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The thick Mesozoic-Cenozoic fill of the Gulf of Mexico basin was deposited on a floor of Paleozoic and older “basement” that is still poorly known. Fewer than 250 wells out in the basin, away from the structural rim, have penetrated pre-Mesozoic rocks; fewer than a dozen of these wells drilled as much as 1,000 m of this older section. Basinwide, the average penetration of these older rocks is only 220 m, and data from many of the wells are sketchy and incomplete. The pre-Triassic has long been considered “economic basement” by the oil industry, and stratigraphers have paid little attention to these rocks. Whereas a great part of the pre-Triassic rocks was subjected to strong deformation and metamorphism, it is now known that unaltered sediments of Cambrian(?) to Devonian age underlie the Mesozoic in the southeastern U.S. states of Alabama, Georgia, and Florida. Sediments of early Pennsylvanian to Permian age, some possibly older, are known to underlie the Mesozoic in the northwestern part of the basin, and seismic data suggest thicknesses of 5 km or more in some areas for these young Paleozoic sediments. Unmetamorphosed pre-Triassic rocks are also known from the western flank of the Gulf of Mexico basin, in eastern Mexico, and from its southern flank in southern Mexico, Guatemala, and Belize.

One of the earliest recorded penetrations of pre-Triassic “basement” out in the basin was reported in 1928. A well drilled in Marion County, Florida (location 1, Fig. 1), encountered rocks first called “basement schist and quartzite.”

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