Print this pageCLASSIFICATION OF ENVIRONMENTS OF THE NORTHWESTERN GULF OF MEXICO
The northwestern Gulf of Mexico region is a model paralic province of Recent depositional environments1 in which clastic sediments are being deposited. (See Figure 1 and Figure 2.) The sediments laid down in these environments are believed to be representative of much of the geologic section (LeBlanc, 1950).
The environments are conveniently divided into three major groups-- continental, transitional, and marine. Most of this brief summary is devoted to the continental and transitional environments, since these two contain, to a large extent, the better developed sand bodies of the late Recent standing sea-level stage.2 The early Recent sand facies deposited during low and rising sea-level stages, between 6000 and 30,000 years ago, will not be discussed, but the reader is referred to Fisk (1947), Fisk and McFarlan (1955), and LeBlanc (1950) for details.
1. By far the most common morphological features to be observed in the Gulf Coastal Plains are depositional in origin. The texture, fabric, sedimentary features, directional features, geometry, trends, etc. of the sediments associated with these features are controlled by the nature of the geological or depositional processes. The morphological expression of these sediments controls the floral and faunal distribution and also may affect the nature of the secondary geological processes operating in this area. Therefore, the principal basis for classifying the depositional environments in the Gulf coastal area is morphology.
2. See LeBlanc and Bernard (1954) for a summary of the history of Recent sea-level stages and also page 39 in Clastic Seminar Notes on Recent Depositional Environments and Related Sediments and Fauna, EPR Geol. Misc. l7, 1957 and revised 1958 (in press).
Within the continental environment is the aeolian plain subenvironment of south Texas. At present little is known of the exact nature and geometry of its sands (Figure 1 and Figure 2). Examination of aerial photographs and the results of a brief field study suggest that the individual sand dunes are patchy and elongated NW-SE. LeBlanc (1950), Lohee (1956), and Price (personal communication, 1949) have shown that the dune alignment is parallel to the direction of the prevailing southeast winds. These deposits are Recent and Pleistocene in age; the individual sand bodies vary from a few feet to over 50 feet in thickness and are relatively narrow.
The alluvial meandering stream plain environment and associated sediments are well represented in the late Recent of Texas and Louisiana. Alluvial braided stream plain environments and associated sediments along the Gulf Coast are represented only by a few early Recent streams and are not well known.
The deltaic environments grade landward into the alluvial and seaward into the marine environments. The pro deltaic plain environment, the major subaqueous portion of the deltaic environment, is transitional with the normal marine environment.
Coastwise the deltaic environments grade into the coastal interdeltaic-marine environments which are characterized by barrier islands, beaches, bays, lagoons, and marshes. Small deltas occur in the bays and lagoons of the coastal interdeltaic environment. Given time, and a constant base level, these small deltas will prograde seaward across the bays,lagoons, and barriers into the marine environment. Thus, the coastal interdeltaic-marine environment may be transformed gradually into a deltaic environment. Note the history of the late Pleistocene (Beaumont) coastal interdeltaic and deltaic environments in EPR Geol. Misc. 1 (revised May 1958)1 (Figure 1).
Seaward, the coastal interdeltaic-marine environment grades into the normal marine environment, from which it is usually distinguished by the first major change in bottom slope offshore. A distinct change in lithology usually occurs at this break in slope.
The normal marine environment is subdivided topographically into the shelf, slope, and deep environments. The boundary between the shelf and slope occurs near the 600-foot contour. The boundary between the slope and the deep is near the 6000-foot contour. These may also be distinguished by sedimentary and faunal criteria (Greenman, 1954, and unpublished faunal data). Depending upon the size of the delta and the width of the shelf, a normal marine shelf environment may occur seaward of a deltaic environment (Figure 1 and Figure 2).
