Lithostratigraphy and geochronology of fills in small playa basins on the Southern High Plains, United States

Playa basins are small depressions (typically ≤1.5 km 2 ) on the Southern High Plains of northwestern Texas and eastern New Mexico. There are about 25 000 playas in the region; they lie on the Blackwater Draw Formation (Pleistocene), a widespread eolian deposit, and locally on the Ogallala Formation (Miocene‐ Pliocene). Understanding the lithostratigraphy and chronostratigraphy of the fill in the basins is important because it should (1) provide clues to the origin and evolution of playas, which have been long debated; (2) yield a paleoenvironmental record for the region; and (3) aid in understanding the history and future of the regional aquifer because playas are the principal source of recharge. Data from 19 playa basins, combined with published data from 4 other basins, show that the basin fill is composed of six distinctive facies: (1) lacustrine mud; (2) lacustrine carbonate; (3) lacustrine delta deposits; (4) eolian sand and silt; (5) eolian loam; and (6) accretionary eolian deposits (Blackwater Draw Formation). Mud deposited under ponded conditions is the most common facies and is the surficial deposit on the floors of most playas, often producing Vertisols. The carbonate was precipitated under lacustrine conditions and is another common facies and surface deposit. Delta deposits are common near the basin margins. Well-sorted layers of eolian sand and silt and poorly sorted eolian loam occur locally above, within, or below the lacustrine deposits. The modern basins in all study areas are locally or completely inset into the Blackwater Draw Formation, supporting the interpretation that the basins are at least in part erosional features. In larger basins with thicker fills, generally coincident with thicker Blackwater Draw Formation, the formation interfingers with the lacustrine fill. Dating is based on radiocarbon ages from the fill in 12 basins and from lunettes adjacent to 5 basins. All dated basins were present at the end of the Pleistocene and some were present in some form throughout the Pleistocene. Lacustrine mud and other clastic deposits accumulated in the late Quaternary and locally much earlier, showing that at least some basins contained water throughout the time of human occupation of the region. Dating of eolian sediments supports other data indicative of aridity and wind deflation in the early and late Holocene. The lacustrine carbonate is late Pleistocene or older and its paleoenvironmental significance is unknown. These lithostratigraphic and chronostratigraphic relationships show that some basins have a prolonged history as depressions, persisting in more or less the same location as the High Plains surface aggraded by eolian addition (Blackwater Draw Formation) throughout the Pleistocene. Sizes of the basins varied through time as they were encroached upon by the Blackwater Draw Formation, enlarged by fluvial, lake margin, and eolian erosion, were filled and reexposed, or were buried. Some basins are newly formed on the High Plains surface and have no apparent predecessors. The only evidence for subsidence beneath the basins is gently warped fill in the basins on the northern margin of the region, known to be affected by salt dissolution in Paleozoic bedrock. Pedogenic carbonates typically are absent from or beneath the basin fill, due to focused recharge through the basins. Playa basins probably have been a ubiquitous component of the High Plains landscape through much of the Quaternary.