mainly of reddish brown sandstones and marly and shaly intercala- ..... because of their strong induration, with water contents of -30%. The relicts of the Littoral ...
Netherlands Journal of Sea Research 17 (2-4): 364-384 (1984)
STRUCTURAL AND SEDIMENTARY GEOLOGY THE CONGO AND SOUTHERN GABON CONTINENTAL SHELF; A SEISMIC AND ACOUSTIC REFLECTION SURVEY
OF
by J. n . E JANSEN,
Netherlands Institute for Sea Research, t70 Box 59, 1790 AB Den Burg 7~xel, The Netherlands
R G I R E S S E * and G. M O G U E D E T * * Dipartement de G~olagie, Universitg Marien NGouabi, Brazzaville, PR. of Congo
CONTENTS 1. 2. 3. 4. 5. 6. 7.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bathymetry and morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pre-Quaternary geology and seismic records . . . . . . . . . . . . . . . . . . . . . . . . . . Quaternary geology and acoustic records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R6sum6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
364 366 368 374 381 381 382
1. I N T R O D U C T I O N T h e present investigation forms part of a project to study the geology of the continental shelf b o r d e r i n g the People's Republic of C o n g o (GIRESSE, KOU¥OUMONTZAKIS & MOGUEDET, 1979). Before this prog r a m started m u c h , was already k n o w n of the surficial sediments of this area, due to the collection of n u m e r o u s grab samples and short cores. Nevertheless the data c o n c e r n i n g the extent and distribution of sediment thickness showed m a n y gaps, and the spatial relationships were insufficiently understood. In A p r i l - M a y 1978 a joint expedition was u n d e r t a k e n by the N e t h e r l a n d s Institute of Sea Research a n d the G e o l o g y D e p a r t m e n t of the University of Brazzaville to carry out continuous acoustic and shallow seismic reflection surveys off C o n g o and s o u t h e r n G a b o n on b o a r d the R.V. "Andr6 N i z e r y " (Fig. 1). T h e * Present address: Centre de Recherches de S6dimentologie marine, Universit6 de Perpignan, Avenue de Villeneuve, 66025 Perpignan, France. ** Present address: Laboratoire de G~ologie, Universit~ d'Angers, Boulevard Lavoisier, Belle-Beille, 49045 Angers Cedex, France.
365
G E O L O G Y C O N G O - G A B O N SHELF
,
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366
J.H.E JANSEN,
P GIRESSE
& G. M O G U E D E T
acoustic equipment consisted of four 3.5 kc transducers (ORE 137), a 2 kW Raytheon transceiver, and a Raytheon recorder. The seismic operations were undertaken with a small air gun (50 cm3), a single channel streamer with 32 hydrophones (EI 100), a filter, transceiver and recorder (Barr & Stroud, O R E 140, EPC 3200). In addition the records were included of two seismic and acoustic lines from the cruise of the R.V. "Tyro" in 1978 by the Netherlands Institute for Sea Research. Acknowlegdements.--We wish to thank Dr B. Piton, director of the O R S q l ) M Centre at Pointe Noire, who kindly made available the R.V. "Andr6 Nizery". We are indebted to Captain L. Plessis and the crew for their pleasant co-operation on board, and to the technicians T.C.J. Buisman and F.J. Schilling for their indispensible contributions to the expedition. R.ED. Aggenbach and H. Hobbelink are gratefully acknowledged for drawing the illustrations, and MrsJ. Hart for typing the manuscript. 2. BATHYMETRY AND MORPHOLOGY The majority of the coasts in this area are lowland coasts formed under the influence of an important coastal drift from SE to NW. In Congo a series of small lagoons are found which are nearly completely isolated and acting as lakes. There are also several large lagoons such as those of Benge, Conkouati, and Banio. Behind the shoreline 2 or 3 barriers rise to nearly 20 m above sea level which are due to aeolian action during the last Pleistocene regression period when the climate was more arid than at the present time. The continental shelf shows only little relief. The bathymetry map (Fig. 2) was constructed from the acoustic profiles, based on an assumed sound velocity in sea water of 1500 m-s -1 and using supplementary information from GIRESSE & KOUYOUMONTZAKIS(1973). Most isobaths run markedly parallel to the shoreline. Down to 100-105 m water depth the slope is very uniform, averaging 7'. Beyond the 110 m depth line the slope decreases, forming probably a marine abrasion level which is well known from several Atlantic shelves of Africa and originates from the last Weichselian and older regression phases (GIRESSE & KOUYOUMONTZAKIS,1973). The shelf is on the average 50 to 60 km wide and passes into the continental slope between 120 and 140 m where the slope increases to 1% The slope amounts to 2 ° at 150 to 180 m and to - 10 ° beyond. Further southward, towards the mouth of the river Zaire (Congo), the shelf becomes smaller because the Congo canyon crosses the shelf diagonally. A deviating direction of - 110°
GEOLOGY
CONGO-GABON
367
SHELF
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368
J.H.E ,JANSEN, E GIRESSE
& G. M O G U E I ) E T
