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 locally ranging up to 17 km thick. Beneath the sedimentary sequence a layer with velocities ranging from 5.2 to 6.0 km/sec probably consists of high-velocity sedimentary rocks (salt and carbonates) possibly overlying a thin upper crustal layer. The combined thickness of the sedimentary sequence and the 5.2-6.0 km/sec layer is between 15 and 20 km. The deeper crust is 12-20 km thick beneath the interior of the coastal plain and thins seaward. The velocity of this crust (6.45-6.9 km/sec) is comparable with that of oceanic crust.

From available data for the north Gulf Coast, we have constructed 3 profiles across the coastal plain to the Sigsbee Deep. From these profiles, we have reconstructed former Gulf coastal margins for 3 epochs of the Mesozoic and Cenozoic, based on the assumption that the region has remained in close isostatic equilibrium. We have postulated the evolution of the Gulf Coast geosyncline, the continental shelf, and the transition from continental crust to oceanic crust at this margin since the Mesozoic.

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