Thermoluminescence ages of loess and associated sediments in central Nebraska, USA

Abstract

Abstract 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 field sampling and thermoluminescence dating has provided new age estimates for the younger aeolian stratigraphic units in the upper part of the sequence. Dates from aeolian sands below the Gilman Canyon Formation at two sites indicated ages ranging from 64 ± 7 to 48 ±5 ka, providing a maximum age for this unit. A date of 35 ± 3 ka was obtained from near the top of a truncated Gilman Canyon sequence in one section, but radiocarbon evidence from other sites suggests that slow accumulation of dust, accompanied by pedogenesis and syn-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 Peoria 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, apparently due to a reduction in sediment supply from the Great Plains river valleys, thereby allowing the widespread development of a soil (the Brady Soil). After 9 ka ago the rate of dust deposition again accelerated, resulting in the accumulation of up to 5 m of Bignell Loess. The main source of Bignell Loess was probably also the valleys of ephemeral Great Plains rivers during periods when the region experienced a drier climate than present. Dating evidence suggests that some time after 3 ± 2 ka ago the rate of dust accumulation again slowed, allowing development of the modern soil. It is suggested that this change reflected a reduction in dust availability, related to a change towards slightly wetter climate and associated changes in river regime.