Abstract

Gross grain shape and mineralogical analyses were used to determine the source, distribution, and mixing of late Pleistocene and Holocene sands on the South Texas continental shelf. Two major gross grain-shape types are found to be present in varying proportions in the sands of this shelf. The first sand type is associated with sands which are relatively enriched in plutonic-metamorphic quartz, volcanic quartz, feldspar and igneous and metamorphic rock fragments, and thus is considered to represent first-cycle sands derived from the Llano Uplift and the silicic volcanic rocks of the Southwest U.S. The second sand type is associated with more quartzose sands rich in sedimentary quartz and sedimentary rock fragments, and is considered to represent multicyclic sands eroded from the Texas Coastal Plain and High Plains. Variations in the relative proportions of these sand types in the sands of the nearshore/inner shelf region reflect both the changing sources of the sand and their transport, mixing, and convergence by modern nearshore processes. Three distinct sedimentary petrologic provinces, corresponding to three provinces previously defined by heavy mineral analysis (Van Andel and Poole 1960), are defined herein on the basis of their relative content of first- and multicyclic sand. Modern sands of the first of these provinces, the Rio Grande, are derived from the drainage basin of the Rio Grande and characteristically contain high proportions of first-cycle sand; such sands are presently found in the nearshore/inner shelf region between the mouth of the Rio Grande and the 27 degrees N latitude to the north of the mouth of the river. Modern sands of the second province, the Texas Coast, are derived largely from the drainage basins of the South Texas coastal plain rivers and characteristically contain high proportions of multicyclic sand; such sands are presently transported southwestward by longshore currents to the 27 degrees N latitude, where they converge with the northward-transported sands of the Rio Grande Province. Modern sands of the third province, the Western Gulf, are derived from the drainage basins of the Colorado and Brazos rivers and characteristically contain high proportions of first-cycle sands', such sands are also transported by longshore currents towards the southwest, where they are gradually diluted by Texas Coast Province sands. Sands of these three provinces are also found in the relict late Pleistocene fluvial and deltaic sediments on the outer part of the South Texas shelf. Previous studies had considered the fluvial and deltaic sediments on the northern part of the shelf to be entirely derived from the Western Gulf Province; however, this study shows that the relict fluvial deposits of certain South Texas coastal plain rivers which occur on the northern part of the shelf should be assigned to the Texas Coast Province. In all, multicyclic sands constitute approximately 48% of the late Pleistocene and Holocene sand of the South Texas continental shelf.

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