The geophysical data on the Gulf of Mexico, combined with the known geology, make possible the preparation of a number of maps which define the modern geosyncline and provide some evidence as to its inception. The velocities and depths of interfaces observed in seismic reflection and refraction profiles reflect the characteristics and thicknesses of the layers of rock in the syncline, and the gravity and magnetic data indicate its tectonic framework. The inception of the geosyncline appears to be related to widespread collapse in Triassic time, which had been preceded by the deposition of Paleozoic sediments and the possible extension Gulfward of the Appalachian orogeny. The problems dealt with in this paper are: (1) the shallow Jurassic and Cretaceous aspect of the Gulf, (2) the widespread extent of the salt and the resultant domes, (3) the lateral or wrench faults and the restoration of transposed elements, (4) the pattern of shifting depocenters, (5) the tremendous acceleration of depositional rates in Tertiary time culminating in the rapid present day rate of 24.4 cm/century determined by Hardin and Hardin, (6) the enigmatic Atlantic trench, (7) the unexpected axial directions of magnetic anomalies, and (8) the intermediate (between continental and oceanic) depth of the Moho below the Gulf. The cumulative result of the complex history is a Mesozoic and Cenozoic geosyncline with a sedimentary thickness of perhaps 60,000 feet. The final problem is why this great prism of rocks does not fold into a mountain range after exceeding the accepted depth limit or other geosynclines.

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