can be observed superimposed upon the NW-SE (135 °) trend of the isobaths. This direction, which is also visible in the shape of the coastline, is the same as the trend of the Mesozoic and Cenozoic outcrops on land and subcrops on the shelf (Fig. 4; GIRESSE & TCmKAYA, 1975), and is therefore attributed to subsurface structure. 3. P R E - Q U A T E R N A R Y GEOLOGY AND A I R - G U N RECORDS The first part of this section outlines the geology of the Congo-Gabon region, mainly based on VINCOTTE (1938), HIRTZ (1964), GIRESSE & KOUVOUMONTZAKIS (1971, 1973), HOURCQ (1966), REYRE (1966a, 1966b), FRANKS & NAIRN (1973), BRINK (1974), GIRESSE & TCHIKAYA (1975), GIRESSE & CORNEN (1976), CORNEN et al. (1977), and on data from petroleum exploration boreholes (Elf-Congo, Petrango, personal communication). The Atlantic coastal plain is predominated by the relief of the Precambrium Maycombe Chain which shows an intensive Pan-African folding, granitisation, metamorphism, overthrusting, etc. Near Mayumba, outcrops stretch as far as the beach and form the boundary between the rather deep Gabon Basin in the north and the Congo Basin in the south which extends over the enclave Cabinda, Zaire, and north Angola. Schematically the two basins show the following development: (a) The lower sequence of Late Jurassic to Aptian age is mainly terrestrial. Thick fluvio-lacustrine deposits were followed by sediments of the first marine episodes reflecting the initial opening of the South Atlantic Ocean. At the outcrops, the oldest transgressive layers are fluvio-marine levels that filled up the depressions at the borderland early in the Cretaceous. (b) The middle, Aptian, part has a strongly evaporitic character, especially in the eastern area. When the incumbent load was large enough, an intensive diapirism arose. The salt domes are believed to have reached their present form during the Oligocene erosional period (BRINK, 1974). Diapirs occur in the subsurface of the Gabon mainland, while in Congo their presence is only demonstrated on the shelf and continental slope. (c) In the upper, post-salt sequence marine deposits dominate from the Late Aptian to the Holocene. -During the Albian, the environment became open marine. Albian layers, recognized from seismic investigations and borings, are absent from the exposures in Congo. - D u r i n g the Cenomanian, continental conditions prevailed at the present inner shelf area of the Gabon Basin. The deposits consist mainly of reddish brown sandstones and marly and shaly intercala-
GEOLOGY CONGO-GABON SHELF
369
tions with anhydrite and gypsum (BRINK, 1974). Towards the present outer shelf they merge laterally into fully marine siltstones and shales. Several outcrops are present at about - 2 5 metres near the boundary between Congo and Gabon (GIRESSE & TCHIKAYA, 1975). Cenomanian deposits are also reported from the shallow subsurface of the bay of Pointe-Noire at the "Plage mondaine" (GIRESSE & KOUYOUMONTZAKIS, 1971). - T h e Turonian formations of the Gabon Basin are characterized by a dominantly carbonate facies (BRINK, 1974). Exposures are found at various coastal locations, but also between the 25 and 50 m isobaths off Mayumba, Pointe Banda, and Cabinda (GIRESSE & TCHIKAYA, 1975). - T h e Senonian to Eocene sequence is transgressive at the base and was followed by an almost continuous regression. In the region off Gabon at least part of this sequence can be subdivided into several sedimentation cycles, containing a variety of deposits ranging from estuarine sandy deposits to deeper marine shales and carbonates. Locally, channel and deltaic sandstones are present as well (BRINK, 1974). Senonian layers are known from outcrops at the coast and in river beds of the hinterland where the Maastrichtian top is phosphatized. At the sea floor they are only discovered off Pointe-Noire (GIRESSE & TCHIKAYA, 1975). Subaerial exposures of Palaeogene age are exclusively found in Cabinda. On the continental shelf, Palaeocene carbonates are observed at three locations at - 1 0 0 m water depth off Pointe Banda, while Eocene limestones are found at the same depth off M a y u m b a and at - 4 0 m off Djeno. The latter occurrence is rich in phosphatized coproliths of selachians (GIRESSE & CORNEN, 1976). - A f t e r a long period of emergence and deep fluvial erosion during Late Eocene and Oligocene times, strong subsidence occurred during the beginning of the Miocene, followed by Late Miocene and Pliocene regressions. In the Gabon Basin, shales with some carbonate intercalations were followed by an alternation of shales and sandstones and finally by sandstones. Strong subsidence in the westernmost part of the basins resulted in up to 2000 m thick deposits, wedging out towards the coast. Numerous Miocene outcrops of diverse lithologic character, varying from limestones to marls and sandstones, occur all over the shelf from M a y u m b a (beyond - 8 0 m) to Djeno (beyond - 3 0 m), and on land in Cabinda. The succession ended with the continental "S6rie des Cirques", a Plio-Pleistocene piedmont deposit, in which clays, sands, and gravels, like the "sables ocres", are locally found down to 85 m depth. On the seismic profiles four structural units can be distinguished: two
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C;EOLOGY CONG()-GABON SItEI,F
371
