Regional geochemistry of cryptic geology: variations in trace element distribution across the Southern Uplands terrane, Scotland

摘要

Stream-sediment geochemistry is commonly used to provide a regionally averaged, background composition against which may be judged the anomalies caused by unusual geology, mineralisation or pollution. Its interpretation may be based on assumptions of relative compositional uniformity within broad categories of bedrock lithology. This approach is undermined by the dramatic variations in stream-sediment chemistry that occur over some large areas of superficially homogeneous geology. In these cases, regional geochemistry may pick-out subtle compositional variations in bedrock that are not otherwise apparent. Thick greywacke sequences present a particular problem of geological interpretation by virtue of their general uniformity, with a good example provided by the Southern Uplands terrane. There, some stratigraphical variations in greywacke composition, reflecting changes in sedimentary provenance, are known from petrographic and analytical work but are not visible macroscopically. However, contoured stream sediment geochemistry maps, supplemented by cross-strike element abundance profiles, reveal a remarkable level of detailed variation pertinent to stratigraphical sub-division and a much-improved provenance interpretation. Of particular value, are the transition metals Cr-Ni-Mg-V and the feldspar-related elements Sr-Rb-K-Ba, the latter defining a major structural break within the terrane. An important corollary to the improved geological understanding arising from regional geochemical survey is the parallel improvement in understanding of the surface processes influencing element distribution in the stream-sediment environment.