Editing Geology and landscape of Upper Teesdale - an excursion

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== Geological background ==
 
== Geological background ==
  
Upper Teesdale is situated on the southern margin of the Alston Block, part of a structural block and trough system developed in Northern England following the '''Caledonian Orogeny'''. The foundation of the block in the Teesdale area comprises Ordovician '''meta-sediments''' (exposed in the Teesdale '''inlier''' at Cronkley Pasture; NY 846296) and '''volcaniclastic''' deposits (Borrowdale Volcanic Group) into which the Weardale '''Granite''' has been emplaced.
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Upper Teesdale is situated on the southern margin of the Alston Block, part of a structural block and trough system developed in Northern England following the Caledonian Orogeny. The foundation of the block in the Teesdale area comprises Ordovician meta-sediments (exposed in the Teesdale inlier at Cronkley Pasture; NY 846296) and volcaniclastic deposits (Borrowdale Volcanic Group) into which the Weardale Granite has been emplaced.
  
This buoyant structural unit is bounded on its southern margin by the Lunedale–Butterknowle '''Fault''' zone, separating it from the Stainmore Trough, a substantial depositional basin during the Carboniferous period. In the subsiding Stainmore Trough some 2850 m of cyclic Carboniferous sediments are still preserved; equivalent strata on the more stable Alston Block are appreciably thinner. In Teesdale and adjacent areas each Yoredale cyclothem commences with a marine '''transgression''' (usually a limestone which gives a generic name to the whole unit; e.g. Five Yard Limestone Cyclothem) followed by sediments of increasing deltaic influence (shales, silt-stones, flagstones, sandstones or grits, '''seatearths''' and perhaps thin coals). The terrestrially derived sediments represent the infilling of a shallow marine gulf by a southerly '''prograding''' deltaic complex. The number of such Yoredale cyclothems in the North Pennine area suggests that sedimentary basin infill was followed by regional subsidence and/or increase in sea-level, with many repetitions of the sedimentary cycle over many millions of years.
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This buoyant structural unit is bounded on its southern margin by the Lunedale–Butterknowle Fault zone, separating it from the Stainmore Trough, a substantial depositional basin during the Carboniferous period. In the subsiding Stainmore Trough some 2850 m of cyclic Carboniferous sediments are still preserved; equivalent strata on the more stable Alston Block are appreciably thinner. In Teesdale and adjacent areas each Yoredale cyclothem commences with a marine transgression (usually a limestone which gives a generic name to the whole unit; e.g. Five Yard Limestone Cyclothem) followed by sediments of increasing deltaic influence (shales, silt-stones, flagstones, sandstones or grits, seatearths and perhaps thin coals). The terrestrially derived sediments represent the infilling of a shallow marine gulf by a southerly prograding deltaic complex. The number of such Yoredale cyclothems in the North Pennine area suggests that sedimentary basin infill was followed by regional subsidence and/or increase in sea-level, with many repetitions of the sedimentary cycle over many millions of years.
  
The end-Carboniferous Variscan Orogeny led to reactivation of basement faults and general uplift of the Northern Pennines area. A late orogenic extensional phase resulted in the injection of basic '''magma''' into deep-seated fault zones, some of which acted as feeder dykes for the extensive Whin Sill, a very important '''doleritic''' intrusion in North East England. Upper Teesdale, the southern limit of this '''igneous''' body, was where the intrusive nature of a sill was first recognized by Adam Sedgwick in 1827. The important Northern Pennines mineralization is chronologically associated with this reheating of the basement rocks of the block areas by these basic magmas (Dunham 1990). '''Mineral veins''' (base metals lead, zinc and iron with a little copper, silver and cobalt) are to be found in fault zones cutting the Carboniferous sediments and indeed replacing some of the limestones in particular. The legacy of intensive mining in the past litters the Teesdale landscape.
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The end-Carboniferous Variscan Orogeny led to reactivation of basement faults and general uplift of the Northern Pennines area. A late orogenic extensional phase resulted in the injection of basic magma into deep-seated fault zones, some of which acted as feeder dykes for the extensive Whin Sill, a very important doleritic intrusion in North East England. Upper Teesdale, the southern limit of this igneous body, was where the intrusive nature of a sill was first recognized by Adam Sedgwick in 1827. The important Northern Pennines mineralization is chronologically associated with this reheating of the basement rocks of the block areas by these basic magmas (Dunham 1990). Mineral veins (base metals lead, zinc and iron with a little copper, silver and cobalt) are to be found in fault zones cutting the Carboniferous sediments and indeed replacing some of the limestones in particular. The legacy of intensive mining in the past litters the Teesdale landscape.
  
 
Post-Variscan faulting in the Teesdale area is complicated by subsequent tectonic episodes (Triassic, intra-Jurassic, early Tertiary and even late Quaternary '''eustatic''' movements) so many faults show different phases and directions of movement and there are some cases of fault inversion. Into one such fault zone were injected basic magmas believed to have originated from the early Tertiary (Palaeocene) Mull volcanic centre. This Cleveland-Armathwaite Dyke '''echelon''' is well represented in Upper Teesdale.
 
Post-Variscan faulting in the Teesdale area is complicated by subsequent tectonic episodes (Triassic, intra-Jurassic, early Tertiary and even late Quaternary '''eustatic''' movements) so many faults show different phases and directions of movement and there are some cases of fault inversion. Into one such fault zone were injected basic magmas believed to have originated from the early Tertiary (Palaeocene) Mull volcanic centre. This Cleveland-Armathwaite Dyke '''echelon''' is well represented in Upper Teesdale.

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