The Atlas system of Morocco: studies on its geodynamic evolution

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Lecture Notes in Earth Sciences Edited by Somdev Bhattacharji, Gerald M. Friedman, Horst J. Neugebauer and Adolf Seilacher

15 Volker H. Jacobshagen (Ed.)

The Atlas System of Morocco Studies on its Geodynamic Evolution

Springer-Verlag Berlin Heidelberg NewYork London Paris Tokyo

Editor Prof. Dr Volker H. Jacobshagen Frele Unlversltat Berhn FB Geowissenschaften, Instltut fur Geologle Altensteinstrasse 34a, D-1000 Berhn 33, FRG

ISBN 3-540-19086-4 Sprmger-Verlag Berhn Heidelberg N e w York ISBN 0-387-19086-4 Springer-Verlag New York Berlin Heidelberg

Ltbrary of Congress Catalogmg-tn-PubhcahonData Geodynamlc evoluhon of the Atlas system/ Morocco / Volker H Jacobshagen, ed p cm -(Lecture notes in earth scrences, 15) "One of the sources of this book was a conference on the geodynamJcaspects of the Atlas system, organtzed m Berltn in 1986" by the Forschungsgruppe "Mobrhtat akhver Kontlnentalrander"-Pref ISBN 0-387-19086-4 (U S ) 1 Geology-Morocco-Atlas Mountains 2 Geology-Morocco-Antt-Atlas Mountains 3 Geodynamics I. Jacobshagen, Volker II Senes QE339 MSG36 1988 556 4'4-dc 19 88-12329 Thrs work is subject to copyright All rights are reserved, whether the whole or part of the material ts concerned, specifically the rights of translation, reprmtrng, re-use of Jllustrattons, recttatlon, broadcasttng, reproduction on mrcrofilms or in other ways, and storage m data banks Duphcat~on of thrs pubhcahen or parts thereof rs only permitted under the provtsJonsof the German Copynght Law of September g, 1965, in its versron of June 24, 1985, and a copyright fee must always be paid Violations fall under the prosecution act of the German Copyrtght Law @ Spnnger-Veriag Berhn Hetdelberg 1988 Pnnted rn Germany Prmhng and binding. Druckhaus Beltz, Hemsbach/Bergstr 2132/3140-543210

PREFACE

The aim of this volume is to r e f l e c t the current state of geoscient i f i c a c t i v i t y focused on the geodynamic evolution of the Atlas system and to discuss new results and ideas. The volume provides a selection of papers on the geological history, structural development, and geophysical data of Morocco. I t was not possible to cover all areas of geoscientific i n t e r e s t , however, we hope to shed some l i g h t on the major geodynamic problems. One of the sources of this book was a conference on the geodynamic aspects of the Atlas system, organized in Berlin in 1986 by the Forschergruppe "Mobilit~t aktiver Kontinentalr~nder". The financial support for this project is being provided by the Deutsche Forscht:!~gsgemeinschaft and by the Freie Universit~t and the Technische Universi o t~t of Berlin (West). Furthermore we are very glad to mention that we have received a great number of publications by authors from d i f f e r e n t countries who did not take part in the Berlin conference. I t should be noted that, at present, a l o t of international geos c i e n t i f i c a c t i v i t y is concerned with Morocco. This is not only due to the fascinating complexity of Moroccan geology and the beautiful outcroups, that may provide the key to many general problems, but also t e s t i f i e s to the l i b e r a l and prescient science p o l i t i c s of the Moroccan a u t h o r i t i e s , most of all the Ministery of Energy and Mining in Rabat. Therefore, on behalf of all the authors from abroad, I take the opportunity to express our deepest gratitude to our host country, especially to the leaders of the Division de la G~ologie, the general director Dr. M. Bensaid and the directors M. Dahmani and M. Hammouda. Thanks are, furthermore, due to Springer Verlag (Berlin-HeidelbergNew York) for offering the opportunity to publish a comprehensive volume and especially to Dr. Wolfgang Engel for his patience and helpful advices. V. Jacobshagen

CONTENTS

INTRODUCTION JACOBSHAGEN, V . : Geodynamic M o r o c c o : an i n t r o d u c t i o n

evolution

of

the A t l a s

System, 3

ANTI-ATLAS WALLBRECHER, E.:

11 The A n t i - A t l a s

system t a n

overview

13

WALLBRECHER, E.: A d u c t i l e shear zone in t h e P a n a f r i c a n basement on t h e n o r t h w e s t e r n m a r g i n of t h e W e s t - A f r i can c r a t o n ( S i r w a dome, c e n t r a l A n t i - A t l a s )

19

BASSIAS, Y . , WALLBRECHER, E. and WILLGALLIS, A . : T e c t o n o t h e r m a l e v o l u t i o n o f t h e L a t e P a n a f r i c a n o r o g e n y in the Central Anti-Atlas ( S o u t h e r n Morocco)

43

BRABERS, P.M.: A p l a t e t e c t o n i c model f o r o r o g e n y in the A n t i - A t l a s , Morocco

61

BUGGISCH, W. and FLOGEL, E.: b o u n d a r y in t h e A n t i - A t l a s new r e s u l t s SDZUY, K. and GEYER, G.: Morocco

the

Panafrican

The P r e c a m b r i a n / C a m b r i a n ( M o r o c c o ) . D i s c u s s i o n ~nd 81

The base o f

t h e Cambrian

in 91

BUGGISCH, W. and SIEGERT, R,: P a l e o g e o g r a p h y and f a c i e s o f t h e d ~ r ~ s t e r m i n a u x ' ~ ( u p p e r m o s t Lower C a m b r i a n , Anti-Atl as/Morocco)

107

BUGGISCH, W.: D i a g e n e s i s and v e r y l o w - g r a d e metamorphism of the Lower Cambrian r o c k s in t h e A n t i - A t l a s (~4orocco)

123

HIGH AND MIDDLE ATLAS

129

SCHAER, J . - P . : Deformation of a r o u n d the T i c h k a g r a n i t e ,

i g n e o u s d i k e s i n and High A t l a s - Morocco

LORENZ, J . C . : S y n t h e s i s o f L a t e P a l e o z o i c s i c redbed s e d i m e n t a t i o n in Morocco WARME J . E . : J u r a s s i c c a r b o n a t e and E a s t e r n High A t l a s r i f t ,

f a c i e s of Morocco

131

and T r i a s 139 the Central 169

BRECHBQHLER, Y . A . , BERNASCONI, R. and S C H A E R , J . - P . : J u r a s s i c s e d i m e n t s o f t h e C e n t r a l High A t l a s o f Morocco: Deposition, b u r i a l and e r o s i o n h i s t o r y

201

FROITZHEIM, N., STETS, J. and WURSTER, P.: A s p e c t s of !qestern High A t l a s t e c t o n i c s

219

JACOBSHAGE,~I, V , BREDE, R , HAUPTMA~IH, M HEINITZ,W and 7YLKA, R : S t r u c t u r e and p o s t - P a l a e o z o i c evolution o f the c e n t r a l High A t l a s

245

Vl FRAISSINET, C., ZOUINE, E.M., MOREL,J.-L., POISSON, A . , ANDRIEUX, J . and FAURE-MURET, A.: S t r u c t u r a l e v o l u t i o n of the southern and n o r t h e r n Central High A t l a s in Paleogene and M i o - P l i o c e n e times

273

DRESNAY, R. du: Recent data on the geology of the M i d d l e A t l a s (Moro, cco)

293

HERBIG, H.-G.: Synsedimentary t e c t o n i c s in the Northern Middle A t l a s (Morocco) during the Late Cretaceous and Tertiary

321

SCH!,/ARZ, G. and WIGGER,P.J. Geophysical s t u d i e s of the e a r t h ' s c r u s t and upper mantle in the A t l a s system of Morocco

339

CENOZOIC BASINS

359

G~RLER, K., HELMDACH, F . - F . , GAEMERS, P., HEISSIG, K., HINSCH, W., M#DLER= K., SCHWARZHANS, ~I. and ZUCHT, M. The u p l i f t of the c e n t r a l High A t l a s as deduced from Neogene c o n t i n e n t a l sediments of the Ouarzazate prov i n c e , Morocco

361

HELMDACH, F . - F . : The ostracode fauna of the A i t region: systematic description

405

Kandoula

ST~BLEIN, G.: Geomorphological aspects of the Quaternary e v o l u t i o n of the Ouarzazate basin, Southern Morocco

433

SCHMIDT, K . - H . : Rates of s c a r p - r e t r e a t : neotectonic activity

445

a means of d a t i n g

SYNTHESES

463

DURAND-DELGA, M. and OLIVIER,Ph.: E v o l u t i o n of the Aiboran block margin from E a r l y Mesozoic to Early Miocene time

465

JACOBSHAGEN, V., G~RLER, K. and GIESE, P.: Geodynamic evol u t i o n of the A t l a s System (Morocco) in p o s t - P a l a e o z o i c times

481

Introduction

GEODYNAMIC EVOLUTION OF THE ATLAS SYSTEM, MOROCCO: AN INTRODUCTION Volker Jacobshagen I n s t i t u t fur Geologie, Freie Universit~t Berlin, A l t e n s t e i n s t r . 34 A, D-lOO0 Berlin

33

The Atlas System The northwestern margin of Africa, between the A t l a n t i c Ocean and the Lesser S y r t i s , is characterized by high mountain ranges, the elevation of which surmounts 3.000 m in the western part. Jbel Toubkal (4.165 m) in the High Atlas is the highest mountain of North Africa. These ranges have been synchronously u p l i f t e d since the Miocene. They d i f f e r , however, both in structure and geological

history. The Maghre-

bides consisting of the Rif and Tell Atlas along the Mediterranean coast are a typical Alpidic orogen with complex nappe structures. The ranges in t h e i r southern foreland, called the Middle and High Atlas, are anorogenic intracratonic mountain belts separated by elevated plateaus. The eastern prolongation of the High Atlas is formed by the Saharan and Tunisian Atlas. The Anti-Atlas, however, which l i e s south of the High Atlas in Morocco, originated by simple updoming of the Precambrian basement of the Sahara craton. As a l l these ranges are geographical

andgeological related to each other they have been named

the Atlas system by M. RICHTER (1970). Thus, we find a mobile zone in the southern foreland of the Maghrebide orogen, which in southern Morocco extends up to 500 km into the African plate. As the southern border of this zone diverges more and more from the Maghrebides to the southwest, we may assume that i t s geological evolution cannot be solely deduced from the Mediterranean orogeneses, but seems to be more complex.

Geotectonic subdivision of Morocco The Moroccan segment of the A t l a s For an o v e r v i e w , the p u b l i c a t i o n s FAURE-MURET (1962,

system is r e l a t i v e l y

of MICHARD (1976),

well

known.

CHOUBERT and

1971, 1973) and FAURE-MURET and CHOUBERT (1971) are

recommended. The g e o t e c t o n i c

s u b d i v i s i o n of the Moroccan A t l a s

system

is shown in f i g .

1 and is b r i e f l y described here:

SEA

0 0 \

0 o

x,,

£o$~bic((Ica

on

o

o

o

o

ii;iC~i~

~

o

re=l

o

~

I

~~

•

'~'

:~$~Z1

HAUTES PLATEAUX

"v

x,, "t"

ZOO*

~!iiiiiiii!!iiiiy

' ....

[3 ......

~

e=aeoz=c

[~

UpperProterozoic

Upp,r Ce,nz0ic vat0n,,0es

R.-uoont=°,

Post- Horcynioncover:

f--I

pi,te,u,

(~

Highend Middlektles

Precambrian [~

UpperCeonzoicbasins

Fig. I : Geotectonic sketch-map of the Atlas system of Morocco, based on MICHARD (1976) The narrow arc of the Rif orogen in the north is overthrust towards the S on an Upper Cenozoic marginal trough. To the SW, the orogenic

belt is bordered by the h i l l s and plains of the Moroccan Meseta. Its Variscan basement is exposed in large areas from under a cover of f l a t - l y i n g Mesozoic and Palaeogene sediments. To the SE, the Meseta and the Rif are bordered by the High and the Middle Atlas. Both ranges developed from early Mesozoic r i f t grabens which originated in correlation to the opening of the Western Tethys and of the North Atlant i c Ocean. Rifting was accompanied by mafic volcanism, which culminated in the Upper Triassic/Liassic and in the Dogger. Some intrusions, however, are of Cretaceous and even Palaeogene age. The structure of the High and Middle Atlas generally correspond to each other, both showing a basement of Palaeozoic or Precambrian rocks that is covered by thick Triassic and Jurassic sediments. Prevailing structures are steep f a u l t s , monoclines and s t r i k e - s l i p f a u l t s . In places, especially in the central and eastern High Atlas, overthrusts, box folds and even overturned folds e x i s t . However, essential c r i t e r i a of orogens such as: ophiolites, nappes, flysches, regional metamorphism and granitoids are not developed. Both ranges are accompanied by a few small Cenozoic basins. The Middle Atlas forks o f f from High Atlas in a northeastern d i r e c t i o n , enclosing the so-called High Plateaus of Oran. The Variscan basement of these plateaus is covered by Mesozoic sediments, which are in the southwest overlain by the Cenozoic of the Haute Moulouya. The High Atlas is separated from the Anti-Atlas by the Souss and the Ouarzazate basins ( " s i l l o n pr~africaine"). As suggested by RUSSO and RUSSO (1934), many geologists believe that the High Atlas is bordered to the S by an important f a u l t zone, known as the South Atlas Marginal Fault zone ("accident sud-atlasien"). Although several authors doubt that this f a u l t r e a l l y exists, others have already introduced i t to plate tectonic concepts. Some of them even believe i t to be the northwestern boundary of the African plate (e.g. DEWEY e t a l . , 1973). The Anti-Atlas was formed by wide-spanned updoming of marginal parts of the Sahara craton. Precambrian rocks form the core of major domes ("boutonnieres") in the axial zone of the range. Normally, this basement is covered by poorly deformed sediments of Uppermost Proterozoic or Lower Palaeozoic age. In the T a f i l a l t region (Southeast Morocco), the Anti-Atlas dips to the E beneath undeformed Cretaceous sediments of the Sahara platform.

Pliocene or even Quaternary volcanoes of alkaline character are widespread in Morocco, with the exception of the High Atlas. Their geodynamic meaning is not yet c l e a r l y understood. The structural development of Morocco south of the Rif and probably also the Mesozoic magmatism were preordained by a pattern of major f a u l t s in the pre-Mesozoic basement, the main directions of which are NE and ENE (DUBOURDIEU 1962, MATTAUER et a l . 1972) and, in addition, NNW and ESE. According to these authors, the f a u l t s are of late Palaeozoic o r i g i n . In the central Anti-Atlas, however, the same pattern o r i ginated as soon as the end of the Panafrican cycle (HEINITZ 1984).

Aspects of the geodynamic evolution Our present geoscientific knowledge of the four ranges of the Atlas system d i f f e r s widely. The Maghrebides have been intensely investigated for many decades. Usually they have been considered within the context of the alpine orogens of the Mediterranean region. To describe t h e i r structure and geologic history with some detail would f i l l

a separate

volume. The present book aims, however, mainly to shed some l i g h t on the development of the intracontinental A~las ranges, and to define the general relations between them. Most of the authors have interpreted t h e i r geodynamic evolution with respect to major plate tectonic events of the Mesozoic and Cenozoic periods. Under this aspect the Rif orogeny was only b r i e f l y reviewed, here, in connection with the alpine development of the Western Mediterranean region by DURAND-DELGA and OLIVIERo The geodynamic history of the Atlas system started very early. Within the Precambrian cores of the Anti-Atlas, traces of several orogenies are preserved (see WALLBRECHER, this v o l . ) . Recent studies have been mainly focussed on the Panafrican cycle for which plate tectonic models have been proposed by LEBLANC (1981), SCHERMERHORN et a l . (1986), and BRABERS ( t h i s v o l . ) . But also many new results concerning the Late Proterozoic and Lower Palaeozoic cover of the Anti-Atlas have been produced, which are referred to by BUGGISCH and FLOGEL, SDZUY and GEYER and by BUGGISCH and SIEGERT in the f i r s t part of this book. The very important Caledonian-Hercynian history, which is documented in the basement of the High and Middle Atlas and of the Moroccan Meseta, has recently been discussed during a symposium "Morocco and

Palaeozoic Orogenesis (I.B.C.P. Project No. 27, The Caledonide Orogen), held in Rabat in 1983. This epoch is, therefore, neglected in the present volume. A great many of recent investigations have been concentrated on post-Palaeozoic

stratigraphy, facies and structural evolution of the

Western and Central High Atlas mountains, but our knowledge of the Middle Atlas is at present r e l a t i v e l y poor. As both ranges play a key role in deciphering the Mesozoic and Cenozoic evolution of the Atlas system, the majority of contributions to this volume deal with these topics. Seismologic, geoelectric and magnetotelluric studies on a section across both ranges by SCHWARZ and WIGGER are of special

im-

portance for the understanding of geodynamic processes. The neotectonic history of the Atlas system is clearly reflected by the sedimentary f i l l i n g s of the Upper Cenozoic basins adjacent to the Atlas ranges as demonstrated mainly by the contributions of GUREER et al. and FRAISSINET et a l . , but also by geomorphologic features. Recent geotectonic hypotheses interpreted the evolution of the intracontinental Atlas ranges with respect to plate tectonic configurations and processes (MICHARD et al.

1975, MATTAUER et al. 1977,

COURBOULEIX e t a l . 1981, STETS and WURSTER 1981). All of them referred to the fundamental fractures of the pre-Mesozoic basement. I t is gene r a l l y agreed that these fractures have been repeatedly reactivated since the Triassic, while the orientation of the continental stressf i e l d s changed in time, due to the r e l a t i v e movements of the African and the European plates. MATTAUER et al. (1977) were of the opinion that the post-Palaeozoic development r e f l e c t s two major geotectonic processes of changing influence: the opening of the Atlantic and the collisions of the West Mediterranean regions. Deviating from this view, STETS and WURSTER (1981) proposed a geotectonic model for the High Atlas which is related only to the Atlantic opening and excluded Mediterranean influences. JACOBSHAGEN, GURLER and GIESE (this vol.) support MATTAUER's view in reference to new geological and geophysical data, they also suggest a mechanism to transmit compressional forces from the active northern margin over several hundred kilometers into the African plate. Acknowledgements: The manuscript would not have been realized without the help of W. Jung, G. Lindemann, D. Reich and C. Di Stefano which is g r a t e f u l l y acknowledged.

REFERENCES CHOUBERT, G. and FAURE-MURET, A. (1962): E v o l u t i o n du domaine a t l a s i aue marocain depuis le temps D a l 6 o z o i q u e . - Livre-m~m. P. F a l l o t , M~m. h. ser. Soc. g~ol. France, 1: 447-527, P a r i s . (1971): A f r i q u e o c c i d e n t a l e : socle or#cambrien et zones m o b i l e s . A n t i - A t l a s ( M a r o c ) . - in UNESCO, Tectonique d ' A f r i q u e , Sci. T e r r e , 6: 163-175, P a r i s . (1973): Moroccan R i f . CER ( e d , ) : Mesozoic - Cenozoic orogenic b e l t s : Soc. London.

in A.M. SPEN37-46, Geol.

COURBOULEIX, S., DELPONT, G. and DESTEUCQ, CH. (1981): Un grand d~crochement e s t - o u e s t au nord du Maroc ~ l ' o r i g i n e des s t r u c t u r e s p l i s s # e s a t l a s i q u e s . Arguments g#ologiques et e x p ~ r i m e n t a u x . B u l l . Soc. g#ol. France, ( V I I ) , 23: 33-43, P a r i s . DE~JEY, J . F , , PITMAN, ~J.C., RYAN, W.B. and BONNIN, J. (1973): Plate t e c t o n i c s and the e v o l u t i o n of the a l p i n e system. B u l l . g e o l . Soc. Amer., 84: 3137-3180. DUBOURDIEU, G. (1962): DynamiQue wegen#rienne de l ' A f r i q u e du Nord.L i v r e M~m. P. F a l l o t , M#m. h. ser. Soc. g~ol. France, I : 627644, P a r i s . FAURE-MURET, A. and CHOUBERT, G. (1971): Le Maroc. Domaine r i f a i n et a t l a s i q u e . - in UNESCO, Tectonique de l ' A f r i q u e , Sci. T e r r e , 6: 17-46, P a r i s . HEINITZ, W. (1984): Die Deformation des sediment~ren Deckgebirges im z e n t r a l e n A n t i - A t l a s (SUdmarokko).- B e r l i n e r geowiss. Abh., (A), 55: 1-84, B e r l i n . LEBLANC, M. (1981): O p h i o l i t e s pr6cambriennes et g?tes a r s e n i c s de c o b a l t (Bou Azzer - M a r o c ) . - Notes M#m. Serv. g~ol, Maroc, 280: 1-306, Rabat. MATTAUER, M., PROUST, F. and TAPPONIER, P. (1972): Major s t r i k e - s l i p f a u l t s of Late Hercynian age in Morocco.- Nature, 237: 160162. MATTAUER, M., TAPPONIER, P. and PROUST, F. (1977): Sur les m~canismes de f o r m a t i o n des cha?nes i n t r a c o n t i n e n t a l e s . L'exemple des cha?nes a t l a s i q u e s du Maroc.- B u l l . Soc. g#ol. France, ( V I I ) , 19: 521-526, P a r i s . MICHARD, A. (1976): Elements de g # o l o g i e m a r o c a i n e . - Notes M~m. Serv. g ~ o l . , 252: 1-408, Rabat. MICHARD, A., WESTPHAL, M., BOSSERT, A. and HAMZEH, R. (1975): T e c t o n i oue de blocs dans le socle a t l a s o - m ~ s # t i e n du Maroc: Une n o u v e l l e i n t e r p r e t a t i o n des donn~es g~ologiques et pal~omagn ~ t i a u e s . - Earth P l a n e t . Sci. L e t t e r s , 24: 363-368, Amsterdam. RICHTER, M. (1970): Das A t l a s - S y s t e m in N o r d a f r i k a . 120 (1968): 68-84, Hannover.

Z.dt.geol.

RUSSO, P. and RUSSO, L. (1934): Le grand a c c i d e n t s u d - a t l a s i e n . Soc. g#ol. France, (V), 4: 375-384, P a r i s .

Ges., Bull.

SCHERMERHORN, L . J . G . , WALLBRECHER, E. and HUCH, K.M. (1986): Der Subd u k t i o n s k o m p l e x , G r a n i t o p l u t o n i s m u s und S c h e r t e k t o n i k im Grundgebirge des Sirwa-Doms ( A n t i - A t l a s , Marokko).- Berliner qeowiss. Abh., ( A ) , 66: 301-332, B e r l i n . STETS,. J . , and WURSTER, P. (1981): Zur S t r u k t u r g e s c h i c h t e des Hohen A t l a s in Marokko.- Geol. Rdsch., 70: 801-841, S t u t t g a r t .

Anti-Atlas

[HE ANTI-ATLAS

SYSTEM:

AN OVERVIEW

Eckard Wallbrecher for Geologie und Paleontologic Universit~t Graz Heinrichstr. 26, A-8010 Graz

Institut

General Situation The Anti-Atlas System, (Fig.

which lies between the High Atlas and the Sahara

i) is a part of the perieratonic

(MICHARD 1976).

sis of the High Atlas, time,

zone of the West Sahara Craton

In the Alpine age, which was the time of the orogenethis system acted as a stable crust.

it was transgressed

an Upper Precambrian

only by the flat Cenoman-Turon

to Lower Paleozoic

sea.

At this It has

cover which was partly folded

during Varisean orogeny. The typical structures

of the Anti-Atlas are the numerous~

often deeply

eroded basement domes which break through the covering rocks as "boutonni@res"(button

holes).

These structures

are lined up in a chain

which runs from WSW towards ENE and thus follows the NW margin of the West Sahara Craton. The Anti-Atlas, Ougnate,

including the eastern branches Jbel Saghro and Jbel

forms the "l'@pine dorsale",

with peaks of 2500 m to 2700 m

height in Jbel Saghro and 3300 m in Jbel Sirw@. ever,

is formed by a Ponto-Pliocene

crystalline uplift. The Anti-Atlas is separated molasse basins:

The latter peak, how-

volcanic complex which lies on a

from the High Atlas by young intramontane

the Souss Basin in the west and the Warzazat and Er-

Rachidia Basins in the east. To the south, the Anti-Atlas borders on the Jbel Bani chain which is built up by the cuestas of a flat southerly dipping Cambro-S~lurian series.

Further to the south follows the Dra Plain and the late Paleo-

zoic layers of Jbel Ouarkziz,

which dip under the Meso- and Cenozoic

Hamadas of the West Sahara. The Basin of Tindouf forms a syneclise a sedimentary

on the West Sahara Craton with

filling of more than lO,O00 m depth.

The Boutonni@res The crystalline domes of the Anti-Atlas

form the northern border be-

tween the West Sahara Cr@ton and the Paleozoic

and Cenozoic mountains

14

ALGI[RS

SAF [ANGER

Anh-Atlas

Rif Atlas Touareg Shield

Westsahara Cralon

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HARRAt(EEH

o..J

[talon

~Slo Luis £raton

~,,

•

Brazil

sOO,,,,k, Archean J.B..@el Bani T.6. Tindouf 6asia

Basin Tindouf f

~

/

,b~"~

S.A,F,SoulhAttasFauU CenozoicHotassetrouths ~'T] Upper Paleozoic ~ Lover Pateozoic

~ ~

•~ ~'

_ ~-

~

~0

,,~.~

~G~%,

Hamada

~

t~ '~E 200 km

Upper Infracambrium and Georgian BasementUplifts: B.O.i.BaSlfnOra i X, Kerdous Z. Zenaga Si. Sirra 8.k. Bou Azzer Sa. $argho Ou, Oognal

Figure i: Main structure of the northwestern margin of the West Sahara Craton of North Africa. CHOUBERT (1963) differentiated six orogenic cycles, among them

15

l)

Precambrium 0 This includes,

for example,

(Boutonniere de Kerdous) niere de E1 Graara).

the augengneisses

of Jbel Ouiharem

and the gneisses of Oued Assemlil

According

to CHOUBERT,

(Bouton-

these series belong

to the "Zagorides". 2)

Precambrium

I

This includes the micaschists tites of the Z~naga Series.

of the Kerdous Series and the migmaCHOUBERT calls these series the "Ber-

berides" 5)

Precambrium

II

To this belong epi- anA pyroclastic Azzer,

3bel Sirwa,

layers and greenstones

and Jbel Saghro.

of Bou

CHOUBERT calls these series

"Marrocanides". 4)

Precambrium

III

This includes the Warzazat Series, Infracambrian (Panafrican)

age.

which is of late Precambrian

It is considered

orogenesis.

to

to be a molasse of the last

It forms the base of the non-metamorphic

covering layers.

Radiometric Dating New rubidium/strontium in a separation

dating,

carried out by CHARLOT

between a metamorphic

volcano-sedimentary

and granitic

(1982),

foundation

resulted and a

cover.

Foundation The foundation

is formed by the chain of boutonni@res

and runs parallel

to the Dorsale de L~o in the south and to the Dorsale de Reguibat in the central part of the craton

(Fig. 1).

Only the western margin,

which begins at the western end of the Ivory Coast and leaves the African continent crystalline

somewhere near Rio de Oro, is of Archean age.

domes of the Anti-Atlas

All of the

are east of this line, and, there-

fore, outside of the Archean core of the craton. A second metamorphic granite),

event,

dated at 1.80 - 1.65 x i09 years

(Taznakht

took place as a thermic event which produced the metamorpho-

sis of the Azguemerzi

granite.

to the Dorsale de Requibat

The same age has also been attributed

(VACHETTE et al. 1973).

CHARLOT has pointed out that these events took place at about the same time as the Hudsonian orogeny in North America, Svekofennides Thus,

in Scandinavia,

the Anti-Atlas

the orogeny of the

and the Laxfordian orogeny in Scotland.

might be a link between the West African Craton and

North America or Northern Europe.

16

According to CHARLOT,

the Precambrium

II unit is probably Panafrican,

that is, about 700 - 600 x lO 6 years old. dating on zircons,

CHARLOT showed,

using U/Pb

thit no new thermic event has rejuvenated

these

rocks. The Cover The lowest of the covering layers is the Warzazat Series.

In Jbel

Sirwa and Jbel Saghro this series consists of ignimbrites, rhyolites, and conglomerates.

Granites and volcanites

have been dated at 604 and

560 x lO 6 years (CLAUER 1976, CLAUER & LEBLANC

1977, CHARLOT 1982).

Above the Warzazat Series follows the Adoudounien Series, an angular unconformity. to Georgian age. erates

It is considered

sometimes with

to be of late Infracambrian

The Adoudounien Series consists of a base of conglom-

followed by carbonates,

marls,

sandstones,

and,

finally,

carbon-

ates again. Volcanites

which are embedded in the lower carbonate

Azzer have been dated at 534 x lO 6 years. known. however,

In the Boutonni@re

layers at Bou

Later thermic events are not

de Kerdous and in the Boutonni@re

d'Ifni,

Variscan closing ages for biotite of 325 x lO 6 and 260 x lO 6

years, respectively,

have been measured.

Variscan elements of impor-

tance can only be found south of the Antiatlas chain in the monoclines of Jbel Bani and Jbel Ouarkziz.

They show NE-SW striking structures

the southern slope of the Anti-Atlas

on

in the SW and NW-SE striking struc-

tures on the slope of the Ougarta chain in the SE. Permian

to early Mesozoic extensional

opening of the Atlantic,

contributed

tectonic events,

caused by the

long doleritic dyke systems run-

ning SWINE. The longest of these systems shows its southernmost

outcrop

in the Jbel Ouarkziz and can be followed for more than 400 km as it cuts through the Bou Azzer structure and ends in the northeastern part of Jbel Saghro.

The youngest geodynamic

Tertiary-Quaternary

activity seems to have been a

elevation of the central part of the Anti-Atlas

an important Ponto-Pliocene

and

volcanism of Jbel Sirwa and eastern Jbel

Saghro. REFERENCES CHARLOT, R. (1982): Caract@risations des @v@nements @burn@ens et panafricains dans l'Anti-Atlas marocain. Apport de la m@thode g@ochronologique Rb-Sr. Notes et m@moires du Service G@ologJque du Maroc 313:106 p., Rabat. CHOUBERT, G. (1963): Histoire g@ologique du Pr@cambrien de l'AntiAtlas. (Tome 1). Notes et m@moires du Service G@ologique du Maroc 162:352 p., Rabat.

17

CLAUER, N. (1976): G@ochimie isotopique du strontium des milieux s@dimentaires. Application ~ la g@ochronologie de la couverture du craton ouest-africain. Th@se Doct. Etat, Univ. Strasbourg. CLAUER, N. & LEBLANC, U. (1977): Implications stratigraphiques d'une @tude g@ochronologique Rb-Sr sur m@tas@diments pr@cambriens de Bou Azzer (Anti-Atlas, Maroc). Notes et m@moires du Service G@ologique du Maroc 268: 7-22, Rabat. MICHARD, A. (1976): El@ments de G@ologie Marocaine. Notes et m@moires du Service G@ologique du Maroc 2 5 2 : 4 0 8 p., Rabat. VACHETTE, M; SOUGY, J.; CARON, J.P.; MARCHAND, J.; SIMON, B.; TEMPIER, C. (1973): Ages radiom~triques Rb/Sr de 2000 ~ 1700 M.A. de s@ries m@tamorphiques et granites intrusifs pr@cambriens dans la partie N e t NE de la dorsale Reguibat (Mauritanie septentrionale). C. R. 7e Collogue G@ologie Africaine, Florence.

A DUCTILE

SHEAR ZONE IN THE PANAFRICAN

MARGIN OF THE WEST AFRICAN CRATON

BASEMENT

ON THE NORTHWESTERN

(SIRWA DOME, CENTRAL ANTI-ATLAS

Eckard Wallbrecher fdr Geologie und Pal~ontologie Oniversit~t Graz Heinrichstr. 26, A-8010 Graz

Institut

ABSTRACT A wrench-fault Precambrian

type shear zone of at least 6 km width was found in the

basement

It is explained

of the Jbel Sirwa

by lateral

mass along the northern

slip of an (unknown)

folds with axes that steepen to a (dextral

in granitic

continuously

tachylitic

veins indicate

whole complex

high strain rates. ly Paleozoie).

Morocco).

continental

seismic

gliding

until they are The grain direction,

in superplastic

of the quartzes.

events after the plastic by either

the end of shearing,

The defor-

producing

in a northern

culminated

This must be explained towards

northwards

shear stress.

that increased

parent rocks,

mation with prism and grain boundary of the fabric.

northern

from south to north,

?) horizontal

fabric suffered mylonitization and that,

Anti-Atlas,

margin of the West African Craton.

mation in the shear zone increases perpendicular

(Central

deforPseudo-

softening

an elevation

or by occasionally

of the very

The age of the shear zone is late Panafrican

(Ear-

RESUME Dans le socle pr@cambrien une zone de cisaillement faille verticale mouvement

lateral

drant des plis, perpendiculaires

dana cette zone s'intensifie dont les axes se redressent @ une contrainte

prismatiques

litiques

indiquent

Marne),

de 6 km a produit

une

comme le r~sultat

do

(inconnue)

le long de la

vers le nord jusqu'& (?) dextrale.

du sud au nord.

pour atteindre

@tre

De m@me,

I1 s'intensifie

au

le degr@ de d@formations

sur lea joints des grains et sur les

des min@raux

l'existence

du sud au nord, engen-

horizontale

augmente

avee glissement

surfaces

central,

du craton ouest-africain.

des roches granitiques

superplastiques

On l'explique

d'une masse continentale

le degr@ de mylonitisation centre

(Anti-Atlas

d'une largeur minimale

de d@crochement.

bordure septentrionale La d@formation

du Jbel Sirwa

de quartz.

d'&v@nements

Des veines pseudotachy-

sismiques

apr@s la d@for-

20

mation plastique. Ceci est explicable soit par une @l@vation du complex entier vers la fin du cisaillement soit par des d@formations occasionelles tr@s rapides. L'@ge de la tectonique est tardi-panafricain (Pal@ozoique inf@rieur). INTRODUCTION The crystalline dome of the northeastern Jbel Sirwa belongs to a chain of Preeambrian basement uplifts, arranged in SW-NE direction,

parallel

to the northwestern margin of the West African Craton (Fig. i). First mapping of this area, the

f

geographic position of ~hich is indicated by the arrow in Fig. l, was carried out by CHOUBERT (1965).

Nonmetamorphic sediments

in the south of the area were investigated by JEANNETTE & TISSERANT (1977).

A comparison

of this uplift with the structure of Bou Azzer was carried

: ]:

~~00,,

out by LEBLANC (1981), who also described tectonic observations and distinguised two tectonic

Figure l: Precambrian uplifts of the Antiatlas and position of the research area in the Sirwa Dome

phases, From these works and from our own observations (SCHERMERHORN et al., 1986) the following sequences have been separated. l)

These are, from south to north (Fig. 2)

The Lmakhzan Formation,

a foreland sequence of epl- and pyroelas-

tic sediments, containing basaltic dykes and sills. 2) The Tachoukaoht Gneiss Formation, a sequence of intermediate gneisses, amphibolites and blastomylonites. 5) The Tourtit Ophiolite Complex, an imbricated and tectonically deformed metamorphic ophiolite sequence of ultrabasic,

basic and, to a

small extent, acid rocks, containing large lenses of the highly de~ formed Tourtit Granite-Mylonite. 4) The sedimentary and volcanic cover which forms the Uarzazat Grou~, a sequence which consists mostly of conglomerates,

ignimbrites,

and

rhyolites. 5) Postteetonic granitic intrusions and the N-S striking Daroufarnou k ~ D

of porphyric granite.

21

"~"

• • Anmid

-e

• • :K~

•

• .Ait N e b d a s

•

•

•

e

•

/ J

/

e

e

•

/, • /•

•

:_ , . [ .

!

.

. < '

.

.

,

•

II

[;i~;'--I Ooleritic dyke

I

o

ot

Oorogfornou dykes ,

---:~ A m O S S I R

~

Volconic rocks(Ptiocene)

m4-'x-~T] POSt t ect onic gronites

,

- ~

'

I~TI •

%

x

x

!:

~

!

!

!

t

!

Ouarzoz.te system

I

I

I For~lllnd series Gronite mylooites

-.. •

"

~ "

"

'

V_"=- "

~

'

x

x

x

x

."

x

I~'10phioLites

".

~---------'d Tochoukocht gneiss /Asdr emt foult

....

• ::•

. .

•

•

/ - / .

/(~) Locntion of equol oreo diogrom /~f~rood Anzel- Askaoun

5: ,

.

.

•

.

.

.

. "

I0

7o25'1

17o35 '

I

t

I

I

15kin

Figure 2: Tectonic and lithological units (SCHERMERHORN eL al., 1986, simplified). Numbers indicate the position of the fabric diagrams.

The petrography

and geochemistry

tonic implications (1986).

These authors

quences

in the following

l)

Opening

continent

Initiation Development

Continent

the fore-arc

of a volcanic

collision, ophiolites, Craton.

in the ophiolites Lateral

of the se-

and an unknown

zone.

Ophiolite

Gneiss

Complex)

Formation),

and a back-arc

basin. which thrusted

the northern

under the island Metamorphism

arc,

continent

and under

and imbrication

and in the Tachoukacht

Gneisses

under

the margin

of

are generated

(island

geosuture

shear zone which will be described

tectonical

horizontal

are separated

upthrust

et al.

arc).

which gives rise

in this paper.

SETTING

The general Generally

Craton

(the Tachoukacht

slip along the thus developed

to a 6 km wide

TECTONICAL

subduction

arc

(the Tourtit

Formation)

the West African

ments

evolution

the West African

of a south-dipping

by a fore-arc

(the Lamakhzan

5)

the geotectonic

and their geotec-

by SCHERMERHORN

steps:

of an ocean between

2)

4)

describe

in detail

north of it.

3)

flanked

of these sequences

have been described

style of the area

or only weakly by a steeply

from the albite-gneisses

(Fig. 2) is rather

inclined

northward

southern

dipping

simple.

foreland

sedi-

and E-W striking

of the Tachoukacht

Gneiss Formation

22

in the north. Tourtit

A similar

Ophiolite

strongly

Complex

imbricated,

Granite-Mylonite

further

contains

the ophiolites

the imbrication.

this formation

north.

This complex,

the E-W striking

part of the area

cover of late Precambrian farnou

separates

which has a thickness

In the northern salan Valley),

thrust

from the which is

layer of the Tourtit

of lO0 to 300 m.

(generally

are overlain

to Infracambrian

parallel

to the Assif Am-

by a younger age

which

horizontal

did not suffer

All these units are cut by the N-S striking

Darou-

Oikes.

Numerous

posttectonic

these units.

granite

A penetrative

intrusions

foliation

from south to north and culminates nounced

vortex

structure

are also

pattern

found in all of

develops

in the ophiolites

continuously

producing

and extreme shear deformation

a pro-

in the Tourtit

Granite-Mylonite.

Macroscopic l)

Tectonic

The Lmakhzan

The layers

Observations

Formation

of the foreland

sediments

generally

gles of about 30 to 60 degrees

(Diagr.

folding

a

unique

in the area of Tizwat, fold pattern

cernable

(degree

distribution

of great circle

= 21%) ++

Up to now,

a)

Here we f i n d siltites

++)

+++)

(sheet

of thin-bedded

w h i c h are deformed slip.

1 km

T a g h d o u t ) +++.

NNW-SSE s t r i k i n g

layers

bands ( F i g .

dextral

+)

at three places.

of Tizwat

vertical

in

from south to north

in the Assif Tamazirt,

north

kink

strike

could be observed

these series T h e s ~ are,

NE at an-

is not dis-

shear zones with brittle separation

dip towards

l) +. In spite of some local

3),

to

indicating

The o r i e n t a t i o n

of

Diagram

1

The position of places of fabric measurements for the diagrams are marked by the numbers in Fig. 2. All parameters describing quantitatively preferred orientation and fabric shape are derived from WALLBRECHER ~979). Numbers 1 to 3 are the eigenvectors of the orientation tensor; s is the center of gravity. All map names refer to the "Carte du Maroc"~ scale 1:50.000.

