RIFT BASIN SEDIMENTOLOGY AND STRATIGRAPHY, FAULT INFLUENCE AND INTERACTIONS WITH THE HUECO BASIN AQUIFER, EL PASO, TEXAS

The aquifer that supplies the largest proportion of groundwater for the city of El Paso, Texas is found within Tertiary-Quaternary rift basin sediments of the Hueco Basin. The aquifer is approximately 270 m thick and is confined to a fresh water lens found within only the western 1/3 of the basin. Below and to the east, more saline water has prevented development of water supplies. Most recently, the city has drilled 16 new wells along the brackish-water boundary of the aquifer to supply the world’s largest inland desalination plant. Logs and cuttings from these wells provide new data about the depositional environments that make up the aquifer and the controls on hydrology that define the saline and fresh water aquifers. The wells are located in an arc that extends eastward away from the mountain front and then south, along the basin axis. Three major features are evident in the facies. There is a unit of exotic gravels and heavy mineral-rich sand in the upper 50 to 150 m that expresses the influx of the Rio Grande. There is a well defined upward coarsening that is defined by progradation of fans and coarse grained axial facies along the mountain front. Finally, although westward tilting of the basin introduced fine grained playa facies almost to the mountain front, there is an eastward transition from a high percentage of mountain front alluvial fans to fine grained playa and distal fan facies of the basin floor. The strata are offset on several synthetic and antithetic faults that offset strata. The best correlation markers are 1 to 6 m thick red clays that are interpreted as deposits of playa or playa lake beds. Less extensive white siltstones and thin sandstones are interpreted as coarser lacustrine deposits. Fine grained sands and thin beds containing locally derived gravels are interpreted as distal alluvial fan and sheet flood deposits.

This rift-basin aquifer is notable for its internal heterogeneity and the abundance of clay intervals which can potentially segment the aquifer. Faults are interpreted to provide boundaries that separate the saline and fresh-water aquifers. An important feature is the progressive upward coarsening which reflects filling of lacustrine basins by alluvial fans, which were initially restricted to within 2 km of the mountain front.