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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Stratigraphic Development of
Proto-South Atlantic Rifting
in Cabinda, Angola
By
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 Lower Cretaceous syn-rift stratigraphy. The rift sequence of West Africa contains basinal lake shales that are the source rocks for most of the produced oil, not only in Angola, but in Gabon, Congo, and Zaire as well.
Rift sediments can be subdivided into tectonostratigraphic
packages that correspond to the structural history
of the rift. The FAULT PHASE
(NEOCOMIAN) is
characterized by rapid subsidence adjacent to major faults
early in the development of the rift so that the rift is
structurally subdivided into sub-basins. Each sub-basin
developed a stratified lake and had a similar subsidence
history although the stratigraphy of each sub-basin varied
depending on the influx rate of detrital sediment. Rapid
transitions from alluvial, lake-margin sands to lacustrine
shales are typical of this
phase
. In lacustrine units,
diamictites, turbidites, and contorted bedding are common.
In the EARLY SAG PHASE
(EARLY BARREMIAN)
fault-related subsidence gradually ended and was replaced
by regional rift-basin-scale subsidence. Separate lakes expanded,
submerged former alluvial deposits and coalesced
to form a single lake. Synchronous lake level fluctuations
can be recognized in all sub-basins in Cabinda. During
low-stands, carbonate deposition expanded into basinal
settings and organic carbon content decreased. During
high-stands, the amount of organic carbon increased and
deposition of basinal organic shales expanded across former
sites of carbonate deposition. Influx of coarse extrabasinal
clastics into the rift virtually stopped.
In 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 its 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 -poor
mudstones. Thick carbonates accumulated in the shallowest
parts of the lake.
DRIFT PHASE
(APTIAN): 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 the influx rate of sediment. Superimposed on the tectonic evolution of the basin were climatic variations which affected lake level and chemistry to produce depositional events essential for correlation.
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