folded units with several faults at the inner shelf, Units 1 and 2, below two oceanwards dipping monoclinal units at the outer shelf, Units 3 and 4, separated by an angular unconformity (Figs 3a and 4). Unit 1 comprises more intensively folded strata with a relatively high density of synclines and anticlines, while Unit 2 is rather gently folded with a limited number of coastwards dipping limbs. Based on the shapes of their sections, most folds are traced over more than one seismic line. Apparently two major systems can be noticed: in the first one the directions of the fold axes vary from 110° to 120 ° and in the second one from 90 ° to 100 °. These systems coincide roughly with the seismic Units 1 and 2 respectively. Discordantly upon Unit 2 lies the monoclinal Unit 3. It dips to the southwest at angles of about 2 °. Also Unit 4 shows southwestern inclinations, but at larger angles around 4 °. By comparison with the geological outline map by GIRESSE & TCHIKAYA(1975) the boundary between Units 2 and 3 evidently forms the lower limit of the Miocene deposits. Consequently, the unconformity in question matches the also in this region well-documented Oligocene regression (M' BoRo, ANGLADE• JOUVAL, 1980). According to VAIL, MITCHUM & THOMPSON(1977) it represents a global fall of sea level. In almost the whole of the African continent, however, the so-called "African denudation surface" was formed at that time, and followed by Late Cenozoic cyclic episodes with uplift phases at the end of the Oligocene and later at about the end of the Miocene (CAHEN, 1954; KING, 1962). So the hiatus has a tectonic or a mixed tectonic-eustatic cause. In the shallow 15 to 30 m deep strip between the seismic lines and the coast, several Upper Cretaceous outcrops are present (GIRESSE & TCHIKAYA, 1975). The majority belong to the Senonian although older deposits also occur, including the exposures of Turonian limestones off M a y u m b a at - 3 5 m water depth. Nearby, at a distance of 15 km from the seismic section, the M a y u m b a n coast contains outcrops of the Precambrian continental basement. It follows that Units 1 and 2 together are of Upper Cretaceous to Eocene age. Since the important folding phases are known to have occurred during the Cretaceous-Palaeocene transition and during Middle to Late Eocene (Lutetian) times, Unit 1 is attributed to the Upper Cretaceous (starting from the Cenomanian) and Unit 2 to the Palaeogene (up to the Eocene). At many locations on the outer shelf, between 90 and 120 m water depth, Miocene rocks are exposed on either side of the limit between the Units 3 and 4 (GIRESSE& TCHIKAYA,1975; GIRESSE, 1980). As a consequence, this unconformity is bound to represent an intraMiocene regression, and most likely it occurred around the Middle to Late Miocene transition, just as on the Congolese mainland (REYRE,
372
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GEOLOGY CONGO-GABON SHELF
373
1960o), and the shelves of Cameroun (REYRE, 1966a) and north and central Gabon (BouRGOIN et al., 1963; BRINK, 1974). Again, like in the case of the Oligocene, this regression corresponds to both kind of events: the subsequent eustatic fall of sea level (VAIL, MIlX:rIUM & THOMPSON, 1977) and the subsequent peneplanation surface (the "Coastal plain surface"; CAI4EN, 1954; KINC, 1962). Therefore the uncorformity is ascribed to the end of the Middle Miocene (Tchengue Formation). Founded on this interpretation and the previous geological description, the following succession of structural events is proposed: (1) During the Late Cretaceous opening of the South Atlantic Ocean, tensional forces resulted in a system of fractures perpendicular to the present coastline (CORNEN et al., 1977). The fractures are obscured by subsequent folding. The spreading was relatively slow (LF, PICHON, 1968). (2) At the beginning of the Palaeocene, undoubtly very moderate compression tectonics prevailed, which were responsible for the distinctive fold orientation of the Cretaceous belt (Unit 1). This tectonic feature is also known from the Benou6 Chain (Cameroun, Nigeria) where Palaeocene sands and sandstones lie uncorformably upon Albian to Maastrichtian formations, and where in the northeastern part a major folding phase took place at the end of the Cretaceous (REYRE, 1966a; BENKHELIL& GUIRAUD, 1980). The folding is also recorded in seismic profiles from the upper continental slope off central Gabon (BECK & LEHNER, 1974: fig. 8). The compressive action had apparently a more regional character and was not only confined to Nigeria as was thought before. (3) The compression attained its climax during the Eocene (Lutetian), a remote consequence of the Pyrenean-Atlantean phase, caused by the anticlockwise rotation of the African plate (BELLION & GUIRAUD, 1980). The folds at the Congolese shelf form its southernmost expression to the present knowledge. (4) During the Miocene, the compression terminated and gave place to a tensional regime reflecting a new plate movement directed to the north and a larger spreading rate. An intensive alkaline volcanism accompanied these movements. In the Congo region, the Cretaceous fracture system was probably reactivated and a positive epirogenesis, increasing in intensity towards the southeast, is demonstrated by the present locations of Miocene shorelines at - 8 0 m off M a y u m b a and at 0 m in Landana (Cabinda). This rise is underlined by the upheaval of the resistent layers of the successive capes along the coast (Pointe Banda, Pointe Kounda, Pointe Indienne, Pointe Noire, and others fur-
374
.].H.F..JANSEN, P. GIRESSE & G. M~)GUEDFT
