Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
American Association of Petroleum Geologists Annual Meeting, Dallas, April 18-21, 2004
Influence of Precursor Salt Structures on Thrust Faulting, Deep-water Lower Congo Basin, Gabon
Michael R. Hudec, Martin Jackson, and David Jennette
Detailed restorations were constructed from two sections across a single salt-detached thrust structure in the deep-water Lower Congo Basin. Although the two sections are only 4 km apart along strike, their structural style and deformation timing are very different. The northern section intersects a thrust fault overriding a low salt anticline, whereas the southern section intersects a large, pinched-off salt diapir. The northern section displays little or no Tertiary deformation, but the southern section shows all Tertiary units thinning over the arched crest of the diapir, and a large normal fault offsetting the seafloor. Restoration of the northern section suggests that the preexisting salt anticline controlled the location and trend of the subsequent thrust fault. Formation of the thrust fault can be dated by change from symmetric to asymmetric depositional patterns. Bed rotation and stratigraphic thinning in the hanging wall were caused by translation up the thrust ramp. Late salt expulsion from the buried anticline folded the thrust. Restoration of the southern section shows that the preexisting diapir localized the thrust fault. Bed rotation and stratigraphic thinning during diapir shortening were caused by salt flow, not translation up a thrust ramp. Diapir pinch-off was recorded by the end of salt extrusion. The Tertiary deformation that was absent to the north was probably not due to shortening of the structure. Instead, we interpret a period of late salt inflation, probably fed by salt escaping from severe shortening to the south of the study area. Not all thick arched roofs are contractional!