Upper Carboniferous (Namurian), Bristol and Gloucester region

Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)

Outcrop of the Dinantian and Namurian rocks in the region. (P948961)

In the last century, the lithostratigraphical subdivision of the Upper Carboniferous sequence into Millstone Grit and Coal Measures was found to be applicable to virtually the whole of the British Isles. Since the 1920s, however, biostratigraphical research on the Millstone Grit and productive Coal Measures has shown that the lithostratigraphical boundary between them is of different ages in different areas. The term Millstone Grit is now used informally and the lower division of the Upper Carboniferous, defined largely in terms of subzones, zones and stages by means of the rapidly evolving goniatite faunas, has been redefined chronostratigraphically as the Namurian Series (Ramsbottom et al., 1973).

The St George’s Land-Brabant landmass persisted from Dinantian into Namurian times. The main area-of marine sedimentation was south of the Bristol Channel. Farther north, in South Wales and the Bristol area, there appear to have been separate basins in Namurian times with only intermittent marine connections via the deeper sea area to the south. Marine influences were minimal in the Bristol basin, where sedimentation was predominantly in a deltaic and swamp environment, and the detailed zonal subdivision based on goniatites cannot be applied here.

Quartzitic sandstone formation

In the Bristol area, Namurian rocks constitute the Quartzitic Sandstone Formation (Kellaway and Welch, 1955). The main outcrop occurs on the northern and north-eastern rim of the Bristol Coalfield between Chipping Sodbury and Cromhall (P948961), where the succession attains its maximum known thickness of about 300 m. Elsewhere, the outcrops are mainly small inliers, partly or wholly surrounded by Triassic rocks. At Bristol, the Quartzitic Sandstone gives rise to high ground in the Clifton–Tyndall’s Park area; hence the local name Brandon Hill Grit, which was formerly used for these rocks. In the Wick inlier, farther south, the succession is about 180 m thick, and the Ashton Park Borehole near the southern end of the Avon Gorge proved a true thickness of 155 m. To the west of Bristol and across the River Severn, the formation is absent or strongly attenuated, due to intra-Carboniferous earth movements and erosion. In the Mendips, the thickness varies between 45 m and 65 m. The most southerly known occurrences of Namurian strata in the district are small inliers north-west of Bridgwater.

Bristol area

The formation is very poorly exposed and detailed knowledge of the succession mainly derives from the Ashton Park Borehole and the Limekilns Lane Borehole at Yate; however, only in the former is the complete sequence seen. At Bristol and Winford, the base of the formation is marked by a distinctive development of chert and cherty mudstone, up to 15 m thick, with a marine fauna mainly of brachiopods and bivalves, but also including fish fragments and conodonts. At Winford, these beds yielded the only goniatite so far found in the formation, a Eumorphoceras of a type found in the earliest Namurian stage, the Pendleian (E1 zonal index). A much attenuated representative of the chert is present in part of the coalfield area north of Bristol, but it appears to be absent elsewhere. The top of the formation is taken at the base of the Subcrenatum Marine Band (Westphalian). Much of the succession, particularly in the upper part, is lithologically indistinguishable from the Coal Measures and was laid down under similar conditions.

Above the basal cherts, the formation comprises varying proportions of mudstone, seatearth and sandstone, with occasional marine mudstone layers and thin carbonaceous and coaly deposits. The rocks show much lateral variation. The sandstone beds commonly have erosive bases, and some may be pebbly or conglomeratic; a high proportion are very hard quartzite. Most of the pebbles are of white quartz, but also include chert, quartzite, siderite ironstone and mudstone. The sandstone beds are best developed at Bristol and north-eastwards to the western side of the coalfield around Tytherington and Cromhall; elsewhere, north-east and east of Bristol, the succession is more argillaceous. Thus, in the Ashton Park Borehole, arenaceous rocks account for three-fifths of the succession, of which two-thirds are quartzitic types; whereas, in the Limekilns Lane Borehole, in a similar thickness of strata, the corresponding proportions are only one-fifth and one half respectively. Some inferior quality coal seams have been worked locally near Barrow Gurney on the northern side of Broadfield Down, at Wick, and in the Tytherington–Cromhall area, where the crop of a coal, known as the Tapwell Bridge Seam, is shown on the Malmesbury (251) sheet.

The only biostratigraphical subdivision of the main mass of the formation has been by means of fossil plants. In the Ashton Park Borehole, the lower half of the succession contains plants having affinities with late Visean to early Namurian (Pendleian–Arnsbergian) floras; in the upper half they have affinities with floras of the two highest Namurian stages (Marsdenian and Yeadonian) and the early Westphalian (see also Ramsbottom et al., 1973, p.16). Deposits of the Chokierian, Alportian and Kinderscoutian stages may not have been represented in this area.

Southern areas

On the north side of the Mendips, between Ashwick and Mells, the Quartzitic Sandstone forms a narrow outcrop on the northern limb of the Beacon Hill Pericline, where it is estimated to be about 45 m thick. The only exposures are in red-stained quartzitic sandstones. Apart from other small outcrops in the Mendips, the full thickness is only again seen at Ebbor Rocks, some 4 km north-west of Wells. Here, the Subcrenatum Marine Band overlies about 65 m of strata above the Hotwells Limestone. This thickness includes an unexposed gap of about 12 m at the base, apparently mainly in mudstone, possibly comparable to the lowest marine shales seen farther north. Although incompletely exposed, the bulk of the overlying strata appear to be quartzitic sandstone.

An interesting discovery of Namurian strata was made in the Cannington Park area, north-west of Bridgwater (Edmonds and Williams, 1985). Here, an inlier of Rodway Beds (now named Rodway Siltstones), hitherto considered to be Devonian in age, was proved by drilling to contain the goniatite Gastrioceras cancellatum indicative of the G. cancellatum Marine Band at the base of the Yeadonian Stage. A second inlier of Rodway Siltstones, nearby, is presumed also to be of Namurian age. The rocks are siltstone, sandstone and shale, too disturbed for any reliable estimates of thickness to be made.

Western areas

Vestigial occurrences of possible Namurian strata in the coastal strip on the west side of the River Severn between Portskewett and Ifton have been described by various authors (Welch and Trotter, 1961). These deposits comprise masses of hard, partly quartzitic sandstone and soft shale, which infill steep-sided channels in the eroded top surface of the Drybrook Limestone (Dinantian) and connect with pipes and cavities in the underlying limestone. In the same area, a shaft sunk near the western end of the Severn Tunnel during its construction proved brecciated Carboniferous Limestone overlain by thin ‘Millstone Grit’ which was, in turn, unconformably overlain by Upper Coal Measures. More recently, exploratory boreholes at Portskewett have yielded palynological evidence for a Namurian (Yeadonian) age for shales overlying limestone there.

On the opposite side of the River Severn, other occurrences of lenticular masses of quartzitic sandstone, unconformably overlying brecciated Carboniferous Limestone of possible late Visean age and unconformably overlain by Upper Coal Measures, have been recorded during tunnelling operations at Kings Weston, north-west Bristol, and at Portishead. Lenticular and piped masses of quartzitic sandstone are present at the top of the Clifton Down Limestone at Tynesfield, 1 km west of Wraxall. The strong unconformity and erosion associated with these vestigial remnants of proved and possible late Visean–Namurian strata provide evidence for activity along the line of the Lower Severn Axis during Carboniferous times.