The interpretation and application of regional geochemistry: lessons from the Paratectonic Caledonides

Abstract

Synopsis The Southern Uplands (southern Scotland) and Lake District (northern England) terranes contain thick and extensive sequences of macroscopically homogeneous, Lower Palaeozoic wacke-type sandstone. There is a close spatial relationship between the outcrop of biostratigraphically defined rock units and distinctive patterns of element distribution in stream sediments derived from those units, with the stream sediment patterns indicating cryptic compositional variations that are not otherwise apparent. Analysis of the stream sediment composition relative to whole-rock values shows that it is not a direct surrogate for the underlying bedrock. Instead, elevation and depletion trends for any given element are controlled by the nature of its mineral host(s) and provide a previously unappreciated means of assessing the detrital mineralogy, and hence provenance of the bedrock sandstones. Stream sediment composition also provides a valuable alternative means of evaluating possible correlation between different stratigraphical sandstone units. Further, since the fine-grained matrix to the wacke sandstones makes a major contribution to the stream sediment, an assessment of the latter allows variations in matrix composition to be taken into account. Examples are given from the Silurian turbidite sequences of the central and southern Southern Uplands (Gala, Hawick and Riccarton groups; the affinity of the Ross Formation with the Hawick Group is demonstrated) and the southern Lake District (Windermere Supergroup).