Editing Geology of the Llanidloes area: Geological description - Late Ordovician and Silurian

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Marine regression during the Aeronian introduced the '''Claerwen Group''' (Davies et al., 1997<ref name="Davies 1997">Davies, J R, Fletcher, C J N, Waters, R A, Wilson, D, Woodhall, D G, and Zalasiewicz, J A. 1997. Geology of the country around Llanilar and Rhayader. Memoir of the British Geological Survey, Sheets 178 and 179 (England and Wales).</ref>), which sharply overlies the the Cwmere Formation ([[Media:P775111.jpg |'''Plate P775111''']]).  It comprises a slope-apron facies of predominantly oxic, pale grey-green, colour-banded turbiditic and hemipelagic mudstone, thinly interbedded on a scale of 2 to 3&nbsp;cm, with subordinate thin siltstones and sandstones.  The turbiditic and hemipelagic mudstones are mostly burrow-mottled, and there are horizons of diagenetic phosphate nodules at intervals.  Individual turbidite units commonly display thin (1&nbsp;mm or less) silt laminae at their bases, and were generally deposited from low-density, fine-grained turbidity currents.  Thin units of anoxic mudstone, which occur in places throughout the succession, are thought to record brief transgressive pulses in response to fluctuations of the Gondwanan ice sheet (Page et al., 2007<ref name="Page 2007">Page, A A, Zalasiewicz, J A, Williams, M, and Popov, L E. 2007. Were transgressive black shales a negative feedback, modulating eustasy in the Early Palaeozoic icehouse? 123–156 ''in'' Deep-time perspectives on climate change: marrying the signal from computer models and biological proxies. Williams, M, Haywood, A M, Gregory, F J, and Schmidt, D N (editors). The Micropalaeontological Society, Special Publication. (London: The Geological Society.)</ref>). The Claerwen Group envelops earlier formations around the margins of the Ordovician inliers and crops out over much of the area around Llanidloes.  It is comprises the '''Derwenlas Formation '''('''DlF'''; up to 140&nbsp;m thick) and the overlying '''Rhayader Mudstones Formation''' ('''Rhs'''; up to 550&nbsp;m thick) separated by a widespread anoxic unit, the '''Monograptus Sedgwickii Shales Member (lh<sup>s</sup>)''', at the base of the latter formation ([[Media:P775112.jpg |'''Plate P775112''']]). This anoxic level correlates with a major sequence boundary in shelfal areas bordering the Welsh Basin, and equates with a well-documented global marine transgression (Davies et al., 2013<ref name="Davies 2013">Davies, J R, Waters, R A, Molyneux, S G, Williams, M, Zalasiewicz, J A, Vandenbroucke, T R A, and Verniers, J. 2013. A revised sedimentary and biostratigraphical architecture for the type Llandovery area, central Wales, UK. Geological Magazine, Vol. 150, 300–332.</ref>and 2016<ref name="Davies 2016">Davies, J R, Waters, R A, Molyneux, S G, Williams, M, Zalasiewicz, J A, and Vandenbroucke, T R A. 2016. Gauging the impact of glacioeustasy on a mid-latitude early Silurian basin margin, mid Wales, UK. Earth Science Reviews, Vol. 156, 82-107.</ref>; Johnson, 2010<ref name="Johnson 2010">Johnson, M E. 2010. Tracking Silurian eustasy: alignment of empirical evidence or pursuit of deductive reasoning? Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 296, 276–284.</ref>) which probably began during the late Aeronian ''convolutus'' graptolite Biozone; graptolites of the latter have been recorded in anoxic units within the upper part of the Derwenlas Formation.
 
