Gulf coast geosyncline

The presence of a geosyncline along the Gulf Coast of Texas and Louisiana is indicated both by geologic and geophysical data. The formations which are exposed at the surface or in drilling are known in general to dip gulfward. The known stratigraphic thickness of formations plus shrewd extrapolation below reach of the drill indicate that the depth to the basement at Houston is greater than 20,000 feet, at Jennings, Louisiana, greater than 25,000 feet, and south of New Orleans greater than 30,000 feet. Seismic prospecting indicates that the basement certainly is deeper than 15,000 feet and, less definitely, that it is deeper than 20,000 feet. The torsion-balance data indicate that the base of the salt core of the salt domes lies at a depth of 17,000-20,000+ feet in the Houston district. The depth of the Gulf of Mexico in the Sigsbee Deep is 12,500, and throughout most of its area, is less than 10,000 feet. Tne thickness of the post-Lower Cretaceous sediments in the central part of the Gulf of Mexico presumably is not more than a very few thousand feet. The basement of the post-Lower Cretaceous beds in the Gulf Coast must be warped down at least 5,000 feet at Houston and at least 15,000 feet in the area south of New Orleans. A regional trough of gravity minimum lies axially along the Gulf Coast. Calculations suggest that it is best explained by a geosyncline on the basement plus a progressive character of the basement from granitic under the land to basaltic under the Gulf of Mexico. A study of the depression of the basement to compensate sedimentation suggests that the trough line of the geosyncline should lie nearly at the present coast line; and that is approximately the position which the gravitational calculations indicated for the actual position of the trough line. Although isostatically negative, the area lias been one of continued subsidence; and the rate of subsidence seems to have kept even pace with the rate of sedimentation.