Isopach maps of the Wilcox, Claiborne, Jackson, and total Eocene; of the Vicksburg, Frio-Anahuac, and total Oligocene; and of the Miocene-to-Recent; projected speculatively beyond well control, indicate that the Cenozoic sediments reach thicknesses of 45,000 feet off the South Texas Coast and 35,000 feet just west of the Mississippi River delta in Louisiana. Thickest deposition took place in Texas during the Paleogene, shifting to Louisiana in the Neogene. Cross sections, projected beyond well data into the Gulf of Mexico by the isopachs, show the form of the Gulf Coast geosyncline.

The Cenozoic history of the Gulf Coast is essentially a conflict between land and sea for possession of the area of the present Coastal Plain. Along the northern and western sides of the Gulf of Mexico, with which this paper is principally concerned, ample supplies of mud and sand have been available to build out the continent, with many interruptions by invasions of marine waters. The character of the sedimentary units which have been isopached are controlled in large part by the sequence of marine transgressions and regressions. The intricate sedimentary patterns produced by these repeated oscillations have provided unlimited variations in stratigraphic conditions for trapping oil and gas.

The contact of Neogene on Paleogene is placed in the Oligocene at the base of the Nodosaria blanpiedi zone because an unconformity here appears to be the most centrally located and extensive in the transitional wedge of Frio sediments.

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