Coal Measures, Carbonifererous, Southern Uplands
|Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
Coal bearing sequences
The economic importance of the coal-bearing parts of the Carboniferous succession, particularly those of Westphalian age, has led to a plethora of research. Some general characteristics are worth considering here in their regional context since they apply equally to the different successions described in subsequent sections of this chapter.
The Westphalian coal-bearing strata now seen spanning the border between north-west England and south-west Scotland, originally accumulated along the northern margin of the Northumberland–Solway Basin, itself the northern sector of the Pennine Basin, a depositional province that was continuous with the north-west European paralic belt, from which it became separated by later Variscan folding. The basin was bounded on its northern side by the Southern Uplands and associated small scale landmasses such as the Cheviot Block (P912346). Farther to the north, a separately subsiding basin incorporated the Midland Valley of Scotland. The dominant transtensional tectonic regime opened up several smaller basins within the Southern Uplands massif that show stratigraphical links with either the Midland Valley succession to the north or the Pennine Basin succession to the south.
During much of the Carboniferous Period, southern Scotland occupied an equatorial position and experienced a humid, tropical climate characterised by high precipitation rates. This combination of factors provided ideal conditions for the development of a high water table, poorly drained palaeosols, peat swamps and the eventual formation of coal seams. The coal-bearing sequences mostly accumulated in a fluviolacustrine environment, a low-lying and largely waterlogged plain that was subjected both to intervals of emergence and to intermittent marine transgressions. Across north-west England and the south-west Scottish border area, the earliest Westphalian strata were deposited in a gently subsiding, lower delta plain environment under the dominant influence of mainly fluvial delta systems. Southward progradation of this depositional system through time resulted in middle Westphalian strata being deposited in a more proximal, upper delta plain environment, dominated by river distributary channels and lacustrine deltas, which was particularly conducive to the development of coal swamps. Later in the Westphalian, a waning fluvial influence caused re-establishment of lower delta plain conditions. Accordingly, the maximum coal development is seen in the middle part of the succession, the upper Langsettian and lower Duckmantian.
The Coal Measures strata contain an abundant and varied fossil fauna that includes both nonmarine and marine species. Nonmarine invertebrates include worms, gastropods, bivalves, eurypterids, crustaceans and fish, whilst the marine faunas include brachiopods, goniatites, foraminifera and conodonts P913696. The mussel bands formed by nonmarine bivalves (P693035) are of particular importance for biostratigraphical purposes and bivalves are the basis of a widely used biozonal scheme (P912387). Nonmarine fossils are normally concentrated in the few metres of argillaceous strata that form the basal zone of the various cyclothems, but many cyclothems have no preserved fauna or contain only a small range of undiagnostic invertebrate fossils. Marine fossils are naturally restricted to the marine bands.
Remains of the coal swamp vegetation are common fossils in the Coal Measures. Masses of compressed plant material make up the coal seams, roots are found in situ in seatearths, drifted leaf fronds and plant stems occur in mudstone and siltstone beds, and chaotic ‘logjams’ of broken tree trunks and branches are a feature of some of the thicker sandstones. The Westphalian flora of the floodplains was dominated by pteridosperms with some ferns, sphenopsids (P688593) and lycopods, and that of the peat-forming swamps by lycopods.
Today, the lycopod group is only represented by low-growing plants, but during the Westphalian some lycopods were tree sized with Stigmaria root systems, the most familiar being Lepidodendron and Sigillaria with their distinctive bark patterns of rhomboidal scales (P688051). Calamites, a giant relative of the present-day horsetail Equisetum was also common and grew around lakes and on point bars, whilst a range of pteridosperms grew on the levées alongside meandering rivers.
Clastic lithologies and cyclothems
The Coal Measures are built-up from a repetition of cyclothems in each of which the main lithologies commonly follow one another in an ascending order of mudstone at the base (overlying the coal at the top of the underlying cycle), siltstone, sandstone, seatearth, coal. The nature and origin of the cyclothems have been much discussed, and it appears that no single explanation will suffice. Such cyclicity is a natural reflection of the interplay of sedimentary processes, and the only external mechanism needed to produce them is continuous subsidence. However, it is generally accepted that the periodic changes in sea level leading to marine flooding events are related to global glacial events, in this case the late Carboniferous glaciation of southern Gondwana which at that time lay over the South Pole. In contrast to circumstances earlier in the Carboniferous, contemporaneous fault activity is not considered to have been a major influence on sedimentary patterns, and only local fault-controlled effects have been recorded.
The inter-coal sequences were deposited during the gradual infilling of shallow interdistributary bays and lakes by shallow-water delta complexes. They are interbedded with a number of prominent sandstone bodies that were deposited by the low-sinuosity, distributary channels feeding crevasse-splay systems (P693035). These bodies can be stacked, one above another, to produce substantial thicknesses of sandstone locally.
Mudstones are generally well bedded and grey, but darker and more fissile when carbonaceous. Siderite commonly occurs in thin layers or as flattened nodules. Many of the mudstones contain a shelly fauna and this is usually indicative of a brackish-water depositional environment; marine mudstones are rare. Siltstones are also grey, commonly laminated and with a range of bioturbation structures. They grade imperceptibly into both mudstone and sandstone, the latter being mostly fine grained and quartzofeldspathic.
Seatearths, as preserved palaeosols, may be developed from any of the clastic lithologies and are gradational into the underlying facies. In contrast, the contact with any overlying coal seam is sharp. The seatearths contain abundant carbonaceous plant material (most commonly Stigmaria rootlets), and bright coaly stringers with disseminated pyrite. The rootlets increase in abundance upwards through the seatearth and disrupt any original lamination that might have been present. The seatearths typically break along irregular fracture surfaces that may be either covered with slickensides, or curved and polished.
Coal swamps formed as mires developed across low-lying alluvial plains and abandoned lacustrine delta systems (P693035). Individual abandoned delta systems are thought to have been up to 10 km wide and 20 km long. The associated peat-forming mires developed over prolonged periods of time and were not necessarily contemporaneous from one delta system to another. Lateral continuity and synchroneity would have been particularly unlikely along the more active, marginal parts of the basin, as preserved in southern Scotland, where variations in coal seam thickness and localised seam splits would be expected. Peat-forming mires thrive under waterlogged conditions of rising base level and are able to maintain themselves for thousands of years in water depths of up to about 1 m, probably the optimum water depth for swamp growth. A lowered water table leads to oxidation and destruction of organic matter as the swamp environment gives way to better-drained conditions; no coals form, only overthickened palaeosols. An accelerating rise in water level allows the mire to be buried by clastic sediment. Across parts of southern Scotland, peat-forming environments ranged from low-lying and brackish mires to seasonally flooded forest swamps. The resulting coal seams range up to about 2 m in thickness and so a peat:coal compaction ratio in the order of 10:1 would indicate that peats were originally up to 20 m thick; autocompaction during peat growth would reduce that decompacted thickness.
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