Recognising different sediment provenances within a passive margin setting: Towards characterising a sediment source to the west of the British late Carboniferous sedimentary basins

Abstract

To investigate whether sediments with subtly different provenances could be recognised within a passive margin setting, samples were taken from Westphalian sedimentary rocks of the Pennine Basin, South Wales Coalfield, Culm Basin of Britain and from Northern Ireland. Although previous work has shown that 3 major source areas were responsible for the infill of the Pennine Basin, this study is restricted to sediments believed to have been derived from the west. Their nominally similar western provenance was confirmed using Sm–Nd isotope techniques and their deposition in a passive margin setting was established using elemental discrimination diagrams. While the overall sediment provenance may have been from the west, the trace element geochemistry indicates that at least two major sediment pathways can be distinguished. Plots of Eu/Eu*–Gd N/Yb N, along with ternary plots of Cr/V–Ti/Zr–Gd N/Yb N or Sc/Cr suggest different source areas for the Culm Basin and the South Wales Coalfield. In the Culm Basin the Cr/V ratio generally exceeds 1.25, with a small Eu anomaly and a low Gd N/Yb N ratio. The Cr/V ratio is less than 1.25 in the South Wales with a larger Eu ratio and a higher Gd N/Yb N than the Culm Basin. Significant separation of the Culm Basin and the South Wales Coalfield during the early Westphalian by the Bristol Channel–Bray Fault may have been a factor contributing to the provenance differences. Late Carboniferous dextral displacement along this fault brought these basins into close juxtaposition, and it is unlikely that the ‘Bristol Channel Landmass’ formed a common source area for these two basins.