ENVIRONMENTS OF AEOLIAN DEPOSITION IN SOUTH-CENTRAL NEBRASKA DURING THE LAST GLACIAL MAXIMUM

Abstract

The Peoria loess of south-central Nebraska, deposited approximately during the last Glacial Maximum, can be subdivided into three zones: an upper laminated loess and a lower massive loess, separated by a dark gley zone in which bison bones and spruce charcoal are found. The lower Peoria unit is characterized by high organic matter content, relatively high bulk density, and common root channels. The upper Peoria unit has rhythmic patterns of two orders: strongly gleyed and weakly gleyed zones alternating in couplets about 2-5 m thick, and coarse- and fine-grained laminae of second order, about 2-4 mm thick. The strongly gleyed zones are relatively low in magnetic susceptibility and contain lenses with abundant fossil plant debris. They may have formed in active zones on top of permafrost during relatively cold and probably moist intervals, with magnetic susceptibility secondarily lowered in the redox environment. Magnetic susceptibility also indicates the possible existence of first-order rhythmic patterns in the lower massive loess. Each of the ten susceptibility oscillations within Peoria loess has a mean duration of about 1000 years, assuming that the loess was deposited between approximately 20,000-22,000 and 10,000-10,500 yr BP. The second-order laminae, close to the expected mean annual accumulation rate of Peoria loess, may be due to seasonal variation in wind intensity or to secondary grain sorting of niveoeolian deposits.