A reappraisal of the stratigraphy and depositional development of the Upper Greensand (Late Albian) of the Devizes district, southern England

Abstract

Three members are recognized within the Upper Greensand Formation of the Devizes district on the basis of outcrop, newly acquired cored borehole and petrographical data. These are, in ascending stratigraphical order, Cann Sand Member, Potterne Sandstone Member and Easterton Sandstone Member. Compared to the imprecise historical subdivisions, the members provide a much clearer indication of lithological variation through the Upper Greensand and this, in turn, provides clues to its depositional development. The biostratigraphy of each member was determined using macrofossils and microfossils. The new biostratigraphical data clarify the relationship of the Potterne Rock to the traditionally named ‘Ragstone’, which caps the Shaftesbury Sandstone in the Shaftesbury district, and suggest that the correlation of the Potterne Rock and ‘Ragstone’ is less straightforward than suggested previously. There are some distinct contrasts with the stratigraphy of the Upper Greensand southwest of Devizes (Shaftesbury and Wincanton districts). Whilst tectonic influences have been demonstrated to affect coeval strata in parts of the eastern Weald, these may not be the dominant control on the Devizes succession, which seems to be influenced more strongly by its palaeogeographical setting with respect to sediment source areas, and the effect this had on the volume and timing of sediment infill. Palaeogeography may also be indirectly responsible for the absence of cherts in the Upper Greensand of the Devizes area, in contrast to their conspicuous development in the Upper Greensand of southwest England and the Weald.