54°N South Pembrokeshire 50°N Volcanic rocks Solway Basin North Cumbria Central Coalfield Basin CHAPTER 4: MIDLAND VALLEY OF SCOTLAND CHAPTER 6: CENTRAL ENGLAND & NORTH WALES Pendine Tenby Gower Vale <strong>of</strong> Glamorgan Culm Basin (northern margin) Bide<strong>for</strong>d Sub-basin Central Devon Sub-basin West Cumbria Merthyr Launceston Sub-basin South Devon Basin Craven Basin North Wales Weston Furness & Cartmel Culm Supergroup Warwickshire Group Clydach Newport Mendips Fife-Lothian Basin Monmouth Chepstow CHAPTER 7: SOUTH WALES & BRISTOL Cromhall Avon Gorge CHAPTER 8: SOUTH-WEST ENGLAND (CULM BASIN) CHAPTER 5: NORTHERN ENGLAND Coal Measures groups Namurian strata Tournaisian & Visean strata Cheviot Block / North-East Northumberland Derbyshire High Widmerpool Trough Warwickshire Northumberland Trough Alston Block Ox<strong>for</strong>dshire Coalfield 4°W 0° Figure 4 extent <strong>of</strong> <strong>Carboniferous</strong> deposits and location <strong>of</strong> regions discussed in text 10 0 N Stainmore Trough Askrigg Block 100 km Regions covered by named chapters
4 midland valley <strong>of</strong> scotland The group and <strong>for</strong>mational nomenclature <strong>for</strong> the midland valley <strong>of</strong> scotland proposed by Browne et al. (1999) has been retained. The boundaries between the main groups are defined by marker horizons that are widespread within the midland valley. There are four main sedimentary groups, in ascending order these are the inverclyde (partly devonian in age), strathclyde, Clackmannan and scottish Coal measures groups (Figure 5), with the addition <strong>of</strong> the Bathgate Group. The Bathgate Group, together with part <strong>of</strong> the strathclyde Group encompasses the major occurrences <strong>of</strong> <strong>Carboniferous</strong> extrusive rocks, which range from thin local basalts to widespread thick volcanic accumulations. 4.1 INVERCLYDE GROUP The inverclyde Group, <strong>of</strong> Continental and peritidal facies, comprises, in ascending order, the Kinnesswood, Ballagan, and Clyde sandstone <strong>for</strong>mations (paterson and Hall, 1986) (Figure 5). The <strong>for</strong>mations equate to variations between the ‘Cornstone’ and ‘Cementstone’ subfacies. The inverclyde Group is about 1500 m thick. The base <strong>of</strong> the inverclyde Group is transitional, typically recognised by the appearance <strong>of</strong> cornstones, though it is locally marked by an uncon<strong>for</strong>mity in the west <strong>of</strong> the midland valley <strong>of</strong> scotland. The Kinnesswood Formation is characterised by purplered, yellow, white and grey-purple, fine- to coarse-grained sandstone, mostly cross-bedded, with characteristic pedogenic carbonate nodules and horizons (‘cornstones’). The Ballagan Formation is typified by grey silty mudstone containing nodules and thin beds <strong>of</strong> dolostone and limestone (‘cementstone’). There may be some lateral passage between the two subfacies. The Clyde sandstone Formation comprises white, fine- to coarse-grained sandstone, commonly pebbly, with beds <strong>of</strong> red-brown or grey mudstone and nodules or beds <strong>of</strong> pedogenic limestone. The Kinnesswood and Clyde sandstone <strong>for</strong>mations were laid down in fluviatile environments ranging from braided stream to floodplain with pedogenetically altered overbank deposits. The intervening Ballagan Formation <strong>for</strong>med in a peritidal environment associated with intermittent emergence. The discovery <strong>of</strong> miospores <strong>of</strong> Tournaisian age (LN–pC biozones) from near the base <strong>of</strong> the Kinnesswood Formation (smith, 1996) confirms that this <strong>for</strong>mation may straddle the devonian–<strong>Carboniferous</strong> boundary, but that most <strong>of</strong> it is <strong>of</strong> early <strong>Carboniferous</strong> age. The Ballagan Formation typically contains miospores indicative <strong>of</strong> the Cm biozone, although smith (1996) also recorded a sample from the pC biozone. (The <strong>Carboniferous</strong> chronozones in current use are shown in appendix 2.) 4.2 STRATHCLYDE GROUP The strathclyde Group, mainly <strong>of</strong> Heterolithic clastic and nonmarine carbonate facies, was introduced by paterson and Hall (1986) in the west <strong>of</strong> the midland valley <strong>of</strong> scotland, where the ascending sequence comprises the 11 Clyde plateau volcanic, Kirkwood and Lawmuir <strong>for</strong>mations (Figure 5). The group has been extended to the east, with five sedimentary <strong>for</strong>mations at Fyfe: the Fife Ness, anstruther, pittenweem, sandy Craig and pathhead <strong>for</strong>mations. These units were originally described by Forsyth and Chisholm (1977) and first called <strong>for</strong>mations by Browne (1986). in the Lothians the group includes both volcanic and sedimentary <strong>for</strong>mations (Figure 5), established by Chisholm et al. (1989). The Gullane Formation is overlain either by the west Lothian Oil shale Formation or the laterally equivalent aberlady Formation. Two volcanic <strong>for</strong>mations are present; the arthur’s seat volcanic Formation and the Garleton Hills volcanic Formation. The strathclyde Group is in excess <strong>of</strong> 1250 m thick. The base <strong>of</strong> the group is generally taken at a sharp irregular uncon<strong>for</strong>mity at the base <strong>of</strong> the Clyde plateau volcanic Formation in the west <strong>of</strong> the midland valley <strong>of</strong> scotland, or the mainly con<strong>for</strong>mable base <strong>of</strong> the Gullane Formation or arthur’s seat volcanic Formation in the east. The group comprises typically mildly alkaline lavas, tuffs and volcaniclastic sedimentary rocks and interbedded sandstone, siltstone and mudstone, with common seatearth, coal and sideritic ironstone. a similar range <strong>of</strong> lithologies is present in each <strong>for</strong>mation, but the emphasis varies between one <strong>for</strong>mation and the next. Thin bioclastic limestones occur within the uppermost part <strong>of</strong> the group. There is commonly interdigitation between sedimentary and volcaniclastic rocks. The sedimentary strata were deposited in fluviatile, deltaic and lacustrine environments. Oil shales <strong>for</strong>med in large freshwater lagoons, rich in algae and other organic matter. The uppermost part <strong>of</strong> the group is characterised by the incoming <strong>of</strong> Yoredale-type cyclothems representing increasing marine conditions (Francis, 1991a). The group ranges in age from late Chadian to Brigantian. 4.3 CLACKMANNAN GROUP The Clackmannan Group comprises a variable succession <strong>of</strong> Yoredale facies, Millstone Grit facies and Coal Measures facies. The name was first used in the airdrie district by Forsyth et al. (1996) to include the Lower Limestone Formation, the Limestone Coal Formation, the upper Limestone Formation and the passage Formation. The base <strong>of</strong> each <strong>for</strong>mation, except the passage Formation, is generally taken at the base <strong>of</strong> a prominent and laterally extensive limestone. The group is up to 1800 m thick in the Clackmannan area. Condensed sequences in the Lower Limestone Formation are locally associated with areas <strong>of</strong> earlier or contemporaneous volcanicity (Francis, 1991b). The con<strong>for</strong>mable base <strong>of</strong> the group is taken at the base <strong>of</strong> the Hurlet Limestone. minor uncon<strong>for</strong>mities are present near the top <strong>of</strong> the upper Limestone Formation (Francis, 1991a) and in the passage Formation (Read, 1981) in areas <strong>of</strong> greatest thickness. Three major discon<strong>for</strong>mities are recognised in the passage Formation. The lowermost occurs within the Cravenoceratoides nitidus Chronozone, the middle discon<strong>for</strong>mity is associated with the absence <strong>of</strong> strata
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