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Local structuring in the basins that rim the Gulf of Mexico is largely the result of gravity acting on sedimentary sections deposited on an unstable base of abnormally pressured shales and/or salt. The resulting deformation takes two primary forms, salt-flow structures and listric-normal faults that sole out at various levels above the basement.

Salt flow in this nonorogenic environment is the result of pressure gradients created by the sediments that overlie, or load, the salt. When differential loading occurs, pressures vary laterally within the salt layer, and the salt tends to move away from areas of higher pressure toward areas of lower pressure. Structures resulting from such flow have a wide variety of forms. These include low-relief anticlines and pillows, which simply deform the overlying beds; high-relief plugs and walls, which have a piercement relation with the sediments and often breach the surface; and extensive salt sheets, which have been emplaced laterally in clastic sediments deposited in a continental slope environment.

Listric-normal faults are the result of coherent, differential basinward movement of the sediments above some decollement layer. In the Gulf of Mexico basin, this layer may be either salt or abnormally pressured shale. The term “listric” comes from the Greek word for shovel and aptly describes the curved, three-dimensional geometry of the fault itself (Bally and others, 1981). The term was originally used to describe thrust faults; thus the adjective “normal” is needed to describe their extensional counterparts. The principal driving force for such faulting is gravity acting on

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