Geology of the Bath area: Carboniferous
| This topic provides a summary of the geology of the Bath area – covered by the British Geological Survey
1:50k geological map sheet 265.
Authors: A J M Barron, T H Sheppard, R W Gallois, P R M Hobbs and N J P Smith (BGS).
The Carboniferous rocks of the district formed when the British Isles occupied a broadly equatorial setting. The Lower Carboniferous (Mississippian) succession is traditionally referred to as the ‘Carboniferous Limestone’ (now a formal supergroup), and in southern Britain comprises a thick sequence of limestone, dolomite and subordinate siliciclastic strata. They were deposited following major sea-level rise in the early Tournaisian and the drowning of the coastal floodplains of southern Laurussia. A shallow, southward-dipping carbonate ramp became established, and active tectonism led variously to periods of emergence and submergence. By the late Visean the ramp had been largely drowned and open shelf conditions prevailed.
During the Namurian, climate change, coupled with uplift of the Wales–Brabant High to the north (Besly, 1987), led to southward progradation of deltas which occluded the marine environments of the Mississippian. At the end of the Carboniferous, the closure of the Rheic Ocean to the south saw the uplift of a fold belt in the region of northern France, and the onset of the Variscan Orogeny in southern Britain. During the early part of the Westphalian, fluviatile and lacustrine depositional environments with coal mires became established on an open coastal plain subject to sporadic marine incursions from the east. By mid Westphalian times, however, marine incursions had ceased and continental red beds were deposited in the Bath district.
Mississippian (Tournaisian to Visean)
At outcrop in the Bath district, occurrences of the Tournaisian to Visean Carboniferous Limestone Supergroup (CL) are confined to the Chipping Sodbury railway cutting and a series of north to south-trending inliers within the Mesozoic outcrop at the foot of the Cotswold escarpment. In the subsurface, Carboniferous Limestone is also found at relatively shallow depths beneath the city of Bath (see Applied geology). These occurrences lie on the flanks of the Bath Axis; Carboniferous strata are absent over its culmination (P785914), and in the subcrop they are inferred to stretch east and north from Bath to the Lucknam Borehole (ST87SW 1 [8338 7071]). This borehole proved 90.5 m of probable Carboniferous Limestone underlying the Penarth Group, but beyond this its eastern extent and structure are uncertain. The axis appears to be sinistrally offset by the Bitton–Tadwick Fault and other east–west faults in the Wick area (P785915), but this displacement maybe the result of complex movements including thrust and normal faulting.
The Carboniferous Limestone Super-group is divided into the Avon Group (Av) and the overlying Pembroke Limestone Group. The Avon Group is poorly known in the district, being exposed only at the eastern end [732 816] of the Chipping Sodbury railway cutting, where it is approximately 25 m thick and rests conformably upon the Tintern Sandstone. The basal part of the group comprises coarse bioclastic and ooidal limestone with subordinate mudstone beds, which is probably equivalent to the Shirehampton Formation recognised in the adjacent Bristol district (Barton et al, 2002). These strata are disconformably overlain by greenish grey mudstone with subordinate black crinoidal limestone. The lowermost 58 m of strata in the Lucknam Borehole include many beds described as shale and probably also represent the Avon Group.
The lowest division of the succeeding Pembroke Limestone Group is the Black Rock Limestone Subgroup (BRL), present in the Chipping Sodbury cutting and in a small inlier [724 781], near Codrington. The uppermost 32.5 m of Carboniferous Limestone in the Lucknam Borehole may also represent this subgroup. The Black Rock Limestone comprises a unit of dark grey or black, well-bedded crinoidal limestones with a rich fauna of corals and brachiopods. It attains a thickness of 180 m in the district, and the uppermost 40 m are widely dolomitised.
In the outcrops at Chipping Sodbury and Codrington, the Black Rock Limestone is disconformably overlain by the Gully Oolite Formation (GuO), comprising white-weathering, thick-bedded, pale grey oolite, approximately 30 m thick and generally unfossiliferous. The formation represents the development of migratory ooid shoals in a high-energy, shallow-water environment following sea-level rise in the early Visean. The sharp, erosional contact between the Gully Oolite and the overlying Clifton Down Mudstone Formation (CDM) is exposed in the extensively quarried inlier [7105 7315] at Wick (Kellaway and Welch, 1993). The basal part of the Clifton Down Mudstone Formation here comprises up to 3.5 m of brecciated and conglomeratic limestone with clasts of Gully Oolite and greenish grey mudstone, overlain by some 13 m of interbedded calcareous and non-calcareous mudstone that is typical of the Clifton Down Mudstone. At Wick, this facies is interrupted by a prominent 8.5 m-thick succession of hard, grey crinoidal and ooidal limestone, representing the Goblin Combe Oolite Formation (GCO). Above this, a further 18 m of lime mudstone and dolomitic limestone are regarded as the upper leaf of the Clifton Down Mudstone Formation. Stromatolitic algae are common in the Clifton Down Mudstone, but otherwise these rocks are poorly fossiliferous, and probably represent subtidal to peritidal deposits developed in a lagoon. In contrast, the Goblin Combe Oolite has yielded the gastropod Bellerophon at Wick, and a rich brachiopod fauna at localities in the Bristol district (Kellaway and Welch, 1993). It represents a higher-energy shoreface environment, and may be indicative of mid Visean transgression.