ther south), and is thought to endure into the Holocene (CoRNEN et al., 1977). In Fig. 4 the proposed fractures are indicated by fault scarps in the present Congolese coast like Pointe Noire and Pointe Kounda, and most probably also in Gabon (Mayumba, Pointe Panga). Their hypothetical orientation by CO~NEN et al. (1977) is more precisely defined now at 10° to 40 ° perpendicular to the observed directions of the folds. The fracture system explains also the offsetting of the submarine strata off Pointe Kounda and Pointe Panga. The deformational tectonics and regression at the Middle to Late Eocene transition were two closely related phenomena on the western African border (REYRE, 1966b). The most accidented reliefs are due to the appearance of normal faults preceeding a compressional period. However, penetrating into the south, the compressive deformation became less intense. On the other hand, the majority of the fold axes show directions between 160 ° and 180 ° (BENKHELIL& GUIRAUD,1980). So two questions arise from our observations on the Congo-Gabon continental shelf: (1) How is it possible that at such a large distance from the supposed origin of the tensional forces the folds could have such a dense occurrence? (2) Supposing that the compression was relatively uniform on this scale, how can we reconcile a shortening perpendicular to 160 ° to 180 °, and widely recognized in western Africa, with the fold system oriented 90 ° to 120 ° on the Congo-Gabon shelf?. The internal stresses of the African plate are certainly still insufficiently understood, and the investigations by the exploration geophysicists are guided by the scale of the deep structures, so that superficial and relatively small sized phenomena remain undiscovered. The example of the intracontinental Benou6 Chain (BURKEel al., 1972; BENKnELIL & GUIRAUD, 1980) shows the complexity of a 1000 km long told bundle which divides into two branches in Tchad, one directed NS, the other E-W. Towards the south of the continent, the tensional activity of the southwestern prolongation of the East African Rift stretches to the Mogamedes-Walvis sector according to the seismic activity maps, but its function is not yet defined (FAIRHEAD & GIRDLER, 1972). 4. QUATERNARY GEOLOGY AND ACOUSTIC RECORDS The Quaternary stratigraphical sequence of the Congo-Gabon shelf is rather well-known from numerous grab samples, soundings, and short cores (GIRESSE, KOUYOUMONTZAKIS& MOGUEDET, 1979; GIRESSEet al., 1981).
GEOLOGY CONGO-GABON SHELF
375
T h e oldest distinguished deposit consists of indurated Weichselfan marine clays, which present o c c u r r e n c e is mainly c o n c e n t r a t e d at the outer shelf a r o u n d the 110 m isobath. T h e y belong to a period of relatively high sea water t e m p e r a t u r e and sea levels. These "Vases m 6 d i o - w f i r m i e n n e " (GIRESSE, KOUYOUMONTZAKIS& MOGUEDET, 1979) are tentatively attributed to the Middle Weichselian substage, a b o u t 40 000 to 30 000 y B E T h e regression, leading to a m i n i m u m relative sea level of 100 to 115 m depth n e a r 18 000 yBP, led to the f o r m a t i o n of littoral, fluvial, and aeolian deposits. T h e Littoral deposits have been preserved at the o u t e r shelf, where they occasionally have thicknesses of up to 6 m. Along the inner shelf continental deposits are rarely observed, but r e m n a n t s of an i m p o r t a n t talus of aeolian clayey sands are frequently present in depressions between - 5 0 m and the shore and further inland. Recent corings d e m o n s t r a t e that these "Sables ocres" occur down to 85 m water depth. T h e first a c c u m u l a t i o n of the Late Weichselian to H o l o c e n e transgression p r o d u c e d the "Cordon coquillier", a clastic deposit with fragments of molluscs, bryozoans, and m a d r e p o r e s (GIRESSE, KOUYOUMONTZAKIS & MOGUEDET, 1979). Its presence is restricted to a strip between - 1 0 0 and -120 m where it attains thicknesses of up to 2 m. Holocene marine clays form the major transgressive deposit of the continental shelf. T h e y are found at depths less than 110 m. T h e 3.5 kc e c h o s o u n d e r records show an accoustically t r a n s p a r e n t surface layer (Fig. 3b). A sharp basal reflector usually forms the top of the u n d e r l y i n g Mesozoic and Cenozoic rocks. T h e layer is subdivided by a distinct internal reflector into an U p p e r and a Lower Unit. Starting from an estimated sound velocity in the sediments of 1700 m.s -1, isopach maps of these units are constructed (Figs 5 and 6). Thicknesses of less than 1 m are not indicated because they are below the limit of detection. T h e Lower Acoustic U n i t has a thickness of less than 5 m throughout. Northwest of M a y u m b a it forms a b a n d confined to water depths of 60 to 100 m and further southeast it divides into two strips, an inner strip at 60 to 70 m directed to Pointe K o u n d a , and an outer strip m e a n d e r i n g a r o u n d the 110 m isobath. T h e U p p e r Acoustic U n i t forms an elongate lobe extending from the Zaire river (GIRESSE, 1980) to the G a b o n shelf even further northwest than the area covered by the present expedition. In o u r observations, the lobe is at m a x i m u m thickness off Pointe-Noire (15 m) and thins out gradually to the northwest. F r o m the C a b i n d a - C o n g o bound a r y to Pointe B a n d a the u p p e r unit covers the area from less than 30
376
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GEOI.OGY
CONGO-GABON
SHELF
377
