Abstract: Influence of Major Steps in the Base of Salt on the Structural Style of Overlying Thin-skinned Structures in Deep Water Angola

Peel, Frank - BHP; Martin Jackson - Bureau of Economic Geology, The University of Texas at Austin; and David Ormerod - BHP

In the mid to lower slope of the Lower Congo and Lower Kwanza Basins, (deep water Angola), the thickness of the Aptian autochthonous salt varies from zero to several kilometres. This variable geometry creates strong lateral velocity gradients and severe imaging problems, so it is difficult to map the depth to base of autochthonous salt by conventional means. We present an alternative mapping approach that enables the base-salt structure to be defined on a regional scale across the Angolan margin.

This reveals the location of a set of major basement faults, parallel to the basin margin, and mostly stepping down to the west. The major displacement on these faults occurred during the Neocomian to Aptian rifting stages, which had terminated prior to the deposition of the Aptian salt, and most of the fault offsets were buried by syn-rift sediments. However, significant topographic expression of the fault offsets remained at the time of salt deposition. Although there is no evidence of any post-Aptian movement on the basement faults, they exerted a strong influence on the later structural development of the margin, in two main ways:

(i) They commonly define the boundaries between zones of different structural style (see Fig. 1); in the Lower Kwanza, the Atlantic Flexure faults mark the boundary between the zone dominated by extension plus salt withdrawal, and the zone dominated by contractional folding plus withdrawal and diapirism. In the Lower Congo Basin, another basement fault defines the limit of the zone dominated by contractional folding plus allochthonous salt.

(ii) The movement of the sediment carapace over basement steps generates characteristic growth synclines. The stratal patterns within these synclines are controlled by two factors, the rate of sediment aggradation and the rate of downslope sliding (see Fig. 2). The structures produced as the sediment carapace slid over basement steps provide strong evidence of the amount, direction, and timing of downslope translation.

Detailed sequential structural restoration of key lines in the Lower Kwanza Basin reveals a consistent amount of downslope translation and demonstrates that the structures in the mid to lower slope are dominantly created by contractional folding, with subordinate salt-withdrawal and diapirism.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil