Clovelly Coast

Highlights

The Clovelly Coast shows the best fossiliferous sequence through the upper Crackington Formation, and combines extensive outcrop with good biostratigraphical control.

Introduction

Foreshore exposures between Wood Rock and Gallantry Bower, 1 km north-west of Clovelly, Devon [SX 314 255]–[SX 305 262], show part of the upper Crackington Formation. It includes a number of biostratigraphically significant horizons, and is part of a complex east–west trending anticlinorial structure (Figure 3.6). A detailed account of the geology is provided by Edmonds et al. (1979).

Description

Lithostratigraphy

The exposed sequence consists of 360 m of shales, siltstones and sandstones (Figure 3.7). The strata appear to represent the latter phases of basin infill by turbidites, typical of the upper Crackington Formation. The sandstones are thin to medium bedded and the siltstones predominantly laminated. Ripple-drift cross lamination is common and, at some points, slumping and sandstone volcanoes are present (Burn, 1969, 1970). Petrographic analysis has revealed a lower proportion of lithic fragments than in the overlying Bude Formation, which may explain the greater resistance to weathering of the former (Edmonds et al., 1979).

The sequence includes six fossil-bearing shales, which have been identified in a number of the coeval exposures in north Devon and north Cornwall. These are assigned names as shown in (Figure 3.7).

Biostratigraphy

The lowest horizon in this sequence to yield fossils is the Clovelly Court Shale, from which Edmonds et al. (1979) report the ammonoids Verneuilties sigma (Wright), Cancelloceras cf. lineatum (Wright) and an unnamed anthracoceratid. They also mention conodonts (Hindeodella sp.), ostracods (Cypridina?) and bivalves (Dunbarella sp.). According to Ramsbottom et al. (1978) and Edmonds et al. (1979) the ammonoids belong to the Donetzoceras sigma Subzone, indicating the topmost Marsdenian Stage.

The Skittering Rock Shale yields poorly preserved fossils, reportedly including ammonoids of the Cancelloceras cancellata Zone (Freshney and Taylor, 1972; Edmonds et al., 1975, 1979; Ramsbottom et al., 1978). However, the only published taxonomic list merely records the bivalves Caneyella sp. and indeterminate ammonoids possibly belonging to Gastrioceras (Edmonds et al., 1979).

Another poor fossil assemblage occurs in the Deer Park Shale, including the bivalves Caneyella sp. and Dunbarella sp., and ammonoid fragments with fine lirae (Edmonds et al., 1979). Ramsbottom in Edmonds et al. states that the ammonoid fragments are consistent with the Cumbriense Marine Band in South Wales, but no specific or even generic identifications are given.

A rather more diagnostic assemblage occurs in and immediately below the Embury Shale. Edmonds et al. (1979) record Gastrioceras subcrenatum (Frech) and G. spp. nov., and it is widely assumed that this level correlates with the Subcrenatum Marine Band in the South Wales and Pennines coalfields. If correct, the Embury Shale is the level of the Namurian–Westphalian boundary in the Crackington Formation.

The Gull Rock Shale yields another diverse fossil assemblage, including the ammonoids Gastrioceras listeri (Sowerby), G. circumnodosum Foord and G. coronatum Foord and Crick, together with the bivalves Dunbarella papyraceae (Sowerby) and Caneyella cf. multirugata (Jackson) (Edmonds et al., 1979). This clearly invites comparison with the deeper-water assemblages from the Listed Marine Band of the Pennines coalfields (Calver, 1968), and thus suggests a position in the lower part of the Langsettian Stage.

Towards the top of the sequence is the Hartland Quay Shale. At Clovelly, it has only yielded a single calcareous nodule with anthracoceratid ammonoids. Elsewhere, however, this bed has yielded Gastrioceras cf. amaliae Schmidt (e.g. Elmscott Beach south of Hartland Point — Freshney et al., 1979) which suggests that it can be correlated with the Amaliae Marine Band of South Wales.

Interpretation

The Clovelly Coast exposes a complete and fossiliferous sequence through the upper Crackington Formation, ranging from the upper Marsdenian to lower Langsettian. Of the other exposures of this interval, the best are to be found near Embury Beach, between Hartland Point and Bude (Freshney et al., 1979). At Embury Beach, there are fossiliferous exposures similar to the Westphalian part of the Clovelly section, but the lower (Namurian) strata appear to be poorly represented there.

Coastal exposures at Westward Ho! are probably partly coeval with the Clovelly sequence, but yield little in the way of fossils (Edmonds et al., 1979). They are probably shallower water sediments than those seen near Clovelly and De Raaf et al. (1965) classified them as the Westward Ho! Formation. However, the Westward Ho! Formation is now usually included within the Crackington Formation (Edmonds, 1974).

The Instow Fish Bed, exposed along the Torridge estuary near Bideford, is probably a correlative of the Gull Rock Shale at Clovelly and has yielded a more diverse fossil assemblage (Prentice, 1960a). However, the rest of the upper Crackington Formation is only poorly exposed at Instow.

Conclusions

The Clovelly Coast provides the best fossiliferous exposures of rocks known as the upper Crackington Formation, thought to be about 315 million years old. They represent the later phases of the haling of a marine basin known as the Clam Trough, which extended from Ireland through south-west Britain to northern Germany. The rocks exposed here are especially important as they include beds containing diverse assemblages of marine animal fossils, which allow detailed correlations with other successions of similar age in the rest of Britain, and elsewhere in northern Europe and eastern North America.

References