Editing Geological history of Yorkshire

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Carboniferous sedimentation was dominated by cycles of transgression and '''regression''' on several scales. The six stages of the Dinantian (early Mississippian) are based on '''mesothem''' cycles with calcareous shales and richly fossiliferous, sometimes bituminous limestones characteristic of the transgressive phase, and '''oolitic''' limestones, '''algal''' limestones, '''dolomites''' and in places sandstones, pebble beds and '''disconformities''' marking the regressive phase in shallow waters. In the Craven Basin, where the Dinanthian sequence is some 3 km thick, early sediments are bioclastic limestones and calcareous shales but, episodic earth movements from mid-Dinantian into Namurian times resulted in northeast–southwest folding and increased subsidence in which '''goniatite'''-bearing, deep-water shales predominated. In the transition zone between the Craven Basin and the Askrigg Block, and around Clitheroe, marginal reef limestones were developed between basin and block in mid and late Dinantian times ([[Craven Fault Zone — Malham to Settle - an excursion|Excursion 2]]). In the Stainmore Trough, subsidence and sedimentation more nearly kept pace and neither significant deep water '''facies''', nor reef facies, were developed. The Askrigg Block, on which the Dinantian is less than 500 m thick, was not completely inundated by the sea until late Dinantian times ([[Craven Fault Zone — Malham to Settle - an excursion|Excursion 2]], [[Carboniferous rocks of upper Nidderdale - an excursion|Excursion 5]], [[Dinantian and Namurian rocks of Bolton Abbey and Trollers Gill - an excursion|Excursion 6]]). Minor sedimentary cycles become increasingly apparent within the topmost mesothem, with many repetitions of marine limestone succeeded by shale, sandstone and in places '''seatearth''' and coal. These are '''Yoredale''' cycles, a term derived from the old name for Wensleydale, where they are classically developed. The limestone component dominates at lower levels and towards the southern part of the Askrigg Block where open marine conditions prevailed. The elastic sediments increased in proportion in younger cycles and to the north, reflecting the increasing influence of southward '''prograding''' deltas. The Dinantian limestones are commonly rich in '''corals, brachiopods''' and '''foraminifera''', all of which help to date and correlate the sequences.
 
Carboniferous sedimentation was dominated by cycles of transgression and '''regression''' on several scales. The six stages of the Dinantian (early Mississippian) are based on '''mesothem''' cycles with calcareous shales and richly fossiliferous, sometimes bituminous limestones characteristic of the transgressive phase, and '''oolitic''' limestones, '''algal''' limestones, '''dolomites''' and in places sandstones, pebble beds and '''disconformities''' marking the regressive phase in shallow waters. In the Craven Basin, where the Dinanthian sequence is some 3 km thick, early sediments are bioclastic limestones and calcareous shales but, episodic earth movements from mid-Dinantian into Namurian times resulted in northeast–southwest folding and increased subsidence in which '''goniatite'''-bearing, deep-water shales predominated. In the transition zone between the Craven Basin and the Askrigg Block, and around Clitheroe, marginal reef limestones were developed between basin and block in mid and late Dinantian times ([[Craven Fault Zone — Malham to Settle - an excursion|Excursion 2]]). In the Stainmore Trough, subsidence and sedimentation more nearly kept pace and neither significant deep water '''facies''', nor reef facies, were developed. The Askrigg Block, on which the Dinantian is less than 500 m thick, was not completely inundated by the sea until late Dinantian times ([[Craven Fault Zone — Malham to Settle - an excursion|Excursion 2]], [[Carboniferous rocks of upper Nidderdale - an excursion|Excursion 5]], [[Dinantian and Namurian rocks of Bolton Abbey and Trollers Gill - an excursion|Excursion 6]]). Minor sedimentary cycles become increasingly apparent within the topmost mesothem, with many repetitions of marine limestone succeeded by shale, sandstone and in places '''seatearth''' and coal. These are '''Yoredale''' cycles, a term derived from the old name for Wensleydale, where they are classically developed. The limestone component dominates at lower levels and towards the southern part of the Askrigg Block where open marine conditions prevailed. The elastic sediments increased in proportion in younger cycles and to the north, reflecting the increasing influence of southward '''prograding''' deltas. The Dinantian limestones are commonly rich in '''corals, brachiopods''' and '''foraminifera''', all of which help to date and correlate the sequences.
  
