Lower Lias, 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 Lias (including Pre-planorbis Beds)
General description
Although other fossils are common in the Lower Lias, the chronostratigraphy is based almost entirely on the ammonite faunas. The only formal lithostratigraphical units widely recognised within the Lower Lias of the district are the Blue Lias and the overlying Lower Lias clay. A number of informal local units have been recognised in the Radstock area (P948996) and subdivision of the Blue Lias has been made in the Keynsham–Bath area and along the west Somerset coast where exposure has been, or is very good.
The range of stratigraphical variation in the Lower Lias successions of the district is given in P948996 and the thickness variation in P948977. Fewer details are available in cols. 1 and 5 of the table than for the other columns due to the relative lack of exposure; these sequences may include as yet unidentified, though probably small, nonsequences. The most striking differences are between the Central Somerset (col. 2) and Cotswold (col. 5) deep basinal areas and their flanking positive areas (cols. 3, 6) which show much attenuation and many nonsequences. In the eastern Mendips, between Frome and Binegar, Lias rocks are entirely absent (P948977), and the Upper Inferior Oolite rests directly on the Carboniferous Limestone. The Central Somerset Basin continues westwards under the Bristol Channel, where seismic reflection and other evidence indicates very thick sequences, with about 530 m attributed to the combined Lower and Middle Lias and about 90 m to the Upper Lias.
Blue Lias
William Smith adopted the old West Country quarryman’s term ‘lias’ for thin compact limestone beds and distinguished, in a stratigraphical sense, the White Lias below from the darker-coloured Blue Lias above in the Somerset Coalfield. Within the present region, the proportion of limestone to interbedded shale and mudstone varies from a preponderance of limestone in the more condensed sequences, such as in the original type coalfield area, to a ratio of 1:4 or 5 in the thick basinal successions. Wildly varying figures for the thickness of the Blue Lias given by different workers in the past are due mainly to differences of opinion as to the proportion of limestone present needed to define the formation, but also to poor exposure of all except the lowest part in most areas. When long, continuous sections are available, the contrast between the strata containing numerous limestone beds and those containing relatively few beds is usually clear. The Blue Lias commonly extends from the base of the Lias to about midway up the semicostatum Zone. In the west Somerset coast sections (P211323) it can be shown that the variations of thickness in the Blue Lias are determined by differences in thickness in the mudstones; the intervening limestones retain their identity and thickness over great distances.
Three main types of limestone can be recognised in the Blue Lias. The most common is blue-grey (due to around 25 per cent sulphide-bearing clay content), finely crystalline, hard and splintery, with bedding tops and bottoms either level or wavy. The thicker beds may attain 0.3 to 0.4 m, but the majority are thinner. There is every gradation between the wavy-bedded types, through nodular limestones to layers of limestone nodules. Secondly, beds of flaggy limestone composed of finely comminuted remains of oysters and other bivalves in a limy-mud matrix are commonly present in the Pre-planorbis (or Ostrea) Beds, but in the Mendip–Radstock area this type may also be present at higher horizons. Thirdly, a distinctive type of limestone is commonly represented in the upper part (johnstoni Subzone) of the planorbis Zone; this has level upper and lower surfaces and is very fine grained, with a porcellanous texture, and may be laminated. The intervening mudstones range from grey, calcareous, blocky, often bioturbated types, to darker-coloured, less calcareous, finely laminated, bituminous types. The latter weather into paper shales and tend to occupy the middle part of the mudstone intervals, and are best represented in the basinal areas.
The cause of the rhythmic alternation of limestone and mudstone has long been debated. Recent opinion favours a cyclical deposition of lime-rich and lime-poor layers, with subsequent diagenetic concentration of the lime in the former to give rise to the majority of the limestones. Hiatuses in sedimentation, which may represent no more than a fraction of an ammonite zone, may be marked by limestone hardgrounds and remanié accumulations of bored and encrusted limestone fragments. These show that limestone formation must have proceeded penecontemporaneously and faunal evidence indicates that a soft muddy bottom was the norm during Blue Lias times so that limestone formation must have proceeded below, rather than on, the sea bed.
Pelagic animals, apart from the ammonites, are represented by belemnites, fishes such as Pholidophorus and Dapedius, and giant marine saurian reptiles including Ichthyosaurus and Plesiosaurus. Next to the ammonites, however, the most conspicuous fossils are bivalves, with about 25 genera being represented, including bysally attached, free-swimming and burrowing forms. Brachiopods are locally abundant, especially Calcirhynchia calcaria and, in the younger beds, Spiriferina and Piarorhynchia. Echinoid spines are locally abundant. The prolific faunas appear to reflect rather well-aerated conditions, but fossils may be much reduced in number or absent in the finely laminated beds, which reflect stagnant deep-water conditions. The recovery from time to time of almost complete skeletons of saurians shows that the sea floor must have been below the wave base.
