Vt. Holliday et al., LITHOSTRATIGRAPHY AND GEOCHRONOLOGY OF FILLS IN SMALL PLAYA BASINS ONTHE SOUTHERN HIGH-PLAINS, UNITED-STATES, Geological Society of America bulletin, 108(8), 1996, pp. 953-965
Citations number
Categorie Soggetti
Geosciences, Interdisciplinary
ISSN journal
Year of publication
953 - 965
SICI code
Playa basins are small depressions (typically less than or equal to 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 depos it, and locally on the Ogallala Formation (Miocene-Pliocene), Understa nding 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 pal eoenvironmental record for the region; and (3) aid in understanding th e history and future of the regional aquifer because playas are the pr incipal source of recharge. Data from 19 playa basins, combined with p ublished data from 4 other basins, show that the basin fill is compose d of six distinctive facies: (1) lacustrine mud; (2) lacustrine carbon ate; (3) lacustrine delta deposits; (4) eolian sand and silt; (5) eoli an loam; and (6) accretionary eolian deposits (Blackwater Draw Formati on), Mud deposited under ponded conditions is the most common facies a nd is the surficial deposit on the floors of most playas, often produc ing Vertisols, The carbonate was precipitated under lacustrine conditi ons and is another common facies and surface deposit, Delta deposits a re common near the basin margins, Well-sorted layers of eolian sand an d silt and poorly sorted eolian loam occur locally above, within, or b elow the lacustrine deposits. The modern basins in all study areas are locally or completely inset into the Blackwater Draw Formation, suppo rting the interpretation that the basins are at least in part erosiona l features. In larger basins with thicker fills, generally coincident with thicker Blackwater Draw Formation, the formation interfingers wit h the lacustrine fill. Dating is based on radiocarbon ages from the fi ll in 12 basins and from lunettes adjacent to 5 basins. All dated basi ns were present at the end of the Pleistocene and some were present in some form throughout the Pleistocene. Lacustrine mud and other elasti c deposits accumulated in the late Quaternary and locally much earlier , showing that at least some basins contained water throughout the tim e of human occupation of the region, Dating of eolian sediments suppor ts other data indicative of aridity and wind deflation in the early an d late Holocene, The lacustrine carbonate is late Pleistocene or older and its paleoenvironmental significance is unknown. These lithostrati graphic and chronostratigraphic relationships show that some basins ha ve 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 ba sins varied through time as they were encroached upon by the Blackwate r Draw Formation, enlarged by fluvial, lake margin, and eolian erosion , were filled and reexposed, or were buried, Some basins are newly for med 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 affect ed hy salt dissolution in Paleozoic bedrock, Pedogenic carbonates typi cally are absent from or beneath the basin fill, due to focused rechar ge through the basins, Playa basins probably have been a ubiquitous co mponent of the High Plains landscape through much of the Quaternary.