Igneous rocks, Lower Carboniferous, Bristol and Gloucester region

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

Igneous rocks

Lower Carboniferous (Dinantian) volcanic rocks at Weston-super-Mare — Volcanic tuff at Middle Hope. (P211372)

Generalised horizontal section to illustrate facies and thickness changes in the Carboniferous Limestone between the Forest of Dean and Cannington Park. For location of section see P948961. The Dinantian stages in the right-hand column refer only to Cannington Park; for the relationship of the stages to the successions elsewhere refer to P948962 and P948964. (P948963)

Lower Carboniferous (Dinantian) volcanic rocks at Weston-super-Mare — Pillow lava at Spring Cove. (P006679)

Minor submarine volcanic activity is recorded at two stratigraphical levels in the Carboniferous Limestone of the district. The earlier episode occurred in the later Courceyan rocks of the Middle Hope area north of Weston-super-Mare, where up to 32 m of tuffs with some interbedded tuffaceous limestones are exposed on the coast (P211372). The tuffs show slight signs of reworking and may be adjacent to the volcanic source. A single flow of basaltic pillow lava about 4 m thick occurs near the top of the sequence. The tuffs rest on the ‘Main Chert’ of the Black Rock Limestone at about the (local) middle of the Caninophyllum patulum Biozone of late Courceyan age (P948963). At the same stratigraphical level as the Middle Hope volcanics, a thin basalt ‘lava’, formerly exposed in a quarry adjacent to the railway at Uphill, is now considered on mineralogical evidence to be a sill.

Later activity, in the early part of the Arundian, is recorded at four localities south-west of Bristol, stratigraphically about 15 m above the top of the Gully Oolite (P948963). The best exposures are on the coast at Spring Cove, Weston-super-Mare (P006679), within the Birnbeck Limestone. Here, about 12 m of reddened tuffaceous limestone are overlain by a 15 m-thick flow of basaltic pillow lava, with a further 8 to 9 m of reddened and tuffaceous limestones lying above this. The tuffaceous material has been reworked and the flow contains much partly assimilated and baked limestone debris. Some 3 km inland, the flow locally reaches a maximum thickness of 35 m. About 13 km further to the east, basaltic lava and tuff crop out in Goblin Combe, on the south side of Broadfield Down, and extend over 3 km eastwards along the strike. The volcanic sequence is about 12 m thick and lies entirely within the Clifton Down Mudstone, which is the lateral equivalent of the Birnbeck Limestone. The remaining two localities, at Tickenham and Cadbury Camp, east of Clevedon, comprise small isolated outcrops of basalt within the Clifton Down Mudstone. The relatively small thickness of the lava flows, combined with their relatively wide area of occurrence (some 250 km²), suggest that there were several small volcanic vents. Re-examination of the petrology of the lavas (Whittaker and Green, 1983) shows that they are all alkaline olivine basalts which have undergone various degrees of autometasomatism by alkali-rich residual fluids.

West of the River Severn, two small intrusions of monchiquite have been dated as post-early Dinantian. One, at Guest House, 6.5 km south-east of Usk, appears to be a small volcanic neck at least 46 m across and mainly filled with agglomerate. This consists of blocks of decomposed monchiquite, along with rocks that can be matched with limestones in the Lower Limestone Shale and dolomites in the Lower Dolomite. The other occurrence is of a small monchiquite dyke intrusive into Lower Devonian nearer Usk and just beyond the limits of the region. It is not known whether they are contemporaneous with the Bristol–Weston occurrences.