Lower Lias Clay
Exposures are scanty in the Lower Lias Clay and detailed knowledge of these beds depends largely on cored boreholes. In general terms, the lower part consists of dark mudstones and shales with rare, thin, dark argillaceous limestone beds or nodules, which roughly correspond to the Shales-with-Beef and the Black Ven Marls of the Dorset Coast. Next follows a lighter-coloured, more calcareous group, roughly equivalent to the Belemnite Marls of Dorset, ranging in age from within the raricostatum Zone through to the jamesoni and ibex zones. It includes more limestone beds than the underlying darker mudstone group. At the top, rather pyritous, silty and micaceous mudstone, with scattered clay ironstone nodules, characterises the davoei Zone and corresponds to the Green Ammonite Beds of Dorset. In most areas the upper beds pass up without appreciable break into the Middle Lias. The fauna of the Lower Lias Clay is again dominated by bivalves, accompanied by brachiopods, ammonites, belemnites and gastropods of considerably greater diversity than in the Blue Lias.
Regional variations
South Somerset
The main part of the Lower Lias outcrop comprises the low-lying clay lands of the vales of Ilchester and Sparkford. These are bounded to the north by the strong north-west-facing escarpment formed by the basal Blue Lias and White Lias limestones, and to the south by the higher ground formed by the Upper Lias sands and the Inferior Oolite. The east-west ridge of Camel Hill, Sparkford, is due to a faulted upfold of Penarth Group to Blue Lias strata. There has been a tendency in this area to limit the term Blue Lias to the flaggy limestone of the Pre-planorbis Beds and the planorbis Zone, which were much used in the past for building, paving and monumental stone. Evidence from the adjacent Sparkford railway cutting and elsewhere shows, however, that the strata with numerous limestone beds extend much higher. The lower flaggy limestones can still be seen in quarries on Camel Hill. The remainder of the Lower Lias succession is rarely exposed, but a study of scattered temporary exposures over the years has shown that the ammonite succession is probably more or less complete. The obtusum Zone at Marston Magna is notable for its richly fossiliferous nodules packed with Promicroceras marstonense and other ammonites. The limestone was formerly cut and polished under the name of ‘Marston Marble’. The ibex Zone, no more than 2 to 3 m or so thick, at the top of the relatively pale coloured ‘Belemnitiferous Marls’, probably represents a condensed deposit, as on the Dorset coast.
Central Somerset Basin
This area is flanked on the south by the White Lias–basal Blue Lias limestone scarp known as the Polden Hills and on the north by the Mendip Hills. The centre of the basin contains the thickest known Lower Lias sequence in the region. Much of the area is covered by alluvial deposits of the Somerset Levels, but the beds up to the middle of the semicostatum Zone are magnificently exposed in the west Somerset coast sections between Blue Anchor and Hinkley Point. The remainder of the sequence is known in detail from the Burton Row Borehole at Brent Knoll.
The Lower Lias succession can be matched, almost bed-by-bed, throughout the coastal outcrops and with the Brent Knoll succession, a total distance of more than 30 km. The thickness of the strata remains remarkably constant, thinning only in the Blue Ben area, close to the Quantocks, which is believed to have formed a landmass in Lias times. There are, however, no littoral deposits preserved here.
Five divisions in the Lower Lias have been mapped in the coastal area (P948996, col. 2) using distinctive limestone marker beds. The limestones in the Blue Lias are most abundant in Divisions 1 and 3. Division 2 is mainly shale and mudstone, and approximately corresponds to similar strata recognised in the Bristol area and South Wales. At the base of the Lias, paper shales, nearly 2 m thick in the Watchet area, were formerly classified as ‘Watchet Beds’, but are now included on lithological grounds with the overlying beds. Inland exposures in the Lower Lias are mainly flaggy limestones in the lower part of the Blue Lias (Division 1 of the coast). These were much quarried in former times along the Polden Hills, between Dunball and Keinton Mandeville. Westwards from Brent Knoll, even greater thicknesses of Lower Lias are present beneath the floor of the Bristol Channel than were proved in the Burton Row Borehole.
Mendips–Radstock–Broadfield Down
In early Lias times much of the Mendips formed a large island in a string of smaller islands, including Broadfield Down, stretching westwards into South Wales. The land was fringed in places by a sublittoral zone, 1 to 3 km wide, where the Lias directly overlies the Carboniferous Limestone (P006740). The sublittoral facies, known informally as Downside Stone in the Mendips and Brockley Down Limestone on Broadfield Down, consists of pale-coloured, massive, coarse, shelly, and commonly pebbly limestones in which the pebbles and much of the coarse debris are derived from the Carboniferous Limestone. The rocks were deposited in offshore shoals in strongly agitated water. The thickest development, around 30 m, is in the Shepton Mallet area. The rocks are mainly Hettangian in age, but as the old coastline is approached near Maesbury Castle, lateral passage of the facies extends up into the jamesoni–ibex zones. The southwards passage of the sublittoral facies into the Blue Lias of the Central Somerset Basin is fairly rapid, whereas north of the Mendip island, the rocks pass into the Radstock facies, representing sedimentation in shallow water on the Radstock Shelf. The limits of the shelf correspond approximately to the 50 m isopach in P948977. Tutcher and Trueman recognised eight subdivisions below the clays of the davoei Zone (P948996, col. 3) in this classic area. They are mostly too thin to map, but are useful for correlation purposes. Apart from beds of Hettangian age, the succession consists of mainly condensed sequences and nonsequences, locally with a wealth of ammonites and other fossils which may be rolled and phosphatised.
