The northern Salt Basin in West Texas and New Mexico is a closed hydrologic system in which discharge of ground-water flow occurs in a series of playas, or salt flats. Ground water originating in peripheral consolidated rocks and alluvial fans flows toward the center of the basin and discharges by evaporation from the salt flats. Progressive increases in salinity are characteristic of the waters moving down gradient and are primarily attributed to evaporative concentration.

This compilation of potassium-argon (K-Ar) ages of igneous rocks of the Tertiary volcanic field of Trans-Pecos Texas includes all published ages known to us as well as many unpublished results. Almost all age determinations and all published ages of Trans-Pecos rocks are by the K-Ar method, so this compilation essentially includes all isotopic age information. It expands upon an earlier summary by McDowell (1979) by including the considerable number of analyses done since then and by tabulating available rock type, analytical data, and location information.

The Big Wells (San Miguel) reservoir in Dimmit and Zavala Counties, South Texas, produces from a broadly lenticular, wave-dominated deltaic sandstone encased in prodelta and shelf mudstones. An updip porosity pinch-out coincides with a gentle undulation on a uniformly gulfward-dipping monocline and forms a structurally modified stratigraphic trap. The reservoir is relatively tight and has average porosity of 21 percent and average permeability of 6 md; wells require fracturing to stimulate production.

This folded transparent map is at the same scale as its corresponding Geologic Atlas of Texas (GAT) sheet and can be superimposed on it to compare gravity information with surface geology.

This folded transparent map is at the same scale as its corresponding Geologic Atlas of Texas (GAT) sheet and can be superimposed on it to compare gravity information with surface geology.