Calciferous Sandstone Measures, Dinantian, 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.|
Calciferous Sandstone Measures (now known as the Strathclyde Group)
The Calciferous Sandstone Measures is the lower and thicker of the two divisions of the Dinantian in the Midland Valley. It succeeds the Upper Devonian rocks apparently with conformity in most areas.
As stated previously (p. 50), the position of the boundary between Devonian and Carboniferous rocks cannot be defined in the region. A major facies change occurs, probably in the basal part of the Carboniferous and the rocks involved do not contain diagnostic fossils. The Upper Old Red Sandstone facies passes up into the cementstone facies of the Carboniferous and locally the two facies alternate. The base of the Carboniferous has been taken arbitrarily at the base of the lowest group of cementstones and this is unlikely to be synchronous throughout the area.
The Calciferous Sandstone Measures is the most variable of the subdivisions of the Carboniferous both in terms of its range of lithologies and in its thickness. The lithology varies from fluvial sandstones and lagoonal cementstones to marine limestones and shales, and includes non-marine limestones, oil-shales and thick volcanic formations. There is a striking contrast between the succession in the west where there are extensive thick piles of lava and the succession in the east where there is a thick sedimentary sequence (P915528). The thickness in east Fife is known to exceed 2 km.
The top of the Calciferous Sandstone Measures is at the base of the Hurlet Limestone which can be recognised in most parts of the Midland Valley. The sequence is subdivided on the basis of local lithology, usually into three parts. The limits of the subdivisions are not defined palaeontologically and are unlikely to be laterally equivalent
In the west-central part of the Midland Valley the sequence is dominated by the thick lava piles of the Renfrewshire and Kilpatrick hills and the Campsie Fells. The sequence has been subdivided into a Cementstone Group underlying the lavas, the lavas themselves and an Upper Sedimentary Group above the lavas. A revision in progress will replace these terms with formal lithostratigraphical divisions.
The Cementstone Group consists of interbedded cementstones, mudstones and sandstones (P219226). The cementstones are pale grey, fine-grained, dolomitic limestones containing a variable amount of mud, silt and fine sand. Individual beds occur up to 1m thick, but are usually less than 0.3m thick and are of uniform thickness. They occur in groups interbedded with mudstones.
Cementstones are broadly of two types. Layered cementstones have a sharply defined top and base and some may show signs of internal stratification. They may also have sedimentary features and desiccation cracks. Nodular cementstones show a transition into calcareous mudstones above and below and may form a more or less persistent layer or a row of nodules. The layered cementstones are thought to be primary in origin and the nodular cementstones of secondary origin.
The mudstones interbedded with the cementstones are dark grey or greenish grey, and are mottled brown or purplish brown in places. They are calcareous, silty, poorly bedded and range in thickness from a few millimetres to several metres.
Sandstones and siltstones also occur in the sequence and are very variable.
The Cementstone Group can be subdivided in some areas into a lower division consisting of a cementstone sequence and an upper division which is a fluviatile sandstone. The cementstones around the Campsie Fells are named the Ballagan Beds. The overlying fluviatile sandstone resembles in some respects the Cornstone Beds of the Upper Old Red Sandstone and is known as the Spout of Ballagan Sandstone. It is probably equivalent to the Downie’s Loup Sandstones in outcrops west of Stirling.
The thickness of the Cementstone Group varies considerably in both its divisions. The Ballagan Beds range in thickness up to about 380m and the sandstone varies from about 30 to 100m. Locally one or both divisions are absent and the lavas rest directly on Upper Old Red Sandstone facies rocks.
The Cementstone Group is succeeded by the Clyde Plateau lavas whose outcrop forms a semi-circle of hills extending from Stirling south-westwards to Greenock and from there south-eastwards to Strathaven. The lavas are also known to underlie the Central Coalfield at least as far east as Salsburgh. The lava pile is up to 900 m thick. It rests with apparent conformity on the Downie’s Loup Sandstones which contain volcanic detritus in the upper part.
