Upper Limestone 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. |
Upper Limestone Group (now known as the Upper Limestone Formation)
The Upper Limestone Group strata show a return to similar marine conditions as existed during Lower Limestone Group times and which were absent during the deposition of the intervening Limestone Coal Group. This is marked by the return of thick limestones and beds of mudstone containing rich and varied marine faunas. Coals are generally poorly developed, in comparison with those of the Limestone Coal Group, but thick beds of sandstone are a feature of the group, particularly in the lower part. The group attains a maximum development of about 590 m in the Kincardine Basin.
The limits of the group are from the base of the Index Limestone up to the top of the Castlecary Limestone. In relation to the goniatite zonation the group is partly in the Pendleian (E1) Stage and partly in the Arnsbergian (E2) Stage.
Lithology
The Upper Limestone Group succession is cyclothemic in character. There are up to nine marine limestones in the group with varying degrees of development and persistence throughout the area (P915533). The thickest and most persistent of these beds are the Index, Orchard, Calmy and Castlecary limestones, in ascending order. The other limestones, which are not everywhere developed are the Huntershill Cement, Lyoncross and Pleans Nos. 1, 2 and 3 limestones.
The Index Limestone, at the base of the group, is a fine-grained grey or brownish grey rock usually rather argillaceous at the top and bottom. Its thickness varies between 1 and 2 m but it is locally thicker in the Muirkirk and Douglas areas. The limestone has not been found in east Fife and in some parts of north Ayrshire it has been removed by pene-contemporaneous erosion.
The Orchard Limestone shows more lateral variation than the Index Limestone. In north Ayrshire and the Glasgow area it consists of well-bedded, fine-grained, dark argillaceous limestone, split in places into two or three beds. In Midlothian and Fife, the same horizon is called the Orchard Beds and they consist of fossiliferous marine shales which locally include thin limestone bands. The Orchard Limestone in north Ayrshire, where it is called the Lower Linn Limestone, is about 6m thick and the Orchard Beds of Midlothian are about 15 m thick. The horizon can be recognised in successions throughout the outcrop in the Midland Valley.
The Calmy Limestone lies about the middle of the Upper Limestone Group in complete sequences. Typically it is pale grey and argillaceous (hence the name), usually occurring in two beds, but locally three or four. The limestone itself contains only fragmented brachiopod shells and crinoid columnals but the shales above and below are more fossiliferous. The limestone is well developed in north Ayrshire where it reaches about 15 m and is known as the Upper Linn Limestone. An exceptionally thick development of about 30 m occurs locally in south Ayrshire. There are also thick developments in the Muirkirk and Douglas areas. Elsewhere, it is usually less than 4 m thick.
Near the base of the mudstones underlying the Calmy Limestone, there is a thin bed of carbonaceous shale in which the bivalve Edmondia punctatella occurs in abundance. The Edmondia punctatella Band is only a few centimetres thick in central and western areas but it has a very wide distribution in the Midland Valley.
The Castlecary Limestone marks the top of the Upper Limestone Group, but in many places in the Midland Valley it has been removed by erosion prior to deposition of the Passage Group. It is not present in Ayrshire, nor in the Muirkirk and Douglas areas. It is also absent in parts of the Central, Fife and Midlothian coalfield areas. The limestone ranges in thickness from 1.5 to 5.5 m and tends to occur in at least two beds. The character of the limestone varies from fine-grained to crystalline and is commonly dolomitised. The rock is mottled light and dark grey in places, but north-east of Glasgow, contemporaneous weathering has reduced the limestone to a green clay and calcareous nodules.
All the limestones, except the Castlecary, are overlain by shales with a marine fauna. Despite the obvious geological interest and correlative importance of the limestones, they form only a very small proportion of the lithological sequence. The predominant rock type of the group is sandstone and considerable thicknesses are developed at several levels. These sandstones, which are usually white to pale yellow or pale grey in colour, frequently have an erosive base and rest unconformably on the underlying strata. In the Glasgow area, one such sandstone, the Barrhead Grit, originates in the sequence above the Huntershill Cement Limestone, but it transgresses downwards in the sequence and rests unconformably on strata below the limestone. In north Ayrshire a coarse sandstone at a similar horizon locally cuts out the Index Limestone.
The coals of the Upper Limestone Group are mostly thin but some have been worked locally. The Upper Hirst Coal, which occurs below the Calmy Limestone, is worked extensively in the Kincardine Basin where it is up to 2.5 m thick. It has a rather high ash content which makes it suitable for power station use.
Correlation
The four main limestones which can be recognised throughout most of the Midland Valley form the essential framework of correlation and they are supplemented by up to five other less persistent limestones. In addition, there are numerous other marine horizons with a more restricted fauna. These tend to be better developed where the sequence is thickest and are poorly developed or absent in areas where the group is attenuated.