Tutcher and Trueman thought that folding had occurred during late Hettangian times along east–west axes and that erosion prior to the deposition of the Bucklandi Bed planed off the crests of the folds. The data, however, are inconclusive and a reconstruction over a wider area than they considered gives a different interpretation (P948978). Although erosion undoubtedly occurred, it is now thought that most of the differences in thickness below the erosion surface are depositional rather than erosional. The Armatum Bed is the lowest part of the Jamesoni Limestone and, like it, consists of cream-coloured, fine-grained limestone speckled with ‘ironshot’ (ferruginised fossil fragments). The Valdani Limestone is a coarsely crystalline rock, ironshot at the base. These limestones are limited to the central parts of the Radstock Shelf (P948978); elsewhere they pass laterally into grey calcareous mudstones. The silty mudstones of the succeeding davoei Zone, which mark the end of the peculiar conditions of the shelf, occur in the Lias successions throughout the region.
The course of sedimentation over most of the Mendips and Broadfield Down in later Liassic times is little known because of the subsequent removal of the deposits. The presence of scattered Lias fissure deposits in the Carboniferous Limestone, however, shows that the sea must have been steadily encroaching upon the land areas, and it is probable that by the end of the period the greater part of the area was submerged.
Bristol–Severn area
The succession given in P948996 (col. 4) refers mainly to Dundry Hill, where it is most complete and probably thickest (P948977). The Blue Lias thickness is comparable to that of the type area at Saltford and Keynsham, although the proportion of interbedded argillaceous strata at Dundry is rather higher. In the type area the Blue Lias has been subdivided into four divisions (A–D). The Saltford Shales (B) comprise part of the planorbis and most of the liasicus zones. The uppermost division (D) is marked at the base by the Calcaria Bed, a prominent limestone with abundant Calcirhynchia calcaria and vermiceratid ammonites, and at the top by the Scipionianum Bed, a thick limestone with phosphatised fossils including Arnioceras spp. on its upper surface. These subdivisions can be recognised over quite a wide area, although only the Saltford Shales are mappable. This latter unit thickens from 5 m in the Keynsham area to about 12 m at Chipping Sodbury.
Outliers of Lower Lias occur on either side of the River Severn upstream as far as Chepstow, but only those immediately north of Bristol include strata as late as the semicostatum Zone. The subdivisions of Blue Lias at Keynsham can be broadly recognised but, with the exception of Division A (Pre-planorbis Beds, planorbis Zone), the strata become markedly more argillaceous.
Cotswolds and Vale of Gloucester
The Stowell Park Borehole between Northleach and Cirencester has provided detailed information of the Lower Lias at its maximum development in the Cotswold Basin (P948996, col. 6). Compared with the Central Somerset Basin, the Blue Lias includes fewer biozones and the thickness of the individual limestones is less. In contrast to other parts of the region, the Pre-planorbis Beds and the planorbis Zone are entirely in a very dark almost black, shaly mudstone facies which forms paper shales at outcrop, and which extends northwards beyond the confines of the district.
When traced westwards away from the basin centre, the Lower Lias shows a marked decrease in thickness (P948977). Throughout the greater part of the Vale of Gloucester, the basal flaggy and shelly limestones of the Pre-planorbis Beds and planorbis Zone maintain their distinctive characters, but the overlying part of the Blue Lias, comprising clays with numerous limestone bands and nodule horizons, cannot locally be separately mapped from the Lower Lias Clay. The details in P948996 (col. 5) mainly refer to recent work in the Tewkesbury area (Worssam et al., 1989)[1]. Detailed information about the Lower Lias Clay at outcrop is rarely available, though the presence of many of the ammonite zones and subzones has been confirmed in temporary exposures.
The nature of the eastern edge of the Cotswold Basin is relatively well known, chiefly from borehole information. It is defined by the Moreton ‘Axis’, across which the Lower Lias shows rapid eastwards attenuation (P948977). Study of the zonal distribution shows that much of the attenuation is due to the onlap of progressively younger beds of the Lias onto the pre-Lias strata of the London Platform (Donovan et al., 1979)[2] this onlap is accompanied by persistent, though modest thinning of individual units in the same direction. The transgression was not continuous and regressional stages are marked by nonsequences in the succession of the shelf area (P948996, col. 7). Thin marker horizons, usually highly calcareous beds, can be recognised in boreholes over many tens of kilometres by their characteristic geophysical log signatures. Small faunal and lithological variations can be traced within each of the markers and these, together with the constancy of the intervening strata, are witness to a remarkable uniformity of sedimentation over the shelf.
References
- ↑ Worssam, B C, Ellison, R A, and Moorlock, B S P. 1989. Geology of the country around Tewkesbury. Memoir of the Geological Society of Great Britain.
- ↑ Donovan, D T, Horton, A, and Ivimey-Cook, H C. 1979. The transgression of the Lias over the northern flank of the London Platform. Journal of Geological Society of London, Vol. 136, 165–174.