Evolution of the conjugate East African - Madagascan margins and the western Somali Basin

Evolution of the conjugate East African - Madagascar!

margins and the western Somali Basin

Millard F. Coffin and

Philip D. Rabinowitz

226

G. S. A. ARCHIVES

Evolution of the conjugate East African-Madagascan margins

and the western Somali Basin

Millard F. Coffin* Lamont-Doherty Geological Observatory

of Columbia University

Palisades, New York 10964

Philip D. Rabinowitz Ocean Drilling Program

Texas A&M University

College Station, Texas 77843

SPECMJWLPAPEP 226

•Present address: Bureau of Mineral Resources, Geology and Geophysics,

G.P.O. Box 387, Canberra, ACT 2601, Australia.

© 1988 The Geological Society of America, Inc.

All rights reserved.

Copyright is not claimed on any material prepared

by government employees within the scope of their

employment.

All materials subject to this copyright and included

in this volume may be photocopied for the noncommercial

purpose of scientific or educational advancement.

Published by The Geological Society of America, Inc.

3300 Penrose Place, P.O. Box 9140, Boulder, Colorado 80301

GSA Books Science Editor Campbell Craddock

Printed in U.S.A.

Library of Congress Cataloging-in-Publication Data Coffin, Millard F., 1955-

Evolution of the conjugate East African-Madagascan margins and the

Western Somali Basin / Millard F. Coffin, Philip D. Rabinowitz.

p. cm. — (Special paper; 226)

Bibliography: p.

ISBN 0-8137-2226-8

1. Geology—Indian Coast (Africa) 2. Continental margins—Indian

Coast (Africa) 3. Submarine geology—Indian Ocean. I. Rabinowitz,

Philip D. II. Title. I I I . Series: Special papers (Geological

Society of America); 226.

QE326.C64 1988

556—del 9 88-24588

CIP

10 9 8 7 6 5 4 3 2

ii

Contents

Acknowledgments v

Abstract 1 Introduction 1 Stratigraphy and structure; Surface geology and borehole results 2

Pre-Jurassic 12

Diego Basin 12

Majunga Basin 12

Somali Coastal Basin 12

Lamu Embayment 12

Tanzanian coastal basins 12

Morondava Basin 15

Western Somali Basin 15

Summary 15

Lower Jurassic 15

Diego Basin 15

Majunga Basin 15


Lamu Embayment 18


Monrondava Basin 18


Summary 18

Middle Jurassic 18

Diego Basin 18

Majunga Basin 18


Lamu Embayment 20


Morondava Basin 20

iii

IV Contents Western Somali Basin 21

Summary 21

Upper Jurassic/Lower Cretaceous 21

Diego Basin 21

Majunga Basin 21


Lamu Embayment 21


Morondava Basin 23


Summary 24

Upper Cretaceous 24

Diego Basin 24

Majunga Basin 24


Lamu Embayment 24


Morondava Basin 26


Summary 26

Paleocene 26

Diego Basin 26

Majunga Basin 26


Lamu Embayment 26


Morondava Basin 28


Summary 28

Eocene 28

Diego Basin 28

Majunga Basin 28


Lamu Embayment 28


Morondava Basin 30


Summary 30

Oligocene 30

Diego Basin 30

Majunga Basin 30


Lamu Embayment 32


Morondava Basin 32


Summary 32

Miocene 32

Diego Basin 32

Majunga Basin 32

Contents v


Lamu Embayment 32


Morondava Basin 34


Summary 34

Pliocene 34

Diego Basin 34

Majunga Basin 34


Lamu Embayment 34


Morondava Basin 36


Summary 36

Quaternary 36

Diego Basin 36

Majunga Basin 36


Lamu Embayment 36


Morondava Basin 36


Summary 36

Stratigraphy and structure; Offshore acoustic stratigraphy studies 36

Correlation with DSDP results with multichannel seismic data 36

Margins bordering the Western Somali Basin 43

Acoustic stratigraphy 43

Depth to basement 43

Jurassic Sediment 44

Jurassic through mid-Cretaceous sediments 49

Mid-Cretaceous through upper Oligocene sediment 52

Upper Oligocene through Quaternary sediments 57

Total sediment thickness 64

Concluding Discussion 73 Conceptual and global implications 75

References 76

Acknowledgments

We thank the officers, crew, and scientists aboard the research vessel Vema for their

support and cooperation in gathering the multichannel seismic data employed in this study.

We are grateful to James Hays, the late Brian Baker, Campbell Craddock, James Cochran,

and Dennis Hayes for reviews. Joyce Alsop, Peter Buhl, and John Mutter were invaluable in

helping process those data, and Greg Mountain and Peter Naumoff played vital roles in

digitizing the seismic data. E. T. Bunce, L. Little, W. Okoth, and the late E.S.W. Simpson

graciously provided additional seismic data, and O. Fox (Esso), C. Gaynor (Mobil), B. Katz

(Texaco), F. Keith (Occidental), L. Luebke (Amoco), A Boxall (Marathon), and H. Wories

(Union) generously supplied industrial data. M.F.C. thanks G. Flores, K. Kelts, M. Esteban,

and the late P. Kent for fruitful discussions, and M. de Buyl (Western Geophysical) and

W.B.F. Ryan for arranging and providing funding for a productive seminar addressing East

African margin development. A. M. Alvarez, C. Brenner, the late M. Braun, M. Giarratano, J.

Kovacs, and M. A. Stage ably supplied technical services. This work was supported by

National Science Foundation Grant OCE-79-19389. Lamont-Doherty contribution #4266.

vii

Geological Society of America

Special Paper 226

1988

ABSTRACT The geologic evolution of the conjugate sedimentary basins and margins produced

during the early breakup of Gondwanaland by the relative motion between Madagascar and Africa is reconstructed utilizing interpretations drawn from outcrop, industrial onshore drilling, Deep Sea Drilling Project (DSDP) offshore drilling, Lamont-Doherty multichannel seismic (MCS) data, and single-channel seismic data. Herein we present (1) maps displaying lithological columnar sections for Karroo (Permo-Carboniferous through Early Jurassic) to Quaternary time slices, (2) depth-to-basement and sediment isopach maps, and (3) acoustic stratigraphy studies based on MCS data. Formation of the conjugate sedimentary basins began in Permo-Carboniferous time, and extension recurred intermittently over a 150-m.y. span until the initiation of sea-floor spreading between Madagascar and Africa in Middle Jurassic time. Occasional marine incursions and the resulting deposition of salt in isolated Tanzanian grabens, and in the conjugate Somali Coastal and Majunga basins, highlight the pre-breakup stratigraphy.

At the initiation of sea-floor spreading, facies changed throughout the basins from dominantly continental to overwhelmingly marine, and volcanic activity and faulting occurred. The mid-Cretaceous was marked by the beginning of vigorous abyssal circu-lation in the Western Somali Basin, and the Late Cretaceous was a time of widespread regional volcanism. During the Paleogene, rifting was renewed in the Tanzanian Coastal Basins, extending to the Davie Fracture Zone, and all of the basins record numerous hiatuses in the Paleocene and Oligocene sections. A vast sediment slide offshore Somalia and Kenya occurred in mid-Tertiary time, demonstrating that the formation of olisto-stromes characterized by significant internal deformation (including thrust faults) may occur in passive margin settings. An intense erosional event in the Western Somali Basin marked the end of Paleogene time. Frequent volcanism affected the Diego Basin throughout the Cenozoic Era and the Comoros Islands during Neogene and Quaternary time. Folding and faulting of onshore and offshore strata of the Tanzanian margin continued through Neogene and Quaternary time to the present. We observed a major network of late Cenozoic canyons and channels on both the East African and Mada-gascan margins and in the Western Somali Basin. Accumulations of sediment on the Madagascan and East African margins total 5+ and 8+ km, respectively, for Middle Jurassic to Holocene time.

INTRODUCTION The passive rifted and transform margins created by the

separation of Madagascar and Africa (Fig. 1) offer an opportu-

nity to investigate the geologic development and evolution of

conjugate margins. Until the recent identification of marine mag-

netic anomalies in the Western Somali Basin (Ségoufin and Pa-

triat, 1980; Parson and others, 1981; Rabinowitz and others,

1983), considerable controversy had arisen concerning the tec-

tonic relations of East Africa and Madagascar. The pioneering

geological research in this region of (primarily) Kent (1971,

1972, 1973Ab, 1974, 1977, 1982) and Besairie (1971, 1972),

among others, was largely completed prior to the general accep-

tance of the plate tectonic model by about 1970.

M a n y new data involv ing the stratigraphie and structural

development of the continental margins bordering the Western

Somali and Comoros Basins have become available in the past 15

years; the aim of this study is to synthesize these new data with

previous results and document the geologic evolution of the con-

jugate East African and Madagascan margins. To this end we

have assembled available stratigraphy and structure studies, drill-

ing results both onshore and offshore, and geophysical (seismic,

magnetic, gravity) data for the region. We first summarize the

drilling results and plot lithological columnar sections from avail-

able wells on geographic base maps for time intervals encompass-

ing the entire sedimentary history of the region (Permo-Carbon-

iferous through Quaternary). Then, for intervals following the

breakup of East and West Gondwanaland (presumably Early to

1

2 Coffin and Rabino witz Middle Jurassic), we report on the results of acoustic stratigraphy

studies by presenting depth-to-basement and sediment isopach

maps, and also some of the offshore seismic data from which they

were compiled. Through our analysis we hope to establish a

regional stratigraphic framework, document and highlight impor-

tant events in the region's geologic evolution, foster ideas for

more detailed studies, and draw conclusions applicable to passive

rifted and transform margins in general.

The area of study encompassing portions of the western

Indian Ocean and East Africa is displayed in Figure 2, a free-air

gravity map of the oceanic domain derived from SEASAT radar

altimetry data (Haxby and others, 1983). Magnetic anomalies of

the Mesozoic sequence identified in the Western Somali Basin

appear in Figure 3 (Rabinowitz and others, 1983); these anoma-

lies led to the tectonic scenario (Fig. 4) developed for the region

(Coffin and Rabinowitz, 1987). Sea-floor spreading evidently

commenced between Madagascar and Africa sometime during

the time of the Jurassic Magnetic Quiet Zone; if we assume a

constant spreading rate for the entire phase of opening, then drift

was initiated at -165 Ma. Relative motion between Madagascar

and Africa ceased by the time of anomaly M9 (Rabinowitz and

others, 1983), or -130 Ma (Kent and Gradstein, 1985). Two

types of conjugate passive margins, evolved from sedimentary

basins, thus exist on Madagascar and East Africa as manifesta-

tions of this relative motion.

Southeastern Somalia (Somali Coastal Basin) and northern

Madagascar (Majunga and Diego Basins) are conjugate rifted

margins, and eastern Tanzania (Tanzanian coastal basins) and

western Madagascar (Morondava Basin) are conjugate sheared

margins (Fig. 1). Note that the Madagascan basins are named

after major cities situated within those basins, as indicated in

Figure 1. The transition between the two types of margins occurs

in the Lamu Embayment in Kenya (considered an aulacogen by

Cannon and others, 1981, and Reeves and others, 1987), and at

the northwestern corner of Madagascar, the St. Andre arch (Fig.

1). Precambrian basement outcrops define the landward edge of

the sedimentary basins fairly well except in the case of the Lamu

Embayment (Fig. 1). Notable is the fact that each of the major

basins on both the African and Madagascan margins has been

subjected to significant tectonic deformation subsequent to the

cessation of relative motion between Madagascar and Africa.

Volcanism, minor faulting, and diapirism have affected Madagas-

car, whereas severe faulting and diapirism are documented on the

East African margin. The margins have not evolved passively

since drift ended, but have subsequently been tectonically

rejuvenated.

STRATIGRAPHY AND STRUCTURE; SURFACE GEOLOGY AND BOREHOLE RESULTS

A comparison of the stratigraphy and structure of the conju-

gate basins of Madagascar and East Africa leads to a better un-

derstanding of the development of the rift processes that

culminated in their separation, and of the postrift evolution of

conjugate rift and transform margins. In examining the onshore

geology of the region, we employed the following references,

listed in order of importance, for each country: Kenya: Walters

and Linton (1973), Cannon and others (1981); Madagascar: Be-

sairie (1971), Boast and Nairn (1982), Radelli (1975); Somalia:

Barnes (1976), Beltrandi and Pyre (1973); Tanzania: Kent and

others (1971), Kent and Perry (1973), Kajato (1982). For sum-

maries of the regional geology, we utilized the data and interpre-

tations of Kamen-Kaye (1978, 1982, 1983), Kamen-Kaye and

Barnes (1978, 1979), Kent (1972, 1973a,b, 1974, 1977, 1982),

Blant (1973), Forster (1975), and Pallister (1971). Well informa-

tion was provided by McGrew (1983), Hartman (1987), Petracca

(1985), and by Petroconsultants L. Luebke of Amoco, O. Fox of

Esso, A. Boxall of Marathon, F. Keith of Occidental, and B. Katz

of Texaco. B. Okoth of Kenya and C. Gaynor of Mobil provided

multichannel seismic as well as borehole data.

Before we examine the geology of the basins, in which sedi-

ments prograde oceanward, a description of their setting is in

order. The Diego Basin is situated on the northern tip of Mada-

gascar, extending as far south as the Ampasindava Peninsula at

~13.5°S (Fig. 1), where it borders the Majunga Basin. To the

west, north, and east is the Indian Ocean, and to the southeast are

Precambrian basement rocks. Sedimentary rocks become

younger and thicken to the northwest away from basement out-

crop. The only deep borehole in the Diego Basin is Ambilobe 1

(Ml in Fig. 1). The Majunga Basin extends from the Ampasin-

dava Peninsula to the St. Andre arch (just to the east of a line

between wells M6 and M9, Fig. 1), a structural feature dating

from at least Early Jurassic time. Its shape is triangular, widening

from 50 km in the north to 200 km at Cap St. Andre. As in the

Diego Basin, the sedimentary strata dip seaward, and also thicken

and become younger away from basement outcrop marking the

southern and eastern boundary of the basin.

The Somali Coastal Basin lies to the southeast of the "Bur"

basement outcrop in Somalia (Fig. 1), away from which sedi-

ments young and thicken. Its southeastern boundary with the

Lamu Embayment is ill-defined, although a dramatic increase in

Cenozoic sediment thickness toward the south of Somalia near

the Oddo Alimo well (SI7 in Fig. 1) may be taken for the

transition. The Lamu Embayment proper extends from Oddo

Alimo south through Kenya to the Tanzanian border, and inland

from the Kenyan coast to Precambrian basement outcrop.

The Tanzanian coast basins are situated landward of the

Davie Fracture Zone (Figs. 1, 3, 5), bounded to the north by the

Lamu Embayment, to the south by the eastern basement promon-

tory of Mozambique, and to the west by the Precambrian base-

ment outcrop. The Morondava Basin extends for 1,000 km along

Figure 1. Geologic sketch map of the conjugate East African and

Madagascan continental margins. Borehole locations, volcanics, base-

ment, salt, and cities are indicated. Only boreholes penetrating deeper

than 1,000 m are included onshore; all offshore drill sites are included.

See Table 1 for borehole summary data. Scale, 1° of latitude =111 km.

East African—Madagascan margins and western Somali Basin

v" , ) Ml7

Moiondava ¿m »-M46 1 M̂45 I M21

.M22f •M23

' M26, .M28

M39-

\M43 J

Coffin and Rabinowitz TABLE 1. SELECTED EXPLORATORY WELLS*

Map Label Well Name Coordinates

Total Depth (m)

Oldest Strata Penetrated Year Operator

Deep Sea Dril l ing Project

DSDP234 same

DSDP235 same

DSDP 240 same

DSDP 241 same

DSDP 242 same

Kenya

K1

K2

K3

K4

K5

K6

K7

K8

K9

K10

K11

K12

K13

K14

K15

K16

An za 1

Bahati 1

Wal Mere

Garisa 1

Hargaso 1

Mararani

Walu 2

Dodori 1

Pate 1

Pandangua 1

Kipini 1

Maridadi B1

Ras Kaluj

Simba 1

Kofia 1

Kencan 1

Madagascar

M1 Ambilobe 1

M2 Mahajamba 1

4°28.96'N 51°13.48'E

3°14.06'N 52°41,64'E

3°29.28'N 50°03.42'E

2°22.24'S 44°40.77'E

15°50.65'S 41°49.23'E

0°55'10.0"N 39°41 '42.0"E

0°26'18.0"N 39°47'02.0"E

0°06'35.0"S 40°35'05.0"E

0°21'56.0"S 39°44'57.0"E

0°47'43.4"S 40°26'40.5:E

1 °37'30.0"S 41°14'15.0"E

1°38'02.0"S 40°15'11,0"E

1°48'54.0"S 41°11'04.0"E

2°04'00.0"S 41°05'00.0"E

2°06'45.0"S 40°30'30.0"E

2°29'23.5"S 40°35'51.3"E

2°53'08.8"S 40°24.07.9"E

3°49'00.0"S S g ^ ' O O ^ ' E

4°00'07.0"S 40°34'04.0"E

2°32'33"S 40°56'19"E

0°18'57.4"S 39°46'16.6"E

13°03'00.0"S 48°52'40.0"E

15°06'33.0"S 46°55'10.0"E

247

684

202

1,174

676

3,662

3,420

3,658

1,240

3,091

1,991

3,728

4,310

4,187

1,981

3,662

4,197

1,537

3,604

3,628

3,863

1,670

3,001

Oligocene 1972

Upper Cretaceous 1972

Upper Paleocene 1972

Upper Cretaceous 1972

Upper Eocene 1972

Cretaceous

Cretaceous

Lower Cretaceous

Middle Jurassic

Lower Cretaceous

Paleogene

Lower Cretaceous

Upper Cretaceous

Lower Eocene

Paleogene

Upper Cretaceous

Upper Oligocene

"Karroo" (see text)

Upper Cretaceous

Cretaceous

1976

1976

1967

1968

1975

1962

1963

1964

1971

1959

1971

1982

1963

1978

1985

1986

Triassic 1964

mid-Cretaceous 1971

DSDP

DSDP

DSDP

DSDP

DSDP

Chevron

Chevron

British Petroleum

British Petroleum

Texas Pacific Kenya

?

British Petroleum

British Petroleum

British Petroleum

?

