Great Oolite Group, Middle Jurassic, Cirencester—North Cotswolds, Bath—Cotswolds Province
| Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.) |
Cirencester–North Cotswolds
Lower Fuller’s Earth
The Lower Fuller’s Earth diminishes in thickness due to lateral passage of its upper part into the Stonesfield Slate Beds between Minchinhampton and Cirencester (P948984). Its lower part persists for some distance, but is absent north of Condicote and east of the Moreton ‘Axis’ where it is replaced by the Chipping Norton Limestone and the Sharp’s Hill Formation.
Chipping Norton Limestone
The Chipping Norton Limestone, which may include the underlying Hook Nor- ton Limestone of Oxfordshire, reaches a maximum thickness of about 12 m in the Hornsleasow (formerly Snowshill) Quarry, about 4 km west-south-west of Bourton-on-the-Water. This formation exhibits considerable lithological variation. Typically, it consists of buff, hard, rather sandy and splintery oolite, often cross-bedded, containing minute specks of lignite; elsewhere the beds are flaggy and have been worked in the past for flagstones. Decalcification has in some cases reduced it to sand.
Fossils are not common. The bivalve Plagiostoma cardiiformis occurs in most exposures, while obscure plant remains and occasional saurian bones of Megalosaurus and the crocodile-like Teleosaurus and Steneosaurus are not infrequently found. Ammonites are rare, but those that occur suggest that the Chipping Norton Limestone falls within the zigzag Zone.
Sharp’s Hill Formation
Overlying the Chipping Norton Limestone north of Condicote in the north Cotswolds, there are up to 2 m of shelly clays known as the Sharp’s Hill Formation, which may include many corals, brachiopods, bivalves and gastropods. They are well exposed at Hornsleasow (formerly Snowshill) Quarry, 4 km west-south-west of Bourton-on-the Water, where a total of 1.7 m of beds includes a coral bed containing abundant well-preserved compound corals such as Cyathopora, Isastraea and Thamnastraea.
Stonesfield Slate Beds
The Stonesfield Slate Beds comprise varied passage strata, 4 to 10 m thick, between the Lower Fuller’s Earth and Taynton Stone north-east of Cirencester. They are most completely seen in the Hampen Cutting of the now disused Cheltenham–Banbury railway line east of Cheltenham. Commonly, the main facies consists of fissile, sandy, shelly, and oolitic limestones comprising the Eyford Member. They were formerly worked for roofing tiles and known as ‘Cotswold Slates’. The ‘Slates’ were obtained from the Slate Bed or ‘Pendle’, an impersistent bed, 0.6 m or less in thickness, of oolitic sandy limestones near the base of the member. The bed was dug out and exposed to frost action which split the rock along the closely spaced relatively permeable laminae formed of ooliths. The fauna is well known mainly due to the long continued collecting of the slates. Characteristic bivalves include Gervillella ovata, Vaugonia impressa and P. acuminata, but the best-known fossils are vertebrate remains including fragments of pterodactyl, dinosaurs, crocodiles and primitive mammals. The zonal index ammonite Procerites progracilis occurs rarely. The thickness varies between 4 m and nearly 10 m.
In the Northleach area a massive freestone member similar to the Taynton Stone and known as the Farmington Freestone is locally developed near the top of the formation.
Taynton Stone
In the Hampen Cutting the Taynton Stone characteristically comprises some 9 m of cream-coloured, cross-bedded oolites with abundant and characteristic seams and wisps of shell detritus. In the Taynton–Burford area it was much worked for freestone and widely used for buildings in Oxford from the Middle Ages onwards; here the thickness reaches up to 12 m. In the area of the Moreton ‘Axis’ the base of the formation oversteps the Sharp’s Hill Formation to rest on the Lower Fuller’s Earth. Rare ammonites indicate a progracilis zonal age for the Taynton Stone.
Hampen Marly Formation
In the north-eastern extremity of the region, the Hampen Marly Formation, from 5 to 9 m in thickness, consists of grey, green and buff, shelly clays and marls with interbedded buff, grey-hearted silty and sandy limestones, which are locally oolitic. The fauna is dominated by bivalves, with locally conspicuous lumachelles (reefs) of Liostrea hebridica; ‘nests’ of the brachiopod Kallirhynchia concinna are common.
Marly limestones become increasingly dominant south-westwards towards Cirencester, where only a few thin marl beds persist. There is a concomitant thinning of the sequence to 3 to 4 m. Beyond Cirencester the formation passes laterally into the Minchinhampton ‘Shelly Beds and Weatherstones’.
