosira species).

This was

replaced by a m i d - t o - o u t e r s h e l f environment w i t h development o f a t r a n s i t i o n zone o f m i x i n g (40-50 cm; T. nitzschioides and b e n t h i c s ) .

F i n a l l y (above 40 cm)

t h e modern o u t e r - s h e l f t r a n s i t i o n zone was e s t a b l i s h e d , w i t h a well-developed r e g i o n of l a t e r a l m i x i n g by f i n e - s t r u c t u r e ,

and t h e Alaskan Stream/Bering water

impinging on t h e outermost s h e l f (6119) and dominating t h e waters seaward (RC14-121)

.

CONC L US I 0NS

1.

The f i v e l i t h o l o g i c u n i t s of RC14-121 probably correspond t o i s o t o p i c Stages 1 through 4, r e p r e s e n t i n g a continuous sequence from t h e end o f t h e l a s t i n t e r g l a c i a l (80,000 ybp) t o t h e Holocene.

2.

These f i v e u n i t s a r e probably e q u i v a l e n t t o JousC's (1967) Horizons I - I V f o r t h e western Bering Sea. vised

--

Ifso, JousC's age estimates must be r e -

her " i n t e r g l a c i a l " Horizon 111 i s more l i k e l y t o be t h e Wiscon-

343 s i n i n t e r s t a d i a l , w h i l e Horizon I V would be t h e beginning o f t h e Wisconsin. 3.

During U n i t V t i m e (about 80,000

-

75,000 ybp) t h e e a s t e r n Bering Sea was

somewhat i s o l a t e d from exchange w i t h t h e N o r t h P a c i f i c due t o lowered sea l e v e l .

The Kuskokwim R i v e r was e r o d i n g t h e c o n t i n e n t a l s h e l f , and

some sea-ice formed over t h e b a s i n d u r i n g t h e w i n t e r . 4.

-

The p e r i o d represented by U n i t I V (75,000

60,000 ybp) saw an i n t e n s i -

f i c a t i o n o f U n i t V c o n d i t i o n s , w i t h s u b s t a n t i a l e r o s i o n by t h e Kuskokwim (minimum sea l e v e l ) and prolonged e x t e n t o f sea-ice w i t h f o r m a t i o n o f dichothermal water.

The h a l o c l i n e may have been s t r o n g e r , as s u r f a c e

s a l i n i t i e s appear t o have been lower than those o f today. 5.

During U n i t I 1 time ( t h e Wisconsin i n t e r s t a d i a l , 60,000

-

20,000 ybp)

s e a s o n a l i t y was h i g h due t o a g r e a t e r c o n t r a s t between w i n t e r and summer conditions.

The environment o f t h e Bering b a s i n waters was somewhat

l i k e t h a t o f t h e modern Sea o f Okhotsk.

I n w i n t e r sea-ice formed so

t h a t t h e water was v e r t i c a l l y homogenized.

During s p r i n g m e l t i n g , mix-

i n g and p r o d u c t i v i t y was v e r y high, and as sumner progressed a sharp h a l o c l i n e and c o l d subsurface waters produced e x t e n s i v e dichothermal water.

On t h e o u t e r c o n t i n e n t a l s h e l f sea-ice may have p e r s i s t e d f o r

most o f t h e year, w i t h a much s h o r t e r p e r i o d f o r t h e " s p r i n g " bloom. Surface s a l i n i t y on s h e l f and i n b a s i n waters was a t l e a s t l o / lower oo t h a n a t present, and t h e 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 may have been steeper than a t present. 6.

A t t h e end o f t h e Wisconsin (20,000

-

11,000 ybp) c o n d i t i o n s d e t e r i o -

r a t e d again, w i t h lowered sea-level,

increased erosion, and prolonged

d u r a t i o n o f s h e l f and b a s i n sea-ice.

S a l i n i t y o f t h e s u r f a c e waters

appears t o have been lower, and surface temperature cooler. 7.

The Pleistocene-Holocene t r a n s i t i o n was a t i m e o f decreasing occurrence of sea-ice and r e - e n t r y o f t h e Alaskan Stream through t h e A l e u t i a n passes. I n 6119, t h e gradual r i s e o f sea-level and increase i n d e f i n i t i o n o f t h e o u t e r - s h e l f t r a n s i t i o n zone i s p a r t i c u l a r l y w e l l recorded.

ACKNOWLEDGEMENTS

I would l i k e t o thank J.V.

Gardner, USGS, who provided t h e m a t e r i a l and

dates f o r core 6119, and S. Robinson, USGS, who r a n t h e 14C a n a l y s i s on RC14121; T. Kinder, Naval Oceanographic Lab, has been most h e l p f u l i n discussions

o f hydrography.

W. Ruddiman, J. Morley and J. Hays, a l l o f LDGO, reviewed

t h e manuscript. T h i s research was supported under NSF g r a n t s 0CE79-06368 and OCE80-07301. T h i s i s L a m o n t - D o h e r t y C o n t r i b u t i o n 3306.

344

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Sea: A review o f Japanese work.

I n : D.W. hood and E.J. K e l l e y ( E d i t o r s )

Oceanography o f t h e Bering Sea, Occ. Publ. 2, I n s t i t u t e o f Marine Science, U n i v e r s i t y o f Alaska, Fairbanks, pp. 39-58.

347

CHAPTER 20 SEDIMENTARY ENVIRONMENTS OF NEOGENE DIATOMACEOUS SEDIMENTS, WEST COAST OF JAPAN

I . KOIZUMI I n s t i t u t e o f Geological Sciences, Osaka U n i v e r s i t y , Osaka (Japan)

ABSTRACT D i a t o m i t e cropping o u t along t h e west coast o f Japan i s l i m i t e d i n age t o middle and e a r l y - l a t e Miocene. The diatom blooms, r e s p o n s i b l e f o r t h e d i a t o mites, r e s u l t e d from t h e convergence o f c o l d and warm c u r r e n t s over t h e c o n t i n e n t a l slope. S i l l e d and aerated small basins adjacent t o i s l a n d s were t h e s i t e s o f d e p o s i t i o n . Two hiatuses, between 14 and 13 Ma and between 7 and 6 Ma, a r e recognized throughout t h e Neogene diatomaceous sequences. The hiatuses appear t o be a consequence o f c l i m a t i c c o o l i n g . 1

INTRODUCTION Sedimentary basins c o n t a i n i n g t h i c k s e c t i o n s o f Neogene s i l i c e o u s sedimentary

rocks a r e developed e x t e n s i v e l y along t h e west coast o f Japan (Ishiwada and Ogawa, 1976).

Outcrops o f s i l i c e o u s deposits, however, a r e w i d e l y s c a t t e r e d .

P r e c i s e c o r r e l a t i o n s and age-assignments have been accomplished by means of diatom b i o s t r a t i g r a p h y .

The diatomaceous s e c t i o n s p r o v i d e a complete Neogene

r e c o r d o f changing sedimentary environments. The purpose o f t h i s paper i s t o i d e n t i f y t h e paleoenvironmental c o n d i t i o n s i n t h e diatomaceous s e c t i o n s and t o e x p l a i n t h e reasons f o r d e p o s i t i o n o f diatomaceous sediments along t h e west coast o f Japan.

2

GEOLOGIC SETTING Subsidence d u r i n g Neogene t i m e produced l i t h o l o g i e s t h a t a r e d i f f e r e n t i n

areas marginal t o t h e Sea o f Japan and marginal t o t h e P a c i f i c .

The Japanese

I s l a n d s a r e d i v i d e d i n t o a r e g i o n o f subsidence c o n t a i n i n g abundant v o l c a n i c rocks ("Green T u f f " r e g i o n ) along t h e coast b o r d e r i n g t h e Sea o f Japan, and an area c h a r a c t e r i z e d by l e s s subsidence and l e s s volcanism ("Non-Green T u f f " r e g i o n ) along t h e P a c i f i c coast (Tanaka and Nozawa, 1977) .(Fig. 1 ) . The Neogene d e p o s i t s i n t h e "Green T u f f " r e g i o n show a major c y c l e of s e d i mentation.

Sedimentation began i n t h e l a t e - e a r l y Miocene w i t h r a p i d subsidence

and ended d u r i n g t h e l a t e Pliocene t o Pleistocene.

T y p i c a l Neogene s t r a t a

deposited d u r i n g t h i s major c y c l e occur on t h e Oga Peninsula and adjacent A k i t a City i n northwest Honshu, Japan where marine t r a n s g r e s s i o n commenced w i t h

d e p o s i t i o n o f t h e l i t t o r a l and n e r i t i c Nishikurosawa Formation i n t h e l a t e - e a r l y Miocene.

I n t h e e a r l y - m i d d l e Miocene, t h e r a t e o f subsidence increased r e s u l t -

348 I i8'

Ii6'

140'

HIR TSUZUR YORINO

AKITA

40'

OGA'

SUGOTA

d 38

-38O

NOT0

I

OK1

NO T O PEN. I ' =

OK1 IS.

,

36

AClFlC ICEAN

2

32'

1

Fig. 1. L o c a t i o n o f t h e diatomaceous sequences d e a l t w i t h i n t h i s paper. I n t h e s t r i p p e d areas t h e green t u f f rocks a r e t h e basement rocks f o r t h e Neogene siliceous strata. i n g i n d e p o s i t i o n o f s i l i c e o u s sediments assigned t o t h e Onnagawa Formation. I n m i d d l e Miocene subsidence was g r e a t e s t .

F o l l o w i n g t h i s t i m e o f g r e a t sub-

sidence, t h e backbone range (ou Mountains) l o c a t e d i n Northeast Honshu, began t o be u p l i f t e d .

During l a t e Miocene, 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

and l i t h o l o g i c types occurred.

R e l a t i v e l y pure diatomaceous sediments ( d i a t o -

m i t e s ) occur i n t h e uppermost p a r t o f t h e Onnagawa Formation and t h e lowermost p a r t o f t h e Funakawa Formation. By t h e Pliocene, t u r b i d i t e sands and s i l t s capped and d i l u t e d t h e diatomaceous sediments, as e x e m p l i f i e d by f r e q u e n t i n t e r c a l a t i o n o f sandstone i n t h e upper p a r t o f t h e Funakawa Formation.

Bathyal

t u r b i d i t e sequences and n e r i t i c d e p o s i t s r a p i d l y f i l l e d basins d u r i n g t h e e a r l y P l e i s t o c e n e ( K i t a u r a and Wakimoto Formations), which was f o l l o w e d by an episode o f u p l i f t and f o l d i n g o f t h e Neogene b a s i n a l sequences d u r i n g l a t e r Pleistocene t i m e ( I n g l e , 1975).

349

3

MATERIALS AND METHODS

Diatomaceous sequences are divided into three lithologic parts; (1) banded, hard, siliceous shale alternating with diatomite, (2) diatomite, and (3) diatoTI ME AGE M,SCALE

D IATOM DATUM LEVELS

ZONES

- Rhizosolenio

D. seminae R.curvirostris

curvirostris

-Actinocyclus oculotus

T 0.97

-Tholossiosiro

ontiquo

T 1.75

- Denticulopsis

komtschotico T 2.48

- Denticulopsis

seminoe v.

A. oculo t u s D.seminoe v. D.seminoe v. D. k o m t z h o t i c o

T 0.27

B 4.3-3.1

b

D. komtschotico

--- FT K-Ar 1FT

-- FTAr -

- - Rouxio colifornico T 5.3 a - Denticulopsis komtschotico Coscinodiscus morginotus

T 6.4 A B 6.4

bl

- K-Ar

D. hustedtii

Coscinodiscus 'kobei"

Denticulopsis louto

IO-

-- K-Ar

T 8.2

T 9.7

Denticulopsis dimorpho 0 I I. I Denticulopsis proedimorpha T I I. I

0. hustedtii

- K-Ar '

Denticulopsis proedimorpho 0 13. I

.Denticulopsis I 5-- K-Ar

hustedtii

-Denticulopsis louto

-

B 14.4

D. louto A. inaens

I Kisseleviello corina

B 15.5 A T 16.5

A. ingens

- PF

-.lctinocyc/us K. corino

ingens

B I 7.4

Fig. 2. Diatom biostratigraphy based on paleomagnetically and radiometrically dated diatom datum levels in the North Pacific region. The precise correlation and age-assignment of the Neogene diatomaceous sediments have been accomplished for western Japan. PF=Planktonic foraminifera1 datum level ; T=Top datum level ; AT=Top abundant or the highest level of the acme of the species; B=Base datum level; BT=Base abundant or the lowest level of the acme of the species.

350

maceous mudstone. Thin layers of tuff or pumiceous sandstone are intercalated with these lithologies in places. Most of diatomites and diatomaceous mudstone are bluish grey, massive, homogeneous, and bioturbated. Diatom frustules flake off the surface as true surface dries, and turning to be a yellowish white. Samples were processed with hydrogen peroxide, not with hydrochloric acid. All diatom species were identified and 200 valves per sample were counted.

4

AGE AND BIOSTRATIGRAPHY Recent advances in diatom biostratigraphy and correlations with paleomagnetic stratigraphy and radiometric dates (Burckle and Opdyke, 1977; Koizumi, 1977; Barron, 1980) provide a chronostratigraphic framework that allows reconstruction of the paleoenvironmental history of Neogene diatomaceous sediments. In this paper, however, Subzones of Denticulopsis hustedtii - D. lauta Zone are modified slightly from Koizumi (1977, 1981a) and Barron (1980,1981) because of the sporadic and local occurrences along the west coast of Japan of previously used key species. The top of Subzone a and the base of Subzone b are defined by the first occurrence of D. praedimorpha (Barron, 1980). The boundary between the top of Subzone b and the base of Subzone c is defined by the last occurrence of D. praedimorpha (Fig. 2). Most diatom assemblages from diatomites in the Akita district represent the T O C H l NAIGAWA

Y

TSUZUREKO

ORINOBE

A G E DIATOM ZONES

0

0

0

ae 0--0

$ 0

E

mf=&n 2 ! a

V

O W

pJm1n NOT E X P O S E D

I-

z-

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THICKNESS (180ml

0 I-

0

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0

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-no

ae 0

-

0

f$g&@iy

ae

o o 0-mo - n-o

zo 0

-

(30ml

g(40m)

LITHOLOGY VOLCANIC ASH

0 a SANDSTONE DIATOMACEOUS

DIATOMITE SlLlCEOUS HARD SHALE

SILTSTONE

Fig. 3. Age-assignment, correlation, variation of diatom biofacies, species diversity, and paleoclimate of the diatomaceous sections in Akita district.

351 e a r l y - l a t e Miocene Subzone a o f t h e 0. h u s t e d t i i Zone.

Banded, hard, s i l i c e o u s

shales a r e e x t e n s i v e l y developed i n t h e lower and u n d e r l y i n g p a r t o f t h i s d i a t o mite (Fig. 3 ) .

The lower h a l f o f t h e Tochinaigawa Formation, south o f H i r o s a k i

City, i s c o r r e l a t e d t o t h e m i d d l e Miocene Subzone b o f t h e D. h u s t e d t i i

-L

l a u t a Zone because o f t h e occurrences o f D. praedimorpha and D. l a u t a .

The

middle p a r t o f t h i s f o r m a t i o n i s n o t exposed along t h e Tochinai R i v e r .

I n the

uppermost p a r t o f t h e Onnagawa Formation (Shinzan Diatomaceous Mudstone Member) i n t h e Oga Peninsula, t h e Subzone b o f t h e D. h u s t e d t i i Zone assemblage i s missing.

I n Iwamisannai and Sugota areas, l a t e Miocene diatomaceous sediments,

which a r e somewhat s i l t y o r sandy, a r e c o r r e l a t e d w i t h Subzone b of t h e h u s t e d t i i Zone and Subzone a o f D. kamtschatica Zone assemblages. The d i a t o m i t e s o c c u r r i n g i n t h e Noto Peninsula have two d i f f e r e n t ageassignments ( F i g . 4).

The lower p a r t o f t h e H o j u j i Formation i s c o r r e l a t e d t o

t h e l a t e - e a r l y Miocene A c t i n o c y c l u s ingens Zone, and t h e upper p a r t o f t h i s f o r m a t i o n and t h e I i d a Formation a r e placed i n t h e e a r l y - m i d d l e Miocene D. l a u t a Zone.

On t h e o t h e r hand, t h e diatom assemblage from t h e Tsukada Formation

represent t h e Subzone b o f t h e D. h u s t e d t i i

- D.

l a u t a Zone, those from t h e

Wakura Formation c o n t a i n t h e upper p a r t o f t h e Subzone b and t h e Subzone c of t h e D. h u s t e d t i i - D. l a u t a Zone and Subzone a o f t h e D. h u s t e d t i i Zone, and t h e I i z u k a Formation i s placed i n t h e l a t e Miocene Subzone a o f t h e 0. h u s t e d t i i

IW A M I S A N N A I

OGA

0

d a

0

z ae 0--0 0

!no

0

H I ATUS

MARINE ENVIRONMENT

0S U B L I T T O R A L

0N E R l T l C OCEANIC

PALEOCLIMATIC I NT E R PRE TAT1 ON

0C O L D W A T E R

W A R M WATER

S UGOTA

352

suzu

WAKURA

AGE

WAJ I M A

DIATOM ZONES 0

0 In

z0 o_

z

m o o

0

0

z0-

z

mo 0 0

0 -

Fig. 4. Age-assignment, c o r r e l a t i o n , v a r i a t i o n o f diatom b i o f a c i e s , species d i v e r s i ty, and p a l e o c l i m a t e o f t h e diatomaceous s e c t i o n s i n t h e Noto Peninsula and Oki I s l a n d s . Symbols as i n F i g . 3. Zone.

Banded, hard, s i l i c e o u s shales a r e v i r t u a l l y l i m i t e d t o t h e l a t e - e a r l y

Miocene. The diatom assemblages from t h e Dogo Group o f t h e Oki I s l a n d s r e p r e s e n t t h e

0. l a u t a Zone assemblage ( F i g . 4 ) . 5

DIATOM BIOFACIES AND PALEOENVIRONMENTS The diatom assemblage o f each sample i s composed m a i n l y of marine diatoms.

They a r e f u r t h e r subdivided i n t o two groups; b e n t h i c and p l a n k t i c . diatoms l i v e i n t h e euphotic area i n a s u b l i t t o r a l sea. capable o f some movement along t h e bottom. t h e p h o t i c zone o f t h e water column.

Benthic

They a r e attached o r

P l a n k t i c diatoms l i v e f l o a t i n g i n

They a r e e i t h e r n e r i t i c o r oceanic.

N e r i t i c ( m o s t l y m e r o p l a n k t i c ) forms a r e found near c o a s t l i n e s .

Since many forms

pass through t h e b e n t h i c stage once i n t h e i r l i v e s , t h e y r e q u i r e shallow s h e l f environments.

Oceanic ( h o l o p l a n k t i c ) forms spend t h e i r e n t i r e e x i s t e n c e i n t h e

open ocean and pass through t h e v a r i o u s phases o f t h e i r l i f e c y c l e i n t h a t environment (Hendey, 1964; Burckle, 1978) ( F i g . 5). The h a b i t a t and mode o f l i v i n g o f each member of f o s s i l assemblages a r e p o s t u l a t e d on t h e b a s i s o f t h e i n f o r m a t i o n a v a i l a b l e from t h e i n v e s t i g a t i o n o f l i v i n g diatoms.

The h a b i t a t o f an e x t i n c t species can be assumed t o be t h e same

as t h a t o f a l i v i n g species o f t h e same genus.

I f a l l members o f t h e genus are

e x t i n c t then t h e h a b i t a t and mode o f l i v i n g of t h e f o s s i l species a r e estimated

353

by use o f s t a t i s t i c a l methods (e.g.,

Sawamura, 1973).

Marine sediments i n places a l s o c o n t a i n fresh-water diatoms t r a n s p o r t e d t o t h e s i t e v i a r i v e r s and c u r r e n t s .

Fresh-water diatoms were r a r e l y found i n t h e

samples s t u d i e d . The i n c r e a s e o f s u b l i t t o r a l diatoms c o i n c i d e s w i t h t h e i n c r e a s e o f t e r r i g e nous d e b r i s , and t h i s d e b r i s g r e a t l y d i l u t e s diatomaceous sediment as e x e m p l i f i ed by t h e s e c t i o n s from Iwamisannai, Sugota ( F i g . 3), and Wajima ( F i g . 4). The p r o p o r t i o n o f t h e t o t a l number o f n e r i t i c diatoms t o oceanic ones i n d i c a t e s t h a t assemblages a r e c h a r a c t e r i s t i c a l l y mixed ones, such as those described by Jous6 (1962) from t h e Okhotsk Sea.

It i s i n f e r r e d t h a t s i m i l a r

environmental c o n d i t i o n s p r e v a i l e d d u r i n g d e p o s i t i o n o f l a t e Miocene d i a t o maceous sediments i n Japan as a r e found on t h e c o n t i n e n t a l slopes i n t h e present day Seas o f Japan and Okhotsk.

According t o paleogeographic i n f o r m a t i o n (e.g.,

F i g . 7), t h e areas where diatomaceous sediments accumulated were slopes adjacent t o islands. Trends t h a t appear common through each o f t h e s t r a t i g r a p h i c s e c t i o n s a r e n o t d i s t i n g u i s h e d i n t h e v e r t i c a l changes o f t h e s u b l i t t o r a l , n e r i t i c , and oceanic categories.

E c o l o g i c a l c o n d i t i o n s appear t o be l o c a l and a r e i n h e r e n t t o t h e

h i s t o r y o f each sedimentary basin.

354

S U B LIT T O R A L

Actinocyclus ehr Actinoptychus se Cocconeis spp. Diploneis spp. Grammatophora spp. Melosira sulcata T r i c e r a t i u m spp.

Fig. 5.

NERITIC

OCEANIC

B i d dul phi a spp. Oenticulopsis spp. Goniothecium tenue Rhaphoneis Spp. R o u x i a spp. Stephanogonio honzawae S t e p ha nopy x i s sPP.

PLAl N Actinocyclus ingens Coscinodiscus spp. Nitzschia spp. R hiz os ole nia SPP. Thalassionema spp. Tha la s s ios ir o spp. Thalassiothrix spp. ~

Major marine environments and fossils diatoms assigned to them.

DIATOM DIVERSITY AND PALEOENVIRONMENTS The number of diatoms per gram of sediment for most samples is comparable to modern diatom ooze of pelagic sediments. The high concentration of diatom valves reflects either a slow supply of terrigenous debris or prolificity of diatoms. As mentioned, most samples are highly diatomaceous, essentially undiluted by terrigenous debris. Diatom diversity has been used as an indicator of surface water productivity. The Shannon-Wiener Equation is used as a Diversity Index: 6

S

Hs =

-x Pi log, Pi i =1

where s is the number of species in a random count of 200 valves; Pi is the relative abundance of the ith species measured from 0 to 1.0; and log, Pi is the logarithum of Pi to the base e. High values o f H, which indicate a highly diversified assemblage, c'haracterize the lower part o f the Tochinaigawa Fomation, the middle part of the Funakawa Formation in the Iwamisannai area and the upper part of the Funakawa Formation in the Sugota area (Fig. 3 ) , the lower part of the Wakura Formation, and all of the Tsukada Formation (Fig. 4).

7

PALEOCLIMATIC INTERPRETATIONS Living cold and warm water diatom species were separated for paleoclimatic analysis. Climatic fluctuations determined from the percentages between numbers

355 o f t h e c o l d and warm water species show f o u r horizons o f increased warm water species t h a t c l e a r l y stand o u t from t h e predominance o f c o l d water species; warm water species dominate near t h e boundary o f e a r l y - m i d d l e Miocene (15.5 Ma), and l a t e Miocene (9.5, 8.5,

and 7.0 Ma).

Although t h e beginnings o f t h e l a t e - e a r l y Miocene and e a r l y - l a t e Miocene warm periods a r e u n c e r t a i n , t h e d u r a t i o n s a r e s h o r t , about 1 t o 2 m.y. ( F i g s . 3 and 4).

The predominance o f warm water faunas and f l o r a s a t e a r l y - m i d d l e Miocene

time i s t h e most d i s t i n c t i v e event i n t h e b i o l o g i c a l h i s t o r y o f t h e Neogene i n Japan (e.g.,

Tsuchi, 1981).

The l a t e Miocene warming c h a r a c t e r i z e d by t h e predominant occurrences of such warm water diatom species as Coscinodiscus n o d u l i f e r , C. vetustissimus,

and

T h a l a s s i o s i r a leptopus i s recognized a t a l l s e c t i o n s d e a l t w i t h i n t h i s paper. The e a r l y - l a t e Miocene warming i s a l s o e x e m p l i f i e d by t h e warm water molluscs A t u r i a c f . minoensis, Argonauta tokunagai

, and

N a u t i l u s izumoensis (Okamoto,

1981). 8

SUMMARY AND D I S C U S S I O N

D i s t r i b u t i o n s i n space and t i m e o f t h e Neogene diatomaceous sediments a r e summarized i n r e l a t i o n t o paleoceanographic, v o l c a n i c , and p a l e o c l i m a t i c events (Fig. 6).

Pure diatomaceous sediments, mined as d i a t o m i t e s , a r e l i m i t e d t o t h e

m i d d l e and e a r l y - l a t e Miocene i n areas adjacent t o t h e Sea o f Japan. Two s h o r t h i a t u s e s between 14 and 13 Ma, and between 7 and 6 Ma occur ext e n s i v e l y i n t h e Neogene diatomaceous sequences b o t h on l a n d and below t h e deep sea f l o o r o f t h e North P a c i f i c r e g i o n (Barron, 1980; Koizumi, 1981b).

These

h i a t u s e s appear t o be a consequence o f c l i m a t i c c o o l i n g , which causes i n t e n s i f i ed deep c i r c u l a t i o n and increased carbonate d i s s o l u t i o n ( K e l l e r , 1980; K e l l e r and Barron, 1981). Most o f t h e diatomaceous sediments o f Japan a r e assumed t o have formed on t h e c o n t i n e n t a l slope, a t a depth range o f approximately 500 t o 1500 in.

T h i s depth

range i s supported by t h e paleodepth curve based on paleobathymetric i n t e r p r e t a t i o n o f b e n t h i c f o r a m i n i f e r a from t h e Neogene sediments i n t h e Oga Peninsula and Sad0 I s l a n d ( I n g l e , 1975). D e p o s i t i o n o f t h e ' l a t e - e a r l y Miocene diatomaceous sediments c o i n c i d e s w i t h increased v o l c a n i c ash beds from DSDP S i t e 438 on t h e deep sea t e r r a c e o f f n o r t h e r n Japan.

But t h e mere supply o f s i l i c a by v o l c a n i c sources, w i t h o u t

being accompanied by o t h e r n u t r i e n t s , i s n o t l i k e l y t o be t h e cause o f t h e sudden blooming o f t h e diatom p o p u l a t i o n .

I t i s n o t reasonable t o p o s t u l a t e

a cause and e f f e c t r e l a t i o n s h i p between i n c r e a s i n g volcanism and development i n t h e l a t e Miocene diatomaceous sediments. Major c o o l i n g and warming events, as i n d i c a t e d from t h e f l o r a l and faunal

356

cj

W (3

a

W

J ‘

3lVl1 A l M V 3

3N3301ld

.

3 1 V l 3

I N

3

3 i a 3 0

ai 1

HI

IAlMV3

R

Fig. 6. Biostratigraphic, paleoceanographic, volcanic, and paleoclimatic summary of the Neogene diatomaceous sequences in the “Green Tuff” region, Japan. Diagonal lines mark intervals of cool events (Keller, 1979, 1980). W represents intervals o f warm events. Paleodepth based on paleobathymetric interpretation of benthic foraminifera (Ingle, 1975) and variations of lithology. Number of volcanic ash beds from Fujioka (personal comnunication) and Kennett and Thunell (1977). Oxygen isotope curve of benthic foraminifers from the western equatorial Pacific DSDP Site 289 following Woodruff et al. (1981).

357

analyses, correspond p r e c i s e l y w i t h c o o l i n g and warming peaks i n t h e oxygen i s o t o p e curve from western e q u a t o r i a l P a c i f i c DSDP S i t e 289 (Woodruff e t a l . , 1981).

An abrupt change from a warm t o a c o l d water p e r i o d occurred d u r i n g

e a r l y - m i d d l e Miocene, approximately 14 Ma.

T h i s change c o i n c i d e s w i t h t h e

beginning o f a major c l i m a t i c event marked by massive i c e accumulation i n Anta r c t i c a , steepening o f t h e l a t i t u d i n a l temperature g r a d i e n t , and i n c r e a s i n g l y vigorous s u r f a c e c i r c u l a t i o n and primary p r o d u c t i v i t y (Kennett, 1977; I n g l e , 1981). The l a t e Miocene i s a cool p e r i o d through i t s d u r a t i o n w i t h o s c i l l a t i n g cool t o temperate c o n d i t i o n s and a major cool event a t about 9.5 Ma ( K e l l e r and Barron, 1981).

I n t h e e a r l y - l a t e Miocene, about 8.5 Ma, warm and c o l d c u r r e n t s

i n t h e o v e r l y i n g sea waters a p p a r e n t l y converged over t h e c o n t i n e n t a l slopes adjacent t o t h e Japanese I s l a n d s ( F i g . 7 ) . The s t r a t i f i c a t i o n o f water masses i n t h e l a t e Miocene t i m e i s assumed from present-day s t r a t i f i c a t i o n o f sea water around t h e Japanese I s l a n d s .

And present-day convergence o f t h e warm Kuroshio

and c o l d Oyashio Currents w i t h seasonal f l u c t u a t i o n s occurs a t about 36'N l a t i tude along t h e P a c i f i c coast o f Japan.

A submerged tongue o f c o l d water,

Oyashio Undercurrent, has been recognized under t h e Kuroshio Current a t a depth o f about 300 t o 1000 m o f f t h e e a s t coast o f c e n t r a l Japan (Omori, 1967; Okutani , 1972). ACKNOWLEDGMENTS

I thank Professor Sakuro Honda o f A k i t a U n i v e r s i t y and D r . Kayo Tanimura o f t h e N a t i o n a l Science Museum, Tokyo, f o r k i n d l y p r o v i d i n g some o f t h e samples from A k i t a d i s t r i c t and t h e s e r i e s o f samples from t h e Oki I s l a n d s .

I also

thank Drs. James R. Hein and John A. Barron o f t h e U. S. Geological Survey f o r r e v i e w i n g t h e manuscript.

T h i s research was sponsored i n p a r t by a Grant i n

A i d f o r S c i e n t i f i c Research o f t h e M i n i s t r y o f Education (C-454265, 1979). APPENDIX Diatom Taxa A c t i n o c y c l us ehrenbergi i Ral f s A c t i n o c y c l u r ingens R a t t r a y A c t i n o c y c l u s o c u l a t u s Jous6 Actinoptychus undulatus (Bai 1ey) Ral f s Coscinodiscus marginatus Ehrenberg Coscinodiscus n o d u l i f e r A. Schmidt Coscinodiscus vetustissimus Pantocsek Coscinodiscus yabei Kanaya D e n t i c u l o p s i s dimorpha (Schrader) Simonsen

358 D e n t i c u l o p s i s h u s t e d t i i (Simonsen and Kanaya) Simonsen D e n t i c u l o p s i s kamtschatica (Zabel i n a ) Simonsen Denticulopsis

lauta ( B a i l e y )

Simonsen

D e n t i c u l o p s i s praedimorpha (Akiba) Barron D e n t i c u l o p s i s seminae (Simonsen and Kanaya) Simonsen

a NW

SE

COLD CURRENT SURFACE WATER

0

-

-E

1

<

WARM CURRENT S U R F A C E WATER

=COLD

U N D E RWCAUTRERRE N T

500 L

I

w

D

I000

AKITA BO T T O M W A T E R

/

F i g . 7. Estimated paleogeography o f Japan d u r i n g t h e e a r l y - l a t e Miocene, about 8.5 Ma, and schematic model o f water mass s t r a t i f i c a t i o n across n o r t h e a s t Japan based p r i m a r i l y on data provided by Chinzei (1978).

359

Denticulopsis seminae var. fossil is (Schrader) Simonsen Goniothecium tenue Brun Ki ssel eviell a carina Sheshuk. Melosira sulcata (Ehrenberg) Kitzing Rhi zosolenia curvirostris Jous'e Rouxia californica Peragallo Stephanogonia hanzawae Kanaya Thalassiosira antiqua (Grunow) Cleve-Euler Thalassiosira leptopus (Grunow) Hasle and G. Fryxell Moll uscs Taxa Argonauta tokunagai Yokoyama Aturia minoensis Kobayashi Nautilus izumoensis Yokoyama REFERENCES Barron, J. A., 1980. Lower Miocene to Quaternary diatom biostratigraphy of DSDP Leg 57, off northeastern Japan. Init. Repts. Deep Sea Drilling Project, 57: 641-686. Barron, J. A., 1981. Late Cenozoic diatom biostratigraphy and paleocenography of the middle-latitude eastern North Pacific, Deep Sea Drilling Project Leg 63. Init. Repts. Deep Sea Drilling Project, 63: 507-538. Burckle, L. H., 1978. Marine diatoms. In: B. U. Haq and A. Boresma (Eds.), Introduction to Marine Micropaleontology. Elsevier, Amsterdam, 245-266. Burckle, L. H. and Opdyke, N., 1977. Late Neogene diatom correlations in the circum-Pacific. In: T . Saito and H. Ujiie (Eds.), Proceedings of the First International Congress on Pacific Neogene Stratigraphy, Tokyo 1976. Kaiyo Shuppan, Tokyo, 255-284. Chinzei, K., 1978. Neogene molluscan faunas in the Japanese Islands: An ecologic and zoogeographic synthesis. The Veliger, 21: 155-170. Hendey, N. I., 1964. Introductory Account of the Smaller Algae of British Coastal Waters. Fish. Invest., Ser. 4, Part 5, Bacillariophyceae (Diatoms). Her Majesty's Stationery Office, London, 317 pp. Ingle, J. C., Jr., 1975. Sununary of Late Paleogene-Neogene insular stratigraphy, paleobathymetry, and correlations, Philippine Sea and Sea of Japan region. Init. Repts. Deep Sea Drilling Project, 31: 837-855. Ingle, J. C., Jr., 1981. Origin, depositional history, and correlation of Miocene diatomites around North Pacific margin. In: R. E. Garrison et al. (Eds.), The Monterey Formation and Related Siliceous Rocks of California. Pacific Section SOC. Econ. Paleontol. Mineral. , Los Angeles, Calif., 159-179. Ishiwada, Y. and Ogawa, K., 1976. Petroleum geology of offshore areas around the Japanese Islands. United Nations ECAFE CCOP. Tech. Bull., 10: 23-34. Jous6, A. P., 1962. Stratigraphic and paleogeographic studies in the northwestern part of the Pacific Ocean. Izd. Akad. Nauk SSSR, MOSCOW, 258 pp. Keller, G., 1979. Late Neogene paleoceanography of the North Pacific DSDP Sites 173, 310, and 296. Mar. Micropaleo., 4: 159-172. Keller, G., 1980. Middle to late Miocene planktonic foraminifera1 datum levels and paleoceanography of the North and southeastern Pacific Ocean. Mar. Micropaleo. , 5: 249-281. Keller, G. and Barron, J. A., 1981. Integrated plantonic foraminiferal and diatom biochronology for the northeast Pacific and the Monterey Formation. In: R. E. Garrison et al. (Eds.), The Monterey Formation and Related Siliceous Rocks of California. Pacific Section SOC. Econ. Paleontol.

360

Mineral., Los Angeles, C a l i f . , 43-54. Kennett, J. P., 1977. Cenozoic e v o l u t i o n o f A n t a r c t i c g l a c i a t i o n , t h e circumA n t a r c t i c Ocean, and t h e i r impact on g l o b a l paleoceanography. J. Geophys., 82: 3843-3860. Kennett, J. P. and Thunell, R. C., 1977. On e x p l o s i v e Cenozoic volcanism and c l i m a t i c i m p l i c a t i o n s . Science, 196: 1231-1234. Koizumi, I.,1977. Diatom b i o s t r a t i g r a p h y i n t h e North P a c i f i c r e g i o n . I n : T. S a i t o and H. U j i i e (Eds.), Proceedings o f t h e F i r s t I n t e r n a t i o n a l Congress on P a c i f i c Neogene S t r a t i g r a p h y , Tokyo 1976. Kaiyo Shuppan, Tokyo, 235-253. Koizumi, I.,1981a. Paleoceanography o f e a r l y - m i d d l e Miocene i n Japan by means of diatom f o s s i l s . F o s s i l s , 30: 87-100. Koizumi, I . , 1981b. The h i a t u s i n t h e N o r t h P a c i f i c o c e a n - f l o o r based on diatom b i o s t r a t i g r a p h y . Mar. Science, 13: 95-100. Okamoto, K., 1981. Paleo-Tsushima S t r a i t i n f e r r e d from t h e Miocene mollusca i n t h e San-in area. F o s s i l s , 30: 49-53. Okutani, T., 1972. The Drobable s u b a r c t i c elements found i n t h e bathval meqalobenthos i n Sagami Bay: J. Oce. SOC. Japan, 28: 95-102. Omori, M., 1967. Calanus c r i s t a t u s and submergence o f t h e Oyashio water. DeepSea Res., 14: 525-532. Sawamura, K:, 1973. Diatom a n a l y s i s on t h e environment o f l a t e Neogene along t h e Japan Sea coast, n o r t h e a s t Japan. B u l l . Geol. Survey Japan, 24: 193-213. Tanaka, K. and Nozawa, T., 1977. Geology and Mineral Resources o f Japan. v o l . I,Geology. Geol. Survey Japan, 430 pp. Tsuchi, R., 1981. Symposium on "Marine biogeography o f Japan d u r i n g t h e Neogene p e r i o d " ___ Preface: A t o p i c o f marine biogeography o f Japan i n t h e e a r l y - m i d d l e Miocene. F o s s i l s , 30: 1-5. Woodruff, F., Savin, S . M . , and Douglas, R. G., 1981. Miocene s t a b l e i s o t o p e record: A d e t a i l e d deep P a c i f i c Ocean study and i t s p a l e o c l i m a t i c i m p l i c a t i o n s . Science, 212: 665-668.

