Conditions of formation of the Carboniferous Coal Measures and their fauna and flora, Bristol and Gloucester region
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
Conditions of formation of the Coal Measures
Sedimentation and uplift during the later part of Lower Carboniferous times finally converted most of the region into land. In a wide area east of the line of the Lower Severn Axis, the surface was marginally above the level of the sea, which lay far to the south. This area, into which the rivers from the higher ground discharged their sediments, was a vast swamp. Here flourished the great forests whose rotting debris accumulated as thick layers of peat, which ultimately formed the coal seams.
The climate of the period appears to have been warm, with high perennial rainfall, so that in the low-lying areas a wet substratum with a high water table was permanently maintained. This produced stagnant swamp conditions in which aerobic decay of the vegetable matter was arrested. In the more elevated regions bordering the swamp, where the water table lay below the ground surface, downward percolation of rain water supplied oxygen, which enabled complete bacterial decay of the vegetation and removal of the humic acids produced by fermentation. The resultant downward movement of the acid-charged waters hastened the decomposition of the underlying substrate and produced lateritic soils of a dominantly red colour, in contrast to the grey or black soils produced by the reduction of the ferric oxides in the anaerobic swamp substrates.
The higher ground along the Lower Severn Axis probably formed the western margin of the swamp: its eastern boundary may have been a ridge of pre-Carboniferous rocks coincident with the Malvern ‘Axis’, extending as far south as the Sodbury area (see P948966). Farther south, the older rocks along the ‘Axis’ plunged beneath the swamp which probably extended westward into Wiltshire.
The most striking feature about Coal Measure sedimentation is its cyclicity. The deposition of the organic debris that formed each coal seam was normally followed by flooding of the swamp, which on some occasions involved ingress of the sea. Mud and sand were then deposited until the swamp resumed its original level and, once again, vegetation grew. The soil, still preserving traces of rootlets, is represented by the underclay or seatearth that lies beneath a coal seam.
Swamp conditions persisted into mid-Westphalian C (late Bolsovian) times, with occasional temporary incursions of the sea caused by minor changes in sea level. Towards the end of this period, general uplift with folding and erosion of the surrounding areas was followed by a widespread change in sedimentation. Marine incursions ceased and in place of shales, fireclays and coal seams, a great thickness of deltaic sediment, comprising coarse-grained, grey, current-bedded, feldspathic, subgreywacke-type sandstone, known as the Pennant Formation (or Measures) was laid down in a belt across the district and in South Wales.
In the Forest of Dean and north Bristol coalfields, beds of red measures, including conglomerates and seatearths without coals, immediately beneath the Pennant sandstones heralded the change in sedimentation. The onset of arenaceous sedimentation was earliest in the south, both in the present district and in South Wales, and it is now generally considered that the sediment was derived from an uplifted landmass of Devonian rocks to the south, in the general latitude of the Bristol Channel. Sometimes there was local silting up of the basins, which allowed the establishment of swamp conditions, so that coal seams, usually of limited extent, were formed. In general, the cycles in the Pennant sandstones are thicker than in the measures above and below. The arenaceous sediments filled up all the low ground, creating deltas that spread over the surrounding higher ground, with resultant overstep onto older formations. P948966 shows diagrammatically the formations on which the arenaceous part of the Coal Measures (or more strictly beds of the phillipsii Biozone) is believed to rest. When the rivers had reached their base level, the supply of sand decreased, swampy conditions were re-established on the delta top and the Supra-Pennant Formation (or Measures) was laid down.
In contrast with most of the lower strata, the Supra-Pennant rocks include thick seatearths associated with red or barren measures containing poorly preserved plants and shells. The red measures may indicate arid or semiarid conditions, but the plant and shell fossils indicate that there was sufficient rainfall to support plants and provide freshwater conditions conducive to the existence of nonmarine bivalves. The absence of coal above the seatearth suggests a periodic lowering of the water table, permitting the rapid decay and erosion of the dead vegetation and oxidation of the substrate; but it cannot be ruled out that, in some cases, the red coloration of the measures may be due to a later, secondary cause.
Plants of the Coal Measures
The Upper Carboniferous was the age of the great arborescent lycopods and the pteridosperms whose descendants now occupy a humble place in today’s floras, but which in Coal Measure times contributed to the equivalent of today’s rain forests. The chief groups of plants were: Lycopodiales e.g. Lepidodendron and Sigillaria, which were tree-like ‘clubmosses’ up to 30 m in height; Equisetales e.g. Annularia and Calamites, which were tall ‘horsetails’ of similar height; Sphenophyllales, e.g. Sphenophyllum, which were slender and herbaceous, with leaves in whorls; Pteridosperms or ‘seed ferns’ of numerous and varied form which include most of the so-called Coal Measure ‘ferns’ such as Neuropteris, Alethopteris, Linopteris, Mariopteris, Sphenopteris and Lonchopteris; Filicales or ‘tree ferns,’ including pecopterids, e.g. Asterotheca.
The higher measures of Bristol, Somerset and the Forest of Dean contain some of the youngest fossiliferous Coal Measures of the British Isles. In these there is a great abundance of pecopterids, whilst Calamites, Lepidodendron and Sigillaria, so abundant in the lower measures, are greatly reduced in numbers.
Fauna of the Coal Measures
The most interesting faunas occur in the marine bands which may show both lateral and vertical faunal variation (Calver, 1969) and which are attributed to open-marine conditions. Nearshore mudstones were dominated by the horny brachiopods Lingula and Orbiculoidea. They gave way farther offshore to a productoid facies dominated by a benthonic assemblage of calcareous and horny brachiopods, bivalves and gastropods, crinoids and ostracods. Even farther offshore a pectinid facies was characterised by semi-planktonic forms such as Dunbarella and Posidonia, which were capable of surviving unfavourable bottom conditions either by periodic swimming or attachment to seaweed. Farthest offshore of all was a fauna characterised by nectonic goniatites. The Subcrenatum Marine Band, about 7 m thick in the Ashton Park area of Bristol, has a rich and varied fauna including several species of goniatites, but as it thins to the north and east the fauna becomes impoverished. The Aegiranum Marine Band is well developed throughout the district and shows a comparable thickness and faunal richness. The other marine bands mostly contain species of Lingula only.
Apart from those in the marine bands, the only other fossils that occur in any abundance are nonmarine bivalves. They are, however, less common than in South Wales where their use as zonal fossils was first developed. Other nonmarine fossils include ostracods, small branchiopod crustaceans such as Leaia and, more rarely, the cheliceratan Eurypterus and the remains of insects such as cockroaches and dragonflies.
- Calver, M A. 1969. Westphalian of Britain. 6th International Congress of Carboniferous Stratigraphy and Geology, Sheffield, 1967, Vol. 1, 233–254.