The San Andres evaporitic sequence in the Palo Duro Basin comprises several thick carbonate units in its lower part and many thin units in its upperpart. To the south, across the Northern Shelf of the Midland Basin, evaporites pinch out and carbonates predominate. Six lithofacies were differentiated in the Palo Duro and Northern Shelf carbonates: dolomudstone, pellet-oolite packstone-grainstone, filamentous (Girvanella-like) grainstone, sponge spicule packstone, wispy-laminated crinoid packstone, and skeletal packstone-grainstone.
The Oligocene-Miocene fluvial uranium host aquifers of the South Texas uranium province were deposited principally as syndepositionally oxidized sands and muds. Early intrusion of reactive sulfide- enriched waters produced large intrastratal islands of epigenetic sulfidic alteration, which contain isotopically heavy pyrite exhibiting unique replacement textures. The only known reservoir containing such sulfidic waters is the deeply buried Mesozoic carbonate section beneath the thick, geopressured Tertiary basin fill.
The Oakville Sandstone (Miocene) of the Texas Coastal Plain comprises a major sand-rich fluvial system composed of deposits of several major and minor rivers that originated within Texas and surrounding states. Broad bed-load fluvial axes, including the Hebbronville, George West, and New Davy trends, lie south of the San Marcos Arch and host significant reserves of uranium. To the north, the Moulton streamplain consists of deposits of numerous small, flashy to ephemeral streams that drained the inland margin of the Coastal Plain.
Environmental geologic maps of the Texas lignite belt were prepared in response to renewed production of lignite in Texas and enactment of Federal and State laws governing coal and lignite surface mining. The map area of this report encompasses the outcrop of the Yegua Formation and Jackson Group from south-central Texas to the Texas-Louisiana border. Yegua sediments are fluvial in outcrop, and deltaic in the deeper subsurface. Jackson deposits are predominantly of deltaic origin. Mapping involved interpretation of black-and-white, large-scale aerial photographs and extensive field work.