361

CHAPTER 21 LATE PALEOZOIC AND MESOZOIC RADIOLARIANS FROM SOUTHWEST JAPAN

A. YAO Department o f Geosciences, Osaka C i t y U n i v e r s i t y , Osaka (Japan)

ABSTRACT Twenty-one r a d i o l a r i a n assemblages a r e recognized i n Permian through Cretaceous rocks. Study o f L a t e Paleozoic and Mesozoic r a d i o l a r i a n s c o n t r i b u t e d much t o the age determinaticjn o f c h e r t s and mudstones o f the g e o s y n c l i n a l complex o f Southwest Japan. 1

INTRODUCTION R a d i o l a r i a n remains occur abundantly i n L a t e Paleozoic and Mesozoic s i l i c e o u s

sedimentary rocks o f Southwest Japan. Yehara (1926) was t h e f i r s t t o i l l u s t r a t e r a d i o l a r i a n s from c h e r t o f the Chichibu and Shimanto B e l t s . Huzimoto (1938) d i s covered " J u r a s s i c " r a d i o l a r i a n s from calcareous s c h i s t o f t h e Sambagawa metamorp h i c rocks. Kimura (1944) and Ichikawa (1950) described r a d i o l a r i a n s from soc a l l e d L a t e Paleozoic and Mesozoic sequences. These s t u d i e s were made on t h i n s e c t i o n s o f s i l i c e o u s rocks, and were n o t successful i n e l u c i d a t i n g t h e b i o stratigraphy o f radiolarians. Since 1969, I have s t u d i e d L a t e Paleozoic and Mesozoic r a d i o l a r i a n s , espec i a l l y T r i a s s i c and J u r a s s i c ones, on t h e b a s i s o f specimens e x t r a c t e d from c h e r t and mudstone. I n a d d i t i o n , i m p o r t a n t c o n t r i b u t i o n s have been p u b l i s h e d r e c e n t l y on t h e Permian, T r i a s s i c , and Cretaceous r a d i o l a r i a n s from Southwest Japan (Nakaseko e t a l . ,

1979; Nakaseko and Nishimura, 1979, 1981; I s h i g a and

Imoto, 1980; I s h i g a e t a l .

, 1981,

1982; Takemura and Nakaseko, 1981a,b).

The r e -

s u l t s o f these s t u d i e s revealed twenty-one r a d i o l a r i a n assemblages i n Permian through Cretaceous rocks t h a t were used f o r d a t i n g c h e r t s and mudstones whose ages were unknown (see Fig. 1). Here, I review t h e r e s u l t s o f r e c e n t s t u d i e s o f Permian through Cretaceous r a d i o l a r i a n s i n Southwest Japan and r e p o r t t h e c h a r a c t e r i s t i c s o f n i n e r a d i o l a r i a n assemblages r e p r e s e n t a t i v e o f Middle T r i a s s i c through L a t e J u r a s s i c time. R a d i o l a r i a n species a r e described i n o t h e r papers. 2

GEOLOGIC SETTING The Chugoku, Mino-Tamba, and Chichibu B e l t s o f Southwest Japan (Fig. 1 ) con-

s i s t o f e x t e n s i v e L a t e Paleozoic and Mesozoic non-metamorphic sedimentary rocks

362 i n c l u d i n g c l a s t i c rocks, limestone, c h e r t , as w e l l as submar ne v o l c a n i c rocks. These sequences o f g e o s y n c l i n a l f a c i e s were f o r m e r l y lumped

ogether as "Late

Paleozoic Honshu ( o r Chichibu) g e o s y n c l i n a l deposits". As o f t e n years ago, T r i a s s i c conodonts have been discovered from t h e s o - c a l l e d L a t e Paleozoic c h e r t s i n several areas ( c f . Koike, 1979). Recently, Middle t o L a t e J u r a s s i c r a d i o l a r i a n s were found from t h e s o - c a l l e d Late Paleozoic mudstones i n t h e Mino-Tamba and Chichibu B e l t s (Yao, 1972, 1979b; Yao e t al., z a k i e t al.,

1981; M i z u t a n i e t al.,

1980a; Sugano e t al.,

1981; etc.).

1980; I s o -

Furthermore, Permian r a d i o l a r -

i a n s were e x t r a c t e d from c h e r t i n t h e Tamba B e l t ( I s h i g a and Imoto, 1980; I s h i g a e t al.,

1981, 1982; Takemura and Nakaseko, 1981b; etc.).

Thus t h e Honshu geosyn-

c l i n a l d e p o s i t s i n Southwest Japan are now known t o be composed o f Carboniferous t o Permian greenstones, limestone, and c h e r t , Middle T r i a s s i c t o E a r l y J u r a s s i c c h e r t , L a t e T r i a s s i c greenstones and limestone, and Middle t o L a t e J u r a s s i c mudstone and sandstone. J u x t a p o s i t i o n o f rocks o f d i f f e r e n t ages i n these B e l t s i s i n t e r p r e t e d t o be t h e p r o d u c t o f t e c t o n i c and/or sedimentary rearrangement. PreMiddle J u r a s s i c rocks a r e regarded as t h r u s t sheets and o l i s t o l i t h s i n Middle t o L a t e J u r a s s i c c l a s t i c sequences. I n t h e n o r t h e r n t e r r a n e of t h e Shimanto B e l t , outermost b e l t o f Southwest Japan (Fig. l),Cretaceous s t r a t a a r e extensive. Valanginian t o Cenmanian c h e r t associated w i t h greenstones occur as l e n t i c u l a r bodies i n a muddy m a t r i x t h a t y i e l d e d Cretaceous r a d i o l a r i a n s younger than those o f t h e c h e r t s (e.g.

Taira,

1981). From t h e l i t h o f a c i e s and f o s s i l contents, f o r m a t i o n o f t h e muddy m a t r i x associated w i t h c h e r t and greenstone bodies i s a t t r i b u t e d t o m i x i n g d u r i n g sedimentary process. 3

OCCURRENCE OF RADIOLARIAN ASSEMBLAGES Twenty-one r a d i o l a r i a n assemblages a r e recognized i n Permian through Creta-

ceous rocks of Southwest Japan (Table 1). They a r e grouped i n t o f o u r segments, namely Permian, Middle T r i a s s i c

-

E a r l y J u r a s s i c , Middle

-

L a t e J u r a s s i c , and

Cretaceous. 3.1

Permian assemblages I s h i g a and Imoto (1980) discovered Permian r a d i o l a r i a n s i n bedded c h e r t s from

a few l o c a l i t i e s (Lacs. 11 and 13 i n Fig. 1 ) i n t h e Tamba B e l t . Pseudoalbailassemblage and F o l l i c u c u l l u s assemblage were s e t up by them i n ascending order. The P s e u d o a l b a i l l e l l a assemblage was subdivided i n t o t h r e e sub-assemblages, namely sp. A

- p.

u-forma

-

elegans,

p.

lomentaria

- p.

l o n g i c o r n i s , and

p.

rhombothoracata sub-assemblages i n ascending order. The age on these

assemblages ranges from Wolfcampian t o l a t e Guadalupian ( I s h i g a and Imoto, 1980). Though t h e Permian c h e r t y i e l d i n g t h e two assemblages successively i s a p p a r e n t l y

363

38OFJ’

I

I

SEA

I

I

PACIFIC

OCEAN

I

1

OF

JAPAN

I

I

32V

Fig. 1. L o c a l i t i e s of Permian t o Jurassic r a d i o l a r i a n s i n Southwest Japan. I:Hida B e l t , 11: Hida Marginal B e l t , 111: Sangun-Chugoku Belt, I V : Maizuru B e l t , V: Mino-Tamba B e l t , V I : Ryoke B e l t , V I I : Sambagawa B e l t , V I I I : Chichibu B e l t , I X : Shimanto Belt. 1: Takayama (Kojima and Adachi, 1981), 2: Gujo-hachiman (Wakit a and Okamura, 1982), 3: Samondake (Wakita, 1981), 4: Kanmuri-yama and Nanjo ( I t o and Shiratake, 1980; I t o and Matsuda, 1980), 5: Imajo (Yoshimura e t al., 1982), 6: Kanayama (Piizutani, 1981), 7: H i c h i s o (Nakaseko and Nishimura, 1979; Kido and Mizutani, 1981), 8: Inuyama (Yao, 1972, 1979a,b, 1982; Yao e t al., 1980 a,b; Yao and Matsuoka, 1981; Ichikawa and Yao, 1976; Nakaseko and Nishimura, 1979), 9: Yoro (Kawaguchi and Shibata, 1981), 10: Taga ( I s h i g a e t al., 19821, 11: Keihoku-Yagi ( I s h i g a and Imoto, 1980; I s h i g a e t al., 1981, 1982; Takemura and Nakaseko, 1981a) , 12: Kameoka (Takemura and Nakaseko, 1981b), 13: Sasayama ( I s h i g a and Imoto, 1980; I s h i g a e t al., 1982), 14: Misakubo (Matsushima e t al., 1981), 15: Shima (Nakaseko and Nishimura, 1979; Sugano e t al., 1980), 16: Omine (Yamato k i n e Research Group, 1981), 17: Yuasa (Isozaki e t al., 1981), 18: K i i yura (Yao, 1980), 19: W a j i k i (Nakaseko and Nishimura, 1979), 20: Kenzan ( I s o z a k i e t al., 1981), 21: Sambosan (Yamato Omine Research Group, 19811, 22: Sakawa (Matsuoka, 1981, 1982; Matsuoka and Yao, 1981), 23: Higashitsuno ( A i t a , 1981), 24: Shirokawa (Nakatani, 1981; Nakatani and Yao, 1980, 1981), 25: Kuma (Nishizono e t al., 1981).

364

conformable, t h e F o l l i c u c u l l u s assemblage has no a l b a i l l e l l a r i a n species cmmon w i t h the P s e u d o a l b a i l l e l l a assemblage. Two new assemblages, P a r a f o l l i c u c u l l u s and A l b a i l l e l l a sp. D assemblages, were found from t h e h o r i z o n between t h e above-mentioned two assemblages i n o t h e r c h e r t s o f t h e Tamba B e l t (LOC. 11: I s h i g a e t al.,

1981).

Takemura and Nakaseko (1981b) recognized a new r a d i o l a r i a n assemblage,

K-

a l b a i l l e l l a assemblage, from t h e Tamba B e l t (LOC. 12). The age o f assemblage i s Late Permian, probably Guadalupian o r younger according t o them. I s h i g a e t a l . (1982) found t h i s assemblage above t h e h o r i z o n o f t h e F o l l i c u c u l l u s assemblage i n bedded c h e r t s o f t h e Mino-Tamba B e l t (Locs. 10, 11, and 13). Sme species o f t h e Permian assemblages occur s p o r a d i c a l l y i n c h e r t s from t h e Mino B e l t (Locs. 1 and 2) and t h e Chichibu B e l t (Locs. 16 and 24). 3.2

Middle T r i a s s i c t o E a r l y J u r a s s i c assemblages Yao e t a l . (1980a,b)

and Yao (1982) s t u d i e d t h e b i o s t r a t i g r a p h y o f conodonts

and r a d i o l a r i a n s i n t h e g e o s y n c l i n a l sequence o f the Mino B e l t (LOC. 9). Yao (1982) d i s t i n g u i s h e d f o u r successive assemblages, Triassocampe deweveri,

e-

socampe nova, Canoptum t r i a s s i c u m , and Parahsuum simplum Assemblages, i n a continuous sequence o f c h e r t . The c h a r a c t e r o f these assemblages w i l l be described i n a f o l l o w i n g p a r t o f t h i s paper. Middle and L a t e T r i a s s i c r a d i o l a r i a n s were found i n several areas (Locs. 1, 2, 4, 5, 7, 8, 15, 16, 18, 19, 20, 21, 22, 24, and 25). Nakaseko and Nishimura (1979) d i s t i n g u i s h e d t h r e e L a t e T r i a s s i c assemblages, namely Capnuchosphaera t h e l o i d e s , T r i p o c y c l i a c f . acythus, and Emiluvia

( ? ) cochleata assemblages. 3.3

M i d d l e t o L a t e J u r a s s i c assemblages Yao (1972, 1979a) and Ichikawa and Yao (1976) described Mesozoic r a d i o l a r i a n

fauna from manganese nodules and h o s t mudstone o f t h e Mino B e l t (LOC. 8). The fauna was c a l l e d

Unuma echinatus

assemblage (Yao e t al.,

1980a). The character-

i s t i c species o f t h i s assemblage have been found i n many places o f Southwest Japan (Locs. 1, 2, 3, 7, 8, 9, 15, 16, 18, 21, 22, 23, 24, and 25). Yao and Matsuoka (1981) found sp. B Assemblage below t h e h o r i z o n o f t h e echinatus Assemblage i n mudstone o f t h e F i n o B e l t (LOC. 8). Lithocampe(?) nudata Assemblage was s e t up by Matsuoka (1981: LOC. 22) and Yao and Matsuoka (1981: LOC. 8 ) above t h e h o r i z o n o f the

echinatus Assemblage.

Matsuoka and Yao (1981) e s t a b l i s h e d Gongylothorax sp. C Assemblage and D i c t y o m i t r a sp. B

-

-

Stichocapsa sp. C

D i c t y o m i t r a sp. A Assemblage w i t h i n Upper

J u r a s s i c formations o f t h e Chichibu B e l t . The species c a l l e d Gongylothorax sp. C was described as

5.

sakawaensis by tlatsuoka (1982). The c h a r a c t e r o f t h e above-

mentioned f i v e assemblages w i l l be described i n a f o l l o w i n g p a r t .

365

TABLE 1 Radiolarian assemblages o f Permian through Cretaceous age i n Southwest Japan

Amphipyndax cf. t y l o t u s Assemblage

I I

Amphipyndax e n e s s e f f i Assemblage

I

RADIOLARIAN ASSEMBLAGES

Period

Artostrobium urna Assemblage Holocryptocanium barbui Assemblage

U1tranapora p r a e s p i n i f e r a Assemblage ~~~~

~

~

E u c y r t i s micropora Assemblage

I

Obesacapsula rotunda Assemblage

Unuma echinatus Assemblage Hsuum sp. B Assemblage

Parahsuum simplum Assemblage

?I?I

Canoptum t r i a s s i c u m Assemblage Triassocampe nova Assemblage Triassocampe deweveri Assemblage

N e o a l b a i l l e l l a Assemblage F o l l i c u c u l l u s Assemblage P a r a f o l 1icucul 1us Assemblage A l b a i l l e l l a sp. D Assemblage P s e u d o a l b a i l l e l l a Assemblage

?

366

M i z u t a n i e t a l . (1981) recognized t h r e e J u r a s s i c r a d i o l a r i a n assemblages i n t h e Flino B e l t , namely D i c t y o m i t r e l l a sp. A

-

Pantanellium sp. A,

echinatus,

and M i r i f u s u s b a i l e y i Assemblages i n ascending order. Takemura and Nakaseko (1981a) reported, p r e l i m i n a r i l y ,

f i v e assemblages o f F i d d l e t o Late J u r a s s i c

age, t h a t i s Archeodictyomitra d i r e c t i p o r a t a l i u m sp. A

-

-

Eucyrtidium(?) sp. A, Pantanel-

Cecropus(?) sp. A, W i r i f u s u s guadalupensis, M i r i f u s u s b a i l e y i , and

P a r v i c i n g u l a a l t i s s i m a assemblages. 3.4

Cretaceous assemblages Nakaseko e t a1

(1979) found abundant Cretaceous r a d i o l a r i a n s i n s i l i c e o u s

and c l a s t i c sedimentary rocks from several places i n t h e n o r t h e r n t e r r a n e o f t h e Shimanto B e l t . Subsequently, Nakaseko and Nishimura (1981) d i v i d e d t h e faunas i n t o seven assemblages, Obesacapsula rotunda, E u c y r t i s micropora, Ultranapora p r a e s p i n i f e r a , Holocryptocanium barbui, Artostrobium

e, Amphipyndax

enesseffi,

and Amphipyndax c f . t y l o t u s assemblages i n ascending order. The f i r s t t h r e e assemblages a r e from c h e r t and t h e l a s t t h r e e from mudstone. The c h a r a c t e r i s t i c species o f t h e Holocryptocanium barbui assemblage occur e x t e n s i v e l y i n c h e r t , t u f f , and mudstone. These assemblage zones were c o r r e l a t e d by them t o t h e r a d i o l a r i a n zones proposed by Riedel and S a n f i l i p p o (1974) and Foreman (1977). 4

ASSEMBLAGES OF MIDDLE TRIASSIC TO LATE JURASSIC AGE

4.1

Triassocampe deweveri Assemblage References: Yao e t a l . (1980a) and Yao (1982). C h a r a c t e r i s t i c species: Triassocampe deweveri (Nakaseko and Nishimura) (Fig.

2-3),

I. sp, A, 1.sp. B y I.(?) sp. D ( F i g . 2-2), I.(?) sp. F, L.(?)sp. G (Fig. I.(?) sp. H (Fig. 2-4), I.(?) annulata (Nakaseko and Nishimura) (Fig. 2-5),

T.(?)

j a p o n i c a (Nakaseko and Nishimura) , Yeharaia elegans Nakaseko and Nishimura,

2-1),

Poulpus a f f . c u r v i s p i n u s Dumitrica, Kozur, and Mostler, Hozmadia(?) sp. A, E p t i n g i u m c f . rnanfredi Dumitrica, S i l i c a r m i g e r sp. A, Pentactinocarpus f u s i f o r m i s Dumitrica, Archaeospongoprunum j a p o n i c a Nakaseko and Nishimura,

A. tenue Naka-

seko and Nishimura, and Staurosphaera(?) sp. B. L o c a l i t i e s : Bedded c h e r t i n LOCS. 2 , 4, 5, 7, 8, 15, 16, 18, 19, 21, 22, 24, and 25. C o e x i s t i n g conodonts : Gladigondolel l a t e t h y d i s (Huckriede) and C a r i n e l l a

k-

g a r i c a (Kozur and Vegh) i n LOC. 8 ( I s o z a k i and Matsuda, 1982). Age: L a d i n i a n ( l a t e L a d i n i a n and e a r l i e r ) . Remarks: T h i s assemblage i s renamed from t h e D i c t y o m i t r e l l a sp. A assemblage (Yao e t a l . ,

1980a) because t h e nominal species, D i c t y o m i t r e l l a sp. A, i s iden-

t i f i e d w i t h T r i a s s o c a m E deweveri (Nakaseko and Nishimura). I t i s probable t h a t t h e T r i p o c y c l i a c f . acythus and E m i l u v i a ( ? ) cochleata assemblages (Nakaseko and

367

Fig. 2. C h a r a c t e r i s t i c species o f Middle T r i a s s i c t o E a r l y Jurassic r a d i o l a r i a nS. 1: Triassocam e deweveri (Nakaseko and Nishimura), 2 : Triassocam e(?) sp. D, 3: T r i d sp. G, 4: Triassocam e ( ? ) sp. H, 5: T r d ) annulata &d Nishimura), 6: Triasso!arnpe sp. C, 7: TriassocamPe nova Yao, 8: Triassocam e(?) sp. E, 9: Euc r t i d i u m ? sp. A, 10: d a m s p . A, 11: sp. A, 14: Canoptum t l i a s s i c u m Yao, 12: ~ i c t y m i ~ r ~sp. l l aC, ?)1Dre e r i c r t i u m sp. A, 15: Parahsuum simplum Yao, 16: P a m ) sp. C, 17:&c y r t i d i u m 7 sp. C, 18: Syringocapsa sp. C. A l l f i g u r e s : x142. A l l specimens: LOC. 8.

*

368

Nishimura, 1979) correspond to the Triassocampe deweveri Assemblage on account of common occurrence of 1.deweveri in both assemblages. Some species of this assemblage resemble the species from the Middle Triassic rocks of the Southern Alps (Dumitrica et al., 1980); for example, Pentactinocarpus fusiformis is common to both areas, while Triassocampe deweveri is closely allied to TrJassocampe scalaris Dumitrica, Kozur, and Mostler. 4.2

Triassocampe nova Assemblage References: Yao et al. (1980a) and Yao (1982). Characteristic species: Triassocampe Yao (Fig. 2-7), 1. sp. C (Fig. 2-6), T.(?) sp. E (Fig. 2-8) , Eucyrtidium(?) pessagnoi (Nakaseko and Nishimura) , E.(?) sp. A (Fig. 2-9), Syringocapsa batodes De Wever, Sauinabolella(?) sp. A (Fig. 2-10) , Lithomelissa(?) sp. A, Siphocampium(?) sp. A, Palaeosaturnalis sp. A, --Capnuchosphaera triassica De Wever. C. theloides De Wever, Capnodoce sarisa De Wever, and C. venusta Pessagno. Localities: Bedded chert in Locs. 7, 8, 16, 21, 22, 24, and 25. Wudstone in LOC. 24. Coexisting conodonts: Gondolella polygnathiformis Budurov and Stefanov, gondolella abneptis (Huckriede), E. postera (Kozur and Mostler), E bidentata Mosher, and Parvigondolella andrusovi Kozur and Mock in LOC. 8 (Isozaki and Matsuda, 1982). Age: Carnian to Norian (possibly up to middle Norian or Alaunian). Remarks: This assemblage represents the lower half of the Dictyomitrella sp. B assemblage proposed by Yao et al. (1980a). This assemblage includes species that were described from the Upper Triassic rocks o f Greece (De Wever et al., 1979) and Baja California (Pessagno et al. , 1979). The Capnuchosphaera theloides assemblage (Nakaseko and Nishimura, 1979) is probably involved in the Triassocampe nova Assemblage.

_.

4.3

w-

Canoptum triassicum Assemblage References: Yao et al. (1980a) and Yao (1982). Characteristic species: Canoptum triassicum Yao (Fig. 2-11), Squinabolella sp. C, Dreyericyrtium sp. A (Fig. 2-14), Haeckelicyrtium(?) sp. A, Poulpus(?) sp. A, Palaeosaturnalis gracilis (Kozur and Mostler), p. bifidus (Kozur and Mostler) , and p. multidentatus (Kozur and Hostler). Localities: Bedded chert in Locs. 8, 18, and 21. Coexisting conodonts: Epigondolella postera (Kozur and Mostler) , bidentata Mosher, Parvigondolella andrusovi Kozur and Mock, Misi kella hernsteini (Mostler), and M. posthernsteini Kozur and Mock in LOC. 8 (Isozaki and Matsuda, 1982). Age: late Norian and Rhaetian (or Sevat-Rhaetian).

r.

369 Remarks: T h i s assemblage i s separated from t h e D i c t y o m i t r e l l a sp. B assemblage (Yao e t al.,

1980a), and corresponds t o t h e P a l a e o s a t u r n a l i s g r a c i l i s and

Poulpus(?) sp. sub-assemblages (Yao e t a l .

, 1980b).

Palaeosaturnalids o f t h i s

assemblage a r e s i m i l a r t o those described by Kozur and M o s t l e r (1972) from t h e upper Norian Poetschenkalk i n A u s t r i a .

4.4

Parahsuum simplum Assemblage References: Yao e t a l .

(1980a) and Yao (1982).

C h a r a c t e r i s t i c species: Parahsuum simplum Yao (Fig. 2-15), 2-16),

Syringocapsa sp. B y

( ? ) sp. C (Fig. 2-17),

5.

sp. C (Fig. 2-18),

and P a l a e o s a t u r n a l i s sp.

p.(?) sp.

C (Fig.

Stichocapsa so. B y E u c y r t i d i u m

D.

L o c a l i t i e s : Bedded c h e r t i n Locs. 4, 5, 7, 8, 16, 18, and 22. Mudstone and t u f f i n Locs. 17 and 20. Age: e a r l y E a r l y Jurassic. Remarks: T h i s assemblage was p r i m a r i l y c a l l e d t h e D i c t y o m i t r e l l a sp. C

- E-

A assemblage (Yao e t a l . , 1980a). However, t h e species f o r m e r l y c a l l e d Archaeodictyomitra sp. A i s named Parahsuum simplum (Yao, 1982).

chaeodictyomitra

SP.

D i c t y o m i t r e l l a sp. C (Fig. 2-12) and Parahsum(?) sp. A (Fig. 2-13) a r e obtained

from t h e upper h o r i z o n o f t h e Canoptum t r i a s s i c u m Assemblage as w e l l as lower p a r t o f t h e Parahsuum simplum Assemblage. The n a s s e l l a r i a n s o f t h i s assemblage a r e more c l o s e l y a l l i e d t o t h e p r e v i o u s l y r e p o r t e d J u r a s s i c forms than t o t h e T r i a s s i c ones. I n t h e Inuyama area (LOC. 8), c h e r t c o n t a i n i n g t h i s assemblage o v e r l i e s conformably Late T r i a s s i c c h e r t t h a t y i e l d e d M i s i k e l l a p o s t h e r n s t e i n i and r a d i o l a r i a n s o f t h e Canoptum t r i a s s i c u m Assemblage (Yao e t al.,

1980a; Yao,

1982). Hsuum sp. B Assemblage

4.5

Reference: Yao and Matsuoka (1981). C h a r a c t e r i s t i c species: Hsuum sp. B (Fig. 3-1) and Spongocapsula(?) sp. C (Fig. 3-2). L o c a l i t i e s : Mudstone and t u f f i n Locs. 8 and 18. Age: l a t e E a r l y o r e a r l y M i d d l e J u r a s s i c .

Unuma , T r i c o l o c a p s a ( ? ) fusi-

Remarks: T h i s assemblage c o n t a i n s species c m o n w i t h members o f t h e echinatus Assemblage, t h a t i s Archicapsa sp. A (Fig. 3-3) formis Yao,

sp. C (Fig. 3-4),

P a r v i c i n g u l a sp. C (Fig. 3-6),

and Zartus

sp. A. 4.6

Unuma echinatus Assemblage References: Yao (1972, 1979a), Yao e t a l . (1980a), Yao and Matsuoka (1981),

and Ichikawa and Yao (1976).

370

5 echinatus

C h a r a c t e r i s t i c species:

cus Ichikawa

Ichikawa and Yao (Fig. 3-5),

u.

tVpi-

and Yao, Diacanthocapsa normalis Yao, Cyrtocapsa

( ? ) k i s o e n s i s Yao, Stlchocapsa t e g i m i n i s Yao,

Hsuum sp.

C (Fig. 3-10),

Parvicin-

gula sp. F (Fig. 3-7), Andromeda sp. C, and Podobursa sp. B. L o c a l i t i e s : Mudstone and t u f f i n Locs. 1, 2, 3, 7, 8, 9, 14, 15, 16, 18, 21, 22, 23, 24, and 25. Bedded c h e r t i n Locs. 15 and 22. Age: M i d d l e J u r a s s i c . Remarks: T h i s assemblage comprises a l a r g e number o f species c l o s e l y r e l a t e d t o t h e L a t e J u r a s s i c forms. I t i s accompanied by probable ancestors o f t h e Late J u r a s s i c n a s s e l l a r i a n s , such as non-cryptothoracic t r i c y r t i d s and s p i n y p a r v i c i n g u l i d s , Based on t h e p a l e o n t o l o g i c a l c h a r a c t e r i s t i c s and t h e b i o s t r a t i g r a p h i c a l p o s i t i o n , t h i s assemblage i n d i c a t e s probably Middle J u r a s s i c age. Baumgartn e r (1981, p. 1050) found t h e

echinatus Assemblage near t h e t o p o f t h e

Sogno Formation ( C a l l o v i a n age) i n t h e Lombardian Alps o f n o r t h e r n I t a l y . I t i s probable t h a t a t i m e gap i s p r e s e n t between t h e Parahsuum simplum Assmeblage and the 4.7

Hsuum sp.

B

- Unuma echinatus

Assemblage (Table 1).

Lithocampe(?) nudata Assemblage References: Hatsuoka (1981) and Yao and Matsuoka (1981). C h a r a c t e r i s t i c species: Lithocampe( ? ) nudata Kocher, Tricolocapsa sp. N.

Pro-

tunuma sp. J, Stichocapsa a s i a t i c a Ichikawa, and Cyrtocapsa sp. C. L o c a l i t i e s : Mudstone and t u f f i n Locs. 8, 18, and 22. Age: l a t e Middle Jurassic. Remarks: T h i s assemblage i n c l u d e s species, Lithocampe(?) nudata Kocher,

&-

c y r t i d i u m p t y c t u m Riedel and S a n f i l i p p o , and P a r v i c i n g u l a ( ? ) a l t i s s i m a (Ruest), t h a t were described from t h e Upper J u r a s s i c rocks (Baumgartner e t al., On t h e o t h e r hand, some species are common w i t h members o f t h e

1980).

Unuma echinatus

Assemblage, such as Tricolocapsa r u e s t i Tan and Gongylothorax oblongus Yao. 4.8

Gongylothorax sakawaensis

-

Stichocapsa sp. C Assemblage

References: Matsuoka and Yao (1981) and flatsuoka (1982). C h a r a c t e r i s t i c species: Gongylothorax locapsa catenarum Matsuoka,

s.( ? )

Matsuoka ( F i g . 3-11),

a-

s p i r a l i s Matsuoka, Cyrtocapsa a f f . k i s o e n s i s

Yao, and Stichocapsa sp. C ( F i g . 3-12). L o c a l i t i e s : Mudstone and t u f f i n Locs. 16, 18, 21, 22, and 24. Age: e a r l y L a t e J u r a s s i c . Remarks: T h i s assemblage i s obtained above t h e h o r i z o n o f t h e Lithocampe(?) nudata Assemblage i n t h e Upper J u r a s s i c formations o f t h e Chichibu B e l t (Matsuoka and Yao, 1981; Matsuoka, 1982). The assemblage c o n t a i n s some comnon species w i t h members o f t h e

ecninatus and Lithocampe(?) nudata Assemblages, t h a t

371

Fig. 3. C h a r a c t e r i s t i c species o f Middle t o L a t e J u r a s s i c r a d i o l a r i a n s . 1: sp. B, 2: Sponqocapsula(?) sp. C, 3: Archicapsa s p . A, 4: Vnuma sp. C, 5: Unuma echinatus Ichikawa and Yao, 6: P a r v i c i n g u l a sp. C, 7: P a r v i c i n g u l a sp. F, 8: M i r i f u s u s a f f . guadalupensis Pessagno, 9: D i c t y o m i t r e l l a sp. D, 10: sp. C, 11: Gongylothorax sakarraensis Matsuoka, 12: Stichocapsa sp. C, 13: Dict y o m i t r a sp. C, 14: D i c t y o m i t r a sp. B, 15: Pseudoeucyrtis sp. A. 8: x70. 1 7 , 6, 7, 10, and 15: x142. 3-5, 9, 13, and 14: x180. 11: x225. 12: x300. 1, 2, and 5-10: LOC. 8. 3, 4, and 11-15: LOC. 18.

372 i s Tricolocapsa p l i c a r u m Yao,

1.p a r v i p o r a Tan,

Stichocapsa convexa Yao, Eucyr-

t i d i m ( ? ) unumaensis Yao, D i c t y o m i t r e l l a sp. D (Fig. 3-9), guadalupensis Pessagno ( F i g . 3-8).

sima (Ruest)

*-

and M i r i f u s u s a f f .

Some species, such as P a r v i c i n g u l a ( ? )

and Tricolocapsa a f f . p l i c a r u m Yao, a r e comnon w i t h members of t h e

Lithocampe(?) nudata Assemblage. T h i s assemblage was p r e l i m i n a r i l y subdivided i n t o two sub-assemblages;

Gongylothorax sp. C

(5. sakawaensis:

Matsuoka, 1982)

and Stichocapsa sp. C sub-assemblages (Matsuoka and Yao, 1981). 4.9

D i c t y o m i t r a sp. B

-

D i c t y o m i t r a sp. A Assemblage

References: Nakatani and Yao (1980), Yao (1980), and Matsuoka and Yao (1981). C h a r a c t e r i s t i c species: D i c t y o m i t r a sp. B (Fig. 3-14), l l .sp. A, Pseudoeucyr-

t i s sp.

A (Fig. 3-15)

, Parvicingula

b o e s i i (Parona), Podocapsa amphitreptera

Foreman, Solenotryma sp. A, and Spongocapsula sp. A. L o c a l i t i e s : rludstone and t u f f i n Locs. 6, 16, 18, 22, and 24. Age: l a t e L a t e J u r a s s i c ( T i t h o n i a n ) . Remarks: Two assemblages, D i c t y o m i t r a sp. A assemblage (Nakatani and Yao, 1980) and D i c t y o m i t r a sp. B assemblage (Yao, 1980), combined t o form t h e

B

- g.

D.

sp.

sp. A Assemblage, and were r e c l a s s i f i e d as sub-assemblages (Matsuoka and

Yao, 1981). The comnon species w i t h t h e Gongylothorax sakawaensis

-

Stichocapsa

sp. C Assemblage a r e E u c y r t i d i u m ptyctum Riedel and S a n f i l i p p o , D i c t y o m i t r a sp. C ( F i g . 3-13), and M i r i f u f u s m e d i o d i l a t a t u s (Ruest). T i t h o n i a n amnonite, Param e l l i c e r a s ( P a r a s t r e b l i t e s ) sp. ( i d e n t i f i e d by T. Sato), was r e p o r t e d from t h e h o r i z o n o f t h i s assemblage i n LOC. 18. The H i r i f u s u s b a i l e y i Assemblage (Mizut a n i , 1981; M i z u t a n i e t al., responds t o t h e

D.

sp. B

- 9.

1981; Takemura and Nakaseko, 1981a) probably c o r sp. A Assemblage on t h e b a s i s o f presence o f cm-

mon species.

5

GENERAL REMARKS Extensive study o f L a t e Paleozoic and Mesozoic r a d o l a r i a n s i n Southwest

Japan has j u s t s t a r t e d . Work i s progressing r a p i d l y ,

nd a l a r g e volume o f data

have accumulated. Recognition and age-assignment o f assemblages a r e s t i l l i n t h e pioneer stage. Some i m p o r t a n t species r e p r e s e n t i n g t h e assemblages g i v e n i n t h i s paper a r e n o t y e t described, and t h e i n f o r m a l name, such as

Hsuum sp.

B etc.,

is

given. The f a u n a l change f r a n t h e N e o a l b a i l l e l l a Assemblage o f L a t e Permian age t o t h e Triassocampe deweveri Assemblage o f Middle T r i a s s i c age i s remarkable, b u t t h e c h a r a c t e r i s t i c s o f t h e l a t e s t Permian, E a r l y T r i a s s i c , and A n i s i a n r a d i o l a r i a n s from Japan remain t o be c l a r i f i e d . Middle T r i a s s i c t o E a r l y J u r a s s i c c h e r t i s e x t e n s i v e i n Southwest Japan, and contains i n many places t h e f o u r assemblages mentioned i n t h e preceding section.

373 These c h e r t s were considered t o be formed i n p e l a g i c

or

hemipelagic environ-

ments on t h e b a s i s o f paleogeography, range i n age, and l i t h o f a c i e s (Yao, 1981). Although t h e

Hsvum sp.

B

-

Unuma echinatus Assemblage o f Middle J u r a s s i c age

c o n t a i n s some common genera w i t h members o f t h e Parahsuum simplum Assemblage o f e a r l y E a r l y J u r a s s i c age, a t i m e gap probably occurs between them. A continuous sequence w i t h r e p r e s e n t i v e r a d i o l a r i a n s between t h e two assemblages i s n o t found i n Southwest Japan. The formations c o n t a i n i n g t h e

Hsuum sp.

B

- Unuma echinatus

Assemblage a r e m a i n l y composed o f c l a s t i c sedimentary rocks, i n d i c a t i n g t h a t a remarkable change o f sedimentary environment occurred i n E a r l y t o M i d d l e Jurass i c time. L a t e J u r a s s i c r a d i o l a r i a n s o f post-Unuma echinatus Assemblage occur e x c l u s i v e l y i n mudstone and a c i d i c t u f f . I n Southwest Japan, Cretaceous r a d i o 1 a r i a n s occur abundantly i n t h e Shimanto B e l t , and s p o r a d i c a l l y i n t h e southern p a r t o f t h e Chichibu B e l t and i n t h e L a t e Cretaceous marine b a s i n (Izumi Group) along t h e Median Tectonic Line. ACKNOWLEDGEMENT

I thank Prof. K. Ichikawa o f Department o f Geosciences, Osaka City U n i v e r s i t y and Dr. D. L. Jones o f U. S. Geological Survey f o r v a l u a b l e d i s c u s s i o n and c r i t i c a l reading o f t h e manuscript. I am indebted t o Mr. T. Matsuda, Mr. Y.