British Petroleum

Citco

Mehta

Total

Union

PetroCanada

Société Petroles de Madagascar

Agip


TABLE 1. SELECTED EXPLORATORY WELLS* (continued)

5


Total Depth (m)


Madagascar (continued)

M3 Mariarano 1

M4

M5

M6

M7

M8

M9

M10

M11

M12

M13

M14

M15

M16

M17

M18

M19

M 20

M21

M22

M 23

M24

M25

M 26

Sofia 1

Tuilerie 1

C. St. Andre 1

C. St. Andre 2

Chesterfield 1

Bemolanga 1

Eloise 1

Belinta 1

Belinta 2

Belinta 3

Ankamotra 2

Maroaboaly 1

Vaucluse 1

Serinam 1

East Serinam 1

Eponge 1

West Kirindy 1

Ankasofotsy

Mandabe 1

Manja 1

Morombe 1

Andavoaka 1

Stkily 1

15°14'49.2"S 46°36'49.8"E

15°29'12.0"S 47°09'00.0"E

15°52'00.0"S 46°30'00.0"E

16°ir00.0"S 44°33'00.0"E

16°11'00.0"S 44°28'30.0"E

16°22'00.0"S 43°57'00.0"E

17°41'24.4"S 44°48'00.0"E

17°58'17.0HS 43°24'57.0"E

18°21'51.9"S 44°52'00.0"E

18°18'16.8"S 45°03'00.0"E

18°20'00.0"S 45°10'24.0"E

18°28.12.0"S 44°18'45.0"E

18°33'00.0"S 45°09'00.0"E

19°28'00.0"S 43°59'20.0"E

19°37'00.0"S 44°43'00.0"E

19°36'00.0"S 44°49'00.0"E

30°56'01.0"S 43°42'00.0"E

21°00"57.2"S 44°13"00.0"E

20°57'00.0"S 44°25'30.0"E

21°06'47.7"S 44°57'00.0"E

21°18'55.0"S 44°12'00.0"E

21°31'24.0"S 43°16'32.0"E

22°01'20.5"S 43°15'00.0"E

21°51'00.5"S 44°07'00.0"E

5,211 Jurassic 1971

3.026 Lower Jurassic 1972

2,659 Lower Jurassic 1965

1,668 Triassic 1960

2,153 Permian 1960

4.774 Triassic 1970


4,490 Middle Jurassic 1971

2,529 Permian 1959

2,513 Permian 1960

1,201 Upper Triassic 1960

3,506 Triassic 1971

1.986 Upper Triassic 1960

4.027 Lower Jurassic 1971


3,048 Triassic 1974

4,300 Lower Cretaceous 1971

2.775 ? 1975




3,458 Upper Cretaceous 1971

3.987 Upper Jurassic 1957


Agip

Conoco




Agip


Cie Petroles Total Madagascar




Conoco



Conoco

Chevron


Conoco




Chevron



6 Coffin and Rabinowitz TABLE 1. SELECTED EXPLORATORY WELLS* (continued)


Total Depth (m)


Madagascar (continued)

M27 Befandriana 1

M 28

M29

M30

M31

M32

M33

M34

M35

M36

M37

M38

M39

M40

M41

M42

M43

M44

M45

M46

M47

M48

M49

M 50

Mamakalia 1

Ampandramitsetak

Amabalabe

Be ravi 1

Mandevy 1

Lambosina 1

Manera 1

Sakaraha 1

Vohidolo 1

Vohidolo 2

Vohidolo Bis

Tulear Bis

Bezaha Bis

Saloanivo 1

Antsokaky 1

Lac 1

Morondava 1

Namakia 1

Saronanala 1

Manambolo 1

Antaotao 1

Ambanasa 1

Vohibasia 1

22°05'14.4"S 43°51'00.0"E

22°00'55.2"S 44°23'00.0"E

22°07'00.0"S 44°39'00.0"E

21°56'51.2"S 45°16'00.0"E

22°20'51.6"S 43°43'00.0"E

22°22'51.3"S 44°12'00.0"E

22°38'06.4"S 44°34'00.0"E

22°54'00.0"S 44°18'00.0"E

22°51'00.0"S 44°31'00.0"E

22°51'00.0"S 44°50'00.0"E

22°51'00.0"S 44°50'00.0"E

22°49'33.4"S 44°54'00.0"E

23°18'54.4"S 43°40'00.0"E

23°27'15.8"S 44°31'30.6"E

23°3r00.0"S 44°47'00.0"E

23°15'00.0"S 45°12'00.0"E

24°10'00.0"S 43°56'00.0"E

18°53'53"S 43°59'23"E

20°35'S 44°39'E

20°13'S 44°46'E

19°18'S 44°37'E

18°53'S 44°52'E

22°31'18"S 44°40'03"E

21°47'55"S 45°40'10"E

1,630

3,153

2,701

2,181

2,250

2,662

2,589

3,911

3,813

2,733

3,464

3,426

2,195

2,714

2,650

1,139

2,449

4,004

4,481

2,385

4.115

3,379

4,670

2,878

Cretaceous

Jurassic

Lower Jurassic

Permian

Middle Jurassic

Lower Jurassic

Permian

Lower Jurassic

Triassic

Upper Triassic

Cretaceous

Permian

Permian

Upper Triassic

Triassic

Jurassic

Upper Permian-Lower Triassic

Upper Permian-Lower Triassic

1957 Société Petroles de Madagascar

1973 Chevron







1974 Chevron








1974 Chevron

1985 Mobil

1985 Amoco

1985 Amoco

1985 Amoco

1985 Amoco

1986 Occidental

1986 Occidental




Total Depth (m)


Somalia

51

52

53

54

55

56

57

58

59

510

511

512

513

514

515

516

517

518

519

520

521

Marai Ascia 1

El Cabobe 1

Mereghl

Gal Tardo 1

Duddumai 1

Uarsceik 1

Afgoi 1

Merca 1

Coriole 1

Coriole 2

Dobei 1

Dobei 2

Brava 1

Lach Bissigh 1

Lach Dera 1

Giamma 1

Oddo Alimo

Obbe 1

Kudha 1

Agfoi 2

Agfoi 3

4°31'00.0"N 47°26'00.0"E

4°14'48.0"N 47°40'42.0"E

3°43'11.6"N 47°32'05.4"E

3°10'00.0"N 45°50'50.0"E

2°37'14.0"N 44°53'57.0"E

2°14'00.0"N 45°30'00.0"E

2°06'52.0"N 45°04'10.0"E

1°52'21.0"N 44°53'28.0"E

1°50'39.0"N 44°33'16.0"E

1°49'43.0"N 44°33'52.0"E

1°48'31.0"N 44°31'29.0"E

2°42'44.0"N 44°28'25.0"E

1°04'00.0"N 43°31'00.0"E

0°49.54.0"N 41°21 '07.0"E

0°29'48.0"N 41°35'34.0"E

0°06'09.0"N 42°49'13.0"E

0°04'16.0"N 42°25'08.0"E

0°39'11.0"S 41°31'07.0"E

0°56'27.5"S 41°53'00.8"E

2°05'20"N 45°04'52"E

Vicinity ot Agfoi 1 and 2

4,115

4,428

4,303

2,438

3,380

4,101

4,164

3,998

3,518

4,069

2,122

3,830

3,810

3,086

2,867

4.126

4,465

4,865

4,972

3,353

4,359

Middle Jurassic

Upper Triassic

Middle Jurassic

Jurassic

Jurassic ?

Jurassic

Upper Cretaceous

Lower Jurassic (?)

Triassic (?)

Tertiary

Tertiary

?

Tertiary

Upper Jurassic

Upper Cretaceous

1958

1980

1982

1967

1960

1968

1966

1959

1961

1965

1961

1961

1963

1965

1965

1965

1964

1982

1982

1985

1985

Sinclair

Arco

Esso

Sinclair

Sinclair

Sinclair

Sinclair

Sinclair

Sinclair

Sinclair

Sinclair

Sinclair

Sinclair

Gulf

Gulf

Sinclair

Sinclair

Deutsche Texaco

Deutsche Texaco

Government of Somalia

Government of Somalia

8 Coffin and Rabino witz



Total Depth (m)


Tanzania

T1

T2

T3

T4

T5

T6

T7

T8

T9

T10

T11

T12

T13

T14

T15

T16

Pemba 5

Ras Machiusi

Zanzibar 1

Tancan 1

Kimbiji 1

Kisarawe 1

Kisangare 1

Mafia 1

Songo Songo 1

Songa Songa 1

Songa Songa 2

Kizimbani 1

Mandawa 7

M'Nazi Bay 1

Lukuliro 1

Kiwangwa

5°16'00.0"S 39°42'00.0"E

6°00'55.0"S 38°51'19.0"E

6°03'00.0"S 39°13'00.0"E

6°56'58.1"S 39°36'39.4"E

6°59'20.0"S 39°32'18.0"E

7°00'18.0"S 39°05'31,0"E

7°29'09.0"S 38°32'42.0"E

7°53'00.0"S 39°45'00.0"E

8°28'37.0"S 39°28'33.0"E

8°30'00.0"S 39°32'00.0"E

8°30'00.0"S 39°32'00.0"E

9°02'30.0"S 39°22'32.0"E

9°25'00.0"S 39°25'00.0"E

10°19'45.2"S 40°23'27.5"E

8°21'31"S 38°25'4rE

6°21'43"S 38°32'56"E


3,370 Upper Cretaceous (?) 1974


4,685 Tertiary 1985

4,326 Tertiary (?) 1982

4,002 Middle Jurassic (?) 1976

3,296 Middle Jurassic (?) 1976


4,426 Middle-Upper 1974 Jurassic (?)

1,006 ? 1976

1,829 ? 1977


4,065 Permian ?

3,489 ? 1982

2,367 "Karroo" (see text) 1985

3,860 Jurassic 1985

British Petroleum

Agip

British Petroleum

Petrocanada

Sonatrach

Agip

Agip

British Petroleum

Agip

Tanzanian Petroleum Development Corp.

Tanzanian Petroleum Development Corp.

Agip

British Petroleum

Agip

Shell

International Energy Development Corporation

'Including all DSDP wells, and onshore wells deeper than 1,000 m.

East African—Madagascan margins and western Somali Basin 30° 35° 40° 45° 50° 55°

Figure 2. Seasat-derived free-air gravity map of the western Indian Ocean. The contour interval is 5

mGal with major shade changes every 10 mGal; black <-50 mGal; white, >50 mGal. Map, courtesy of

W . F. Haxby. Scale, 1° of latitude = 111 km.

10 Coffin and Rabinowitz

4 0 4 5 5 0

Figure 3. Magnetic anomaly identifications and tectonic elements in the Western Somali and Comoros

Basins (Rabinowitz and others, 1983). D H O W , VLCC and ARS are fracture zones defined by Bunce

and Molnar (1981). Scale, 1° of latitude = 111 km.

20 55

East African—Madagascan margins and western Somali Basin 41 42 43 44 45

Figure 4. Reconstruction of Madagascar to Africa employing a pole at 10°N, 150°E (Coffin and

Rabinowitz, 1987). The intermediate position (Jurassic-Cretaceous boundary, marine magnetic

anomaly M17) involves a rotation of 4.6°; the pre-drift configuration, a rotation of 14.2°. Scale, 1° of

latitude =111 km.

12 Coffin and Rabino witz the west coast of Madagascar landward of the Davie Fracture

Zone (Fig. 5), and exhibits a horst and graben structure created

by faults trending N20°W or N20°E. The northern terminus of

the basin occurs at the St. Andre arch, its eastern boundary is

marked by Precambrian basement outcrop, and its southern

boundary is in the vicinity of the Lac 1 well (M43 in Fig. 1).

All of the onshore basins are mature, and geophysical inves-

tigations have indicated a total sediment accumulation in excess

of 10 km. The Western Somali Basin is the ocean basin created

by the separation of Madagascar and Africa (Fig. 4); its bathy-

metry appears in Figure 6. The Davie Fracture Zone marks its

western boundary, and geophysical parameters (gravity and

magnetics) help define the continent-ocean boundary (Rabino-

witz and LaBrecque, 1977) on the rifted East African and Mad-

agascan margins (Fig. 5).

Pre-Jurassic (Fig. 7) Diego Basin. The oldest sedimentary rocks found in the

Diego Basin are Middle Permian marine beds; marine conditions

persisted into the earliest Triassic. The three major subbasins of

the Diego Basin each record different stratigraphic sequences for

the interval basement through Upper Triassic, although all are

similar to the Sakamena facies of the Majunga Basin. The stratig-

raphy of the best known subbasin includes (from base to Upper

Permian and dated by microfossils): tectonic breccia overlying

basement; massive coarse-grained, dolomite-cemented sandstone

(to 500 m thick), containing coral and other fossils, with angular

conglomerate bands at the base; gray, yellow, or black fossil-

bearing shale (100 to 250 m thick) with dolomitic or quartzite

bands; green sandstone (50 to 100 m thick); interbedded sandy

and massive shale (120 to 150 m thick). Vertical crustal move-

ment during the Late Permian and Early Triassic is recorded as a

discordance between Permian and Triassic strata, and it preserved

Permian sediments in grabens. The Early Triassic was dominated

by the deposition of marine shale (20 to 100 m thick). This shale

is separated by an angular unconformity from overlying continen-

tal sandstone (>2,000 m thick) correlative with the Isalo facies of

the Morondava Basin. The sandstone varies greatly in grain size,

contains a few conglomerate bands and sandy shale horizons, and

continues into the upper Lower Jurassic. Ammonites, fish, am-

phibians, and lamellibranches have been used to date the pre-

Jurassic sediment.

Majunga Basin. Three hundred meters of Upper Permian

through Lower Triassic sedimentary rock, consisting of two prin-

cipal sandstone layers separated by shale, are found in the Ma-

junga Basin. The lower arkose contains large pebbles of granite.

The fossil-bearing shale is fissile, locally sandy, and contains fos-

siliferous carbonate lenses and nodules. The upper sandstone is

fine grained, micaceous, and argillaceous, with nodules of

carbonate-cemented sandstone. Disconformably overlying the

previous sequence is continental sandstone of the Isalo group, the

deposition of which probably commenced in the Middle Triassic

and continued through Late Triassic time. This fossil-poor sand-

stone is generally coarse grained, cross-bedded, mineralogically

homogeneous, and poor in argillaceous material; it attains a

thickness of several hundred meters.

Somali Coastal Basin. No pre-Jurassic sedimentary rock

has been reported from outcrop in the Somali Coastal Basin.

However, the Brava 1 well (SI3 in Fig. 1) bottomed in quartz

sandstone 120 m thick, which bears resemblance to the continen-

tal Triassic-Lower Jurassic Adigrat Formation described in other

wells. Both Kamen-Kaye (1978) and Kamen-Kaye and Barnes

(1978) noted that palynomorphs of Permo-Triassic age were

found in the shale overlying the quartz sandstone in the Brava 1

well. Nevertheless, Beltrandi and Pyre (1973) considered both

units to be Jurassic in age. Until more documentation becomes

available, we consider the basal sandstone to be the top of the

Adigrat Formation, the lower part of which is Triassic in age.

Lamu Embayment. The Karroo makes up the oldest sed-

imentary unit in the Lamu Embayment. At the bottom of the

Karroo sequence are the Taru Grits, fluviatile (with possible ma-

rine horizons), "fresh" feldspathic grit and sandstone derived

from basement to the west. Deposition of the Karroo beds was

initiated by major faulting in Late Carboniferous or Early Per-

mian time. The Taru Grits are Late Carboniferous to Late Per-

mian in age (dated by a fresh-water bivalve) and attain a

thickness of 2,700 m. At the very base of the Taru Grits are

tilloids comparable in age to tillites found in the Morondava

Basin. Overlying the Taru Grits are the Maji-ya-Chumvi beds,

which represent a change from fluviatile to lacustrine conditions.

These beds, totaling 1,200 m in thickness, consist of silty shale,

siltstone, and flaggy sandstone that commonly exhibit ripple

marks, cross-bedding, sun cracks, and rain pits. The lower half

(550 m) of the beds is continental, containing plant fossils, and is

overlain by a thin sequence of Lower Triassic marine shale con-

taining fish fossils. The upper part (650 m) of the unit indicates by

the presence of a fresh-water brachiopod—a fresh or possibly

brackish lacustrine environment. The Mariakani Sandstone suc-

ceeds the Maji-ya-Chumvi beds, and represents a fresh cycle of

coarse deposition. The sandstone, containing local micaceous silt-

stone, silty shale, and plant remains, is clean, fine grained, and

flaggy. The environment of deposition was deltaic, indicated by

current beds and ripple marks. The sandstone totals 2,900 to

3,400 m in thickness, and is assumed to be Middle and Late

Triassic in age. Significant faulting occurred between deposition

of the Mariakani and the overlying Mazeras Sandstone, which

tops the Karroo sequence. This unit ranges in age from Late

Triassic through Early Jurassic, and consists of 450 m of clean,

coarse-grained, cross-bedded deltaic sandstone and grit with in-

terbedded siltstone and silicified wood fragments.

Tanzanian coastal basins. The onset of Karroo (Permo-

Carboniferous to Early Jurassic) deposition was marked by the

Carboniferous activation of systems of faults trending north-

northeast, north-northwest, and one trending east-west; major

faulting continued intermittently into Jurassic time. Karroo sand-

stone, siltstone, and conglomeratic arkose overlying a basal con-

glomerate (resting on basement) attain thicknesses of 400 to


Figure 5. Tectonic elements of the East African-Madagascan margins and Western Somali Basin,

including fracture zones, region of steep gradient in the free-air gravity anomaly field and axis of positive

magnetic anomaly, and the seaward limit of diapirs. The location of magnetic anomaly M17 on both the

north and south flanks of the paleo-spreading ridge are plotted. Scale, 1 ° of latitude =111 km.


C o n t o u r s in m e t e r s

Somalia

L I B A S I N

Tanzania

isjnoledo —¿»roup 1

Comoros Islands

Mozambique C O M O R O S BAS>**I

Madagascar

East African—Madagascan margins and western Somali Basin 41

42 43 44 45

2,200 m in northern Tanzania; as much as 3,000 m of limestone,

sandstone, and mudstone are preserved in the south. The Man-

da wa 7 borehole (T13 in Fig. 1) penetrated 3,000 to 4,000 m of

predominantly Triassic (dated by fossils, including a fish) evapo-

rites in a Karroo rift graben. No pre-Jurassic sediment has been

encountered in outcrop or by drilling in central Tanzania or

offshore.

Morondava Basin. Rocks of the Sakoa Group, consisting

of four divisions, unconformably overlie faulted, horst-and-

graben Precambrian basement terrain. Thicknesses vary dramati-

cally from subbasin to subbasin, and also from north to south.

The Sakoa is 2,000 m thick in the southern Morondava Basin,

and thins gradually to Cap St. Andre where none is preserved.

The lowest unit of the Sakoa is a 50- to 450-m-thick glacial

sequence of tillite and black, locally varved shale; it is of Late

Carboniferous age. Immediately overlying the glacial sediment is

a coal-bearing sequence, 100 to 150 m thick, that begins with

coarse, cross-bedded sandstone and conglomeratic horizons. Rare

shale is associated with coal horizons, and the entire sequence is

Early Permian in age. Significant occurrences of the tillite and

coal-bearing sequences are limited to the southern portion of the

Morondava Basin, whereas the two younger sequences of the

Sakoa Group extend into the central and northern portions.

Above the coal sequence are the Lower Red Beds, from 20 to 400

m thick. Sand and shale dominate this sequence, and the sand

becomes coarser and conglomeratic toward the top of the section.

Two weak marine episodes are recorded in the sequence. The top

of the Sakoa Group is marked by the Middle Permian Vohitolia

marine limestone, 20 to 30 m thick. The limestone may be oolitic,

reefal, or brecciated, and the sequence was strongly eroded prior

to the deposition of younger rock preserved in the Sakamena

Group.

The Sakamena Group rests with a 12° unconformity on

either basement or the Sakoa Group, and is more widely distrib-

uted than the Sakoa. The Sakamena thins from greater than

4,000 m in the south to 20 m in the north, and is divided into

three units. The basal conglomerate of the lower unit, totaling

several tens of meters, is a torrential deposit of rounded boulders

derived from either basement or the Vohitolia Limestone. A

shale-sandstone succession overlies the conglomerate; the sand-

stone is hard and siliceous, cross-bedded at certain horizons, and

locally conglomeratic. There are some marine limestone intervals,

but the Lower Sakamena (Upper Permian), 2,000 to 3,000 m

thick, is a dominantly continental section. The Middle Sakamena,

Figure 6. Bathymetry of the East African continental margin, Mada-

gascan insular margin, Western Somali Basin, and Comoros Basin. Con-

tour interval, 500 m. Primary data sources for the compilation are

Hydrographie Office (South Africa), Hydrographie Office (United

Kingdom), Lamont-Doherty Geological Observatory, Scripps Institution

of Oceanography, U.S. Naval Oceanographic Office, University of Cape

Town, and Woods Hole Oceanographic institution. Scale, 1° of latitude

= 111 km. From Coffin (1985).

