Editing Geological history of Northumbria

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In Northumbria, the Yoredale cycles extend up into the Namurian as the Upper Limestone Group ([[Carboniferous rocks of the Howick shore section - an excursion|Excursion 7]], [[Carboniferous rocks of the Roman Wall and Haltwhistle Burn - an excursion|Excursion 11]], [[Carboniferous of the Wear Valley and Derwent Gorge, County Durham - an excursion|Excursion 15]], [[Geology and landscape of Upper Teesdale - an excursion|Excursion 16]], [[Carboniferous and Permian rocks in southern County Durham - an excursion|Excursion 17]]). Although the thickest of all the Yoredale limestones, the Great, marks the base of the Namurian, upwards the limestones thin and the sandstones increase in thickness. Towards the top of the Namurian, marine influences and the limestones die out and the succession is dominated by cycles of erosive, coarse-grained, fluvial sandstones with interbedded fine sandstones, siltstones and mudstones. This is a thin northern equivalent of the Millstone Grit of the Central Pennines (Excursion 7, Excursion 15). There, the grit facies is developed throughout the Namurian and exceeds 2000 m in thickness. In Northumbria, the Namurian thins from just over 500 m in the subsurface near Newcastle to about 270 m on the Northumberland coast, of which only the top 50 m or so is of Millstone Grit facies.
 
In Northumbria, the Yoredale cycles extend up into the Namurian as the Upper Limestone Group ([[Carboniferous rocks of the Howick shore section - an excursion|Excursion 7]], [[Carboniferous rocks of the Roman Wall and Haltwhistle Burn - an excursion|Excursion 11]], [[Carboniferous of the Wear Valley and Derwent Gorge, County Durham - an excursion|Excursion 15]], [[Geology and landscape of Upper Teesdale - an excursion|Excursion 16]], [[Carboniferous and Permian rocks in southern County Durham - an excursion|Excursion 17]]). Although the thickest of all the Yoredale limestones, the Great, marks the base of the Namurian, upwards the limestones thin and the sandstones increase in thickness. Towards the top of the Namurian, marine influences and the limestones die out and the succession is dominated by cycles of erosive, coarse-grained, fluvial sandstones with interbedded fine sandstones, siltstones and mudstones. This is a thin northern equivalent of the Millstone Grit of the Central Pennines (Excursion 7, Excursion 15). There, the grit facies is developed throughout the Namurian and exceeds 2000 m in thickness. In Northumbria, the Namurian thins from just over 500 m in the subsurface near Newcastle to about 270 m on the Northumberland coast, of which only the top 50 m or so is of Millstone Grit facies.
  
By the beginning of the Westphalian, Northumbria was part of a broad Central Pennine Basin, in which maximum subsidence was in the Manchester area ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]d). There, the Coal Measures are over 3000 m thick, compared to some 875 m in the Northumberland and Durham Coalfield ([[Carboniferous and Permian rocks between Tynemouth and Seaton Sluice - an excursion|Excursion 8]], [[Carboniferous of the Wear Valley and Derwent Gorge, County Durham - an excursion|Excursion 15]], [[Carboniferous and Permian rocks in southern County Durham - an excursion|Excursion 17]]). Cyclic sedimentation of shale/mudstone–sandstone–'''seatearth'''–coal, continued under the influence of deltaic processes, often as small fluvially dominated deltas prograding into fresh to brackish water flood plain lakes. Major distributaries are marked by ribbons of erosive, cross-bedded sandstones cutting down into the coal-bearing sequences. Marine bands are most common in the lower part of the succession, and where present can be correlated over large areas. There is a gradual shift to more fluvial dominated conditions higher in the Westphalian. The thickest and most productive coals occur in the lower half of the Middle Coal Measures. Coal mining in Northumbria was at its peak in the 19th and early 20th centuries. The last 30–40 years have seen a drastic reduction in underground working as first the inland mines became exhausted or uneconomic and most recently the larger coastal pits have been closed.
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By the beginning of the Westphalian, Northumbria was part of a broad Central Pennine Basin, in which maximum subsidence was in the Manchester area ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]d). There, the Coal Measures are over 3000 m thick, compared to some 875 m in the Northumberland and Durham Coalfield ([[Carboniferous and Permian rocks between Tynemouth and Seaton Sluice - an excursion|Excursion 8]], [[Carboniferous of the Wear Valley and Derwent Gorge, County Durham - an excursion|Excursion 15]], [[Carboniferous and Permian rocks in southern County Durham - an excursion|Excursion 17]]). Cyclic sedimentation of shale/mudstone–sandstone–seatearth–coal, continued under the influence of deltaic processes, often as small fluvially dominated deltas prograding into fresh to brackish water flood plain lakes. Major distributaries are marked by ribbons of erosive, cross-bedded sandstones cutting down into the coal-bearing sequences. Marine bands are most common in the lower part of the succession, and where present can be correlated over large areas. There is a gradual shift to more fluvial dominated conditions higher in the Westphalian. The thickest and most productive coals occur in the lower half of the Middle Coal Measures. Coal mining in Northumbria was at its peak in the 19th and early 20th centuries. The last 30–40 years have seen a drastic reduction in underground working as first the inland mines became exhausted or uneconomic and most recently the larger coastal pits have been closed.
  