Marine regression during the Aeronian introduced the '''Claerwen Group''' (Davies et al., 1997<ref name="Davies 1997">Davies, J R, Fletcher, C J N, Waters, R A, Wilson, D, Woodhall, D G, and Zalasiewicz, J A. 1997. Geology of the country around Llanilar and Rhayader. Memoir of the British Geological Survey, Sheets 178 and 179 (England and Wales).</ref>), which sharply overlies the the Cwmere Formation ([[Media:P775111.jpg |'''Plate P775111''']]).  It comprises a slope-apron facies of predominantly oxic, pale grey-green, colour-banded turbiditic and hemipelagic mudstone, thinly interbedded on a scale of 2 to 3&nbsp;cm, with subordinate thin siltstones and sandstones.  The turbiditic and hemipelagic mudstones are mostly burrow-mottled, and there are horizons of diagenetic phosphate nodules at intervals.  Individual turbidite units commonly display thin (1&nbsp;mm or less) silt laminae at their bases, and were generally deposited from low-density, fine-grained turbidity currents.  Thin units of anoxic mudstone, which occur in places throughout the succession, are thought to record brief transgressive pulses in response to fluctuations of the Gondwanan ice sheet (Page et al., 2007<ref name="Page 2007">Page, A A, Zalasiewicz, J A, Williams, M, and Popov, L E. 2007. Were transgressive black shales a negative feedback, modulating eustasy in the Early Palaeozoic icehouse? 123–156 ''in'' Deep-time perspectives on climate change: marrying the signal from computer models and biological proxies. Williams, M, Haywood, A M, Gregory, F J, and Schmidt, D N (editors). The Micropalaeontological Society, Special Publication. (London: The Geological Society.)</ref>). The Claerwen Group envelops earlier formations around the margins of the Ordovician inliers and crops out over much of the area around Llanidloes.  It is comprises the '''Derwenlas Formation '''('''DlF'''; up to 140&nbsp;m thick) and the overlying '''Rhayader Mudstones Formation''' ('''Rhs'''; up to 550&nbsp;m thick) separated by a widespread anoxic unit, the '''Monograptus Sedgwickii Shales Member (lh<sup>s</sup>)''', at the base of the latter formation ([[Media:P775112.jpg |'''Plate P775112''']]). This anoxic level correlates with a major sequence boundary in shelfal areas bordering the Welsh Basin, and equates with a well-documented global marine transgression (Davies et al., 2013<ref name="Davies 2013">Davies, J R, Waters, R A, Molyneux, S G, Williams, M, Zalasiewicz, J A, Vandenbroucke, T R A, and Verniers, J. 2013. A revised sedimentary and biostratigraphical architecture for the type Llandovery area, central Wales, UK. Geological Magazine, Vol. 150, 300–332.</ref>and 2016<ref name="Davies 2016">Davies, J R, Waters, R A, Molyneux, S G, Williams, M, Zalasiewicz, J A, and Vandenbroucke, T R A. 2016. Gauging the impact of glacioeustasy on a mid-latitude early Silurian basin margin, mid Wales, UK. Earth Science Reviews, Vol. 156, 82-107.</ref>; Johnson, 2010<ref name="Johnson 2010">Johnson, M E. 2010. Tracking Silurian eustasy: alignment of empirical evidence or pursuit of deductive reasoning? Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 296, 276–284.</ref>) which probably began during the late Aeronian ''convolutus'' graptolite Biozone; graptolites of the latter have been recorded in anoxic units within the upper part of the Derwenlas Formation.
  
[[Image:P775111.jpg|thumb|center|500px|  '''Plate P775111'''&nbsp;&nbsp;&nbsp;&nbsp;Rusty weathering anoxic mudstones of the Cwmere Formation overlain in the upper part of the scarp by more resistant bioturbated mudstones of the Derwenlas Formation, Uwch-y-coed [SN 8300 9502].          ]]
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[[Image:P775111.jpg|thumb|center|500px|  '''Plate P775111'''&nbsp;&nbsp;&nbsp;&nbsp;Rusty weathering anoxic mudstones of the Cwmere Formation overlain in the upper part of the scarp by harder bioturbated mudstones of the Derwenlas Formation, Uwch-y-coed [SN 8300 9502].          ]]
  
 
[[Image:P775112.jpg|thumb|center|500px|  '''Plate P775112'''&nbsp;&nbsp;&nbsp;&nbsp;Feature (slack) formed by Monograptus Sedgwickii Shales Member at the base of Rhayader Mudstones Formation (right of picture), with the underlying Derwenlas Formation exposed in left foreground, Uwch-y-coed [SN 8300 9513].    ]]
 
[[Image:P775112.jpg|thumb|center|500px|  '''Plate P775112'''&nbsp;&nbsp;&nbsp;&nbsp;Feature (slack) formed by Monograptus Sedgwickii Shales Member at the base of Rhayader Mudstones Formation (right of picture), with the underlying Derwenlas Formation exposed in left foreground, Uwch-y-coed [SN 8300 9513].    ]]

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