3-Sb. Near the sea-bottom, the interval is cut by an ero- ...... The data was then corrected for drift and ...... for Teknisk Utveckling, Slutrapport STU 75-5084.
THE CONTINENTAL MARGIN OF WESTERN COTE D'IVOIRE: STRUCTURAL FRAMEWORK INHERITED FROM INTRA-CONTINENTAL SHEARING
Guy C de Caprona
GEOLOGISKA INSTlTUTlONEN I'uhl. A 69 1992
THE CONTINENTAL MARGIN OF
WESTERN COTE D'IVOIRE:
STRUCTURAL FRAMEWORK INHERITED
FROM INTRA-CONTINENTAL SHEARING
GUY C de CAPRONA
AKADEMISK AVHANDLING som for avlaggande av filosofie doktorsexamen yid Goteborgs universitet kommer att forsvaras offentligt fredagen den 22 maj 1992 kl. 13.15 i A3-salen, Chalmers tekniska hogskola, Sven Hultins gata 6, Goteborg. Fakultetsopponent: Jean-Claude Sibuet, Ifremer Centre de Brest, Frankrike Examinator: Professor Sven Ake Larson, Goteborg
THE CONTINENTAL MARGIN OF WESTERN COTE D'IVOIRE: STRUCTURAL FRAMEWORK INHERITED FROM INTRA-CONTINENTAL SHEARING Guy C de Caprona, Department of Geology, Chalmers University of Technology and University of Goteborg, S-412 96 Goteborg, Sweden ABSTRACT The continental margin of western cote d'Ivoire, in the northern Gulf of Guinea, extends along the continental termination of the Saint Paul Fracture Zone. The objective of this thesis is to understand if the faults, horsts and grabens identified on this former plate boundary were formed in the Early Cretaceous intra-continental shearing between West Africa and northern Brazil. Secondary objectives are to understand the effects on this margin of the subsequent oceanic-continental shearing and of the present passive stage. For this purpose, the present-day intra-continental shearzones of the San Andreas Fault and of the Dead Sea Transform Fault are reviewed. Active oceanic-continental wrenching and the following passive phase of sheared margins are discussed, with examples taken from the Gulf of California. The multi-channel reflection seismic, gravity and magnetic data, from a 2,370 km non-exclusive survey is used for the seismic stratigraphic and structural interpretation of this margin. These data were recorded by GECO (today GECOPRAKLA) in 1986 on the continental shelf and upper slope of western Cote d'Ivoire (from 5 deg 30 min W to 7 deg 30 min W, i.e. the Liberian border) . Three stages of evolution of sheared margins are observed. (a) The west Ivorian margin was first structured during an Albo-Aptian intra-continental shearing of essentially transtensional character. (b) foundering of blocks, thermal upheaval of blocks and a regional reduction in subsidence rates are likely expressions of the subsequent Cenomanian Lower Senonian continent-oceanic shearing. (c) From the Senonian to Present, the margin is passive. It is was subject to a reduced thermal upheaval until the Paleocene, and thereafter to post-shearing flexural subsidence. The structural interpretation and the proposed stratigraphic sequence of the western Cote d'Ivoire margin are in agreement with those of the rest of the African margin controlled by the Saint Paul Fracture Zone; with those of the Brazilian conjugate margin; and, with those of the western Ghanaian margin, controlled by the Romanche Fracture Zone. Key words: Equatorial Atlantic margins, Cote d'Ivoire, gravimetry, fracture zones, Ivory Coast, magnetometry, marginal ridges, reflection seismic, seismic stratigraphy, shear motion, structural geology, transform margins. ISSN 0348-2367 ISBN 91-7032-685-1
Publ. A 69, 1992
Chalmers tekniska hogskola och Giiteborgs universitet GEOLOGISKA INSTITUTIONEN S-412 96 G6teborg Te!' 031-722040
Guy C de Caprona
THE CONTINENTAL MARGIN OF WESTERN COTE D'IVOIRE: STRUCTURAL FRAMEWORK INHERITED FROM INTRA-CONTINENTAL SHEARING
ISBN 91-7032-685-1 ISSN 0348-2367
Pub!. A 69 Dissertation G6teborg 1992
i
THE CONTINENTAL MARGIN OF WESTERN COTE D'IVOIRE: STRUCTURAL FRAMEWORK INHERITED FROM INTRA-CONTINENTAL SHEARING Guy C de Caprona, Department of Geology, Chalmers University of Technology and University of Goteborg, S-412 96 Goteborg, Sweden ABSTRACT The continental margin of western Cote d'Ivoire, in the northern Gulf of Guinea, extends along the continental termination of the Saint Paul Fracture Zone. The objective of this thesis is to understand if the faults, horsts and grabens identified on this former plate boundary were formed in the Early Cretaceous intra-continental shearing between West Africa and northern Brazil. Secondary objectives are to understand the effects on this margin of the sUbsequent oceanic-continental shearing and of the present passive stage. For this purpose, the present-day intra-continental shearzones of the San Andreas Fault and of the Dead Sea Transform Fault are reviewed. Active oceanic-continental wrenching and the following passive phase of sheared margins are discussed, with examples taken from the Gulf of California. The multi-channel reflection seismic, gravity and magnetic data, from a 2,370 km non-exclusive survey is used for the seismic stratigraphic and structural interpretation of this margin. These data were recorded by GECO (today GECOPRAKLA) in 1986 on the continental shelf and upper slope of western Cote d'Ivoire (from 5 deg 30 min W to 7 deg 30 min w, i.e. the Liberian border) . Three stages of evolution of sheared margins are observed. (a) The west Ivorian margin was first structured during an Albo-Aptian intra-continental shearing of essentially transtensional character. (b) foundering of blocks, thermal upheaval of blocks and a regional reduction in subsidence rates are likely expressions of the sUbsequent Cenomanian Lower Senonian continent-oceanic shearing. (c) From the Senonian to Present, the margin is passive. It is was subject to a reduced thermal upheaval until the Paleocene, and thereafter to post-shearing flexural subsidence. The structural interpretation and the proposed stratigraphic sequence of the western Cote d'Ivoire margin are in agreement with those of the rest of the African margin controlled by the Saint Paul Fracture Zone; with those of the Brazilian conjugate margin; and, with those of the western Ghanaian margin, controlled by the Romanche Fracture Zone. Key words: Equatorial Atlantic margins, Cote d'Ivoire, gravimetry, fracture zones, Ivory Coast, magnetometry, marginal ridges, reflection seismic, seismic stratigraphy, shear motion, structural geology, transform margins. ISSN 0348-2367 ISBN 91-7032-685-1
Publ. A 69, 1992
ii ACKNOWLEDGEMENTS
I am in great debt to the seismic contractor GECO (today GECO-PRAKLA) for allowing me to use and pUblish the seismic, gravity and magnetic data in this thesis; and, to GECO, Oslo, for their assistance in gravity and magnetic modeling. The geophysical data in this thesis are from a 2,370 km non-exclusive seismic survey conducted by GECO (today GECOPRAKLA) in 1986 on the continental shelf of western Cote d'Ivoire, West Africa. The covered area is 10,000 km 2 • I was personally in charge of this project, which included: the planning and the carrying-out of the survey; the quality control of the processing of the seismic profiles; and the quality control of the modeling of the gravimetric and magnetic information. Before leaving the company in 1987, I interpreted the seismic data. I am also grateful to the Department of Geology of the UNIVERSITY OF GOTHENBURG, Professors K. GOSTA ERIKSSON, SVEN AKE LARSON, Docent GUSTAF LIND and Docent JIMMY STIGH, for having me as a graduate student since 1986, despite the fact that I have continued to be employed in the private sector. They, and others of the Department, have reviewed and analyzed my work. Not in the least, I wish to thank ANITA SVAN for a professional drafting of all my figures. Finally, I wish to thank all my colleagues and ex-colleagues for bearing with my non-economic work. Doctor JEAN MASCLE, directeur de recherches CNRS, whom I contacted already in 1987 to have his views on my study area, was extremely kind in inviting me for two months in 1989 to visit the Laboratoire de Geodynamique sous-marine, universite Paris-VI at Villefranche-sur-mer, France. This stay was most valuable, as it gave me the chance of meeting and working together with fellow researchers dedicated to the adjacent transform margins of Cote d'Ivoire-Ghana and Guinea. It is not an understatement to say that I would not have been able to control the theoretical side of my work without Jean Mascle's guidance. In return, I hope this thesis may provide a few additional clues to current research on the equatorial margins which is carried out in Villefranche-sur-mer. Finally, Jean's support gave me the courage to complete my work as I had reached a stand-still. Last but not least, I am extremely grateful to my wife. She has had to bear for many years with my studies at night, while I worked daytime in the industry. My wife, parents (my father proofread the text), mother in-law and late father in-law have supported me throughout my studies. Goteborg, March 1992 GUy C de Caprona
iii TABLE OF CONTENTS
ABSTRACT ACKNOWLEDGEMENTS
Page i ii
TABLE OF CONTENTS
iii
LIST OF FIGURES
vii
LIST OF TABLES PART I: THE AFRICAN CONTINENTAL MARGIN IN PROLONGATION OF THE SAINT PAUL FRACTURE ZONE 1. 1.1 1. 1. 1 1.1.2 1.1.3 1.1.4
THE EQUATORIAL ATLANTIC AND ITS CONTINENTAL MARGINS Western Cote d'Ivoire - southeastern Liberia margin: its position within the Equatorial Atlantic margins Introduction Origin of the present data and past research and petroleum exploration work Major tectonic units along the West African Equatorial Atlantic margin The Saint Paul Fracture Zone and associated marginal ridges on the West African continental margin
1.2 1. 2.1 1. 2.2
Models of passive continental margins continental rift margins continental transform margins
1.3 1. 3.1 1. 3.2
Kinematic evolution of the Equatorial Atlantic continental fit Kinematics of the opening of the Equatorial Atlantic A. Initial rifting in the Neocomian to Late Aptian B. oceanic communication in the Late Albian/ Early Cenomanian C. End of transform motion in the Lower Senonian D. Passive phase from the Lower Senonian to Present
ix 1 1
1 1 1 4 6 6
7 8 9
10
12 12 12 13 13
2.
TRANSCURRENT PLATE MOTION
14
2.1 2.1.1
Intra-continental shearing stage Fault patterns along transcurrent zones A. Controls on the development of structural patterns along strike-slip faults Patterns at divergent plate boundaries Pull-apart basins A. Evolution of the structural pattern
15 15
2.1. 2 2.1. 3
16 18
20 20
iv B. Basin ridges C. Subsidence and sedimentation D. Tectonic activity and geothermal gradient
21 21 22
2.2
continental - oceanic shearing stage
22
2.3
Passive sheared continental margins
24
3•
STRUCTURE AND STRATIGRAPHIC SEQUENCE OF THE AFRICAN CONTINENTAL MARGIN IN PROLONGATION OF THE SAINT PAUL FRACTURE ZONE
26
3.1 3.1.1 3.1. 2 3.1. 3 3.2 3.2.1
3.2.2
Generalities Physiography of the continental margin Basement shield A. Southwestern C6te d'Ivoire B. Lineaments Continental margin controlled by the Saint Paul Fracture Zone western part of the African tranform margin of the Saint Paul Fracture Zone Structure and stratigraphic sequence of the continental margin of southeastern Liberia A. continental slope and rise B. Continental shelf C. Liberian basins north of the marginal ridges Structure and stratigraphic sequence of the western Ivorian continental margin
3.3.1 3.3.2 3.3.3
Eastern part of the African termination of the Saint Paul Fracture Zone: continental margin of C6te d'Ivoire - western Ghana Structure stratigraphic sequence Seismic facies
3.4 3.4.1 3.4.2
Regional conclusions Tectonic framework stratigraphic sequence
3.3
PART II: ANALYSIS OF THE WESTERN IVORIAN TRANSFORM MARGIN
26 26 26 28 28 29 30
31 31
36 36 37
39 40
42 45 48 48
49
51
4.
DATA BASE
51
4.1
Objectives and methods
51
5.
BATHYMETRY
52
6.
MAGNETIC AND GRAVITY DATA
54
6.1 6.1.1 6.1. 2
Qualitative interpretation Magnetic anomaly map Gravity anomaly map
54 54 57
v
6.2 6.2.1 6.2.2 6.2.3
Modeling Estimation of parameters Magnetic modeling Gravity modeling
61 61 64 66
7.
