Lower and Middle Inferior Oolite, Middle Jurassic, Bristol and Gloucester region
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
Lower and Middle Inferior Oolite
S S Buckman, at around the turn of the century, subdivided the Middle Inferior Oolite and defined the relationship of the Lower, Middle and Upper parts to each other. In the Cotswold area (P948961, col. 1), the Lower Inferior Oolite is primarily an oolite freestone; the Middle Inferior Oolite has been described as ‘Ragstone Beds’. The use of the term ‘grit’ for some of the members is misleading because the rocks, though comprising hard, shelly and rubbly limestones with a fine-grained ‘gritty’ bioclastic matrix, lack any coarse quartzose sand or ‘grit’ content.
The Lower Inferior Oolite of the area has recently been re-examined by Mudge (1978), who proposed a new nomenclature, to which, however, some objection has been raised (Cope et al., 1980). The following description takes cognisance of the former account and the new names are added in parenthesis where appropriate. For purposes of description the long established names are retained, but the new work enables them to be applied with a precision and consistency that was often lacking in the older accounts.
Scissum Beds (Leckhampton Limestone)
These yellow to brown, rather fine-grained, sandy limestones and calcareous sandstones extend throughout the area shown as Lower and Middle Inferior Oolite (P948982). Although the Scissum Beds usually rest with nonsequence on the Upper Lias and locally include a basal conglomerate, the lower limit may, in the absence of exposures, be difficult to locate where they directly overlie Cotteswold Sands. The rocks are bioturbated and their somewhat sparse fauna is mainly of bivalves and rhynchonellids, especially Homoeorhynchia cynocephala and Rhynchonelloidea subangulata. The upper part is appreciably more calcareous than the lower and may contain limonitised pellets and scattered ooliths. The thickness is rarely more than a few metres.
In the Stroud-Wotton-under-Edge area, the Lower Limestone comprises up to 10 m of unfossiliferous, massive, current-bedded oolite (Frocester Hill Oolite) which rests on a planed, oyster-encrusted surface of the Scissum Beds. It may include an appreciable coarse sand fraction and, locally, even rounded grey quartzite pebbles, as in the Stroud district, where it is termed ‘Dapple Beds’. It passes eastwards and northwards into well-bedded, oolitic, bioclastic limestones (Crickley Limestone) that show a downward passage into the underlying Scissum Beds.
The beds which give this member its name are the very coarsely pisolitic limestones of the Crickley Hill (P210783)–Leckhampton Hill–Cleeve Hill sections (Crickley Oncolite). The distinctive pisoliths are disc-shaped, algal-coated grains (oncoliths) in which the micrite coatings include moulds of algal tubes and filaments. The beds, which measure about 2 to 5 m in thickness, are buff, marly and rubbly with an abundant fauna in which micromorphic brachiopods, gastropods and regular echinoids are notable. Bivalves are abundant and the normal-sized brachiopods include Stroudithyris pisolithica, Plectothyris plicata and Epithyris submaxillata.
The term Pea Grit, or Pea Grit ‘Series’, has generally been extended to include the variable group of rubbly, pelloidal, coralliferous and sparsely pisolitic limestone and interbedded oolitic rocks (Fiddler’s Elbow Limestone) that intervene between the main pisolite (Crickley Oncolite) and the typical oolite-freestones of the succeeding member. Mudge considers that this variable group is replaced laterally by a much bioturbated, dominantly oolitic facies (Cleeve Hill Oolite) in the Cheltenham area, which the older accounts generally include in the Lower Freestone. The Pea Grit, in its wider sense, measures about 8 to 12 m in thickness and is recognised at outcrop between Cleeve Hill and the Stroud area; it continues in a south to south-easterly direction beneath the cover of younger rocks. To the west of this belt, the greater part of the Lower Inferior Oolite comprises cross-bedded oolites which cannot be subdivided with certainty, while eastwards the Pea Grit apparently passes laterally into a warm brown freestone known as ‘Guiting Stone’ (Jackdaw Quarry Oolite), which has been extensively worked at Temple Guiting and Stanway on the western margin of the Cotswolds, and to the east at Bourton-on-the-Hill. This rock is known informally as ‘Yellow Guiting’, the overlying ‘White Guiting’ being the equivalent of the Lower Freestone.
Lower Freestone (Devil’s Chimney Oolite)
The Lower Freestone is the thickest and one of the most distinctive members of the Lower Inferior Oolite. It is important in Cheltenham as a building stone and was once extensively quarried on Leckhampton and Cleeve hills, where its thickness and massive, uniform oolitic nature and relative freedom from shells made it ideal for dimension stone and carving. The beds are strongly current-bedded. It is said by Mudge (1978) to be nearly 40 m thick at Leckhampton, where most of the underlying Cleeve Hill Oolite is included. Recent BGS work at Cleeve Hill, however, shows that this section is faulted. A borehole on Cleeve Cloud has proved a total thickness, together with the Cleeve Hill Oolite, of 51 m. The Lower Freestone has long been worked under the name ‘Campden Stone’ on the hill above Chipping Campden. It is still worked at Westington Hill Quarry and, farther south-west, in Jackdaw Quarry at Stanway.
Oolite Marl — Upper Freestone (Scottsquar Hill Limestone)
Recent detailed work confirms Buckman’s original contention that the Oolite Marl and Upper Freestone represents a single sedimentary unit of interdigitating facies (Baker, 1981). This highly distinctive member rests nonsequentially on the Lower Freestone, whose upper surface is an oyster-encrusted, planed and bored hard-ground, although the time gap represented is probably quite short. Three main facies are represented. First, a marl-dominated ‘trough’ facies, corresponding to the Oolite Marl, which comprises marls with subordinate calcite-mudstones (micrites) that may contain abundant pellets (pelmicrite). Second, an oolite-dominated ‘shoal’ facies which comprises ooliths in a matrix ranging from calcite mud (oomicrite) to clear calcite (oosparite), which corresponds to the Upper Freestone. Finally, a mainly micritic marginal facies which shows a range of lithologies transitional to the two main types and dependent for its characters on proximity to either shoal or trough deposits; this facies shows evidence of much reworking of the sediments. The finer-grained rocks are often highly fossiliferous. Brachiopods are particularly abundant and, as originally recognised by Buckman, show stratigraphical zonarion. The latest work distinguishes a lower Zeilleria–Flabellirhynchia lycetti fauna, associated with micromorphic brachiopods, and an upper Globirhynchia–Plectothyris fimbria fauna. Reworking has led to a mixture of forms in a few localities.
Although the oolite facies overlies the marl facies over the greater part of the area, this is not always so, for the oolite dominates the entire succession in the Stroud area, and the marly-micritic facies dominates in the area of the Painswick and Cleeve Hills troughs. The best exposure of the beds is in Westington Hill Quarry above Chipping Campden.
Harford Sands, Snowshill Clay and Tilestone
This variable group of beds occupies a restricted area in the north Cotswolds and is best seen in the region between Winchcombe and Broad Campden. Although the members are described as an ordered tripartite sequence, this is only an approximation and the members should more accurately be regarded as facies types.
The lowest member, the Harford Sands, consists of up to 3 m of pale brown quartz sand, which is frequently hardened to form doggers or sand-burrs. Mineralogically, they are characterised by abundant grains of sphene and rare kyanite. The sands were formerly dug on Cleeve Hill and carried to the Staffordshire potteries.
The Snowshill Clay comprises some 4 to 5 m of stiff, chocolate-coloured clay, which reaches its maximum thickness at Blockley. The overlying Tilestone is also best developed in the Blockley area, where it consists of sandy, oolitic, flaggy limestone, locally containing rolled pebbles of oolite.
Lower Trigonia Grit
This basal member of the Middle Inferior Oolite was deposited upon the eroded surface of underlying strata, and its base is frequently conglomeratic. The deposit consists of rubbly, commonly ironshot limestone crowded with fossils, particularly bivalves such as Trigonia. A coral bed with Latomeandra occurs near the base.
Buckmani and Gryphite Grits
This block of yellow and brown, sandy, marly rubbly, shelly and commonly iron-shot limestones was divided by Buckman into a lower part characterised by Lobothyris buckmani and an upper part with an abundance of the oyster Gryphaea.
The Notgrove Freestone has a wide extent in the north and central Cotswolds, where it attains a thickness of 4.5 to 8 m. It consists of hard, white, fine-textured oolitic limestone, locally crowded with shells of Propeamussium cf. laeviradiatum (formerly Variamussium pumilum).
The Witchellia Grit consists of thin, grey-brown, ironshot limestone containing ammonites in greater abundance and in a better state of preservation than the lower beds.
Phillipsiana and Bourguetia Beds
These beds, representing the highest part of the Middle Inferior Oolite preserved in the Cotswolds, are confined to the Cleeve Hill Syncline. They consist of hard, shelly limestones yielding ‘Terebratula’ phillipsiana and the gastropod Bourguetia striata. Many of the brachiopod shells are beekitised (silicified).