White Limestone
In recent years subdivision of the White Limestone has been established as far as the eastern edge of the present region (Table 7, col. 7; Sumbler, 1984[1]). Correlation and subdivision is by means of widespread and distinctive lithological marker beds, including four main gastropod-bearing, hardground, micritic limestone beds. The latter are distinguished by different species of the high-spired nerineid gastropod Aphanoptyxis. The most important section between Cirencester and the Hampen Cutting is provided by the Stony Furlong Cutting, 1.5 km south-east of Chedworth, on the disused Cirencester-Cheltenham railway line. The White Limestone ranges from cross-bedded, ‘millet seed’, pelloidal oolites to level-bedded, white, splintery calcite mudstone or siltstone with a variable proportion of ooliths and pellets. Clay and marl layers are confined to the lower part of the formation. The most distinctive rocks are the ‘Dagham Stone’ hardground horizons (see Great Oolite Group) of which up to five are present; their precise relationship to the hardgrounds recognised in the east remains to be confirmed. However, one persistent bed about midway in the sequence has been identified in the Stony Furlong Cutting and elsewhere as equivalent to the Excavata Bed, which marks the top of the Shipton Member of the Oxford district. The top of the overlying Ardley Member of the Oxford district is defined by the hardground known as the Bladonesis Bed. In the Chedworth Cuttings the beds equivalent to the lower part of the Ardley Member (for long known as the ‘Ornithella Beds’) are characterised by numerous terebratulid brachiopods, notably Stiphrothyris and Digonella digonoides.
Beds corresponding to the uppermost member of the eastern Oxfordshire sequence, the Bladon Member, occur between Cirencester and Burford, and are known as the Signet Member. This member is locally absent because of pre-Forest Marble erosion. The rocks mainly consist of rubbly, finely oolitic limestone with a micritic and finely shell-detrital matrix. The lowest beds, which may be marly, commonly contain abundant specimens of the large brachiopod Epithyris oxonica. Locally, a fossiliferous coralline facies, known as the Fairford Coral Bed, is developed, in which there is an abundance of well-preserved corals such as Isastraea and Montlivaltia.
Forest Marble
The term Forest Marble, first used by William Smith, originally referred only to the distinctive shell-detrital limestones lying between the Great Oolite and the Cornbrash, but it was later extended to include all the associated rocks in this position. These form a variable but nevertheless readily recognisable lithological group. Attempts to define the rocks on the basis of the ‘Bradford Clay’ fauna have led to confusion in the past. Rare ammonites ascribed to the hollandi Subzone have been found in the lower part of the formation.
The dominant facies is argillaceous, but arenaceous and carbonate facies are locally important. In the latter there is complete gradation from pure shelly oolites and calcareous shelly sandstones to sands with doggers. The name ‘Hinton Sands’ was applied by William Smith to the sandy facies after the type locality of Hinton Charterhouse near Bath where they are about 10 m thick. Like other units in the Forest Marble they are lenticular and impersistent, and the term has little stratigraphical significance. The usual limestone type is hard, blue-hearted, rather sandy, shell-fragmental, flaggy limestone, which weathers to shades of brown and buff; it commonly contains ochreous weathering clay galls, specks of lignite and scattered ooliths. The shell debris is of varied provenance but is dominated by oysters.
The various rock types range from wisps and laminae of a few millimetres to lenticular masses up to 10 m or so in thickness. With the exception of the purer shelly oolites, which tend to be confined to thick massive beds in the more northern areas, every degree of interbedding of the various facies occurs. Small-scale current structures such as ripple marks are widespread and are presumed to have been preserved because of the absence of burrowing organisms. The clays, which are typically unbedded, are grey or green when fresh and weather to whitish and brown respectively.
In Somerset, the Forest Marble consists of a clay series with a middle division of shelly, conspicuously false-bedded limestones, largely made up of oyster fragments, and associated interdigitating sandy facies. From south of the Mendips to the Dorset Coast a thin bed of white-weathering marl known as the Boueti Bed forms the dividing line between the Frome Clay (formerly ‘Upper Fuller’s Earth’) and the Forest Marble. The fossils of the Boueti Bed include abundant Goniorhynchia boueti and other brachiopods including Acanthothiris, Avonothyris, Digonella and Dictyothyris. The ossicles of the ‘pear encrinite’ Apiocrinus are also recorded. The fossils of the Boueti Bed are usually covered with serpulids and the marl composing it sometimes contains pellets of clay apparently derived from the underlying beds. It marks a period when there was pause in sedimentation, probably accompanied by erosion. On the Dorset coast, about 18 m above the Boueti Bed, there is a second fossil band known as the Digona Bed from the abundance of Digonella digona in it. There is borehole evidence to suggest that this bed extends northwards into the present district; it is associated with a further pause in sedimentation, for the underlying stratum is strongly burrowed.
