EOLIAN DEPOSITIONAL RECORDS FROM SOUTHWESTERN KANSAS AND SOUTHEASTERN COLORADO, A POTENTIAL LANDSCAPE RESPONSE TO DROUGHTS IN THE PAST 10,000 YEARS

The Great Plains is dominated by presently stabilized dune fields that are indicators of extreme drought in the late Holocene. This study focused on evaluating geomorphological, stratigraphic and chronological evidences of dune reactivation for stabilized ergs in southwestern Kansas and southeastern Colorado adjacent to the Arkansas and Cimarron Rivers. Dunes occur on multiple-aged terraces, with modified star dunes at the upper valley margin and reverse and transverse dunes at lower topographic positions. Parabolic and blow dunes are also common and appear to reflect the latest period of dune reactivation. This complex of eolian landforms reflects a variety of paleowinds with dominant directions from the northwest and northeast and secondary directions from the southeast and southwest. Multiple stratigraphic exposures reveal eolian sand separated by up to three paleosols indicating repeated eolian activity followed by landscape stability. Most paleosols show minimal horizonation with either a sole A horizon and/or a weak cambic B, indicating pedogenesis for < 1 ka. The chronology of eolian-sand depositional events is constrained by optical stimulated luminescence, single aliquot regeneration method (SAR) on 150-250 micron quartz grains. The oldest eolian sand stratigraphically above high fluvial terrace gravels yielded a SAR age of 7900 ± 610 yr BP, indicating significant eolian activity in the early Holocene. Basal eolian sand from a stratigraphic exposure within Arkansas River dune field yielded the SAR age of 1490 ± 130 yr BP, indicating pervasive late Holocene reactivation. Widespread dune reactivation occurred at 230 ± 20 and 190 ± 15 yr BP, possibly coincident with a multi-decadal (?) tree-ring identified drought in the 19th century. There is at least one additional eolian event potentially between 200 and 40 years old, associated with dune reactivation from the northeast and secondary movements from the northwest. The complex eolian landscape adjacent to the Cimarron and Arkansas Rivers reflects large-scale dune reactivation with extreme droughts in the past ca. 2000 years.