Stratigraphic relations of the Lower Greensand (Lower Cretaceous) of the Calne area, Wiltshire

The Lower Greensand (Aptian-Albian) around Calne, lies on Jurassic Lower Kimmeridge Clay (mid-Aulacostephanoides mutabilis Zone) and comprises a lower, relatively clean, fine to medium grained sand, overlain by iron-rich poorly-sorted, medium to coarse grained sandstone. The upper beds are identified as the Carstone, but the lower sands are lithologically different from the closely adjacent Lower Greensand occurrences on the margins of the Wessex Basin. They are here named the Calne Sands Formation of the Lower Greensand Group (equivalent to part of the Folkestone Beds Formation of SE England). The Calne Sands form an areally-restricted sequence with sand-grade sediment coarsening upwards and a concomitant increase in clay content. Palynological evidence indicates that they are referable to the Hypacanthoplites jacobi Zone. The age and sedimentary facies contrast with the nearby Seend Ironsand (Parahoplites nutfieldiensis Zone). Both occurrences however are interpreted as transgressive deposits infilling NW-SE palaeovalleys cut into Jurassic basement. -from Author