The lavas are mainly olivine-basalts with some trachytes and they are described in greater detail in Chapter 12.
The Upper Sedimentary Group rests on an irregular, weathered surface of the lavas. In many places the earliest sediments are conglomerates, sandstones and mudstones composed mainly of volcanic material. The upper part of the group consists of mudstones, sandstones, coals and up to three marine limestones.
The variation in thickness of the group reflects the topography of the lava surface on which it was deposited. Some areas of the lava outcrop remained above the level of deposition until Lower Limestone Group times, but in other areas a considerable thickness of sediment accumulated.
The thickest sequence occurs in the Paisley area where it is about 360m thick. The lower part of the succession consists of volcanic detritus which is overlain by sandstones with some coals. The upper part of the sequence consists of mudstones, sandstones, seatclays and coals with up to three thin limestones. Two of the limestones, the Blackbyre and the Hollybush, attain a reasonable thickness and persistence and have been correlated over much of the region.
An extraordinarily thick coal occurs in the sequence to the south-west of Paisley. Several seams thicken and come together by attenuation of the intervening strata to form the Quarrelton Thick Coal. It is up to 15m thick but is locally doubled in thickness, perhaps by contemporaneous gravity sliding. The coal is riddled with old workings some of which were as early as 1634.
North of the Clyde, in the Campsie area, the general succession is similar but rather thinner. It is exceptional in that the lower part of the sequence consists of a quartz-conglomerate and sandstone which is largely free of volcanic detritus.
In north Ayrshire, the sequence above the lavas probably does not exceed about 30 m, but two limestones are present and are correlated with those in the Paisley district.
South-west Midland Valley
In the southern part of the Midland Valley, between south Ayrshire and Douglas, the Calciferous Sandstone Measures consists of red, white, yellow or pink sandstones, red and grey-green mudstones with subordinate conglomeratic beds and rarely thin impersistent coals with seat clays. In places cornstone nodules occur and the predominantly arenaceous sequence is locally interdigitated with the cementstone-shale facies similar to the Ballagan Beds. The strata are not well exposed. They are apparently marginal to the main area of sedimentation and were possibly also affected by contemporaneous fault movement.
In the Douglas area the upper part of the sequence contains two marine limestones which can be correlated with the Blackbyre and Hollybush limestones of the Paisley area. At Muirkirk there is only one limestone in the corresponding part of the succession. More detailed correlation between sequences below the Lower Limestone Group is not possible.
One of the thickest sequences in this area occurs around Dailly where the thickness is estimated to be about 510m. However, the thickness tapers off to a few metres in parts of the Douglas area south of the Kennox Fault where only the uppermost beds were deposited.
Edinburgh area and West Lothian
The Calciferous Sandstone Measures in this area are made up of the Cementstone Group and the Lower and Upper Oil-Shale groups and the sequence is one of the thickest in the region.
The base of the Cementstone Group is taken at the lowest occurrence of the cementstone-shale facies. The sediments consist of sandstones and sandy shales, cementstones and mudstones and locally conglomerates at the base. The sandstones may be red, brown, grey or white and the argillaceous beds red, brown or greenish grey. Nodular cornstones occur in places and there are thin layers of gypsum in some beds. The sequence is predominantly arenaceous but there is an interdigitation of the sandstone and cornstone facies with the cementstone-shale facies. The thickness ranges from about 760 to about 1120 m.
The Lower Oil-Shale Group consists of the Arthur’s Seat Volcanic Rocks at the base overlain by a sequence consisting principally of pale sandstones and dark grey shales. The thickness of the group has been calculated to be over 1000m in Midlothian but thins to about 700m in West Lothian.
The sediments overlying the Arthur’s Seat Volcanic Rocks consist of dark grey shales, bituminous in places, with ironstone nodules and interbedded with mainly fine-grained, pale yellow, grey or pinkish sandstones. The thicker sandstone units are named the Craigleith, Ravelston and Hailes Sandstones and were formerly worked for building stone. Thin limestones and rare, thin, impersistent coal seams also occur.
