Carboniferous of the Tweed and Northumberland–Solway basins: Difference between revisions

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== Introduction ==
== Introduction ==
In the Scottish Borders, along the northern margin of the Northumberland–Solway Basin and underlying the Tweed Basin, the oldest Carboniferous rocks, of Tournaisian age, belong to the Inverclyde Group. They include the basaltic lavas of the Kelso Volcanic Formation (‘Kelso Traps’) and the Birrenswark Volcanic Formation which overlie red-bed strata assigned to the Kinnesswood Formation. The latter rests either conformably on red-bed strata of Devonian age (Stratheden Group) or with marked angular unconformity upon an irregular surface eroded across the steeply dipping, Lower Palaeozoic turbidite sequence. Stratigraphically above the lavas lies the Ballagan Formation, which forms the upper part of the Inverclyde Group. This earliest Carboniferous sedimentary succession of the Tweed and Solway basins is distinct from the rest of the Carboniferous succession in being lithologically similar to coeval successions in the Midland Valley of Scotland. Accordingly, the same lithostratigraphical formational terminology is applied ([[Media:P912386.png|P912386]] and [[Media:P912348.png|P912348]]).
 
In the Scottish Borders, along the northern margin of the Northumberland–Solway Basin and underlying the Tweed Basin, the oldest Carboniferous rocks, of Tournaisian age, belong to the Inverclyde Group. They include the basaltic lavas of the Kelso Volcanic Formation (‘Kelso Traps’) and the Birrenswark Volcanic Formation which overlie red-bed strata assigned to the Kinnesswood Formation. The latter rests either conformably on red-bed strata of Devonian age (Stratheden Group) or with marked angular unconformity upon an irregular surface eroded across the steeply dipping, Lower Palaeozoic turbidite sequence. Stratigraphically above the lavas lies the Ballagan Formation, which forms the upper part of the Inverclyde Group. This earliest Carboniferous sedimentary succession of the Tweed and Solway basins is distinct from the rest of the Carboniferous succession in being lithologically similar to coeval successions in the Midland Valley of Scotland. Accordingly, the same lithostratigraphical formational terminology is applied ([[Media:P912386.jpg|P912386]] and [[Media:P912348.jpg|P912348]]).


Higher parts of the Carboniferous succession are not present in the Scottish sector of the Tweed Basin, with the exception of a structurally isolated coastal outcrop north of Berwick. Elsewhere, along the north-west margin of the Nothumberland–Solway Basin, there is a marked departure from the Midland Valley lithofacies, reflected in the application of different lithostratigraphical terminology. There (and across northern England) the Inverclyde Group is succeed by the Border Group, which includes the Lyne Formation, partly coeval with the Ballagan Formation and largely restricted to the Solway area, and the Fell Sandstone Formation.
Higher parts of the Carboniferous succession are not present in the Scottish sector of the Tweed Basin, with the exception of a structurally isolated coastal outcrop north of Berwick. Elsewhere, along the north-west margin of the Nothumberland–Solway Basin, there is a marked departure from the Midland Valley lithofacies, reflected in the application of different lithostratigraphical terminology. There (and across northern England) the Inverclyde Group is succeed by the Border Group, which includes the Lyne Formation, partly coeval with the Ballagan Formation and largely restricted to the Solway area, and the Fell Sandstone Formation.


The Border Group is largely of early Visean age but marine lithofacies are rare within it and there are few stratigraphically useful fossils. Consequently there are difficulties in making a detailed correlation of the succession across the region, particularly toward the bottom of the sequence. For much of this early Carboniferous interval the basins formed narrow gulf-like extensions of the open sea, widening to the south-west, and their sedimentary rock successions reflect the interplay of fluviodelatic and paralic depositional systems ([[Media:P912351.png|P912351]]). The northern emergent margins of the Northumberland and Solway basins were sources of clastic sediment during the early period of deposition, but for much of Dinantian time axial drainage systems were dominant, building from the north and east towards a shallow sea in the west. However, marginal clastic deposition was relatively persistent in the Solway Basin adjacent to the active North Solway Fault system. The variations in local lithostratigraphy for this ‘Dinantian’ interval are summarised and correlated in [[Media:P912352.png|P912352]].
The Border Group is largely of early Visean age but marine lithofacies are rare within it and there are few stratigraphically useful fossils. Consequently there are difficulties in making a detailed correlation of the succession across the region, particularly toward the bottom of the sequence. For much of this early Carboniferous interval the basins formed narrow gulf-like extensions of the open sea, widening to the south-west, and their sedimentary rock successions reflect the interplay of fluviodelatic and paralic depositional systems ([[Media:P912351.jpg|P912351]]). The northern emergent margins of the Northumberland and Solway basins were sources of clastic sediment during the early period of deposition, but for much of Dinantian time axial drainage systems were dominant, building from the north and east towards a shallow sea in the west. However, marginal clastic deposition was relatively persistent in the Solway Basin adjacent to the active North Solway Fault system. The variations in local lithostratigraphy for this ‘Dinantian’ interval are summarised and correlated in [[Media:P912352.jpg|P912352]].


The Border Group is succeeded by the Yoredale Group, a Visean to Namurian succession characterised by repeated upward-coarsening sedimentary cycles from limestone through mudstone to sandstone, capped by seatearth and coal. The ‘Yoredale’ cycles range in thickness up to about 35 m with an average of about 20 m; some are incomplete, and the proportions of the different lithologies vary in response to subtle changes in depositional environment ([[Media:P912353.png|P912353]]). The Yoredale Group is divided, in upward sequence, into the Tyne Limestone, Alston and Stainmore formations based largely on the relative abundance of the different rock types within cycles. The base of the group (and of the Tyne Limestone Formation) is of early Asbian age on the northern margin of the Northumberland–Solway Basin, but becomes diachronously younger southwards. The Alston Formation is mostly of Brigantian age but its uppermost strata are Pendleian so that the formation straddles the Visean–Namurian chronostratigraphical boundary. The Stainmore Formation (which is only sparsely represented in southern Scotland) extends upwards to the top of the Namurian.
The Border Group is succeeded by the Yoredale Group, a Visean to Namurian succession characterised by repeated upward-coarsening sedimentary cycles from limestone through mudstone to sandstone, capped by seatearth and coal. The ‘Yoredale’ cycles range in thickness up to about 35 m with an average of about 20 m; some are incomplete, and the proportions of the different lithologies vary in response to subtle changes in depositional environment ([[Media:P912353.jpg|P912353]]). The Yoredale Group is divided, in upward sequence, into the Tyne Limestone, Alston and Stainmore formations based largely on the relative abundance of the different rock types within cycles. The base of the group (and of the Tyne Limestone Formation) is of early Asbian age on the northern margin of the Northumberland–Solway Basin, but becomes diachronously younger southwards. The Alston Formation is mostly of Brigantian age but its uppermost strata are Pendleian so that the formation straddles the Visean–Namurian chronostratigraphical boundary. The Stainmore Formation (which is only sparsely represented in southern Scotland) extends upwards to the top of the Namurian.


The Yoredale Group is succeeded in the Langholm–Canonbie area of the Scottish Borders by Westphalian fluviodeltaic strata of the Pennine Coal Measures Group. The group includes the Pennine Lower, Pennine Middle and Pennine Upper Coal Measures formations and comprises repeated mudstone–sandstone–coal cyclothems that individually range up to about 15 m in thickness ([[Media:P912353.png|P912353]]). Late in the Westphalian, the Pennine Coal Measures Group was succeeded in the Solway Basin by the red-bed succession of the Warwickshire Group.
The Yoredale Group is succeeded in the Langholm–Canonbie area of the Scottish Borders by Westphalian fluviodeltaic strata of the Pennine Coal Measures Group. The group includes the Pennine Lower, Pennine Middle and Pennine Upper Coal Measures formations and comprises repeated mudstone–sandstone–coal cyclothems that individually range up to about 15 m in thickness ([[Media:P912353.jpg|P912353]]). Late in the Westphalian, the Pennine Coal Measures Group was succeeded in the Solway Basin by the red-bed succession of the Warwickshire Group.




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The lavas of the Kelso and Birrenswark volcanic formations interfinger with, and are overlain conformably by the Ballagan Formation; where it does not overlie the lavas the Ballagan Formation conformably follows the underlying Kinnesswood Formation. The Ballagan Formation is of late Tournaisian to early Visean (Courceyan–Chadian) age and crops out in the Tweed Basin, the Langholm area, and on the Solway Coast. In the Tweed Basin, underlying the Merse of Berwickshire, the Ballagan Formation comprises about 430 m of mudstone with thin interbeds of argillaceous dolostone (‘cementstone’) and sandstone ([[Media:P667218.jpg|P667218]]), with sporadic lenticular bodies of cross-bedded, channel-fill sandstone up to 30 m thick. Locally, the sequence succeeding the Kelso Volcanic Formation also contains concretionary calcrete, the most notable example being the Carham Limestone Bed (‘Carham Stone’), a cherty magnesian limestone up to 7.5 m thick, thought to have accumulated by chemical precipitation from waters enriched in lime by showers of volcanic dust.
The lavas of the Kelso and Birrenswark volcanic formations interfinger with, and are overlain conformably by the Ballagan Formation; where it does not overlie the lavas the Ballagan Formation conformably follows the underlying Kinnesswood Formation. The Ballagan Formation is of late Tournaisian to early Visean (Courceyan–Chadian) age and crops out in the Tweed Basin, the Langholm area, and on the Solway Coast. In the Tweed Basin, underlying the Merse of Berwickshire, the Ballagan Formation comprises about 430 m of mudstone with thin interbeds of argillaceous dolostone (‘cementstone’) and sandstone ([[Media:P667218.jpg|P667218]]), with sporadic lenticular bodies of cross-bedded, channel-fill sandstone up to 30 m thick. Locally, the sequence succeeding the Kelso Volcanic Formation also contains concretionary calcrete, the most notable example being the Carham Limestone Bed (‘Carham Stone’), a cherty magnesian limestone up to 7.5 m thick, thought to have accumulated by chemical precipitation from waters enriched in lime by showers of volcanic dust.