Isozaki,

Mr. C. Kurimoto, Hr. T. Nakatani, Mr. A. Matsuoka, and Mr. H. I s h i g a f o r t h e i r i m p o r t a n t c o n t r i b u t i o n s t o t h i s study. REFERENCES A i t a , Y., 1981. S t r a t i g r a p h y on t h e Sambosan Group i n t h e neighborhood o f Higashitsuno-mura, Takaoka-gun, Kochi P r e f e c t u r e . Abst. Program, 1981 Annual Weet. Geol. SOC. Japan, 161. Baumgartner, P. O., 1981. Eurorad 11, 1980 Second European Meeting o f Radiol a r i a n P a l e o n t o l o g i s t s : Current research on Cenozoic and Mesozoic r a d i o l a r i a n s . Eclogae geol. Helv., 74:1027-1061. Baumgartner, P. O., De Wever, P., and Kocher, R., 1980. C o r r e l a t i o n o f Tethyan Late Jurassic E a r l y Cretaceous r a d i o l a r i a n events. Cah. Micropal., 1980-2: 23-72. De Wever, P., S a n f i l i p p o , A., Riedel, W. R., and Gruber, B., 1979. T r i a s s i c r a d i o l a r i a n s from Greece, S i c i l y and Turkey. Micropaleontology, 25:75-110. Dumitrica, P., Kozur, H., and Mostler, H., 1980. C o n t r i b u t i o n t o t h e r a d i o l a r i an fauna o f t h e Southern Alps. Geol. Palaeont. M i t t . Innsbruck, 1O:l-46. Foreman, H. P., 1977. Mesozoic R a d i o l a r i a from t h e A t l a n t i c Basin and i t s Borderlands. I n : F. M. Swan ( E d i t o r ) , S t r a t i g r a p h i c Micropaleontology o f A t l a n t i c b a s i n and borderlands, E l s e v i e r , 305-320. Huzimoto, H., 1938. R a d i o l a r i a n remains discovered i n a c r y s t a l l i n e s c h i s t o f t h e Sambagawa system. Proc. Im. Acad. Japan, 14:252-254. Ichikawa, K., 1950. A study on t h e r a d i o l a r i a n fauna o f M t . M i t a k e i n t h e southe a s t e r n p a r t o f t h e Kwanto Mountainland, Japan. Jour. Fac. Sci. Univ. Tokyo, sec. 2, 7:281-315. Ichikawa, K. and Yao, A., 1976. Two new genera o f Mesozoic c y r t o i d r a d i o l a r i a n s f r m Japan. In: Y. Jakayanagi and J , Saito /Editors), Progress i n MicropaJeontology, Micropaleontology Press, 110-117. 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 ,

-

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374 Southwest Japan. E a r t h Science (Chi kyu Kagaku), 34:333-345. I s h i g a , H., K i t o , T., and Imoto, N., 1981. Permian r a d i o l a r i a n b i o s t r a t i g r a p h y i n Tamba D i s t r i c t . Proc. Kansai Branch, Geol. SOC. Japan, 89:2-3. I s h i g a , H., K i t o , T., and Imoto, N., 1982. L a t e Permian r a d i o l a r i a n assemblages i n t h e Tamba d i s t r i c t and a d j a c e n t area, Southwest Japan. E a r t h Science (Chi kyu Kagaku), 36:lO-22. I s o z a k i , Y., Maejima, W . , and Faruyama, S . , 1981. Occurrence o f J u r a s s i c r a d i o l a r i a n s f r o m t h e pre-Cretaceous rocks 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 , Wakayama and Tokushima P r e f e c t u r e s . Jour. Geol SOC. Japan, 87:555-558. I s o z a k i , Y. and Matsuda, T., 1982. M i d d l e and L a t e T r i a s s i c conodonts from bedded c h e r t sequences i n t h e Mino-Tamba B e l t , Southwest Japan. P a r t I:Epigond o l e l l a . Jour. Geosci., Osaka City Univ., 25:103-136. I t o , M. and Shiratake, T., 1980. R e i n v e s t i g a t i o n o f t h e Paleozoic f o r m a t i o n by means o f r a d i o l a r i a n f o s s i l s found from M t . Kanmuri i n t h e Fukui-Gifu Pref e c t u r a l Border, C e n t r a l Japan. B u l l . Assoc. Like-minded Persons Fukui Munic. Nat. Mus., 27:l-6. I t o , M. and Matsuda, T., 1980. Discovery o f 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 and T r i a s s i c Conodonts from t h e Nanjo Mountains, Western Mino B e l t , C e n t r a l Japan. Ibid., 27:7-12. Kawaguchi, I. and Shibata, K., 1981. On t h e g e o l o g i c age o f J u r a s s i c s i l i c e o u s mudstone i n t h e c e n t r a l p a r t o f t h e Yoro Mountains. Abst. Program, 1981 Annual Meet. Geol. SOC. Japan, 153. Kido, S . and M i z u t a n i , S . , 1981. Mesozoic f o r m a t i o n o f 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 o f Mino-kamo. I b i d . , 151. Kimura, T., 1944. Some r a d i o l a r i a n s i n Nippon. Jap. J. Geol. Geogr., 19:285-288. Koike, T., 1979. 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. B i o s t r a t i g r a p h y of Permian and T r i a s s i c Conodonts and H o l o t h u r i a n S c l e r i t e s i n Japan (Prof. M. Kanuma Memorial volume), 21-77. Kojima, S . and Adachi, M., 1981. Paleozoic and Mesozoic f o r m a t i o n s i n t h e easte r n p a r t o f Takayama. Abst. Program, 1981 Annual Meet, Geol. SOC. Japan, 150. Kozur, H. and M o s t l e r , H., 1972. B e i t r a e q e z u r Erforschung d e r mesozoischen R a d i o l a r i e n . T e i l I: R e v i s i o n d e r O b e r f a m i l i e Coccodiscacea Haeckel 1862 emend. und Beschreibung i h r e r t r i a s s i s c h e n V e r t r e t e r . Geol. Palaeont. M i t t . Innsbruck, 2:l-60. Matsuoka, A., 1981. M i d d l e t o L a t e J u r a s s i c r a d i o l a r i a n assemblages i n t h e Sakawa area o f t h e southern s u b b e l t o f t h e Chichibu B e l t . Proc. Kansai Branch, Geol. SOC. Japan, 90:3-4. Matsuoka, A., 1982. J u r a s s i c two-segmented Nassel l a r i a n s ( R a d i o 1 a r i a ) from Shikoku, Japan. Jour. Geosci., Osaka City Univ., 25:71-86. Matsuoka, A. and Yao, A., 1981. J u r a s s i c r a d i o l a r i a n assemblages from t h e Sakawa area, Kochi P r e f e c t u r e . Proc. Kansai Branch, Geol. SOC. Japan, 89:4-5. Matsushima, PI., Miyata, T., Kitamura, K., and Takeuchi, Y., 1981. I n v e s t i g a t i o n o f t h e Misakubo f o r m a t i o n i n t h e A k a i s h i Mountains. Abst. Program, 1981 Annual Meet. Geol. SOC. Japan, 148. M i z u t a n i , S . , 1981. A J u r a s s i c Formation i n t h e Hida-Kanayama Area, C e n t r a l Japan. B u l l . Mizunami F o s s i l Mus., 8:147-190. M i z u t a n i , S., H a t t o r i , I.,Adachi, M., Wakita, K., Okamura, Y., Kido, S., Kawaguchi, I . , and Kojima, S. , 1981. J u r a s s i c Formations i n t h e Mino Area, Cent r a l Japan. Proc. Japan Acad., 57(B):194-199. Cretaceous R a d i o l a r i a i n t h e Nakaseko, K., Nishimura, A., and Sugano, K.,1979. Shimanto B e l t , Japan. News o f Osaka M i c r o p a l e o n t o l o g i s t s , Spec. vol., 2:l-49. Nakaseko, K. and Nishimura, A., 1979. Upper T r i a s s i c R a d i o l a r i a from Southwest Japan. Sci. Rep., Col. Gen. Educ. Osaka Univ., 28:61-109. Nakaseko, K. and Nishimura, A., 1981. On t h e Holocryptocanium b a r b u i zone. Abst. Program, 1981 Annual Meet. Geol. SOC. Japan, 172. Nakatani, T., 1981. M i d d l e t o Upper J u r a s s i c f o r m a t i o n s i n t h e Chichibu B e l t of t h e Shirokawa area, Ehime P r e f e c t u r e . I b i d . , 162. Nakatani, T. and Yao, A., 1980. R a d i o l a r i a n assemblages i n t h e e q u i v a l e n t t o t h e Torinosu Group, Western Shikoku. Proc. Kansai Branch, Geol. SOC. Japan, 86: 5-6.

375 Nakatani, T. and Yao, A., 1981. T r i a s s i c r a d i o l a r i a n s from t h e M i t a k i area ( t h e n o r t h e r n s u b b e l t o f Chichibu B e l t ) , Western Shikoku. I b i d . , 88:3-4. Nishizono, Y., Nakaseko, K., and Murata, M., 1981. Mesozoic s t r a t i g r a p h y and r a d i o l a r i a n assemblages i n t h e Kuma Mountains, Kyushu. Abst. Program, 1981 Annual Meet. Geol. SOC. Japan, 169. Pessagno, E. A,, Jr., Finch, W., and Abbott, P. L., 1979. Upper T r i a s s i c Radiol a r i a from t h e San H i p o l i t o Formation, Baja C a l i f o r n i a . Micropaleontology, 25 :160-1 97. Riedel, W. R, and S a n f i l i p p o , A., 1974. R a d i o l a r i a from t h e southern I n d i a n Ocean, DSDP Leg 26. I n i t i a l Reports of t h e Deep Sea D r i l l i n g P r o j e c t , 26: 771-81 3. Sugano, K., Nakaseko, K. , and Wakimoto, R., 1980. R a d i o l a r i a n s from t h e T s u i j i Group i n t h e e a s t e r n p a r t o f t h e Shima Peninsula, Mie Prefecture, Japan. Mem. Osaka Kyoiku Univ., ser. 3, 2 8 : l l l - 1 2 1 . T a i r a , A., 1981. Process o f f o r m a t i o n o f t h e Shimanto B e l t . Kagaku, 51:516-523. Takemura, A. and Nakaseko, K., 1981a. 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 the c e n t r a l p a r t o f t h e Tamba B e l t . Abst. Program, 1981 Annual Meet. Geol. SOC. Japan, 154. Takemura, A. and Nakaseko, K., 1981b. A new Permian R a d i o l a r i a n genus from t h e Tamba B e l t , Southwest Japan. Trans. Proc. Palaeont. SOC. Japan, N. S . , 124: 208-214. Wakita, K., 1981. R a d i o l a r i a n f o s s i l s from t h e Samondake f o r m a t i o n i n t h e border area between Fukui and G i f u Prefectures. Abst. Program, 1981 Annual Meet. Geol. SOC. Japan, 149. Wakita, K. and Okamura, Y. , 1982. Mesozoic sedimentary rocks c o n t a i n i n g a l l o c h thonous blocks, Gujo-hachiman, G i f u P r e f e c t u r e , c e n t r a l Japan. B u l l . Geol. Surv. Japan, 33:161-185. Yamato Omine Research Group, 1981. Paleozoic and Mesozoic System i n t h e c e n t r a l area o f t h e K i i Mountains, Southwest Japan. Guide Book f o r t h e Excursion prepared f o r t h e 35th Annual Meet. Asso. Geol, Collabo. Japan, 88 pp. 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 o f t h e Inuyama Area, C e n t r a l Japan. P a r t I: Spongosaturnalids. Jour. Geosci., Osaka City, Univ., 15:21-64. Yao, A., 1979a. 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 o f 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. Yao, A., 1979b. 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 Honshu g e o s y n c l i n a l sequences. Abst. Program, 1979 Annual Meet. Geol. SOC. Japan, 148. Yao, A., 1980. J u r a s s i c r a d i o l a r i a n s i n t h e K i i - y u r a area. Proc. Kansai Branch, Geol. SOC. Japan, 87:lO-11. Yao, A. , 1981. Space t i m e d i s t r i b u t i o n and sedimentary environment o f Paleozoic and Mesozoic r a d i o l a r i a n c h e r t s i n Japan. Abst. Program, 1981 Annual Meet. Geol. SOC. Japan, 55-56. Yao, A., 1982. M i d d l e T r i a s s i c t o E a r l y 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. Jour. Geosci., Osaka City Univ., 25:53-70. Yao, A., Matsuda, T., and I s o z a k i , Y., 1980a. 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. Yao, A., Matsuda, T., and I s o z a k i , Y., 1980b. 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 i n Inuyama o f the.Mino B e l t . Abst. Program, 1980 Annual Meet. Geol. SOC. Japan, 221. Yao, A. and Matsuoka, A., 1981. Unuma echinatus Assemblage i n t h e Inuyama Area o f t h e Mino B e l t . Proc. Kansai Branch, Geol. SOC. Japan, 90:5-6. Yehara, S., 1926. On t h e Monobegawa and Shimantogawa s e r i e s o f Southern Shikoku. Jour. Geography, 38:l-10. Yoshimura, M., Kido, S . , and H a t t o r i , I . , 1982. S t y l o l i t i c Cherts and R a d i o l a r i an F o s s i l s i n t h e Imajo Area o f t h e Nanjo Massif, Fukui Prefecture, Central Japan. Mem. Fac. Educ. Fukui Univ., 11, 31:65-77.

CHAPTER 22 SEDIMENTARY STRUCTURES OF PERMIAN-TRIASSIC CHERTS I N THE TAMBA DISTRICT, SOUTHWEST JAPAN

N. IMOTO Department o f E a r t h Sciences, Kyoto U n i v e r s i t y o f Education, Kyoto (Japan) ABSTRACT I n t h e Tamba d i s t r i c t , Southwest Japan, Permian and T r i a s s i c c h e r t s a r e most abundant i n c h e r t s t h a t ranges i n age f r o m M i d d l e C a r b o n i f e r o u s t h r o u g h E a r l y J u r a s s i c . Permian c h e r t i s commonly u n d e r l a i n by greenstone. Between c h e r t and greenstone o c c u r many s m a l l - s c a l e , i n t e r c a l a t e d j a s p e r d e p o s i t s . T r i a s s i c c h e r t o c c u r i n b l a c k s h a l e s t h a t a r e accompanied by many s m a l l manganese d e p o s i t s . Permian c h e r t i s p r e d o m i n a n t l y r e d o r p u r p l e and c o n t a i n s m a i n l y s p u m e l l i n e r a d i o l a r i a n t e s t s w i t h v a r i o u s amount o f s i l i c e o u s sponge s p i c u l e s , and f i n e grained s i l i c e o u s m a t e r i a l s w i t h sparsely dispersed well-preserved r a d i o l a r i a n t e s t s and sponge s p i c u l e s . On t h e c o n t r a r y T r i a s s i c c h e r t i s u s u a l l y g r a y t o b l a c k and i s m a i n l y composed o f r a d i o l a r i a n t e s t s w i t h a s m a l l amount o f sponge s p i c u l e s and i n p l a c e s w i t h no sponge s p i c u l e s . The t h i c k n e s s o f l a y e r s i n t h e s e c t i o n o f Permian c h e r t v a r i e s w i d e l y ( 1 3 t o 0.5 cm), b u t most T r i a s s i c c h e r t layers a r e l e s s than 6 centimeters. B o t h Permian and T r i a s s i c c h e r t s c o n t a i n s e d i m e n t a r y s t r u c t u r e s such as p a r a l l e l and c r o s s - l a m i n a t i o n s , c u t - a n d - f i l l s t r u c t u r e , graded bedding, and p r e f e r r e d o r i e n t a t i o n o f sponge s p i c u l e s and c o n i c a l - s h a p e d r a d i o l a r i a n s h e l l s . These s t r u c t u r e s suggest t h a t bedded c h e r t s were d e p o s i t e d by t u r b i d i t y c u r r e n t s and/or bottom water c u r r e n t s c a r r y i n g mostly s i l i c e o u s d e b r i s . I propose t h a t t h e environment o f d e p o s i t i o n was above o r n e a r a submarine v o l c a n i c r i d g e o r an a r r a y o f seamounts i n L a t e P a l e o z o i c t i m e and i n r e l a t i v e l y deep o c e a n i c b a s i n s i n T r i a s s i c t i m e . Some l e n t i c u l a r c h e r t b o d i e s o f L a t e P a l e o z o i c and T r i a s s i c ages were r e a r r a n g e d as o l i s t o l i t h s i n J u r a s s i c t i m e . 1

INTRODUCTION The r e c e n t advances i n conodont and r a d i o l a r i a n b i o s t r a t i g r a p h y have c o n t r i b -

u t e d t o d e t e r m i n a t i o n o f g e o l o g i c ages o f c h e r t i n t h e Honshu Geosyncline,Japan. I n t h e Tamba d i s t r i c t o f Southwest Japan, an a r e a c o v e r e d by t h i c k sedimentary r o c k s o f t h e Honshu Geosyncline, age d e t e r m i n a t i o n s o f m a i n l y c h e r t b u t a l s o s u r r o u n d i n g c l a s t i c r o c k s have been c a r r i e d o u t f o r a decade. R e s u l t s show t h a t t h e ages o f c h e r t c o v e r two d i f f e r e n t t i m e i n t e r v a l s , namely L a t e P a l e o z o i c r a n g i n g f r o m M i d d l e C a r b o n i f e r o u s t o L a t e Permian, and Mesozoic r a n g i n g f r o m M i d d l e T r i a s s i c t o E a r l y J u r a s s i c (Tamba B e l t Research Group, 1979a, b, 1980; Yoshida and Wakita, 1975; I s o z a k i and Matsuda, 1980; I s h i g a and Imoto, 1980; Imoto e t a l . ,

1980).

R e s u l t s a l s o show t h a t c h e r t l e n s e s o f v a r i o u s s i z e s and ages o c c u r as o l i s t o l i t h s i n c l a s t i c r o c k s o f J u r a s s i c age ( I s h i g a and Tamba B e l t Research Group, 1981). Whether t h e s e o c c u r r e n c e s o f c h e r t l e n s e s a r e due t o t h e t r a n s p o r t a t i o n o f c h e r t s t r a t a f o r l o n g d i s t a n c e s by p l a t e movement and a c c r e t i o n i n an

378 a n c i e n t subduction zone (e.g.

Chipping, 1971) o r m i x i n g caused by submarine

s l i d i n g o r slumping o f c h e r t s t r a t a t h a t accumulated o r i g i n a l l y i n t h e geos y n c l i n a l b a s i n (e.g. Suyari e t a l . ,

1981) remains a c o n t r o v e r s i a l question.

Analyses o f sedimentary processes and d e p o s i t i o n a l environments o f c h e r t , e s p e c i a l l y bedded c h e r t , nave progressed. The main questions a r e whether bedded c h e r t s a r e t h e product o f t u r b i d i t y c u r r e n t s c a r r y i n g s i l i c e o u s d e b r i s , and whether c h e r t s represent d e p o s i t i o n i n deep o r shallow water ( N i s b e t and P r i c e , 1974; Garrison, 1974; Imoto and Fukutomi, 1975; I i j i m a e t a l . ,

1978; Folk and

McBride, 1978; McBride and Folk, 1979, e t c ) . Here, I describe and compare t h e occurrences, c o n s t i t u e n t s , and sedimentary s t r u c t u r e s o f bedded c h e r t s o f two d i f f e r e n t ages, Permian and T r i a s s i c , i n the Tamba d i s t r i c t and then endeavor t o i n t e r p r e t t h e sedimentary process and d e p o s i t i o n a l environments. 2

GEOLOGIC SETTING The Tamba d i s t r i c t i s s i t u a t e d on t h e i n n e r s i d e o f Southwest Japan adjacent

t o t h e Maizuru B e l t on t h e n o r t h and t o t h e Ryoke Metamorphic B e l t on t h e south ( F i g . 1 ) . This d i s t r i c t i s covered by t h i c k non-calcareous s t r a t a named t h e Tamba Group (Sakaguchi, 1961), composed m a i n l y o f shale, sandstone, bedded c h e r t , and greenstone. The Tamba Group i s d i v i d e d i n t o t h r e e formations, t h e lower, middle, and upper formations (Imoto e t a l . ,

1980). The lower f o r m a t i o n i s m a i n l y

greenstone accompanied by p h y l l i t e a t t h e base. The thickness o f t h e lower f o r mation ranges from 500 t o 700 m. The geologic age according t o f u s u l i n i d s t h a t a r e contained i n s m a l l - s c a l e limestone lenses and h y a l o c l a s t i t e , i s c h i e f l y E a r l y t o Middle Permian b u t p a r t l y Carboniferous. The base o f t h e f o r m a t i o n i s c u t by a reverse f a u l t . The middle f o r m a t i o n i s m a i n l y bedded c h e r t , 200700 m i n thickness. Conodonts and r a d i o l a r i a n s show t h e f o r m a t i o n ranges i n age from M i d d l e Carboniferous t o L a t e Permian. The upper f o r m a t i o n c o n s i s t s c h i e f l y o f b l a c k shale, i n t e r c a l a t e d T r i a s s i c t o J u r a s s i c bedded c h e r t i n t h e lower p a r t , and sandstone and sandstone-shale a l t e r n a t i o n i n t h e upper p a r t o f t h e formation. I n t h e upper p a r t , t h i n l a y e r s o f conglomerate a r e i n t e r c a l a t e d and o l i s t o l i t h s o r blocks c o n s i s t i n g o f c h e r t and sandstone predominate. The t h i c k ness o f t h e f o r m a t i o n ranges from 500 t o 2000 m o r more. Recently J u r a s s i c r a d i o l a r i a n s were e x t r a c t e d from s i l i c e o u s shale from t h e upper p a r t o f t h e f o r m a t i o n (Shimonishi and Tamba B e l t Research Group, 1981). The t o t a l thickness of t h e Tamba Group ranges from 1200 t o 3500 m. S t r a t a show f o l d s t r e n d i n g E-W and p l u n g i n g g e n t l y westward, t h e wave lengths of which range from 5 t o 20 km. S t r a t a a r e repeated as t h e r e s u l t o f several f a u l t s t r e n d i n g i n an E-W d i r e c t i o n . I n t h e western p a r t o f t h e d i s t r i c t , t h e Tamba Group i s covered d i s c o r d a n t l y by Cretaceous l a c u s t r i n e s t r a t a o f t h e Sasayama Group and mainly a c i d p y r o c l a s t i c

379

Figure 1 . Location of the Tamba d i s t r i c t . S = Sasayama and F = Funaeda. rocks of the Arima Group. Several g r a n i t i c stocks intruded the Tamba Group during Cretaceous time. OCCURRENCE OF CHERT There are three types of chert in the Tamba d i s t r i c t , bedded, massive, and nodular, bedded chert being dominant. 3

3.1

u s i v e u d u l a r chwb Massive chert occurs i n two ways. Massive chert can have a jasper-like appearance, several to ten meters thick, occurs on the top o r within the greenstone, and was formerly mined as raw materials f o r f i r e bricks. Massive chert can also underlie bedded manganese deposits intercalated i n Triassic and Jurass i c bedded cherts. T h i s type i s black in color and mostly 0.5-2 m thick. Both types o f massive chert have similar colloform and/or brecciated textures made of colored chert and white quartz veins. Colored chert i s chiefly composed of microcrystalline and/or cryptocrystalline quartz with disseminated f i n e iron oxides i n jasper-like massive chert and carbonaceous matter i n massive chert occurring in manganese deposits. White quartz veins consist mainly of megaquartz in the sense o f McBride and Thomson (1970) and a subordinate amount of chalcedonic quartz. Radiolarians and sponge spicules a r e rare. Nodular chert occurs rarely as irregular shaped nodules o r concretions of

380 cobble to, i n places, boulder s i z e s i n T r i a s s i c m i c r i t i c limestone. I t i s dusky w h i t e i n appearance and i s m a i n l y composed o f m i c r o c r y s t a l l i n e and/or c r y p t o c r y s t a l l i n e q u a r t z . T h i s type o f c h e r t a l s o c o n t a i n s few s i l i c e o u s skeletons. 3.2

Bedded c h e r t

( i ) Late Paleozoic bedded c h e r t . According t o t h e newly developed r a d i o l a r i a n zonation (Ormiston and Babcock, 1979; Holdsworth and Jones, 1980; I s h i g a and Imoto, 1980; Kozur, 1981; Takemura and Nakaseko, 1981; I s h i g a e t a l . , 19821, most of t h e Late Paleozoic c h e r t i n t h e Tamba d i s t r i c t a r e Wolfcampian-Guadalupi s n and i n places lower p a r t s may p o s s i b l y be M i d d l e Carboniferous on t h e b a s i s o f conodonts. No continuous succession from Permian through T r i a s s i c time has been r e p o r t e d . Two types o f occurrences o f L a t e Paleozoic c h e r t a r e recognized. One t y p e l i e s d i r e c t l y on greenstone. Sometimes j a s p e r - l i k e massive c h e r t i s i n t e r c a l a t e d between bedded c h e r t and greenstone. The o t h e r type occurs as o l i s t o l i t h s i n t h e c l a s t i c rocks o f t h e upper f o r m a t i o n o f t h e Tamba Group. Chert on greenstone ranges i n thickness from 300 t o 750 m and continues 10 km o r more l a t e r a l l y . I s h i g a e t a l . (1981) estimated t h a t t h e thickness o f bedded c h e r t r a n g i n g i n age from Wolfcampian through Guadalupian i s no more than 50 m, t h i c k e r outcrops being t h e r e s u l t o f r e p e t i t i o n due t o t e c t o n i c t h i c k n i n g . Yao e t a l . (1980) r e p o r t e d t h e same t y p e o f r e p e t i t i o n o c c u r r i n g i n T r i a s s i c bedded c h e r t a t Inuyama, c e n t r a l Japan. Permian c h e r t i n t h e Tamba d i s t r i c t , a r a t h e r t h i c k f o r m a t i o n o f bedded c h e r t may a l s o c o n s i s t o f stacked t h i n n e r b i o stratigraphic units.

I n some s e c t i o n s b i o s t r a t i g r a p h i c u n i t s a r e missing. I n t h e Sasayama area, western p a r t o f t h e Tamba d i s t r i c t , occurs a c l e a r break i n r a d i o l a r i a n fauna between t h e P s e u d o a l b a i l l e l l Q and F o l l i c u c u l l u s assemblages w i t h o u t any t e c t o n i c d i s t u r b a n c e ( I s h i g a and Imoto, 1980). A t t h e Funaeda s e c t i o n about 30 km e a s t o f t h e Sasayama area, a continuous succession e x i s t s i n c l u d i n g two assemblages, A l b a i l l e l l a sp. D and P a r a f o l l i c u c u l l u g assemblages from&. (Ishiga e t al.,

to

a.assemblages

1981). The thickness o f s t r a t a encompassing these two assem-

blages was 12 m o r more. The c o l o r of L a t e Paleozoic c h e r t i s g e n e r a l l y red, chocolate brown, o r i n places p u r p l i s h r e d near t h e border o f greenstone f o r m a t i o n and changes graduall y t o gray, l i g h t gray, o r b l u i s h gray upward over an i n t e r v a l o f several tens

o f meters. Chert occurs as o l i s t o l i t h s o r b l o c k s i n J u r a s s i c c l a s t i c racks i n t h e upper f o r m a t i o n o f t h e Tamba Group. Thus f a r a few b l o c k s ranging i n thickness from several meters t o several tens o f meters o f L a t e Paleozoic age have been i d e n t i -

fied. Pal eozoi c ages in c l ude M i d d l e Carboni ferous , Wol fcampi an-Guadal u p i an or l a t e r , and Guadalupian proper ( I s h i g a and Tamba B e l t Research Group, 1981).

381

0

5

10km

Figure 2. Geologic map o f t h e c e n t r a l and southern p a r t s o f t h e Tamba d i s t r i c t . Cherts a r e red, chocolate brown, dark gray, l i g h t gray, and o t h e r colors.

(ii) Mesozoic bedded c h e r t . The ages determined by conodonts range from Middle through Late T r i a s s i c , E a r l y T r i a s s i c conodonts being rare. Coexisting radiol a r i a n s w i t h Middle-Late r r i a s s i c conodonts resemble fauna examined a t Inuyama, c e n t r a l Japan, by Yao e t a l . (1980). Though most bedded c h e r t formations i n t h e Tamba d i s t r i c t are T r i a s s i c , Jurassic bedded c h e r t has r e c e n t l y been found based on a r a d i o l a r i a n assemblage

382 s i m i l a r t o Parahsuum simplum assemblage i n t h e sense o f Yao (1982). Most Mesozoic c h e r t i s i n t e r c a l a t e d w i t h shale, w i t h a few exceptions accompanied by greenstone. I n some s e c t i o n s c h e r t s a r e a p p a r e n t l y up t o about 700 m t h i c k and extend more than 10 km l a t e r a l l y . Continuous s t r a t i g r a p h i c s e c t i o n s from T r i a s s i c c h e r t t o J u r a s s i c s i l i c e o u s shale were r e p o r t e d ( I s o z a k i and Matsuda, 19801, however, many o f t h e s e c t i o n s from l e n t i c u l a r bodies being l e s s than about a hundred meters t h i c k and l e s s than a few k i l o m e t e r s i n l a t e r a l e x t e n t . I n places many s m a l l - s c a l e T r i a s s i c c h e r t lenses were i n c l u d e d as o l i s t o l i t h s i n J u r a s s i c shale ( I s h i g a and Tamba B e l t Research Group, 1981). Except f o r a few r e d c h e r t lenses, most Mesozoic c h e r t s a r e gray, dark gray, o r l e s s commonly b l a c k i n c o l o r i n t h e c e n t r a l and southern p a r t s o f t h e Tamba d i s t r i c t . These c h e r t s a r e i n s t r i k i n g c o n t r a s t t o t h e Permian c h e r t s i n t h e Tamba d i s t r i c t and t h e r e d T r i a s s i c - J u r a s s i c c h e r t s a t Inuyama, c e n t r a l Japan described by Yao e t a l . (1980). Many s m a l l - s c a l e manganese o r e deposits composed o f m a i n l y rhodochrosite, hausmannite, t e p h r o i t e , and rhodoni t e a r e i n t e r c a l a t e d conformably i n t h e c h e r t . 4

LITHOLOGY Next, I d e s c r i b e c o n s t i t u e n t s and sedimentary s t r u c t u r e s o f c h e r t according

t o o b s e r v a t i o n o f HF etched surfaces. 4.1

Constituents The p r i n c i p a l c o n s t i t u e n t s o f bedded c h e r t i n t h e Tamba d i s t r i c t a r e r a d i o -

l a r i a n t e s t s , sponge s p i c u l e s , and very f i n e s i l i c e o u s m a t e r i a l s . Minor c o n s t i t uents a r e conodonts, f i s h t e e t h , s m a l l e r f o r a m i n i f e r a , and c l a y s . I n o r d e r t o compare c h e r t s I determined t h e r a t i o o f t h e number o f r a d i o l a r i an t e s t s l a r g e r than 0.125 mn i n diameter versus these r a d i o l a r i a n t e s t s p l u s sponge s p i c u l e s l o n g e r than 0.25 mn i n l e n g t h , and t h e percentage o f m a t r i x t h a t i n c l u d e s s m a l l e r s i l i c e o u s skeletons and f i n e grained s i l i c e o u s and a r g i l l a c e o u s m a t e r i a l . Other m a t e r i a l s such as conodonts, f i s h teeth, q u a r t z veins, e t c . were n o t measured. A f t e r g r i n d i n g by # l o 0 0 abrasive, c h e r t s were t r e a t e d w i t h 10% HF f o r 1-2 hours. The o b s e r v a t i o n and measurement were made on t h e etched surfaces p a r a l l e l t o a bedding plane o f c h e r t u s i n g a b i n o c u l a r microscope a t 80 m a g n i f i c a t i o n w i t h an attached square ocular-micrometers 5 m on a s i d e and each d i vided i n t o f i v e equal p a r t s . 200 t o 300 skeletons and about 1000 i n t e r s e c t i n g p o i n t s were counted t o determine t h e r a t i o o f r a d i o l a r i a n s versus sponge s p i c u l e s and t o e s t i m a t e t h e amount o f m a t r i x r e s p e c t i v e l y . The m a t r i x as d e t e r mined by t h i s technique i s l e s s than t h a t determined from t h i n s e c t i o n o r p o l i s h e d surfaces.

Thus, I c a l l t h i s an apparent m a t r i x .

The s i l i c e o u s p a r t of t h e apparent m a t r i x c o n t a i n s detached spines and f r a g ments o f r a d i o l a r i a n skeletons and/or d i s c r e t e sponge s p i c u l e s , and very f i n e

383

% n 0 a

0 0

8

3

0O

0

0

0

0 0 0

I

50

100%

APPARENT MATRIX Figure 3. Constituents o f Permian and T r i a s s i c bedded cherts i n the Tamba district

.

grains o f quartz. As well-preserved r a d i o l a r i a n remnants a r e f r e q u e n t l y d i s persed s p o r a d i c a l l y i n the quartz o f Permian cherts, t h e s i l i c e o u s . f i n e p a r t i c l e s probably accumulated i n i t i a l l y as s i l i c e o u s muds. Whether the s i l i c e o u s f i n e p a r t i c l e s o r i g i n a t e d from biogenic skeletons i s n o t known. The c o n s t i t u e n t s o f T r i a s s i c cherts a r e confined w i t h i n narrow l i m i t s compared w i t h Permian cherts (Fig. 3 ) . I t i s noteworthy t h a t i n Permian cherts t h e number o f sponge spicules i n some rocks exceeds r a d i o l a r i a n skeletons and a l a r g e number o f r a d i o l a r i a n s a r e densely aggregated i n a few rocks.

384

C l a s s i f i c a t i o n o f c h e r t according t o t h e i r c o n s t i t u e n t s has been u s u a l l y simple such as r a d i o l a r i a n c h e r t , s p i c u l i t i c c h e r t , and diatomaceous c h e r t . However, r a d i o l a r i a n skeletons and sponge s p i c u l e s mix e m p i r i c a l l y w i t h each o t h e r i n various p r o p o r t i o n s w i t h i n Late Paleozoic and Mesozoic c h e r t s i n t h e Tamba d i s t r i c t . The proposed measurement can be u s e f u l f o r t h e d e t a i l e d c l a s s i f i c a t i o n o f chert. 4.2

Sedimentary s t r u c t u r e s Some sedimentary s t r u c t u r e s , such as p a r a l l e l and cross-laminations,

graded

bedding, c u t - a n d - f i l l s t r u c t u r e s , p r e f e r r e d o r i e n t a t i o n o f sponge s p i c u l e s , detached r a d i o l a r i a n spines, and conical-shaped r a d i o l a r i a n skeletons were seen i n b o t h Paleozoic and Mesozoic bedded c h e r t s i n t h e Tamba d i s t r i c t . Also a few kinds o f c u r r e n t marks were seen on t h e soles o f bedded c h e r t s (e.g.

Imotoetal.,

1974). (i)

P a r a l l e l and cross-laminations.

P a r a l l e l l a m i n a t i o n i n bedded c h e r t s a r e

c h i e f l y made up o f s i l i c e o u s and clayey l a y e r s ( F i g . 4). The thickness o f each l a y e r i s a few m i l l i m e t e r s o r l e s s . The boundaries o f adjacent l a y e r s a r e genera l l y i n d i s t i n c t b u t when a h i g h c l a y c o n t e n t occurs boundaries a r e m o r e d i s t i n c t .

F i g u r e 4. P a r a l l e l l a m i n a t i o n and c u t - a n d - f i l l s t r u c t u r e .

385

Figure 5. Cross-lamination. I n places cross-laminations formed by t h e alignment o f s i l i c e o u s skeletons a r e seen (Fig. 5). Most o f cross-laminations are tabular-shaped w i t h dips of beds from about 10" t o 30". ( i i ) Graded bedding. Two types o f graded bedding e x i s t . F i r s t , s p h e r i c a l shaped spumelline t e s t s d i m i n i s h i n s i z e and amount i n one d i r e c t i o n through a c h e r t l a y e r (Fig. 6a), and the boundary between a c h e r t l a y e r and a shale p a r t i n g i s i n d i s t i n c t and t h e a r g i l l a c e o u s content i n a c h e r t l a y e r tends t o i n crease near t h e boundary. The second type shows symmetrical grading i n both t e s t s i z e and amount from t h e center o f a c h e r t ' l a y e r outward i n both d i r e c t i o n s . I n symmetrical graded beds e i t h e r t h e t e s t s i z e may increase (Fig. 6b; I i j i m a e t al.,

1978), or de-

crease. Beds showing an increase i n t e s t s i z e have a sharp and even contact w i t h shale p a r t i n g s and cherts where the t e s t s i z e decreases t h e c h e r t l a y e r grades i n t o t h e shale p a r t i n g w i t h o u t c l e a r boundaries. (iii)C u t - a n d - f i l l s t r u c t u r e . I n places small-scale c u t - a n d - f i l l s t r u c t u r e s a r e seen. A t Sasayama, western Tamba d i s t r i c t , c u t - a n d - f i l l s t r u c t u r e s (Fig. 4) occur near where a break e x i s t s w i t h o u t any apparent l i t h o l o g i c d i s c o n t i n u i t y between the P s e u d o a l b a i l l e l l g and f o l l i c u c u l l u s r a d i o l a r i a n assemblages. This

386

F i g u r e 6. Two types o f graded bedding. Explanation i s i n t h e t e x t . r e l a t i o n suggests t h a t t h e break may have been caused by c u r r e n t erosion. (iv)

P r e f e r r e d o r i e n t a t i o n o f elongated and conical-shaped t e s t s . Imoto and

Fukutomi (1975) described t h e p r e f e r r e d o r i e n t a t i o n o f sponge s p i c u l e s on surfaces p a r a l l e l t o bedding planes o f bedded c h e r t s . Apexes o f conical-shaped r a d i o l a r i a n t e s t s , such as theoperids i n t h e T r i a s s i c c h e r t and f o l l i c u c u l l i d s i n t h e Permian c h e r t , i n places appear i n concent r a t i o n s a l i g n e d i n o n l y a few d i r e c t i o n s on t h e surfaces o f bedded c h e r t s ( F i g . 7). Such a1 ignments suggest s t r o n g l y t h a t c u r r e n t s t r a n s p o r t e d o r reworked these t e s t s . Analysis o f t e s t o r i e n t a t i o n and cross-laminations should be u s e f u l i n determining a n c i e n t c u r r e n t d i r e c t i o n .

4.3

Bedding f e a t u r e s Rhythmic s t r a t i f i c a t i o n o f s i l i c e o u s l a y e r s and t h i n n e r shale p a r t i n g s char-

a c t e r i z e s bedded c h e r t formations. McBride and F o l k (1979) summarized p o s s i b l e mechanisms f o r f o r m a t i o n o f t h e rhythmic bedding. N i s b e t and P r i c e (1974) suggested t h a t p a r t i a l e r o s i o n o f s h a l e p a r t i n g s r e s u l t e d from g e n t l e ocean bottom winnowing o r s u b t u r b i d i t e e r o s i o n i n Neraida c h e r t s , c e n t r a l Greece. F o l k and McBride (1978) a l s o r e p o r t e d t h e e x i s t e n c e o f a s t r o n g c o r r e l a t i o n between t h e thickness o f shale p a r t i n g s and u n d e r l y i n g c h e r t beds f o r t h e L i g u r i a n c h e r t

381

Figure 7. Preferred alignment of JollicucullyS a l a s t i E t e s t s . beds in I t a l y . Authors of both papers interpreted these relations t o mean that bedded chert sequences were deposited by turbidity currents t h a t carried mostly siliceous debris. Iijima e t a l . (1978) recognized no relation between the thickness of chert beds and shale partings in the Triassic bedded chert in central Japan and therefore they suggested that slow and steady accumulation of radiolarian t e s t s accounted f o r the chert beds. I measured the thickness of chert layers and shale partings along some sections of bedded chert formations in the Tamba d i s t r i c t ( F i g . 8 ) . Among the Permian s e c t i o n s , sections G and J a r e situated near greenstones while sections H and I a r e not. The thickness of chert layers i n section J and the lower one t h i r d of section G varies widely, from 13 cm to about 1 cm, i n an irregular way. I n sections H , I , and the upper two-thirds of section G , the thickness of chert layers is commonly l e s s t h a n about 3 cm and shows regular s t r a t i f i c a t i o n . Shale partings in the sections t h a t include thick chert layers are thick also. In Triassic sections, except section F , most chert layers have a thickness of less than 6 cm and follow a regular pattern. However, the thickness of shale

388

F i g u r e 8. R e l a t i o n between t h e thickness o f c h e r t l a y e r s ( r i g h t columns) and shale p a r t i n g s ( l e f t columns) i n some s e c t i o n s o f bedded c h e r t formations. p a r t i n g s v a r i e s over a wide range. I n t h e lowest p a r t s o f s e c t i o n s

B and C t h e

shale p a r t i n g s a r e u s u a l l y t h i c k regardless o f t h e avarage thickness o f c h e r t l a y e r s . I t i s noteworthy t h a t t h e thickness o f b o t h c h e r t l a y e r s and shale p a r t i n g s i n s e c t i o n B diminishes g r a d u a l l y i n an upward d i r e c t i o n , although i n t h e upper p a r t of t h e s e c t i o n t h i c k c h e r t l a y e r s a r e found i n places. The d i s t r i b u t i o n o f c h e r t and shale beds ( s e c t i o n

B, F i g . 8) shows a r e l a t i o n

between t h e maximum thickness of c h e r t l a y e r s and s h a l e p a r t i n g s (Fig. 91, s i m i l a r t o t h e d i s t r i b u t i o n p a t t e r n noted by N i s b e t and P r i c e (1974). According t o t h e i r hypothesis t h e bedded c h e r t s accumulated i n an environment t h a t was dominated by t u r b i d i t y c u r r e n t s c a r r y i n g mostly s i l i c e o u s d e b r i s . The t h r e e d i f f e r e n t symbols on F i g u r e 9 tend t o be segregated from t h e lower t o upper p a r t s o f t h e s e c t i o n , w i t h t h e exception o f some s e t s o f beds having t h i c k c h e r t l a y e r s and t h i n shale p a r t i n g s i n t h e upper p a r t o f t h e s e c t i o n . One p o s s i b l e i n t e r p r e t a t i o n i s t h a t t h e change i n p a t t e r n o f d i s t r i b u t i o n represents a t r a n s i t i o n i n t h e sedimentary environment from proximal t o d i s t a l t u r b i d i t y c u r r e n t

389

A

A

A

A A A A

A

A A A

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3 4 5 8 THICKNESS OF C H E R T LAYERS 2

7

.

