NEW INSIGHTS OF AN ANCESTRAL PECOS (?) RIVER VALLEY BELOW MONAHANS DUNES, WARD AND WINKLER COUNTIES, WEST TEXAS

The Monahans Dune Field is one of the southernmost Quaternary aeolian depositional systems on the Great Plains sourced from a major drainage system, the Pecos River. Previous studies document for the nearby Blackwater Draw Formation at least six well developed paleosols intercalated within aeolian sediments, spanning at least the past 1.4 ma, but the stratigraphic base and associated paleoenvironments for the early Quaternary are unknown. New insights from 141 exploratory borings in the area indicate that these aeolian deposits are infill sediments of a large-scale ancestral river valley with at least an 18 km width and broader landscape relief of >75 m. This buried landscape is sedimentologically distinct from the superjacent successions and reflects different depositional processes. The buried river valley is composed of unlithified, pedogenically modified red clay and fluvial clastics. Carbonate nodules and wedge-shaped peds can be found within the red, clay-rich paleosols, suggestive of floodplain weathering. Optically stimulated luminescence dating of the overlying aeolian deposits above well-developed carbonate-rich paleosols provide a minimum age of > 300 ka and is correlative to the Blackwater Draw Formation. However, interfingering of the alluvial and aeolian deposits within some cores indicates that the diverse depositional environments occurred contemporaneously for some time, such that this unconformity may reflect variable periods of stability dependent on facies. This buried, ancestral valley is oriented NNW-SSE with drainage towards the south. The nearest major drainage system in the region is the modern-day Pecos River, which is approximately 60 km towards the west with a roughly subparallel orientation. These new insights can help address questions about the source of the Monahans Dune Fields, the morphology of the subsurface architecture, and the history of the position/migration of the Pecos River during the Pleistocene.