Résumé of Jurassic to Recent Sedimentation History of Gulf of Mexico Basin: ABSTRACT

ABSTRACT The oldest marine sediments that are known to occur on all sides of the Gulf of Mexico basin are of Late Jurassic age. These sediments, mainly carbonates, overlie generally unfossiliferous sandstone, and shale, anhydrite, and salt of unknown age—possibly Jurassic, Triassic, or Permian. The Late Jurassic was deposited only after a probable shield area (which is postulated to have occupied the Gulf of Mexico basin) sank. The Late Jurassic Smackover Limestone and equivalents were deposited above a very thick salt section. As the bordering lands rose, gravel, sand, silt, and mud of the Cotton Valley (up to 4,000 ft thick) were deposited. Deposition continued unbroken into Early Cretaceous time, beginning with up to 4,000 ft of Hosston and equivalents. In the southeastern part of the Gulf of Mexico, Early Cretaceous equivalents include carbonate, evaporite, and some black shale. After Hosston deposition, carbonates were deposited around the entire Gulf basin, though terrigenous clastics also were deposited in the northern region. There is no significant regional depositional break between the Lower and Upper Cretaceous. In medial Late Cretaceous time, the sea transgressed farthest, coinciding with a worldwide eustatic sea-level rise. The Mississippi embayment formed and the sea reached southern Illinois. Mountain-building activity and uplift at the end of Cretaceous time forced the sea from much of the continent. Carbonate deposition continued in the southeast from Paleocene through Miocene, but fine terrigenous clastics were deposited in the north and west (Midway-Velasco). As the mountains of the interior rose, clastic debris supplied to the basin was more abundant and coarser. In general the northwestern gulf shoreline was pushed seaward, though important transgressions occurred in early, middle, and late Eocene times and during the Oligocene. Deltas formed, and the local shifts in Miocene deltaic depocenters caused numerous local transgressions and regressions during that time. Sedimentation rates increased steadily. The Pleistocene deposits reach a thickness of 15,000 ft in the northern Gulf of Mexico. Although a marine basin has occupied the Gulf of Mexico since Late Jurassic time, the existing gulf may be a very young feature. In fact, parts of the gulf may have been land during much of Jurassic-Pleistocene time. The depositional and tectonic history, insofar as it is known, of the large gulf basin is described briefly. The need for more information on the geological history of all parts of the basin is apparent to anyone charged with exploring for minerals in this area.