8 CY

F i g u r e 9. R e l a t i o n between t h e thickness o f c h e r t l a y e r s and shale p a r t i n g s along s e c t i o n B i n F i g u r e 8. T r i a n g l e s , open c i r c l e s , and s o l i d c i r c l e s represent c h e r t - s h a l e couplets i n t h e lower, middle, and upper p a r t s o f t h e formation respectively. a c t i v i t y and a l s o suggests a deepening o f t h e sedimentary basin.

5

SEDIMENTARY ENVIRONMENTS OF THE PERMIAN AND TRIASSIC BEDDED CHERTS The s i l i c a content i n t h e c h e r t l a y e r s was d e r i v e d from accumulation o f

mainly r a d i o l a r i a n t e s t s w i t h subordinate q u a n t i t i e s o f sponge s p i c u l e s i n t h e T r i a s s i c c h e r t . I n Permian c h e r t sometimes sponge s p i c u l e s dominated, although t h e o r i g i n o f t h e f i n e s i l i c e o u s m a t r i x i s unknown. Although no e s t i m a t i o n o f major sources o f s i l i c a has been completed, submar i n e volcanism and hydrothermal e x h a l a t i o n c o n t r i b u t e d t o t h e formation of j a s per deposits i n Permian t i m e and manganese o r e deposits associated w i t h massive c h e r t i n T r i a s s i c time. A h i g h topography t h a t o r i g i n a t e d from t h e p i l i n g up of p i l l o w lavas and h y a l o c l a s t i t e s p o s s i b l y presented a convenient s i t e f o r upwelli n g o f sea water causing r a d i o l a r i a n "blooms" and an increase o f s i l i c e o u s

Supply o f s i l i c a and n u t r i e n t from background

Hemipelagic and/or pelagic materials

I -

11 11

,.

Upwe11ing

11 d t t Pyroclastics

.

\

,!; \,:and

Increas of sponges supply of spicules

\

b

b 1 Deposition and s e t t l i n g as shale partings o f bedded cherts

/ -

Formation o f limestone

\

\

B1oomi ng of radiolarians

b

x

\

SA . .i-1Li ceous 4..111 IA2C..

111 11) Redi s trib u t i on by -bottom current

Submarine basic volcanism and exhalation o f hydrothermal solution and emanation

+

Overturning o f sea water; Supply o f nutrients, s i l i c a , and a small amount o f iron, manganese, and o f metals

F/

bedded chert

greenstone

Figure 10. Schematic i l l u s t r a t i o n o f the depositional environment and processes o f formation of Permian bedded chert i n the Tamba d i s t r i c t .

supply of silica and nutrient from background

Hemipelagic and/or pelagic materials *

\

\

\

i

e

Deposition and settling as shale partings of bedded cherts

-

Blooming of t t radiolarians 11

* *

Upwelling

*A

Redi s tri but i on by om current-

1'

".Pyroclastics

bbl 11 I

Terrigenous clastics

r

't

/

l

Ic

r

/

1

/

Siliceous turbidity current and slumping

8

/

Addition of sponge spicules

< Precipitation of

I

micritic limestone

bedded chert *.--.-.--

m/ ..- . . .w ::...

sandstone

..

Figure 1 1 . Schematic illustration of the depositional environment and processes of formation of Triassic bedded chert in the Tamba district.

-

Submarine basic volcanism and exhalation of hydrothermal solution and emanation Overturning of sea water; supply of nutrient, silica and a large amount of manganese

w

s

392 sponges p a r t i c u l a r l y i n Permian time. The s i t e a l s o became a source area f o r t u r b i d i t y c u r r e n t s . A f t e r b i o g e n i c s i l i c a was deposited as t u r b i d i t e s , i t was winnowed o r r e d i s t r i b u t e d by bottom c u r r e n t s and slumping ( F i g . 10 and 11). The p r i n c i p a l change from t h e Permian sedimentary basin t o t h e T r i a s s i c one i n t h e Tamba d i s t r i c t m i g h t be explained as a t r a n s i t i o n from r e l a t i v e l y s h a l low, o x i d a t i v e , and h i g h energy environment t o a deeper, reducing, and low energy environment. Whether t h i s s o r t o f t r a n s i t i o n was caused by s i n k i n g due t o p l a t e movement o r t o s i n k i n g i n s i t u i s n o t known. Some p a r t s o f b o t h Permian and T r i a s s i c bedded c h e r t s were rearranged as o l i s t o l i t h s i n J u r a s s i c time. ACKNOWLEDGEMENTS

I would l i k e t o thank D r . James R. Hein, U. S. Geological Survey, f o r h i s c r i t i c a l review o f t h e manuscript and suggestions f o r i t s improvement.

M r . H. I s h i g a , M r . T.Omura, M r . T. A n y o j i , M r . A. Tamaki, and many colleagues o f Tamba B e l t Research Group a r e thanked f o r t h e i r v a l u a b l e assistance. Special thanks a r e due t o M r . Robert J. Shannon f o r h i s reading o f t h e manuscript. REFERENCES Chipping, D. H., 1971. Paleoenvironmental s i g n i f i c a n c e o f c h e r t i n t h e Franciscan f o r m a t i o n o f western C a l i f o r n i a . Geol. SOC. Amer., B u l l . , 82:1707-1712. 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 subjac e n t "oceanic c r u s t " i n L i g u r i a , I t a l y . J. Sed. P e t r o l . , 48:1069-1102. Garrison, R. E., 1974. R a d i o l a r i a n c h e r t s , p e l a g i c limestones, and igneous rocks i n eugeosynclinal assemblages. I n : K. J. Hsu and H. C. Jenkyns ( E d i t o r s ) , P e l a g i c Sediments:on Land and under t h e Sea. Spec. Publs I n t . Ass. Sediment., 1:367-399. Holdsworth, B. K. and Jones, D. L., 1980. P r e l i m i n a r y r a d i o l a r i a n zonation f o r L a t e Devonian through Permian time. Geology, 8:281-285. I i j i m a , A., Kakuwa, Y., Yamazaki, K., and Yanagimoto, Y., 1978. Shallow-sea 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. S c i . Univ. Tokyo, sec. 2, 19:369-400. Imoto, N. and Fukutomi, M., 1975. Genesis o f bedded c h e r t s i n t h e Tamba B e l t , Southwest Japan. Monograph Ass. Geol. Collab. Japan, 19:35-42. Imoto, N., Shimizu, D., S h i k i , T., and Yoshida, M., 1974. Sole-markings observed i n bedded c h e r t s from t h e Tamba B e l t , Japan. B u l l . Kyoto Univ. Educ. B, 44: 19-26. Imoto, N., Shimizu, D., and Tamba B e l t Research Group, 1980. Reexamination o f Paleozoic-Mesozoic s t r a t i g r a p h y o f t h e Tamba d i s t r i c t . B u l l . Ass. S t r u c . Geol. Japan, 25:25-31. 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 Science (Chikyu Kagaku), 34:332-345. I s h i g a , H., K i t o , T., and Imoto, N., 1981. Permian r a d i o l a r i a n b i o s t r a t i g r a p h y i n t h e Tamba B e l t . Proc. Kansai Branch Geol. SOC. Japan, 89:2-3. I s h i g a , H., K i t o , T., and Imoto, N., 1982. L a t e Permian r a d i o l a r i a n assemblages i n t h e Tamba d i s t r i c t and an adjacent area, Southwest Japan. E a r t h Science (Chikyu Kagaku), 36:lO-22. I s h i g a , H. and Tamba B e l t Research Group, 1981. Geologic ages o f s m a l l - s c a l e c h e r t lenses i n c l u d e d i n 1-formation of t h e Tamba Group. Proc. Kansai Branch Geol. SOC. Japan, 90:4-5.

393 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 l i n e " , western h i l l s o f Kyoto, Southwest Japan. J. Geosci. Osaka C i t y Univ., 23:115-134. Kozur, H . , 1981. A l b a i l l e l l i d e a ( R a d i o l a r i a ) aus dem Unterperm des V o r u r a l s . Geol. Palaeont. M i t t . Insbruck. 10:263-274. McBride, E. F. and F o l k , R. L., 1979. Features and o r i g i n o f I t a l i a n J u r a s s i c r a d i o l a r i t e deposited on c o n t i n e n t a l c r u s t . J. Sed. P e t r o l . , 49:837-868. McBride, E. F. and Thomson, A., 1970. The Caballos N o v a c u l i t e , Marathon r e g i o n , Texas. Geol. SOC. Amer. Spec. Paper, 122:l-103. N i s b e t , E. G. and P r i c e , I.,1974. S i l i c e o u s t u r b i d i t e s : bedded c h e r t s as r e deposited, ocean r i d g e - d e r i v e d sediments. I n : K. J. Hsu and H . C. Jenkyns ( E d i t o r s ) , P e l a g i c Sediments:on Land and under t h e Sea. Spec. Publs I n t . ASS. Sediment., 1:351-366. Ormiston, A. and Babcock, L., 1979. F o l l i c u c u l u s , new r a d i o l a r i a n genus from t h e Guadalupian (Permian) Lamar Limestone o f t h e Delaware Basin. J. Paleontology, 53: 328-334. Sakaguchi, S., 1961. S t r a t i g r a p h y and palaeontology o f t h e South Tamba D i s t r i c t . P a r t I, S t r a t i g r a p h y . Mem. Osaka Gakugei Univ., 10:35-76. Shimonishi, S. and Tamba B e l t Research Group, 1981. Occurrence o f Monotis(Ent0monotis) from 1-sandstone f o r m a t i o n o f t h e Tamba Group. Proc. Kansai Branch Geol. SOC. Japan, 89:3-4. Suyari, K., Kuwano, Y . , and I s h i d a , K., 1981. Sedimentary environments i n t h e Sambagawa and Chichibu B e l t s d u r i n g t h e T r i a s s i c . J. S c i . Univ. Tokushima, 14~139-161. Takemura, A. and Nakaseko, K., 1981. A new Permian r a d i o l a r i a n genus from t h e Tamba B e l t , Southwest Japan. Trans Proc. Palaeont. SOC. Japan, N. S., 124: 208-21 4. Tamba B e l t Research Group, 1979a. Paleozoic and Mesozoic Systems i n t h e Tamba B e l t ( P a r t 4 ) , L i t h o f a c i e s and g e o l o g i c s t r u c t u r e o f t h e Tamba Group a t t h e northwestern h i l l s o f Kyoto City. t a r t h Science (Chikyu Kagaku), 33:137-143. Tamba B e l t Research Group, 1979b. 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 Science (Chikyu Kagaku), 33:247-254. Tamba B e l t Research Group, 1980. Paleozoic and Mesozoic Systems i n t h e Tamba Be1 t ( P a r t 6 ) , Geology o f southeastern p a r t o f Kei hoku-cho, K i takuwada-gun, Kyoto P r e f e c t u r e . E a r t h Science (Chikyu Kagaku), 34:ZOO-204. Yao, A., 1982. M i d d l e T r i a s s i c t o E a r l y 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. J. Geosci. Osaka City Univ., 25:53-70. Yao, A., Matsuda, T. , and I s o z a k i , Y., 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. J. Geosci. Osaka City Univ., 23:135154. Yoshida, M. and Wakita, M., 1975. T r i a s s i c conodonts from t h e Tamba B e l t a t t h e northwest o f Kyoto, Southwest Japan. Monograph Ass. Geol. C o l l a b . Japan, 19: 43-48.

395

CHAPTER 23 ENVIRONMENT OF DEPOSITION OF CRETACEOUS CHERT FROM THE SHIMANTO BELT, KII PENINSULA, SOUTHWEST JAPAN K. NAKAZAWA',

F. KUMONl, K. KIMURA',

H. MATSUYAMAl, and K. NAKAJ02

Department of Geology and Mineralogy, Kyoto University (Japan) 2 Higashi-Sumiyoshi High School, Osaka (Japan)

ABSTRACT Chert from the Shimanto Belt in Kii Peninsula, Southwest Japan is mostly red, bedded, radiolarian chert. These rocks are commonly associated with greenstone. Besides chert-greenstone, flysch units occur repeatedly in the area studied. Radiolarians date the chert as Early Cretaceous, older than the surrounding argillaceous rocks or flysch unit which are of Late Cretaceous age. Ages and field occurrences indicate a slump origin for chert and greenstone, presumably related to subduction. However, some of these rocks formed at the same sites where argillaceous units occur. Sedimentary structures in the bedded cherts suggest formation by transportation and accumulation of siliceous materials, mostly radiolarian tests, on or near a submarine volcanic seamount by low density currents something like a bottom nepheloid layer. The frequent inflow of volcanic materials is indicated by intercalated black tuffaceous partings. INTRODUCTION The Shimanto Belt is one of the major geologic provinces extending from southern Kwanto to southern Kyushu along the Pacific coast in Southwest Japan. Thick geosynclinal sedimentary rocks ranging in age from latest Jurassic? to early, early Miocene are well developed in this belt. They are collectively called the Shimanto Supergroup, which is divided into several groups of different ages in different belts (Fig. 1 ) In Kii Peninsula occupying the central part of the Shimanto Belt, three smaller belts can be recognized from north to south, the Cretaceous Hidakagawa belt, the Eocene? Otonashigawa belt, and the Oligocene - early, early Miocene Muro belt. The rocks of the latter two belts are composed mostly of flysch-like alternation of sandstone and mudstone, sandstone, mudstone, and conglomerate. The late, early Miocene to middle Miocene Tanabe and Kumano Groups of shelf-slope

396

&: I lz/

.

Area o f Fia. 7

34"

w

136.

4 Miocene

I g n e o u s Rocks

Tanabe F, Kumano Groups ( l a t e E a r l y - Middle Miocene)

a

$= m

Muro Group

(O

0

Nyunokawa F o n

Y . . . .v .

3

Fig. 1. Generalized g e o l o g i c a l map o f t h e Shimanto B e l t i n K i i Peninsula. Numbers represent l o c a t i o n s shown i n Figures 2 - 5, 1: Komori, Ryujin-mura; 2: Second c h e r t greenstone u n i t along Takatsuo River; 3: T h i r d chert-greenstone u n i t along Takatsuo R i v e r ; 4: I d a n i , Miyama-mura; 5: Anego, Nakatsu-mura.

f a c i e s o v e r l a y t h e t h r e e groups w i t h a remarkable angular unconformity i n t h e peninsula ( F i g . 1 ) . Cenozoic s t r a t a .

N e i t h e r s i l i c e o u s rocks nor greenstones occur i n these

Here we a r e concerned w i t h t h e Cretaceous Hidakagawa Group

which c o n t a i n s c h e r t and greenstone.

HIDAKAGAWA GROUP The Hidakagawa Group i s d i v i d e d i n t o f o u r formations i n t h e c e n t r a l p a r t o f t h e peninsula, from n o r t h t o south, Yukawa, Miyama, Ryujin, and Nyunokawa Formations; a l l formations a r e i n f a u l t c o n t a c t .

I n t h e western p a r t o f t h e penin-

sula, t h e Hidakagawa Group i s d i v i d e d i n t o t h e Terasoma and Shirama Formations. Yukawa Formation The Yukawa Formation i s composed e x c l u s i v e l y o f bedded sandstone, a l t e r n a -

397

tions of sandstone and shale, and shale. Exotic blocks of chert are rarely found, derived presumably from the Chichibu Belt to the north of the Shimanto Belt. The Aptian-Albian shallow-sea bivlaves (Spondylus aff. decoratus, Plicatula aff. hanaii, Amphidonte cf. subhaliotoidea, etc.) and a coral (Camalophyllia sp.) were reported from sandstone and the Cenomanian Inoceramus concentricus from shale. Aptian to Cenomanian radiolarian assemblages were found in places. Thus, this formation is assigned an age range of Aptian to Cenomanian. The megafossils together with a predominance of proximal turbidites suggest a relatively shallow-water environment at the margin of a basin (nakazawa et al., 1979). No greenstones occur in this sequence.

-

Miyama Formation The Miyama Formation consists of two lithologic units, flysch and chertgreenstone units ("preflysch" unit). Flysch is composed of alternations of sandstone and shale, shale, and sandstone. The latter unit consists of shale, and pebbly shale associated with bedded chert, basalt lava, hyaloclastite, ana siliceous sandstone. Acidic tuff, and greenish or red shale occur in both units. Megafossils are rare; only fragments of Inoceramus have been found in black shale of sandstone-shale alternations. Five radiolarian assemblages indicating ages from Tithonian to Santonian were found in many places, however, the age of deposition of this formation is considered to be from Turonian? to Santonian, as the result of reworking of the older sediments. Ryuji n Formation The Ryujin Formation is composed mostly of argillaceous rocks and intercalated alternations of sandstone and shale, greenstone, and acidic tuff, but no chert. The greenstones are traceable for a long distance. The field occurrence of greenstone suggests eruption in situ (Shiida et al., 1971; Kimura, 1978). Sometimes, acidic tuff is coarse-grained and attains to several tens of meters in thickness. Recently, the Campanian-Maestrichtian Amphipyndax tylotus assemblage was found in a few places. Nvunokawa Formation The Nyunokawa Formation is composed of flysch-like alternations of sanastone and shale, shale, sandstone, and conglomerate named the Nyunokawa Conglomerate (Tokuoka et al., 1981). Small greenstone bodies occur in the lower part of this unit. Chert is not present. Late Cretaceous radiolarians were found in shale, however, the exact age could not be determined.

398

i

1

tone

:uf f

lens

lens

alt.

Fig. 2. Cliff exposure of lenticular chert and greenstone in the Miyama Formation at Komori (location number 1 in Fig. 1). CHERT AND GREENSTONE IN ‘THE MIYAMA FORMATION As mentioned already, the Miyama Formation consists of the flysch and intercalated chert-greenstone units (Fig. 7). In places chert and greenstone occur isolated from each other, but more commonly they occur together. Four types of occurrences were distinguished in the field. (1) Relatively small lenses or blocks in argillaceous sedimentary rocks (Fig. 2 ) . ( 2 ) Overlain conformably by sandstone-shale alternations or other clastic

399

Fig. 3. Route map and sketch o f the second chert-greenstone unit along Takatsuo River (location number 2 in Fig. 1 ) . Legend; 1 : sandstone, 2 : sandstone-rich alternations of sandstone and shale, 3: alternations o f sandstone and shale in equal proportion, 4: shale-rich alternations o f sandstone and shale, 5: shale, 6: chert, 7: basalt lave, 8: hyaloclastite, 9: red shale. Numbers of soild c i r c l e ; 1 : Mirifusus sp.-Parvidngula s p . Assemblage, 2: Thanarla conica - Ultranapora sp. A . , 4: Dictyoniitra formosa A., 5: Artostrobium urns A., a : Late Cretaceous Assemblage.

Fig. 4. Route map o f the third chert-greenstone unit along Takatsuo River (location number 3 in Fig. 1 ) . Symbols same as in Fig. 3.

400

rocks, but in fault contact with underlying strata (Figs. 3 and 5). (3) Conformable sequences with adjacent strata (Fig. 4). (4) Tectonic blocks in flysch deposits (Fig. 6). In Fig. 2, the boundary between chert-greenstone blocks and the surrounding rock is not a fault plane, and seems to be sedimentary, the blocks are a kind of slump deposits. The second type of occurrence is illustrated in Fig. 3. The strata in the southern part (Fig. 3a right) dip nearly vertically or very steeply to the northwest, however, the grading of sandstone beds indicates southeast younging. A red, bedded chert, about 10 m thick, is overlain by intercalated siliceous sandstone, acidic tuff, and greenish shale, 1.8 m thick, and sandstone-shale alternations about 15 m thick. The chert i s underlain successively by greenish 75

Fig. 5. Route map o f the chert-greenstone unit at Idani (location number 4 in Fig. 1). Sketch in circle shows intrusion of dolerite dike near the boundary between pebbly shale and lava. At the opposite side the lave conformably overlies the pebbly shale.

401

Fig. 6. Fault contact between basalt lava and shale-rich alternations of sandstone and shale at Anego (location number 5 in Fig. 1). shale with thin acidic tuff beds, sandstone-shale alternations with carbonaceous shaly partings, black shale, sheared slump beds containing block of chert

and sandstone, and sheared shale-rich alternations of sandstone and shale, in descending order. The chert and the greenish shale are in contact with the underlying sandstone-shale alternations by a minor fault plane, however a great fault is thought to mark the northern border of the sequence. The northern part (Fig. 3a left) and Fig. 4 show a seemingly conformable succession of chert-greenstone with surrounding beds, but in many place the chert-greenstones are lenticular bodies as determined by tracing outcrops laterally (Fig. 4). At Idani (Fig. 5) a doleritic dike intruded near the contact between the chert-greenstone complex and the underlying pebbly shale, and other two sheet-like basalts, about 150 cm and 40 cm thick respectively, occur in the alternations of acid tuff and shale. Furthermore, pebbly shale, similar to the underlying one, is found as a block in greenstone breccia. This clearly shows that the eruption of the greenstones took place at the site of sedimentation of the pebbly shale and alternations of acid tuff and shale. The last type is not comnon (Fig. t i ) . In this outcrop the greenstone is crushed showing a blocky fracture. OCCURRENCE AND AGE OF RADIOLARIAN IN THE MIYAMA FORMATION Cretaceous radiolarian biostratigraphy has advanced greatly in recent years (Riedel and Sanfilippo, 1974; Foreman, 1975; Pessagno, 1976, 1977; etc.). In Japan, Nakaseko et al. (1979) and Nakaseko and Nishimura (1981)discriminated seven radiolarian assemblages in the Shimanto Belt as shown by Yao (this vol ume) Chert, shale, hyaloclastite, green or red shale, and acidic tuff were sam-

.

402

pled for radiolarians. Of 806 samples from the Miyama Formation 50 were useful for age-determination. Six radiolarian assemblages were distinguished, the last one being confined to the Ryujin Formation. (1 Mirifusus sp.-Parvicingula sp. Assemblage, (2) Thanarla conica-Ultranapora sp. A., (3) Holocryptocanium barbui A., (4) Dictyomitra formosa A., (5) Artostrobium urna A., and (6) Amphipyndax tylotus A. (see Plate 1). The first assemblage is found at two places, one from red chert and the other from red shale. In addition to Mirifusus cf. baileyi and Parvicingula cf. boesii, Thanarla conica, Archaeodictyomitra apiara, Pantanellium sp., Protunuma sp., etc. are found together. This assemblage is comparable to that of Tithonian-Valaginian of Pessagno (1977). This is the oldest assemblage in the Shimanto Belt. The second assemblage is characterized by Thanarla conica and Ultranapora sp. in association with Eucyrtis sp., Pseudodictyomitra spp., Dictyomitra sp., Acaenyotyle sp., etc. This assemblage corresponds to the Eucyrtis micropora and Ultranapora praespinifera Assemblages of Nakaseko and Nishimura (1981). The age is considered to be Hauterivian-Albian. This assemblage is common and always comes from red chert. H. barbui, H. The third, Holocryptocanium barbui Assemblage, consists of -geysersensis, Pseudodictyomitra pseudomacrocephala, Tharnarla veneta, - - - T. elegantissima, Novixitus weyli, etc. This assemblage is widely known and is considered a good key of late Albian-Cenomanian age (Nakaseko and Nishimura, 1981). In kii Peninsula it i s found from the Shirama Formation and the east extension of the Miyama Formation. The Fourth, Dictyomitra formosa Assemblage, has no characteristic species. D. duodecimcostata, Archaeodictyomitra squinabol and Hemicrypto-D. -formosa, capsa polyhedra occur. It has no diagnostic species of the underlying Holocryptocanium barbui Assemblage nor o f the overlying Artostrobium urna Assemblage and is tentatively considered to be Turonian in age. The fifth, Artostrobium urna Assemblage, is characterized by A. -urna, - Dictyomitra koslovae, Pseudoaulophacus floresensis, Alievium gallowayi, etc. A -large amount of Discoidea occur. This assemblage is most commonly found in black shale of.the Miyama and Terasoma Formations. The Terasoma Formation yields many Coniacian to Santonian inoceramids, such as Inoceramus sublabiatus, -I. cf. rhomboides, -I . cf. cycloides, I. cf. amakusensis etc. (Morozumi, 1970; Matsumoto and Yoshimatsu, in preparation). Therefore, this assemblage definitly indicates a Coniacian-Santonian age as suggested by Foreman (1975). The last assemblage contains Amphipyndax tylotus, - - A. stocki, Artostrobium urna (rare), Dictyomitra lamellicostata, Diacanthocapsa acuminata, Gogylothorax A. tylotus Zone of Foreman verbeeki, etc. This is comparable to that of the (1977) which is considered to range from Campanian to Maestrichtian and repre-

-

-

-

-

-

-

403

sent t h e youngest r a d i o l a r i a n zone o f t h e Cretaceous i n t h e Shimanto B e l t . FIELD OCCURRENCE OF RADIOLARIAN ASSEMBLAGE AND THE GEOLOGICAL SIGNIFICANCE Most E a r l y Cretaceous r a d i o l a r i a n s a r e from r e d c h e r t w h i l e t h e L a t e Cretaceous f o s s i l s a r e from shale, tuffaceous shale, a c i d i c t u f f and r e d shale as shown i n t h e n e x t t a b l e and Fig. 7b.

chert

5. 4. 3. 2. 1.

A r t o s t r o b i u m urna A. D i c t y o m i t r a formosa A. Holocryptocanium barbui A. Thanarla conica-Ultranapora sp. A. C l i r i f u s u s sp.-Parvicingula sp. A.

0 0 1 16 1

r e d & green shale

acidic tuff

black shale

1 10 2

0 1

Nakaseko (1979) noted t h e same r e l a t i o n f o r rocks from t h e n o r t h e r n Shimanto B e l t i n e a s t e r n Shikoku, which i s t h e western e x t e n s i o n o f t h e Hidakagawa B e l t . To i l l u s t r a t e these r e l a t i o n s , F i g . 3 shows greenish shale o f Turonian age between H a u t e r i v i a n - A l b i a n r e d c h e r t and a slump bed. c h e r t l a y e r i s Coniacian-Santonian.

Black shale above t h e

Judging from these occurrences o f r a d i o -

l a r i a n s , i t i s concluded t h a t resedimentation o f Lower Cretaceous c h e r t took p l a c e d u r i n g t h e Turonian and were covered by Coniacian-Santonian f l y s c h sediments.

S i m i l a r l y , i n t h e n o r t h e r n p a r t o f t h e same route, a Tithonian?-Valang-

i n i a n chert-greenstone complex r e s t s on Upper Cretaceous sandstone-shale a l t e r n a t i o n s and i s o v e r l a i n conformably by Coniacian-Santonian f l y s c h . The chert-greenstone u n i t i n many places i s considered t o be a k i n d o f o l i sthostrome o r sedimentary mclage, because o f i t s l e n t i c u l a r o r b l o c k y shapes and accompanied pebbly shales.

S i m i l a r greenstone and c h e r t s e c t i o n s a r e noted

by T a i r a e t a l . (1980) i n t h e melage zone i n Kochi Prefecture, Shikoku. Based on t h e d e t a i l e d f i e l d survey around Takatsuo and M i s o i R i v e r s ( F i g . 7), t h e chert-greenstone u n i t was t r a c e d f o r a l o n g d i s t a n c e b u t i t does n o t show a d e f i n i t e s t r a t i g r a p h i c succession, such as i s c h a r a c t e r i s t i c o f an o p h i o l i t e sequence, and i n columnar s e c t i o n s (Fig. 8) t h e succession and t h e thickness o f c h e r t o r greenstone v a r y 'from p l a c e t o place.

These a r e i n accordance w i t h an

olisthostrome o r i g i n . On t h e o t h e r hand, t h e I d a n i exposure ( F i g . 5 ) s t r o n g l y i n d i c a t e s t h e autochthonous f e a t u r e s o f t h e chert-greenstone complex as p r e v i o u s l y stated.

This

i s i n d i c a t e d by r a d i o l a r i a n f o s s i l s , t h a t i s , b o t h t h e c h e r t and r e d shale o f t h e complex and t h e shale o f t h e u n d e r l y i n g pebbly shale have t h e same r a d i o l a r i a n assemblage o f L a t e Cretaceous age. I n conclusion, most o f t h e chert-greenstone complex o f t h e Miyama Formation i s considered t o be allochthonous bodies t h a t s l i d i n t o younger c l a s t i c sedi-

.4

"

..

-c

/ H f l

Figure 7b

,Occurrence of radiolaria

\

8

chert

A red shale

black o r greenish sh.

405

R ...... .. .... ..........

f-

1L

._. . .. . U Fig. 8. Columnar sections o f the second chert-greenstone u n i t . Legend; 6: pebbly shale, 9: p i l l o w lava, 13: limestone, others same as i n Fig. 3. ments.

Some are t e c t o n i c a l l y i n t e r c a l a t e d i n t h e f l y s c h u n i t , b u t more common-

l y are conformably covered by f l y s c h deposits.

However, t h e r e i s evidence t h a t the chert-greenstone formed o r i g i n a l l y a t t h e s i t e o f d e p o s i t i o n o f terrigenous c l a s t i c materials. SEDIMENTARY ENVIRONMENT Kanmera and Sakai (1975) and Sakai and Kanmera (1981) d i s t i n g u i s h e d two sedimentary u n i t s , f l y s c h and p r e f l y s c h u n i t , i n the Shimanto B e l t i n Kyushu. They suggested t h a t the f l y s c h u n i t formed on t h e submarine t e r r a c e o r slope, overl y i n g w i t h a s t r u c t u r a l discordance t h e accreted p r e f l y s c h u n i t which consisted o f t r e n c h - f i l l and oceanic f l o o r sediment.

A s i m i l a r o p i n i o n was expressed by

Fig. 7. Geological map (Fig.7a) and occurrence o f r a d i o l a r i a n f o s s i l s (Fig. 7b) o f t h e Miyama Formation around Takatsuo and Misoi Rivers. Location i n Fig. 1. Numbers i n Fig. 7b represent t h e r a d i o l a r i a n assemblages as shown i n Fig. 3.

406

Suzuki e t a l . (1978) who r e f e r r e d t o the p r e f l y s c h u n i t as a t e c t o n i c melange intercalated i n the flysch unit.

T a i r a e t a l . (1980) a l s o explained t h a t the

chert-greenstone o f the melange zone i n Kochi Prefecture as a deep ocean f l o o r deposit t h a t s l i d i n t o and mixed w i t h t r e n c h - f i l l sediments by subduction.

On

the o t h e r hand, Kishu Shimanto Research Group (1975), Kimura (19771, and Yanai (1981 ) considered t h a t the Shimanto Supergroup formed i n an " i n t e r c o n t i n e n t a l " basin o r forearc basin w i t h a s i a l i c basement. Greenstone Most o f the chert-greenstone complex occurs as e x o t i c blocks, and thus i t i s d i f f i c u l t t o r e c o n s t r u c t t h e o r i g i n a l environment o f deposition.

However,

Sugisaki e t a l . (1979) p o i n t o u t the s i m i l a r i t y o f the chemical composition o f greenstone w i t h t h a t o f oceanic t h o l e i i t i c basalt.

The f i e l d occurrence o f the

greenstone a t I d a n i i n K i i Peninsula (Fig. 5) i n d i c a t e s t h a t e r u p t i o n took place a t the s i t e where much terrigenous m a t e r i a l was supplied and most probably the greenstone i s n o t a b a s a l t i c l a y e r o f oceanic c r u s t . Tsuchiya e t a l . (1979) and Sakai and Kanmera (1981) concluded t h a t greenstone i n Kyushu i s t h e product o f o f f - r i d g e volcanism where terrigenous materia l s could be supplied.

Among the several examples o f o f f - r i d g e volcanism exa-

mined by DSDP, S i t e 446 o f Leg 58 i n the Northern P h i l l i p p i n e Sea (The Shipboard S c i e n t i f i c Party, 1980) may be analogous.

The lower 230 m o f the 628.5 m

s e c t i o n i s composed o f b a s a l t i c s i l l s and a l t e r n a t i n g claystone, s i l t s t o n e , sandstone, and ash o f e a r l y Eocene age.

The s i t e i s l o c a t e d i n the basin bet-

ween the D a i t o Ridge t o the n o r t h and the Oki-Daito Ridge t o the South.

I n the

Shimanto B e l t o f K i i Peninsula, the presence o f a paleoland mass t o the south o f t h e b e l t i s presumed from sedimentologic studies o f the Upper Cretaceous Nyunokawa Formation and the 01 igocene Muro Group. The presence o f limy f i l l i n g o f i r r e g u l a r shape i n t h e interspaces o f p i l lows and h y a l o c l a s t i t e s and a f o r a m i n i f e r a 1 m i c r i t i c limestone lens immediately above t h e l a v a a t I d a n i suggests t h a t e r u p t i o n may have taken place above t h e CCD, although most o f p i l l o w s are devoid o f vesicules.

Chert Chert o f the Hidakagawa b e l t i s mostly bedded r a d i o l a r i a n c h e r t o f reddish c o l o r o c c u r r i n g as lenses w i t h i n h y a l o c l a s t i t e s o r a l t e r n a t i n g w i t h o r r e s t i n g on greenstones, unless they a r e i s o l a t e d blocks i n olisthostrome. Two d i f f e r e n t microscopic clayey p a r t i n g s e x i s t i n bedded chert.

One i s

very fine-grained, brown i n c o l o r , w i t h undeterminable primary mineral compos i t i o n ( P l a t e 2, Figs. 3, 5, and 8 ) .

The p a r t i n g has a sharp boundary w i t h the

o v e r l y i n g c h e r t y l a y e r b u t are gradational t o t h e underlying one ( P l a t e 2, Fig.

407

5). Nisbet and Price (1974) described similar structures from Greece, and suggested a turbidite origin for the chert layers. The transitional part of the Japanese parting is commonly thin, less than 1 mn (Plate 2, Fig. 3). The other type of parting is black and opaque containing many lath-shape crystals or fragments of feldspar (Plate 2, Fig. 2 and 4) and in some rocks, radiolarian test (Plate 2, Fig. 7). These partings are considered to be tuffs. The black partings are sharply bounded by adjacent layers. In places the black partings lie on the brown partings with a sharp boundary, too. Preliminary EPMA analyses show that both kinds of partings are characterized by much Fez03 and Al2O3. The content of Mn and Ti may indicate either a deep sea environment or the effects of submarine volcanism. The black partings contain more alkalies and Mn than the brown ones. The chert layers are similar to the radiolarian silica siltite of Nisbet and Price (1974). They are massive or more comnonly parallel-laminated suggesting the influence of currents during deposition. This is clearly seen on surfaces etched by HF (Plate 2, Fig. 1). Lamination seems to be related to the amount of hematite; radiolarian tests commonly do not contribute to the lamination (Plate 2, Fig. 6). Except for thin graded beds, no sedimentary structures indicative of turbidites (Imoto et al., 1974) are observed. Because the origin of matrix materials of chert is unkown, it is difficult to estimate the mechanism of sedimentation. But we presume that radiolarian tests and siliceous materials probably of volcanic origin were transported by a very low density current possibly as a bottom nepheloid layer as observed by Baker (1976) on the slope and submarine canyon off Washington, and deposited on the slope and basin as presumed by Nisbet and Price. Chert sections in the area studied are thin, less than 30 m, and are limited laterally as noticed by Sakai and Kanmera (1981) in Kyushu. Synsedimentary microfaults (Plate 2, Fig. 3) and flowage structures are frequently seen, but slump folds are rare. This may indicate only a slightly unstable depositional environment. We speculate that the cherts of the Miyama Formation formed by transport of siliceous materials, mostly radiolarian tests, by weak density currents something analogous to the bottom nepheloid layer, which resulted in slow deposition on or near a submarine volcanic seamount free from terrigenous supply. Volcanism occurred on the sea floor where much terrigenous material was supplied. Deposition was interrupted frequently by volcanic ash which occurs as black partings in bedded cherts.

ACKNOWLEDGEMENTS We are much obliged to the members of the Kishu Shimanto Research Group who helped in sampling and offered important geologic data. Thanks are due to Dr.