SEISMIC INTERPRETATION
68
7.1 7.1.1 7.1. 2 7.2 7.2.1 7.2.2 7.2.3
Seismic data Acquisition and processing parameters Seismic section quality Sequence and reflector identification Meso2oic - Ceno2oic eustatic cycle charts Seismic sequences Interpretation difficulties A. Discrimination criteria between the acoustic basement and the Top Albian unconformity B. Reflector correlation
68 68 68 69 69 77 84 84 85
7.3.3 7.3.4
Seismic mapping Acoustic basement Top Albian unconformity A. continental shelf B. Graben south of Sassandra C. continental slope Upper cretaceous unconformities Ceno2oic unconformities
7.4 7.4.1 7.4.2 7.4.3
Seismic velocities and facies Albo-Aptian Upper Cretaceous - Paleocene Eocene - Neogene
95 95 97 100
7.5 7.5.1 7.5.2
Timing of faulting and subsidence evolution Timing of faulting along the margin Subsidence evolution in relation to tectonic activity
100 100
7.6 7.6.1 7.6.2
Tectonic interpretation Structural trends Structural interpretation and comparisons A. Intra-continental shearing stage B. continental - oceanic shearing stage C. Passive stage
104 104 105 105 107 109
7.7 7.7.1 7.7.2 7.7.3
Conclusion on the data interpretation Tectonic framework Stratigraphic sequence Suggestions for future research
109 110 111 111
7.3 7.3.1 7.3.2
86 86 88 88
91 91 93 93
101
PART Ill: COMPARISONS OF THE WESTERN IVORIAN MARGIN WITH THE CONTINENTAL TERMINATIONS OF THE SAINT PAUL FRACTURE ZONE AND WITH PART OF THE AFRICAN TERMINATION OF THE ROMANCHE FRACTURE ZONE 113 8.
SOUTHEASTERN LIBERIA AND EASTERN COTE D'IVOIRE
113
8.1
Saint Paul marginal Ridges
113
vi 8.2
Cote d'Ivoire/Ghana Basin
114
8.3
Stratigraphic sequence
115
9.
NORTH BRAZILIAN CONJUGATE MARGIN OF COTE D'IVOIRE: ILHA DE SANTANA PLATFORM
116
9.1 9.1.1
Generalities Physiography of the continental margin Basement shield
116 116
structure and stratigraphy Western part of the platform and mouth of the Amazon River Eastern part of the platform: Para-Maranhao Basin
118
119
9.3 9.3.1 9.3.2
Comparison with western Cote d'Ivoire structural pattern Tectonic evolution
122 122 122
10.
CONTINENTAL MARGIN OF WESTERN GHANA
124
10.1
Physiography of the continental margin
124
10.2
structure and stratigraphy
124
10.3
Comparison with western Cote d'Ivoire
126
9.1. 2
9.2 9.2.1 9.2.2
116
118
Conclusion on the regional comparisons 10.4 10.4.1 structural patterns and evolution 10.4.2 stratigraphic sequences
127 127 127
11.
129
GENERAL CONCLUSION
SUMMARY OF: THE CONTINENTAL MARGIN OF WESTERN COTE D'IVOIRE - STRUCTURAL FRAMEWORK INHERITED FROM INTRA-CONTINENTAL SHEARING
131
Regional geology Seismic interpretation Seismic sequences Timing of faulting and subsidence evolution Tectonic interpretation Regional comparisons Conclusion
139
LIST OF REFERENCES
141
131 132 132 134 135 137
vii LIST OF FIGURES
1-1: 1-2: 1-3: 1-4: 1-5: 2-1: 2-2: 2-3: 2-4: 2-5:
Principal oceanic structures in the Equatorial Atlantic Geco non-exclusive seismic survey program map, western Cote d'Ivoire structural elements of the continental margin and craton of West Africa Main characteristics of transform margins Paleoreconstructions of the opening of the Equatorial Atlantic Evolution of transform margins Diagrammatic fault map of the Salton Trough area, California Divergent strands along transform faults structures associated with divergent wrenching Multi-channel seismic, magnetic and gravity profile across the northern margin of the Guaymas Basin, Gulf of California
Page 2 3 5 8 11 14 17 19 20 23
3-1:
Tectonic, bathymetric and location map of Liberia and Cote d'Ivoire 27 3-2: structural trends of the southeastern continental margin of Liberia 31 3-3: Total magnetic anomaly field map of southeastern Liberia 32 3-4: Gravity maps of the southeastern continental margin of Liberia 33 3-5: Seismic lines across the southeastern continental margin of Liberia 34-35 3-6: Seismic line across the continental margin of western Cote d'Ivoire 38 3-7: Structural map of the Albian - Cenomanian unconformity in the deep offshore Cote d'Ivoire Ghana Basin 41 3-8: Schematic geological section in the Cote d'Ivoire Basin 43 3-9: Tectonic subsidence curves for three wells offshore Cote d'Ivoire 45 3-10: Seismic lines in the offshore Cote d'Ivoire Basin 46-47 5-1: 6-1: 6-2: 6-3: 6-4: 6-5:
Water depth and section location map of the western Ivorian margin Magnetic anomaly map of the western Ivorian margin Magnetic anomaly map of the margins of western Cote d'Ivoire and of southeastern Liberia Free-air gravity anomaly map of the western Ivorian margin Bouguer gravity anomaly map of the western Ivorian margin Free-air anomaly map of the margins of western Cote d'Ivoire and of southeastern Liberia
53 55 57 58 59 61
viii 6-6: 6-7: 6-8: 7-1: 7-2: 7-3:
7-4: 7-5: 7-6: 7-7:
7-8: 7-9: 7-10:
7-11: 7-12: 7-13:
7-14: 7-15: 7-16: 7-17: 7-18: 7-19: 7-20: 8-1:
9-1: 9-2:
Western Cote d'Ivoire, onshore: geologic map. Offshore: qualitative interpretation of the magnetic and gravity profiles Magnetic models Gravity models
62 65 67
Seismic dip line across the eastern part of the surveyed area, south of Sassandra 70 Seismic dip line across the central part of the surveyed area, south of San Pedro 71 continental shelf south of Sassandra: prograding Upper Cretaceous - Paleocene sequence on top of sub-horizontal Albian - Cenomanian bedding 72 Syn-sedimentary faulting and slumping at shelfedge and upper slope, south of Sassandra 73 Deep slope south of Sassandra: Upper CretaceousPaleocene sediment in-filling 74 Elongated, faulted and eroded ridge in deep waters off the central and western parts of the surveyed area 75 Monotonous monocline under the western continental shelf: a possible forced monocline 76 Mesozoic - Cenozoic sea level cycle charts 78-79 seismic sequence analysis of Figs 7-1 and 7-2 81 Strike line across the continental shelf south of Sassandra: basin floor flexuring 82 Seismic expression of the acoustic basement and of the Top Albian unconformity 85 Acoustic basement map of the eastern part of the surveyed area 87 Top Albian unconformity map of the western Ivorian margin 89 Rotated fault block at the eastern end of the basin, located on the shelf south of Sassandra: a releasing fault junction in a pull-apart 90 Paleocene paleo-canyon of the Sassandra River, on the shelf in the eastern part of the studied area 94 Seismic interval velocities across the margin 96 Geological section across the western Ivorian margin 98 Seismic expression of the Albian-cenomanian paleoshelf south of Sassandra, with possible carbonate build-ups on top of a basement ridge 99 Depth conversions across the western Ivorian margin 103 Reconstruction of the tectonic evolution of the western Ivorian margin 108 Comparison of the tectonic trends of the western Ivorian margin with the trends on the margin of southeastern Liberia Bathymetry, structural and location map of the Brazilian conjugate margin of Cote d'Ivoire Basement structure map of the Para-Maranhao Basin
114 117 119
ix 9-3: 9-4:
Geologic section through the transtensional leg of the Para-Maranhao Basin Stratigraphic columns along the Brazilian shelf controlled by the saint Paul Fracture Zone
120 121
10-1: Schematic structural map of the margin of western Ghana and of the deep Cote d'Ivoire Basin 124 10-2: Seismic section across the Ghanaian continental margin, at the wedge-out of the deep Ivorian Basin 125 LIST OF TABLES
1-1: 6-1:
Comparative characteristics of rifted and sheared margins Estimation of gravity and magnetic modeling parameters
7 63
x
1 PART I: THE AFRICAN CONTINENTAL MARGIN IN PROLONGATION OF THE SAINT PAUL FRACTURE ZONE 1.
THE EQUATORIAL ATLANTIC AND ITS CONTINENTAL MARGINS
1.1
Western Cote d'Ivoire - southeastern Liberia margin: its position within the Equatorial Atlantic margins
1.1.1 Introduction
The ocean floor of the present-day Equatorial Atlantic is dissected by several major fracture zones that offset the Mid-Atlantic Ridge. The fracture zones extend from Africa to South America (Saint Paul and Romanche in particular) (Fig 1-1) and are inherited from transform plate motion. The continental margins in their prolongation are consequently a prime area for the study of the tectonic evolution of transform margins. The studied area is the continental margin of western Cote d'Ivoire (*), located on the African termination of the Saint Paul Fracture Zone (Fig 1-1). The objective of this thesis is to analyze, with multi-channel reflection seismic, gravity and magnetic data, if the interpreted faults, horsts and grabens were created in an intra-continental shearing stage between northern Brazil and Cote d'Ivoire. A secondary objective is to study the effects on the margin of the subsequent continental-oceanic shearing and of the present passive stage. The present dissertation is more detailed than previously pUblished studies on the African Equatorial Atlantic margins. The study consequently, provides the opportunity, within the studied area, to analyze the strain regimes that were active during the Equatorial Atlantic opening phases. The data interpretation also allows the comparison of the formed structures with modern examples. 1.1.2 Origin of the present data and past research and petroleum exploration work
The data interpreted in this dissertation are from a nonexclusive survey from 1986 by the geophysical contractor firm GECO (today GECO-PRAKLA) on the continental shelf and slope of western Cote d'Ivoire. The survey was conducted between the latitudes 4 deg 10 min Nand 5 deg 00 min N, or the coastline, and the longitudes 5 deg 40 min Wand 7 deg 30 min W (i.e. the Liberian border) (Fig 1-2). (*) At the request of the government of the Republic of Cote d'Ivoire in 1986, and following the apparent adherence by the United Nations, World Bank etc., all references to the Ivory Coast are replaced by Cote d'Ivoire (Petroconsultants, 1986).
10'N 10'N
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o
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U
SUbmarine, Qutcropping or buried ridges
,
Mid - Atlantic Ridge, separated by fracture zone
An3L. __ Extent of magnetic sea-floor anomalies, dated in Fig 7-8
200m __ Water depth
Figure 1-1: Principal oceanic structures in the Equatorial Atlantic (modified after Gorini, 1981, Emery and Uchupi t 1984 and Gouyet, 1988).
Profiles ___ Seismic Seismic, Gravity
7000'
+
Seismic, Gravity and Magnetic
Well
SAN PEDRO
«
COTE D'IVOIRE
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Principal Bathymetry Contours Contoured by: Guy de Caprona
7"30'
Figure 1-2: Geea non-exclusive seismic survey program map, western Cote d'Ivoire. KI-IX and Kl-2X are industry wells. See location in Figs 1-1, 1-3, and 3-1.
w
4
The data comprise 2,370 km of multi-channel seismic lines, 815 km of gravity profiles and 670 km of magnetic profiles. Academic work was carried out offshore western Cote d'Ivoire and southeastern Liberia, in the late sixties and early seventies, with predominantly single channel reflection seismic data and gravity and magnetic profiles. The lines concentrate, however, on the abyssal plain and continental rise and very few are recorded along western Cote d'Ivoire (Arens et al., 1971, Behrendt et al., 1974, Delteil et al., 1974, Schlee et al., 1974, Emery et al., 1975 and Mascle, 1977). During these surveys, sea-bottom corings were taken off Liberia but not offshore western Cote d'Ivoire. No Deep Sea Drilling Program (DSDP) nor Ocean Drilling Program (ODP) wells have been drilled on the northern shore of the Gulf of Guinea between Nigeria and Sierra Leone. Petroleum exploration was conducted, offshore western Cote d'Ivoire, in the early seventies, by a group of companies led by Esso who recorded the first industrial seismic survey. Thereafter, the two Ivorian state companies, Petroci and Sodemi, acquired several surveys. No exploration wells have been drilled. The closest borings to the study area are K1-1X and Kl-2X and lie to the east, 40 and 52 km respectively (Fig 1-2). They were drilled in 1984 to a depth of 3,525 m for K1-1X and 3,512 m for Kl-2X. To the west in Liberia, the closest well, Cestos-1, is drilled to a depth of 3,170 m, 250 km northwest of the border with Cote d'Ivoire (Stewart and Kromah, 1987) (Fig 1-3). The information on these wells is not in the public domain. Previous interpretations of this continental margin, of both academic and industrial seismic data, indicate that the shelf consists of a shallow basement (Arens et al., 1971 and Gooma, 1990) with a sedimentary cover of 0 to 150 m (Brancart, 1977). However, an unpublished report (Soquip, unpubl.) proposes 2,400 m of sediments on the shelf south of Sassandra. Previous seismic data is of poor quality, and was therefore of limited interest in designing the present survey. Instead, an unpublished satellite gravity map was used as it shows the presence of a previously not reported basin. 1.1.3 Major tectonic units along the West African Equatorial Atlantic margin
The major African tectonic units in west Africa, along the Gulf of Guinea are, following Affaton et al. (1980) (Fig 13) :
*
The West African craton. A Precambrian basement (1,8003,000 Ma) which includes, in Ghana, a Late Precambrian and Paleo2oic cover.
