Lower Jurassic and its classification, 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 marine transgression at the end of Triassic times, during which the Penarth Group was deposited, was followed by the establishment of open-sea conditions, first indicated by the early Liassic strata but which continued for much of the Mesozoic era. Liassic rocks, comprising the whole of the Lower Jurassic, crop out as a wide north-north-easterly trending belt, much of it low-lying heavy clay land, that traverses the region. The succession continues eastwards beneath a cover of younger rocks. Sedimentation in the Central Somerset Basin and beneath the Cotswolds was more or less continuous. Between these areas, in the Bristol–Mendip area, the succession is thinner and sedimentation was interrupted. The eastern edge of the main Cotswold Basin (a continuation southwards of the Worcester Basin) is formed by the Moreton ‘Axis’, which also defines the edge of the gently and intermittently subsiding London Platform, a low-lying landmass in Jurassic times, which extended eastwards beyond the Oxford area as far as Belgium.
The base of the Jurassic System is defined by the first appearance of ammonites of the genus Psiloceras and, therefore, the lowest beds of the Lias (Pre-planorbis Beds), which do not contain them, are of Triassic age. The appearance of these ammonites is not accompanied by an appreciable change in lithology, yet it marks the establishment of a connection with a southern ocean known as the Tethys from which the British area was separated during parts of Triassic time. From that period onwards, successive waves of ammonites entered the Lias seas in large numbers, and spread rapidly over wide areas. Some families survived longer than others, but all include a variety of species, many combining a short vertical range with wide distribution, features which make them ideal ‘zonal’ fossils.
The ammonite zones and subzones, of which no fewer than 54 are recognised in the Lias, have proved to be of great value, both in the demonstration of non-sequences and in showing how conditions of deposition varied from place to place at any given period. A three-fold subdivision of the Lias into Lower, Middle and Upper parts has been made since 1829, although the definition of the term Middle Lias has varied with different workers. During the last hundred years the Middle Lias has most commonly been defined as comprising the Amaltheus margaritatus and the Pleuroceras spinatum zones, a chronostratigraphical rather than a lithostratigraphical division. However, the Lower/Middle Lias boundary thus defined rarely coincides with a change in lithology and the requisite fossil evidence is not always available. In practice, in the field, the boundary is taken below the Marlstone Rock Bed at a lithological change from sand and siltstone above to mudstone below, which may come within the margaritatus, davoei or even the upper part of the ibex Zone.
The top of the Lower Jurassic is taken at the base of the Leioceras opalinum Zone. In times past this has included the Pleydellia aalensis Subzone, but this has now been reclassified as the uppermost subzone of the preceding (Lower Jurassic) Dumortieria levesquei Zone. In the southern part of the present region, the topmost two metres or so of the Upper Lias Yeovil and Bridport Sands include part of the opalinum Zone and are therefore Middle Jurassic in age, though for convenience of description they are here included with the Lower Jurassic.