23

the bedding planes and of corrugation

linea-

tions in these beds are plotted

;~Ni

?:

in Dia-

gramm 2 a) and b) + b)

near Bou Wals

(trail from Souq at Tlata to Azib-n-lriri, sheet Tachoukacht). Here, there is a sequence of vertical SW-NE striking

shear

planes with distances in the dm range between which some

Figure 3: Kink bands in siltites of the Lmakhzan Formation, north of Tizwat.

rocks of 1 to 2 m width were dextrally c)

in the Assif Lmakhzan,

kacht).

(sheet Tachou-

layers of the Lmakhzan Formation

in

and shows a dextral

along kink bands.

Diagramm 2a

+)

300 m south of the car trail

Here we find a lens of serpentinite which is imbricated

the sedimentary movement

rotated.

Diagramm 2b

In all diagramms inner small circles are the cones of confidence for a level of 99% and the outer small circles are the spherical aperture, a deviation measure (WALLBRECHER, 1979).

24

2)

The Tachoukacht-Gneisses

Steep upthrusts served

of these gneisses

in the Assif Lmakhzan,

Izgaran

near Amassin

onto the southern

in the Assif Tamazirt,

foreland

~as ob-

and at the Tizi

(Fig. 4).

At their southern margin,

the Tach-

oukacht

Gneisses

"

L!]

r200m

S

begin with a fold pattern which sketched

is

in Fig.

T. Gneiss

5.

8osnlt

Conglomerotes

Lmakhzon Form.

The fold axes are Figure

4:

Profile

at Tizi Izgaran

flat and arranged in E-W direction

.

The folds S

are strongly

asymetric

long vertical

or steeply

ward dipping

limbs.

ing enveloping steeply

surface

plane surfaces indicate

south-

The result-

surface

northward.

veloping

and have

dips

L

~

I

This en-

and the axial

of the folds

a southward

tectonic

movement. In equal area diagrams,

Figure 5: Fold pattern of the Tachoukacht Gneisses, Assif Lmakhzan, south of the car trail

the

fold axes of cm to dm range form

a

flatly

dipping

eastward ...............

cluster

with

a center of gravity at 970/27 ° (direction of dip and angle of dip, Diagram The foliation

3a).

strikes

E-W and shows a center of gravity

at

174o/66 ° and a beginning transition the cluster

from

distribu-

tion to a girdle distribution

(Diagram 3b).

Since eigenvector which belongs

c

i,

to the

Figure 6: Fold with flat axis, Gneiss, south of the car trail

Tachoukacht

25

80 O A T E N

17g O A T E N

i

]

~

• .

'~

Diagram 3a smallest foliation pattern,

,IL-

o

.

Diagram 3b

eigenvalue

of the orientation

with the center is arrBnged

of gravity

tautozonally

which is established

about 200 m width, transition,

is followed

tensor

(SCHEIDEGGER,

tautozonality

to the minor

fold axes.

in the Lmakhzan

Valley

to the north,

probably

the same (Diagram

of the foliation

1965),

of the fold axes cluster,

by a zone in which the center of gravity

poles is approximately coincides

°

4"

I

coincides

"

.

4a).

with an also steeper cluster

This fold

in a zone of with a floating of foliation

However,

is much steeper

the

the axis of

(67o/430 ) and again

of the minor

fold axes

(71°/49 °,

Diagram 4b).

\.

oi

.....

."

.

.

2 "

"

"/

26

An even steeper structures

arrangement

with beginning

lel to the foliation circle distribution Diagrams

of the fold axes in the style of vortex

transition

88

o

poles is demonstrated

the contact

6a and b (Lmakhzan

ation shows almost

of the ophiolites,

5b

this pattern

of foliation

Valley north of the car trail).

the same picture

as before,

..

6a

"

continu-

and fold axes shown in The foli-

but the fold axes now

""

Diagram

by

DATEN

Diagram

ously turns into the arrangement Diagram

paral-

(great

•

5a

As we approach

pattern

near Tichkiwin.

2ZZ D A T E N ~

Diagram

cluster

= 40%) of the foliation

5a and b, measured

•

to a girdle distribution

plane and a stronger

.

o

Diagram 6b

.-

. .

.'.- .....

.

~'~

~2

"I,

.

"

o

27

are scattered

to a girdle distribution

gree of great circle distribution A similar

setting

drem Pass

(Diagram

of foliation

in the plane of foliation

= 66%, circular

7a and b).

The flatter

tion and fold axes fabrics must probably

3211 D A T E N

~ _

.'..

"

Diagram

2

"

aperture

and fold axes was measured

-" ""

arrangement

be explained

(de-

= 99%). at the As-

of both foliaby a tilting

of

-!

7 a

Diagram 7 b

the Asdrem bloc along the younger which raised the high plateau

Asdrem

fault

(CHOUBERT,

of Asdrem upwards

1965)

in relation

to the

plain of Tamazirt. A true vortex

style of fold axes was observed

the Assif Tamazirt, This extremely plained 5)

NNE of Tizwat,

southern

position

sheet Taghdout

of vortex

by a dextral bloc movement

The Tourtit

The ophiolite lization cumulates

Ophiolite

sequence

parallel resisted

formed by extreme

Complex

the schistosity. structures

Diagram

of Amassin.

The foliation

also strike E-W.

The schistosity with vertical

intrusion

fold measures

suffered

ophiolites,

and recrystal-

planes are de-

fold axes (Fig. 7).

of schistosity

and fold axes part of the

from the region west

planes of the Tourtit

This granite

fault.

Only the cores of basic

lOa and b in the northern

ll contains

as did the surrounding

schistosity

planes.

The vergency of most folds is dextral. Diagrams 9a and b show the arrangement in the southern, and Diagrams

might be ex-

and the Tourtit Granite-Mylonite

shows a very strong

ophiolites.

(Oiagram 8a and b).

structures

along the young Asdrem

to the schistosity vortex

E of the Asdrem Pass in

Granite-Mylonite

the same tectonical

treatment

which proves that the granitic

must be older than the shear tectonic

event.

28

10~ OATEN

Diagram 8a

i~

Diagram 8b

Figure 7: Vertical folds in serpentinites, Assif Tasrwin south of Tourtit

1 17 OATEN

S i':.-:.:' Diagram £ a

Diagram 9b

--~~'

x

29

258

DATEN ~.~_

•

Diagram 4)

Diagram

lOa

The Warzazat

The younger northern

191

OATEN

margin on top of the crysuplift shows the same tec-

tonical

deformation

the basement, (Diagram

10b

Group

cover which lies at the

talline

nounced

,%

patterns

and less penetrative 12).

schistosity

The distance

planes

tex structures

as

yet in a less proform between

is greater,

vor-

are not found so

D

often,

and are concentrated

certain

zones.

In the conglomerates, tion of the pebbles observed ure 8).

Diagram

12

in

a disseccould be

in some outcrops

(Fig-

30

Figure 8: Sheared pebbles in the conglomerates of the Warzazat Group south o¢ Ait Nebdas

Macroscopic

tectonic

observations:

The two most important ferred orientation, G%,were Diagrams

derived

statistical

other in Figure

9, as proposed the development

very narrow clusters

by BEER

in a zone of transition

an-

4 to 7), and

ending in vertical

This plot very clear-

with a tectonical (Diagram

style of 3), pass-

along a girdle

1--

®i,

I~

Southern morgm

folds

distributions

8 to ll).

(Diagrams

each

Cluster

/

cluster

To explain

this tectonic

we presume

a system of lateral

pattern

which moved a

bloc along the margin

of the West

African

thus causing

a (dextral?)

fault zone. The Tachoukacht Tourtit

(1981).

against

in which

which again show narrow

shear stresses

were plotted

beginning

gles of dip in the foliation

northern

of fold axes in each of the

of the fold axes distribution

the axes lie with various

finally

the degree of pre-

of flat fold axes in the south

an elongation

(Diagrams

parameters,

These two parameters

ly demonstrates

plane

and conclusions

R%, and the degree of great circle distribution,

from the distributions

1 to ll.

ing through

summary

Gneisses

Ophiolites,

ready suffered

Craton wrench and the

-6%-~ ]20

I.~0

160

tO0

which al-

upthrusting

onto

Figure 9: R% - G%, measures fold axes, Diagrams i to Ii

of

81

the southern

foreland

(SCHERMERHORN striking

et al.,

mobile

developed southern a certain

extension

planes

which

then served

continent

transformed

of folds,

deformation

the system

into an E-W

overprinted

(in connection of N-S striking

to

of south-

The steep thrust

in the gneisses for lateral

and thus,

pattern

and imbrication.

the stress

of this zone

while the more marginal

the older tectonic

generated

collision

The center

less strongly

as shear planes

in E-W direction

caused

porphyry

pattern

of upthrusting

the end of the shear

lantic)

vortex

could preserve

planes

and foliation

were hereby

of the zone were

degree,

pentinites

1986),

during

zone of at least 6 km width.

a distinct parts

ern directed

and imbrication

and in the ser-

displacement.

At

field was rebuilt, and an

with the opening dykes

of the At-

of the Daroufarnou

granites.

THE GRAIN FABRIC l)

The Tachoukacht

At the microscopic patterns. Rests

Gneiss scale,

It occurs

of the former

southern

margin

the foreland.

of the Tachoukacht

the garnets

turing.

The old grains

cated

reacted

to the foliation.

in a small

rim at the

close to the upthrust

quartz brittle

to this stress

to schistose

+ plagioclase deformation pattern

onto fab-

+ bio-

caused

by external

quartz and primarily

textures

(Fig-

with a diaph-

which was indi-

in the new grains

the parsgenesis albite

fabric

This deformation

went together toresis,

At first,

all older

i0) and frac-

feldspar

ure ll).

gneisses

had the paragenesis

+ amphibolite.

(Fig.

formed mortar

parallel

can only be observed

In this small zone we find a gneissic

tite + garnet cata61asis;

superimposes

as a grain reduction fabric

ric which originally

rotation

mylonitization

+ epidote

quartz

by +

+ chlor-

ite. Towards

the north,

deformation

rapidly

came more plastic, Tachoukacht into ductile

the beand the

Gneisses

turned

protomylites

Figure lO: Externally rotated garnet clast with asymmetric pressure shadows (Tachoukacht Gneiss, 60 x, Nic. X)

32

and blastomylonites (sensu HIGGINS,

1971).

At the beginning, tic deformation restricted

plas-

was

to single

shear bands with characteristically

elonga-

ted quartzes whlch show low angle grain boundaries.

Between

these

shear bands there is an old grain assemblage of quartz and feldspar

clasts.

Recovery

patterns

the Quartz

They form mosaics

triple junctions

the genetically stadium

preceding

strain heterogeneity spar paleoclasts

grains with charac-

structures

with lobate grain boundaries.

1978).

point 4, there is an intensive went through

an external

This

of rigid

In areas with unusually

The blasts are packed very densely,

about 5 mmo and obviously

with

and with the following

must be caused by the distribution

(PRICE,

near measuring

albite blastesis.

of isometric

and exist in the same sample together mortar

of recrystallization

ing, i.e.,

in Tach-

fabric can

also be observed. teristic

Figure ll: Mortar quartz structures oukacht Gneiss, 30 x, Nic. X)

of

strong

feldfold-

synkinematic

reach a size of

rotation

(Fig. 12).

Figure 12: Synkinematic albite blast (Tachoukacht Gneiss, 80 x, Nic X)

33

2)

The Tourtit Ophiolite

In the ultrabasic ed metamorphic

Complex and the Tourtit Granite. Mylonite

and basic rocks of this complex,

recrystallization

which follows the foliation planes

and which covers a probably preexisting In contrast,

the leucocratic migmatite

parallel to the foliation ness of some centimeters

of the serpentinites

1980).

ation culminate The genuine

or mylonitization. which are arranged

and which have a thick-

are characterized

and strain homogeneity.

by a

In these bands of

rocks most of the strike-slip movement must have taken

place under the conditions (WHITE et al.,

cataclasis schlieren,

to a few meters,

strong ductile deformation quartz-feldspar

there is an extend-

of strain softening

The fabric homogeneity

and superplasticity and superplastie

deform-

in the lens of Tourtit Granite.

granitoid grain fabric first developed

into cataclasites,

remnants of which survived in some nests within the Tourtit Granite. Plastic deformation

turned these cataclasites

mylonites with very fine subgrain aggregates of feldspar.

Only a few feldspar

marginal disintegration

into mylonites

clasts still exist.

into subgrains

and ultra-

of quartz and neoblasts These show a

and a schlieren-like

arrange-

ment of very small new grains in the pressure shadows of the clasts. The quartz grains are strongly elongated of >5:1.

Only low angle grain boundaries

An intercalation lonitic

with an average ratio of axes are visible.

of layers of elongated new quartz grains with a my-

fabric of very small feldspar and quartz aggregates

can be observed.

Mylonites

(Fig.

15)

of this type are generated at high temper-

atures and relatively low stresses and strain rates

(BOULLIER & GUE-

GUEN,

1975).

Accord-

ing to these authors the ratio T/Tm between the temperature

T at

the time of deformation and the melting temperature Tm of the mineral phase

(feldspar)

must be >0.5.

In the

entire area investigated~the geometric

pat-

tern of the grain fabric is characterized

Figure 13: Intercalation of elongated quartzes and feldspars with strong grain reduction (Tourtit Granite-Mylonite, 13 x,Nic~X)

34

by the perpendicular is manifested

relationship

by elongation

of the stretching

lineation,

of quartz and by feldspar

to the fold axes of the vortex structures

which

lineation

in the neighboring

rods,

serpen-

tinites. This means that the serpentinites folding and forming dough"

of B-tectonites

deformation

shear stress,

reacted

(SANDER,

(LISTER & PRICE,

whereas

1950) in a "rolling

1978) with b-axes vertical

most of the shear movement

dered as a simple shear deformation relatively

to the shear stress only by

small leucocratic

which can be consi-

must have been concentrated

migmatite

the

to the in the

bands and in the Tourtit

Granite. In the Tourtit which contain ed

contacts

optical

Granite-Mylonites, rounded

grains of parent rock.

to the host rock

microscopic

glassy material

(Fig.

14).

They show sharp,

sinuat-

Even though we have only

data up to now, and there is no indication

in these bands,

pseudotachylitic

1-2 mm wide dark veins were found

the observations

of

point strongly

to

bands

with their characteristic injection

veins

(of. SIBSON,

1975;

ALLEN,

PASSCHIER,

1979;

1982).

Pseudotachy-

lites are explained

as

former rock melts generated by rapid melting which is caused by friction

fusion along

seismically

active

fault

(ALLEN,

zones

19797 ERMANOVICS et a l . , 1972; FRANCIS; 1972;

GROCOTT, 1977;

SIBSON, 1 9 7 5 , 1 9 7 7 ; The f a c t less

that

the

vertically

pseudotachylites servation zone.

F i g u r e 14: Pseudotachylite vein Granite-Mylonite, 10 x, N i c . / / )

through

veins

the mylonite

were g e n e r a t e d

1979).

are undeformed and c u t more or

after

bands c l e a r l y

shows t h a t

the m y l o n i t i z a t i o n .

should begin with

brittle

behaviour

may d e v e l o p and s h o u l d be f o l l o w e d

w h i c h can be s u p p o r t e d caused by h y d r a t i o n

by a r e d u c t i o n

along a fault

during

zone.

a shear

w h i c h pseudo-

by a d u c t i l e

of the effective

the

T h i s ob-

does n o t c o n f o r m to t h e d e v e l o p m e n t e x p e c t e d a t

This

tachylites

WATTS & WILLIAMS, injection

(Tourtit

reaction

normal s t r e s s

85

These observations a)

The entire ductilely deformed

crustal TULLIS

level with transition and YUND

is mostly b)

may be explained

(1972)

temperature

An aseiamic

fabric

zone was elevated

to brittle

showed

rate was interrupted

deformation

Quartz-c-axes

by seismic

into a higher

and plastic

In experiments

of rock material

with a transition

which took place in the deeper

ways:

behaviour.

that this behaviour

controlled,

ductile

in two different

at about 400 ° C.

softening

of the

crust at a very low strain

events

in greater

depth.

patterns

Method The quartz-c-axes tively

distribution

in two separate

section

were measured

tions of defined BRECHER,

1986),

tation which

ways. with

On the one hand,

orientation.

Using

the thin sections were plotted parameters

to this method 1978)

was used

distributions caused

an integral

and to register

field of view,

which

ing the polarizer, the analyzer

the microscope. intensity

photometric

to obtain

analyses

the thin section,

is measured

The method

of SANDER

at each degree

system

fields

may be comIn the

of the microafter pass-

fixed on the phototube

is turned

through

of

360 ° , and

of Fotometer

In order to carry out these measurements the apparatus

of PRICE

fied as shown in Figure measurements

ties and storage by a computer K6nigswinter, were plotted

(1980) 15.

Movement

Gypsum Plate

of

Thin Section

of the intensi-

,J-

Microscope Polarizer

for this purpose by

BOro C. H. HILGERS,

Germany.

The intensities

as rose diagrams

field of view.

Analyzer

was modi-

of data are controlled

built

the Technisches

represent

of the

plate of 550 nm, and

a gypsum

stage

method

(1950).

turning.

the stage,

and

of the strain

light transmitted

by a photodiode

The microscope

measuring

a quick overview

of rigid clasts.

of monochromatic

is measured

into the orien-

area projection

is fixed by the optical

the intensity

(WALL-

In this orientation

the heterogeneity

pared to the axial distribution scope,

were then rotated

in sec-

program

were calculated.

in order

by the distribution

of a thin

stage method

a small computer

into an equal

quantita-

200 grains

they would have had in the field.

the quartz-c-axes

(PRICE,

were registered

the usual universal

their statistical Parallel

patterns

These diagrams

the distributions

of quarz-c-

(517 rim)

/

for each thus

Filter

<

Stage/ Motor

/ Figure 15: Device for integral measurement of quartz-c-axes azimuths

36 axes azimuths in the thin section.

The method is quite quick (ap-

proximately one minute per rose diagram) and has the advantage that the distribution of quarz-c-axes in the individual domains of the fabric can be analysed. Results of the universal stage measurements Diagrams 13 and 14 show the distributions of quartz-c-axes in two samples of the Tachoukacht gneisses.

In addition,

the mylonitic fol-

iation (shear plane sf) of the samples and the axes of the finite ~

D

A

T

~

"~/~""

X

X

Diagram 13

Diagram 14

strain ellipsoid

(X>Y>Z) are plotted into the diagrams.

The Y-axis

is the direction of axes of minor folds at the place where the sample was t a k e n . Both

diagrams

show a p a r t i a l

a goodness-of-fit

to

a great

girdle

distribution

circle

distribution,of

(solid

lines)

with

G% = 61% and

G% = 89%, respectively. This means that they most closely fit the model "quartzite I with axial elongation" of LISTER et al. (1978).

Theoretically,

ric type should form a girdle around the X-axis.

this fab-

LISTER et al.

(1978) explain this distribution by lattice gliding with both basal slip along (O001) and slip along the rhomboedric planes{Olil I •

Since

the girdle distributions are not around X, gliding must have taken place on other lat~ee planes, too. Following the method of SIMPSON (1981), the position of the girdle distribution to the orientation of the finite strain ellipsoid is marked by the angles O(angle between Z and the direction of strike of the great circle) and ~ (angle between Y and the great circle,

37

measured

in (XY)),

distribution

in order

compared

to determine

with the

the obliquity

(YZ) plane

(LAURENT

of the girdle

& ETCHECOPAR,

1976). The divergence ally expected ferred

orientation

This means ple shear

( 5 5 o and

angle girdle

58 ° , respectively)

in (YZ) might

on which

the elongation

that the true deformation alone

(SIMPSON,

1981).

The older

during

the period

~hich

preceded

tectonic

respectively)

around

X which

became

so weak that only the mobile

forced

to arrange

mica

(LISTER,

accordance a quartz

took place

period.

could

itself

1977;

between

VOLLBRECHT,

fabric

is dependant

preferred

quartz

and in which

fabric

rotation the stress

was influenced

clasts

(1985)

on the content

(4 ° and

by a later

and

of feldspar

This explanation

of LISLE

tectonics

The declination stage

by sim-

orientation

of imbrication

the unmobile 1981).

with the observation

be explained

be explained

in a very late

pre-

in X was superimposed.

have been generated the shear

by a preexisting

path cannot

could

20 ° sinistrally,

from the theoretic-

be explained

and

is also in

that the strength

of other

minerals

of

in the

sample. A totally ite.

different

situation

Here we found cluster

was found

distributions

(Diagram

1S) and more pronounced

(Diagram

16).

have

tozonality a great

represented

DAT~

~

Diagram

with R% = 64% and G% = 46%

these

parameters

G% is especially

and less sensitive

(WALLBRECHER,

1986).

to equal

The preferred

by eigenvector

~3is,

X3

Diagram

correctly

sensitive

16

we

to tau-

distribution orientation

in Diagram

×

15

Granite-Mylon-

with R% = 51% and G% = 48%

clusters

to interpret

that the measure

in general

circle

quartz-c-axes,

2ZZ

In order

to consider

in the Tourtit

15,

along of

38

parallel

to the X-axis of the finite strain ellipsoid,

gram 15, at least close to the X-axes. represented

by the eigenveetors XI,

the strain ellipsoid quartz-c-axes

MAINPRICE Spain,

(1986)

sample

granitic

Since the same pattern can assume

et al.

The differing distributions tial girdles

(1984)

found a transition

temperature

were reached

(superplastic

The

of the crystals.

which ~ere,

at least,

of the fold pattern

and oblique

on the one side,

1975).

girdle and par-

Granite-My-

clear by synoptic

is rotated

high

Granite-Nylon-

& GUEGUEN,

in X in the Tourtit

is made especially foliation

we

that grain boundary

sensu 80ULLIER

gneisses,

strain

Granite-Mylonites,

of the quartz-c-axes

in the Tachoukacht

on the other,

by

from basal

or decreasing

It is also possible

around Y ~ith a center

in ~hich the mylonite

recently

In experiments,

This ~ould mean that the Tourtit

orientation

to the direc-

(750 ° C), and from

by the water content

that temperatures

ires are SP mylonites

As

veins in the Vosges Massif.

is seen in the Tourtit

enough to start prism slip. gliding took place.

of the

of 550 ° C to 750 ° C and a hydrostatic

with increasing

rate which is controlled

to

of the Oj@n nappe of southern

at high temperature

a temperature

(1975) and LINKER

lineation

parallel

of PH20 = 350 MPA for the second sample.

to prism gliding

lonites,

The elongation

of quartz-c-axes

from migmatites

from sheared

authors derive pressure

ellipsoid,

is parallel

(X) has only been described

which were deformed

another

BLACIC

elongation

et al.

%311X.

dots),

is around the Y-Axis in these cases.

far as we know, a clustering tion of maximal

~3(large

with ~llIY, ~ J Z ,

distribution

and in Dia-

The orientation

diagrams

in an E-W plane and the b-

into a vertical

position

(Diagrams

17

and 18).

880 OATEN

40B OATEN ~ Z

.__ ..-..-::-;..--..-:.:...

- , -

_

,_

:;/ . "....

7:~-" . 7 .

.

rl,

:.

,

\'" "<'~

Diagram 1.7 ~

Z

"

-

".

.

.

..'-;i~:- ~×

.... . " r . ; . . - ~ . 3 ; , '

•

:

•

•

.

Diagram 18 ' ~ X 2 0 - ~" -~

..

39

Results of the photometric The same thin sections the universal (15 domains

measures

in ~hich the quartz-o-axes

stage were also measured

per section).

ted as rose diagrams

~ere measured

~ith the photometric

The light transmission

(Fig.

16 and 17).

tion of the finite strain ellipsoid

~ith

method

spectra ~ere plot-

In these plots the orienta-

is the same as in Diagrams

17 and

18.

-

I~4 -

17

~

-

:17 - 17

-

-

sl -

L~

- IS

-

-

@©©©©

QQQQ Figure 16: Rose diagrams of integrally c-axes, Tachoukacht Gneiss.

measured

azimuths

of quartz-

©©©©© ©©©©© ©©©©© © © © © © ©©©©© ©©©©© 75-125-2@

75-121-2@

75-I l'7-Ig

75-113-2@

75-105-2@

75-124-7

75-I

75-i

75-IZ6-7

75-I@3-6

75-125-3

75-118-3

75-1~6-2

75-101-2

18-8

I@-7

75-1 tg-1

~S -

T@

- le5

7g

-

Figure 17: Rose diagrams of integrally c-axes, Tourtit Granite-Mylonite. In each domain, be explained selves differ ogeneity

the Tachoukacht

by oblique

girdles

from each other.

due to a high content

gneisses (Fig.

Iz5

. 17

-

1~

.

II

-

4

?s .

lls .

I~ .

7s

7@ -

l~g -

13

76

.

It~ .

76

llg

4

7@

-

113

-

measured

-

~12

17

-

7s

~ .

7~ .

I ~ 7.

I

7@

lAB

azimuths

-

of clasts of feldspar,

-

zs-

~.

N

I

7@

I~

-

I ~

them-

to strain hetermica,

and

garnet. In contrast,

the orientation

Tourtit Granite-Mylonites tion (Fig.

17).

tic softening

of quartz-c-axes

is almost parallel

distributions

This proves much higher strain-homogeneity

of the fabric.

in the

to the mylonitic

- :e

l~

which can

The orientations

This points strongly

t5

of quartz-

show distributions

16).

- l*e -

folia-

and plas-

q.

-

4

40

GENERAL CONCLUSIONS In the Sirra area of the Antiatlas,

a segment of the collision zone

between the West African Craton and a northern continental mass is exposed

(Fig. 18).

A first phase of deformation tion tectonics.

and metamorphism

brication of rock sequences. The enveloping With increasing

was caused by imbrica-

This caused a steep E-W striking schistosity and im-

surfaces

Fold axes were flat and stroke E-W.

of these folds dip steeply to the north.

lateral shear,

a wrench

fault system developed which

caused a broad shear zone of at least 6 km width.

The strike slip

shear stresses caused different strain patterns

in

N

the three lithological units involved.

Cataclas-

ites to protomylonites were generated in the marginal gneisses.

The mech-

. . . .

x

~lUmllnlM

~

I~

.

.

.

.

.

~ 6 r a n i r e

[ou~t~t

anism of initiation of ductile deformation

in

these rocks was mainly

z

basal slip in the quartz fabric and external

rota-

tion of the clastic minerals.

The serpentinites

reacted only by forming vortex structures

verti-

cal to the shear stress.

H.V.

/igmafiri£

P.T. F. G. M. O.

pseudotachylite fe|dspar |arnet mica quartz

wi.s

x.y,z: axes of finile s;'rain ellipsoid

Numerous E-W arranged migmatite

veins of quartz-

Figure 18: Synoptic sketch of structure patterns of the shear zone in the Sirwa

feldspar compositi.on and

the elongated band of area. Tourtit granite in the center of the zone transported most of the shear movement.

This was made possible by plastic to superplastic

simple shear deformation

of the quartz

probably caused a dehydration

assume a very high water pressure. tectic conditions

fabric.

This,

al strain softening

reinforced

In the entire shear zone,

so we have to

together with almost ana-

and very high flow stress,

and grain boundary mechanisms

High temperatures

of the serpentinites,

activated prism gliding

in the quartz fabric. the shear movement

these movements

This geometric-

in these zones.

added up to a considerable

41

lateral dislocation

of the northern continental

the plastic softening which slip movements bands.

Finally,

of the material might have reached a point at with seismic velocities

These formed pseudotachylite migmatitic

block.

became possible.

bands in the superp!astically

The geographic

range of this Panafrican

deformed collision

zone and of the ductile shear zone is not yet known. REFERENCES ALLEN HA (1979): Mechanism of frictional J. Struct. Geol. I: 231-293

fusion in fault zones.

BEER W (1981): Die strukturelle Entwicklung der Metamorphite Bayerischen Waldes. Diss. Univ. G~ttingen, 186 pp BLACIC JD (1975): Plastic deformation mechanism effect of water. Tectonophysics 27:271-294 BOULLIER AM; GUEGUEN Y (1975): Sp-mylonites: ires by superplastic flow. Contributions 50:93-104

in quartz:

des The

Origin of some mylonMineral. Petrol.

CHOUBERT G (1963): Histoire g@ologique du Pr@cambrien de l'AntiAtlas. Notes et m@m. du service g@ol. du Maroc 162:352 pp ERMANOVICS JF; HELMSTAEDT H; PLANT AG (1972): Archean pseudotachylite from southeastern J. Earth Sci. 9:257-265 FRANCIS PW (1972): The pseudotachylite Sci. (Geophys.) 3:35-53

An occurence of Manitoba. Canadian

problem.

Comments on Earth

GROCOTT J (1977): The relationship between Precambrian shear belts and modern fault systems. J. Geol. Soe. London 133:257-261 HIGGINS MW (1971): Cataclastic Pap. 687:97 pp

rocks.

U. S. Geol.

Surv. Prof.

JEANETTE D; TISSERANT D (1977): Les @pisodes tectoniques et intrusifs du Pr@cambrien sup@rieur de l'Anti-Atlas occidental. Estudios geol. 33:315-326 LAURENT P; E T C H E C O P A R A (1976): Mise en @vidence ~ l'aide de la fabrique du quartz d'un cisallement simple ~ d@versement ouest dans le Massif de Dora Maira (Alpes oceidentales). Bull. Soc. G@ol. France (S@rie 7) 18:1387-1393 LEBLANC M (1981): Ophiolites pr@cambriens et gites ars@ni@s de cobalt (Bou Azzer, Maroc). Notes et m@m. Serv. G@ol. du Maroc 280:306 pp Effects of compresLINKER MF; KIRBY SH; ORD A; CHRISTIE JM (1984): sion direction on the plasticity and rheology of hydrolytically weakened synthetic quartz at atmospheric pressure. J. Geophys. Res. 89:4241-4255 R (1985): The effect of composition and strain on Quartz fabric intensity in pebbles from a deformed conglomerate. Geol. Rundsch. 74:657-663 LISTER GS (1977): Discussion: Crossed girdle c-axes fabrics in quartzites plastically deformed by plane strain and progressive simple shear. Tectonophysics 3~9:51-54

LISLE

42

LISTER GS; PETERSON MS; HOBBS BE (1978): The simulation of fabric development in plastic deformation and its application to quartzite: The model. Tectonophysics 45:107-158 LISTER GS; PRICE GP (1978): Fabric development mylonite. Tectonophysics 49:37-78

in a quartz-feldspar

MAINPRICE D; BOUCHEZ JL; BLUMENFELD P; TUBIA JM (1986): Dominant c slip in naturally deformed quartz: Implications for dramatic plastic softening at high temperature. Geology 14:819-822 PASSCHIER CW (1982): Pseudotachylite and the development of ultramylonite bands in the Saint-Barth@imy massif, French Pyrenees. 3. PRICE

....

Struct.

Geol.

4:69-79

GP (1978): Study of heterogenous fabric and texture within a quartz-feldspar mylonite using the photometric method. Geol. Soc. Am. Bull. 89:1359-1372 (1980): The analysis of quartz c-axes metric method. J. of Geol. 88:181-195

fabrics by the photo-

SANDER B (1950): EinfOhrung in die GefOgekunde K~rper. 409 pp, Springer, Wien

der geologischen

SCHEIDEGGER AE (1965): On the statistics of the orientation of bedding planes, grain axes, and similar sedimentological data. U. S. Surv. Prof. Pap. 525C:C164-C167 8CHERMERHORN LJG; WALLBRECHER E; HUCH M (1986): Der Subduktionskomplex, Granitplutonismus und Schertektonik im Grundgebirge des Sirwa-Doms (Anti-Atlas, Marokko). Berliner Geowiss. Abh. (A) 66:301-322 SIBSON RH (1975): Generation of pseudotaehylite by ancient seismic faulting. Geophys. J. R. Astron. Soc. 4~3:775-794 SIMPSON C (1981): Ductile shear zones: A mechanism of rock deformation in the orthogneisses of the Maggia Nappe, Ticino, Switzerland. Diss. ETH ZOrich, 265 pp TULLIS J; YUND RA (1977): granite. J. Geophys.

Experimental deformation Res. 82:5705-5718

on dry Westerly

VOLLBRECHT A (1981): Tektogenetisehe Entwicklung der MOnchberger Gneismasse (QuarzgefOge-Untersuchungen und Mikrothermometrie an Fl~ssigkeitseinschl~ssen). G6ttinger Arb. Geol. Pal~ont.2_44:122 pp WALLBRECHER E (1979): Methoden zum quantitativen Vergleich von Regelungsgraden und -formen strukturgeologischer Datenmengen mit Hilfe von Vektorstatistik und Eigenwert-Analyse. N. Jb. Geol. Pal~ont. Abh. 159:113-149 .... (1986): Tektonische und gefOgeanalytische Arbeitsweisen: Grafische, rechnerische und statistische Verfahren. 240 pp, Ferdinand Enke, Stuttgart WATTS MJ; WILLIAMS GD (1979): Fault rocks as indicators of progressive shear deformation in the Guigamp region, Brittany. O. Struct. Geol. 1:323-332 WHITE SH; BURROW SE; CARRERAS J; SHAW ND; HUMPHREY FJ (1980): On mylonites in ductile shear zones. J. Struct. Geol. Z:175-188

TECTONOTHERMAL

EVOLUTION

IN T H E C E N T R A L

OF THE LATE P A N A F R I C A N

ANTI-ATLAS

Yannis Institut

fur Geologie

Altensteinstr.

(SOUTHERN

OROGENY

MOROCCO)

Bassias

der Freien

Universit~t

34 A, D-1000

Berlin

Berlin,

33

Eckard Wallbrecher Institut

fur Geologie

und P a l ~ o n t o l o g i e ,

Heinrichstr.

26, A - 8 0 1 0

Alexander Institut

fur M i n e r a l o g i e Takustr.

Universit~t

Graz

Graz

Willgallis

der F r e i e n

Universit~t

6, D-IoOO B e r l i n

Berlin

33

ABSTRACT Medium

grade

gneisses

and m e t a p e l i t e s

Mountains

show c o m p l e x

a lateral

slip of a n o r t h e r n

of the W e s t A f r i c a n

retrograde

Craton.

by h e a t i n g

and s u b s e q u e n t

generation

is w e a k l y

garnets

show growth

Early

Muscovites

to p o s t - m y l o n i t i c .

nets

are pre-

and p o s t - m y l o n i t i c .

while

shear

continental

at about

zone w h i c h

masses

the p r e s s u r e

paths

Those

Sirwa

related

the n o r t h e r n

An older p r e - m y l o n i t i c chloritized, inclusion

to

margin

interrupted

whereas trails

from rocks w i t h both g a r n e t

garnet younger

and texand bio-

from rocks w h i c h b e a r only gar-

The s t r o n g m y l o n i t i z a t i o n developed

and seems

8 kb in which

along

and e r o s i o n w e r e

over older p l a g i o c l a s e s ,

are syn-

of the

uplift

mass

mylonitization.

tite

path

pressure-temperature

continental

zoned and p e r v a s i v e l y

tural u n c o n f o r m i t i e s .

by an e a s t - w e s t

from the n o r t h e a s t e r n

during

to appear

temperatures

caused

at the end of an i s o b a r i c

decreased

P t o t - PH20 was m a i n t a i n e d

was

the lateral m o v e m e n t

from 600 to 500 °C,

between

2 and 4 kb.

44

RESUME

Dans

la partie n o r d - e s t de la m o n t a g n e de Sirwa,

m~ta-p41ites complexes.

des gneiss et des

de moyen degr~ e n r e g i s t r e n t des trajectoires retrogrades

Le s o u l ~ v e m e n t et l'~rosion prlcoce,

m e n t prograde,

~ la suite d'un ~v~ne-

sont interrompus par un ~ c h a u f f e m e n t et une m y l o n i t i -

sation li~s ~ un m o u v e m e n t lateral entre une masse continentale au nord et la marge nord du craton ouest-africain. mylonitiques

Des gr~nats ant~-

sont zones et i n t e n s i v e m e n t chloritis~s.

post-mylonitiques

Des gr~nats syn-

se c r i s t a l l i s e n t sur des anciens plagioclases,

c o n t i e n n e n t des inclusions et m o n t r e n t des d i s c o n t i n u i t ~ s texture.

situ~e

ils

dans leur

Des m u s c o v i t e s p r o v e n a n t des roches ~ gr~nat et biotite sont

syn- ~ p o s t - m y % o n i t i q u e s .

Celles p r o v e n a n t des roches ~ gr~nat sans bio-

tite sont ant~- et p o s t - m y l o n i t i q u e s .

La m y l o n i t i s a t i o n intense est

li~e a une zone de c i s a i l l e m e n t E-W, qui se produit, p r o b a b l e m e n t ,

~ la

fin d'une p~riode de conditions isobares ~ 8 kbar environ pour des temperatures

comprises entre 600 et 500°C et pour

Ptot - PH20 m a i n t e n u

au moins entre 2 et 4 kb.

INTRODUCTION

The A n t i - A t l a s m o u n t a i n chain in Southern M o r o c c o consists of a Precambrian b a s e m e n t covered by P a l e o z o i c

layers.

The crystalline b a s e m e n t

crops out in several windows, w h i c h were formed by uplift and erosion. They are arranged in a SW-NE direction, margin of the W e s t African Craton

p a r a l l e l to the n o r t h w e s t e r n

(see W A L L B R E C H E R

Three other papers in this volume

1987a,

this vol.)

(WALLBRECHER 1987a, b and BRABERS

1987) p r e s e n t a review of older and recent works

on the g e o l o g i c a l

setting and g e o d y n a m i c context of the P r e c a m b r i a n b a s e m e n t of the AntiAtlas. We carried out an analysis of the m e t a m o r p h i c conditions under w h i c h the g e n e r a t i o n of gneisses, metapelites,

and amphibolites

ment

(Fig.

e x p o s e d in n o r t h e a s t e r n Jbel Sirwa

I)

took place. This

analysis includes p r e s s u r e - t e m p e r a t u r e - d e f o r m a t i o n paths geothermometric-geobarometric

calibrations.

in the base-

derived from

45

Taurtit

5 km

Amassin Q$ 00

~gah+

garz+ mus

~gah+

mus + plag

'l"

bio ,I" ptag

00~ 010 I

QI

" ~ ' i ~ ' . ". • .- •

fremol. + Fe - Mg - hornblende ~mus

Tamazirt

+ bio + pLag

(~

samplenumber o Tizwaf

Fig.

I The T a c h o u k a c h t Gneiss and g e o l o g i c a l setting

GEOLOGICAL

SETTING

According

to S C H E R M E R H O R N

the T a c h o u k a c h t Formation, contains

lenses

an epi-

authors

that

continental

TECTONIC

came

affected

mylonitic

ophiolite

with

and basic

granite, that

sequence w h i c h

ophiolite

rocks,

O h~h~sequen-

containing

to the north. the m e t a m o r p h i s m

and gneiss

an u n d e r t h r u s t i n g

are part of

the L m a k h z a n

and the T o u r t i t

and m e t a m o r p h i s e d

of u l t r a b a s i c

to the c o n c l u s i o n the

foreland

to the south,

deformed

rocks

thrust b e t w e e n

sequences northern

were

and imbricaused by a

continent.

SETTING

Gneisses

points

deformed

the studied

sedimentary

and sills,

essentially

(1986)

which was

and p y r o c l a s t i c dykes

collision

An analysis kacht

et al.

Formation,

an imbricated,

consists

of h i g h l y

These cation

Gneiss

basaltic

lite Complex, ce w h i c h

F o r m a t i o n of Jbel Sirwa (for location see W A L L B R E C H E R 1987b, Fig. I, this vol.)

of the

fabric

is given by W A L L B R E C H E R

can be s u m m a r i z e d

- Imbrication

(1987b,

evolution

of the Tachou-

this vol.) . Three main

as follows:

tectonics

fold p a t t e r n with sequences.

and the tectonic

caused

flat E-W

a steep E-W s t r i k i n g

fold axes,

schistosity,

and a r e p e t i t i o n

of rock

a

46

- A lateral shearing and

low strain

within

PETROGRAPHIC

elevation

a wide

associated

shear with

zone with

SP-mylonites

high

temperature

in anatexites,

sequence. caused

re-establishment

of

a brittle

regime.