In the upper part of the sequence oil-shales are developed at two horizons. The more important development is known as the Pumpherston Oil-Shales which were formerly mined in West Lothian. Immediately underlying them is the Pumpherston Shell Bed which is the only horizon in the Lower Oil-Shale Group in this area which contains a fauna which enables correlation with other successions.
The Upper Oil-Shale Group extends from the base of the Burdiehouse Limestone to the base of the Lower Limestone Group and is characterised by the occurrence of nine or ten oil-shale seams which were formerly mined extensively in West Lothian. The group is thickest around West Calder in Midlothian where it is about 850 m thick but it thins eastwards and southwards with a reduction in the number of oil-shale seams. In the west side of the Midlothian coalfield it is about 450m thick and to the south around Carlops it is about 150 m thick.
The Burdiehouse Limestone at the base of the Group is up to 15 m thick and is thought to be of freshwater or estuarine origin. Its fossil content of plants, fish and ostracods is not diagnostic and its lateral equivalence to other limestones is based mainly on lithological similarity.
Most of the sequence consists of argillaceous rocks and contains seams of oil-shale up to 5 m thick. They are hard and usually very dark brown in colour and are minutely laminated. They tend to grade into bituminous or carbonaceous shale.
The sequence also includes the Houston Coal and the Two Foot Coal. They are both thin and of inferior quality and only the former was mined.
Thick beds of ‘marls’ occur at several levels in the sequence. They are very poorly bedded mudstones with thin calcareous bands and are greenish grey in colour. Thick sandstones are also present in the lower part of the sequence and were worked for building stone at one time.
In the upper part of the Upper Oil-Shale Group there are three horizons containing marine fossils which are useful for correlation. The oldest of the three is the Raeburn Shell Bed which occurs below the Raeburn Shale and is widely distributed in West Lothian and parts of Midlothian. Higher in the sequence a marine band in West Lothian is correlated with the Basket or Cot Castle Shell Bed of Lanarkshire and with the Cephalopod Limestone of the Midlothian Coalfield. The third marine horizon is thought to be equivalent to the Under Limestone of Lanarkshire and the Bone Bed Limestone in Midlothian.
Beds of volcanic ash and lava occur at several horizons in the Lower and Upper Oil-Shale groups in addition to the Arthur’s Seat Volcanic Rocks at the base. The lavas of the Bathgate Hills make their earliest appearance just above the Two Foot Coal in the Upper Oil-Shale Group and they extend up into the Upper Limestone Group of that area.
In East Lothian the Calciferous Sandstone Measures are subdivided into the Lower and Upper Lothian groups. The division is made at the base of a group of marine bands known as the Macgregor Marine Bands which are thought to be equivalent to the Pumpherston Shell Bed in West Lothian.
The Lower Lothian Group is predominantly arenaceous, red at the base but interdigitated with cementstone-shale facies at least two levels. It also includes the Garleton Hills Volcanic Rocks. The Upper Lothian Group includes several marine limestones and the Macgregor Marine Bands. It also includes a few thin coals but oil-shale seams are absent.
The Upper Lothian Group is about 140 m thick and the Lower Lothian Group is at least 730m thick near Pencaitland.
The Calciferous Sandstone Measures in east Fife have been subdivided into an ascending succession of Fife Ness, Anstruther, Pittenweem, Sandy Craig and Pathhead beds. The strata consist mainly of pale sandstones, grey mudstones and siltstones, many showing faunal evidence of non-marine deposition.
Numerous thin impersistent coal seams and seat beds are also developed and thin dolomitic limestones occur throughout the sequence.
Marine limestones and mudstones are present throughout the succession and become more common in the upper part of the sequence.
The maximum known thickness is over 2000 m, but miospore evidence indicates that the oldest beds seen are of Viséan age. It is possible that a basal cementstone facies with the older Tournaisian miospore zones is present at depth.
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