m, the depth of the inner acoustic lines, to -110 m. Further northwestwards it narrows, so that off M a y u m b a and beyond, it is bound by the 55 and 85 m depth lines. The "crest line" of the upper unit, connecting the points of maximum thickness of all sections, follows closely the 70 m isobath. Superimposed on these trends, the isopatch patterns of both units show directions of - 110°, which is about the direction of the fold axes of the Mesozoic and Palaeogene seismic Units 1 and 2 and of the strike of the monoclinal Miocene Units 3 and 4 (Fig. 4). Conspicuous examples are the thick strip in both acoustic units off Pointe Banda to M a y u m b a and the thickest occurrence of the Upper unit off Pointe Noire. The Upper Acoustic Unit is unquestionably identical with the Holocene marine clays; it forms the superficial deposit, and its outer limit of distribution follows closely the 110 m isobath. The " C o r d o n coquillier" is too thin to be recognized in the acoustic records. The Marine and Littoral deposits remain, and are considered to comprise the Lower Acoustic Unit. Both deposits are concentrated near 110 m depth, and also both were locally preserved below the Holocene clays. However, the more precise information from the cores indicates that the Lower Unit represents the Middle Weichselian marine clays. They are essentially suited to have produced the distinct reflector at the top because of their strong induration, with water contents of - 3 0 % . The relicts of the Littoral deposits are limited to a thin layer with occasional pockets a few metres thick. The shape of the Holocene lobe, with m a x i m u m thickness in the southeast and diminishing with increasing distance to the Zaire river mouth, indicates that its sediments were mainly supplied by the Zaire. This is in agreement with the complicated present local current system, which is controlled by three major currents, the northward Benguela Current and Tradewind Drift along the coast, and the eastward South Equatorial Counter Current and Equatorial Under Current (DuFouR & STRETTA,1973; VAN BENNEKOM & BERGER, 1981). Their frequently shifting courses result at the Congo-Gabon shelf in a dominant northwest current parallel to the shore, with a coastward component. The latter component tends to move the Holocene deposits towards the central and inner shelf. A strong southwest oceanic swell causes a northwest littoral drift with important sand movements, so that sedimentation of the Congo mud occurs mainly at the quiet central shelf. These are the conditions for a "midshelf mud belt" of MCCAVE (1972) and is analogous to the mud belt on the Washington shelf of Columbia river mud (NITTROUER, 1979). The area
378
J.H.E JANSEN, P. GIRESSE & G. MOGUEDET
of the greatest thickness coincides with the zone of most fine-grained sediments (GIRESSE ~Z KOUYOUMONTZAKIS, 1973; GIRESSE el al., 1981), which indicates that the distribution of the Holocene deposits is in concordance with the present current and wave regime. The origin is also demonstrated by the high kaolinite contents (at m i n i m u m 70% of the clay mineral composition): clays from samples from the Congo estuary consist almost exclusively of kaolinite (GIRESSE, KOUYOUMONTZAKIS~Z MOGUEDET, 1979; VAN DER GAAST & JANSEN, 1984). The strips in the isopach map oriented - 110° are not recognizable in the bathymetric pattern, The acoustic records also show that they have only slightly affected sea floor topography, and are related to depressions in the bedrock. These depressions were probably formed in exposed layers which are more succeptible to weathering during successive marine and subaerial phases. The 5 m thick strip off Pointe K o u n d a is situated upon the Upper Cretaceous seismic unit just near the Cretaceous-Palaeogene boundary and is probably determined by the presence of a Cenomanian or Turonian subcrop. The strip off Pointe Banda to M a y u m b a is oriented in the direction of the Palaeogene folds, which may have favoured the exposure of less resistant layers. A structural influence upon the isopachs may be observed in the monoclinal Miocene zone as well. The superposition of its low dipping strata made possible the formation of small cuestas and cliffs facing southwest. At their feet various sediment accumulations are found of several metres thick. The most remarkable example is formed by the strip off Pointe Noire. Towards the northwest, at the Gabon shelf, the Holocene is also controlled and limited by the structure of the Cretaceous and Tertiary rocks. There are many exposures, and the topography in this sector is almost entirely formed by the Tertiairy surface (HORN, 1978). The seismic and acoustic lines recorded off the mouths of the various rivers and close to the coasts at about 30 m water depth, do not indicate relict channels. All observed depressions are related to the differential erosion as discussed above, and so the river runoff probably made use of these depressions during the regressions. Moreover, in borings in the estuary of the Kouilou no indications of channels deeper than 30 m were found (GIRESSE8Z MOGUEDET, 1980). The information deduced from the acoustic records combined with the knowledge obtained from the cores and other sediment samples leads to the following synopsis of the Quaternary sedimentary history. The oldest recorded Quaternary sediments (Fig. 6) represent the Middle Weichselian transgression, and correspond probably to
(;EOLOGX,"
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Fig. 6. Thickness in metres of the Upper Acoustic Unit (Holocene marine clays).