The volcanic and intrusive activity that had continued throughout much of the Carboniferous further south in England was absent in the north. Northumbria, however, was distinguished by a single, Permo–Carboniferous, intrusive event of considerable volume. This is the Whin '''Sill''' complex, a '''tholeiitic dolerite''' fed by '''dykes''' emplaced along approximately east–west extensional fractures formed at this time ([[Geology and landscape of Holy Island and Bamburgh - an excursion|Excursion 6]], [[Geology of the North Tyne and Saughtree - an excursion|Excursion 10]], [[Carboniferous rocks of the Roman Wall and Haltwhistle Burn - an excursion|Excursion 11]], [[Geology and landscape of Upper Teesdale - an excursion|Excursion 16]]). The sill, in composite form, reaches 100 m thick and extends from Teesdale to the Scottish borders, abruptly changing its stratigraphical level via faults and '''joints''' from the mid Dinantian to the Lower Coal Measures. It is lowest in the sequence at its northern and southern extremities and highest around Alnwick (up to Namurian) and in the Midgeholme Coalfield ([[:File:YGS_NORTROCK_FIG_00_1.jpg|Figure 1]]). The term 'sill' originated locally to describe any persistent hard bed (e.g. the Firestone Sill — a Namurian sandstone) and only subsequently took on its modern restricted meaning as an igneous rock.
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The volcanic and intrusive activity that had continued throughout much of the Carboniferous further south in England was absent in the north. Northumbria, however, was distinguished by a single, Permo–Carboniferous, intrusive event of considerable volume. This is the Whin Sill complex, a tholeiitic dolerite fed by dykes emplaced along approximately east–west extensional fractures formed at this time ([[Geology and landscape of Holy Island and Bamburgh - an excursion|Excursion 6]], [[Geology of the North Tyne and Saughtree - an excursion|Excursion 10]], [[Carboniferous rocks of the Roman Wall and Haltwhistle Burn - an excursion|Excursion 11]], [[Geology and landscape of Upper Teesdale - an excursion|Excursion 16]]). The sill, in composite form, reaches 100 m thick and extends from Teesdale to the Scottish borders, abruptly changing its stratigraphical level via faults and joints from the mid Dinantian to the Lower Coal Measures. It is lowest in the sequence at its northern and southern extremities and highest around Alnwick (up to Namurian) and in the Midgeholme Coalfield ([[:File:YGS_NORTROCK_FIG_00_1.jpg|Figure 1]]). The term 'sill' originated locally to describe any persistent hard bed (e.g. the Firestone Sill — a Namurian sandstone) and only subsequently took on its modern restricted meaning as an igneous rock.
  
 
Carboniferous, particularly Lower Carboniferous sediments of the Alston Block, also host the many '''mineral veins''' and '''flats''' of the North Pennine Orefield ([[Northern Pennine Orefield: Weardale and Nenthead - an excursion|Excursion 14]]). '''Galena''' was the main mineral, extracted for lead, but '''sphalerite''' was also common together with, in the central area, minor amounts of '''pyrite''', '''marcasite''' and occasionally '''chalcopyrite''' and '''pyrrhotite'''. The '''gangue''' minerals are zoned, with '''fluorite''' predominating in the central part of the Alston Block, surrounded by '''baryte''', '''witherite''' or '''calcite'''. The zoning was temperature related, with fluids reaching 220°C in the fluorite area, dropping to as low as 60°C on the margins of the orefield. Ore-bearing fluids are thought to have originated as brines forced out of the thick surrounding sedimentary basins, stripping out metals as they migrated through the Carboniferous, Lower Palaeozoic and granite rocks, and channelled towards the block along its bounding faults. High heatflow from the Weardale Granite, a Caledonian intrusion in the Lower Palaeozoic basement of the Alston Block, set up a convection cell with the hottest brines rising through the granite and the Dinantian sediments of the block at its centre. The main phase of mineralization was most likely Permian, although potentially mineralizing fluids are still circulating in the area. The deposits have been worked probably since Roman times and extraction peaked in the 19th century. Today the gangue minerals, fluorite and baryte, are the main resource; lead and zinc ores are produced as by-products.
 
Carboniferous, particularly Lower Carboniferous sediments of the Alston Block, also host the many '''mineral veins''' and '''flats''' of the North Pennine Orefield ([[Northern Pennine Orefield: Weardale and Nenthead - an excursion|Excursion 14]]). '''Galena''' was the main mineral, extracted for lead, but '''sphalerite''' was also common together with, in the central area, minor amounts of '''pyrite''', '''marcasite''' and occasionally '''chalcopyrite''' and '''pyrrhotite'''. The '''gangue''' minerals are zoned, with '''fluorite''' predominating in the central part of the Alston Block, surrounded by '''baryte''', '''witherite''' or '''calcite'''. The zoning was temperature related, with fluids reaching 220°C in the fluorite area, dropping to as low as 60°C on the margins of the orefield. Ore-bearing fluids are thought to have originated as brines forced out of the thick surrounding sedimentary basins, stripping out metals as they migrated through the Carboniferous, Lower Palaeozoic and granite rocks, and channelled towards the block along its bounding faults. High heatflow from the Weardale Granite, a Caledonian intrusion in the Lower Palaeozoic basement of the Alston Block, set up a convection cell with the hottest brines rising through the granite and the Dinantian sediments of the block at its centre. The main phase of mineralization was most likely Permian, although potentially mineralizing fluids are still circulating in the area. The deposits have been worked probably since Roman times and extraction peaked in the 19th century. Today the gangue minerals, fluorite and baryte, are the main resource; lead and zinc ores are produced as by-products.

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