Gulf of Mexico - Geology

Geology

The consensus among geologists, who have studied the geology of the Gulf of Mexico, is that prior to Late Triassic, the Gulf of Mexico did not exist. Before the Late Triassic, the area now occupied by the Gulf of Mexico consisted of dry land, which included continental crust that now underlies Yucatan, within the middle of the large supercontinent of Pangea. This land lay south of a continuous mountain range that extended from north-central Mexico, through the Marathon Uplift in West Texas and the Ouachita Mountains of Oklahoma, and to Alabama where it linked directly to the Appalachian Mountains. It was created by the collision of continental plates that formed Pangea. As interpreted by Roy Van Arsdale and Randel T. Cox, this mountain range was breached in Late Cretaceous times by the formation of the Mississippi Embayment.

Geologists and other Earth scientists agree in general that the present Gulf of Mexico basin originated in Late Triassic time as the result of rifting within Pangea. The rifting was associated with zones of weakness within Pangea, including sutures where the Laurentia, South American, and African plates collided to create it. First, there was a Late Triassic-Early Jurassic phase of rifting during which rift valleys formed and filled with continental red beds. Second, as rifting progressed through Early and Middle Jurassic time, continental crust was stretched and thinned. This thinning created a broad zone of thick transitional crust, which displays modest and uneven thinning with block faulting, and a broad zone of uniformly thinned transitional crust, which is half the typical thickness, 35 kilometers, of normal continental crust. It was at this time that tectonics first created a connection to the Pacific Ocean across central Mexico and later eastward to the Atlantic Ocean. This flooded the subsiding basin created by rifting and crustal thinning to create the Gulf of Mexico. While the Gulf of Mexico was a restricted basin, the subsiding transitional crust was blanketed by the widespread deposition of Louann Salt and associated anhydrite evaporites. Initially, during the Late Jurassic, continued rifting widened the Gulf of Mexico and progressed to the point that sea-floor spreading and formation of oceanic crust occurred. At this point, sufficient circulation with the Atlantic Ocean was established that the deposition of Louann Salt ceased.

During the Late Jurassic through Early Cretaceous, the basin occupied by the Gulf of Mexico experienced a period of cooling and subsidence of the crust underlying it. The subsidence was the result of a combination of crustal stretching, cooling, and loading. Initially, the combination of crustal stretching and cooling caused about 5–7 km of tectonic subsidence of the central thin transitional and oceanic crust. Because subsidence occurred faster than sediment could fill it, the Gulf of Mexico expanded and deepened.

Later, loading of the crust within the Gulf of Mexico and adjacent coastal plain by the accumulation of kilometers of sediments during the rest of the Mesozoic and all of the Cenozoic further depressed the underlying crust to its current position about 10–20 km below sea level. Particularly during the Cenozoic, thick clastic wedges built out the continental shelf along the northwestern and northern margins of the Gulf of Mexico.

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 Paleogene periods in a compressive deformation episode, the Laramide Orogeny, which created the Sierra Madre Oriental of eastern Mexico.

In 2002 geologist Michael Stanton published a speculative essay suggesting an impact origin for the Gulf of Mexico at the close of the Permian, which could have caused the Permian–Triassic extinction event. However, Gulf Coast geologists do not regard this hypothesis as having any credibility. Instead they overwhelmingly accept plate tectonics, not an asteroid impact, as having created the Gulf of Mexico as illustrated by papers authored by Kevin Mickus and others. This hypothesis is not to be confused with the Chicxulub Crater, a large impact crater on the coast of the Gulf of Mexico on the Yucatan Peninsula.

Today, there are 7 main areas of the gulf:

• Gulf of Mexico Basin, which contains the Sigsbee Deep and can be further divided into the continental rise, the Sigsbee Abyssal Plain, and the Mississippi Cone.
• Northeast Gulf of Mexico, which extends from just east of the Mississippi Delta near Biloxi to the eastern side of Apalachee Bay.
• South Florida Continental Shelf and Slope, which extends along the coast from Apalachee Bay to the Straits of Florida and includes the Florida Keys and Dry Tortugas.
• Campeche Bank, which extends from the Yucatan Straits in the east to the Tabasco–Campeche Basin in the west and includes Arrecife Alacran.
• Bay of Campeche, which is an isthmian embayment extending from the western edge of Campeche Bank to the offshore regions just east of the port of Veracruz.
• Western Gulf of Mexico, which is located between Veracruz to the south and the Rio Grande to the north.
• Northwest Gulf of Mexico, which extends from Alabama to the Rio Grande.