Quaternary Evolution of Playa Lakes on the Southern High Plains--A Case Study from the Amarillo Area, Texas

This report documents the stratigraphy beneath playa lakes and associated lake-margin, playa-basin, and upland settings. Genetic facies interpretation of the observed lithologies was undertaken to better predict the nature of interplaya stratigraphic variation and to extract information about the long-term geologic and paleoclimatic evolution of the area.ABSTRACTPlaya lakes are abundant small ephemeral lakes (generally ~0.5 km in area) that occur in shallow depressions (generally <11 m deep) on the surface of the Southern High Plains. This study, based on analysis of excavations and 63 hollow-stem auger cores taken from 10 lake basins in the study area around the Pantex Plant northeast of Amarillo, Texas, resolves longstanding controversies regarding origin, evolution, and recharge behavior of playa lakes. Origin of playa lakes has been debated for decades because the lakes are abundant and the processes that form them obscure. All of the lake basins studied have had a long history, originating as topographic lows before or during the early phases of Blackwater Draw deposition. The origin of some topographic lows can be related to dissolution of underlying Permian salt beds. Other basins appear to have been formed by other geomorphic processes. Stacked depositional cycles identified in lake sediments record repeated phases of (1) initial highstand, (2) ephemeral lake conditions, and (3) lake shrinkage and prolonged subaerial exposure. Although sedimentary structures show that the lakes were ephemeral during all phases, duration and frequency of flooding varied, thus changing relative amounts of sediment accumulation, deflation, and soil formation. Playa lake basins in the study area contain 5 to 18 m of Quaternary lake sediments, including gray clays, oxidized red-brown clays, heterogeneous lacustrine delta deposits, fine lacustrine-eolian sand and silt beds and laminae, and admixed sand and clay. Lake sediments interfinger with calcic soils and red-brown loam of the Blackwater Draw Formation near the edges of the playas. As many as 12 calcic soil horizons are identified in the Blackwater Draw Formation in the upland. A sand unit underlies both playas and uplands at depths of 10 to 20 m. Although the ephemerally ponded playa lakes are floored by clay soils, ground-water and unsaturated zone investigations show that playa lakes serve as sites of focused recharge. Description of a spectrum of playa basins of various sizes and recharge behaviors documents the long-term maintenance of the seasonally ephemeral lakes and their responses to past climatic changes. All of the lakes are underlain by thick but laterally and vertically heterogeneous clay sections. However, evidence of preferential pathways controlling flow is abundant beneath playas, including shrink-swell cracks and roots in clay soils and gleying, illuviated clay, leached carbonate, or mineralized fractures in older sediments. Sand interbeds within the lacustrine deposits may also influence flow rates. Vertical fractures served as conduits for both oxidizing and reducing fluids in high-permeability, well-sorted sands, as well as in low-permeability sediments. The observed lateral and vertical heterogeneity within, and the range of variation among, playas can be used to constrain assumptions that will be made about subsurface stratigraphy of the playas at the Pantex Plant. Preferential pathways similar to those observed in all the studied playas can also be inferred to influence recharge behavior at the plant.