Distribution, petrology and mode of development of silcretes (sarsens and puddingstones) on the eastern South Downs, UK

This paper presents the first intensive integrated field and laboratory study of an area of typical silcrete occurrence in the UK, and enables parallels to be drawn with in situ silcretes in the neighbouring Paris Basin and other parts of northern Europe. Silcrete distribution in the eastern South Downs is localized and discontinuous, with occurrence principally as dislocated boulders on the Chalk. These boulders mainly occur between the Cuckmere and Goldstone valleys, with greatest concentrations around Stanmer, Falmer, Rottingdean, and in the Goldstone valley. Many occur at higher levels in the landscape, on or near interfluves usually peripheral to outliers of Palaeocene sediments and in close association with Claywith-Flints. At lower levels they occur on valley floors and on the coastal plain within Quaternary sediments. Boulders commonly have a-axis dimensions of 0·5–2 m and thicknesses of 0·3–0·6 m. Silcretes in higher positions typically exhibit angular tabular or prismatic shapes whilst those in derived settings are more rounded, suggesting weathering and erosion during transport from a localized high level silcrete lens (or lenses). Three varieties of silcrete have been identified. Pale grey ‘saccharoid sarsens’ are most widespread, and exhibit a simple grain-supported (GS-) fabric with predominantly sand-sized quartzose sediment cemented by optically continuous quartz overgrowths and minor micro- and cryptocrystalline silica. Brown ‘hard sarsens’ occur as smaller blocks, mainly in the Seaford Head area. These also exhibit a GS-fabric but with more fine sediment and a greater variety of cement types. Textural and geochemical evidence suggests there is gradation between the saccharoid and hard varieties of sarsen. Conglomeratic ‘puddingstone’ is common in the Goldstone valley but rare elsewhere, and shows closest similarity to saccharoid sarsen in texture and cementation. Silcrete micromorphology is universally simple and uniform, with fabrics suggestive of groundwater or drainage-line silicification. The uniformity of sediments within all three types of silcrete indicates a common host, suggested to be the Upnor Formation of the Lambeth Group (Palaeocene). Deposition of the host sediment appears to have been followed by illuviation of clay-rich material via the primary fabric and along cracks to form geopetal caps, drapes and vein-like structures. During silicification this finer material has been replaced by less well-ordered silica whilst optically continuous quartz overgrowths characterize ‘purer’ areas. The silcrete was also affected by late-stage influxes of iron oxides, clay minerals and occasionally calcite. On the basis of distributional and other evidence it is suggested that silicification occurred during the Neogene or Quaternary, in association with acid leaching of Lambeth Group sediments probably under temperate conditions. The timing of silcrete formation in the eastern South Downs is thus much later than proposed for other parts of the UK.