Editing Geological history of Yorkshire

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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 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.
 
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.
  
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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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 1 gth century, whilst most recently, limited mining activity has concentrated on the former gangue minerals baryte and fluorite.
  
 
== Permian ==
 
== Permian ==
  
Renewed subsidence in Permian times placed Yorkshire near the western margin of a broad basin extending across northern Europe into Poland, situated in the hot, dry belt about 12–13° north of the equator. Basal and marginal piedmont '''breccia''' wedges and sheets are associated with patchy continental dune sands of presumed late Cisuralian age ([[:File:YGS_YORKROCK_FIG_03_00.jpg|Figure 3]]c) ([[Permian rocks of south-central Yorkshire - an excursion|Excursion 14]]), resting with gentle unconformity on the Carboniferous ([[:File:YGS_YORKROCK_FIG_01_00.jpg|Figure 1]], [[:File:YGS_YORKROCK_FIG_02_00.jpg|Figure 2]]). At the beginning of the Guaclalupian, the basin, then well below sea level, was flooded, extensively reworking the sands and introducing a period of marine and hypersaline conditions in a fluctuating '''epicontinental''' sea. Four major cycles of limestones, later altered to dolomites, and succeeding '''evaporites''', including '''gypsum''' (now '''anhydrite'''), '''halite''' and potash salts, were developed. These cycles resulted from periodic recharge of the basin by normal marine waters, from which limestones, 'many oolitic and locally fossiliferous, were formed around the margins. Only the limestones and dolomites are prominent at outcrop in a narrow north–south belt bisecting the county, the evaporites being reduced to thin, silty dissolution residues ([[Permian and Carboniferous rocks of Knaresborough - an excursion|Excursion 13]], [[Permian rocks of south-central Yorkshire - an excursion|Excursion 14]]). In the first cycle, fossiliferous '''bryozoan''' and '''stromatolitic''' patch reefs formed, but in the third cycle the limestones contain only a few plant and invertebrate species. Limestones of the second and fourth cycles do not crop out in Yorkshire but are present in thicker sequences in the subsurface to the east, where potash is mined at Boulby near the coast west-northwest of Whitby. Towards the end of the Permian, continental, water-lain red sandstones, siltstones and mudstones gradually filled the basin from the west.
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Renewed subsidence in Permian times placed Yorkshire near the western margin of a broad basin extending across northern Europe into Poland, situated in the hot, dry belt about 12–13° north of the equator. Basal and marginal piedmont breccia wedges and sheets are associated with patchy continental dune sands of presumed late Cisuralian age ([[:File:YGS_YORKROCK_FIG_03_00.jpg|Figure 3]]c) ([[Permian rocks of south-central Yorkshire - an excursion|Excursion 14]]), resting with gentle unconformity on the Carboniferous ([[:File:YGS_YORKROCK_FIG_01_00.jpg|Figure 1]], [[:File:YGS_YORKROCK_FIG_02_00.jpg|Figure 2]]). At the beginning of the Guaclalupian, the basin, then well below sea level, was flooded, extensively reworking the sands and introducing a period of marine and hypersaline conditions in a fluctuating epicontinental sea. Four major cycles of limestones, later altered to dolomites, and succeeding evaporites, including gypsum (now anhydrite), halite and potash salts, were developed. These cycles resulted from periodic recharge of the basin by normal marine waters, from which limestones, 'many oolitic and locally fossiliferous, were formed around the margins. Only the limestones and dolomites are prominent at outcrop in a narrow north–south belt bisecting the county, the evaporites being reduced to thin, silty dissolution residues ([[Permian and Carboniferous rocks of Knaresborough - an excursion|Excursion 13]], [[Permian rocks of south-central Yorkshire - an excursion|Excursion 14]]). In the first cycle, fossiliferous bryozoan and stromatolitic patch reefs formed, but in the third cycle the limestones contain only a few plant and invertebrate species. Limestones of the second and fourth cycles do not crop out in Yorkshire but are present in thicker sequences in the subsurface to the east, where potash is mined at Boulby near the coast west-northwest of Whitby. Towards the end of the Permian, continental, water-lain red sandstones, siltstones and mudstones gradually filled the basin from the west.
  
 
These continental clastic deposits are unfossiliferous and span the Permo-Triassic boundary, which is consequently difficult to define. In addition, the soft sandstones, '''marls''' and evaporites of the Triassic, deposited along the western margin of an epicontinental North Sea Basin are rarely seen at outcrop, forming low ground largely mantled by glacial deposits in the Vales of Mowbray and York. The early Triassic (Sherwood Sandstone) consists of fluvial sandstones, with some flood-plain mudstones and siltstones and, in the south of the county, pebble beds deposited by north-flowing river systems. These are overlain with gentle unconformity by a thin concentrate deposit and then by dolomitic and silty mudstones (often red) and evaporites (principally halite) of the Mercia Mudstone, the result of waters from the Tethys Ocean flooding into the North Sea Basin, forming a shallow hypersaline marine environment. Macrofossils are rare but plant spores allow dating and correlation of the lower part of the sequence. Renewed transgression at the end of the Triassic produced thin, shaly mudstones with a '''bivalve''' fauna, bone beds and argillaceous limestones overlain by soft lagoonal mudstones. The total thickness of Triassic deposits increases from about 400 m at outcrop to some 700 m in the subsurface beneath east Yorkshire.
 
These continental clastic deposits are unfossiliferous and span the Permo-Triassic boundary, which is consequently difficult to define. In addition, the soft sandstones, '''marls''' and evaporites of the Triassic, deposited along the western margin of an epicontinental North Sea Basin are rarely seen at outcrop, forming low ground largely mantled by glacial deposits in the Vales of Mowbray and York. The early Triassic (Sherwood Sandstone) consists of fluvial sandstones, with some flood-plain mudstones and siltstones and, in the south of the county, pebble beds deposited by north-flowing river systems. These are overlain with gentle unconformity by a thin concentrate deposit and then by dolomitic and silty mudstones (often red) and evaporites (principally halite) of the Mercia Mudstone, the result of waters from the Tethys Ocean flooding into the North Sea Basin, forming a shallow hypersaline marine environment. Macrofossils are rare but plant spores allow dating and correlation of the lower part of the sequence. Renewed transgression at the end of the Triassic produced thin, shaly mudstones with a '''bivalve''' fauna, bone beds and argillaceous limestones overlain by soft lagoonal mudstones. The total thickness of Triassic deposits increases from about 400 m at outcrop to some 700 m in the subsurface beneath east Yorkshire.

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