Molluscan palaeoecology and biostratinomy in a Lower Cretaceous meanderplain succession (Wessex Formation, Isle of Wight, southern England)

Abstract

Five categories of shelly sediment are documented from the upper part of the alluvial Wessex Formation (Barremian) on the Isle of Wight, southern England. They represent biostratinomic ‘windows’ within an essentially corrosive and intermittently depositional, warm to very hot meanderplain environment. The fauna is dominated by unionoid bivalves with rarer viviparid and physid gastropods. These molluscs are considered to have adopted similar life modes to their present-day freshwater counterparts. Thick, regularly spaced growth lines in some bivalves probably reflect alternating wet and dry phases and solution-pitted shells indicate acidic surface waters on the floodplains. Autochthonous concentrations characterize well-oxygenated pond or lake deposits and consist of Margaritifera preserved in their inferred life position. Such water bodies were subjected to wet-period sediment influx and disturbance and to dry-period desiccation. Poorly sorted muddy conglomerates representing allochthonous mudflow sediments contain closed, variably oriented unionoids, indicating their catastrophic exhumation and subsequent rapid burial during periodic floods. Channel sediments are poorly fossiliferous. Sand bodies containing viviparid gastropods are possibly crevasse splay deposits.