The U.S. Atlantic and Gulf of Mexico continental margins are thickly sedimented passive margins that formed when Pangea split apart during Middle Jurassic time to create the Atlantic Ocean and Gulf of Mexico. Keys to understanding the process of continental breakup and its relation to preexisting structure are found in the structure of the crust beneath the sediment-filled basins and adjacent platforms and embayments that outline the margins. Because of the great thickness of post-rift sedimentary rock in the basins and the presence of massive reef carbonates and salt layers and diapirs, the crustal structure beneath the basins in the region of the transition between oceanic and continental crust is poorly known at present. Recent advances in seismic reflection and refraction data collection techniques (both sources and receivers), however, are just beginning to yield new data to look at the crustal structure in this important region. One of the most important results on the deep structure of continental margins obtained in recent years is recognition of a thick, high-velocity (7.2 to 7.5 km/sec) layer at the base of the crust beneath the U.S. Atlantic continental margin as well as beneath several other margins worldwide. This layer is observed beneath both extended continental crust and early oceanic crust and has been interpreted to indicate that extensive intrusive magmatism was associated with the late stage of rifting and early sea-floor spreading. Only a weak suggestion of such a layer has been observed beneath the Gulf of Mexico margin, although this may be in part due to the difficulty of observing lower crustal arrivals because of the extensive presence of salt in the shallow section.

Abstract The accumulation of abundant geologic and geophysical information about the Gulf of Mexico Basin during the last few decades has contributed significantly to the understanding of the geologic history of this important geologic province—its birth during Late Triassic-Jurassic time, and the subsequent geologic events that have given the basin its present configuration. Much of this information has been the product of extensive surface and sub-surface geologic studies, geophysical surveys, and drilling around the periphery of the basin, mainly in connection with the exploration for oil and gas. More recently, invaluable data have been obtained in the previously little-known central part of the basin by regional geophysical surveys—particularly seismic reflection and refraction—and the results of the Deep Sea Drilling Project. The geophysical surveys were conducted by academic institutions, the U.S. Geological Survey, and the petroleum industry. The results of these geophysical surveys have shed much light on the seismic nature of the crust underlying the Gulf of Mexico Basin—“oceanic” in the deep central part and “transitional” around the periphery of the deep basin. (The term “transitional” is applied to crust intermediate in thickness between oceanic and continental crust, possibly the result of stretching during episodes of rifting or pull-apart.) The surveys have also for the first time contributed information concerning the distribution, thickness, and structural configuration of the extensive salt deposits, as well as of the thick sedimentary column that fills the central part of the basin. This large volume of geologic and geophysical information is now scattered throughout the extensive literature on the Gulf of Mexico Basin.