The role of peri-glacial active layer development in determining soil-regolith thickness across a Triassic sandstone outcrop in the UK

Abstract

This paper examines the weathering processes that have combined to produce the distribution of soil-regolith (SR) thickness across the Triassic Sherwood Sandstone Group outcrop (750km 2) in Nottinghamshire, UK. Archive borehole logs (n=282) taken across the outcrop showed that SR thickness had mean and median depths of ~1·8 and 1·5m, respectively. Cores were taken from a forested site to depths ~3m for geochemical analysis. At this site the SR thickness was ~1·7m. Analysis of the loss of elements, compared to bedrock using mass balance calculations (τ) showed that all the calcite and gypsum cement had been removed to depths of >3m. Thus the major difference between the SR and the underlying saprolite was that the former exists as loose sand as opposed to a semi-durable rock. Scanning electron microscopy (SEM) analysis of core samples suggested that the non-durable rock or saprolite had greater cementation of clay particles. We propose that the mechanism through which the clay cement (and other interlocking grain bonds) was eased apart was through freeze-thaw processes associated with the summer 'active layer development (ALD)' during the last glacial activity in the UK. We tested this theory by developing a Monte Carlo simulation based on a simplified version of the Stefan equation. Current Arctic datasets of air and ground temperatures were obtained to provide reasonable starting conditions for input variables. These were combined with known data for thermal conductivity, bulk density and moisture content of the Sherwood Sandstone regolith. Model predictions (n=1000) of the distribution of SR thickness accurately reflect the observed distribution thickness from the borehole logs. This is strong evidence that freeze-thaw and 'ALD' processes are major factors in determining the thickness of SR across this outcrop. British Geological Survey © NERC 2012.