The working group divided the Penarth Group into two formations, the Westbury Formation and the overlying Lilstock Formation named after a locality on the north Somerset coast.
The limestone facies is represented by a single thin 0. These sections are separated by outcrops of younger and older strata over a distance of 5 km of coastline. It is not surprising, therefore, given the shallow- water nature of the sediments, that the succession at all three localities differs in detail.
Geoscience in South-West England,12, The name Penarth Group was introduced by the Triassic Working Group of the Geological Society to describe a laterally variable succession of brackish and fully marine sedimentary rocks that form a transition from the terrestrial, red-bed facies of the Triassic Mercia Mudstone Group to the fully marine conditions of the Jurassic Lias Group.
The current nomenclature is in a confused state. In the Severn Estuary area, the Langport Member comprises calcareous mudstones former Watchet Beds , but throughout much of its outcrop and sub- crop it comprises limestones former White Lias. The Penarth Group is an attenuated succession that was deposited in shallow-water brackish to marine environments. Each of the formations proposed here is bounded by erosion surfaces, and each contains numerous sedimentary breaks.
This needs to be taken into account in any discussion of possible global events, such as sea-level changes, mass extinctions and bolide impacts, close to the Triassic-Jurassic boundary. These descriptions did not, however, clarify the Triassic Working Group nomenclature, but simply highlighted the problem of applying it to the successions exposed in the Severn Estuary area. The aims of this paper are to describe and correlate the Penarth Group successions exposed in the Severn Estuary area and the adjacent regions, and to use the results to propose a lithostratigraphical nomenclature that is applicable to the whole of the UK outcrop and subcrop.
At the base of the group the Westbury Formation is lithologically relatively uniform throughout its onshore outcrop. The present account concentrates, therefore, on the lithostratigraphy of the Cotham Beds, White Lias and Watchet Beds and their replacement, the Lilstock Formation.
Gallois 72 Figure 1. Distribution of the Triassic rocks of the Severn Estuary area showing the positions of the principal outcrops and localities referred to in the text. Evolution of the nomenclature of the Penarth Group. All three formations are lithologically distinctive, and are separated by erosion surfaces. Comparison of the Devon and inland successions with those exposed on the Severn Estuary coast in Somerset and South Glamorgan has shown that the Westbury and Cotham formations remain lithologically relatively unchanged, but that the limestones of the White Lias Formation either pass laterally into, or are much reduced in thickness, and are overlain by calcareous mudstones.
These last, the Watchet Beds of Richardson , are described here under the proposed new name Watchet Mudstone Formation. The proposed type sections and a selection of reference sections are shown in Table 2.
The formation crops out discontinuously between the Devon coast near Lyme Regis and the North Yorkshire coast near Redcar, and it has an extensive subcrop in the adjacent onshore and offshore areas.
The mudstones maintain their distinctive lithological character over the whole of its outcrop. Garden Cliff, Westbury-on-Severn is proposed as the type section for historical reasons. There are detailed published descriptions of the full thickness of the formation exposed at all the principal exposures in the Severn Estuary area including those at Westbury Garden Cliff Richardson, ; Benton et al.
In south-west Britain the formation rests non-sequentially on strata that range in age from Carboniferous to late Triassic Wilson et al. At the type section and elsewhere in the Severn Estuary area, the base of the formation is taken at an erosion surface that separates green and grey thinly and thickly bedded silty mudstones and siltstones at the top of the Blue Anchor Formation from overlying laminated dark grey pyritic mudstones Warrington et al.
The thickest succession recorded in the Severn Estuary is that at Blue Anchor where Richardson described The Cotham Formation is no longer well exposed in the type area, but it remains wholly exposed in the cliffs and foreshores at numerous localities in the Severn Estuary area Table 2.
It has also been wholly exposed from time to time at Culverhole on the Devon coast and, less commonly, at nearby Pinhay Bay. Inland, the mudstones rapidly weather to clay and natural exposures are rare.
The full thickness has been exposed in excavations in Avon and Somerset, mostly in road and railway cuttings Rendle Short, ; Richardson, This is an extensive exposure [ST to ] on a wave-cut platform that includes all the principal lithological and sedimentary features that characterise the formation in the Severn Estuary area. The published descriptions of the formation exposed in the Severn Estuary area are the same as those for the Westbury Mudstone.
All of these placed the base of the formation at an erosion surface that marks an upward lithological change from dark grey mudstones to greenish grey mudstones.
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In most areas the formation comprises 2 to 4 m of greyish green mudstone with thin interbeds of limestone and sandstone that were deposited in shallow and very shallow brackish-water and restricted-marine environments. The proportion and composition of the non-mudstone component varies from area to area. In the Severn Estuary area, the formation can be divided into two parts separated by a prominent desiccated erosion surface D in Figures 3, 5 and 6.
The lower part, here named the Lower Cotham Member, comprises lenticular-bedded greenish grey mudstones and siltstones with dewatering structures, and slump structures that locally involve all or part of the beds below the erosion surface. The slumped beds are laterally impersistent over distances of tens to hundreds of metres, are not all at the same stratigraphical level with respect to the base and top of the member, and are not present at every locality Figures 3 and 5.
On the Glamorgan coast the whole of the Lower Cotham Member and the top part of the Westbury Mudstone is disturbed by slump and dewatering structures. Similarly, at Lilstock, disturbed beds in the upper part of the Lower Cotham Member pass laterally into undisturbed beds over a distance of m.
