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33 ␥ crust, which displays modest thinning and typically lies at subsea depths between 2 and 10 km. Much of the present lower coastal plain, shelf, and continental slope is underlain by thin transitional crust, which is generally less than half of the 35-km thickness typical of continental crust and is buried more than 10 km below sea level. Basement may lie below 20 km in the depocenter beneath the southern Louisiana coastal plain and adjacent continental shelf (Peel et al. 1995). The broad history of plate tectonic movements that culminated in the Gulf basin is generally understood (Marton and Buffler 1999; Pindell and Kennan 2001; Jacques and Clegg 2002; Harry and Londono 2004), if not agreed upon in detail. The Gulf of Mexico opened by the separation of the North and South American plates. Triassic through early Jurassic tensional deformation created a series of basement grabens and half grabens, which are filled with terrestrial red beds and volcanics. Crustal stretching in Bathonian and Callovian time initiated a broad sag, which opened initially to the Pacific Ocean. Widespread deposition of thick Louann salt and associated evaporites, a defining event for the later structural evolution of the Gulf sedimentary fill, spread across the shallow, hypersaline basin centered above the thinned continental crust. A regional unconformity beneath the evaporite layer separates localized syn-rift from blanket post-rift deposits (Sawyer et al. 1991; Buffler and Thomas 1994). Opening of the Gulf entailed approximately 500 km of extension accompanied by southward migration and counterclockwise rotation of the Yucatan block The Gulf of Mexico is a small ocean basin lying between the North American Plate and the Yucatan block. It contains within its depocenter a succession of Jurassic through Holocene strata that is as much as 20 km thick. Sediment supply from the North American continent has filled nearly one-half of the basin since its inception , primarily by offlap of the northern and northwestern margins. The Gulf of Mexico basin is a world-class repository of hydrocarbons (Nehring 1991). It has been actively explored for nearly 100 years, creating a three-dimensional well and reflection seismic database of unique abundance, extent, and diversity. Because of this history, the northern Gulf has served, for more than 50 years, as a natural laboratory for understanding the sedimentary processes, facies, stratigraphy, and gravity tectonics of prograding continental margins. This chapter will focus on the history of this northern fill, with emphasis on the area beneath the present continental shelf of the northern Gulf. Crustal Structure and Basin Origin The Gulf of Mexico basin was created by crustal extension and seafloor spreading during the Mesozoic breakup of Pangea (Sawyer et al. 1991). Most of the structural basin is underlain by transitional crust that consists of continental crust that was stretched and attenuated primarily by Middle to Late Jurassic rifting (Fig. 3.1). The northernmost basin margin is underlain by thick transitional 3 Pre-Holocene Geological Evolution of the Northern Gulf of Mexico Basin William E. Galloway 34 ~ Galloway The northern margin is a relatively simple divergent margin with a broad zone of stretched continental crust separatingoceanicandcontinentalcrust.TheYucatanmargin juxtaposes thick transitional and oceanic crust. This pronounced asymmetry suggests a simple-shear model for extension (Marton and Buffler 1993; Watkins et al. 1995). Additional Mesozoic and Cenozoic tectonic phases have influenced subregional subsidence history of the Gulf. Several of the marginal highs, including the San Marcos arch, Sabine arch, and Monroe uplift, display and by extensive north northwest–south southeast shear along the east and west flanks of the basin (Marton and Buffler 1993, 1999; Pindell and Kennan 2001). Crustal rupture began by the Oxfordian and continued until the termination of spreading in the latest Berriasian or early Valanginian. By the end of the Early Cretaceous, combined deposition and subsidence had created the modern morphology of the Gulf basin (Winker and Buffler 1988). Late Cretaceous and, especially, Cenozoic history was dominated by loading subsidence. MEXICO YUCATAN 0 100 km CUBA 12 12 12 12 1 4 8 8 8 1 0 10 10 10 6 6 2 2 2 1 1 1 1 6 6 6 12 8 8 4 2 4 14 14 4 10 12 1 2 10 10 2 4 1 0 1 2 6 8 8 14 14 16 RGE TA 90 Ouachita Mountains Appalachian Mountains MU ETB NLSB SA SMA MSB THICK TRANSITIONAL CRUST Llano Uplift WA AE MGA TE SrA THICK TRANSITIO NAL CRUST THIN TRANSITIO NAL CRUST O C E A N...

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