408

M. Musashino o f Kyoto U n i v e r s i t y o f Education f o r h i s i n s t r u c t i o n on EPMA analyses. REFERENCES Baker, E. T., 1976. D i s t r i b u t i o n , composition, and t r a n s p o r t o f suspended m a t t e r i n t h e v i c i n i t y o f W i l l a p a submarine canyon, Washington. Geol. SOC. Amer. B u l l . , 87: 625-632. Foreman, H. P., 1975. R a d i o l a r i a from t h e N o r t h P a c i f i c , DSDP Leg 32. I n i t . Rep. DSDP, 32: 579-676. Foreman, H. P., 1977. Clesozoic R a d i o l a r i a from t h e A t l a n t i c Basin and i t s borderlands. Development i n Palaeontology and S t r a t i g r a p h y 6: (Scoan, F. M., ed.) S t r a t i g r a p h i c micropaleontology o f A t l a n t i c b a s i n and borderlands: 305-320. Imoto, N., Shimizu, D . , S h i k i , T., and Yoshida, M., 1974. Sole-markings observed i n bedded c h e r t from t h e Tamba b e l t , Japan. Mem. Kyoto Educat. Univ., Ser. 6, 44: 19-26. Kanmera, K. and Sakai, T., 1975. Correspondence o f t h e f o r m a t i o n p l a c e o f t h e Shimanto Group t o t h e p r e s e n t s e a - f l o o r . GDP Rep., 11-1 ( l ) , S t r u c t u r a l Geol., 3: 55-64. Kimura, K., 1978. Geology o f t h e Hidakagawa Group i n t h e c e n t r a l p a r t o f t h e K i i Peninsula, Southwest Japan. Master t h e s i s o f Kyoto Univ. (Manuscript) Kimura, T., 1977. S t r u c t u r a l developments o f Japan and P l a t e - t e c t o n i c s . J. Geosci., 88: 54-67. Kishu Shimanto Research Group, 1975. Development o f t h e Shimanto geosyncline. Problems on geosynclines i n Japan. Ass. Geol. Collab. Japan, Monogr. 19: 143-1 56. Morozumi, Y., 1970. Upper Cretaceous Inoceramus from t h e Shimanto B e l t of K i i Peninsula. B u l l . Osaka Mus. Nat. H i s t . , 23: 19-24. Nakaseko, K., 1979. Some problems on t h e g e o l o g i c a l h i s t o r y o f Japanese I s l a n d s viewed from r a d i o l a r i a n f o s s i l s . J. Osaka Micropal. Ass., 7: 19-26. Nakaseko, K., and Nishimura, A., 1981. On t h e Holocryptocanium b a r b u i Zone. Abst. 8 8 t h Ann. Meet., Geol. SOC. Japan, p. 172. Nakaseko. K.. Nishimura.. A.. . andSuaano. K..1979. Study on r a d i o l a r i a n f o s s i l s o f t h e Shimanto B e l t . J. Osaka-Micropal-. Ass., Spec: No.2: 1-49. Nakazawa, K., Kumon, F., and Kimura, K., 1979. Occurrence o f Cretaceous s h a l low-sea b i v a l v e s from n o r t h e r n border o f Shimarito T e r r a i n , K i i Peninsula, Southwest Japan. Trans. Proc. Palaeont. SOC. Japan, N.S. 113: 15-29. Nisbet, E. G., and P r i c e , I.,1974. S i l i c e o u s t u r b i d i t e s : bedded c h e r t s as r e d e p o s i t e d ocean r i d g e - d e r i v e d sediments. I n Hsu, K. J. and Jenkyns, H. ( E d i t o r s ) , P e l a g i c sediments: on Land and under t h e Sea. I n t . Assoc. Sediment. Spec. Publ. 1: 351-366. Pessagno, E. A. Jr., 1976. R a d i o l a r i a n z o n a t i o n and s t r a t i g r a p h y o f t h e Upper Cretaceous p o r t i o n o f t h e Great V a l l e y sequence, C a l i f o r n i a Coast Range. Micropal. Spec. Paper 2: 1-95. Pessagno, E. A. Jr., 1.977. Lower Cretaceous r a d i o l a r i a n b i o s t r a t i g r a p h y of t h e Great V a l l e y sequence and Franciscan complex, C a l i f o r n i a Coast Ranges. Cushman Found. Foram. Res. Spec. Publ. 15: 1-63. Riedel, W. R. and S a n f i l i p p o , A., 1974. R a d i o l a r i a f r o m t h e southern I n d i a n Ocean, DSDP Leg 26, I n i t . Rep. DSDP, 26: 771-813. Sakai, T. and Kanmera, K., 1981 S t r a t i g r a p h y o f t h e Shimanto t e r r a i n and t e c t o n o s t r a t i g r a p h i c s e t t i n g o f greenstones i n t h e n o r t h e r n p a r t o f Miyazaki Pref e c t u r e , Kyushu. S c i . Rep. Dept. Geol. Kyushu Univ., 14: 31-48. Shiida, I.,Suwa, K., Sugisaki, R., Tanaka, T., and Shiozaki, H., 1971. Greenstones o f t h e Cretaceous Hidakagawa B e l t o f t h e Shimanto t e r r a i n i n t h e Totsukawa area, Nara P r e f e c t u r e , C e n t r a l Japan. Geol. SOC. Japan, Monogr. 6: 137-149. Sugisaki , R., Suzuki, T., Kanmera, K., Sakai, T., and Sano, Y., 1979. Chemical Composition o f green r o c k s i n t h e Shimanto B e l t , Southwest Japan. J. Geol.

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SOC. Japan, 85: 455-466. Suzuki, T., Hada, S., Umemura, H., Kado, Y., Sakamoto, Y., and Nakagawa, H., 1978. Genetic consideration of the green rocks in the Shimanto Belt - With special reference to mode of occurrence. Earth Sci. (Chikyu Kagaku), 32: 321 -330. Taira, A., Okamura, M., Katto, J., Tashiro, M., Saito, Y., Kodama, K., Hashimoto, M., Chiba, T., and Aoki, T., 1980. Lithofacies and geologic age relationship within dlange zones of Northern Shimanto Belt (Cretaceous), Kochi Prefecture, Japan. In Taira, A. and Tashiro, M. (eds.), Geology and Paleontology of the Shimanto Belt, 179-214. Rinyakosaikai Press, Kochi, Japan. The Shipboard Scientific Party, 1980. Site 446 Daito Basin, DSDP Leg 58. Init. Rep. DSOP, 58: 401-545. Tokuoka, T., Harata, T., Inouchi, Y., Ishigami, T., Kimura, K., Kumon, F., Nakajo, K., Nakaya, S., Sakamoto, T.,, Suzuki, H., and Taniguchi, J., 1981. Geology of the Ryujin district. Quadrang. Ser., 1:50,000, Geol. Surv. Japan, 1-69. Tsuchiya, N., Sakai, T., and Kanmera, K., 1979. Mode of occurrence and petrological characteristic of greenstones of the Shimanto terrain in the Mimi river area, Kyushu. Geol. SOC. Japan, 85: 445-454. Yanai, S., 1981. The stratigraphical and paleontological situation of the Upper Cretaceous Uwajima Group of the shelf-facies within the Shimanto Supergroup, western Shikoku, Japan. J. Geol. SOC. Japan, 87: 339-352. EXPLANATION OF PLATE 1 Radiolarian fossils of the Hidakagawa Group in Kii Peninsula 1: Archaeodictyomitra apiara (Rust), x 245, 2: Parvicinqula cf. boesii (Parona), x 245, 3: Mirifusus cf. baile i Pessagno, x 100, 4: Thanarla cf. conica (Aliev), x 175, 6: Holocryptocanium barbui Dumix 175, 5: T. cf. ulchra&abol), trica, x 175, 7. k r i e n s i s Pessagno, x 210, 8: Thanarla vene-quinabol), 9: Pseudodictyomitra seu omacrocephala (Squinabol), x 175, 10: Dict omitra formosa S q u i n a b z b k o s l o v a e Foreman, x 245, 12: A r b u r n a Foreman, x 245, 13: Alievium rae allowa i Pessagno, x 245, 14: Am hi ndax tylotus Foreman, x 2 ~ D ~ a m e l l i c o s t a Foreman, t a

+-

EXPLANATION OF PLATE 2 Photo (Fig. 1) and phtomicrograph (Figs. 2-8) of bedded chert 1. Etched surface of three sets of bedded chert showing parallel lamination. a: red clayey parting, b: black tuffaceous parting. 2. Enlargement of parting b in Fig. 1, showing many fragments of feldspar and sharp boundary with chert layer. 3. Enlargement of parting a of Fig. 1, showing upper sharp boundary and lower graded boundary and microfaul t. 4. Tuffaceous parting. .Lower left is clayey red chert. 5. Graded structure of chert layer made by decrease of radiolarian shells and increase of muddy material. 6. Parallel lamination made by different contents of haematitic material. Radiolarian tests are uniformly distributed. 7. Black parting containing many radiolarian shells. Both the upper and lower boundaries are sharp. 8. Red clayey parting showing sharp upper boundary and gradational lower boundary. The lower half corresponds to the upper half of Fig. 5.

All samples collected from Sumomo, Miyama-mura, Scale bars, 1 mn.

410

413

CHAPTER 24 MESOZOIC ACCRETION OF SILICEOUS DEPOSITS IN SOUTHWEST JAPAN Y. OGAWA, K. NAKASHIMA and H. SUNOUCHI

INTRODUCTION Recent identification of Triassic and Jurassic microfossils, especially Jurassic radiolarians, within chert and other siliceous sedimentary rocks in "eugeosynclinal" facies of what was known as the Chichibu Paleozoic Group (Kobayashi, 1941) in Southwest Japan has fostered a great revolution in regards to reconstruction of tectonic events for the Paleozoic and Mesozoic. We present the basic stratigraphy and structure of some areas of the Chichibu Terrane, particularly in the southern half of the "Chichibu Geosynclinal Area", and we present new biostratigraphic data from siliceous sedimentary rocks in selected parts of Shikoku and Kyushu (Fig. 1). We also discuss important observations on the mode of occurrence of chert and siliceous claystone. We suggest that chert and siliceous claystone were oceanic or pelagic sediments that were deposited on basaltic rocks and were accreted together with arc- or continentderived Middle and Upper Jurassic terrigenous clastic sediments along two subduction zones during the Late Jurassic. This tectonic interpretation differs from the conservative interpretations of Kimura et al. (1975), Ogawa (1978) and Murata (1381), who did not have assess to the radiolarian biostratioraphy. GEOLOGIC SETTING OF THE CHICHIBU TERRANE The Chichibu Terrane, about 20 km wide on the average, runs from Kyushu to Central Japan in parallel to the major structures of Southwest Japan, bordered on the north by the Sambagawa Metamorphic Belt and on the south by the Shimanto Belt (Fig. 1). The former belt lies structurally below the Northern Chichibu Zone (Ogawa, 1978) and the latter belt is interpreted to be a Cretaceous to lower Tertiary accretionary belt (Taira, Okada et al., 1981). The Chichibu Terrane is ,characterized by weakly metamorphosed "eugeosyncl inal I' sedimentary and volcanic rocks that yield Upper Paleozoic fossils as well as Mesozoic ones (Kobayashi, 1941 ; Tanaka and Nozawa, 1977). The Chichibu Terrane is subdivided by faults into the northern, middle, and southern parts, which are called here the Northern Chichibu Zone, the Kurosegawa Tectonic Zone, and the Sanbosan Zone respectively (Fig. 1). Northern Chichibu Zone This structural unit is interpreted as a part of the northern Sambagawa high

414 pressure metamorphic b e l t (Ogawa, 1978) , and i s composed o f s t r o n g l y deformed Upper Paleozoic t o Cretaceous sedimentary rocks Pre-Cretaceous sedimentary rocks form a so-called sedimentary melange, and some p a r t s a r e metamorphosed ~

t o greenschist o r pumpel l y i t e facies.

Pre-Cretaceous rocks a r e unconformably

o v e r l a i n by Lower Cretaceous brackish t o marine sedimentary rocks. The main phase of metamorphism and deformation ended b e f o r e the E a r l y Cretaceous, though r a t h e r extensive deformation has continued a f t e r t h e Cretaceous (Ogawa, 1973). Upper Paleozoic f o s s i l s , mostly f u s u l i n i d s , are contained w i t h i n limestone associated w i t h b a s a l t i c rocks. some from limestone,

Some conodonts a r e mostly from bedded chert,

Jurassic r a d i o l a r i a n s a r e found i n bedded c h e r t o r s i l i -

No terrigenous sedimentary rocks a r e found i n sections o l d e r

ceous claystone. than the Jurassic.

Kurosegawa Tectonic Zone This zone i s characterized by southward verging t i g h t recumbent f o l d s and t h r u s t f a u l t s (Ogawa, 1974b; Maruyama, 1981). The zone comprises a v a r i e t y o f rocks of S i l u r i a n t o Cretaceous age. S i l u r i a n and Devonian rocks a r e d a c i t i c t o r h y o l i t i c welded t u f f and c o r a l r e e f limestone, and Upper Paleozoic rocks c o n s i s t o f limestone and slump deposits. Mesozoic rocks are shallow marine sediments i n c l u d i n g the Upper Jurassic c o r a l r e e f 1imestone.

G r a n i t i c rocks

dated as 390 Ma (Ishizaka, 1972; r e c a l c u l a t e d by the new decay constant), h i g h

L

I

I

0

~

0

0zpo km

Fig. 1. Index map o f the study areas, showing t h e c o n s t i t u t i o n o f the Chichibu Terrane. A, Nakaoi-Ishimi area; B, Yoshio area.

415

pressure metamorphic rocks dated as 298-240 Ma (Maruyama et al., 19781, and high grade gneiss probably older than Silurian are contained within the serpentinite lined fault zone, forming serpentinite melange (Maruyama, 1981). Rocks in this zone are considered to represent an originally much wider terrane before the Cretaceous, which was either an exotic terrane from far to the south or a detached Eurasian terrane. The terrane became a complicated collage zone after the Cretaceous, forming an imbricate tectonic zone (Ogawa, 1978). Sambosan Zone This zone was considered to be a portion of the Chichibu "geosyncline" by Ogawa (1974a) and Kimura et al. (1975). This zone is made of three different lithological units in Kyushu. They are from north to south, the Yonaku Formation, Yoshio Formation, and Konose Group. The Yonaku Formation is composed of a large amount of pebbly mudstone which includes huge blocks of basalt, chert, and fusulina limestone. The chert contains Permian and Triassic conodonts, and the claystone contains Jurassic radiolarians (Nishizono, 1981 , personal communication). The northernmost part, which is called the Haki !?etamorphic Zone (Matsumoto and Kanmera, 1964), is metamorphosed at greenschist facies. The Yoshio Formation is composed largely of alternations of sandstone and mudstone, which overlie chert, siliceous claystone, limestone, and a small amount of basaltic rocks. The chert yields mostly Triassic conodonts and Lower Jurassic fossils, and rare Permian ones. The Konose Group is composed of many thrust sheets, comprising basaltic rocks, limestone, chert, and claystone in ascending order. Limestone yields rare Middle Permian fusulinids, and mostly Triassic corals, conodonts and bivalves (Kanmera, 1964; Kanmera and Furukawa, 1964; Tamura et al., 1981). Chert yields Triassic conodonts and claystone yields Cretaceous radiolarians (Oishi and Murata, 1980; Nishizono, 1981 personal communication). No diagnostic Jurassic radiolarians have been found in chert or siliceous claystone of the Konose Group yet, but in places Upper Jurassic limestone blocks are in contact with the Triassic limestone (Tamura et al., 1981).

OCCURENCE OF CHERTS AND OTHER SILICEOUS DEPOSITS In this chapter we describe new stratigraphic observations in the selected parts in the Chichibu Terrane; the Nakaoi-Ishimi area in the Northern Chichibu Zone, Shikoku and the Yoshio area in the Sambosan Zone, Kyushu ( A and B in Fig. 1). Nakaoi-Ishimi area The Nakaoi-Ishimi area occupies the majority of the central part of the

416 Northern Chichibu Zone, northwest o f Kochi (K i n Fig. 1). The Takabagamori Group occurs south of t h e Kamiyakawa Formation and n o r t h o f t h e I n o Formation, those adjacent formations a r e both schistose (Fig. 2).

The age o f t h e Taka-

bagamori Group has l o n g been known t o be L a t e Paleozoic based on Upper Carboniferous t o Middle Permian f u s u l i n i d s from limestone blocks ( K a t t o and Kawasawa, 1958). But one o f us (H.S.) found T r i a s s i c conodonts from bedded c h e r t and Jurassic r a d i o l a r i a n s from s i l i c e o u s claystone, showing t h e age o f sedimentation t o be as young as Jurassic (Sunouchi, 1980). The Takabagamori Group i s f u r t h e r d i v i d e d i n t o t h e Nakaoi Formation i n t h e n o r t h and t h e I s h i m i Formation i n the south. The Nakaoi Formation i s composed o f bedded c h e r t and a r g i l l a c e o u s sedimentary rocks w i t h sandstone and scarce b a s a l t i c rocks. Some a l t e r n a t i o n s o f sandstone and mudstone a r e deformed i n t o

Fig. 2. Geologic map and cross s e c t i o n o f t h e Nakaoi-Ishimi area. @ ,@ ,@ and @ show t h e f o s s i l l o c a l i t i e s o f Carboniferous, Permian, T r i a s s i c and Jurassic respectively. Based on Sunouchi (1980).

417

a broken formation (sensu HsL, 1974), and the argillaceous sedimentary rocks are transformed into phyllite or slate. The Ishimi Formation is characterized by blocks of various lithologies included within phyllitic, argillaceous matrix. The blocks comprise basalt, chert, limestone, siliceous claystone, sandstone, and mudstone. The ages of those blocks cover the range of ages mentioned above. On this basis the Ishimi rocks are considered to be on the whole of olistostromal origin. Cherts in the Ishimi and Nakaoi Formations are radiolarian and entirely recrystallized into aggregates o f microcrystalline quartz. Red cherts in the Ishimi Formation commonly occur between basalt lava flows. Upper Carboniferous to Fiddle Permian fusulinids are found in the limestone layers which also alternate with the basalt (Katto and Kawasawa, 1958; @ and @ in Fig. 2); therefore the red chert ages probably range from the Late Carboniferous to Middle Permian. Gray-colored bedded cherts have rare fossils. In the Ishimi Formation occurs a Triassic conodont (NeohindeodeZZa sp.; K. Watanabe, 1980 personal communication), and in the Nakaoi Formation occur Middle to Upper Triassic conodonts (NeohindeodeZla navicuZa steinbergensis and N. a e q u i m o s a ; Sunouchi, 1980) ( @ in Fig. 2). The Triassic cherts are contained within argillaceous sedimentary rocks as blocks or layers. The chert of the Nakaoi Formation is apparently very thick on account of repetition by folding and thrusting. Some imbricate sheets both in the Nakaoi and Ishimi Formations contain blocks o f less deformed siliceous claystone (0 in Fig. 2) bordered by faults. Claystones include abundant Jurassic radiolarians. Nakaoi sections range to about 80 m thick, contains no terrigenous detrital grains, but does contain manganiferous layers. The nassellarian radiolarian assemblages give an Late Jurassic age throughout the sequence. Siliceous claystone o f the Ishimi Formation far south of Nakaoi also yields the same assemblages of radiolarians. The argillaceous matrix between the siliceous claystone blocks has not so far yielded fossils, but the claystone blocks may be coherent equivalents to the sheared argillaceous matrix. Varied lithologic blocks of various ages in the Ishimi Formation suggest formation as an ol istostrome after Late Jurassic time, then was subsequently deformed into sedimentary melange (sensu Gucwa, 1975). The lithologies and field relations in the northern Chichibu Zone in the Nakaoi-Ishimi area are very similar to the Sawadani-Kenzan area in eastern Shikoku (Ogawa, 1973). In the south large masses (400 m thick at least, Ogawa, 1974a) made of alkali and tholeiitic basalts probably of seamount origin are apparently tectonically repeated or alternated depositionally limestone, chert, and terrigenous clastic rocks (Kanmera, 1969; Ogawa, 1974a; Maruyama and Yamasaki, 1978). The basalt is intercalated with Upper Carboniferous to Lower

418

Permian limestone and dolomite and i s a l s o capped by t h i c k Middle Permian limestone (Kanmera, 1969). Around t h e b a s a l t blocks a r e T r i a s s i c c h e r t (Yokoyama e t al., 1979) and undated terrigenous c l a s t i c rocks c o n t a i n i n g abundant slump pebbles o f v a r i e d l i t h o l o g i e s . The l i t h o l o g i c a l assemblage i s named as the Sawadani Group (Kanmera, 1969). To t h e north, bedded c h e r t blocks ranging i n age from Late

Carboniferous t o E a r l y Jurassic through Permian and T r i a s s i c

are i n t e r c a l a t e d w i t h i n mudstone and sandstone (Suyari e t a1

, 1979;

Isozaki

e t al., 1981). This assemblage i s c a l l e d the Kenzan Group. The v a r i e d l i t h o l o g i e s i n t h e Sawadani-Kenzan area a r e probably o l i s t o s t r o m a l as i n t h e HakaoiI s h i m i area.

Paleogeographic changes d u r i n g t h e Mesozoic a r e shown i n Fig. 6.

Yoshio area The age and occurrence o f the s i l i c e o u s sedimentary rocks i n the Sambosan Zone a r e w e l l analyzed i n the Yoshio Formation (Fig. 3 ) .

The formation i s com-

posed o f sandstone and c h e r t w i t h s i l i c e o u s claystone, limestone, and a small amount o f b a s a l t . The i n t e r n a l s t r u c t u r e o f t h e formation i s n o t f u l l y understood, b u t r e c e n t discoveries o f conodonts and r a d i o l a r i a n s by Nakashima (1979), Nishizono e t a l . (1981), and Ogawa (1981 unpublished data) suggest t h a t i t i s a continuous succession o f bedded c h e r t from Lower T r i a s s i c t o t h e Lower Jurassic, covered by Middle and Upper Jurassic terrigenous c l a s t i c rocks.

The

succession i s repeated many times by f o l d i n g and t h r u s t i n g , mostly w i t h southward vergence. Fig. 3 shows t h e d i s t r i b u t i o n o f s t r a t a and f o s s i l l o c a l i t i e s , and Fig.4 shows the generalfzed columnar sections. A l l c h e r t s a r e r a d i o l a r i a n ones, and those y i e l d i n g conodonts a r e mostly T r i a s s i c and r a r e l y Permian, whereas those barren o f conodonts c o n t a i n Lower Jurassic r a d i o l a r i a n s .

Chert

i s o v e r l a i n unconformably by Middle Jurassic r a d i o l a r i a n - b e a r i n g claystone o r mudstone.

I n outcrops i t seems t h a t chert, claystone, and sandstone a l t e r n a t e

t o form a very t h i c k sequence shown by Ogawa (1974a), I s h i d a (1977) and Murata (1981). R a d i o l a r i a n b i o s t r a t i g r a p h y , however, shows t h i s i s n o t t h e case. Our new observation i n d i c a t e s t h a t d e p o s i t i o n a l sequences a r e very t h i n ranging t o a hundred o r a few hundred meters t h i c k ; and a r e t e c t o n i c a l l y repeated many times by f a u l t s and f o l d s t o g i v e the apparent t h i c k sequences seen i n outcrop. Furthermore, the c h e r t never a1 ternates w i t h terrigenous c l a s t i c s , b u t appears always t o be i n c o n t a c t w i t h c l a s t i c s by f a u l t s . I n some cases, sandstone dykes i n t r u d e the c h e r t sections, p r o v i d i n g f u r t h e r complications.

Figs. 4 and

5 show examples o f such occurrences and Fig. 3 i n d i c a t e s t h e l o c a l i t i e s of

sandstone dykes.

I n Fig. 5-6, f o l d e d bedded c h e r t , which y i e l d s a Norian cono-

dont (NeohindeodeZta mvicula steinbergensis) , i s i n t r u d e d by sandstone dykes. The dykes commonly c u t t h e a x i a l surfaces o f the f o l d s . Bedded c h e r t containi n g Lower Jurassic r a d i o l a r i a n s i s a l s o i n t r u d e d by sandstone dykes o r s i l l s as

419 shown i n Fig. 5-A.

North o f the f a u l t shown, s i l i c e o u s claystone w i t h t h i n

sandstone l a y e r s i s brought i n t o contact w i t h massive sandstone by a t h r u s t f a u l t . The two sandstone, the dyke sandstone and t h e t h i c k massive sandstone, are both medium grained f e l d s p a t h i c (40-60 %) l i t h i c a r e n i t e , and commonly cont a i n heavy minerals such as zircon, garnet, and b i o t i t e . The dyke sandstone has no sedimentary s t r u c t u r e such as l a m i n a t i o n and graded bedding, b u t has c h e r t fragments presumably derived from t h e dyke w a l l .

The sandstone dyke

boundaries a r e g e n e r a l l y s t r a i g h t , c l e a r l y c u t t i n g bedding planes. Sometimes, however, the boundaries a r e very i r r e g u l a r , a f e a t u r e best understood i f the s o f t sand i n j e c t e d i n t o already consolidated bedded chert. I n several outcrops, t h e T r i a s s i c bedded c h e r t i s eroded i n t o and o v e r l a i n by a sequence made o f conglomerate, sandstone, and claystone. The sequence i s c a l l e d t h e "Ebirase Formation" (Matsumoto and Kanmera, 1964).

Jurassic 1 ime-

Fig. 3. Geologic map and cross s e c t i o n o f t h e Yoshio area, showing the conodont l o c a l i t i e s and sandstone dyke l o c a l i t i e s . P, S, A, L, C, and N i n d i c a t e the ages o f the f o s s i l s ; Permian, Skytian, Anisian, Ladinian, Carnian, and Norian r e s p e c t i v e l y . Numbers i n d i c a t e t h e l o c a l i t i e s o f columnar sections i n Fig. 4. Based on Nakashima (1979) and Ogawa (1981 unpublished data).

420

stone, which i s characterized by abundant c o r a l fragments, i s included as We a l s o i d e n t i f i e d Middle Jurassic r a d i o l a r i a n s i n the claystone o f Nishizono e t a l . (1981) f u r t h e r the "Ebirase Formation" (Fig. 4 - 0 ,

blocks.

a).

found Tithonian r a d i o l a r i a n s i n mudstone associated w i t h huge blocks o f limestone i n the south o f the Yoshio Formation outcrop (near @ of Fig. 3 ) . These s t r a t i g r a p h i c observations within the Yoshio Formation (which includes a l s o the "Ebirase Formation") i n d i c a t e t h a t T r i a s s i c t o Lower Jurassic c h e r t was eroded i n t o by processes responsible f o r i n t r o d u c i n g terrigenous sediment d u r i n g the Middle Jurassic. The t h i c k massive sandstone o f the Yoshio Formation i s compos i t i o n a l l y d i s s i m i l a r t o the sandstone o f the "Ebirase Formation"; and does n o t y i e l d any index f o s s i l s , b u t judging from the f o s s i l evidence i n Middle Shikoku by Yao (1981), the t h i c k sandstone o f the Yoshio Formation i s synchronous t o the middle p a r t of t h e Middle t o Upper Jurassic sequence.

Fig. 4. Generalized columnar sections o f the Yoshio Formation. are shown i n Fig. 3.

Localities

421

5s

N

&

%%e 8 claystone

bedded chert 8 sandstone dyke

folded bedded chert 8 sandstone dyke

altcrnation,ot bedded c k t

8 claystone In contict with sa&tOrW by fault cd. 2Om

-

A

H

Sm D

,

Sm

,

*

Fig. 5. Occurrence of sandstone dykes and s i l l s i n the south of Kajiki. indicates the f o s s i l l o c a l i t y ; 1-5 and 1-7, Carnian; 5-1, Norian; 3-8 and 3-7, Lower Jurassic; 3-4, 3-1 and 2-6, Middle Jurassic. In the Yoshio Formation also occur small blocks of basalt covered by Lower Permian fusulinid-bearing dolomite and red chert, succeeded by Triassic conodont-bearing bedded chert (Fig. 4 - 0 ) . One horizon i s rich i n blocks of Upper Carboniferous fusul ina 1 imestone ( F i g . 4-@ ). Two Upper Paleozoic formations, the "Amatsuki" and "Shizo" Formations were defined by Matsumoto and Kanmera (1964). B u t the limestone blocks seem t o be originated as o l i s t o l i t h s into the Nesozoic Yoshio Formation. Slump deposits o r pebbly mudstone a l s o occur around the chert and sandstone beds. The occurrence of slump deposits and other sedimentological features mentioned above suggest that the ocean floor deposits, ranging i n age from the Late Carboniferous to Early Jurassic and made u p of basaltic rocks and limestone i n the lower part and chert above, were mixed w i t h the Middle and Upper Jurassic terrigenous c l a s t i c sediments to form some k i n d of accretionary bodies. A t the time of mixing several phenomena may have occurred. The intrusion Of sandstone dykes o r s i l l s into the chert. The formation of slump deposits, which include Carboniferous to Jurassic rocks. The erosion of the Triassic chert by Jurassic terrigenous c l a s t i c sediments may have occurred a t this time. And, southward imbricate structure containing overturned folds w i t h southward vergence occurred.

422

To summarize the three parts of the Sambosan Zone in Kyushu, the north Yonaku Formation is rich in argillaceous sedimentary rocks containing Permian limestone with subordinate Triassic and Jurassic sedimentary rocks, and the middle Yoshio Formation is rich in sandstone containing Triassic and Jurassic chert and other siliceous sedimentary rocks with subordinate Upper Paleozoic limestone, chert, and basalt. The basaltic rocks of the two formations are of unknown age, but according to those in adjacent sedimentary rocks, they are Upper Carboniferous to Permian. On the other hand the south Konose Group is rich in basalt, limestone, and chert, and the ages are mostly Triassic with subordinate Jurassic and Cretaceous strata. In general the last sedimentation occurred in the Yonaku and Yoshio in the Middle Jurassic and Late Jurassic respectively, while in the Konose it occurred in the Cretaceous. Thus the ages of basalt and of the last sedimentation both shifted southward. The Sambosan Zone in Kyushu, therefore, comprises a large belt of ocean-floor-type rocks arranged in imbricate structures and having overall southward younging. For these reasons it best merits interpretation as a product of accretion processes TECTONIC SIGNIFICANCE OF THE CHERT AND OTHER SILICEOUS DEPOSITS As described above, Upper Paleozoic to Jurassic oceanic lithologies are diversely developed in two separate zones to the north and south of the Kurosegawa Tectonic Zone. They comprise cherts and other siliceous sedimentary rocks that accompany either abundant basalt and limestone or sandstone and mudstone. Cherts can be classifeid into two distinct types on the basis of field relations. One is red chert associated directly with basaltic rocks; the other is usually gray-colored bedded chert associated with sandstone and mudstone. The red chert is intercalated within or rests on the basaltic rocks, which probably represent the oceanic crust itself or seamount-1 ike bodies thereon, because the chemistry of some rocks is similar to those of the present oceanic crust or seamounts (Sugisaki et al., 1972; Maruyama and Yamasaki, 1978). Such cherts are found along the southern halves of the Northern Chichibu Zone and the Sambosan Zone as described above. The thick basalt bodies date back to the Upper Carboniferous to Middle Permian in the Northern Chichibu and to the Triassic in the Sambosan (Kanmera, 1974). The gray-colored bedded chert is usually separate from the basaltic rocks, and is associated with sandstone and mudstone. The basal boundaries of such cherts are mostly faulted, but very rarely they rest on basaltic rocks conformably (e.g. Fig. 4-@ ; Ogawa, 1974a). We speculate that the gray chert may therefore also represent original oceanic sediments deposited on the basaltic rocks and were detached from them by faulting or gravity sliding at the trench during tectonic juxtapositioning with the terrigenous sediments. The ages of such cherts range from Late Carboniferous to Jurassic in the Northern

423

N

LATE CARB.

-

5 MID. PERM.

distance unkriown

LATE PERM.

/

1 I -x..A x x x TRIAS.-EARLY

JURA. bedded ralioralian chcrl

MID.-LATE

JURA.

S

SAMBOSAN

a metamorphic r x k j

CRETAC. .

.:-:

-

.-

..

,

_.,,_),

brackish B X X X shallow marine ctastic sediments

-

LATEST CRETAC. PALE0G.- EARLY MIOC. S SHIMANTO

.

ca. lOkm

,

Fig. 6. Paleogeographic prcfiles around the Chichibu Terrane, Southwest Japan. Only showing the related lithology of each age except for the basement rocks in the Kurosegawa Zone. Chichibu (Isozaki et al., 1981) and from Triassic to Jurassic in the Sambosan (Nakashima, 1979; Nishizono et al., 1981). A small amount of Upper Paleozoic gray-colored bedded chert is reported in the Sambosan. We consider the facies associations and tectonic significance of these chertbearing fOrmations in the two zones to be as follows. Chert was originally deposited on the oceanic crust or seamounts in the oceanic region, because no terrigenous clastics are recognized within the chert formations as described in the previous chapters, and rather they rest on the basalt of oceanic crust or seamounts, and the chert in places alternates with pelagic sediments such as siliceous claystone comnonly associated with manganese deposits. The earliest sedimentation of the two zones began at the same time, in the Late Carboniferous. However the ages of the seamount-type basalts are much older in the Northern Chichibu than in the Sambosan. The red chert may have been deposited

424

during and after the sea-floor volcanism either on the active ridge or on a seamount (Kannera, 1969, 1974; Sano, 1982). We stress that the red cherts commonly occur in the Ishimi and Konose, repectively in the southern halves of the two zones. On the other hand, the gray-colored chert is not so closely associated with sea-floor volcanism and may have developed in the open-ocean away from a spreading center, some intercalations of claystone films occur forming the bedded aspect (Sano, 1982). This chert is developed in the northern halves of each zone, the Nakaoi and Yoshio. As the ocean floor area having such lithologies came close to the convergent continental margin or island arc area through ocean-floor spreading, the oceanic rocks and sediments, or at least some of them, may have been trapped, scraped off or accreted to the inner trench slope side to form an accretionary prism during a subduction phase. Recently similar tectonism has been well analyzed in the Ordovician-Silurian accretion in the Southern Uplands, Scotland, by Leggett et al. (1979) and Leggett (19801, and in the Cretaceous to Tertiary Nicoya Complex, Costa Rica, by Schmidt-Effing (1979) and Gursky and Schmidt-Effing (1982). In Japan subduction probably occurred during the Late Mesozoic (Uyeda and Miyashiro, 1974; Ono, 1980). Yhen a seamount mass moves down the outer trench slope by subduction, and becomes trapped, filling the trench as described for the present Japan trench area by Mogi and Nishizawa (19801, the chert and other oceanic sediments above the seamount could easily be incorporated into the accretionary prism together with arc- or continent-derived terrigenous sediments deposited in the trench. This is what we speculate happened in the Ishimi and Konose where a large amount of oceanic rocks and sediments are mixed to form an imbricate structure of melange-like formations. When the chert-bearing sections rest not on the seamount but directly on the seafloor distant from the seamount, possibly the sediments only above the oceanic crust may be scraped off to form an accretionary prism like the lithologic associations fround in the Nakaoi and Yoshio Formations. Paleogeographic profiles from the Late Carboniferous to early Miocene are reconstructed in Fig. 6. The accretion of the Northern Chichibu Zone occurred during the Sambagawa subduction, probably during the Jurassic (Ono, 1980), and that of the Sambosan occurred during the formation of the Haki Metamorphic Rocks. The two separate subduction and accretion events in Northern Chichibu and Sambosan, which occurred nearly simultaneously, could be explained by the strike-slip tectonisms around the northeastern Eurasian continental margin as presented by Taira, Saito, and Hashimoto (1981). ACKNO'ALEDGEMENTS We are grateful to Professor K. Kanmera for his kind guidance and sugges-

426

tions. Professors K. Nakaseko and M. Murata and rlr. Y. Nishizono gave us many suggestions on radiolarian biostratigraphy. Professor A. Iijima, Drs. J. R. Hein and J. K. Leggett and Mr. H. Sano are appreciated for improvement of the manuscript. REFERENCES Gucwa, P o R., 1975. Middle to Late Cretaceous sedimentary melange, Franciscan Complex, northern California. Geology, 3: 105-108. Gursky, H.-J. and Schmidt-Effing, R., 1982. Sedimentology of the radiolarites within the Nicoya Ophiolite Complex (Costa Rica, Central America).(this volume) Hsi, K.J., 1974. Melanges and their distinction from olistostrome. 1n:R.H. Dott,Jr. and R.H. Shaver (Editors), Modern and Ancient Geosynclinal Sedimentation. SOC. Econ. Paleont.. Mineral. Spec. Pap. , 19: 321-333. Ishida, K., 1977. Reexamination of the Palaeozoic and Mesozoic formations in the southern zone of the Chichibu belt in eastern Shikoku by means of conodonts and fusulinids. J. Geol. SOC. Japan., 83: 227-240. Ishizaka, K., 1972. Rb-Sr dating on the igneous and metamorphic rocks of the Kurosegawa Tectonic Zone. 3. Geol. SOC. Japan, 78: 569-575. Isozaki, Y., Maejima, W. and Maruyama, S., 1981. Occurrence of Jurassic radiolarians from the pre-Cretaceous rocks in the northern subbelt of the Chichibu Belt, Wakayama and Tokushima Prefectures. J. Geol. SOC. Japan, 87: 555-558. Kanmera, K., 1964. Triassic coral fauna from the Konose Group in Kyushu. Mem. Fac. Sci., Kyushu Univ., Ser.D., 15: 117-147. Kanmera, K. , 1969. Upper Paleozoic stratigraphy of the northern Chichibu Belt in eastern Shikoku. Sci. Rep., Fac. Sci., Kyushu Univ., Geol., 9: 175-186. Kanmera, K. , 1974. Paleozoic and Mesozoic geosynclinal volcanism in the Japanese Islands and associated chert sedimentation. In: R.H. Dott, Jr. and R.H. Shaver (Editors) , Modern and Ancient Geosynclinal Sedimentation. Soc. Econ. Paleont. Mineral. Spec. Pap., 19: 161-173. Kanmera, K. and Furukawa, H., 1964. Stratigraphy of the Upper Permian and Triassic Konose Group in Kyushu. Sci. Rep. , Fac. Sci. , Kyushu Univ. , Geol. , 6: 237-258. Katto, J. and Kawasawa, K., 1958. Paleozoic system to the north of Ino Town, Kochi Prefecture. Sci. Rep., Kochi Univ., 7: 1-8. Kimura, T., Yoshida, S. and Toyohara, F., 1975. Paleogeography and earth movements of Japan in the late Permian to early Jurassic Sambosan stage. J. Fac. Sci., Univ. Tokyo, Sec. 2, 19: 149-177. Kobayashi , T., 1941. The Sakawa orogenic cycles and its bearing on the origin of the Japanese Islands, J. Fac. Sci., Univ. Tokyo, Sec. 2, 5: 219-578. Leggett, J.K., 1980. The sedimentological evolution of a Lower Palaeozoic accretionary fore-arc in the Southern Uplands of Scotland. Sedimentology, 27: 401 -417. Leggett, J.K., McKerrow, W.S. and Eales, M.H., 1979. The Southern Uplands of Scotland: A Lower Palaeozoic accretionary prism. 3. Geol. SOC. , 136: 755-770. Maruyama, S., 1981. The Kurosegawa melange zone in the Ino district to the north of Kochi City, central Shikoku. J. Geol. SOC. Japan, 87: 569-583. Maruyama, S., Ueda, Y. and Banno, S., 1978. 208-240 m.y. old jadeite-glaucophane schists in the Kurosegawa tectonic zone near Kochi City, Shikoku. J. Japan. Assoc. Min. Petrol. Econ. Geol., 73: 300-310. Maruyama, S. and Yamasaki , M., 1978. Paleozoic submarine volcanoes in the highP/T metamorphosed Chichibu system of eastern Shikoku, Japan. 3. Volc. Geotherm. Res., 4: 199-216. Matsumoto, T. and Kanmera, K., 1964. Geological Map o f Hinaku (1/50,000) and its explanatory text. Geol. Surv. Japan, 147pp. Mogi, A. and Nishizawa, K., 1980. Breakdown of a seamount on the slope of the

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Japan Trench. Proc. Japan Acad., 56: Ser.B. 257-259. Murata, A., 1981. A l a r g e o v e r t h r u s t and paleogeography o f the Kurosegawa and i n the case o f the Gokase area i n c e n t r a l Kyushu. J. Sambosan Terrains Geol SOC. Japan, 87: 353-357. Nakashima, K., 1979. S t r a t i g r a p h y o f the Southern B e l t o f the Chichibu Terrane, Southern Kyushu. Master Thesis, Kyushu Univ. (MS) Nishizono, Y., Nakaseko, K. and Murata, M., 1981. Mesozoic s t r a t i g r a p h y and r a d i o l a r i a n assemblages o f t h e Kuma Mountains, Kyushu. Abs. Ann. Meet. Geol. SOC. Japan, p. 169. Ogawa, Y., 1973. Tectonic development o f the Chichibu T e r r a i n i n eastern Shikoku, Japan, w i t h special reference t o the deformational stages. J. Fac. Sci., Univ. Tokyo, Sec. 2, 18: 475-506. Ogawa, Y., 1974a. S t r a t i g r a p h y and paleogeography o f t h e Chichibu T e r r a i n i n eastern Shikoku, Japan. Proc. I n s t . Nat. Sci., Nihon Univ., Earth Sci., 9: 15-48. Ogawa, Y., 1974b. Geologic s t r u c t u r e s o f the Chichibu T e r r a i n i n eastern Shikoku, Japan. J. Geol. SOC. Japan, 80: 439-455. Ogawa, Y., 1978. S t r u c t u r a l c h a r a c t e r i s t i c s and tectonisms around t h e microc o n t i n e n t i n the o u t e r margin o f t h e Paleozoic-Mesozoic geosyncl i n e of Japan. Tectonophysics, 47: 295-310. Oishi, A. and Murata, M., 1980. New views o f the southernmost p a r t o f t h e Konose b e l t , Kuma River area. Abs. Ann. Meet. Geol. SOC. Japan, p.147. Ono, A., 1980. A model o f the formation o f t h e Ryoke-Sambagawa p a i r e d metamorphic b e l t . J. Japan. Assoc. Hin. P e t r o l . Econ. Geol., 75: 31-37. Sano, H., 1982. Bedded c h e r t s associated w i t h greenstones i n t h e Sawadani and Shimantogawa Groups, Southwest Japan. ( t h i s volume) Schmidt-Effing, R., 1971. A l t e r und Genese des Nicoya-Komplexes, e i n e r ozeanischen Palzokruste (Oberjura b i s Eozzn) i n s i d l ichen Zentralamerika. Geol Rundsch. , 68: 457-494. Sugisaki, R., Mizutani, S., H a t t o r i , H., Adachi, M. and Tanaka, T., 1972. Late Paleozoic geosynclinal b a s a l t and tectonism i n t h e Japanese Islands. Tectonophysics, 14: 35-56. Sunouchi , H. , 1980. Geology o f the northern Chichibu B e l t o f c e n t r a l p a r t of Kochi Prefecture. Graduation Thesis, Kyushu Univ. (MS) Suyari, K., Kuwano, Y. and Ishida, K., 1979. R e l a t i o n between the Sambagawa and Chichibu b e l t s i n Shikoku. I n : I.Hara ( E d i t o r ) , Studies on Late Mesozoic Tectonism i n Japan, 1: 39-49. Taira, A . , Okada, H., Whitaker, J.H.McD. and Smith, A.J., 1981. The Shimanto B e l t o f Japan: Cretaceous-Lower Miocene sedimentation. I n : J.K. Leggett ( E d i t o r ) , Trench-Forearc Geology o f Modern and Ancient Continental Margins. Geol. SOC. London, Spec. Pap., 10: 481-501. Taira, A., Saito, Y. and Hashimoto, M., 1981. Basic process f o r the formation o f the Japanese Islands. Kagaku (Science), Iwanami-Shoten, Tokyo, 51 : 508515. Tamura, M., Watanabe, K. and Hirokawa, H., 1981. Some new data on t h e geology o f t h e Sambosan zone i n Kyushu. Abs. West. Japan Branch, Geol. SOC. Japan, 72: 6-7. Tanaka, K. and Nozawa, .T. ( E d i t o r s ) , 1977. Geology and Mineral Resources of Japan. Vol,. 1, Geology, Geol. Surv. Japan, 430 pp. Uyeda, S. and Miyashiro, A., 1974. P l a t e t e c t o n i c model and Japanese Islands: A synthesis. Geol. SOC. Am. B u l l . , 85: 1159-1170. Yao, A., 1981. Late Paleozoic and Mesozoic r a d i o l a r i a n s from southwest Japan. Abs. 2nd I n t e r n a t . Conf. S i l i c . Dep. P a c i f i c Region, pp. 49-50. Yokoyama, T., Tominaga, R., Hara, I.and Kuwano, Y., 1979. S t r u c t u r a l a n a l y s i s northernmost zone o f t h e Kurosegawa zone i n the Sawadani area, Tokuof shima Prefecture. I n : I.Hara ( E d i t o r ) , Studies i n Late Mesozoic Tectonism i n Japan, 1: 9-20.

