Doulting Railway Cutting, Somerset

[ST 645 424]–[ST 652 424]

B.M. Cox

Introduction

Doulting Railway Cutting, near Shepton Mallet, Somerset, exposes both Bajocian and Bathonian strata (Figure 2.45), and features in both the Aalenian–Bajocian and Bathonian GCR blocks. The strata comprise the Doulting Conglomerate, Garantiana Beds, Doulting Stone and Anabacia Limestone, overlain by the Fullonicus Limestone and Knorri Beds of the Fuller's Earth Formation (see (Figure 2.3) and (Figure 2.4)). The Bajocian-Bathonian stage boundary lies within the Anabacia Limestone. The cutting is the type section for the Fullonicus Limestone (named by Torrens (1980b) after a species of ammonite; (Figure 2.46)C) which is here the basal unit of the Lower Fuller's Earth Member, Fuller's Earth Formation and Great Oolite Group. The cutting also lies within the type area of the Doulting Conglomerate, Doulting Stone, Anabacia Limestone (named by Richardson (1907a) after a genus of button coral (now Chomatoseris); (Figure 2.46)A) and Knorri Beds (named by Richardson (1916a) after a species of small oyster; (Figure 2.46)B). The Anabacia Limestone and Fullonicus Limestone have yielded ammonite faunas indicative of the Lower Bathonian Zigzag Zone and its component subzones. The underlying part of the Inferior Oolite Formation has yielded Upper Bajocian ammonite faunas. As elsewhere in the Mendips area (see Vallis Vale GCR site report, this volume), the Aalenian and Lower Bajocian successions are missing; the Doulting Conglomerate unconformably overlies the Lower Jurassic (Toarcian) Lias Group.

Description

The section was described by Richardson (1907a) and Torrens (in Donovan (1969)) on which the following details are largely based (Figure 2.45). The lithostratigraphical classification has been amended following Parsons (1975a, 1980a) and Bristow et al. (1999) such that the lower part of the Doulting Stone as recognized by Richardson (1907a) and Torrens in Donovan (1969) (Bed 1a herein) is reclassified as Garantiana Beds (= Ragstone of Parsons, 1975a; Ragstones of Parsons, 1980a). The strata dip gently eastwards such that the stratigraphically lowest are exposed in the western part of the cutting, which totals c. 730 m in length. Exposure is presently patchy owing to vegetation cover.

Interpretation

When Richardson (1907a) first described the section, he referred to the conglomeratic bed at the base of the Inferior Oolite Formation as the 'Upper Trigonia Grit', believing that it was the same as the well-known bed of that name in the Cotswolds (see Chapter 3). Richardson (1916a) maintained this correlation but Parsons' (1975a) subsequent reassessment of the ammonite fauna, including specimens not seen by Richardson, concluded that it indicated the Upper Bajocian Subfurcatum Zone rather than the next youngest Garantiana Zone to which the Upper Trigonia Grit belongs; correlation of the Doulting Conglomerate with the Upper Trigonia Grit of the Cotswolds was therefore considered to be untenable. According to Parsons (1975a), the ammonite fauna of the Doulting Conglomerate comprised Cadomites deslongchampsi (d'Orbigny), Leptosphinctes aff. davidsoni (S.S. Buckman), Orthogarantiana sp., Stephanoceras sp., Strenoceras (S.) cf. subfurcatum (Zieten) and Teloceras banksi (J. Sowerby), and could be reconciled only with the Banksi Subzone of the basal Subfurcatum Zone in which the co-occurrence of stephanoceratid and perisphinctid ammonites is typical. The Banksi Subzone is generally accepted as marking the base of the Upper Bajocian Substage (Callomon and Chandler, 1990; see (Figure 1.3), Chapter 1). In Richardson's defence, Parsons (1975a) reported that there was little reason to doubt Richardson's (1907a, 1916a) assessment of the ammonites as belonging to the Garantiana Zone on the basis of the specimens available to him at that time, if one assumed that a specimen of Stephanoceras was reworked. The fact that the ammonite fauna of the Upper Trigonia Grit in the Cotswolds indicates the upper part of the Garantiana Zone (Acris Subzone) implies that the Late Bajocian transgression north of the Mendips occurred at a slightly later date than south of the Mendips (Parsons, 1975a).

