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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Kwanza Arcana: A New Look at Angolan Salt
Tectonics, Tectonostratigraphy, and Differential Uplift
By
Bureau of Economic Geology
John A. and Katherine G. Jackson
School of Geosciences
The University of Texas at Austin
We highlight results of a three-year research project on two basins: the Inner Kwanza Basin (IKB, onshore) and the Outer Kwanza Basin (OKB, offshore). The best-known region is the extensional province on the continental shelf, which has been intensively explored for its Albian reservoirs. To appreciate the entire Kwanza system, we focused instead on its lesser known, more arcane, regions. One such obscure region is obviously the deep-water and ultradeep-water frontiers of the OKB, which are now being explored. The other region is the IKB, which is mysterious not because it is new to exploration but because it is old; it has not been explored for several decades.
The Kwanza region has a far more complex history than might
be expected on a passive margin. The following themes illustrate
these complexities and surprises. (1) The IKB was an interior salt
basin enclosed by basement highs, dominated by halokinesis,
and having only minor translation, even though this region is
often described as the type area for highly extensional raft tectonics;
in contrast, the OKB was an open continental margin
that deformed as a gravity-driven system dominated by large
translation; each basin evolved separately and was more or less
tectonically isolated. (2) A translation distance of ~25 km is
robustly constrained by restoration of the extensional, translational,
and contractional provinces of the OKB in a
360-km-long regional profile across both basins. (3) The IKB is
segmented from north to south by diverse tectonic provinces,
which run the gamut from salt withdrawal to basement-involved
contraction in three fold belts. (4) Uplift of both basins varied
greatly in space and time; Neogene coastal uplift in hinged segments
destabilized the continental margin by triggering renewed
gravity spreading; erosional unroofing rejuvenated the sediment
supply. (5) Since the Eocene, five major hiatuses formed in the
IKB; where the Tertiary cover is thickest, the basin remained
close to sea level, which refutes the idea of a massively elevated
coastal plateau. (6) The style, cause, and location of shortening
varied greatly; in the Albian, a rind of overburden wrinkled to
form small buckle folds over a wide area; from the Late
Cretaceous to Miocene, virtually all shortening in the OKB was
accommodated by advance of the Angola Salt Nappe across the
distal limit of the salt basin; finally, Plio-Pleistocene burial of the
nappe toe
locked its advance and compressed the entire deepwater
salt basin. (7) Apatite fission-track analysis suggests at least
three thermal events, which we link to volcanism, erosional
unroofing, and passage of hot fluids; the youngest thermal event
coincided with Neogene uplift.
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