SETTING

From samples

collected

an i m p o r t a n t

trend

each

studied.

section

sher'

rate,

the ophiolite

- Subsequent

mylonitic

caused

texture,

metamorphic

This

to north

area of exposure

in the

lithology

trend is c h a r a c t e r i z e d

increasing

quartz

assemblages,

garnet + biotite Muscovites,

from the entire

from south

content

ranging

of the gneisses,

was

observed

by i n c r e a s i n g

in

blasto-

and the a p p e a r a n c e

of

from the garnet + m u s c o v i t e

'freto the

zone.

biotites,

toids

and p l a g i o c l a s e s

grams

of their

garnets,

were

chemical

amphiboles,

chlorites,

a n a l y z e d by microprobe.

features

epidotes,

Representative

chloridia-

are shown below.

MUSCOVITE

Muscovite

is p r e s e n t

it is an uncommon and after

the m y l o n i t i z a t i o n .

growth with

chlorite

is common

zation

time of low grade

a weak

variation

the sense

6.6.

and dates From

A trend

towards

with b i o t i t e a latest

syn-

towards

substitution

Texturall~

essentially is rare;

inter-

proportions,

the p h e n g i t e

composi-

of the o c t a h e d r a l

is o b s e r v e d

during

to p o s t - m y l o n i t i -

the S i - A l - ( F e t o t + Mg)

substitution

sites

in

on the Si - A I I V / A I VI -

2). The

Si

Fig. o •

6.4

.$$

"- •o %

z

°o a "2 ?*. ~ . . .

&2

ool •

°

o

•

,

,

,

S.0

5.2

S.~

2

S i / ( A I I V / A I vI) d i a g r a m for muscovites. Open symbols: garnetbiotite-muscovite-assemblages (Samples: squares 69, diamonds 77). F i l l e d symbols: g a r n e t - m u s covite-assemblages (Samples: diamonds 50, large dots 86, triangle~ 49). Small dots sample 134 (biot i t e - m u s c o v i t e assemblage).

.~. .... o00Oo ~0 0

6.0

and gneisses.

It c r y s t a l l i z e s

Intergrowth

facies.

of the T s c h e r m a k

(Fig.

all m e t a p e l i t e s

from the ideal m u s c o v i t e

tion is observed.

diagram

in almost

part of the matrix.

, A L I"V A I V ["

47

tetrahedral data

Si varies

from 6.13

to 6.59

lie at the m u s c o v i t e - p h e n g i t e

nent is small, Most

fluence

atomic

with

data

a 2:1

tions

the

assemblage;

solutions

of p a r a g o n i t e

B a o was

as high

substitutions

varies

as 0.76%.

86

range

u

These compo-

increasing

of ferric

al-

that m u s c o v i t e varia-

and/or m e t a m o r p h i c

than in d i f f e r e n t in the XII

up to 16 mol% (Fig.

spots

sites.

of

Solid

for the rims

3). The

amount

only a minor

role

of in

Fig. 3 .4+ ~. and $I v a r i a t i o n s in m u s c o Vml~es. G r a p h i c d e n s i t i e s refer to assemblages as in Fig. 2. Arrows indicate c o m p o s i t i o n a l v a r i a t i o n s from older to y o u n g e r m u s c o v i t e s .

N=9

n

so

B,

i N=2 13/.

f

.O

,1

•Z

r],

&O

3,1

[], 3~

,

3.3

BIOTITE Biotites mediate

are m o s t l y

between

pleochroic

and iron-bearing.

the Mg and Fe end members.

ions

in the o c t a h e d r a l

tion

from t r i o c t a h e d r a l

join.

iron;

and i n t r a s a m p l e

Ca plays

'grade'.

to the p h e n g i t e

The data show,

grade

and a

2, the in-

sites.

~ _-

B

inter-

of grains

Si

L

amounts iron.

uncertainty,

no more

In contrast,

in the XII

from Fig.

with

is the v a r i a t i o n

in m u s c o v i t e

the p h e n g i t e

plot p a r a l l e l

ferrous

unit.

of iron and m a g n e s i u m

on m e t a m o r p h i c

for the centers

XNa

grade

has minor

depending

An e x c e p t i o n

and up to 13 mol%

amounts

formula

is possible.

As can be seen

of a n a l y t i c a l

the c o m p o s i t i o n

sample.

However,

component

significant

than

layer c o m p o s i t i o n

of c o m p o s i t i o n

the same

show

mo~ferric limits

per

content w e a k e n s

from each

formulae

is sometimes

though w i t h i n

content.

ferri-muscovite

projected

Normalized there

show m i n o r

in the alkali

of the

However,

join.

and a f e r r i - m u s c o v i t e

of the analyses

deficiency

atoms

sites

remains

below

stoichiometry.

This

They

are inter-

The total n u m b e r 6.0,

indicating

implies

of a devia-

substitution

of

48

the type

(Mg,Fe)2+-A13+ in o c t a h e d r a l sites and/or F e 3 + - A 1 3 + in

t e t r a h e d r a l sites. A d e v i a t i o n towards a 5% to 20% s o l u t i o n of d i o c t a h e d r a l m i c a in b i o t i t e is probable. V a r i a t i o n s

in XMg are,

at a minimum, b e t w e e n 0.43 and O.5 for m a t r i x b i o t i t e s and for biotites in contact with garnet. For the latter the A1%~ varies from 0.43 to 0.98 and is, on the average, h i g h e r than that of the biotite ~ m u s c o v i t e

'grade'

T i t a n i u m amounts

in the garnet + biotite

'grade'

are limited to

1.5 w e i g h t % TiO 2 or O.18 atoms per six o c t a h e d r a l sites. A c c o r d i n g to GUIDOTTI

(1984),

these values are consistent w i t h the g r e e n s c h i s t

grade for m e t a p e l i t e s

and metabasites.

v o l v e d in s u b s t i t u t i o n s has been suggested. + muscovite

(Fig.

However,

t i t a n i u m is not in-

4). A positive c o r r e l a t i o n with XMg

There is an AI-Ti s u b s t i t u t i o n for the b i o t i t e

'grade'. S o d i u m b i o t i t e s

(Na=O.O1 - 0.078) have h i g h e r

XMg ' i m p l y i n g a p r o b a b l e e x c h a n g e b e t w e e n Fe and Na at o c t a h e d r a l sites. B e c a u s e the total of A1 and Si is always h i g h e r than 8, the p r e s e n c e of Fe 3+ at the t e t r a h e d r a l sites is improbable. mate ratio F e 2 + / F e 3+, ca.

The approxi-

13.67 for pelites and s e m i - p e l i t e s

(GUIDOTTI

1984), is c o n f i r m e d by the n o r m a l i z e d atomic formulae. Fig.

ALvl 1,0

4

AIVI/Ti diagram for biotites. Symbols are the same as in Fig. 2. Open symbols indicate c o m p o s i t i o n of the cores. F i l l e d symbols indicate rim composition. Diamonds are y o u n g e r biotites.

o 0 0 []

oo

o

,6

oe

Olb •

me

• 1

ri i

,2

,

!

,i

P o t a s s i u m is the p r e d o m i n a n t cation at XII sites. B a r i u m varies O.O1

from

to 0.07 atoms and, in the presence of K - f e l d s p a ~ c h a r a c t e r i z e s high

grade

(TRACY & ROBINSON,

1976). A slight p o s i t i v e

correlation between

XII totals and XMg was also observed.

GARNET The garnets show p o l y m e t a m o r p h i c patterns.

Two m a i n g e n e r a t i o n s can

49

be distinguished.

The first g e n e r a t i o n garnets

w i t h o u t inclusions

are s m a l ~ s u b a u t o m o r p h i c ,

and p e r v a s i v e l y chloritized.

occur a s s o c i a t e d w i t h high X

plagioclase

an brown to b r o w n i s h green biotite.

In the m a t r i x they

(anorthite 30-40 mol%)

These garnets show a fair increase in

Mg from the core to the rim and have flat patterns They are rotated and their main internal v e r t i c a l l y to the m y l o n i t i z a t i o n generation

for Fe, Mn and Ca.

fractures are o r i e n t e d sub-

foliation.

are large and strongly

epidote and quartz.

and

Garnets of the second

zoned with inclusions

They grew in two stages,

of plagioclase,

the e a r l i e r of w h i c h

is

m a n i f e s t e d by: I) ' p u s h i n g aside'

forms and d i s p l a c e d b r o w n - g r e e n biotite

2) cores of garnets s u r r o u n d e d by b r o w n - g r e e n biotite an i n t e r m e d i a t e p r o g r a d e reaction,

flakes.

as a result of

followed by final r e t r o g r a d e

r e a c t i o n and rim crystallization. 3) garnet c r y s t a l l i z i n g over p l a g i o c l a s e w i t h high Xan

(0.35-0.40).

A l t h o u g h m a t r i x m u s c o v i t e seems to be a s s o c i a t e d with these garnets, it was n e v e r found in contact w i t h them. The latest stage of crystallization concerns garnets with almost i d e n t i c a l chemical composition. H o w e v e r these have h i g h e r g r o s s u l a r and spessartine crystallize on the m a t r i x together w i t h flakes.

content,

and they

large b r o w n - g r e e n biotite

The p r e s e n c e of b o t h these stages indicates p o l y m e t a m e r p h i e

or complex retrograde patterns.

Such patterns

are generally attri-

b u t e d to e i t h e r d i f f u s i o n during p r o g r a d e m e t a m o r p h i s m or to retrograde r e - e q u i l i b r a t i o n . rarely observed: fractures.

There is another special feature that has been

h o m o g e n o u s garnet 'stringers'

These garnets

filling m y l o n i t i z a t i o n

are not fractured and may be syn- to post-

mylonitic.

AMPHIBOLE

Two kinds of amphiboles have b e e n found in the amphibolites calated in the gneisses:

actinolite

and hornblende.

green and have a b r o w n i s h M g - h o r n b l e n d e rich in rotated epidote

are

core. They crystallize in rocks

(allanite ?) and they coexist w i t h y o u n g e r fine

g r a i n e d epidote and w i t h low Xan plagioclase. 0.56

Actinolites

inter-

Their XMg varies

from

to 0.72.

Hornblendes

are p l e o c h r o i c

from b l u e - g r e e n to g r a s s - g r e e n and show,

from core to rim, r e v e r s a l patterns These reversals

a c c o r d i n g to the Si-XMg projection.

are m a t e r i a l i z e d in the join

b e t w e e n Mg- and Fe-horn-

50

blende

(Fig. 5). W h e n both amphiboles are plotted,

h o r n b l e n d e -~ actinolite trend which,

1~0 C

I'

presumably, Fig.

ra

E

A

5

Open circles: sample 37, filled circles: sample 79 A= ferro-actinolite, B= actinolite, C= tremolite, D= ferro-hornblende, E= M g - h o r n b l e n d e , F= ferrotschermakite, G = t s c h e r m a k i t e

D

8

is a retrograde one.

XM~A1VI-diagram for amphiboles

°~-~

>(

they define a Fe-

S~

EPIDOTE

Epidote

appears in gneisses

as w e l l as in amphibolites.

m y l o n i t i c g e n e r a t i o n in the gneisses is rich in Si and in Fe 2+ for low amounts of A1 VI amphibolites

The pre-

(38.9 w e i g h t % SiO 2)

(2.43 atoms per formula unit).

the grains have subhedral,

In

cross fractured and rotated cores

w h i c h are o v e r g r o w n by fine grained rims of i r o n - p o o r and Ca-rich epidote. Y o u n g e r p o s t - m y l o n i t i c epidote fills large fractures. the p u m p e l l y i t e - p r e h n i t e

It is related

to

facies. This epidote is frequently o b s e r v e d near

the N E - S W faults that a f f e c t e d the studied area

(Fig.

I).

CHLORITE

Analyses

of chlorites

are p l o t t e d in Fig. 6. The c o m p o s i t i o n varies

b e t w e e n ripidolite and p y c n o c h l o r i t e of Fe/(Fe+Mg).

High amounts of MnO

in younger chlorites.

(Si=5.3 to 6.2), with a small range

(0.50 to 0.84 w e i g h t %) were d e t e c t e d

In c o - e x i s t i n g m u s c o v i t e - c h l o r i t e p a i r ~ the amount

of MnO varies around 0.75 wt%.

CHLORITOID

TWO d i f f e r e n t generations of c h l o r i t o i d have been found:

the first one

is p l e o c h r o i c and forms large dark green lustrous crystals w i t h straight regular sides and rounded angles. mylonitic

crystallization.

It is strongly t e c t o n i z e d and dates a pre-

The second g e n e r a t i o n is p o s t - m y l o n i t i c w i t h

small crystals growing in all directions,

often in 'rosettes'

51 12

Fig.

1.0

6

Fe/(FeMg-(Fe2++Fe3+)-diagram f o r c h l o r i t e s (HEY 1954): A= c o r u n d o p h i l i t e , B= pseudothuringite, C=sheridanite, D= ripidolite, E= daphnite, F= chlinochlore, G= pycnochlorite, H= b r u n s v i g i t e , I= penninite, J= diabantite, K= talc-chlorite.

+ ÷

~6.

:£

u-

.2 i

0

l

Si

6 FELDSPAR

Plagioclase K-feldspar

Plagioclase Very

often

previous grained

is the d o m i n a n t

the

trails

forms

porphyroblasts

has

also been

in the garnet

patterns

have not been

+ muscovite

varying

have b e e n

observed

which

pure

generation The

varies

studied

from

has

rocks. thin section.

associations.

of quartz w h i c h

of garnets.

anorthite

Very

content

define fine-

of the

44 to 19% and from core

found. Large implies

albite

assemblages

of the s t u d i e d

inclusions

observed.

assemblages

(BUCHER et ai.1983) . L a t e s t

content

have

as those of the second

to rim no reversal

the b i o t i t e

for m o s t

in almost every

or m i c r o p o r p h y r o b l a s t i c

large p o r p h y r o b l a s t s

plagioclase

plagioclase

crystal

mineral

is not a b u n d a n t but exists

variations

low to m e d i u m also been

show p l a g i o c l a s e

within

grade

observed. with

a single

conditions Rocks

from

an a n o r t h i t e

from 20% to 0%.

TURMALINE

Turmaline

is a common

and few inclusions. crystals

which

could

accessory

In sample imply

mineral with blue-grey

80 an i n t e r n a l

a probably

rim

detrital

was

homogenous

observed

colour

in some

origine.

QUARTZ

Quartz are

is u b i q u i t o u s

assumed

in the assemblages

to have e q u i l i b r a t e d

with

it.

studied.

All m i n e r a l

assemblages

52

M E T H O D S OF P R E S S U R E - T E M P E R A T U R E C A L I B R A T I O N S

T e m p e r a t u r e d e p e n d e n t Fe-Mg p a r t i t i o n i n g has b e e n used on c o e x i s t i n g garnet-biotite

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

Serious changes in the K D of each

c a l i b r a t i o n are c a u s e d by the Fe 3+ which is c o m m o n l y p r e s e n t in micas. For this p u r p o s e n o r m a l i z e d atomic formulae have been used a c c o r d i n g to LAIRD & ALBEE

(1981)

for the e s t i m a t i o n of Fe 2+. For g a r n e t - b i o t i t e pairs

the c a l i b r a t i o n of HODGES et al. PERCHUK

(1981), GOLDMAN & ALBEE

(1977)

and

(1970) have been applied. A c c o r d i n g to the r e l e v a n t literature

the first c a l i b r a t i o n s have an a n a l y t i c a l error of 20°C. The larger s c a t t e r that can appear w i t h i n a n a r r o w m e t a m o r p h i c

zone may be r e l a t e d

w i t h the scatter b e t w e e n the Mg-Fe e x c h a n g e t e m p e r a t u r e and the thermal peak from sample to sample.

The second c a l i b r a t i o n obtains

of the lower limit and overcorrects of HODGES & S P E A R

(1982)

accounts

temperatures

garnet with high XCa" The c a l i b r a t i o n

for n o n - i d e a l i t y in garnet solid solu-

tion. However,

it is s u p p o s e d that at the low a m p h i b o l i t e facies, b e t w e e n

450 and 6OO°C,

the s p e s s a r t i n e component has an ideal m i x i n g b e h a v i o u r

w i t h the g r o s s u l a r - p y r o p e

component. HOINKES

(1986) p r o p o s e d a c o r r e c t i o n

of the g r o s s u l a r c o m p o n e n t in the lower amphibolite

facies.

In the case

of our data this does not greatly affect the e s t i m a t e d temperatures. For g a r n e t - m u s c o v i t e pairs the c a l i b r a t i o n of GREEN & H E L L M A N

(1982)

was used. The c a l i b r a t i o n for pelitic and b a s a l t i c rocks w i t h high CaO content was used in order to b o r d e r the variations

in temperat-

ure. An i n t e r p o l a t e d temperature has also been e s t i m a t e d according to the Mg value

(MgO/(MgO+FeO)

mol.prop.)

of the c o n c e r n e d muscovite.

However,

this i n t e r p o l a t i o n involves p o s s i b l e errors if the n o n - i d e a l i t y of the exchange is not linear and suggests that these i n t e r p o l a t e d t e m p e r a t u r e s c a n n o t be taken into a c c o u n t as absolute values. A n o t h e r u n c e r t a i n t y is caused by the p r e s e n c e of high X s p e s s a r t i n e w h i c h could lead t e m p e r a t u r e s of the second g e n e r a t i o n of garnets to h i g h e r ranges. P r e s s u r e e s t i m a t e s have been b a s e d on the e q u i l i b r i a

3 anorthite = grossular + 2 Al-silicate

according to GHENT

(1976). The g r o s s u l a r activity

lated from the e x p r e s s i o n of GANGULY & KENNEDY s p e s s a r t i n e c o m p o n e n t into account. 0.50,

~an is given by ORVILLE

Pressure

(~ gros ) was calcu-

(1974) w h i c h takes the

For an anorthite content lower than

(1972) as log10 ~ an = O.1060.

and t e m p e r a t u r e for g a r n e t - m u s c o v i t e

and p l a g i o c l a s e w e r e

53

derived by s i m u l t a n e o u s l y solving the g a r n e t - m u s c o v i t e g e o t h e r m o m e t e r and the g a r n e t - p l a g i o c l a s e geobarometer.

W a t e r fugacities were c a l c u l a t e d

according to the e q u i l i b r i u m

paragonite

(in m u s e o . ) + quartz = albite

according to GHENT CHENEY & GUIDOTTI

(in plag.)+ A l - s i l i c a t e + H20,

(1976). ~par was e s t i m a t e d from the e x p r e s s i o n of (1979) which is only a function of the temperature.

The

p a r a m e t e r s A, B and C for p l a g i o c l a s e b a r o m e t r y and m u s c o v i t e w a t e r fugacities w e r e chosen a c c o r d i n g to the 5 kb - t e m p e r a t u r e i n t e r s e c t i o n for g a r n e t - b i o t i t e pairs,

and according to the pressure - temperature inter-

section for g a r n e t - m u s c o v i t e pairs. As b a r y c e n t e r s

for g a r n e t - b i o t i t e pairs

the m i d d l e values of three p r e s s u r e - t e m p e r a t u r e values derived from the t e m p e r a t u r e c a l i b r a t i o n were used.

For g a r n e t - m u s c o v i t e pairs b a r y c e n t e r s

were d e d u c e d from the i n t e r p o l a t e d t e m p e r a t u r e according to the Mg value of the muscovite. Pressure-temperature

paths were d e d u c e d from the rectangles of error

w h i c h derive from separate c a l i b r a t i o n for each pair of minerals. pressure and w a t e r respectively,

Total

fugacities m u s t be c o n s i d e r e d as m a x i m u m and minimum,

since no a l u m i n i u m - s i l i c a t e was o b s e r v e d in the thin sections.

A p p l i c a t i o n of the c o r r e c t i o n of HODGES et al.

(1982)

increases all tem-

p e r a t u r e s o b t a i n e d by other calibrations d e r i v e d from natural

(Ca + Mn)

b e a r i n g garnets by ca. 80°C and all pressures by I kb. Thus another probable path could be p a r a l l e l to that p r o p o s e d in Fig. tively lower pressures

and temperatures.

does not affect the conclusions

However,

7a and 7b for rela-

the choice of the path

in the p r o p o s e d g e o d y n a m i c scheme since

part of the r e t r o g r a d e m e t a m o r p h i s m was almost isobaric.

CONDITIONS OF M E T A M O R P H I S M

On the basis of e x i s t i n g e x p e r i m e n t a l studies of phase equilibria, following e s t i m a t e s can be made: dote b e a r i n g rocks

(curve 1, Fig.

7) is r e s t r i c t e d to lower pressures

and t e m p e r a t u r e s a c c o r d i n g to the s t a b i l i t y of m u s c o v i t e + quartz 2, Fig.

the

The upper stability of epidote for epi-

7). The u p p e r s t a b i l i t y of c h l o r i t o i d

(curve 3, Fig.

(curve

7) r e s t r i c t s

the t e m p e r a t u r e even more. The s t a b i l i t y of M g - c h l o r i t e + m u s c o v i t e is given by the curve 4. A l t h o u g h the products of the assemblage are not all found

(phlogopite + k y a n i t e is missing)

m o s t common a s s e m b l a g e bility for m u s c o v i t e s

the reactants make up part of the

in the studied area. M i n i m u m t e m p e r a t u r e s (paragonite is not associated)

of sta-

were e s t i m a t e d from

54

the K/(K+Na)

ratio

(EUGSTER et al.

w i t h o u t any p r e s s u r e correction.

1972)

and may be higher than 400°C

The presence of K - f e l d s p a r s

absence of p r o g r a d e m u s c o v i t e in samples

and the

from the garnet + b i o t i t e

assemblages indicates that the b r e a k d o w n of earlier m u s c o v i t e s

is pro-

able,

This

and partial m e l t i n g occured prior to the m y l o n i t i z a t i o n .

hypothesis.is covites,

also supported by the very small variations

compared w i t h that of the garnet + m u s c o v i t e s

ditionally, plagioclases

of XNa in mus-

assemblages.

Ad-

small m e l a n o s o m a t i c horizons w h i c h contain rounded i s o l a t e d (highly albitic)

These horizons

have been found in six localities

are always in contact with amphibolites

(Fig. I).

and seem to sepa-

rate the n o r t h e r n border of the garnet I + m u s c o v i t e q u a r t z i t i c gneisses from the southern border of the garnet I + g a r n e t 2 + m u s c o v i t e ~ b i o t i t e gneisses. The p-t- path from samples w i t h garnet 2 + m u s c o v i t e

shows a complete

pressure retrograde - temperature retrograde e v o l u t i o n in the stability Fig. 7 P r e s s u r e - t e m p e r a t u r e stability fields for the analyses samples. a) g a r n e t - b i o t i t e - m u s c o v i t e plagioclase-assemblages (sample 69) ..... p r o b a b l e equilibria b) g a r n e t - m u s c o v i t e - p l a g i o c l a s e (samples 50, 86, 49) sample 50, ..... sample 86, .... = sample 49

12 -

Kb

%%

]0/],%

107O/Z&

-">'-

81

/ /

~"

8

6-

L,-

-

2

I: Upper stability of epidote (LIOU 1973). 2: M u s c o v i t e + quartz=andalusite+sanidine+H20 (CHATTERJEE & JOHANNES 1974). 3: C h l o r i t o i d = A i - F e - a n t h o p h y l lite+staurolite+hercynite (GRIEVE & FAWCETT 1974). 4:Mgchlorite+muscovite=phlogopite + k y a n i t e + q u a r t z + H 2 0 (BIRD & FAWCETT 1973). 5: Upper stability of muscovite. 6: A l - s i l i cate p o l y m o r p h s (RICHARDSON et al° 1969). 7: A l - s i l i c a t e polymorphs (HOLDAWAY 1971). 8: Field of p r e h n i t e - p u m p e l l y ite facies for basalt andesite systems.

~7 l

10-

1 500

i 6,00

~

I&121

Kb

1 700 0/11

9111

/-

2/3

820111

. -

6 "'"

& 2 ~ i

<,00

I

500

?

I

I

I

soo

700

800

field of m u s c o v i t e and thus can well explain the large variations of m u s c o v i t e

1

800

in ~ a

(Fig. 3). The p-t-time path from samples w i t h garnet 2 +

biotite + m u s c o v i t e shows a similar e v o l u t i o n which enters into the

55

stability

field of the m u s c o v i t e

grade history, Biotite colour

brown

Microprobe ones

a XMg v a r y i n g

(0.14 to 0.18

However,

atoms

from the b r e a k d o w n in the e a r l i e s t

garnet

THOMPSON

reactions (1981)

which

could

proposed

reaction

in w h i c h

later

(1977)

chloritoid

before

of the g r o s s u l a r the d e c r e a s i n g covite,

and the y o u n g e r

are p l o t t e d creasing final

- Mg/Fe

younger

Mn-rich -

content

events

which

is d e p l e t e d

prograde

role,

consuming

+ chloritoid

(Fig.

chlorite. (Fig.

8) garnets

and show,

is i n d e p e n d e n t

the

well

the

large v a r i a t i o n s as w e l l

the appearance

as

of mus-

of the second g e n e r a t i o n

from

of their

in Mn could well

+ H20 +

+ chlorite

relatively

7),

ZEN

area a

as follows:

of the second generation,

diagram

of the g a r n e t

to form is also

to m y l o n i t i z a t i o n .

+ muscovite

in p l a g i o c l a s e ,

profiles

This

or p o l y m e t a m o r p h i s m .

can explain peak

Mn-rich

which

event.

prior

+ biotite

reaction

the thermal

of garnets

from d i f f e r e n t

Mn/Fe-ratio

enrichment

10

content

anorthite

In the Mn/Fe

this

and after

grew e i t h e r

to those of the s t u d i e d

+ high-ab-plagioclase

In the case of our study,

at very

for both biotites.

intermediate

an i m p o r t a n t

+ high-an-plagioclase

and the green

is retained

to form green b i o t i t e

tectonic

is

to 0.57.

or the garnet I reacted

chloritization

plays

the b i o t i t e

green b i o t i t e

retrograde

similar

to a green

from 0.50

are e n r i c h e d

unit)

The

have used

represent

= high-Ca-garnet

evolution

formula

of the

for assemblages

low-Ca-garnet quartz

reacted

et al.

ones

of garnetl,

strong

from garnets,

the T i - c o n t e n t

of Ti.

stages

by the o b s e r v e d

this

of Ti.

per

depleted

directly

Thus,

and has

show that the brown

in A1 VI.

are totally

part of the retro-

is a l t e r e d

Away

chlorite supported

of garnets

to b r o w n - g r e e n

analyses

Garnets

vicinity

a low XMg of ca. 0.45.

are d e p l e t e d

low levels

the isobaric

the heating.

in the i m m e d i a t e

and has

commonly

long after

during

core

to rim,

Ca-variations.

explain

an enThis

the p r e s e n c e

of

chlorites.

H.

Fig. 8 ( M g / F e ) / ( M n / F e ) - d i a g r a m for garnets. Mn/Fe i n c r e a s e s c o n s t a n t l y from core to rim. Symbols are the same as in Fig. 2

,

,

,

T

,

,,,] 1.0

56

GEODYNAMIC E V O L U T I O N

A normal retrograde path,

convex towards the temperature,

is the classic

e v o l u t i o n of a m e t a m o r p h i c

field after uplift and erosion

RICHARDSON

the retrograde h i s t o r y of the T a c h o u k a c h t Gneis-

1978). However,

ses and m e t a p e l i t e s

(ENGLAND &

(Fig. 9) shows other c h a r a c t e r i s t i c s wl%ich can be

grouped in two stages as follows: -

An increase in temperature seems to be m a n i f e s t e d in b o t h g a r n e t - b i o t i t e and g a r n e t - m u s c o v i t e pressures

'grade' with peaks of about 750 ° and 650 ° for

of 10 kb and 8 kb, respectively.

- this thermal effect is not only a result of uplift and e r o s i o n because it is followed by an almost isobaric decrease of temperature b e t w e e n 680°C and 620°C for the g a r n e t - b i o t i t e

and the g a r n e t - m u s c o v i t e

'grade',

respectively. F i e l d observations

and m a p p i n g show that the thermal effect could be

related to the intrusive granites situated on the norhern b o r d e r of the gneisses and m e t a - p e l i t e s p r e s e n c e of migmatites, the early retrograde

(see W A L L B R E C H E R 1987b, Fig.

high molar volume of w a t e r in the m u s c o v i t e s

stage and the absence of retrograde

Kb ~

~00

i

,

,

s00

60o

2, this vol.). The

~00

of

large garnets

~ig. 9 p - t - d e f o r m a t i o n paths for g a r n e t - b i o t i t e m u s c o v i t e - p l a g i o c l a s e (heavy symbols) and g a r n e t - m u s c o v i t e - p l a g i o c l a s e , a+a'=prograde paths (first garnet-generation), b+b' = p r o g r a d e t e m p e r a t u r e paths (first garnet generation), c+c' = thermal peak metamorp h i s m after uplift, erosion, and i n t r u s i o n related w i t h m i g m a t i z a t i o n of the neighb o u r i n g ophiolites. Early stage garnets of the second generation, d+d' = isobaric retrograde paths, m y l o n i t i z a t i o n and late •c stage garnets of the second generation. e+e' = i s o t h e r m a l - l i k e path for garnet + 800 biotite + m u s c o v i t e + p l a g i o c l a s e assemblages, m o d e r a t e to high g e o t h e r m a l gradient for the garnet + m u s c o v i t e + plagioclase assemblages.

on the s o u t h e r n part of the studied profiles r e i n f o r c e

the p r o p o s e d model

of a thermal effect caused by the presence of a hot intrusive body. The s u b s e q u e n t i m p o r t a n t decrease in t e m p e r a t u r e at almost constant p r e s s u r e is i l l u s t r a t e d in the p r e s s u r e - t e m p e r a t u r e a concave-towards-the-temperature

(Fig.

7) as

path. This shape is translated as a

p e r i o d of i m p o r t a n t tectonic activity ture,

diagram

(mylonitization).

The isobaric fea-

for i m p o r t a n t decrease of temperature, may also mean that the dura-

tion of this period was r e s t r i c t e d in time.

Field observations

and

m i n e r a l o g i c a l data suggest also a short duration since p s e u d o t a c h y l i t i c

57

features w e r e o b s e r v e d frequently because small v a r i a t i o n s

(WALLBRECHER 1987b,

this vol.)

in the XMg of chlorites, m u s c o v i t e s

and

and bio-

tites as w e l l as in the XMg of bulk rock imply rates of r e - e q u i l i b r a tion not slower than those of the d e s t a b i l i z a t i o n

of the paragenses.

The Xmola r volume of H20 during the retrograde isobaric path

(Fig. 9)

is r e t a i n e d at high levels b e t w e e n 0.90 and 0.60 and p r o b a b l y has fav o u r e d the strain energy.

The P t o t a l - P w a t e r is b e t w e e n 2 and 4 kb for

the same p e r i o d and increases d e c r e a s i n g temperature. zation was r e l a t i v e l y

towards the end of the isobaric path for

The p a r t i a l p r e s s u r e of w a t e r during m y l o n i t i -

low at 3 kb, since the earlier h i g h e r grade m u s t

have r e m o v e d m o s t of the available water.

During m y l o n i t i z a t i o n

the rocks

w e r e u p l i f t e d to m i d - c r u s t a l level. A l t h o u g h the r e t r o g r a d e e v o l u t i o n can be r e l a t i v e l y w e l l defined, i n f o r m a t i o n about the prograde event

(or events)

no

is available. A l s o the

s t a r t i n g point of the r e t r o g r a d e m e t a m o r p h i s m of Fig.

7 cannot be pro-

p o s e d as the initial one. The g r a n i t i c i n t r u s i o n near the n o r t h e r n border a f f e c t e d both the g n e i s s e s O p h i o l i t e Complex

and the m e t a - p e l i t e s

(BASSIAS & W A L L B R E C H E R

and the T o u r t i t

1987). Thus they are y o u n g e r

than the collision w h i c h t r a n s p o r t e d the T o u r t i t Series and the Tachouk a c h t Series.

It seems that these intrusions

followed an uplift and

e r o s i o n period, p r o b a b l y a rapid one, w h i c h c r e a t e d shorter and easier paths for the u p r a i s i n g hot m a t e r i a l but also, antithetically, advantages

created

for its cooling.

ACKNOWLEDGEMENT

We w o u l d

like to thank the Deutsche F o r s c h u n g s g e m e i n s c h a f t for fin-

ancial support for field excursions

in A p r i l

1986 and O k t o b e r

1986, as

w e l l as the I n s t i t u t e for M i n e r a l o g y of the Freie U n i v e r s i t ~ t Berlin, e s p e c i a l l y Ms. Siegmann,

for the ARL e l e c t r o n m i c o r p r o b e

The w o r k was carried out w h i l e one of the authors

analyses.

(Y.B.) held R e s e a r c h

F e l l o w s h i p from t~e A l e x a n d e r von H u m b o l d t Stiftung. We are g r a t e f u l to this o r g a n i z a t i o n for generous support. We thank Prof.Dr. morphique,

C. TRIBOULET,

L a b o r a t o i r e de P e t r o g r a p h i e

U n i v e r s i t ~ Pierre et Marie Curie,

M~ta-

Paris France for c r i t i c a l l y

r e a d i n g the manuscript. F i n a l l y we would

like to thank the Service G ~ o l o g i q u e du M a r o c for the

friendly c o o p e r a t i o n and logistic help w i t h o u t which this p r o j e c t could not have b e e n carried out.

58

REFERENCES

BASSIAS I. & W A L L B R E C H E R E. (1987): P r e s s u r e - t e m p e r a t u r e path of late P r o t e r o z o i c gneisses (Central Anti-Atlas, M o r o c c o ) . - EUG 4th B i e n n i a l Meeting, 13-16 April, Strasbourg, 7 (2-3): 290. BIRD GW. & FAWCETT JJ. (1973): Stability relations of M g - c h l o r i t e M u s c o v i t e and Quartz b e t w e e n 5 and 10 kb w a t e r pressure. J. Petrol. 14: 415-428. BRABERS P. (1987): A plate tectonic m o d e l for the P a n a f r i c a n orogeny in the Anti-Atlas, Morocco.- (this volume). B U C H E R K., FRANK E. & FREY M. (1983): A model for the p r o g r e s s i v e regional m e t a m o r p h i s m of m a r g a r i t e - b e a r i n g rocks in the Central Alps.- Am.J.Sci., 283: 370-395. C H A T T E R J E E N.D. & JOHANNES W. (1974): T h e r m a l stability and s t a n d a r d t h e r m o d y n a m i c p r o p e r t i e s of synthetic 2 M 1 - m u s c o v i t e , K A I 2 ( A I S i O10(OH)2) ~ Contr. M i n e r a l o g y and Petrology, 48: 89-114. 3 CHENEY J.T. & GUIDOTTI C.V. (1979): M u s c o v i t e + p l a g i o c l a s e e q u i l i b r i a in s i l l i m a n i t e + quartz b e a r i n g metapelites, Puzzle M o u n t a i n area, n o r t h w e s t Maine.- Am.J.Sci., 279: 411-434. E N G L A N D P.C. & RICHARDSON S.W. (1977): The influence of erosion upon the m i n e r a l facies of rocks from d i f f e r e n t m e t a m o r p h i c environm e n t s . - Geol. Soc. London J., 134: 201-213. E U G S T E R H.P., ALBEE A.L., BENCE A.E., T H O M P S O N J.B. JR. & W A L D B A U M D.R. (1972): The two-phase region and excess m i x i n g p r o p e r t i e s of parag o n i t e - m u s c o v i t e c r y s t a l l i n e s o l u t i o n s . - J. Petrol., 13: 147-179. GANGULY J. & KENNEDY G.C. (1974): The energetics of natural garnet solid s o l u t i o n I. M i x i n g of the a l u m i n o s i l i c a t e e n d - m e m b e r s . Contr. Mineral. Petrol., 48: 137-148. GHENT E.D. (1976): P l a g i o c l a s e - g a r n e t - A l 2 S i O 5 - q u a r t z : a p o t e n t i a l geob a r o m e t e r - g e o t h e r m o m e t e r . - Am. M i n e r a l o g i s t , 61: 710-714. GOLDMAN D.S. & A L B E E A.L. (1977): C o r r e l a t i o n of M g / F e p a r t i t i o n i n g between garnet and b i o t i t e w i t h ~O/ ~ O p a r t i t i o n i n g b e t w e e n quartz and m a g n e t i t e . - Am.J.Sci., 277: 750-767. GREEN T.H. & H E L L M A N P.L. (1982): Fe-Mg p a r t i t i o n i n g b e t w e e n c o e x i s t i n g garnet and p h e n g i t e at high p r e s s u r e and comments on a g a r n e t - p h e n gite g e o t h e r m o m e t e r . - Lithos, 15: 253-266. GRIEVE R.A.F. & FAWCETT J.J. (1974): The s t a b i l i t y of chloritoid b e l o w 10 kb P H 2 0 . - J o u r n a l of Petrology, I~5: 113-139. GUIDOTTI C.V. (1984): Micas in m e t a m o r p h i c rocks.- in: S.W. B A I L E Y Reviews in Mineralogy, M i n e r a l . S o c . A m e r . , 13: 357-456. HEY M.H.

(1954): A new review of chlorites.- Min. Mag.,

(ed.),

30: 277.

HODGES K.V. & S P E A R F.S. (1982): G e o t h e r m o m e t r y , g e o b a r o m e t r y and the A I 2 s i o 5 triple p o i n t at Mt. Moosilauke, New H a m p s h i r e . - Am. Mineral., 67: 1118-1134.

59

HOINKES G. (1986): E f f e c t of g r o s s u l a r - c o n t e n t in garnet on the partitioning of Fe and Mg b e t w e e n garnet and b i o t i t e . - Contr. Mineral. Petrol., 92: 393-399. H O L D A W A Y M.J. (1971): S t a b i l i t y of andalusite and the a l u m i n u m silicate phase diagram.-Am. Jour.Sci., 271: 97-131. LAIRD I. & ALBEE A.L. (1981): High p r e s s u r e m e t a m o r p h i s m in mafic schists from N o r t h e r n V e r m o n t . - Am.J.Sci., 281: 97-126. LIOU J.G. (1973): Synthesis and stability relations of epidote, F e S i 3 0 1 2 ( O H ) . - J o u r n a l of Petrology, 14: 383-419.

Ca2AI 2

ORVILLE P.M. (1972): P l a g i o c l a s e cation exchange e q u i l i b r i a w i t h aqueous chloride solution: Results at 700oc and 2000 bars in the p r e s e n c e of quartz.- Am.J.Sci., 272: 234-272. L.L. (1970): E q u i l i b r i u m of b i o t i t e with garnet in m e t a m o r p h i c rocks.- Geochem. Int. 1970: 157-179.

PERCHUK

R I C H A R D S O N S.W., GILBERT M.C. & BELL P.M. (1969): E x p e r i m e n t a l determ i n a t i o n of k y a n i t e - a n d a l u s i t e and a n d a l u s i t e - s i l i m a n i t e equilibria: the a l u m i n o s i l i c a t e triple point. Am.J.Sci. 259-272. S C H E R M E R H O R N L.J.G., W A L L B R E C H E R E. & HUCH K.M. (1986): Der S u b d u k t i o n s komplex, G r a n i t p l u t i o n i s m u s und S c h e r t e k t o n i k im G r u n d g e b i r g e des Sirwa-Doms (Anti-Atlas, M a r o k k o ) . - B e r l i n e r geowiss. Abh. (A). 66: 301-322. TRACY R.J. & ROBINSON P. (1976): Garnet c o m p o s i t i o n and zoning in the det e r m i n a t i o n of temperature and p r e s s u r e of m e t a m o r p h i s m , central M a s s a c h u s e t t s . - A m . M i n e r a l . , 61: )762-775. T H O M P S O N A.B., TRACY R.J., LYTTLE P. & T H O M P S O N J.B. (1977): P r o g r a d e r e a c t i o n histories d e d u c e d from c o m p o s i t i o n a l zonation and mineral inclusions in garnet from the Gassetts schist, V e r m o n t . - Am.J.Sci., 277: 1152-1167. W A L L B R E C H E R E.