Above the Doulting Conglomerate and representing the Garantiana Zone, Parsons (1975a, 1980a) separated a unit of less massive limestones with marl partings (Bed la of section) from the base of the overlying Doulting Stone. Referred to as the 'Ragstone' or 'Rag Bed' by Parsons (1975a) and the 'Ragstones' by Parsons (1980a), this unit is herein called the 'Garantiana Beds' (Richardson, 1916a) following Bristow et al. (1999). Parsons (1975a) reported an ammonite fauna of Prorsisphinctes sp. and Spiroceras sp. in the Doulting area and deduced these to be forms of the upper part of the Garantiana Zone because of the close similarity of P. ('Glyphosphinctes')glyphus (S.S. Buckman), of which the Ragstone is the alleged type horizon (Buckman, 1925), and P. ('Stomphosphinctes') stomphus (S.S. Buckman), which is known to characterize the upper Garantiana Zone elsewhere (see Burton Cliff and Cliff Hill Road Section GCR site report, this volume). Much of the Subfurcatum and Garantiana zones (equal to six subzones) is thus missing beneath the Garantiana Beds (see (Figure 1.3), Chapter 1).

The overlying Doulting Stone has been quarried extensively hereabouts since at least the Middle Ages and was used in the building of Wells Cathedral, Glastonbury Cathedral and all of the older buildings of Doulting village (Savage, 1977). Parsons (1975a, 1980a) implied that both the Doulting Stone and overlying Anabacia Limestone had yielded ammonite faunas indicative of the Parkinsoni Zone but the only ammonites specifically mentioned were those that Torrens (in Donovan, 1969) reported from his beds 2c and 2d of the Anabacia Limestone where the macroconch/microconch pair Parkinsonia convergens (S.S. Buckman) and P. pachypleura (S.S. Buckman) in the lower part of Bed 2d indicate already the basal Lower Bathonian Zigzag Zone, Convergens Subzone (Torrens, 1974; Page, 1996a). The ammonite fauna in the highest part of Bed 2d, including Bigotites sp., Morphoceras sp. (including 'Ebrayiceras'cf. jactatum S.S. Buckman), Oxycerites yeovilensis Rollier and Zigzagiceras plenum Arkell, indicates the next youngest Macrescens Subzone (Torrens in Donovan, 1969; Page, 1996a). The Bajocian-Bathonian stage boundary is arbitrarily taken at the base of Bed 2d. Richardson (1907a) had used the term Anabacia Limestone' in a more restricted sense than herein, preferring to recognize the upper part as a separate unit that he called the 'Rubbly Beds'. However, Torrens (1980b) proposed that this term should be abandoned because the beds were not lithologically distinct from Richardson's Anabacia Limestone and they also contained the latter's characteristic button coral.

The overlying Fullonicus Limestone, at the base of the Fuller's Earth Formation, is distinguished from the Anabacia Limestone by a total lack of ooids and a micritic matrix (Torrens, 1980b). The erosive nature of its basal boundary is indicated by pebbles of the Inferior Oolite Formation in its basal bed. Its perisphinctid ammonite fauna of macroconch and microconch variants of Procerites fullonicus (S.S. Buckman) (the latter referred to as 'Siemiradzkia') is one of the two main ammonite faunas recognized in the Yeovilensis Subzone, the youngest of the three subzones of the Zigzag Zone in Britain (Torrens, 1974; Page, 1996a). This fullonicus fauna is associated with the small oyster Catinula knorri, which occurs in abundance in the overlying Knorri Beds. According to Torrens (1980b), the latter have yielded no ammonites, but they have been tentatively assigned to the Tenuiplicatus Zone on the basis of a specimen of Asphinctites recinctus S.S. Buckman that possibly came from the Knorri Beds of Midford, near Bath (Torrens, 1980b).

Conclusions

The section at Doulting Railway Cutting exposes the Bajocian–Bathonian stage boundary in ammonitiferous limestone facies, and provides one of the most important Lower Bathonian exposures in southern England. At the top of the Anabacia Limestone, a hardground, which is probably correlatable over wide areas, marks the boundary between the Inferior Oolite Formation and the Great Oolite Group. The cutting is the type locality for the Fullonicus Limestone, at the base of the Great Oolite Group, and lies within the type area of several of the other exposed stratal units. It is thus an important section for local and regional lithostratigraphy. The fauna that it has yielded, including ammonites characteristic of the oldest documented British Bathonian ammonite assemblage (Parkinsonia convergens Biohorizon of the Convergens Subzone and Zigzag Zone; see (Figure 1.4), Chapter 1), enables correlation with areas further afield, and thus endows the site with national and international significance. The influence of the Mendip Axis on sedimentation in the Mid Jurassic Epoch is clearly demonstrated here not least by the absence of Aalenian and Lower Bajocian strata.

References