Recent Marine Sediments and Environments of Northwest Gulf of Mexico

Abstract

This paper presents the results of a megascopic study of eighty-five sediment cores, ranging in length from 3 to 10 feet and averaging 7 feet, taken by the Woods Hole Oceanographic Institution in the northwest Gulf of Mexico. The study has led to the following conclusions. 1. Recent sediments in this region are accumulating in five major marine environments, each with its characteristic sedimentary facies. These are: (a) the continental shelf, extending seaward to a depth of about 600 feet, with a coarse-textured facies consisting of sand, silty clay, and shell clay; (b) the rises, a group of isolated topographic highs at the outer edge of the shelf and upper part of the slope, with a coarse-textured facies consisting of sandy and silty glauconitic foraminiferal ooze, calcareous biostromal deposits, and other deposits similar to those of the shelf facies; (c) the upper slope, from depths of 600 to 3,000 feet in the central and western parts of the region but extending to 8,000 feet in the eastern part, with a green-gray to brown-gray homogeneous clay fa ies; (d) the lower slope-western Sigsbee Deep, from depths of 3,000 to 12,000 feet in the central and western parts of the region, with a light yellow-brown mottled clay facies; and (e) the eastern Sigsbee Deep, from depths of 8,000 to more than 11,000 feet in the eastern part of the region, with a foraminiferal ooze facies. 2. Most of the continental slope and Sigsbee Deep cores show a three-fold zonation. The intermediate zone is reddish brown and/or dark olive-gray clay in slope cores and red or red-streaked clay in Sigsbee Deep cores. Because changes from warm- to cold-water faunas generally occur at the stratigraphic End_Page 813------------------------------ level of this zone, the base of the reddish brown, red, or red-streaked clay is taken to be the contact between the Recent and the last glacial stage of the Pleistocene. There is no evidence of the Recent-Pleistocene contact in shelf cores or, with one possible exception, in rise cores. 3. The sedimentary facies distribution pattern is the result of two factors: (a) the great volumes of sediment carried into the Gulf by a system of several major and many minor rivers with a large combined drainage area and (b) the transport of this sediment by the counter-clockwise prevailing current. Because of factor (a), most of the northwest Gulf is an area of clastic deposition. Because of factor (b), the non-clastic ooze facies, an extension of the large eastern Gulf calcareous area, terminates southwest, rather than directly south, of the Mississippi Delta. 4. The thickness of the Recent, as shown by the isopach pattern, appears to be controlled by the same two factors. Though variable, it decreases on the average from slope to deep and from west to east within the deep. Averages for the upper slope, lower slope, western deep, and eastern deep are 56, 42, 37, and 20 inches, respectively. The corresponding average sedimentation rates are 360, 480, 540, and 1,000 years per inch, on the assumption that the Recent epoch began 20,000 years ago. 5. On the slope and in the deep, the good to excellent core-to-core correlation and the absence of graded bedding indicate that turbidity current activity has been non-existent, or at most of minor importance, during the time represented by the cored strata. 6. Coarse-textured deposits similar to those of the shelf are found in the Pleistocene sections of Sigsbee Deep cores. Evidence for turbidity current origin, such as graded bedding, is lacking. It is postulated either that they accumulated in a shelf environment and were downfaulted to their present depth during late Pleistocene or early Recent time, or that they are products of the coarser loads contributed to the Gulf by the rejuvenated streams of the last glacial stage.