ABSTRACT

Seismic refraction data from the western part of the Northern Gulf Coast of the United States indicate that the uppermost crust of the Gulf region consists of a thick sequence of sedimentary rocks with thicknesses locally ranging up to 17 km. Beneath the sedimentary sequence a layer with velocities ranging from 5.2 to 6.0 km/sec probably consists of high velocity sedimentary rocks comprised mainly of evaporites and carbonates. The combined thickness of the sedimentary sequence and the 5.2-6.0 km/sec layer is between 15 and 20 km. The igneous and metamorphic continental crust has a velocity of 5.8-6.0 km/sec, and is 12-20 km thick beneath the interior of the coastal plain. It thins seaward and disappears beneath the shelf. The lower crust is 15-20 km thick beneath the coastal plain, has a velocity of 6.45-6.9 km/sec and grades into normal oceanic crust somewhere beneath the shelf or slope.

From all available data, we have constructed three profiles from the North Gulf Coastal Plain to the Sigsbee deep. From these profiles, making what we consider reasonable interpolations where data is sparse or missing, we have derived structure cross sections of the former Gulf Coast at the end of Cretaceous, Oligocene and Pliocene. Our reconstructions are based on the assumption that the region has remained closely in isostatic equilibrium as it is today. We have thus arrived at a first order approximation to the post-Mesozoic evolution of the Gulf Coast Geosyncline, the Continental Shelf and the transition from continental crust to oceanic crust at this continental margin.