Ryewater, Corscombe, Dorset

[ST 513 062]–[ST 514 067]–[ST 506 064]

K.N. Page

Introduction

The GCR site known as 'Ryewater', near Corscombe, comprises intermittent stream-bed and bank exposures in two streams near Rye Water Farm, c. 1 km north-west of the village of Corscombe in Dorset. The western 'limb' of the site is the stream section from near Redland Coppice to just north of the bridge over Rye Water Lane, and the eastern 'limb' is the stream section from near Lovelands to the confluence with the western stream, just south of the Rye Water Lane bridge (Figure 2.21). In addition, a small section of the stream that runs parallel with, and to the north-east of, Rye Water Lane is also included. These exposures of mainly the Kellaways and Oxford Clay formations were probably amongst those referred to by Wilson et al. (1958) as 'an occasional stream section' in an area of otherwise poor exposure but they were not specifically noted until exceptionally heavy rainstorms at the end of June 1968 flushed out the streams to leave areas of clear exposure and well-preserved fossils in the stream-bed shingle (Cope and Cox, 1970). The site includes a more-or-less permanent exposure of the Kellaways Clay Member of the Kellaways Formation and has yielded important ammonite faunas of the Lower Callovian Koenigi Zone and Subzone. Fossiliferous exposures of the underlying Upper Cornbrash are also present as well as the overlying Kellaways Sand Member and probably the lowest part of the Oxford Clay Formation.

Description

The succession at Ryewater occupies the core of a narrow syncline as a result of which exposures of the Upper Cornbrash and Kellaways Formation repeat themselves.

Hard, shelly limestone, with bivalves including Meleagrinella echinata (J. Sowerby), belonging to the Lower Cornbrash, is exposed upstream of the ford at Lovelands (c. [ST 5135 0625]), and the lower part of the overlying Upper Cornbrash, which is at least 5 m in total thickness, has been reported nearby (Page, 1988). The latter comprises sandy, nodular and bedded limestones with locally abundant specimens of the terebratulid brachiopod Microthyridina siddingtonensis (Walker). Traces of similar M. siddingtonensis-rich sandy concretions have been recorded west of Redland Coppice (c. [ST 5045 0650]). Higher levels of the Upper Cornbrash include massive beds of sandy limestone interbedded with sandstones, sometimes with limestone concretions. These can be seen in the stream bed immediately downstream of Lovelands Ford, and again immediately east of the Rye Water Lane bridge [ST 5140 0675], in Redlands Coppice [ST 5062 0643] and possibly in a faulted outcrop at [ST 5167 0676]. A fragment of the ammonite Macrocephalites ex gr. terebratus (Phillips) collected near Redlands Coppice appears to be from these levels. The highest beds of the Upper Cornbrash crop out close to the eastern side of Rye Water Lane bridge and include grey sandy clay overlain by 0.4 m of muddy sand with an abundant bivalve fauna including Goniomya, Modiolus, Pholadomya and Pleuromya, and the ammonite Macrocephalites ex gr. kamptus (S.S. Buckman).

The overlying Kellaways Clay Member of the Kellaways Formation is seen in several small exposures a short distance west of Rye Water Lane (c. [ST 512 067])–[ST 513 067]–[ST 514 066] and repeated farther west [ST 506 064]. It is represented by grey, sandy clay with one or more bands of septarian limestone concretions and has yielded the ammonite Proplanulites cf. koenigi (J. Sowerby) and a bivalve fauna including Catinula, Modiolus, Myophorella and Protocardia. The bulk of the ex-situ fauna recorded by Cope and Cox (1970), including macroconch and microconch Proplanulites koenigi, Cadoceras sp. nov. A of Callomon and Page (in Callomon et al., 1989), Chamoussetia buckmani Callomon and Wright and Macrocephalites lophopleurus (S.S. Buckman), probably came from these levels.

Greyish, sandy clay with a band of harder, more calcareous lenses is exposed in the stream bank and bed around [ST 5115 0665]. They are the sandiest lithologies seen in the stream sections and probably represent the Kellaways Sand Member. Bivalves include Gryphaea, Myophorella and ?Pholadomya but Sigaloceras sp. is the only ammonite to have been recorded in situ (Page, 1988). Around [ST 5108 0660], c. 0.6 m of blue-grey sandy clay has yielded abundant aragonitic fossils including the bivalves Gryphaea, Oxytoma, Thracia and Trautscholdia, and ammonites including macroconch and microconch Sigaloceras (Catasigaloceras) enodatum (Nikitin) trans a of Callomon et al. (1989) (specimens figured as Sigaloceras (Catasigaloceras) sp. nov. by Page, 1991, pl. 13, figs 9,10) and rare Cadoceras sp. and ?Anaplanulites sp.. This may also be part of the Kellaways Sand Member or, alternatively, part of the locally developed Mohuns Park Member (Bristow et al., 1995) of the basal Oxford Clay Formation. The highest beds so far detected in situ occur at around [ST 5100 0655] and may also belong to the Mohuns Park Member. They are grey clays, silty in part, with harder, more calcareous lenses containing crushed ammonites including macroconch and microconch Kosmoceras (Gulielmiceras) medea Callomon and ?Homoeoplanulites sp.. A band of Gryphaea is present in one of the exposures and other bivalves (including Pinna) are also recorded. The Kellaways Formation is probably at least 20 m thick here.