(1987 a) : The A n t i - A t l a s

System,

an overview.-

(this vol.)

W A L L B R E C H E R E. (1987 b): A ductile shear-zone in the P a n a f r i c a n b a s e m e n t on the n o r t h w e s t e r n m a r g i n of the W e s t A f r i c a n craton (Sirwa Dome, Central A n t i - A t l a s ) . - (this volume). ZEN E. (1981): M e t a m o r p h i c m i n e r a l assemblages of slightly caleic pelitic rocks in and around the T a c o n c i c Allochtone, S o u t h w e s t e r n Massachusetts and adjacent C o n n e c t i c u t and New York.- Geol. Surv. Prof. Papers 1113, Washington.

60

APPENDIX SAMPLE 69 SPOT Xa.~

~..

r

c

r

c

r

c

1

3

2O

15

29

28

32

33

35

0,670

0,669

0,674

0,670

0,700

668

0,688

r 3 8 ....... 0,664

0,685

0,687

Xpyr

O,171

O,157

0,158

0,172

172

O,163

O,174

O,168

0,189

0,171

Xg r

O,101

O,102

0,088

O,O98

114

O,109

0,t10

O,~O9

O,103

0,114

Xsp T Mg

0,042

O,043

0,046

0,043

046

0,048

0,048

0,050

0,517

O,O51

0,200

0,202

0,184

0,197

205

O,193

0,206

0,200

0,202

0,205

4~

50

86

SAMPLE SPOT

1~

' 20

e

r

c

r

8

9

2 ~ 24

.

23

r

r

2

16

18

c

r

'6

.............. 1

Xa1

0,629

0,635 0,588

0,588 ~O,559

0,520

0,580

0,581

0,591

0,555

0,538

Xpyr

0,157

O,164 iO,~52

O,~49

O,152

O,136

O,147

0,158

0,150

O,162

O,159

Xgr

0,173

O,124

0,142

O,139

0.159

0,154

0,139

O,138

0.140 IO 1 6 ~ . 0 , 1 6 7

Xsp

O,042

0,077

0,118

O,124

0,130

O,189

0,124

0,122

O,119

0,121

0,136

XMg

O,189

O,196

0,203

O,198

O,214

0,208

O,195

0,209

0,203

0,226

0,228

TABLE 1 : Garnet mole fractions for used cores and rims. Xal Xpr = Mg/(Fe+Mg+Ca+Mn}, Xgr = Ca/IFe+Mg+Ca+Mn), Xsp al = almandlne, py = pyrope, gr= grossular,

50

Fe/(Fe+Mg+Ca+Mn), Mn/[£e+Mg+Ca+Mn),

sp = spessartlne.

l-----~-[

..... 86 1

XMg = Mg/(Fe+Mg)•

69,

1 2, I 1TI 28 1.0 1 31 1 0,36

0,35

TABLE 2: Xsg of biotites XMg and Xmg = MgO/(MgO+FeO) of muscovltes used for calibratlons].

0,33

0,54

0,49

(molar proDortZons

fH20~I~XH20~ _ SAMPLE

SPOTS

69

Xgr

)

r - mica 88 34 bl 38 31 m 29 30 m

~ 690 720 757

~~

I"

-

29

O,114 I ( O,114 I (

35 32 20

31 m 34 bi

708 698

I

-

26

O,108 I (

I

26 b i

62O

87 8

0,098

(

0,838

3 35 18

4 bi 37 b£ 14 b£

593 619 568

86 19 2

O,102 0,109 0,09

( (

0,855 { 0,820 0,835

60 27

0,114 I (

0,004

6151

0,019

4556 4834 4834 5~67 5~67

0,009 O,OOO

571

0,78

194c 0,72 113~ 0~80 64

0,67

.------4--

i 86

50

~

49

20

18 m

15

23 m

6s7

25

18 8

21 m 11 m

585

21 22

2 24

3 m 4 m

~

911 m

582

21

6

15 m

566 415

21

t

TABLE

3:

26 m

593

O,124

.

I o , 1.7. .5. . .I~

i

617 23 643 25 593 "23

--

in

pond

O~/S

O,139 O,159 O,158 0,138

0,802 0,767 O,?SO

5~ 5958 5~ 4387 4 ~66g

0,804

4, 3086

0,55

0,162 I 0,192 i

0,841 0~82J

1195 1 188

O,2t

15

garnet

~

water biotite

that

samples to

7~26 87{)0

O,851 0,820

indicates for

0,55

7~

O,140 I 0,134 I

Pressure-temperature, data

1776 17

0,829

the

fuqacity

= muscovite

muscovites

86,

50,

Xmg

value

49

are derive

of

used

8 b4o8

(fH20) +

not from

and

molar

plagioclase in

muscovites.

volume

0,93

O,S3 0,72

0,3[

o9

water

(XH20)

assembla<jes.

equilibrium.

pressures

0,89 u,8]

and

Water

fugacity

temperatures

values

which

corres-

A P L A T E T E C T O N I C M O D E L FOR THE P A N A F R I C A N O R O G E N Y IN THE A N T I - A T L A S , M O R O C C O Peteralv M. B r a b e r s * Fysico-Chemische Geologie, Katholieke Universiteit Celestijnenlaan 200C, 3030 Heverlee, Belgium

Leuven

Combined field observations and petrological evidence from Taznakht block, in the Anti-Atlas, Morocco, have led to a s t r a t i g r a p h i c c l a s s i f i c a t i o n of the Precambrian, applicable in other P r e c a m b r i a n b l o c k s in the A n t i - A t l a s and High Atlas as well.

the new the

Lithostratigraphic and geochronological correlations made between Precambrian formations in the different blocks, have allowed to propose a plate tectonic model explaining the Panafrican Orogeny in the Atlas by the opening and closing of an ensialic marginal basin associated with southward subduction of an oceanic plate below the northern edge of the West-African craton.

RESUME Une combinaison d ' o b s e r v a t i o n s de terrain et de donn~es p6trographiques de la boutonniere de Taznakht a men~ ~ une classification nouvelle de la stratigraphie pr~cambrienne, qui est valable aussi dans les autres boutonnieres de l'Anti-Atlas et du HautAtlas.

Des c o r r e l a t i o n s l i t h o s t r a t i g r a p h i q u e s et g ~ o c h r o n o l o g i q u e s entre lea f o r m a t i o n s p r ~ c a m b r i e n n e s des d i f f ~ r e n t e s b o u t o n n i e r e s nous ont permis de proposer un module g~odynamique expliquant l'orog~n~se panafricaine par l'ouverture et fermeture (obduction) d'un bassin m a r g i n a l e n s i a l i q u e c o n t e m p o r a i n e de s u b d u c t i o n d'une p l a q u e o c d a n i q u e vers le sud a u - d e s s o u s de la b o r d u r e s e p t e n t r i o n a l e du c r a t o n ouestafricain.

Introduction Geographically, chain

the

in South Morocco,

the south by the Precambrian

M o r o c c a n Sahara.

blocks

sedimentary cover Lower-Dra,

Ifni

and

extension

the

Saghro

Jbal

or

(Fig.

northern

"accident

Anti-Atlas

bordered

blocks.

majeur

de

zone e r o s s c u t t i n g * p r e s e n t address:

The

a

F r o m west blocks;

the

geology

An

it (fig.

the

important

trending

the High

and

in of

feature major

by

the m a i n

Jbal the

mountain

is m a r k e d by a n u m b e r

surrounded

to east

High-Atlas;

l'Anti--Atlas", a

WSW-ENE

north by

"boutonnieres", I).

Kerdous into

is

in the

Sirwa

of

the

a

Palaeozoic

b l o c k s are: block

Taznakht,

east-west

Atlas

with

the it's

Bou--Azzer and

Anti-Atlas trending

is

I).

Geosurvey,

Blijde

Inkomststraat

the

fault

31, B 3 0 0 0 L e u v e n

62

.q

4J

I

0

a -,'4

0 ,-4 r~

0 0 P

5 o°

0,

t~Z

63

It

may

be

represented

northwest are

of

the

PIII)

Their

orientation quartzites

northwest.

Such

an

is

neg]igeable

environment

PI],

formations, detritic

indicate

and

even

grade. the

E1

the The

of

are

is known

shows

that

sediments

of

Jbal

the

because

of and

The m i s i d e n t i f i c a t i o n PII-III

described in fact,

by

Choubert

is not of

from

suitable

the

(near in

the

end

of

important

by

abundant devoid

been

of

erupted

of

the

turfs

quartzites

may

all

Azguemerzi with

part

the

PIIa

is h i d d e n

lower

(quartzitie)

part

is

cut

off

the

Eburnean

to

the

granites

basement

of

and

most

PII-III

always

1963).

This

mistake

similarities shelf

defining

by

the

is

between

the

PIIa

near

Taghdout

Taghdout

profile,

the

PII

because

unconformity

a reverse

been

quartzites.

recognizing

type-locality

below

the

1986).

have

The

as

study

equivalents

by not

and show

(Pl)

petrographical

continental

Pl

metamorphic

gneisses,

(Brabers

(1951).

for

upper

high

Taghdout)

PII

Termier

assigned

their

lithological

the

Tiffadine,

general,

been caused

at

an

Assemlil,

metamorphic

the PIIa

and

in

of

the

turfs,

been

and v o l c a n i c s

at

Jbal

Post-tectonic

shelf

A detailed

the

the

has

unconformity

onset at

have

and

(Choubert

quartzites

the

marked

of

associated

Mimount

PII

have

Taznakht

Anti-Atlas

the

the

environment.

underwent

Cued

because

plain.

shelf

to

always

different

represent

understandable

-

I)

Zenaga

degrees

deposited

orientation

Imghlay,

the

in

were

Amsemsa

similar

1963)

they

in

the PII-III

of

the

quartzites

quartzites assigned

Jbal

them.

as well.

ultramafics

quite

in

continental 4. The

(fig.

30

erosion.

continental

origin

(Choubert

absence

they

probably

the

of

continental

phase

probably

the

m north

before

series from

mountainless

tectonic

tuffs

Ouarzazate distinct

dipping

necessarily

Tamazight,

Hara

P0

presence

that it

of

a

Amsemsa

Amsemsa

pretectonic

existed

the

pretectonic

a preteetonic

the

probably

main

are

while

Jbal

area,

intensive

Moreover,

the

gneisses

Ourika or

in

have

Jbat

intercalations

erosion

the

of

400

the

ruffs

intercalations,

to

of

same

Anti-Atlas

therefore,

pretectonically. compared

as

detritic

with

sediments,

sedimentary

3. The

the

associated

The

quite

these

only

ruffs

exposed

the

of

During

however,

uplift

2).

same

that

can

Orogeny.

(Fig. volcanics

in

absence

of

flow

PIII)

the

shows

Panafrican the

(Lower

ruffs,

period

by

ash

lithologically

(PIIa)

The

Amsemsa

plain

are

series

shelf

the

overlain

and

Sirwa-Saghro

a

Zenaga

unconformably

(Upper

by

fault

and

the the

separating

64

This the

paper

northern

part

is

based

of

the

mainly

on

Taznakht

results

block

from

(fig.

•

a

detailed

study

2).

Cretaceous, Ter~iary, Quaterna~f

!!ii!iiiiiii?!iil i!;ii!!iii ~

~

.....

ar~rlan and Infra-Carbri~n: marine sediments calcalkalLne %~icanics

~

calcalkallne granltoids

~

calcalkaline diorltes

~

detrltic sedirents (PII-III)

~

h loWly deformed qnelsses (PIla) with c~/~iolite fra~e-qts (riCh) ~HH~ c~hiolit/c rod,s (pITh)

Ta~n~ht.

~

tholeltic gabbros (PIIa)

~

oontir~ntal shelf formation (PIIa): dolomltes, quartzltes, tho~ei~es amd pelltas ~--~granltes, gneisses and micaschists (Pl)

•iiiiiii!i!i!!!i!! iiiii!iiiiiii!iii':'::::ii!':::~i!~i

Taznak~ht Zenaga plain

C~

-

@ km

Figure 2: ~ e

Taznakht block.

in

65

]. S t r a t i g r a p h y l.l New c l a s s i f i c a t i o n A

new

proposed

of

classification

in fig.

the

Precambrian

in

the

Anti-Atlas

is

3.

Cambrian and Infra-Can~rian: conglomerates, shales, dolomites, sandstones, calcalkaline volcanic intercalations

PIII: Sirwa-Saghro series(l) and Ouarzazate series(2): calca/kaline volcanic andesites and ash flow ruffs with their plutonic equivalents and sedimentary intercalations

0r ' u~

&

rr.v:.v.v,':,v:,v::v.v::.

o cO

..4

PII-III: detritic sediments with minor calcalkaline volcanic intercalations

PIIb

pIIa

PIIc

|--__. . . . . .

E

"}:::5::::::::::i

PIIa: continental shelf formation: quartzites and dolomites(1), tholeitic volcanics and plutonic rocks(2), sericite-schists with spilitic and keratophyric tuffs (3) ophiolite sequence: ultramafics(1), tholeitic volcanlcs and plutonic rocks (2), spilites, kerato~yres and ~=~dments(3) continental facies: C" dnyolitic ash flow tuffs and granites

PI: Eburnean basement (Lower-Proterozoic) micasd%ists and migmatic gneisses(a) intruded by Azguemerzi(b) and Taznakht(c) granites ¢

Fig. 3: Stratigraphy of the Precanbrian in the Anti-Atlas.

The

metamorphic

unconformably three an

facies

oceanic

facies

covered may

by

shelf

basement

formations

distinguished:

(ophiolitic)

(PIIc).

continental

be

Eburnean

North

of

(PIIb)

the

accident

of

have

Precambrian

a continental

facies

sediments

the

(Precambrian

and

a

shelf

II

majeur

de

in

facies

continental

been g n e i s s i f i e d

I)

is which

(PIIa),

(volcanic)

l'Anti-Atlas

the

due to o v e r t h r u s t i n g

66

of

ophiolites

tholeitic

from

the

volcanics,

transformed

into

amphibolites

ophiolitic

metamorphosed.

It

quartz-diorites

associated

sediments.

Ophiolite continents]

The

next

level

formations.

This

trending

overturned

the

by erosion The

series,

Infra-Cambrian

with

different

placed

volcanoand

over

the

sequence

1963)

a

have

huge

northwest-

been

bimodal

may

marine

minor

older

vertical

be

or

even

generated

overlain

sequence

angular

calcalkaline

recognized

unconformably

A

detritic

faults.

of

1963).

a

and

set

probably

reverse

folded

a

is

covering

east-west

a

other

the

of

(the by

the

Cambrian

unconformity

to

with

the

Zenaga

ectinites

covering have

level

erosion

as

and

the

intrusions.

granite

granites

and

Taznakht

in

the

the

of

facies

the

and

been

to

has

any they

been

PI

I.

The

differences schemes. granites

at

However, granites,

be

the

they

consist and

than

of the

them.

volcanism

recognized

as

influence

younger

covering

in

same

gneisses

metamorphic

oaloalkaline

never

number

misinterpreted

the

must

a

table

Azguemerzi

unconformably

pretectonic

in

essential

classification

basement

Therefore,

assigned

schemes

some

past

undergo

in fact,

(PIIc)

been

PI micaschists.

of

didn't

they are,

existence

continental

they

(1981)

belonging

products

micaschists

shows

the

(orthoschists)

stratigraphic

have

classification scheme

and Leblanc's

plain

classifications

formations

different

(1963)

I. The m e t a - a r k o s e s

the

stratigraphic

classification

with C h o u b e r t ' s

2. The

all

column

have

consists

Precambrian

into

proposed

of

sediments

Choubert

shows

scattered

to

that

along

(Choubert age

a

series.

1.2 C o m p a r i s o n

the

related

The

III

and

and

keratophyres

unconformably

faults

which

and

trondhjemites

metsgabbros

stratigraphic

is

uplifted

in

series

Ouarzazate

and

the

reverse

PrecambrJan

garnet-bearing disrupted

spilites, found

(PII-III)

sediments.

complex

Sirwa-Saghro

newly

in

are

been

formation.

sequence

of blocks

Ouarzazate

The

shelf

sequence

southeast

volcanic

fragments

of

have

fine-grained

lenses, highly

syntectonic

tholeitic

consisting

gneissified

gneisses,

is

and

dolomites,

sericite-schists

amphibolite

prewith

Quartzites,

scspolite

formation

includes

sequence

sedimentary

dolerites,

chlorite-schists,

The

sedimentary

tholeitic

facies.

metaquartzites,

and

gneisses.

oceanic

in

the

in

a

past.

67

i I

A O

~

t.,l

I

.,,.i ~

~

1,~

•. t o o

1~1

"° ~

m

Cn-,~

I I

I

§ | N

O

!

1.-4 {-~ e l:u

..O

|

I--t ~ ~.

N ~

v r~

.,H u~

(J E)

O

II I-4

I-4

I-4

n

.,M 4J A

v

N ,.,.4

g.l

,< N

,-4 O

t-.4 I

I H

1

I

I

68

a southern Jbal

uplifted

Mimount

the uplifted associated explains

block

quartzites block

with

that

have

reverse

our view

%

from

a northern

undoubtedly been

% % !

downfaulted the

erosion

set vertical

faulting

on the Taghdout

are

(see

block.

The

products

of

due to d r a g f o l d i n g

section

2.3).

Figure

4

profile.

•

quartzites and eonglon~rates (PII-III)

~

dolerite sills (PIIa)

~

skarns (PIIa)

~

dolomites (PIIa)

~

Taznakht granite (PI) Jbal Mimoun1:

1

,

S

Oi

N

3• 0 0 m

Fig. 4: The Taghdout profile.

The

original

general

confusion

Useful

criteria

misinterpretation concerning to

are:

I) PII-III

quartzites

faults.

a consequence,

As

overturned.

quartzites, reverse 2)

PIIa

while

are

This,

unless

they

with

usually

generally, are

all

profile over

between

associated

these

this

quartzites

distinguish

quartzites

even

of

has

the

PIIa

and

bounded

been

set

the

case

situated

PII-III

by

not

accidentally

to a

Anti-Atlas. and

have

is

led

reverse

vertical with

near

or

PIIa

such

a

fault. quartzites

PII-III

quartzites

are

intercalated

quartzites

are overlain

can

be

traced

about

10

or

overlain

by shales. kilometers

by

dolomites,

The Jbal westward

Mimount where

69

shales

are

should 5.

The

be

the

Arguments

this

PII-III may

similar

the

as

There

series

kilometers

3)

2.

The

to and it

Alpine difficult

fault.

classified

and

Choubert.

both

series:

detritic

have

still

reverse

active

in

vertical

set

the

been

because

both

sediments

undergone:

have

been

PIII, a

to

some

a

while

cases close

the

to

some

clear

detritic

are

series,

Sirwa-Saghro

between

series.

north

of

series

of

Tamezzarra

This

the

the

Sirwa-Saghro

unconformity

Zenaga

northwest

of

is

plain the

well

and

Zenaga

3

plain

sheet).

important

while

the

unconformity

PII-III

Taghdout

ealcalkaline

classify

because:

Imdghar-n-Izdar

lithological

volcanics in

the

occur detritic

differences

between

abundantly PII-III

in

the

series,

both Sirwa-

they

are

insignificant. detritic while

the

PII-III

series

Sirwa-Saghro

is

always

series

may

associated occur

with

elsewhere

reverse

as

well.

evolution

Precambrian

numerous

has

prefer

northeast

There

Tectonic The

Lower

1/50,000

faults

as

both

PlII,

we

the

at

Saghro

between

well

may

series

exists

and

exposed

rather

was

Leblanc

faults.

separately

2)

history

Lower

Nevertheless,

series:

as

PII-III,

volcanosedimentary

(see

similarities

tectonic

during

series

by

be:

volcanics

the

undoubtedly

PII-III. Sirwa-Saghro

during

I)

therefore,

series

faulting

reverse

They,

voleanosedimentary

calcalkaline

the

the

lithological

contain

2)

to

them.

detritic for

the

covering

assigned

folded

with

I)

found

tectonic orogenies. or

formations movements

in

during

Reactivation

impossible

to

the

of

assign

Anti-Atlas the older a

have

been

submitted

Panafrican,

the

Hemcynian

fault

correct

systems age

to

often every

makes single

70

2.1 T h r u s t i n g The

first

important

thrusting

during

phase

lost

are

during

the

foliation reaching

in

the

in

disappearing

ophiolite

south

of

the

thrust

grade

in

the

tectonic

intensive What

rocks;

2)

majeur;

4)

accident

in

i.

2 0'.', 0 ,

1900

i u

'~'~

Adrar Asfal

the

majeur

north

to

50 of

..'"

deformed

Ar~

NtQ°b

•

.

©

Lower-PIII: cc~zglomerates and sandstones PIl-III: oDnglommrates, quartzltes and shalem calcalkallne granites (PII) ~holeltlc gabbros (PIIa) PIIa: metaoonglomerates and quartzites(1) dol~mltes and scapolite ~neisses(2) tholeitic mmtavolcanlcs {3 ) shales, schists and gnelsses(4)

x

@

clear

Pana frlcan faults 1~rcgni~m faults Alpine faults

Fig. 5: b e "accident majeur de l'Anti-Atlas" near N'Qob.

area,

area

and

by

the

northward (Fig.

,: ."

a

amphibolite

degrees N'Qob

this

metamorphism

Imghlay

,Ti~arnina

il.il::::!.

I)

Tamazight

Jbal

I

of

faulting

smaller

highly

40

..... ~

is:

the

the

single

Jbal Assakkoumd

southward

features

regional

in

fragments, a

$

2100

a

3)

is

strike-slip

remains

southward

ophiolite and

movement

conditions

accident

sheet

fault

phase.

facies

larger

ophiolite

of

related

metamorphic the

Most

overprint

amphibolite

of

tectonic

obduction.

the

tectonic

south

overriding dipping

to

second

decreasing

lenses

ophiolite due

lower

Panafriean

N

5).

71

2.2

Strike-slip PI

The

and

PII

formations

intensive

strike-slip

movements

The

of

can

sense

Jbal

movement

Amsemsa The

the

ash

most

is

remobilized the

An

rare

primary

example de

of

faults

2.3

Reverse

faulting

by

synsedimentary

overturned exactly

the

at

substratum; substratum,

by

have

it,

this

before,

the

reverse

age

(Fig.

PII-III

have

faults

are

between

the

PII-III

they

may

seen

the

sediments

be

erosion le at the Asdrem aplites ~

erosion near

in

~

\

level

that

of

Another which

it

has

obscured

1986). is

the

accident

appear

(Fig.

""'"%~ ..... ~ \ ~

usually

set

contact

on the e r o s i o n

most

to

be

5).

the

depending

2).

that

it,

faults

formations

that

These

with

zone

most

the

(Fig. is

(Brabers

fault

where

zones.

vertically.

extent

patterns

faults

stratification.

such

fault

by

crosscutting

phase

rotated

strike-slip

deformed

fault

associated

to

N'Qob,

fault

tectonic

been

elements

near

a

left-lateral

retromorphism

typical

highly trending

from

lithogeoehemical a

are

east-west

a

of

of P a n a f r i c a n

mentioned

is

effect

earth

l'Anfi-Atlas

strike-slip

As

this

greenschist

the

original

majeur

affected

the

along determined

tuffs:

pronounced

formations

feature

flow

be

usually

are

vertical

not

even

necessarily

sediments as

bounded or

well

and as

seen their

in

the

6).

,!'"

" """~" .... :'" '

level

Fig. 6: Synsedim~ntary PII-III reverse faulting.

In representing III)

the

Taghdout the

is s i t u a t e d

southern in the

profile, limit

for of

substratum

example,

the

Jbal

(Fig.

5).

the

Mimount

reverse

fault

quartzites

(PII-

72

The

other

PII-III (fig.

sediments, 2).

A

crosscutting aplites and

configuration,

the

can be

shales The

before

may

PIla

traced

sudden

sediments

is

number

be

found of

with on

the

the

Asdrem

north-south

gneisses 50 to

in

the into

of

these

terms

of

///a.

to

reverse erosion

When,

during

erosion The

faulting than

started

aplites,

the

reverse

(Fig.

however,

7).

The

surrounding faulting,

attacking

PII-III

the

Taehaokcht

aplites

are

formation.

The

the P I I - I I I

sandstones

aplites

in

the

erosion

PII-III

associated

plane

at the Asdrem aplites.

aplitic

gneisses the

gneisses being

of

in

before erosion

Fig. 7: PII-III faulting and ~ n t a t l o n

with

south

granitic

differential

~/~ t /~

situated

abruptly.

disappearance explained

of

southward

fault

plateau

striking

north

80 m

disappearing

reverse

more

dikes and

northern

and

aplites

resistant

are

stick block in to

the

more out was

resistant in

uplifted,

uplifted

erosion

relief.

block.

than

the

73

gneisses,

were

certain

period

sediments. the

of

Thus,

impression

fact of

eroded

older the

2.4

of

slowly were

aplitic

intruded

sediments.

edge

of

the

than

at

can

after

the

first

give

they

be

a

PII-III

sight

formation,

fault

(Fig.

and

by

the

PII-]II

reverse

aplites

gneisses

surrounded

dikes

the

The

the

are

situated

in

south

7).

Folding to

PIII

volcanics

west

trending

style

of

gentle

folds.

the

as

Hercynian

subhorizontal

well

folds.

folding

The

the

may

vary

PII-III on

volcanics,

sediments

the

from

tectonics

"accident and

southern

D~collement unfolded

the

Upper-PIII

are

competence

tight

Lower

in

east-

folded

of

isoclinal

the

the

folds

rocks, to

the

extended

tectonics

downfaulted

uplifted

as

Depending

Hercynian

alongside was

the

having

the

southern

more

sedimentation,

although

than

Contrary

2.5

much

majeur

the

block

structures

Precambrian

are de

marked

slipped

basement

vertical

l'Anti-Atlas".

The

sedimentary

strata

Palaeozoic

showing

by

downward folded

rocks,

onto

the

Palaeozoic are

movements

northern

block

covering

downfaulted

formations

found n o r t h e a s t

the

block.

on

top

of the

of

Zenaga

plain. 2.6 Alpine

tectonics

During northern and

the

and

where

blocks.

the A s d r e m

by such a fault Both

(Fig.

faults

phonolites

orogeny,

northwestern

southeastern

Asdrem,

5).

Alpine

of Jbal

blocks

were

Examples

of

plateau

2) and

have

which

in

fact,

uplifted Alpine

is

relative

faults

are

is

separated

from

in the

"accident

majeur"

displaced

Late

still

Tertiary

to seen

active, southern at

the T a m a z i g h t

(or

near Early

N'Qob

Tizi plain (Fig.

Quaternary)

Sirwa.

3. G e o c h r o n o l o g ~ The

abundance

High-Atlas

allows

Precambrian

blocks.

geochronological stratigraphic all other

of

geochronological

correlations A

data

be

data

in

made

between

reinterpretation leads

classification

Precambrian

to

blocks

to

set up

the

the

of

for the

Taznakht

the

that block

and

different

stratigraphic

conclusion

in the A n t i - A t l a s

Anti-Atlas

the is valid

and H i g h - A t l a s

and new in

as well.

74

Geochronological data

and

correlations

between

the

different

b l o c k s a r e d i s c u s s e d e l s e w h e r e ( B r a b e r s 1986). 4.

P l a t e t e c t o n i c mode] The

the

(Fig.

stratigraphic,

former

sections

8)

tectonic

may

and

petrographic

n o w be t r a n s l a t e d

data

into plate

discussed

tectonic

in

terms.

I) R i f t i n g At

the

onset

Anti-Atlas, dolomites. opening

the

PII,

subsidence

may

have

in

the

deposition

of

PIIa

The

of

dipping The

of

resulting

subsequent

an

subduction

subduction

exposed

at

zone

and m a y

tholeitie

ensialic

volcanism

back-are

the

and

basin

northern

the

be s i t u a t e d

north

of

arc

could

the

in

the

quartzites have

associated

edge

volcanic

occurred

marked

with

the

southward

West-African

associated

and

with

craton.

it

are

not

of the A t l a s .

2) E x p a n s i o n As basin

expansion

and

pelites result that

a

were of

deposited.

seaward

more

produced

(Vlaar

and

at

The

1976, situation

lithosphere

marginal of

midocean

a

is s u b d u c t e d

crust

probably

lithosphere the

Wortel

such

oceanic

shelf

migration

oceanic

amount

1980),

proceeded,

continental

basin

subduction

was

consumed

and

is

to

fast and

may

trench

as

the

a

fact

than

by s e v e r a l

where

PIIa

seen

to

England

occur

marginal the

be

due

the

1978,

the

which

trench in

Atwater

in

on

As d i s c u s s e d

likely

easily,

formed

expansion

the

ridge.

Molnar

was

existed

the

authors

and old

Wortel oceanic

steeply

dipping.

trench

becomes

younger,

subduction

angle

3) C o m p r e s s i o n As

the

subduction decrease. reached produced the

oceanic process

After that at

as the

marginal

a

lithosphere will

slow

certain

much

basin

period

oceanic

midocean

at

the

down

and

of

the

time

then,

lithosphere

ridge.

increases

As

is

a

a consequence,

and

expansion

situation

consumed

in

as

lateral the

the will

will

the

be

amount

pressure

marginal

on

basin

ceases. Due

to

volcanism

the

is

West-African subduction and

decreasing

shifted

continent.

volcanism

keratophyres

subduction

southward

may

In be

associated

the

over

angle the

marginal

represented with

the

by

the

focus

of

marginal

basin

basin,

this

the

ophiolite

calcalkaline towards

preteotonic

trondhjemites, suite.

A

the

spilltes

mechanism

for

75

N

/

a. Rifting (788+9 m.y.)

Expansion P[!c Plla PIIb

0phiolie~en

ion (724+50 m.y.) PII-III

e. ~ l ~ s i ~

T~Post-collision

volcanism (PIII)

Jbal ~oho

g. Transgression (Cambrian- Infra-Canbrian)

Fig. 8: Plate tectonic evolution of the Atlas during the Panafrican Orogeny

76

generating (Arth

trondhjemitic

]979;

nebu]itic ]98]).

Payne

of

tholeitic

fluids

rising

basin.

The

the

thus, in

the

ash

obduction

volcanic

the

have

tuffs

of

of t h o l e i t i c

slab

by

due

subducted

to

the

and

Amsemsa

volcanism

occurrence

of

marginal

pelitic

(Pllc)

may have

partial

influx

the

the

of

(Leblanc

by

an

below

in

(PIla)

gabbro

area

generated

crust

intercalations

Jbal

the

Bou-Azzer been

oceanic

formation

subduction

melting

supported in

may

the

oceanic

shelf

flow

shifting

is

rocks

gabbros

from

partial

1979)

magmas,

calcalkaline

subaerial

by

similar

continental

southward before

up

Strong

in

Trondhjemitic

melting

of

and

textures

magmas

series

pretectonie

show

reached

that

the

the

continent

started.

4) O b d u c t i o n As

obduction

calcalkaline that the

the

obduction

contact

due

to

this

magmatie

over

The

the

shelf

residual

The

Oued

and

The

as

movement

absorbed

by

calcalkaline lubricant

a

in

for

trondhjemitic

by

the

appearance is

not

to

the

marginal

of

unlikely

the w e a k e n i n g

continental

have

facies

the

generated

basin

of

lithosphere

pressure

been

at

the

in

the

ophiolites

were

marking

the

metamorphism

been

overriding

the

in

the

south

of

the

to the

absorbed

marginal

strike-slip magmatism. strike-slip magmas.

is

caused

by

ophiolite

between and

about

tectonics

result

was

distance the n o r t h

for the o r i g i n a l

relative

movement of

initiated

and

response

may have

north-south

strike-slip

obduction

it

could

and

first

shelf.

the

the O u r i k a

a minimum

interpreted

this

zone

of

was

In

planes

formation heat

Assemlil

considered

of

activity.

contact

the margin,

frictional slab

heating

(cfr.

Woodcock

ophiolitic

m~langes

1977).

maximum

of E 1 H a r a

plate

fact, oceanic

greenschist-amphibolite

and Eobertson

and

in

thrust

after

continental

the

the c o n t i n e n t a l

continental and

weak

the

between

trench,

mechanically

shortly

at

process,

zone

subduetion

thrust

started

volcanism

70

active

by

movements Granitic movements

in

magmas while

I).

this

plate.

the

of

of

thus

area may

thrusting

may

be

oceanic

was

be

oceanic

component plate

and

component

was

weakened have

may

the

normal

tangential

the

This

period

The the

Imghlay

width.

component

subduction while

(Fig.

basin

in

continental

of T i f f a d i n e ,

km

marginal

tangential

basin,

the

those

by

served

the as

lubricated

a by

77

5)

Collision

(PII-I]I)

Obduction collision

the

subduction,

plate

during

the

After as

well.

The

the

of

As one

that

the

have

meant

movements.

As

The

7)

has

Marine In

the

and

areas

of

be

Late

the the

deposited The

a

oceanic

high

to

be

on

the

zone

was

sedimentation

lithosphere associated

pre-collision due

to

the

strike-slip

the

continued with

it

volcanics

intensive

between

the

is

as the

erosion

in

the

of

in t h i s

period,

crust

during

continental against

strike-slip

strike-slip tectonics

may

zones.

Lower-PIII

tectonic outside

tectonics

resistance

focus

weaker

plate sought

of

higher

Precambrian,

Adoudounian Anti-Atlas later

numerous

formations 1960,

collision

and

and

model.

and A

Upper-PIII

possible

is

not

explanation

Morocco.

transgression

Atlas

were

this to

had

pressure

the

volcanism

thickening

towards

unconformity

probably

and

the

obduction

component

faulting

the

the

of

consequence,

elsewhere

by

by

and

component

zone.

disappearance

a

of

with

may

explained

normal

normal

consequence,

reverse

intercalations

argue

shifted

absorbed

a

calcalkaline

collision

have

the

untill

(PIII)

difference

the

fragment

increased

subduction the

collision

for

can

and

detritic

uplifted

continental collision

entire As

the

volcanism

main

abundance

continued

was

the

have

explain

collision,

before

before

probably

PIJ-III.

Post-collision

just

northern

alone.

may

may

basin

movement

collision

margin This

the

Whereas

subduction

continental

6)

between

after

by

uplifted.

marginal

continent.

relative

absorbed

rates

the

occurred

West-African of

of

et

1961,

Van

Viland

1972)

show

that

craton

was

still

High

the

calcalkaline

(Boudda

Choubert

active.

marine

sediments and

during

Proust

a

transgression

were

deposited.

Atlas,

Middle

In

first

over

the

marine

the

northern sediments

Cambrian.

volcanic 1972,

the

came

intercalations

Buggisch

Looy

1980-1985,

Van

the

subduction

zone

1986,

Pouit

Guestaine north

in

of

et

the

et

Van

the

marine

Jouravski Looy

1983,

West-African

78

I. A

new

stratigraphic

Precambrian The

applicable

stratigraphic

three

classification

facies

in

in

the

are

shales

and

applied different 3.

In

reinterpret

quartzite

addition

the

to

oceanic

(Plla),

facies

4. All g n e i s s e s the

gneissified

of the

by

strike-slip

tectonics m~lange.

from

Precambrian

the

towards 5. For a

the

m e l t i n g of subducted

is

the

I,

of

in

facies in

a

as

l'Anti-Atlas", the

Oued

They

they

mafic

and

Panafrican

be

called

is

accepted

the

by

may

position

to be

formation,

contain

disrupted

generally

Assemlil,

interpreted platform

such

thus in

an

changed

the

past,

II. associated to

with

explain

with

the

as

them,

ophiolites,

their

by

are

genesis

influx of

well

as

thus

with

the

partial

fluids

the

from

spilites

interpreted

in an o c e a n i c

e x p l a i n e d by

associated

of

the A n t i - A t l a s

i n v o l v i n g an

trondhjemites

The P a n a f r i c a n o r o g e n y i s marginal basin

PIIa

therefore,

of s u b d u c t i o n v o l c a n i s m

n o r t h e r n edge

are

known

shelf

recognized

de

are

stratigraphic

manifestation

a

is

ophiolite

associated

the

with

position

volcanics

gneisses,

keratophyres

of

quartzites

criteria

continental

obduction. an

proposed

The

the

activity

of

tholeitic gabbros plate.

PII-

associated

These

calcalkaline

Imghlay

and,

trondhjemites

mechanism

Plla

stratigraphic

Hara gneisses,

Their

the P r e c a m b r i a n

between

"accident m a ~ e u r

of

ophiolitic

Precambrian

I,

III.

faulting.

and

ophiolite

fragments

blocks.

as well.

equivalents

ultramafic

Atlas

the

in the A n t i - A t l a s .

volcanic

and E1

for

Precambrian

quartzites

correct

(Pllb)

and

Ourika

metamorphic

reverse the

(PIIc)

north

detritic

PIl-III

pretectonic

Tamazight

Tiffadine,

6.

the

facies

a

High

Eburnean

distinguish and

formations

pretectonic

continental

as

to

dolomites,

synsedimentary

to

II,

proposed

and

an

volcanic Precambrian

proposed

a s s o c i a t e d with

Anti-Atlas

comprises

Precambrian

III and a c a l c a l k a l i n e 2. C r i t e r i a

all

sequence

is

a

and as

a

facies.

creation

and c l o s u r e

s o u t h w a r d s u b d u c t i o n below

West-African craton.

An

e v o l u t i o n of

e x p a n s i o n t o w a r d s c o m p r e s s i o n was c o u p l e d w i t h s o u t h w a r d s h i f t i n g of s u b d u c t i o n c l o s u r e of

volcanism

the m a r g i n a l

due

to a d e c r e a s i n g

basin

resulted

s u b d u c t i o n angle.

in o p h i o l i t e

obduction

The and

79

finally

in

collision

the W e s t - A f r i c a n

of

the

northern

continental

fragment

with

continent.

Acknowledgements This paper has been the subject of a Ph.D. thesis at the Katholieke Universiteit Leuven in Belgium. It w o u l d not have been p o s s i b l e w i t h o u t the help of Prof. D.E. Vogel, the thesis promotor, who has been c o n d u c t i n g and s t i m u l a t i n g g e o l o g i c a l research work in the A n t i - A t l a s and H i g h - A t l a s for the last 13 years until his u n t i m e l y decease on May 18th 1986. His enthusiasm and the stimulating d i s c u s s i o n s with him are h i g h l y a p p r e c i a t e d by the author.

.R...e.!e.r.~ n_~ .e..~ Arth

J.G. 1979. Some trace elements in trondhjemites. Their implications to magma genesis and p a l a e o t e c t o n i c setting. In: Trondhjemites, dacites and related rocks (F. Barker), pp. 1 2 3 132.

Boudda A. et C h o u b e r t G. 1972. Sur la limite Marco. C.R.Acad. Sc. Paris, 275, D, 5-8.

inf~rieure

Brabers P.M. 1986. Een p l a a t t e c t o n i s c h model voor de o r o g e n e s e in de Anti-Atlas, Marokko. Ph.D. thesis U n i v e r s i t e i t Leuven.

du C a m b r i e n

au

Panafrikaanse - Katholieke

B u g g i s c h W. 1986. The L o w e r - C a m b r i a n of the Anti-Atlas: facies and p a l a e o g e o g r a p h y . Workshop: O r o g e n y on c o n t i n e n t a l margins, Berlin 19/2/1986. Charlot R. 1982. Caract~risation des ~v~nements panafricains dans l'Anti-Atlas marocain. Apport g ~ o c h r o n o l o g i q u e Rb/Sr. N.M.Serv. G ~ o l . M a r o c 313.