-200 m thick, is a nodular, septarian shale sequence regarded as

lagoonal or marine, which correlates with the Lower Triassic of

the Diego Basin. The 500-m-thick Upper Sakamena is an alterna-

tion of white, cross-bedded sandstone and red shale, representing

mixed continental and marine conditions.

The Sakamena Group is separated by a weak angular dis-

cordance from the overlying Isalo Group, which attains a thick-

ness of 5,000 to 6,000 m in the south and 1,700 m in the north.

The Isalo ranges in age from Middle Triassic through Early Ju-

rassic, and consists of a fine-grained, poorly consolidated,

argillaceous-cemented, ocherous sandstone containing feldspar,

mica, lenses of variegated shale and coal, and tar sands.

Western Somali Basin. No pre-Jurassic rock has been

recovered from the Western Somali Basin, nor is it suspected that

there is any rock of that age, according to the tectonic scenario

(e.g., Coffin and Rabinowitz, 1987) developed for the region.

Summary. Mixed-facies rock sediments of the Diego and

Majunga Basins demonstrate subsidence occurring from Permo-

Carboniferous time through the Late Triassic in grabens striking

northeast. The conjugate Somali Coastal Basin has no well-

documented rock for this time interval, but the "Bur" basement

outcrop strikes northeast as well. Mixed-facies rocks, including

tilloid and salt, of the Lamu Embayment, Tanzanian Coastal

Basins, and Morondava Basin, document subsidence in the Car-

boniferous through Late Triassic interval along faults trending

generally north-northeast or north-northwest. A major problem

with Karroo-age rocks of East Africa and Madagascar is the lack

of paleontologic and other information to determine facies and

age.

Lower Jurassic (Fig. 8) Diego Basin. The thick (>2,000 m) continental Isalo

Sandstones with conglomeratic beds and thin, sandy shale hori-

zons extend upward into the Lower Jurassic. By the end of Early

Jurassic time, however, the facies changed dramatically. North of

the Ambilobe 1 well (Ml in Fig. 1), marine marl and limestone

(—50 m thick) of Toarcian and Aalenian age are present as the

fossiliferous Marivorahana Series. To the southwest a mixed fa-

cies represents this time interval, and the stratigraphic succession

is as follows: gray limestone with some shale and sandstone inter-

calations overlying the Isalo Sandstones; mixed facies of inter-

bedded marine and continental shale, limestone, siltstone, and

sandstone. Interestingly, an island offshore the Diego Basin,

Nosy Be, exposes 3,000 m of Lower Jurassic sandstone and

arkose.

Majunga Basin. Deposition of the fossil-poor Isalo conti-

nental cross-bedded sandstone, described above with pre-Jurassic

rock, continued from the Triassic into the Early Jurassic. The

total thickness of the Middle Triassic through Pliensbachian sec-

tion, which is cemented by carbonate in its upper portion, is 500

to 600 m in the southern Majunga Basin. At the end of Early

Jurassic time, the Sahondralava-Ihopy horst became emergent

and resulted in differing sedimentary histories to its north and


Unconformity

Figure 7. Pre-Jurassic plate tectonic reconstructon and stratigraphie sections. Scale, 1° of latitude =

111 km; both key and scale apply to this and subsequent maps.


42 43 44 45


south. In the northern Majunga Basin, the Middle Triassic

through Bajocan interval is -200 m thick. The Toarcian, identi-

fied only in the southern part of the basin, is represented by a

distinctive unit of interbedded marly limestone and shale. Toward

the end of Early Jurassic time, lateral facies changes became

significant, as did the development of structural highs and flexures

within the Majunga Basin. Marine conditions gradually became

established from south to north during Early and Middle Jurassic

time.

Somali Coastal Basin. Deposition of the Adigrat Forma-

tion continued uninterrupted from Triassic through Pliensbachian

time. The formation consists of as much as 130 m of quartz

sandstone with intercalations of gypsum and dark shale. Interest-

ingly, the Coriole 1 well (S9 in Fig. 1) bottomed in extrusive

igneous rock after penetrating at least part of the Adigrat Forma-

tion. This rock may be a manifestation of the rifting process that

ultimately led to the separation of Madagascar and Africa in

Middle Jurassic time (Segoufin and Patriat, 1980; Parson and

others, 1981; Rabinowitz and others, 1983). Overlying this sand-

stone and extending to the top of the Middle Jurassic are basinal

dark gray shale and dark gray argillaceous fossiliferous limestone

that grade to pure limestone seaward. At the Marai Ascia well

(SI in Fig. 1), this section—the Hamanlei Formation—is at least

1,525 m thick; just to the north at the Obbia well (not on map),

the thickness is at least 2,175 m.

Lamu Embayment. Deposition of the Mazeras Sandstone

probably continued from Late Triassic through Early Jurassic

time. These beds consist of 450 m of clean, coarse-grained, cross-

bedded deltaic sandstone and grit containing interbedded siltstone

and silicified wood. At the end of the Early Jurassic, a major

episode of faulting accompanied the end of the predominantly

continental Karroo deposition.

Tanzanian coastal basins. The deposition of Karroo

conglomerate and sandstone, siltstone, shale, and evaporites lo-

cally (Mandawa) continued unimpeded from Triassic through

Toarcian time. Similarly, faulting of the region continued through

the Early Jurassic to the end of Middle Jurassic time. The total

thickness of Karroo sediment ranges from 400 to 2,200 m in the

north, and to as much as 3,000 m in the south. Evaporites en-

countered in the Mandawa 7 drillhole (T13 in Fig. 1) extend

through the Toarcian, although most of the 3,000- to 4,000-m-

thick section is of Triassic age. The transitional Ngerengere and

correlative rock, consisting of bedded feldspathic calcareous

sandstone with local limesoone beds (some oolitic) and shale, and

appearing to be reworked Karroo sediment, are sandwiched be-

tween the Karroo and Middle Jurassic rock and are thus presum-

ably of Toarcian and Aalenian age. The total thickness of the

Ngerengere sequence ranges from 300 to 760 m in the north, to

230 m in the south. No Lower Jurassic sedimentary beds have

been sampled in outcrop or by drilling in central Tanzania or

offshore. The Jurassic section thins inland.

Morondava Basin. Deposition of the undifferentiated

Isalo continental sandstone continued from Middle Triassic

through Early Jurassic time, attaining a total thickness of 5,000 to

6,000 m in the south, and 1,700 m in the north of the Morondava

Basin. A slight exception to this is in the extreme north, where a

marine Toarican limestone horizon was deposited synchronous

with a change to marine facies in the Majunga Basin. The Isalo

Group is a fine-grained, poorly consolidated, argillaceous-

cemented ocherous sandstone containing feldspar, mica, lenses of

variegated shales, and coal. At the Chesterfield 1 well (M8 in Fig.

1), 1,670 m of Lower Jurassic interbedded limestone and shale,

with basalt flows near the base, were recovered. The East Seri-

nam 1 well (Ml8 in Fig. 1) recovered 330 m of massive shelf

limestone, and to the south the Mamakiala 1 borehole (M28 in

Fig. 1) recovered 621 m of Lower Jurassic thin limestone, shale,

and minor sandstone.

Western Somali Basin. No rock of Early Jurassic age has

been recovered from the Western Somali Basin. The tectonic

scenario developed for the region (Coffin and Rabinowitz, 1987)

argues against the existence of rock of that age in the basin.

Summary. The conjugate rifted basins—Diego, Majunga,

and Somali—along with the extreme northern part of the Mo-

rondava Basin, all show a major facies change from continental to

marine near the end of Early Jurassic time. In the Majunga Basin

the marine conditions proceeded from south to north during

Early and Middle Jurassic time. Extrusive igneous rock, possibly

rift volcanics, has been found in the Somali Coastal Basin, as well

as in the northern portion of the Morondava Basin. Continental

facies dominated the Lamu Embayment until a major episode of

faulting at the end of the Early Jurassic Epoch. Mixed facies

prevailed in the conjugate Tanzanian and Morondava Basins,

including evaporites at Mandawa, and faulting was intense in

Tanzania.

Middle Jurassic (Fig. 9) Diego Basin. The Diego Basin must be divided into two

contrasting provinces for the Middle (and Upper) Jurassic. The

extreme westernmost section of the basin, including the island of

Nosy Be and the peninsula extending from Madagascar toward it,

subsided rapidly during this time. Thick (3,000 to 6,000 m) de-

posits of paralic, silty, fossiliferous sediment, commonly calcare-

ous, record episodes of coarse-grained and calcareous sediment

deposition, and rapid vertical and horizontal facies changes. The

region to the east contains epicontinental, fauna-rich, sandy marl

and limestone ranging from 400 to 550 m thick.

Majunga Basin. In the northern Majunga Basin, deposi-

tion of the Isalo continental cross-bedded sandstone continued

until Bajocian time; the total Middle Triassic through Aalenian

interval is -200 m thick. The Lower Calcareous Sandstone series,

consisting of fine-grained, carbonate-cemented sandstone inter-

calated with shale and local Isalo-type sandstone, is assigned to

the Bajocian from its stratigraphic position (no paleontologic

markers are present). The Upper Calcareous Sandstone of Batho-

nian age succeeds the Lower series; it comprises 450 m of Isalo-

type cross-bedded fossiliferous (including dinosaur remains)

sandstone with intercalations of calcareous sandstone and lime-


35° 40" 45° 50°

Figure 9. Middle Jurassic plate tectonic reconstruction and stratigraphie sections.

20 Coffin and Rabino witz stone. Alternating marine and nonmarine phases dominated the

Bajocian and Bathonian in the northern Majunga Basin, and

marine conditions became firmly established in the Callovian,

from which are preserved 100 m of fossiliferous clay, marl, sand-

stone, and limestone. At the Sofia 1 borehole (M4 in Fig. 1),

590 m of Middle Jurassic limestone and shale were encountered.

In the southern portion of the Majunga Basin, the Aalenian con-

sists of 60 m of fine-grained fossiliferous shale with continental

sandstone layers and local lignite horizons. Marking the onset of

marine conditions above are 250 m of fossiliferous Bajocian sed-

iment, primarily massive and oolitic limestone, but including sev-

eral shale horizons. The Bathonian is represented by 150 m of

interbedded limestone and shale. Finally, 60 m of fossiliferous

Callovian marl, with limestone bands, are preserved. Stratigraph-

ic evidence from the northern and southern portions of the

Majunga Basin thus indicate a northward migration of marine

facies during Middle Jurassic time.

Somali Coastal Basin. Deposition of the undifferentiated

Hamanlei Formation, consisting of basinal dark gray shale and

dark gray argillaceous fossiliferous limestone that grades to pure

limestone seaward, continued through the end of Callovian time.

At the Marai Ascia well (SI in Fig. 1), the Hamanlei is at least

1,525 m thick, and a minimum thickness of 2,175 m has been

reported just to the north. Major normal faults trending northeast

in the Somali Coastal Basin were active during Middle Jurassic

time, and these faults step down to the southeast.

Lamu Embayment. Marine conditions became established

in the Lamu Embayment during Middle Jurassic time. The Bajo-

cian and Bathonian are represented by 150 to 600 m of the

Kambe Formation, a dark gray oolitic limestone with abundant

fauna, especially coral and ammonoids (Westermann, 1975) and

interbedded shale. A deep-water facies is present in the south, and

a shallow-water environment is indicated to the north. The

Kambe limestone is succeeded by 180 m of sandy micaceous

shale with thin, fine-grained sandstone. These Kibiongoni beds

rest conformably on the Kambe, and represent a continuation of

shallow-water deposition. The beds grade into the "Upper" Ju-

rassic shale. The lower shale is Callovian in age, and is probably

deltaic. It contains numerous ammonites, often in septarian nod-

ules. The total thickness of the "Upper" Jurassic shale is 1,700 m,

most of which is Late Jurassic in age.

Tanzanian coastal basins. Deposition of the transitional

Ngerengere (north), Pindiro (south), and correlative units, appar-

ently reworked Karroo sediment, continued from the Early Ju-

rassic through the Aalenian in northern and central Tanzania, and

through the Bajocian in the south. The Ngerengere consists of 300

to 760 m of bedded feldspathic calcareous sandstone with local

limestone beds (some oolitic) and shale, whereas the Pindiro

comprises as much as 230 m of shale with local coarse, con-

glomeratic feldspathic sandstone, red mudstone, and thin oolitic

limestone. At the Mandawa 7 borehole (T13 in Fig. 1), the thick

evaporite sequence continues to the end of the Aalenian. The

transitional units are overlain unconformably in all locations ex-

cept the Mandawa borehole by marine limestone of Bajocian or

Bathonian through Callovian age. (At Mandawa the Aalenian

evaporites pass conformably upward to shale and limestone of

Bajocian and Bathonian age, respectively.)

In the north the Amboni Limestone, a dense, well-bedded

rock, commonly sandy, partly oolitic or pisolitic, attains a maxi-

mum thickness of 340 m. The limestone is poorly fossiliferous,

and is dated as Bajocian and Bathonian (and sometimes Callo-

vian) based on limited molluscs and the presence of well-dated

(ammonites) overlying Callovian shale in certain sections. In cen-

tral Tanzania, the correlative Lugoba, Kidugallo, and Kidunda

limestones of Bajocian through Callovian age are preserved. In-

terfingered with these limestones are the Posidonia Shales of the

same age. To the south, the Bathonian Mtumbei Limestone—a

massive sequence of oolitic sandy limestone and calcareous sand-

stone containing a varied fauna of bivalves, gastropods, brachio-

pods, corals, and algae—attains a thickness of 150 m. In the

south, in outcrop and at the Kisarawe and Kisangare boreholes

(T6 and T7, respectively, in Fig. 1), the Callovian is represented

by the bottom of the Mandawa Series, which extends into the

Upper Jurassic. The entire series consists of -600 m of inter-

layered buff sandstone and red clay with marine fossils. Finally, at

the Kizimbani borehole (T12 in Fig. 1), evaporites of Callovian

age were recovered above Bathonian limestone, but no thick-

nesses have been reported. The Jurassic section thins inland. The

basal transgressive limestone coincides with the phase of major

faulting that defined the inland margins of Tanzania's coastal

basins.

Morondava Basin. The Aalenian beds in the Morondava

Basin are a continental facies, undifferentiated from the Middle

Triassic to Lower Jurassic Isalo Sandstone. Marine conditions

developed during Bajocian and Bathonian time, and dominated

the northern part of the basin, whereas in the central and southern

portions, mixed facies with localized marine incursions were the

rule. Facies varied markedly in Middle Jurassic time, as did basin

geometry, and there were important periods of nondeposition.

To document these variations, we examine five sections de-

rived from outcrops in a north-south orientation along the axis of

the Morondava Basin. In the north the Bajocian through Callo-

vian interval is represented by -300 m of limestone. In the north-

central portion of the basin is -1,200 m of this interval, consisting

of three units approximately equal in thickness.

The lower unit consists of interbedded limestone and shale,

the middle unit of interbedded shale and limestone with common

sandstone horizons, and the upper unit of limestone. The central

basin displays 2,000 m of Bajocian through Callovian strata. At

the base of the section are -100 m of limestone, succeeded by

1,900 m of sandstone with some shale horizons in the middle, and

rare limestone horizons in its upper section. The East Serinam 1

well (Ml8 in Fig. 1) documents 732 m of Middle Jurassic mas-

sive shelf limestone. In the south-central portion of the basin lies

1,500 m of the interval, with limestone and shale (100 m) at the

base of the column succeeded by 1,400 m of sandstone with

oolitic limestone in the center of the section and some marl and

shale stringers near the top. At the Mamakiala 1 borehole (M28


42 43 44 45

in Fig. 1), 625 m of deltaic shale, siltstone, and thin sandstone

were recovered. No Middle Jurassic sedimentary rock is pre-

served in the southernmost part of the Morondava Basin. Off-

shore, the Chesterfield 1 well (M8 in Fig. 1) records 414 m of

Middle Jurassic basalt flows interbedded with limestone and

sandstone. The Eloise 1 well (M10 in Fig. 1) recovered 244 m of

calcareous shale and thin limestone stringers.

Western Somali Basin. The rift/drift transition marking

the initiation of sea-floor spreading between Madagascar and

Africa occurred during Callovian time, so a small section of

Middle Jurassic age should be present. None, however, has been

recovered from the Western Somali Basin to date.

Summary. Both the Diego and Somali Coastal Basins re-

cord active faulting and subsidence during Middle Jurassic time.

In the Majunga Basin the migration of marine facies from south

to north continued from Early Jurassic time. Marine conditions

were established in the Lamu Embayment, as well as in the

conjugate Tanzanian Coastal and Morondava Basins. Major

faulting and subsidence (and the continuation of evaporite depo-

sition) are recorded in Tanzania, and changing subbasin geome-

tries and basalt flows highlighted the Middle Jurassic Epoch in

the Morondava Basin.

Upper Jurassic/Lower Cretaceous (Fig. 10) Diego Basin. The Upper Jurassic stratigraphy of the Diego

Basin is not differentiated from the Middle Jurassic section. As

previously discussed, the two subbasins of the Diego Basin dis-

play very different stratigraphies. The Nosy Be and peninsular

province is characterized by paralic, silty, commonly calcareous,

fossiliferous sediment (3,000 to 6,000 m thick), recording influxes

of coarse-grained sediment, calcareous intervals, and sudden fa-

cies changes. To the west are epicontinental, marine, fauna-rich,

sandy marl and limestone ranging from 400 to 550 m in

thickness.

The Lower Cretaceous is represented by fossiliferous marl,

sandstone, and shale. The entire stratigraphie sequence consists of

-250 m of Upper Valanginian through Upper Hauterivian marl

and shale separated by an angular unconformity from 600 m of

overlying sandstone, which is overlain in turn by 190 to 250 m of

Lower Albian shale and Middle and Upper Albian marl.

Majunga Basin. Sedimentation in the Majunga Basin from

Callovian through the end of Valanginian time was, with minor

exceptions, predominantly marine. In both the northern and

southern portions of the basin, 10 m of fauna-rich Lower Oxford-

ian marl and limestone are present, succeeded by 30 m of

Kimmeridgian marl, glauconitic or gypsiferous, and clay. In the

north the Tithonian consists of 70 m of marl and gypsiferous clay,

whereas in the south this interval is represented by 25 m of

glauconitic marl. Upper Jurassic shale totaling 333 m in thickness

was recovered from the Sofia 1 (M4 in Fig. 1) borehole.

Berriasian (containing ammonites) and Valanginian (con-

taining belemnites and oysters) strata consist, respectively, of

150 m of clay with gypsum, and 100 m of shale and marl, in the

northern portion of the basin. The south records 180 m of marly

limestone and marl for the interval. Hauterivian and Aptian time

is marked by a change to predominantly continental conditions.

To the north the Hauterivian is a marine epicontinental facies 400

m thick consisting of shale (yielding indeterminable fauna),

commonly containing ferruginous nodules or pellets, and rare

glauconitic sand with some lignite horizons. The Hauterivian

comprises 250 m of continental cross-bedded sandstone in the

south. The Aptian records several marine horizons (faunally) in a

predominantly continental section. To the north, 25 m of glauco-

nitic sandstone are found beneath 340 m of poorly consolidated,

continental cross-bedded sandstone. The Aptian interval in the

south, from bottom to top, consists of 110 m of continental

lignitic sandstone, 55 m of continental sandstone, 20 m of glau-

conitic sandstone, and 30 m of gypsiferous marl. The Albian is an

entirely marine facies, rich in fossils, made up of 150 to 250 m of

shale and marl with calcareous or ferruginous nodules and com-

monly glauconite in the north, and 50 to 100 m of marl in the

south. The Sofia 1 well (M4 in Fig. 1) recovered 1,165 m of

Lower Cretaceous massive shale with thin sandstone beds.

