Silurian, Bristol and Gloucester region
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
Rocks of Llandovery to Ludlow age occupy a number of small inliers extending from north of the Severn near Newnham southwards to the eastern Mendips (P948958). The most extensive, and best known, is on the northern margin of the Bristol Coalfield in the Tortworth area. Although rocks of Silurian age have so far been proved only at depth under the Cotswolds, they are presumed to be present under much of the remaining part of the district. The rocks fall into four main divisions:
- Pridoli (formerly Downton) Series (part)
- Ludlow Series
- Wenlock Series
- Llandovery Series
- Pridoli (formerly Downton) Series (part)
The lower three series form a natural grouping of marine strata which, until relatively recently, comprised the Silurian system as understood in Britain. However, international agreement in 1972 to define the base of the overlying Devonian System at an horizon coincident with the base of the Monograptus uniformis graptolite biozone has meant that part or all of the overlying Pridoli Series must now also be included in the Silurian. No graptolites are, however, known from the Pridoli of Britain and no consensus of opinion has yet emerged as to where the top of the Silurian should be placed within the series.
The Llandovery–Ludlow rocks of this district comprise shallow-water, arenaceous and argillaceous, marine sedimentary rocks with some limestone. They accumulated in the southern part of a wide, intermittently, but gently subsiding shelf region that separated the rapidly subsiding Welsh Basin to the north-west from a land area called the Midland Block that lay to the east and the south. An important volcanic episode occurred in Somerset during the Wenlock.
The post-Ludlow strata of Silurian age fall within the lowest part of the Old Red Sandstone. Classically, in this country, the Old Red Sandstone has been accorded system status synonymous with the Devonian. In this district the Old Red Sandstone forms a natural grouping of continental facies sedimentary rocks, which, because it is largely Devonian in age, will be treated as a whole in the next chapter.
Widespread marine transgressions occurred in mid and late Llandovery times in Britain. The Llandovery Series of the type area at Llandovery has been divided, on the basis of brachiopod assemblages and lineages, into substages of which six, informally symbolised as C1 to C6, refer to the late Llandovery. The course of the marine transgressions across the shelf areas may be followed by the aid of these substages. In the region only C5 and C6 are present, represented by the Damery Beds and Tortworth Beds respectively. The strata, with a thin basal conglomerate, rest with slight angular discordance on the Tremadoc Series with no trace of Ordovician strata intervening. Relative, but not absolute water depths of deposition in the Llandovery and Wenlock, and hence bathymetric conditions, have been inferred by the study of marine, mainly brachiopod shelly communities in the Welsh Borderland areas (e.g. Hurst et al., 1978). By this and sedimentological criteria, it has been possible to deduce in general terms that the shelf seas deepened through late Llandovery times in the region, and that the Wenlock was heralded by marked shallowing followed by more variable conditions, leading to marked deepening again in early Ludlow times. Shallowing through the Ludlow led eventually to the establishment of continental conditions in the Pridoli.
The most complete succession of Silurian rocks has been established in the Tortworth and immediately adjacent Charfield inliers (P948959), though exposures are very limited. These rocks have long been recognised as Silurian, and Curtis (1972) has given an account of the history of research; modern knowledge mainly stems from his work and that of Cave (1977). The general succession is as follows:
|Wenlock Series||Brinkmarsh Beds
Mudstone and sandstone
Mudstone and sandstone
up to 30
|Llandovery Series||Tortworth Beds
Mudstone and sandstone
Upper Trap: basalt
Mudstone and sandstone
Lower Trap: basalt
The Llandovery sedimentary strata thicken westwards and the thickest sequence, about 500 m, is present in the north-western part of the inlier around Stone. Conversely, the igneous strata are absent in the west but thicken south-eastwards (P948959).
The Lower Trap and Upper Trap
These volcanic rocks were originally thought to be intrusions, but the balance of evidence now favours an extrusive origin. They are both contaminated enstatite-bearing basalts, with or without olivine. Both may be strongly amygdaloidal and are much altered, with plagioclase completely or partially albitised, enstatite replaced by bastite and olivine altered to chlorite and opaque oxides. In the Upper Trap the presence of quartz xenocrysts, which are clearly visible with a hand lens, usually provides a ready means of distinguishing the two flows in the field.
