Arenaceous facies in the Clifton Down Group and Hotwells Group, 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.)

Stratigraphy

Generalised horizontal section to illustrate facies and thickness changes in the Carboniferous Limestone between Cromhall and the eastern Mendips. For location of section see P948961. (P948964)

In the following account the Dinantian rocks are described in ascending stratigraphical order for the whole region, except for those at Cannington Park, which are treated separately.

Arenaceous facies in the Clifton Down and Hotwells groups

The collective name given to the arenaceous facies within the Clifton Down and Hotwells groups is the Cromhall Sandstone east of the River Severn and the Drybrook Sandstone west of the River Severn. These sandstone successions are thickest in the north; southwards they split into several units separated by calcareous sequences.

The Lower Cromhall Sandstone, which separates the Clifton Down Mudstone from the Clifton Down Limestone, extends as far south as Chipping Sodbury and is equivalent to the Lower Drybrook Sandstone west of the River Severn. The Middle Cromhall Sandstone separates the Clifton Down and Hotwells limestones and extends south to beyond Wick. The Upper Cromhall Sandstone replaces the Hotwells Limestone from the top downwards in a northerly direction. This is the thickest and most extensive of the arenaceous developments in the Dinantian sequence and at its southern limit reaches the Mendips at Compton Martin. West of the River Severn, the Upper Drybrook Sandstone rests on the Drybrook Limestone but, in the absence of any recognisable Hotwells Limestone above, it has not been possible to determine its exact equivalence to the sequence on the east side of the river. The Lower and Upper Drybrook sandstones come together at about the middle of the Forest of Dean due to the northwards wedging out of the Drybrook Limestone.

The best documented sequence of the Upper Cromhall Sandstone is in the Ashton Park Borehole (Kellaway, 1967), a short distance south of the Avon Gorge. There, the Hotwells Group is 221 m thick and can be divided into fourteen sedimentary cycles. Those typical of the Hotwells Limestone comprise, in upwards succession, black mudstone-limestone-sandy seatearth, and those typical of the Cromhall Sandstone comprise limestone-banded sandy and silty beds with thin shales-seatearth. The latter cyclothems are similar to those of the Yoredale facies of northern England and the change in character of the sediments between the two types of cyclothem indicates a change from the predominantly carbonate platform conditions of the Hotwells Limestone to the deltaic conditions of the Upper Cromhall Sandstone. More restricted marine conditions are suggested by the replacement of crinoidal limestone by oolitic limestone in the later cyclothems. The Mollusca Band at Wick, and the Tanhouse Limestone in the country to the north, are thin limestones in the uppermost part of the Upper Cromhall Sandstone and are useful markers for separating this formation from the overlying Quartzitic Sandstone Formation.

The Hotwells Group is thickest at the northern end of the Bristol Coalfield, at Yate, where the Upper Cromhall Sandstone is 250 m thick and overlies some 70 m of Hotwells Limestone, which includes beds of Middle Cromhall Sandstone in the middle and at the base. The succession at Wick is intermediate between this and Bristol (P948964). The thickest Drybrook Sandstone in the Forest of Dean is only 105 m, apparently due to intra-Carboniferous erosion. The Hotwells Group may also be represented amongst relict brecciated beds along the line of the Lower Severn Axis.