The Clifton Down Mudstone is overlain by the Lower Cromhall Sandstone (LCS) in the Chipping Sodbury railway cutting (where it is 4 m thick) and in quarries [centred at 7252 8263] immediately north of the district. It is the lowermost of three tongues of sandstone (Kellaway and Welch, 1993, fig. 9) constituting the Cromhall Sandstone Formation, which spread southward and interrupted carbonate deposition in the late Visean (Cave, 1977). However, no evidence of arenaceous strata has been found at this level at Wick, where the 240 m-thick Clifton Down Limestone Formation (CDL) occurs above the Clifton Down Mudstone. The basal 60 m comprises splintery limestone with algal beds and mudstone partings, overlain by bedded lime mudstone which passes upwards into ooidal limestone. The Clifton Down Limestone represents the transition from the Tournaisian–early Visean carbonate ramp to a more open marine shelf setting in the late Visean.
At both the Chipping Sodbury cutting and Wick, the Clifton Down Limestone is succeeded by the Middle Cromhall Sandstone (MCS). This interval is relatively thick where seen in a quarry [7232 8420] north of the district, with some 27 m of sandstone, dolomitised limestone and mudstone (Murray and Wright, 1971). However, at Wick, only around 5.5 m of rippled sandstone, overlying 2 m of nodular mudstone, is exposed. Up to 20 m of strata may be present in the district, but southwards the beds have pinched out completely at Grandmother’s Rock [7090 7118], north-east of Beach.
Above the Middle Cromhall Sandstone at Wick is the Oxwich Head Limestone Formation (OHL; formerly known as the Hotwells Limestone Formation), where it is approximately 75 m thick. It also forms most of the Grandmother’s Rock inlier where it may be over 100 m thick. The formation comprises massive, grey, crinoidal and ooidal limestone, with the most varied Visean fauna of the district, notably corals including Syringopora, and brachiopods including gigantoproductids, athyrids, chonetids and spiriferids. This rich fauna indicates that deposition took place in fully open sea, shelf conditions.
The uppermost tongue of Visean sandstone, the Upper Cromhall Sandstone (UCS), overlies the Oxwich Head Limestone Formation at outcrop, and is more than 210 m thick at Wick. It largely comprises interbedded sandstone and mudstone of fluviodeltaic origin, with several beds of crinoidal and ooidal limestone. The lowermost of these is the 20 m-thick Castle Wood Limestone (CWL), which may correlate with the Rownham Hill Coral Bed of Bristol (Kellaway and Welch, 1993). A thin bed of fossiliferous limestone, named the Mollusca Bed (Mo), lies about 15 m below the top of the formation.
Mississippian to Pennsylvanian (Namurian)
Namurian rocks are represented by the Marros Group (Mar), laid down in an isolated basin with relatively little marine influence. They crop out only in the Wick inlier, although they are present at depth throughout the west of the district (P785914). Near the base is a sequence of distinctive chert and cherty mudstone beds, up to 15 m thick, which in the Bristol district (Barton et al., 2002) has yielded an early Namurian (Pendleian) age fauna (Kellaway and Welch, 1993., p.63) correlated with the Aberkenfig Formation of South Wales (Waters et al., 2009). Above these, the Quartzitic Sandstone Formation (QS) comprises fluviodeltaic sandstone and mudstone with seatearth beds and thin coal seams, between 70 and 185 m thick.
Westphalian rocks crop out widely in the western part of the district, as part of the Bristol–Somerset Coalfield. The coalfield is divisible into three structural areas (P785914), two of which are at surface: the Kingswood Anticline running east to west from Wick towards Kingswood, and the Coalpit Heath Syncline, running north to south from Iron Acton towards Mangotsfield. Coal-bearing strata also occur in the Pensford–Radstock Syncline in the south-west, generally subcropping beneath Mesozoic rocks.
