Publication Date:
2008-04-01
Description:
The Lower Congo Basin contains the greatest salt-based fold and thrust belt off Africa's Atlantic margin. Our study area in the Anton Marin and Astrid Marin exploration blocks is in the northern part of the basin. Gravity-driven tectonic shortening began soon after the Aptian salt deposition, forming gentle, west-trending, salt-cored anticlines, which, together with salt diapirs, created a template for later thrusting. In the Late Cretaceous, a thrust front propagated landward into the study area, and thrusts formed above salt anticlines and diapirs. Formation of a hanging-wall wedge of growth strata was recorded when each thrust fault ruptured the seabed. Thrusting began after widespread salt thinning, as autochthonous salt was expelled into older, passive diapirs. Thinning stiffened the detachment, so that thrusts verge strongly seaward. Structural restorations, dip-corrected isochron maps, and fault-activity graphs all show that the landward edge of the thrust belt propagated landward. Three main pulses of shortening episodically reactivated thrust faults as the thrust front broke landward. As thrusting culminated, precursor passive diapirs were squeezed and extruded small allochthonous sheets. Translation culminated in major erosional scouring, from which we infer epeirogenic slope steepening in the Late Cretaceous. As shortening spread updip into the previously extensional domain during the Late Cretaceous to Paleogene, older extensional faults were inverted, and new extensional faults formed orthogonally, parallel to the regional paleoslope. The structural pattern, created in the Late Cretaceous when the paleoslope dipped southward, remains recognizable in the little-deformed Neogene strata, although the present continental slope dips westward. Martin Jackson received his Ph.D. from the University of Cape Town in 1976. After teaching, he joined the Bureau of Economic Geology in 1980 and founded the Applied Geodynamics Laboratory in 1988. He received AAPG's Sproule Award, Matson Award, and Dott Award for his research on salt tectonics, which focuses on the Gulf of Mexico, Mediterranean, Arctic Canada, and Mars. Mike Hudec received his Ph.D. from the University of Wyoming in 1990. He worked for Exxon Production Research and taught at Baylor University. He joined the Bureau of Economic Geology in 2000, where he codirects the Applied Geodynamics Laboratory. His current research interests include palinspastic restoration of salt structures, salt-sheet emplacement mechanisms, and minibasin initiation. Dave Jennette is currently the geology manager of E&P Technology at Apache Corporation, Houston, Texas. Previously, he held lead researcher positions at the Bureau of Economic Geology and ExxonMobil Upstream Research Company, where he focused on integrated analysis of turbidite depositional systems. Richard Kilby received a B.S. degree in geology from Washington and Lee University in 2003 and an M.S. degree in geology from the University of Texas at Austin in 2005. He works for Shell Exploration and Production in Houston, Texas, as a structural geologist in the Gulf of Mexico new ventures team. His research interests include salt tectonics and forward modeling of fault-controlled folds.
Print ISSN:
0149-1423
Electronic ISSN:
1943-2674
Topics:
Geosciences
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