380
,J.H.E , J A N S E N . P. G I R E S S E
& G. M O G U E I ) E T
mangrove deposits discovered at - 3 0 m in the present estuary of the river Kouilou, dated =>35 000 yBP (GIRESSE & MOGUEDET, 1980). The marine clays of this period were mainly found at the outer shelf, concentrated around the 110 m isobath. Their induration limited the penetration by corings to 60 cm or less. Apart from the inner strip off Pointe Banda to Mayumba, bound to depressions of the underlying Palaeogene, the clays in the more shallow regions were only locally encountered in borings. It is not likely that they are included in the Upper Acoustic Unit without being perceived because of their marked top reflector, so they were either eroded or buried subsequently. This burial possibly took place at the inner border where the "sables ocres" were formed during the regression, while in the deeper regions littoral erosion had probably left only relicts of the original cover whose extension may have been comparable to the later Holocene one. If this is true, the strongest erosion has taken place in the southeastern part of the study area, which may be due to vertical movements of the sea floor or to differences in the hydrodynamic regime. Compared to the northwestern area, the southeastern area underwent an upheaval of roughly 80 m since Miocene times (section 3). If the process occurred linearly, the differential movements between the Middle Weichselian and the following Holocene transgression has been roughly 30 cm, not enough to account for the differences in erosion in the order of metres. Consequently, the cause must have been different hydrodynamic conditions during the preglacial regression or Holocene transgression. Beyond the 110 m depth line, at the continental slope, a continuous cyclic accumulation took place during the Quaternary, with velocities of 40 to 80 cm.103 y-1 and 15 to 20 cm-103 y-1 during humid and arid periods respectively (GIRESSE, unpublished data). The subsequent regression involved an important erosion of the freshly deposited marine clays. The accompanying littoral deposits at the outer shelf, about 18 000 years old, are rich in glauconitized faecal pellets. Their composition is not uniform; stagnations in the sealevel movements just before and after 18 000 yBP are mirrored by an alternation of more marine (more pelitic) and more nearshore (more glauconite and shells) beds. At larger water depths (175 m) the accumulation of glauconite was more regular. The first transgressive deposit is the "Cordon coquillier", dated 12 000 y g P (GIRESSE, KOUYOUMONTZAKIS& MOGUEDET,1979). It was followed by the Holocene deposit (Fig. 5), the marine clays which composition and distribution are closely related to the vicinity of the river Congo and to the structure of the Cretaceous and Tertiary bedrock. Core samples demonstrate coarse remnants of a nearby stagnating
GEOI,OGY
CONGO-GABON
SHELF
381
shoreline at the base, and after that a clayey deposit with an increasing content of faecal pellets towards the top corresponding to the rise of sea level. At the Gabon shelf the influence of the Congo material is very limited, and as a consequence the Holocene sediments are restricted to the sandy and gravelly relicts of the passage of the shore, comparable to the recent deposits between the coast and 35 m water depth. After the present situation was attained about 5000 yBP, some minor sealevel oscillations and climatic fluctuations were recorded in the sediments of the Congo-Gabon continental shelf. 