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

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The marked, rapid rise in sea level that followed the Late Ordovician glacial maximum is recognised across the Welsh Basin.  It began during the late Hirnantian and continued into the Silurian, leading to the widespread re-establishment of mudstone-dominated slope-apron facies. The onset of this Hirnantian postglacial deepening is recorded by the '''Bryn-glâs Formation (BGF)''', a mudstone succession up to 300&nbsp;m thick that sharply overlies the Pencerrigtewion Member and is unfossiliferous and commonly thinly laminated, but contains units of slumped mudstone (Cave and Hains, 1986<ref name="Cave 1986">Cave, R, and Hains, B A. 1986. Geology of the country between Aberystwyth and Machynlleth. Memoir of the British Geological Survey. Sheet 163 (England and Wales).</ref>; Davies et al., 2009<ref name="Davies 2009">Davies, J R, Waters, R A, Williams, M, Wilson, D, Schofield, D I, and Zalasiewicz, J A. 2009. Sedimentary and faunal events as revealed by a revised correlation of postglacial Hirnantian (Late Ordovician) strata in the Welsh Basin, UK. Geological Journal, Vol.&nbsp;44, 322–340.</ref>). Locally, in the westernmost inliers, the Pencerrigtewion Member is overlain by the '''Lluest-y-Graig Member (LyG),''' a sequence of thinly interbedded turbiditic mudstone and sandstone up to 50&nbsp;m thick, which represents a basal division of the Bryn-glâs Formation.
 
The marked, rapid rise in sea level that followed the Late Ordovician glacial maximum is recognised across the Welsh Basin.  It began during the late Hirnantian and continued into the Silurian, leading to the widespread re-establishment of mudstone-dominated slope-apron facies. The onset of this Hirnantian postglacial deepening is recorded by the '''Bryn-glâs Formation (BGF)''', a mudstone succession up to 300&nbsp;m thick that sharply overlies the Pencerrigtewion Member and is unfossiliferous and commonly thinly laminated, but contains units of slumped mudstone (Cave and Hains, 1986<ref name="Cave 1986">Cave, R, and Hains, B A. 1986. Geology of the country between Aberystwyth and Machynlleth. Memoir of the British Geological Survey. Sheet 163 (England and Wales).</ref>; Davies et al., 2009<ref name="Davies 2009">Davies, J R, Waters, R A, Williams, M, Wilson, D, Schofield, D I, and Zalasiewicz, J A. 2009. Sedimentary and faunal events as revealed by a revised correlation of postglacial Hirnantian (Late Ordovician) strata in the Welsh Basin, UK. Geological Journal, Vol.&nbsp;44, 322–340.</ref>). Locally, in the westernmost inliers, the Pencerrigtewion Member is overlain by the '''Lluest-y-Graig Member (LyG),''' a sequence of thinly interbedded turbiditic mudstone and sandstone up to 50&nbsp;m thick, which represents a basal division of the Bryn-glâs Formation.
  
The Bryn-glâs Formation is succeeded by the late Hirnantian to Aeronian '''Cwmere Formation (CeF)''', which crops out in a complex pattern of folds around the flanks of the Ordovician inliers.  It consists of 220&nbsp;m to 550&nbsp;m principally of thinly interbedded turbiditic and anoxic hemipelagic mudstones, generally in units about 5&nbsp;cm thick, with thin beds and laminae of turbiditic siltstone and sandstone at intervals throughout.  At its base is the '''Mottled Mudstone Member (MMb)''', comprising up to 25&nbsp;m of pale grey burrow-mottled mudstone ([[Media:P775109.jpg |'''Plate P775110''']]) that, in its lowest part, includes units of laminated hemipelagite containing latest Hirnantian ''persculptus'' Biozone graptolites ([[Media:P775109.jpg |'''Plate P775109''']]).  The Mottled Mudstone Member sharply overlies the Bryn-glâs Formation and records the subsequent, postglacial recolonisation of the basin prior to the creation of a deep thermally stratified water column and the widespread deposition of the anoxic facies (Davies et al., 2009<ref name="Davies 2009"></ref>).
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The Bryn-glâs Formation is succeeded by the late Hirnantian to Aeronian '''Cwmere Formation (CeF)''', which crops out in a complex pattern of folds around the flanks of the Ordovician inliers.  It consists of 220&nbsp;m to 550&nbsp;m principally of thinly interbedded turbiditic and anoxic hemipelagic mudstones, generally in units about 5&nbsp;cm thick, with thin beds and laminae of turbiditic siltstone and sandstone at intervals throughout.  At its base is the '''Mottled Mudstone Member (MMb)''', comprising up to 25&nbsp;m of pale grey burrow-mottled mudstone that, in its lowest part, includes units of laminated hemipelagite containing latest Hirnantian ''persculptus'' Biozone graptolites ([[Media:P775109.jpg |'''Plate P775109''']]).  The Mottled Mudstone Member sharply overlies the Bryn-glâs Formation and records the subsequent, postglacial recolonisation of the basin prior to the creation of a deep thermally stratified water column and the widespread deposition of the anoxic facies (Davies et al., 2009<ref name="Davies 2009"></ref>).
  
 
In the west of the district, around the Bryn Mawr, Mynydd-y-groes and Plynlimon inliers, the Cwmere Formation is largely replaced by the '''Glaslyn Formation (Gly)''', a succession of abundant, thin, parallel- and cross-laminated turbiditic sandstone arranged in characteristic Bouma-type units, interbedded with turbiditic and hemipelagic mudstone.  The sandstones form up to 50 per cent of the succession and are typically 1 to 3&nbsp;cm thick, although they may locally reach thicknesses of 30&nbsp;cm. The formation thins eastwards, splitting into two leaves that interdigitate with the Cwmere Formation around the flanks of the Van Inlier. The lower leaf crops out around the eastern side of the inlier, where it replaces the lower 30 to 60&nbsp;m of the Cwmere Formation, including the uppermost part of the Mottled Mudstone Member; the upper leaf thins eastwards and passes into the Cwmere Formation north and south of the inlier.  The Glaslyn Formation records deposition from mixed sand and mud turbidity currents, and was probably deposited as a number of small turbidite lobes that amalgamated on the margin of the Rhuddanian slope apron.
 
In the west of the district, around the Bryn Mawr, Mynydd-y-groes and Plynlimon inliers, the Cwmere Formation is largely replaced by the '''Glaslyn Formation (Gly)''', a succession of abundant, thin, parallel- and cross-laminated turbiditic sandstone arranged in characteristic Bouma-type units, interbedded with turbiditic and hemipelagic mudstone.  The sandstones form up to 50 per cent of the succession and are typically 1 to 3&nbsp;cm thick, although they may locally reach thicknesses of 30&nbsp;cm. The formation thins eastwards, splitting into two leaves that interdigitate with the Cwmere Formation around the flanks of the Van Inlier. The lower leaf crops out around the eastern side of the inlier, where it replaces the lower 30 to 60&nbsp;m of the Cwmere Formation, including the uppermost part of the Mottled Mudstone Member; the upper leaf thins eastwards and passes into the Cwmere Formation north and south of the inlier.  The Glaslyn Formation records deposition from mixed sand and mud turbidity currents, and was probably deposited as a number of small turbidite lobes that amalgamated on the margin of the Rhuddanian slope apron.

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