The Gulf Coastal province of the United States is a segment of the Mesozoic-Cenozoic coastal geosyncline of eastern North America which can be traced continuously from Newfoundland to Guatemala. The geosyncline is roughly lens-shaped in cross section; approximately equal parts exist (1) submerged beneath the waters of the Atlantic Ocean and Gulf of Mexico, and (2) partly emerged adjacent to their shores.
The Gulf Coastal portion of the geosyncline has an area of more than 150,000 square miles and contains about 50,000 feet of predominantly arenaceous-argillaceous, marginal to shallow-marine strata, although calcareous materials predominate in the Florida Peninsula and in the Cretaceous in Texas. The geosynclinal mass overlies Precambrian-Paleozoic rocks of variable facies, structure, and degree of metamorphism; • their top surface possesses an over-all slope toward the Gulf of Mexico.
Strata of the Gulf geosyncline also possess a general gulfward slope, at least as far basinward as the edge of the continental slope, and thus constitute a great sedimentary structural arc from Florida to Mexico. Deformation from primary compressional movements is presently unknown. Instead, deformation appears to be principally a result of vertical movements due mainly to isostatic adjustments, density differences between a thick bed, or thick beds of salt and superjacent strata, and igneous emplacements. Larger structural anomalies, such as the Sabine and Monroe uplifts and the East Texas embayment, constitute incipient growing “welts and furrows” in the developing geosynclinal province.
Axes of the larger anomalies trend primarily northwest and northeast but secondary north and east orientations also exist. Some evidence suggests that certain of these larger anomalies are, in fact, related to and probably positioned and controlled by Paleozoic structures and trends.
Major systems of normal, strike faulting, or of pronounced downflexing, believed to be associated with (1) down warping of the margins of the Gulf of Mexico basin in the earlier stages of its development, and (2) subsidence related to rapid sedimentation, break the general continuity of the strata constituting the geosynclinal prism. The faults are of variable orientations; their dip ranges from 35° to 70°. Faults of younger, gulfward systems ordinarily dip more steeply than those of older, landward systems.
Pre-Cretaceous, Coahuilan, Comanchean, and early Gulfian rocks are predominantly red-bed elastics in the eastern and up-dip portions of the Gulf province of the United States; westward and down dip marine equivalents of these beds predominate. Extensive marine deposits constitute the middle and late Gulfian, arenaceous facies predominating in the east and argillaceous-calcareous materials being prevalent westward and down dip. Tertiary deltaic sediments center in Mississippi, Louisiana, and Texas; marine facies are prevalent eastward and down dip. Fluviatile and deltaic Quaternary deposits occur as a surficial mantle over much of the region; down dip they are replaced by marine equivalents.
Stratigraphic studies indicate that major sedimentary units are arranged in belts subparallel to the modern northern shore line of the Gulf of Mexico. They have the general shape of flattened link sausages, the thicker lenses being centers or loci of deposition (depocenters) for a particular depositional epoch. Depocenters of the same age constitute a regional axis of deposition (depoaxis). Sedimentary materials have accumulated repeatedly in depocenters to thicknesses of 2,500 feet, or more, after which the depositional locale has shifted. Depocenters are not generally known to occur vertically above immediately older ones. Depoaxes have generally shifted gulfward from the Jurassic to the modern day; major reversals of this gulfward progression occurred in the Cretaceous and Tertiary and resulted in the formation of widespread cyclic depositional sequences (cyclothems).
During Jurassic and Cretaceous time the major source of sediments for the region was apparently eastern and central United States. In the Cenozoic appreciable quantities of material appear to have come from western United States.
Figures & Tables
All but one of the papers contained in this volume represent a symposium summarizing the results of work carried on in Project 51 of the American Petroleum Institute. This study of modern sediments along the northwest margin of the Gulf of Mexico contains 14 papers plus a consolidated bibliography. Paper titles are: Geologic framework of Gulf coastal province of United States; Sources and dispersion of Holocene sediments; Mississippi delta; Delta building and the deltaic sequence; Phytoplankton production in the Mississippi delta; Bays of central Texas coast; Sediments of Laguna Madre; Gulf Coast barriers; Sediments and history of Holocene transgression; Sedimentary patterns of microfaunas; Ecology and distributional patterns of marine macro-invertebrates; Rise of see level; Regional aspects of modern sedimentation; and Recent sedimentology.