Somali Coastal Basin. The Oxfordian-Kimmeridgian Ua-

randab Formation consists of yellowish, marly limestone contain-

ing belemnites and ammonites in southern Somalia. The

formation is represented in boreholes by basinal dark gray shale

and gray marly limestone stringers, and in the Marai Ascia well

(SI in Fig. 1) a total thickness of 538 m was recorded. The

remainder of the Upper Jurassic (late Kimmeridgian-Tithonian)

section is expressed as basinal dark gray and dark brown shale,

with some gray, finely crystalline foraminifera-bearing limestone,

of the Gabredarre Formation, which attains a maximum thick-

ness of 350 m.

In south-central Somalia the Lower Cretaceous crops out as

a series of gypsum and limestone with interbedded shale. In the

subsurface a fore-reef limestone and medium-depth neritic shale

make up the Cotton Formation, which is entirely of Early Cre-

taceous age (dated by foraminifera). At Marai Ascia (SI in

Fig. 1), the section is 130 m thick and is in unconformable

contact with both Upper Jurassic and Upper Cretaceous rock.

Lamu Embayment. The deposition of the Upper Jurassic

shale continued from Callovian into Neocomian time, and 1,700

m of dark gray or brown shale, silty to sandy, with ammonites

and thin lenticular beds of gray limestone, are preserved. A 100-

m-thick Oxfordian limestone layer contains brachiopods. The

Garissa 1 borehole (K4 in Fig. 1) bottomed in 205 m of Jurassic

mudstone, shale, siltstone, and sandstone.

The uppermost 30 m of shale lacks concretions and is Neo-

comian in age, as dated by ammonites. More than 1,500 m of

Neocomian and 250 m of Aptian sediment are preserved in the

Lamu Embayment proper. The Neocomian consists of quartzite,

sandstone, siltstone, and dark gray shale, and the Aptian is mostly

sandstone and siltstone. These clastic rocks grade to shallow-

water limestone and mudstone near the edge of the embayment.

The thick clastic section is succeeded by more than 1,000 m of

Albian dark gray shale, generally calcareous, in which planktonic


Figure 10. Plate tectonic reconstruction for the Jurassic-Cretaceous boundary (magnetic anomaly M17),

and stratigraphie sections for Late Jurassic-Early Cretaceous time (black bars across the columns

indicate the Jurassic-Cretaceous boundary).


42 43 44 45

foraminifera and ammonite fragments are abundant. Near the

margin of the embayment, the rock types are the same, but thick-

nesses are drastically reduced to several tens of meters. At the

Wal Merer borehole (K3 in Fig. 1), the Neocomian consists of

1,445 m of interbedded sandstone (commonly orthoquartzitic)

and shale of shallow-marine facies. The overlying Aptian, of

similar lithology and facies, is 250 m thick, and the Albian is a

marine limestone with varying amounts of interbedded shale. At

Walu 2 (K7 in Fig. 1), the Aptian-Albian interval is represented

by 110 m of dense argillaceous and calcareous siltstone.

Tanzanian coastal basins. In the north the Oxfordian

beds represent a marked facies change from underlying massive

limestone. Between 700 and 1,050 m of Oxfordian and Kimmer-

idgian sandstone, mudstone, marl, and limestone are preserved,

and Tithonian rock is absent. A similar transition from Middle to

Upper Jurassic is recorded in central Tanzania. To the south, the

Mandawa Series continues uninterrupted from the Callovian, and

the Upper Jurassic marine sequence is as follows: fossiliferous

sandstone and nerinella beds (Bathonian to Oxfordian), 170 to

480 m thick; septarian marl (lower Kimmeridgian), 200 to 310 m

thick; sandstone (middle-upper Kimmeridgian), 225 to 250 m

thick; oolite (upper Kimmeridgian to lower Tithonian), 105 to

115 m thick; and sandstone and grit (upper Kimmeridgian to

lower Tithonian), 225 to 300 m thick. The Upper Jurassic section

thins rapidly landward.

The Upper Cretaceous is notable for rapidly changing rock

types and regressive facies throughout Tanzania. In northern

Tanzania, every age of the epoch is represented, and the section

consists of sandstone, in places conglomeratic with interbedded

limestone. The only exception is the upper Albian, which is shale.

In central Tanzania, the Neocomian consists of 145 to 210 m of

mauve, red, and green variegated fossiliferous shale, commonly

silty and slightly calcareous with cross-bedded, friable, fine-

grained sandstone beds in places. This facies is estuarine with

marine intercalations. The Aptian section comprises a 9-m-thick

basal sandstone (brown, conglomeratic, fine to medium-grained,

with boulders) overlain by 27 m of a gray silty mudstone with

thin sandstone bands. At the top of the Aptian are 25 m of

brown, fine-grained sandstone. The lower Albian consists of a

lower shale unit and an upper sandstone section. The shale, 70 m

thick, is a gray calcareous mudstone with a reddish sandstone

band. Toward the west the shale is red and green. The 90-m-thick

sandstone is brown, massive, fine- to medium-grained, and peb-

bly near its base, with flaggy sandstone and calcareous shale in

the east. Finally, 275 m of blue-gray calcareous shale, and some

calcareous sandstone and grit, compose the middle and upper

Albian.

To the south the Jurassic cycle of sedimentation persisted

into Berriasian time, as represented by 110 m of white coralline

limestone spanning the Jurassic-Cretaceous boundary; in some

areas, sandstone with local ammonite-bearing limestone spans the

boundary. The middle of the Berriasian shows a break in sedi-

mentation; subsequent Neocomian rocks—predominantly sand-

stone, siltstone, and in places limestone grading to mudstone

eastward—were deposited only locally during a general regres-

sion. By the end of Aptian time, another phase of sedimentation

commenced, beginning with sandstone, conglomerate, and silt-

stone fining eastward. By latest Aptian time, reef limestone,

changing laterally to marl with thin detrital limestone in places,

was deposited, and Aptian-Albian thicknesses range from 150 to

360 m.

Morondava Basin. The Late Jurassic was an epoch of

extreme facies variation in the Morondava Basin. Marine condi-

tions prevailed in the north, whereas mixed environments domi-

nated the central and southern portions of the basin. Significantly,

periods of nondeposition occurred during Late Jurassic time.

Generally, the northern regions experienced no sedimentation

during Oxfordian time, whereas in the south the lower Oxfordian

is represented by a 30-m-thick marine limestone, and the middle

Oxfordian by a mixed facies variable in thickness. In the north

the Kimmeridgian and Tithonian consist of marine deposits,

which are absent in the south. Generalized sections from the basin

for the Upper Jurassic are as follows: in the north, -200 m of

limestone were deposited during the epoch, while in the north-

central portion of the basin a 200-m-succession (bottom to top)

of limestone, marl, and sandstone was preserved, in the center of

the basin, a dominantly sandstone section 500 m thick with some

shale and marl near its base represents the Upper Jurassic, and the

East Serinam 1 well (Ml8 in Fig. 1) records 100 m of Upper

Jurassic calcareous shale. In the south-central segment of the

basin, a 330-m-thick sequence (bottom to top) of shale and marl,

limestone, sandstone, shale and marl, and limestone with shale

horizons is recorded. At the Mamakiala 1 borehole (M28 in

Fig. 1), 924 m of paralic Upper Jurassic interbedded limestone,

sandstone, siltstone, and shale were recovered. Finally, in the

extreme south, no sediment of Late Jurassic age is present. Off-

shore, the Chesterfield 1 well (M8 in Fig. 1) recovered 218 m of

Upper Jurassic limestone and rhyolite. The Eloise 1 borehole

(M10 in Fig. 1) penetrated 556 m of calcareous shale and thin

limestone stringers, with volcanics near the top of the section.

In the northern Morondava Basin, Valanginian through Al-

bian strata —150 m thick consist of dark marine shale (Duvalia)

and/or continental sandstone (Sitampiky Formation) overlain by

marine shale. In the central basin, the East Serinam 1 well (Ml8

in Fig. 1) recovered 538 m of Lower Cretaceous calcareous shale

with thin sand stringers. No rocks of Valanginian through middle

Albian age have been found in the south of the basin, although a

thin sequence of upper Albian shaly sandstone continues in the

Upper Cretaceous. Offshore, the Chesterfield 1 well (M8 in

Fig. 1) records 740 m of Lower Cretaceous interbedded sand and

shale, with basaltic flows. The Eloise 1 well (M10 in Fig. 1)

penetrated 978 m of shale, silty shale, and volcanics. The Vau-

cluse 1 borehole (Ml6 in Fig. 1) recovered 2,094 m of undiffer-

entiated pyroclastics, tuffs, and ignimbrites. The Eponge 1 well

(Ml9 in Fig. 1) encountered 826 m of Lower Cretaceous shale

and siltstone.

Western Somali Basin. No rock of Late Jurassic or Early

Cretaceous age has been sampled in the Western Somali Basin.


Summary. The Diego and Majunga Basins record alternat-

ing marine and nonmarine periods during Late Jurassic and Early

Cretaceous time, whereas the conjugate Somali Coastal Basin is

wholly marine for the entire interval. In the Lamu Embayment,

mixed facies predominate inland of the present coastline, whereas

marine facies are found in the offshore wells. The conjugate

Tanzanian coastal basin and Morondava Basin show mixed facies

for the two epochs, and volcanics are present in the latter. Off-

shore marine facies predominate in each basin.

Upper Cretaceous (Fig. 11) Diego Basin. The Upper Cretaceous is well represented in

the Diego Basin. Cenomanian strata include 100 to 150 m of

marl rich in microfauna, and the lower Turanian is marked by a

sudden change to fossiliferous continental sandstone (60 m thick).

Furthermore, the presence of trachyte in conglomerates marks an

important episode of volcanism commencing in early Turonian

time. A return to marine conditions in late Turonian time is

indicated by sandstone succeeded by shaly limestone, with a total

thickness of 10 to 20 m. The Senonian section (dated by lamelli-

branches and echinoids) consists of sandstone and sandy marl

with some limestone bands; the entire section is 150 to 180 m

thick. Finally, the Maastrichtian is represented by 40 to 80 m of

unfossiliferous sand and sandstone.

Majunga Basin. The Cenomanian was an age of transition

(although not synchronous basin-wide) from marine to continen-

tal facies. As much as 100 to 120 m of shale with limestone

horizons underlies as much as 370 m of coarse, cross-bedded

pebbly sandstone. By the end of Cenomanian time, the

Sahondralava-Ihopy horst's role as a north-south partition be-

tween subbasins ceased. The Turonian is marked by extensive

lava flows, averaging 50 m but ranging to as much as 200 m in

thickness. Most of the flows are subaerial, although some were

extruded in lacustrine and marine environments. In the southern

Majunga Basin, the flows are underlain by 20 m of coarse-

grained, unfossiliferous sandstone, and overlain by 5 m of fossilif-

erous clay, marl, and limestone. The Senonian is predominantly

continental, but it does contain thin, well-defined marine inter-

vals. Lagoonal-continental beds, consisting (bottom to top) of

limestone, sandy clay, argillaceous limestone, and cross-bedded

sandstone with some marine intervals, compose the 180-m-thick

Coniacian section. The Santonian is represented by cross-bedded

sandstone overlain by lagoonal conglomerates and argillaceous

sandstone (containing dinosaur remains), which in turn are over-

lain by well-dated sandy limestone and gray shale; it totals 170 m

in thickness. Continental cross-bedded sandstone with some cal-

careous horizons records the Campanian. Offshore, the interval

Coniacian through Campanian is represented by 220 m of marl

and sandy marl, and is unconformably overlain by middle and

upper Eocene marine sediments. Finally, the Maastrichtian, a 50-

to 70-m-thick unit of very fossiliferous marly limestone or chalky

marl, in places dolomitic toward its base, marks a change to

marine conditions that persisted through Eocene time. Offshore,

the Maastrichtian is usually absent. At the Sofia 1 (M4 in Fig. 1)

borehole, 875 m of Upper Cretaceous thick sandstone with inter-

bedded shale were recovered.

Somali Coastal Basin. At Marai Ascia (SI in Fig. 1), the

Upper Cretaceous is represented by 1,025 m of deep-water gray

shale and marl (Sagaleh Formation), and the section thins sea-

ward. Farther to the south, at the Merca well (S8 in Fig. 1), the

entire Upper Cretaceous column was not penetrated, but 360 m

of dark gray shale of that age with interbedded spilitic basalt

flows were recovered. In southeastern Somalia, the Upper Cre-

taceous section is approximately the same thickness as at Marai

Ascia, but consists of open marine sandstone and siltstone with

shale intercalations. The Sagaleh Formation is rich in

foraminifera.

Lamu Embayment. Deposition of deep-water marine

shale (-1,200 m) containing planktonic foraminifera persisted

from Early Cretaceous to the beginning of Cenozoic time. The

sequence comprises gray to gray-green calcareous mudstone with

thin bands of fine-grained argillaceous sandstone and micritic

limestone. At the Walu 2 well (K7 in Fig. 1), 967 m of Cenoman-

ian through Campanian gray and gray-green calcareous mud-

stone and shale with thin bands of dense micritic limestone

were recovered. The lower section is of deep-water facies; the

upper, of shallow-water facies. The Dodori 1 well (K8 in Fig. 1)

bottomed in 210 m of Upper Cretaceous silty shale, gray-black

bituminous coal, and carbonaceous interbedded sandstone and

siltstone, all of shallow-water marine facies. The Kipini 1 well

(K l l in Fig. 1) bottomed in 580 m of Upper Cretaceous silty

shale and mudstone, with some sandstone, of shallow-water ma-

rine facies. These rocks are unconformably overlain by middle

Eocene strata. Offshore, the Simba 1 borehole (K14 in Fig. 1)

bottomed in 852 m of Upper Cretaceous dark gray to dark brown

shale and light gray to dark gray sandstone, with a band of dense

limestone. Microfossils suggest a deep-water environment of dep-

osition, and the Upper Cretaceous rock is separated from

Paleocene strata by an unconformity.

Tanzanian coastal basins. The Upper Cretaceous sedi-

mentary record in Tanzania is dominated, with minor exceptions,

by transgressive marine clay. Sandstone and limestone totaling

900 m comprise the Cenomanian, Coniacian, and Maastrichtian

in northern Tanzania. Central Tanzania records 2,000 m or more

of silty shale and marl, rich in microfauna, which were deposited

in a subparalic environment. The Cenomanian is 625 m thick; the

Turonian, 10 to 170 m; the Coniacian, 60 m; the Santonian,

undetermined; the Campanian, 635 to 705 m; and the Maastrich-

tian, 30 m. A representative Upper Cretaceous section from the

south includes 90 m of Cenomanian gray silty shale with thin,

hard, calcareous sandstone bands; Turonian black silty claystone

and shale with calcareous sandstone beds, totaling 190 m; 90 m

of Coniacian gray to black silty clay sandstone and some septaria,

unconformably overlain by 105 m of Santonian-Campanian

gray silty clay and thin sand; and Maastrichtian gray silty clay

and thin sand totaling 380 m. Deep wells on the offshore islands

all bottom in Upper Cretaceous rock.

Figure 11. Stratigraphic sections of Late Cretaceous age.

2 6 Coffin and Rabino witz

On Pemba Island (T1 in Fig. 1), 816 m of Upper Senonian

interbedded gray and gray-brown mudstone, containing forami-

nifera, with thin bands of gray calcareous clay were recovered.

The mudstone is hard, pyritic, micaceous, silty, and commonly

contains fine lignite fragments; rare thin bands of white silty marl

and finely granular limestone also occur. On Zanzibar Island (T3

in Fig. 1), 150 m of Maastrichtian rock were penetrated, consist-

ing of black shale and claystone with some siltstone and a few

fine calcareous sandstone beds. To the south, the Mafia Island

deep borehole (T8 in Fig. 1) encountered 590 m of Danian and

older rock. The Campanian, from bottom to top, consists of 1 m

of white, pyritiferous quartzite; 22 m of green igneous dike or sill

rock (trachyte or phonolite) of undetermined age; 3 m of veined

sedimentary rock and hornfels; 16 m of green igneous dike or sill

rock (trachyte or phonolite) of undetermined age; 258 m of hard

quartz sandstone overlying sandstone with some black mudstone,

purple marlstone, and red-brown beds, grading into predomi-

nantly dark gray less-calcareous mudstone and rare red mud-

stone. The Maastrichtian-Danian is represented by silty gray and

some red-brown claystone (187 m) with common sandstone but

no limestone, underlain by 40 m of pale gray marlstone and red

mudstone with foraminifera.

Morondava Basin. The Cenomanian and Turonian sec-

tion ranges in thickness from 100 to 500 m, and in southern and

central Morondava is represented by thick continental to marine

sand grading rapidly offshore to shale. Marine conditions domi-

nated the northern Morondava Basin by the end of Turonian

time. Prominent igneous activity continued from the Aptian and

peaked during late Turonian time. Basaltic lava flows, microgab-

broic intrusions, and dolerite dikes are found throughout the

basin. The flows are as much as 100 m thick, but average 30 m,

and are interbedded with sediment. Coniacian through Maas-

trichtian strata, dominantly marine shale with sandstone, silt-

stone, limestone, and marl, but in places becoming continental in

origin higher in the section, total 150 to 200 m. Volcanic and

sedimentary rocks are interbedded within the Santonian and

Campanian interval. At the East Serinam 1 well (Ml8 in Fig. 1),

1,069 m of Upper Cretaceous thick deltaic sand with interbedded

shale were recovered.

Offshore, Upper Cretaceous shale and marl with inter-

bedded volcanic rocks may attain several thousand meters in

thickness, but are usually on the order of several hundred meters.

The Chesterfield 1 borehole (M8 in Fig. 1) penetrated 710 m of

interbedded shale, sand, and basalt flows. Eloise 1 (M10 in Fig.

1) recovered 1,128 m of volcanics, pyroclastics, shale and silty

shale. Vaucluse 1 (Ml6 in Fig. 1) encountered 2,094 m of undif-

ferentiated Cretaceous pyroclastics, tuffs, and ignimbrites overlain

by 183 m of Upper Cretaceous marl, and Eponge 1 (Ml9 in Fig.

1) 1,166 m of shale with thin siltstone and sandstone beds.

Western Somali Basin. The oldest rock encountered by

drilling and coring in the Western Somali Basin is Senonian and

Maastrichtian in age, respectively. DSDP Site 241 (Fig. 1) pene-

trated at least 450 m of foram- and nanno-bearing Senonian

claystone and silt-rich claystone, and piston coring on the Davie

Fracture Zone—reported by Segoufin and others (1978) and

Segoufin (1981)—encountered several meters of Campanian-

Maastrichtian nanno and foram chalk. DSDP 235 yielded 38 m

of upper Maastriachtian basalt with sediment intercalations.

Summary. Late Cretaceous volcanism occurred in all of the

sedimentary basins except the Lamu Embayment. The Diego and

Majunga Basins contain rocks of marine, nonmarine, and mixed

facies, whereas each of the remaining basins was dominantly

marine for the epoch.

Paleocene (Fig. 12)

Diego Basin. There is no sedimentary rock in the Diego

Basin that has been unambiguously identified as Paleocene in age.

However, dolomite found at the base of the Eocene could be

Paleocene in age.

