TEDBURY CAMP QUARRY  A geological gem in the Mendip Hills

The Jurassic

Some 4-6m of sub-horizontal Jurassic strata is preserved around all but the eastern edge of the quarry, the remainder having been removed during quarrying operations. It is assigned to the uppermost part of the Inferior Oolite Formation which is of Upper Bajocian (Middle Jurassic) age (Bristow et al., 1999). Beyond the quarry the formation is preserved in outliers across the eastern half of the Beacon Hill pericline where it is up to 20m thick and known locally as Doulting Stone, which is one of Britain’s most important building stones. The oldest part of the succession comprises a poorly bedded, occasionally rubbly and nodular, oolitic grainstone unit 1.00 -1.20m thick. In contrast to many other localities in the Mendips, the basal part is not conglomeratic and contains no reworked fragments of Carboniferous Limestone. Fossils are particularly abundant at the unconformity surface and in the overlying few centimetres of well-cemented oolite (see ‘Palaeontology’ section below). The top of the unit shows evidence of boring (Figure 9) and it is encrusted with fragmented bivalves (mainly Liostrea and Ctenostreon). This is interpreted as a hardground, and it can be traced around the extent of the quarry face (as Hardground ‘A’) where it is often expressed as a resistant, bench-like feature (Panorama NW). Hardgrounds are produced during periods of reduced sedimentation when the dissolution of aragonite shells on or just below the sea floor increase the amount calcium carbonate that is available to be precipitated as inter-granular cement.
This resource developed by Dr Martin Whiteley & realised by Peter Williams for ESTA (The Earth Science Teachers’ Association)

Figure 10. Prominent bluff of Inferior Oolite

Above Hardground B, the uppermost 2-4m of limestone are poorly bedded, locally oolitic grainstones with occasional highly resistant, dark grey, cemented horizons. Lenticular beds and vertical borings are developed at the base, and bivalve, brachiopod and gastropod fragments are common throughout. Weakly developed cross-bedding with a WNW dip orientation is seen in the southern part of the quarry (Figure 11).

Figure 11. Cross-bedding in the Inferior Oolite

The entire Jurassic succession is disturbed by numerous near-vertical faults and shatter zones (Figure 12). These align with displaced features on the unconformity surface (see ‘Unconformity’section below) and are often infilled with brown clay, presumably derived from the soil horizon above. In the region of Location 5 (see Figure 5), a fault plane bearing dog-tooth calcite crystals clearly displaces the bench produced by Hardground A by a few centimetres.

Figure 9. Bored surface of Hardground A

A thin (2-5cm), brown, claystone horizon overlies Hardground A and separates it from a massive, pale grey (cream weathering), lenticular bed that is 50-80cm thick and ooltic in parts. The base of the bed is smoothly scalloped, perhaps as a result of loading into the claystone and enhancement by solutional weathering. Numerous horizontal burrows that are weathered out as holes and tubes up to 25mm in diameter characterise this bed and they are particularly common in the middle part (Figure 10). The top of the bed is often convex upwards and assumes a lenticular, bank-like form with a bored surface and encrusting bivalves. This is identified as Hardground ‘B’ and it can also be traced around the quarry Panorama NW).

Figure 12. View of the Inferior Oolite succession, looking west

Palaeontology.The following faunal list for the Jurassic strata has been compiled from publications, unpublished field notes (principally from Charles Copp) and material collected by various individuals. However, these fossils have not been systematically recorded or archived in the past, making it impossible to verify the assemblage. The fauna is dominated by bivalves (Ctenostreon sp., Liostrea sp., Lithophaga sp., Pholadomya sp., Pleuromya sp., Trichites sp. and Trigonia sp.) and brachiopods (Acanthothyris spinosa, Kallirhynchia sp. and Sphaeroidothyris sphaeroidalis). Occasional echinoids (Crotoclypeus sp. and Nucleolites sp.) have been found in life position on the unconformity surface and above Hardground B, whilst Hardy (1999, p.82) records the ammonite Parkinsonia sp. Poorly preserved pleurotomariid and procerithiid gastropods, corals and serpulid worms are also known.   In order to help the non-specialist identify the fossils that are most likely to be found here, a montage of typical Inferior Oolite fossils is shown in Figure 13. Whilst none of these particular specimens originate from Tedbury Camp, the Acanthothyris spinosa (Figure 14) and Crotoclypeus sp. (Figure 15) do.

Figure 13. Typical fossils from the Inferior Oolite

Clockwise from bottom left: Pleuromya uniformis; Trigonia costata; Liostrea hebridica; Ctenostreon pectiniforme; Sphaeroidothyris sphaeroidalis; Pholadomya lirata. Images reproduced from sources created by the Natural History Museum, London; the scale bar represents 1cm

Figure 14. Acanthothyris spinosa from Tedbury Camp

Specimen found 10cm above the unconformity surface at the base of the prominent Jurassic bluff (Location 4 on Figure 5)

Figure 15. Crotoclypeus sp. from Tedbury Camp

Specimen found 25cm above Hardground B towards the top of the prominent Jurassic bluff (Location 4 on Figure 5)
The oyster Liostrea and the thick-shelled, scallop-like Ctenostreon are found mainly in life position in small patches on the unconformity surface, but they were probably much more widely distributed before the surface was mechanically cleared. Liostrea typically lived in very shallow water and cemented itself to stable substrates, whilst Ctenostreon was bysally attached when young and free-lying on the substrate when older. These bivalves are therefore well adapted to surviving in conditions where the unconformity surface might have been repeatedly swept clear of sediment by current action, and then buried again. The other common bivalve, Trigonia, was a suspension-feeding, shallow burrower in soft substrates. It is usually preserved as either an internal or external mould at Tedbury Camp, the aragonite shell having been dissolved away. Pholadomya and Pleuromya were vertically-orientated, fairly deep burrowers in mud and sand; they are occasionally found at Tedbury in orientations that suggest that they have been eroded from their domicile and re-deposited.
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