The Damery Beds consist of weakly calcareous, thin-bedded, sandstone, siltstone and mudstone, with occasional layers of impure limestone, which are usually associated with fossil-rich bands. The colours range from grey to green, with ?Triassic staining giving rise to reds and purples. For mapping purposes it is usually possible to subdivide the strata into thick members, each dominated by either sandstone or mudstone. Fossils are abundant throughout, with brachiopods, often in great numbers, being by far the commonest forms. Crinoid columnals, corals and trilobites are plentiful; gastropods and bivalves less so. No faunal subdivisions of the Damery Beds have been established, but there is a well-marked change in the composition of the fauna towards the top, thought to be related to increasing water depth. In the lower part of the Damery Beds the brachiopod Eocoelia curtisi is abundant, accompanied by small rhynchonellids. These forms continue into the middle and upper parts of the formation but become less conspicuous. Other forms, like Leptaena rhomboidalis, Costistricklandia lirata alpha, Atrypa reticularis and Howellella anglica become more numerous. Some species such as Leptostrophia compressa and the trilobite Dalmanites weaveri are fairly common throughout the succession.
The Tortworth Beds are less well known than the underlying strata. They appear to be of generally similar lithology, but tend to be less fossiliferous. Commonly present at the base is the Palaeocyclus Band, a thin decalcified sandstone characterised by the button coral Palaeocyclus porpita, the tabulate coral Favosites, abundant brachiopods and Dalmanites weaveri. Where the Upper Trap is present there is commonly a metre of ashy limestone with a large fauna, including abundant tabulate corals intervening between the top of the lava and the Palaeocyclus Band. The fossils from the lower horizons indicate the Coslistrietlandia community (that is a group of animals living in the same habitat), while higher levels probably contain representatives of the shallower water Eocoelia community.
The Brinkmarsh Beds, of Wenlock age, consist predominantly of grey mudstone with thin layers and beds of siltstone and fine-grained sandstone. The sandstone is calcareous and the mudstone may occasionally include layers of calcareous nodules. The sandy beds may show ripple marks, cross-bedding, drag marks and ‘ball-like’ load structures. The succession includes three units of argillaceous limestone which are usually crinoidal and very fossiliferous. They are thin-bedded, rubbly in texture and grey in colour, except where stained red due to proximity of the Triassic rocks. The limestone units give rise to ridges in the otherwise somewhat subdued topography determined by the intervening mudstone.
The base of the Brinkmarsh Beds is taken at the base of the lowest limestone unit or, where this is absent, beneath a series of red-stained calcareous sandstone beds containing abundant crinoid columnals, apparently a local facies variation of the limestone. The basal limestone unit and its associated sandstone appear to be practically continuous throughout the inlier, in contrast to both the middle and upper limestone units, particularly the latter, which tend to be lenticular in development. The lowest limestone unit is clearly equivalent, in part at least, to the well-known Woolhope Limestone of the areas to the north, but the others appear to be local developments not represented elsewhere. The highest exposed Wenlock strata are in the Milbury Heath area, at the southern end of the inlier, where they are unconformably overlain by rocks of late Devonian age. They contain a fauna that is common to both the Wenlock and Ludlow elsewhere.
The total thickness of exposed Brinkmarsh Beds approaches 300 m, but due to the late Devonian unconformity the full succession is not present at outcrop. It is probable that the unexposed strata at the top of the succession lie beneath rocks of Ludlow age to the north-west of the inlier and contain a representative of the Wenlock Limestone, so widespread to the north.
A narrow, faulted outcrop of rocks of Ludlow age extends inland from Tites Point for some 5 km on the downthrow (western) side of the north-south-trending Berkeley Fault to within 2 km of the Silurian outcrops of the Tortworth inlier. On the foreshore at Tites Point, 22 m of ‘Whitcliffe Beds’ overlying perhaps a further 11 m of ‘Leintwardine Beds’ are exposed in a faulted, plunging anticline. These rocks, of Ludfordian age, are overlain by strata attributed to the Downton (now Pridoli) Series (Cave and White, 1971) on both sides of the anticline.