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CHAPTER 25 BEDDED CHERTS ASSOCIATED WITH GREENSTONES I N THE SAWADANI AND SHINANTOGAWA GROUPS , SOUTHWEST JAPAN HIROYOSHI SANO

INTRODUCTION I t i s w e l l known t h a t bedded c h e r t s commonly o c c u r i n a s s o c i a t i o n w i t h

o p h i o l i t i c sequence (Grunau, 1965). Many c h e r t s e c t i o n s a r e regarded as b e i n g d e p o s i t e d i n t h e deep ocean b a s i n on p i l l o w l a v a t h a t makes u p t h e uppermost l a y e r o f t h e o c e a n i c c r u s t . T h i s i n t e r p r e t a t i o n has been p r e s e n t e d f o r s t u d i e s o f Mesozoic c h e r t s i n r l e d i t e r r a n e a n and A l p i n e r e g i o n s (e.3. Robertson, 1977; F o l k and :lcBride,

1978). N e d i t e r r a n e a n and A l p i n e c h e r t s

a r e t h i c k , commonly u p t o 200 m y and a r e u s u a l l y covered by p e l a g i c carbona t e rocks. Bedded c h e r t s t h a t a r e c l o s e l y a s s o c i a t e d w i t h greenstones a r e found i n t h e Sawadani Group, L a t e C a r b o n i f e r o u s t o E a r l y Permian i n age, and t h e Shimantogawa Group, m o s t l y o f Cretaceous age, b o t h i n t h e Outer Zone o f Southwest Japan. They d i f f e r f r o m t h e c h e r t s r e c o v e r e d f r o m t h e open-ocean by t h e DSDP w i t h r e s p e c t t o s t r a t i g r a p h i c r e l a t i o n t o greenstones, l i t h o l o 3 i c associations,

t h i c k n e s s , and s e d i m e n t a r y s t r u c t u r e s ( s e e H e i n and K a r l ,

t h i s volume), P e t r o g r a p h i c c h a r a c t e r i s t i c s o f t h e Sawadani and Shimantogawa c h e r t s a r e s i m i l a r t o t h o s e o f t h e Tethyan Mesozoic c h e r t s . GEOLOGIC SETTING The Sawadani Group i s d i s t r i b u t e d i n t h e upper reaches o f t h e Naka R i v e r o f Tokushima P r e f e c t u r e , e a s t e r n Shikoku, and g e o t e c t o n i c a l l y o c c u p i e s t h e n o r t h e r n p a r t o f t h e C h i c h i b u Terrane ( F i g . 1 ) . The s t r a t i g r a p h i c succ e s s i o n , d e t e r m i n e d b y Kanmera (1969), i s as f o l l o w s : Takano Formation (500 m): m a i n l y mudstone w i t h some b a s a l t i c l a v a , h y a l o c l a s t i t e , and c h e r t . Sawadan i Format i o n a. upper p a r t (200-650 m): m a i n l y h y a l o c l a s t i t e w i t h l i m e s t o n e i n t h e upper p a r t . b. m i d d l e p a r t (270-800 m): m o s t l y b a s a l t i c l a v a and h y a l o c l a s t i t e . c. l o w e r p a r t (500 m): m a i n l y mudstone w i t h some b a s a l t i c l a v a and c h e r t . The f u s u l i n i d s from l e n t i c u l a r 1 imestones a t s e v e r a l s t r a t i g r a p h i c l e v e l s i n d i c a t e t h a t t h e Sawadani Formation ranges i n age f r o m L a t e Carb o n i f e r o u s t o E a r l y Permian. The t h i c k v o l c a n i c sequence o f t h e m i d d l e

428

Fig. 1. Map showing l o c a l i t i e s of t h e i n v e s t i g a t e d areas. 1, Ryujin; 2 , Mihama; 3, Sawadani; 4, Aki; 5, Hinokage. sa, Sambagawa Metamorphic Terrane; ch, Chichibu Terrane; s h y Shimanto Terrane. P a r t has been considered t o be t h e remnant o f a seamount (Maruyama and Yamasaki , 1978). Bedded c h e r t s commonly occur i n t h e lower p a r t o f t h e Sawadani Form a t i o n and r a r e l y i n t h e Takano Formation. Noteworthy i s t h a t c h e r t s occur on greenstones o r a r e interbedded between flows. The Shimantogawa Group occurs i n t h e n o r t h e r n p a r t o f t h e Shimanto Terrane which occupies t h e southern margin o f t h e Outer Zone o f Southwest Japan ( F i g . 1 ) . Recently, t h e Shimantogawa Group has been i n t e n s i v e l y i n v e s t i g a t e d i n some areas ( K i i Peninsula: Kishu Shimanto Research Group, 1975; c e n t r a l Shikoku: Sano e t a l . , e t al.,

1979; Suzuki and Hada, 1979; T a i r a

1980; e a s t e r n Kyushu: Sakai, 1978; Sakai and Kanmera, 1981), and

n a j o r s u b d i v i s i o n s were recognized: Upper f o r m a t i o n i s a l t e r n a t i n g sandstone and s h a l e w i t h some i n t e r c a l a t i o n s o f s h a l e i n sandstone. Lower f o r m a t i o n i s m a i n l y mudstone, m o s t l y converted t o s l a t e o r p h y l l i t e , , w i t h greenstones and c h e r t . The lower formation has been i n t e r p r e t e d as being p a r t o f t h e a c c r e t i o n a r y prism o f t r e n c h - f i l l

d e p o s i t s (Kanmera and Sakai, 1975; Sakai, 1978; Sakai

and Kanmera, 1981; Suzuki and Hada, 1979; T a i r a e t al.,

1980).

Cherts occur e x c l u s i v e l y i n t h e lower f o r m a t i o n and a r e a s s o c i a t e d w i t h greenstones t h a t i n c l u d e l a v a and h y a l o c l a s t i t e . T h i s l i t h o l o g i c a s s o c i a t i o n i s common through t h e Shimantogawa Group from t h e Kanto Mountains o f c e n t r a l Japan t o t h e Ryukyu I s l a n d s . Cherts y i e l d Cretaceous and L a t e Jurassic r a d i o l a r i a n assemblages (Nakagawa e t a1 FIELD OBSERVATI O

., 1980;

Okamura, 1980).

M

Sawadani Group Sawadani c h e r t s a r e grouped i n t o t h r e e types; a. bedded c h e r t o v e r l y i n g

429

-

m s h e e t lava =pillow =massive

lava W h y a I o c h s t i t e =claystone chert =bedded chert =black slate

Fig. 2. Sketches showing the mode o f occurrence o f the B-type chert. Arrows indicate the face o f beds. A, 700 m NW of Higashiura, Sawadani area: lower part o f the Sawadani Formation; B, the left bank o f the Sakashu River in the vicinity of Setsu, Sawadani area: middle part o f the Sawadani Formation. greenstones (B-type: Fig. 2 ) , b. lenticular, massive chert embedded in basaltic lava (M-type: Fig. 3-A, B, C), c. chert blocks in pelitic matrix (0-type: Fig. 3-0). B-type chert occurs as thin laminae or beds, 1 to 3 cm thick, interbedded in the upper part of hyaloclastite (Fig. 2) which rests on basaltic lava. Chert increases up section in number and thickness o f beds, becomes rhythmically interbedded with claystone beds that are less than 1 cm thick in the middle part o f the section, and is finally covered by siliceous mudstone or black slate (Fig. 5-A, B). Chert sections are generally 5 to 20 m thick and rarely attain 30 m. Basaltic lava, hyaloclastite, and chert in ascending order exhibit a distinct stratigraphic sequence. In a few localities chert rests directly on a rugged floor o f pillow lava (Fig. 2-81, and the chert is coarsely crystalline for several centimeters above the contact with lava. Near faults, stratification appears to be thick-bedded or massive owing to intense recrystallization, but faint vestiges o f the original bedding surfaces are visible. Cherts are commonly red to brownish red, but in some places are greenish. Darker laminae o f less than 0.5 cm thick are commonly visible in a single chert bed. Claystone interbedded with chert is generally less than 1 cm thick, extremely fine-grained, fissile, and dark red or green. It is distinguished from black argillite not only by its color but also by the absence of dis-

430

t i n c t coarser terrigenous m a t e r i a l s . C h e r t o f M-type i s found i n a few o u t c r o p s . I t i s c h a r a c t e r i z e d b y t h e absence o f c l a y s t o n e i n t e r b e d s , and no d i s t i n c t s e d i m e n t a r y s t r u c t u r e s a r e v i s i b l e . I t i s c o m p l e t e l y embedded i n b a s a l t i c l a v a ( F i g . 3 - A ,

6 , C ) as

l e n t i c u l a r b o d i e s s e v e r a l m e t e r s t h i c k , and l i e s on uneven s u r f a c e o f p i l l o w l a v a . C h e r t o f t h i s t y p e i s r e d t o r e d d i s h brown w i t h o u t e x c e p t i o n and coarsely c r y s t a l l i n e . No t e r r i g e n o u s beds o c c u r i n 8 - and M-type c h e r t sequences.

F i g . 3. Sketches showing t h e mode o f o c c u r r e n c e o f t h e M- and 0 - t y p e c h e r t s . A , B, and C, M-type c h e r t . Note t h e i r r e g u l a r l y rugged boundary o f c h e r t and p i l l o w l a v a a t t h e basal p a r t o f c h e r t . A and C, t h e same l o c a l i t y as t h a t o f F i g . 2-8; B, r o a d s i d e o f t h e Kamagatani Tunnel, Sawadani a r e a : m i d d l e p a r t o f t h e Sawadani Formation. D, 0 - t y p e c h e r t , a b o u t 2 km NE o f Kamisawadani, Sawadani a r e a : Takano Formation. Symbols e x c e p t f o r t h o s e o f sandstone and b l a c k s l a t e a r e same as t h o s e o f F i g . 2. C h e r t o f 0 - t y p e o c c u r s as d i s r u p t e d b l o c k s i n i n t e n s e l y sheared and f o l i a t e d p e l i t i c r o c k s t h a t a l s o i n c l u d e b l o c k s o f sandstone, greenstones, l i m e s t o n e , and o t h e r s r a n g i n g f r o m s e v e r a l m e t e r s t o c e n t i m e t e r s i n s i z e ( F i g . 3-0).

The c h e r t b l o c k s a r e h i g h l y c o n t o r t e d and f r a c t u r e d , and a r e

bounded w i t h p e l i t i c r o c k s by shear o r f a u l t planes ( F i g . 3 - 1 3 ) .

Chert i s

p a l e green t o grey, o r p a r t l y s e c o n d a r i l y pigmented t o b r i g h t r e d d i s h , and commonly c o a r s e l y c r y s t a l 1 i n e . Shimantogawa Group

A s Kanmera (1974) and Sano e t a1 . (1979) have d e s c r i b e d t h e mode o f o c c u r r e n c e o f c h e r t and o t h e r s i l i c e o u s r o c k s o f t h e Shimantogawa Group, b r i e f n o t e s a r e g i v e n h e r e on newly o b t a i n e d d a t a .

431

The Shimantogawa c h e r t e x c l u s i v e l y o c c u r s above o r between greenstones t h a t a r e composed m a i n l y o f b a s a l t i c l a v a and h y a l o c l a s t i t e ( F i g . 4 ) . The sequence f r o m greenstones t o c l a y s t o n e and c h e r t i n ascending o r d e r i s most common ( F i g . 4-C).

C l a y s t o n e d i r e c t l y l i e s on b a s a l t i c l a v a o r h y a l o -

c l a s t i t e , and i s commonly s e v e r a l m e t e r s t h i c k . I n p l a c e s , c l a y s t o n e i s s i l i c e o u s and i n c l u d e s t h i n beds and l a m i n a e o f c h e r t , commonly l e s s t h a n 1 cm t h i c k , i n t h e upper p a r t . C h e r t b e g i n s t o o c c u r i n t h e upper p a r t o f c l a y s t o n e , g r a d u a l l y i n c r e a s e s u p s e c t i o n i n number and t h i c k n e s s o f beds, and f i n a l l y becomes r h y t h m i c a l l y i n t e r b e d d e d w i t h t h i n beds o r f i l m s o f c l a y s t o n e . I n some p l a c e s b a s a l t i c l a v a i s i n t e r l a y e r e d w i t h c h e r t beds. C h e r t u n d e r l y i n g l a v a i s t h e r m a l l y metamorphosed f o r s e v e r a l c e n t i m e t e r s above t h e c o n t a c t . Thickness o f c h e r t s e c t i o n s r a r e l y a t t a i n s 30 m, b u t g e n e r a l l y a r e 5 t o 20 m ( F i g . 5-C). The l a t e r a l change o f t h e t h i c k n e s s o f c h e r t s e c t i o n s has n o t been f u l l y c l a r i f i e d because o f t h e i n t e n s e p o s t d e p o s i t i o n a l d i s r u p t i o n ( F i g . 6 ) . B l a c k s l a t e most commonly c o v e r s t h e g r e e n s t o n e - c h e r t sequence.

F i g . 4. Sketches showing t h e mode o f o c c u r r e n c e o f t h e Shimantogawa c h e r t . A, Kue, A k i a r e a : Susaki Formation; 6 , Sakurahana, A k i a r e a : Susaki Form a t i o n ; C, T e r a u c h i , A k i a r e a : Susaki Formation; D, E, Toyanohira, Hinokage a r e a : Makimine Formation. Symbols e x c e p t f o r t h o s e o f m e t a l l i f e r o u s c h e r t and umber a r e same as t h o s e o f F i g . 2. M e t a l l i f e r o u s c l a y s t o n e t h a t i s p r o b a b l y analogous t o umber o r o c h r e of t h e Troodos M a s s i f (Robertson, 1975; Robertson and Hudson, 1974) o c c u r i n h o l l o w s o f t h e p i l l o w l a v a s u r f a c e i n a few l o c a l i t i e s ( F i g . 4-A). T h i s c l a y s t o n e i s g e n e r a l l y 1 0 t o 30 cm t h i c k and r a r e l y a t t a i n s 40 cm. I t i s c h o c o l a t e brown t o b r o w n i s h p i n k , and i s f a i n t l y l a m i n a t e d .

432

r

F

0 A

B

F i q . 5. Columnar s e c t i o n s showing t h e s t r t i a r a D h i c r e 1 t i o n o f a r e e n s t o n S a n i c h e r t s . A, t h e r i g h t bank o f - t h e Sakashu-River i n t h e v i c i n i i y o f Setsu, Sawadani a r e a : m i d d l e p a r t o f t h e Sawadani Formation; B, Shimokagedani, Sawadani a r e a : Takano Formation; C, t h e same l o c a l i t y as t h a t o f F i g . 4-A. An a r r o w i n d i c a t e s t h e s t r a t i g r a p h i c h o r i z o n o f t h e umber shown i n F i g . 4-A.

F i g . 6. A s k e t c h showing t h e d i s r u p t i o n o f greenstones and a s s o c i a t e d s e d i m e n t a r y r o c k s . Note t h a t t h e t h i c k n e s s o f greenstones and a s s o c i a t e d s e d i m e n t a r y r o c k s a b r u p t l y changes f r o m a b o u t 1 8 m t o l e s s t h a n 0.5 m i n t h e d i s t a n c e o f 100 m a l o n g g e n e r a l t r e n d o f s u r r o u n d i n g a r g i l l a c e o u s s e d i m e n t a r y r o c k s . The a s s o c i a t e d s e d i m e n t a r y r o c k s a r e a l s o c o m p l i c a t e d by many f a u l t s . A c o a s t a l exposure i n t h e v i c i n i t y o f S h i o f u k i i w a , Mihama a r e a : t h e l o w e r p a r t o f t h e Hidakagawa Group.

433

T h e S h i m a n t o g a w a c h e r t , 4 t o 8 cm t h i c k b e d s , i s c o m m o n ly r h y t h m i c a lly i n te rb e d d e d w i th c la y s to n e p a r ti n g s l e s s t h a n 1 cm t h i c k , a lth o u g h c h e r t c o m m o n ly s h o w s a p i n c h a n d s w e ll s tr u c tu r e . T h e b e d d i n g s u rf a c e o f c h e r t a n d c la y s to n e i s c le a r . N C R O S C O P IC O B S E R V A T IO N S T h e p e tro g ra p h i c c h a r a c te r i s ti c s o f t h e 8 - a n d M -ty p e c h e r t s o f th e S a w a d a n i G ro u p a n d th e S h i m a n to g a w a c h e r t a r e g i v e n h e re , a n d th o s e o f th e 0 -ty p e c h e r t a r e o m i tte d s i n c e i t i s c o n s i d e ra b ly r e c r y s ta lli z e d . T h e B -ty p e c h e r t a n d S h i m a n to g a w a c h e r t c o n s i s t o f r a d i o la r i a n s h e lls ( F i g . 7 -1 , 3 ) , s p o n g e s p i c u le s (F i g . 7 -2 ), a n d th e i r f ra g m e n ts , a n d a n e x tre m e ly f i n e -g ra i n e d m a tri x . T h e r a d i o la r i a n re m a i n s a r e g e n e ra lly 1 0 0 to 3 0 0 pm i n s i z e ; a n d a r e p o o rly p re s e rv e d a n d f i lle d i n o rd e r o f a b u n d a n c e w i th n o n -f i b ro u s q u a r tz , c h a lc e d o n y , c h lo r i te s , a n d h e m a ti te . R a d i o la ri a n s a r e m o re o r l e s s f la tte n e d , i n p a r a lle l w i t h f o li a ti o n p la n e s th a t n e a rly p a r a lle l th e b e d d i n g s u r f a c e s , a n d h a v e a c c o m p a n y i n g p re s s u re sh a d o w s. T h e m a tri x i s co m p o sed o f c r y p to c r y s ta lli n e q u a r t z 5 t o 1 0 pm i n s i z e , c la y n re d c h e r t. O p ti c a lly , m i n e ra ls , a n d h e m a ti te ; h e m a ti te i s f o u n d o n ly i c la y m i n e ra ls i n th e m a tr i x h a v e n o t b een i d e n ti f i e d . H e m a ti te i s c o m n o n ly t s v a r i a ti o n i n a m o u n t c a u s e s e x tre m e ly f i n e -g ra i n e d , le s s th a n 5 p m , a n d i th e m a tri x o f r e d c h e r t t o b e c l o u d y . No t e r r i g e n o u s g r a i n s w e re d e te c te d . A b u n d a n t p la g i o c la s e i s com m on i n c h e r t a n d p a r ti c u la r ly i n c la y s to n e a s d e te rm i n e d b y X -ra y d i f f r a c ti o n a n a ly s i s . P la g i o c la s e w as n o t o b s e rv e d i t s e x tre m e ly f i n e g ra i n s i z e . u n d e r th e m i c ro s c o p e b e c a u se o f S i li c e o u s s k e le to n s , c la y m i n e ra ls , a n d h e m a ti t e c o m m o n ly f o rm p a r a lle l la m i n a ti o n s th a t a r e th e m o st s i g n i f i c a n t s e d i m e n ta ry s tr u c tu r e i n c h e r t. c k , a r e m o st c m o n (F i g . 7 -4 ). R a d i o la ri a -ri c h la m i n a e , 0 .5 t o 1 .5 m m th i c k a r e f o u n d f re q u e n tC la y m i n e ra ls a n d h e m a ti te -ri c h la m i n a e 0 . 5 t o 2 mn t h i l y a t th e to p a n d b o tto m o f c h e r t b e d s (F i g . 7 - 4 ) . C la y m i n e ra ls a n d h em ath t i t e a r e a ls o c o n c e n tra te d a lo n g s t y lo li te s a rra n g e d n e a rly p a r a lle l w i b e d d i n g s u rf a c e s . B e s i d e s th e b e d d i n g p a r a lle l s tr u c tu r e s , n o o th e r s tr u c tu r e s , i n c lu d i n g g ra d e d b e d d i n g , w e re o b s e rv e d . C la y s to n e i n te rb e d d e d w i th c h e r t c o n ta i n s s p a rs e r a d i o la r i a n re m a i n s th a t a r e m o s tly f i lle d w i t h n o n -f i b ro u s q u a r tz , s o m e ti m e s w i th c h a lc e d o n y , a n d a r e f la tte n e d a s th e y a r e i n th e c h e r ts (F i g . 7 -5 ). P a r a lle l la m i n a ti o n s r i c h i n r a d i o la r i a n re m a i n s a r e r a r e ly i n te r c a la te d . S ty lo li te s w i th c o n c e n tr a ti o n s o f c la y m i n e ra ls a n d h e m a ti te o f le s s th a n 2 0 pi n th i c k a r e co m n o n . N o c o a rs e r te rri g e n o u s g ra i n s o c c u r. T h e M -ty p e c h e r t o f th e S a w a d a n i G ro u p i s c o m p o se d m o s tly o f m i c ro c ry s ta lli n e q u a r tz g e n e r a lly 1 0 to 5 0 pm i n s i z e , a n d a s m a ll a m o u n t o f h e m a ti te (F i g . 7 - 6 ) . I t s h o u ld b e n o te d t h a t th e g r a i n s i z e o f q u a rtz i s r e la ti v e ly

434

435

F i g . 7. Photomicrographs o f c h e r t and c l a y s t o n e . 1, Red r a d i o l a r i a n c h e r t . The same l o c a l i t y as t h a t o f F i g . 2-8; 2, Etched s u r f a c e o f r a d i o l a r i a n F i g . 2-8; 3, c h e r t w i t h some sponge s p i c u l e s . The same l o c a l i t y as t h a t o f Red r a d i o l a r i a n c h e r t , Terauchi, A k i a r e a : Susaki Formation; 4, P a r a l l e l c o n c e n t r a t i o n s o f r a d i o l a r i a n remains and h e m a t i t e , Kamaqatani Tunnel, Sawadani a r e a : m i d d l e p a r t o f t h e Sawadani Formation; 5 , C l a y s t o n e w i t h sparse r a d i o l a r i a n remains. The same l o c a l i t y as t h a t o f F i g . 4-E; 6, massive c h e r t o f t h e 1-1-type. No d i s t i n c t o r g a n i c remains a r e found. The same l o c a l i t y as t h a t o f F i g . 3-C. A l l s c a l e b a r s r e p r e s e n t 1 mm. l a r g e r t h a n i n t h e B-type c h e r t . No d i s t i n c t o r g a n i c remains were f o u n d i n t h i n s e c t i o n s o r on s u r f a c e s e t c h e d by HF, a l t h o u g h some r e l i e f on t h e l a t t e r c o u l d be d o u b t f u l v e s t i g e s o f o r g a n i c remains. No t e r r i g e n o u s g r a i n s occur. X-RAY MINERALOGY A n a l y s i s was made on b u l k samples t h a t were n o t o r i e n t e d . C h e r t s and claystones o f B - t y p e o f t h e Sawadani Group and t h o s e o f t h e Shimantogawa Group a r e m a i n l y composed o f q u a r t z w i t h s u b o r d i n a t e p l a g i o c l a s e , illi t e , c h l o r i t e s , and h e m a t i t e ; h e m a t i t e was found i n r e d c h e r t and c l a y s t o n e . X-ray a n a l y s i s was a l s o c a r r i e d o u t t o examine t h e v e r t i c a l change o f cons t i t u e n t m i n e r a l s i n i n d i v i d u a l c h e r t beds. The a n a l y t i c a l procedure i s as f o l l o w s : A r o c k s l i c e , c u t p e r p e n d i c u l a r t o t h e bedding s u r f a c e , i s d i v i d e d

436

Fig. 8. Peak h e i g h t diagrams o f selected samples o f c h e r t and claystone. 1, greenish grey c h e r t , lower p a r t o f t h e Sawadani Formation; 2, d a r k r e d c h e r t , m i d d l e p a r t o f t h e Sawadani Formation; 3, brownish r e d c h e r t , m i d d l e p a r t o f t h e Sawadani Formation; 4-5, brownish r e d c h e r t and claystone, t h e Miyama Formation: R y u j i n area. q, quartz; p, plagioclase; i, i l l i t e , c. c h l o r i t e s ; h, hematite. R e f l e c t i o n s o f q u a r t z were measured u s i n g 4 ~ 1 0 ~ c.P.s., and those o f t h e o t h e r m i n e r a l s u s i n g 1 ~ l O ~ c . p . s . . f u l l scale. i n t o 5 t o 15 i n t e r v a l s (each 3 t o 5

m n t h i c k ) along t h e v i s i b l e laminae.

The d i v i d e d pieces a r e crushed and powdered. Results a r e shown i n Fig. 8 as peak h e i g h t diagrams t h a t i n d i c a t e t h e r e l a t i v e i n t e n s i t i e s o f c o n s t i t u e n t m i n e r a l s . Mesured peaks a r e as f o l l o w s : quartz, 101; p l a g i o c l a s e , 002; i l l i t e , 001; c h l o r i t e s , 002; hematite, 104. The amount o f quatrz v a r i e s

437

i n v e r s e l y w i t h t h e amount o f subordinate m i n e r a l s , p l a g i o c l a s e , i l l i t e , c h l o r i t e s , and hematite. The i n t e r v a l s o f maximum q u a r t z correspond w i t h those o f minimum subordinate m i n e r a l s , and l i e approximately i n t h e c e n t r a l p a r t o f c h e r t beds. Subordinate m i n e r a l s v a r y s y m p a t h e t i c a l l y and i n c r e a s e towards t h e t o p and bottom o f c h e r t beds.

D IS C U S S I ON Bedded c h e r t s on p i l l o w l a v a i n o t h e r areas, e s p e c i a l l y t h e Mediterranean and A l p i n e regions, have been considered t o be t h e uppermost l a y e r o f t h e oceanic c r u s t (Abbate and Sagri, 1970; Garrison, 1974; Robertson, 1977; Folk and McBride, 1978). These c h e r t s a t t a i n a c o n s i d e r a b l e thickness, commonly u p t o 200 m, and a r e covered by t h i c k p e l a g i c carbonate r o c k s ( G a r r i s o n and Fischer, 1969). Petrographic s t u d i e s show t h a t these c h e r t s formed from r a d i o l a r i a n ooze t h a t accumulated e x t e n s i v e l y on t h e oceanic basement. Furthermore, some o f these c h e r t s may have been deposited by t u r b i d i t y c u r r e n t s generated on oceanic spreading r i d g e ( N i s b e t and P r i c e , 1974; P r i c e , 1977). The bedded c h e r t h e r e i n described d i f f e r s from t h e ocean f l o o r c h e r t s dcs c r i b e d from o t h e r places: 1 ) The Sawadani and Shimantogawa c h e r t s and ass o c i a t e d r o c k s show a d i f f e r e n t l i t h o l o g i c succession t h a t do t h e Tethyan c h e r t s . The Sawadani and Shimantogawa c h e r t s e c t i o n s a r e covered by t e r r i g e n o u s r o c k s c o n s i s t i n g o f b l a c k s l a t e and s i l i c e o u s mudstone t h a t a r e presumed t o be hemipelagic sediments. 2 ) The r e p e t i t i o n o f b a s a l t i c l a v a and c h e r t i s found i n some l o c a l i t i e s . 3) The t h i c k n e s s o f t h e Sawadani and Shimantogawa c h e r t s e c t i o n s do n o t exceed 40 m, and a r e much t h i n n e r compared w i t h presumed ocean f l o o r c h e r t s . These f a c t s i m p l y t h a t t h e d e p o s i t i o n o f

,

b i o g e n i c m a t e r i a l s t h a t formed t h e Sawadani and Shimantogawa c h e r t s occured s h o r t l y a f t e r c e s s a t i o n o f submarine v o l c a n i c a c t i v i t y , o r d u r i n g i t s waning stages. The t h i n sequences o f c h e r t s under c o n s i d e r a t i o n a l s o suggest a short duration o f chert deposition. The mode o f occurrence o f t h e greenstones o f t h e Shimantogawa Group has been i n t e n s i v e l y i n v e s t i g a t e d r e c e n t l y i n some areas (Sakai, 1978; Tsuchiya e t al.,

1979; Sakai and Kanmera, 1981 ; Suzuki and Hada, 1979). The most

s i g n i f i c a n t f i n d i n g i s o f t h e occurrence o f mudstone x e n o l i t h s captured i n l a v a f l o w s and s i l l s and t h e conformable r e l a t i o n o f greenstones and underl y i n g a r g i l l a c e o u s r o c k s (Sakai, 1978; Sakai and Kanmera, 1981). Based on these evidences Tsuchiya e t a l . (1979) and Sakai and Kanmera (1981) demons t r a t e d t h a t greenstones of t h e Shimantogawa Group a r e t h e product of o f f - r i d g e volcanism t h a t occured i n places where a r g i l l a c e o u s sediments were being deposited. Greenstones o f t h e Sawadani Group, based on t h e v o l c a n o s t r a t i g r a p h i c and petrochemical s t u d i e s , a r e n o t a fragment o f oceanic

438

c r u s t , b u t i s more l i k e a seamount formed on a s e d i m e n t a r y sequence near a c o n t i n e n t o r an i s l a n d a r c (Maruyama and Yamasaki, 1978). From f i e l d o b s e r v a t i o n s on c h e r t s and a s s o c i a t e d r o c k s , t h e Sawadani and Shimantogawa c h e r t s a r e c o n s i d e r e d t o have accumulated i n s m a l l t o p o g r a p h i c d e p r e s s i o n s on a seamount o r o t h e r v o l c a n i c mound. The mode o f o c c u r r e n c e o f t h e 11-type c h e r t o f t h e Sawadani Group f i t s t h i s i n t e r p r e t a t i o n w e l l . The M-type c h e r t was n o t c h a o t i c a l l y c a u g h t u p i n b a s a l t i c f l o w s , b u t was d e p o s i t e d i n s m a l l t o p o g r a p h i c ponds on t h e l a v a f l o w s . Four h y p o t h e s i s have been proposed f o r t h e f o r m a t i o n o f r h y t h m i c l a y e r i n g o f c h e r t and c l a y s t o n e : 1 ) d i a g e n e t i c s e g r e g a t i o n o f s i l i c a and c l a y (Davis, 1918), 2 ) e p i s o d i c s u p p l y o f s i l i c e o u s organisms and c o n t i n u o u s and s l o w d e p o s i t i o n o f c l a y ( G a r r i s o n and F i s c h e r , 1 9 6 9 ) , 3 ) c o n t i n u o u s d e p o s i t i o n o f s i l i c e o u s organisms and e p i s o d i c s u p p l y o f c l a y ( C a l v e r t , 1966), 4 ) t u r b i d i t y c u r r e n t d e p o s i t i o n o f s i l i c e o u s organisms and c l a y ( N i s b e t and P r i c e , 1974). T u r b i d i t y c u r r e n t d e p o s i t i o n i s s u p p o r t e d by s t u d i e s on t h e Mesozoic c h e r t s o f t h e M e d i t e r r a n e a n r e g i o n s which have been c o n s i d e r e d t o have accumulated on o c e a n i c basement. Ftlajor i n d i c a t i o n s f o r t u r b i d i t y f l o w depos i t i o n a r e graded bedding, c r o s s and p a r a l l e l l a m i n a t i o n s , s c o u r i n g , r h y t h m i c bedding, and s o l e marks. I n t h i s t h e o r y s i l i c e o u s organisms o f s i l t s i z e a r e c o n c e n t r a t e d i n t h e l o w e r p a r t o f c h e r t beds, and g r a d u a l l y grade i n numbers upward i n t o c l a y i n t e r v a l s , as i l l u s t r a t e d b y P r i c e ( 1 9 7 7 ) . I n c o n t r a s t t o t h e s e presumed ocean f l o o r c h e r t s , p a r a l l e l l a m i n a t i o n s which a r e c o n c e n t r a t i o n s o f r a d i o l a r i a n remains, c l a y m i n e r a l s , and hemat i t e a r e most e s s e n t i a l i n t h e Sawadani and Shimantogawa c h e r t s , and no sedimentary s t r u c t u r e s occur t h a t i n d i c a t e t u r b i d i t y c u r r e n t d e p o s i t i o n o f c h e r t . I n a d d i t i o n t o t h e s e d i m e n t a r y f e a t u r e s , t h e symmetrical changes o f c o n s t i t u e n t m i n e r a l s a b o u t t h e c e n t r a l p a r t o f c h e r t beds a r e i n c o n s i s t e n t w i t h t h e graded changes o f l i t h o l o g i c components i n t h e t u r b i d i t y f l o w d e p o s i t e d c h e r t . To c l a r i f y t h e process of r h y t h m i c l a y e r i n g o f t h e Sawadani and Shimantogawa c h e r t s , some o t h e r mechanism m i g h t be necessary t o consider. ACKNOW LEDGE#ENTS I am i n d e b t e d t o P r o f . K. Kanmera o f Kyushu U n i v e r s i t y f o r h i s c r i t i c a l

reading o f t h e m a n u s c r i p t and f o r h i s h e l p f u l s u g g e s t i o n s and t o D r . Y. Ogawa and M r . T. Sakai o f t h e same u n i v e r s i t y f o r t h e i r u s e f u l d i s c u s s i o n s . I am a l s o g r a t e f u l t o Dr. T. Watanabe o f t h e same u n i v e r s i t y f o r h i s v a l u -

a b l e a d v i c e s on t h e X-ray m i n e r a l o g y .

