Penarth Group, Permo-Triassic, Bristol and Gloucester region
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
The Penarth Group is named after the coast sections at Penarth, South Glamorgan, and is virtually synonymous in a lithological sense with the former ‘Rhaetic’. The group contains the Westbury and Lilstock formations, comprising thin, very distinctive rock sequences uniformly developed throughout much of Britain.
The Westbury Formation takes its name from Garden Cliff, Westbury-on-Severn. It is better developed in the Central Somerset Basin where it reaches a thickness of nearly 14 m. Here, the Blue Anchor Formation is succeeded by dark grey to nearly black shale with thin beds of limestone and sandstone. The junction is usually a nonsequence. The colour change from grey and green to black is striking, and the presence of fragments or pebbles of the underlying rocks in the base of the Westbury Formation shows that the former suffered erosion prior to the deposition of the black shales.
Thin-shelled bivalves are abundant in some beds of the black shales and include Rhaetavicula contorta, Eotrapezium concentricum, Chlamys valoniensis and Tutcheria cloacina; small gastropods, fish remains and other fossils also occur. Thin limestones full of Chlamys valoniensis, Placunopsis alpina and other forms can be traced locally, and form useful but limited aids to correlation. In this respect, one of the most interesting beds is the Ceratodus Bone Bed, a conglomeratic sandy limestone a few centimetres thick, which passes locally into ginger-coloured sandstone. The rock is packed with vertebrate remains, mainly the scales, teeth and spines of Acrodus, Hybodus, Gyrolepis and other fishes, together with the bones and teeth of marine saurians such as Ichthyosaurus and Plesiosaurus. Locally, the most characteristic remains are the large palatal teeth of Ceratodus, a relative of the modern lung-fish. Except in the Bristol–Mendip area, where the lowest beds have been overlapped against the basement, this bone bed occurs at the base of the Westbury Formation. In the Central Somerset Basin, where the formation is thickest, up to four or five thin bone beds, separated by black shales, are developed in the lower part of the sequence. In the Wedmore area, the lowest part of the formation includes a hard, shell-fragmental limestone, the Wedmore Stone, 0.8 to 1.4 m in thickness, that was formally much used as a local building stone.
In the Mendip area and the country to the north, the erosion that preceded the deposition of the Westbury Formation was more severe than in the south, and the widespread submergence that followed it led to the deposition of fine, dark grey muds on an eroded surface composed of a wide variety of formations (P948977). Fragments of green and red Mercia Mudstone Group sediments, and insoluble residues such as rolled phosphatic pellets and quartz pebbles are abundant in the basal bone bed or, where this is missing, may be seen lying on the eroded surface beneath the black shales.
There is no extensive littoral deposit of the type found in Glamorgan, but at Butcombe, north of the Mendips, the rocks pass into a conglomerate packed with well-rounded pebbles of Carboniferous Limestone. A Mendip shoreline may have contributed to this deposit, and the occurrence of terrestrial vertebrates, including reptiles and early mammals in late Triassic deposits infilling fissures in the Carboniferous Limestone of the Mendips area proves the existence of one or more island land areas at this time.
The Lilstock Formation comprises a lower argillaceous unit, the Cotham Beds (now Cotham Member) and an upper dominantly limestone unit, the Langport Beds (now Langport Member); the latter is synonymous with the White Lias sensu William Smith in the Bristol district.
A nonsequence marks the junction of the Westbury Formation and the overlying Cotham Member. The latter consists of soft, greenish grey, silty, calcite mudstones at the top of which, in the Bristol district, lies the well-known Cotham Marble. The total thickness of the member is rarely more than 2 m. The location of the Cotham Member, and to some extent of the Westbury Formation also is, however, of importance to the engineer since these rocks have low bearing strength, particularly when weathered.
The fauna of the Cotham Member is generally impoverished, and in west and central Somerset very few fossils are to be found, except at the base. North of the Mendips it is slightly less barren and a thin bed of banded calcite mudstone in the lower part of the member has both plant- and shell-bearing layers. This, the Naiadita Bed, yields Naiadita lanceolata, a fresh- or brackish-water liverwort (Bryophyta) and its associated spores, together with algae, insect larvae and occasional specimens of the crustacean Euestheria minuta. The occurrences of Euestheria, of plants and of marine shells, including Chlamys valoniensis and other bivalves, are usually in separate distinct beds. Mud-cracks and, more rarely, ripple-marks and worm-tracks, also occur in the Naiadita Bed.
The Cotham Marble takes its name from the type locality at Cotham, Bristol. It is a hard, splintery calcite-mudstone with an irregular mammillated top and a smooth flat base. In Gloucestershire it passes locally into a hard, fissile limestone with Meleagrinella fallax, but elsewhere animal fossils, apart from fish scales, are rare. The bed is seldom more than 0.2 m thick and is notable for the tree-like markings with rounded cloud-like forms above, which are seen when the rock is broken in a vertical plane; hence the popular name ‘Landscape Marble’. In the past it was used as an ornamental stone, both cut and polished for indoor use and, undressed, for outside walls and ornamental rockeries. It is now generally agreed that the ‘Landscape’ is an association of algal growths occurring in convex masses. Other types known as ‘False’ or ‘Crazy’ Cotham Marble were formed by the penecontemporaneous breaking up of partly consolidated calcite mud; these contain shell detritus including bivalves and echinoderm fragments, and may occupy channels between the algal ‘buns’ as lag deposits.
The Langport Member, or the White Lias as it has long been known in this district, consists of pale grey and cream limestone with mudstone partings. The limestones tend to be irregular and rubbly in the lower part of the succession and finer grained, even bedded and very hard in the upper part. The upper beds have been widely quarried in the Somerset Coalfield for use as a building stone. The top bed, locally called the ‘Sun Bed’, may show signs of emergence in the Somerset area, where it is typically penetrated from above by U-shaped tubes. The member has a restricted fauna of bivalves, gastropods and locally abundant ostracods, mainly in the lower part; echinoid fragments and simple corals occur sporadically. Some bivalves, such as Modiolus and Protocardia, are common in the underlying beds, but a few genera including Astarte, Plagiostoma and Pleuromya make their earliest regional appearance in the White Lias. Most of the genera and species persist into the Lower Lias. Small fronds and leaves, including those of the bennettitalean plant Otozamites obtusus and miospores of pteridophytes and gymnosperms, represent the land flora. The fauna indicates that the sea was very shallow, with the sea bed being intermittently exposed.
The Langport Member is thin or absent north and west of a line joining Stratford-on-Avon to Bristol thence running southwards to the Mendips and turning westwards along the northern margin of the Central Somerset Basin. At or near outcrop, the thickness varies from just under a metre to rather more than 5 m, but scattered borehole evidence suggests that it thickens eastwards and south-eastwards down dip to 8 m or more within a gulf extending northwards beneath the Cotswolds and southwards to the Wessex Basin. The thickness variations are due partly to depositional causes and partly to the effects of pre-Liassic and intra-Liassic erosion in different parts of the district. The remaining part of the Triassic System, which comprises the lowest few metres (at the most) of the Lias, is considered in the next chapter.