Majunga Basin. Marine conditions continued from Maas-

trichtian through Paleocene time in the Majunga Basin. The

microfauna-rich Paleocene section (bottom to top) consists of 60

m of chalky marl and marly limestone, 80 m of argillaceous

sandstone, and 120 m of dolomite and dolomitic limestone com-

monly covered by red clay. Offshore, the Mahajamba 1 well (M2

in Fig. 1) recovered 287 m of calcareous shale.

Somali Coastal Basin. Paleocene rock is abundant in

boreholes of the Somali Coastal Basin. In the Marai Ascia well

(SI in Fig. 1), the Sagaleh Formation—a foraminifera-bearing

deep-water gray shale and marl—continues into the Paleocene

from the Upper Cretaceous; Paleocene thickness of the unit is

~ 100 m. Overlying the shale is a 200-m-thick transitional zone,

the fossiliferous (foraminifera) Marai Ascia Formation, between

the shale and the overlying Auradu Limestone. The Auradu

Formation, 320 m thick in the Marai Ascia borehole, is a finely

crystalline, compact, hard, tan to light brown limestone with

local, thin, gray shale; the unit grades to a deeper water facies

toward the Somali continental margin. Rich in foraminifera, the

formation continues into the Eocene. At the Merca 1 well (S8 in

Fig. 1), the Paleocene section consists of 960 m of dark gray to

brown shale with local dark gray to brown limestone layers, and

some light gray to brown, fine- to medium-grained, calcareous,

well-cemented quartz sandstone beds. Sills of spilitic basalt in-

trude this section. Farther to the south, the epoch (plus the early

Eocene) is represented by 2,745 m (Oddo Alimo, S17 in Fig. 1)

of predominantly terrigenous quartz sandstone interbedded with

shale, mudstone, and some anhydrite.

Lamu Embayment. Dense, micritic limestone interbedded

with dark, gray-brown shale and fine-grained sandstone makes

up the ~ 200-m-thick Paleocene section. The environment of

deposition was shallow water. The Dodori well (K8 in Fig. 1)

bottomed in 1,200 m of Paleocene limestone, sandstone, and

shale, all of marine facies. At the Kipini 1 borehole (Kl l in

Fig. 1), the epoch is represented by an unconformity. Offshore,

the Simba 1 well (K14 in Fig. 1) recovered 295 m of shale and

sandstone unconformably overlying Upper Cretaceous rock.

Figure 12. Paleocene stratigraphic sections.

28 Coffin and Rabino witz Tanzanian coastal basins. The best-documented Paleo-

cene sections are in southern Tanzania and on the offshore is-

lands, where significant thicknesses are observed. In southern

Tanzania, the base of the Paleocene occurs in the middle of a

thick, fossiliferous series of gray clay with sandy and silty layers,

and local bands of limestone and marl. The thickness of the unit

ranges from less than 90 m to 600 m. Folding and faulting of the

beds on the offshore islands occurred in Paleogene time, as dem-

onstrated by severe deformation of sedimentary rock of that age.

On Pemba Island (T1 in Fig. 1), the section (dated by foraminif-

era) consists of 217 m of interbedded mudstone and siltstone with

thin, marly limestone and clay-bound sand, all overlying gray-

brown mudstone. Zanzibar (T3 in Fig. 1) records 133 m of

fossiliferous Danian-Paleocene black siltstone with shale, clay-

stone, and sporadic thin calcareous sandstone. Above are 598 m

of Paleocene strata, consisting primarily of argillaceous black

siltstone containing foraminifera, grading from shale and clay-

stone, with some hard calcareous sandstone and thin nodular

ironstone. Deep drilling on Mafia Island (T8 in Fig. 1) recovered

94 m of Danian-Paleocene transitional beds consisting of gray

and red-brown claystone, with a few detrital and algal limestone

bands, and minor sand except near the base of the unit. Above are

539 m of gray claystone containing foraminifera, with some silt-

stone and sandstone, a few thin silty limestone layers, and some

thicker current-bedded sandstone. Detrital algal limestone inter-

bedded with sandstone occurs near the base of the Paleocene

section. Paleocene rocks onshore generally dip to the east or

east-northeast.

Morondava Basin. Much of the Paleocene is missing in

the northern Morondava Basin. In the southern and central por-

tions, as much as 750 m of limestone, dolomite, and marl are

preserved. Offshore, the Chesterfield 1 well (M8 in Fig. 1) en-

countered 23 m of Paleocene dolomite. An unconformity at

Eloise 1 (M10 in Fig. 1) results in Eocene rock overlying the

Upper Cretaceous section. At the Vaucluse 1 well (M16 in Fig.

1), 50 m of Paleocene marl were recovered, and the Eponge 1

well (Ml9 in Fig. 1) records 53 m of Paleocene shale and

siltstone.

Western Somali Basin. DSDP Site 241 (Fig. 1) pene-

trated -50 m of suspect (based on foraminifera assemblage) Pa-

leocene brown claystone, and DSDP Site 240 recovered 5 m of

basalt (containing chalk inclusions) of latest Paleocene or earliest

Eocene age. Site 235 penetrated Upper Cretaceous rock, but no

Paleocene section was identified biostratigraphically; the maxi-

mum thickness would be a few tens of meters if it does exist at the

drill site.

Summary. The Paleocene strata are noted for widely vary-

ing thicknesses and numerous unconformities. The Diego Basin

contains no known rock of that epoch. The conjugate Majunga

and Somali Coastal Basins record mainly marine facies for the

epoch, as does the Lamu Embayment. Severe folding and faulting

of the marine sediment and sedimentary rock now composing the

offshore islands of the Tanzanian Coastal Basins occurred during

Paleogene time (see Coffin and Rabinowitz, 1984). Paleocene

rocks are commonly absent or thin in the Morondava Basin, but

those present are marine. The Paleocene section is quite thin in

the Western Somali Basin as well.

Eocene (Fig. 13) Diego Basin. The Eocene Series consists of two units, a

lower dolomite and basaltic tuff sequence (110m thick) and an

upper karst limestone section (150 m thick) of Lutetian age.

There was an important development of volcanism at this time,

indicated by alkaline intrusives in the western part of the basin.

Majunga Basin. A fossil-rich sequence of Ypresian age,

consisting of 35 m of limestone, marl, and sandy horizons, is

succeeded by a fossiliferous Lutetian limestone section 75 to 150

m thick in the Majunga Basin. Offshore, 850 m of middle and

upper Eocene mixed marl, sandy marl, and limestone uncon-

formably overlie Campanian marl. In the Mahajamba 1 well (M2

in Fig. 1), 1,563 m of Eocene fossiliferous limestone and shale

overlie Paleocene shale.

Somali Coastal Basin. Deposition of the Auradu Forma-

tion, a finely crystalline, compact, hard, tan to light brown,

foraminifera-bearing limestone with local, thin, gray shale hori-

zons, continued uninterrupted from Paleocene through Ypresian

time. As previously discussed, the formation grades seaward to a

deeper water shale facies. At Marai Ascai (SI in Fig. 1), the

undifferentiated Paleocene-lower Eocene is represented by

320 m of the Auradu Formation. The Taleh Formation,

consisting of 117 m of pink, very fine-grained, hard, calcareous

quartz sandstone, immediately overlies the Auradu. To the south,

the Merca 1 borehole (S8 in Fig. 1) records 430 m of lower

Eocene dark gray to brown shale of the Auradu Formation. The

shale contains some dark gray to brown limestone layers, and

also some light gray to brown, fine- to medium-grained, calcare-

ous, well-cemented quartz sandstone beds. Overlying the Auradu

are 174 m of the Taleh Formation, consisting of dary gray to dark

green, calcareous, finely micaceous shale containing glauconite

and pyrite, and a few thin sandstone beds. A very similar fossilif-

erous shale (Karkar Formation), 268 m thick, lies above the

Taleh Formation and extends to the top of the Eocene section. As

previously mentioned, 2,745 m of undifferentiated Lower Ter-

tiary clastic sediments, extending through the lower Eocene, are

preserved in the Oddo Alimo borehole (SI7 in Fig. 1) in extreme

southeastern Somalia.

Lamu Embayment. Deposition of dense, micritic lime-

stone interbedded with dark, gray-brown shale and fine-grained

sandstone continued from Paleocene through early Eocene time.

The shallow-water lower Eocene rock attains a thickness of 1,500

m. In the southeastern portion of the Lamu Embayment, the

middle and upper Eocene are represented by poorly sorted, fria-

ble, argillaceous sandstone with variable development of lime-

stone, mudstone, and lignite bands. The environment of

deposition was fluvio-littoral and deltaic. To the northwest the

marine influence disappears, resulting in a fossiliferous sequence

of variegated red-green mudstone and poorly sorted friable sand-


Figure 17. Quaternary stratigraphie sections


stone. The total thickness of the middle and upper Eocene section

is —1,000 m. The Mararani 1 borehole (K6 in Fig. 1) bottomed

in 677 m of middle Eocene limestone and mudstone, which were

overlain by 140 m of upper Eocene sandstone. Just to the south,

the Dodori 1 well (K8 in Fig. 1) penetrated a thick Eocene

section composed of 600 m of lower Eocene limestone, sand-

stone, and shale of shallow-marine facies; 1,260 m of similar

middle Eocene sediment; and 140 m of upper Eocene limestone

and sandstone. A similar sequence was recorded at the nearby

Pate 1 well (K9 in Fig. 1); the well bottomed in 1,070 m of lower

Eocene shale and limestone, and the middle and upper Eocene

are represented by 1,660 and 200 m, respectively, of limestone

and sandstone. At the Kipini 1 well (K11 in Fig. 1), 1,020 m of

middle Eocene sandstone and limestone unconformably overlie

Upper Cretaceous sediment, and underlie 310 m of similar upper

Eocene sediment. Offshore the Simba 1 well (K14 in Fig. 1)

records 495 m of lower Eocene deep-water shale with sandstone

beds; 392 m of middle Eocene deep-water clay, marl, and sand;

and 62 m of upper Eocene deep-water marl, clay, and sand.

Tanzanian coastal basins. Southern Tanzania and the

offshore islands record the most complete Eocene sections in

Tanzania, and in all locations the lower Eocene is absent. On-

shore the middle Eocene consists of less than 90 to 1,000 m of

fossiliferous green clay or marl with subordinate limestone and

thin sandstone beds topped by a possible angular unconformity.

The upper Eocene consists of 65 to 1,140 m of fossiliferous green,

gray, and brown clay, coarse-grained and reefal limestone, and

siltstone. Severe deformation of Paleogene rocks on the offshore

islands, contrasted with the relatively undeformed Neogene and

Quaternary rocks, points toward significant tectonic activity prior

to the Neogene. On Pemba Island (T1 in Fig. 1), 1,821 m of

fossil-bearing middle Eocene littoral to deltaic coarse sand, silt,

mudstone, and thin limestone overlying more argillaceous and

silty beds are present; the unit becomes increasingly neritic

downward.

The upper Eocene consists of 132 m of fossiliferous shallow-

shelf limestone with a few shaly partings, underlain by sand with

some mudstone and thin limestone beds. The Zanzibar borehole

(T3 in Fig. 1) contains 907 m of fossil-bearing middle Eocene

rock, comprising (bottom to top) 314 m of black siltstone and

shale with rare thin calcareous sandstone, a gray marly limestone

horizon, a glauconitic sandy algal limestone bed, and thin red-

brown claystone; 151 m of black shaly siltstone and silty marl

interbedded with calcareous sandstone and black silty shale; 300

m of massive detrital limestone underlain by sandy and silty

limestone, with some thin black siltstone near its base; 143 m of

black silty shale and marl, with some sandy limestone and sand

interbedded as thin stringers in the upper part of the section, and

deeper as thicker limestone horizons in the shale.

Finally, on Mafia Island (T8 in Fig. 1) the fossiliferous

middle Eocene is 994 m thick. The unit is primarily claystone; the

basal 180 m contains some sandstone and conglomerate; detrital

and algal limestone bands increase upward, and silty claystone

grades upward into gray-green argillaceous marlstone. The upper

Eocene rock is 394 m thick. The base of the section consists of

shallow-water marly siltstone with thin limestone bands, plant

fragments, and fine pyrite, and passes upward into green marl-

stone interbedded with silty (lower section) and sandy (upper

section) limestone. Above the marlstone is coral and sugary fossil-

iferous limestone, and topping the sequence is hard, gray

cavernous dolomite. Eocene rock onshore generally dips to the

east or east-northeast.

Morondava Basin. The Eocene marine carbonate se-

quence, with interbedded shale, thins from 400 to 450 m in the

south to 10 to 15 m in the north, where it also loses its marine

character. There was minor volcanic activity during the epoch.

The Chesterfield 1 well (M8 in Fig. 1) recovered 345 m of

Eocene shelf limestone and dolomite. Eocene calcareous shale

with thin limestone stringers totals 434 m at the Eloise 1 well

(M10 in Fig. 1), and unconformably overlies Upper Cretaceous

rock. At Vaucluse 1 (Ml6 in Fig. 1), 188 m of Eocene shallow-

water marl was penetrated, and farther to the south at Eponge 1

(Ml9 in Fig. 1), 1,169 m of Eocene marl and limestone were

recovered.

Western Somali Basin. DSDP Site 241 encountered 150

m of lower and middle Eocene clay and silty clay. Middle Eocene

sediment is unconformably overlain by upper Oligocene sedi-

ment. DSDP Site 240 penetrated 5 m of basalt with chalk inclu-

sions, of latest Paleocene or Eocene age, overlain by a few tens of

meters (at most) of lower Eocene nanno ooze, silty clay, sandy

silt, and sand. The middle and upper Eocene are absent. Sim-

ilarly, Site 235 penetrated, at most, a few tens of meters of clay.

Summary. All of the basins contain marine Eocene sedi-

ment; in addition, mixed facies are found in the Somali Coastal

Basin and Lamu Embayment. Volcanic episodes are recorded as

tuff in the Diego and Morondava Basins. The Paleogene rock of

the offshore islands of the Tanzanian Coastal Basins is strongly

deformed.

Oligocene (Fig. 14) Diego Basin. No Oligocene sedimentary rock has been

documented in the Diego Basin, although there are indications of

volcanic activity continuing from Eocene time.

Majunga Basin. On an island (Nosy Kalakajava), lime-

stone followed by fossiliferous marl, both beneath a trachyte

flow, make up the Oligocene section. As elsewhere in Madagas-

car, this epoch was marked by volcanic activity. The lower and

mid-Oligocene is represented offshore by 660 m of limestone,

with some marl near the base of the section. An unconformity

separates mid-Oligocene rock from the lower Miocene section.

At the Mahajamba 1 well (M2 in Fig. 1), 583 m of Oligocene

sandstone and thin dolomite were recovered.

Somali Coastal Basin. Oligocene rock totaling 936 m in

thickness was encountered at the Merca 1 well (S8 in Fig. 1). A

gray, silty, pyritic, calcareous, and partly glauconitic fossiliferous

shale 122 m thick forms the lowest portion of the section. Overly-

ing the shale are 814 m of white to gray, granular fossiliferous

limestone, bearing gypsum and anhydrite in the lower 457 m. In

32 Coffin and Rabino witz southeastern Somalia the Oligocene is represented by several

hundred meters of neritic/deltaic sandstone with shale and lime-

stone horizons.

Lamu Embayment. The Oligocene section is extremely

thin in the Lamu Embayment. Some rock of this epoch may be

present in the unfossiliferous, variegated, red-green mudstone and

poorly sorted friable sandstone of the northwestern Lamu Em-

bayment. To the southeast, an Oligocene shallow-water marine

limestone section some tens of m thick exists, representing a

complete change of environment from the underlying fluvio-

littoral and deltaic sediment. This limestone continues upward

into Miocene rock. The Mararani 1 well (K6 in Fig. 1) penetrated

30 m of Oligocene gray sandstone, locally argillaceous. Dodori 1

(K8 in Fig. 1) records 80 m of limestone, and the epoch is

represented by a few tens of meters of argillaceous sand and

sandstone in the Pate 1 borehole (K9 in Fig. 1). The Kipini 1 well

(K l l in Fig. 1) encountered 100 m of Oligocene calcareous

shale, and offshore the Simba 1 well (K14 in Fig. 1) encountered

no Oligocene beds.

Tanzanian coastal basins. The Oligocene Series is best-

preserved in southern Tanzania, and on the offshore islands. To

the south the Oligocene consists of 70 to 90 m of algal-detrital

limestone and fossiliferous buff-gray to brown-gray silty clay. On

Pemba Island (T1 in Fig. 1), 425 m of shallow marine, claybound

sand with some shale, calcareous sandstone, and thin detrital

limestone were penetrated. Deep drilling on Zanzibar (T3 in Fig.

1) recovered no rocks of Oligocene age, but the Oligocene section

on Mafia Island (T8 in Fig. 1) consists of 129 m of fossiliferous

detrital limestone commonly with coral and algae, and very little

sand, succeeded by saccharoidal dolomite, and topped by fossilif-

erous sandy detrital limestone. Oligocene strata onshore generally

dip to the east or east-northeast.

Morondava Basin. The Oligocene is absent from the

northern Morondava Basin, but as much as 400 m of marl and

limestone are present in the south. Offshore, the Series is missing

at the Chesterfield 1 well (M8 in Fig. 1). At the Eloise 1 borehole

(M10 in Fig. 1), 379 m of calcareous shale and limestone were

recovered, and the Vaucluse 1 well (Ml6 in Fig. 1) penetrated

69 m of Oligocene sand and calcareous shale. The Eponge 1 well

(Ml9 in Fig. 1) encountered 210 m of Oligocene limestone and

marl.

Western Somali Basin. At DSDP Site 241 (Fig. 1), an

extremely thin Oligocene section totaling less than 10 m of clay-

rich nanno ooze, foram nanno chalk, and nanno clay was drilled.

DSDP Site 240 (Fig. 1) demonstrated a hiatus between early

Eocene and Miocene sediments. Site 234 (Fig. 1) yielded 53 m of

lower Oligocene dark clay and nanno ooze and 32 m of upper

Oligocene blue, green, and gray clay. Site 235 (Fig. 1) recovered

no definitely Oligocene sediment; at most, a few tens of meters

could be present. Finally, Site 242 (Fig. 1) penetrated -45 m of

lower Oligocene, brown, clayey nanno chalk, and 120 m of

upper Oligocene, brown, clayey nanno chalk.

Summary. The Oligocene Series varies widely throughout

the study area; commonly it is absent or contains unconformities.

The Diego and Majunga Basins contain Oligocene volcanic rock,

and the latter marine strata as well. The other basins contain

marine Oligocene sections of varying thicknesses. The Paleogene

of the offshore islands of the Tanzanian Coastal Basins is highly

deformed.

Miocene (Fig. 15) Diego Basin. A sequence (50 to 200 m thick) of inter-

layered limestone, sandstone, and basaltic tuff composes the Mio-

cene (Aquitanian to Burdigalian) section. Lava flows on top of

this rock are considered late Miocene in age.

Majunga Basin. The Miocene in the Majunga Basin con-

sists of a richly fossiliferous marine marl and limestone interval in

excess of 200 m thick. Offshore, 350 m of lower Miocene dolo-

mite and limestone unconformably overlie mid-Oligocene rock.

At the Mahajamba 1 borehole (M2 in Fig. 1), 274 m of Miocene

dolomite and thin sandstone are preserved.

