Changes in Location and Style of Deepwater Contraction
in the Kwanza Basin, Angola

Michael R. Hudec and Martin P. A. Jackson
Bureau of Economic Geology
John A. and Katherine G. Jackson School of Geosciences
The University of Texas at Austin, Austin TX 78713-8924

ABSTRACT
The position of deepwater foldbelts is generally thought to be controlled by the toe of the continental slope and/or the basinward limit of salt deposition. Analysis of contractional deformation in the Kwanza Basin, Angola (Fig. 1), suggests that the locus of downdip shortening has varied considerably through time. Although the toe of slope and the basinward limit of salt have both played a role, foldbelt location is not a simple function of either variable.


Figure 1.
Depth section across the northern Kwanza Basin, Angola. VE ´ 7.

Regional restoration shows that sediments in the Kwanza Basin have been gravitationally transported more than 25 km basinward above a salt detachment. The translation has been absorbed at the downdip end of the system on three sets of contractional structures. These contractional belts are geographically distinct and differ in age and structural style. The oldest set of contractional structures formed during the Albian, when only a few hundred meters of sediment covered Aptian salt. Differential thermal subsidence produced a seaward tilt on the margin, forming a carbonate ramp. Salt and sedimentary cover moved basinward, forming extensional structures updip and a belt of short-wavelength buckle folds in deeper water. Most of these folds formed well updip of the basinward salt pinchout, suggesting that the sediment cover was too thin and weak to transmit stress all the way to the downdip end of the linked system. The second stage of contractional deformation, by contrast, was localized at the basinward limit of salt. Seaward salt flow formed a thick, uplifted salt plateau in deep water by the Late Cretaceous. The salt broke through its roof and began to extrude over the abyssal plain. Overlying sediments were carried along with the Angola Salt Nappe as it advanced, duplicating section above and below salt. From Late Cretaceous through the Miocene virtually all of the downdip contraction in the Kwanza Basin was accommodated by ~25 km of advance on the nappe. Regions that had buckled during the Albian became inactive and were carried along passively above the salt.

The third stage of contraction was triggered by burial of the toe of the Angola Salt Nappe in the Pliocene. Now pinned, the nappe could no longer accommodate the shortening needed to balance updip extension. Instead, a broad zone extending from the base of the continental slope to the leading tip of allochthonous salt began to contract. In this 150-km-wide zone, most of the deformation is expressed through contraction of preexisting salt structures, especially salt domes, walls, and anticlines. Shortening is most intense at the landward edge of the belt near the foot of the continental slope. Structures there include squeezed diapirs and overthrust salt anticlines. Farther outboard, deformation is limited to gentle buckling. The deformational sequence described above illustrates the importance of sediment thickness, bathymetry, buttressing, and salt distribution in controlling the expression of deepwater contraction. During the earliest stage of deformation, the sediment was too thin to have much mechanical significance. Albian sediments formed a thin rind above a thick salt layer and wrinkled above salt as it flowed downdip. After enough sediment had accumulated to make the overburden more resistant to deformation, strain was concentrated at the basinward limit of salt. Unconstrained advance of the Angola Salt Nappe provided a mechanism for large-scale contraction that involved little sediment deformation. Finally, sedimentary burial pinned the toe of the nappe, placing the entire deepwater region in compression. Deformation was concentrated at the toe of the continental slope, but the broad zone of shortening extended all the way out to the tip of the Angola Salt Nappe.