Some aspects of the geochemistry, provenance and palaeoclimatology of the Torridonian of NW Scotland

Abstract

Geochemical data, mostly from mudstones, are used in an attempt to investigate some aspects of the palaeoclimatology and provenance of the Torridonian succession in NW Scotland. The basal part of the Stoer Group, which has been interpreted as glaciogenic is thought to have formed in a warm arid setting. The presence of oscillation ripples and desiccation cracks in mudstones containing isolated clasts is thought to preclude their interpretation as ice-rafted debris. Some aspects of the major element geochemistry of the Stoer Group mudstones suggest deposition in an environment with little weathering, but chemical weathering is inhibited in both frigid and arid settings and sedimentological evidence favours the latter. The Stoer Group mudstones are enriched in Mg, Ca and Na relative to the Torridon Group samples, which are much closer in composition to an average shale estimate (PAAS). The Cr and Ni contents of the Stoer mudstones are significantly higher than those of the Torridon Group samples. There is, however, no evidence that enrichment in these elements was due to contamination with volcanic material like that of the Stac Fada Member. The Scourian basement appears to be an adequate source of the Ni and Cr in the Stoer Group sediments. Th/Sc ratios are higher in the Torridon than in the Stoer mudstones. This difference is interpreted as indicating that the Stoer mudstones were largely derived from local Archaean sources but that the Torridon Group mudstones incorporated materials from a much wider provenance, including younger and recycled materials. Evidence from rare earth elements also provides evidence for different source terrains for the Stoer and Torridon Groups. The Torridon Group mudstones are more weathered, have a higher REE content and display a more prominent Eu anomaly (lower value for Eu/Eu*) than do the Stoer samples. These results support evidence from Th/Sc ratios, that the Torridon Group was derived from a more varied, recycled and granite-rich terrain than the Stoer, much of which appears to have relatively local provenance. The geochemistry of mudstones can shed considerable light on problems of provenance and palaeoclimatology, because many useful trace elements tend to be concentrated in such fine grained rocks and some aspects of their major element chemistry may be related to climatic factors.