LATE QUATERNARY EOLIAN STRATIGRAPHY AND CHANGING ENVIRONMENTS OF THE MONAHANS DUNE FIELD, TX

The Monahans dune field is a Quaternary eolian system on the Southern High Plains in West Texas. Previous work characterizing the Monahans is limited by the depth and spatial coverage of stratigraphic analyses. Ongoing study of the dune system has been facilitated by the collection and analysis of 80 deep boreholes (>40 m) and 14 Geoprobe cores (5-15 m). 3-D modeling of borehole data reveals a complex depositional past, which can be subdivided into three major units. This study analyzes the most recent strata deposited in the past 67 ka. This upper sequence is 5 to 10 m thick and composed of well sorted eolian sands and intercalated paleosols. The sand infills and is over-thickened in depressions formed in the underlying strata of correlative paleo-lake and a buried-soil complex, with stage 2-4 carbonate morphology, > 100 ka in age, that resulted in a topographic reversal in the modern dune surface. Direct push core samples yielded >95% recovery and preserve intact pedo-sedimentary structure and viable material for optically stimulated luminescence (OSL) dating. Samples were characterized including description of lithology and pedology, granulometry, gasimetric carbonate content, and micromorphology of pedo-sedimentary features. Absolute chronology of the stratigraphy has been derived from Single Aliquot Regeneration OSL protocols on quartz grains. OSL dating reveals three major periods of deposition within the upper sequence between 67 and 40 ka, 26 and 12 ka, and Holocene deposition <12 ka. The intermediate period, correlative with the last glacial maximum, is marked by an abatement in eolian accretion and prolonged pedogenesis. The spatial variability of deposition is associated with landscape variability, and the stabilizing influence of even weakly developed surface soils. An increase in pedogenesis, and associated landscape stability is observed during glacial and stadial periods, with increased eolian reactivation and deposition during interglacials. The results of this study provide new insight into sedimentary structures and processes within the Monahans dune system, and relationship with large scale climate variability of the past 67 ka.