Alkali metals in the Permo-Triassic as geochemical indicators of surficial processes

Abstract

Techniques established as most successful in the examination of widely variable but mainly oxidised geological materials require separation and isolation of components prior to analysis. These methods reduce the number of variables involved in the production of clastic debris and enable conclusions to be drawn from a practical reversal of rock manufacture. Alkali metals prove successful as indicators of the surficial processes operative during Permo-Triassic times. They demonstrate enrichment by igneous material during early breccia production, and subsequent dilution by wider sedimentation in basins to the east of Cornubia, England. Alkalis in Recent soils, river, estuarine and beach sediments of southwest England are concentrated in the fine-grained (< 2 μm) fraction. Soils developed over Permo-Triassic bedrock, modern river, estuarine sediments, and beach sands, all show anomalous enrichment of trace alkali metals. They reflect the abundance of Li, Rb and Cs in parent material. Geochemical analyses of clays from deep boreholes in the Wessex and Central Somerset Basins enable inter-basin comparisons with the high-alkali deposits of Devon. Absolute trace alkali levels are broadly similar to those of correlated beds at outcrop. Concentrations of Cs prove higher in the Upper Permian than in the Triassic of the Wessex Basin, whilst values show Devonian (Old Red Sandstone) sediments to be un-enriched in Li and Cs. Enrichment of Li over Rb in Triassic Mercia Mudstones of the south Devon coast suggests some Li capture from sabkha brines by clays during evaporite production.