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+
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?=? ----/--
---
1
2000 m
WD4000m
Fold Inferred fault
o
M
11°00'
•
M
6°00'
Figure 3-2: Structural trends of the southeastern continental margin of Liberia (modified after Schlee et al., 1974). Three ridges (Grand Cess, Cape Palmas and Saint Paul), of up to 20 km across, are identified. They appear to offset the Liberian slope southeastwardly. Sediments have accumulated below the continental slope against the northern side of the Grand Cess Ridge. Similarly, the two southern ridges have acted as dams for sediments carried down the slope forming small "pocket basins" 10-30 km wide, several kilometers deep, within two broad valleys that cut into the continental slope. Lines 26, 30, 34 are found in Fig 3-5a.
A. continental slope and rise On the slope and rise, the magnetic anomalies (Fig 3-3) are in continuation with the Eburnean trend onshore and on the continental shelf and have consequently probably reused
3"00'
32 this Precambrian strike (Behrendt et al., 1974) (chapter 2.1.3B). Northwards, the trend shifts to a coast parallel orientation. Beneath the slope, the single channel seismic sections show large blocks up to 20 km across, with no internal reflectors and with dips as high as 45 deg (Fig 3-5a). The multichannel line reveals probable internal beddings (Fig 3-5b). The associated magnetic curves reveal high amplitude anomalies, at the seismic ridges, due either to intrusions or
/ Eburnean
;/"Age V$ \00
.................................................. 6'00'
~.:o-~C6TE
D'IVOIRE Sassandra
km i
11°00'
10'00'
S'OO'
7'00'
Figure 3-3: Total magnetic anomaly field map of southeastern Liberia
(modified after Behrendt et al., 1974). The map has been corrected for the International Geomagnetic Reference Field (IGRF). The map has not been integrated with the available anomaly map of southwestern Cote
d'Ivoire (Strangway and Vogt, 1970), as the latter is too sketchy. Lines 26, 30 and 34 are found in Fig 3-5a.
to the juxtaposition of crystalline basement rocks with contrasting susceptibilities. The Bouguer map (Fig 3-4b) confirms the presence of mass excesses at the location of the ridges in Fig 3-2. The blocks consist likely of continental basement with locally pre- or syn-tectonic indurated rocks. In plan view (Fig 3-2) the blocks align into three elongated ridges which trend in a WSW direction. The ridges extend into the Cape Palmas area and have an expression on the sea-bottom topography (Fig 3-1). The first and principal zone, Grand Cess, runs continuously into the continental shelf, west of the Ivorian border. The second, Cape Palmas Ridge, is a buried feature below the continental
33
km .................................... . L
12°00'
WOO'
8°00'
.c
..J3'OO'
7°00'
A: Free-air map
o ..................J 6'00'
"
COTE D'IVOIRE
km
......................~ 3'00' 12'00'
7°00'
B: Bouguer map Figure 3-4: Gravity maps of the southeastern continental margin of Liberia (modified after Behrendt et al., 1974). The applied Bouguer correction is 2.67 g/cm 3 •
34 rise south of Cape Palmas. The ridge coincides with the shelf-edge at the border. The third transverse ridge, Saint Paul, stays in deep waters and is a topographical high as far east as 10 deg W, and can be traced, buried under the continental rise as far east as 7 deg W (Behrendt et al., 1974) (Fig 3-1). All three ridges have trapped several kilometers of sediments (Fig 3-2). On the upper slope, Cretaceous and younger sediments pinch out abruptly on the steep northern walls of the valleys (Schlee et al., 1974) and accumulate against the southern ones, with a more gentle topography (Fig 3-5a). Further seawards, the sedimentary section thickens to several kilometers, thinning gradually in the abyssal plain. An indication of an oceanic/continental crustal contact is given by a gravity model (Fig 1-4) where the Saint Paul Ridge may be of oceanic origin. Multi-channel seismic sections show that it has the shape of a cone (Soquip, unpubl.). The Saint Paul Ridge could be a seamount.
Figure 3-5: Seismic lines across the southeastern continental margin of Liberia. See location in Figs 3-1,
3-2 and 3-3.
A:
Interpreted single-channel seismic profiles across the southeastern continental margin of Liberia (modified after Schlee et al., 1974). The margin is controlled by three ridges which can be followed from line to line. The sedimentary cover on the shelf ("Xli) is very thin. The total magnetic intensity profiles are IGRF corrected. A gravity model of line 30, with an onshore extension is in Fig 1-4.
'\ifpo
0"
I.\/x
2-'
hi 1\ et'
A3
..•. ,-;~./ --.. ':-' o 40km
34
Saint Paul Ridge
VERTICAL EXAGGERATION OF TOPOGRAPHY ABOuT x9: SU8SURFACE REFLECTORS x5 OR LESS
On the single-channel profiles (Fig 3-5aj the sediments are 2 seconds TWT (two-way time) thick in the basins. This seems to correspond to the I and II units on the multichannel section (Fig 3-5b). Thereby, the third unit with strong reflectors would have acted as the acoustic basement on the first lines. Below this unit however, a deeper basement is not visible. To match the magnetic depth estimate of at least 4 km below sea-bottom (Fig 3-2), it would
35 come in 0.5 to one second below the 11/111 unconformity. From gravity constraints, Behrendt et al. (1974) interpret this third unit as a dense, thick non-magnetic sequence, concealing the true basement. It may thus consist of sheet like volcanic flows within the sediments or thick sequences of limestone. Finally, this third unit shows strong faulting and folding and should thus be a syn-tectonic Lower Cretaceous bounded by a strong unconformity, possibly of Late Albian/Early Cenomanian age, chapter 3.1.3.
o
sw
NE
o
GRAND CESS RIDGE
1 1I 5
5 ]JI
BENIN PROFll 11
Figure 3-58: Multi-channel seismic line (modified after Delteil et al., 1974). Basement probably outcrops on the continental shelf and is block faulted below the slope where it is rapidly overlain by over 3 seconds TWT (two-way time) of sediments. The seismic resolution on the
slope is much better that on the adjacent single channel lines (Fig 3Sa), as the section reveals a faulted syn-tectonic sequence Ill, in particular on the sides of the horst that appears to be the Grand Cess Ridge. An acoustic basement is not visible. By comparison with the lines in the Cote d'Ivoire Basin (Fig 3-10) and off western Cote d'Ivoire (chapter 7), unit Ill, with a series of strong reflectors, is likely a Lower cretaceous sequence topped by the Top Albian tectonic unconformity. Unit 11, with low amplitude reflectors, may consist of Upper Cretaceous and Paleocene sediments and unit I, of the Neogene, starting with a strong erosional surface, possibly of Paleocene age. See location in Figs 3-1 and 3-2. The length of the section is approximately 30 km and the ridge is 2 to 4 km across.
Above this lower unit, the sequence 11, characterized by low amplitudes, is of a mean thickness of 0.6 seconds TWT and fills the paleolows. It can be compared with the Upper Cretaceous - Paleocene in the sections from the Cote d'Ivoire offshore Basin (Fig 3-10). The interval is diffi-
36
cult to follow towards the upper slope where it seems to be truncated by a major erosional surface, which forms the base of the uppermost sequence. Off the shelf-edge, unit I pinches out against a paleoshelfbreak with a dip of 13-45 deg with no internal reflectors or only mUltiple energy (Fig 3-5b). This youngest interval has been slumped down, as shown by the irregular, hummocky character of the reflectors. In deeper waters, above the ridge, it seems that the unit is dissected by a second cenozoic erosional surface at approximately 500 ms (milliseconds) TWT below the sea-bottom. The continental slope has been dredged on the southern flank of the Grand Cess and Cape Palmas Ridges where the sedimentary cover is thin. On both locations the samples are marine, possibly Paleogene, sediments (dolomites and clastics derived probably from metamorphic and volcanic material, Schlee et al., 1974). The erosion at the base of section I should then be related to a post-Paleogene regression. It may be the trace of the major regressional phase off West Africa which took place during the Oligocene (chapter 3.1.3) and which is described off eastern Cote d'Ivoire (chapter 3.3.2). B. continental shelf
Onshore southeastern Liberia, the coast is barren of sedimentary formations. On the continental shelf, the singleand multi-channel seismic profiles, interpreted by Schlee et al. (1974) and Delteil et al. (1974) (Figs 3-5a and -b) give evidence to a thin or locally non-existent sedimentary cover (less than 0.5 seconds two-way time) resting on the acoustic basement. The interpretation by Behrendt et al. (1974) of the gravity (Fig 1-4) and magnetic surveys point to a crystalline nature for this acoustic basement. There is consequently no geophysical expression of marginal ridges on the shelf except for a reported aeromagnetic continuation of a Precambrian lineament, in trend with the Grand Cess Ridge ("0" in Fig 3-1). The lineament could be related to the fracture zone (Behrendt et al., 1974). C. Liberian basins north of the marginal ridges
North of the Grand Cess Ridge, three basins, with a coast parallel strike, are located on the Liberian continental margin (Stewart and Kromah, 1987). The basins have between 0.5 and 4 km of sediments on the shelf and 2 to 8 km on the slope. Underneath, the acoustic basement is block faulted down to the ocean (Behrendt et al., 1974). On the basement horsts offshore, from the latitude of Greenville northwardly, coast-parallel magnetic lineaments
37
of high spatial frequency are associated with doleritic dykes which have a parallel trend onshore (Behrendt et al., 1974). Similar dykes have been encountered in wells. They are of Jurassic and Early cretaceous age (Schlee et al., 1974) . On top of Lower Paleozoic sediments deposited in the coastal area, the first sediments encountered are in the basins formed by the initial Jurassic/Early Cretaceous rifting. Most of the sedimentary section found in borings is syntectonic and of Albian and pre-Albian age. The section consists mainly of continental and marine clastics with smaller amounts of siltstone and limestone. It is up to 2,000 m thick (Schlee et al., 1974). The column lacks the coarse clastics deposited in Cote d'Ivoire (chapter 3.3.2). The post-rift section consists of thin, non-consistent marine Upper Cretaceous clastics that have been truncated by a Tertiary er2sion. The overlying cenozoic (with Eocene at the base) is thin. Unrestricted marine conditions prevailed throughout the Cenozoic, although subsidence was limited (Schlee et al., 1974). In addition to the rifting phase, Schlee et al. (1974) report a last faulting and folding event at the end of the Mesozoic to the beginning of the Tertiary. Conclusively for the marginal ridges, although the amount of seismic data limits the possibility to identify structural trends within the basins (Fig 3-2), they are nonetheless tensional holes with a rapid syn-tectonic subsidence located on a transform margin. The depositional pattern and the restricted sedimentation within these small basins indicate a syn-tectonic, of possibly Late Albian/Early Cenomanian age for the marginal ridges. The present-day pronounced topography of the ridges gives evidence to continued post-tectonic vertical movements. North of the marginal ridges, extensive magmatic activity occurred in the Jurassic/Early Cretaceous prior to the onset of sedimentation in the opening basins. Tectonic activity and rates of sedimentation were very high throughout the Lower Cretaceous. The age of the post-rift unconformity is not more detailed than between the Lower and Upper Cretaceous. 3.2.2 structure and stratigraphic sequence of the western Ivorian continental margin
In prolongation of the Liberian southeastern continental margin, the western Ivorian shelf and slope show an ENE-WSW orientation, parallel with the coastline and in prolongation of the Cape Palmas and Saint Paul Ridges (Fig 3-2). The margin has been regionally described by Arens et al. (1971), Delteil et al. (1974), Emery et al. (1975) and Mascle (1977).
38
The coast of western Cote d'Ivoire until Sassandra is barren of sedimentary formations until Fresco where the Ivorian coastal Basin wedges out (Fig 3-1). On the shelf, at the KI-IX and KI-2X petroleum wells (Fig 3-1), Gooma interprets a basin to be of pull-apart origin. From there to the Liberian border, on the basis of seismic data (Fig 3-6) and magnetic anomalies, Arens et al. (1971) interpret the shelf as consisting of shallow basement. The interpretation is confirmed by Gooma (1990). From unpublished seismic data, Brancart (1977) puts up to 150 m of sediments between the Liberian border and Fresco. The upper part of the slope, west of 5 deg W, is interpreted by Arens et al. (1971) as directly cut into crystalline basement. The major NE-SW trending strike-slip lineament of Dimbokro intersects the shoreline a few tens of kilometers east of the studied area (chapter 3.1.2B, Fig 3-1). The parallel Eburnean foliation strikes at an angle with the continental margin and is followed locally by the coastline. There is, at this stage, no indication of a structural heritage during the Atlantic opening of this Precambrian framework.
®
®
-------0
, 6
6
t MAJOR IVORY COAST FAULT
Figure 3-6: Seismic line across the continental margin of western Cote
d'Ivoire (modified after Arens et al., 1971). In·the Tertiary (T) sequence, the shallow Oligocene erosional surface is clearly visible.
