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The lithostratigraphy of the Penarth Group (Late Triassic) of the Severn Estuary area

By R.W. Gallois


The name Penarth Group was introduced by the Triassic Working Group of the Geological Society to describe a laterally variable\ud succession of brackish and fully marine sedimentary rocks that form a transition from the terrestrial, red-bed facies of the Triassic\ud Mercia Mudstone Group to the fully marine conditions of the Jurassic Lias Group. The Penarth Group is well exposed in cliff and\ud foreshore outcrops near Penarth, South Glamorgan, on the Somerset coast between Blue Anchor and Lilstock, and on the upper\ud reaches of the Severn Estuary at Aust Cliff and Westbury-on-Severn. The group includes strata formerly known as the Westbury\ud Beds, Cotham Beds, Langport Beds, Watchet Beds and White Lias. The current nomenclature is in a confused state. The Westbury\ud Beds were renamed the Westbury Formation by the working group, and the Cotham Beds and White Lias were grouped together\ud to form the Lilstock Formation, in which the renamed Cotham Member is overlain by the Langport Member. In the Severn Estuary\ud area, the Langport Member comprises calcareous mudstones (former Watchet Beds), but throughout much of its outcrop and subcrop\ud it comprises limestones (former White Lias). A revision of the lithostratigraphical nomenclature is proposed in which the name\ud Lilstock Formation is abandoned and the Cotham Formation, Watchet Mudstone and White Lias are defined as lithologically distinct\ud formations that can be recognised throughout southern Britain. The Penarth Group is an attenuated succession that was deposited\ud in shallow-water brackish to marine environments. Each of the formations proposed here is bounded by erosion surfaces, and each\ud contains numerous sedimentary breaks. This needs to be taken into account in any discussion of possible global events, such as\ud sea-level changes, mass extinctions and bolide impacts, close to the Triassic-Jurassic boundary

Publisher: Ussher Society
Year: 2009
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