The limestones form one of the most prominent of the Jurassic escarpments in south-east Somerset. Starting in the south of the area the Forest Marble outcrop forms the ridge of Birts Hill and runs to Sutton Bingham and Hardington. The greatest thickness of the formation is at Birts Hill where it exceeds 50 m. From here it extends north-east past Sherborne, where it is 40 m or more thick, to Wincanton. At Bowden the limestone was extensively quarried as ‘Bowden Marble’. From Wincanton to Frome the Forest Marble is 30 to 40 m or so thick and gives rise to the high ground upon which Frome is built.
The abrupt passage northwards from argillaceous basin facies into shallow-water carbonate shelf facies that characterises the Frome Clay-Great Oolite transition, also affects the lower third of the Forest Marble which passes northwards into the Upper Rags (included on the Bath (265) and Frome (281) sheets with the Great Oolite) just south of Bath. Here the thickness of the Upper Rags ranges between 4.5 and 9 m and the dominant lithology is cream-coloured, shell-detrital, cross-bedded oolite, locally used as a freestone. A widespread patch reef development at the base is called the Corsham Coral Bed and a more limited development at the top is known as the Bradford Coral Bed. The former is named after the section in the eastern approaches cutting of the Box Tunnel where five patch reefs, together with their associated apron deposits, are well exposed over a distance of 300 m directly underlain by the bored top of the Bath Oolite.
Farther north the Upper Rags were mapped separately as the Acton Turville Beds by tracing field brash from the bored hardground on the top of the Bath Oolite together with coralline debris above it. Northwards from Hawkesbury Upton the Acton Turville Beds are no longer separately distinguished on the Geological Survey maps from the remainder of the Forest Marble.
East of Bath the Bradford-on-Avon district is the type area for the celebrated Bradford Clay. Here the clay is about 3 m thick including a fossil-bed, 0.3 to 0.6 m thick at the base, and is separated from the Upper Rags by about 2 to 3.5 m of rather massive shelly Forest Marble limestone. When well developed the brachiopod fauna is rich and distinctive including such forms as Digonella digona, Eudesia cardium, Dictyothyris coarctata and Epithyris bathonica. In its type locality in the Canal Quarry, on the south side of Bradford, groups of Apiocrinus parkinsoni have been found attached in position of growth to the underlying limestone floor. The fossil assemblage has a number of forms in common with the Boueti Bed. Detailed work in this and surrounding areas has shown, however, that the ‘Bradford Clay Bed’ is merely one of several impersistent fossiliferous clay beds interbedded with limestones in the lower third or so of the Forest Marble. The Bradford Clay fauna occurs over a few metres of strata and not just as a thin marker band.
Between Hawkesbury Upton and Cirencester there is a considerable but variable development of limestone, mainly cross-bedded, shell-detrital oolite, between the top of the Athelstan Oolite (or White Limestone) and the base of the more typical terrigenous facies of the Forest Marble. Boreholes in the Malmesbury–Kemble area have proved from 9 to 14 m of these beds, and also a variable thickness of the underlying Great Oolite.
Defining the base of the Forest Marble within these oolitic limestones has proved difficult. Earlier accounts have taken it at the base of the ‘Bradford Clay’ and the name ‘Kemble Beds’ applied to the underlying oolites, but the discontinuous nature of the Bradford Clay has made consistency of application impossible. Survey officers on the Malmesbury (251) and Swindon (252) sheets have regarded the local representative of the Corsham Coral Bed as the basal member of the Forest Marble. The latter rests on a planed and bored surface which can commonly be recognised even when the coral bed itself is absent. However, even these criteria are locally difficult to apply.
The thickness of the Forest Marble, including the Acton Turville Beds or the Upper Rags, varies between about 26 to 28 m between Bath and Malmesbury, but it thins to about 20 m at Cirencester and more dramatically to about 5 to 8 m in the Burford area. The thickness variations in the latter area are due to the strong channelling of the Forest Marble into the White Limestone. Similar channelling has been noted in the railway cuttings north-east of Cirencester.
Between Cirencester and Fairford the Forest Marble locally furnished tilestones which are known as ‘Poulton Slates’.
References
- ↑ Sumbler, M G. 1984. The stratigraphy of the Bathonian White Limestone and Forest Marble of Oxfordshire. Proceedings of the Geologists’ Association, Vol. 95, 51–64.