The Yoredale facies extends up into the Namurian, but from south to north across the county is replaced at progressively younger horizons by the thick and often coarse-grained fluviatile and deltaic sandstones of the Millstone Grit ([[Carboniferous rocks of upper Nidderdale - an excursion|Excursion 5]], [[Dinantian and Namurian rocks of Bolton Abbey and Trollers Gill - an excursion|Excursion 6]], [[Millstone Grit of Almscliff Crag and Harlow Car, near Harrogate - an excursion|Excursion 7]], [[Carboniferous (Namurian and Westphalian) of the Cliviger Valley, Todmorden - an excursion|Excursion 8]], [[Upper Carboniferous of the Halifax area - an excursion|Excursion 9]], [[Middle and Upper Carboniferous rocks (Millstone Grit and Coal Measures) of the Sheffield region - an excursion|Excursion 10]]). By early Namurian times, the clearly defined basin and block topography had largely disappeared, although subsidence rates remained highest, and sedimentary sequences therefore thickest, in the basinal areas. In the Craven Basin, the succession reaches 2.5 km in thickness, in contrast to a mere 370 m on the Askrigg Block and 500 m at Stainmore. These sediments were deposited from river systems flowing predominantly southwestwards into the area. As the deltas advanced, turbidites locally formed on basinal slopes in front of them, over which the deltaic and fluviatile sediments prograded, often building up to sea level to form forested flood plains and swamps. Episodic transgressions of the sea across the delta top resulted in the deposition of thin marine bands with goniatites, vital for dating and correlation.
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The Yoredale facies extends up into the Namurian, but from south to north across the county is replaced at progressively younger horizons by the thick and often coarse-grained fluviatile and deltaic sandstones of the Millstone Grit ([[Carboniferous rocks of upper Nidderdale - an excursion|Excursion 5]], [[Dinantian and Namurian rocks of Bolton Abbey and Trollers Gill - an excursion|Excursion 6]], [[Millstone Grit of Almscliff Crag and Harlow Car, near Harrogate - an excursion|Excursion 7]], [[Carboniferous (Namurian and Westphalian) of the Cliviger Valley, Todmorden - an excursion|Excursion 8]], [[Upper Carboniferous of the Halifax area - an excursion|Excursion 9]], [[Middle and Upper Carboniferous rocks (Millstone Grit and Coal Measures) of the Sheffield region - an excursion|Excursion 10]]
 +
). By early Namurian times, the clearly defined basin and block topography had largely disappeared, although subsidence rates remained highest, and sedimentary sequences therefore thickest, in the basinal areas. In the Craven Basin, the succession reaches 2.5 km in thickness, in contrast to a mere 370 m on the Askrigg Block and 500 m at Stainmore. These sediments were deposited from river systems flowing predominantly southwestwards into the area. As the deltas advanced, turbidites locally formed on basinal slopes in front of them, over which the deltaic and fluviatile sediments prograded, often building up to sea level to form forested flood plains and swamps. Episodic transgressions of the sea across the delta top resulted in the deposition of thin marine bands with goniatites, vital for dating and correlation.
  
