Limestone Coal Group, Namurian, Carboniferous, Midland Valley of Scotland
|Cameron, I B, and Stephenson, D. 1985. British regional geology: The Midland Valley of Scotland. Third edition. Reprint 2014. Keyworth, Nottingham: British Geological Survey.|
Limestone Coal Group (now known as the Limestone Coal Formation)
The Limestone Coal Group varies greatly in its development from a thin sandy sequence about 30 m thick with thin coals in parts of Ayrshire, up to 550m in the northern part of the Central Coalfield, where there are about fifteen workable coal seams (P915532). A large part of the Scottish coal-mining industry has been, and continues to be, based on the coals of the Limestone Coal Group.
The base of the group is taken at the top of the Top Hosie Limestone, the uppermost bed of the Lower Limestone Group. The top of the group is drawn at the base of the Index Limestone so named because it indicated that the valuable coals lay underneath. The strata are all of Pendleian (E1) age.
The strata consist principally of sandstones, siltstones and mudstones with seatearths or seatclays and coals. The sandstones are normally fine-grained and grey or pale yellow in colour but coarse-grained pebbly sandstones occur in parts of the sequence. The latter are channel-fill deposits with an erosive transgressive base which may cut down into and replace older deposits. The mudstones are grey to dark grey, locally carbonaceous, often silty and in places inter-laminated with siltstone or fine sandstone. Seatearths and seatclays are fossil soils penetrated by the roots of the vegetation which gave rise to the coal seams. The sediment consists of sandstone or mudstone and the bedding may remain obvious or be destroyed by soil-forming processes. Small ironstone nodules are common.
Nodular and bedded clayband ironstones occur within the more argillaceous parts of the succession and were formerly worked. Blackband ironstones were also of economic importance in the early years of the iron and steel industry in Scotland. These consist of iron carbonate with enough coaly material to make them virtually self-smelting.
Coal seams are rather variable in their development. At their thickest they exceed 2m but this occurs only locally. There are, however, many seams over 1m thick. Correlation of coals from one coalfield to another is seldom possible as the rapid changes in thickness, and splitting of seams makes comparison difficult. This difficulty has resulted in the use of different sets of names for coals in the same part of the succession in adjacent areas.
Although the sediments are mainly of fluvio-deltaic, non-marine origin, two marine incursions affected most of the region. These are recorded by the Johnstone Shell Bed and the Black Metals Marine Band which occur in the lower part and about the middle of the group respectively. Relatively rich marine assemblages are present in both marine bands over most of the region but in some marginal areas the fauna is reduced to Lingula or is absent (P915532). An extra marine band also occurs between the major ones in north Ayrshire and the Glasgow area and numerous bands with Lingula only are present in most areas. The marine fossils are mainly found in mudstones and more rarely in ironstones.
Correlation within the group is based on the two principal marine bands and the Lingula bands are useful for local correlations.
Lateral variation within the group is considerable both in thickness of sediment between one area and another and in the lithological sequence (P915531, P915532). Differences in thickness are due to differential subsidence in sedimentary basins and contemporaneous movement on north-easterly trending faults. The differences in the sediments record the changing environment of deposition within the delta area.
In the Glasgow area and north Ayrshire the lower part of the group consists of a relatively high proportion of mudstone compared to sandstone, and clayband ironstone occurs at several levels. The coals are relatively few and thin, but the two main marine bands are present and a third marine horizon is also present in at least part of the area.
The upper part of the group contains a greater proportion of sandstone, and coals are more numerous and thicker. There are also about 15 bands containing Lingula above the Black Metals Marine Band.
As the strata are traced eastwards towards the Kilsyth area, the group as a whole increases in thickness from about 200 m in north Ayrshire to 330m in the Glasgow area and to 400 m near Kilsyth. The increase in thickness is mainly due to additional beds of siltstone and sandstone in the succession. The number of coals also increases.
In the west, the Johnstone Shell Bed consists of about 22 m of dark mudstones with a varied marine fauna. Further east the shell bed splits into separate leaves of fossiliferous mudstone separated by beds of siltstone and sandstone. Rooty horizons and thin coals have formed locally at the top of the sandy beds. In the Kilsyth area, the Johnstone Shell Bed is split into as many as five leaves. The richest fauna is restricted to the bottom leaf and the fauna in the other leaves is reduced to Lingula and bivalves.