5. SUMMARY Seismic and acoustic reflection profiles demonstrate the presence of 4 seismic and 2 acoustic units. From the inner shelf outwards they are successively: Unit 1, interpreted as Upper Cretaceous starting from the Cenomanian, and a Palaeocene and Eocene Unit 2. Both units are folded, the latter more gently. The Oligocene regression induced the discordant boundary with two monoclinal units 3 and 4 representing the Miocene, and separated by a Middle to Late Miocene regressional unconformity. Late Cretaceous tensional tectonics, accompanying the early opening of the South Atlantic, was followed by at least 2 phases of compressional tectonics during Palaeocene to Eocene times. The orientation of the axes, 90 ° to 120 °, is different from the known directions in western Africa. Fractures belonging to a Miocene tensional regime are proposed to have caused offsetting of the mapped strata. The thickness distributions of 2 acoustic units were mapped. The Lower Unit, less than 5 m thick, consists of Middle Weichselian marine clays. Their relicts are concentrated at the outer shelf, due to the fall of the sea level (100 to 115 m) around 18 000 yBR The Upper Unit forms an at m a x i m u m 15 m thick Holocene lobe of clays originating from the Congo River. Both units show strips oriented 110°, controlled by the direction of the Cretaceous and Tertiary subcrops. 6. RI~SUMI~ GI~OLOGIE STRUCTURALE CONTINENTAUX
DU CONGO RI~FLECTION
ET S E D I M E N T A I R E
DES PLATEAUX
ET DU SUD G A B O N ; U N E E X P L O R A T I O N SISMIQUE
DE
ET ACOUSTIQUE
Les profils de r6flections sismique et acoustique dfimontrent le presence de 4 unit~s sismiques et de deux unit~s acoustiques. Du plateau interne vers le large, on trouve successivement: l'unit~ 1, inter-
382
, l . H v JANSEN. P. G I R E S S E & G. M O G U E D E T
pr6t6e c o m m e d u C r ~ t a c ~ s u p 6 r i e u r c o m m e n ~ ; a n t au C ~ n o m a n i e n ; l ' u n i t 6 2 Pal6oc~ne et E o c e n e . C h a c u n e de ces unit6s sont pliss6s, l'unit6 2 m o i n s f o r t e m e n t . Le r6gression oligoc~ne i n d u i t u n e surface de d i s c o r d a n c e avec les unit~s m o n o c l i n a l e s 3 et 4 repr6sentatives d u M i o c e n e et s f p a r 6 e s p a r u n e d i s c o n t i n u i t 6 li~e p a r u n e r6gression entre le M i o c e n e m o y e n et sup6rieur. Les distensions t e c t o n i q u e s d u C r 6 t a c ~ s u p 6 r i e u r qui a c c o m p a g n e n t le d 6 b u t de l ' o u v e r t u r e de l ' A t l a n t i q u e Sud, sont suivies p a r au m o i n s d e u x p h a s e s de t e c t o n i q u e c o m p r e s s i v e s p e n d a n t les t e m p s pal6oc~nes 5 6oc~nes. I2orientation des axes des plis 90 ° ~a 120 °, est diff~rente des d i r e c t i o n s r e c o n n u e s en Afriq u e de l ' O u e s t . D e s f r a c t u r e s a p p a r t e n a n t au r ~ g i m e distensif du M i o c e n e sont suppos6s avoir d6cal~ les c o u c h e s c a r t o g r a p h i 6 e s . Les i s o p a q u e s des d e u x unit~s a c o u s t i q u e s sont trac~s. I2unitfi inf6rieure, m o i n s de 5 m d ' f p a i s s e u r , c o r r e s p o n d 5 des vases m a r i n e s d u milieu du W i i r m . Ses t 6 m o i n s sont c o n c e n t r 6 s s u r la b o r d u r e e x t e r n e d u plateau, en f o n c t i o n de la r6gression (100 5 115 m) vers 18 000 ans B R I2unit~ s u p f r i e u r e consiste en des vases holoc~nes issues du fleuve C o n g o de 15 m m a x i m u m d'6paisseur. C h a q u e unit6 se pr6sente sous f o r m e de b a n d e s allong6es selon 110 °, cette o r i e n t a t i o n est contr616e p a r la d i r e c t i o n des a f f l e u r e m e n t s cr~tac6s et tertiaires. 7. R E F E R E N C E S BECK, R.H. & R. LEHNER, 1974. Oceans, new frontier in exploration.--Bull. Am. Ass. Petrol. Geol. 58 (3): 376-395. BELL[ON, Y. & R. GUmAUl~, 1980. Tectonique intraplaque: mise en 6vidence sur le littoral s~n6galais de dfiformations li~es ?a la phase tectorog~nique pyr~n~oatlasique. 8e Rfiunion ann. Sc. Terre, Marseille 1980. Soc. Gdol. Fr., Paris: 33. BENKHELIL,J. & R. GUIRAUD, 1980. La Benou~ (Nigeria): une ehMne intracontinentale de style atlasique. 8e R6union ann. Sc. Terre, Marseille 1980. Soc. G~ol. Fr., Paris: 37. BENNEI~OM, AJ. VAN & G.W. BERCER, 1984. Hydrography and silica budget of the Angola Basin.--Neth. J. Sea Res. 17 (2-4): 149-200. BOURGOIN, J., D. REYRE, l~ MAGIA)IRE & M. KRICHEWSHY, 1963. Les canyons sousmarins du Cap Lopez (Gabon).--Cah. oc6anogn 15: 372-387. BRINK, A.H., 1974. Petroleum geology of Gabon Basin.--Bull. Am. Ass. Petrol. Geol. 58 (2): 216-235. BURKE, K.C., T.F.J. DESSAUVaG[E& A.J. WHITEMAN, 1972. Geological history of the Benue Valley and adjacent areas. In: T.F.J. DESSAt:VAGIE& A.J. WmTEMAN. African geology, Ibadan 1970. Geol. Dept. Univ. Ibadan, Nigeria: 187-205. CAHEN, L., 1954. G6ologie du Congo belge. Vaillant-Carmanne, Liege: 1-577. CORNEN, G., P. GIRESSE, G. KOUYOUMONTZAKIS & G. MOGUEDET, 1977. La fin de la transgression holoc~ne sur les littoraux atlantiques d'Afrique ~quatoriale et australe (Gabon, Congo, Angola, Sao Thom6, Annobon)--R&les eustatiques et n6otectoniques.--Ass, s6n6gal Et. Quatern. Afr., Bull. Liaison, S~n6gal 50: 59-83. DtJvour~, P. & J.M. STgETXA, 1973. Production primaire, biomasses du phytoplancton
