Bruton Railway Cutting, Somerset

[ST 688 347]

B.M. Cox

Introduction

The cutting adjoining the railway station at Bruton, Somerset, exposes the Fuller's Earth Rock Member of the Fuller's Earth Formation. The Milborne Beds, Ornithella Beds and Rugitela Beds, into which the Fuller's Earth Rock Member is divided in Somerset (see (Figure 2.4)), are all present (Figure 2.43). Bruton Railway Cutting is famous for its ammonite fauna of tulitids, perisphinctids and rare oppeliids. The tulitid genera Tulites and Morrisiceras are, respectively, diagnostic of the Middle Bathonian Subcontractus and Morrisi zones (see also Troll Quarry GCR site report, this volume). When Torrens (1974) proposed the south side of the cutting at Bruton Railway Station as the type section of the Morrisi Zone (Torrens, 1965), the zonal fauna was known in detail only from southern England. He considered Bruton Railway Cutting to be an ideal type section because of its richness in ammonites and its probable permanence (the cutting lies on the line between Westbury and Taunton). The abrupt changes in the succession of ammonite faunas here enable the lower and upper boundaries of the Morrisi Zone to be placed precisely. As elsewhere along its outcrop in southern England, the zone is only 0.6 m thick, but there is no evidence of any major non-sequence at the top of the zone in this region (Torrens, 1974).

Description

The Bruton Railway Cutting GCR site includes both sides of the now partly overgrown cutting at Bruton Railway Station, but it is the longer section on the south side that was recorded by Torrens (1974) and proposed as the type section of the Morrisi Zone. The following section is based on his account together with that given in Torrens (1966). Bed numbers follow Torrens (1974). The presence of the Fuller's Earth Rock Member in the cutting had been noted earlier by Davidson (1878), Parsons (1879) and Richardson (1909c). Most recently, the section has been described briefly by Bristow et al. (1999).

Interpretation

The tripartite division of the Fuller's Earth Rock Member in Somerset has already been discussed (see Shepton Montague GCR site report, this volume). Shepton Montague is sited c. 2.5 km south of Bruton Railway Cutting.

At both sections, the ammonite fauna enables recognition of three of the standard ammonite-based zones of the Bathonian Stage (Torrens, 1974, 1980b; Page, 1996a). At Bruton Railway Cutting, the presence of Tulites in the basal bed identifies the Subcontractus Zone. The base of the overlying Morrisi Zone is taken at the base of Bed 2. This zone is characterized by the genus Morrisiceras, the range of which does not overlap with that of Tulites. Page (1996a) referred the Morrisiceras from beds 3 and 4 at Bruton Railway Cutting to M. ex gr. morrisi, in which he included M. sphaera. Both Tulites and Morrisiceras are tulitids, the most typical and common forms of which are the cadicone–sphaerocone macroconchs. The corresponding microconchs are quite different, being small, lappetted, planulate serpenticones (Donovan et al., 1981). For these 'partners' of Tulites and Morrisiceras, Torrens (1970) introduced, respectively, the names Trolliceras (see Troll Quarry GCR site report, this volume) and Holzbergia. Although Torrens (1974) proposed the site as type section for the Morrisi Zone, Page (1996a) referred to it as a reference section, the type area being southwest Germany.

The base of the next highest zone is taken at the base of Bed 5. When Torrens (1974) first published a description of the section, he assigned beds 5 and 6 to a 'Retrocostatum' Zone, which was tentatively based on the index taxon Probecticoceras retrocostatum (de Grossouvre). He considered that this zone generally seemed to be characterized more by the absence of certain genera and species of the zones above and below than by its own characteristic assemblage. Nevertheless, he reported that in southern England the basal part of the zone was crowded with large perisphinctids (notably Procerites) such as occur in the rubbly limestone fades of the Ornithella Beds of the Fuller's Earth Rock Member. Such a fauna is relatively common in Bed 5 at Bruton Railway Cutting (see also Shepton Montague GCR site report, this volume). The naming of this zonal interval has since undergone two changes. First, Torrens (1980b) replaced the index taxon by Procerites hodsoni Arkell mainly because, in France, Prohecticoceras retrocostatum was used as an index taxon of a zone of partly different age, and because the species was less common in its eponymous zone than in the overlying zone.

Torrens (1980b) listed a number of reasons why Procerites hodsoni was a suitable replacement index species not least of which was the fact that it seemed to be a distinctly recognizable 'proceritid' in a group that often shows morphological diversity at any one level as great as it is in successive beds. The Hodsoni Zone thus became well established in the British Bathonian succession. Secondly, recognition by Callomon and Cope (1995) of subzones in common with those of the slightly different zonation used farther south in Europe (Figure 2.44) prompted Page (1996a) to extend the usage of the latter zonation to the British succession. Thus, the interval covered by the Bullatimorphus and Fortecostatum subzones of the Hodsoni Zone (Callomon and Cope, 1995) can be referred instead to the Bremeri Zone (Page, 1996a). By analogy with the ammonite records from the Bullatimorphus Subzone in France and despite the lack of the subzonal index species, Page (1996a) concluded that the common Procerites and rare Oxycerites found 0.45 m above the base of Bed 5 at Bruton Railway Cutting could be accommodated in that subzone.

Conclusions

The Fuller's Earth Rock Member in Dorset and Somerset shows one of the best developments of Middle Bathonian rocks in Europe (see also Troll Quarry GCR site report, this volume). The section at Bruton Railway Cutting, where the upper part of the Milborne Beds has yielded the ammonite Morrisiceras, provides a primary reference — if not the type — section for the Morrisi Zone. The lower and upper boundaries of the zone, which is here only 0.6 m thick, are well constrained because of marked and abrupt changes in the ammonite succession; these make this the best section in England for demonstrating the palaeontological differentiation of the Subcontractus and Morrisi zones. The site is thus a most important one for regional, national and international correlation.

References