Тип бассейна: Платформ
Подтип бассейна: Пассивных окраин (перикратонно-океанический)
Класс бассейна: Периокеанический
Возраст бассейна: Зрелый - Мезозойский
Тип полезных ископаемых:
Геологический возраст начало:
Геологический возраст конец:
Площадь: 147894.53 км²
Lower Congo Basin
The Lower Congo Basin is located on the West African continental margin between latitudes 5°S and 8°30'S of Angola (Fig. 1). The area is a part of the 'Aptian Salt Basins' (Clifford, 1986) that extend between the Cameroon Volcanic Line to the north and the Walvis Ridge to the south (Fig. 1). Since the Cretaceous the Aptian Salt Basins have been subjected to 'raft tectonics' , a term first introduced by Burollet (1975) in his summary of the Kwanza Basin (Fig. 1).
Fig. 1. Location of the study area within the southern part of the Lower Congo Basin offshore Angola (upper left). In the lower part 2D seismic grid, wells andseismic sections cited in text are marked.
More recently this deformation process has been described more fully by other workers in the area (e.g. Lundin, 1992; Mauduit & Brun, 1998; Vendeville & Jackson, 1992a,b). Raft tectonics is a process whereby rigid blocks of strata separate along listric faults that sole out in a ductile detachment horizon. In the Lower Congo Basin the decoupling horizon consists of Aptian salt. The open areas between the separating blocks are successively filled with syn-kinematic strata (Fig. 2). The Lower Congo Basin is a quickly growing hydrocarbon province where turbidity deposits are important reservoir units (Anderson, Cartwright, Drysdall, & Vivian, 2000). It has been shown that fault growth due to salt tectonics has acted in periods as the fundamental control on the flow patterns of the turbidity currents (Anderson et al., 2000; Valle, Sperrevik, & Gjelberg, 2001). Therefore one of our objects for the present work has been to achieve precise time constraints for the raft tectonics and the associated tectonostratigraphic development.
Fig. 2. The principle of raft tectonics (Duval et al., 1992). Pre-rafts remain in mutual contact while rafts separate so far that there is no longer contact between the original footwalls and hanging walls. The basement remains undi sturbed by faultjng since the salt layer acts as a decoupling horizon.
In the Kwanza Basin immediately to the south of the Lower Congo Basin (see Fig. 1) Duval, Cramez and Jackson (1992) recognized two distinct phases of raft tectonics (rupturing in the Late Early Cretaceous and in the Early Eocene) separated by an inactive period (Late Cretaceous- Early Tertiary). During the inactive period the Cretaceous raft-blocks were 'yoked' together by Late Cretaceous and Early Tertiary sediments (Duval et al., 1992). On the other hand, Lundin (1992), in his study of the northern Kwanza Basin, described rafting as a continuous process that started in the Cretaceous and is active today.
The driving mechanism of raft tectonics has also been debated. Most authors emphasize the importance of extension in the initiating of raft tectonics, and it is a common opinion that the necessary extension is governed by gravity processes on the inclined slope (Lundin, 1992; Vendeville & Jackson, 1992b). In the literature the gravity-induced deformation is attributed to two principal factors: gravity gliding and gravity spreading (Duval et al., 1992; Lundin, 1992). Duval et al. (1992) define gravity gliding as: "translation of fault blocks down a gentle slope driven by the downslope shear stress" and gravity spreading as "vertical collapse and lateral spreading of a rock mass under gravity".
Shultz-Ela (2001) reviews the use of these terms, and states that if there is rigid translation down a slope then it is gravity gliding, but if there is any internal strain within the sliding sheets, then it is gravity spreading. According to Lundin (1992), it is difficult to determine whether gravity gliding dominated over gravity spreading or vice versa off the Angolan margin. However, in the present work we show that gravity spreading probably was the dominant mechanism.
