Optically stimulated luminescence (OSL) dating of loessic sediments and cemented scree in northwest England

Abstract

Optically stimulated luminescence (OSL) dates are reported for silts and very fine sands believed to be loessic sediments from northwest England. At three sites loessic sediments were initially interpreted as primary aeolian deposits, and at two other sites as loess incorporated into the matrix of cemented scree. However, the results of OSL dating indicate a more complex pattern of accumulation than originally hypothesized and have prompted reconsideration of these materials. Whatever the process(es) and underlying cause(s), it is evident that significant amounts of soil erosion occurred on the limestone uplands earlier than previously thought. All but one of the ages fall entirely within the Holocene period and suggest that these deposits contain reworked, rather than primary loess. Four of the five sites are characterized by non-Gaussian dose distributions, and consequently equivalent doses have been estimated using a range of appropriate age models. The implications of differences in the ages derived from the fine silt and fine sand fractions of the samples are considered. Three processes, namely aeolian transport, overland flow and subsoil piping, are invoked to account for the reworking of loess, although their relative contributions cannot be quantified. At one site the inclusion of limestone clasts within the reworked loess strongly suggests that the sediment can be regarded as loess-derived colluvium. Human impacts on the landscape and climate shifts, either separately or in combination, are considered to have been the most likely mechanisms that triggered loess erosion.