PLAYAS AND LUNETTES ON THE SOUTHERN HIGH-PLAINS - MORPHOMETRIC AND SPATIAL RELATIONSHIPS

The surface of the Southern High Plains is dotted with thousands of small (<5 km(2)), circular, seasonally dry depressions, or playas, some of which are accompanied by a crescentic dune ridge, or lunette. The playas are inset into the sheets of eolian sediment that blanket the region (Blackwater Draw Formation) and the lunettes rest on the eolian mantle. Approximately 25,000 small playas, 1,100 of which have a lunette, were counted from all 540 topographic quadrangles covering the region (verified using data from soil surveys). Statistical analyses of the physical and spatial relationships of playas and lunettes were undertaken to better understand the origins and development of playas, and their relationship to lunettes. Data were collected from a subset of 40 topographic maps, digitized on a CIS, and analyzed using a nonparametric statistical procedure. Variation in texture of the Blackwater Draw Formation exerts considerable control on the distribution and size of playas. Playas are most numerous in the coarsest (sandy), most easily deflated substrate, followed by the finest substrate which produce erodible, sand-size aggregates. The deepest playas and those with largest area also are in the finest, easily erodible substrate. These data support the hypothesis of playa development by erosion rather than by dissolution and subsidence. Lunettes most often are associated with deeper and wider playas. Presence or absence of lunettes probably is related to the nature of the playa fill and is not a function of substrate texture. Lunettes form from deflation of calcareous playa fill which is more common in larger playas. Otherwise, the controls on the distribution of this fill are unknown.