Abstract

Grain shape and mineralogical analyses were used to determine the sources of sands on the South Texas shelf. Two grain shape types were found in the sands using the Fourier grain shape technique. One type, associated with sands rich in igneous and metamorphic rock fragments and quartz varieties as well as feldspars, is considered to represent first-cycle sands eroded from igneous and metamorphic rocks of the Llano Region and of West Texas and New Mexico. The second type, associated with sands rich in sedimentary quartz and sedimentary rock fragments, represents sand eroded from the Texas Coastal Plain and High Plain. Variations in the proportions of these sand types in the nearshore sediments reflect both the changing sources of nearshore sand and their transport and mixing by nearshore processes. Three sedimentary provinces, corresponding to those defined by heavy mineralogy, are defined on the basis of their content of first- and multi-cyclic sand. Modem sands of the Rio Grande Province are derived from the drainage basin of the Rio Grande and contain high proportions of first cycle sand; such sands are found in the nearshore zone between the Rio Grande and the 27° latitude line to the north of the river mouth. Modern sands of the Texas Coast Province are reworked deposits of the coastal plain rivers and are dominated by multicyclic sands. Such sands are presently transported southwestwards by longshore currents to the 27° line where they converge with northward-transported Rio Grande sands. Modern sands of the Western Gulf Provinces come from the drainage basin of the Colorado River and contain high proportions of first-cycle sand; such sands are also transported toward the southwest, where they arc gradually diluted by Texas Coast Province sands. Ancient sands of these provinces are also found in relict deltaic deposits on the outer shelf. Previous studies had considered the southernmost outer shelf deltaic sands to be of the Rio Grande Province, and our data agrees with this. However, previous studies had considered the northernmost outer shelf deltaic sand to be only of the Western Gulf Province, but our data reveals that to the west of the relict Colorado delta these sands belong to the Texas Coast Province. Seismic data supports this conclusion by showing that these outer shelf sands of the Texas Coastal Province overlie coastal plain river valleys.