Shallow-Seismic Evidence for Playa Basin Development by Dissolution-Induced Subsidence on the Southern High Plains, Texas

The origin of thousands of playa basins on the Southern High Plains of Texas and New Mexico has been attributed to eolian deflation, evaporite or carbonate dissolution and subsidence, piping, or animal activity. Shallow-seismic data from three ephemeral lake (playa) basins in the Texas Panhandle, collected as part of a hydrogeological study of High Plains playa and interplaya environments, demonstrate that subsidence has figured prominently in the formation of these three basins. Sevenmile Basin, Pantex Playa 3, and Pantex Lake, playas that occupy basins varying from 1.2 to 5.5 km in diameter, have 4 to 9 m of surface relief and contain lacustrine and eolian sediments that interfinger with the Quaternary Blackwater Draw Formation. Below these sediments lies the upper Tertiary Ogallala Formation, which overlies Permian or Triassic bedrock. We collected seismic reflection and refraction data across these basins to investigate their geological history and hydrogeological framework and to help explain the mechanism by which they recharge the regionally important Ogallala aquifer.Sevenmile Basin, 25 km east of Amarillo, Texas, is the largest of the three playa basins studied. The reflection section shows a middle Ogallala reflector, another reflector produced by an erosional unconformity at the top of Permian or Triassic bedrock, and reflectors within bedrock that indicate a structural low beneath Sevenmile Basin. Relief increases with depth, ranging from 9 m at the surface to 70 m on the middle Ogallala reflector to 110 m at the unconformity at the base of the Ogallala. Reflection data at Pantex Playa 3, a small playa about 25 km northeast of Amarillo, reveal the presence of four major reflecting horizons. Each horizon mimics surface topography, and relief increases with depth: the playa floor lies 4 m below the upland; the Ogallala caprock, absent directly under the playa, has 16 m of relief; an upper Ogallala fine-grained zone has 30 m of relief; a stratigraphic boundary between lower Ogallala coarse fluvial deposits and finer middle Ogallala lacustrine and fluvial deposits has 35 m of relief; and the erosional Permian or Triassic bedrock surface has 75 m of relief.At Pantex Lake, which is situated in a large, irregularly shaped basin 34 km northeast of Amarillo, five strong reflecting horizons can be correlated to recurring geophysical log patterns in nearby water wells. The shallowest reflector, not visible directly beneath Pantex Lake, is thought to be from the Ogallala caprock. Deeper reflectors are interpreted to be (1) the top of a conductive, fine-grained (dominantly clay to sandy clay) section in the middle of the Ogallala Formation; (2) the boundary between a lower Ogallala resistive and coarse-grained (clayey sand to gravelly sand) zone and the conductive, fine-grained zone composing the middle Ogallala section; (3) a stratigraphic boundary within the lower Ogallala coarse-grained zone; and (4) the top of eroded Permian or Triassic bedrock. Relief on each surface increases with depth: the playa floor lies 6 m below the upland, the top of the middle Ogallala fine-grained zone has 28 m of relief, the boundary between lower Ogallala resistive and middle Ogallala conductive zones has 31 m of relief, and the lower Ogallala and Permian or Triassic bedrock reflectors have 45 to 50 m of relief.Relief increasing with age indicates that subsidence, probably caused by dissolution of underlying Permian evaporites, has played a role in the formation of each basin. A combined Ogallala Formation and Blackwater Draw Formation thickness greater than bedrock relief suggests that subsidence began before or during Ogallala deposition and may continue to this day. Pedogenic carbonate is less abundant directly beneath the playas than it is beneath uplands adjacent to the playas. Either pedogenic carbonate formed beneath these playas and was subsequently removed by erosion or dissolution, or it never formed beneath perennially wet basins that have probably existed since Ogallala deposition.