In several parts of the Tweed Basin the Ballagan Formation contains miospores and a fossil fauna that includes environmentally tolerant bivalves and ostracods, and at Foulden Burn in Berwickshire (NT 921 553) has yielded twelve or more species of fresh- to brackish-water fish from the mudstone of the Foulden Fish Bed. Farther west, at the margin of the Solway Basin around Langholm, the Ballagan Formation contains plant fossils and a sparse faunal assemblage that includes gastropods, ostracods, modiolid bivalves and fish remains. The fossils occur in discrete associations of alluvial, fluvial and fluviodeltaic sedimentary rocks, intercalated with lacustrine, coastal plain and sporadic marine deposits ([[Media:P912351.png|P912351]]).
In several parts of the Tweed Basin the Ballagan Formation contains miospores and a fossil fauna that includes environmentally tolerant bivalves and ostracods, and at Foulden Burn in Berwickshire (NT 921 553) has yielded twelve or more species of fresh- to brackish-water fish from the mudstone of the Foulden Fish Bed. Farther west, at the margin of the Solway Basin around Langholm, the Ballagan Formation contains plant fossils and a sparse faunal assemblage that includes gastropods, ostracods, modiolid bivalves and fish remains. The fossils occur in discrete associations of alluvial, fluvial and fluviodeltaic sedimentary rocks, intercalated with lacustrine, coastal plain and sporadic marine deposits ([[Media:P912351.jpg|P912351]]).


In its outcrop from the Langholm area, westward along the Solway coast, the Ballagan Formation is in the order of 100 to 150 m thick (thinning westward) and is made up of thinly interbedded sandstone, mudstone and lagoonal dolostone (‘cementstone’) with some anhydrite ([[Media:P579357.jpg|P579357]]). It is a terrestrial/fluvial to peritidal assemblage known in its Solway coastal outlier as the Kirkbean Cementstone Member. It was deposited in shallow channels within mudflats in a low-lying, coastal environment but with only a limited and intermittent marine influence ([[Media:P912351.png|P912351]]). The Kirkbean Cementstone Member contains plant fragments and a quasi- to nonmarine fauna of bivalves, gastropods and ostracods, although fully marine bivalves are present locally. Elsewhere, in Annandale around Dalton (NY 104 740), boreholes prove the Kirkbean Cementstone Member to be at least 100 m thick and to contain abundant fibrous gypsum (satin spar) bands up to 4 cm thick. The ‘cementstones’ are underlain by dark red, fine-grained to conglomeratic sandstone, the Annandale Sandstone Beds. The anhydrite (or its precursor, gypsum) formed either subaqueously in shallow water or subaerially in coastal sabkhas, but following burial to depths greater than a few hundred metres, CaSO4 is usually preserved as anhydrite. Both gypsum and anhydrite are readily removed in the surface weathering zone and so are rarely seen at outcrop.
In its outcrop from the Langholm area, westward along the Solway coast, the Ballagan Formation is in the order of 100 to 150 m thick (thinning westward) and is made up of thinly interbedded sandstone, mudstone and lagoonal dolostone (‘cementstone’) with some anhydrite ([[Media:P579357.jpg|P579357]]). It is a terrestrial/fluvial to peritidal assemblage known in its Solway coastal outlier as the Kirkbean Cementstone Member. It was deposited in shallow channels within mudflats in a low-lying, coastal environment but with only a limited and intermittent marine influence ([[Media:P912351.jpg|P912351]]). The Kirkbean Cementstone Member contains plant fragments and a quasi- to nonmarine fauna of bivalves, gastropods and ostracods, although fully marine bivalves are present locally. Elsewhere, in Annandale around Dalton (NY 104 740), boreholes prove the Kirkbean Cementstone Member to be at least 100 m thick and to contain abundant fibrous gypsum (satin spar) bands up to 4 cm thick. The ‘cementstones’ are underlain by dark red, fine-grained to conglomeratic sandstone, the Annandale Sandstone Beds. The anhydrite (or its precursor, gypsum) formed either subaqueously in shallow water or subaerially in coastal sabkhas, but following burial to depths greater than a few hundred metres, CaSO4 is usually preserved as anhydrite. Both gypsum and anhydrite are readily removed in the surface weathering zone and so are rarely seen at outcrop.


Another ‘cementstone’ sequence is seen in the Langholm–Newcastleton area, where it is up to 200 m thick and passes downwards into fine-grained, massive, fluviatile and deltaic sandstones, derived from the north, which are collectively referred to the Whita Sandstone Beds. These ‘cementstones’ were formerly regarded as part of a ‘Lower Border Group’ ([[Media:P912354.png|P912354]]) and demonstrate the uncertainties that bedevil correlation between the various cementstone–sandstone sequences. The ‘cementstones’ above the Whita Sandstone could equally well be regarded as part of the Ballagan Formation or the Lyne Formation (Border Group — see below), whilst the Whita Sandstone is assigned to the Ballagan Formation.
Another ‘cementstone’ sequence is seen in the Langholm–Newcastleton area, where it is up to 200 m thick and passes downwards into fine-grained, massive, fluviatile and deltaic sandstones, derived from the north, which are collectively referred to the Whita Sandstone Beds. These ‘cementstones’ were formerly regarded as part of a ‘Lower Border Group’ ([[Media:P912354.jpg|P912354]]) and demonstrate the uncertainties that bedevil correlation between the various cementstone–sandstone sequences. The ‘cementstones’ above the Whita Sandstone could equally well be regarded as part of the Ballagan Formation or the Lyne Formation (Border Group — see below), whilst the Whita Sandstone is assigned to the Ballagan Formation.


The most westerly outcrop of the Ballagan Formation forms part of the Rerrick Outlier on the north coast of the Solway Firth. There, the Wall Hill Sandstone Member rests with angular unconformity on Wenlock strata of the Raeberry Castle Formation. It has a maximum thickness of about 360 m and comprises upward-fining sequences of conglomeratic sandstones, siltstones and mudstones. The fine-grained rocks were laid down on a floodplain by a braided fluvial system of low to moderate sinuosity, with the coarse-grained rocks deposited in channels. Palaeocurrent data indicate a predominately north and north-westerly source from the Southern Uplands terrane, and the conglomerates include clasts derived from the roof of the Criffel–Dalbeattie pluton. Local but intense sediment fluidisation developed at a number of localities, perhaps influenced by synsedimentary activity on the nearby North Solway Fault.
The most westerly outcrop of the Ballagan Formation forms part of the Rerrick Outlier on the north coast of the Solway Firth. There, the Wall Hill Sandstone Member rests with angular unconformity on Wenlock strata of the Raeberry Castle Formation. It has a maximum thickness of about 360 m and comprises upward-fining sequences of conglomeratic sandstones, siltstones and mudstones. The fine-grained rocks were laid down on a floodplain by a braided fluvial system of low to moderate sinuosity, with the coarse-grained rocks deposited in channels. Palaeocurrent data indicate a predominately north and north-westerly source from the Southern Uplands terrane, and the conglomerates include clasts derived from the roof of the Criffel–Dalbeattie pluton. Local but intense sediment fluidisation developed at a number of localities, perhaps influenced by synsedimentary activity on the nearby North Solway Fault.
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=== Border Group ===
=== Border Group ===
The Border Group has a more-or-less continuous outcrop along the northern margin of the Northumberland–Solway Basin but in the east this lies entirely on the English side of the border ([[Media:P912313.png|P912313]]). The outcrop spans the border to the north of Keilder and thence extends westward to Newcastleton, Ecclefechan and the Southerness area of the Kirkbean outlier.
The Border Group has a more-or-less continuous outcrop along the northern margin of the Northumberland–Solway Basin but in the east this lies entirely on the English side of the border ([[Media:P912313.jpg|P912313]]). The outcrop spans the border to the north of Keilder and thence extends westward to Newcastleton, Ecclefechan and the Southerness area of the Kirkbean outlier.


==== Lyne Formation ====
==== Lyne Formation ====
In general, the Ballagan Formation becomes more marine in character, both upward and laterally towards the south-east, as it passes conformably and diachronously up into the Lyne Formation of the Border Group. The Lyne Formation is late Tournaisian–Arundian in age and in the main part of its outcrop, which lies south of the English border, it comprises cyclical sequences of fine-grained subarkosic sandstone, siltstone, mudstone, thin beds of limestone or dolostone, and anhydrite: the Lynebank, Bewcastle, Main Algal, Cambeck, and Easton Anhydrite members are recognised ([[Media:P912354.png|P912354]]). At outcrop, the Lyne Formation is up to about 1000 m thick but the Easton Anhydrite Member has been proved in boreholes to comprise anhydrite abundantly interbedded through about 1300 m of clastic strata at about the level of the Lynebank and Bewcastle members. Deposition took place in a fluctuating network of peritidal, deltaic and fluvial environments subject to occasional marine incursions, with the anhydrite probably accumulating under sabkha conditions.
In general, the Ballagan Formation becomes more marine in character, both upward and laterally towards the south-east, as it passes conformably and diachronously up into the Lyne Formation of the Border Group. The Lyne Formation is late Tournaisian–Arundian in age and in the main part of its outcrop, which lies south of the English border, it comprises cyclical sequences of fine-grained subarkosic sandstone, siltstone, mudstone, thin beds of limestone or dolostone, and anhydrite: the Lynebank, Bewcastle, Main Algal, Cambeck, and Easton Anhydrite members are recognised ([[Media:P912354.jpg|P912354]]). At outcrop, the Lyne Formation is up to about 1000 m thick but the Easton Anhydrite Member has been proved in boreholes to comprise anhydrite abundantly interbedded through about 1300 m of clastic strata at about the level of the Lynebank and Bewcastle members. Deposition took place in a fluctuating network of peritidal, deltaic and fluvial environments subject to occasional marine incursions, with the anhydrite probably accumulating under sabkha conditions.