The strong reflectors 800 ms (milliseconds) TWT underneath are likely on top of the Paleocene unconformity. The Upper Cretaceous (CS) interval may correspond to unit 11 in Fig 3-5b. The syn-tectonic
Albo-Aptian (eI) section is masked by peg-leg multiples (deep reflector mapped) between 6 to 8 seconds in the southern part of the section. See location in Fig 3-1.
39 The seismic section (Fig 3-6) shows that the shallow acoustic basement on the shelf is bounded at the shelf-edge by an important steep dipping fault system with a throw exceeding 2 seconds TWT. A comparable faulting is known in the COte d'Ivoire Basin where the "Faille des Lagunes" has an aggregate throw of over 4,000 m. As very few seismic lines are available, the eastern prolongation of the Cape Palmas and saint Paul Ridges cannot be traced. The westernmost profile in COte d'Ivoire (Fig 3-6) runs east of the studied area, i.e. approximately 280 km, east of Cape Palmas (Fig 3-1). It shows only a possible small buried horst beneath the slope, at the foot of the major shelf-edge fault. The acoustic basement is masked by important peg-leg multiples (deepest marked reflector at the southern end of the line). The one second interval above this mUltiple consists of low amplitudenreflectors which can be compared with the Upper Cretaceous~Paleocene sequence in Figs 3-10 and 3-Sb. The interval pinches out towards the upper slope where it is truncated by a strong series of reflectors. These would be the base of the Paleocene to Eocene sequence which displays slumping features by the small ridge, as in Fig 3-Sb. Near the sea-bottom, the interval is cut by an erosional surface, likely of Oligocene age. As for southeastern Liberia, this is an extrapolation from the Ivorian Basin. In this case because the information on the Kl wells has not been released and nothing has been pUblished on the stratigraphic sequence west of the Ivorian Basin. 3.3
Eastern part of the African termination of the saint Paul Fracture Zone: continental margin of Cote d'Ivoire - western Ghana
The Ivorian Basin is emplaced at the eastern end of the saint Paul transform margin. The predominant feature is the major coastal "Faille des Lagunes" which trends more or less east-westerly until the Ghanaian border. The fault system shifts southeastwardly thereafter and extends, off Cape Three Points, towards the COte d'Ivoire-Ghana Ridge, which is the continental extension of the Romanche Fracture Zone (Figs 1-3 and 3-7). The basin is thus trapped between three structural units: (1) to the north, the "Faille des Lagunes" , which ties into the Saint Paul Fracture Zone in western COte d'Ivoire (Arens et al., 1971); (2) to the northeast, the coastal fault which branches towards the COte d'Ivoire-Ghana Ridge; (3) the COte d'Ivoire - Ghana Ridge to the southeast, with a northeasterly trend. The basin is open-ended southwestwardly and oceanwardly between the marginal ridges in prolongation of the two transform faults.
40 3.3.1
structure
The setting of the basin can be compared with Fig 2-1. The northern flank appears to be the far end of a transform margin with a short wrenching evolution of transtensive character (Blarez, 1986). It faces a northwesterly striking rift margin which is, in turn, bound by the transform margin controlled by the parallel Romanche Fracture Zone. These units indicate a large pull-apart opening, as described by Blarez (1986), with shearing regimes along the edges and rifting in the axis of the basin (Mascle and Blarez, 1987). Tectonic maps have not been published but according to Blarez (1986) the principal faults should overall be eastwesterly with strikes equivalent to the "Faille des Lagunes". In the central part of the basin, Grillot et al. (1986) describe, in a detailed structural map over the Espoir oil field (Fig 3-7), frequent faulting with a NNWSSE trend, in what can be interpreted an en echelon arrangement in respect to the principal direction of wrenching. The age of the faulting is Albian (Figs 3-8 and 3-10a) (for the chronostratigraphy, see Fig 7-8). The main, east-west trending faults, affect also the rest of the sedimentary section (up to the Quaternary) (Brancart, 1977). The cumulate throw of the coastal fault system is 4,000 to 5,000 m on the east-westerly transtensional stretch (Fig 3-8). From drilling information, the throws are again very important on the divergent part of the basin, in Ghana (Blarez, 1986). The stratigraphic sequence of the continental slope of western Cote d'Ivoire should therefore be linked to the Ivorian divergent Basin starting from the initial oceanic communication in Late AlbianjEarly Cenomanian. Onshore, a thin section of up to 300 m of Upper Cretaceous to Quaternary sediments covers only approximately 8,000 km', or 2.5% of the territory (Spengler and Delteil, 1966). The basin is limited to the coastal eastern half of the
Figure 3-7: Structural map of the Albian - Cenomanian unconformity in the deep offshore Cote d'Ivoire - Ghana Basin (modified after Blarez,
1986 and Lati1-Brun et al., 1988). Insert 1: Strain Ellipse from Fig 2-4. Insert 2: Structural Interpretaton (Lati1-Brun et al., 1988). See location in Fig 3-1. Isochrons are in seconds TWT. Note the structural offsets on the upper slope, corresponding to basement offsets on the
map, insert 2. The steep gradient along the shelf-edge is likely to be a water wedge effect and not an expression of the tectonic style. "E" and "B" locate the oil fields Espoir and B~lier. Circles mark petroleum industry wells referred to in the text. Insert 1: See legend in Fig 2-4. Insert 2: Continental crust onshore and on the Ghanaian shelf (crosses) is flanked to the SW by the stretched crust of the Ivorian
Basin, with the Cote d'Ivoire/Ghana Ridge at the rim (both in hatchings). To the south the two areas are in contact with oceanic crust (stippled). Faults are drawn as rectilinear lines, arrows indicate the direction of motion or extension. Curvilinear arrows show shear sedimentary folds, developed as a consequence of the right lateral
transform motion along the Cote d'Ivoire/Ghana Ridge (chapter 10).
"
+
T
+
+
i-
+.....:.: t'
Insert 1
~
C
\~
----." ----
~
Insert 2
I I
',{U
100 km
zone de fracture
1 de Saint Paul
2
I
"00
"00
"~
zone de fracture de la Romanche
42
country, in a narrow strip that pinches out 35 km or less northward. The structural map of the deepest horizon published is of the Albo-Cenomanian unconformity (Fig 3-7). The isochrons cover the divergent part of the basin and have a general ENE-WSW strike. The unconformity is gently dipping oceanwards to the southwest and gives an indication of damming along the Cote d'Ivoire-Ghana Ridge. 3.3.2 stratigraphic sequence The following description of the offshore stratigraphy is in agreement with Brancart (1977), with additional information from sources referred to in the text. The stratigraphic sequence is broken into a syn- and a post-tectonic series by a Late Albian/Early Cenomanian unconformity (AL) (Mascle et al., 1988) (Fig 3-8). In addition to this un conformity, two major erosions, of Upper Paleocene and of Oligocene age, cut through the post-tectonic sequence. The syn-tectonic sequence starts in the basin with a series of red deposits of continental origin which has been recognized in boreholes overlying the Precambrian shield. Its thickness exceeds 2,000 m in Ghana, where basement has not been reached, and decreases westward. The age of this series is believed to be Neocomian with its upper part probably reaching into the Albo-Aptian. No intrusive nor extrusive activity is known to have occurred in the Cote d'Ivoire Basin. Albo-Aptian sequences of conglomerates and fluvial and marine clastics lie discordantly on top of the preceding interval. Their thickness is in excess of 2,600 m in the center of the basin, where the marine influence is more important, and thins east- and westward. Upper Albian sandstones, fine clastics and dolomites are reported by Brancart (1977) to be deposited along the fringe of the basin in a shallow water, deltaic environment as indicated by the reservoir of the Espoir field (Fig 3-8). This interval is separated from the Albo-Aptian strata by a Middle Albian unconformity which ends the syn-tectonic sequence (Brancart, 1977). The distinction remains unclear with the overlying Albian-cenomanian erosion which has generally worked down to the lower unconformity (Blarez, 1986). The important Lower Cretaceous syn-tectonic subsidence rates are illustrated by Fig 3-9. The post-tectonic sedimentation, starting with the Cenomanian-Turonian, is marked by a sudden slow down of the subsidence on the transform end of the basin (Fig 3-9). This burial slow down lasts throughout the Upper Cretaceous to the Paleocene, with low or no sedimentation. The Cenomanian is widely represented in the basin and rests discordantly on top of the Albian (Fig 3-8). It is typically regressive and of constant thickness (600 to 700 m)
43
(Brancart, 1977 and Spengler and Delteil, 1966). The presence of the Turonian is still debated as the interval is reported to be partially eroded. The interval is predominantly shaly in its lower part (Lower Cenomanian) and sandy-dolomitic in the Upper Cenomanian. At the edges of the basin, the facies are coarser. Limestones, often dolomitic, are generally found in wells.
•
ESPOIR
s
°ii
BElIER
(PROJECTED)
•
N
;;
........: '::. :::-:.:-:.:. :':':'::::::::::::::::::::::X::::::::::::::::::::::::::::::::::::::::::::::::::::::::.:.:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::':':':':':',' ..:
-------------
.................... ............
-----------------
--
10
.................. ... . .......................... .............
... :::::.:::::::: ....::::-::-:-:-:-:-:-:.::: .R.IF.! .S~Q~.E.~'i
KM
[= ] [ · ..
a
YE ·31
Fi ne CIa sties 1
Coarse Clasties Limestones Crystall ine Basement
Figure 3-8: Schematic geological section in the Cote d'Ivoire Basin (modified after Clifford, 1986). The unconformities are labelled with the abreviations used in chapter 7: Top Albian (AL), CenomanianTuronian (CE). The section does not show the Paleocene and Oligocene unconformities. Espoir and Belier are two oil fields offshore Cote d'Ivoire, located in Fig 3-7.
The Lower Senonian (Coniacian-santonian-Campanian, as defined by Brancart, 1977) is unconformably transgressive with sediments of bathyal origin (Brancart, 1977 - Gooma
44
(1990) proposes that the transgression starts already in the Turonian). This is the first interval to locally reach north of the large coastal faults. The rate of subsidence is, however, much lower than during the Albo-Aptian (Spengler and Delteil, 1966). At the center of the basin the sediments are coarse and detritic; in the west they are predominantly shaly. The reservoir of the Belier oil field are Lower Senonian turbidites (Fig 3-8). The Upper Senonian is characterized by neritic to lagoonal deposits of Maastrichtian age, separated, on well logs, by an angular unconformity from the Lower Senonian. The stratigraphy of the Senonian is still disputed due to the lack of paleontological studies and the relative absence of datable material (Brancart, 1977). At the end of the Maastrichtian a widespread emersion takes place (Spengler and Delteil, 1966 and Brancart, 1977) but is not followed py an erosion. The overlying Paleocene, of deeper water facies, transgresses from the central parts of the basin, with an angular unconformity offshore, concordantly in the coastal basin. In Fig 3-9, it appears that from the Paleocene, the subsidence progressively increases for a short period on the transform end of the basin and decreases then assymptotically as in the rifted part. Offshore wells show the progradation, from the central parts of the basin to its fringes, of a bathyal Paleocene. In Upper Paleocene, an important erosion cuts down to the Lower Maastrichtian and transports the sediments to the eastern parts of the basin. The facies in the Eocene grade from bathyal-neritic in the lower part to benthic-neritic in the Upper Eocene and are composed of sandy glauconitic clays with limestone streaks in the lower parts (Spengler and Delteil, 1966 and Brancart, 1977). The last sedimentary section was deposited on the continental shelf after the last major transgression. This followed an important withdrawal of the sea occurring in Upper Eocene and Lower Oligocene. The drop in sea level implied local emersions and an intense erosion (M'Boro et al., 1980), which extends into the Middle Cretaceous (Spengler and Delteil, 1966). It is followed by a transgression in the Upper Oligocene (M'Boro et al., 1980) which reaches the coastal basin in the Lower Miocene (Bacchiana et al., 1982). The regression may be reflected by the halt in subsidence in wells IVCO-1 and -3 (Fig 3-9). The Oligocene corresponds thus to a slow-down in subsidence. In the Neogene, the Lower Miocene follows up on the important marine invasion from the Upper Oligocene. It is present in the central part of the basin. Middle and Upper Miocene are much thinner in the west and absent east. The middle interval corresponds to a halt in sedimentation, whereas the upper interval is related to a second Neogene transgression (Brancart, 1977). Overall, the rates of
45
subsidence remain very low in Upper the Tertiary to Present (Fig 3-9). Fi~ure
3 9: Tectonic su s~dence curves for three wells offshore Cote d'Ivoire (Latil-
Brun et al., 1988). The wells are located in Figs 3-1 and 3-7. The tectonic subsidence curves are corrected for sediment loading. Well IVCO-3 is located in the divergent basin and displays exponentially decreasing subsidence rates characteristic of extensional basins (Sleep, 1971 ). Wells !VCO-l and -2, emplaced closer to the transform end of the
basin, rev-eal subsi~ dence halts typical
CRETACE INFERIEUR 140
120
CRETACE
TERTIAIRE
SUP 100
80
60
40
20
OMa
o km-1--l._ _l--,-----+_ _L-_...L_-l._ _l-_....l
Cenomanien
Santonlen
____ I
Paleocene (66.4 Ma I
!
