Lower Carboniferous (Dinantian), its classification and sedimentation, 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 Dinantian rocks of this district have long been known as the Carboniferous Limestone Series. Used in a lithostratigraphical sense, Carboniferous Limestone remains acceptable as a collective name for this group of rocks.
The Carboniferous Limestone has the most extensive outcrop of any Palaeozoic rocks within the district and forms much of the high ground and the most striking scenery. In the north of the district, its outcrop (P948961) surrounds the Forest of Dean Coalfield, except in the south-east, where it is concealed by an overstep of the Coal Measures and extends south-westwards to Chepstow in the narrow Tiddenham Chase Syncline, and then westwards to Magor in the broad Caerwent Syncline. Carboniferous Limestone rims the northern part of the Bristol Coalfield, extending from Over north-eastwards to Tortworth and thence southwards to Chipping Sodbury. Along the strike south of here, it is concealed by newer rocks, except in small inliers near Codrington and Wick.
West of Bristol, Carboniferous Limestone forms the high ground, cut by the Avon Gorge, extending from Penpole Point through King’s Weston to Durdham Downs and thence by Failand to Clevedon. It also gives rise to the Clevedon–Portishead ridge. Carboniferous Limestone constitutes the dome-like structure of Broadfield Down and, farther south, is the principal rock group in the Mendip Hills, with an outcrop extending 50 km from Frome westwards to Brean Down. Beyond there, it continues out to sea in the islands of Steep Holm and Flat Holm.
Except for small inliers protruding through the Mesozoic cover under the southern lee of the Mendips, and the isolated inlier of Cannington Park near Bridgwater, no Carboniferous beds are exposed south of the Mendips in the region.
The Geological Survey officers who carried out the original survey of the district, and also subsequent workers, notably Wethered, working in the Forest of Dean, and Lloyd Morgan, in the southern part of the Bristol Coalfield, classified the Carboniferous rocks on a lithostratigraphical basis. This method was abandoned following Vaughan’s classic paper on the Avon Gorge section in 1905. Vaughan grouped the Lower Carboniferous rocks into the Avonian Series, which he sub-divided into five zones based on what he considered were evolutionary lineages, especially of corals. The zones were symbolised by the letters K, Z, C, S and D. Application of this zonal scheme by Vaughan and many other workers was undertaken with enthusiasm over much of Britain in the succeeding decades, but in the process, both the underlying concept and the zones themselves underwent extensive alteration in an attempt to achieve consistency between different areas. When the district was remapped by the Geological Survey, starting in the Forest of Dean in 1933, Vaughan’s scheme was abandoned and replaced by a new, lithostratigraphical nomenclature (Kellaway and Welch, 1955).
Following pioneer work on the early Carboniferous rocks by Dixon and later workers, mainly in South Wales, the concept of ‘bathymetric cycles’ (i.e. cyclical alternations of shallower- and deeper-water sedimentation) evolved. This was developed by Ramsbottom in several papers between 1973 and 1976. He proposed a classification based on six major transgressive events, separated by periods of regression. These were recognised on the basis of lithology and fossil content. Ramsbottom’s major cycles approximately coincide with stages proposed by the Dinantian Working Group of the Geological Society of London (George et al., 1976) because each major transgression was accompanied by the migratory faunas that are used to recognise the different stages. These stages have been defined in terms of type sections (stratotypes) and are applicable only in Britain. The application of the different classification schemes to the Avon section (P948962) shows the close relationship between conditions of deposition, lithology and faunal content of the strata.
The open sea, which during late Devonian times lay to the south and east of the district, spread over the whole area during earliest Carboniferous times. In the Mendips, the change from continental to marine conditions was abrupt, whereas to the north and south, passage beds, in which Old Red Sandstone facies rocks are interbedded with marine shale and limestone, indicate a period of fluctuating shorelines. After this relatively short initial period, however, the low-lying Old Red Sandstone alluvial plains were submerged by the sea, leaving a land mass, known as the St George’s Land–Brabant massif, stretching from Ireland across Britain to Belgium, some distance to the north of the present region. It was in the tropical shelf seas bordering this massif that the Carboniferous Limestone was deposited. At first, much terrigenous material, represented by parts of the Lower Limestone Shale, was carried into the sea and deposited, but with the change to a high energy environment, deposition of fine clastic material ceased, the sea water cleared and carbonate sedimentation became dominant, both by the accumulation of the hard parts of the teeming marine life and by chemical precipitation.
The general east–west trend of the land mass to the north was paralleled by an elongated sedimentary prism in the form of a southwards-sloping carbonate ramp (P948963). The sediments deposited on the ramp are mainly of shallow-water facies; epeirogenic movement, either uplift to the north or depression to the south, and eustatic effects played an important part in determining sediment type and distribution.
Over most of the area, deposition was on broad flats submerged to only a few metres; thus minor changes in sea level or in the rate of elevation or of depression caused large effects. Sometimes, large areas of the carbonate ramp were exposed and eroded, or inundated by clastic material derived from the St George’s Land–Brabant massif.
