It is thought that the Gulf of Mexico formed approximately 300 million years ago. Many theories exist as to the exact mechanism of formation, but most scientists agree that the Gulf was formed as a result of seafloor subsidence.
The Gulf of Mexico basin is a relatively simple, roughly circular structural basin approximately 1,500 km in diameter, filled in its deeper part with 10 to 15 km of sedimentary rocks that range in age from Late Triassic to Holocene (approximately 230 m.y. to present). Little is known about the geologic history of the Gulf of Mexico Basin before Late Triassic time. Since pre-Triassic rocks are known from only a few widely separated outcrop areas and wells, much of the geologic history of the basin during Paleozoic time needs to be inferred from the study of neighboring areas. Some authors have postulated the presence of a basin in the area during most of Paleozoic time, but most evidence seems to indicate that Paleozoic rocks do not underlie most of the Gulf of Mexico basin and that the area was, at the end of Paleozoic time, part of the large supercontinent of Pangea, the result of the collision of several continental plates. The present Gulf of Mexico basin, in any case, is believed to have had its origin in Late Triassic time as the result of rifting within the North American Plate at the time it began to crack and drift away from the African and South American plates. Rifting probably continued through Early and Middle Jurassic time with the formation of "stretched" or "transitional" continental crust throughout the central part of the basin. Intermittent advance of the sea into the continental area from the west during late Middle Jurassic time resulted in the formation of the extensive salt deposits known today in the Gulf of Mexico basin. It appears that the main drifting episode, during which the Yucatan block moved southward and separated from the North American Plate and true oceanic crust formed in the central part of the basin, took place during the early Late Jurassic, after the formation of the salt deposits.
Since Late Jurassic time, the basin has been a stable geologic province characterized by the persistent subsidence of its central part, probably due at first to thermal cooling and later to sediment loading as the basin filled with thick prograding clastic wedges along its northwestern and northern margins, particularly during the Cenozoic. To the east, the stable Florida platform was not covered by the sea until the latest Jurassic or the beginning of Cretaceous time. The Yucatan platform was emergent until the mid-Cretaceous. After both platforms were submerged, the formation of carbonates and evaporites has characterized the geologic history of these two stable areas. Most of the basin was rimmed during the Early Cretaceous by carbonate platforms, and its western flank was involved during the latest Cretaceous and early Tertiary in a compressive deformation episode, the Laramide Orogeny, which created the Sierra Madre Oriental of eastern Mexico.
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