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.

Oakwood salt dome in Leon and Freestone Counties, Texas, has a core composed of a diapiric salt stock at a depth of 355 m. A vertical borehole in the center of the salt stock yielded 57.3 m of continuous rock-salt core overlain by 137 m of anhydrite-calcite cap rock. The lower 55.3 m of rock salt exhibits a strong, penetrative schistosity and parallel cleavage dipping at 30� to 40� and more than 60 variably dipping layers of disseminated anhydrite. Anhydrite constitutes 1.3 � 0.7 percent of the rock-salt core.

The Oakville Formation consists of sediments deposited by several major fluvial systems that traversed the Texas Coastal Plain during the Miocene Epoch. Facies geometry and composition, together with superimposed structure and topography, are important determinants of ground-water flow, aquifer transmissivity, and regional hydrochemical evolution.