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

Geochemical data, mostly from mudstones, are used in an attempt to investig ate some aspects of the palaeoclimatology and provenance of the Torridonian succession in NW Scotland. The basal part of the Steer Group, which has be en interpreted as glaciogenic is thought to have formed in a warm arid sett ing. The presence of oscillation ripples and desiccation cracks in mudstone s containing isolated clasts is thought to preclude their interpretation as ice-rafted debris. Some aspects of the major element geochemistry of the S teer Group mudstones suggest deposition in an environment with little weath ering, but chemical weathering is inhibited in both frigid and arid setting s and sedimentological evidence favours the latter. The Steer Group mudston es are enriched in Mg, Ca and Na relative to the Torridon Group samples, wh ich are much closer in composition to an average shale estimate (PAAS). The Cr and Ni contents of the Steer mudstones are significantly higher than th ose of the Torridon Group samples. There is, however, no evidence that enri chment in these elements was due to contamination with volcanic material li ke that of the Stac Fada Member. The Scourian basement appears to be an ade quate source of the Ni and Cr in the Steer Group sediments. Th/Sc ratios ar e higher in the Torridon than in the Steer mudstones. This difference is in terpreted as indicating that the Steer mudstones were largely derived from local Archaean sources but that the Torridon Group mudstones incorporated m aterials from a much wider provenance, including younger and recycled mater ials. Evidence from rare earth elements also provides evidence for differen t source terrains for the Steer and Torridon Groups. The Torridon Group mud stones are more weathered, have a higher REE content and display a more pro minent Eu anomaly (lower value for Eu/Eu*) than do the Steer samples. These results support evidence from Th/Sc ratios, that the Torridon Group was de rived from a more varied, recycled and granite-rich terrain than the Steer, much of which appears to have relatively local provenance. The geochemistr y of mudstones can shed considerable light on problems of provenance and pa laeoclimatology, because many useful trace elements tend to be concentrated in such fine grained rocks and some aspects of their major element chemist ry may be related to climatic factors.