Estimated total volume of sediments contained in certain Mesozoic and Cenozoic stratigraphic units in the Mexican Gulf Coastal Plain (excluding the Yucatán Peninsula) is of the order of 303,900 cubic miles. Breakdown of this volume by distribution and thickness of individual units is shown on an accompanying table and by isopachous maps.

During the Mesozoic era, most of the Mexican Gulf Coastal Plain apparently was a submerged shelf or foreland area along the eastern border of a great geosyncline that extended from Central America through Mexico, the United States, and Canada. Tremendous thicknesses of sediments accumulated in parts of this geosyncline, and relatively thin deposits formed on the shelf area except locally where large organic reefs grew. It is unknown whether the Mesozoic sediments thicken again to the east or continue thinning.

During Eocene time, several basins or foredeeps along the western margin of the present Coastal Plain received thick deposits of geosynclinal sediments (flysch and molasse), derived by erosion of the uplifted Mesozoic geosynclinal block. Lesser thicknesses of sediments were deposited east of these foredeeps. Whether the Eocene sediments thicken farther eastward under the Gulf of Mexico is at present unknown.

During Oligocene and later periods of the Cenozoic, it appears that the present Mexican Gulf Coastal Plain consisted of a number of embayments that extended off a great sedimentary basin. The deepest part of this basin was in the present Gulf of Mexico, similar to the conditions existing today.

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