~burn~ens et de la m ~ t h o d e

Choubert G. et T e r m i e r G. 1951. Les c a l c a i r e s p r ~ c a m b r i e n s de Taghdout et leurs o r g a n i s m e s p r o b l ~ m a t i q u e s . N.M.Serv. G ~ o l . M a r o c 85, 9-33. Chouhert G., F a u r e - M u r e t A., G a u t h i e r H., H i n d e r m e y e r J., H o l l a r d H. 1960. Carte g ~ o l o g i q u e du flanc nord de l'Anti-Atlas Central dans la r~gion de O u a r z a z a t e (I/I00.000). Choubert G,, Faure-Muret A., g ~ o l o g i q u e de la plaine des

Morin Zenaga

C h o u b e r t G. 1963. H i s t o i r e g @ o l o g i q u e N.M.Serv. G ~ o l . M a r o c 162.

P., Hollard (I/I00.000). du

precambrien

H.

de

1961.

Carte

l'Anti-Atlas.

C o l e m a n R.G. and Donato M.M. 1979. Oceanic p l a g i o g r a n i t e revisited. In: Trondhjemites, dacites and related rocks (F.Barker), pp 149168. E n g l a n d P. and Wortel R. 1980. Some c o n s e q u e n c e s young slabs. E.P.S.L. 47, 403-415.

of

the

subduction

of

Leblanc M. et Lancelot J.R. 1980. Interpretation g~odynamique du d o m a i n e p a n a f r i c a i n de l ' A n t i - A t l a s (Marco) ~ partir de donnees g 4 o l o g i q u e s et g ~ o c h r o n o l o g i q u e s . Can. J.E.Sc. 17, I, 142-155.

80

Leblanc M. cobalt

1981. Ophiolites pr~cambriennes (Bou-Azzer, Maroc). N.M.Serv. Geol.

et g~tes ars~ni~es Maroc 280.

de

M o l n a r P. and A t w a t e r T. 1978. Interarc s p r e a d i n g and C o r d i l l e r a n t e c t o n i c s as a l t e r n a t e s r e l a t e d to the age of s u b d u c t e d o c e a n i c lithosphere. E.P.S.L, 41, 330-340. Pouit

G. et J o u r a v s k i G. 1960. Le g i s e m e n t de m a n g a n e s e de Isdid (r4gion de l'Ounein, H a u t - A t l a s ) . Mines et G ~ o l o g i e 30.

Tizi-nII, 21-

Payne

J.G. and Strong D.F. 1979. Origin of the Twillingate t r o n d h j e m i t e , N o r t h - C e n t r a l N e w f o u n d l a n d : p a r t i a l m e l t i n g in the roots of an island arc. In: T r o n d h j e m i t e s , d a c i t e s and r e l a t e d rocks (Barker F.), pp 489-516.

Proust F. 1961. Etude s t r a t i g r a p h i q u e , p e t r o g r a p h i q u e et s t r u c t u r a l e du Bloc O r i e n t a l du M a s s i f A n c i e n du H a u t - A t l a s (Maroc). Th~se de d o c t o r a t - U n i v e r s i t 4 de M o n t p e l l i e r . Saunders A.D., Tarney J., Stern C.R., Dalziel J.W.D. 1979. G e o c h e m i s t r y of M e s o z o i c m a r g i n a l basin floor. Igneous rocks from s o u t h e r n Chili. Geol.Soc. Am. Bull. part I, 90, 37-258. V a n g u e s t a i n e M. et Van Looy J. 1983. A c r i t a r c h e s du C a m b r i e n Moyen de la v a l l ~ e de T a c h e d d i r t (Haut-Atlas, Maroc) dana le cadre d ' u n e n o u v e l l e z o n a t i o n du Cambrien. Ann. Soc. G e o l . B e l g i q u e T.I06, 6985. Van Looy J . 1 9 8 0 . S t r a t i g r a f i e en p a l e o n t o l o g i e van de P a l e o z o i s c h e s e d i m e n t a i r e g e s t e e n t e n van h e t dal van T a c h e d d i r t ( B o g e A t l a s , Marokko). L i c e n t i a a t s t h e s i s - K a t h o l i e k e U n i v e r s i t e i t Leuven.

Van

Looy J. 1985. Het kaartblad Tazenakht Marokko. Kartering, lithostratigrafie, P r e c a m b r i u m tot Tremadoc.

Viland J.C. 1972. Lea m i n 4 r a l i s a t i o n s G ~ o l o g i e 36, 5-32.

1/100.000 Anti-Atlas, biostratigrafie van

cuprif~res

de

Tanfit.

Mines

Vlaar N.J. and Wortel M.J.R. 1 9 7 6 . L i t h o s p h e r i o a g i n g , i n s t a b i l i t y s u b d u c t i o n . T e c t o n o p h y s i c s 32, 331-351.

et

and

W o o d c o c k N.H. and R o b e r t s o n A.H.F. 1977. O r i g i n s of some o p h i o l i t e r e l a t e d m e t a m o r p h i c rocks of the "Tethyan" belt. Geol. 5, 373376.

THE PRECAMBRIAN/CAMBRIAN BOUNDARY IN THE ANTI-ATLAS (MOROCCO) DISCUSSION AND NEW RESULTS Wemer Buggisch and Erik FI0gel Institut f0r Geologie, Universit&t Erlangen-N0rnberg, SchloBgaden 5, D-8520 Erlangen and Institut for Pal~tontologie, Universit~_tErlangen-N0rnberg, Loewenichstr. 28, D-8520 Erlangen

Abstract Controversial theories of the age of the Latest Precambrian and Early Cambrian are discussed based on published radiometric and paleontological data. New fossil findings of calcareous algae

(Kundatia composita KORDE) in the

S~rie des calcaires inferieurs give evidence of the Early Cambrian age of the Lower Adoudounien, which was formerly correlated with the Latest Precambrian.

Introduction The basement of the Anti-Atlas consists of Precambrian rock-units with multiple deformation and metamorphism. The stratigraphy of the basement is still under discussion (CHOUBERT 1963; CLAUER et al. 1982; LEBLANC & LANCELOT 1980) and will not be dealt with in this paper. The Panafrican Orogeny affected the Anti-Atlas during the Late Precambrian. The Precambrian metamorphics are unconformably overlain by sequences which can be interpreted as molasse of the Panafrican Orogeny: the S~rie d'Ouarzazate in the central Anti-Atlas and the S6rie d'Anezi in the anticline of Kerdous in the western AntiAtlas. These sequences are characterized by coarse-grained, padly volcano-detritic sediments and andesitic to rhyolitic volcanic rocks and ignimbrites. The molasse sediments locally grade either conformably or with low angular discordance into the overlying sediments of the 'Adoudounien' which overlap the older Precambrian rocks of the Anti-Atlas. The ages of the S6rie d'Ouarzazate and of the Adoudounien are under discussion as well. These units were established by CHOUBERT (1952; 1963) and the lithostratigraphic succession is still generally accepted by most geologists.

Middle Cambrian:

Schistes & Paradoxides Niveau d'Ourmast

Lower Cambrian: (G~orgien)

Gr(~s terminaux Complexe schisteux S6rie schisto-calcaires S6rie des calcaires sup~rieurs

'lnfracambrian': (Adoudounien)

S~rie lie de vin S~rie des calcaires inf~rieurs (S6rie de base)

'Precambdan II1':

Serie d'Ouarzazate

Tab. 1. Lithostratigraphic succession of the Latest Precambrian and Early Cambrian in the Anti-Atlas after CHOUBERT (1952; 1963).

82

During the Adoudoun~en and G~orgien the sea prograded with two regressive megacycles from the region of Agadir into the Guff of Anti-Atlas. Except for a short regression during the Upper Cambrian, a marine environment prevailed in the Anti-Atlas from the Middle Cambdan to the Lower Carboniferous. Geochemical analyses of lavas and pyroclastics intercalated in the S@rie des calcaires inf@rieurs south of Tazenakht indicate an intracontinental volcanism with affinities to the analytical data of the Afra volcanics (GASSN ER 1986). Therefore, the transgression of the Adoudounien sea was probably combined with the subsidence of an 'Anti-Atlas-Graben' (aulacogen linked with the Japetus Ocean?). During the H e r c y n i a n O r o g e n y, the sedimentary sequences of the central Anti-Atlas were hardly affected by deformation; the 'metamorphism' did not exceed the stage of diagenesis. The Lower Paleozoic strata of the western Anti-Atlas, however, show increasing strain and metamorphism (see BUGGISCH, this volume).

Discussion of previous work Although the lithostratigraphy has been generally accepted, the theories on the stratigraphic correlation of the S~rie d'Ouarzazate and the Adoudounien diverge considerably. The first biostratigraphical subdivision based on trilobites was established by HUPE (1952). New fossils - archaeocyathids (DEBAENNE & DEBRENNE 1978) and trilobites (SDZUY 1978) verified the Lower Cambrian age of the G#orgien (upper part of the S~de des calcaires sup~rieurs to Gr~s terminaux). The Adoudounien was hence regarded to be older than the Lower Cambrian. Therefore, the Adoudoudien and the S~rie d'Ouarzazate were assigned to the 'Infracambrian' and to the Precambrian 111. The first r a d i o m e t r i c d a t i n g of volcanics from the S@ried'Ouarzazate and of rhyolites from the High Atlas revealed K/At-ages of 315 ma and 215 ma and scattered Rb/Sr whole-rock ages with a mean value of about 420 - 430 ma (JUREY et al. 1974). These radiometric ages were obviously rejuvenated because the strata are conformably overlain by fossiliferous Lower Cambrian sediments. U/Pb dating of zircons from magmatic rocks - previously assumed to be of 'Late Precambdan' or 'lnfracambrian' age - revealed unexpectedly young ages. Geochronological correlations by different authors are summarized in Tab. 2 and Fig. 1 ;new data from the literature (after 1974) will be discussed below (numbers refer to Fig. 1): (1)An U/Pb zircon age of 615 _+ 12 ma was determined for the granodiorite of Bleida, which corresponds to the time of emplacement (LEBLANC & LANCELOT 1980). In relation to the older Precambdan basement the intrusion is postkinematic. It predates the deformation which affected the Precambrian 113. (2) CLAUER (1974) and CLAUER & LEBLANC (1977) established a Rb/Sr age of 623 _+ 18 ma (recalculated by DUCROT & LANCELOT, 1977) for the fraction < 21~mof metapelites of the Precambrian 113.This age is interpreted as the age of the very low grade to low grade metamorphism during the Panafrican Orogeny. (3) A Rb/Sr whole-rock age of 618 + 22 ma was determined for trachytes at the base of the Sede d'Ouarzazate (YAZID11976: after CLAUER et al. 1982, recalculated). (4) The 'ignimbrite 3' of the upper part of the S@rie d'Ouarzazate revealed an U/Pb zircon age of 563 +_ 10 ma. The Rb/Sr whole-rock age of the ignimbdtes is about 489.4:.18 ma (MIFDAL & PE UCAT t 985). (5) The 'ignimbrite complex 5' of the S@rie d'Ouarzazate yielded an U/Pb zircon age of 586 +- 20 ma (or 574 + 5 ma). The discordance of the age increases with the decreasing grain size of the analyzed zircons (MIFDAL & PEUCAT1985).

83

(6)

Subvolcanic rocks of the S~rie d'Ouarzazate were dated by BENZIANE (1974: after CLAUER et aL 1982,

recalculated). A Rb/Sr whole-rock age of 573 _+21 ma was determined. (7)

An U/Pb zircon age of 534 _+ 10 ma was established by DUCROT et al. (1976; recalculated by DUCROT &

LANCELOT 1977) from the syenite of the Jebel Boho (= Alougoum). The Rb/Sr whole-rock age is on the order of 310_+10ma. Western Anti-Atlas: Several rocks from the lfni anticline (dome-like structure = 'boutonniere') range between 660 and 554 ma ( J E A N E ~ E & TISSERANT 1977). CHARLOT (1976) reported a Rb/Sr whole-rock age of 530 _+ 10 ma (not recalculated) in sampled from the postkinematic Tafraoute granite of the Kerdous anticline. BQNHOMME &

N

S I I R m

z.-~c, z

Fig, 1. Schematic section across the central Anti-Atlas during the Lower Cambrian (not to scale). Units A to C according LEBLANC & LANCELOT (1980),

Series a f t e r CHOUBERT 1952;1953 G

calcaires de bose

Etage d' Amouslek

BOUDDA et al. 1979

BOUKHAR[ et al. 1979

AmousEekien

G~,orgien 570 - 500 m.a.

C B A

caicaires sup~rieurs fie de vin calcaires in f~rieurs s

Adoudoun{en (: Infracambrian}

S&rie d'0uarzazate

Precambrian Ill

Pre cambrian older

II

Tioutien Taliouinien ...... 550- 680 m. a. Adoudounien

Taiiouinie~ Adoudounien

800 - 900 m.a. , Precambrian III

Precambrian III

DESTOMBES et al. 1985 slate and limestone Formation Upper limestone Fro. lie de vin Fm, lower limestone and dolomite Formation Precambrian III

lO00m.a. Precambrian II

Precambrian II3

]

Precambrian I13 1450m.a,

Pre

c ambria

n

basement

Tab. 2, Subdivision and correlation of the Uppermost Precambrian and Early Cambrian according to several authors (Ato G compare with Fig. 1).

84

HASSENFORDER (1985) analyzed the fraction < 2p.m from the pelites of the Precambrian and the Adoudounien. Rb/Sr ages are scattered between 371 + 8 and 364_+24 ma; K/Ar ages range from 357 + 9 to 291 + 7 ma. CHOUBERT et al. always questioned the credibility of the radiometric data (CHOUBERT & FAURE-MURET 1971 ; 1977; CHOUBERT et al. 1979; BOUDDA et al. 1979). They favored an earlier age for the Precambrian, the S@rie d'Ouarzazate and the Adoudounien (Tab. 2) based on compared successions and paleoclimatologic and paleontologic arguments.

Paleontologic arguments SDZUY (1978) described olenellid trilobites (Eofallotaspis) from

the Sede des calcaires sup@rieurs of the

western Anti-Atlas (-Rout). He correlated this fauna with the lowermost Atdabanian of the Siberian platform. In the High Atlas the oldest trilobites were found immediately above the lie de vin facies, but the lithostratigraphic units of the High Atlas and the Anti-Atlas cannot be compared with complete certainty for the reason that they are separated by the Atlas boundary fault. Based on the same trilobites, BOUDDA et al. (1979) correlated the S@rie lie de vin (Taliouinian) with the 'Riph@en terminal = Vendian'. The S@rie des calcaires sup@rieurs was compared with the Tommotian, the 'Niveau de Rout' containing Eofallotaspis was regarded to represent transition beds and the S~de schisto-calcaires was correlated with the Atdabanian. DEBRENNE & DEBRENNE (1978) argued against these correlations on the basis of archaeocyathids found 10m above the first trilobites in the same section (Tiout). They compared the upper S@rie de calcaires sup~rieurs with the Atdabanian, but not with the lower Atdabanian. According to these authors the Precambrian/Cambrian boundary ranges lower in the section. Stromatolites from the S6rie lie de vin and the S@rie des calcaires sup@rieurs were described by SCHMITT (1978; 1979). Based on these stromatolites, SCHMI]q- correlated the S@rie lie de vin with the Vendian to Riphean III. This correlation was clearly contradicted by BERTRAND-SAFATI (1981) due to the fact that SCHMITT had neglected all the arguments for a Cambrian dating: the abundant algal bioherms of thrombolites and Renalcisand Epiphyton-like microstructures. According to TIMOVEIEV (in CHOUBERT et al. 1979), acritarchs are evidence of a Middle Riphean age for the Precambrian 113and an Upper Riphean age for the S@ried'Ouarzazate and the Adoudounien. Based on analyses of amino-acids, PRASHNOVSKY (in CHOUBERT & FAURE-MURET 1977) estimated the age of the S@riedes calcaires inf@deursat 700 - 800 ma.

Paleoclimatological arguments The regressive facies of the S@rie lie de vin was explained by glacio-eustatic sea-level changes in conjunction with the Varanger glaciation (CHOUBERT & FAURE-MURET 1977; BOUKHARI et al. 1979; BOUDDA et al. 1979). MONNINGER (1979: PI. 17, Fig. 6) even thought he had found pseudomorphs after ice crystals in thin sections of the S~rie lie de vin (the pseudomorphs are probably due to Ca-sulfates). However, in contrast to the glaciationmodel, all the distinctive features indicate a hot and dry climate and a frequently hypersaline environment: casts of halite, stromatolites and red beds. New data and fossil findings Complete sections of the Adoudounien were studied E of the road from Tazenakht to Foum Zguid at the SW margin of the El Graara anticline (GASSNER 1986). These sections expose the marginal facies of the Adoudounien which is strongly influenced by volcanism (Fig. 2).

85

W

E

coJcalres de bose

[~

algae

S~rie des colcaires sup&rieur

[~

trilobites

[~

limestones

[~

dolostones

S~rie lie de vin

siltstones

sandstones

S6rie des

[~

conglomerates

[~

volcanic rocks

calcaires inf~rieur 100 m

S~rie d' Ouarzazate

5o L 0

Fig. 2. Columnar sections of the Adoudounien at the SW margin of the Bou Azzer- El Graara anticline. Distance between the two sections about 4 km (after GASSNER 1986).

The S6rie d'Ouarzazate predominantly consists of rhyolites, ignimbdtes, tufts and conglomerates. The deposition of the Adoudounien begins above a minor unconformity and contains thin-bedded partly dolomitic red mudstones and siltstones. Poorly sorted greywackes are intercalated. They contain volcanic lithoclasts of acid and intermediate composition and marginally resorbed quartz grains. These basal beds are overlain by massive dark conglomerates with clasts of ignimbrites, rhyolites and quartz within a dolomitic matrix. Typical stromatolitic carbonates were deposited above this first unit of the S~rie des calcaires inf~deurs. Owing to the lava-flows and volcanoclastics, they can be subdivided into a lower and an upper dolomite sequence. Distinct algal structures were found within the lower dolomite sequence of the S~rie des calcaires inf6deurs. The S~rie lie de vin is exposed in the 'gr6s de Tikirt' facies. It is overlain by the S6rie des calcaires sup~rieurs containing thrombolites, oolites and early trilobites in the upper part.

Lower Cambrian Calcareous Algae Distinct algal structures occur in sample X 1 ci. D e s c r i p t i o n: The algal structures are represented by calcareous micritic areas (maximum dimension 10 mm x 20 mm) which contain tubular elements (Plate 1, Figs. 6,8). The outline of the structures is irregularly lobate. The lumina of the tubes are tilled with sparry calcite and surrounded by differentiated, concentric micrite zones which include very small spar-filled cells (Plate 1, Fig. 5). Longitudinal sections show inflated 'tubes' (Plate 1, Fig. 7); tangential and oblique sections exhibit peripheral spine-like processes which are sometimes truncated (Plate 1, Figs. 3, 5). Some tubes are constricted, some are

86

subdivided by densely spaced, bent and thin micritic partitions. The following pattern can be recognized within the 'wall' of the tubes (starting with the innermost part): a) a verythin black and undifferentiatied micrite zone bordering the lumina of the tubes (Plate 1, Fig. 3); b) a zone (width up to 35 p.m) with closely spaced or isolated cross-sections of cells which may occur in one or two layers (Plate 1, Fig. 2). The diameter of the cells is between 12 and 20 ~Lm;larger cells are found in the outer layer; c) a variably thick, very irregularly bounded zone exhibiting irregular concentric laminations and also isolated cells (diameter up to 15 tLtm). This zone consists of up to five lamellae which are characterized by alternating thin micritic or pelmicritic layers and sparry layers in between. This marginal zone is gray in transmitted light. Quartz-grains incorporated within this zone indicate a weaker calcification than in the central zones. The inner diameter of the tubes varies between 100 and 240 pm. D i s c u s s i o n:

The existence of cell structures, differentiated according to size and distribution, as well as the

tubular pattern indicates the algal nature of these structures. The algae were sometimes strongly calcified. The first occurrence of calcareous algae is known near the base of the Tommotian (RIDING & VORONOVA, 1984). The Cambrian algal flora is domintated by cyanophyceans and possible chlorophyceans (CHUVASHOV & RIDING 1984) which can be attributed to six morphological groups (RIDING & VORONOVA 1985). A classification of the alga described using these groups is not possible because of the occurrence of characteristics typical of the dendritic, tubiform and the tuberous form groups. The morphological classification does not embrace all Cambrian algae. This is also true for Kundatia composita KORDE, 1973 - a species considered to be a red alga. The criteria of this species correspond to the criteria of the alga described with regard to the differentiation of the thallus, the morphology and distribution of the cells, as well as the size of the cells. The species is characterized by encrusfing thalli with chain-like cells arranged along the axis of the thalli (corresponding to the 'tubes'). A g e:

Kundatiacomposita KORDE has been described until now only from the Bagradskian Horizon of the Lower

Cambrian. The species was found near the river Kiya in the Alatau region, Kuznets basin. Together with Kundatia the following species occur: Kordephyton crinitum (KORDE), Epiphyton leptosum, Razumovskia sp., Renalcis sp. and Siphonomorpha sp.. According to RIDING & VORONOVA (1984: Fig. 2) the first occurrence of Kordephyton in Siberia and Mongolia is Tommotian Ill, of Epiphyton Tommotian II, of Razumovskia Middle Atdabanian (perhaps Lower Atdabanian), and of Renalcis Nemakitdaldynian (a time interval prior to Tommotian). The Bagradskian horizon is attributed to the higher Lower Cambrian by KORDE (1971: p. 86); it forms the upper unit of the Aldanskian Group. The Bagradskian horizon overlies limestones with archaeocyathids.

Plate 1 : Lower Cambrian Algae, Anti-Atlas

Kundatia composita KORDE. East of the road Tazenakht to Foum Zguid, southwestern margin of the El Graara anticline. Sample X lci from the lower dolomite sequence of the S~rie des calcaires inf6rieurs. Fig. 1. Fig. 2. Fig. 3. Fig. 4. Fig. 5. Fig. 6. Fig. 7. Fig. 8.

Cross-section. The wall of the algal tube consists of micrite with closely spaced small cells (arrow). x 42 Cross-section, exhibiting closely-spaced cells occurring in two layers. Larger cells are concentrated within the outer layer, x 70 Cross-section with spine-like projections (arrow). x 70 Several 'tubes' with differentiated walls. Oblique and longitudinal sections of the ceils are indicated by arrows, x 42 Cross-section. Concentrical wall structure and truncated spine-like projections, x 70 Algal thallus with cross-sections and longitudinal sections of 'tubes', separated by dark, inhomogenous micrite, x 10 Cross-section and longitudinal section, showing inflated 'tubes' (arrow = x 30 Algal thallus with irregularly distributed 'tubes' surrounded by concentrical wall structures, x 10

87

88

Algae, most probably identical with Kundatia, are also known from the Lower Cambrian of Sardinia: SELG (1985) and BECHSTADT et al. (1985) describe algal/archaeocyathid reef limestones from the Matoppa Member of the Nebida Formation in southwestern Sardinia (upper Atdabanian). A re- investigation of the thin-sections shows that these limestones yield archaeocyathids together with algae of the Kundatia type, Epiphyton, Renalcis and

Razumovskia.

Conclusions In summarizing all the radiometric data, the following interpretation appears to be reasonable: The intrusion of the granodiorite of Bleida (615 + 12 ma) predates the last strong deformation which affected the central Anti-Atlas. This deformation is more or less of the same age a~ the veFy low grade to low grade metamorphism which affected the sediments of the Precambrian 113 (623 +_18 ma). Therefore, the age of this deformation is about 620 ma (602 627 ma) which can be correlated with the Panafrican Orogeny of the Pharusian Chains (Ahaggar: ALLEGRE & CABY 1972). During the postorogenetic crustal extension, the molasse basins of the Ouarzazate and Anezi subsided along synsedimentary faults. These troughs were filled with conglomerates, volcanic and subvolcanic rocks between 618 + 22 and 563 + 10 ma. In the western Anti-Atlas intrusions continued up to 530 + 10 ma (granite of Tafraoute). The syenite of the Jebel Boho (534 + 10 ma) is intercalated in the S~rie des calcaires inf@rieurs (formerly 'lnfracambrian'). Therefore, the correct interpretation of the age of this syenite (syngenetic or later intrusion?) is difficult. Most of the Rb/Sr whole-rock ages were obviously rejuvenated. The rejuvenation in the western Anti-Atlas can be explained by the Hercynian metamorphism (360-290 ma). However, the temperature did never exceed the 300 o C isotherme in the central Anti-Atlas (see BUGGISCH, this volume). This contradicts the views of LEBLANC & LANCELOT (1980) and ODIN et al. (1983). Probably devitrification and alteration were important factors for the rejuvenation in this area. The radiometric age of the Precambrian/Cambrian boundary is still under discussion. ODIN et al. (1983) proposed an age of 520 - 540 ma. If the Precambrian/Cambrian boundary is defined at 570 ma, the radiometric data of the AntiAtlas suggest a correlation of the Adoudounien with the Lower Cambrian. The occurrence of Kundatia composita KORDE in the S@rie des calcaires inf~deurs of the central Anti-Atlas provides evidence of a Lower Cambrian age for the entire Adoudounien. This is in accordance with the U/Pb zircon ages dicusssed above.

References ALLEGRE, C.J. & CABY, R (1972): Chronologie absolue du Pr@cambrien de I'Ahaggar occidental. - C. R. Acad. Sci., 275-D, 2095-2098, Paris BECHST,~DT, T., BONI, M. & SELG, M. (1985): The Lower Cambrian of SW-Sardinia: From a clastic l]dal Shelf to an Isolated Carbonate Platform, Facies, 12, 113-140, PI. 11-13, 5 Figs., Erlangen BERTRAND-SARFATI; J. (1981): Probl@me de la limite Pr@cambrien-Cambrien: la section de Tiout (Maroc); les stromatolites et leur biostratigraphie (SCHMI-FI 1979): critique et observations. - Newsl. Stratigr, 10, 20-26, Berlin BONHOMME, M.G. & HASSENFORDER, B. (1985): Le m~tamorphisme hercynien dans les formations tardi- et post-panafricaines de rAnti-Atlas occidental (Maroc). Donn@es isotopiques Rb/Sr et K/Ar des fractions fines. -Sci. Geol. Bull., 38, 175-183, StraBburg BOUDDA, A., CHOUBERT, G & FAURE-MURET, A. (1979): Essai de stratigraphie de la couverture s@dimentairede rAnti-Atlas: Adoudounien - Cambrien Inf~deur. - Not. [email protected]. Geol. Maroc, 271,96 pp, Rabat

89

BOUKHARI, A., CHOUBERT, G., FAURE-MURET, A. & TIMOFEIEV, B. (1979): Sur la microflore des couches terminales du Pr~cambrien III et sur rb.ge de I'Adoudounien au Maroc. - C. R. Acad. Sci., 288, 13-14, Paris CHARLOT, R. (1976): The Precambrian of the Anti-Atlas (Morocco): a geochronological synthesis. - Precambrian Res., 3,273-299, Amsterdam CHOUBERT, G. (1952): Histoire g6ologique du domaine de I'Anti-Atlas. - Not. M~m. Serv. Geol. Maroc, 100, 77194, Rabat --

(1963): Histoire g~ologique du Pr~cambrien de rAnti-Atlas. - Not. M6m. Serv. Geol. Maroc, 162,352 pp. ,Rabat

CHOUBERT,

G.

&

FAURE-MURET, A. (1971): Afrique occidental: Socle pr~cambrien et zones mobiles. Anti-Atlas

(Maroc). - UNESCO, Tectonique de I'Afrique, Sci. Terre, 6, 165-175, Paris - - (1977): Difficult~s g~ochronologiques dans le Pr~cambrien de I'Anti-Atlas (Maroc). - C. R. Somm. Soc. Geol. Fr., 2, 72-74, Paris CHOUBERT, G., FAURE-MURET, A. & TIMOFEIEV, B. (1979): Premiers resultats de r~cherche d'Acritarches dans le Pr~cambrien de I'Anti-Atlas. - C. R. Acad. Sci. Pads, 288 D, 191-194, Paris CHUVASHOv, B. & RIDING, R. (1984): Principal Floras of Paleozoic Marine Calcareous algae. - Palaeontology, 27'/3,487-500, 9 Figs., Oxford CLAUER, N. (1974): Utilisation de la m6thode rubidium-strontium pour la datation d'une schistosit~ de s~diments peu m~tamorphis6s: application au Precambrien II de la boutonniere de Bou Azzer-EI Graara (Anti-Atlas, Maroc). - Earth Planet. Sci. Lett., 22,404-412, Amsterdam CLAUER, N. & LEBLANC, M. (1977): Implications stratigraphiques d'une ~tude g~ochronologique Rb-Sr sur m~tas~diments pr~cambriens de Bou Azzer (Anti-Atlas, Maroc). - Not. M~m. Serv. Geol. Maroc, 38, 7-12, Rabat CLAUER, N., CABY, R., JEANETTE, D. & TROMPETE, R. (1982): Geochronology of sedimentary and metasedimentary rocks of the West African Craton. - Precambrian Research, 18, 53-71, Amsterdam DEBRENNE, F. & DEBRENNE, M. (1978): Archaeocyathid fauna of the lowest fossiliferous levels of Tiout (Lower Cambrian, Southern Morocco). - Geol. Mag., 115, 101-119, Cambridge DESTOMBES, J., HOLLARD, H. & WILLEFERT, S. (1985): Lower Paleozoic rocks of West Africa and the western part of central Africa. - In: HOLLAND, C.H. (ed.): Lower Paleozoic rocks of the world, 4, 91-336, Chichester (Wiley) DUCROT, J. & LANCELOT, J. (1977): Probl~me de la limite Pr~cambrien-Cambrien: 6tude radiochronologique par la m6thode U/Pb sur zircons du volcan du Jbel Boho. - Canad. J. Earth Sci., 14,2771-2777, Ottawa DUCROT, J., LANCELOT, J.R. & LEBLANC, M. (1976): Age panafricain de la diorite de Bleida (Anti-Atlas, Maroc) et consequences.- 6e R6un. ann. Sci. Terre, p. 150, Orsay GASSNER, W. (1986): Das Infra- und Unterkambrium am S0dostrand des Pr&kambriumaufbruches von Beu Azzer/EI Graara (Zentraler Anti-Atlas, Marokko). - Unpubl. Dipl. Arb., 92 pp, Erlangen HUPE, P. (1952): Contribution & retude du Cambrien inferieur et du Pr6cambrien III de I'Anti-Atlas marocain. - Not.. M~m. serv. g~ol. Maroc, 103,362 pp, Rabat JEANETTE, D. & TISSERANT, D (1977): Les ~pisodes tectoniques et intrusifs du Pr~cambrien Sup6rieur de I'AntiAtlas occidental. - Estud. Geol., 33,315-326, Madrid JUREY, A., LANCELOT, J.R., HAMET, J., PROUST, F. & ALLEGRE, C.J. (1974): L'&ge des rhyolites du Pr~cambrien Ill du Haut Atlas et le probl#me de la limite Pr~cambrien-Cambrien. - 2e R6un. ann. Sci. Terre, P. 230; Nancy KORDE, K.B. (1971): Vodorosli kembriya. - Akad. nauk SSSR, Trudy paleont, inst., 139,349 pp., 49 PIs., 65 Figs., Moskva (Nauka) LEBLANC, M. & LANCELOT, J.R. (1980): Interpretation g6odynamique du domain panafricain (Pr6cambrien terminal) de I'Anti-Atlas (Maroc) & partir de donn~es g~ologiques et g~ochronologiques. - Can. J. Earth Sci., 17, 142-155, Ottawa MIFDAL, A. & PEUCAT, J.-J. (1985): Datations U-Pb et Rb-Sr du volcanisme acide de I'Anti-Atlas Marocaine et du

90

MIFDAL, A. & PEUCAT, J.-J. (1985): Datations U-Pb et Rb-Sr du volcanlsme acide de I'Anti-Atlas Marocaine et du socle sousjacent dar, s la r~gion de Ouarzazate. Apport au probl~me de la limite Pr~cambrien- Carnbrien. Sci. Geol. Bull., 38, 185-200, Stra6burg MONNINGER, W. (1979): The section of Tiout (Precambrian/Cambrian boundary beds, Anti-Atlas, Morocco): An environmental model.- Arb. Pal&ont. Inst. W0rzburg, 1,289 pp., W0rzburg ODIN, G.S., GALE, M.H., AUVRAY, B., BIELSKI, M., DORE, F., LANCELOT, J.R. & PASTEELS, P. (1983): Numerical dating of Precambrian-Cambdan boundary. - Nature, 301,21-23, London RIDING, R. & VORONOVA (1984): Assemblages of calcareous algae near the Precambrian/Cambrian boundary in Siberia and Mongolia.- Geol. Mag., 121/3,205-210, 3 Figs., London - - & - - (1985): Morphological Groups and Series in Cambrian Calcareous Algae. - I n : TOOMEY, D.F. & NITECKI, M.H. (eds.): Paleoalgology.-56-78, 17 Figs., Berlin-Heidelberg-New York-Tokyo (Springer) SCHMITT, M. (1978): Stromatolites from the Tiout section. Precambrian- Cambrian boundary beds, Anti-Atlas, Morocco.-Geol. Mag., 115, 95-100, Cambridge --

(1979): The section of Tiout (Precambrian/Cambdan boundary beds, Anti-Atlas, Morocco): stromatolites and their biostratigraphy. - Unpubl. Thesis, University WOrzburg

SELG, M. (1985): Die siliziklastisch-karbonatische Wechsellagerung der unterkambrischen Nebida-Formation (SWSardinien): Entstehung einer Karbonat-Plattform. - Unpubl. Thesis, Geowiss. Fak. Univ. Freiburg, 105 pp., 9 PIs., 15 Figs., Freiburg SDZUY, K. (1978): The Precambrian-Cambrian boundary beds in Morocco (preliminary report).- Geol. Mag., 115, 83-94, Cambridge

T h e B a s e o f t h e C a m b r i a n i n Morocco

K. S d z u y & G. Geyer I n s t i t u t ffir Pal{iontologie, U n i v e r s i t i i t W/irzburg, Pleicherwall

1, D - 8 7 0 0 Wiirzburg

Abstract

The

paper

summarizes problems

of d a t i n g s

of L a t e s t

r o c k s of Morocco. The k n o w n p a l a e o n t o l o g i c a l d a t a a r e and

correlatability

of t h e

earliest

trilobites

Precambrian

to

Early

Cambrian

reviewed briefly. C h a r a c t e r i s t i c s

of Morocco

are

discussed. Additional re-

m a r k s c o n c e r n t h e a p p e a r a n c e of h a r d p a r t a n i m a l s a t t h e b e g i n n i n g of t h e C a m b r i a n .

1. I n t r o d u c t i o n

In Morocco Lower C a m b r i a n rock s u c c e s s i o n s a r e c o m p l e t e , well e x p o s e d , r i c h in f o s s i l s , and there

is no b r e a k b e t w e e n

P r e c a m b r i a n a n d C a m b r i a n d e p o s i t s . The h i s t o r i c a l i m -

p o r t a n c e of t h e Moroccan C a m b r i a n b a s e s m a i n l y on p u b l i c a t i o n s of Pierre Hup6. Hup6 erected

a detailed biostratigraphical

scheme based

on t r i l o b i t e s

(Hup6,

1952), a n d h e

p u b l i s h e d a c l a s s i c a l m o n o g r a p h on Lower C a m b r i a n t r i l o b i t e s (Hup6, 1953). He i m p r o v e d t h e l a t t e r in a n i n t e r n a l t h e s i s s t r a t i g r a p h i c a l f r a m e in Hup6,

(Hup6, 1959), w h i c h n e v e r w a s p u b l i s h e d ( r e v i s e d b i o 1960). However, h i s f i r s t m o n o g r a p h led to a n i m p r o v e d

i n t e r n a t i o n a l c o r r e l a t i o n of t h e E a r l y C a m b r i a n a n d i t s p u b l i c a t i o n

c a n be r e g a r d e d a s

t h e b e g i n n i n g of m o d e r n r e s e a r c h on E a r l y C a m b r i a n . Good r e v i e w s on t h e Boudda,

Choubert

h i s t o r y of i n v e s t i g a t i o n s on t h e Moroccan C a m b r i a n a r e g i v e n by

& Faure-Muret

(1979)

and

Hollard

(1985),

respectively.

Although

C a m b r i a n s u c c e s s i o n s a r e k n o w n from t h e Moroccan M e s e t a a s well a s from J b i l e t , High A t l a s

Mountains and

the Anti-Atlas,

t h e l a t t e r b e c a m e "type"

area

t i g r a p h y a n d for b i o s t r a t i g r a p h y of t h e Moroccan E a r l y C a m b r i a n u n t i l

the

for l i t h o s t r a -

t o d a y . T h i s was

i n d u c e d by good o u t c r o p s a s well a s by h u g e r e s e a r c h a c t i v i t y in t h i s r e g i o n , a b o v e all by Georges

Choubert and

lithostratigraphy

Pierre

of t h e f i n a l

Hup6.

Choubert published

Precambrian and

Early

most

important

C a m b r i a n (e.

articles

g. C h o u b e r t ,

on

1942;

C h o u b e r t in Hup6, 1953; C h o u b e r t , 1968). A c c o r d i n g to Hup6 t h e b a s e of t h e C a m b r i a n is m a r k e d by a p p e a r e n c e of a r c h a e o c y a t h a . T r i l o b i t e s were s a i d to a p p e a r s o m e w h a t l a t e r , b u t Hup6 h a d good r e a s o n s to

believe the early trilobites

of Morocco b e i n g d i s t i n c t l y

t h e o l d e s t k n o w n on e a r t h so f a r . L a t e r , t r i l o b i t e s c l e a r l y older t h a n t h o s e from Hup6's o l d e s t z o n e I a n d p r o v i s i o n a l l y r e f e r r e d to a n EofM]otaspis z o n e h e r e i n were d i s c o v e r e d in a s e c t i o n n e a r

T i o u t (Sdzuy,

section.

the section

Therefore

1978), n o t f a r from t h e s e c t i o n of Hup~'s T a z e m m o u r t of T i o u t is t h e one

where most information about

the

92

Precambrian-Cambrian

boundary

in Morocco may be

e x p e c t e d . In

this

p a p e r we will

d i s c u s s p o s s i b i l i t i e s to d a t e t h i s b o u n d a r y using b i o s t r a t i g r a p h i c a l m e t h o d s . We will n o t d i s c u s s l a r g e l y t h e use of r a d i o m e t r y or m a g n e t o s t r a t i g r a p h y b e c a u s e no c l e a r l y r e l i a b l e d a t a are a v a i l a b l e to us. A c t i v i t i e s of t h e Wfirzburg I n s t i t u t e s t a r t e d in 1964, when one of us (K. S.) was i n t r o duced by J. P. S c h a e r (Neuch~tel) to s e v e r a l c l a s s i c a l Cambrian s e c t i o n s of t h e A n t i A t l a s and t h e High A t l a s One of t h e s e a r e a s ,

(e. g. Tizi

n'Tichka, Siksawn region, L e m d a d - O u n e J n area).

t h e Lemdad s y n c l i n e , was s t u d i e d l a t e r by A. Boudda (1968) in a

d i p l o m a - m a p p i n g of t h e Wfirzburg i n s t i t u t e . On l a t e r f i e l d t r i p s a d d i t i o n a l s u c c e s s i o n s were e x a m i n e d and sampled for s h e l l y f a u n a s . Worth m e n t i o n i n g are t h o s e n e a r Aqdz and Bou Azzer in t h e c e n t r a l A n t i - A t l a s . In 1974 W. Monninger and M. Schmitt s t a r t e d field work in t h e Tiout s e c t i o n (comprising all members from t h e Calcaires i n f ~ r i e u r s up to t h e t r i l o b i t e - b e a r i n g h o r i z o n s of Hup~'s zone III), Monninger s t u d y i n g s e d l m e n t o l o g y , Schmitt working on s t r o m a t o l i t e s (Monnlnger & Schmitt,

1975; Monninger

Archaeocyatha (Debrenne,

from

the

1979;

same

Schmitt

section

1975; Debrenne & Debrenne,

1978,

were 1976;

1979a;

Schmitt & Monninger,

collected

and

Debrenne,

studied

1976;

by

F.