Much of the Scottish succession is assigned to the Southerness Limestone Member, about 135 m of thinly interbedded calcareous mudstone, siltstone and limestone. The member is best seen in the Kirkbean Outlier where it is richly fossiliferous, with a varied marine fauna of Chadian to Arundian age, and contains two distinctive algal stromatolite bands each about 1 m thick ([[Media:P220451.jpg|P220451]]). The stromatolites resemble dome types from algal beds of the Lyne Formation in the Northumberland Basin. Individual domes are up to 30 cm in diameter with a relief of 10–15 cm, and are set in a calcareous mudstone matrix. They probably formed in a low intertidal to shallow subtidal depositional environment since there are signs of limited reworking. The algal bands are developed in two of three sedimentary cycles identified in the upper part of the member as follows:
Much of the Scottish succession is assigned to the Southerness Limestone Member, about 135 m of thinly interbedded calcareous mudstone, siltstone and limestone. The member is best seen in the Kirkbean Outlier where it is richly fossiliferous, with a varied marine fauna of Chadian to Arundian age, and contains two distinctive algal stromatolite bands each about 1 m thick ([[Media:P220451.jpg|P220451]]). The stromatolites resemble dome types from algal beds of the Lyne Formation in the Northumberland Basin. Individual domes are up to 30 cm in diameter with a relief of 10–15 cm, and are set in a calcareous mudstone matrix. They probably formed in a low intertidal to shallow subtidal depositional environment since there are signs of limited reworking. The algal bands are developed in two of three sedimentary cycles identified in the upper part of the member as follows:
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Around Newcastleton and Ecclefechan, the member contains an Arundian fauna of brachiopods, bivalves, foraminifera and ostracods that suggests a stratigraphical level equivalent to the Cambeck Member. Also present, both there and in the Kirkbean Outlier, are limestone beds (respectively the ‘Harden Beds’ and Syringothyris Limestone Bed) of possible equivalence to the Whitberry Marine Band that farther south marks the base of the Fell Sandstone Formation. However, stratigraphically above the Syringothyris Limestone Bed at Southerness are the two distinctive algal stromatolite bands of ‘Lyne Formation type’. If these stratigraphical correlations are correct, then the Southerness Limestone Member ranges diachronously higher than the top of the Lyne Formation elsewhere in the Solway and Northumberland basins.
Around Newcastleton and Ecclefechan, the member contains an Arundian fauna of brachiopods, bivalves, foraminifera and ostracods that suggests a stratigraphical level equivalent to the Cambeck Member. Also present, both there and in the Kirkbean Outlier, are limestone beds (respectively the ‘Harden Beds’ and Syringothyris Limestone Bed) of possible equivalence to the Whitberry Marine Band that farther south marks the base of the Fell Sandstone Formation. However, stratigraphically above the Syringothyris Limestone Bed at Southerness are the two distinctive algal stromatolite bands of ‘Lyne Formation type’. If these stratigraphical correlations are correct, then the Southerness Limestone Member ranges diachronously higher than the top of the Lyne Formation elsewhere in the Solway and Northumberland basins.


In the Rerrick Outlier, the Orroland Member of the Lyne Formation comprises about 280 m of red beds formed under subaerial conditions on the margin of the Solway Basin against the syntectonically active North Solway Fault ([[Media:P912351.png|P912351]]). The fining-upward cycles of conglomerate, sandstone, siltstone and calcrete–palaeosol are sporadically interrupted by marine beds of greyish red, fossiliferous limestone, which is locally oolitic. A mostly detrital fauna of corals, brachiopods and bivalves suggests correlation with the Syringothyris Limestone in the Southerness succession.
In the Rerrick Outlier, the Orroland Member of the Lyne Formation comprises about 280 m of red beds formed under subaerial conditions on the margin of the Solway Basin against the syntectonically active North Solway Fault ([[Media:P912351.jpg|P912351]]). The fining-upward cycles of conglomerate, sandstone, siltstone and calcrete–palaeosol are sporadically interrupted by marine beds of greyish red, fossiliferous limestone, which is locally oolitic. A mostly detrital fauna of corals, brachiopods and bivalves suggests correlation with the Syringothyris Limestone in the Southerness succession.


==== Fell Sandstone Formation ====
==== Fell Sandstone Formation ====
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The earliest fluvial deposits accumulated in the north-east. There, to the south of Burnmouth (NT 956 610) on the Berwickshire coast, some 200 m of pale-coloured, cross- and con-volute-bedded sandstone contain bands of quartz pebbles and lenses of red sandy mudstone. The sandstone is overlain by some 75 m of interbedded mudstone, sandstone and seatearth with a few thin coals (worked at Lamberton, just north of the border, in the 19th century) and argillaceous dolostone. Near the top of this sequence lies the 1.5 m thick ‘Lamberton Limestone’ which is a likely correlative of the Cove Marine Bands, part of the Aberlady Formation (Strathclyde Group), farther north in the Oldhamstocks Basin (see below).
The earliest fluvial deposits accumulated in the north-east. There, to the south of Burnmouth (NT 956 610) on the Berwickshire coast, some 200 m of pale-coloured, cross- and con-volute-bedded sandstone contain bands of quartz pebbles and lenses of red sandy mudstone. The sandstone is overlain by some 75 m of interbedded mudstone, sandstone and seatearth with a few thin coals (worked at Lamberton, just north of the border, in the 19th century) and argillaceous dolostone. Near the top of this sequence lies the 1.5 m thick ‘Lamberton Limestone’ which is a likely correlative of the Cove Marine Bands, part of the Aberlady Formation (Strathclyde Group), farther north in the Oldhamstocks Basin (see below).


To the south-west, in the vicinity of Langholm, a coarsening-upwards siliciclastic sequence culminates in a fluvial sandstone unit up to 250 m thick, known locally as the ‘Larriston Sandstone’, overlain by a more mixed, thinly bedded sandstone–mudstone succession. The ‘Larriston Sandstone’ was derived from the north-east as channel-fill deposits within a braided river system, which by the mid Arundian was of increasing regional importance and fed into a delta system farther to the south-west. There, around Ecclefechan, the sequence comprises sandstone (some conglomeratic, with quartz pebbles), mudstone and sporadic seatearth with roots, the latter suggesting intermittent emergence of the delta surface. In the Newcastleton outcrop there is evidence for a syndepositional, extensional tectonic regime from the presence, in mid sequence, of about 30 m of porphyritic olivine basalt lava: the Kershopefoot Basalt ([[Media:P912354.png|P912354]]).
To the south-west, in the vicinity of Langholm, a coarsening-upwards siliciclastic sequence culminates in a fluvial sandstone unit up to 250 m thick, known locally as the ‘Larriston Sandstone’, overlain by a more mixed, thinly bedded sandstone–mudstone succession. The ‘Larriston Sandstone’ was derived from the north-east as channel-fill deposits within a braided river system, which by the mid Arundian was of increasing regional importance and fed into a delta system farther to the south-west. There, around Ecclefechan, the sequence comprises sandstone (some conglomeratic, with quartz pebbles), mudstone and sporadic seatearth with roots, the latter suggesting intermittent emergence of the delta surface. In the Newcastleton outcrop there is evidence for a syndepositional, extensional tectonic regime from the presence, in mid sequence, of about 30 m of porphyritic olivine basalt lava: the Kershopefoot Basalt ([[Media:P912354.jpg|P912354]]).


In the Kirkbean Outlier, on the north Solway coast, the 120–150 m-thick Gillfoot Sandstone Member comprises quartzose sandstone, conglomerate and red mudstone, with a few thin beds of sandy limestone containing detrital fossils. Conglomerates form about 20 per cent of the succession and have a calcareous matrix. They contain intraformational fragments in addition to pebbles of Silurian wacke-type sandstone and Devonian dyke rocks. The member is sparsely fossiliferous and an Arundian age is likely. The lithologies indicate a marginal coastal depositional setting with the conglomerates deposited by sheetfloods flowing over low-lying supratidal areas.
In the Kirkbean Outlier, on the north Solway coast, the 120–150 m-thick Gillfoot Sandstone Member comprises quartzose sandstone, conglomerate and red mudstone, with a few thin beds of sandy limestone containing detrital fossils. Conglomerates form about 20 per cent of the succession and have a calcareous matrix. They contain intraformational fragments in addition to pebbles of Silurian wacke-type sandstone and Devonian dyke rocks. The member is sparsely fossiliferous and an Arundian age is likely. The lithologies indicate a marginal coastal depositional setting with the conglomerates deposited by sheetfloods flowing over low-lying supratidal areas.
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==== Tyne Limestone Formation ====
==== Tyne Limestone Formation ====
The lowest division of the Yoredale Group is the Tyne Limestone Formation, a sequence of highly variable, marine to deltaic, ‘Yoredale’-type upward-coarsening cycles in which a thin but extensive bed of marine limestone is overlain by mudstone and sandstone commonly topped by a seatearth and a thin coal seam ([[Media:P912353.png|P912353]]); there is much lithostratigraphical variation both vertically and laterally. The formation is largely Asbian in age and locally exceeds 2000 m in thickness. It has a wide outcrop across Northumberland and northern Cumbria and extends across the border into Scotland from south-west Liddesdale to Ecclefechan and the north Solway coast.
The lowest division of the Yoredale Group is the Tyne Limestone Formation, a sequence of highly variable, marine to deltaic, ‘Yoredale’-type upward-coarsening cycles in which a thin but extensive bed of marine limestone is overlain by mudstone and sandstone commonly topped by a seatearth and a thin coal seam ([[Media:P912353.jpg|P912353]]); there is much lithostratigraphical variation both vertically and laterally. The formation is largely Asbian in age and locally exceeds 2000 m in thickness. It has a wide outcrop across Northumberland and northern Cumbria and extends across the border into Scotland from south-west Liddesdale to Ecclefechan and the north Solway coast.


The base of the Tyne Limestone Formation is locally marked, between Langholm and Canonbie, by the volcanic rocks of the Glencartholm Volcanic Member, about 150 m of tuffs and basaltic lavas. Sedimentary rocks associated with the tuffs contain a marine fauna of brachiopods and bivalves, but are notable for rich arthropod and fish faunas, the former including crustaceans, eurypterids and scorpions. The fossils indicate a level near the base of the Asbian Stage. Above the Glencartholm Volcanic Member, the Tyne Limestone Formation comprises ‘Yoredale’ cyclical sequences consisting predominantly of marine mudstone and limestone with subordinate sandstone and a few seatearths and thin coal seams. Many of the beds are fossiliferous with a fauna noted for extensive coral colonies and layers of large brachiopods. More arenaceous strata follow, succeeded in turn by 30 m of marine mudstone, the Dinwoodie Beds, which are notable for their rich and varied fauna of corals, brachiopods and bryozoans, perhaps the most diverse fauna of the entire Carboniferous sequence.
The base of the Tyne Limestone Formation is locally marked, between Langholm and Canonbie, by the volcanic rocks of the Glencartholm Volcanic Member, about 150 m of tuffs and basaltic lavas. Sedimentary rocks associated with the tuffs contain a marine fauna of brachiopods and bivalves, but are notable for rich arthropod and fish faunas, the former including crustaceans, eurypterids and scorpions. The fossils indicate a level near the base of the Asbian Stage. Above the Glencartholm Volcanic Member, the Tyne Limestone Formation comprises ‘Yoredale’ cyclical sequences consisting predominantly of marine mudstone and limestone with subordinate sandstone and a few seatearths and thin coal seams. Many of the beds are fossiliferous with a fauna noted for extensive coral colonies and layers of large brachiopods. More arenaceous strata follow, succeeded in turn by 30 m of marine mudstone, the Dinwoodie Beds, which are notable for their rich and varied fauna of corals, brachiopods and bryozoans, perhaps the most diverse fauna of the entire Carboniferous sequence.
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=== Pennine Coal Measures Group ===
=== Pennine Coal Measures Group ===
Westphalian Coal Measures strata crop out in the Canonbie Coalfield, a limited area of such strata that lies mainly to the north of the border in south-west Scotland, but has a concealed extention southwards into Cumbria, linking with the Cumbrian Coalfield ([[Media:P912355.png|P912355]]), as described in the companion volume for Northern England. The Canonbie Coalfield is situated at the north-
Westphalian Coal Measures strata crop out in the Canonbie Coalfield, a limited area of such strata that lies mainly to the north of the border in south-west Scotland, but has a concealed extention southwards into Cumbria, linking with the Cumbrian Coalfield ([[Media:P912355.jpg|P912355]]), as described in the companion volume for Northern England. The Canonbie Coalfield is situated at the north-


west margin of the north-east to south-west-trending Solway Syncline, beneath which some coal seams may persist to reappear in Cumbria. Although their continuation has not been proved by deep boreholes, it is supported by seismic surveys. Application of both of these techniques in the 1980s indicated that the concealed coalfield is larger and has more economic potential than was previously envisaged. The exposed Canonbie coalfield is relatively small and was exploited on a limited scale in the 19th and early 20th centuries; deep mining ceased in the 1920s
west margin of the north-east to south-west-trending Solway Syncline, beneath which some coal seams may persist to reappear in Cumbria. Although their continuation has not been proved by deep boreholes, it is supported by seismic surveys. Application of both of these techniques in the 1980s indicated that the concealed coalfield is larger and has more economic potential than was previously envisaged. The exposed Canonbie coalfield is relatively small and was exploited on a limited scale in the 19th and early 20th centuries; deep mining ceased in the 1920s
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Deposition in much of the Solway Basin continued unbroken from the Namurian into the Westphalian, but along the northern margin of the basin there is an unconformity at the base of the Pennine Coal Measures Group, such that strata high in the Pennine Lower Coal Measures Formation rest unconformably on Yoredale Group rocks. There is also evidence for at least one unconformity in the overlying, late Westphalian ‘red beds’ that in the Canonbie Coalfield form part of the Warwickshire Group (see below).
Deposition in much of the Solway Basin continued unbroken from the Namurian into the Westphalian, but along the northern margin of the basin there is an unconformity at the base of the Pennine Coal Measures Group, such that strata high in the Pennine Lower Coal Measures Formation rest unconformably on Yoredale Group rocks. There is also evidence for at least one unconformity in the overlying, late Westphalian ‘red beds’ that in the Canonbie Coalfield form part of the Warwickshire Group (see below).


The Pennine Coal Measures Group is predominantly nonmarine and is characterised by coarsening-upward cycles, commonly up to 15 m thick, composed of mudstone, siltstone and sandstone capped by a seatearth and coal, though coal forms only a minor part of the total sequence ([[Media:P912353.png|P912353]]); clay ironstone occurs within some mudstones. The Solway Basin and its Westphalian strata were subject to some uplift and deformation during the late Carboniferous to early Permian Variscan tectonic episode, with development of open folds ([[Media:P221694.jpg|P221694]]) and much faulting, whilst regionally the strata of the Solway Basin were compressed into the major Solway Syncline ([[Media:P912349.png|P912349]]). Uplifted areas were soon eroded and Westphalian beds are now preserved only in the downwarped, coalfield areas. Subsequent sedimentation produced an unconformable cover of Permo-Triassic rocks over the Westphalian beds, which are commonly reddened to a depth of more than 100 m beneath the sub-Permian unconformity. Whilst erosion has since removed parts of the cover, much of the Canonbie Coalfield remains concealed beneath Permo-Triassic strata ([[Media:P912355.png|P912355]]).
The Pennine Coal Measures Group is predominantly nonmarine and is characterised by coarsening-upward cycles, commonly up to 15 m thick, composed of mudstone, siltstone and sandstone capped by a seatearth and coal, though coal forms only a minor part of the total sequence ([[Media:P912353.jpg|P912353]]); clay ironstone occurs within some mudstones. The Solway Basin and its Westphalian strata were subject to some uplift and deformation during the late Carboniferous to early Permian Variscan tectonic episode, with development of open folds ([[Media:P221694.jpg|P221694]]) and much faulting, whilst regionally the strata of the Solway Basin were compressed into the major Solway Syncline ([[Media:P912349.jpg|P912349]]). Uplifted areas were soon eroded and Westphalian beds are now preserved only in the downwarped, coalfield areas. Subsequent sedimentation produced an unconformable cover of Permo-Triassic rocks over the Westphalian beds, which are commonly reddened to a depth of more than 100 m beneath the sub-Permian unconformity. Whilst erosion has since removed parts of the cover, much of the Canonbie Coalfield remains concealed beneath Permo-Triassic strata ([[Media:P912355.jpg|P912355]]).


In the Scottish part of the Canonbie Coalfield, the Pennine Lower Coal Measures Formation is about 110 m thick, the Middle Coal Measures about 250 m and Upper Coal Measures about 150 m ([[Media:P912355.png|P912355]] and [[Media:P912356.png|P912356]]): the sequence thickens into the hinge zone of the Solway Syncline. Coals in the Pennine Lower Coal Measures Formation are commonly unnamed and difficult to correlate, with borehole information showing them to be generally less than 0.8 m in thickness. The main seams are from the Duckmantian part of the Pennine Middle Coal Measures Formation, with only a few thin coals present in its uppermost, Bolsovian section. The few coals in the Pennine Upper Coal Measures are mostly located close to the base of the formation but an exception is the aptly named High Coal, which occurs about 150 m above the base of the Upper Coal Measures. It forms a convenient marker for the base of the overlying Warwickshire Group.
In the Scottish part of the Canonbie Coalfield, the Pennine Lower Coal Measures Formation is about 110 m thick, the Middle Coal Measures about 250 m and Upper Coal Measures about 150 m ([[Media:P912355.jpg|P912355]] and [[Media:P912356.jpg|P912356]]): the sequence thickens into the hinge zone of the Solway Syncline. Coals in the Pennine Lower Coal Measures Formation are commonly unnamed and difficult to correlate, with borehole information showing them to be generally less than 0.8 m in thickness. The main seams are from the Duckmantian part of the Pennine Middle Coal Measures Formation, with only a few thin coals present in its uppermost, Bolsovian section. The few coals in the Pennine Upper Coal Measures are mostly located close to the base of the formation but an exception is the aptly named High Coal, which occurs about 150 m above the base of the Upper Coal Measures. It forms a convenient marker for the base of the overlying Warwickshire Group.


==== Pennine Lower Coal Measures ====
==== Pennine Lower Coal Measures ====
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=== Warwickshire Group ===
=== Warwickshire Group ===
In the Canonbie Coalfield, the Pennine Coal Measures Group is overlain by a red-bed succession that, together with other, similar upper Carboniferous sequences elsewhere in northern and central England, make up the Warwickshire Group ([[Media:P912387.png|P912387]]). A key characteristic is reddening that occurred soon after sediment deposition, but which is not uniformly developed throughout; as a result, red beds alternate with unreddened, grey-green intervals. This phenomenon introduces local uncertainty as to the exact age of the Warwickshire Group strata and their stratigraphical relationship to apparently coeval, but unreddened rocks in the Pennine Coal Measures Group.
In the Canonbie Coalfield, the Pennine Coal Measures Group is overlain by a red-bed succession that, together with other, similar upper Carboniferous sequences elsewhere in northern and central England, make up the Warwickshire Group ([[Media:P912387.jpg|P912387]]). A key characteristic is reddening that occurred soon after sediment deposition, but which is not uniformly developed throughout; as a result, red beds alternate with unreddened, grey-green intervals. This phenomenon introduces local uncertainty as to the exact age of the Warwickshire Group strata and their stratigraphical relationship to apparently coeval, but unreddened rocks in the Pennine Coal Measures Group.


The Canonbie outcrop of the Warwickshire Group consists of interbedded mudstones, siltstones and sandstone similar to those of the underlying Pennine Coal Measures but mostly reddened; coal is rare. The group is late Bolsovian to Asturian in age. Its base at Canonbie is conformable, grading up from the grey mudstones of the Pennine Upper Coal Measures Formation into the overlying red beds. The sharp, unconformable base of the Permian succession defines the top of the group. At outcrop, about 290 m of Warwickshire Group red beds are exposed along the banks of the River Esk, but the maximum proved thickness is the 530 m seen in the Becklees Borehole, close to the central axis of the Solway Syncline. Seismic reflection data indicate that elsewhere in the centre of the syncline the group could be up to about 700 m thick.
The Canonbie outcrop of the Warwickshire Group consists of interbedded mudstones, siltstones and sandstone similar to those of the underlying Pennine Coal Measures but mostly reddened; coal is rare. The group is late Bolsovian to Asturian in age. Its base at Canonbie is conformable, grading up from the grey mudstones of the Pennine Upper Coal Measures Formation into the overlying red beds. The sharp, unconformable base of the Permian succession defines the top of the group. At outcrop, about 290 m of Warwickshire Group red beds are exposed along the banks of the River Esk, but the maximum proved thickness is the 530 m seen in the Becklees Borehole, close to the central axis of the Solway Syncline. Seismic reflection data indicate that elsewhere in the centre of the syncline the group could be up to about 700 m thick.


Three formations have been recognised within the group, each with distinctive geophysical log signatures that allow them to be readily correlated in the subsurface. The lowermost, Eskbank Wood Formation (late Bolsovian to Asturian), ranges in thickness from 145 m to 175 m. Its base is gradational (and probably diachronous) across alternations of grey and primary red-bed strata but can conveniently be taken at the first appearance of pedogenic carbonate nodules just below the High Coal, which forms a prominent marker horizon ([[Media:P912356.png|P912356]]). The formation comprises red mudstone and some sandstone, calcrete palaeosols, thin beds of ''Spirorbis'' limestone and ''Estheria''-bearing mudstone. In the lower part there are a few thin coals and beds of grey mudstone, some of which contain nonmarine bivalves. The overlying Canonbie Bridge Sandstone Formation (Asturian) ranges in thickness from 131 m to 154 m. The base of the formation is sharp, marked by the incoming of thick units of medium- and coarse-grained, cross-bedded channel sandstone. A noticeable feature of these sandstones is their greenish grey colour, which can be related to the presence of abundant lithic grains. There are sporadic interbeds of mudstone and calcrete. The Becklees Sandstone Formation (Asturian) is the highest unit recognised from the Warwickshire Group of Canonbie and is overlain unconformably by Permian strata. Its full thickness is not known, but about 200 m is proved in the Becklees Borehole. Fine-grained sandstone with a distinct orange-brown colour is the dominant lithology, with some thin beds of mudstone and calcrete. Borehole cuttings suggest limestones and thin coals may also be present, though rare.
Three formations have been recognised within the group, each with distinctive geophysical log signatures that allow them to be readily correlated in the subsurface. The lowermost, Eskbank Wood Formation (late Bolsovian to Asturian), ranges in thickness from 145 m to 175 m. Its base is gradational (and probably diachronous) across alternations of grey and primary red-bed strata but can conveniently be taken at the first appearance of pedogenic carbonate nodules just below the High Coal, which forms a prominent marker horizon ([[Media:P912356.jpg|P912356]]). The formation comprises red mudstone and some sandstone, calcrete palaeosols, thin beds of ''Spirorbis'' limestone and ''Estheria''-bearing mudstone. In the lower part there are a few thin coals and beds of grey mudstone, some of which contain nonmarine bivalves. The overlying Canonbie Bridge Sandstone Formation (Asturian) ranges in thickness from 131 m to 154 m. The base of the formation is sharp, marked by the incoming of thick units of medium- and coarse-grained, cross-bedded channel sandstone. A noticeable feature of these sandstones is their greenish grey colour, which can be related to the presence of abundant lithic grains. There are sporadic interbeds of mudstone and calcrete. The Becklees Sandstone Formation (Asturian) is the highest unit recognised from the Warwickshire Group of Canonbie and is overlain unconformably by Permian strata. Its full thickness is not known, but about 200 m is proved in the Becklees Borehole. Fine-grained sandstone with a distinct orange-brown colour is the dominant lithology, with some thin beds of mudstone and calcrete. Borehole cuttings suggest limestones and thin coals may also be present, though rare.


Warwickshire Group sedimentation in the Canonbie area largely took place on an alluvial plain drained by braided river systems, and was characterised by an early, primary oxidation of the strata. Palaeocurrent data from channels in the Canonbie Bridge Sandstone show that the rivers flowed towards the north. Overbank and floodplain mud was deposited between the channels, where soils were able to form during intervals of low aggradation. The ‘Spirorbis’ limestones of the Eskbank Wood Formation are most probably of lacustrine origin.
Warwickshire Group sedimentation in the Canonbie area largely took place on an alluvial plain drained by braided river systems, and was characterised by an early, primary oxidation of the strata. Palaeocurrent data from channels in the Canonbie Bridge Sandstone show that the rivers flowed towards the north. Overbank and floodplain mud was deposited between the channels, where soils were able to form during intervals of low aggradation. The ‘Spirorbis’ limestones of the Eskbank Wood Formation are most probably of lacustrine origin.

Revision as of 10:30, 27 July 2015

From: 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.

Introduction

In the Scottish Borders, along the northern margin of the Northumberland–Solway Basin and underlying the Tweed Basin, the oldest Carboniferous rocks, of Tournaisian age, belong to the Inverclyde Group. They include the basaltic lavas of the Kelso Volcanic Formation (‘Kelso Traps’) and the Birrenswark Volcanic Formation which overlie red-bed strata assigned to the Kinnesswood Formation. The latter rests either conformably on red-bed strata of Devonian age (Stratheden Group) or with marked angular unconformity upon an irregular surface eroded across the steeply dipping, Lower Palaeozoic turbidite sequence. Stratigraphically above the lavas lies the Ballagan Formation, which forms the upper part of the Inverclyde Group. This earliest Carboniferous sedimentary succession of the Tweed and Solway basins is distinct from the rest of the Carboniferous succession in being lithologically similar to coeval successions in the Midland Valley of Scotland. Accordingly, the same lithostratigraphical formational terminology is applied (P912386 and P912348).

Higher parts of the Carboniferous succession are not present in the Scottish sector of the Tweed Basin, with the exception of a structurally isolated coastal outcrop north of Berwick. Elsewhere, along the north-west margin of the Nothumberland–Solway Basin, there is a marked departure from the Midland Valley lithofacies, reflected in the application of different lithostratigraphical terminology. There (and across northern England) the Inverclyde Group is succeed by the Border Group, which includes the Lyne Formation, partly coeval with the Ballagan Formation and largely restricted to the Solway area, and the Fell Sandstone Formation.

The Border Group is largely of early Visean age but marine lithofacies are rare within it and there are few stratigraphically useful fossils. Consequently there are difficulties in making a detailed correlation of the succession across the region, particularly toward the bottom of the sequence. For much of this early Carboniferous interval the basins formed narrow gulf-like extensions of the open sea, widening to the south-west, and their sedimentary rock successions reflect the interplay of fluviodelatic and paralic depositional systems (P912351). The northern emergent margins of the Northumberland and Solway basins were sources of clastic sediment during the early period of deposition, but for much of Dinantian time axial drainage systems were dominant, building from the north and east towards a shallow sea in the west. However, marginal clastic deposition was relatively persistent in the Solway Basin adjacent to the active North Solway Fault system. The variations in local lithostratigraphy for this ‘Dinantian’ interval are summarised and correlated in P912352.

The Border Group is succeeded by the Yoredale Group, a Visean to Namurian succession characterised by repeated upward-coarsening sedimentary cycles from limestone through mudstone to sandstone, capped by seatearth and coal. The ‘Yoredale’ cycles range in thickness up to about 35 m with an average of about 20 m; some are incomplete, and the proportions of the different lithologies vary in response to subtle changes in depositional environment (P912353). The Yoredale Group is divided, in upward sequence, into the Tyne Limestone, Alston and Stainmore formations based largely on the relative abundance of the different rock types within cycles. The base of the group (and of the Tyne Limestone Formation) is of early Asbian age on the northern margin of the Northumberland–Solway Basin, but becomes diachronously younger southwards. The Alston Formation is mostly of Brigantian age but its uppermost strata are Pendleian so that the formation straddles the Visean–Namurian chronostratigraphical boundary. The Stainmore Formation (which is only sparsely represented in southern Scotland) extends upwards to the top of the Namurian.

The Yoredale Group is succeeded in the Langholm–Canonbie area of the Scottish Borders by Westphalian fluviodeltaic strata of the Pennine Coal Measures Group. The group includes the Pennine Lower, Pennine Middle and Pennine Upper Coal Measures formations and comprises repeated mudstone–sandstone–coal cyclothems that individually range up to about 15 m in thickness (P912353). Late in the Westphalian, the Pennine Coal Measures Group was succeeded in the Solway Basin by the red-bed succession of the Warwickshire Group.


Inverclyde Group

Kinnesswood Formation

Along the southern margin of the Southern Uplands massif the Upper Devonian Stratheden Group passes conformably up into the Inverclyde Group. The base of the latter is taken at Kinnesswood the first appearance of calcrete palaeosol and so is likely to be diachronous; in many places Formation it remains ill defined. The oldest strata of the Inverclyde Group belong to the Kinnesswood Formation of Late Devonian (Famennian) to early Tournaisian (Courceyan) age. The best biostratigraphical resolution is provided by a Courceyan miospore microflora from the Tweed Basin near Eyemouth, but fish macrofaunas from the Eyemouth and Langholm areas could be either Famennian or Courceyan in age.

The Kinnesswood Formation comprises red and yellow-brown sandstones and siltstones with locally developed conglomerate and thin beds of palaeosol containing concretionary carbonate nodules (calcrete or ‘cornstone’); thicker beds of calcrete appear near the top of the formation. The clastic rocks were deposited by ephemeral streams that drained the Southern Uplands landmass under semi-arid, seasonally wet, weathering conditions. In the Tweed Basin, the Cheviot volcanic massif was the source of the conglomerates, whilst farther west their provenance lay in the Caledonian igneous and Lower Palaeozoic sedimentary rocks of Galloway.

Birrenswark and Kelso volcanic formations

The initially rapid, extensional fault-controlled subsidence of the Northumberland–Solway Basin was accompanied by extrusion of basaltic lavas along its northern margin. In the west, around Langholm, the Birrenswark Volcanic Formation comprises thin flows of microphyric, feldspar-rich basalt totalling 100 m in thickness at Burnswark but reducing to some 15 m at Kirkbean. An olivine basalt flow at Watch Hill (NY 435 908), north-east Langholm, has given a K-Ar whole-rock age of 361 ± 12 Ma. Several small volcanic vents containing agglomerate and/or intrusive material occur in the vicinity of Langholm, and whilst no direct genetic link has been established, it is likely that they acted as feeders for the lavas of the Birrenswark Volcanic Formation, or possibly for the slightly younger Glencartholm Volcanic Member of the Tyne Limestone Formation (see below). Sedimentary debris in the agglomerate-filled vents includes Lower Palaeozoic lithologies and dolostone and chert of probable Late Devonian age; the principal igneous component is olivine-basalt.

Farther east, in Roxburghshire, the Kelso Volcanic Formation comprises at least six flows, and possibly as many as twelve, of alkaline olivine basalt together with subordinate tuff beds. The formation is about 120 m thick. A scattering of small basaltic intrusions is spatially associated with the lavas and one, from Mellerstain Hill (NT 641 397) has given a K-Ar whole-rock age of 361 ± 7 Ma. Like the date from the Birrenswark lavas, this suggests volcanism at around the Devonian–Carboniferous boundary. The outcrop of the Kelso Volcanic Formation extends southward across the English border near Carham (NT 799 384), whilst a very similar volcanic assemblage, the Cottonshope Volcanic Formation, crops out just to the south of the border on the south-west flank of the Cheviot Hills.

Ballagan Formation

The lavas of the Kelso and Birrenswark volcanic formations interfinger with, and are overlain conformably by the Ballagan Formation; where it does not overlie the lavas the Ballagan Formation conformably follows the underlying Kinnesswood Formation. The Ballagan Formation is of late Tournaisian to early Visean (Courceyan–Chadian) age and crops out in the Tweed Basin, the Langholm area, and on the Solway Coast. In the Tweed Basin, underlying the Merse of Berwickshire, the Ballagan Formation comprises about 430 m of mudstone with thin interbeds of argillaceous dolostone (‘cementstone’) and sandstone (P667218), with sporadic lenticular bodies of cross-bedded, channel-fill sandstone up to 30 m thick. Locally, the sequence succeeding the Kelso Volcanic Formation also contains concretionary calcrete, the most notable example being the Carham Limestone Bed (‘Carham Stone’), a cherty magnesian limestone up to 7.5 m thick, thought to have accumulated by chemical precipitation from waters enriched in lime by showers of volcanic dust.

In several parts of the Tweed Basin the Ballagan Formation contains miospores and a fossil fauna that includes environmentally tolerant bivalves and ostracods, and at Foulden Burn in Berwickshire (NT 921 553) has yielded twelve or more species of fresh- to brackish-water fish from the mudstone of the Foulden Fish Bed. Farther west, at the margin of the Solway Basin around Langholm, the Ballagan Formation contains plant fossils and a sparse faunal assemblage that includes gastropods, ostracods, modiolid bivalves and fish remains. The fossils occur in discrete associations of alluvial, fluvial and fluviodeltaic sedimentary rocks, intercalated with lacustrine, coastal plain and sporadic marine deposits (P912351).

In its outcrop from the Langholm area, westward along the Solway coast, the Ballagan Formation is in the order of 100 to 150 m thick (thinning westward) and is made up of thinly interbedded sandstone, mudstone and lagoonal dolostone (‘cementstone’) with some anhydrite (P579357). It is a terrestrial/fluvial to peritidal assemblage known in its Solway coastal outlier as the Kirkbean Cementstone Member. It was deposited in shallow channels within mudflats in a low-lying, coastal environment but with only a limited and intermittent marine influence (P912351). The Kirkbean Cementstone Member contains plant fragments and a quasi- to nonmarine fauna of bivalves, gastropods and ostracods, although fully marine bivalves are present locally. Elsewhere, in Annandale around Dalton (NY 104 740), boreholes prove the Kirkbean Cementstone Member to be at least 100 m thick and to contain abundant fibrous gypsum (satin spar) bands up to 4 cm thick. The ‘cementstones’ are underlain by dark red, fine-grained to conglomeratic sandstone, the Annandale Sandstone Beds. The anhydrite (or its precursor, gypsum) formed either subaqueously in shallow water or subaerially in coastal sabkhas, but following burial to depths greater than a few hundred metres, CaSO4 is usually preserved as anhydrite. Both gypsum and anhydrite are readily removed in the surface weathering zone and so are rarely seen at outcrop.

Another ‘cementstone’ sequence is seen in the Langholm–Newcastleton area, where it is up to 200 m thick and passes downwards into fine-grained, massive, fluviatile and deltaic sandstones, derived from the north, which are collectively referred to the Whita Sandstone Beds. These ‘cementstones’ were formerly regarded as part of a ‘Lower Border Group’ (P912354) and demonstrate the uncertainties that bedevil correlation between the various cementstone–sandstone sequences. The ‘cementstones’ above the Whita Sandstone could equally well be regarded as part of the Ballagan Formation or the Lyne Formation (Border Group — see below), whilst the Whita Sandstone is assigned to the Ballagan Formation.

The most westerly outcrop of the Ballagan Formation forms part of the Rerrick Outlier on the north coast of the Solway Firth. There, the Wall Hill Sandstone Member rests with angular unconformity on Wenlock strata of the Raeberry Castle Formation. It has a maximum thickness of about 360 m and comprises upward-fining sequences of conglomeratic sandstones, siltstones and mudstones. The fine-grained rocks were laid down on a floodplain by a braided fluvial system of low to moderate sinuosity, with the coarse-grained rocks deposited in channels. Palaeocurrent data indicate a predominately north and north-westerly source from the Southern Uplands terrane, and the conglomerates include clasts derived from the roof of the Criffel–Dalbeattie pluton. Local but intense sediment fluidisation developed at a number of localities, perhaps influenced by synsedimentary activity on the nearby North Solway Fault.


Border Group

The Border Group has a more-or-less continuous outcrop along the northern margin of the Northumberland–Solway Basin but in the east this lies entirely on the English side of the border (P912313). The outcrop spans the border to the north of Keilder and thence extends westward to Newcastleton, Ecclefechan and the Southerness area of the Kirkbean outlier.

Lyne Formation

In general, the Ballagan Formation becomes more marine in character, both upward and laterally towards the south-east, as it passes conformably and diachronously up into the Lyne Formation of the Border Group. The Lyne Formation is late Tournaisian–Arundian in age and in the main part of its outcrop, which lies south of the English border, it comprises cyclical sequences of fine-grained subarkosic sandstone, siltstone, mudstone, thin beds of limestone or dolostone, and anhydrite: the Lynebank, Bewcastle, Main Algal, Cambeck, and Easton Anhydrite members are recognised (P912354). At outcrop, the Lyne Formation is up to about 1000 m thick but the Easton Anhydrite Member has been proved in boreholes to comprise anhydrite abundantly interbedded through about 1300 m of clastic strata at about the level of the Lynebank and Bewcastle members. Deposition took place in a fluctuating network of peritidal, deltaic and fluvial environments subject to occasional marine incursions, with the anhydrite probably accumulating under sabkha conditions.

Much of the Scottish succession is assigned to the Southerness Limestone Member, about 135 m of thinly interbedded calcareous mudstone, siltstone and limestone. The member is best seen in the Kirkbean Outlier where it is richly fossiliferous, with a varied marine fauna of Chadian to Arundian age, and contains two distinctive algal stromatolite bands each about 1 m thick (P220451). The stromatolites resemble dome types from algal beds of the Lyne Formation in the Northumberland Basin. Individual domes are up to 30 cm in diameter with a relief of 10–15 cm, and are set in a calcareous mudstone matrix. They probably formed in a low intertidal to shallow subtidal depositional environment since there are signs of limited reworking. The algal bands are developed in two of three sedimentary cycles identified in the upper part of the member as follows:

1. Flaggy, micaceous sandstone with ripple marks and plant remains
5. Interbedded limestone and mudstones
4. Nodular algal band (in two of the three cycles)
3. Argillaceous limestone interbedded with calcareous mudstone
2. Fine grained, rippled sandstone
1. Flaggy, micaceous sandstone with ripple marks and plant remains

Each cycle commences with sandstones deposited in a littoral environment. These are succeeded by calcareous beds, formed under shallow subtidal conditions, but with some sandy limestones containing oolites as an indication that the sediments were at times affected by wave action. The cyclicity may be attributed to variation in subsidence rate and terriginous sediment input, both perhaps related to movement on the basin boundary faults.

Around Newcastleton and Ecclefechan, the member contains an Arundian fauna of brachiopods, bivalves, foraminifera and ostracods that suggests a stratigraphical level equivalent to the Cambeck Member. Also present, both there and in the Kirkbean Outlier, are limestone beds (respectively the ‘Harden Beds’ and Syringothyris Limestone Bed) of possible equivalence to the Whitberry Marine Band that farther south marks the base of the Fell Sandstone Formation. However, stratigraphically above the Syringothyris Limestone Bed at Southerness are the two distinctive algal stromatolite bands of ‘Lyne Formation type’. If these stratigraphical correlations are correct, then the Southerness Limestone Member ranges diachronously higher than the top of the Lyne Formation elsewhere in the Solway and Northumberland basins.

In the Rerrick Outlier, the Orroland Member of the Lyne Formation comprises about 280 m of red beds formed under subaerial conditions on the margin of the Solway Basin against the syntectonically active North Solway Fault (P912351). The fining-upward cycles of conglomerate, sandstone, siltstone and calcrete–palaeosol are sporadically interrupted by marine beds of greyish red, fossiliferous limestone, which is locally oolitic. A mostly detrital fauna of corals, brachiopods and bivalves suggests correlation with the Syringothyris Limestone in the Southerness succession.

Fell Sandstone Formation

The Fell Sandstone Formation has an arcuate outcrop around the flanks of the Cheviot massif from Burnmouth in south-east Scotland through Northumberland and northern Cumbria, crossing back into Scotland in the Newcastleton area. Thence the outcrop continues westward along the northern shore of the Solway Firth, where the Gillfoot and Powillimount sandstone members are identified in the Kirkbean Outlier, and the Rascarrel Member in the Rerrick Outlier. The formation is mostly of Chadian–Holkerian age, though the highest strata may range up into the early Asbian. The lower boundary in Northumberland is taken at the Whitberry Marine Band, but this fails westward as the lithostratigraphical boundary becomes diachronously younger. It is Chadian in the Cheviot district where the Fell Sandstone directly overlies the Ballagan Formation, and Chadian to Holkerian in the north Solway Basin where it overlies the Lyne Formation.

Subarkosic sandstone is the dominant lithology and in its thickest development of about 350 m in central Northumberland, the formation consists almost entirely of sandstone. Northwards, the proportion of sandstone to mudstone varies considerably and in the vicinity of Berwick-upon-Tweed the Fell Sandstone Formation may contain up to 40 per cent of finer-grained lithologies. It was laid down by a fluvial depositional system that advanced from an uplifted source area to the north and east within the Grampian and Fenno–Scandinavian structural blocks. A continuous steady uplift of the source area provided a constant supply of submature clastic sediment, with complementary subsidence of the basin maintaining the fluviodeltaic depositional environment. Deposition was probably effected by several braided river systems that were constrained to the axial region of the Northumberland Basin by intrabasinal faulting. Temperate humid conditions prevailed during deposition. The succession is largely unfossiliferous, although ostracods and the large bivalve Archanodon jukesi (Bailey) have been recorded together with some plant fossils. Along the axis of the Northumberland Basin the fluvial sandstone passes westward and diachronously into a succession of fluviodeltaic and shallow marine deposits.

The earliest fluvial deposits accumulated in the north-east. There, to the south of Burnmouth (NT 956 610) on the Berwickshire coast, some 200 m of pale-coloured, cross- and con-volute-bedded sandstone contain bands of quartz pebbles and lenses of red sandy mudstone. The sandstone is overlain by some 75 m of interbedded mudstone, sandstone and seatearth with a few thin coals (worked at Lamberton, just north of the border, in the 19th century) and argillaceous dolostone. Near the top of this sequence lies the 1.5 m thick ‘Lamberton Limestone’ which is a likely correlative of the Cove Marine Bands, part of the Aberlady Formation (Strathclyde Group), farther north in the Oldhamstocks Basin (see below).

To the south-west, in the vicinity of Langholm, a coarsening-upwards siliciclastic sequence culminates in a fluvial sandstone unit up to 250 m thick, known locally as the ‘Larriston Sandstone’, overlain by a more mixed, thinly bedded sandstone–mudstone succession. The ‘Larriston Sandstone’ was derived from the north-east as channel-fill deposits within a braided river system, which by the mid Arundian was of increasing regional importance and fed into a delta system farther to the south-west. There, around Ecclefechan, the sequence comprises sandstone (some conglomeratic, with quartz pebbles), mudstone and sporadic seatearth with roots, the latter suggesting intermittent emergence of the delta surface. In the Newcastleton outcrop there is evidence for a syndepositional, extensional tectonic regime from the presence, in mid sequence, of about 30 m of porphyritic olivine basalt lava: the Kershopefoot Basalt (P912354).

In the Kirkbean Outlier, on the north Solway coast, the 120–150 m-thick Gillfoot Sandstone Member comprises quartzose sandstone, conglomerate and red mudstone, with a few thin beds of sandy limestone containing detrital fossils. Conglomerates form about 20 per cent of the succession and have a calcareous matrix. They contain intraformational fragments in addition to pebbles of Silurian wacke-type sandstone and Devonian dyke rocks. The member is sparsely fossiliferous and an Arundian age is likely. The lithologies indicate a marginal coastal depositional setting with the conglomerates deposited by sheetfloods flowing over low-lying supratidal areas.

Succeeding the Gillfoot Sandstone are about 160 m of strata, of Arundian to Holkerian age, that make up the Powillimount Sandstone Member. Lithologies include calcareous and Fault plane cuts Early Devonian microdiorite intruded into Silurian strata of the Ross Formation quartzose sandstones, sandy limestone and mudstone, with sporadic thin coals and associated rooted seatearth. Sandstone beds range in thickness from 0.3 to 3 m and are laterally extensive. They are well sorted and commonly exhibit ripple cross lamination, contain abundant carbonaceous plant remains and are extensively bioturbated. Limestones range from arenaceous to argillaceous types and contain detrital fossil remains, oolites and rolled algal nodules (or oncolites). The characteristic lithologies, and the evidence of oncolites and detrital fragments, point to deposition in a shallow marine environment subject to wave action. Conversely, the presence of thin developments of coal and associated seatearths indicates periodic shallowing of lagoonal waters and the development of highly vegetated coastal plains and low lying supratidal flats.

It is likely that the sediments of the Powillimount Sandstone Member were deposited in a tidal lagoon protected from the effects of severe storms by some form of offshore sand barrier. That barrier may now be represented by the prominent and thickly bedded Thirlstane Sandstone Beds, 25 m of quartzose sandstone, which form the top of the member. At the base of the Thirlstane Sandstone, the contact with the underlying strata is irregular and intraformational fragments and wood remains are present in the lowest beds. The rocks are characterised by large-scale cross-bedding (P220455) and an increasing number of penecontemporaneous liquefaction deformation structures from south to north along the strike of the outcrop (P001161). The magnitude and frequency of the liquefaction structures may perhaps be attributed to seismic activity on the nearby and syndepositionally active North Solway Fault.

Farther west, in the isolated faulted blocks of the Rerrick and Colvend outliers, the Fell Sandstone Formation is represented by its Rascarrel Member. At Colvend, east of Castlehill Point, the spectacular basal Carboniferous unconformity is represented by a veneer of carbonate cemented breccia, overlain by conglomerate, clinging to cliffs cut in Silurian strata and a microdiorite dyke (P220436). The breccia clasts comprise Lower Palaeozoic sandstone and igneous rock derived from the hinterland to the north. The breccia and conglomerate dip seawards at high angles as a result of syndepositional faulting on the line of unconformity, and the conglomeratic sandstone grades upwards into mudstone; thin inter-beds of sandy limestone contain algal fragments and a sparse marine fauna of Arundian to Holkerian aspect. At both Rerrick and Colvend, at least 350 m of much-faulted strata are present, comprising arkosic and conglomeratic fluvial sandstone, siltstone, calcareous mud-stone and a few coals, making this potentially the thickest development of the Fell Sandstone Formation, a situation strongly influenced by the proximity of the syndepositionally active North Solway Fault.


Yoredale Group

The Yoredale Group does not crop out in the Scottish sector of the Tweed Basin sensu stricto, but does appear on the North Sea coast to the north of Berwick, separated from the Tweed Basin succession by the Eyemouth outcrop of Silurian strata. The following account therefore refers mostly to the northern margin of the Northumberland–Solway Basin.

Tyne Limestone Formation

The lowest division of the Yoredale Group is the Tyne Limestone Formation, a sequence of highly variable, marine to deltaic, ‘Yoredale’-type upward-coarsening cycles in which a thin but extensive bed of marine limestone is overlain by mudstone and sandstone commonly topped by a seatearth and a thin coal seam (P912353); there is much lithostratigraphical variation both vertically and laterally. The formation is largely Asbian in age and locally exceeds 2000 m in thickness. It has a wide outcrop across Northumberland and northern Cumbria and extends across the border into Scotland from south-west Liddesdale to Ecclefechan and the north Solway coast.

The base of the Tyne Limestone Formation is locally marked, between Langholm and Canonbie, by the volcanic rocks of the Glencartholm Volcanic Member, about 150 m of tuffs and basaltic lavas. Sedimentary rocks associated with the tuffs contain a marine fauna of brachiopods and bivalves, but are notable for rich arthropod and fish faunas, the former including crustaceans, eurypterids and scorpions. The fossils indicate a level near the base of the Asbian Stage. Above the Glencartholm Volcanic Member, the Tyne Limestone Formation comprises ‘Yoredale’ cyclical sequences consisting predominantly of marine mudstone and limestone with subordinate sandstone and a few seatearths and thin coal seams. Many of the beds are fossiliferous with a fauna noted for extensive coral colonies and layers of large brachiopods. More arenaceous strata follow, succeeded in turn by 30 m of marine mudstone, the Dinwoodie Beds, which are notable for their rich and varied fauna of corals, brachiopods and bryozoans, perhaps the most diverse fauna of the entire Carboniferous sequence.

In the lower part of the Tyne Limestone Formation there is a lateral eastward transition into a sequence of lacustrine–deltaic cycles of limestone, mudstone, sandstone and thick coal seams (up to 2 m) traditionally known in Northumberland and Berwickshire as the ‘Scremerston Coal Group’. In Berwickshire, this division is about 300 m thick (though much thicker in Northumberland) and is succeeded by a return of the marine–deltaic lithofacies. In this, typically, the sedimentary cycles comprise a limestone overlain by mudstone, which commonly shows evidence for storm-driven reworking of the sediment, followed by shallow-marine sandstone. Some of the cycles are topped by a terrestrial development of calcrete, seatearth and coal, perhaps indicating a semi-arid but seasonally wet environment.

The Tyne Limestone Formation strata that crop out along the north Solway coast in the Kirkbean Outlier are assigned to the Arbigland Limestone Member. Strata include thickly bedded, bioturbated, calcareous sandstones with coalified plant casts, thin sandy limestones, locally with ooliths and algal debris, dark grey carbonaceous mudstones and thin coal partings. The limestones and mudstones have an abundant and diverse fauna that includes brachiopods, bivalves, gastropods, crinoids, bryozoa, orthocones and corals, the latter including colonies of Lithostrotion. An Asbian age is likely and the sedimentary features are consistent with deposition in a restricted, lagoonal environment in which there was only limited reworking of sediment. A more active depositional regime is shown by the stratigraphically highest part of the member, where thickly bedded, bioturbated, ripple cross-laminated sandstone is interbedded with calcareous mudstone and some argillaceous limestone. These strata have been much affected by slump folding and are steeply inclined and locally overturned. Shallow, sandstone filled scours and washouts are common.

Alston Formation

The Alston Formation is the lowest division of the Yoredale Group present on the south side of the Northumberland–Solway Basin, where it succeeds the platform limestone of the Great Scar Limestone Group. The base is less well defined on the north side of the basin, where Brigantian ‘Yoredale’ cycles conformably succeed those of Asbian age that are assigned to the Tyne Limestone Formation. The top of the Alston Formation is defined by the Great Limestone Member and its correlatives, which are of Pendleian (earliest Namurian) age. The Alston Formation has a relatively restricted outcrop on the northern side of the Northumberland and Solway basins, but is present between Langholm and Canonbie and near Ecclefechan; from these localities it dips beneath the Canonbie Coalfield (Westphalian — see below) where its concealed strata have been proved in boreholes.

Between Langholm and Canonbie the limestone–mudstone–sandstone, ‘Yoredale’ cyclicity is well developed. Nine or ten cycles are developed over an interval of about 120 m, with the highest of the fossiliferous limestones, the Catsbit Limestone, being a correlative of the Pendleian Great Limestone Member; it therefore marks the top of the Alston Formation. A lower limestone, the Penton Limestone, is exposed in the hinge zone of the anticline illustrated in P221694. All of the limestones contain a relatively rich fauna with corals and brachiopods (Plate 34), and the overlying mudstones yield many brachiopods and molluscs. Assemblages of foraminifera occur in places.

Stainmore Formation

Across parts of northern England the Stainmore Formation comprises a largely deltaic, cyclical succession of sandstone, mudstone and poor-quality coal: a few limestone beds occur, mostly in the lower part of the formation. Some of the sandstones occur as large, channel-fill deposits, fining upwards and with erosional bases cross-cutting the underlying strata. Although the general term ‘Millstone Grit’ has been used for this Namurian sucession, it has long been recognised that the lithological assemblage in the Northumberland–Solway Basin is distinct from the thick development of coarse ‘gritstone’ and marine mudstone of the Millstone Grit Group’s type area in the south Pennines. Rather, across northern England and into the Canonbie area of southern Scotland, ‘Yoredale’ cyclicity dominates, albeit limestone beds are relatively scarce. This northern Namurian sequence is now defined as the Stainmore Formation. Its base overlies the Pendleian Great Limestone Member (and correlatives); the top of the formation is defined at the base of the Subcrenatum Marine Band, which marks the base of the Westphalian Pennine Coal Measures Group.

Strata of the Stainmore Formation in the Canonbie area are about 400 m thick but, in a further lateral variation in lithofacies, limestone and calcareous mudstone generally take the place of the dark goniatite-bearing mudstone that forms the basis for the traditional biostratigraphical zonation farther south. At Canonbie, goniatites are rare and the macrofossils, though abundant at many horizons, are not particularly diagnostic of stratigraphical position. Rather, the fauna is an impoverished continuation of that seen in the Alston Formation. Towards the top of the formation at Canonbie, the ‘Yoredale’ cycles pass up into a sequence of sandstones, seatearths and thin coals, collectively called the Penton Coals, which have been worked locally. This division also contains several interbedded units of mudstone with Lingula, whilst a few thin beds of limestone and calcareous mudstone have marine faunas.


Pennine Coal Measures Group

Westphalian Coal Measures strata crop out in the Canonbie Coalfield, a limited area of such strata that lies mainly to the north of the border in south-west Scotland, but has a concealed extention southwards into Cumbria, linking with the Cumbrian Coalfield (P912355), as described in the companion volume for Northern England. The Canonbie Coalfield is situated at the north-

west margin of the north-east to south-west-trending Solway Syncline, beneath which some coal seams may persist to reappear in Cumbria. Although their continuation has not been proved by deep boreholes, it is supported by seismic surveys. Application of both of these techniques in the 1980s indicated that the concealed coalfield is larger and has more economic potential than was previously envisaged. The exposed Canonbie coalfield is relatively small and was exploited on a limited scale in the 19th and early 20th centuries; deep mining ceased in the 1920s

Deposition in much of the Solway Basin continued unbroken from the Namurian into the Westphalian, but along the northern margin of the basin there is an unconformity at the base of the Pennine Coal Measures Group, such that strata high in the Pennine Lower Coal Measures Formation rest unconformably on Yoredale Group rocks. There is also evidence for at least one unconformity in the overlying, late Westphalian ‘red beds’ that in the Canonbie Coalfield form part of the Warwickshire Group (see below).

The Pennine Coal Measures Group is predominantly nonmarine and is characterised by coarsening-upward cycles, commonly up to 15 m thick, composed of mudstone, siltstone and sandstone capped by a seatearth and coal, though coal forms only a minor part of the total sequence (P912353); clay ironstone occurs within some mudstones. The Solway Basin and its Westphalian strata were subject to some uplift and deformation during the late Carboniferous to early Permian Variscan tectonic episode, with development of open folds (P221694) and much faulting, whilst regionally the strata of the Solway Basin were compressed into the major Solway Syncline (P912349). Uplifted areas were soon eroded and Westphalian beds are now preserved only in the downwarped, coalfield areas. Subsequent sedimentation produced an unconformable cover of Permo-Triassic rocks over the Westphalian beds, which are commonly reddened to a depth of more than 100 m beneath the sub-Permian unconformity. Whilst erosion has since removed parts of the cover, much of the Canonbie Coalfield remains concealed beneath Permo-Triassic strata (P912355).

In the Scottish part of the Canonbie Coalfield, the Pennine Lower Coal Measures Formation is about 110 m thick, the Middle Coal Measures about 250 m and Upper Coal Measures about 150 m (P912355 and P912356): the sequence thickens into the hinge zone of the Solway Syncline. Coals in the Pennine Lower Coal Measures Formation are commonly unnamed and difficult to correlate, with borehole information showing them to be generally less than 0.8 m in thickness. The main seams are from the Duckmantian part of the Pennine Middle Coal Measures Formation, with only a few thin coals present in its uppermost, Bolsovian section. The few coals in the Pennine Upper Coal Measures are mostly located close to the base of the formation but an exception is the aptly named High Coal, which occurs about 150 m above the base of the Upper Coal Measures. It forms a convenient marker for the base of the overlying Warwickshire Group.

Pennine Lower Coal Measures

At Canonbie, the Pennine Lower Coal Measures (Langsettian) rest unconformably on Namurian strata of the Yoredale Group. In common with the situation seen in other coalfields of Formation the region, the Subcrenatum Marine Band, which defines the base of the Langsettian, is not present. Instead, correlation rests on the likely equivalence of the stratigraphically higher Langley Marine Band of west Cumbria and Templeman’s Marine Band, identified at Canonbie near the bottom of the Becklees Borehole (NY 3517 7158). Mudstone–siltstone–seatearth cyclothems are most common, but substantial sandstone bodies occur as likely channel fills, and several thin coals are present. A distinctive mudstone ‘mussel band’, with nonmarine bivalves and ostracods lies near the top of the formation some 4–6 m below the Vanderbeckei (Queenslie) Marine Band.

Pennine Middle Coal Measures Formation

The Pennine Middle Coal Measures (Duckmantian to lower Bolsovian) at Canonbie include strata between the Vanderbeckei (Queenslie) Marine Band and the Cambriense Marine Band, both of which can be identified. The Vanderbeckei Marine Band contains mega-spores, the inarticulate brachiopod Lingula, and fish remains; slightly higher is a ‘mussel band’ in about 7.5 m of mudstone with ironstone bands that also contains nonmarine bivalves and fish material. Mudstone–sandstone–seatearth cyclothems tend to be well developed on a scale of 3 to 30 m, with a few nonmarine mussel bands in the mudstone. Several coals are also well developed so that the lower half of the Pennine Middle Coal Measures has been the most economically important part of the Canonbie succession. A ‘mussel band’ just below the Archerbeck Coal has a particularly varied fauna of at least 20 species of non-marine bivalves. In the upper part of the Pennine Middle Coal Measures, above the Archer-beck Coal, marine bands become more common and there is a commensurate reduction in coal development.

The Aegiranum (Skelton) Marine Band, marking the base of the Bolsovian Substage is developed in mudstone with foraminifera, inarticulate brachiopods, fish remains and conodont elements. Above this, two further marine bands appear in the succession before the Cambriense (Riddings) Marine Band marks the onset of the Pennine Upper Coal Measures. It appears as a bioturbated mudstone with ironstone nodules and contains foraminifera, inarticulate brachiopods, fish fragments, and conodont elements.

Pennine Middle Upper Coal Measures Formation

The Pennine Upper Coal Measures Formation (upper Bolsovian) at Canonbie largely comprises mudstone–siltstone cyclothems with few substantial sandstones and only sporadic coals. Those that are present are relatively thin and located close to the base of the formation. The exception is the High Coal, which occurs about 150 m above the base and marks the onset of the overlying Warwickshire Group.


Warwickshire Group

In the Canonbie Coalfield, the Pennine Coal Measures Group is overlain by a red-bed succession that, together with other, similar upper Carboniferous sequences elsewhere in northern and central England, make up the Warwickshire Group (P912387). A key characteristic is reddening that occurred soon after sediment deposition, but which is not uniformly developed throughout; as a result, red beds alternate with unreddened, grey-green intervals. This phenomenon introduces local uncertainty as to the exact age of the Warwickshire Group strata and their stratigraphical relationship to apparently coeval, but unreddened rocks in the Pennine Coal Measures Group.

The Canonbie outcrop of the Warwickshire Group consists of interbedded mudstones, siltstones and sandstone similar to those of the underlying Pennine Coal Measures but mostly reddened; coal is rare. The group is late Bolsovian to Asturian in age. Its base at Canonbie is conformable, grading up from the grey mudstones of the Pennine Upper Coal Measures Formation into the overlying red beds. The sharp, unconformable base of the Permian succession defines the top of the group. At outcrop, about 290 m of Warwickshire Group red beds are exposed along the banks of the River Esk, but the maximum proved thickness is the 530 m seen in the Becklees Borehole, close to the central axis of the Solway Syncline. Seismic reflection data indicate that elsewhere in the centre of the syncline the group could be up to about 700 m thick.

Three formations have been recognised within the group, each with distinctive geophysical log signatures that allow them to be readily correlated in the subsurface. The lowermost, Eskbank Wood Formation (late Bolsovian to Asturian), ranges in thickness from 145 m to 175 m. Its base is gradational (and probably diachronous) across alternations of grey and primary red-bed strata but can conveniently be taken at the first appearance of pedogenic carbonate nodules just below the High Coal, which forms a prominent marker horizon (P912356). The formation comprises red mudstone and some sandstone, calcrete palaeosols, thin beds of Spirorbis limestone and Estheria-bearing mudstone. In the lower part there are a few thin coals and beds of grey mudstone, some of which contain nonmarine bivalves. The overlying Canonbie Bridge Sandstone Formation (Asturian) ranges in thickness from 131 m to 154 m. The base of the formation is sharp, marked by the incoming of thick units of medium- and coarse-grained, cross-bedded channel sandstone. A noticeable feature of these sandstones is their greenish grey colour, which can be related to the presence of abundant lithic grains. There are sporadic interbeds of mudstone and calcrete. The Becklees Sandstone Formation (Asturian) is the highest unit recognised from the Warwickshire Group of Canonbie and is overlain unconformably by Permian strata. Its full thickness is not known, but about 200 m is proved in the Becklees Borehole. Fine-grained sandstone with a distinct orange-brown colour is the dominant lithology, with some thin beds of mudstone and calcrete. Borehole cuttings suggest limestones and thin coals may also be present, though rare.

Warwickshire Group sedimentation in the Canonbie area largely took place on an alluvial plain drained by braided river systems, and was characterised by an early, primary oxidation of the strata. Palaeocurrent data from channels in the Canonbie Bridge Sandstone show that the rivers flowed towards the north. Overbank and floodplain mud was deposited between the channels, where soils were able to form during intervals of low aggradation. The ‘Spirorbis’ limestones of the Eskbank Wood Formation are most probably of lacustrine origin.


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