Goff, Douglas1, Greg Schoenborn2
(1) ChevronTexaco Overseas Petroleum, Bellaire, TX
(2) ChevronTexaco, San Ramon, CA
ABSTRACT: Origin and Structure of The Cretaceous Toe-Thrust Belt, Offshore Northern Angola: Possible Control on Sedimentary Patterns
Middle Cretaceous (Albian – Cenomanian) extensional tectonics above a salt
detachment are well known from shallow water areas off northern Angola. These extensional
faults have a compressional counterpart 60-150Km farther down the slope. Remarkably, this
compressional belt did not form at the southwestern end of the salt basin which is another
100Km farther SW, but may have localized around small salt welds, or a locally salt
deficient basin. This thrust belt forms the core of a regional Tertiary structural high
which contains three known oil accumulations.
Many classic “thrust-belt” features have developed above the relatively thin salt horizon. Fault-propagation folds and detachment folds with breakthrough faults created a series of nine west-vergent duplexes with about 55% of shortening. Backthrusts occur occasionally in the thrust belt. Some of the duplexes were completely decapitated by erosion. Older growth strata were re-eroded and redeposited again. Some new structural features have been identified; most notable among them are circular synclines that appear to be salt-withdrawal features, or “anti-diapirs.”
Although the Pinda and Vermelha equivalent sediments were very young at the time of deformation, the thrusts created a remarkably accentuated topography at the seafloor. Some of the anticlines stood more than 750m above synclines that are only 2Km away. This corresponds to a topographic angle at the seafloor of more than 20° certainly influencing local currents and stratigraphy. The influence of this paleo-topography continued through to the earliest Tertiary, and may control the depositional patterns of an unconfined Oligocene (?) basin floor fan system.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.