Lateral changes in thickness are also seen in the Kincardine Basin where the Limestone Coal Group reaches its maximum thickness of about 550m. The proportion of sandstone, particularly in the lower part of the sequence, increases across the region towards the east as does the total thickness of coal. There is also a decrease in the number of Lingula bands in the sequence from west to east. In the Kincardine Basin Lingula bands are more common in the central parts of the basin than they are on the flanks.
In Fife and Midlothian the group contains a considerable proportion of sandstone and there are numerous coals, many of economic thickness throughout the succession. The Johnstone Shell Bed and the Black Metals Marine Band are both present but in places they are split into two leaves by a bed of sandstone. The number of Lingula bands is reduced to about eight in Midlothian and four or five in the Leven area of Fife.
In Ayrshire, south of the Dusk Water Fault, the Limestone Coal Group succession is attenuated and in places shows a marginal development. Sedimentation has been controlled by contemporaneous movement on north-easterly trending faults. The most important of these are the Dusk Water, Inchgotrick, Kerse Loch, Drumgrange and Southern Upland faults.
The thickness of the group is reduced to about 50m on the south side of the Dusk Water Fault near Kilwinning, although this increases to about 90 m north of Kilmarnock. Coals, thick enough to have been worked, are only locally developed. North of Darvel, the base of the group overlaps the Lower Limestone Group and rests unconformably on Lower Carboniferous lavas. At several localities in the outcrop from Kilwinning eastwards, a coarse erosive sandstone in the overlying Upper Limestone Group has eroded the Index Limestone and rests unconformably on rocks in the upper part of the Limestone Coal Group.
Further attenuation occurs on the south side of the Inchgotrick Fault. The group is represented in this area by as little as 15 m of sandstone which is pebbly in places and locally reddened. The reddening suggests local contemporaneous emergence. The sequence thickens towards the southeast where the pattern of cyclic sedimentation becomes re-established and the two marine bands and several workable coals appear in the succession at Sorn. In the vicinity of Ayr it is probable that there was no deposition during Limestone Coal Group times.
In the area south-east of Galston, the Limestone Coal Group strata rest on rocks of Upper Old Red Sandstone facies.
There is a marked increase in thickness in the strata on the south side of the Kerse Loch Fault. The strata change from marginal sandstone facies on the north side of the fault to coal-bearing deltaic cycles on the south side. In the Dailly coalfield, the sediments are predominantly arenaceous with several workable coals. Further north around Patna, where the group is about 80 m thick, the sequence consists of cyclic repetitions of sandstones, mudstones and coals. The two marine bands contain only Lingula, but locally the Black Metals Marine Band contains a more varied fauna. In Dailly a Lingula band is perhaps the local equivalent of the Black Metals Marine Band.
The strata thin from Patna towards the south and south-east and the facies reverts from coal-bearing cycles to marginal sandstones and conglomerates which are reddened in places. The thinning is abrupt across the line of the Drumgrange Fault south-east of Patna.
The arenaceous facies around Dalmellington indicates proximity to the margin of the depositional area during Limestone Coal Group times. South-west of Dalmellington the base of the sequence rests on Lower Devonian lavas.
At Muirkirk the maximum thickness of the group is about 110 m, but this is reduced to 34 m a few miles to the south-west. Several coals of economic importance are developed in the sequence, but they are fewer and thinner where the group as a whole is thinner. The Johnstone Shell Bed and the Black Metals Marine Band are both present throughout the area, but the latter contains little more than Lingula in areas where the succession is attenuated.
In the Douglas area the Limestone Coal Group ranges in thickness from 60 m in the south-west part of the coalfield to 220 m in the north-east. The development in the north-east contains eight workable coals, but this is reduced to five on the west side of a north-easterly trending zone of minimum thickness. To the south-west the coal content is further reduced in thickness and the sequence is more arenaceous. On the south-east side of the Kennox Fault erosion in later Carboniferous times caused younger Carboniferous sediments to overstep a thin development of Limestone Coal Group strata.
The Johnstone Shell Bed is present throughout the Douglas coalfield but the Black Metals Marine Band is poorly developed and contains only Lingula.
Contemporaneous volcanic rocks
In north Ayrshire, in an area west and south of Dalry, south of the Dusk Water Fault, beds of volcanic tuffs, up to 25 m thick are interbedded with normal sediments at several levels. In places the Dalry Blackband Ironstone is replaced laterally by volcanic tuffs.
In Fife similar pyroclastic rocks and, more rarely, lavas occur locally in the succession.
A sequence of basalt lavas in the Bathgate to Linlithgow area of West Lothian replaces most of the Limestone Coal Group sediments and in the Bo’ness area lavas interdigitate with the sedimentary rocks.
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