The Bajocian transgressions in the Cotswolds
During Bajocian times the Cotswold area was affected by two main periods of earth movement and their associated intraformational erosion and transgression. The Lower, Middle and Upper Inferior Oolite deposits reflect the effects of the movements.
Deposition of the Inferior Oolite in the Cotswolds commenced with the formation of the Scissum Beds and continued without major interruption until the close of Lower Freestone times. Subsidence during this period appears to have been greatest in the Cheltenham–Cleeve Hill area, which formed the centre of a broad basin of deposition where the maximum thickness of the sediments was built up.
After a break in sedimentation at the end of Lower Freestone times, the Upper Freestone and oolite marl were laid down. Facies variation within these rocks provides the first clear evidence of the initiation of the Painswick Syncline or Trough and the Birdlip Anticline or ‘High’, as well as the continuing development of the Cleeve Hill Syncline. The ‘high’ areas are distinguished by thinner sedimentation and oolitic shoal facies, the ‘low’ areas by thicker sedimentation with micritic ‘trough’ facies. Slight general uplift, followed by a change in conditions, led to the deposition of the Harford Sands, which apparently spread from the east. They were overlapped first by the Snowshill Clay and then by the Tilestones. The full extent of the area of deposition, particularly to the west of the Cleeve Hill Syncline, cannot be estimated owing to the effect of later erosion.
Deposition may have been continuous in the centre of the Cleeve Hill Syncline, though the nature of the sediments suggests that the waters were shallow and landlocked. The deposition of the sandy limestone known as the Tilestone, with its low-diversity marine fauna, took place in more open water. Thereafter, the whole area was submerged beneath the waters of the Lower Trigonia Grit sea. This extension of the area of deposition constitutes the Lower Bajocian Transgression. During deposition of the Middle Inferior Oolite, thickening of the various divisions, as they are traced from Painswick into the Cleeve Hill Syncline, suggests that once again subsidence was greatest in the latter area.
In the Cotswolds, the date of the second, major phase of warping can only be fixed as later than the Phillipsiana Beds and earlier than the Upper Trigonia Grit, since no intermediate strata are represented north of the Mendips. This warping, which was more widespread and of greater intensity than that of the earlier Aalenian–Lower Bajocian movements, completed the shaping of the Painswick and Cleeve Hill synclines and the intervening Birdlip Anticline. Following this episode, the entire Cotswold area was elevated and subjected to erosion and the Middle Inferior Oolite was completely removed from the area of the Birdlip Anticline.
Erosion and planation were followed by subsidence of all the country from the Mendips to Moreton-in-Marsh, and the limestones of the Upper Trigonia Grit were laid down upon the bored and eroded surface of the older rocks.
Dundry and the area south of the Mendips
The main outcrop of the Inferior Oolite from Old Sodbury to Doulting shows Upper Inferior Oolite resting upon an erosional surface of Upper Lias or older strata (P948991). At Dundry Hill (P948997, col. 2), however, in the most westerly outlier north of the Mendips, up to about 6 m of Lower and Middle Inferior Oolite are preserved in a shallow syncline beneath the Upper Inferior Oolite in the western half of the hill. The Upper Inferior Oolite rests directly on Upper Lias at the eastern end of the outlier. The close similarity between the Dundry succession and contemporaneous strata in Dorset and east Somerset indicates that these areas were within the same depositional and faunal province during Lower and Middle Inferior Oolite times. Sandy ferruginous beds and hard limestones with limonitic ooliths (‘ironshots’) are the typical Dundry rock-types. Of the fossils, brachiopods are common, while all groups of the mollusca are abundant, with numerous well-preserved ammonites.
Middle and Lower Inferior Oolite rocks are also preserved south of the Mendips in the shallow Cole Syncline at Bruton, in east Somerset (P948991). Here, rocks of the blagdeni Subzone are present beneath the Upper Inferior Oolite. These were thought to represent the youngest beds preserved beneath the Upper Bajocian transgression, but in the Doulting area the conglomeratic limestone below the base of the Upper Inferior Oolite (Doulting Stone) is now considered to belong to the younger, lowest subzone of the subfurcatum Zone.
There is evidence that the Mendip ‘High’ and its north-eastward continuation through Trowbridge and thence along the line of important anticlinal structures in the Cretaceous rocks, was bounded to the south by an important growth fault, which was active throughout much of the Mesozoic (see P948989). In Bajocian times, the most dramatic known movement along this structure occurred at Kingsclere to the east of the present region where one of the thickest known Inferior Oolite successions, some 108 m, occurs a short distance to the south of successions only 10 m or so thick. In the present region this structure is known as the Vale of Pewsey Fault; the Westbury Borehole provides evidence for its activity in Middle Jurassic times. The borehole proved a thick succession of Inferior Oolite (39 m) immediately south of the wide belt in which only relatively thin Upper Inferior Oolite is present (P948982). Unfortunately, owing to very poor core recovery in the borehole, the thickness of Lower and Middle Inferior Oolite is not known, though this must have been considerable.