Somali Coastal Basin. The undifferentiated Miocene sec-

tion recovered from the Merca 1 well (S8 in Fig. 1) is 837 m

thick. At the base are 91 m of multicolored shale, overlain by

213 m of sandstone. Red, green, and gray silty shale totaling 107

m succeeds the sandstone, and at the top of the section are 426 m

of white to gray, hard, fine- to medium-grained, calcareous sand-

stone with some gray-green and brown soft clay layers in the

lower 91 m, along with one cream to white, finely crystalline,

gypsum-bearing fossiliferous limestone bed. At Brava 1 (SI3 in

Fig. 1), 914 m of Miocene marly limestone interbedded with

calcareous shale, and a few sandstone beds were encountered,

unconformably overlying Lower Cretaceous rock. The Oddo

Alimo (SI7 in Fig. 1) borehole records Miocene calcareous shale;

farther to the south, the facies changes to marine limestone sev-

eral hundred meters thick.

Lamu Embayment. The Miocene Series is dominated by a

shallow-water limestone sequence -1,400 m thick. This lime-

stone is locally dolomitic with veins of anhydrite. At the margin

of the embayment, the limestone becomes sandy, and to the

northwest it grades to unfossiliferous red-green variegated mud-

stone with interbedded argillaceous sandstone. The Mararani 1

borehole (K6 in Fig. 1) penetrated 893 m of lower and middle

Miocene dense limestone overlain by 175 m of upper Miocene

sandy limestone. At Dodori 1 (K8 in Fig. 1), the lower Miocene

consists of 340 m of limestone with thin dolomite; the middle

Miocene, 490 m of dolomite and limestone; and the upper Mio-

cene, 160 m of limestone. The Pate 1 well (K9 in Fig. 1) pene-

trated a similar section consisting of 380 m of lower Miocene

limestone, 460 m of middle Miocene limestone and shale, and

220 m of upper Miocene limestone. At Kipini 1 (Kl l in Fig. 1),

590 m of lower Miocene limestone and shale, 710 m of middle

Miocene shale and limestone, and 160 m of upper Miocene lime-

stone were encountered. Offshore, the Simba 1 well (K14 in

Fig. 4) contains an unconformity for the entire epoch.

Tanzanian coastal basins. All Tertiary geology of north-

ern Tanzania is very poorly known. In central Tanzania the Mio-

3 4 Coffin and Rabino witz cene Series consists of quartz sand and sandstone. These rocks are

soft, richly kaolinitic, and locally feldspathic, and are locally

current-bedded with red and green clay partings. The Miocene

rocks are commonly faulted and dip gently; they range in thick-

ness from 300 to 750 m. Miocene rock in southern Tanzania is of

shallow-water facies, and commonly shows evidence of contem-

poraneous faulting. The section consists of 55 to 1,000 m of gray

and brown clay; coral, detrital, chalky, or sandy limestone; and

silt. The lower Miocene section on Pemba Island (T1 in Fig. 1)

begins with 382 m of deltaic sandy and detrital limestone, clay-

bound sand, calcareous sandstone, marl, and lignite, with some

pebbles near its base. Overlying that is marine limestone with

sand and marl bands. Above the lower Miocene are 66 m of

middle Miocene (Langhian) calcareous silty mudstone, detrital

limestone, and sand, with traces of lignite. Because the well was

drilled on a horst, no dated younger rocks were penetrated. An

extremely thick Miocene section, totaling 2,546 m, was drilled on

Zanzibar (T3 in Fig. 1).

At the base of the lower Miocene are 265 m of black pyritic

shale interbedded with unsorted coarse or pebbly sand, kaolinitic

sandstone, and siltstone, all of paralic facies. Next are 1,165 m of

claystone and siltstone interbedded with claybound (kaolinitic)

sand, the upper 268 m of which are partially of lagoonal facies.

Deltaic claybound sand with some thin lignite beds, together with

variable amounts of thin, sandy limestone, clay, and siltstone,

make up the upper 1,116 m of the Miocene section; this includes

124 m more of lower Miocene strata and 992 m of middle

Miocene rock. No upper Miocene section was recovered from

this horst setting. The lower Miocene on Mafia Island (T8 in

Fig. 1) is represented by 205 m of detrital sugary limestone, with

sandy intervals near its base and some greenish marl. The middle

Miocene section is 386 m thick; massive detrital limestone with

an interbedded black calcareous marlstone composes the lower

301 m, and the upper 85 m consist of algal detrital sandy lime-

stone, sandy clay, and quartz sand. No upper Miocene rock was

recovered, presumably because the drill site is situated on a horst.

Miocene faulting is evident throughout Tanzania and on the off-

shore islands.

Morondava Basin. The Miocene Series is present as car-

bonate rock in small embayments in southern and northern Mor-

ondava, where it attains thicknesses of 275 m. Offshore, the

Chesterfield 1 well (M8 in Fig. 1) recovered 395 m of shallow-

water limestone and dolomite; Eloise 1 (M10 in Fig. 1) recovered

529 m of similar rock. The Vaucluse 1 borehole (Ml6 in Fig. 1)

penetrated 579 m of Miocene shale and thin sand; Eponge 1

(Ml9 in Fig. 1) encountered 628 m of dolomite, limestone, and

thin sandstone.

Western Somali Basin. All of the DSDP sites (Fig. 1) in

the Western Somali Basin and vicinity recovered sediment of

Miocene age. At DSDP Site 241, 65 m of lower Miocene clayey

nanno ooze, silty clay, and nanno clay were recovered. The mid-

dle Miocene consists of 115 m of clay, locally nanno-rich, and

clayey nanno ooze. The upper Miocene records 120 m of clayey

and clay-rich nanno ooze. Site 240 records more than 150 m of

undifferentiated Miocene through Quaternary silt, clay, and

nanno ooze. At Site 234, 91 m of lower Miocene green clay and

nanno clay, 60 m of middle Miocene green clay, and 11 m of

upper Miocene-Pliocene nanno clay and nanno ooze were pene-

trated. Site 235 recovered no lower Miocene sediment; a few tens

of meters could be present. The middle Miocene, 148 m thick,

and the upper Miocene, 161m thick, consist of dark nanno ooze

and nanno clay. At Site 242, the lower Miocene consists of 75 m

of brown clayey chalk, the middle Miocene 95 m of clayey nanno

chalk, and the upper Miocene -170 m of foram-bearing clayey

nanno ooze.

Summary. The basins record dominantly marine facies

within the Miocene Series, save for local continental facies in the

Diego, Somali Coastal, and Tanzanian coastal basins. Volcanic

rock is preserved in the Diego Basin, and extremely thick Mio-

cene sections are found on the Tanzanian islands.

Pliocene (Fig. 16)

Diego Basin. No Pliocene sedimentary rock has been re-

ported in the Diego Basin, although volcanism probably con-

tinued from Miocene time if one assumes the hot-spot model of

Emerick and Duncan (1982) to be true.

Majunga Basin. Cross- or irregularly bedded gray or red

sand and sandstone, intercalated with silty clay or rare sandy

lacustrine limestone, compose a Pliocene section 50 to 150 m

thick in the Majunga Basin.

Somali Coastal Basin. At the Merca 1 well (S8 in Fig. 1),

11 m of Pliocene clastic and carbonate deposits were recovered.

In the south, more than 500 m of limestone, clay, and sandstone

are present just to the north of the Lamu Embayment.

Lamu Embayment. The thin, undifferentiated Pliocene/

Quaternary section consists of unfossiliferous red-brown, argil-

laceous sand, and subordinate clay with limestone stringers. Dep-

ositional environments range from fluviatile and aeolian inland to

partly marine near the coast. The Mararani 1 borehole (K6 in

Fig. 1) recovered 76 m of undifferentiated Pliocene-Quaternary

gray sandstone. At Dodori 1 (K8 in Fig. 1), 160 m of Pliocene

limestone with subordinate clay and marl were encountered. At

Pate 1 (K9 in Fig. 1), sand totaling several tens of meters predom-

inates, with local fine-grained limestone. The Kipini 1 borehole

(K11 in Fig. 1) recovered several tens of meters of Pliocene sand.

Tanzanian coastal basins. In northern Tanzania, thin

marine Pliocene sediment, consisting of sandy clay, marl, marly

limestone, and sand, flanks the coast. Estuarine and fluviatile

conglomerate, sand, clay, silt, and gravel attaining thicknesses of

more than 100 m are present in central Tanzania. Both marine

and continental Pliocene deposits are present in southern Tanza-

nia; reefal limestone, rubbly sandstone, clay, and gravel as thick

as 30 m are preserved. No unambiguous Pliocene sediment was

found by drilling the offshore island horst blocks, although ex-

posed limestone beds totaling less than a few tens of meters on

Zanzibar and Mafia Islands have been dated as Pliocene.

East African—Madagascan margins and western Somali Basin 44 40°


36 Coffin and Rabino witz Morondava Basin. Red continental sandstone masking the

coastal lowlands attains a thickness of 50 to 100 m. The Vaucluse

1 well (Ml6 in Fig. 1) offshore recovered 526 m of Pliocene

shale.

Western Somali Basin. Every DSDP site in the Western

Somali Basin (Fig. 1) recovered Pliocene sediment. Site 241

penetrated 90 m of nanno-rich clay and clay-rich nanno ooze;

Site 240, 150 m of undifferentiated Miocene through Quaternary

silt, clay, and nanno ooze; Site 234, 11 m of upper Miocene-

Pliocene nanno clay and nanno ooze; Site 235, 72 m of lower

Pliocene and 79 m of upper Pliocene nanno ooze and nanno clay;

and Site 242, 60 m of lower Pliocene and 40 m of upper Pliocene

foram-bearing nanno ooze.

Summary. Thin sediment or sedimentary rock of both ma-

rine and nonmarine facies are found in all of the basins except the

Diego, where no Pliocene sediment has been identified. However,

some Pliocene volcanic rock is preserved in the Diego Basin.

Quaternary (Fig. 17)

Diego Basin. This period is dominated by coastal dune and

reef development, along with volcanism (basalt flows) that is

probably continuous since Eocene time.

Majunga Basin. Quaternary strata consist of vast alluvial

deposits in estuaries and in valleys strongly affected by Pleisto-

cene sea-level changes.

Somali Coastal Basin. The Quaternary Period is marked

by reef development and clastic deposition in the Somali Coastal

Basin.

Lamu Embayment. Unfossiliferous red-brown argilla-

ceous sand and subordinate clay, with limestone stringers, com-

pose the thin, undifferentiated Pliocene-Quaternary section. The

environments of deposition range from fluviatile and aeolian in-

land to partly marine near the coast. Thicknesses are several tens

of meters at maximum.

Tanzanian coastal basins. Thin marine sand, well-

developed limestone, and clay, totaling less than 100 m in thick-

ness, are found along the coast and on the offshore islands.

Morondava Basin. The deposition of continental sand-

stone continued from the Pliocene Epoch, and thicknesses locally

may attain several tens of meters. In the offshore area, shale

predominates.

Western Somali Basin. Quaternary beds are present at

most of the DSDP sites (Fig. 1) in the Western Somali Basin. Site

241 encountered 75 m of clayey nanno ooze, nanno ooze, and

minor sand-silt-clay; Site 240,10 m of silt-, clay-, and nanno-rich

radiolarian ooze and diatom-bearing, radiolaria-rich detrital silty

clay; Site 234, no Quaternary; Site 235,45 m of nanno ooze; and

Site 242, 40 m of foram-rich clayey nanno ooze.

Summary. Pliocene marine, nonmarine, and mixed facies

depositional conditions generally continued into Quaternary

time.

STRATIGRAPHY AND STRUCTURE; OFFSHORE ACOUSTIC STRATIGRAPHY STUDIES

The preliminary acoustic stratigraphic framework for the

Western Somali Basin and East African continental margin has

been established by Coffin and Rabinowitz (1982, 1983). These

studies have documented four major reflecting horizons, one of

which was penetrated by drilling at DDSP Site 241 (Fig. 1), the

only deep well useful for seismic correlation in the Western So-

mali Basin. The studies also displayed seismic evidence for major

events in the sedimentary history of the East African continental

margin: salt diapirs, a massive sediment slide, and an abyssal

channel system. Results of further investigations into the acoustic

stratigraphy of the region based on analysis of additional seismic

data (Fig. 18) are considered below.

The primary data base for our studies is the -6,000 km of

12-fold multichannel seismic (MCS) data collected aboard R.V.

Vema and processed at Lamont-Doherty Geological Observa-

tory, indicated by dotted lines in Figure 18; the ship's navigation

data appear in Figure 19. The data consist of 10 East African

margin transects, spaced roughly every 100 km, three across the

rifted segment (Fig. 20a) and seven across the transform portion

(Fig. 20b), and of one long tie line connecting five of the transects

(Figs. 18, 19). These high-quality data cover slightly more than

1,000 km of the -3,500 km of conjugate East African and Mad-

agascan margins. The various reflectors, described below, were

digitized using the techniques of Mountain (1981). Secondary

data, displayed in solid black lines in Figure 18, consist of single-

channel seismic data collected by the Lamont-Doherty Geologi-

cal Observatory, Woods Hole Oceanographic Institution, Deep

Sea Drilling Project, Scripps Institution of Oceanography, and

the United States Navy. Supplementary proprietary data not

shown in Figure 18 include MCS data collected by Total offshore

Kenya (-3,500 km) and by Mobil offshore Madagascar

(-10,000 km).

Correlation of DSDP results with multichannel seismic data

The stratigraphic sequence derived from correlating MCS

data with drilling results from DSDP Site 241 appears in Figure

21 (see Fig. 19 for the location). DSDP Site 241, as previously

mentioned, is the only regionally useful stratigraphic well offshore

East Africa. The well was drilled in a water depth of 4,054 m,

penetrated 1,174 m sub-bottom, and recovered 137 m of sedi-

ment, or 12 percent (Schlich, Simpson and others, 1974). The

principal drilling objective was to establish the bio- and lithostrat-

igraphic sequence with particular emphasis on (1) the post-

Karroo epeirogenic movement of East Africa; and (2) seismic

horizon identification, composition, and age. Four major reflec-

tors, the "green," "purple," "red," and "blue" horizons, are ob-

served and traced in the Western Somali Basin. The top of

oceanic basement is indicated by the blue horizon; because the

East African—Madagascan margins and western Somali Basin 3 7


47 Coffin and Rabino witz 54" 55*

Figure 18. Ship track lines and drill holes, indicating control for acoustic stratigraphy studies. Scale, 1°

of latitude =111 km.


41 42 43 4 4 4 5 4 6

Figure 19. Multichannel seismic reflection (MCS) navigation data for R / V Verna cruise 3618. The

numbers along the profiles indicate common depth points (CDP), which are indicated on the actual

MCS data. Scale, 1° of latitude = 111 km.


as

a

Figure 20. MCS data-derived transects of the East African margins with prominent reflectors indicated,

a, Rifted margin transects, b (facing page), Transform margin transects aligned along the Davie Fracture

Zone (Coffin and Rabinowitz, 1987).


42 43 44 45

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42 43 44 45

drill site is situated in the Jurassic Magnetic Quiet Zone just

landward of magnetic anomaly M25, here the reflector is of

presumed Middle Jurassic age. Rabinowitz and others (1983)

have identified marine magnetic anomalies M10 through M25 in

the Western Somali Basin, and have extrapolated derived spread-

ing rates to the Jurassic Magnetic Quiet Zone, concluding that

oceanic crust in the region is between 165 and 130 m.y. old (Kent

and Gradstein, 1985). Thus the blue reflector is time-trans-

gressive.

The "red" reflector immediately overlies oceanic basement,

and is believed to be of Middle to Late Jurassic age. It is a

particularly prominent, high-amplitude reflector that commonly

fills in basement troughs, and is marked by a stepped velocity

increase to 4.58 km/s from the overlying sediments (Coffin and

others, 1986). As previously discussed by Coffin and Rabinowitz

(1982, 1983), our age interpretations for the purple and green

horizons are modified from those reported by Schlich and others

(1974) based on new seismic velocity information (Coffin and

others, 1986). The green reflector, observed as a prominent reflec-

tor ~0.5 s subbottom, correlates with a middle Eocene through

late Oligocene hiatus at -470 m subbottom, and may be related

to a major mid-Oligocene sea-level drop documented by Vail and

others (1977) and Haq and others (1987). Above this unconform-

ity, the dominant lithologies are clay, clay-rich nannofossil ooze,

and silty clay. Beneath the unconformity, clay/claystone and silty

clay/claystone prevail. Site 241 bottomed at 1,174 m below the

sea floor in silt-rich claystone of Early Senonian age. We do not

believe that drilling at Site 241 penetrated the prominent purple

horizon observed at 1.0 to 1.1 s subbottom. By employing a

velocity of 2.2 km/s derived from multichannel velocity analysis

and sonobuoy measurements, and by extrapolating sedimentation

rates, we estimate the age of the purple horizon to be mid-

Cretaceous (Cenomanian-Albian), possibly correlating with the

major Cenomanian drop in sea level proposed by Vail and others

(1977) and Haq and others (1987).

Margins bordering the Western Somali Basin

On a gross morphological and geophysical scale, the differ-

ences between the transform and rifted margins of East Africa

were first noted by Rabinowitz (1971), and subsequently by Cof-

fin and Rabinowitz (1982, 1983). Generally, both the East Afri-

can and Madagascan rifted margins (Figs. 5,6,20a) demonstrate

typical rifted margin profiles, i.e., from land to sea, a continental

shelf (quite narrow offshore Somalia, usually 25 to 50 km wide,

and broader offshore Madagascar, as much as 100 km wide) that

extends to the shelf/slope break, a rather steep continental slope,

a well-developed continental rise, and finally the abyssal plains of

the Western Somali and Comoros Basins. Seismic transects of the

rifted passive margins have been investigated by Coffin and Ra-

binowitz (1982,1983), and in Figure 20a we present line draw-

ings of the three Vema MCS profiles across this portion of the

margin. These and the transform profiles to follow were termi-

nated landward where none of the four major reflecting horizons

could be identified on the MCS data.

In contrast to the rifted margin transects, in the vicinity of

the intersection of the Davie Fracture Zone with the East African

(Figs. 5, 6; between ~3°S and ~8°S) and Madagascan (Fig. 5;

between ~20°S and 22°S) margins, profiles across the transform

margins (Fig. 20b) essentially show a ramp from the coast to

abyssal depths. Intermediate, between ~8°S and ~20°S, the

Madagascan and East African margins appear topographically

similar to rifted margins, except that the Davie Fracture Zone

truncates the continental rise before it attains abyssal depths (Figs.

5, 6, 20b). It appears that a phase of east-west extension, or at

least crustal thinning, prior to the Middle Jurassic north-south

separation of Madagascar and Africa affected this section of the

margin. In this zone the East African shelf is <50 km wide,

whereas the Madagascan shelf is somewhat wider, in the range of

50 to 125 km. The geophysical character of the rifted East Afri-

can and Madagascan margins, and of the East African transform

margin, have been presented and discussed by Coffin and Rabin-

owitz (1987); a dearth of data prevents similar analysis of the

Madagascan transform margin. The East African transform mar-

gin has been severely affected by Tertiary tectonic activity appar-

ently related to East African rifting, but that topic is beyond the

scope of this study (Coffin and Rabinowitz, 1984).

Acoustic stratigraphy The identification of four major reflecting horizons (Fig. 21)

observed throughout much of the Western Somali Basin allowed

us to create depth-to-basement, sediment isopach, and total sedi-

ment thickness maps. The first step was to trace the various

reflectors, where present, on the various seismic profiles (Fig. 18),

beginning with high-quality MCS data (Fig. 19). Then these data

were digitized in time, enabling creation of line drawings as in

Figure 20. Depth conversion was accomplished through the ap-

plication of velocity functions derived for the region from reflec-

tion and refraction data by Coffin and others (1986), employing

the techniques of Mountain (1981). The data were then plotted

and contoured. For each map we indicate the seismic control, i.e.,

those lines on which we observed the various reflectors. In inter-

preting the seismic data we have employed the terminology of

Vail and others (1977).

