Micromorphology and geochemistry of groundwater silcrete boulders in the eastern South Downs, UK

Groundwater silcretes have been recognised recently as major terrestrial silicon sinks and yet their origins are poorly understood. This paper aims to further understanding of the micro-fabric, geochemistry and formation of groundwater silcretes, through detailed analyses of silcrete boulders from the South Downs, Sussex, UK. In-depth petrological investigations of silcrete blocks identified three varieties of silcrete across the study area (saccharoid, hard and pebbly conglomeratic or silcrete breccia), similar to those elsewhere in England. Silcrete fabrics were universally simple and preserved host material structures. Optically-continuous quartz overgrowths were the dominant cement and developed on framework grains in the absence of silt- and clay-sized particles. Finer grained silica cements occurred in isolated patches and as vein- or cap-like geopetal features. These caps and veins formed through the silicification of illuviated clay-rich material that entered the host sediment prior to, or in the early stages of, overgrowth formation. Titanium contents were related to the amount of fine-grained silica and appear to reflect variations in host material chemistry. Subsequent to silicification, the silcretes were altered by at least two phases of ferrunginisation, characterised by the ingress of iron minerals and partial fabric dissolution or replacement. The study reveals strong similarities in the micromorphology and geochemistry of groundwater silcretes in the study area and those developed in sandy host sediments in neighbouring parts of northwest Europe. Variations that do occur could be explained by differences in the host sediment, geological setting or tectonic history of the respective basins (or sub-basins), suggesting that there may be a common mechanism for silcrete formation across northwest Europe. Silcrete development appears to be related to the release of silica accompanying acid leaching of the host material or adjacent strata. In the South Downs, this probably resulted from oxidisation of pyrite in the argillaceous and lignitic sediments overlying the host sands. It is envisaged that palaeoenvironmental conditions were of limited importance for silcrete development and that there was no specific 'era' of formation, with phased development accompanying landscape evolution through the Neogene into the Pleistocene.