From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

Houston Geological Society International Symposium, September 34, 2003, Houston

Effects of Basement Uplift in Deep Water: Kwanza Basin, Angola

Michael R. Hudec and Martin P. A. Jackson

Abstract:

The Thickened Salt Plateau and Angola Salt Nappe in the deepwater Kwanza Basin together form one of the largest salt massifs in the world, measuring roughly 400 ´ 75 km in area. In most places, the autochthonous salt is 2-4 km thick beneath the sediments that blanket the salt plateau. The plateau formed during the Campanian (~75 Ma), as recorded by a change to condensed sedimentation in the uplifted suprasalt section.

Significantly, the top of salt is generally at least 1 km above the equivalent stratigraphic level on the basement highs that flank the plateau. That is, salt is almost everywhere above its regional in the deepwater Kwanza Basin. How can this be? Normally salt will rise above its regional only if it is displaced by an equivalent volume of sediments that sink below regional. However, on our regional section the area of salt above regional exceeds the area of sediment below regional by a ratio of more than 13:1. This observation holds true for all eight of our regional seismic profiles, and it appears to be characteristic of the entire Thickened Salt Plateau.

The only plausible explanation for this widespread uplift of salt and overburden is basement uplift beneath the plateau. Accordingly, we propose ~1 km of uplift beneath the salt plateau, starting in Campanian time. Uplift may have been a byproduct of the Santonian plate reorganization that reactivated basement structures in the onshore Kwanza Basin, or it could be related to the Late Cretaceous magmatism both onshore and offshore.

Uplift of the Thickened Salt Plateau has a major effect on the salt tectonics. Elevation of the overburden relative to the adjacent abyssal plain created a 400-km-long bathymetric escarpment at the seaward end of the salt system. Salt then broke through this slumping and eroding escarpment to form the extrusive Angola Salt Nappe. Advance of the nappe since the Late Cretaceous has carried sediments more that 20 km across the abyssal plain, making it by far the largest contractional structure in the Kwanza and Benguela basins.