The early history of the Gulf basin is conjectural. It once was believed the basin formed by late Paleozoic foundering of Llanoria, a postulated large offshore landmass occupying much of the present basin area.

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Currently, there are two schools of thought: (1) the basin has existed since late Precambrian; or (2) it was formed by early Mesozoic sea-floor spreading in the Gulf, a product of the general breakup of Pangea into continental blocks.

Late Paleozoic orogeny, in phase with or a part of a west-southwestward continuation of Appalachian folding, created a northern structural rim for the basin which strongly influenced subsequent sedimentation and structural trends. Postorogenic tension faulting along and south of this rim was particularly active during the Triassic. Jurassic sediments along the flank and gulfward from the structural rim overlie this faulted basin floor and are in unconformable contact with rocks ranging in age from Triassic to Mississippian.

Triassic sediments are fluvial to deltaic redbeds. Paleozoic deposits include both "Ouachita facies" and unmetamorphosed fluvial to offshore marine clastic rocks and highly fossiliferous shallow-water carbonate rocks. Seismic data suggest Triassic and/or late Paleozoic sediments underlie the Jurassic throughout the Gulf basin. These pre-Jurassic rocks comprise a large, very sparsely tested frontier for oil and gas.

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