Deep-Sea Drilling into the Challenger Knoll, Central Gulf of Mexico

Abstract

During the recent pioneering cruise of a deep-sea coring project, a series of holes was drilled into the deep floor of the Gulf of Mexico and the western North Atlantic. One of the holes, in the Sigsbee Knolls area of the central Gulf of Mexico, encountered salt-dome caprock saturated with oil, gas, and sulfur. The objectives of this coring project are scientific and the discovery of petroleum was not an intended part of the program, even though the possibility had been recognized. The results of drilling into the Challenger Knoll indicate that the Sigsbee Knolls and adjacent buried domes are diapirs derived from a mid-Mesozoic salt deposit, comparable in age and lithology with other major evaporite sequences within the Gulf of Mexico area. Marine geophysical data show that these diapirs are part of a belt which extends southwest into the saline basin of the Isthmus of Tehuantepec. Cores of caprock, recovered from more than 12,000 ft below sea level, contained a typical salt-dome caprock mineral assemblage of calcite, gypsum, and free sulfur (up to 19 percent). The petroleum is sulfur-rich and of low gravity, and geochemical data indicate that these hydrocarbons are immature, of fairly young age, and derived from marine organic material. Residual mineral assemblages--including quartz crystals--are comparable with those found in caprock elsewhere in the Gulf of Mexico region; similar quartz crystals are common in the mid-Mesozoic Louann Salt. All sediments above the caprock are pelagic clays and calcareous organic oozes of late Tertiary age (late Miocene and younger). The presence of these sediment types indicates that the Challenger Knoll has been topographically above the surrounding sea floor at least since late Miocene time. In contrast, the turbidite-rich sequence cored beneath the adjacent abyssal plain shows that pelagic sediments constitute only a fourth to less than half the total late Miocene and younger section. The late Cenozoic pelagic deposits of Challenger Knoll and the turbidites appear to reflect major geologic events and significant changes in sediment provenance within the land area surrounding the Gulf of Mexico. The regional problem which probably will be debated most earnestly as a result of the Challenger Knoll coring is whether the salt underlying the knoll was deposited originally in a deep-sea environment, or on a crust which once was much shallower. This problem is of considerable geologic importance, and it can now be approached with a background of firm data.