Depth to basement (Fig. 22) It is illuminating to compare the depth-to-basement map

with the bathymetry of the study area (Fig. 6). Prominent physio-

graphic features—Chain Ridge (and associated features), the

western edge of the Seychelles Bank, the Amirante Arc, the Far-

quhar Group, the Cosmoledo Group, Wilkes Rise, the Comoros

Islands, and the Davie Fracture Zone (from ~9°S to ~18°S)—

are also manifested as positive or negative anomalies on the

depth-to-basement map because they are basement-controlled.

The ages of these basement features are mostly inferred. The


Chain Ridge and associated lineaments were created by relative

motion commencing ~80 Ma (McKenzie and Sclater, 1971) be-

tween the Indian and African plates. Bunce and others (1967)

reported a Cretaceous date (K-Ar) from a rock dredged from the

flank of the Chain Ridge, and DSDP Site 235 (Fisher and others,

1974) bottomed in basalt overlain by Upper Cretaceous sedi-

ment. The Seychelles are continental in origin; Proterozoic gran-

ites are common (Baker, 1963). Igneous and sedimentary rock

samples from the Amirante Ridge (Fisher and others, 1968) and

nearby sea floor (Johnson and others, 1982; Masson and others,

1982) have yielded Late Cretaceous dates, supporting recent tec-

tonic models (Masson, 1984). Basement underlying the atolls and

islands of the Farquhar and Cosmoledo groups has yet to be

sampled.

The age of the Wilkes Rise, as discussed by Coffin and

Rabinowitz (1987), is probably not significantly younger than the

oceanic crust on which it lies, although it has not been geologi-

cally sampled. A comprehensive study of the Comoros Islands

(Emerick and Duncan, 1982) concluded that the chain, along

with the Tertiary volcanic province of northern Madagascar,

make up a hotspot trace active over the past 10 million years.

Finally, Rabinowitz and others (1983) have dated the creation of

the Davie Fracture Zone as being between 165 and 130 Ma,

although analysis of seismic data in a later section documents

continuing deformation along and landward of the fracture zone

near East Africa.

The features that become more vivid on the depth-to-

basement map are those masked by sediment. Proximal to the

East African margin we observe a continuation of the Davie

Fracture Zone north of ~9°S, represented by a series of basement

ridges, peaks, troughs, and deeps parallel to the margin. This

northern extension, indicated in Figure 20 by a dotted line usually

connecting basement anomalies, is also observed on magnetic and

gravity data, and is more fully documented by Coffin and Rabin-

owitz (1987). In some of the troughs and deeps along the frac-

ture zone, the basement descends to depths greater than 10 km

below sea level. Also apparent just to the west of the southwest-

ern terminus of the Chain Ridge is a series of roughly north-

south-trending basement ridges and troughs, presumably related

to the separation of Africa and Greater India (Coffin and Rabin-

owitz, 1987). The great thicknesses of sediment on both the East

African and Madagascan margins unfortunately precludes any

discrimination of the continent-ocean boundary based on MCS

data.

Acoustic, or igneous, basement is difficult to clearly discern

on MCS data at DSDP Site 241 (Fig. 21); farther into the West-

ern Somali Basin it becomes much easier to identify it unambigu-

ously. In Figure 23a, we show a segment of line 85 (Fig. 19) on

which basement is manifested as a typical package of hyperbo-

lated reflectors. The relief of the basement surface on line 85 is

typical of the study area, with that surface usually varying be-

tween 8 and 9 s of two-way travel time. The line drawings of

margin transects (Fig. 20) corroborate this observation. Where

basement was difficult or impossible to identify by reflection

character, we used the results of 118 sonobuoy wide-angle reflec-

tion and refraction experiments to locate the sediment-igneous

crust interface by the velocity of the top of layer 2,5.4 ± 0.4 km/s

(Coffin and others, 1986). These particular velocity data were

especially important in identifying basement south of line 88 (Fig.

19), e.g., on the seaward end of line 94 (Fig. 23b). Figure 20

summarizes the landward extent of identified basement (blue) for

the MCS data.

Evidence of igneous activity is present on two of the MCS

lines proximal to the margin, as well as on single-channel seismic

data in the Western Somali Basin. The features display strong

positive magnetic signatures, and are probably basaltic. Line 90

(see Fig. 19 for the location) transected the edge of a probable

volcanic structure that rises from typical basement depths of ~8

to ~9 s of two-way travel time to just pierce the sea floor (Fig.

24a). Line 100 (Fig. 19) crossed an igneous intrusion that de-

formed sediments during its emplacement (Fig. 24b). Some indi-

cation of the age of the feature comes from the observation that

deeper reflectors show far more deformation than shallower re-

flectors. Unfortunately, the intrusion is -800 km from DSDP

241, and stratigraphic correlations proved impossible.

Jurassic Sediment (Fig. 25) The high-amplitude red reflector is most prominent prox-

imal to the East African margin, and lies so deeply buried as to be

discernible only on MCS data (see Fig. 20 for its distribution

along MCS transects). It is commonly flat-lying, although faulted

in places (Fig. 38b), and conformable with sediment below and

above; thus it probably represents a lithologic boundary as op-

posed to an unconformity (Fig. 26). As noted by Coffin and

others (1986), the red reflector correlates with the top of a high-

velocity (4.6 ± 0.3 km/s) layer overlying acoustic basement.

Offshore drilling has not yet penetrated the red reflector, so we

cannot absolutely identify the cause of the velocity discontinuity.

This high velocity, however, is not diagnostic for any particular

rock type. Keen and Cordsen (1981), for example, reported ve-

locities in that range for sandstone, shale, dolomite, and salt on

the Nova Scotian margin at depths of burial and ages similar to

those encountered for the layer on the East African margin.

However, we have tentatively identified the layer as the offshore

equivalent of a massive Middle to Upper Jurassic limestone en-

countered by drilling onshore in both East Africa and Madagas-

car (see the section on Stratigraphy and structure).

The red-blue interval consists of a regular series of low-

frequency parallel reflections, and thins from a value far in excess

of 1,000 m beneath the rise, where both reflectors disappear

beneath thick sediment, to zero thickness in the basin. The data

for this interval are probably too sparse to enable drawing signifi-

cant conclusions from the isopach map other than landward

thickening; however, as previously mentioned, the unit fills in

basement troughs, and its narrow, linear distribution may high-

light the importance of basement-controlled, margin-parallel sed-

imentation processes in the early phases of continental separation.


42 43 44 45

Figure 22. Depth (from sea level) to basement map compiled and contoured from digitized MCS data

(after Coffin and others, 1986). The basement features masked by sediment stand out on this map when

compared to the bathymetry (Fig. 6). Scale, 1° of latitude =111 km.


102

sw n e 6

Figure 23. a, Typical oceanic basement, displaying numerous diffractions, on MCS line 85. See Figure

19 for location of profile, b, Example from MCS line 94 on which igneous basement is not readily

observed; velocities derived from sonobuoy data were used to determine the location of basement (blue

horizon). See Figure 19 for location of profile.


42 43 44 45

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Figure 24. a, Igneous emplacement, probably volcanic, on M C S line 90. See Figure 19 for location of

profile, b, Igneous intrusion, indicated by seismic, gravity, and magnetic data, on M C S line 100. See

Figure 19 for location of profile.


Figure 25. Isopach map, blue reflector (basement) to red reflector (Jurassic?) compiled and contoured

from digitized MCS data. The names of physiographic features appear in Figure 6. Scale, 1° of latitude =

111 km.

Figure 26. Typical red (Jurassic?) reflector character, usually flat-lying or draping basement topography,

on line 88. See Figure 19 for location of profile.


Presumably somewhere in the red-blue interval lies the salt,

which is manifested as diapirs on the East African rifted margin

(Coffin and Rabinowitz, 1982; Rabinowitz and others, 1982).

We document the probable existence of salt on the conjugate

Madagascan rifted margin, as well as provide additional data

supporting the presence of salt on the East African rifted margin.

Figures 27a and b display Mobil MCS data in the subsurface salt

zone (near the Mahajamba 1 well, M2 in Fig. 1) offshore north-

ern Madagascar. A deformation front and scarp reminiscent of

the Sigsbee Scarp (Buffler and others, 1978; Buffler, 1983) are

striking in Figure 27a. By analogy with the Sigsbee Scarp, the

front represents the seaward limit of halokinetic deformation on

the Madagascan margin; very few coherent reflections are present

landward (southeast) of the front. A parallel profile a few tens of

kilometers to the southwest shows diapir and pillow structures

similar to those observed on the East African margin (Fig. 27b).

The diapir in the center of the figure demonstrates increasing

deformation downward in the sediment, although the salt layer is

not apparent. To the northwest (downslope) of the diapir is a

probable salt pillow similar to those observed in the North Sea

(Owen and Taylor, 1983). In Figure 28, we display an industrial

MCS profile located a few tens of kilometers northeast of line 86

(Fig. 19) on which prominent salt pillows appear seaward of the

diapirs. On additional closely spaced industrial profiles individual

diapir ridges can be traced for many tens of kilometers. The

similarities between the diapir and pillow structures, occurring at

comparable depths on the conjugate rifted margins separated by

-1,500 km, are truly remarkable.

Jurassic through mid-Cretaceous sediments (Fig. 29) Our data for the purple-red interval lie primarily along the

rifted portion of the East African continental margin. The unit

generally thickens landward, with two exceptions: a plume ex-

tends offshore from Kenya at ~4°S (possibly a Mesozoic fan

deposit), and a thick, linear trend of the interval parallels the

Somali margin between ~1°S and ~3°S (perhaps representing a

Mesozoic sedimentary ridge. These anomalies are visible in the

line drawings of Figure 20. The reflection character of the purple

reflector varies considerably throughout the study area, lending

mystery to its regional significance. A few kilometers to the

southeast of DSDP Site 241 (Fig. 21), the reflector forms a sharp,

angular unconformity with overlying sediment that demonstrates

onlap. A few kilometers to the northwest of the drill site (Fig. 21),


10 k m

Figure 27. a, Mobile MCS data from offshore northern Madagascar, showing a deformation front at the

northwest end of the line similar to the edge of the salt deformation zone (Sigsbee Scarp) observed in the

Gulf of Mexico (Buffler and others, 1978; Buffler, 1983). b, Mobil MCS data from offshore northern

Madagascar showing folding of the sedimentary section, which may be attributed to salt diapirs and

pillows.

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the horizon is conformable with the sedimentary sequence, al-

though it serves as a boundary between sequences with distinctive

reflection characters. The purple-red interval in the vicinity of the

drill site (Fig. 21) gives little hint as to its nature; the reflection

character is disrupted with a few intermittent reflections. Along

line 81 (Fig. 30a), the "purple" reflector may be seen as a hum-

mocky, rough, locally hyperbolated horizon; it probably repre-

sents an unconformity, but yet it is difficult to identify positively

as such because of the regular underlying reflection section. In

places the purple reflector appears to be an erosional unconform-

ity, as the small channel feature with onlap fill at ~6.5 s two-way

travel time demonstrates in Figure 30b. Just beneath 7 s two-way

travel time in the purple-blue interval (Fig. 30b) is a lenticular

reflection configuration, presumably representing a time of vigor-

ous bottom circulation. Farther to the south, the purple reflector

loses any character that would cause it to be identified as an

unconformity (Fig. 30c); in fact, identification of it as a unique

horizon proves impossible south of ~6.5°S. As observed in Fig-

ures 21 and 30, the reflector typically separates overlying sedi-

ment displaying even, parallel, closely spaced, fairly continuous

reflection character. Like the red-blue interval, the purple-red (or

blue) interval reveals few clues from its reflection character about

sedimentary processes, although in the former unit normal pelagic

sedimentation may be indicated. The dominantly reflection-free

character of the unit, especially in the south, suggests a uniform

shale lithology. The possible Mesozoic fan and sedimentary ridge

both trend to the northeast toward the major ocean basin of the

time, the Tethys (Norton and Sclater, 1979), and thus may be

used to infer paleocirculation patterns. This circulation probably

intensified with proximity to Tethys, and may account for the

conformity of the purple horizon in the south and its more uncon-

formable nature in the north.

Mid-Cretaceous through upper Oligocene sediment (Fig. 31) The mid-Cretaceous to late Oligocene section shows a

general increase in thickness toward the margin, attaining a max-

imum observable thickness in excess of 2,000 m offshore the

Kenya-Somalia border (see MCS data line drawings in Fig. 20).

Its distribution is most interesting in the vicinity of the intersec-

tion of the Davie Fracture Zone, with the Kenyan margin be-

tween ~3°S and ~5°S; the well data discussed in the previous

section indicate renewed subsidence landward of the Davie Frac-

ture Zone during the green-purple interval, which is corroborated

by the isopach map. In contrast to the underlying purple-red

interval, the reflection character of the unit indicates an energetic

sedimentation regime. The purple reflector, as previously noted

and here more fully documented, becomes unconformable with

overlying sediment toward the north of the study area, i.e., along

the rifted portion of the East African margin. A prime example of

this unconformable nature is displayed in Figure 32, in which

parallel, even reflectors (perhaps turbidites) onlap the purple ho-

rizon to the southeast of DSDP Site 241. The green reflector most

often forms an unconformity with overlying sediment on the

continental rise, but with increasing water depth it becomes con-

formable with sediment both above and below.

Along the East African rifted margin, the purple horizon

acted as a décollement surface for a major sedimentary event, a

huge (-20,000 km2) sediment slide discussed by Coffin and Ra-

binowitz (1982,1983). In Fig. 33 (a and b), we further document

this olistostrome that involves sediment of practically the entire

green-purple interval. Farther to the south, sliding on a smaller

scale (observed only on line 88) involved the purple horizon and

underlying sediment as well (Fig. 33c). The faulting within the

slides is thrusting of upslope sediment over downslope, with in-

tensity of deformation decreasing downslope. The major sedi-

ment slide (Fig. 33, a and b) merges upslope with the diapir

province, with no clear demarcation between the two styles of

deformation. On both lines 82 and 84, the toe of the olistostrome

is marked by a clear transition from a chaotic and contorted

reflection configuration within the slide to a parallel or subparal-

lel, even configuration seaward of the slide. The red and blue

reflectors cannot be traced for more than a few tens of kilometers

landward of the toe of the slide. On line 84 (Fig. 33b), some

faulting of the red reflector is observed circa CDP 3600, although

the reflection-free and chaotic reflection zones above allow dating

of this faulting only as definitely pre-late Oligocene (green) and

probably pre-mid-Cretaceous (purple).

The upper surface of the olistostromes on all three of the

seismic lines (Fig. 33) lies either at or just beneath the green

reflector that forms a prominent unconformity with sediment

both above (onlapping) and below (truncated). It appears that

sediment filled in depressions in the irregular upper surface of the

olistostromes following gravity sliding; then, some of these

deposits—and in places the olistostrome itself—were truncated

by a major erosional event that resulted in the green reflector,

perhaps correlating with a proposed major mid-Oligocene sea-

level drop (Vail and others, 1977; Haq and others, 1987). As

discussed by Coffin and Rabinowitz (1983), the age and cause of

the sliding is problematic. The best determination for the age is

middle Eocene to early Oligocene, and the two most probable

causes are halokinesis and tectonic activity in East Africa associ-

ated with initiation of the present phase of rifting. We consider it

important to note that olistostromes marked by severe internal

deformation, including thrust faults, may form on passive conti-

nental margins.

The conformity of the green reflector with sediment beneath

and above in deeper portions of the Western Somali Basin is

demonstrated in Figure 34a. The green-purple interval consists of

parallel, low-amplitude, even reflections. However, at continental

rise depths (Fig. 34b), the reflector becomes a striking unconfor-

mity. In the center of Figure 34b, the green-purple interval thins

to essentially zero thickness; the evidence for vigorous sedimenta-

tion and sediment transport (bedforms and wavy and lenticular

reflection configurations), as well as erosional (canyons and

channels) regimes, within the unit is overwhelming. At similar

water depths on a profile —100 km to the north (Fig. 34c), the

unconformities are not as marked, yet there is a wide variety of


Figure 29. Isopach map, purple reflector (mid-Cretaceous?) to red reflector (Jurassic?), compiled and

contoured from digitized MCS data. The names of physiographic features appear in Figure 6. Scale, 1°

of latitude =111 km.


Figure 30. a, The purple reflector, a hummocky, rough, locally hyperbolated horizon, is probably an

unconformity on MCS line 81. See Figure 19 for location of profile, b, A small channel (CDP-8750) on

the purple reflector, representing an erosional unconformity, appears on MCS line 88. See Figure 19 for

location of profile, c, In the south, on MCS line 90, the purple reflector is conformable. See Figure 19 for

location of profile.


42 43 44 45

88 NW SE

10 K M

2700 C D P 3 2 0 0


Figure 31. Isopach map, green reflector (upper Oligocene) to purple reflector (mid-Cretaceous?),

compiled and contoured from digitized MCS data. The names of physiographic features appear in

Figure 6. Scale, 1° of latitude =111 km.


84 DSDP 241

N W ^ E 5

1 0 K M

Figure 32. Sediments of the green-purple interval lapping onto the purple horizon. See Figure 19 for

location of MCS profile 84.

reflection configurations within the purple-green interval, indica-

tive of high-energy sedimentary processes. Contorted, chaotic,

hummocky, wavy, and shingled clinoforms are apparent. Similar

reflection configurations are observed in places at slightly greater

depths (Fig. 34d) within the interval, again indicating an ener-

getic bottom environment during Late Cretaceous and Paleogene

time.

Upper Oligocene through Quaternary sediments (Fig. 35) The upper Oligocene sea-floor sediment package attains a

maximum thickness of 2,000 m landward of the Davie Fracture

Zone, a region of active Tertiary and Quaternary subsidence. The

overall distribution of the interval reflects a combination of fairly

well-known basement structure (see Figs. 20,22) and much more

poorly known abyssal circulation. Piston cores document nonde-

position and/or severe erosion, and hence an extremely vigorous

physical oceanographic regime, by recovering Cretaceous sedi-

ment exposed on the Davie Fracture Zone ridge (Segoufin and

others, 1978; Segoufin, 1981). Upper Miocene and upper Plio-

cene sediment has been recovered in piston cores from the floor

of the Western Somali Basin (L-DGO unpublished data). Upper

Cretaceous sediment has been sampled from the floor of the

Amirante Passage (Johnson and others, 1982; Masson and oth-

ers, 1982; Masson, 1984). Thick deposits of the unit in the south

(Fig. 25) may represent material carried through the Amirante

Passage into the Western Somali Basin. Seismic reflection data

also indicate that the Neogene and Quaternary have been periods

of energetic circulation in the Western Somali Basin.

The continental shelves of Somalia, Kenya, Tanzania, and

Madagascar are incised by deep (as much as 700 m) canyons

presumably eroded severely during glacial epochs, and remaining

relatively quiescent during interglacial times. In Figure 36 (a and

b), we display two strike profiles offshore Somalia and Tanzania,

respectively, documenting these canyons. Lack dense seismic cov-

erage, especially strike lines, prevents us from tracing the canyons

onto the slope, rise, and abyssal plain provinces. We do, however,

observe broad channels on the lower rise and abyssal plain (Fig.

37, a and b), which are in all probability related to sediment

erosion and transport from shallower depths. In Figure 37a, the

wavy and subparallel reflections and hummocky clinoforms be-

tween the sea-floor channel and the green reflector point to an

active Neogene and Quaternary depositional environment.

Farther to the south, along line 99 (Fig. 37b), the entire sea


5 « j > i *->-ro>Q)— Eo ) « 3ro>< ->-"-ro>a»— —• — E® 3

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89 sw NE

M.*" V";. n!.

