THERMOLUMINESCENCE AGES OF LOESS AND ASSOCIATED SEDIMENTS IN CENTRAL NEBRASKA, USA

Thick deposits of Quaternary loess (maximum 60 m, more typically 20-30 m) occur in the Loess Hills and south of the Platte River in central Nebraska. A programme of held sampling and thermoluminescence dating h as provided new age estimates for the younger aeolian stratigraphic un its in the upper part of the sequence. Dates from aeolian sands below the Gilman Canyon Formation at two sites indicated ages ranging from 6 4+/-7 to 48+/-5 ka, providing a maximum age for this unit. A date of 3 5+/-3 ka was obtained from near the top of a truncated Gilman Canyon s equence in one section, but radiocarbon evidence from other sites sugg ests that slow accumulation of dust, accompanied by pedogenesis and sy n-depositional reworking, continued in some areas until at least 23 ka ago. The Gilman Canyon Formation is overlain by up to 25 m of grey Pe oria Loess which accumulated between c. 22 ka and 10.5 ka ago. Between c. 10.5 ka and 9 ka ago the rate of dust accumulation decreased, appa rently due to a reduction in sediment supply from the Great Plains riv er valleys, thereby allowing the widespread development of a soil (the Brady Soil). After 9 ka ago the rate of dust deposition again acceler ated, resulting in the accumulation of up to 5 m of Bignell Loess. The main source of Bignell Loess was probably also the valleys of ephemer al Great Plains rivers during periods when the region experienced a dr ier climate than present. Dating evidence suggests that some time afte r 3+/-2 ka ago the rate of dust accumulation again slowed, allowing de velopment of the modern soil. It is suggested that this change reflect ed a reduction in dust availability, related to a change towards sligh tly wetter climate and associated changes in river regime.