About 2 km inland, the Brookend Borehole proved a nearly complete Ludlow succession beneath the Pridoli. The formational names used in it are derived from the type Ludlow area, with which correlation was made mainly by means of the abundant shelly faunas, dominated by brachiopods. The dip of bedding in the borehole varies from an average of 56° above 177 m depth to 35° below, and the apparent thicknesses given in the borehole log have been corrected to give true thicknesses, calculated to the nearest metre and shown in brackets after the borehole thicknesses. A total of about 160 m of Ludlow strata was proved. The succession is as follows:
The lowest strata contain a fossil assemblage indicative of deep-shelf conditions and thought to be characteristic of the lowest part of the Ludlow Series. The succeeding rocks reflect progressive shallowing of the shelf sea, with depositional pauses marked in the ‘Leintwardine Beds’ by conglomerates with bored limestone pebbles, believed to represent hard-grounds. Sporadic layers of bentonite throughout the successions appear to indicate volcanic activity, though the sediment source is not known at present. The Downton Castle Sandstone testifies to the final shallowing of the sea before the onset of dominantly continental conditions represented by the overlying Thornbury Beds.
A small inlier of Ludlow rocks is situated about a kilometre south-west of Newnham, north of the River Severn. The beds, seen in two old quarries, are inverted and dip to the east between 59° and 75°; Cave and White (1971) found that the best section was in the southernmost quarry. This showed some 20 m of strata consisting of 11 m of ‘Whitcliffe Beds’ and the uppermost 9 m of the ‘Leintwardine Beds’. The rocks are similar to those already described in the Sharpness inlier to the south.
West of Wickwar, in the valley of the Little Avon, there are small exposures of Wenlock strata showing dips of around 30° in a general south-westerly direction. The rocks consist of pink dolomitic siltstone and argillaceous limestone. An adjacent shaft proved about 8 m of very fossiliferous limestone.
To the north-east of Shepton Mallet, Silurian rocks form a narrow belt in the core of Beacon Hill Pericline and are overlain with low angular unconformity by late Devonian (Old Red Sandstone) strata. In the middle of the inlier the rocks are exposed in several large quarries where the strata are vertical or very steeply dipping and young to the north. The area has recently been described in detail by Hancock (1982), who disagreed with the interpretation of the structure as an anticline and gave a complete succession across the inlier as:
|8||Tuff and bedded agglomerate||20–29|
|6||Tuff with red and green mudstone||18|
|4||Tuffs, locally fossiliferous, sandy tuffs and red mudstones||105–135|
|1||Fossiliferous Wenlock shales||95|
The total thickness is about 600 m. It has now been shown that the fossiliferous Wenlock shales are the oldest part of the succession, not the youngest as had hitherto been thought. They have been dated by the brachiopod Eocoelia angelini as being of early Wenlock age. Shallowing of the sea during the succeeding volcanism is suggested by the shelly assemblages present in Bed 4, and there is evidence for both subaerial and submarine eruption in different parts of the succession. The lavas are massive flows of highly altered pyroxene-andesite. Lithologically, the pyroclastic rocks range from fine-grained, well-bedded, water-lain tuffs to massive, coarse agglomeratic beds with boulders and bombs of lava up to 30 cm across; colours range from blue-black to green and red, but with shades of buff and brown predominating. A large agglomerate mass that cuts all the members of the succession to the east of the main quarries is interpreted as a vent-fill.
Batsford (Lower Lemington) borehole
The extent of the Silurian rocks beneath the Upper Palaeozoic and Mesozoic cover east of the inliers is not known but they were found in the Batsford Borehole, situated 2 km south-east of Moreton-in-Marsh (Williams and Whittaker, 1974). This borehole proved more than 52 m of limestone, shale and sandstone, with dips of 10° to 49°, beneath gently dipping Coal Measures (see also Other coal measures). The shelly fauna (Substage C6) indicates a late Llandovery age.
- Cave, R. 1977. Geology of the Malmesbury district. Memoir of the Geological Survey of Great Britain.
- Hurst, J M, Hancock, N J, and McKerrow, W S. 1978. Wenlock stratigraphy and palaeogeography of Wales and the Welsh borderland. Proceedings of the Geologists’ Association, Vol.89, 197–226.
- Curtis, M L K. 1972. The Silurian rocks of the Tortworth Inlier, Gloucestershire. Proceedings of the Geologists’ Association, Vol.83, 1–35.
- Cave, R, and White, D E. 1971. The exposures of Ludlow rocks and associated beds at Tites Point and near Newnham, Gloucestershire. Geological Journal, Vol.7, 239–254.
- Hancock, J M. 1982. Stratigraphy, palaeogeography and structure of the East Mendips Silurian inlier. Proceedings of the Geologists’ Association, Vol.93, 247–262.
- Williams, B J, and Whittaker, A. 1974. Geology of the country around Stratford-upon-Avon and Evesham. Memoir of the Geological Survey of Great Britain.