Rippled calcareous sandstones and calcarenites are present at one or more different levels in the Cotham Formation in the Bath-Bristol, inland Somerset and Devon areas. In the absence of palaeontological evidence, it is not known if these were deposited contemporaneously. At Culverhole, Devon, the most prominent sandstone bed and desiccation surface lies at the base of the formation Gallois, The upper part of the Cotham Formation in the Severn Estuary area contains laterally variable amounts of tabular bedded and nodular muddy limestone and porcellanous limestone within a predominantly mudstone succession.
One of the limestone beds is locally lithologically similar to parts of the White Lias, with the result that different authors have placed the upper boundary of the Cotham Formation at different stratigraphical levels. This bed has R.
Gallois 74 Figure 2. Examples of Penarth Group lithologies in the Severn Estuary area.
The cliff is about 30 m high: The cliff is about 7. Lenticular bedded greenish grey Lower Cotham Formation mudstones and siltstones rest with sharp lithological contrast on an irregular surface cut into dark grey Westbury Mudstone.
View east towards Barry Island and town. The cliff is about 20 m high: Photograph uncorrected for parallax. The base of the sandy Upper Cotham Member is marked by a desiccation surface. Lithostratigraphy of the Penarth Group 75 a widespread distribution in the eastern part of the outcrop between Bath and the east Devon coast, but the distribution is patchy on both local and regional scales.
Cotham Marble is well developed at Aust Cliff, but has not been recognised in the sections south of there on the Somerset and Glamorgan coasts. In the present work the top of the formation has been taken at an erosion surface at the top of a prominently bioturbated limestone hardground. This marks a sharp upward change to a porcellanous limestone that is lithologically similar to parts of the White Lias Formation. According to Arkell and Tomkeieff the word lias was adopted from the Old French word liois, meaning a compact type of limestone.
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The name White Lias remained in geological use until when it was abandoned by the Triassic Working Group. Proposed type and reference sections for the Penarth Group formations. Gallois 76 Figure 3. Limestones of the sort that characterise the White Lias of the Bath area crop out discontinuously between the east Devon coast and Nottinghamshire Donovan et al.
In Devon Gallois, and inland Somerset Richardson, , the junction with the Cotham Formation appears to be everywhere marked by an irregular erosion surface, with or without Cotham Marble.
In the Severn Estuary area, where the bulk of the limestone passes laterally into calcareous mudstone, the White Lias is represented by a single thin bed mostly 0. The White Lias exposed on the Devon coast contains channels, slumps and fractured hardground surfaces that are atypical of the formation as a whole Gallois, Almost all published accounts of exposures of White Lias in southern England describe the top of the Lithostratigraphy of the Penarth Group 77 Figure 4.
Examples of Penarth Group lithologies in Somerset and Devon. Arrow indicates axis of NW plunging fold. The highest bed of the Cotham Formation C10 is a highly bioturbated limestone with abundant Thalassinoides burrows that is capped by a mineralised hardground surface.
The White Lias is represented by a single bed of porcellanous limestone throughout the Somerset and South Glamorgan coastal sections. The Watchet Mudstone at Lilstock Bay is a bioturbated, shelly calcareous mudstone with laterally variable beds of muddy limestone. The junction with the overlying Blue Lias is conformable. Numerous Diplocraterion burrows descend from the unconformity surface. The highest limestone bed of the White Lias is overlain by up to 40 mm of bioturbated, shelly calcareous mudstone that is lithologically similar to that of the Watchet Mudstone.
This bed is conformably overlain by laminated organic-rich mudstones at the base of the Blue Lias. Gallois formation as a desiccated erosion surface from which characteristic Diplocraterion borings descend the Sun Bed auctt. In the Severn Estuary area, the White Lias is represented by a single thin bed 0.
In inland Somerset, in the Langport area, White Lias limestone is locally overlain by a thin bed of bioturbated shelly mudstone Figures g, h, i. He noted that the marls were distinct from the limestones of the White Lias and that they decreased in thickness eastwards as the limestones increased in thickness in the same direction. However, at some localities notably Lilstock Bay Richardson included part of the overlying laminated organic-rich mudstones in the Watchet Beds. The beds for which the name Watchet Mudstone Formation is proposed here are lithologically distinct from the White Lias and from the laminated organic-rich mudstones of the basal Lias.
The formation is thicker on the South Glamorgan coast than on the Somerset coast, but the name Watchet is retained for continuity with a proposed type section at Doniford Bay, Watchet. The Watchet Mudstone comprises thinly and thickly bedded, pale and medium grey calcareous mudstones and silty mudstones with variable carbonate and shell mostly bivalve contents. On the Somerset coast, the formation thickens westwards from 0.
The sections at Lilstock Bay show marked lateral variations in lithology over a distance of about m with one or more sedimentary breaks. They include porcellanous limestone, calcareous mudstone and ripple-marked calcareous sandstone and siltstone. The more westerly successions on the Somerset coast consist of calcareous mudstone with variable amounts of muddy limestone.
In South Glamorgan, where the formation is up to 2. The boundary is planar at all the localities examined. It has been interpreted as a change from an aerobic to a dysaerobic or anaerobic environment Hesselbo et al. In southern Britain these rest with lithological contrast either conformably on the Watchet Mudstone this paper or disconformably on the White Lias Donovan and Kellaway, The continuous cliffs between Pinhay Bay and Monmouth Beach, Lyme Regis, permanently expose the full thickness of a Blue Lias succession that is lithologically similar to that at the Bath type section.
It should be considered as a possible replacement type section.
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