By Westphalian times, although the cyclicity was undiminished, the marine incursions were fewer and shorter in duration though still widespread. The sandstones were finer grained and thinner and periods of soil formation and swamp vegetation more frequent and prolonged, resulting in thicker coal seams. These Coal Measures ([[Carboniferous (Namurian and Westphalian) of the Cliviger Valley, Todmorden - an excursion|Excursion 8]], [[Upper Carboniferous of the Halifax area - an excursion|Excursion 9]], [[Middle and Upper Carboniferous rocks (Millstone Grit and Coal Measures) of the Sheffield region - an excursion|Excursion 10]]) are now exposed in the Leeds–Sheffield industrial belt of south–central Yorkshire where the sequence is 1500 m thick, and are present in the subsurface to the east. Yorkshire was now part of a broad subsiding area called the Pennine Basin, which in turn was part of a vast belt of tropical Coal Measures sedimentation extending from eastern U.S.A. to Poland. To the south of this was a rising landmass, developing as a result of the '''Variscan Orogeny'''. Towards the end of the Westphalian, sediments from this landmass caused the infill of the Pennine Basin with cyclic continental red-beds, followed by a period of rather gentle folding, extensive faulting and uplift, particularly in the former basinal areas. For 30 m.yrs., during the late Carboniferous and early Permian, the resulting upland landscape was deeply weathered and peneplaned in a largely hot, dry climate.
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By Westphalian times, although the cyclicity was undiminished, the marine incursions were fewer and shorter in duration though still widespread. The sandstones were finer grained and thinner and periods of soil formation and swamp vegetation more frequent and prolonged, resulting in thicker coal seams. These Coal Measures ([[Carboniferous (Namurian and Westphalian) of the Cliviger Valley, Todmorden - an excursion|Excursion 8]], [[Upper Carboniferous of the Halifax area - an excursion|Excursion 9]], [[Middle and Upper Carboniferous rocks (Millstone Grit and Coal Measures) of the Sheffield region - an excursion|Excursion 10]]) are now exposed in the Leeds–Sheffield industrial belt of south–central Yorkshire where the sequence is t500 m thick, and are present in the subsurface to the east. Yorkshire was now part of a broad subsiding area called the Pennine Basin, which in turn was part of a vast belt of tropical Coal Measures sedimentation extending from eastern U.S.A. to Poland. To the south of this was a rising landmass, developing as a result of the Variscan Orogeny. Towards the end of the Westphalian, sediments from this landmass caused the infill of the Pennine Basin with cyclic continental red-beds, followed by a period of rather gentle folding, extensive faulting and uplift, particularly in the former basinal areas. For 30 m.yrs., during the late Carboniferous and early Permian, the resulting upland landscape was deeply weathered and peneplaned in a largely hot, dry climate.
  
 
The Carboniferous rocks, principally the Dinantian and Namurian of the Pennines, are host to economic deposits in the form of numerous '''mineral veins''' ([[North Swaledale Mineral Belt around Gunnerside - an excursion|Excursion 4]]). The principal metalliferous minerals are '''galena''' and '''sphalerite''', with some '''chalcopyrite, pyrite''' and '''bornite''', and scattered occurrences of several other minor components. These occur as localized masses or are dispersed in veins in which the main constituent is usually '''baryte''', occasionally '''witherite''' and locally '''fluorite'''. Mineralization probably occurred in the latest Carboniferous as a result of the circulation of low-temperature hypersaline brines, possibly expelled from the thick sedimentary sequences of the basinal areas (so called Mississippi Valley-type mineralization), through '''joint''' fracture systems imposed by Variscan earth movements. Vein fractures are widest and cleanest in sandstones and limestones, and locally the latter have been replaced by ores. The Romans certainly mined lead and pre-Roman exploitation has been suggested. The peak of mining activity was in the late 18th and first half of the 19th century, whilst most recently, limited mining activity has concentrated on the former '''gangue''' minerals baryte and fluorite.
 
The Carboniferous rocks, principally the Dinantian and Namurian of the Pennines, are host to economic deposits in the form of numerous '''mineral veins''' ([[North Swaledale Mineral Belt around Gunnerside - an excursion|Excursion 4]]). The principal metalliferous minerals are '''galena''' and '''sphalerite''', with some '''chalcopyrite, pyrite''' and '''bornite''', and scattered occurrences of several other minor components. These occur as localized masses or are dispersed in veins in which the main constituent is usually '''baryte''', occasionally '''witherite''' and locally '''fluorite'''. Mineralization probably occurred in the latest Carboniferous as a result of the circulation of low-temperature hypersaline brines, possibly expelled from the thick sedimentary sequences of the basinal areas (so called Mississippi Valley-type mineralization), through '''joint''' fracture systems imposed by Variscan earth movements. Vein fractures are widest and cleanest in sandstones and limestones, and locally the latter have been replaced by ores. The Romans certainly mined lead and pre-Roman exploitation has been suggested. The peak of mining activity was in the late 18th and first half of the 19th century, whilst most recently, limited mining activity has concentrated on the former '''gangue''' minerals baryte and fluorite.

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