Michael J. Barker

Palaeoenvironmental significance of storm coquinas in a Lower Cretaceous coastal lagoonal succession (Vectis Formation, Isle of Wight, southern England)

Thin bioclastic limestone beds (‘coquinas’) in the Vectis Formation (Wealden Group, Lower Cretaceous) of the Isle of Wight, southern England, exhibit a range of biofabrics and internal stratigraphies. These features are attributed to both simple and complex storm deposition of allochthonous biogenic and siliciclastic materials in coastal lagoons and on adjacent mudflats. These modes of deposition facilitated preservation of dinosaur trackways, desiccation cracks, shallow-tier trace fossils and in situ bivalve colonies through rapid burial. The coquinas thus preserve a record of surficial muds, commonly lost through reworking. The principal components of the coquinas comprise dispersed elements from within the argillaceous ‘background’ facies. Some of these beds are laterally traceable for up to 27 km, providing the foundations for a high-resolution event-stratigraphic framework.

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

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.

Stratigraphy, palaeontology and correlation of the Vectis Formation (Wealden Group, Lower Cretaceous) at Compton Bay, Isle of Wight, southern England

A stratigraphie section of the Vectis Formation (Wealden Group, Lower Cretaceous) at Compton Bay, Isle of Wight is presented, including lithological and palaeontological data. The sequence is relatively thin, indicating either attenuation towards the faulted northern margin of the Wessex sub-basin or proximity to an active growth fault associated with this structure. The succession is closely comparable to the Atherfield ‘type’ section in terms of lithofacies and fauna, enabling recognition of intrabasinal marker horizons. The salinity tolerances of molluscan fossils from the section are discussed and storm-generated bioclastic marker beds are figured.