1977).

Debrenne

D e b r e n n e & Debrenne,

1978), t r i l o b i t e s by Sdzuy (Sdzuy, 1978; Llfian & Sdzuy, 1978; Sdzuy, 1981). Most of t h e latter

are s t i l l u n d e s c r i b e d .

In 1971

Choubert b r o u g h t

some well p r e s e r v e d t r i l o b i t e s

from below t h e o l d e s t t r i l o b i t e s known to Hup~ from t h e s e c t i o n n e a r Amouslek to t h e a t t e n t i o n of Sdzuy. In 1981

E. B e r n e k e r and t h e j u n i o r a u t h o r s t a r t e d a new campaign of taxonomic, b i o -

stratigraphical

and

palecological

studies

on

Cambrian

faunas,

Berneker

working

on

a r c h e o c y a t h a n s , Geyer mainly on t r i l o b i t e s . S e v e r a l c l a s s i c a l l o c a l i t i e s were r e - s t u d i e d (Lemdad River s e c t i o n , Ounein (Oued Boutergui, Tamguert,

area, Wirgane, Tiout,

Amouslek), new s e c t i o n s e x p l o r e d

A s i f Aouerga in t h e w e s t e r n A n t i - A t l a s ) .

The r e s u l t s are

u n p u b l i s h e d , e x c e p t for a s t u d y on Cambrian molluscs (Geyer, 1986).

2. Rock u n i t s

2a. General r e m a r k s

In Morocco,

there

exist

breakless

s e d i m e n t a r y s e q u e n c e s which

pass

from

Late

Pre-

c a m b r i a n i n t o well d a t e d E a r l y Cambrian. The u s u a l l i t h o s t r a t i g r a p h i c u n i t s of t h e U p p e r P r e c a m b r i a n and t h e Lower Cambrian were c h o s e n in t h e w e s t e r n to c e n t r a l p a r t of the Anti-Atlas

(Choubert, 1942) and are more or l e s s e a s i l y d e l i m i t a b l e in t h i s region.

U n f o r t u n a t e l y a f r e q u e n t c h a n g e of n a m e s and a c a r e l e s s use of l i t h o -

and b i o s t r a t i -

g r a p h i c a l u n i t s led to c o n f u s i o n of t e r m s for many of t h e formations. C h a r a c t e r i s t i c s of t h e rock

u n i t s of i n t e r e s t in t h i s a r t i c l e

are s k e t c h e d

below. For more d e t a i l e d I n -

formation of t h e s e q u e n c e see Boudda e t al. (1979) and Hollard (1985).

93

o

0

~3 0 o o ~

N m

o

~1

~.~

o

¢d

0 • .>

'

o

#

o

N~

i o

0

Q 0

o

o

~._~

0

~

94

2b. Calcaires i n f ~ r i e u r s

The Calcaires i n f ~ r i e u r s (Adoudounien, Choubert, 1958; vs. Boukhari e t al., 1979) are a monotonous complex of a b o u t

1000 m t h i c k n e s s . It is e s s e n t i a l l y composed of shallow

w a t e r c a r b o n a t e s t r a n s g r e s s i n g d i s c o r d a n t l y o v e r v a r i o u s P r e c a m b r i a n rocks. The l i g h t l y coloured m a s s i v e l y bedded dolomites

and l i m e s t o n e s are

locally i n t e r r u p t e d

by m u d -

s t o n e s , p e l i t i c s h a l e s and s a n d s t o n e s . The s i l i c e o u s i n t e r c a l a t i o n s are o f t e n m i n e r a l i z e d by d i v e r s e ores. The

f o r m a t i o n commences with a s o - c a l l e d S~rie

s i l i c l a s t i c t r a n s g r e s s i o n a l s e q u e n c e of a b o u t

100

de b a s e

(Choubert,

m thickness

1952),

(Boudda

et

a mostly al.,

1979),

comprising dolomites, c o n g l o m e r a t e s , and f i r s t of all a u n i t with a r g i l l i t e s and s h a l e s , i n t e r r u p t e d by c o n g l o m e r a t i c or a r e n i t i c - a r k o s i c l a y e r s . Choubert et

al. (1950,

1952a,

1952b), Buggisch e t al.

(1979), and Buggisch & Heinitz

(1984) r e p o r t e d s t r o m a t o l i t e s from t h a t formation, Buggisch & Flfigel ( t h i s volume) d e s c r i b e a c a l c a r e o u s alga. A s t r i k i n g v o l c a n i c a c t i v i t y is r e p o r t e d from t h e c e n t r a l A n t i - A t l a s , were t r a c h y t e s and a n d e s i t e s are i n t e r c a l a t e d i n t o t h e

dolomites. The c e n t r e

of v o l c a n i s m was found

in

Jbel Bokho n e a r Alougoum (Choubert, 1952a; Boudda et al., 1979). V o l c a n o c l a s t i c l a y e r s may c o n t a i n ripple marks and s o f t p e b b l e s (Buggisch e t al., 1979).

2c. S~rie lie de vin

The name

"Lie de

v i n ' was c h o s e n by C h o u b e r t (1942)

b e c a u s e of t h e

t y p i c a l purple

colour of t h e s h a l e s c o n t a i n e d . "Taliwinien" is more or l e s s a s y n o n y m (Boudda e t al., 1979, Boukhari

e t al.,

1979).

A l t h o u g h t h e rocks composing t h i s

f o r m a t i o n v a r y to a

large e x t e n d , t h e y are mostly c o n t i n e n t a l l y - t e r r i g e n o u s l y i n f l u e n c e d , giving i n d i c a t i o n s of t h e r e g r e s s i v e p h a s e of t h e f i r s t p o s t - o r o g e n i c cycle (e. g. C h o u b e r t in Hup~, 1953). The t h i c k n e s s d i f f e r s from 300 m in t h e

Warzazate region in t h e e a s t (Boudda e t al.,

1979) to a b o u t 1000 m s o u t h of T a r o u d a n t (Monninger, 1979). Red s a n d s t o n e s and c o n g l o m e r a t e s d o m i n a t e close to t h e b a s i n a l s h o r e in t h e e a s t . The totally

detritic

succession

of p r e p o n d e r a n t s a n d s t o n e s

which a d v a n c e s

from e a s t

of

Warzazate into t h e c e n t r a l A n t i - A t l a s and i n t e r f i n g e r s t h e middle member of t h e S~rie lie de vin

is called Gr~s de Tikirt.

Its t h i c k n e s s is up to 225 m s o u t h

of t h e Jbel

K i s s a n e (Buggisch e t al., 1979). Towards t h e c e n t r e of t h e b a s i n t h e d e p o s i t s p a s s o v e r into f i n e - g r a i n e d rocks. The most t y p i c a l f a c i e s is found in s e c t i o n s on t h e s o u t h e r n f l a n k of t h e A n t i - A t l a s : The s u c c e s s i o n is l a r g e l y formed by purple

s h a l e s ; dolomites and l i m e s t o n e s are more

o f t e n i n t e r c a l a t e d t o w a r d s t h e top, e s p e c i a l l y in t h e u p p e r t h i r d of t h e s e q u e n c e (Hollard,

1985).

Some t h i c k l i m e s t o n e b e d s

may be

used

cropping out o v e r large a r e a s (e. g. "Barre de Tata").

as l i t h o l o g i c a l m a r k e r

horizons

95

The

well

studied

rather stable

section

conditions

of Tiout

represents

(low w a t e r energy,

cyclic

sedimentation,

shallow w a t e r

range, p a r t l y high s a l i n i t y ; Monninger, 1979).

deposited

depths, insignificant

under tidal

Trace f o s s i l s ("worm burrows") are c o m -

mon. Algal b u i l d - u p s can be u s e d as f a c i e s i n d i c a t o r s , in p a r t i c u l a r columnar s t r o m a t o l i t e s can be u s e f u l i n d e x f o s s i l s .

2d. Calcaires s u p ~ r i e u r s

The d e p o s i t i o n of t h e

Calcaires s u p 4 r i e u r s was i n i t i a t e d by a new t r a n s g r e s s i o n . The

f o r m a t i o n mainly c o n s i s t s of dark l i m e s t o n e s ; l i g h t grey to pink c a r b o n a t e s ( e s p e c i a l l y f i n e l y c r y s t a l l i n e d o l o s t o n e s ) are l e s s common. Obvious d e t r i t a l d e p o s i t s are a b s e n t . The known t h i c k n e s s e s v a r y b e t w e e n 76 m a t Bou Azzer (Boudda e t al.,

1979) and up to

3 0 0 - 4 0 0 m in t h e region of Tiznit. South of t h e

Jbel

Sarhro t h e

facies changes

to t h e

s o - c a l l e d S~rie s c h i s t o - c a l c a i r e

o r i e n t a l e with s h a l e s and l i g h t l i m e s t o n e s . The f o r m a t i o n was d e p o s i t e d u n d e r shallow w a t e r c o n d i t i o n s . Mud cracks and l g n i m b r i t e h o r i z o n s p o i n t to local e m e r s i o n (Buggisch e t al., 1979). Owing to a r e m a r k a b l e f a c i a l c h a n g e in t h e Tiout

s e c t i o n Monninger & Schmitt (1975)

called t h e u p p e r p o r t i o n of t h e f o r m a t i o n " t r a n s i t i o n l a y e r s " . I n s t e a d of black micritic l i m e s t o n e s dark o o l i t e s and more and more m a r l s t o n e s compose t h e s u c c e s s i o n . A similar c h a n g e of f a c i e s is d e s c r i b e d from t h e region s o u t h of t h e J b e l K i s s a n e (Buggisch e t al., 1979). The t h i c k n e s s of t h i s l a t t e r member called " c a l c a i r e s de base" is up to 20 m. Thrombolites occur in t h e b a s a l l i m e s t o n e s ; m o r e o v e r t r i l o b i t e f r a g m e n t s , ooids and i n t r a c l a s t s are r e p o r t e d from t h e c a r b o n a t e beds. Biostromal and domal s t r o m a t o l i t e s are g e n e r a l l y r e p o r t e d from t h e s o u t h e r n Jbel Sarhro (Buggisch e t al., 1979). Schmitt (1979a,

1979b) d e s c r i b e d s t r o m a t o l i t e s and r e p o r t e d t r a c e f o s s i l s from t h e lower

p a r t of t h e Calcaires s u p ~ r i e u r s of Tiout. In some b e d s of t h e u p p e r p a r t a r c h a e o c y a t h a form local b i o h e r m s (Debrenne, 1975, Debrenne, 1976, D e b r e n n e & Debrenne, 1978). B e sides this

Sdzuy (Sdzuy, 1978; Lifian & Sdzuy,

1978; Sdzuy, 1981)

reported trilobite-

bearing horizons.

2e. S~rie s c h i s t o - c a l c a i r e

The c l a s s i c a l E a r l y Cambrian s t u d i e d by Hup~ commences with an a l t e r n a t i o n of g e n e r a l l y dark l i m e s t o n e s and l i g h t s h a l y s l a t e s ,

called S~rie s c h i s t o - c a l c a i r e (Choubert,

1952b). A r a t h e r t y p i c a l l i t h o l o g i c a l s e q u e n c e is e x p o s e d a t Amouslek ( s o u t h w e s t of T a r o u d a n t ) . The s e c t i o n h a s b e e n d e s c r i b e d s e v e r a l times (e. g. C h o u b e r t in Hup~, 1953; Choubert, 1963). The s e r i e s of s h a l e s is i n t e r r u p t e d by l i m e s t o n e beds, t h e l a t t e r l a r g e l y formed

96

by a r c h a e o c y a t h a n

bioherms. The

formation may

be s u b d i v i d e d into

u n i t s which a l t o g e t h e r are more t h a n 200 m thick.

three

lithological

The similar s e c t i o n of Tazemmourt

was of g r e a t i m p o r t a n c e for t h e i n v e s t i g a t i o n s of Pierre Hup~. In t h e s e c t i o n of Oued Boutergui (region of Tiznit; B e r n e k e r & Geyer, in prep.) t h e s e quence has a thickness

of a b o u t 200 m, with

dark l i m e s t o n e s l a r g e l y d i s p l a c i n g t h e

b a s a l s h a l e s . In t h e n o r t h w e s t e r n A n t i - A t l a s (e. g. n o r t h e r n p a r t of A s l f Aouerga) t h e s u c c e s s i o n can be formed e s s e n t i a l l y by l i m e s t o n e s c o n t a i n i n g only t h i n s h a l e l a y e r s (p. p. "Calcaires m a s s i f s de Tiznit" s e n s u Boudda et al., 1979). Some p u b l i s h e d

estimations

of t h i c k n e s s e s

in

the

western

Anti-Atlas

are

possibly

e x a g g e r a t e d . According to C h o u b e r t (1942) t h e formation is up to 390 m t h i c k in t h e Tara area, b u t t h i s might p o s s i b l y not comprise t h e i d e n t i c a l l i t h o s t r a t i g r a p h i c a l u n i t as in c u r r e n t use. S e d i m e n t a t i o n was i n f l u e n c e d by v a r y i n g w a t e r d e p t h s and p e r h a p s s h o r e l i n e o s c i l l a t i o n under

s h a l l o w marine s h e l f c o n d i t i o n s . Towards

the east

the content

of d e t r i t a l e l -

e m e n t s i n c r e a s e s with s h a l l o w i n g w a t e r d e p t h s (Choubert, 1964). The s e q u e n c e o f t e n c o n t a i n s a b u n d a n t s h e l l y f a u n a s . T r i l o b i t e s and a r c h a e o c y a t h a were d e s c r i b e d in d e t a i l by Hup~ (1953) and D e b r e n n e (1958, 1961, 1964, 1973), r e s p e c t i v e l y . The famous a r c h a e o c y a t h a n

l i m e s t o n e s of Sidi Moussa

d'Aglou n e a r

Tiznit

(Bourcart,

1927; Bourcart & Le Villain, 1931; Debrenne, 1964) belong to t h i s formation.

3. S t r o m a t o l i t e s and c a l c a r e o u s a l g a e

3a. O c c u r r e n c e s of s t r o m a t o l i t e s and algae

The only f o s s i l s d i s c o v e r e d below t r i l o b i t e b e a r i n g beds are algal b u i l d - u p s . Low algal mats,

stromatolites,

n o t e s are s p a r s e :

and

t h r o m b o l i t e s are

widespread,

C h o u b e r t e t al. (1950, 1952a,

but

published

palaeontological

1952b) r e p o r t e d s t r o m a t o l i t e s from t h e

Calcalres i n f ~ r i e u r s . S t r o m a t o l i t e s from t h e S4rie lie de vin 'were d e s c r i b e d by S c h m i t t & Monninger, 1977, Schmitt, 1978, Schmitt, 1979a, and Schmitt, 1979b (review in Boudda e t al.,

1979). Columnar s t r o m a t o l i t e s are also known from t h e Calcaires s u p ~ r i e u r s of t h e

Tiout s e c t i o n (Schmitt, 1979a; Schmitt, 1979b) and lithologic e q u i v a l e n t s of t h e Lemdad a r e a (Schmltt, 1979b). (Buggisch e t al.,

1979),

Buggisch & Helnitz (1984),

and Heinitz

(1984) m e n t i o n e d s t r o m a t o l i t e s from t h e Calcaires s u p ~ r i e u r s e a s t of t h e J b e l Sirwa.

A d e t a i l e d s t u d y of t h e s t r o m a t o l i t e s of t h e

Tiout

s e c t i o n was p r e s e n t e d by Schmitt

(1979a) giving t h e following b i o s t r a t i g r a p h i c a l information: The lower s e q u e n c e of t h e S~rie lie de vin f u r n i s h e s a number of h o r i z o n s with poorly p r e s e r v e d s t r o m a t o l i t e s . Schmitt (1979a) d e s c r i b e d a Linella cf. a v i s Krylov, 1967 p o i n t ing to a l a t e Riphean or V e n d i a n age (Schmitt, 1979a). From t h e middle s e q u e n c e of t h e S~rie lie de vin

T u n g u s s i s cf. i n n s Walter, 1972 a n d

97

Patomia fsp. i n d e t , were d e s c r i b e d , i n d i c a t i n g most p r o b a b l y V e n d i a n age. The u p p e r

s e q u e n c e of t h e

1979, N. ? fsp.

indet.,

S~rie lie

Tioutella rupta Schmitt,

1979, Tifounkeia n. fsp. T, a n d

Nlmbophyton fungiforme Schmitt,

de vin y i e l d e d

Igoudinim hemisphaerica Schmitt,

1979,

Tungussia f. i n d e t . ( b e s i d e s t h r o m b o l i t e s and o t h e r u n -

named s t r o m a t o l i t e s ) . From t h e

lower to middle p o r t i o n

of t h e

Calcaires s u p 6 r i e u r s

Schmitt (1979a,

1979b)

d e s c r i b e d Aeaclella cf. angepena Preiss, 1972, which implies an Early Cambrian age. In a d d i t i o n , Tloutella

bouddai Schmitt,

1979 and some h o r i z o n s with t h r o m b o l i t e s are r e -

ported. From t h e Lemdad River s e c t i o n s t r o m a t o l i t e s are only d e s c r i b e d from t r i l o b i t e b e a r i n g beds (Schmitt 1979a, 1979b; "Ounein region").

A s k e l e t a l alga is

d e s c r i b e d by Buggisch & Fl~igel ( t h i s

volume) from

t h e Catcaires

i n f ~ r i e u r s e a s t of t h e Jbel Sirwa a n d s u p p o s e d to i n d i c a t e Early Cambrian age. Frequent

algae

intergrowing with

archaeocyatha

Epiphyton

are

RenMcis, p a r t l y

and

forming p u r e algal b o u n d s t o n e s (e. g. Oued Boutergui, B e r n e k e r & Geyer, in prep.). Gir-

wneHa

appears

in

several

Tolypothrix

localities,

and

Skythonema?

were

found

in

a r c h a e c y a t h a n l i m e s t o n e s of Tiout (Berneker, p e r s . comm.).

3b. Problems of d a t i n g s

The d a t i n g s

of Schmitt

(1979a,

1979b)

were

c r i t i c i z e d by S e m i k h a t o v

(1977),

Clauer

(1980), and B e r t r a n d - S a r f a t i (1981). S e m i k h a t o v m e n t i o n e d a s t r o m a t o l i t e from t h e lower p a r t of t h e S6rie lie de v i n of Tiout and s t r e s s e s t h e s i m i l a r i t y t o t h e S i b e r i a n Par-

mites vlctorius from t h e Early Cambrian. E s p e c i a l l y B e r t r a n d - S a r f a t i s t r e s s e s s i m i l a r i t i e s of t h e forms d e s c r i b e d by Schmitt with Early Cambrian s t r o m a t o l i t e s . A s s o c i a t e d t h r o m b o l l t e s also are t h o u g h t to i n d i c a t e Early Cambrian age by B e r t r a n d - S a r f a t i (1981). On t h e o t h e r h a n d , it is known t h a t t h e o c c u r r e n c e of t h r o m b o l i t e s is c o n t r o l l e d by f a c i e s . The b i o s t r a t i g r a p h i c a l u s a b i l i t y of

s t r o m a t o l i t e s d e p e n d s of course

on

a

sufficiently

broad s p e c t r u m of forms. The d i v e r s i t y of s t r o m a t o l i t e s in t h e Tiout s e c t i o n c e r t a i n l y is n o t s u f f i c i e n t to d a t e rocks w i t h p r e c i s i o n , b u t t h e r e is a good c o n f o r m i t y in t h e s u c cession of the

indicated biostratigraphical levels

find it d i f f i c u l t

to a c c e p t d a t i n g s b a s e d

forms. However,

in s p i t e

of t h e

on t h e

c r i t i q u e we

of Schmitt (1979a). same or

In c o n t r a s t , we

e v e n more poor

were informed by Walter

amount of

(written

pers.

comm.) t h a t S c h m i t t ' s work was i n d e e d c a r r i e d out c a r e f u l l y . Clauer

(1980)

presented

a

comparison

between

biostratigraphic

datings

and

isotopic

g e o c h r o n o l o g y o f t h e Upper P r e c a m b r i a n of t h e A n t i - A t l a s , b a s e d mainly on a n a b s o l u t e d a t i n g of t r a c h y t i c volcanism, which implies t h a t t h e C a l c a i r e s i n f ~ r i e u r s were d e p o s i t e d a f t e r 618_+22 m. y. b. p. However, t h i s r a d i o m e t r i c age does n o t s t r o n g l y c o n t r a d i c t t h e biostratigraphic data

g i v e n by Schmitt. In a d d i t i o n i t s h o u l d be m e n t i o n e d t h a t a large

98

percentage

of recent

U/Pb d a t i n g s

strongly

opposes

provable

biostratigraphical

data

b a s i n g on t r i l o b i t e s (e. g. Ducrot & Lancelot, 1977, Leblanc & Lancelot, 1980). Moreover Tucker (1986) p u b l i s h e d r e c e n t c a r b o n i s o t o p e a n a l y s e s on rocks of t h e Tiout s e c t i o n . Two p o s i t i v e 5~C

e x c u r s i o n s (in t h e u p p e r p a r t of t h e Catcaires i n f ~ r i e u r s and

in t h e lower p a r t of t h e Calcaires s u p 4 r i e u r s ) could be t r a c e d which are i n t e r p r e t e d as i n c r e a s e d l e v e l s of organic p r o d u c t i v i t y , b e l i e v e d to r e f l e c t t h e e v o l u t i o n a r y i n c r e a s e of m i c r o p l a n c t o n close to t h e R i p h e a n / Y e n d i a n b o u n d a r y and t h e i n c r e a s e d organic m a t t e r produced d u r i n g t h e rapid m e t a z o a n e v o l u t i o n a t

t h e v e r y b e g i n n i n g of t h e Cambrian

(Tucker, 1986). A l t h o u g h t h e s i g n i f i c a n c e of some d a t a may be q u e s t i o n e d and n o t h i n g can be said a b o u t t h e p r e c i s e p o s i t i o n s of b o u n d a r i e s we would llke to s t r e s s t h e s i m i l a r i t y of t h e s k e t c h e d s t r a t i g r a p h y with t h a t of Schmitt (1979a). Buggisch & Flfigel (this

volume) r e p o r t t h e

calcareous

alga Kund~ti~ composlta Korde

from t h e Calcaires i n f ~ r i e u r s e a s t of t h e Jbel Sirwa. The f i n d i n g s are t h o u g h t to i n d i c a t e Early Cambrian age, However, Kundatia

composita was only known from a single

a r e a of t h e S i b e r i a n p l a t f o r m before. Its o c c u r r e n c e in Siberia t o g e t h e r with algae which are only known from Early Cambrian and much y o u n g e r s t r a t a , g i v e s in any case no e v i d e n c e for Cambrian age. Moreover, one may d o u b t a s u f f i c i e n t s h o r t time r a n g e of a c a l c a r e o u s alga

to allow r e l i a b l e c o r r e l a t i o n . In a d d i t i o n , t h e r e

Calcaires i n f ~ r i e u r s

in t h e

area

studied

by Buggisch and

his

is no p r o v e t h a t collaborators

are

the time

e q u i v a l e n t s of t h o s e in t h e Tiout s e c t i o n .

4. Cambrian shelly fossils

4a. Occurrence of v e r y e a r l y Cambrian f o s s i l s in Morocco

The e a r l i e s t

trilobites

known

to Hup~ (1953)

were

t h o s e of t h e

zone

of Fallotaspls

tazemmourtensls, which is well d o c u m e n t e d only in t h e Tazemmourt s e c t i o n and t h e only s t i l l e a r l i e r Cambrian f o s s i l s were A r c h a e o c y a t h a . About 1970 e a r l y t r i l o b i t e s were d i s c o v e r e d n e a r t h e Lemdad r i v e r , s o u t h e r n slope of t h e High Atlas, n o r t h of T a f i n e g o u l t and in t h e following y e a r s one of us

(K.S.) collected t r i l o b i t e s from a l a r g e n u m b e r of

horizons. The l o w e s t h o r i z o n which y i e l d e d Lemdade11~ Sdzuy, 1978, is i m m e d i a t e l y on top of s e d i m e n t s t h a t

s t r o n g l y r e s e m b l e t h e Lie de vin of t h e A n t i - A t l a s (see Sdzuy,

1978: fig. 3) and c o n s e q u e n t l y i t was t h o u g h t t h a t Lemd~de]ls might be c o n s i d e r a b l y older t h a n t h e F. tazemmourtensls zone. L a t e r i t t u r n e d out, h o w e v e r , t h a t it p r o b a b l y h a s t h e age of t h a t zone, The s e c t i o n w h e r e t h e in all p r o b a b i l i t y e a r l i e s t Cambrian f o s s i l s of Morocco were found is t h a t n e a r Tiout, n o r t h e r n slope of t h e c e n t r a l A n t i - A t l a s , s o u t h e a s t of T a r o u d a n t . It was c a r e f u l l y s t u d i e d by us (see I n t r o d u c t i o n ) , t h e rock s u c c e s s i o n is t y p i c a l l y

that

d e s c r i b e d in c h a p t e r 2. Regarding t h e problem of t h e b a s e of t h e Cambrian, t h e Tiout s e c t i o n is t h e most i m p o r t a n t one in Morocco.

99

4b. T o m m o t i a n , A t d a b a n i a n ,

The

base

of the

Cambrian. The

Tommotian stage

identification

tigraphy. the

In S i b e r i a t h e

presence

tematic position,

beds

sediments

the correlation

found various

but

they

of the early trilobite

of u n d o u b t e d

section

of this

found

near

z o n e . Here, t h e C a l c a i r e s

the

named

it

is

S~rie

about

well

trilobite

but

sufficient

horizon,

the distance

T

1. We d i d

from the

that

generic

identity

It in

rare.

stage.

however,

Whether section

that

a cor-

is n o t p o s s i b l e ,

dependable. t h e z o n e o f FaHot~spls tazemIt c a n b e d e m o n s t r a t e d distant

are

above

the top

below

the

measure

top of the formation

the

from Tiout

older

than

this

in t h e T i o u t s e c -

of t h e top

of

1978 w h i c h in t h e T i o u t not

shown

horizons of the Tiout

s i m i l a r to t h o s e

Well

fossils

for correlation.

is e x t r e m e l y

schisto-calcaire

40 m e t e r s

documented.

although

A few phosphatic

been found.

s e e m to be v e r y

we f o u n d Hupetina antiqua S d z u y ,

earliest

exactly,

for

the

sys-

None of the

trilobites

w h i c h is j u s t a f e w k i l o m e t e r s

below

F. tazemmourtensis a p p e a r s zone

sup~rieurs the

Tiout

sup~rieurs

of unknown

of the Atdabanian

representing

z o n e k n o w n to Hup~, h a v e

section

and by

of "small shelly

of Monninger have

or a t l e a s t

of Tazemmourt

tion.

are not

bearing

in the classical trilobites

first

methods.

t y p e w o u l d be s u f f i c i e n t l y

no f o s s i l s d o u b t l e s s l y

corre-

is b i o s t r a -

are trilobites. the

below. It m u s t be s t r e s s e d ,

specific

upon the

of trilobites

material partly

the base

mourtensis, t h e e a r l i e s t that

absence

below

investigations

occur near

of the

and especially

Tiout section

been

trilobites

base

well known phyla like molluscs.

with

will be d i s c u s s e d

on the base

Tiout

has

geochemical

trilobites

and only a correlation In t h e

will d e p e n d

t h e Eo£allotaspls z o n e ( s e e b e l o w ) p h o s p h a t e

the earliest

with the Atdabanian relation

with

that

older than

this justifies

above

as the

m e t h o d to do t h i s

by the

of phosphatic

to r e p r e s e n t

investigated

in association

accepted

regions

brachiopods,

C a m b r i a n f o s s i l s in t h e

mentioned

be m e n t i o n e d

In S i b e r i a ,

and

elements

thought

were thoroughly

were found should

are skeletal

undoubtedly

in o t h e r

T o m m o t i a n is c h a r a c t e r i z e d

partly

Tommotian fossils the

base

is g e n e r a l l y

and the only dependable

of some archaeocyathids

fossils". The latter

The first

of Siberia

of this

lation with Siberian successions,

EofaHotaspis z o n e

and the

limestones,

so

the

Calcaires

section

o c c u r s in

Tazemmourt

section

very

s e e m s to be s i m i l a r to t h a t

in

the Tiout section. Trilobites

have

section in

been

f o u n d in t h e

upper

17 h o r i z o n s , T 1 - T 17 ( s e e S d z u y ,

a biostratigraphical use elsewhere to f a c i l i t a t e

unit

c a n be

will be c o m p l e t e l y d e s c r i b e d .

separately

i t is p r o b a b l e

biostratigraphical the

below.

latter

sup~rieurs

of the

named yet

and to

of correlation

Tiout

b e l o n g to

which no name in

(see below).

In o r d e r

we c a l l t h i s u n i t t h e EofaHotaspls z o n e . T h i s n a m e i s s o m e w h a t

the faunas

archaeocyatha,

Calcalres

1978: fig. 2). T h o s e t r i l o b i t e s

a p p l i e d , o w i n g to d i f f i c u l t i e s

discussions because

of

of the

w h i c h i n Morocco i s n o t

provisional

Fossils

part

were

that

value

more than

from

studied

the

one zone

will be

EofaHotsspls z o n e

b y F. D e b r e n n e .

Both

groups

distinguished

are

trilobites

will b e

when

and

discussed

100

4c. Very e a r l y C a m b r i a n t r i l o b i t e s a n d c o r r e l a t i o n

H u n d r e d s of s p e c i m e n s of t r i l o b i t e s the

Tiout section

and

all

have been collected

of them

belong

to

Eofallotaspls zone of (Eofallotaspis

from t h e

two t y p e s :

1)

Olenellids

Sdzuy, 1978) a n d 2) o p i s t h o p a r i a n s w i t h t h i c k o c u l a r r i d g e s a n d m o d e r a t e l y long e y e s . We d e a l h e r e o n l y w i t h t h e s e c o n d , a n d b e c a u s e Bigotin~ Cobbold, 1935 from N o r m a n d y if q u i t e

t y p i c a l for t h i s g r o u p

we i n f o r m a l l y call it

t y p i c a l s p e c i m e n s from t h e T i o u t b y Sdzuy

(1978:

figs. 2b,

the

Bigotlna group. D r a w i n g s of

a n d t h e Lemdad ( s e e a b o v e ) of Bigotin~ a n d

3), p h o t o s

s e c t i o n s were p u b l i s h e d

of s p e c i m e n s from T i o u t

by t h e

s a m e a u t h o r (figs. 1 3 - 2 0 ) . One problem

of t h e

between trilobites

Bigotina g r o u p is t h a t on one h a n d t h e r e e x i s t s g r e a t s i m i l a r i t y

we c o n s i d e r

d i f f e r e n c e s c a n be s e e n ,

to b e l o n g to t h a t

and that

on t h e

we a r e d e a l i n g w i t h a c l o s e l y r e l a t e d g r o u p of

( p e r h a p s f a m i l y group) a n d t h a t

t h e d i f f e r e n c e s a r e t h o s e of

s p e c i e s or a t m o s t of g e n e r a . But it is a l s o q u i t e p o s s i b l e t h a t the stem

group

other hand clear

it is now k n o w n w h a t t h e s i g n i f i c a n c e of s i m i l a r i t y

a n d of d i f f e r e n c e s is. It is p o s s i b l e t h a t r a t h e r low t a x o n o m i c r a n k

group, t h a t

of o p i s t h o p a r i a n t r i l o b i t e s

and

that

phyletic

genetically but not yet strongly morphologically and that

t h e Bigotlna

lineages

were

g r o u p is separated

o n l y l a t e r in C a m b r i a n t i m e s

c h a r a c t e r s of h i g h r a n k i n g t a x a b e c a m e d i s t i n c t . R e p i n a (1969) who r e f e r r e d t r i l o b i t e s , we

would

Early

consider

as

belonging

C a m b r i a n was a p p a r e n t l y

to

Bigotln~ group, to t a x a

the

i n f l u e n c e d by t h e l a t t e r

e x i s t i n g in

philosophy, --

the

late

It is o b v i o u s

t h a t on t h e t a x o n o m i c p r o b l e m s d e p e n d o t h e r s r e g a r d i n g t h e u s e of t h o s e t r i l o b i t e s

as

index fossils. T h e r e a r e s t i l l o t h e r r e a s o n s w h y t h e u s e of v e r y e a r l y

C a m b r i a n t r i l o b i t e s as

index

f o s s i l s is p r o b l e m a t i c a l . It is o b v i o u s t h a t t h e f i r s t t r i l o b i t e s m u s t h a v e d e v e l o p e d from some u n k n o w n a n c e s t o r s in a n u n k n o w n r e g i o n a n d t h a t from t h e r e t h e e a r l y t r i l o b i t e s must

have

spread

over

the

oceans.

If

this

is

true,

then

the

primitive

trilobites

( p e r h a p s t h e Bigotins group) m u s t h a v e a p p e a r e d in d i f f e r e n t r e g i o n s a t d i f f e r e n t t i m e s . T h e r e is s t i l l a n o t h e r

problem: M i g r a t i o n implies f a c t o r s s t r o n g l y i n f l u e n c i n g e v o l u t i o n

like g e o g r a p h i c i s o l a t i o n of s m a l l p o p u l a t i o n s ,

c h a n g e of e n v i r o n m e n t , etc. T h e r e s u l t

m u s t h a v e b e e n a r a p i d g e n e t i c c h a n g e a n d t h e r e f o r e it m u s t be e x p e c t e d t h a t p r i m i tive

trilobites

of

different regions

that

are

closely related

c a n n o t be r e c o g n i z e d a s

s u c h . T h i s , of c o u r s e , is a l s o t r u e for o t h e r f o s s i l g r o u p s u n d e r s i m i l a r c o n d i t i o n s . - The u s e of v e r y e a r l y C a m b r i a n f o s s i l s a s i n d e x f o s s i l s for long d i s t a n c e c o r r e l a t i o n is e v e n more p r o b l e m a t i c a l , i f it is t r u e t h a t c h a n g e s of e n v i r o n m e n t i n f l u e n c e d t h e r e g i o n a l d i s t r i b u t i o n of life n e a r t h e b a s e of t h e C a m b r i a n ( s e e 4e).

4d. A r c h a e o c y a t h a

Archaeocyatha are common fossils in most of the Lower Cambrian rocks of Morocco. Ira-

101

portant

localities

k n o w n from

with

north

of

archaeocyathan the

Jbel

bioherms

from

TaYssa ( D e b r e n n e ,

the

1958;

S~rle

1961),

schisto-calcaire

are

from T a z e m m o u r t ( D e -

b r e n n e , 1960), a n d from m o s t p r o b a b l y e q u i v a l e n t s t r a t a of t h e Lemdad a r e a . Notes on

a r c h a e o c y a t h a from t h e

mourt section

(Debrenne,

1960)

C a l c a i r e s s u p ~ r i e u r s were

a n d for t h e

p u b l i s h e d for t h e

s e c t i o n of Tiout.

From t h a t

part

Tazemof t h e

T a z e m m o u r t s e c t i o n o n l y one s p e c i e s w a s d e s c r i b e d (Cosclnoeyatbus mlnutus D e b r e n n e , 1960), b u t Hup~ (1959) a n d e. g. Boudda e t al. (1979) m e n t i o n e d s e v e r a l h o r i z o n s b e a r ing a r c h a e o c y a t h a . At T i o u t a r c h a e o c y a t h a (Debrenne,

1975;

a p p e a r a l s o in n u m e r o u s h o r i z o n s w i t h i n t h e Eofallotaspls zone

Debrenne

& Debrenne,

1976;

Debrenne,

1976;

Debrenne & Debrenne,

1978). S e v e n s p e c i e s a r e k n o w n o n l y from t h i s s e c t i o n , f i v e s p e c i e s were a l r e a d y d e s c r i b e d from o t h e r Moroccan l o c a l i t i e s ; m o s t of t h e m h a v e

a wide s t r a t i g r a p h i c a l r a n g e .

"Aldanocyathus blohermicus ( Z h u r a v l e v a , 1960)?" is s i m i l a r to a S i b e r i a n s p e c i e s from the Atdaban thus its is a l s o

and

Lena stages,

b u t like

r e c o g n i t i o n is d o u b t f u l . k n o w n from S a r d i n i a ,

Therefore, the

correlation

Dictyocyathus?. sp. v e r y poorly p r e s e r v e d a n d

Protopharetra circula v a r . colonialis ( D e b r e n n e , 1964)

Spain,

a n d Siberia,

with the Siberian

s p e c i e s . D e b r e n n e & D e b r e n n e (1978) r e g a r d t h e Calcaires sup~rieurs

only

a long time r a n g e , a s

well.

genera, not

a r c h a e o c y a t h a b e a r i n g h o r i z o n s of t h e

of T i o u t a s p r o b a b l y n o t v e r y e a r l y

e s s e n t i a l l y on t h e a p p e a r a n c e these genera,

but has

s t a g e s h a s to be b a s e d u p o n

of Neoloculicyathus,

A t d a b a n i a n . T h i s is f o u n d e d

Tumulifungia, a n d Gordonifungia. Of

Tumullfungia w a s d e s c r i b e d from more t h a n one r e g i o n before, b u t

t h e Moroccan s p e c i m e n s d i f f e r l a r g e l y in m o r p h o l o g y from t h e S i b e r i a n s p e c i e s . T h e r e f o r e biostratigraphical correlations

of t h e

Eofallotaspis z o n e b a s e d on a r c h a e o c y a t h a a p p e a r

to be n o t v e r y r e l i a b l e in t h e p r e s e n t s t a t e

of k n o w l e d g e .

4e. E a r l y C a m b r i a n r a d i a t i o n e v e n t a n d / o r c h a n g e of e n v i r o n m e n t ?