Southwards from the Cole Syncline, the Mere Fault appears to have acted as a controlling influence on sedimentation in an analagous way to the Pewsey Fault, with only thin Upper Inferior Oolite to the north of it and a much thicker succession to the south including both the Lower and Middle divisions. At outcrop, the Lower and Middle Inferior Oolite reappear on the south side of the Mere Fault and, within 2 km or less measure 8 to 10 m in thickness. The change is, however, most dramatic at Wincanton (P948982) where the borehole adjacent to the fault proved a thickness of at least 40 m of the lower divisions. Within the basin, the uppermost part of the Middle Inferior Oolite comprises a distinctive sequence, no more than 0.5 to 2 m thick, of fossiliferous, ironshot, glauconitic limestones with abundant ammonites. These beds, which contain local nonsequences and con- densed successions, span the zonal range of subfurcatum to laeviuscula. The most complete sequence at outcrop occurs to the east and north-east of Sherborne (P948997, col. 3). These ironshot limestones correspond to the highly condensed ‘Irony Bed’, often barely more than a few centimetres thick, in the area west of the basin. Within the basin, the ironshot beds overlie fossiliferous, hard grey, variably rubbly and sandy limestone with irregular marly clay partings. These beds are now named the Corton Denham Beds, the upper limit of which is taken at a minor unconformity just above the base of the overlying ironshot beds (P948997, col. 3). The thickness ranges from 6 to 8 m. The Ringens Bed is a widespread fossiliferous marker horizon about 2 m above the base which contains abundant brachiopods, notably Homoeorhynchia ringens.
Westwards from Sherborne, the thickness of the Lower and Middle Inferior Oolite diminishes steadily, and the greatly condensed successions thereabouts have long been famous for the profusion of their fossils. S S Buckman’s work in unravelling the complexity of the ammonite successions of the area has remained classic. At Halfway House, between Sherborne and Yeovil, these strata comprise 1.7 m of mainly ironshot limestone with the ‘Irony Bed’ at the top and the Ringens Bed lying only a few centimetres above the base (P948997, col. 4). This much attenuated thickness for the combined Lower and Middle Inferior Oolite persists for nearly 20 km westwards along the outcrop. Locally, in what may informally be called the Yeovil ‘High’ at Clifton Maybrook and Haslebury Mill, respectively 2 km south-east and 11 km south-west of Yeovil, the beds are absent, and in the Stoford area they range from 0.5 to 0.85 m. Elsewhere within the marginal belt they are only slightly thicker, with the Lower Inferior Oolite being thicker and more widespread than the Middle Inferior Oolite. The latter is absent over most, if not all of the Misterton–Crewkerne–Haselbury area.
Not until the western edge of the Inferior Oolite outcrop near Seavington St Mary and Hinton St George is reached do the strata thicken to any significant degree. Although there are no complete sections, it is probable that the Lower and Middle Inferior Oolite may together attain 3 m or so. Of this thickness approximately the upper third is attributable to the Middle division, which is roughly equivalent to the well-known condensed ferruginous ‘Red Bed’ of the Dorset coast (P948997, col. 4). Further evidence for this westward thickening is provided by a borehole drilled at Seaborough in 1974 and by outcrops in Dorset, to the south of the present region (P948982).
Discussion of the conditions of deposition and sedimentation of the Lower and Middle Inferior Oolite of this southern Dorset–south Somerset area is discussed below (see Upper Inferior Oolite, Middle Jurassic, Bristol and Gloucester region).
- Mudge, D C. 1978. Stratigraphy and sedimentation of the Lower Inferior Oolite of the Cotswolds. Journal of the Geological Society of London, Vol. 90, 133–152.
- Cope, J C W, Duff, K L, Parsons, C F, Torrens, H S, Wimbledon, W A, and Wright, J K. 1980. A correlation of Jurassic rocks in the British Isles. Part 2: Middle and Upper Jurassic. Special Report of the Geological Society of London, No. 15.
- Baker, P G. 1981. Interpretation of the Oolite Marl (Upper Aalenian, Lower Inferior Oolite) of the Cotswolds, England. Proceedings of the Geologists’ Association, Vol. 92, 169–188.