Cenomanlen
of a post-shearing,
thermal upheaval
phase of
~
trans-
Iorm margJ.n.
Eocene
!
IVCO
1
superteur
Latil-Brun et al.
(1988) propose to link the halt in subsidence in the IVCO-1 and -2 wells to tectonic movements associated with the shearing along the Saint Paul transcurrent zone. The onset of subsidence (66 Ma) at the top of the cretaceous, base of the Tertiary may be tied to the end of the continental - oceanic shearing along the margin. The thermal evolution in the Tertiary is similar to rifted margins. The shearing is estimated, however, to have ended in the Santonian (Mascle ana Blarez, 1987). Lati1-Brun et al. (1988) suggest that the continued absence of subsldence reflects an influence of the post-shearing thermal uplift of more westerly-lying parts of the transform margin.
The logs from wells offshore, show the basin to be very shale-rich with a frequent recurrence of bathyal sediments (Figs 3-8 and 3-10b). The sea-level lowstands and unconformities are notated next to the eustatic curves in Fig 7-8. According to Brancart (1977), the four major unconformities are: (1) Mid Albian (or Late Albian/Early Cenomanian); (2) Lower Senonian; (3) Upper Paleocene; and, (4) Miocene. From the above description, the Oligocene (Simon and Amakou, 1984) has to be added as a fifth discordancy. The angular unconformities are: Mid-Campanian, Late Maastrichtian, Lower to Mid-Eocene and Middle to Upper Eocene (Brancart, 1977) . 3.3.3 Seismic facies
The seismic lines in Fig 3-10 show five reflector sequences, starting with (1) an Albo-Aptian unit with strong, laterally coherent, low frequent reflections. This syn-tectonic interval is block-faulted, down to the basin and is unconformably overlain by (2) Lower Senonian down-
46
lapping reflectors of similar amplitude and continuity (Fig 3-10a). The Lower Senonian unconformity is of Turonian or Campanian age. The same section shows that the faults die in the Lower Senonian unconformity which truncates the Albo-Aptian.
sw
NE
o
o
OUGOCENE UNCONFORMITY
(Ol) (Pl)
-~-,:,,-_---
-.:;.-'""'=~-
~
4
oI
KM
_ _.'10
4
2,
Figure 3-10: Seismic lines in the offshore Cote d'Ivoire Basin.
A: 3D Seismic dip line across the Espoir oil field (modified after Grillot et al., 1986). Rotated fault blocks beneath the Albian unconformity (AL). The overlying sequence, topped by the Lower Senonian unconformity would be of Cenomanian-Turonian age. The Paleocene glid-
ing surface (PL) is not mapped by Grillot et al.
(1986) but compares
well with the section in Fig 3-6. The line is comparable with the southern part of the geological schematic section (Fig 3-8). See location of the Espoir oil field in Fig 3-7.
(3) The overlying reflectors are of Senonian - Paleocene age up to the Paleocene (PL) unconformity which is very clear in both sections in Fig 3-10. The Senonian reflectors are of comparatively poorer quality than the over- and underlying sequences. In line 3-10b, the Cenomanian cannot be differentiated from the Senonian - Paleocene which is divided into three members by two minor unconformities: (1) the Cenomanian - Campanian, (2) the Maastrichtian and, (3) the Paleocene intervals (Blarez, 1986).
47 (4) The reflectors between the Paleocene and the Oligocene erosions, constitute a fourth unit and are again of very good amplitudes and lateral continuity. They display slumping patterns above what may be a hinge line on a regional scale with slump faults located over older zones of weakness (Fig 3-l0a). From the description of the stratigraphic sequence, the reflectors are likely Upper Paleocene and Eocene. Sonic Log
1000ft/sec
N
1.I'lc;2~2"""-, :' 1 ---~~-
tlEOGEtJE
rllscnnnANCE Ol.lc,nc:r:Ilt:......
(0 L)
OllGOCEIlE EOCENE I.~OYEN
EOCEIlE
IIIFERIEUR
(PL) P.\LEOCEI1E
(53) I.1AESTAIClmEN
(51 or 52) CENQMf\IJIEN A CM.lf'MllEN
(AL)
AU1IEN SUPERlEUn
..
,
" +++++++++
+++
t·.. ~~
... Figure 3-108: Seismic line crossing well IVCO-2
~
"OI.~·
f ...~o.
(modified after
B1arez, 19B6). Inserted is the 1ithologic column (see Fig 3-8 for legend) and the sonic log. Unlike the previous line, the Albian shows very little seismic layering below an indurated, high-velocity sur-
face. See location of well Ivco-2 in Fig 3-7.
(5) The Oligocene erosional surface displays deep scars. The surface is, in addition, easily identifiable by the reported very low seismic velocities of the uncompacted post-erosional Neogene sediments. These are between 1,700 and 1,900 m/s in the western part of the basin (Simon and Amakou, 1984). The reflectors do not have any lateral continuity. Eocene and Miocene unconformities, reported by Brancart (1977), cannot be identified in Fig 3-10a. The five major discordancies are labeled, in Part II, by the dating of the top of the underlying seismic sequence. They are referred to as: (1) AB, the acoustic basement, (2) AL, the Top Albian (a Mid Albian unconformity has not been identif ied), (3) CE, the Cenomanian-Turonian (probably
48
Lower Senonian in Brancart, 1977), (4) PL, the Upper Paleocene and, (5) OL, the Oligocene (probably the Miocene in Brancart, 1977). Bounded by the major unconformities, four sequences are described, in the seismic sections in Part 11, and are: (1) the Albo-Aptian; (2) the Upper cretaceous-Paleocene (including where identifiable, the Cenomanian-Turonian, the Lower and Upper Senonian and the Paleocene); (3) the Upper Paleocene-Oligocene; and, (4) the Oligocene to Present. 3.4
Regional conclusions
3.4.1 Tectonic framework
The African transform margin, in trend with the Saint Paul Fracture Zone, is characterized from west to east by the following: Offshore southeastern Liberia, a series of marginal ridges; Offshore western Cote d'Ivoire, important shelf-edge faults (chapter 3.2.2); Eastwards, the faults are prolongated by the coastal fault system in eastern Cote d'Ivoire and Ghana (chapter 3.3). The marginal ridges, off Liberia (chapter 3.2.1A), have dimensions similar to the inter-basinal ridges in the East African Rift (chapter 2.1.3B). The interspacing "pocket basins" have dimensions similar to the pull-aparts of the Gulf of Aqaba and of the Salton Sea Trough (chapter 2.1.3A). The southernmost marginal ridge may be of oceanic origin or a seamount (chapter 3.2.1A) . The margin does not show, with the limited amount of available data, any easily identifiable shearing structures (chapter 3). Similarly to the San Andreas Fault and the Dead Sea Transform Fault, the fossil Saint Paul Transform Fault is oblique with respect to theoretical Cretaceous slip-lines (chapter 2.1.2). A transtensive regime should prevail regionally, where negative flower structures may be difficult to identify and principal displacements and synthetic strike-slips may only be identified by their strike. Normal, oblique, en echelon faulting should be well represented in a detailed interpretation. Compressive features, such as thrusting and folding, are not observed and should only be expected in local transpressive areas. In the eastern part of the margin, secondary faulting is limited to the Albo-Aptian interval, i.e. the intra-continental shearing and rifting stage (chapter 3.3.1). Major faults, parallel with the coastal fault, affect sediments up to the Present. These later vertical movements may be effects of the sUbsequent oceanic-continental shearing phase (crustal upheaval) and of the passive phase (flexural sUbsidence) (chapters 2.2 and 2.3). At the western end of the margin, the dating of the faulting from seismic data can only be done by analogy, due to the absence of wells (chapter 3.2.1). The marginal ridges are likely of syn-tectonic age, i.e. Albo-Aptian, followed by differential subsidence until present times. The transform margin may
49
consequently be structured by the end of the intra-continental wrenching phase. Later vertical movements have accentuated this structural pattern. Due to the absence of wells, it is not yet possible to see whether there are any diachronisms of wrenching events along the margin. The marginal ridges in prolongation of the saint Paul Fracture Zone have followed, at their onset off southeastern Liberia, the Eburnean foliation trend and major Precambrian strike-slip lineaments (chapters 3.1.2B and 3.2.1A and -B). Eastward, the margin of western Cote d'Ivoire, however, does not display the same heritage of strike (chapter 3.2.2). Along the Ivorian Basin, the wrench zone shifts to a more eastward orientation, making a wider angle with the northeastern Eburnean foliation trend which has probably had only a minor influence on the orientation of the main Mesozoic structural directions. There is no evid~nce of magmatic activity along this transform margin, except at the southeast Liberian end (chapter 3.2.1A) . 3.4.2 stratigraphic sequence
The litho logic record at both ends of the shearing zone is similar (chapters 3.2 and 3.3.2). The first section consists of a very thick Albo-Aptian clastic section with marine incursions already in the Aptian. Off Liberia, the interval is preceded by volcanism and a Paleozoic section. On seismic data the Upper Albian has a strong reflectivity, and it has been interpreted as carbonates or volcanics off southeastern Liberia. The Albian section is topped by a tectonic unconformity of Late Albian/Early Cenomanian age in eastern Cote d'Ivoire, followed by Upper Cretaceous sediments reflecting a slow down in subsidence. The regressive Cenomanian is contemporaneous of the passage of the accretionary ridge. At the western end, the tectonic unconformity is only known to be located between the Lower and Upper Cretaceous and to be overlain similarly by a thin Upper Cretaceous. At both ends, the Senonian shows low amplitUdes on seismic records. The Upper Cretaceous is truncated at its top by an erosion which could be Paleocene at both ends of the margin. A second major erosion occurred in the Oligocene. On the continental shelf and upper slope of western Cote d'Ivoire (chapter 3.2.2), Part 11, below, brings information as to the litho-acoustic section, its possible thickness, and suggestions for the age of the unconformities.
50
51 PART 11: ANALYSIS OF THE WESTERN IVORIAN TRANSFORM MARGIN
In this Part 11, the multi-channel reflection seismic, gravity and magnetic survey, recorded by the seismic contractor firm GECO (today GECO-PRAKLA) in 1986, is presented and interpreted. The resulting model for this western margin of Cote d'Ivoire is compared with present cases of intra-continental and continental-oceanic shearing models introduced in Part I, chapter 2. The stages in the evolution are dated with the results in chapter 3. 4.
DATA BASE
4.1
Objectives and methods
The objective was to acquire seismic profiles over a stretch of the West African continental shelf where very little oil exploration work had been done. The only background information available consisted of satellite gravimetric maps. As very little academic work has been done as well, this information of a sub-regional character is of interest for the study of the African transform margin, in prolongation of the saint Paul Fracture Zone. Being an industry project, the surveyed area covers only the shelf and the upper and lower slope (Fig 1-2). Therefore, no information is available over the continental rise, so questions related to the oceanic/continental crustal contact cannot be answered. The satellite gravity data over the studied area pointed at a basin on the continental shelf. The basin had not been previously been reported. The dip line density which generally is 6 km, was consequently increased to 2 km (Fig 1-2). At the far western end, which was considered of less potential for oil exploration, the lines are 10 to 19 km apart. The dip lines were drawn from the near shore down to 2,000 - 2,500 m of water depth (WD) , as the upper slope is very steep and was expected to be underlain by chaotic reflectors. The more gentle lower slope, with more uniform structuring, would be better suited to tie the interpretation of the lines. The spacing of the strike lines is 3 km on the outer shelf and down to 1,000 m of water. One line was drawn near shore and a last tie-line at 1,500 to 2,000 m water depth. The acquisition was carried out in two phases with M/V GECO MY, 1,610 km in January 1986 and 760 km in May of the same year. The seismic acquisition parameters are reported in chapter 7.1.1. Gravity and magnetics were not recorded in the first phase. The acquisition parameters in May 1986 are reported in chapter 6.
52 The primary navigation system used was Maxiran. The secondary was by satellite. Seismic processing was done at the GECO center of Sandvika, Norway and is reported in chapter 7.1.1. Gravity and magnetic modeling was done concomitantly in Sandvika with the start of the seismic interpretation which could then provide the necessary modeling constraints. Regional geology has been of significant assistance in understanding this environment which differs significantly from standard petroleum geology. Only two wells could be used for geologic control (K1-1X and Kl-2X, Fig 5-1) so available lines in Liberia and Cote d'Ivoire were used for comparison as well as eustatic curves by Haq et al. (1987) and Petters (1983). Finally, seismic interval velocities were used for the lithoacoustic interpretation. 5.
