Controlling Factors in Different Styles of Deformation Associated with Gravity Sliding, Kwanza Basin, Angola

Steven G. Henry, Larry A. Standlee

The movement of fault blocks above Aptian-age salt in the offshore kwanza Basin, Angola, has resulted in highly disparate styles of deformation. These include (1) small, rotated, asymmetric fault blocks separated by syntectonic half grabens, (2) large, non-rotated, fault blocks, or rafts, bounded by troughs containing rotated Tertiary sediments, and (3) tight folds and thrusts resulting from localized shortening. All of these features resulted from gravity-induced sliding above the salt decollement. A generally westward slope of the salt was caused by (1) thermal subsidence related to the early stages of the opening of the South Atlantic Ocean, in Mid-Late Cretaceous, and (2) younger (Tertiary) uplift of the African craton.

Restored cross sections based on seismic lines and well control demonstrate that the spatial and temporal configuration of the basement and pre-salt strata strongly influenced the original depositional thickness of the salt and overlying sediments, the time of initiation of fault-block movements, and the resulting geometry of structures in the basin. It appears that simple concepts of salt flowage governed by critical thicknesses of salt and post-salt overburden are not valid. Early movement of relatively thin fault blocks above thin salt occurred near the Ambriz Arch, whereas to the south thick salt and overlying sediments remained essentially undeformed until the Tertiary uplift. During the two stages of slope development gravity sliding along an irregular surface resulted in shorte ing folds and local thrust faults.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995