Great Oolite Group, Middle Jurassic, Tormarton—Nailsworth, Bath—Cotswolds Province
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
Lower Fuller’s Earth
Apart from an increase in thickness, from 16 to 25 m between Tormarton and Nailsworth, the Lower Fuller’s Earth continues from the south largely without change in this area. In the southern part of the area, sections between the Inferior Oolite and the Great Oolite were provided by a pipeline trench at Dyrham Park in 1969 and during the construction of the M4 motorway 2 km to the north. These showed that the Acuminata Bed continues from the south (see Great Oolite Group, Middle Jurassic, Bath, Bath—Cotswolds Province) as a thin, discrete bed about 5 m below the base of the Fuller’s Earth Rock. Farther north, due no doubt to increasing estuarine influences in that direction, Praeexogyra acuminata also becomes abundant in the overlying strata, which include shelly limestones, and the name ‘Acuminata Beds’ has been applied to this sequence as in the south (seeGreat Oolite Group, Middle Jurassic, Dorset—Somerset Province).
Dodington Ash Rock
The M4 motorway sections proved that the Dodington Ash Rock, shown on the survey maps as Cross Hands Rock, is equivalent to the Milborne Beds at Bath. The zonal ammonite Morrisiceras morrisi, typical of the latter, has been found in the upper part of the rock near its northern limit, about 5 km west of Minchinhampton.
The Dodington Ash Rock is a compact, rather fine-grained limestone with some shell debris. Scattered ferruginous granules and pisoliths are typically present. The thickness varies from 3 to 8 m. North-east of Nympsfield the formation is thought to pass into the ‘Shelly Beds and Weatherstones’ at Minchinhampton.
The Ornithella and Rugitela beds which, at Bath, form the upper part of the Fuller’s Earth Rock, are represented north of Tormarton by the Hawkesbury Clay, a dominantly mudstone sequence up to 11 m thick. It thins northwards and passes laterally into the limestone succession at Minchinhampton.
Upper Fuller’s Earth
The Upper Fuller’s Earth passes laterally northwards from Tormarton into a limestone sequence by the thickening of two main limestone bands at the expense of the intervening clays (P948984). The limestones change progressively from argillaceous to finely shell-detrital to oolitic. The lower non-oolitic part of the limestone sequence near Hawkesbury is known as the Tresham Rock, and the upper oolitic phase is called the Athelstan Oolite. The latter progressively replaces the Tresham Rock north-eastwards. These oolites were for long confused with those of the younger Great Oolite. The lateral changes are similar to those already noted in the Great Oolite south of Bath, although the complete transition from clay to oolite takes place over a much greater distance (P948984).
The 30 m-thick Great Oolite sequence continues north from Bath as far as Castle Combe. Thereafter, it appears to be progressively overstepped by the Forest Marble (Acton Turville Beds) and is absent from the main outcrop north of Starveall, though farther east it is locally present within a broad belt of oolites extending north-eastwards towards Cirencester. The underlying Athelstan Oolite is generally distinguishable from the Great Oolite by its paler colour, greater purity and eveness of grain.
Along the main outcrop north of Starveall the Coppice Limestone occupies the stratigraphical position of the Great Oolite and is widely overlain by the Forest Marble. Locally, however, outlying patches of Great Oolite are preserved between the two where the Forest Marble overstep has not completely removed it. The Coppice Limestone has been regarded as the top member of the Athelstan Oolite (Cave, 1977), but it could equally well represent a condensed, hardground facies of the Great Oolite beds.
The Coppice Limestone is a distinctive, fawn to cream-coloured, very hard calcite mudstone with a planed, bored and oyster-encrusted surface. The limestone is typical of the micritic hardground beds known as ‘Dagham Stone’ that occur at intervals in the White Limestone farther north. A characteristic feature of both is the presence of ramifying voids, which it has been suggested are due to the solution of either branching corals or, perhaps more likely, burrow systems. The thickness of the Coppice Limestone is usually 0.3 m to 0.6 m but it may reach 1.5 m. The fauna of bivalves and gastropods indicates that the rock is of primary origin, probably deposited in a sheltered lagoon.
- Cave, R. 1977. Geology of the Malmesbury district. Memoir of the Geological Survey of Great Britain.