Geomorphic Processes and Rates of Retreat Affecting the Caprock Escarpment, Texas Panhandle

The interaction of geomorphic and ground-water processes has produced the Caprock Escarpmentthat bounds the eastern margin of the Southern High Plains in the Texas Panhandle. Spring sapping,slumping, and piping at the surface and salt dissolution in the subsurface are some of the many erosionalprocesses affecting the escarpment.Substantial thicknesses of bedded Permian salt (halite) have been dissolved from the Salado, SevenRivers, San Andres, and Glorieta Formations beneath the Caprock Escarpment and the Rolling Plains,east of the escarpment. Dissolution of salt from the Salado and Seven Rivers Formations beneath theCaprock Escarpment has resulted in subsidence and development of a regional dip reversal in theoverlying Alibates Formation. Although most dissolution has taken place beneath the CaprockEscarpment and Rolling Plains, dissolution and subsidence extend westward beyond the escarpment tobeneath the Southern High Plains. Dissolution-induced subsidence apparently accounts for as much as75 m (250 ft) of Caprock Escarpment relief.Complex fracture systems along the escarpment have developed in part because of subsidence.Permian and Triassic strata contain systematic and nonsystematic fractures, extension fractures, and,locally, clastic dikes. The Ogallala Formation contains systematic fractures in basal calcretes andsilcretes and, locally, clastic dikes. Of these, the nonsystematic fractures, extension fractures, and clasticdikes probably result from salt dissolution and subsidence. Ground-water leakage downward toward thesalt-bearing zones is most likely enhanced by fractures.Conceptual and numerical models of ground-water flow beneath the Caprock Escarpment suggest thatlow-salinity ground water flows downward from the Ogallala and Dockum aquifers to dissolve Permiansalt. Brines flow eastward through permeable anhydrites, dolomites, and dissolution breccias beneath theRolling Plains to discharge as springs in topographic lows.Dissolution is active along the Caprock Escarpment; during a 15-yr period it resulted in the loss ofover 15 million m3 (525 million ft3) of salt from the study area. Westward expansion of the active dissolution zone along the Caprock Escarpment and beneath the Rolling Plains occurs at a mean rate of0.02 km/1,000 yr (0.013 mi/1,000 yr).Surface erosion on the hillslopes of the Caprock Escarpment occurs by sheetflooding, slope wash, rillwash, rockfalls, spring sapping, piping, and rotational slumping. Fluvial erosion dominates in canyonbottoms. Sheetflooding, slope wash, and rill wash resulting from runoff from episodic, high-intensityrainfall events have modified the upper margin of the escarpment. Runoff from these same events scourscanyons and their tributaries. Sapping below areas of spring discharge in strata of the Dockum Groupand Ogallala Formation helps to remove sediment and contributes to the development of rockfalls.Numerous rotational slumps and landslides have substantially accelerated slope degradation in PaloDuro Canyon. Large pipes commonly develop along the slip planes of these slumps.Scarp retreat rates based on surface erosion rates were examined and compared with the subsurfacesalt dissolution rates. Scarp retreat rates based on projected former positions of the Caprock Escarpment are probably maximum rates that range from 0.06 km/1,000 yr (0.038 mi/1,000 yr) to 0.19 km/1,000 yr (0.119 mi/1,000 yr). Short-term erosion rates based on semiannual measurement of erosion pins and stream headcuts range from 0.01 to 0.03 km/1,000 yr (0.006 to 0.019 mi/1,000 yr). The mean horizontal dissolution rate for salt in the study region is 0.02 km/yr (0.013 mi/yr). Because the Caprock Escarpment parallels and overlies the zone of salt dissolution, the retreat rates of these two features are comparable and appear to converge in the range of 0.01 to 0.20 km/1,000 yr (0.006 to 0.12 mi/1,000 yr). Discharge from many springs along the Caprock Escarpment has decreased as much as an order of magnitude over the last 50 yr as a result of widespread exploitation of the Ogallala aquifer for irrigation. Because of this trend, associated erosional processes such as spring sapping, slumping, rockfalls, and stream discharge have most likely decreased as well, thus slowing the westward retreat of the Caprock Escarpment.