Regional studies in the sedimentology, mineralogy and geochemistry of the Penarth group ("Rhaetic") of Britain
The Upper Triassic, Penarth Group has been examined, throughout Britain, both at outcrop and in boreholes. It is divided, lithostratigraphically into a lower, Westbury Formation and an upper, Lilstock Formation. Within these units a number of regionally-and locally-applicable members are distinguished. The Penarth Group is a predominantly marine, sedimentary sequence transitionally situated between arid or semi-arid lacustrine and fluviatile sediments (Mercia Mudstone Group and New Red Sandstone) and more fully marine strata (Lias). Early stages of the "Rhaetic" marine incursion, principally represented by the Westbury Member, document the establishment of semi-isolated, brackish basins which were superseded by a less restricted marine environment. This transgressional advance is linked to nutrient supply and thence to phytoplankton productivity in order to explain organic carbon profiles through the Westbury Member, Offshore and nearshore zones of this sea are recognised, the latter including sediments assigned to barrier-inlet, lagoon and tidal=flat environments. Clay mineral studies reveal an enrichment of smectite in offshore areas which is ascribed to differential sedimentation processes. Transgression was interrupted by a regressional phase marked by the lagoonal sediments of the Gotham Member. A shift in the zone of smectite deposition can be related to this regression. The inferred shelf-lagoon carbonates of the Langport Member reflect a renewal of transgression. A local influx of sediment-laden fresh- water is believed to have been responsible for the local change to the Watchet Member Facies. Deeper-water intermittently oxygen-deficient, marine environments are envisaged for the Pre-planorbis Beds. These spread across the Langport Member shelf-lagoon, as a result of continuing transgression, culminating in the widespread establishment of deeper-water conditions in the Lower Jurassic. An increase in kaolinite relative to smectite in these younger strata, reflects increasingly humid climatic conditions. A regional correlation of the lithostratigraphica.1 subdivisions is attempted leading to a series of palaeogeographical reconstructions.