439

REFERENCES Abbate, E. and S a g r i , M., 1970. The e u g e o s y n c l i n a l sequences. I n S e s t i n i , G. ( E d i t o r ) , Development o f t h e n o r t h e r n Appenine Geosyncl i n e . Sediment. Geol 4: 251-340. C a l v e r t , S.E., 1966. Accumulation o f diatomaceous s i l i c a i n t h e sediments o f t h e G u l f o f C a l i f o r n i a . B u l l . Geol. SOC. Amer., 77: 1761-1773. Davis, E.F., 1918. The r a d i o l a r i a n c h e r t o f t h e F r a n c i s c a n Group. Univ. C a l i f . Publs. Dept. Geol., 11: 236-432. 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 subj a c e n t " o c e a n i c C r u s t " i n L i g u r i a , I t a l y . J. Sediment. Geol 48: 10691102. G a r r i s o n , R.E., 1974. R a d i o l a r i a n c h e r t s , p e l a g i c l i m e s t o n e s , and igneous r o c k s i n e u g e o s y n c l i n a l assemblages. I n HSU, K.J. and Jenkyns, H.C. ( E d i t o r s ) , P e l a g i c sediments on l a n d and under t h e sea. Spec. Publ. I n t . Assoc. Sediment., 1 : 367-399. G a r r i s o n , R.E. and F i s c h e r , A.G., 1969. Deep-water l i m e s t o n e s and r a d i o l a r i t e s o f t h e A l p i n e J u r a s s i c . I n Friedman, G.M. ( E d i t o r ) , D e p o s i t i o n a l env i r o n m e n t i n c a r b o n a t e r o c k s . Spec. Publ. SOC. Econ. Paleont. M i n e r a l . , Tulsa, 14: 20-56. Grunau, H.R., 1965. R a d i o l a r i a n c h e r t s and a s s o c i a t e d r o c k s i n space and t i m e . Eclog. Geol. Helv., 58: 157-208. Hein, J.R. and K a r l , S.M., 1982. Comparisons between open-ocean and c o n t i n e n t a l m a r g i n c h e r t sequences. ( T h i s volume). Kanmera, K., 1969. Upper P a l e o z o i c s t r a t i g r a p h y o f t h e n o r t h e r n C h i c h i b u B e l t i n e a s t e r n Shikoku. S c i . Rep. Dept. Geol. Kyushu Univ., 2: 175-186. Kanmera, K., 1974. P a l e o z o i c and Mesozoic g e o s y n c l i n a l v o l c a n i s m i n t h e Japanese I s l a n d s and a s s o c i a t e d c h e r t s e d i m e n t a t i o n . I n D o t t , R.H. and Shaver, R.H. ( E d i t o r s ) , Modern and a n c i e n t geosyncl i n a l s e d i m e n t a t i o n . Spec. Publ SOC. Econ. P a l e o n t . N i n e r a l . , No. 19, 161-173. Kanmera, K. and S a k a i , T., 1975. Sedimentary framework o f t h e Shimanto Group from a c t u a l v i e w p o i n t . R e p o r t o f GDP i n Japan, I I - I - ( 1 ) S t r u c t u r a l Geology, No. 3, 55-64. Kishu Shimanto Research Group, 1975. The development o f t h e Shimanto Geos y n c l i n e . Wonograph Assoc. Geol. C o l l a b . Japan, No. 19, 161-173. Maruyarna, S . and Yamasaki, M., 1978. P a l e o z o i c submarine volcanoes i n t h e h i g h P/T metamorphosed C h i c h i b u System o f e a s t e r n Shikoku, Japan. J. Volcan. Geoth. Res., 4 : 199-216. Nakagawa, C., Nakaseko, K., Kawaguchi, K. and Yoshimura, R., 1980. R a d i o l a r i a j F o s s i l i o j ( e l Marfura Juraso g i s Marfura Kretaceo) de Norda Zone d e l a Simanto F o r m a t i a r o (Generala Aspekto). Mem. Tokushima Univ. , 31: 1-27. N i s b e t , E.G. and P r i c e , I . , 1974. S i l i c e o u s t u r b i d i t e s : bedded c h e r t s as r e d e p o s i t e d , o c e a n - d e r i v e d sediments. I n Hsu, K.J. and Jenkyns, H.C. ( E d i t o r s ) , P e l a g i c Sediments on l a n d and under t h e sea. Spec. Publ. I n t . Assoc. Sediment., 1 : 351-366. Okamura, !I.,1980. R a d i o l a r i a n f o s s i l s f r o m t h e N o r t h e r n Shimanto B e l t ( C r e t a c e o u s ) i n Kochi P r e f e c t u r e , Shikoku. I n T a i r a , A. and T a s h i r o , M. ( E d i t o r s ) , Geology.and P a l e o n t o l o g y o f t h e Shimanto Be1 t. R i n y a k y o s a i k a i Press, Kochi, Japan, pp. 153-178. P r i c e , I . , 1977. Facies d i s t i n c t i o n and i n t e r p r e t a t i o n o f p r i m a r y c h e r t s i n a Mesozoic c o n t i n e n t a l m a r g i n succession, O t h r i s , Greece. Sediment. Geol., 18: 321-335. Robertson, A.H.F., 1975. Cyprus umber: basal-sediment r e l a t i o n i n a Mesoz o i c ocean f l o o r . J. Geol. SOC. London, 131: 511-531. Robertson, A.H.F., 1977. The o r i g i n and d i a g e n e s i s o f c h e r t s f r o m Cyprus. Sedimentology, 24: 11-33. 1974. P e l a g i c sediments i n t h e Cretaceous Robertson, A.H.F. and Hudson, J.D., and T e r t i a r y h i s t o r y o f Cyprus. I n Hsu, K.J. and Jenkyns, H.C. ( E d i t o r s ) , P e l a g i c sediments on l a n d and under t h e sea. Spec. Publ. I n t . Assoc. Sediment., 1 : 403-436.

.,

.,

.

440

Sakai, T., 1978. 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 Shimantogawa Group i n t h e m i d d l e reaches o f t h e Gokase River, Miyazaki P r e f e c t u r e . S c i . Rep. Dept. Geol. Kyushu Univ., 13: 23-38. Sakai, T. and Kanmera, K., 1981. S t r a t i g r a p h y o f t h e Shimanto T e r r a i n and t e c t o n o s t r a t i g r a p h i c s e t t i n g o f greenstones i n t h e n o r t h e r n p a r t o f t h e t l i y a z a k i P r e f e c t u r e . Sci. Rep. Dept. Geol. Kyushu Univ., 14: 31-48. Sano, H., Kanmera, K. and Sakai, T., 1979. Sediments a s s o c i a t e d w i t h greenstones o f t h e Shimanto T e r r a i n . J. Geol. SOC. Japan, 85: 435-444. Suzuki, T. and Hada, S., 1979. Cretaceous t e c t o n i c melange o f t h e Shimanto B e l t i n Shikoku, Japan. J. Geol. SOC. Japan, 85: 467-479. Taira, A., Okamura, M., Katto, J., Tashiro, M., S a i t o , Y., Kodama, K., Hashimoto, M., Chiba, T. and Aoki, T., 1980. L i t h o f a c i e s and g e o l o g i c age r e l a t i o n s h i p w i t h i n melange zones o f Northern Shimanto B e l t (Cretaceous), Kochi P r e f e c t u r e , Japan. I n T a i r a , A. and Tashiro, M. ( E d i t o r s ) , Geology and Paleontology o f t h e Shimanto B e l t . Rinyakyosaikai Press, Kochi, Japan, pp. 179-214. Tsuchiya, N., Sakai, T. and Kanmera, K., 1979. Mode o f occurrence and pet r o l o g i c a l c h r a c t e r i s t i c s o f greenstones o f t h e Shimanto T e r r a i n i n t h e M i m i R i v e r area, Kyushu. J. Geol S O C . Japan, 85: 445-454.

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CHAPTER 26 CHERT-LAMINITE, ONE OF THE PRINCIPAL SEDIMENTS IN A GEOSYNCLINE

S. YOSHIDA Department of Earth Sciences, Nagoya'' University, Nagoya (Japan) ABSTRACT "Chert-laminite" is a newly proposed lithologic term for a sedimentary rock with very thin and rhythmic laminations of chert and claystone. Chertlaminites include many rocks that hitherto have been called phyllite, pelitic schist, and argillite. Chert-laminites are commonly accompanied by bedded radiolarian chert, greenstone, laminated limestone, siliceous mudstone, and distal turbidites. Chert-laminites occur widely in most Japanese geosynclinal sections of various geologic ages, and occur probably widely in geosynclinal sedimentary rocks in many parts of the world. Comparisons with modern and ancient marine varves suggest that chert-laminites are also probably varves. INTRODUCTION "Chert-laminite" is a newly proposed lithologic term for very thinly and rhythmically alternating beds of chert and claystone (Yoshida, 1981). Chertlaminites include many rocks that have hitherto been called phyllite, pelitic schist, black schist, and siliceous argillite. These terms imply that the rocks have been deformed or metamorphosed, but the lamination of chertlaminites is not metamorphic but is of sedimentary origin. Such misunderstandings stem from: ( 1 ) laminations of chert-laminites look like schistosity (Figs. 2a and 3a), (2) chert laminae have been considered to be quartz segregation veins, and (3) when chert-laminites are split with the hammer, the surfaces of very thin light colored chert laminae comnonly are masked by a film of darkrcolored clays and thus tend to be overlooked. The term "laminite" was first proposed by Sander (1936), and was redefined by Lombard '(1963) as a finely laminated detrital rock related to turbidites genetically. Although chert-laminites consist of biogenic and other nonelastic materials as well as clastic grains, I proposed use of this term for the following reasons; ( 1 ) chert-laminites are characterized by a welldefined laminated texture resulting from the presence of thin chert laminae, (2) chert-laminites are commonly associated with the laminites defined by Lombard, and (3) petrographically, chert-laminites differ from laminites and chert, and form a distinct lithological unit within geosynclinal sections.

44 2 R

135"E

H.: Hida terrane S - Y . : Sangun-Yamaguchi terrane R.: Ryoke terrane S-C. : Sambagawa-Chichi bu terrane Sb.: Sambosan terrane Sm.: Shimanto terrane St. : Setogawa terrane T.: Tanzawa terrane K-S. : Kamui kotan-Sorachi terrane

Gi f u

northern Kitakami Mtz

m

0

Jf Yaman as hi Shi zuoka

5*2'?5b'

\

Nara

' kobeoka

300 km u

Fig. 1. Index map f o r t e r r a n e and p l a c e names mentioned i n t e x t . TEXTURE AND COMPOSITION OF CHERT-LAMINITES C h e r t - l a m i n i t e s c o n s i s t o f t h i n l y a l t e r n a t i n g c h e r t and claysone laminae ( F i g . 3, 4, and 5).

Laminae range from several tens o f microns t o one

m i l l i m e t e r t h i c k b u t most commonly a r e 50 t o 100 microns t h i c k (Figs. 3c and 4b).

The thickness o f c h e r t o r c l a y laminae i s v a r i a b l e i n a v e r t i c a l column,

and i s r a t h e r u n i f o r m l a t e r a l l y ( F i g . 3b and c ) unless they s u f f e r e d from deformation o r b i o t u r b a t i o n .

The l i g h t and dark c o n t r a s t and t h i c k n e s s

combination o f c h e r t and c l a y laminae a r e h i g h l y v a r i a b l e (Figs. 3b and 5b). Very t h i n laminae o f v e r y f i n e sandstone w i t h c l a y - o r f i n e - s i l t - s i z e d q u a r t z g r a i n s as a m a t r i x a r e i n places i n t e r c a l a t e d i n c h e r t - l a m i n i t e s . I n places l a m i n a t i o n s a r e d i s t o r t e d and obscure as t h e r e s u l t o f slump f o l d s , f a u l t s , and b i o t u r b a t i o n and a r e o c c a s i o n a l l y associated w i t h b i o t u r b a t e d shales.

Some s i l i c e o u s mudstones w i t h o u t l a m i n a t i o n s b u t w i t h

l i g h t c h e r t and dark c l a y m o t t l e s may be b i o t u r b a t e d c h e r t - l a m i n i t e s .

443

Mineral composition of typical gray to black chert-laminites of Permian, Cretaceous, and Paleogene ages was examined by the X-ray diffraction and under the microscope (Yoshida, 1981). All the rocks examined have identical X-ray diffractograms and have clear peaks of quartz, sericite, chlorite, and plagioclase, though grain sizes, distribution ratios of the four minerals, and plagioclase species vary from rock to rock. Microgranular carbonate minerals are observed in places and in various amounts. A Lower Permian chert-laminite of Sambagawa-Chichibu terrane in Shikoku contains nearly 50 % calcite and may be properly called a calcareous chert-laminite. Euhedral pyrite crystals are disseminated in places and in various amounts (Fig. 5b). Chert 1 ami nae Chert laminae that are fully matured and indurated consist mainly of microgranular or cryptocrystalline quartz like that found in bedded radiolarian cherts, whereas chert laminae of a diagenetically immature chertlaminite (Fig. 4) consist mainly of opal-CT. Laminae also include a small amount of sericite, chlorite, pyrite, and other clastic grains. Quartz grains occlude each other with serrated and indistinct contacts. The grainsize changes in places within a lamina; for example, quartz grains in the marginal part of a lamina are finer than those in the middle part, and vice versa. The coarser grains in both size distributions are several times larger than the finer grains. Grain contacts of the coarser grains are rather sharp and not serrated; presumably such coarser grains have recrystallized from finer grains. Segregated quartz veins often occur obliquely and parallel to laminae (Fig. 3a). Quartz grains in the segregated veins are several tens of times larger than grains in the laminae. Radiolarian remains are rarely found in some chert laminae (Figs. 3c and 4b), and occur more frequently in thicker laminae than in thinner ones. This occurrence may reflect the fact that the diameters of most radiolarians are larger than the thickness of the chert laminae. On the other hand, diatoms, wnose diameters are mostly smaller than the thickness of the chert laminae, are sporadically found in the Miocene chert-laminites. No diatoms were found in the older chert-laminites. Silt-sized clastic grains are sometimes disseminated in chert laminae. They are rounded or subrounded single crystals of quartz, plagioclase, epidote, and tourmaline.

444

Fig. 2. ( a ) T y p i c a l appearance o f c h e r t - l a m i n i t e i n outcrop. The t h i c k l i g h t bed a t t h e p o i n t e d t i p o f the hammer i s very f i n e s i l i c e o u s q u a r t z - f e l d s p a t h i c sandstone w i t h graded bedding. Cretaceous Shimanto Group, n o r t h e a s t o f Nobeoka. (b) Bedded c h e r t w i t h f i n e p a r a l l e l h a i r l i n e laminations. The middle l a y e r has no h a i r l i n e lamination, and c o n t a i n s r a d i o l a r i a n remains. Upper T r i a s s i c bedded c h e r t member, G i f u .

445

F i g . 3. Characteristic textures of chert-lamini t e . Paleogene Kobotoke Group (Setogawa t e r r a n e ) , Yamanashi. ( a ) Polished surface of a hand specimen. Lighter and thicker layers s l i g h t l y oblique t o the laminae are segregation quartz veins. ( b ) Photograph of the thin section from a s p l i t of the hand specimen of the l e f t , transmitted l i g h t . Light laminae consist mostly of micro- o r cryptocrystalline quartz, and dark ones mostly of s e r i c i t e . (c) Photomicrograph of the framed part of Figure 3b, plane l i g h t . The shades of l i g h t and dark laminae vary w i t h the amount of s e r i c i t e . Thicknesses of laminae range from a few tens of microns to a hundred microns. The larger rounded grain a t the upper l e f t i s a radiolarian. Sporadic smaller white grains are c l a s t i c grains of quartz or feldspar.

446

Fig. 4. A Miocene c h e r t - l a m i n i t e c o n t a i n i n g many diatom f r u s t u l e s , Middle Miocene On-nagawa Format i on, Aki t a . (a) P o l i s h e d surface. Note p a r a l l e l p i n - s t r i p e l a m i n a t i o n s . ( b ) Photomicrograph o f a t h i n section, plane l i g h t . I t c o n s i s t m o s t l y o f opal-CT. Dark laminae are a r g i l l a c e o u s . Black dusty b l o t s are p y r i t e . Two l a r g e g r a i n s a t the lower are r a d i o l a r i a n s .

447

Fig. 5. Deformed c h e r t - l a m i n i t e s i n the Sambagawa-Chichibu terrane, t r a n s m i t t e d light. (a) Chert-lamini t e w i t h p e n e t r a t i v e oblique c r e n u l a t i o n cleavages. Note the m i c r o l i t h o n s between cleavages are made up o f c h e r t - l a m i n i t e . Black p e l i t i c " s c h i s t " o f the Sambagawa metamorphic b e l t , Mie. The scale i s i n m i l l i m e t e r . (b) Folded c h e r t - l a m i n i t e . Note very f i n e p a r a l l e l laminae of various thickness and darkness, which c o n s i s t mostly o f m i c r o c r y s t a l l i n e q u a r t z and s e r i c i t e . The l a m i n a t i o n has been c a l l e d " s c h i s t o s i t y " . Oblique cleavage i s more s t r o n g l y formed i n the clayey laminae. Black angular g r a i n s are p y r i t e . Black p e l i t i c " s c h i s t " o f the Sambagawa metamorphic b e l t , Shizuoka. The scale i s i n m i l l i meter.

448

Clay laminae Clay laminae are commonly gray to black and occasionally pale green to dark green. Gray to black laminae consist mainly of sericite and include a small amount of microgranular quartz and chlorite. Pale green to dark green laminae consist mainly of microcrystalline chlolite and sericite and include a small amount of microgranular quartz. Green chert-laminites have often been called as "green tuffs" or "green schists". RELATION OF CHERT-LAMINITES TO BEDDED CHERTS AND "VARVED" CHERT Chert-laminites comnonly grade into bedded radiolarian cherts vertically; chert laminae thicken gradually toward the bedded chert, whereas the thickness of clay laminae remain unchanged. The transitional section between chertlaminite and bedded chert generally has a thickness of several tens of centimeters to several meters. An example of alternating layers of chertlaminites and bedded radiolarian chert is shown in Fig. 6-a. When the thicknesses of chert laminae are more than a few millimeters, radiolarian remains are comnon. Varved cherts (Lowe, 1976) occur in places with bedded radiolarian cherts. A varved chert bed has fine, parallel hairline laminations separated by 0.3 mn to a few millimeters of structureless chert (Fig. 2b). Hairline laminae consist of clayey minerals as well as micro-orcryptocrystalline quartz and are somewhat darker than pure chert. Varved intervals rarely contain radiolarian remains, whereas non-varved intervals contain many. Radiolarian remains are small when they occur sporadically in the varved intervals. The similar texture between chert-laminites and varved cherts suggests that they may have a comnon genesis. ROCKS ASSOCIATED WITH CHERT-LAFINITES In addition to bedded chert, there are several kinds of rocks that are associated with chert-larninites. Comnon chert-laminite-bearing lithologic associations at three localities are shown in Fig. 6. Chert-laminites comnonly grade vertically into non-laminated siliceous mudstone and mudstone (Fig. 6-a). Some siliceous mudstones with indistinct light and dark mottles may be bioturbated chert-laminite as mentioned above. Minerals in siliceous mudstone are similar to those in chert-laminites. Radiolarian remains are also disseminated in siliceous mudstone and in places are abundant. Chert-laminites are commonly interbedded with distal turbidites (Fig. 6c)

Symbol

**

denote CHERT-LMNITE (C-L).

0.1-1 nm t h i c k laminae gray bedded chert, 0.5-2 cm t h i c k beds

_ _-

l i g h t gray massive chert I-

0.1-1 m t h i c k laminae

black s l a t e w i t h s l i g h t l y oblique cleavage

pale green bedded chert. 1-3 t h i c k beds

laminated black and green shale, l e s s than 1 mn t h i c k

gray bedded chert, 1-3 an thickbeds black s i l i c e o u s s l a t e ui t h oblique cleavage

(a)

450

that are thinly laminated, 1 mm to 1 cm thick, and consist mostly of silt and very fine sand. Some beds are graded. These laminated turbidites belong to the laminite defined by Lombard (1963). The matrices of distal turbidites consist of microgranular quartz. Chert-laminites are also interbedded with thicker bedded sandstones, 1 cm to 10 cm thick, that are graded; these beds are muddy and fine-grained, siltstone to fine sandstone with matrices of microgranular quartz. Chert-laminites in places are interbedded with slump breccias (Fig. 6b and 6c) whose clasts consist generally of a single kind of sandstone or siltstone with matrix of microgranular quartz, are angular to subangular, and are a few centimeters to several tens of centimenters in diameter. The "matrix" of some slump breccias is made up of chert-laminites that are disturbed. Such slump breccias occur in the Cretaceous Shimanto Group near Nobeoka in Kyushu and the Upper Permian strata near Obama, Fukui-ken. Some chert-laminites are intercalated with thin sections (less than several meters) of limestone beds that are white to gray, generally laminated to bedded (up to several centimeters a bed). Some limestone in the SambagawaChichibu Terrane rarely yield conodonts. A limestone of the Permian Chichibu Group near Sawadani in Shikoku grades into calcareous chert-laminites. Chert-laminites are especially associated with basaltic rocks including pillow basalts, diabases, gabbroic rocks, and basaltic volcanic sandstones and mudstones. Such associations are well represented in the Mikabu Group of Sambagawa-Chichibu Terrane. Three kinds of stratigraphic relations exist between basaltic rocks and chert-laminites; (1) chert-laminite directly underlies basal tic rock, (2) chert-laminite directly overlies basaltic rock, and (3) chert-laminite and basalt are separated by bedded chert, turbidite, or other rock types. GEOLOGIC DISTRIBUTION OF CHERT-LAMINITES Chert-laminites are present in most of the geosynclinal sedimentary rocks in Japan. The main occurrences are as follows (the names of geotectonic division are after Kimura and Tokuyama (1971); meters in parentheses are rough estimations of total thickness of chert-laminites): Silurian ( ? ) to Devonian strata (Yamagami Formation and Matsugadaira Formation) in Abukuma Mts. (300 m+); Carboniferous to Lower Permian strata in the Sangun-Yamaguchi Terrane (500 to 1000 m); Carboniferous to Middle Permian Sambagawa and Mikabu Group in the Sambagawa-Chichibu Terrane (500 to 1000 m) ;

451

Upper Permian to Middle Triassic strata in the Sambosan Terrane, especially in northern Ki takami Mts. (300 m?) ; Upper Permian to Middle Triassic strata in the Kamuikotan-Sorachi Terrane in Hokkaido. (500 m+); Upper Jurassic ( ? ) to Lower Cretaceous strata in the Shimanto Terrane (Zoom+); Paleogene strata in the Setogawa Terrane (the lower horizon, 200 m+); Miocene strata in the Green Tuff Region (On-nagawa Formation, loom-), in the Tanzawa Terrane (the lower horizon, loom), and in the Kameno-o Formation in the Abukuma Mts. (the lower to middle horizon, 100 to 200 m ) . Chert-laminites have also been found in the Mesozoic ( ? ) strata of Susunai Mts. in Sakhalin which is the northward extension of the Kamuikotan-Sorachi Terrane of Hokkaido. Many other minor occurrences exist in Japan (Yoshida, 1981). It is noteworthy that many of the chert-laminites mentioned above occur at lower horizons in the geosynclinal piles, and that some are associated with basaltic rocks. Chert-laminites are also distributed in southern Alaska (Yoshida, in preparation); in the Cretaceous Uyak Complex (Connelly, 1978), in Jurassic and (or) Cretaceous McHugh Complex (Clark, 1973), and in Paleocene Ghost Rocks Formation (Nilsen and Moore, 1979). I suggest that the chert-laminites may be widely distributed in geosynclinal sedimentary rocks in many places around the world, but have mistakenly been described as cherts and argillites, siliceous argillites, phyllites, or black schists. ORIGIN OF CHERT-LAMINITES Several examples of recent sediment exist that are similar to the chertlaminites in regards to sedimentary texture and mineral composition. Diatomaceous varved sediments in the Gulf of California (Calvert, 1966a) are particularly noteworthy. The varved sediments consist of regularly a1 ternating 1 ight-colored (diatom-rich) and dark-colored (cl ay-rich) laminae, approximately 1 mm thick each. Light-colored laminae consist mostly of opaline silica with abundant diatom frustules and scarce radiolarian tests, whereas dark-colored laminae consist of opaline silica and terrigenous materials consisting mainly of illite (sericite) and chlorite, with accessory quartz, feldspar, and other grains. In places laminae are disturbed by slumping and bioturbation. A couplet of light and dark laminae represents an annual layer (Calvert, 1966a). Calvert (1966b) and Schrader et al. (1980) ascribed the light and dark laminae to changes in ocean currents resulting from seasonal winds. Laminae are well preserved at the sea bottom where low oxygen prevai 1 s (oxygen minimum zone, Cal vert , 1964). The oxygen minimum

452

zone i s present a t i n t e r m e d i a t e depths ( a few hundred meters t o one thousand meters) meeting t h e slopes on b o t h sides o f t h e G u l f o f C a l i f o r n i a .

Laminae

a r e d i s t u r b e d by b i o t u r b a t i o n forming m o t t l e d s i l i c e o u s sediment a t ocean bottom s i t e s o f 1,000 m t o 2,000 m deep where d i s s o l v e d oxygen becomes h i g h e r . The sedimentary t e x t u r e s and t h e occurrences o f diatomaceous varved sediments seem t o be good analogues t o c h e r t - l a m i n i t e s .

Only t h e thicknesses

o f laminae a r e n o t c o r r e l a t a b l e between t h e c h e r t - l a m i n i t e s and t h e varved sediments; laminae o f t h e varved sediments a r e about t e n times as t h i c k as those o f t h e c h e r t - l a m i n i t e s .

The b u l k d e n s i t y o f t h e varved sediments,

however, i s about 1 g/cm3, whereas t h a t o f t h e c h e r t - l a m i n i t e i s around 2.7 g/cm3 except f o r t h e Miocene c h e r t - l a m i n i t e o f t h e Green T u f f Region whose d e n s i t y i s around 2.0 g/cm3.

I t i s probable t h a t compaction reduced t h e

t h i c k n e s s o f laminae i n t h e c h e r t - l a m i n i t e s . Besides t h e varved d e p o s i t s o f t h e G u l f o f C a l i f o r n i a , t h e r e a r e several r e c e n t sediments t h a t have been confirmed t o be n o n - g l a c i a l varves; f o r example, Clyde sea area o f Scotland (Moore, 1931), M l j e t i s l a n d o f A d r i a t i c Sea (Seibold, 1958), Santa Barbara b a s i n o f f Los Angels (Hiilsemann and Emery, 1961), f j o r d o f Vancouver i s l a n d (Gross e t a l . ,

1963), G u l f o f Aden (Olausson

A l l these r e c e n t varved d e p o s i t s were

and Olsson, 1969), and o t h e r s .

deposited i n semi-enclosed basins o f nearshore sea areas where t e r r i g e n o u s coarse m a t e r i a l s a r e scarce.

The laminations, l i g h t and dark, o r i g i n a t e d

from t h e seasonal change o f ( 1 ) supply o f t e r r i g e n o u s m a t e r i a l s and ( 2 ) p r o d u c t i v i t y o f diatoms and o t h e r planktons; r a i n f a l l may p a r t i c u l a r l y a f f e c t t h e former, and sea c u r r e n t s t h e l a t t e r .

Comparisons suggest t h a t t h e c h e r t -

l a m i n i t e s were a l s o p o s s i b l y deposited i n semi-enclosed sea areas, where sea c u r r e n t s change seasonally and where coarse t e r r i g e n o u s m a t e r i a l s a r e scarce. Garrison ( 1 975) suggested t h a t t h e Neogene diatomaceous sediments around t h e c i r c u m - P a c i f i c r e g i o n are o f p e l a g i c f a c i e s deposited i n basins where t e r r i g e n o u s d e b r i s was a minor component, and t h a t they were deposited a t an e a r l y stage of b a s i n formation and then were covered w i t h t r u b i d i t e s and n e r i t i c sediments.

On the o t h e r hand, laminated

f i n e - g r a i n e d c l a s t i c sediments i n c l u d i n g t h i n graded beds a r e known t o be deposited as d i s t a l t u r b i d i t e s i n a deeper sea area (Lombard, 1963; Piper, 1972). An environment where coarse c l a s t i c m a t e r i a l s r a r e l y reach, where d i s t a l t u r b i d i t e s accumulate i n t e r m i t t e n t l y , and where sea water c i r c u l a t i o n c r e a t e s rhythmic beds ( p o s s i b l y annual) i n c l u d e semi-enclosed seas 1 i k e marginal basins;

f o r e a r c basins, and r i f t basins.

The occurrence o f c h e r t - l a m i n i t e s

mentioned e a r l i e r suggests t h a t most were deposited i n such basins and a t an e a r l y stage o f b a s i n formation.

The above-mentioned environments a r e n o t

453

necessarily deep sea areas. Some chert-laminites are possible trench deposits similar to finely laminated sediments found along the southern Kurile and northern Japan Trenches (Hesse, 1977), which include many finely laminated diatomaceous beds (von Huene et a1 . , 1980). An important question is whether the origin of silica of chert-laminites is biogenic. The silica of the Miocene chert-laminites is obviously of biogenic origin because many diatom frustules are present and the silica is mostly opal. All of the chert-laminites older than Neogene seem to contain no diatoms but contain scarce radiolarians, although the appearance of diatoms dates back at least to the Cretaceous period. Micro- tocryptocrystalline quartz with indistinct serrated grain contacts suggests an origin other than clastic grains. Ancient finely laminated beds exist that are believed to be non-glacial marine varves; banded iron ore formation of the Proterozoic Hamersley Group (Trendall , 1972) , lower member o f Devonian Arkansas Novacul ite (Lowe, 1976), Carboniferous Mansfield Marine Band (Spears, 1969), Upper Cretaceous Mowry siliceous shale member (Rubey, 1931) , Miocene Monterey formation (Bramlette, 1946), and others. Most of these finely laminated beds were deposited in semi-enclosed epicontinental sea basins during times when little terrigenous sediment reached. Textural and compositional similarities between these probable marine varves and chert-laminites, especially between the Monterey formation and the Miocene chert-laminite (Fig. 4), and between the Arkansas Novaculite and the finely laminated chert (Fig. 2b) that grades into chertlaminite also suggest that chert-laminites are marine varves. In conclusion, (1) chert-laminites were probably deposited in basins where sea water circulation changed seasonally, and during times when little terrigenous sediment other than in suspension was supplied; such conditions are likely to occur in semi-enclosed basins and at an early stage o f basin formation; (2) comparisons with recent and ancient marine varves suggest that chert-laminites are possibly varves (a couplet, light and dark, representing an annual layer); and (3) laminations were preserved only where no mixing by currents or bioturbation occurred after deposition. DEFORMATION AND METAMORPHISM OF CHERT-LAMINITES Chert-laminites are deformed by three types of displacement; bedding slip, flow predominantly parallel to laminae, and slip along crenulation cleavage. They are often minutely and intensely folded by gravitational creep even without deep burial and tectonic force. Folds created by creep are usually of centimenter-order in wavelength, have axial surfaces dipping gently

4 54

(generally less than 20°), and are usually of kink type. These creep folds are accompanied with systematic and/or random tension fractures filled with quartz, so-called segregation quartz veins. planes resulting from creep folds created polished and striated surfaces parallel to laminae and along which laminae easily flake off. features have sometimes been mistaken for shear zones created by faulting. cleavages and flow of clayey materials along laminae and cleavages occur remarkably (Fig. 5). Penetrative cleavage develops commonly in chertlaminites. appears to be dark laminae and the non-cleaved intervals to be light laminae (Fig. 5a). When chert-laminites are folded in the flow-fold style, they are folded tightly and often isoclinally. in such isoclinal folds are large and sometimes attain a factor of more than ten. Several tectonic lines or faults have been drawn on maps by reason of the presence of chert-laminites that are intensely deformed by creeping or folding. When chert-laminites were regarded as sheared argillite or phyllite forming fault zones, chert laminae were mis-interpreted as segregation quartz veins . Chert-laminites are usually metamorphosed into muscovite-quartz schist, muscovite-chlorite-quartz schist, muscovite-biotite-quartz schist, muscovitea1 bi te-quartz schist and others. Non- or weakly-metamorphosed chert-laminites have long been mistaken for "phyllite", "pelitic schist", or "green schist" recrystallized from argillite or green tuff; the lamination has been considered as being of metamorphic origin, hence schistosity. Metamorphosed chert-laminites are easily distinguishable from non-metamorphosed ones by the following; (1) the grain size of minerals of the former is coarser than that of the latter, (2) large euhedral or anhedral crystals such as muscovite, quartz and albite are common in the former, and

455

laminites, once considered to have developed the laminated texture by metamorphism or deformation, are now understood to be sedimentary in origin. Chert-laminites were commonly deposited in semi-enclosed basins such as forearc basins, marginal basins, and rift basins. Many of chert-laminites occur at lower horizons in geosynclinal piles, and are usually associated with basaltic rocks, bedded radiolarian chert, and distal turbidites. Comparisons with modern and ancient marine varves suggest that chertlaminites are also probably varves; a couplet, light and dark, represents an annual layer. ACKNOWLEDGMENTS Dr. Toshio Kimura of the University of Tokyo first pointed out to me the importance of chert-laminites and gave much instructive advice and discussions in the field and laboratory. Drs. Shinjiro Mizutani of Nagoya University, Yasuo Nakamura of the University of Tokyo, Koichi Nakamura of Geological Survey of Japan provided critical discussions and suggestions. Drs. Akira Iwamatsu of Kagoshima University, and Tanio Ito of the University of Tokyo gave me many helpful suggestions and hand specimens of chert-laminites. Mr. Setsuo Yogoof Nagoya University devised a new method to prepare thin sections and polished specimens of chert-laminites whose preparation was quite difficult with the usual method owing to exfoliation. Drs. Mamoru Adachi of Nagoya University, James R. Hein of U. S. Geological Survey, Azuma Iijima of the University of Tokyo, and David L. Jones of U. S. Geological Survey kindly read and criticized the manuscript. This study was partly supported by a Grant-in-Aid for Overseas Scientific Research from the Ministry of Education, Japan (no. 504130). REFERENCES Bramlette, M. N., 1946. The Monterey Formation of California and the origin of its siliceous rocks. U. S. Geol. Survey Prof. Paper, 212: 57. Calvert, S. E., 1964. Factors affecting distribution of laminated diatomaceous sediments in Gulf of California. In: Andel, T. H. van and Shor, G. G., (eds. ), Marine geology of the Gulf of California. Amer. Assoc. Petrol. Geol. Memoir, 3: 311-330. , 1966a. Origin of diatom-rich, varved sediments from the Gulf of California. J. Geol., 74: 546-565. , 1966b. Accumulation of diatomaceous silica in the sediments of the Gulf of California. Geol. SOC. Amer. Bull., 77: 569-596. Clark, S. H. B., 1973. The McHugh Complex of south-central Alaska. U. S. Geol. Survey Bull., 13724: D1-D10. Connelly, W., 1978. Uyak.Complex, Kodiak Inlands, Alaska: A Cretaceous subduction complex. Geol. SOC. Amer. Bull., 89: 755-769. Garrison, R . E., 1975. Neogene diatomaceous sedimentation in East Asia: a review with recommendations for further study. U. N. ESCAP, CCOP Technical Bull., 9: 57-69. Gross, M. G., Gucluer, S. M., et al., 1963. Varved marine sediments in a stagnant fjord. Science, 141 : 918-919. Hesse, R., 1977. Softex-radiographs o f sliced piston cores from the Japan and southern Kurile Trench and slope areas. In: E. Honza (ed. 1, Geological investigation of Japan and Southern Kurile Trench and slope areas GH 76-2 Cruise April-June 1976. Geol. Survey Japan, Cruise Rep., 7: 86-108. Hulsemann, J. and Emery, K. O., 1961. Stratification in recent sediments of Santa Barbara basin as controlled by organisms and water character. J. Geol . , 69: 279-290.

456

Kimura, T. and Tokuyama, A., 1971. Geosynclinal prisms and tectonics in Japan. Geol. SOC. Japan Mem., 6: 9-20. Lombard, A., 1963. Laminites: a stratum of flysch-type sediments. J. Sedim. Pet., 33: 14-22. Lowe, D. R., 1976. Nonglacial varves in lower member of Arkansas Novaculite (Devonian) , Arkansas and Oklahoma. Amer. Asso. Petrol. Geol . Bull., 60: 2103-2116. Moore, H. B., 1931. The muds of the Clyde Sea area. 111. J. Marine Biol. ASSOC. U. K., 17: 323-358. Nilsen, T. H . and Moore, G. W., 1979. Reconnaissance study o f Upper Cretaceous to Miocene stratigraphic units and sedimentary facies, Kodiak and adjacent islands, Alaska. U. S. Geol. Survey Prof. Paper, 1093: 1-34. Olausson, E., and Olsson, I. U., 1969. Varve stratigraphy in a core from the Gul f of Aden. Pal aeogeo. Pal aeocl i Pal aeoeco. , 6 : 87-1 03. Piper, 0. J. W., 1972. Turbidite origin of some laminated mudstone. Geol. Mag., 109: 115-126. Rubey, W. W . , 1931. Lithologic studies of fine-grained Upper Cretaceous sedimentary rocks of the Black Hills region. U . S. Geol. Survey Prof. Paper, 165A: 1-54. Sander, B., 1936. Bei trage zur Kenntni s der An1 agerungsgefuge (Rhythmische Kalke und Dolomite aus der Trias). Mineral. Pet. Mitt., 48: 27-209. Schrader, H., Matherne, A., et al., 1980. Laminated marine sediments in the central Gulf of California. Abs., Cordilleran Section of Geol. SOC. Amer. 76th. Ann. Meeting, Oregon, 151-152. Seibold, E., 1958. Jahreslagen in Sedimenten der Mittleren Adria. Geol. Rundschau , 47 : 100-117. Spears, 0. A., 1969. A laminated marine shale of Carboniferous age from Yorkshire, England. J. Sedim. Pet., 39: 106-112. Trendall, A. F . , 1972. Revolution in Earth history. J. Geol. SOC. Australia, 19: 287-311. von Huene, R., Nasu, N., et al., 1980. Sites 438 and 439 : Japan deep sea terrace, Leg 57. In: Scientific Party (Editors), Initial Reports of the Deep Sea Drilling Project, 56, 57, P t 1 , Washington (U. S. Govt. Printing Office): 23-191. Yoshida, S., 1981. Chert-laminite: its petrological description and occurrence in Japanese geosynclines. 3. Geol. SOC. Japan, 87: 131-141.

.