The lowermost division is the South Wales Coal Measures Group, which largely comprises rhythmic alternations of mudstone, silty mudstone, sandstone and coal. It was laid down on a coastal plain which stretched from the Bath district west to South Wales, and occupied an area to the south of the Wales–Brabant landmass. The intercalation of marine mudrocks (‘marine bands’) indicates infrequent marine incursions, probably related to eustatic transgressions and valuable for regional correlation. In the area west of Wick, marine mudstone with the gastropod Donaldina ashtonensis (Bolton) overlies Namurian strata (Kellaway and Welch, 1993), and is the local representative of the Subcrenatum (Ashton Vale) Marine Band that marks the base of the South Wales Coal Measures Group. Together with the strata up to the base of the Vanderbeckei (Harry Stoke) Marine Band, these comprise the South Wales Lower Coal Measures Formation (SWLCM) of Langsettian (Westphalian A) age. The formation, which is thought to be around 200 m thick, does not crop out in the district, and is poorly known; it has not been worked for coal and it is uncertain whether representatives of the main seams of the Lower Coal Measures in the Bristol district (the Ashton coals) are present.
Higher in the succession, the Aegiranum (Croft’s End) Marine Band divides the Duckmantian (Westphalian B) and Bolsovian (Westphalian C) successions, and the youngest recorded marine horizon in the European Westphalian is represented by the Cambriense (Winterbourne) Marine Band. The rocks between the base of the Vanderbeckei Marine Band and top of the Cambriense Marine Band comprise the South Wales Middle Coal Measures Formation (SWMCM), which is around 685 m thick in the district. It crops out in the core of the Kingswood Anticline and its subcrop extends to the north and south (P785914). The formation contains a number of economically important coal seams, the Kingswood Great Coal formally being the main productive seam of the Bristol Coalfield and generally about 1 m thick.
In the Bristol Coalfield, the South Wales Middle Coal Measures Formation is conformably overlain by the Pennant Sandstone Formation — the lower part of the Warwickshire Group and here the lateral equivalent of the South Wales Upper Coal Measures Formation. The Cambriense Marine Band marks the base of the group south of the Kingswood Anticline, but north of this axis interpretation is complicated by the absence of a recognisable marine interval at this horizon. The Pennant Sandstone is fully developed in the Coalpit Heath Syncline and in the subsurface in the Pensford–Radstock Syncline, and it is inferred at depth in the east (P785914). It is Bolsovian in age, possibly ranging up to Asturian (Westphalian D), and up to 1100 m thick. During the Bolsovian, the Variscan Orogeny began to exert an influence on the depositional environment and patterns of sedimentation in southern Britain. The Pennant Sandstone is characterised by the development of massive sandstone bodies thought to represent increased sediment supply from orogenic highlands to the south. The lower division of the Pennant Sandstone is the Downend Member (Dn), which comprises thick sandstone and mudstone with coal seams in its lower part, and includes the Mangotsfield Coals (Mng) at the top. From over 650 m thick on the western edge of the district at Downend, it thins north-east to under 150 m at Yate [706 820] (Kellaway and Welch, 1993, p.96). It also comes to crop in an inlier at Corston [696 656]. The overlying Mangotsfield Member (Mg) similarly comprises massive sandstone with mudstone and rare thin coal seams, and is around 450 m thick.
The Pennant Sandstone is succeeded by the Grovesend Formation, which here is Asturian in age (Waters et al., 2009), and occupies the core of the Coalpit Heath Syncline. The High Coal marks the base of the formation, the constituent Farrington and Barren Red members (FaBR) not being differentiated on the map. The strata comprise grey mudstone with sandstone beds and coal seams, passing up into red mudstone and sandstone lacking coal, and may attain 500 m in total thickness.
Younger Westphalian strata are absent from the Coalpit Heath Syncline, and the succeeding Radstock Member (Rad) is found at outcrop only in the far south-west of the district. It comprises up to 100 m of grey mudstone with sandstone lenses and numerous thin, muddy coal seams.
At the close of the Carboniferous, the convergence of the Laurasia and Gondwana continents, closing the Rheic Ocean to form the Pangaean supercontinent, resulted in north-directed thrusting. This is especially evident on the Farmborough and Southern Overthrust faults, collectively causing the uplift of the Bath district, and more widely resulting in the establishment of an arid terrestrial environment. The district lay on the northern margin of the Variscan mountains, and northwards abutted against the Worcester Uplift (Smith and Rushton, 1993) that forms part of the Midlands Microcraton. In early Permian times deep erosion stripped the Pennsylvanian cover from the central part of the Bath district, exposing the Tournaisian to Visean and older rocks which lie along the axis of the Kingswood Anticline and along the Worcester Uplift.
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