SPATIAL AND TEMPORAL PATTERNS OF LATE QUATERNARY EOLIAN DEPOSITION, EASTERN COLORADO, USA

Eolian sediment covers about 60% of Colorado east of the Rocky Mountains; about 30% of the sediment is sand and 70% is loess. Initially, flood plains were the principal sources of eolian sediment, but during the Holocene, dunes formed from older eolian sand and alluvium on uplands. Since latest Pleistocene time, dominant dune-forming winds have been northwesterly in the northern part of the region and southwesterly in the southern part. At present, sand sheets and dunes, mainly parabolic types, are stable and covered with vegetation. In dunes, sand is commonly 20-30 m thick but elsewhere averages < 10 m. Three sand units are recognized on the basis of bedforms, topographic expression, and soil development. Preliminary age limits for the three units, based on 26 numerical ages, are 22.5-9 ka, 8-1 ka, and 1.0-0.15 ka. The middle unit is the product of multiple episodes of eolian activity that are not yet accurately dated. Loess is widespread but thin (generally < 2.4 m). Three units - middle Pleistocene, late Pleistocene, and Holocene - are recognized on the basis of differences in soil-profile development and stratigraphic position; late Pleistocene loess is by far the most common loess.