GSA QUATERNARY GEOLOGY AND GEOMORPHOLOGY DIVISION FAROUK EL-BAZ AWARD FOR DESERT RESEARCH: FOURTH DIMENSIONAL UNDERSTANDING OF LATE PLEISTOCENE EOLIAN DEPOSITIONAL ENVIRONMENTS FOR THE MONAHANS DUNE FIELD, TEXAS USA

The Monahans Dune Field is one of many eolian systems associated with river-systems on the Southern High Plains. Previous studies document the Pleistocene Blackwater Draw Formation with eolian, fluvial and lacustrine facies buried by Holocene eolian sands. However, knowledge of this eolian system is limited because of the lack of stratigraphic data and ages for strata deeper than a few meters from available sections. This study analyzed 80 borings, > 40 m depth, from throughout the dune field and 14 Geoprobe cores >5 m long. OSL dating provides ages for these quartz-rich cores by single aliquot regenerative protocols and thermal transfer (TT-OSL) methods with, an effective dating range of 1 ma. Architectural modeling based on the stratigraphic data from the borings indicate aeolian deposits infilled an ancestral valley at least a 18-km wide and landscape relief of >75 m with potential sinkholes. This buried landscape is sedimentologically distinct with unlithified, pedogenically-modified red clay with fluvial clastics. A thick sequence of eolian sand infills against this fluvial paleo-surface resulting in a topographic reversal with the current dune field. Geoprobe cores reveal a complex stratigraphy that is subdivided into three major pedo-strata sequences. The lowest sequence is a > 8 m-thick eolian sand with a finite TT-OSL age of 620 ka from the upper 4 m and at depth intercalates with the early Pleistocene basal red-fluvial clay. A pedo-complex of stacked, well-developed carbonate paleosols formed from 400 to 100 ka in the top of the lower eolian sand. Correlative to these paleosols is a carbonate-rich, playa-lake deposit immediately southwest of the dune field; dated between 335 and 100 ka. Overlying this carbonate soil complex is 5-10-m thickness of eolian sand that spans the past 67 ka, with distinct depositional periods between 67 and 40 ka and in the Holocene. Eolian accretion appears to be modest or associated with mild pedogenesis between 25 and 12 ka, the last glacial maximum. Eolian deposition occurs preferentially during interglacials, with soil development and playa lake formation mostly confined during glacials. Hydrologic excess appears to persist in SW Texas with the Laurentide Ice Sheet at southward limits, and the southern shift of Jet Stream circulation, particularly during long Stage 6 and 8 glacials.