Bickley Wood

Highlights

Bickley Wood is the best available exposure of the Downend Formation in the Bristol–Somerset Coalfield, and has yielded potentially important biostratigraphical information.

Introduction

The north banks of the Avon near Hanham Green, 5 km ESE of the centre of Bristol, lie in the central part of the Bristol–Somerset Coalfield, sometimes known as the Pensford Basin (Kellaway, 1970). A series of exposures there [ST 644 703] show various facies in the Downend Formation. There are no detailed geological descriptions in the published literature, but fossils from here are listed by Moore and Trueman (1937) and some details of the sedimentology are provided in an unpublished thesis by Stead (1974).

Description

Lithostratigraphy

A coherent lithostratigraphical log has yet to be constructed from the various separate exposures present in this site. However, it is possible to see here most of the lithofacies characteristic of the lower Pennant Formation of the Bristol–Somerset Coalfield, including massive, cross-bedded sandstones and lenticular quartz-conglomerates. There are also some thin shales, which have yielded fossils (see below).

Biostratigraphy

Moore and Trueman (1937) reported that plant and animal fossils had been collected from shales exposed near Elm Tree Inn at Hanham Green. A species list was not given, but it was claimed that the assemblage was similar to that collected from the other side of the river at Chandos Lodge. The latter included non-marine bivalves A. phillipsi, which suggest the upper Bolsovian to Cantabrian.

The list of plant fossils given is difficult to assess and may include some misidentifications. The occurrence together of elements typical of lower to middle Wesphalian assemblages Laveineopteris tenuifolia (Sternberg) Cleal et al, Neuropteris cf. obliqua (Brongniart) Zeiller, Paripteris cf. linguaefolia (Bertrand) Laveine and typical Westphalian D elements N. ovata Hoffmann, N. flexuosa Sternberg, Ptychocarpus unitus (Brongniart) Weiss would seem an unlikely combination. The palaeobotany here is in clear need of revision, but at this this stage tends to suggest the lower or even middle Westphalian D.

Interpretation

This is the best exposure of the Downend Formation in the Bristol–Somerset Coalfield, and may be taken as the type for the interval (Figure 6.4). In particular, it is important because it has yielded indicative (albeit as yet equivocal) biostratigraphical evidence.

In the absence of any detailed published account of the stratigraphy or sedimentology of the formation, it is difficult to place it in a regional geological setting. As it lies only a short distance above the Cambriense Marine Band, there would seem to be an obvious comparison with the lower part of the Pennant Formation in the southern part of the South Wales Coalfield (Llynfi Member). However, the evidence of the plant fossils suggests a rather higher stratigraphical position, nearer to that of the Brithdir or even the Hughes members in South Wales. Unfortunately, the position of the above mentioned fossiliferous shales within the Downend Formation is not clearly stated; Moore and Trueman (1937) merely recorded that they are at a higher level than the shales exposed at St Anne's Park, which themselves are not far above the base of the 'Pennant Series'. If they are not significantly high in the Downend Formation, however, and the tentative biostratigraphical observations given above are correct, then it would seem that there has to be a non-sequence between the Pennant Formation and the underlying Productive Coal Formation in the Bristol–Somerset Coalfield, representing perhaps half a stage or more. This could have considerable implications for understanding the regional development of the Pennant-style depositional environments in the southern part of Britain.

Conclusions

Bickley Wood is the best available exposure of rocks belonging to the Downend Formation in the Bristol–Somerset Coalfield, about 300 million years old. Of particular importance is the presence here of fossils, normally very rare in this formation, that allow an indication of the stratigraphical position of these rocks relative to the rest of the coalfield to be established.

References