Middle Jurassic (Inferior Oolite Group) its classification and depositional pattern, Bristol and Gloucester region
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
The Upper Lias is succeeded by a sequence of shallow-water marine limestones, known as the Inferior Oolite Group, first identified by William Smith. The group extends through the Aalenian and Bajocian stages into the earliest part of the Bathonian stage.
Throughout Gloucestershire the Inferior Oolite forms the great indented scarp of the Cotswolds Hills, which forms a natural boundary between the Severn Valley to the west and the limestone dip slope of the Cotswolds to the east. From Bath to Doulting the Inferior Oolite forms an elevated tract of dissected country that passes athwart the eastern end of the Mendips, and then crowns a low range of hills extending southwards to Sherborne. Thence, the escarpment continues west-south-westwards to Yeovil, beyond which it rapidly diminishes in the belt of attenuated, condensed and much-faulted limestones that runs westwards to Crewkerne and Chard.
East of the main outcrop, the Inferior Oolite underlies the remainder of the district and over the last few decades deep boreholes have much extended our knowledge in these areas. The most westerly onshore outcrop of the Inferior Oolite forms the capping of Brent Knoll where only the Lower Inferior Oolite survives.
The early classifications of the Inferior Oolite were based on lithology and mixed faunal assemblages. At around the turn of the century S S Buckman, in a series of classic papers, showed that the previous work had failed to recognise the complexity of the stratigraphy and that the key to its understanding lay in the ammonite sequences, supplemented in the Cotswolds, where ammonites are comparitively rare, by the use of brachiopods. This pioneer work, which was greeted with much hostility and misunderstanding, has since formed the basis of our present, but still incomplete understanding and classification of these rocks.
P948997 shows present-day usage, in which the Aalenian is represented as a separate stage rather than as the lowest part of the Bajocian, formerly the favoured British practice. The ammonite zonal scheme, though extensively revised, must to some extent be regarded still as being provisional. The position of the Middle/Upper Inferior Oolite boundary coincides with the top of the Strenoceras subfurcatum (formerly S. niortensis) Zone, following the recognition that the lowest beds of the Upper Inferior Oolite, attributable to the garantiana Zone, rest unconformably or in nonsequence, as originally postulated by Buckman, on the beds beneath.
The general characters of the Inferior Oolite indicate that most of the succession was formed in a shallow shelf sea, where deposition was interrupted and/or modified at frequent intervals by earth movements, which caused slight warping of the sea floor, or by changes in sea level. Nondeposition, or erosion of the sediments, sometimes before consolidation, resulted from the sea bed coming within the influence of current action. Although these events were not big enough to cause visible angular discordance at any one outcrop, they often led to appreciable gaps or nonsequences in the succession, some of which can be recognised over very wide areas. Where subsidence or a rise in sea level followed, there was commonly overstep of older by younger rocks.
The presence of a nonsequence is often, but not invariably indicated by the occurrence of planed, bored and oyster-covered surfaces, by limestone conglomerates, or by condensed deposits in which fossils of more than one faunal horizon are to be found. Some fossils are often rolled and have coatings of limonite or phosphatic material, or encrusting serpulids.
The isopach and distribution map (P948982) of the Inferior Oolite clearly shows two main areas of thick deposits, located in the Cotswolds and in south-east Somerset, and separated by a wide belt of relatively thinner deposits in which the major part or the whole part, of the sequence comprises the Upper Inferior Oolite. The successions in these two areas differ markedly. In the Cotswolds, the thickness of the Upper Inferior Oolite is relatively constant; thus variations of the Inferior Oolite as a whole indicate changes in the residual thicknesses of the Lower and Middle divisions. The depositional environment was dominantly one of very shallow-water, high-energy conditions. The limestones are predominantly oolitic and shell-fragmental, and contain relatively few, poorly preserved ammonites. The Lower, Middle and Upper divisions are separated by well-marked unconformities.
By contrast, in the southern area, the Upper Inferior Oolite shows considerable thickness variations, which follow those of the lower subdivisions. The environment was, on the whole, one of deeper water and lower energy conditions than in the north. The clear distinction between Lower and Middle Inferior Oolite is no longer present. The limestones are mostly finer grained and more marly, and are ferruginous at certain horizons, with abundant geothite ooliths (‘ironshots’), pellets and concretions and incrustations of various sorts. Ammonites are abundant and well preserved.
At outcrop, the only reasonably full development of the southern sequence occurs in the Sherborne–Milborne Port area (P948997, col. 3). The thickest sequences occur farther to the east in the subcrop.
This ‘basinal’ area was flanked on the west and south by a broad positive belt, known as the South Dorset High (P948982), in which sedimentation was much reduced and the succession very variable and highly condensed locally; the Lower and Middle Inferior Oolite may be absent altogether (P948997, col. 4). The ferruginisation of the limestones is more widespread and better developed here than in the adjacent basinal area.
The passage of the ‘northern facies’ into the ‘southern facies’ occurs in the Upper Inferior Oolite in the Wincanton area and appears to be related to the development of the Mere Fault. It is not known where the corresponding facies change occurred in the Lower and Middle Inferior Oolite because of the extensive erosion that these beds suffered in earliest Upper Bajocian times. It must, however, have been a considerable distance to the north of where it lay in Upper Inferior Oolite times, because the deposits on Dundry Hill are of ‘southern facies’.
Kellaway and Welch (1948) suggested that the Inferior Oolite suffered attenuation along a narrow north-south belt (the ‘Bath Axis’), that represented the southern prolongation of the Malvern ‘line’ to the Dorset Coast at Burton Bradstock. Subsequent drilling results, however, indicate that south of the Mendips this attenuation appears to be related to broadly east-west trends in the Wessex Basin (P948982).
- Kellaway, G A, and Welch, F B A. 1948. British regional geology: Bristol and Gloucester district (2nd edition). (London: HMSO for Institute of Geological Sciences.)