GEOLOGY CONGO-GABON SHELF
383
et du zooplancton dans l'Atlantique tropical Sud, le long du mfridien 4°W. Cah. O R S T O M , S~r. Oc~anogr. 11 (4): 419-429. FAIRHEAI~,J.D. & R.W. GmDLER, 1972. The seismicity of the East African Rift system. In: R.W. GIRDLER. East African rifts.--Tectonophysics 15 (1/2): 115-122. FRANKS, S. & A.E.M. NAIRN, 1973. The equatorial marginal basins of west Africa. In: A.E.M. NAmN & EG. STEHLI. The ocean basins and margins. I. The South Atlantic. Plenum, New York: 301-350. GAAST, S.J. VAN DER &J.H.F. JANSEN, 1984. Mineralogy, opal and manganese of Middle and Late Quaternary sediments of the Zaire (Congo) deep sea fan; origin and climatic variation.--Neth. J. Sea Res. 17 (2-4): 313-341. GIRESSE, V., 1980. Carte s~dimentologique du plateau continental du Congo. Notice explicative 85: 1-24, ORS~IDM, Paris. GIRESSE, V. & G. CORNEN, 1976. Distribution, nature et origine des phosphates mioc~nes et ~oc~nes sous-marins des plateformes du Congo et du Gabon.--Bull. Bur. Rech. g~ol. Minieres 4 (1): 5-15. GmESSE, P., J.H.F. JANSEN, G. KOU¥OUMONTZAKIS& G. MOGUEDET, 1981. Les fonds de la plateforme congolaise, le delta sous-marin du fleuve Congo. Bilan de huit ans de recherches s~dimentologiques, pal6ontologiques, g~ochimiques et g6ophysiques.--Trav. Doc. O R S T O M 138: 13-45. GIRESSE, P. & G. KOt3VOUMONTZAKIS,1971. G6ologie du sous-sol du port de PointeNoire et des fonds sous-marins voisins.--Ann. Univ. Brazzaville 7: 95-107. , 1973. Cartographie s~dimentologique des plateaux continentaux du Sud du Gabon, du Congo, du Cabinda et du Zaire.--Cah. O R S T O M , S~r. G6ol. 5 (2): 235-257. GIRESSE, R, G. KOUVOUMONTZAKIS& G. MOCUEDET, 1979. Le Quaternaire sup~rieur du plateau continental congolais. Exemple d'~volution pal~o-oc~anographique d'une plate-forme depuis environ 50 000 ans. In: E.M. VAN ZINDEREN BA~KER & J.A. COETZEE. Palaeocology of Africa, I1: 193-217. Balkema, Rotterdam. GIRESSE, R & L. LERIBAULT, 1981. Contribution de l'~tude exoscopique des quartz 5 la reconstitution pal~og~ographique des derniers ~pisodes du Quaternaire littoral du Congo.--Quaternay Res. 15 (1): 86-100. G1RESSE, R & G. MOCUEDEa', 1980. Chronosfiquences fluvio-marines de l'Holoc~ne de l'estuaire du Kouilou et des colmatages c6tiers voisins du Congo. In: E BLASCO, C. CARATINI, A. FREDOU, P. GIRESSE, G. MOGUEDET, C. TISSOT & H. WEISS. Les rivages tropicaux. Mangroves d'Afrique et d'Asie. C N R S / C E G E T , Talence: 23-46. GmESSE, R & J.-B. TCmKAYA, 1975. Contribution 5. la carte g6ologique de la plateforme sous-marine congolaise (Mission N.O. Nizery de janvier 1974).--Ann. Univ. Brazzaville 11: 23-34. HmTz, R, 1964. Travaux d'exploration ex6cut6s par la SPAFE au large des c6tes du Gabon. Publ. Institut Fran~ais du P6tr61e (Coll. et S6m.) 1: 1-19. HORN, R., 1978. La g6ophysique marine: application 5. la reconnaissance du plateau continental africain. C. r. s~m. Ressources min~rales sous-marines.--Bull. Bur. Rech. g~ol. Minieres, Orl6ans: 185-216. HouRcQ, V., 1966. Les grands traits de la g~ologie des bassins c6tiers du Groupe Equatorial. In: D. REYRE. Bassins s6dimentaires du littoral africain 1: Littoral atlantique. Ass. Serv. Gfiol. Afr., Paris: 163-170. KING, L.C., 1962. The morphology of the earth. Oliver and Boyd, Edinburgh: 1-726. LEPICHOr~, X., 1968. Sea floor spreading and continental drift.--J, geophys. Res. 73: 3661-3697. M'BoRo, R., R. ANGLAnE &J. JOUVM, 1980. Les modalit~s de la transgression oligomiocbne dans le Golfe de Guin6e. 8e Rfunion. ann. Sc. Terre, Marseille 1980. Soc, G6ol. Ft., Paris: 248.
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