KARST IN PERMIAN EVAPORITES AT RED BLUFF DAM ON THE PECOS RIVER IN LOVING AND REEVES COUNTIES, TEXAS

Red Bluff Dam and Reservoir, located on the Pecos River between Loving and Reeves Counties in West Texas, has been losing water since impoundment began in early 1937—mainly due to seepage through evaporite-karst features beneath the dam. Water flows through collapse structures and brecciated-karst zones in the Permian-age Rustler Formation, which is the bedrock beneath most parts of the dam, and through Quaternary deposits of weakly to strongly cemented sand and gravel that locally lie between the base of the dam and bedrock. In the Red Bluff area, the Rustler Formation consists mainly of interbedded dolomite, gypsum, and shale, and it probably also contained some layers of salt (halite) that have been dissolved by ground water. The dam has undergone three grouting programs: the most recent being completed in December 2014.

Rocks in the Rustler and overlying Dewey Lake (shale) Formations at Red Bluff are largely brecciated, mainly due to dissolution of salt and gypsum within the Rustler Formation and in the underlying Permian Salado and Castile Formations. The high solubility of these evaporites enables karst features, such as dissolution channels, cavities, sinkholes, breccias, and collapse structures, to form readily in gypsum and salt, and this can result in slow subsidence or sudden collapse of overlying strata.

Collapse breccia is exposed in the Rustler and Dewey Lake Formations in the dam and spillway area, as well as beneath parts of the dam itself. As seepage from the reservoir flows in and around the breccia, it can further dissolve the gypsum and dolomite blocks to enhance karst features that allow even more circulation of ground water. In general, gypsum karst is an important factor in any dam’s ability (or inability) to hold water, because dams built upon gypsum karst generally experience at least some loss of reservoir water.

The loss of water from the reservoir is mainly due to the following: 1) dissolution of salt and/or gypsum beds in the Rustler, Salado, and Castile Formations; 2) collapse and chaotic disruption of gypsum and dolomite units in the Rustler Formation; 3) development of karst features within the gypsum and dolomite units and collapsed blocks of the Rustler Formation; and 4) probable seepage through Quaternary sands and gravels, chiefly beneath the northeast side of the dam.