Milborne Wick Section, Somerset

[ST 662 205]

B.M. Cox

Introduction

The Inferior Oolite Formation section generally known as 'Milborne Wick Lane', south-west of Milborne Wick in Somerset, was first reported by Buckman (1893a, section XVII) and has retained an important place in the study of Bajocian stratigraphy since that time. It is particularly famous for its abundant and well-preserved ammonite faunas of the Romani Subzone (Lower Bajocian Humphriesianum Zone). Fossil material from this section has found its way into numerous museums, abroad as well as in Britain (Parsons, 1976a). The stratigraphy at the site and in adjacent temporary excavations has been reviewed by Huxtable (1996).

Description

The following description, including bed numbers and ammonite names, is based on that recorded by Parsons (1976a) with additional data from Buckman (1893a), Richardson (1916a) and Page (unpublished English Nature records). The lithostratigraphical classification follows Bristow et al. (1995).

Both Buckman (1893a) and Richardson (1916a) recorded a further two metres of beds at the base of the section. Between them, these authors also noted belemnites, nautiloids, lignite and Astarte spissa in Bed 2, but it is not clear if the records relate to Bed 2a or 2b of the above section.

Interpretation

According to Parsons (1976a), there has been some confusion concerning the Milborne Wick section in past literature because Richardson (1916a) considered that Buckman (1893a) had been overzealous in splitting up beds 3–5 of the above section; Richardson believed that they were all of the same age and lithology. However, Parsons (1976a) concluded that there was no doubt that Buckman was correct because, of these three beds, only Bed 3 contained glauconite grains and also its ammonite fauna proved it to be significantly older than the overlying beds. Incidentally, both Buckman (1893a) and Richardson (1916a) numbered the beds from the top down such that, for example, Bed 1 in the above section is their Bed 6 and Bed 2 (a and b) is their Bed 5.

The succession belongs almost entirely to the Lower Bajocian Substage. The three youngest zones of that substage are well substantiated on the basis of the ammonite faunas, specimens of which were figured by Buckman (1909–1930). Beds 1, 2a and 2b are assigned to the Laeviuscula Zone and Subzone, Bed 3 to the Sauzei Zone, and beds 4 and 5 to the Humphriesianum Zone, Romani Subzone (Parsons, 1976a; (Figure 2.39)). The Romani Subzone, with a type area and horizon in south-east France and index ammonite Witchellia romani (Oppel) (Dorsetensia (D.) romani in Rioult et al., 1997), was confirmed in the British zonal sequence by Parsons (1976a, 1980a). Callomon and Chandler (1990) subsequently substituted the more commonly occurring Poecilomorphus cycloides (d'Orbigny) as the index taxon for this interval (following Sturani's (1971) usage) but later, Callomon and Cope (1995) reverted to the earlier nomenclature. The base of Bed 6 marks a non-sequence, which probably spans the younger subzones of the Humphriesianum Zone as well as the Upper Bajocian Subfurcatum Zone (Bristow et al., 1995), and Bed 6 itself may belong to the Garantiana Zone.

The Milborne Wick Section is sited on the gradient known as 'Miller's Hill', which rises from the western side of the village of Milborne Wick. Barton et al. (1993) and Bristow et al. (1995) recognized a distinctive member within the Inferior Oolite Formation in this area, and named it the 'Miller's Hill Beds' ('Miller's Hill Member' herein). Characterized by glauconitic sands, limestones and calcarenites with clasts of phosphatic limestone and abundant ammonite fragments, the base of this member is defined by an erosion surface. It is underlain by the Corton Denham Member (based on Parsons' (1980a) Cotton Denham Beds for Richardson's (1932) 'grey-beds'), and overlain by the Sherborne Building Stone Member (based on the Sherborne Building Stone of Buckman, 1893a). Bed 5 was called the 'Astarte spissa Bed' by Richardson (1916a) but should not be confused with the 'Spissa Bed' of Parsons (1980a), which occurs at a lower stratigraphical level, equivalent to Bed 2b at the GCR site.

The locality of the Milborne Wick Section is the north-easternmost of a cluster of GCR sites near Sherborne (see also Bradford Abbas Railway Cutting, Louse Hill Quarry, Halfway House Cutting and Quarry, Sandford Lane Quarry, Frogden Quarry and Holway Hill Quarry GCR site reports, this volume) where the Inferior Oolite Formation is rather thicker than elsewhere in Dorset and Somerset (see (Figure 2.29)). According to Bristow et al. (1995), this local thickening is related to deposition in a half-graben with a depocentre located approximately 10 km south of the Mere Fault (see (Figure 2.29)). The fault-controlled basin was starved of sediment during deposition of the Miller's Hill Member, and Bristow et al. (1995) considered that the inferred slow rate of deposition and the member's widespread iron mineralization, now contained as glauconite, reflected an important tectonic event.

Conclusions

The Milborne Wick Section has yielded a prolific and well-preserved ammonite fauna characteristic of the Romani Subzone of the Lower Bajocian Humphriesianum Zone. With other sites in the Sherborne area, it constitutes one of the most important localities in Britain for this stratigraphical interval. Known in the geological literature for over 100 years and represented in the collections of many museums both in Britain and abroad, it remains a key site for Bajocian stratigraphy as well as providing an insight into the palaeogeography and depositional history of the Wessex Basin.

References