QUATERNARY GEOMORPHIC EVOLUTION OF THE SOUTHERN HIGH PLAINS

The Southern High Plains (or Llano Estacado) (SHP) is a plateau covering >100,000 sq km in northwestern Texas and eastern New Mexico. The region evolved through a combination of geomorphic processes, dominantly eolian and alluvial, but also including subsidence. The plateau was isolated on the east by development of the Colorado, Brazos, and Red rivers, on the north by the Canadian River, and on the west by the Pecos. The SHP surface is largely constructional, owing to slow, episodic accumulation of eolian sediment through the Pleistocene (on top of the Mio-Pliocene Ogallala Fm) and development of a vast sheet of sand and clay (the Blackwater Draw Formation, BWD). The BWD is heavily modified by pedogenesis. The modern surface soils of the region (most commonly Paleustolls and Paleustalfs) represent the youngest phase of soil development in the BWD. Inset into the High Plains surface are draws and playa basins. The draws are now dry tributaries of the Red, Brazos, and Colorado rivers. Latest incision of these valleys was in the late Pleistocene. The playa basins likely are the result of wind erosion with enlargement by centripetal slopewash. Most basins appear to date to the late Pleistocene. Both the draws and playa basins filled in the terminal Pleistocene and through the Holocene; the playas with paludal and some eolian and colluvial deposits; the draws with alluvial, lacustrine, paludal, and eolian deposits. Sand sheets and dunes, largely of Holocene age, and lunettes formed adjacent to some playas through the late Quaternary, rest on top of the BWD. Limited data from exposures and cores show that playas and valleys were present on the SHP landscapes as the BWD slowly accreted; these landforms built up as the BWD aggraded.