THE RED BED RIDGE AND OTHER GEOLOGIC AND HYDROGEOLOGIC ASPECTS OF THE SOUTHWESTERN FEATHER EDGE OF THE HIGH PLAINS AQUIFER

In Andrews County, Texas the edge of the High Plains aquifer, often interchangeably referred to as the Ogallala aquifer, occurs in the vicinity of a well-developed buried ridge on the paleo-surface of the Triassic Dockum Group red beds. The ridge is capped by Cretaceous Antlers sands and gravels, highly silicified in places. The Antlers may have acted as an erosion-resistant cap throughout late Cenozoic time, maintaining the ridge as a drainage divide while late Tertiary Ogallala and partially age-equivalent Gatuna Formation sediments were deposited on either side of the ridge. The Ogallala, Antlers and Gatuna Formations occupy the same hydrostratigraphic position, hosting the High Plains aquifer immediately above the low permeability claystones and interbedded siltstones and sandstones of the Dockum. The aquifer feathers from saturated to unsaturated conditions, with the transition occurring as the buried ridge is approached from the northeast. The location of the transition from saturated to unsaturated conditions is transient, dependent on rainfall and the amount of playa-focused infiltration that occurs. In relatively dry periods the saturated zone retreats down the northeast slope of the ridge, while during wetter periods the zone of saturation extends further southwest toward its axis.

A thin area of 0 to 10 feet of saturation at a depth of about 45 feet on the northeastern side of the buried ridge is perched above a relatively thick section (over 100 feet) of unsaturated red bed claystones. The regional water table is much deeper, occurring in the Triassic red beds at a depth of about 180 feet. The area of continuous saturation in the High Plains aquifer northeast of the ridge is part of the regional ground water flow system that includes the High Plains aquifer and deeper aquifers in the Dockum Group. The ground water in the High Plains aquifer in this vicinity is modern, based on 14C and tritium analyses. The ground water in the low permeability siltstones and sandstones in the upper Dockum lacks tritium and exhibits radiocarbon-adjusted ages of about 15,000 years BP. The stable isotope signature of the Dockum ground water indicates the water was derived from precipitation in a cooler, moister climate than currently exists in arid west Texas, which is consistent with the late Pleistocene radiocarbon age.