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Although the sediments filling the shallow-water parts of the Gulf of Mexico basin have been extensively explored for hydrocarbons using seismic methods and are reasonably well known, the nature and distribution of the underlying crust and mantle are much less well known. Several characteristics of the basin's sediments have made deep-penetration observations difficult. Thick Cenozoic clastic sediments throughout the Gulf of Mexico basin attenuate seismic energy, while Jurassic to Upper Cretaceous carbonates and evaporites provide a large impedance contrast, through which seismic transmission is further limited. In parts of the basin, mobile salt forms pillows, domes, sills, and other structures, which focus and defocus seismic energy in ways that make imaging difficult. In the southeastern Gulf of Mexico, solution cavities and karst-like paleotopography in shallow-water carbonates severely scatter seismic signals. In spite of these difficulties, reflection seismic, refraction seismic, gravity, magnetic, and subsidence techniques have been used to resolve the gross characteristics of the crust under the Gulf of Mexico basin.

In all cases the terms “crust” and “basement” will be used as synonyms. In areas of normal or modified continental crust, the crust and basement are defined to include all rock lying beneath a widespread unconformity at the base of the marine Mesozoic section. This surface is overlain and onlapped by Middle Jurassic salt (or equivalent rocks) as well as younger sedimentary rocks. Included within “basement” are the Upper Triassic to Lower Jurassic rift sequences (see Chapter 8, this volume). Although these “red-bed” sequences should perhaps be considered a

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