BATHYMETRY
The water depth recordings on the continental shelf are of very poor quality. The majority of the readings were off by up to 50 m, with an average discrepancy of 30 m at intersections. At the slope and continental rise, the values were of fair quality, although the instrument used, a Simrad echosounder, is not designed for water depths over 1,000 m. The posted values were compared with the onset of the sea bottom reflector, using a water velocity of 1,480
m/so On the map (Fig 5-1), echo-sounder values have been used beyond the shelf-edge and corrected only when they were obviously incorrect. The accuracy should be around ± 25 m. On the shelf, very little contouring could be done shallower than 100 m or 135 ms TWT (milliseconds two-way time), as the water bottom reflector on the sections had been muted in the processing. In the studied area, the shelf is 35 km wide at Sassandra and narrows down to 21 km at Cape Palmas (Fig 5-1). The shelf-edge has a very gUllied appearance and breaks at depths between 100 and 150 m, with an average at 120 m (Martin, 1973). The continental slope is very steep down to 2,500 m with a gradient in the order of 7-9 deg on the upper slope down to 1,000 m depth, and of 5-6 deg in deeper waters. The slope is very irregular and cut by many small troughs as seen in the sea-bottom map (Fig 5-1). In addition, the steep slopes have caused important slumpings, as observed in Fig 7-2, 20 km from the origin of the line, where the fault plane at sea-bottom is over 200 m high.
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100 clastics. On the lower slope, the poor internal reflectivity of the downlapping lower sequences indicates a clayey content whereas the upper members and the sequences at the western end are probably of alternating coarse and fine clastics (Figs 7-5 and 7-6). 7.4.3 Eocene - Neogene
The seismic facies of the rest of the section is interpreted as clastics, similar to what is found in the shalerich Cote d'Ivoire Basin (Fig 3-8). The Paleocene Oligocene interval (PL-OL) shows clastic interval velocities (2,000 - 2,700 mjs on the shelf and 1,700 - 2,000 mjs on the slope). The Neogene (post-OL) sequence reveals figures (1,700 to 2,300 mjs on the shelf and 1,600 to 1,800 mjs on the slope) indicative of unconsolidated clastics, as in the Cote d'Ivoire Basin (chapter 3.3.3). On the shelf, the Tertiary for~s an on lapping wedge of possible shallow water deposits on top of the Paleocene and Oligocene erosional surfaces. Off the slope, the sediments are affected by gravitational faulting and slumpings. Down slope, the seismic signature is characteristic of olistostromes of clastic material in the Eocene. On some locations, turbiditic seismic facies can be observed in the Oligocene below the lower slope (Fig 7-5). Minor unconformities can be traced locally (Fig 7-9) and may be the reported Mid-Eocene and Miocene ones (Brancart, 1977). 7.5
Timing of faulting and subsidence evolution
7.5.1 Timing of faulting along the margin
It is mentioned in chapter 7.2 that the dating of the seismic reflectors remains speculative in the absence of well information within the studied area. The dating of the reflectors and of the tectonic events is, however, coherent within the studied area. Beneath the shelf south of Sassandra, the structures are bound by faults that stop at the end of the Albian. This corresponds to the end of the intra-continental shearing phase. By then, the lows are filled and the top of the interval forms a bank (Fig 7-3). After the Lower and Middle cretaceous, major faults continue to be active in the Senonian as shown by the reflector sequences down lapping upon the CE unconformity. Despite the seismic ringing, steep lower dips are visible, which gradually level upwards and give evidence to an initial more important vertical movement, probably after the Cenomanian-Turonian (CE) unconformity. This fault movememt decreases with time up to the Paleocene. The strike line (Fig 7-10), with draping Upper Cretaceous reflectors,
101 confirms this Lower Senonian fault movement. Similarly, the eastern wedge of the basin is down-faulted during Lower Senonian time, along the border fault (Fig 7-14). The paleoshelf-edge of the basin south of Sassandra is affected by Senonian faulting as well (Fig 7-9a). As no Upper Cretaceous sediments are left after the Paleocene slumpings, a more detailed age for the faulting cannot be given than between the Cenomanian-Turonian and the Paleocene. The important gravitational movements in the overlying cenozoic to present sediments and the steepness of the present slope, indicate a persistent and more important subsidence seaward of the shelf-edge than on the continental shelf. The rest of the shelf, westwardly, is affected by faults that die at the end of the Albian or in the early Upper Cretaceous (Fig 7-9b). As for the eastern shelf-edge, the subsidence rates increase markedly across the shelf-edge in the Cenozoic. On the continental slope, Upper Albian blocks are offset by faults that stop at the Top Albian reflector (AL) or in the Lower Senonian. The age of the movements is shown by the Cenomanian-Turonian and Lower Senonian onlaps on the structures formed (Fig 7-9): the movements may have occurred in two stages, in the Late Albian/Early Cenomanian and in the Lower Senonian. Gooma (1990) puts a pre-Cenomanian age for the elongated ridge on the western slope (Fig 7-9b). The rest of the Senonian (Sl-PL intervals) drapes over the structures until the Upper Paleocene (PL) (Fig 7-9). The unconformity shows only limited curvature, probably due to limited differential compaction and/or the end of the upheaval or compression of the block. The draping is particularly visible on top of the western deep water sediment covered ridge. 7.5.2 Subsidence evolution in relation to tectonic activity A calibration of subsidence rates is not attempted. Rates of subsidence may be estimated on the depth converted sections (Fig 7-19). The first interval between the acoustic basement and the Top Albian is the most important. It has a fairly constant 2 km of sediments, except near shore or on the basement highs. If the Albo-Aptian is not underlain by older layers, subsidence is comparable to the eastern Cote d'Ivoire Basin (Fig 3-9). Most faults die within the interval and the Top Albian horizon is regarded as the tectonic unconformity on this margin (chapter 3.1.3). The Cenomanian-Turonian is relatively thin where it is preserved/deposited and can be associated with the regressive phase observed in the eastern Cote d'Ivoire Basin (chapter
102 3.3.2). In the mapped area, the Cenomanian-Turonian corresponds to the onset of the Upper Cretaceous - Paleocene decrease in subsidence and to the upheaval or compression of blocks on the present-day slope (chapter 7.5.1). The decrease in subsidence is likely interrupted in the Lower Senonian by a foundering phase of the blocks defined in the previous chapter 7.5.1, as up to one km of steeply dipping and prograding sediments are interpreted at the foot of the major faults (Figs 7-19 and 7-20). A block foundering phase does not appear in the subsidence curves in the eastern Cote d'Ivoire Basin (Fig 3-9), but it is reported as variations in sea-level in the text (bathyal Lower Senonian sediments unconformibly overlay neritic Cenomanian-Turonian deposits, chapter 3.3.2). On the slope, the block upheavals continue in the Lower Senonian (Figs 79 and 7-20). From the Paleocene onwards, the upheaved blocks seem to be sUbject to a regIonal subsidence (Fig 7-9). The first observable transgression on the shelf is recorded by the on laps on top of the Upper Paleocene erosion. From the age of the first gravitational faults, the modern slope dates back to the Upper Paleocene. The subsidence curves in Fig 3-9 show an increase of rates in the Paleocene. Unlike during the two previous stages, subsidence has not acted within fault blocks but has a regional, flexural character with a break at the shelf-edge. The subsidence has been more important than the sedimentary influx as the slope is still very steep. The Oligocene erosion may mark a second halt in sUbsidence, as in Fig 3-9. The cumulate post-Albian subsidence is illustrated in Fig 7-19, where the Top Albian drops, from one end of the seismic sections to the other, from a depth of a few hundred meters, near shore, to over 4 km, at the lower slope. In summary, the margin acquired its present morphology in three stages (Fig 7-20): (1) In the Albo-Aptian (Middle Cretaceous), a general horst and graben framework is developed; (2) The Cenomanian to Lower Senonian is characterized by a decrease in subsidence rate and by the upheaval or compression of fault blocks on the present-day slope. Crustal founderings are interpreted in the Lower Senonian (large blocks bound by major ENE-WSW faults: the shelf-basins and the structures at the present continental slope); (3) Through the rest of the Upper cretaceous, the studied portion of the margin is SUbject to a continued slow subsidence rate. On the slope the block upheaval slows down. From the Paleocene to the Present, the margin is SUbject to a regional subsidence that has affected the continental slope much more than the shelf.
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profiles are those in Fig 7-16 (Figs 7-1 and 7-2, located in Fig 5-1) and the listed interval velocities have been used for the converSlon. The heavy lines are faults. The unconformities (thin lines) are: AB: Acoustic basement; AL: Top Albian; CE: Cenomanian-Turoniani PL: Upper Paleocene; OL: Oligocene.
104 7.6
Tectonic interpretation
As seen in the above structural analysis, the framework of the shelf results from basement features which form two deep and narrow grabens, parallel with the coast and the shelf-edge, and a central basement horst. The slope plunges very rapidly into deep waters, with faulted blocks in the east and an elongated ridge running along the western margin. This alternation of structures is found along strike and not in the dip direction, as in rift margins. Although data on variations of crusta 1 thickness are only available for the Cape Palmas area (Behrendt et al., 1974 and Fig 1-4), the western Cote d'Ivoire shelf and slope present regional characteristics of transform margins (chapter 1.2.2), in line with the results of the bibliographical synthesis in chapter 3. The eastern part however, has a pronounced tilted fault block structure which can be associated with rifted margins. The trends and structures, in this chapter, are interpreted in a wrench context. 7.6.1 structural trends
All faults observed are normal and the structures parallel with the general trend of the margin. No positive flower structure has been observed. The objective is first to test the transform nature of the margin, then to see whether there is a stress component other than shearing. An ellipse of strains in divergent wrenching is inserted next to the structural maps of the acoustic basement and of the Top Albian maps (Figs 7-12 and 7-13). The major, ENE-WSW faults coincide with the theoretical direction, which was first indicated by Le Pichon and Hayes (1971) and Francheteau and Le Pichon (1972) as the prolongation of the saint Paul marginal Ridges. This represents the principal direction of the right lateral-wrenching between western Cote d'Ivoire and Brazil and is still expressed by the coastline orientation. The second most frequent trend is NW-SE. It corresponds to the only tensional faults expected on the strain ellipse, with a 35-65 deg angle with the direction of main displacement. These trends are very common in the western part, south of San Pedro. There, the trends are arranged obliquely, en echelon, with respect to the main faults and downthrown to the west. They have opened the narrow elongated graben at the flexural hinge line on the eastern shelf (Fig 7-10). They offset parts of the main faults on the eastern slope and they also control the depression east of the studied area on the shelf. These trends are frequent in the eastern Sassandra half-graben but are only rarely expressed in the central basement high.
105 Although a potential normal slip direction, this trend is associated with right-lateral slip at both ends of the graben south of Sassandra. The faults delineate elongated troughs, later used by the Paleocene erosion, such as the Sassandra River canyon (Fig 7-15). In plan view, the faults splay towards land, following, in particular, an antithetic NNE-SSW direction. In section, they look like negative flowers. A third fault direction, oriented WNW-ESE or nearly eastwest, shows a small angle with the direction of wrenching and corresponds likely to synthetic strike-slip movements. This direction is followed by the northern fault closing the basin south of Sassandra. The synthetic fault is steep but normal, with an interpreted very large throw. On the acoustic basement map, the ridge closing off the same basin and the coastal fault are seen to be offset by east-west trends (Fig 7-12). A fourth direction, NE-SW, is secondary synthetic. It is expressed as offsets of the coastline, of the major faults and of the parallel western deep water ridge. It is noticeable that the Dimbokro and the Coastal Fault system have similar trends at their onshore terminations (Figs 7-12 and 7-13) . Finally, a NNE-SSW fault direction is observed on both sides of the master fault, north of the eastern Sassandra graben. If the faults result from lateral movements, the direction is antithetic. (On the ellipse, Figs 7-12 and 713, the antithetic strike-slip direction is oriented NNWSSE. The direction rotates however in the sense of wrenching with increased displacement magnitUde, Crowell and Ramirez, 1979). The elongate western monocline, with the positive structure at its foot, has a trend parallel with the major faults. The eastern slope structures however show a nearly eastwesterly trend. The mapped structural trends are compatible with a divergent shearing interpretation. The western deep water ridge may reflect a local compressional component (chapter 7.6.2B) . 7.6.2 structural interpretation and comparisons A. Intra-continental shearing stage
Fig 2-4, a compilation of features found in divergent wrenching, shows striking similarities with the studied margin mapped at the tectonic unconformity (TOp Albian map, Fig 7-13). The reported features are represented off western Cote d'Ivoire but with a difference in scale.