457

SUBJECT INDEX

A Abukuma Mountains (Japan) accretion

450, 451

5, 424 19, 55, 424, 428

accretionary terrane

3, 26, 36

acid-reducing s o l u t i o n

Africa

barite

70

452

basalt

2

3, 26, 28

147-154, 154 150, 157, 159

428

A k i t a (Japan)

massive pillow

48

26, 35, 36, 109, 451

basalt s i l l

albite

58

bedded c h e r t 213-218,

amphibole

220, 222-224 101

32, 37, 45, 51,

26,

55, 57, 140, 187, 389 113,

geochemistry

123-1 25

53, 143, 175, 179,

193, 299 30

l a y e r i n g types

annual l a y e r

451

A n t a r c t i c Sea

5, 320

A n t h i l l Black Shale (New Zealand) 149

apophyllite

95

101

53

laminar

53

s i ngl e-1 ayered

31, 52

d i agenetic d i f f e r e n t i a t i o n

argillite

Atsunai (Japan)

53-55

mechanism o f f o r m a t i o n

200, 206

3, 6, 35,,36 Ashio Mountains (Japan)

53, 55

53

triple-layered

151, 153, 156

Appenines ( I t a l y )

53, 54

grading

striped

Aponga Shale (New Zealand)

augite

2-5,

d e p o s i t i o n a l environments

58, 149

Angayucham Terrane (Alaska)

apatite

132

448

Amuri Limestone (New Zealand)

andesite

146, 152-154 26, 29, 110, 112, 113,

84, 96, 147, 148, 152, 153, 170,

150, 151, 164

analcime

144,

165, 167,

116, 117, 146, 152-154

Alaska

alkalinity

29, 33-35,

157, 160-162,

170, 450

aggregate e x t i n c t i o n A k i (Japan)

86

149, 150, 154, 156

barium o r e

1

aggl omera t e

85

Bantimala (Indones a )

176

Adoyama Formation (Japan)

Bangka (Indonesia)

93

88

Banda Arc (Indonesia)

a c c r e t i o n a r y prism

A d r i a t i c Sea

B a l l o o n Formation (New Zealand)

model 176

52

doubl e accumulation model

242

52, 53

150

sedimentary d i f f e r e n t i a t i o n

Azuero Peninsula (Costa Rica)

130

model

52

mineral composition B Babar (Indonesia) bacterial reduction

mode o f occurrence

88

225

rhythmic l a y e r i n g r h y t h m i c a l l y bedded

438 413 187 3

458

B u t t Formation (New Zealand)

sedimentation

51

shale p a r t i n g

51-55,

140, 181,

bytowni t e

104

150

184, 187, 406, 407 southwest Japan

5, 6, 361, 377,

395, 413, 427 bedded 1imestone-shale sequence benthic foraminifers Bering Sea

148

151

Cache Creek Formation (Canada)

214, 216, 219, 220,

calcareous organism

B i l l i t o n I s l a n d s (Indonesia)

85

California

317, 318

normative c a l c u l a t i o n

birnessite

317

C a l i f o r n i a Coast Ranges

31, 137

Canada carbonate

Blue Mountains Terrane (Oregon)

Ca

114, 123-125 28

bottom c u r r e n t

5, 53, 187, 188

bottom nepheloid l a y e r braunite brecciation

Ca-Mg

286, 294

Fe-Mn

149

149, 151, 154

166, 225

149 - 152, 154 -1 57 ,

calcite

159, 170, 212, 286, 294

B r i d g e R i v e r Terrane (Canada)

114,

dolomite

123 -1 25

30, 266, 273-276,

286,

294

Broad Pass Terrane (Alaska)

113,

carbonate rock

Broken R i v e r (New Zealand)

286 , 294

rhodochrosi t e

123-125 brown c l a y

286

carbonate m i n e r a l s

149, 150

98

3 286, 294

Mn

407

104

2

Caples Terrane (New Zealand)

149

b l a c k shale

247 -256

Campbell I s l a n d (New Zealand)

149, 151

bixbyite

219

1, 2, 5, 26, 35, 36, 109,

206, 224, 247

157, 164

bioturbation

31, 32

calcareous s i l i c e o u s sediment

58, 317

determination

98

33

Bunguran (Natuna) I s l a n d s (Indonesia) 87

212

Caramines Norte ( P h i l i p p i n e s )

89

C a v a l l i I s l a n d s (New Zealand)

96

Ce anomaly

29, 55, 187, 196

Cedros-Vizcaino Terrane (Baja

b u r i a l and d i a g e n e t i c h i s t o r i e s b u r i a l depth

14

58, 59, 262

b u r i a l temperature burrows

38

calcareous m i c r o f o s s i l

b i o g e n i c opal

2 114,

123 -1 25

224

biotite

131

Cache Creek Terrane (Canada)

5, 333

B-cristobalite

C

cabbage head s t r u c t u r e

4, 5, 58, 262

26, 31, 32, 36, 137,

149-151,

Cenozoic paleoceanographic changes Central America

88

Butsuzo Tectonic L i n e (Japan)

118, 123-125

251

329

157, 159

Buru (Indonesia)

California) c e l adoni t e

chalcedony 47, 48

26, 35, 127

13, 83, 84, 87, 149-151,

159, 220, 433

459

chalk 31 -34, 36, 162 chal k/l imestone 26 Chancet Rocks (New Zealand) 104 chaotic intraformational s l ide mass 135 Chatham Islands (New Zealand) 104 chemical composition 3, 5, 158, 160, 165, 170, 171, 177, 178 chert-mud d i l u t i o n curve 181, 182 Fe-A1 l i n e a r regression 168, 169 interelemental r e l a t i o n 53, 177, 180 - 187 lithogenous contribution and non-lithogenous contribution 183 major elements 165, 175, 177, 178 minor elements 165, 175, 178 principal component analysis 177 ternary diagram 161 - 164, 168, 169 c h e r t 1-5, 25-39, 45, 79, 81 -89, 93, 129, 144-171, 211, 257, 264 - 266 argillaceous 84 banded 83 bedded (see bedded c h e r t ) bedding 26, 134, 302, 303, 314, 386 biogenic 46 black, gray 83, 422 carbonaceous 82 cementation of 31, 59 chemical 46 c l a s s i f i c a t i o n and termi no1 ogy o f 45, 46 diagenetic evolution of 11 diatomaceous 30 dolomitic 101

d u c t i l e behavior of 13 e a r l y 46 later 46, 60 manganiferous 149 massive 134, 148, 152, 181, 185, 379 matrix of 382 nodular 8, 10, 84, 379 opaline 46, 231, 240 pelagic 88 primary 46 quartzose 45 radiolarian 3, 4, 28, 29, 35, 36, 51, 55, 82, 85, 86, 88, 154, 156 r e c r y s t a l l i z a t i o n of 131 , 135 red 422 comparison with red pelagic clays 204 rep1 acement 46 replacement t e x t u r e 26 ribbon 26, 29-32, 35, 38, 39, 134 secondary 46 "varved" 448 c h e r t breccia 153, 154 c h e r t environment 8 c h e r t f a c i e s 49 chert-greenstone u n i t 397, 403 c h e r t laminite 6 , 441 c h e r t l e n s 26, 32, 34-36, 130 c h e r t nodule (see nodule) chert-shale sequence 5, 152, 153, 302, 310, 314 c h e r t i f i c a t ion 60 cherty 1 imestone 83, 84, 95 cherty mudstone 31 chevron f o l d 152, 157 Chichibu (Japan) 48 Geosyncline 47, 55, 69, 175,

460

177, 188 Group

Diplognathodus oertliiNeogondolella pequopensis zone

2, 149

Terrane

4, 47, 48, 61, 71,

413, 427 Inner, Outer

47, 48, 61,

71 Northern, Southern

413

Chico Martinez Creek ( C a l i f o r n i a ) 259, 262, 263 c h i l l e d margin

133

5, 28, 51, 149-151,

156,

157, 433 Chokubetsu Formation (Japan) Chugach Terrane (Alaska)

231

114,

C h u l i t n a Terrane (Alaska) 123-1 25

zone

29-32,

103

307-314

c l a y mineral

4, 155, 156, 158, 165,

170, 317 58, 159

Clyde Sea

452

coccol it h

156

compaction rate o f

60, 186, 266-270 60

60

compensation depth (CCD) 18, 29, 31, 32, 38, 39, 140, 166 148, 151, 154

conodont b i o s t r a t i g r a p h y

66

66

Neogondolella bisselliSweetognathus whitei zone Neogondolella bulgarica zone Neogondolella clarkiIdiognathoides cormgatus zone

67 69

66

Neogondolella excelsaNeogondolella constricta

c o n c r e t i o n (see nodule) conglomerate

zone

zone

differential

66

Idiognathodus sinuosusStreptognathodus expansus Misikella hemsteini zone 69 Misikelk posthemsteini zone 69 Neognathodus bassleri aymetricus

37, 429

clinoptilolite

66

Idiognathoides noduliferusParaganthodus nagatoensis

114,

Clarence V a l l e y (New Zealand)

claystone

Epigondolella nodosa zone 69 EpigondoZeZZa spatulata zone 69 Gnathodus bilineatus-Gnathodus a f f . texanus zone 66 Gnathodus bilineatusParagnathodus nodosus zone 66 Idiognathodus delicatusDiplognathodus atetsuensis zone

123 -1 25

clay

Epigondolella bidentata zone 69 Epigondolella multidentata zone 69

Zone

chlorite

67

3, 65

Anchignathodus typicalisDiplognuthodus sp. fauna 67 Carinella mungoensis zone 69 Dip lognathodus ZanceolatusDiplognathodus nodosus zone 67

zone

69

Neogondolella foliata zone 69 Neogondolella polygnathifonis zone

69

Neospathodus conservativusNeospathodus dieneri zone 69 Neospathodus homeri zone 69

461 Neospathodus timorensis zone

69

kinetics

261, 262

69

influence o f d e t r i t a l clay

Streptogna thodus elonga t u s fauna

58, 59, 260, 261

67

c o n t a c t metamorphism c o n t i n e n t a l slope

zeolitic

131 424

5

diatom b i o s t r a t i g r a p h y

350 351

Actinocyclus ingens

26, 30, 36

Derticulopsis h u s t e d t i i

Cordillera

3

Costa Rica

2-4,

26, 30, 35-37,

127,

Costa Rica R i f t

diatom d i v e r s i t y

26, 31, 33

Coverham (New Zealand)

diatomite

146

cut-and-fill structure

2, 5, 9, 10,

31, 333, 451

50, 129, 361, 395

c r y s t a l l i t e size

351

351, 352

354

diatomaceous sediment

104

351

Denticulopsis karntschatica Denticulopsis lauta

143, 206

Cretaceous

9

diatom assemblage

424

convolute bed

58

diatom abundance

188

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

4, 58, 59, 242, 257-262

i n f l u e n c e o f carbonate

Neospathodus? cotlinsoni zone

18

silica

Neospathodus triangularis-

1, 11, 32, 61, 348, 350,

355

385

diatom

1, 3, 5, 30, 51, 333, 347

D

Daigo Group (Japan)

E

67 East P a c i f i c Rise

Deep Sea D r i l l i n g P r o j e c t (D.S.D.P.) 4, 25, 26, 28-33, 164-168,

177

Eastern Klamath Mountains Terrane

36, 38, 157,

115, 123 -1 25

(Oregon-Cal i f o r n i a )

171, 320

Leg 62

26, 27, 31 -33,

36-39

Eketahuna (New Zealand)

Leg 69

26, 27, 31 -33,

36-39

embri ttl ement

d e p o s i t i o n a l environment

26, 55, 57,

Eocene r a d i o l a r i a n s

148

diagenesis

2, 4, 5, 13, 31, 32, 57,

epidote

309-311,

314

evaporite

burial

58

chemical

’

151

rate o f reaction

9

F

294

a c t i v a t i o n energy

175

213, 214, 216,

219, 220

58, 212, 224, 225, 283, 299-301,

212, 295

f a c i e s model

140

Favosites-Hatisites r e e f limestone

295

r a t e - c o n t r o l l i n g step early

206

Eocene t o Middle Miocene

140, 187, 255, 256, 389 diabase

England

98

13

212,

47

220, 222, 225

fine-grained siliceous deposit

218

f i n e - g r a i n e d s i l i c e o u s rock

45 45, 46,

462

57, 60, 175 aspects o f diagenesis chemical composition

4, 5 175

chemical sedimentology

gravity current

136

gravity sliding

57

graywacke

2, 4

3, 28, 35, 36

Great V a l l e y Terrane ( C a l i f o r n i a )

c l a s s i f i c a t i o n and terminology

117, 118, 123-125

45

Greece

5, 206, 299-303,

d i s t r i b u t i o n i n space and time

greenstone

2, 3 1 it h i f i c a ti on

G u l f o f Aden

Green T u f f Region (Japan) 58 -60

source o f s i l i c a

1

F l i n t Beds (New Zealand)

56

Gumai ( I n d o n e s i a )

86

H

foraminifers

32

Haast S c h i s t Terrane (New Zealand)

Franciscan Complex ( C a l i f o r n i a )

26,

29, 30, 34-39

98 Halmahera ( I n d o n e s i a )

Franciscan Formation ( C a l i f o r n i a )

2

Franciscan Terrane ( C a l i fornia-Oregon) 117, 123-125

heat flow

5, 149-151,

348

hematite o r e

86

H e r r i n g Formation (New Zealand)

47, 55, 69, 175, 177,

188

Hess R i s e ( N o r t h P a c i f i c ) heulandite

g e o s y n c l i n a l sedimentary r o c k geothermal g r a d i e n t g l a c i a l event

450

12

hiatus

451

333

149, 151, 156

355

Go1 conda Terrane (Oregon) 123-125

72

Hidakagawa Be1 t (Japan) Hidakagawa Group (Japan)

G o l f i t o (Costa Rica)

145, 170

136

graded bedding

26, 37, 52, 54, 134,

157, 385

graptol i t i c shale

Hinokage (Japan)

28

Hokkaido (Japan) hollandite

396 83

3, 48, 50-52,

149

Holocene sediment Honshu (Japan)

395

428

Ho Chi Minh City (Vietnam)

30, 52, 54, 134

28

48, 51

Hidaka Group (Japan) 115,

47, 48

Hida metamorphic r o c k s (Japan) Hidaka (Japan)

85, 151, 157

130 101

26, 31, 32

Hida O l d C o n t i n e n t (Japan)

Ghost Rocks Formation (Alaska)

149-153,

155

Herradura Peninsula (Costa Rica)

Garba Mountains ( I n d o n e s i a )

graded bed

32, 161, 162, 437

hemipelagic d e p o s i t i o n

148

gradation

153, 155-157,

157

hemipelagic c l a y

goethite

453

12

159, 170, 433

G

geosyncline

89

Hamersl ey Group ( A u s t r a l i a ) hematite

213

Funakawa Formation (Japan)

gabbro

451

101

3, 29

f r e s h water

347, 451

452

Gulf o f California

f l a t - t o p p e d b a s a l t i c seamount flysch

307, 313

5, 28, 35, 36, 427, 428

333 3

451

463

hornblende 157 Horokanai o p h i o l i t e s u i t e (Japan) Hunua Range (New Zealand) 97 hyaloclastite 154 hydrocarbon 1 , 2 hydrogenous accumulation 185 hydrothermal a l t e r a t i o n 59, 62 hydrothermal d e p o s i t 181 hydrothermal f l u i d 156, 185

55

I IGCP P r o j e c t 115 i , 1 , 2 illite 5 , 28, 51, 149-151, 156, 159, 165, 166 imbrication 79, 82, 88, 89 Indochina 3, 79, 82, 8 3 Indonesia 3, 79, 82 i n d u c t i v e l y coupled emission plasma spectroscopy 146 Innoko Terrane (Alaska) 113, 123-125 I n t e r n a t i o n a l Geological Correlation Programme (IGCP) i intraformational breccia 138 intraformational f o l d 57 Inuyama (Japan) 175 i r o n o r e 2 , 160-162, 164 i s l a n d a r c 424 i s o t o p i c age 292 K-Ar 5 , 283, 292 Rb-Sr 5, 283, 293 Italy 5 Izu Peninsula (Japan) 62

J Jaco (Costa Rica) 145, 170 Japan 1-5, 45, 65, 175, 229, 283, 347, 361, 377, 395, 413, 427, 441 Japan Trench 453 j a s p e r 83, 87, 110, 115, 131, 147, 150, 151, 153, 154, 156-158,

160-162, 164 Java (Indonesia) 86 J u r a s s i c 47, 175, 283, 361 K K-fel dspar 150 Kagvi k-Brooks Range Terrane (A1 aska) 112, 113, 123-125 Kai (Indonesia) 88 Kalimantan (Borneo) 86, 87 Kamenoo Formation (Japan) 451 Kamiyoshida Formation (Japan) 72 Kampuchea (Indochina) 79, 83 Kamui kotan-Sorachi Terrane (Japan) 451 Kanchanaburi S e r i e s (Thailand) 83 Karakelang (Indonesia) 89 Kedah (Malaysia, Thailand) 82 Kelp Bay Group (Alaska) 26, 34-36, 38, 39 kerogen maturation 270-272 Kii Peninsula (Japan) 5, 395 K i takami Mountains (Japan) 451 Kitami (Japan) 48, 52 Klamath Mountains Terrane ( C a l i f o r n i a Oregon) 114, 123-125 Kobotoke Group (Japan) 445 Konose Group (Japan) 73 Kuala Lumpur (Malaysia) 83 Kuching (Borneo) 85 Kurosegawa-Ofunato Be1 t (Japan) 28, 47, 48 Kurosegawa Tectonic Zone (Japan) 413 Kuroshio Current 357 Kuzuu (Japan) 48, 176 L

1 abradori t e 150, 151 laminated 149, 150, 152, 153, 155, 157, 159, 166

464

1ami n a t i o n

manganese carbonate 1ens

cross-lamination

30, 52, 384

microcross-1 amination parallel lamination

136

441 79, 83

Marble Bay (New Zealand)

47, 176, 361, 380

marginal sea

149, 151, 156

l e n t i c u l a r s l i d e body lepisphere

135

marine varve

212, 218, 220, 222-224,

21 2

453

146-148,

3, 26, 28-34, 149-151,

36, 144,

153, 154, 159,

164, 170

151

Matsugadaira Formation (Japan) Mazegawa Formation (Japan) McHugh Complex (Alaska)

lithogenous r e s i d u e

183, 184

113, 114, 117, 118,

125, 131 109, 111,

tectonic

109, 112, 1 4,

117, 118

88 86

47, 50, 299, 300, 361,

381 , 413

113-118 s i l i c i c v o l c a n i c and c h e r t 111, 113-115,

118, 125

subsidence c h e r t

109, 111,

118

109,

metagraywacke

29

metalliferous claystone m e t a l l i f e r o u s sediment metamorphism

volcanogenic c h e r t

98

Ludox (see s i l i c a )

Mexico

Luzon ( P h i l i p p i n e s )

87

89

microfault

151 149-151

mid-ocean r i d g e Mihama (Japan)

149

Mamba Formation (Japan)

, 159

60 12

M i d - P a c i f i c Mountains

66

millipore f i l t e r

26, 31, 32

428

Mikabu Group (Japan)

3, 79-81

161 , 162, 164

206

microstylolite

M

431

156, 165

microbreccia

Lupar V a l l e y (Malaysia)

magnetite

403, 41 7

Mentawai I s l a n d s (Indonesia) Mesozoic

167

406

Meratus Mountains (Indonesia) 109, 110,

48

3, 55, 79, 82, 86-89,

sedimentary

125

o p h i o l i t i c chert

113-115,

101 Median Tectonic L i n e (Japan) melange

melange c h e r t

113,

Mead H i l l Formation (New Zealand)

a1 t e r n a t i n g p i l l o w basal t / c h e r t

113-118,

283, 289

123-125

109, 146, 165, 166

clastidchert

450

451

McKinley Terrane (Alaska)

l i t h o l o g i c association o f chert

109-111,

101

Marl borough (New Zeal and) mass e x t i n c t i o n

limestone

453

96

56, 186, 188

marine sediments

242

Malaysia

149, 152, 156, 170

M a n s f i e l d Marine Band (England)

Laos (Indochina) laumontite

176,

181, 185 manganese oxides

L a t e Paleozoic

175

2, 110, 115, 117, 118,

148, 152, 153, 157, 160-164,

134, 384,

438 laminite

manganese o r e

213

450

465

Mindanao ( P h i l i p p i n e s )

nodule

87

Mino Terrane (Japan)

Mirza Formation (New Zealand) Miyama Formation (Japan)

101

Monotis f a c i e s

Morocco

134, 150, 153 59

North America

224, 247-282,

198, 206

3, 26, 35, 36

North I s l a n d (New Zealand) 115 Northland (New Zealand)

206

Mororimu Stream (New Zealand)

102

Mowry S i l i c e o u s Shale (Wyoming) mudstone 5, 29, 30, 36, 168 m u l t i p l e regression a n a l y s i s Muro B e l t (Japan) 395

453

Noto Peninsula (Japan) Nueva E c i j a ( P h i l i p p i n e s )

ocean f l o o r

89

212

oceanic c r u s t

Nakamura (Japan) nannofossil

28

oceanography

153

41 3

33

o f f s h o r e bank

69

56

o f f s h o r e mud

Nelson (New Zealand)

93

406, 437

56, 188

o f f s h o r e basin

1

.

333

o f f - r i d g e volcanism

N a r u t a k i i s h i (Japan)

Neo (Japan)

155

oceanic sediment

73

nannofossil limestone n a t u r a l gas

139

oceanic c u r r e n t

Nakagawa Group (Japan)

176, 189

Oga Peninsula (Japan) 232, 347

176

48, 51, 59,

Neogene 51, 60, 247, 347 Neogene 1it h o f a c i e s 249 -253 New Zealand 3, 9.3

Okabe Formation (Japan)

New Zeal and Geosyncl ine (New Zeal and) 96

olistostrome 57, 403, 417 Onnagawa Formation (Japan)

Nha Trang (Vietnam)

Oki I s l a n d s (Japan)

Nias I s l a n d (Indonesia)

88

242

1, 2, 52, 59, 232, 242, 348, 446, 451 calcareous

127,

143, 206 Nicoya Peninsula (Costa Rica)

48, 352

ooze

Nicoya Complex (Costa Rica) 2, 4, 26, 34, 35, 37-39,

52

Okoppezawa Formation (Japan)

83

397

2

Obi (Indonesia)

N

89

0 obduction

113, 123-125

96 48, 51, 351

Nyunokawa Formation (Japan)

319

29

M y s t i c Terrane (Alaska)

94

Northern S i e r r a Terrane ( C a l i f o r n i a )

453

58

muscovite

32, 101, 132

tridymite

47

Monterey Formation ( C a l i f o r n i a ) mordenite

154

manganese

88

Montagne N o i r e (France) 1-4,

chert

100

36, 59

60

chalcedony

397, 398

Mokoiwi Formation (New Zealand) Moluccas (Indonesia)

26, 31 -34,

carbonate

5, 284

diatom 127, 143

33, 37

coccol ith-forami n i f e r a l 9, 30, 333

nannofossil

32

154

466

radiolarian

9, 38

siliceous

33-35,

12, 19, 27, 29, 30,

32, 37, 166 31, 33,

34, 37 1, 58, 229

cristobalite

Pahaoa Group (New Zealand)

230 - 233 mixture

46 -49, 216-225

pelagic clay

58, 146, 155-157,

159,

217-220,

222, 225,

p e l i t i c schist

58

d(l0l)-spacing

146, 243, 220, 224 231 -233

242

Peru

1 petroleum generation 272, 273 pH 211, 213, 216, 219, 220 Philippines

2, 3, 79, 87, 89 2

phosphatization

403

phyllite

150, 154 130, 145,

154, 170

252

6

1

phytoplankton

Osa Peninsula (Costa Rica)

p i emont ite-bea r ing metachert pinch-and-swell s t r u c t u r e

Otonashigawa Be1t (Japan)

395

Pindos (Greece)

5, 299, 313, 314

oxygen i s o t o p i c r a t i o

(&loo)

29,

262, 263, 274, 275, 300, 301, 305, 313, 314 Oyashio Undercurrent

32, 451 357

P

2

Pismo s y n c l i n e ( C a l i f o r n i a )

5, 149, 151, 155, 157,

159, 433 p l agiograni t e

148

1

p l a s t i c sediment deformation 30, 31,

270,

271, 273

plankton 25-27,

88 134, 152

300, 307, 311

Pisco Formation (Peru)

plagioclase

oxygen minimum zone

P a c i f i c Ocean

59

1, 2

phosphate o r e 35, 49, 50, 55, 127

oxygen isotopes

79, 82-84

petroleum

1, 4, 58, 59, 231,

o p h i o l i t e sequence

140

p e r c o l a t i n g groundwater

259, 261, 263 thermal p r o p e r t i e s

218

6

Peninsular Malaysia

crystallinity

organic m a t t e r

4, 37, 213, 413

p e l a g i c environment p e l a g i c sedimentation

231, 242

“disordered”

87

29, 31, 32, 37, 161,

pelagic deposit

1, 4, 5, 13, 20, 32,

tridymite

149 -1 51

127

162, 176

1, 4

211 -213,

193

140

Panay ( P h i l i p p i n e s )

1, 4, 5, 30, 32, 58,

211 -214, opal-CT

p a l e o r e l ie f Panama

58

254, 255

paleogeographic zonation palygorski t e

229, 233-242

occurrence

100

79, 82, 85, 89

paleoceanographic s e t t i n g

1, 4, 58, 233, 243

d e f i n i t i o n and i d e n t i f i c a t i o n

ophiolite

85, 86

Palawan ( P h i l i p p i n e s )

opaline s i l i c a

opal-C

i, 1

Padang (Indonesia)

s i l i c e o u s calcareous

opal-A

37, 39, 205

P a c i f i c Region

p l a t e boundary

7

134

467

p l a t e t e c t o n i c environment plate tectonics

7

30, 37, 51, 93, 149, 152-157,

55, 424

p l u t o n i c rock

159, 170, 213, 214, 216-220,

3

porcelanite

222-224,

1, 34, 46, 51, 84, 85,

240, 257, 264-266 porosity

59,

60, 266-270

Amphipyndax enesseffi assemblage 365, 366

porosity reduction

60, 266-270

p r e h n i te-pumpel l y i t e f a c i e s "preflysch" u n i t

156

397

pressure s o l u t i o n

hphipyndm tylotus assemblage 365, 366, 397, 402

Archeodictyomitra directiporata-

60, 299, 307, 314

333

Pulau Banggi (Malaysia)

Eucyrtidim? sp. A assemblage 366 Artostrobim u m a assemblage

86

151, 154, 156

Punta Conchal (Costa Rica)

365, 366, 402 132

Punta Conchal Formation (Costa Rica) 127 pyrite

5

364, 365

p o r o s i t y - b u r i a l depth r e l a t i o n

pumpellyite

r a d i o l a r i a n assemblage

AlbailZeZla sp. D assemblage

59, 60

productivity

361, 433

Canoptm t r i a s s i c m assemblage 364, 365, 368, 369

Capnuchosphaera theZoides 33, 150, 151, 154, 157

p y r i t i c clay pyrolusite pyroxene

33

assemblage

364, 368

Dictyomitra formosa assemblage

149, 150

402

149, 151, 156, 157

Dictyomitra sp. B

-

Dictyornitra

sp. A assemblage 364, 365, 372 q u a n t i t a t i v e emission spectroscopy 146 quartz

1, 28, 32, 148, 149, 151,

153-157,

159, 170, 211, 224, 433

chalcedonic

1

crystallinity

146, 149, 155, 156,

159

qua rt z - c l ays t o n e

k i l u v i a ? cochleata assemblage 364, 366 365, 366 FoZlicucuZZus assemblage

13

Gongylothorax sakawaensis Stichocapsa sp. C assemblage

443

6

365, 370, 372

Costa Rica)

130, 145

Gongylothorax Sp. c sp. C assemblage

-

Stichocapsa 364

Holocryptocaniwn barbui assemblage 365, 366, 402

R

Radi o l a r i a , r a d i o a r i a n s

362,

364, 365

1

segregated v e i n Quepos Peninsula

Pantanellim sp. A assemblage 366

Eucyrtis micropora assemblage

microcrysta11i.ne

m i croquartz

-

DictyomitreZZa sp. A

Q

1, 5, 26,

H S U ~sp.

B assemblage

364, 365,

468 369, 370

361, 413

Hsuwn sp. B - Unwna echinatus assemblage 373

r a d i o l a r i a n mudstone

Lithocampe? nudata assemblage

r a d i o l a r i a n packstone

r a d i o l a r i a n c h e r t (see c h e r t )

364, 365, 370, 372

radiolarite

289, 366, 372 assemblage assemblage

4, 11, 29, 30, 35, 83,

89, 127, 133, 299, 300, 302-304,

Mirifusus guadalupensis

-

306, 307,

309, 312

Mirifusus b a i l e y i assemblage

Mirifusus sp.

305-307

310, 311, 314

366

r a d i o l a r i t e composition

ParvicinguZa sp.

R a n g i t a t a Orogen (New Zealand)

402

r a r e - e a r t h elements (REE)

NeoaZbaiZlelZa assemblage

364,

365, 372 365, 366

-

Cecropus?

sp. A assemblage 366 Paraf o Z 1i c u c u l Zus as sembl age

Rat B u r i Limestone ( T h a i l a n d )

100

" r e d me1ange"

96

Red P a i n t Terrane (Alaska)

Parahsum simpzwn assemblage

83

3, 37, 53, 55,

185, 186, 188, 189 r e d bed

364, 365

4, 28, 29,

86

r a t e o f sedimentation

Pantanelliwn sp. A

3, 96

157, 204 R a t a i Bay (Indonesia)

ObesacapsuZa rotunda assemblage

195

113,

123-125

364, 365, 369, 370, 373

Red Rock P o i n t (New Zealand)

Parvicingula a l t i s s i m a assemblage 366

redeposi t i o n a l f e a t u r e

136

r e f r a c t o r y raw m a t e r i a l

61

98

PseudoalbailZelZa assemblage

r h y o l i t i c t o andesitic volcanic rock

362, 364, 365 Sphaerostylus lanceola

r h y t h m i c bedding

28 133

Staurosphaera septemporata ThanarZa conica assemblage

-

133

UZtranapora sp.

402

252, 253

rhythmic gradation

5, 52, 55, 187

r h y t h m i c sedimentation

Triassocmpe deweveri assembl age 364, 365, 366, 368, 372

rhythmic s t r a t i f i c a t i o n

197 134

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

Triassocmpe nova assemblage

c h e r t - s h a l e sequence

364, 365, 368

35, 148,

152, 170

TripocycZia c f . acythus assemblage 364, 366

rift valley

17

romanechi t e

149

UZtranapora praespinifera

Roti (Indonesia)

assemblage

134

rhythmic l a y e r i n g

365, 366

R y u j i n (Japan)

Unwna echinatus assemblage 365, 366, 370, 373 r a d i o l a r i an b i o s t r a t i graphy

289,

364,

88 428

R y u j i n Formation (Japan)

397

469

secondary carbonate

S Sabah (Malaysia)

79, 82, 86, 88

Sabana Grande (Costa Rica)

35, 146,

147, 155, 160, 162, 169

149

sedimentary b r e c c i a

146, 148

sedimentary s t r u c t u r e

Sabana Grande u n i t (Costa Rica) 153-157,

266, 273-276

secondary mineral

148,

160, 161, 164-167,

170,

171

3, 5, 26, 36,

37, 154, 155, 170, 171, 302, 304, 310, 314 sedimentation diagram

Sakhalin

451

semi-enclosed sea

salinity

4

s e p i o l it e

Sanbagawa- ( o r Sambagawa-) Chichibu Terrane (Japan)

serpentini t e

Sanbosan ( o r Sambosan) Geosyncl i n e (Japan) Group Zone

175, 187, 188, 451 2, 47, 48, 71, 451

shale

4, 26, 31, 34-37,

155, 157-165,

5, 26, 32, 34, 35, 52, 418

Sangun-Yarnaguchi Terrane (Japan) San Juan Terrane (Washington)

450 114,

123 -125

Shannon-Wiener equation

354

Shien Formation (Japan)

73

Shimadani Formation (Japan)

452

Santa Barbara Coast ( C a l i f o r n i a ) 260, 261 , 269, 271

5, 395

Geosyncl i n e Group

50

444, 449

Supergroup

Santa Elena Peninsula (Costa Rica) 129

Terrane

50, 395

28, 48, 50, 428, 451

Shirnantogawa Group (Japan)

Santa Maria Basin ( C a l i f o r n i a ) 262 - 264

Shinzan D i a t o m i t e (Japan)

28, 427 59, 61,

232, 242

Sarawak (Malaysia)

79, 82, 84, 85,

87

siderite silica

150

1

Savu (Indonesia)

88

amorphous

1 , 219

Sawadani (Japan)

428

biogenic

4, 30-32,

Sawadani Group (Japan)

427

scanning e l e c t r o n microscope (SEM) 213, 222

clay-associated Ludox

45

317, 322-324

213, 215, 217-220,

222 - 224

sea f l o o r

1

non-biogenic

317

sea water

213, 225

v o l canogenic

30

seamount

68

Shimanto (Japan) Be1 t

Santa Barbara Basin ( C a l i f o r n i a )

148-153,

167-171

shale p a r t i n g (see bedded c h e r t )

153, 157, 159, 164 sandstone dyke

sea-ice

50

4, 48, 52,

Setogawa Terrane (Japan)

413

sandstone

88

168

Setogawa Geosyncl i n e (Japan) 47

71

Terrane

219

Seram (Indonesia)

447, 450

16

452

333 428

s i l i c a diagenesis (see diagenesis) s i l i c a phases

257, 261

470

silica phase transformation 1 , 60, 243, 262 opal-A to opal-CT 18 silicastone 3, 46, 61 Akashiro-keiseki ore 61 diatomite ore 61 hydrothermal ore 62 silicified chert ore 61 silica zones 58 biogenic opal zone 58 opal-CT zone 58 quartz zone 58 siliceous argillite 96 siliceous biogenic debris 1 , 4, 5 siliceous clay 33, 37 siliceous deposit i , 1, 30, 32, 153 siliceous iron-rich rock 147 siliceous limestone 34, 159 siliceous microfossil 28 siliceous mudstone 29, 82, 448 siliceous organism 1 , 2, 30 siliceous plankton 32, 38 siliceous rock 3, 50, 51, 60, 82, 158 siliceous shale 1 , 3, 5, 46, 82, 83, 85, 86, 101, 154, 159, 283, 351 siliceous shale/mudrock 257, 264-266 siliceous skeletons 51, 53 preservation 60 silicic and intermediate volcanics 49 silicification 26, 61, 62 silicified chalk 31, silicified limestone 148, 157 silicoflagellate 1, 30 silicon 1 silt 30 siltstone 150, 153, 164 Singapore 79, 82, 84 slump breccia 450 slump fold 135

smectite 149, 151, 156, 159, 165, 166 soft-sediment deformation 152 soft-sediment folding 36, 37 sole mark 37, 149 solubility 4 So& Peninsula (Costa Rica) 130 Sorachi (Japan) 72 Sorachi Geosyncline 50 South Island (New Zealand) 95 Southeast Asia 3, 79 Southern Ocean 317 specific surface area (SSA) 211 -215, 218-220, 222, 224, 225 sponge spicule 1 , 30, 51, 104, 159, 433 preferred orientation 386 stilbite 149, 151, 156 stilpnomelane 151 stratiform manganese mineralization 134 stringer 26, 31, 36, 150, 152 stylolite 149, 150, 306-308, 310, 314 subduction 5, 424 subduction zone 12, 413 submarine hydrothermal activity 61 submarine mafic volcanics 49 submarine sliding 57 Sulawesi (Indonesia) 86, 88 Sumatra (Indonesia) 85, 86, 88 Sumulong Diatomite (Philippines) 2 surface charge 220 Susunai Mountains (Alaska) 451 swell 57 T Tanba (or Tamba) (Japan) 5, 61, 378 Tanba Group (Japan) 67, 70, 378 Tanimbar (Indonesia) 88

471

Tanzawa Terrane (Japan) 451 Tatsukobu Formation (Japan) 52 tectonics 299, 307, 308 temperature-time diagram 17 Tenpoku (Japan) 48, 52, 231 Terasoma Formation (Japan) 402 Tertiary 50, 175 Thailand 3, 79, 82-84 Thomson Mountains (New Zeal and) 98 thrust f a u l t i n g 2 Timor (Indonesia) 88 Togano (Japan) 71 Togano Formation (Japan) 73 Torlesse Terrane (New Zealand) 98 t r a c e metals 1 t r e n c h - f i l l deposit 428, 453 Triassic 47, 175, 361, 377 Tucker Cove Limestone (New Zealand) 104 tuff 3, 28, 32, 36 Tupou Formation (New Zealand) 100 t u r b i d i t e 3, 4 , 28, 30, 32, 36, 37, 52, 144, 155, 165, 167, 170 Bouma sequence 136, 155 distal 188, 448 s i l i c e o u s 52 t u r b i d i t y c u r r e n t 5, 34-37, 53, 137, 155, 165, 170 Turkey 193, 198 U

Ugusu s i l i c a deposit ,(Japan) 62 ultramafic rock 144 upwelling 1 , 5, 9, 29, 38, 166 uranium o r e 2 U.S.S.R. 2 Uyak Complex (Alaska) 451 Vancouver Is1 and (Canada) 452 2 Vagompolkian S e r i e s (U.S.S.R.) vernadi t e 149

Vientiane (Indochina) 83 Vietnam 79, 83 volcanic a c t i v i t y 5 volcanic ash 30, 31, 33, 152 volcanic breccia 131, 146 volcanic g l a s s 4, 30, 58, 159 volcanic rock 3 vol cani cl a s t i c sediment 58, 139, 233, 243 v o l c a n i c l a s t i c rock 28 W

Waigeo (Indonesia) 89 Waiheke Group (New Zealand) 97 Waioeka Gorge (New Zealand) 98 Waipapa Terrane (New Zealand) 96 Wairarapa (New Zealand) 100 Wakasugi Group (Japan) 68 Wakkanai Formation (Japan) 52, 231 Washington 2 Whangai Formation (New Zealand) 101 Whangai Range (New Zealand) 101 Wharanui Point Limestone (New Zealand) 104 Woodside Creek (New Zealand) 102 Woolshed Formation (New Zealand) 101 X xenolith 131 X-ray d i f f r a c t i o n 146, 213, 229, 233-240, 433 X-ray fluorescence spectroscopy (XRF) 146, 177 X-ray mineralogy 149, 155, 159, 433 Y Yabuhara Formation (Japan) 71 Yamagami Formation (Japan) 450 Yamaguchi Facies (Japan) 69 Yolla Bolly Belt ( C a l i f o r n i a ) 35

472

Yo1 1a Bol l y Terrane (Cal i f o r n i a ) Yukawa Formation (Japan) 396

Z zeolite 58, 149, 150, 153, 159 z e o l i t e zone 58 zeolitic clay

31, 34, 36

29

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