Interpretation

By comparison with former sections recorded near Corscombe, the Lower Cornbrash at Ryewater is assumed to be of latest Bathonian age (Discus Zone and Subzone; Douglas and Arkell, 1928; Page, 1988, 1989).

The Upper Cornbrash exposures are virtually the only ones in the region. They are of particular significance as they are close to important localities in the Corscombe district referred to by Douglas and Arkell (1928), which yielded the ammonites Macrocephalites cf. jacquoti Douvillé and the very rare basal Callovian index taxon Kepplerites (K.) keppleri (Oppel). This fauna indicates the internationally important Kepplerites keppleri Biohorizon that is used to correlate the base of the Callovian Stage from the Caucasus through Europe to east Greenland, and which is also recognizable in southern Alaska, British Columbia and possibly Japan (Callomon, 1994). The records of this fauna in the Corscombe area suggest that the Ryewater GCR site may include one of the most complete Bathonian–Callovian stage boundary successions in Britain. The site is also only c. 6 km from the sections at Sutton Bingham described by Arkell (1954a) and used by Callomon (1964) to define the base of the Macrocephalus (now Herveyi) Zone and therefore the base of the Callovian Stage. The Corscombe sections are probably stratigraphically more complete than those at Sutton Bingham, where the lower part of the Upper Cornbrash yielded the ammonite Macrocephalites verus S.S. Buckman (index taxon of the second oldest Callovian ammonite biohorizon), although more work is required to obtain in-situ age diagnostic specimens. The brachiopod Microthyridina siddingtonensis characterizes the siddingtonensis Biozone of Douglas and Arkell (1928), which corresponds approximately with the Keppleri Subzone (Lower Callovian Herveyi Zone) (Page, 1988, 1989).

The presence of the ammonite Macrocephalites ex gr. terebratus near Redlands Coppice indicates the Terebratus Subzone of the latter zone (Page, 1988). The sandy lithologies recorded at the top of the Upper Cornbrash at the Ryewater GCR site have been noted elsewhere in the district (e.g. Rampisham; Page, 1988). The presence of Macrocephalites ex gr. kamptus in these highest sandy beds suggests the Kamptus Subzone (youngest subzone of the Herveyi Zone).

The exposure of the overlying Kellaways Clay Member is particularly significant; nowhere else is a fossiliferous sequence now exposed more-or-less permanently; the type section of the member, designated by Page (1989), is a cored borehole near Kellaways in Wiltshire (see Kellaways West Tytherton GCR site report, this volume). The recorded ammonite fauna is indicative of the Koenigi Zone, Gowerianus Subzone and certainly includes elements of the Kepplerites metorchus Biohorizon. This fauna is of particular biostratigraphical significance because it includes probable chorotypes (i.e. specimens from a neighbouring locality but at a similar stratigraphical level to the holotype) of the zonal index ammonite Proplanulites koenigi (J. Sowerby) (Figure 2.22). The type locality of the species is likely to have been the now obliterated hrickpit near Rampisham, c. 6 km to the south-east (Page, 1988).

The sandiest bed in the stream sections is assigned to the Kellaways Sand Member. The single Sigaloceras fragment recorded from it suggests the Calloviense Zone and Subzone. This may be confirmed by a loose specimen of Proplanulites ex gr. petrosus (S.S. Buckman)–crassicosta (S.S. Buckman) in a sandy matrix (originally reported by Cope and Cox (1970) as Reineckeia rehmanni (Oppel)).

The ammonite fauna recorded from the topmost part of the Kellaways Sand Member (beds that might belong instead to the Mohuns Park Member of the basal Oxford Clay Formation) indicates the Sigaloceras enodatum Biohorizon of the Calloviense Zone, Enodatum Subzone. The highest beds exposed, with their fauna of Kosmoceras medea and ?Homoeoplanulites, belong to the Middle Callovian Jason Zone, Medea Subzone.

Conclusions

The outcrops in the stream beds and banks near Rye Water Lane, Corscombe which comprise the Ryewater GCR site, provide the best more-or-less permanent exposure of the Kellaways Clay Member of the Kellaways Formation in Britain. The member there yields the only in-situ ammonite fauna of the Lower Callovian Koenigi Zone, Gowerianus Subzone in England. Associated exposures of the Upper Cornbrash and ?basal Oxford Clay Formation also yield stratigraphically important faunas, and together these may comprise one of the most complete Bathonian–Callovian stage boundary successions in Britain. The ammonite faunas collected from the site can be related to those, including type material, from old, now obliterated exposures in the area that are recorded in the literature. These ammonite faunas are important for international correlations of the Lower Callovian Substage.

References