A textural study of beach and river sediments along the Texas Gulf Coast
Rogers, John J. W.
Master of Arts
The object of this study was to compare the textures of samples from beach and river sands of the Texas Gulf Coast. The main problem involved determining what criteria, if any, could be most useful in detecting the origin of lenticular sand bodies for which small samples, such as well cores, are available. Owing to the differences in the nature of the beach and river environments, and to the difficulty of obtaining representative samples from the rivers, samples which appeared similar in grain size and sorting were chosen from each type of deposit. Beach samples were taken along the foreshore of the barrier islands, and the river samples were taken from bars, terraces, flood deposits, and river bottoms within fifty miles of the coast. Much work has been published on the modern sediments of the Texas Gulf Coast, partially under the stimulus of A, P. I. Project 51. Little attention, however, has been directed toward the Texas rivers which are contributing material to the lagoons and barrier islands. The general facies relations of sediments along the Gulf Coast have been described by several writers. Among these, Greenman and LeBlanc (1956) discussed the major facies present on the floor of the northwestern Gulf of Mexico. Shepard and Moore (1955) studied the characteristics of the sediments of the central Texas coast in the Rockport area, and proposed their "coarse-fraction" method of differentiating the various nearshore and continental shelf environments. An investigation into the significance of various grain size parameters was conducted on a sand bar in the Brazos River by Polk and Ward (1957). Using some of these methods, Mason and Polk (1958) found it possible to differentiate beach, dune, and aeolian flat environments on Mustang Island, Texas, by grain size analysis. The value of laminations and internal features in defining depositional environments has been pointed out by Moore and Scruton (1957). McKee (1957) has also described, with the aid of excellent photographs, the types of lamination and stratification common in beach, lagoonal, and nearshore sediments. Several recent papers deal with the transportation of materials on the Texas beaches. Curray (1956) pointed out that sand grains along beaches and streams tend to be oriented with their long axes parallel to the direction of the depositional agent. Thus, if the origin of such a deposit is known, its trend can be predicted. Beal and Shepard (1956) noted a sharp increase in sand grain roundness going from the beaches into the sand dunes, and attributed this to selective wind transportation of the more rounded grains from the beaches. Bradley (1957)> from a study of three profile traverses across the northern end of Mustang Island, found a tenfold volume Increase of the heavy mineral content in the barrier island dune's over the offshore sediments (0.04$ to 0.45$); he attributed this variation to selective wind transportation of the heavy minerals from the beach. The influence of source rocks on the heavy mineral suites of the major Texas rivers and beaches has been discussed by Bullard (1942). Adams (1958) described the competition and size distribution of heavy minerals on the foreshore and backshore of Galveston Island, Texas. Sedimentation rates in some of the Texas estuaries and lagoons were discussed by Shepard (1953); he arrived at an average rate of three feet per century prior to compaction. The potential application of the results of the present study are discussed in this thesis. The determination of the origin of lenticular "shoestring" sand bodies presents a difficult problem to petroleum geologists, especially where detailed sub-surface control is not available. It is believed by the writer that a method of analysis such as outlined in this thesis may provide important clues for the differentiation between sinuous channel deposits and the more linear offshore bar deposits.