T h e s t r a t i g r a p h i c a l p r o b l e m s we a r e c o n f r o n t e d w i t h w h e n d e a l i n g w i t h t h e b a s e of t h e Cambrian are

c e r t a i n l y c a u s e d or a t

least

i n f l u e n c e d by t h e f a c t t h a t

we a r e

dealing

w i t h a t i m e s p a n t h a t is u n i q u e in t h e P h a n e r o z o i c : It is t h e time w h e n a n i m a l s b e a r i n g hard s k e l e t o n s are

d o c u m e n t e d for t h e f i r s t

time, a n d t h e f a c t t h a t t h e i r

Precambrian

a n c e s t o r s a r e s t i l l u n k n o w n c r e a t e s one of t h e g r e a t e s t e n i g m a s of E a r t h h i s t o r y . T h e y are unknown

in s p i t e

of a t r e m e n d o u s a m o u n t of r e s e a r c h on t h e l a t e

P r e c a m b r i a n in

t h e l a s t d e c a d e s a n d t h i s led to t h e a s s u m p t i o n of a r a p i d e v o l u t i o n , of a r a d i a t i o n of a n i m a l life, n e a r t h e b e g i n n i n g of C a m b r i a n time. We will n o t d i s c u s s t h i s , b u t we w a n t to p o i n t o u t t h a t

t h e mode of a p p e a r a n c e of h a r d - p a r t

a n i m a l s in e a r l y C a m b r i a n t i m e

c a n n o t be e x p l a i n e d by a r a d i a t i o n alone. T h e f i r s t C a m b r i a n f o s s i l s did n o t r e a l l y a p p e a r

a t once,

but obviously they first ap-

p e a r e d in v e r y l i m i t e d r e g i o n s a n d t h e n n e e d e d a c o n s i d e r a b l e time to s p r e a d o v e r t h e

102

early Cambrian seas. Very few sections many in which the and the

existence

m o r e we a s c e n d

of Tommotian sediments

It is o b v i o u s t h a t

in e a r l y C a m b r i a n where

rests

were

was

Monninger's sup6rieurs

environment

water agitation,

The

interrupted,

apparently

makes

were us

appearance

and currents

mal marine

were

were

suspect

very

m e n t is a l s o i n d i c a t e d

of the

or w h e r e regions

that

that

the

basin

water

their

the en-

similar envi-

there

was

and

indicators yet

the and

was

influx

ocean

of t e r -

probably

the extensive

oversaturated

shal-

a r e no s i g n s o f

was little open

Calcaires

long period

for very

there

change

of the

was

development

with

oxygen.

rock composition

of oolitic limestones.

stronger

and the a very

It

to m a r i n e a n i m a l life.

a slight

than

favourable

by the results

Lie de v i n

that

slow and there

the

trilobites

were

likely

There are various

not favourable

markedly

that

i t is q u i t e

shown

being the appearance

conditions

exist

in t h o s e

many stromatolites),

between

of the first

curs, the most notable

regions where sedimenta-

did n o t

is that

periods of hypersalinity,

was an environment

With t h e

clearly

the

apparently

connection

vast

parts

investigations

have

was exceptional:

there

existed

hard

life, a n d

f o s s i l s m a y be s u s p e c t e d ,

time.

obvious being

currents

material.

of stromatolites

tation

for animal

sedlmentological

(the most

with

are

the more widely are fossils spread.

most likely explanation

in late Precambrian

(1979)

in w h i c h t h e

time, there

animals

of the Tiout section

low w a t e r

partly

The

not favourable

existed

rigenous

either

not fossilized.

vironment ronments

without

in t h e C a m b r i a n s u c c e s s i o n s

tion occurred

and

with Tommotian fossils are known, but there

before,

to b e n t h i c

Obviously water

and this probably animals.

l e d to n o r -

A change

of Tucker (1986) who investigated

ocagi-

of environ-

carbon

isotopes

in t h e T i o u t s e c t i o n . Stromatolites sible

that

conditions

true then than

were enormously

reasons

should

ancestors

of

regarding question

unfavourable

the correlation

for the

cambrian

of the

the Cambrian

life conditions

terminated

and

within

Phanerozoic.

isms occurred of Cambrian

there

the atmosphere

benthic

late

Precambrian

animals

above.

Conditions

are

still

unknown.

authors

over

we t h i n k

to i t s p r e s e n t

that

suppose

--

hundreds

state

to

There

this

prove

was not finished

animals

is s t i l l

in the

another

as

is

Pre-

late

as

assimilation before

to i m a g i n e t h a t

the

aspect

It i s a n o p e n

to i t s p r e s e n t

was that

If t h i s

puzzling fact that

of millions of years impossible

too.

pos-

more problematical

stromatolites:

of earth

that

however,

i t is q u i t e

widespread

for the

of the Cambrian and

stromatolites,

and it is quite

will be e v e n

unfavourable

of changing the atmosphere

The

were

of the Cambrian

near the base

are

in the

as a possible explanation

faunas

on a large scale times, and

to base

discussed

be c o n s i d e r e d

when the process

the

widespread

state

was

some

time

by organ-

the beginning the change

of

long before this time.

5. C o n c l u s i o n s

The trilobites

from the

EofaHotaspis z o n e o f T i o u t ( a n d o f T a z e m m o u r t ) a r e t h e e a r l i e s t

103

trilobites

k n o w n i n Morocco a n d

their

appearence apparently

c h a n g e of e n v i r o n m e n t . For r e a s o n s w h i c h we h a v e t i o n of t h e

w a s m a d e p o s s i b l e by a

discussed, a long-distance

correla-

Eofallotaspis zone is n o t p o s s i b l e a n d t h e r e f o r e we c a n n o t d e c i d e w h e t h e r

i t s a g e i s e a r l y A t d a b a n i a n (we would e x c l u d e a y o u n g e r age) or w h e t h e r it g o e s down i n t o t h e T o m m o t i a n . T h e p r e s e n c e of t r i l o b i t e s c e r t a i n l y is no p r o o f a g a i n s t T o m m o t i a n age. Later Early

C a m b r i a n f a u n a s from Morocco a n d

s i m i l a r o n e s from S p a i n (Sdzuy,

1962)

c a n be c o r r e l a t e d w i t h S i b e r i a n o n e s a n d i t c a n be s h o w n t h a t t h e z o n e s IV a n d V of Hup~ a r e u p p e r m o s t A t d a b a n i a n . We t h i n k i t to be p r o b a b l e t h e n t h a t t h e EofaHotaspis z o n e a n d t h e e a r l i e s t f o u r or f i v e z o n e s of ttup~ t o g e t h e r c o m p r i s e a t i m e s p a n t h a t

is

a t l e a s t e q u a l to t h e A t d a b a n i a n . T h e b a s e of t h e C a m b r i a n c a n n o t be l o c a t e d w i t h p r e c i s i o n i n t h e T i o u t s e c t i o n . E v i d e n c e from s t r o m a t o l i t e s p o i n t s to a p o s i t i o n n e a r t h e b a s e of t h e C a t c a i r e s s u p ~ r i e u r s , b u t i t is no f i r m e v i d e n c e . A p o s i t i o n of t h e b a s e o f t h e C a m b r i a n f a r below t h e C a l calres

sup~rieurs has been suggested repeatedly.

We c a n n o t e x c l u d e t h a t ,

but

on t h e

o t h e r h a n d we do n o t t h i n k t h a t i t is of g r e a t i m p o r t a n c e w h e t h e r r o c k s t h a t c a n n o t be well d a t e d a r e r e g a r d e d a s V e n d i a n or a s C a m b r i a n .

6. References

B e r t r a n d - S a r f a t i , J. (1981): Probl~me de la l i m i t e P r ~ c a m b r i e n - C a m b r i e n : la s e c t i o n de T i o u t (Maroc); l e s s t r o m a t o l i t e s e t l e u r b i o s t r a t i g r a p h i e (SCHMITT 1979): c r i t i q u e s e t o b s e r v a t i o n s . - Newsl. S t r a t i g r . , 10(1): 2 0 - 2 6 , 1 Tab.; Berlin, S t u t t g a r t . Boudda, A. (1968): Zur g e o l o g i s c h e n K a r t i e r u n g 1 : 25000 lm G e b i e t y o n O u n e i n ( M a s s i f a n c i e n d u H a u t - A t l a s , Marokko). - M a s t e r s T h e s i s , P a l e o n t . I n s t . Univ. Wfirzburg: 52 pp., 35 figs., 2 pl.; Wfirzburg ( u n p u b l . ) . Boudda, A. & C h o u b e r t , G. (1972): Sur la l i m i t e l n f ~ r i e u r du C a m b r i e n a u Maroc. Acad. Sci. P a r i s , D 275: 5 - 8 ; P a r i s .

C. R.

Boudda, A., C h o u b e r t , G. & F a u r e - M u r e t , A. (1979): E s s a i . de S t r a t i g r a p h i e de l a C o u v e r t u r e s ~ d i m e n t a i r e de l ' A n t i - A t l a s : A d o u d o u n i e n - C a m b r i e n I n f ~ r i e u r . - Notes M4m. S e r v . g~ol Maroc, 2 7 1 : 9 6 pp., 20 figs.; R a b a t . B o u k h a r l , A., C h o u b e r t , G., F a u r e - M u r e t , A. & T i m o f e i e v , B. V. (1979): S u r l a m i c r o f t o r e d e s c o u c h e s t e r m i n a l e s d u P r ~ c a m b r i e n III e t s u r l'~ge de l ' A d o u d o u n i e n a u Maroc. - C. R. Acad. Sci. P a r i s , 288: 1 3 - 1 4 ; P a r i s . B o u r c a r t , J. (1927): D ~ c o u v e r t e d u C a m b r i e n & A r c h a e o c y a t h u s d a n s I ' A n t i - A t l a s c a i n . - C. R. somm. Soc. g~ol. Fr., ( 1 - 2 ) : 1 0 - 1 1 ; P a r i s .

maro-

B o u r c a r t , J. & Le V i l l a i n , G. (1928a): S u r le g i s e m e n t & A r c h a e o c y a t h u s de Sidi Mouca d ' A g l o u ( A n t i - A t l a s m a r o c a i n ) . - C. R. somm. Soc. g6ol. Fr., (10): 115; P a r i s . ---

(1928b): S u r la l a u r i e d e s c a l c a i r e s c a m b r i e n s de Sidi Mouqa d ' A g l o u pr&s de T i z n i t (Sud m a r o c a i n ) . - C. R. Acad. Sci. P a r i s , 187: 1 0 6 6 - 1 0 6 8 ; P a r i s . (1931): La f a u n e d e s c a l c a i r e s c a m b r i e n s de Sidi Mou~a d ' A g l o u ( A n t i - A t l a s m a r o c a i n ) . - Notes M~m. Serv. Mines C a r t e g~ol. Maroc, 1 5 : 4 4 pp., 1 t a b . , pl. I X; Macon.

104

B u g g i s c h , W. & Heinitz, W. (1984): S l u m p f o l d s a n d o t h e r e a r l y d i a g e n e t i c d e f o r m a t i o n s in t h e e a r l y C a m b r i a n of t h e W e s t e r n a n d C e n t r a l A n t i a t l a s (Morocco). - Geol. Rdsch., 73(2): 8 0 9 - 8 1 8 , 8 figs., 1 tab.; S t u t t g a r t . B u g g i s c h , W., Marzela, C. & Hfigel, P. (1979): Die f a z i e l l e u n d p a l ~ o g e o g r a p h i s c h e E n t w i c k l u n g d e r i n f r a k a m b r i s c h e n bis o r d o v i z i s c h e n S e d i m e n t e im M i t t l e r e n A n t i a t l a s um Agdz (S-Marokko). - Geol. Rdsch., 68(1): 1 9 5 - 2 2 4 , 21 figs., 1 tab.; S t u t t g a r t . C h o u b e r t , G. (1942): La c o n s t i t u t i o n e t la p u i s s a n c e de la sArie p r i m a i r e de l ' A n t i - A t l a s . - C. R. Acad. Sci. P a r i s , 215: 4 4 5 - 4 4 7 ; P a r i s . (1943): Sur le G~orgien de l ' A n t i - A t l a s . Paris. ---

-

C. R. Acad. Sci. P a r i s , 216: 6 9 - 7 0 ;

(1952a): Le v o l c a n g ~ o r g i e n de la r~gion d ' A l o u g o u m ( A n t i - A t l a s ) . Sci. P a r i s , 234: 3 5 0 - 3 5 2 ; P a r i s .

C. R. Acad.

(1952b): t t i s t o i r e g~ologique du d o m a i n e de l ' A n t i - A t l a s . In: C h o u b e r t , G. & Margais, J.: GAologie du Maroc. - XIX e Congr~s G~ol. I n t e r n . , A l g e r 1952, Monogr. r~g., Ser. 3, Maroc, 6: 7 7 - 1 9 4 , 16 maps; Alger. (1953): Le P r ~ c a m b r i e n III et le G~orgien de l ' A n t i - A t l a s . I n t r o d u c t i o n s t r a t i g r a p h i q u e . - In: Hup~, P.: C o n t r i b u t i o n ~ l ' ~ t u d e du C a m b r i e n i n f 4 r i e u r e t du P r ~ c a m b r i e n III de l ' A n t i - A t l a s m a r o c a i n . N o t e s M~m. Serv. g~ol. Maroc, 103: 1 7 39, 4 figs.; B a g n o l e t / S e i n e ("1952"). (1958): L ' A d o u d o u n i e n e t le P r e c a m b r i e n III darts l ' A n t i - A t l a s . - In: Les R e l a t i o n s e n t r e P r ~ c a m b r i e n e t Cambrien. Probl~mes d e s s ~ r i e s i n t e r m ~ d i a i r e s . Coll. i n t e r n . C e n t r e Nat. Rech. Sci., Paris, 1957, LXXVI: 1 4 3 - 1 6 2 , 2 f i g s . , 1 tab.; P a r i s . (1963): H i s t o i r e g 4 o l o g i q u e de M~m. Serv. g~ol. Maroc, 1 6 2 : 3 5 2 Rabat.

P r ~ c a m b r i e n de l ' A n t i - A t l a s . Tome 1. - N o t e s pp., 33 figs., 81 Phot., 5 m a p s , 7 c o l o u r m a p s ;

(1964): N o u v e l l e s p r ~ c i s i o n s s u r la s t r a t i g r a p h i e d e s f o r m a t i o n s c a l c a i r e s e t s c h i s t o - c a l c a i r e s du C a m b r i e n i n f ~ r i e u r de l ' A n t i - A t l a s . - C. R. Soc. g~ol. F r a n c e , (8): 3 3 7 - 3 3 8 ; P a r i s . C h o u b e r t , G., Boudda, A & F a u r e - M u r e t , A. (1975): E s s a i de s t r a t i g r a p h i e de la c o u v e r t u r e s e d i m e n t a i r e de l ' A n t i - A t l a s : A d o u d o u n i e n - Cambrien. - N o t e s MAm. Serv. gaol Maroc, 2 7 1 : 9 5 pp., 20 figs.; R a b a t (1979). & R e u n i o n Lim. C a m b r i e n - P r ~ c a m b r i e n , A g a d i r - R a b a t , Dok. 1A: 57 pp.; R a b a t ( u n p u b l . ) . C h o u b e r t , G., D r e s n a y , R. du & H i n d e r m e y e r , J. (1950): Sur l e s c a l c a i r e s ~ C o l l e n i a de la r~.gion de S a f s a f - A i n Chair. - Notes M~m. Serv. g4ol. Maroc, 3, 76: 9 3 - 1 0 3 ; R a bat. C h o u b e r t , G., H i n d e r m e y e r , J. & Hollard, H. (1952a): Sur la p r e s e n c e de C o l l e n i a d a n s le PrAcambrlen III e t le GAorgien de l ' A n t i - A t l a s . - C. R. Acad. Sci. P a r i s , D 234: 543-545; Paris. (1952b): Note pr41iminaire s u r le Collenia de l ' A n t i - A t l a s . g~ol. Maroc, 95: 8 5 - 1 0 2 ; R a b a t .

- N o t e s MAre. Serv.

Clauer, N. (1980): N o u v e l l e s p r ~ c i s i o n s s u r la g 4 o c h r o n o l o g i e i s o t o p i q u e e t b i o s t r a t i g r a p h i q u e du PrAcambrien de l ' A n t i - A t l a s a u Maroc. - C. R. somm. Soc. g~ol. Fr., (3): 8 4 - 8 7 ; P a r i s . D e b r e n n e , F. (1958): Sur q u e l q u e s A r c h a e o c y a t h a du J e b e l T a i s s a ( A n t i - A t l a s tal). - Notes Serv. g~ol. Maroc, 16, 143: 5 9 - 7 3 , 2 figs., 2 pl.; Rabat.

occiden-

105

(1960): Archaeocyatha d e s l e n t i l l e s c a l c a i r e s de T a z e m m o u r t N o t e s Mere. S e r v . G~ol. Maroc, 18, 147: 7 - 2 6 , pl. I - V ; R a b a t . ---

(Anti-Atlas).

-

( 1 9 6 1 ) : N o u v e l l e s d o n n 4 e s s u r la f a u n e d ' A r c h a e o c y a t h a du Jbel Taissa (AntiA t l a s o c c i d e n t a l ) . - N o t e s S e r v . g~ol. Maroc, 20, 152: 7 - 3 7 , pl. I - V I ; R a b a t . ( 1 9 6 4 ) : A r c h a e o c y a t h a . C o n t r i b u t i o n ~ l ' ~ t u d e d e s f a u n e s c a m b r i e n n e s d u Maroc, de S a r d a i g n e e t de F r a n c e . - N o t e s M~m. S e r v . g~ol. Maroc, 1 7 9 : 3 7 1 pp., 52 pl.; Rabat.

---

[Debrenn] (1976): Drevnejshie arkheotsiaty S e r i y a Geol., 12: 1 5 7 - 1 5 9 , fig.; Moscow.

Marokko.

-

Izv.

Akad.

Nauk

SSSR,

D e b r e n n e , F. & D e b r e n n e , M. ( 1 9 7 6 ) : A r c h a e o c y a t h i d f a u n a o f t h e l o w e s t f o s s i l i f e r o u s l e v e l s o f T i o u t (Lower C a m b r i a n - S o u t h e r n Morocco). - R ~ u n . Gr. T r a y . L i m i t e P r ~ c a m b . - C a m b . Maroc, M a r s - A v r i l 1976, C. R. a c t i v i t Y : 22 pp., pl. I - I V ; R a b a t (unpubl.). ( 1 9 7 8 ) : A r c h a e o c y a t h i d f a u n a o f t h e l o w e s t f o s s i l i f e r o u s l e v e l s o f T i o u t (Lower C a m b r i a n - S o u t h e r n Morocco). - Geol. Mag., 115(2): 1 1 0 1 - 1 1 9 , 3 t a b . , pl. 1 - 4 ; London. Etude D u c r o t , J. & L a n c e l o t , J. R. ( 1 9 7 7 ) : P r o b l 6 m e de la l i m i t e P r ~ c a m b r i e n - C a m b r i e n . radiochronologique p a r la m 6 t h o d e U - P b s u r z i r c o n s d u v o l c a n d u j b e l Boho ( A n t i - A t l a s m a r o c a i n ) . - C a n . J. E a r t h Sci., 14: 2 7 7 1 - 2 7 7 7 ; O t t a w a . Geyer,

G. ( 1 9 8 6 ) : M i t t e l k a m b r i s c h e M o l l u s k e n a u s M a r o k k o u n d S p a n i e n . g i a n a l e t h . , 6 7 ( 1 / 4 ) : 5 5 - 1 1 8 , 6 f i g s . , 1 t a b . , 6 pl.; F r a n k f u r t a. M.

Heinitz,

W. ( 1 9 8 4 ) : Die D e f o r m a t i o n d e s s e d i m e n t ~ r e n D e c k g e b i r g e s im z e n t r a l e n A n t i A t l a s ( S f i d m a r o k k o ) . - B e r l i n e r g e o w i s s . A b h . , (A), 5 5 : 8 4 pp,, 64 f i g s . ; B e r l i n .

Hollard,

If. ( 1 9 8 5 ) : P r e c a m b r i a n - C a m b r i a n B o u n d a r y a n d L o w e r C a m b r i a n . - In: D e s t o m b e s , J., H o l l a r d , H. & W i l l e f e r t , S.: L o w e r P a l a e o z o i c R o c k s o f Morocco. In: H o l l a n d , C. H. (ed.): L o w e r P a l a e o z o i c R o c k s o f t h e World, Vol. 4: L o w e r P a l a e o zoic o f n o r t h - w e s t e r n a n d w e s t c e n t r a l A f r i c a : 1 0 1 - 1 5 7 , f i g s . 5 - 2 8 , pl. 1 - 4 ; C h i c h e s t e r , New York, B r i s b a n e , T o r o n t o , S i n g a p o r e (J. Wiley & S o n s ) .

Hup~, P. ( 1 9 5 2 ) : S u r l e s z o n e s de T r i l o b i t e s A c a d . Sci. P a r i s , 235: 4 8 0 - 4 8 1 ; P a r i s .

du Cambrien

inf~rieur

Senckenber-

marocain.

-

C. R.

( 1 9 5 3 ) : C o n t r i b u t i o n ~t l ' ~ t u d e d u C a m b r i e n i n f ~ r i e u r e t d u P r 6 c a m b r i e n III de L ' A n t i - A t l a s m a r o c a i n . - N o t e s M6m. S e r v . g6ol. Maroc, 1 0 3 : 4 0 2 pp., 99 f i g s . , 4 t a b . , pl. I - X X I V ; B a g n o l e t / S e i n e ( " 1 9 5 2 " ) . ( 1 9 5 9 ) : N o u v e l l e c o n t r i b u t i o n & l ' ~ t u d e d u C a m b r i e n m a r o c a i n . - T h 6 s e Sci. Fac. Sci. U n i v . P a r i s , 447 pp., 1 2 + 5 f i g s . , 3 m a p s , pl. A - W , 1 - 1 0 , 1 0 b i s , 1 1 - 3 2 ; P a r i s (unpubl.). ( 1 9 6 0 ) : S u r le C a m b r i e n i n f ~ r i e u r d u Maroc. - Rep. X X I t h i n t . N o r d e n 1960, 8, 8. L a t e P r e - C a m b r i a n and Cambrian stratigraphy: Kopenhagen.

Lifian, E. & S d z u y , K. ( 1 9 7 8 ) : A t r i l o b i t e and its stratigraphical significance. f i g s . , 1 pl,; F r a n k f u r t a. M.

geol. C o n g r . , 75-85, tab.;

from the Lower Cambrian of CSrdoba (Spain) - Senckenberg. leth., 59(4-6): 378-399, 2

M o n n i n g e r , W. ( 1 9 7 9 ) : T h e s e c t i o n o f T i o u t ( P r e c a m b r i a n / C a m b r i a n A t l a s , Morocco): A n e n v i r o n m e n t a l m o d e l . - Arb. P a l ~ o n t . pp., 49 f i g s . , 32 t a b . , 22 pl.; W f i r z b u r g ( u n p u b l . ) .

boundary beds, AntiI n s t . Wfirzburg, 1 : 2 8 9

106

Monninger, W. & Schmitt, M. (1975): The s e c t i o n of Tiout: Lithology and algal s t r u c t u r e s . - R~un. Gr. Tray. Limite Pr~cambrien/Cambrien, A g a d i r - R a b a t , 1 9 7 5 : 8 pp.; Rabat (unpubl.). Repina, L. N. (1969): T r i l o b i t y n i z h n e g o k e m b r i y a i s r e d n e g o kembriya yuga Sibiri ( n a d s e m e j s t v o Redlichioidea). C h a s t ' II. - Akad. Nauk SSSR, Sib. Otdel., Trudy Inst. Geol. Geofiz., 6 7 : 1 0 7 pp., 52 figs., pl. I-IV; Moscow. Schmitt, M. (1978): S t r o m a t o l i t e s from t h e Tiout s e c t i o n , P r e c a m b r i a n - C a m b r i a n b o u n d a r y beds, A n t i - A t l a s , Morocco. - Geol. Mag, Special i s s u e , 115(2): 9 5 - 9 9 , 3 figs., pl.; Cambridge. (1979a): The Section of Tiout ( P r e c a m b r l a n / C a m b r i a n Boundary beds, A n t i - A t l a s , Morocco): S t r o m a t o l i t e s a n d t h e i r B i o s t r a t i g r a p h y . - Arb. Pal~ont. i n s t . Wfirzburg, 2 : 1 8 8 pp., 44 figs., 23 pl.; Wfirzburg (unpubl.). (1979b): New s t r o m a t o l i t e s from t h e Late P r e c a m b r i a n of t h e A n t i - A t l a s and from t h e Lower Cambrian of t h e High Atlas, Morocco. - S e n c k e n b e r g . l e t h a e a , 60(1/3): 3 9 - 4 9 , 7 figs., 2 pl.; F r a n k f u r t a. M. Schmitt, M. O. & Monnlnger, W. E. W. (1977): S t r o m a t o l i t e s a n d T h r o m b o t i t e s in P r e c a m b r i a n / C a m b r i a n Boundary Beds of t h e A n t i - A t l a s , Morocco: P r e l i m i n a r y Results. - In: Flfigel, E. (ed.): Fossil Algae: 8 0 - 8 5 , 2 pl.; Berlin, Heidelberg (Springer). Sdzuy, K. (1962): T r i l o b i t e n aus dem U n t e r - K a m b r i u m der Sierra Morena ( S - S p a n i e n ) . S e n c k e n b e r g . leth., 43(3): 181-229, 4 figs., pl. XVIII-XXIII; F r a n k f u r t a. M.

-

(1978): The P r e c a m b r i a n - C a m b r i a n b o u n d a r y beds in Morocco (Preliminary R e port). - Geol. Mag., 115(2): 8 3 - 9 4 , 6 figs., 1 pl.; Cambridge. ---

(1981): Der Beginn des P h a n e r o z o i k u m s - Pal~tobiologische und s t r a t i g r a p h i s c h e Probleme. - Natur und Museum, 111(11): 3 9 0 - 3 9 9 , 20 figs.; F r a n k f u r t a. M.

Semikhatov, M. A. (1977): M6thode p a l ~ o n t o l o g i q u e d a n s la corr61ation du Pr~cambrien s u p ~ r i e u r des r~gions p ~ r i - a t l a n t i q u e s de l'Europe, de l ' A f r i q u e et de l'Am6rique. Estud. Geologicos, 33: 3 6 5 - 3 7 2 ; Madrid. -

Tucker, M. E. (1986): Carbon i s o t o p e e x c u r s i o n s in P r e c a m b r i a n / C a m b r i a n b o u n d a r y beds, Morocco. - Nature, 319(6048): 4 8 - 5 0 , 2 figs.; New York.

PALEOGEOGRAPHY AND FACIES OF THE 'GRES TERMINAUX' (UPPERMOST LOWER CAMBRIAN, ANTI-ATLAS/MOROCCO)

Werner Buggisch & Rainer Siegert Institut for Geologie der Universit&t, Schlo6garten 5, D-8520 Erlangen

Abstract The facies and paleogeography of the uppermost Lower Cambrian of the Anti-Atlas iS described. Five different paleogeographic units were recognized: Fluviatile sandstones in the east, continental to marine facies in the center, marine sediments and volanic rocks in the northwest and interfingering of shallow marine sediments with the 'Delta of Asrir' in the southwest. The Ait Ounein section is separated from the Anti-Atlas by the High Atlas boundary fault. 1, Introduction

Above the first Lower Cambrian regressive megacycle of the Anti-Atlas (S~rie des calcaires inf~rieurs to S~rie lie de vin), a second sedimentary cycle starts with the transgression of the S6rie des calcaires sup6rieurs, Abundant

Fig. 1. Map of the Anti-Atlas showing the studied sections,

108

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z o N

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,,S c h i s t e s

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& P a r a do x i d e s "

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(GrSs d' Ourmast) .Niveau d' 0uriken-n' 0urmast"

OURMASTtE N I I

Conglomerate of Aguerd N > Z 0 N

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rouges

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o - c a I c a i r es"

AMOUSLEKIEN 1

Tab. 1. Stratigraphy of the uppermost Early Cambrian in the Anti-Atlas. trilobites and archaeocyathid bioherms in these Lower Cambrian sediments are evidence of an open marine environment in the western Anti-Atlas up to the central Anti-Atlas (particularly in the black limestones of the "calcaires de base'). The overlying series (schisto-calcaires, complexe schisteux and gr~s terminaux) mark the second regression in the Lower Cambrian prior to the Middle Cambrian transgression, which covered most parts of the Anti-Atlas. This paper describes the lithology and facies of the clastic sediments below the Middle/Upper Cambrian boundary and outlines paleogeographic maps and facies models of the western and central Anti-Atlas. This investigation is based on 40 sections (Fig. 1) and several hundred samples and thin-sections.

2, Previous work The first survey of the Cambrian in the Anti-Atlas by NELTNER (1938) established the stratigraphic outlines, which are still valid today. Based on trilobites, HUPE(1952) subdivided the Lower Cambrian into different zones and classified the clastic sediments below the Lower/Middle Cambrian boundary as Zone VI to Vllt. The term 'gres terminaux' was established by CHOUBERT (1952) for the sandstones of the trilobite Zone VII. In 1963, he named the sedimentary time point of the gr~s terminaux '6tage d'Asrir', referring to the type locality. The term 'niveau & Micmacca' or 'niveau d'Ourmast' was used for the Lower/Middle Cambrian transition beds. Volcanoclastic intercalations in these sequences were described in the western Anti-Atlas; in the High Atlas they were described by BOUDDA et al. (1974).The biostratigraphy of the Lower to Middle Cambrian boundary beds in the western AntiAtlas was studied in detail by GEYER (1984).

3. Llthostratigraphy The clastic sequences below the Lower/Middle Cambrian boundary are not at all uniform. The variety in lithology

109

~

Serie d'Akerouz

F~

Asririen

Niveau d'0urmast

~

'ssaferfien

0unein j

20,

i

400

S~&rie volcano- d~tritique

Astir

60 i

6,0~

Cornplexe schisteux

Khenig

km i

Aguerd

m

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Tat(]

Air Sedrat

Akerouz

÷ 4- ..-.:-:~. -.-:7.-' Jr • ¢.-1 ~, 44-

-h/j/'~.' 4 f//.z

Fig. 2. Sections across the depositional area of the 'gr@sterminaux' in the Anti-Atlas.

documents a differentiated depositional area with an inferfingering of terrestrial and shallow marine sediments. In order to classify the different units, new terms will be introduced (underlined in Tab. 1) and existing terms will be defined more precisely in this paper. The term 'gr@s terminaux' is applied as a generic term for sandstones of the trilobite ZoneVIl. Five different paleogeographic units were recognized within the trilobite Zone VII (HUPE 1952) in the Anti-Atlas: (1) The eastern Anti-Atlas (between Foum Zguid/Tazenakht and Erfoud). This area is characterized by fluviatiie sandstones. (2) The central Anti-Atlas (between Tata and Foum Zguid) with the transition from continental to madne facies. (3) The northwestern Anti-Atlas (region of Tiznit-Taroudant) distinguished by marine sediments and thick volcanic sequences. (4) The southwestern Anti-Atlas (Tan-Tan, Goulimime, Bou Izakarn) characterized by the interfingedng of the 'Delta of Asrir' with shallow marine sediments. (5) The Ait Ounein section is situated north of the southern Atlas boundary fault and cannot be clearly correlated with the sections of the Anti-Atlas.

3.1. Eastern Anti-Atlas A 70 m thick sequence of white and grey sandstones alternating with red-violet mudstones is exposed in the

110

easternmost section of the Anti-Atlas about 60 km WNW of Erfoud. This S~rie d'Akerouz (Gdorgien continental) consists of laminated or sometimes cross-bedded sandstones. The bed thickness is about one meter. The grainsize parameters (mean = 0.06-1.0 mm; sorting and skewness) indicate transportation and sedimentation in slowly moving water• Pedogenic broken quartz-grains (PI. 1, Fig. 1) floating in a carbonate matrix indicate temporary sedimentation above the ground water table under arid conditions (caliche). The upper Lower Cambrian of the other sections of the eastern Anti-Atlas are characterized by the succession: gr&s d'Ourmast gr~s rouges complexe schisteux Red-violet mudstones, siltstones and thin-bedded fine-grained sandstones are typical of the comDlexe schisteux. Frequent ripple marks, small-scale cross-bedding, desiccation cracl~ and casts of halite argue for a depositional environment with occasional water currents, high salinity and abundant desiccation (lagoon to salt marsh, REINECK 1985).

3.1.1. Gr&s rouges The gr&s rouges are the typical 'gr&s terminaux' in the eastern Anti-Atlas. They consist of light-red, thin-bedded, 20 -

300 cm thick layers of fine-grained sandstones forming up to 15 m thick units. These units wedge out laterally

after short distances (maximum width about 300 m). Erosion channels with a few meters in width (point bars), layers

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Fig. 4. CM-patterns in the Passega-diagram.

with mud pebbles and asymmetric current ripples with a wave length of a few centimeters up to more than one meter are frequently observed. Arched trough-shaped cross-bedded sand bodies, several meters wide and about 50 cm thick, are typical of the gr~s rouges. Statistical measurements of cross-beddings (Fig. 3) indicate a W or WSW directed transport in outcrops at the southern margin of the Jebel Sarhro and W or WNW directed transport in the vicinity of the anticline (boutonniere) of Bou Azzer - El Graara. The sandstones are fine-grained (Md 0.06-0.07 mm), sorting is good to medium with a predominance of the fine-grained fraction; roundness ranges from rounded to subrounded. The pattern of the CM-diagram ranges from 'uniform' transport to 'graded suspension' (Fig. 4). According to the diagram of PETTIJOHN et al. (1973) the sandstones of the gr~s rouges are classified as 'arcosic arenites' or 'subarcoses' (Fig. 5, PI. 1, Figs. 2,3). The quartz content increases and the matrix content decreases from E to W. About 50 - 60% of the sandstones consist of quartz. Metamorphic quartz grains are unimportant in comparison with the non-metamorphic volcanic quartz with sharp extinction and resorption features which make up the bulk of the grains. At the southern margin of the anticline of the Jebel Sarhro, only 25 to 30% of

!12

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gr~'s d' Ourrnast

Fig. 5. Classification of the upper Lower Cambrian sandstones.

the quartz grains exhibit undulatory extinction, whilst in the vicinity of the anticline of Bou Azzer - El Graara the amount of undulatory quartz grains is about 40%. Feldspars (plagioclase, orthoclase and microcline) make up about 20 - 30% of the sandstones. Rock fragments (up to 10%) consist of metamorphics and plutonics. The gr~s rouges are cemented by syntaxial overgrowth of detritic quartz and feldspar grains (PL 1, Figs. 2, 3). Two different source areas probably existed in the NE and SE according to the data from current directions and the modal composition of the gr~s rouges. The sandstones were deposited in braided river systems. The thickness of the sections Tazlaft and Talioulat (Fig. 2) indicates an increased rate of subsidence in this area. The base of these sections consists of bioturbated calcareous arenites; typical cross-bedding ismissing. The thick sequences of the eastern Anti-Atlas probably represent lake deposits. Recent braided rivers are usually characterized by the dominance of coarse-grained sand and gravel (REINECK & SINGH 1980), but fine-grained sediments are described as well, for instance from the Brahmaputra and the Mekong river (COLEMAN 1969; MORGAN 1970).

3.1.2. Gr~s d'Ourmast Dark or black, medium-grained to coarse-grained sandstones interbedded with thin green-black to dark-viobt mudstones occur at the base of the 'niveau d'Ourmast'. This formation is named 'gr~s d'Ourmast' in this paper. It is overlain by light-green mudstones and coquinas of fossils of the 'br~cche ~ Micmacca'. The thickness of the gr~s d'Ourmast varies considerably (Fig. 6). The orientation of cross-bedding suggests a NE - SW directed pattern of usually unimodal paleocurrents ; a NW - SE transport is measured in one section (IG = Ighels 21 Km NE Tazenakht). Asymmetric ripples and mud pebbles are common. Bioturbation and grazing trails can be observed in the upper part of the gr~s d'Ourmast. These horizons are partly cemented by carbonates and they

113

• MARRAKECH

A~(×I

Fig. 6. Thickness of the gr~s d'Ourmast.

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may contain fossil fragments (trilobites, archaeocyathids and echinoderms). A different facies of the gr~s d'Ourmast is exposed in the Targant section (Tr). Lenticular and flaser bedding and small-scale cross-bedding of mudstones and fine-grained sandstones are attributes of a muddy to sandy tidal flat (REINECK 1972). The grain-size is not uniform: rolling and traction transport and transport in uniform to graded suspension is documented in CM-diagrams (Fig. 4). Whereas rolling transport dominated in the eastern Anti-Atlas, water energy decreased towards the central Anti-Atlas. In this area, fluvial transport was obviously unimportant in comparison with transport by waves and marine currents. The gr~s d'Ourmast are composed of dominating undulate quartz grains, not-undulate quartz, rock fragments (20 %)> feldspars (10-20%) and abundant matrix (up to 30%). According to the diagram (Fig. 5), the gr~s d'Ourmast consist of lithic and arcosic arenites and arcosicwackes. The depositional environment of the gr~s d'Ourmast was predominantly fluviatile in the E with increasing marine influence toward the W and toward the top of the sections (beach and delta sediments in the central Anti-Atlas and tidal flat deposits at Targant).

3.2. Central Anti-Atlas The facies of the gr~s rouges interfingers with the sediments of the 6tage d'Asrir in the central Anti-Atlas. A change in color from red to green is combined with the facies change. Cross-bedding measurements give evidence of a SW transport direction in the E, whereas the bimodal paleocurrent patterns in the sections IT and TT are indicative of an estuary or probably tidal-dominateddelta environment. 3.3. Southwestern Anti-Atlas The sedimentation of the southwestern Anti-Atlas is mainly influenced by the 'Delta of Asrir". The following facies units and formations can be distinguished: 1. Basin sediments 'schistes de I'lssaf~ne' 'S~rie de Jebel Tichinchine' 2. Bar and platform sediments 'Calcaires scoriac~s'

114

3. Delta sediments:

'etage d'Asrir' 'conglomerate d'Aguerd'

3.3.1. Schistes de rlssaf~ne The up to 350 m thick sequence consists of 25 m thick units of green mudstones and up to 3 m thick limestones. Crystal tufts occur in some layers. The fine-grained sediments are usually laminated, occasionally crossbedded and frequently bioturbated. According to the CM-diagram (Fig. 4), the sediment transport is to be classified as 'pelagic' to 'uniformly suspended' (PASSEGA 1964). The mudstones and siltstones are composed of quartz and feldspar grains floating in a matrix of carbonate, chlorite and sericite. The cystal tufts consist of plagioclase, sanidine, orthoclase and rock fragments. The feldspars are altered to sericite and carbonates. The reddish brown, massive, meter-thick sandy carbonate units consist of quartz and feldspar containing biosparites with trilobites, echinoderms and archaeocyathids (PI. 1, Fig. 4). Chlorite and oxychlorite are neomorphic in fossil shells. The schistes de I'lssaf~ne are open marine basin sediments deposited belowthe wave base.

3.3.2. Etage d'Asrir The ~tage d'Asrir can be correlated with the gr~s rouges. The up to 70 m thick basal sandstones are overlain by green mudstones in alternation with sandstones. The ~tage d'Asrir is overlain by the conglomerate of Aguerd or by the S~rie volcano-d6tdtique.The transition to the Middle Cambrian 'schistes & Paradoxides' is continuous in some sections of the western Anti-Atlas. While cross-bedding shows bimodal, E-W directed current patterns in the sections IT, T and TZ, no clear direction can be recognized in the western sections. The mean of the grain sizes varies from silt to coarse sand; the sorting is poor to good. All possibilities from rolling transport to uniform suspension are shown in the CM-diagram (Fig. 4). Accordingly, the modal composition is very inhomogeneous. Corresponding to the variability of the (~tage d'Asrir, the sediments were deposited in different environments as follows: ~38~

~

AGAO,R 8S~AL(~001/ / ~62/ ~

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, O TATA

S Anti-Atlas:

predominantlagoon

SWAnti-Atlas:

prodeltato delta plat-

NWAnti-Atlas:

form shelftoshore, pyro-

The source area of the 'delta of Asrir' is situated south of the Anti-Atlas where mainly metamorphic rocks and inter-

/

mediate to basic volcanics were eroded. It is still an open question as to where

/ /

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(3)

e" ~ - ~

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the source area for the sediments of the 0 ~

50 ,

lOOkm

northwestern Anti-Atlas was situated.

,

Local emersion combined with erosion • K [2]

Fig. 7. Thickness of the Serie volcano-d~tritique.

evidence the redistribution of sediments in this area.

115

3.3.3. Conglomerate of Aguerd In the sections Aguerd (A), Asrir (AS), Khenig (K), and Tata (T) the etage d'Asrir is overlain by a 10 - 20 m thick sequence of massive quartzitic coarse sandstones and conglomerates. Planar and through-shaped cross-beds and scour fills indicate NE to NW directed paleocurrents. 65 - 95% of the components consist of quartz (recrystallized metaquartzites, volcanic quartz grains with embayments); feldspars and rock fragments are less abundant (Pi. 1, Fig. 5). The basal units of the conglomerate d'Aguerd were deposited in a littoral environment evidenced by the occurrence of Scolithos. Sedimentary structures and granulometric parameters substantiate a fluviatile environment for the sedimentation of the upper units. Therefore, the conglomerates of Aguerd probably represent the topsets of the Delta of Asrir.

3.4. Northwestern Anti-Atlas and Ait Ounein Only marine environment is known in the northwestern Anti-Atlas for the late Upper Cambrian and the Middle Cambrian period.

3.4.1. Calcaires scoriac#s The approximately 100 m thick reddish brown limy marls contain a rich fauna. A direct terrigenous influx cannot be observed; the few lithoclasts are of volcanic origin. Possibly, the calcaires scoriac~s were deposited on a submarine rise which separated the depositional area of the schistes de I'lssaf6ne from the open marine.