C O P 7300 ^ • 6800

10 K M

Figure 34. a, In deeper portions of the Western Somali Basin, the green reflector is conformable with

both underlying and overlying sediment. See Figure 19 for location of M C S profile 89. b, At continental

rise depths, on M C S line 90, the green reflector is a striking unconformity. See Figure 19 for location of

profile, c and d, The purple-green interval shows reflection configurations indicating high-energy sedi-

mentary processes. See Figure 19 for location of M C S profiles 88 and 81.


42 43 44 45

rWr^SBfS

N'ivf.s.'ss«;.-:

3000

Figure 35. Isopach map, sea floor to green reflector (upper Oligocene), compiled and contoured from

digitized MCS data. The names of physiographic features appear in Figure 6. Scale, 1° of latitude =

111 km.


42 43 44 45


floor-blue interval consists of parallel, though in places discon-

tinuous, reflections, indicating a lower energy environment.

In addition to canyons and channels incising the sea floor,

we observe many such relic features. Toward the southwest end

of line 83 (Fig. 36a) on the Somalian shelf, numerous buried

canyons are apparent. They cannot be dated due to a lack of

stratigraphic control on the shelf. However, as previously dis-

cussed by Coffin and Rabinowitz (1982, 1983), we observed

major analogues at abyssal depths near DSDP Site 241 (Fig.

38a) and elsewhere along the margin (Fig. 38b and c). The relic

channel along line 81 (Fig. 38a) has been dated as Miocene, and

the other relic canyons and channels may very well be of similar

age. The middle Miocene was a time of intense tectonic activity

in East Africa, associated with the present phase of rifting (Kent,

1974), and Vail and others (1977) and Haq and others (1987)

have reported several drops in sea level during Miocene time;

both factors could account for shelf overloading, slope instability,

and generation of turbidity currents to create canyons and

channels.

The relic abyssal channels (Fig. 38) display several hundred

meters of thalweg-to-levee relief, are 10 to 35 km wide, are

buried by several hundred meters of sediment, and display a pair

of high-amplitude reflectors, presumably lag deposits, in their

thalwegs. Above the strong reflectors, the reflection configuration

of the channel fill is usually hummocky with some parallel reflec-

tions, probably representing alternating intervals of downslope

and pelagic sedimentation after significant erosion of the channels

ceased. Generally, the sea floor-green interval outside the relic

channels consists of parallel to subparallel, even reflections indic-

ative of a fairly low-energy depositional environment. In both

Figures 38b and 38c the thalweg cuts into the purple reflector,

and numerous hyperbolae (probably side-diffractions) appear on

the MCS data. The increased thickness of the purple-red interval

beneath the relic channel on line 84 (Fig. 38b) is unusual; the

channel occurs at the apex of the deposit, and the high-amplitude

red reflector at ~8 s two-way travel time remains flat except for

local faults beneath the channel. Between the channel thalweg

and the deep reflectors there are few coherent reflections. The

increased thickness of the purple-red unit may represent a sedi-

ment drift or shale diapir; it is possible that the feature and the

channel are genetically linked.

A more diffuse expression of Neogene and Quaternary sed-

imentary processes is manifested on MCS data to the south of the

well-defined abyssal channels (Fig. 39, a-c). The vigorous abyssal

circulation that prevailed in Late Cretaceous and Paleogene time

in the north of the study area extended to the south during Neo-

gene and Quaternary time (sea floor-green interval). Figure 39a

and 39b adjoin at CDP 3350, and document active Late Tertiary

and Quaternary sedimentation processes on the continental rise.

A wide variety of reflection configurations is observable, repre-

senting high-energy and varying conditions, including numerous

angular unconformities, erosional truncations, hummocky clino-

forms, contorted and chaotic reflection zones, possible migrating

sediment waves, and shingled reflections. To the north, line 92

(Fig. 39c) shows a similar character for the sea floor-green inter-

val, confirming a pervasively energetic continental rise and abys-

sal plain environment during the Late Tertiary and Quaternary

along the East African margin.

The omnipresence of the green reflector throughout the area

for which we have high-quality MCS coverage is quite remarka-

ble. We observe onlap onto the green surface at both abyssal and

rise depths (Fig. 40a and b, respectively). As reported at DSDP

Site 241 (Schlich and others, 1974), the green horizon correlates

with a hiatus: upper Oligocene overlies lower middle Eocene

sediment. Local factors asice (e.g., renewed East African rifting),

our analysis regarding the ubiquity of the green horizon lends

credence to the major mid-Oligocene eustatic sea-level drop re-

ported by Vail and others (1977) and Haq and others (1987). The

conjunction of salt tectonics with a vigorous physical oceano-

graphic regime and active downslope/cross-slope sedimentary

processes produces an extremely complex seismic record, as doc-

umented in Figure 41. From the northwest to the southeast, the

diapir province merges into the sediment slide at depth. Between

the upper surface of the diapirs and sediment slide, and the prom-

inent unconformity marking the green reflector, are examples of

onlap, complex, chaotic, and prograded basin fill. The sea floor-

green unit manifests previously discussed reflection configurations

indicative of high-energy environments.

Between CDP 2050 and 2250 in the interval we observe

what Vail and others (1977) would consider a "complex sigmoid-

oblique" seismic reflection configuration, representing a variable-

energy depositional environment. However, the configuration

faces the wrong direction, i.e., the apparent direction of prograda-

tion is upslope! The horizontal nature of the underlying reflec-

tions indicates that this is an original depositional configuration;

one possibility is that the deposit is a contourite (Heezen and

Hollister, 1964).

Total sediment thickness (Fig. 42) Distribution of sediment in the Western Somali Basin is

strongly influenced by basement structure (see Fig. 22). The most

notable accumulations of sediment occur (1) along the Davie

Fracture Zone, (2) surrounding the Wilkes Rise and Comoros

Islands, (3) along the Chain Ridge and other basement ridges in

the northeast of the study area, (4) along the Amirante Arc, and

(5) on the conjugate rifted Madagascan and East African mar-

gins. The Davie Fracture Zone's northern extension (Fig. 20b,

and Coffin and Rabinowitz, 1987) consists of a north-

south-trending basement ridge flanked by troughs. In places the

ridge is buried by only a kilometer of sediment, whereas the

landward trough may contain in excess of 8 km of sediment.

Figure 43 displays a transect across the landward trough, with the

Davie Fracture Zone ridge to the southeast (CDP 7800) acting as

a sediment barrier and creating a divergent fill basin. The dis-

rupted reflectors on the northwestern flank of the Davie Fracture

Zone indicate normal faulting down to the northwest, but be-

cause stratigraphic control is extremely poor landward of the


42 43 44 45

Figure 37. a and b, Broad sea-floor channels at abyssal depths on the East African margin. See Figure 19

for location of MCS profiles 93 and 99.


42 43 44 45

6 8 Coffin and Rabinowitz

<0

92

NW

S

E4

a'

Fig

ure

39.

a,

b,

and

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Refl

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in th

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ent

lappin

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eflecto

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abyssal depth

s o

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CS line 8

8. See F

igure

19

for

locati

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, b, Sedim

ent

lappin

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CS line 8

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3 84

NW

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CS re

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84.


Figure 42. Total sediment thickens map compiled and contoured from digitized MCS data (after Coffin

and others, 1986). Maximum sediment thickness exceeds 8 km on the East African margin and 5 km on

the Madagascan margin. Scale, 1° of latitude =111 km.


42 43 44 45

90 1 NW SE

10 K M

Figure 43. The sediment-filled trough landward of the Davie Fracture Zone, where total sediment

thickness exceeds 8 km. See Figure 19 for location of MCS profile 90.

Davie Fracture Zone, the age of faulting is unclear. We observe

similar faulting, apparently basement-controlled, associated with

the Davie Fracture Zone on all of our transform margin transects

(Fig. 20b). Significant seismic activity is associated with the

Davie Fracture Zone, with normal faults breaching the sea floor

(Coffin and Rabinowitz, 1984, 1987; Mougenot and others,

1986): perhaps we are witnessing the reactivation of a zone of

weakness.

More intense faulting is observed at rise and shelf depths on

the Tanzanian and Kenyan margins landward of the Davie Frac-

ture Zone, as evidenced by the disrupted reflectors of Figure 44a

and b. In Figure 44a we observe three major faults between CDP

1100 and 1400, all downthrown to the east. Between CDP 1500

and 1700 are two faults with a graben formed between them.

Each of the five faults along line 102 is manifested on the sea

floor. On line 90 we observe more intensely disrupted reflections,

with numerous faults between CDP 9400 and 9900 (Fig. 44b).

These appear to be listric normal faults, with fault planes dipping

to the southeast, as evidenced by the rotation of individual blocks.

The deformation and its implications vis-à-vis East African rifting

are beyond the focus of this study (see Coffin and Rabinowitz,

1984); suffice it here to state that both well and CDP seismic data

point to significant late Tertiary and probably Early Tertiary

tectonic activity (subsidence and associated normal faulting) as-

sociated with the Davie Fracture Zone and the region between it

and the main East African rifts.

The moat surrounding the Wilkes Rise (Figs. 6,42) contains

significant accumulations of sediment, in places exceeding 3 km.

The Comoros Islands have shed volcanic aprons around their

flanks, leading to greater than average sediment thicknesses. The

Amirante Trough is extraordinary for the abyssal realm, far from

any nonmarine sediment source: total sediment accumulation ex-

ceeds 2 km. The Chain Ridge, together with associated ridges and

troughs (Figs. 6, 42), acts as major controls on sedimentation;

total sediment thickness ranges from less than 1 km to more than

4 km.

The gross thickness of sediment on the passive conjugate

rifted margins of East Africa and Madagascar exceeds 8 and 5

km, respectively. These are among the oldest in situ passive mar-

gins, and thus such figures are not surprising. Yet because base-

ment could not be traced very far landward on either margin, it

may be assumed that total sediment accumulations on these mar-

gins are greatly in excess of thicknesses depicted in Figure 42.

CONCLUDING DISCUSSION The salient results of our investigations into the geologic

evolution of the margins and ocean basin created by the separa-

tion of Madagascar and Africa may be summarized as follows:

1. A long (-150 m.y.) episode of recurrent rifting, subsi-

dence, and uplift, and possible crustal extension and thinning as

well, is recorded by the Karroo sedimentary rock of the conjugate

East African and Madagascan margins. The duration of Karroo

sedimentation was therefore longer in the study area than in the

main Karroo basins to the south (Cox, 1970). The sedimentary

basins began forming in Permo-Carboniferous time, with the de-

position of conglomerate and breccia commonly overlying base-

ment. Intermittent rifting along faults trending similar to the


102

g 10 K M

Figure 44. Major faults demonstrating Tertiary and Quaternary extension of the East African continen-

tal margin, a, Faults on continental rise. See Figure 19 for location of MCS profile 102. b, Faults on

continental shelf. See Figure 19 for location of MCS profile 90.


42 43 44 45

earliest observed orientations (northeast in the conjugate Majun-

ga/Diego and Somali Coastal Basins, and north-northwest and

north-northeast in the Lamu Embayment and conjugate Tanzan-

ian Coastal and Morondava Basins) continued until the initiation

of sea-floor spreading between Madagascar and Africa in Middle

Jurassic time.

2. The Karroo sedimentary rock, although predominantly

continental, contains many marine units. Most notable of these

are (1) salt deposits in Tanzania (Permian through Bajocian) and

in what are now the conjugate rift basins of Majunga/Diego and

Somalia (with diapirism continuing to the present day), and (2)

the marine strata of Middle Permian through earliest Triassic age

in the Diego Basin.

3. During latest Early Jurassic and Middle Jurassic time, all

of the basins record a major facies change from dominantly con-

tinental to overwhelmingly marine. Extrusive igneous rocks of

those series have been identified in the Somali Coastal Basin and

in the northern reaches of the Morondava Basin, and intense

faulting has been recorded in all of the basins for that time

interval.

4. From Late Jurassic through the end of Mesozoic time,

mixed facies are encountered in all of the onshore basins. The

Late Cretaceous marked an episode of widespread volcanism in

the region; igneous rocks have been encountered in all basins

except the Lamu Embayment. Offshore on MCS data we observe

two strong reflecting horizons (red and purple), which are judged

to be Mesozoic in age. The former is probably a lithologic boun-

dary; the latter, a major erosional event and/or hiatus that

marked the beginning of vigorous abyssal circulation in the

Western Somali Basin.

5. The hallmarks of the Tertiary System are (1) numerous

unconformities and hiatuses, especially in the Paleocene and

Oligocene series; (2) volcanism in the Diego Basin and the Co-

moros Archipelago; and (3) intense tectonism on the islands and

subbasins offshore Tanzania and Kenya, and on the Davie Frac-

ture Zone ridge. A major sediment slide occurred offshore Soma-

lia and Kenya during mid-Tertiary time; we observe a major

unconformity (green reflector), generally at slope and rise depths.

In Neogene and Quaternary deposits, offshore MCS data reveal a

major network of canyons and channels. An energetic physical

oceanographic regime persisted in the Western Somali Basin

from mid-Cretaceous through much of Cenozoic time.

6. Depth-to-basement and sediment isopach maps reveal

the importance of the Davie Fracture Zone as a barrier to sedi-

ment shed from the transform margin of East Africa; they high-

light the thick stratigraphic sections encountered on the East

African and Madagascan margins (8+ and 5+ km, respectively).

Conceptual and global implications

While the general intent of this study has been to detail the

geologic evolution of the conjugate East African-Madagascan

margins and Western Somali Basin, our results have implications

for more global and conceptual topics, including plate tectonics,

rifting, basin formation and subsidence, sea-level change, and

paleoceanography. The discussion that follows is a qualitative

attempt to place our results in a broader perspective and contri-

bute to an understanding of these subjects.

Geologic data and inferences from both onshore (outcrop

and wells) and Deep Sea Drilling Project sites, until the identifi-

cation of marine magnetic anomalies in the Western Somali

Basin (Ségoufin and Patriat, 1980; Parson and others, 1981; Ra-

binowitz and others, 1983), have been used to support a variety

of paleopositions for Madagascar in Gondwanaland, e.g., north:

du Toit (1937), Smith and Hallam (1970), Craddock (1979),

Cannon and others (1981); autochthonous: Dixey (1960),

Flower and Strong (1969), Darracott (1974), Kamen-Kaye

(1978, 1983), Tarling (1981); and south: Flores (1970, 1984),

Wright and McCurry (1970), Green (1972), Kent (1972, 1973),

Tarling (1972), and Burke and Whiteman (1973). In reviewing

the stratigraphic evidence, we are struck by its ambiguity vis-à-vis

the paleoposition of Madagascar in Gondwanaland (with the

exception of conjugate diapir provinces on the rifted Madagascan

and East African margins); the constancy of structural trends in

the conjugate basins seems far more consistent and reliable. Given

the incompleteness of the geologic record (Ager, 1980) and the

lateral geologic variations along present-day rifts (Mohr, 1982),

we conclude that extreme caution must be exercised in construct-

ing or even fine-tuning a plate tectonic model from stratigraphic

data alone.

The seismic reflection data across the conjugate rifted mar-

gins of East Africa and Madagascar do not provide enough in-

formation at depth to distinguish between pure shear (McKenzie,

1978) and simple shear (Wernicke, 1981) modes of extension.

However, the geological and geophysical data do provide some

constraints for such models. One remarkable aspect of basin de-

velopment recorded by the sediment of the conjugate East Afri-

can and Madagascan margins is its duration of ~ 150 m.y. prior to

obvious sea-floor spreading. Although the rifting accompanying

basin development may not have been continuous throughout the

pre-breakup interval, such a long history could imply significant

weakening, thinning, and stretching of the East African/Mada-

gascan continental crust. Substantial east-west stretching between

Madagascar and Africa prior to breakup is indicated (although

undated) by anomalous crust landward of the Davie Fracture

Zone on both margins and by onshore geophysical data in Kenya

(Reeves and others, 1987). Limited pre-breakup volcanic activity

is recorded in the sedimentary record. Following breakup, the

marginal basins underwent a large amount of subsidence. The

conjugate margins' subsidence history and geologic record are

complicated by widespread regional volcanism (Late Cretaceous)

and tectonism (Tertiary/Quaternary in Tanzania landward of the

Davie Fracture Zone) following breakup.

The East African margin has long been a region of tectonic

instability, as previously discussed. Another interesting aspect of

the margin involving rifting concerns the extensional orientation

and history of the conjugate Morondava and Tanzanian Coastal


Basins. Both experienced recurrent east-west extension from

Permo-Carboniferous through Middle Nurassic time. All such

activity ceased as Madagascar and Africa separated by north-

south drift from Middle Jurassic through Early Cretaceous time.

In the Tertiary Period, east-west extension resumed (after a 100-

m.y. hiatus) in the Tanzanian Coastal Basins, while the Moron-

dava Basin remained quiescent. This long-term instability at the

eastern edge of the present African craton (qualifying as a "zone

of weakness"), and its ability to change orientation and intensity,

hints at a durable underlying mantle process most definitely de-

serving investigation.

Our investigations into the stratigraphy of the margins and

the Western Somali Basin provide several clues about the Paleo-

zoic and Mesozoic Tethys Ocean. The deposition of marine sed-

iment in the Diego Basin from Middle Permian through earliest

Triassic time may indicate an arm of the Tethys extending to the

south during that time. The source of repeated marine incursions

during Karroo time in all of the sampled basins, depositing, for

example, salt in Tanzania, may also have been the Tethys. How-

ever, the Early and Middle Jurassic migration of marine facies

northward across the Majunga Basin from the northern Moron-

dava Basin hints at a southern ocean source during that time.

The tectonic model for the area beginning in Middle Jurassic time

(McKenzie and Sclater, 1971; Norton and Sclater, 1979) allows

for a northern derivation of the sea following the breakup of

Madagascar and Africa.

The major sediment slide that probably occurred in mid-

Tertiary time serves to emphasize that the formation of olisto-

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Beltrandi, M. D., and Pyre, A., 1973, Geological evolution of southwest Somali,

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stromes characterized by severe internal deformation, including

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compressional plate boundaries. This is an especially important

point for investigators working in regions that have undergone

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The acoustic stratigraphy studies of the margins and West-

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MANUSCRIPT ACCEPTED BY THE SOCIETY JANUARY 2 5 , 1 9 8 8

Typeset by WESType Publishers Service, Inc., Boulder, Colorado Printed in U.S.A. by Imperial Printing Co., St. Joseph, Michigan

P Q A ADPU'*-"""̂ u . O . n » n l \ u n E , . . .

THE GEOLOGICAL SOCIETY OF AMERICA 3300 Penrose Place, P.O. Box 9140 Boulder, Colorado 80301

Contents Acknowledgments v

Abstract 1

Introduction

Stratigraphy and structure; Surface geology and borehole results 2 Pre-Jurassic 12 Lower Jurassic 15 Middle Jurassic 18 Upper Jurassic/Lower Cretaceous 21 Upper Cretaceous 24 Paleocene 26 Eocene 28 Ollgocene 30 Miocene 32 Pliocene 34 Quaternary 36

Stratigraphy and structure: Offshore acoustic stratigraphy studies 36 Correlation with DSDP results with multichannel seismic data 36 Margins bordering the Western Somali Basin 43 Acoustic stratigraphy 43 Depth to basement 43 Jurassic Sediment 44 Jurassic through mid-Cretaceous sediments 49 Mid-Cretaceous through upper Oligocene sediment 52 Upper Oligocene through Quaternary sediments 57 Total sediment thickness 64

Concluding Discussion 73 Conceptual and global implications 75

References 76

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Evolution of the conjugate East African - Madagascan margins and the western Somali Basin

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