The continental shelf of the northwest Gulf of Mexico, between the Mexican border and the Mississippi Delta, is a smooth, gently sloping sediment-covered plain interrupted by occasional hills or banks. Many of these banks probably represent the surface expressions of salt domes protruding through the sediment cover. The width to the edge of the shelf at about 65 fathoms ranges from 50 to 130 miles. Drowned barriers relict from lowered sea level are found on the nondepositional part of the shelf.

The Holocene (Recent) sediments are divided into marine transgressive basal nearshore sands and shelf facies muds (silty clays and clayey silts). The basal sands are exposed at the surface near the shore line and across almost the entire shelf off the Rio Grande, off east Texas, and off western-most Louisiana. The shelf facies overlies the basal facies off central Texas and off most of Louisiana. The thickness of the Holocene Series is probably less than 20 feet in much of the area where the basal facies has not yet been buried under the shelf facies, but is several hundred feet thick on the shelf in the vicinity of the Mississippi Delta.

Texturally about half of the surface sediments are polymodal mixtures of thin overlapping sediment masses deposited on the shelf during different periods of postglacial time. The mixing has been by burrowing organisms and the strong wave surges associated with hurricane waves, the former having produced irregularly interlayered and mottled sands and muds and the latter homogeneously mixed polymodal sediments. The sandy sediments are subarkoses everywhere, except off the central Texas coast where they are orthoquartzites. Some of the sediments contain mixed shallow and deeper shelf faunas. Most of the sand-size particles in the outer shelf silty clays are Foraminifera and fragments of echinoids. Glauconite is locally abundant in the relict basal sands of the outer shelf.

The sediments on the shelf are the products of the marine transgression following the Wisconsin glaciation. The chronology of this transgression has been traced back 17,000 years by radiocarbon dates of shells of nearshore organisms. The sequence of events has been interpreted in the light of textural and mineralogical characteristics of the sediments and from the physiography of the drowned barriers of the east Texas shelf area. These ridges, which slope for long distances down the middle and inner shelf at acute angles to the contours, represent barrier spits deposited at the mouths of rivers during brief periods of regression. Two and possibly three periods of temporary regression interrupting the transgression are suggested. One of these, occurring at about the mid-point of the tr nsgression, correlates well with the Two Creeks interstadial and the advance of the Mankato glaciers of North America. The wind pattern appears to have changed during these periods of regression and reversed the current directions in parts of the gulf.