3.4.2. Series of Jebel Tichinchine The depositional area of the series of the Jebel Tichinchine is situated between the Delta of Asrir and the open marine northwestern Anti-Atlas. Green slaty mudstones with intercalated decimeter-thick limestones alternate with thin fine-grained sandstones. The increasing abundance of crystal tuffs in the upper part of the section is indicative of a correlation with the S~rie volcano-d~tritique. Biosparites and oolites (section BI) confirm a sedimentation in an open marine shallow water at the base of these sections. Other sedimentary structures indicate deposition below the wave base and a reduced environment within the sediment (pyrite). The area of deposition probably represents a partly restricted offshore basin.

3.4.3. S6rie volcano-d~tritique The Lower Cambrian section Amouslek (AL) is well known for its abundance of fossils. The section is situated about 30 km SW of Taroudant at the southern border of the Souss Plain. The S6rie volcano-d~tritique d'Jebel Issendalen is exposed at the top of the complete section. This series is known throughout the northwestern AntiAtlas. Some horizons of volcanics can even be traced south of Goulimime. The thickness amounts to approximately 100 m in the type section, decreasing towards the southwest and east (Fig. 7). Multiple eruption cycles characterize this series. Each cycle starts with splintery green tuffites, overlain by up to 10 m thick coarse-grained volcanoclastics. The cycle terminates with a I om thick horizon of precipitated iron oxides. The base o! the tuffitic layers consists of closely packed pyroclastic material (fragments of devitrified volcanic

116

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scorieces"

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118

glasses). Dispersed chlorite, calcite and ore form the matrix. Non-vulcanogenetic compounds (for instance metaquartzites) become more frequent towards the top. The coarse-grained unit (PI. 1, Fig 6) is composed of trachytic rock fragments in a tuffaceous matrix. The S~rie volcano-detritique was deposited in a shallow marine basin by pyroclastic turbidites and ash-falls. The location of the eruption center cannot be definitely determined; CHOUBERT & FAURE-MURET (1956) supposed that this eruption center was situated about 1O0 km east of Agadir.

4. Conclusions 4.1. Serie d'lssaf~ne (Trilobite Zone IV-VI) (Fig. 8) Three lithologic units can be distinguished: the comptexe schisteux in the eastern Anti-Atlas, the schistes de I'lssaf~ne and the calcaires scodac~s in the western Anti-Atlas. The facies of the reddish mudstones of the compiexe schisteux

extends over several hundred kilometers

without a major change. The sediments were deposited in a flat lagoon or flute plain under aride climate. The unit complexe schisteux might have been deposited diachronously. The transition into lithology of the schistes de I'lssaf~ne is located east of Tata. Green mudstones, crystal tufts and intercalated carbonate horizons are predominant west of this boundary. Trilobites indicate an open marine environment for the schistes de I'lssaf~ne. The distinct increase in pyroclastics point to an intensified volcanic activity in the western Anti-Atlas. The carbonates of the calcaires scoriac~s were deposited on top of a wide submarine ridge in the westernmost part of the Anti-Atlas. This ridge probably separated the depostitional area of the schistes de I'lssaf~ne from the open ocean (Japetus ocean?).

4.2. Asririen (Trilobite Zone VII) (Fig. 9) The pattern of sedimentation is completely different in the Asririen. Whereas mudstones prevailed in the S6rie de I'lssat~ne, the Asririen is characterized by fine-grained detrital sediments which were distributed by several river systems into the Anti-Atlas basin. Two river systems can be traced in the eastern Anti-Atlas. The first exhibits a nearly E-W directed flow regime and the second a dominance of SE-NW sediment transport. Both river systems of the gr~s rouges represent the type of 'braided river' with slow water flow. The fluviatile facies of the gr~s rouges gradually passes into the marine sandstones of the ~tage d'Asrir in the central Anti-Atlas. Sediments of a tidal-dominated delta are found in the transitional area. During the same period, the Delta of Asrir developed in the southwestern Anti-Atlas prograding towards the marine basin in the north. The conglomerates of Aguerd represent the littoral to tluviatile topset bed of the Delta of Asrir. They are exposed throughout a width of appr. 300 km at the southern margin of the Anti-Atlas. The area of Issaf~ne is characterized by local erosion and redepostition of the sediments. The mudstones of the ~tage d'Asrir were deposited outside of the influx of sand. The carbonates of the S~rie d'Jebel Tichinchine were deposited in the former sedimentation area of the calaires scoriac~s. The upper Asririen exhibits an intensified volcanic activiy leading to an increased deposition of volcanoclastics.

119

Fig. 12. Model of the upper Lower Cambrian deposition area in the Anti-Atlas.

4.3. Transition Asririen - Ourmastien (Trilobite Zone VII - VIII) (Fig. 10) A new transgressive cycle of sedimentation started with the deposition of the gr~s d'Ourmast. The source area of these series was located in the north, in the region of the present-day High Atlas. The gr#s d'Ourmast wedge out towards the W. They are replaced by only few meters of mudstones and siltstones. The volcanic activity increased considerably in the western Anti-Atlas. The up to 100 m thick S6rie volcanod6tritique was formed by eruptions with submarine currents of pyroclastics and a considerable amount of ash-fall. The Delta of Asdr reached its maximum extent at the Asririen - Ourmastien boundary.

4.4. Ourmastien (Trilobite Zone Viii) (Fig. 11) Coquina horizons intercalated within the increasingly more fine-grained sediments of the gr~s d'Ourmast indicate the transgression of the sea which covered nearly the entire Anti-Atlas - with the exception of some areas in the SE and E. The prograding of the Delta of Asrir terminated. The volcanic activity decreased in the northwestern AntiAtlas. The schistes & Paradoxides are indicative of the Middle Cambrian transgression which considerably exceeded the former area of sedimentation in the Anti-Atlas.

120

•

~

•

~,i~!~~~ii!i~i~

........ ~,~' ~!

~ii~ ~

~, ~ 4 : , ~

~, i.,~

~i~i~~.~!,~ ~'! '~'ii~

i!I

~ ~:~

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~i~i

~

,',~

121

Plate 1: Uthology of the 'gr~s terminaux' (thin sections of Figs. 1 - 3, and 5 with crossed nicols;Figs. 1,4 - 6: scale = 1 mm, Figs. 2, 3: scale = 0.1 mm) Fig. 1.

Pedogenic broken quartz within a carbonate matrix. S(~rieof Akerouz (sample AK 11).

Fig. 2. Gres rouges. Detrital quartz cemented by secondary quartz overgrowth (sample AM 32). Fig. 3. Grl~,srouges. Detrital feldspar with secondary feldspar overgrowth (sample AZ). Fig. 4. Schistes de I'lssaf~ne. Sandy biosparite with echinoderms and trilobites (sample A 11). Fig.&

Congbmerate d'Aguerd. Component of partly recrystallized metaquartzite (sample T 16).

Fig. 6. S~rie volcano-d~tritique. Lithic arenite composed of reworked volcanic rock fragments (sample AL 18).

Acknowledgment The authors are indebted to the authorities of Morocco - particularly to Dr. Bensaid and M. Dahmani of the Minist~re de I'Energie et des Mines - for the permission to work in the Anti-Atlas and for support during fieldwork. We,gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (Bu 312-10/1-2). Furthermore, we wish to thank H. Bogle for the translation and proof reading.

References BOUDDA, A., CHOUBERT, G. & FAURE-MURET, A. (1974): Note au sujet des 'gres terminaux' du Cambrien inf~rieur de rAnti-Atlas et de la transgression du Cambrien moyen. - C. R. somm. Soc. g~ol. France, 5, 138140; Paris CHOUBERT, G. (1952): Histoire g~ologique du domaine de rAnti-Atlas. - Not. M6m. Serv. g~ol. Maroc, 1 0 0 , 7 7 194; Rabat (1963): Histoire geologique du Precambrien de I'Anti-Atlas. - Not. M~m. Sew. g6ol. Maroc, 162,352 pp.; Rabat CHOUBERT,

G. &

FAURE-MURET, A. (1956): L'activit6 volcanique de la fin du G(~orgien dans I'Anti-Atlas et le Haut

Atlas occidentaux. - C. R. Acad. Sci. Pads, 242, 2735-2738; Paris COLEMAN, J. M. (1969): Brahmaputra river, channel processes and sedimentation. - Sed. Geol., 3, 129-239; Amsterdam GEYER, G. (1984): Biostratigraphische und systematisch- pal~ontologische Untersuchungen im Grenzbereich Unter-/Mittelkambrium von Marokko. - Diss. Univ. W0rzburg, 229 pp.; W0rzburg HUPE, P. (1952): Contribution & I'~tage du Cambrien inf(~rieur et du Pr6carnbrien Ill de I'Anti-Atlas marocain. - Not. M~m. Sew. g{}ol Maroc, 103, 402 pp.; Rabat MORGAN, J. P. (1970): Depositional processes and products in the deltaic environment. - In: MORGAN, J. P. (ed): Deltaic sedimentation. - SEPM Spec. Paper, 15, 31-47; Tulsa NELTNER, L. (1938): Etudes g~ologiques dans le sud Marocain. - Not. M6m. Serv. g0ol. Maroc, 42, 298 pp.; Rabat PASSEGA, R. (1964): Grain size representation by CM-pattern as a geological tool. - J. Sed. Petrol., 34, 830-847; Menasha PETTIJOHN, F. J., POTTER, P. E. & SIEVER, R. (1973): Sand and Sandstones. - 618 pp.; Heidelberg (Springer) REINECK, H. E. (1972): Tidal flats. - SEPM Spec. Paper, 17,146-159; Tulsa --

(1985) Aktuogeologie klastischerSedimente. - 348 pp.; Frankfurt (Kramer)

REINECK, H. E. & SINGH, I. B. (1980): Depositional sedimentary environments. - 451 pp.; Heidelberg (Springer)

DIAGENESIS AND VERY LOW-GRADE METAMORPHISM OF THE LOWER CAMBRIAN ROCKS IN THE ANTI-ATLAS (MOROCCO)

Werner Buggisch Institut f0r Geologie, Universit&t Erlangen-N0rnberg, SchloBgarten 5, D-8520 Erlangen

Abstract The diagenesis and the very low-grade metamorphism of the 'gr~s terminaux' in the Anti-Atlas were studied on the basis of the 'crystallinity' of illite and the deformation of quartz. The illite crystallinity improves from E to W. The quartz cements of the quartzitic sandstones are not deformed in the eastern Anti-Atlas. Undulation of quartz cement is observed in the central Anti-Atlas. The deformation increases towards the WSW where subgrains of quartz are found in quartz veins.

Introduction The diagenesis and the very low-grade metamorphism (WINKLER 1967) were studied in the strata of the AntiAtlas on the basis of clay minerals, 'illite crystallinity', authigenic minerals and the grade of quartz deformation. The stratigraphy, paleogeography and facies of the Lower Cambrian strata are discussed by BUGGISCH & FLOGEL and BUGGISCH & SIEGERT (this volume). The illite crystallinity was measured in one section containing strata from the 'S@de des calcaires inf@rieurs' to the 'gr@s terminaux' of the central Anti-Atlas, in order to analyse the influence of the sedimentary environment. The quartz deformation and the illite crystallinity were studied in the gr~s terminaux between Erfoud in the E and Agadir and Tiglit in the W in order to establish the regional pattern of the diagenesis and very low-grade metamorphism.

Phyllosilicates The clay mineral association of the eastern Anti-Atlas is dominated by illite and mixed layer minerals. Furthermore montmorillonite and kaolinite were detected in the X-ray diffraction records. The stability of kaolinite and its replacement by illite and chlorite depends considerably on the K+/H + and (Mg++ + Fe++)/H+ concentration ratios (KISCH 1983). Therefore, the disappearance of kaolinite is documented in many deep wells at depths between 2000 m (90o C) and more than 5100 m (180o C), but not at as great a depth as the appearance of anchizonal illite "crystallinities" (references summarized in KISCH 1983). An overburden pressure has already resulted from more than 3000 m thick Middle Cambrian and Ordovician sediment in the region south of Ouarzazate. Therefore, the new formation of kaolinite during weathering (Cretaceous or later) must be taken into consideration as well. Chlorite is the main phyllosilicate in the upper Early Cambrian sediments of the western Anti-Atlas. The chlorites which were frequently altered to oxychlorites replaced detrital micas as well as fossil shells. The replacement of mica by chlorite is known from the diagenesis and the 'early metagenesis' (KARPOVA 1969 after KISCH 1983: Fig. 5-9). The matrix of the clastic sediments in the western Anti-Atlas is composed of authigenic chlorite and illite. Intergrowth of quartz, chlorite and illite/sericite forming 'beards' can be obsewed in thin sections of rock samples from the southwesternmost Anti-Atlas (PI. 1, Fig. 6, 7).

124

UNI

T S

<2p thick

<2~ t hin Z deviotion

F• 2 e]

meon

GRES TERMINAUX

('4

vv

"I

CALCAIRES

°1

I

SUPERIEURS

E :L c~

I

c

~-~E

LIE DE

cg v

:m

:r :L :1

VIN

CALCA[RES

I NFERIEURS ,

3

2

i

3

4

Fig. 1. 'lllite crystallinity' in the Lower Cambrian rocks of the section Ait Mrabt (AM of Fig. 1 in BUGGISCH & SIEGERT, this volume) in the central Anti-Atlas with alternating marine carbonates and mudstones and fluviatile sandstones.

Formation of authigenic minerals The overburden was sufficient to mobilize quartz by pressure solution. In this way, enough SiC 2 was available for cementation. Authigenic overgrowth of feldspar (plagioclase and orthoclase) is also common in the central and eastern Anti-Atlas. GASSNER (1986) described the alteration of olivine to chrysotile along fissures in volcanic rocks which are intercalated in the 'S~rie des calcaires inf~rieurs' west of the 'boutonniere of Bou Azzer-EI Graara'. Chrysotile was formed due to hydrothermal solutions which were probably autometamcrphically produced during the cooling of the lava. Chrysofile becomes instable in the greenschist facies. Therefore, later heating never exceeded the temperatures of the very low-grade metamorphism in this area. 'lllite crystallinity' The sharpness of the illite (001) diffraction peak at 10 A becomes more pronounced with an increasing degree of burial diagenesis. Therefore, the sharpness of this peak was used as a parameter of the incipient metamorphism by WEAVER (1961). Later, the width at half peak was used as a scale of the 'illite crystallinity' (KUBLER 1967, WEBER 1972). In this paper, the delta 2 theta value of the width at half diffraction peak of the 10 .~ peak is measured (CuKO~).

125

Marrokech •

~

G ~

~

Agadir~AAz~

~. ix

~

A

-

•

AmTL

/

/

G a

~/

~.

~-~ /--~-

/

~

'/~(~

.....

[] ---(~--13 Q

~

A AA A ~~

__ ~weQR orno undu[ationofquartz

O

Marrakech •

z~ Erfoud

A~h Z~

~I~O~Y ~'O~Y

/

S /

Ouarzazate A A

~ ~

--

ix ~

"~

ix

flattened ooids

L ~

"~

Ouo.rzozate

ix ~

s

/

[~ Erf °eu

[]

.O

100 Km

ILLITE CRYSTALLINITY;

[]

A 2e

<20 /

x.v

A

onty few data

2O

Fig. 2. 'lllite crystallinity' of the fraction < 2 p.m and quartz deformation in the upper Early Cambrian 'gr~s terminaux' of the Anti-Atlas. IlUtevalues measured as delta 2 theta of the (001 ) peak.

The value of 'illite crystallinity' depends considerably on the experimental methods, e.g. the preparation of samples and X-ray diffraction conditions (KISCH 1980, BUGGISCH 1986).The fractions < 2 p.m and << 2 #m were pipetted off from 0.01 n ammonia solutions after suitable settling periods, and sedimented on glass slides. Preparations of varied thickness (TEICHMOLLER et al. 1979) were used because of the dependence of the peak width on the thickness of the preparations. The samples were measured with a Philips X-ray diffractometer (generator PW 1729: fine focus tube 20 KV, 30 mA; diffractometer PW 1840 with automatically variable divergenceslit: speed 0.005/2 theta/min, chart 50 mm/2 theta, range lx103 - 5xl 03, TC 5 sec, slit 0.2 mm). The measured peak widths are narrower than those of KUBLER (1967), WEBER (1972) or even KISCH (1980) depending on the thickness of the preparations and diffractometer conditions. The peak width depends on the grain-size of the preparations as well. Broadening of the peaks is observed when very small grain sizes are used. Some detrital mica might still remain present in the fraction <2 p.m. The mode of preparation is always a compromise between different sources of errors. Therefore, the value of the 'illite crystallinity' is taken only as a relative parameter in this paper.

126

The 'illite crystallinity' is poor in the eastern Anti-Atlas and improves in the western Anti-Atlas (Fig. 2). The transition zone is relatively narrow. The best sharpness of the 10 ~, peaks is observed in the southwesternmost Anti-Atlas. The improvement of the illite crysfallinity coincides with the transition from fluviatile to marine facies of the sediments. The crystallinity was studied in a section with alternating continental and marine strata (Fig. 1) in order to examine whether the improvement of the illite crystallinity is dependant on the sedimentary environment or on the burial metamorphism. The illite (001) peak of thin < 2#m preparations is a little bit broader in the fluviatile strata of the studied section compared with the marine sediments. The change in the sharpness of the illite peak from E to W is much more significant than the dependency on the environment. Therefore, the improved 'illite crystallinity' is due to a higher overburden or to an increased stress in the western Anti-Atlas.

Deformation and subgrain formation of quartz The behavior of quartz depends on stress and temperature (VOLL 1960, 1969; MAJORIBANKS 1976; BELL & ETHERIDGE 1976 and ETHERIDGE & WILKIE 1981). The quartz grains of the gr#s terminaux are affected only by a weak pressure solution in the eastern Anti-Atlas. Only weak or no undulation of volcanic quartz grains or of quartz cement is observed in thin sections from this area. Undulation of the quartz grains and of the authigenic overgrowth of quartz is common west of the line TazenakhtFoum Zguid (Fig. 2). This boundary coincides more or less with the area in which the 'illite crystallinity' distinctly improves. Increasing stress is documented by the deformation lamellae of quartz and by the loss of quartz cement due to pressure solution. These phenomena are restricted to competent layers. The strain is buffered by the matrix within the incompetent rocks. Boundary migration and formation of quartz subgrains are restricted to quartz veins within the gr@s terminaux of the southwestern Anti-Atlas. Recrystallization of quartz is not observed in the studied thin-sections of the gr~s terminaux. Schistosity A distinct schistosity is not developed in the Cambrian rocks of the Anti-Atlas. Flattening of oolites is observed in the section Bou Izakarn (BI) in the southwestern Anti-Atlas (PI. 1, Fig. 8). 'Beards' of illite and chlorite and new growth of chlorite and illite in the matrix with distinct orientation are also restricted to the southwestern Anti-Atlas (PI. 1, Fig. 6, 7).

Plate 1 (Fig. 1 - 7 w i t h crossed nicols) Fig. 1. Undeformed quartz grains with authigenic overgrowth (sample AM, scale = 0.5 mm) Fig. 2. Weak undulation of quartz in detritic grains and authigenic overgrowth (sample AM, scale = 0.1 ram) Fig. 3. Undulation and deformation lamellae in detritic grains and cements (sample K, scale = 0.5 mm) Fig. 4. Strong pressure solution affecting detritic quartz grains (sample TT, scale = 0.5 mm) Fig. 5. Formation of subgrains and incipient boundary migration in quartz veins (sample AS, scale = 0.1 ram) Fig. 6. 'Beards' of chlorite, illite and quartz (sample A, scale = 0.5 mm) Fig. 7. 'Beards' of chlorite, illite and quartz ( sample A, scale = 0.1 mm) Fig. 8. Flattened oolites (sample BI, scale = 0.5 mm)

127

128

Conclusion The eastern Anti-Atlas is characterized by broad illite diffraction peaks and by weak deformation of quartz. The Lower Cambrian sediments still exhibit the diagenesis stage. West of Tazenakht-Foum Zguid, the 'illite crystallinity' improves and the quartz grains and the authigenic quartz overgrowth reveal undularory extinction. The quartz deformation and the 'illite crystallinity' increases towards the southwestern Anti-Atlas. The boundary of diagenesis and very low-grade metamorphism coincides with the mobility of the socle. The eastern Anti-Atlas is characterized by low rates of subsidence and by the dominance of continental-fluviatile sedimentation during the Lower Cambrian. High rates of subsidence and marine sediments prevailed in the more mobile western Anti-Atlas. While the Paleozoic sediments are only weakly deformed in the eastern Anti-Atlas, a foldbelt was formed during the Hercynian Orogeny in the (south)western Anti-Atlas.

Acknowledgement The author is indebted to Dr. W. v. Gosen (Erlangen) for many discussions of the results.

References BELL, T.H. & ETHERIDGE, M.A. {1976): The deformation and recrystallization of quartz in a mylonite zone, central Australia. - Tectonophysics, 32: 235-267; Amsterdam BUGGISCH, W. (1986): Diagenese und Anchimetamorphose aufgrund von Conodontenfarbe (CAI) und "lllitKristallinit~t" (IC). Methodische Untersuchungen und Daten zum Oberdevon und Unterkarbon der Dillmulde (Rheinisches Schiefergebirge).- Geol. Jb. Hessen, 114:181-200; Wiesbaden ETHERIDGE, M.A. & WlLKIE, J.C. (1981): An assessment of dynamically recrystaliized grainsize as paleopiezometer in quartz-bearing mylonite zones. - Tectonophysics, 78: 475-508; Amsterdam GASSNER, W. (1986): Das "lnfra"- und Unterkambrium am SfJdostrand des Pr~_kambriumaufbruchs yon Bou Azzer/ El Graara (zentraler Antiatlas, Marokko). - Unpubi. Dipl. Arb., 92 pp, Erlangen KISCH, H.J. (1980): Incipient metamorphism of Cambro-Silurian clastic rocks from the Jamtland Supergroup, Central Scandinavian Caledonids, Western Sweden: illite crystallinity and 'vitrinite' reflectance. - J. geol. Soc. London, 137:271-288; London -(1983): Mineralogy and petrology of burial diagenesis (burial metamorphism) and incipient metamorphism in clastic rocks.- In: LARSEN, G. & CHILINGAR, G.V. (eds): Diagenesis in sediments and sedimentary rocks, 2: Development in Sedimentology, 25B: 289-493; Amsterdam (Elsevier) KUBLER, B. (1967): La cdstallinit~ de I'illite et des zones tout & fait supedeurs du m(~tamorphism. - In: Colloque Etage Tectoniques. A la Baconni6re: 105-122; Neuch&tel MAJORIBANKS, R.W. (1976): The relation between microfabric and strain in a progressively deformed quartzite sequence from central Australia. -Tectonophysics, 32: 269-293; Amsterdam TEICHMOLLER, M., TEICHMOLLER, R. & WEBER, K. (1979): Inkohlung und lllit-Kdstallinit~t. Vergleichende Untersuchungen im Mesozoikum und Pal~lozoikum von Westfalen. - Fortschr. Geol. Rheinl. Westf., 27: 201276; Krefeld VOLL, G. (1960): New works on petrofabrics. - Liverpool and Manchester Geol. J., 2:503-567 -(1969): Klastische Mineralien aus den Sedimentserien der Schottischen Highlands und ihr Schicksal bei aufsteigender Regional- und Kontaktmetamorphose.- Habil. Schr., 206 pp, Berlin WEAVER, C.E. (1961): Clay minerals of the Ouchita structural belt and adjacent foreland. - Univ. Texas Publ. 6120: 147-160 WEBER, K (1972): Kdstallinit&t des Illits in Tonschiefern und andere Kriterien schwacher Metamorphose im n(Srdlichen Rheinischen Schiefergebirge. - Jb. Geol. Pal~ont. Abh., 141: 333-363; Stuttgart WlNKLER, H.G.F. (1967): Die Genese der metamorphen Gesteine.- 2nd ed., 237 pp; Berlin (Springer) -

High and Middle Atlas

DEFORMATION OF IGNEOUS DIKES IN AND AROUND THE TICHKA GRANITE, HIGH ATLAS - MOROCCO J e a n - P a u l Schaer Institut de G@ologie, U n i v e r s i t 6 de N e u c h A t e l ii, rue E . - A r g a n d , C H - 2 0 0 7 N e u c h A t e l

In Morocco, the

north

of

the

non-metamorphic

often with

pierced well

western ceived which

is

metamorphic

small

(10-15

km

Atlas

great due

metamorphic south

deal

to

as in the rest of N o r t h Africa,

weakly

developed

High a

by

Anti-Atlas,

or

of

the

diameter)

aureoles.

of M a r r a k e c h attention

complexity

q u a l i t y of the o u t c r o p s

in

Paleozoic

The

granitic Tichka

is one

from

formations

of

of the i n t r u s i o n

and

and

Fig. 1. granite

The to

Generalized

formations be

weakly and with

with

metamorphosed

considerable few

lavas)

in other nal

of

deformed

thickness

of

especially

facies

variations.

Cambrian,

the

locally

a

the

in an arid climate.

i~ ."i)

Marrakech •

i{

~

. : ;y

~

as

there

We

to

Morocco

igneous of

are

often m.

it

with

I

are

Tichka

considered

thickness

As

is

1 km,

In h a r m o n y

whereas with

lapilli the

case

the origi-

lithological

of i m p o r t a n t of

These

limestones

(tuffs,

different

indications

exceeding

I

the

schists,

material

the

/ .

Infra-Cambrian.

composition.

of

total

thick.

.

~

is not easy to deduce

or

many a

\

complex

some

graywackes,

dacitic

pile

propose

hundred

SW

volcano-detrital

total

volcanics few

Tichka

sequences,

the

/

of

contain

of

\

:S

.

map

the

andesitic

rock

.

possibility

rocks

volumes

groups,

are

geological

surrounding

Cambrian

.

it has re-

petrologists,

L< - ' .

in the

and to the e x c e l l e n t

d i s s e c t e d by deep v a l l e y s

Cenozoic I~'~:i~ mesozoic ~J Paleozoicand older L~l wifh granite

intrusions,

granite

them

geologists

are

3-5

km

lateral for

the

the

limestone

other

Hercynian

132

granites

in M o r o c c o

boniferous give

age.

305

and

remarkable rites, are

often sion

sharp there

its

but

On

the

they

range

whole,

trusion

(Lagarde

well

marked

they

can

are

from

and

also

the

be

Roddaz

visible

at

planar

structure

domains,

whereas

in m a n y

late

The

their

elliptical

enclosing

structure

rocks

and

however,

the

vertical;

in the N.E.

thick

extend

meters be

of and

regional

(1983)

schist

proposed from the

and

(1976)

differentiation analysis.

has

close and

with

f r o m the

evidence

can

Scott

not

form

an of

interthe

structures

more

or

inare

1981);

less

in

lacking

pro-

dioritic

or

so

small

weakly

in

the

adjacent granite

all

received

a

totally

Walker

(1975)

be

ruled

out

on

and

Vogel

(1980)

and

have

the have

schists.

some

that of

The from

biotite) stauroand Roddy the

intru-

transcurrent

magmatic

rocks

of

the

satisfactory

solution.

revealed

contempo-

basic shown

basis

may

segre-

contact,

Lagarde

domain

the

margin

sillimanite,

patterns

of

generally

migmatitic

the

a

the

detail,

or two k i l o -

scale, the

and garnet.

acidic

and

actinolite,

in

with In

up to I00 m e t e r s

for one

with

strain

(1980)

scale.

sharp

andalousite,

of

concordant

rocks

origin

of

recogni-

a kilometer.

part

apophyses

epidote,

Vogel

than

general

towards

study

association and

a

pyroxene

and

a

and)

a

emplacement

the

Vogel

Scott

On

amphiboles,

diapiric

moment

intrusion field

raneity

RE

with

on

material grade

assimilation still

expressed

are

transitional

(chlorite,

or to m a r b l e s

they

country

1955).

metamorphic

as

cross-cutting,

the

intru-

is u n c e r t a i n .

sheet-like

sometimes

greenschist

For

into

the

are

inclusions,

(G. and H. T e r m i e r

well

is

gabbros

rocks

Throughout

concentric

(parallel

often

Ziegler

have

Tichka Using

is

occasional

is

results

shear.

al.

in

garnet

sion

also

quartzofeldspathic

coarser

lite,

and and

granites

structure

horizontally

(Bizard

gations

to

contact

irregular

country

is

their

igneous

northern

method

granodio-

whereas

undergone

scale

often

identification

diorites,

generally

on m a p s

outcrop

vertical

that

intrusion

inclusions

the

These

and

nounced

developed

in

1983).

the

Rb/Sr

to m o r e

metasedimentary

Car-

The

have are

of

the

of m e t a s e d i m e n t a r y

centimeters

photographs

be

by

different

they

centered

to

1972)

proportion,

lithologies

several and

al.

transitional. slabs

south;

the

pattern

on a e r i a l

between

or

the

considered

heterogeneity; in

also

but

et

equal

lenses

in

magmatic

elliptical

about

is

determinations

(Termier

Contacts

numerous

locked

age

m.a.

are

degree,

They

intrusion

petrographical

abundant

a varying

zable.

i0

rare.

are

especially to

~

granites

extremely

Tichka

Radiometric

318

for

and

the

of

suggested

the

magmas. that major that

an

Vogel

et

origin

by

element the

and

granite

133

may

represent

gabbro

The

A

marked

feature

dikes

The

Paleozoic

from

the

diorite

a mixture

of

low

SiO 2 g r a n i t e

of

and

outcrop

km

or

has

a

have

no

a N/S

may

Atlas,

which

to

the

and

acid. some

its

envelope

been

made

second

but

with

group

particular

Most

it

at

dikes

where

is

the

that

general

from

angles,

relationship

wihh

the

of

the

granite,

the

a

few

meters

km

and

of v a r y -

(with o l i -

vertical

(the

in the v i c i n i t y numerous,

of

great

their

only

structure

they

orienta-

majority

(folds,

whereas the

be

are m o r e

structure

70

dikes

basic

to

analysis

direction

about

however,

they

systematic

right

for

appear

in this);

seems

the

n'Fiss

is cut b y n u m e r o u s

part

No

in an W . - E .

compositions

especially

horizontal.

parallel

with

to

extends

river

direction,

have

granite,

often

less

intrusion

relationship,

Tichka

with

High

pyroxenes)

also

tion

Tichka

d'Argana

in

pattern

the

are

the

couloir 30

structural

vine

the dikes.

in the e n v e l o p e

the

about

ing

of

of n u m e r o u s

The

of

melts,

origin.

dikes

presence

by

crustal

of m a n t l e

are

more

schistosity)

a

small

of

number

the

enclosing

dikes

is v a r i a -

rocks.

In

the

ble,

vicinity

in

general

ficient

data

sequence

tural med

development

(many Fig.

do

deformed, see

dikes sion

not

maintain

folded

that

they

intersect, deformation,

most It

that

dikes is

not

and

dikes 2). their

even

across the

the

moment

the

andesitic

occurred folds,

marbles,

Many

the

character. it is s t i l l

very

post-magmatic

folds

(Fig.

presents severe

2)

related

struc-

and defor-

of

the

They

are

possible

two

sets

of

post-intru-

2) N e a r

mineralogical

modifications

When

evidence (Fig.

obser-

granite,

post-deformation but

time

the m a i n

schistosity near

a

Nevertheless,

ones.

after

insuf-

of

into boudins,

youngest

separate

For

establishment

any confidence.

across

the

older

occasionnally

to

cut

cut In

the

with

intrusion

stretched

cut

present easy

always

dike

of the

decimeters.

prevent

emplacement

apparently

demonstrate

conglomerates

dikes

to

dikes

or

intersections

of a t h r u s t f u l

the b a s i c vations

on

thickness

the

granite

re-equilibrations. to

normal

post-

134

B A

C~

D

Fig. 2 A. Basic dike cutting folded m e t a s e d i m e n t s near the margin of the Tichka granite. Assif n'Ait Tament, N of Tisgane B. Two generations of folded basic dikes (b i n t e r s e c t i n g c) in a large calcareous xenolith with streched p y r o x e n i c horizon (a), near the southern contact of the Tichka granite N of Kma, Ida Oukays. C. In the Tichka granite, an old dike (probably g e n e r a t i o n H) is cut by a granitic vein (a) which has c a r r i e d small fragments of the dike. Amendrech. D. Dike of the oldest g e n e r a t i o n (H), with non planar geometry without c h i l l e d margins cutting granodiorite. Asif n'Fiss, SE of Azmou n'Guenfis. In each drawing the scale is given by the ham.

magmatic

and h y d r o t h e r m a l

activities

mal

metamorphism.

In

tes

partly

expense

at

monstrably the or

to

main

intrusion.

in the

schists

post-magmatic in

the

due

harmony

dikes w i t h i n ed rocks provoke

some

thermal

and

it

the

that were

and

seems to

intensity

the granite

from those

however,

of magmatic

Elsewhere,

paragenesis

with

cases,

dikes

logical contact

relate

of

deformation. it is clear

some m i n e r a l o g i c a l

deformed,

at t e m p e r a t u r e s

and structural

dikes

is

dewith

in the marbles metamorphic,

which

In c o n c l u s i o n that

ther-

of bioti-

associated

their

metamorphism

envelope,

already

in the

activity

are deformed to

with

development

amphibole

metasomatic the

associated

wide

they have

develops for

high enough

transformation.

the

intrudto

135

The dikes

in the granite

In the

Tichka

sition,

from

granite granitic

rally vertical, tic

the

classes,

on

structure.

the

of

generation) coloured

generation).

tendency

to

of

dioritic

pyroxene firmed.

These

present,

and

was

linear,

not

shows

from

that

to

classes is

are

but

all

not

were

while

the main

from

mush

to

a

magmas ce

of

amoebic

rocks cut

crystal

of different

(Vogel by

1976).

against

tic

which

closing nough

granite.

rigidity

material, dike

granite

the

was

of

but

the

still

granite,

from

of

veins

granitic and

is

con-

types

acid

to

Field

are basic

evidence

of

plutonic

body was

evolving

or

presence

(see

vertical

of

occuren-

granodioritic

dikes

(H)

also

is often Vogel

dike with

and

sharp

to be cut by a late graniof

the

intrusion, contained cut

but

work.

olivine

stages

granitic

fragments dike

of

by the common

set

observed

away

time

fracture

intruded

and

granitic

field

simultaneous

older

finely

andesites

late

a microdioritic,

carried

At to

and

dikes

(H

light-

grey,

with

to granitic The

esta-

then by darker

rocks

recurrences.

rock.

the

generation

intermediate

the

a few

classification

evolution

in

and

to

the

dike the

nearby dike

into

the

granite

and

en-

has

enough removed

e-

fluid some

chips.

In the logic rocks. sharp

which

case,

the has

the

The

allow

during

rocks

is i l l u s t r a t e d

1976).

pegmatitic

In one

contacts vein

solid

lithology

light

this

vitric

dioritic

between

Walker

aplitic,

Walker

a

main

dominantly

suspected

several

composition

contacts and

confirm

emplaced

crystallization

by

homogeneous;

included

dikes

dikes

three

oldest of

non-porphyritic

that

The

into

i0 meters

microcrystalline

and

probable

The

group

are geneand diori-

latter.

(K generation),

not

basic,

porphyritic

than

followed

sections

the

grouped

intersections

porphyries

and transition

They

granitic

relationships,

variable

rocks,

Thin

with be

intrusion.

a

and in compo-

doleritic.

more

many

of

evolution

it

may

from

with

of

composition,

through

parallel

varies

including

differences

to

cross-cutting

sequence

in structure

inclusions,

terms,

micro-crystalline

(L

The

of

composed

dikes

andesitic

also

exposures

time

crystallised

reveal

are

thickness

is

vary

sedimentary

general

basis

Good

blishment

some

in

Their

decimeters.

ones

but

granite,

dikes

through

intersect

foliation,

in

numerous

granite

pluton

evolution

is

The

youngest

parallel

as

a whole

illustrated dike

contacts

the by

generation and

well

last

dike is

stage

of thermal

behaviour nearly

developed

in

the

and rheogranitoid

always

vertical

with

chilled

margins;

they

136

can or

easily in

be

swarms

they

trend

its

followed

of

envelope.

their the

8 to

E/W,

either

Within

are

towards non

the

planar

of

the

to

post-intrusion

granite,

Berger

to

(1972)

of

folding

believe

conditions

of

to

be

zation

history

dikes

granite.

in

in

material

the

are

not

after

material

(H)

structure due

Pitcher

and

wavy

their

as-

margins

emplacement; related of

to

the

body was

geometry

and

we the

dikes

not rigid crystalli-

in r h e o l o g y

and host

their

decreases

the

of

are

of

alignment

by

assign

contrast

it;

commonly

temperatures

their

low

to size

their

after

disposition,

mineral

The cooling

that

the

the i n t r u d i n g

do

where

similar.

believe to

the

and

marbles

internal

crystallization

a medium

related

the

granite

the

grain

described

We

dikes,

deformed

in

non-parallelism

granite

and

were

We

the

been

generation to

been

the

parallel

the

exhibit

single

regular

or

oldest

has

and

geometry

brittle.

between

not

that

Donegal

intrusion

and of intruded

The

not

fairly

subparallel

does

movements their

enough

perature

it

events

Tichka

or

that

but

a

irregular;

contacts.

of

situation

structure

or

geometry,

in the

early

granite

have

the

as

of the Tichka P l a t e a u

structure

they

to

display

straight

sometimes

In the region

granite,

dikes

the

either

slightly

pect

K

km

internal

opposition

The

presents

the

the

in

transecting

margins

several

i0 members.

cutting

intrusion, envelope.

for

and

tem-

rocks.

Conclusion

The

synplutonic

envelope late

can

kinematic

placed,

the

tallized; cut

by

dikes which

be

used and

the

acquired In the

last

respect

to

and

dike

more

stage

of

its

dike

emplacement,

with

intense

the

is

Paleozoic

takes

formations.

the

and

the These

a

form

brittle

the

time,

became

the

marbles, regime. zones

structures

for

its

were

em-

due

dikes

to

granite

more

planar. with

deformation. to

their

the

mar-

Late

de-

rheolo-

marbles

cutting not

are

the

contact.

Where

are

crys-

still mobile

internal

is c o n f i n e d

near

fault

dikes

and

With

without

thermal of

first

the pluton was

deformation

the

references

irregular

boudinage

within

such

and its s e d i m e n t a r y

had not totally

geometry

ascended

and

the

veins.

dike

ductile

folding

under

When

often

evolution,

concentrated

properties deformation

are

pluton

temporal

material

granitic

and

envelope,

bles,

formation

evolution.

contacts

rigidity

its

and

granodioritic

pegmatitic,

After

gical

cut the Tichka

geometric

thermal

granite

aplitic,

as

across easily

lack, the sepa-

137

rated

from

Triassic Range

those

age

during

Tichka

from

the

granite

deformation, No

or

related

regional

those

moved

dextral

1983)

of the

first

has

During

upwards ductile ENE

been

the

development

accompanying

Cenozoic.

while

Roddaz

to

as

its

a rigid

structures

normal

formation phase

piston formed

transcurrent

detected

the last

of

shear

of

of

granite

been

undertaken

the

with

little

within

its

after

the

of

Atlas

intrusion,

component

in the

faults

the

internal envelope.

(Lagarde

and

emplacement

dikes.

Acknowledgements

This

reasearch

support

of

Dr.

and M. Dahmani, I would version

like

to

could M.

not

Bensaid,

chef

have

director

du Service

thank

St Ayrton

of the manuscript.

of

the

de la Carte

without

Division

de

the

active

la G@ologie,

g@ologique.

who has greatly

improved

the english

138

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T.A., W I L L I A M S ; E.R., P R E S T O N , J.K. & W A L K E R , O r i g i n of the late P a l e o z o i c p l u t o n i c m a s s i f s - Bull. geol. Soc. A m e r i c a 87, 1 7 5 3 - 1 7 6 2 .

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B.M. (1976) in M o r o c c o .

: