The Gulf Coast Regional Aquifer-System Analysis includes all major aquifer systems in Cenozoic deposits in the Gulf Coastal Plain in the States of Arkansas, Illinois, Kentucky, Louisiana, Mississippi, Missouri, Tennessee, Texas, and small areas in Alabama and Florida (western panhandle area), an area of about 290,000 square miles. The Gulf Coast geosyncline and the Mississippi embayment were the major depocenters for the Tertiary and Quaternary deposits that form the framework for the aquifer systems.

FoiiDation of the Gulf Coast geosyncline and the Mississippi embayment began with downwarping and downfaulting at the end of the Paleozoic Era. Sedimentation caused the geosyncline to continue to subside throughout Mesozoic and Cenozoic time. During the Late Cretaceous, at the close of the Mesozoic Era, the sea advanced northward and eventually inundated the Mississippi embayment. Marine cycles persisted throughout the Paleocene and Eocene Epochs of the Tertiary Period, as the sea alternately advanced and retreated over the Mississippi embayment. The resulting sediments form a series of dense marine clays separated by terrigenous sands. The Gulf Coast geosyncline remained submerged during marine regressions in the embayment. After withdrawal of the last Tertiary sea from the Mississippi embayment at the end of the Eocene Epoch, deposition continued along the Gulf Coastal Plain under a shifting variety of nonmarine, marine, near-marine, and deltaic environments. Deposition resumed in the Mississippi embayment during the Quaternary with glacially related terraces and aggradation of streams. Fluvial deposition continues.

Structural features in the Gulf Coastal Plain and Mississippi embayment significantly affected Cenozoic deposition. The Desha basin, for example, is a pronounced Tertiary synclinal depocenter in southeastern Arkansas. Three large uplifts are approximately aligned along the latitude of the northern boundary of Louisiana. These are, from west to east, the Sabine, Monroe, and Jackson uplifts. A belt of three major fault zones, the Luling-Mexia-Talco, Arkansas, and Pickens-Gilbertown, generally follows the strike of sediments across the Coastal Plain and more or less forms the northern updip extent of the Gulf Coast geosyncline. An alternating series of gentle synclines and anticlines is oriented perpendicular to the coastline along the Gulf Coast in Texas. Beginning at the southwestern end, these are the Rio Grande embayment, San Marcos arch, Houston embayment, and Sabine arch. The Wiggins anticline is oriented approximately along strike of the sediments in southern Mississippi. Salt domes are numerous in the Gulf Coastal Plain and may penetrate thousands of feet of sediments. Although the degree of salt intrusion can be very great, disruption of adjacent strata is limited to the vicinity of the dome.

The physiography of the Gulf Coastal Plain is a direct result of the nature of the strata at land surface and physical forces that act upon them. Different terrains typify the lithologies that underlie them. The sands and clays that are the predominant rock types each produce characteristic geomorphologic patterns; sands tend to produce ridges, and clays produce topographic lows.

Although Cenozoic deposits are not uniformly differentiated, interstate correlations of major Paleocene and Eocene units are generally established throughout the area. Younger deposits are not as well differentiated. Some stratigraphic designations made at surface exposures cannot be extended into the sub-surface, and the scarcity of distinct geologic horizons has hampered differentiation on a regional scale. The complexities of facies development in Oligocene and younger coastal deposits preclude the development of extensive recognizable horizons needed for stratigraphic applications. Coastal deposits are a heterogeneous assemblage of deltaic, lagoonal, lacustrine, palustrine, eolian, and fluvial clastic facies and local calcareous reef facies. Even major time boundaries, as between geologic series, are not fully resolved. Surficial Quaternary deposits overlie the truncated subcrops of Tertiary strata and generally are distinguishable, although some contacts between Pleistocene and underlying Pliocene deposits have been a ?lstoncal source of controversy. Glacially related terraces are characteristic of the Pleistocene Epoch, and alluvium of aggrading streams typifies the Holocene.