Abstract

The value of correlations based on heavy minerals is tested on the sands now being deposited on the beaches of the Texas Gulf Coast. Each of the principal rivers carries a distinct suite of heavy minerals. That igneous and metamorphic rocks are a source of a portion of the Rio Grande material is indicated by an assemblage of heavy minerals, exclusive of the opaque black minerals, containing 60 per cent basaltic hornblende and pyroxene, 10 per cent green hornblende, and only 30 per cent of the more stable minerals such as garnet, rutile, staurolite, tourmaline, and zircon. The Nueces, San Antonio, Brazos, Trinity, and Neches rivers derive their load from sedimentary rocks giving a high content of more stable minerals in the heavy-mineral residue with only minor amounts of hornblende and pyroxene. The Colorado River derives its load from igneous, metamorphie, and sedimentary rocks. Its suite of heavy minerals, exclusive of the opaque black minerals, is more than 60 per cent green hornblende with the remainder the more stable minerals.

The heavy-mineral residues of the Gulf Coast beach sands commonly contain augite, basaltic hornblende, enstatite, epidote, garnet, green hornblende, hypersthene, kyanite, leucoxene, monazite, rutile, zircon, and from 25 to 50 per cent of opaque black minerals. From the Rio Grande northeastward along Padre Island beach the characteristic Rio Grande suite of heavy minerals, rich in basaltic hornblende and pyroxene, gives way rather rapidly to the Colorado suite rich in green hornblende. The influence of the Nueces, San Antonio, and Brazos rivers is shown by an increased ratio of the more stable minerals.

Along the southern two-thirds of Padre Island beach the modal grain diameter is ⅛ mm., and the mode constitutes 90 per cent of the total. A less pronounced, but nevertheless high, degree of sorting persists northeastward along the beaches with ⅛ mm. the modal grain diameter as far as Galveston Island, where the mode shifts to the 1/16 mm. grain diameter size, but the ⅛ mm. grain diameter size is almost as abundant.

The distribution of material along the Texas Gulf Coast beaches is effected by a southward longshore current carrying sediment brought to the Gulf of Mexico. The offshore bars on the Texas Gulf Coast may have been formed in this manner.

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