106 The basins mapped on the shelf display typical rhomboidal pull-apart orientations. The half-graben south of Sassandra (up to 15 km wide and 45 km long) is closed to the east by a junction of large basement faults (chapter 2.1.3A). The Albo-Aptian section thickens against these border fault (Fig 7-14). This fault movement may result from a release of stresses at the fault junction (Crowell, 1974a). From the west, the basin floor drops down from the western high, with a thickening overlying Albo-Aptian wedge. This second drop occurs in the direction of extension, against boundary faults (Fig 7-10). A similar example is described in the Dead Sea by Arbens (1984, in Reches, 1987) with a boundary fault "soling out" at 15 km depth. Such depths are beyond the limit of resolution of the seismic data in this survey. Pull-apart basins are often very deep despite their limited lateral size, and are rapidly filled (chapter 2.1.3C). It is possible that better seismic data will increase the depth of the present acoustic basement (4 km). The depth to magnetic basement is 7 to 9 km. In the Albo-Aptian margin reconstruction (Fig 7-20), the 3 km wide basement ridge parallel to the shelf-edge, and closing off the graben from the ocean side, may correspond to an inter-basinal ridge (chapter 2.1.3B). It occurs between non-overlapping, opposing, and opening basins as defined by Rosendahl et al. (1986) and Rosendahl (1987). The western basin has much larger dimensions and a more developed pull-apart than the one south of Sassandra (Mann et al., 1983). The fault-bound, 100 km long, northern flank dips as a forced monocline towards the ocean. The other flank may be on the Brazilian margin. The depression is closed towards the east by a set of en echelon faults forming releasing step-over faults (Harding et al., 1985). The western end lies outside of the surveyed area, in Liberian waters. Faulting and bUlging at the elongated positive structure may have initiated during the intracontinental shearing phase as an inter-basinal ridge. Such ridges can be of limited dimensions (500 m half wavelength, 100 m amplitude, chapter 2.1.3B), much smaller than the mapped positive structure (5 km across, 800 m amplitude, Fig 7-19b). Inter-basinal ridges may be masked, in the faulted part of the ridge, by the subsequent development during the Upper Cretaceous continental-oceanic shearing. A common feature of transform zones is the heritage of bedrock orientations (chapter 2.1.1A). Figs 3-1 and 6-2 show that the Saint Paul Ridges are oriented along with the Eburnean foliation trend and prolongate at least one Precambrian lineament. Off western Cote d'Ivoire, the break-up framework does not reflect preceding orientations. However, the eastward-dipping monocline at the eastern end of the studied area lies in the prolongation of the Dimbokro lineament (Figs 7-12 and 7-13) (chapter
107 3.1.2B). It may be controlled by the fault, forming part of its convergence with the zone of principal shearing. The breaking-up of the basement nearby into the graben, south of Sassandra, could be a consequence of such convergence. There is however, no available data, from the monocline eastwards. The coastal fault system, an eastern prolongation of the transform margin, reaches shore 40 km east of the studied area. Its onshore termination points towards the same convergence area. B. continental - oceanic shearing stage
The horst and graben framework of the margin, acquired during the intra-continental shearing stage, is reused in the following oceanic-continental shearing, in the Upper Cretaceous (Cenomanian to Lower Senonian, chapter 1.3.2). The phase is characterized by a decrease in subsidence rates, by the upheaval or compression of fault blocks and by crustal founderings (chapter 7.5.2). The crustal founderings, along the major, coast-parallel, faults are likely to occur in the Lower Senonian (chapter 7.5.2), with the formation of marginal plateaus (Fig 7-20). This foundering phase may be caused by isostatic adjustments, an indirect evidence of rapid variations in crustal thicknesses, as mode led at Cape Palmas (Fig 1-4). The marginal plateau of the Guaymas Basin is similarly bound by basement faults with important throws (chapter 2.2). Rotations, started in the Albo-Aptian, continue in the Lower Senonian in the pull-apart graben south of Sassandra. The Top Albian (AL) and the Cenomanian (CE) reflectors are flexured down into the graben from the western high. At the junction of the border faults of the graben (Fig 7-14), the Top Albian (AL) and Cenomanian (CE) are dropped and tilted against the border fault. The continental slopes, and in particular the western slope, are subject to block upheavals from the Cenomanian to the Lower Senonian, slowing down in the Upper Cretaceous and Paleocene (Figs 7-9, 7-19 and 7-20). These vertical movements may be mechanically induced, by a local compressional shearing component between opening basins (chapter 2.1.3B), and/or by a thermal impact from a hot oceanic crust (chapter 2.2). The continental slope south of Sassandra experienced, during this shearing phase, extension along dip, by block faulting (Fig 7-20: 4 km of the present total 28 within the line), similarly to what is described in the Gulf of Aqaba (chapter 2.1.3A). This would rule out a compressional shearing component, in favor for a thermal cause for the crustal upheaval. In the western part of the studied area, the elongated ridge, or very large drag fold, may indicate that the continental slope has been sUbject to more parallel shearing than the slope south of Sassandra.
108
0E===;==;;10~=~~2o:0'-_ _-'l30 Block
Neocomian to
End Albion
5
,
40
KM
faUI;:-~~--:~he ~argin ~'·I eo_ bottom::: Al
Senonian Extension
\
Int ra - Continental Shearing
\ \
AB r---~ -0- - - - - , - - - - - - - - , - - - - - - - - - , - - - - - - - - - - ,
..
Block Foundering ............. (Marginal Plateau)
"
Cenomanian to Lower Senonian
............
Block Upheaval (')
CE
AL
ContinentalOceanic Shearing
CE
AB Senonian
Beginning of Passive Phase ( Continued Block Upheaval)
c===:..::::---r
1
r------------------,----,
Differential Subsidence
I
I
Paleocene
= Onset of
PL
Differential Subsidence
CE L
PL
Present
Morphology
'-
-_--"CE~~
AL
Sea
-bottom OL
2
3
PL
AB
CE AL AB
4 5
KM
Figure 7-20: Reconstruction of the tectonic evolution of the western Ivorian margin. The reconstruction is focused on the eastern part of the studied area, Fig 7-1, located in Fig 5-1. The layers have not been decompacted. The heavy lines are faults. The unconformities (thin lines) are: AB: Acoustic basement; AL: Top Albian; CE: CenomanianTuronian; PL: Upper Paleocene; OL: Oligocene.
109 The elongated ridge (5 km half wavelength and 800 m amplitude at the Top Albian) is of similar dimension to the ridge on the active rim of the Guaymas Transform Fault (chapter 2.2), but it consists seemingly only of compressed, or upheaved sediments and basement. There is no indication of oceanic crust. The elongated ridge is very similar to the ridge on the passive side of the Guaymas Transform Fault (chapter 2.3). This may be an indication of a thermal origin for the Ivorian ridge. Both this ridge and the upheaved fault blocks south of Sassandra are not located far from oceanic crust (i.e. the saint Paul Ridge, chapter 3.2.1A, Fig 8-1). As the amount of erosion on the upheaved blocks is limited, the amplitude of the thermal effect seems small in comparison with the model by Todd and Keen (1989) (2 km of upheaval, followed by erosion, chapters 2.2 and 3.1.3). The Ivorian elongated ridge is of much larger dimension than the ridge cJosing the eastern shelf basin. Compared to this deep water ridge, the absence of folding on top of the eastern ridge may be due to a thin sedimentary cover or to limited and divergent wrenching (chapter 2.1.3B). The present interpretation has not revealed any magmatic activity during the active shearing phases. C. Passive stage
The low subsidence rates and the block upheavals continue in the passive phase, in the Upper Senonian. The beginning of this last phase may be contemporaneous with a slow-down of block upheavals. The continued vertical motion may be caused by lateral conduction of heat from a hot newly emplaced, but now stationary oceanic crust, to the cold continental crust (chapter 2.3). It is only from the Upper Paleocene that the margin is subject to a regional subsidence, in accordance with Fig 39. This new development starts with an unconformity which coincides with a major worldwide sea level drop. The subsidence is differentail on the shelf and the continental slope (Fig 7-20) (chapter 7.5.2). This may be a further indirect evidence of important crusta 1 variations, a characteristic feature of transform margins (Scrutton, 1982a). 7.7
Conclusion on the data interpretation
The disclosure of the structures on the west Ivorian margin, required gravity and magnetic data for the determination of a depth to crystalline basement. Reflection seismic was required for a detailed description of this portion of the margin and for a proposed lithologic column. Regional geology was necessary for the description of the geologic characteristics of the margin.
110 In summary, the interpretation of the data from the GECO (today GECO-PRAKLA) geophysical survey fulfills the objectives set in the introduction (chapter 1.1.1). The interpretation confirms and develops the assumptions on the tectonic framework and the stratigraphic sequence set in the regional conclusions (chapter 3.4). In the next part (Part Ill), the interpretation of the west Ivorian margin, is compared with the regional geology and tectonic environment. 7.7.1 Tectonic framework
As discussed in the regional geology (chapter 3), the middle part of the saint Paul African transform margin, appears to be structured in the Albo-Aptian by an intracontinental shearing, topped by a Top Albian unconformity of possibly Late Albian/Early Cenomanian age (chapter 7.5.1). As predicted from regional data, the shearing regime, which has formed the structures observed, was transtensional with a coast-parallel dominant faulting. The expected fault pattern corresponds to the theoretical model (chapter 2.1), and to modern examples (chapters 2.1 and 7.6). No transpressive structures have been observed. The following oceanic-continental shearing in the Upper Cretaceous (Cenomanian to Lower Senonian) is contemporaneous with a drop in subsidence rates and with the upheaval of crustal blocks. In the Lower Senonian, a phase of foundering of large crusta I blocks occurred along the major faults. The low subsidence rates and block upheavals may be caused by thermal exchanges with the oceanic crust. The elongated ridge may also be mechanically induced, by parallel shearing. The thermal exchanges slow down in the passive phase, from the Santonian to the Paleocene. From the Paleocene, the margin has been subject to a differential subsidence of a flexural character (see also Fig 3-9). No compressional structures have been identified. The block founderings and the differential subsidence are indirect indications of rapid crustal thickness variations, a common character of sheared margins. The heritage of Precambrian trends is not visible on seismic sections and appears as a weak ENE trend on the shelf on the gravity and magnetic anomaly maps. The possible convergence of the Precambrian Dimbokro lineament with the transform margin may be responsible for the structures in the eastern part of the studied shelf. Although the saint Paul Ridge is located at the edge of the studied area (Fig 3-1), and is probably of oceanic nature or a seamount, no magmatic activity has been traced in the mapped area.
111 7.7.2 stratigraphic sequence From the seismic character, the entire section seems to consist of clastics. They are probably coarse in the syntectonic Albo-Aptian section and shale-rich from the Upper Cretaceous to the Present. There is no indication of older sediments. In the Upper Albian a carbonate bank may be present. 7.7.3 suggestions for future research within the studied area, the interpretation in this Part 11 has left, in particular, the following questions unanswered:
* *
*
*
The evolution of the tectonic subsidence (as in Fig 3-9, offshore eastern Cote d'Ivoire); The possible reuse of Precambrian lineaments and structural trends in the intra-continental shearing. Only minor effects are mapped at this stage, while offshore southeastern Liberia, the lineaments appear to control the marginal ridges (Fig 3-1). The variations in crustal thickness across the margin (as in Fig 1-4, offshore southeastern Liberia); The location of the oceanic crust which is suspected to be next to the stUdied area (i.e. the Saint Paul Ridge, Fig 3-1).
The interpretation covers only a portion of the African continental margin in prolongation of the Saint Paul Fracture Zone. It would be of interest to expand the work from southeastern Liberia to Ghana, to calibrate the interpretation with existing wells, and to prolongate the seismic, gravity and magnetic profiles into deeper waters. Questions that could be answered are:
*
* * *
* * *
The location of the transtensive and transpressive parts along the margin. The Saint Paul marginal Ridges, offshore southeastern Liberia, are schematically mapped. It is not known if they are formed by transtensive or transpressive shearing; The effect of Precambrian structural trends and lineaments on the development of the margin; The location and the characteristics of the oceaniccontinental contact along the margin; The effects of the oceanic-continental shearing along the margin; The thickness of the continental crust across the margin. The crust should be thick as the margin is bounded onshore by a Precambrian shield (Figs 1-4 and 3-1); The possible diachronism of unconformities and faulting along the margin; The relative dating and geographical extent of the two mentioned tectonic unconformities (chapter 3.3.2): the rift and the shearing unconformities. Only one tectonic
112 unconformity is identified offshore western Cote d'Ivoire (chapter 7.2.2). These answers would provide the elements for a reconstruction of the shearing phases along the entire saint Paul controlled African margin.
113 PART Ill: COMPARISONS OF THE WESTERN IVORIAN MARGIN WITH THE CONTINENTAL TERMINATIONS OF THE SAINT PAUL FRACTURE ZONE AND WITH PART OF THE AFRICAN TERMINATION OF THE ROMANCHE FRACTURE ZONE
The first control of the structural and litho logic interpretation of the western margin of Cote d'Ivoire (Part II above) is a comparison with the reviewed structural geology and lithologic sequence along the African transform margin in prolongation of the saint Paul Fracture Zone (chapter 3). A second control is to compare with the conjugate margin, the Ilha de Santana Platform, off northern Brazil, at the western end of the saint Paul Fracture Zone (Figs 11 and 1-5). Thirdly, the tectonic evolution of the west Ivorian margin is matched with the evolution observed on the continental margin controlled by the Romanche Fracture Zone, off eastern Cote d'Ivoire and Ghana (Fig 1-1). 8.
