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The AAPG/Datapages Combined Publications Database

AAPG Special Volumes


Pub. Id: A133 (1990)

First Page: 307

Last Page: 326

Book Title: M 50: Lacustrine Basin Exploration: Case Studies and Modern Analogs

Article/Chapter: Stratigraphic Development of Proto-South Atlantic Rifting in Cabinda, Angola--A Petroliferous Lake Basin: Chapter 19

Subject Group: Basin or Areal Analysis or Evaluation

Spec. Pub. Type: Memoir

Pub. Year: 1990

Author(s): Tim R. McHargue


During the Early Cretaceous, a thick section of nonmarine sediment accumulated in the developing rift between South America and Africa. Extensive lake systems occupied much of the rift throughout its history. The rift sequence of west Africa is economically important because the basinal lake shales include exceptional source rocks for petroleum. A large majority of the oil reserves of Gabon, Congo, Zaire, and Angola are sourced from this lacustrine section. Cabinda is a small, detached enclave of Angola that produces most of that country's oil. An understanding of this important oil province requires an understanding of its stratigraphy. Abrupt facies changes among fluvial and alluvial sandstone, lake-margin sandstone, and carbonate and basinal lacustrine shale, diamictit s, and turbidites are crucial to successful exploration in the basin. Synrift stratigraphy developed in response to active faulting, geographically variable subsidence rates, and variable topographic relief during deposition. Rift strata can be subdivided into tectono-stratigraphic packages that correspond to the rift's structural history.

Early rifting (fault phase, Neocomian) was characterized by rapid subsidence adjacent to major faults such that the rift became structurally subdivided into subbasins. Each subbasin developed a stratified lake and had a similar subsidence history, although the stratigraphy of each subbasin varied depending on influx of detrital sediment. Rapid transitions from alluvial, lake-margin sand to lacustrine shale are typical of this phase. In lacustrine units, diamictites, turbidites, and contorted bedding are common.

During the early sag phase (early Barremian), as fault-related subsidence gradually ended, regional rift-basin subsidence ensued. Lakes expanded and submerged former alluvial deposits, coalescing to form a single lake. Synchronous lake-level fluctuations can be recognized in all subbasins in Cabinda. During lowstands, carbonate deposition extended into basinal settings, and organic-carbon content decreased. During highstands organic carbon increased, and deposition of basinal organic shale expanded across former sites of carbonate deposition. Influx of coarse-grained extrabasinal clastics into the rift virtually stopped.

During the late sag phase (late Barremian), rates of regional subsidence gradually diminished, and faulting became rare. The lake became shallower as it filled with sediment, and the water column became fully oxygenated. Rhythmic fluctuations of lake level and chemistry are preserved as laterally persistent cyclical alternations (typically 10 m thick) of carbonate-rich and carbonate-poor mudstone. Thick carbonates accumulated in the shallowest parts of the lake.

During the drift phase, near the beginning of the Aptian, the entire region was uplifted and subjected to erosion. Uplift is attributed to rebound of the rift shoulders after crustal rupture. Following uplift, the pattern of renewed subsidence was that of a passive margin rather than a rift.

Throughout the history of the rift, sedimentation was controlled by the rate of subsidence relative to sediment influx rate. Superimposed on the tectonic evolution of the basin were climatic variations that affected lake level and chemistry to induce depositional events essential for correlation.

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