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12/07/2013

A Brief Tectonic and Depositional History of the Northern Gulf of Mexico


The Gulf of Mexico (GOM) is the 9th largest body of water on earth, covering an area of approximately 1.6 million km2 with water depths reaching 4,400 m (14,300’). The basin formed as a result of crustal extension during the early Mesozoic breakup of Pangaea. Rifting occurred from the Late Triassic to early Middle Jurassic. Continued extension through the Middle Jurassic combined with counter-clockwise rotation of crustal blocks away from North America produced highly extended continental crust in the subsiding basin center. Subsidence eventually allowed oceanic water to enter from the west leading to thick, widespread, evaporite deposition. Seafloor spreading initiated in the Late Jurassic eventually splitting the evaporite deposits into northern (USA) and southern (Mexican) basins. Recent work suggests that this may have been accomplished by asymmetric extension, crustal delamination, and exposure of the lower crust or upper mantle rather than true sea floor spreading (or it could be some combination of the two). By 135 Ma almost all extension had ceased and the basic configuration of the GOM basin seen today was established. The Laramide Orogeny was the last major tectonic event impacting the GOM. It caused uplift and erosion for the NW margin from the Late Cretaceous to early Eocene.

Sedimentation in the GOM can be divided into five megasequences: Rifting to Upper Jurassic, Lower Cretaceous, Upper Cretaceous, Paleogen, and Neogene. The oldest sediments are clastics in the Upper Triassic known only from peripheral rift basins onshore. In the basin center evaporites of the Middle Jurassic Louann Formation are the oldest deposits encountered. Deformation and movement of the Louann salt affects almost all the overlying strata and plays a very important role in all aspects of the basin’s petroleum systems. Above the salt, Upper Jurassic marine shales of Oxfordian and Tithonian age comprise two of the most important petroleum source beds. In the Lower Cretaceous megasequence the Aptian age Sligo and Albian age Stuart City carbonates established basin rimming reef margins that divided shelf from deep water. These reefs sit above the structural hinge between thick and thin continental crust. In the Upper Cretaceous megasequence the Cenemanian age Woodbine-Tuscaloosa system represent the first coarse clastics to advance beyond the Lower Cretaceous shelf margin. The megasequence is capped with tsunami deposits from the Chicxulub impact on the Yucatan peninsula. The Paleogene and Neogene megasequences are dominated by major clastic inputs of the lower Wilcox, upper Wilcox, Vicksburg, and Frio, lower Miocene, middle Miocene, upper Miocene, Pliocene, and Pleistocene. These progradational episodes not only advanced the shorelines and shelf margins significantly but also deposited thick sands (major reservoirs) in the deep GOM. The Neogene progradational episodes are strongly influenced by glacio-eustatic cycles of increasing frequency and amplitude.