SOUTHEASTERN LIBERIA AND EASTERN COTE D'IVOIRE
8.1
Saint Paul marginal Ridges
Figs 6-2 and 6-5 show that the gravity and magnetic anomaly fields of the margins of southeastern Liberia and western Cote d'Ivoire display similar trends. The present interpretation and the structural sketches of Schlee et al. (1974) and Mascle (1977) have a good coherency (Fig 8-1). The faults at the westernmost Ivorian shelfedge are in prolongation of the Liberian faults at the shelf-break. Additionally, the western deep water ridge may be in continuation of the Cape Palmas Ridge. The southernmost, Saint Paul Ridge, lying off southeastern Liberia, which may be of oceanic origin or is a seamount (chapter 3.2.1A), is outside of the studied area. The two grabens are comparable to the Liberian "pocket basins". They have similar size, shape and depths to acoustic and magnetic basement. The western deep water ridge is similar to the sediment covered horst, which is part of the Cape Palmas Ridge (Fig 3-5b). Their widths and paleoreliefs are comparable. The ridges in Fig 3-5a still have a sea-bottom topography, whereas those offshore western Cote d'Ivoire are buried under sediments. Detailed structural information on the margin of southeastern Liberia is not published. No comparisons can therefore be made on the intra-continental shearing regimes which have been longest offshore southeastern Liberia. Similar variations in sedimentation rates are reported both offshore southeastern Liberia and offshore western Cote d'Ivoire, with a very thick sequence of Lower Cretaceous and a thin Upper Cretaceous. The still pronounced sea-
114
bottom topography of the marginal ridges off southeastern Liberia is indicative of a continued differential subsidence in the passive stage. A similar differential subsidence is found across the shelf-edge of western Cote d'Ivoire. The inter-basinal basinal ridges have followed the regional subsidence of the Ivorian margin.
o .........................290km
i !
j
11"00'
10'00'
3"00'
5"00'
Figure 8-1: Comparison of the tectonic trends of the western Ivorian margin with the trends on the margin of southeastern Liberia. The eastern part of the map is from Fig 7-13, the western part from Fig 3-2. Normal faults are drawn with notches in the direction of throw. In hatching, small basins with more than 2 km of sediments on top of acoustic basement, and 4 km on top of magnetic basement (the magnetic basement of the westernmost Ivorlan margin is not known). water depth curves (WD) are dashed.
Unlike western Cote d'Ivoire, volcanism is reported off shore southeastern Liberia in the Late Jurassic/Neocomian. In the basins north of the marginal ridges, faulting and folding is reported at the end of the Mesozoic or beginning of the Cenozoic (chapter 3.2.1C). It is not specified whether the movements are gravitational, as in western Cote d'Ivoire, or tectonic. 8.2
Cote d'Ivoire/Ghana Basin
Pull-apart basins and inter-basinal highs similar to those on the west Ivorian margin (Fig 8-1) are not reported in the Cote d'Ivoire/Ghana Basin. The east-westerly strikes at the Top Albian unconformity, in the eastern part of the studied area (Fig 7-13), may be in trend with the parallel strikes in the Cote d'Ivoire Basin (Fig 3-7). This orientation disappears westwards, at the foot of the central basement high, and gives place to the predominant ENE-W8W strike.
115 The upper slope, south of Sassandra, has probably been subject to shearing with a predominant tensional component, along the edge of the Ivorian rift margin. To the west, the trend has likely been parallel with shearing. The margin south of Sassandra is thus a hinge area between transtensive and more purely transform motion. The eastern coastal fault system turns seaward 40 km east of the studied area and is interpreted to join the shelfedge faults (Schlee et al., 1974). It is probable that the monocline, at the eastern end of the studied area, is part of the fault system, as Gooma (1990) interprets it to be a pUll-apart basin (chapter 3.2.2). A shallow basement high is bound there by a NNW trending, ENE dipping monocline, that dips into a deep shelf basin (Fig 7-13). The fault system would then merge with the interpreted shelf-edge faults at the convergence with the Dimbokro lineament (chapter 7.6.2). As in western Cote d'Ivoire, faulting is Albo-Aptian. Continued movements at major faults is reported through the Upper Cretaceous and the Ceno2oic (Brancart, 1977). It is not specified whether it is gravitational from the Paleocene onwards. Subsidence slowed in the Senonian at both locations but block founderings are not reported and can only be supposed, from lithologic variations, in the eastern Cote d'Ivoire Basin. 8.3
stratigraphic sequence
The stratigraphic sequence proposed, from the seismic data offshore western Cote d'Ivoire, is very similar to those offshore Liberia and eastern Cote d'Ivoire. The entire sections consist of clastics which are likely shale-rich. Like in the Ivorian Basin, there is, off western Cote d'Ivoire, no indications of a Paleo2oic section, nor of volcanics, as offshore Liberia. Possible Upper Albian carbonates in western Cote d'Ivoire may be represented off southeastern Liberia. Due to the absence of wells, the age of the unconformities cannot be dated along the entire margin controlled by the Saint Paul Fracture Zone. Diachronisms of wrenching events cannot yet be studied. The Late Albian/Early Cenomanian shearing unconformity off Liberia is known to be dated between the Lower and upper Cretaceous. The erosional phases during the passive phase (Upper Paleocene and Oligocene) may be of the same age along the entire margin.
116 9.
NORTH BRAZILIAN CONJUGATE MARGIN OF COTE D'IVOIRE: ILHA DE SANTANA PLATFORM
Fig 1-5a shows the conjugate margin of western Cote d'Ivoire along the northern Brazilian shelf in a preopening position. The Gulf of Cote d'Ivoire is occupied by the Ilha de Santana Platform, also called Para-Maranhao or Para (for commodity reason, called Ilha de Santana). 9.1
Generalities
9.1.1 Physiography of the continental margin
Like on the West African side, the saint Paul Fracture Zone is expressed off northern Brazil by elongate, discontinuous ridges and seamounts (Fig 9-1). They have a ESE-WNW or more east-westerly trend. The southernmost one is in trend with the northern edge of the Ilha de Santana Platform. From 44 deg W, or from the town of Sao Luis, the continental shelf widens significantly from approximately 60-80 km along northeastern Brazil to over 200 km over the platform. Further west, off the Amazon River, the shelf-edge runs over 300 km from the coast line. The continental slope is fairly steep and gUllied except for the Amazon Cone that has an extremely gentle slope. 9.1.2 Basement shield
The most prominent structural feature onshore is the ENEWSW trending Amazon Trough, with a Paleozoic to Recent infill. The trough reaches the coast next to the conjugate margin of the studied area (Fig 9-1). The trough separates the north-lying Guiana shield from the Brazilian craton, which is mostly covered by a Paleozoic basin. At outcrops in coastal areas, the two cratons consist of metamorphosed formations and granites from the equivalent of the Eburnean orogeny (2,000 Ma, Gouyet, 1988) (chapter 3.1.2). Overall, foliation and lineaments follow NNW to WNW trends or the WSW to SW orientation of the Amazon Trough (Fig 9-1). On the continental shelf, the magnetic anomalies in the mouth of the Amazon and on the Ilha de Santana Platform are oriented NE-SW. They have the same wavelength and amplitUde patterns as trends, found onshore Brazil, which are of Eburnean equivalent age (Milliman, 1979). The pattern is the same as off southeastern Liberia and Cote d'Ivoire. The margin is thus considered by Milliman (1979) to be under-
Figure 9-1: Bathymetry, structural and location map of the Brazilian conjugate margin of Cote d'Ivoire (compilation of Almeida, 1978;
Gouyet, 1988; Schobbenhaus et al., 1981).
50 0 W
45°W
0
40 W 4°N Ir--/I ~I Precambrian Basement with foliation \ trend, lineaments, dolerite dykes Sedimentary Basins Onshore Offshore Platforms 1,..,....1 Normal Faults Oceanic Fracture Zone with ass. ~""' Marginal Ridges, with .Outcropping mRldg_~!>m
o
D
ts:::'3
,--,-~~~S:::;a;,in;.:t Pa u I Fr a ctu re >-1'-"--'1 " Zone~-...J
Paleozo ic Basin
n
L -_ _-'-'--'--_ _-'-------ll-----'L-
o ---"-L--'--
100
200 km ---'--'--L-_-'---L-----'
4° S
118
lain by a basement of Eburnean equivalent age. Northwards, the magnetic anomalies on the shelf, north of the mouth of the Amazon, are associated with anomalies of Liberian equivalent age (2,700 Ma, Milliman, 1979) (chapter 3.1.2). 9.2
structure and stratigraphy
9.2.1 western part of the platform and mouth of the Amazon River Little is published on the conjugate margin of western Cote d'Ivoire, most likely because of the poor seismic data quality (Gouyet, 1988). A thick Tertiary carbonate bank masks, on the seismic sections, the underlying basement structures. Information is available from the adjacent depocenter of the mouth of the Amazon, broken up into several individual grabens (Fig 9-1). The northern flank of the platform and the continental rise are oriented parallel with the saint Paul Fracture Zone. To the northwest, the platform is defined by faults with a NEsw heading. They bound the mouth of the Amazon depocenter and may take up the Paleozoic weakness trend of the Amazon Trough (Fig 9-1). Regionally, the throws involved are significant oceanwards (over 2,000 m, Rezende and Ferradaes, 1971) but are small towards the depocenter, towards which the basement plunges gently (Gouyet, 1988). Beyond the platform, the Saint Paul trend is prolongated by seaward-dipping faults and ridges beneath the Amazon Cone (Rezende and Ferradaes, 1971 and Kumar et al., 1976). On the northern seaward edge of the platform, reverse faulting and folds appear to be absent. Along the western and northwestern flank of the platform, faults have normal throws and trend WSW-ENE to SW-NE and E-W (following possibly secondary synthetic or antithetic directions) and oblique NNW-SSE faults, controlling the opening of grabens in the depocenter. This gives evidence to a tensional component in the opening phase of the Equatorial Atlantic (Gouyet, 1988). The tectonic movements, leading to the opening of the Equatorial Atlantic, are preceded in the Triassic - Jurassic by basaltic magmatism (Rezende and Ferradaes, 1971). The sediments up to Paleocene age are coarse fluvio-deltaic to slope clastics. Their deposition is controlled by tectonism (Fig 9-4). The sequence is a transgressive megacycle, with a lower portion characterized by extensive terrigenous sedimentation. The Upper cretaceous, starting with the Cenomanian resting discordantly (rift unconformity) on the underlying sediments, is still continental with minor marine intercalations. The Paleocene is marked by a more extensive marine transgression (Carneiro de Castro et al., 1978).
119
The area is affected through the Upper Cretaceous by late tensional or transtensional movements (Gouyet, 1988). From the Eocene the entire mouth of the Amazon area is subject to regional subsidence and to the spreading of an extensive Eocene to Middle Miocene carbonate bank on top of a second major Meso-Cenozoic unconformity (Fig 9-4). From the Upper Eocene to the Present, the area is sUbject to tectonic movements which may be linked to the Caribbean tectonic evolution (Campos et al., 1987). Over the entire area, the Middle Miocene to Recent unit consists of fluvio-deltaic, submarine fan and slope systems which cover the mouth of the Amazon and the platform. This unit is separated from the older sequence by a third regional unconformity linked to the uplift of the Andes (Carneiro de Castro et al., 1978). 9.2.2 Eastern paFt of the platform: Para-Maranhao Basin On the eastern part of the Ilha de Santana Platform, the Para-Maranhao Basin is bound to the south by the platform and westward by the mouth of the Amazon area (Fig 9-1). The basin is dog-leg shaped on the continental shelf (Fig 9-2). In the west, the basin is dominated by east-west trending faults and on its southeastern leg by a NW-SE striking
PARTE OESTE
Fig9-3 t--l SEGAO GEOLOG1CA CONTOR NO ESTRUTURAL "'3 .. "
TEMPO S{SM1CO DUPLO (SEGUNDOS)
09
PN;OS
o ,
GRABEN DEI LHA DE SANTANA
\0
20
~o
'
100
zone de fracture de Saint Paul zone de fracture de la Romanche 3'N
k'==d' 3'W
2'W
l'W
0'
Figure 10-1: Schematic structural map of the margin of western Ghana and of the deep Cote d'Ivoire Basin {Modified after Latil-Brun et al., 198B}. The basin is shown in hatchings. Enlargement of insert 2 in Fig 3-7. Legend is found there.
125 by corridors of nearly vertical, normal and reverse faults, including flower structures and transverse faults (Popoff et al., 1989). The deformations rapidly decrease northwards in the deep Ivorian Basin (C), where reflectors are continuous and sub-horizontal. There is low amplitude folding (NEE-SSW trending) and few normal faults.
C B N -----=-----~.o_-_-=
5
ST
•
A
-.----'--'---~.-
'I .,
:....
