Middle Jurassic (Great 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 Great Oolite Group embraces all the formations between the Inferior Oolite and the Kellaways Clay. The pioneer work on this group was done by William Smith in the Bath district in the 1790s and the names Fuller’s Earth, Fuller’s Earth Rock and Great Oolite derive from this time. Bath is the type area for these rocks which, apart from the Upper Cornbrash and with the addition of the topmost beds of the Inferior Oolite, are assigned to the Bathonian Stage (P948998).
Although the succession is predominantly clay south of the Mendips, the Fuller’s Earth Rock and, more particularly the Forest Marble, form well-marked escarpments. Northwards from the southern environs of Bath, the outcrop broadens with the incoming of thick oolitic limestones, which form the steep scarp and wide uplands of the southern Cotswolds. In the mid and north Cotswolds, where the Inferior Oolite thickens and gives rise to the highest ground, the Great Oolite forms the wide tablelands to the east of the main escarpment that slope gently eastwards and southwards to the low-lying claylands of the Oxford Clay outcrop. The freestones, ragstones and tilestones (‘slates’) of the group have long been widely used in the Cotswolds for the buildings and dry-stone walls that contribute so much to the beauty of the area.
The correlation of this extremely varied group of rocks is difficult due to the changeability of their sedimentary facies and their faunas, coupled with the rarity of ammonites. Nevertheless, correlation has advanced much in recent years, partly due to new borehole information.
Arkell and Donovan in an important paper in 1952 on the Fuller’s Earth and Great Oolite of the Cotswolds wrote that ‘the rocks between the Inferior Oolite and the Cornbrash are the least understood in the Jurassic’. Until then the traditional view, originally put forward by William Smith and developed in detail by Woodward in the latter part of the 19th century, was that the Lower Fuller’s Earth, Fuller’s Earth Rock, Upper Fuller’s Earth, Stonesfield Slates, Great Oolite, Bradford Clay and Forest Marble were independent sequential formations. However, as early as 1901 Buckman pointed out that, on the evidence of ammonite faunas, the Fuller’s Earth Rock was contemporaneous with the lower part of the Great Oolite of Minchinhampton in Gloucestershire. The stratigraphical implications of this interpretation, however, were not fully appreciated until the investigations by Arkell and Donovan cited above, and by others, notably the British Geological Survey, later.
The name Fuller’s Earth has been used since the time of William Smith for the clays that occur between the top of the Inferior Oolite and the base of the Great Oolite or, where this is absent, the Forest Marble. It derives from the occurrence in the Bath area of a bed of commercial fuller’s earth within the clay sequence. The more recent extension of the term to include the dominantly limestone successions of the mid-Cotswolds, which are considered to be the lateral equivalents of the clay farther south, is not followed here and the term Fuller’s Earth is used in a lithostratigraphical sense to refer to the clay facies.
The name Great Oolite is used for the main oolite sequence of the Bath area within the Great Oolite Group. The term Great Oolite Limestone, which has been loosely used synonymously with Great Oolite, refers to strata of different ages both in the north Cotswolds and in the Midlands; it is therefore, not used in this account.
The ammonite zones of the Bathonian in Britain are not subdivided, apart from those of Zigzagiceras zigzag (three subzones) and Clydoniceras discus (two subzones). The Prohecticoceras retrocostatum Zone of earlier accounts has been renamed the Procerites hodsoni Zone. Unfortunately, ammonites are rare in the Great Oolite Group except in parts of the Dorset Basin, so that even today, after a trickle of discoveries in the last two decades, the limits of the majority of the zones can only be regarded as provisional.
The Great Oolite Group of the district falls into two main provinces. South of the Bath–Mendip area, and covering Dorset and part of Somerset (P948985), the sequence is mainly argillaceous and comprises the Fuller’s Earth, the Frome Clay, the Forest Marble and the Cornbrash. Together, these formations thicken southwards into the Wessex Basin where the thickest sequences, in excess of 300 m, lie in a growth faulted belt beyond the margins of the district. Northwards and north-eastwards from this clay-dominated basinal area lies a wide zone covering much of Gloucestershire and Wiltshire, in which most of the clays pass laterally north-eastwards into fine-grained detrital limestones and oolites. These in turn give way to the thinner but varied sequences of Oxfordshire and north Gloucestershire in which micritic limestones form an important part, and which are themselves marginal to the low-lying London Platform landmass east of the borders of the district. Only the overlying Forest Marble and Cornbrash extend across the entire district without substantial facies changes, though the former is notably thicker and more argillaceous in the basinal area.
The successions are broadly interpreted in terms of a relatively deeper-water open sea to the south, which passed northwards and north-eastwards into a shallow marine shelf with tranquil conditions on its deeper water margins and turbulent water farther inshore, where there were shifting carbonate sand banks and shoals. Yet farther inshore, the shoals gave way to shallow lagoonal conditions with evidence of periodic brackish and freshwater influence, and even occasional emergence. The position of the edge of the shelf fluctuated between north and south in Bathonian times; each successive transgression, represented by the clays, penetrated less far north, while the intervening phases, represented for instance by the oolite shoals, migrated progressively farther south (P948984, P948985).
As the facies belts moved backwards and forwards in response to the relative changes in sea level, they left their mark at any one place by vertical cyclical alternations of sediment. Starting with a transgressive phase, a generalised ideal cycle would comprise: marine clay passing up into fine-grained detrital limestone with or without pisoliths (e.g. Fuller’s Earth Rock, Tresham Rock, Twinhoe Beds), passing up into oolites (e.g. Bath Oolite), passing up into micrites (Coppice Limestone, part of the White Limestone) and then into lagoonal and estuarine terrigenous sedimentary rocks (e.g. Sharp’s Hill Formation, Hampen Marly Formation, parts of the Forest Marble). This full sequence, however, was rarely achieved.
Comparison of the isopach map of the group (P948986) with that for the Inferior Oolite (P948982) shows a general resemblance in the eastern Mendip–Wincanton area and in the north-east, where the limit of the Lower Fuller’s Earth approximates to the line of the Moreton ‘Axis’ (P948986). West of this ‘axis’, although the pattern of thickness variation (not shown separately) in the Lower Fuller’s Earth in the Cotswolds follows that of the Inferior Oolite, the variations for the Great Oolite Group as a whole are quite different.
- Cave, R. 1977. Geology of the Malmesbury district. Memoir of the Geological Survey of Great Britain.