The present study covers an area of approximately 15,500 km2 of the Lower Congo Basin, and it is interpreted 204 2D seismic sections with a total length of about 10,000 km (Fig. 1).The post-rift package has been subdivided by means of biostratigraphic markers identified in exploration wells. For the Tertiary interval the biostratigraphic markers were tied to the seismic sections in order to obtain a set of structural maps for the different time surfaces. The time surfaces form the basis for isochron maps which allow 'monitoring' of fault growth and depocenter distribution along the faults through time.
Tectonic setting
The Lower Congo basin belongs to a family of sedimentary basins occurring both onshore and offshore on both sides of the Southern Atlantic Ocean. The modern South Atlantic margins started their evolution as a part of a continental rift system that developed in the southern parts of the Gondwana super-continent at the Jurassic-Cretaceous boundary (Ala & Selley, 1997; Brice, Cochran, Pardo, & Edwards 1982). According to Nuernberg and Mueller (1991), rifting started at approximately 150 Ma at the southernmost tip of the Southern Atlantic and propagated northwards, to reaching the area immediately south of the Walvis Ridge at 126.5 Ma (Fig. 1), and the Benue Trough at 118.7 Ma. The active rifting has been further divided into three separate phases: Berriasian, Hauterivian and Late Barremian-Early Aptian (Karner & Driscoll, 1999), the first phase being regarded as the most important (Coward, Purdy, Ries, & Smith 1999). Subsequently southern Gondwana split into the South American and African continental plates. The margins became passive when active seafloor spreading started between the two continents. The post-rift period was dominated by thermal subsidence as from the Late Early Cretaceous (Ala & Selley, 1997; Brice et al., 1982).
The post-rift-present interval is dominated by regional subsidence associated with deposition of a thick regressive package, especially during the Oligocene and Miocene (Ala & Selley, 1997; Brice et al., 1982). Seranne, Seguret and Fauchier (1992) subdivided the post-rift package into an aggradational super-unit (Albian-Eocene) and progradational super-unit (Oligocene-present), separated by an Oligocene erosional event of 10-20 Ma duration. It is a common opinion among workers in the area that the progradation and erosion were governed by epiorogenic motions.
Sahagian (1988) relates them to hot spot activity reflecting the Late Cretaceous-present mantle convection regime underlying the African lithosphere. The latter author has quantified the motion by means of ancient shoreline deposits, and suggests uplift in the range 0.5-1.0 km for the continental areas east of the study area. However, the time constraint for the uplift is poor (in the range Cenomanian- Pliocene). Uplift calculations, based on maturity data from onshore exploration wells, suggest 1.0- 2.0 km of uplift of the western African continent from Miocene to present (Lunde, Aubert, Lauritzen, & Lorange, 1992).
Stratigraphical and structural description
Stratigraphy
The sedimentary infill of the Aptian salt basins is traditionally divided into three megasequences (Ala & Selley, 1997; Brice et al., 1982; Teisserenc & Villemin 1990): (1) 'the syn-rift megasequence' (Late Jurassic-Early Cretaceous), (2) 'the transitional/early drift megasequence'
(Aptian) and (3) 'the drift megasequence' (AlbianHolocene).
A generalized stratigraphic column for the Lower Congo Basin is shown in Fig. 3. In accordance with the general subdivision for the Aptian Salt Basins, the column is subdivided in a pre-salt unit (syn-rift megasequence), a transitional unit and a drift unit. The stratigraphic information is compiled from Abilio (1986), Ala and Selley (1997), Brognon and Verrier (1966), Coward et al. (1999) and Franks and Nairn (1973). In addition we have had access to data from unpublished exploration wells and associated oil company reports (see Fig. 1 for location of wells).
Fig. 3. Generalized stratigraphic column for the Lower Congo Basin. The seismic markers cited in text are marked in the right column. (Compiled from Abilio (1986), Ala and Selley (1997), Brognon and Verrier (1966) and Franks and Nairn (1973) and unpubli shed oil company reports).
