Cliff-foot sandy cones: A proxy to study the time frames, patterns and rates of sandstone caprock decay?

Abstract

Constraining time frames, patterns and rates of sandstone caprock disintegration in tablelands under temperate climate remains challenging. This is due to the scarcity of material in the appropriate geomorphological context that could be dated. Sandy cones are widespread below the sandstone plateaus and may serve as a direct sedimentological evidence of caprock disintegration via subsurface erosion. However, they have not been utilised to their full potential. The paper presents the results of investigation of two sandy cones in the Stołowe Mountains tableland, SW Poland, and soil profiles developed within them. The study also included collecting material for charcoal ¹⁴C and single grain Optically Stimulated Luminescence (OSL) dating. Both soil profiles (Dystric Arenosols (Humic)) are dominated by sand fraction. The lens–like laminae of light-coloured sand and darker material enriched in organic matter result from changing sedimentation dynamics. Charcoals were found to be widespread and occur down to the bottom of the studied soil profiles. While they record some of the oldest fire events ever documented in the tableland (mostly from Preboreal and Boreal, in one case from the Younger Dryas), we show that they are inappropriate to determine the timing of sediment deposition because of long residence time in the fissure network. Contrarily, single grain OSL dating is far better suited to trace the sedimentological history of the cones that initiated 10.5 ± 1.5 ka ago. The overall sedimentation trends are similar, likely reflecting climate fluctuations throughout the Holocene and the impact of deforestation. Although sandy cones may serve as a universal geochronological proxy in sandstone tablelands under temperate climate, the patterns and rates at which they develop are site-specific and controlled primarily by local geomorphological and topographical circumstances, governing the efficiency of the sediment supply. This finding was supported by a novel approach to the analysis of FMM components.