Individual limestones are recognised by a combination of lithological and faunal characteristics. In many cases the faunal content of the limestones themselves is rather poor, the richest and most abundant faunas occurring in the mudstones associated with limestones.
The fossil assemblages show lateral changes which are more or less consistent at each horizon and these may be related to differing rates of subsidence and sedimentation. The differences are evident between east and west on either side of the Burntisland Anticline and there are also variations between south-east and north-west affecting the lower part of the group in the Central Coalfield.
Lateral variation
The pattern of variation in thickness in the Upper Limestone Group is similar to that of the Limestone Coal Group. The maximum thickness occurs in the Kincardine Basin and there is notable thinning over the Burntisland Anticline. In Ayrshire, north-easterly trending faults exerted a control on the thickness of preserved sediment in the same manner as occurred in the Limestone Coal Group, but to a lesser extent.
The greatest known thickness of sediment in the group occurs near Clackmannan, in the Kincardine Basin where the complete succession is about 590 m thick. All the previously mentioned limestone horizons are developed including the three Plean Limestones. In addition there are numerous other beds of marine shale and Lingula bands.
In the lower part of the succession there are coarse erosive sandstones which locally cut down through older beds. Similar sandstones occur at comparable levels in the Glasgow, north Ayrshire and Douglas areas. Statistical studies of sections between the Calmy Limestone and Plean No. 1 Limestone in the Kincardine Basin suggest that sand entered the basin from the north-west, north and east, transported by more than one river.
The thickness of the group as a whole is reduced over the Burntisland anticline to about 210 m but this increases again further east in Fife and Midlothian. It is about 500 m thick near Leven in east Fife. In Midlothian the maximum thickness of about 330 m occurs at the west margin of the syncline, around Loanhead and Gilmerton. From there it diminishes towards the south and east.
From the Kincardine Basin there is a general thinning to the south and south-west. The thinning, however, is interrupted by a narrow east-north- easterly basin on the south side of the Campsie Fault, near Kirkintilloch, where a thickness of 405m has accumulated in the centre of the basin.
In the Glasgow area, the complete succession is about 300 m thick and at several horizons above the Calmy Limestone coarse channel sandstones are present. Further thinning occurs south-westwards, into north Ayrshire, where the thickness is reduced to about 100 m. The latter represents little more than half of the complete sequence as it is known in the Glasgow area. A large proportion of the succession above the equivalent of the Calmy Limestone was removed by erosion prior to deposition of the Passage Group.
The thickness of the group is further reduced abruptly across the line of the Dusk Water Fault from 100m in the Dalry area to 40m near Kilwinning. Most of the succession above the Lower Linn Limestone (= Orchard) has been eroded prior to Passage Group sedimentation. In addition, the Index Limestone is locally absent due to erosion by an overlying channel sandstone.
Further attenuation occurs southwards across the Inchgotrick Fault where the thickness below the unconformity is about 8 m. The group recovers towards the south and east on the south side of both faults, so that in the Kilmarnock area about 80 m of strata remain and near Sorn the lower part of the succession, including possibly the Orchard Limestone, is preserved and is about 36 m thick. The correlation of the limestones above the Index Limestone in the area south of the Dusk Water and Inchgotrick faults is uncertain. It is probable that no deposition took place in Upper Limestone Group times in the area south of Ayr and that the group may be represented in places by a marginal sandstone facies.
The succession increases in thickness again on the south side of the Kerse Loch Fault where it is about 210m thick and the Index, Lyoncross, Orchard and Calmy limestones are present. The number of limestones decreases and the overall thickness of the group diminishes towards the Southern Upland Fault.
The succession at Douglas shows the same pattern of variation in thickness as the Limestone Coal Group. The maximum thickness of 320m occurs in the north-east part of the area and there is a north-easterly trending zone of minimum thickness between Douglas and Coalburn where it is only 150m thick.
The Castlecary Limestone is not present in the Douglas area, but the Plean Limestones are represented. The top of the group is taken at the unconformity at the base of the Passage Group which transgresses from a horizon above the Plean Limestones to one just above the Calmy Limestone.
The marine horizons tend to be thicker in the outcrops in the southern part of the Midland Valley. The Index Limestone and particularly the Calmy Limestone with thin associated marine shales are well developed in Douglas and Muirkirk, and in the Dalmellington area. The local equivalent of the Calmy Limestone is about 24 m thick and is characterised by the presence of corals.
Contemporaneous volcanic rocks
Contemporaneous volcanic rocks in the Upper Limestone group are limited to parts of Fife and West Lothian and various occurrences in the Dalry area of north Ayrshire. In the Bo’ness to Bathgate area basaltic lavas and tuffs occupy much of the sequence below the Orchard Limestone. In Fife basaltic tuffs occur at several levels in the succession and lavas occur less commonly.
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