Among the available wells, the shallow water well C (Fig. 4) offers the most complete general overview of the Lower Congo stratigraphy since it has fully penetrated all the megasequences mentioned above and terminates in basement. In well C the lowermost 40 m above the termination depth consists of basement, or more specific crystalline and metamorphic rocks of early-mid Proterozoic age (2000-1100 Ma) (Teisserenc & Villemin, 1990). The basement is unconformablyoverlain by the Pre-salt Group that contains syn-rift (and pre-rift?) sediments. Lithologically, the Pre-salt Group is dominated by lacustrine shales interrupted by several fluviatile sand deposits, especially near the base.
Fig. 4. Exploration wells from the Angolan shelf. (A) and (B) are located in the deepwater area, while C-E all are located in shallower water (see Fig. I for location of wells).
The base of the transitional unit is marked by an approximately 100 m thick sandstone that appears immediately underneath the Loeme Group. In well C the Loeme Group is circa 150 m thick and consists basically of halite occasionally interrupted by 1-5 m thick layers of silt and clay.
However, due to post depositional movement, the salt package shows extreme thickness variations, from near zero thickness in the east to thousands of meters further west due to doming.
The drift unit consists of the Pinda, Iabe, Landana and Malembo groups of Late Early Cretaceous (Albian)-present age (Fig. 3). In well C the Pinda Group consists of a 200 m thick bipartite package with limestone in the lower half and shale in the upper half. The limestones represent a shallow carbonate platform that developed during a relative sea level rise along the margin of west Africa during the Aptian (Ala & Selley, 1997), while the shale in the upper half marks the establishment of more open marine conditions.
The remaining 1300 m of well C (Iabe, Landana and Malembo groups) shows a very uniform lithology consisting of marine shale interrupted by minor silt layers. The upper part of the Iabe and the Landana Groups are associated with a distinct gamma ray peak. This, together with the very small thickness of the Landana Group (approximately 20 m) suggests that the Upper Cretaceous and Lower Tertiary should be regarded as a condensed section.
The deepwater well A (Fig. 4) terminates within the Iabe Group, here represented by approximately 300 m of sediments.
Like in well C Iabe consists of a mixture of limestone and shale, and the upper part of the Iabe together with the lower part of Malembo is also associated with a distinct gamma ray peak. However, the Landana Group is better developed in the deepwater well and has a thickness of almost 400 m. Well A is totally dominated by the Malembo Group that is more than 3000 m thick, consisting of marine shale, with several intervals of up to 25 m thick sandstone packages. The sandstone packages represent debris flowand turbidite units (Dupont, Masse, Moron, Pourtoy, &
Gerard, 2000) deposited under relatively deep marine conditions.
Structural domains
The main post-rift deformation mechanism in the area is raft tectonics (Fig. 2). Following the terminology of Jackson and Talbot (1991) the term 'raft' is applied on normal faulted blocks that are so widely separated that there is no longer contact between the original footwall and hanging wall after faulting. On the other hand, ' pre-rafts' represent an early stage of rafting where the hanging wall is still resting on the original footwall after faulting (Fig. 2).
Based on the post-rift structuring we sub-divide the study area into four domains where the names of the domains refer to the dominant structural elements within each of them. From the east towards the west they are (1) Cretaceous raft blocks/grabens, (2) Tertiary raft locks/grabens, (3) Tertiary pre-rafts and (4) Salt diapirs (Fig. 5).
Fig. 5. Structural domains governed by post-rift deformation. Domain 1: Cretaceous raft blocks nd grabens, Domain 2: Tertiary raft blocks and grabens, Domain 3: Tertiary pre-rafts and Domain 4: Tertiary salt diapirs. Faults that were active during the Cretaceous are marked with thin solid line while the Tertiary faults affecting the El level are shown with thick solid line.
Source: Tectonostratigraphic development in the eastern Lower Congo Basin, offshore Angola, West Africa. Paul J. Valle, John G. Gjelberg, William Helland-Hansen. Marine and Petroleum Geology 18 (2001) 909- 927
Следующий Бассейн: Outeniqua