Shallow Seismic Data Acquisition, Processing, and Interpretation at Playa 5, Carson County, Texas

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Seismic methods were used to determine the physical properties and geological development of Playa 5, a playa basin located on the U.S. Department of Defense's former Pantex Ordnance Plant, for comparison with results from other basins (Sevenmile Basin, Pantex Playa 3, and Pantex Lake) as well as with results from seismic data collected in interplaya areas. These studies have led to a better understanding of stratigraphic differences between playa basins, which serve as preferential recharge points for the Ogallala aquifer, and between playa and unaltered interplaya areas, where little Ogallala recharge is thought to occur. Playa 5 is a nearly circular playa that is 0.7 to 0.9 km across. It is enclosed by a basin that is about 2 km across and has 5 m of relief between the highest and lowest closed elevation contours. Refraction surveys show that the surface layer at Playa 5 is a few meters thick and has typical seismic velocities of 420 to 440 m/s. This layer is underlain by a layer with higher seismic velocities of 808 to 910 m/s that has similar texture but more pedogenic carbonate. Refraction methods also detected a layer at more than 60-m depth with significantly higher seismic velocities of about 2000 m/s. This layer probably represents a competent horizon above the modern Ogallala water table that has been cemented by either pedogenic or hydrologic processes. Reflection data collected across Playa 5 show that relief on seismic horizons increases with age. Modern surface relief is 6 m, which increases to 30 m on a horizon that is interpreted to be correlative to a fine-grained zone that perches groundwater beneath parts of the Pantex Plant. Relief increases to 50 m on a horizon that is interpreted to be the top of Permian or Triassic bedrock. Internal bedrock reflectors dip toward the basin center beneath the playa, suggesting that subsidence related to dissolution of underlying Permian salt has contributed to the development of the Playa 5 basin. Playa 5 subsidence has occurred at average rates of 0.33 to 0.42 m per meter of deposition, rates that are similar to those at Playa 3 and Pantex Lake and are less than half those inferred for Sevenmile Basin.


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