Interpretation of palaeobathymetric facies has to be undertaken with caution since similar rock types can be deposited in more than one environment, and different rock types can be deposited at the same water depth. Furthermore, material accumulated in one environment can be swept into another to its final depositional resting place. Nevertheless, after making allowances for these complicating factors, some generalisations can usefully be made. The shallowest facies are considered to be the calcite- and dolomite-mudstones (‘chinastones’ of earlier accounts). Such rocks are thought to represent deposition under peritidal conditions. Where they include abundant stromatolitic algal mats and little other sign of life, they are interpreted as supratidal or intertidal in origin. Similar rocks with a very sparse shelly fauna, though locally with immense numbers of a single brachiopod species, have been interpreted as lagoonal deposits. Nowadays, this term is restricted to deposits laid down behind an offshore barrier, and it has been suggested (Wilson et al., 1988) that the oolite shoals that were forming contemporaneously farther south provided such a barrier (e.g. the Burrington Oolite of the Mendip area).
The carbonate mudstones are extensively recrystallised and opinion remains divided as to how much of the lime mud represents finely comminuted organic mainly algal, debris and how much was chemically precipitated in the very shallow warm water. The dolomite in these and other rocks may be primary or secondary in origin, having been formed as a result of conditions of intense evaporation of either groundwater or very shallow sea water.
Another distinctive facies is oolite, which represents chemical precipitates formed in strongly agitated shallow water, probably no more than 5 m in depth. Such high energy conditions occur, for instance, in channels and on shoals, bars and underwater deltas, both within and outside the lagoonal areas. Cross-bedding is often conspicuous in these rocks, but the macrofauna is sparse due to the unstable substrate. Associated with the oolites, grainstones formed of micritised pellets and various intraclasts are widespread and probably represent the winnowed remnants of the muddier sediments. More open-sea or deeper-water conditions are represented by massive, pale grey, bioclastic limestones ranging from well-sorted grainstone, often finely cross-bedded, to less well-sorted packstone, often strongly bioturbated. The energy regimes range from high to medium. Crinoidal debris is ubiquitous, and fossil frequency and degree of sorting are inversely related. Colonial and solitary corals, and thick-shelled brachiopods are characteristic. Deeper water conditions are thought to be represented by the well-bedded, dark grey to almost black, bioclastic limestone predominating in the Black Rock Limestone. This is a poorly sorted wackestone with an appreciable carbonate mud matrix, and is usually highly fossiliferous. The limestone may be bituminous and include chert in layers and nodules.
The deepest water carbonate facies, and hence the farthest downslope from the shoreline, is represented by the Waulsortian ‘reefs’ or bioherms (steep-sided lime-mud mounds) (P948963). Lees and Miller (1985) have suggested that the pioneer faunal community responsible for the establishment of this facies lived under aphotic conditions below wave base, at a depth of over 200 m. The pioneer community was replaced by successive faunal and floral communities as carbonate was generated and accumulated, reflecting the growth of the bioherm above the surrounding sea floor. A belt of Waulsortian facies may contain many bioherms at different stages of growth. Although these bioherms have often been termed reefs, they lack any obvious skeletal framework and hence are not true reefs. The term Waulsortian, adopted from Belgium where this facies is well exposed, is used, therefore, to distinguish this unique Dinantian facies from other types of bioherm. Although this facies has only recently been described in the region from the Knap Farm Borehole at Cannington Park, it provides valuable additional evidence for the presence, during an appreciable part of Tournaisian times, of a belt of Waulsortian reefs stretching in an approximate east-west direction between Dinant in Belgium to the east and southern Ireland to the west.
The basin margin is marked west of Taunton by a belt of limestone turbidites and shales, the former presumably derived from the shelf carbonates. If this evidence to the west of the region is combined with seismic evidence (P948971) a short distance to the east of it, the basin margin can be predicted to trend at depth across the region more or less at the latitude occupied by Yeovil. Beyond this, to the south, within the sediment-starved basinal areas, the early Carboniferous is represented by an attenuated sequence of dark coloured, non-calcareous mudstones with chert and a mainly planktonic fauna including conodonts, goniatites and radiolaria.
- Kellaway, G A, and Welch, F B A. 1955. The Upper Old Red Sandstone and Lower Carboniferous rocks of Bristol and the Mendips compared with those of Chepstow and the Forest of Dean. Bulletin of the Geological Survey of Great Britain, No.9, 1–21.
- George, T N, Johnson, G A L, Mitchell, M, Prentice, J E, Ramsbottom, W H C, Savastopulo, G D, and Wilson, R B. 1976. A correlation of Dinantian rocks in the British Isles. Special Report of the Geological Society of London, No.7.
- Wilson, D, Davies, J R, and Waters, R A. 1988. Structural controls on Upper Palaeozoic sedimentation in south-east Wales. Journal of the Geological Society of London, Vol.145, 901–914.
- Lees, A, and Miller, J. 1985. Facies variations in Waulsortian buildups, Part 2; Mid-Dinantian buildups from Europe and North America. Geological Journal, Vol.20, 159–180.