Cenozoic Growth-Fault Trends as Secondary Indicators for Deep Basement Structures in the Northwestern Gulf of Mexico Basin

Barry E. Bradshaw, Joel S. Watkins

The northern Gulf of Mexico is a rifted continental margin in which deep-seated tectonic forces have been essentially absent since Mesozoic times. A massive influx of siliciclastic sediments into the northern Gulf triggered by the late Cretaceous Laramide Orogeny has infilled the rift basin with a maximum thickness of up to 15 km of strata. This enormous thickness of post-rift strata in conjunction with extensive salt and overpressured shale masses generally prevents imaging of basement structures on seismic reflection data. Indirect evidence for deep-seated basement structures is possible through a comprehensive analysis of post-rift salt and shale withdrawal structures. Such indirect evidence is applied to the Texas shelf margin in the northwestern Gulf of Mexico. We be ieve that areas of extensive shale withdrawal indicate relative basement highs in which mid-Jurassic salt was thin or absent. In contrast, areas of extensive salt deformation represent deep salt basins formed over highly attenuated crust. A distinct transition from shale to salt withdrawal structures is evident on the Texas shelf, this transition systematically decreasing in age and occurring further basinwards from the southwestern to northeastern Texas shelf margin. Such observations are consistent with the presence of a series of northwest-southeast trending strike-slip transfer faults that right laterally offset rifted basement. Inferred locations for transfer faults are supported by regional gravity and crustal extension (ß-value) data, and line up with onshore structural tren s such as the San Marcos Arch, Llano Uplift, Marathon Uplift, Rio Grande Embayment and Houston Embayment. Our interpretations are also consistent with previously inferred locations of Permo-Triassic rift transforms by researchers in the Texas Gulf coast.

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