Silurian, introduction, Wales: Difference between revisions

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'''From: Howells, M F. 2007. [[British regional geology: Wales|British regional geology: Wales]]. Keyworth, Nottingham: British Geological Survey.'''
'''From: Howells, M F. 2007. [[British regional geology: Wales|British regional geology: Wales]]. Keyworth, Nottingham: British Geological Survey.'''
[[File:P916228.jpg|thumbnail|Silurian stages, series and biozones. P916228.]]


The end-Ordovician, glacio-eustatic low-stand of sea level was reversed during early Silurian times when an ice cap, centred on the south pole, then located on what is now North Africa, began to melt, causing a worldwide marine transgression. Subsequent fluctations in sea level were mainly the result of changes in the volume of ice at the poles, but probably there was also some local tectonic contribution related to the closure of Iapetus Ocean. By late Llandovery (Telychian) times, the Midland Platform was totally flooded although slight changes in platform and basin distribution continued to affect the patterns of sedimentation across Wales. Broadly, it was a period of comparative tectonic stability and warm climate.
The end-Ordovician, glacio-eustatic low-stand of sea level was reversed during early Silurian times when an ice cap, centred on the south pole, then located on what is now North Africa, began to melt, causing a worldwide marine transgression. Subsequent fluctations in sea level were mainly the result of changes in the volume of ice at the poles, but probably there was also some local tectonic contribution related to the closure of Iapetus Ocean. By late Llandovery (Telychian) times, the Midland Platform was totally flooded although slight changes in platform and basin distribution continued to affect the patterns of sedimentation across Wales. Broadly, it was a period of comparative tectonic stability and warm climate.
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WOOD, A, and SMITH, A J. 1959. The sedimentation and sedimentary history of the Aberystwyth Grits (Upper Llandoverian). ''Quarterly Journal of the Geological Society of'' ''London'', Vol. 114 (for 1958), 163–195.
WOOD, A, and SMITH, A J. 1959. The sedimentation and sedimentary history of the Aberystwyth Grits (Upper Llandoverian). ''Quarterly Journal of the Geological Society of'' ''London'', Vol. 114 (for 1958), 163–195.
[[Category:Wales]]
{{British regional geology of Wales - contents}}
[[Category:Regional Geology of Wales]]

Latest revision as of 09:53, 5 May 2016

From: Howells, M F. 2007. British regional geology: Wales. Keyworth, Nottingham: British Geological Survey.

Silurian stages, series and biozones. P916228.

The end-Ordovician, glacio-eustatic low-stand of sea level was reversed during early Silurian times when an ice cap, centred on the south pole, then located on what is now North Africa, began to melt, causing a worldwide marine transgression. Subsequent fluctations in sea level were mainly the result of changes in the volume of ice at the poles, but probably there was also some local tectonic contribution related to the closure of Iapetus Ocean. By late Llandovery (Telychian) times, the Midland Platform was totally flooded although slight changes in platform and basin distribution continued to affect the patterns of sedimentation across Wales. Broadly, it was a period of comparative tectonic stability and warm climate.

Silurian strata occupy the core of the Central Wales Syncline and can be traced in a narrow outcrop between the Harlech and Berwyn domes into the Denbigh moors and Clwydian Hills in north Wales. They also crop out on the eastern limb of the Twyi Anticline, and eastwards towards the Welsh Borderland. Elsewhere, they occur in small inliers, as at Cardiff and Usk, and in folded and thrust slices within the Variscan zone of west Pembrokeshire. It was from these Welsh outcrops and their continuation into the Welsh Borderland that Murchison in 1835 proposed the name of the system, from the ancient tribe, the Silures, and from where the names of three of the four series, Llandovery, Wenlock and Ludlow, were taken. The biostratigraphical subdivisions of the sequence, based on both graptolites and shelly faunas, are well founded (P916228). It is estimated to span approximately 28 Ma, from 444 to 416 Ma, and throughout this time, faunal provincialism on either side of Iapetus diminished because of the closure of the ocean. Closure of Iapetus probably occurred in the mid to late Silurian, but related deformation events continued into Devonian times.

Silurian rocks through most of central Wales are mainly of mudstone and silty mudstone with interbedded sandstones, which on cursory examination appear to be remarkably uniform or repetitive. The sedimentary structures and faunal content determine their basinal character, and a traverse to the east clearly demonstrates their contrast with penecontemporneous rocks in the vicinity of the shelf in the Welsh Borderland. This marked contrast in facies inhibited a clear understanding for many of the early surveyors, and it was not until O T Jones presented his perceptive Presidential Address, ‘On the Evolution of a Geosyncline’, to the Geological Society in 1938, that the basis was laid for modern interpretation.

More recently, following a long period of research into the shelf areas, there has been a concerted effort by the British Geological Survey and others to unravel the structure of central Wales and to trace the sedimentological changes from shelf to basin. The elucidation of the basinal sequence has been profoundly influenced by the determination of the thickness and sandstone content of the turbidite units, and the amount and type of hemipelagic mudstone. The hemipelagites represent background sedimentation; they comprise black and dark grey laminated mudstone deposited beneath anaerobic bottom waters and pale grey bioturbated mudstone deposited under oxygenated conditions. Many of the formational subdivisions are based on these variations and are thus markedly diachronous.

Bibliography

CAVE, R, and HAINS, B A. 2001. Geology of the country around Montgomery and the Ordovican rocks of the Shelve area. Memoir of the British Geoloical Survey, Sheet 165 Montgomery with part of Sheet 151 Welshpool (England and Wales).

CAVE, R, and LOYDELL, D K. 1997. The eastern margin of the Aberystwyth Grits Formation. Geological Journal, Vol. 32, 37–44.

COCKS, L R M, WOODCOCK, N H, RICKARDS, R B, TEMPLE, J T, and LANE, P D. 1984. The Llandovery Series of the type area. Bulletin of the British Museum, Natural History, Vol. 38, 131–182.

COCKS, L R M, HOLLAND, C H, and RICKARDS, R B. 1992. A revised correlation of Silurian rocks in the British Isles. Geological Society of London Special Publication, No. 21.

COPE, J C W, INGHAM, J K, and RAWSON, P F. 1992. Atlas of palaeogeography and litho-facies. Memoir of the Geological Society of London, No.13.

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).

DAVIES, J R, WILSON, D, and WILLIAMSON, I T. 2004. The geology of the country around Flint. Memoir of the British Geological Survey, Sheet 108 (England and Wales). (London: HMSO.)

ELLES, G L. 1900. The zonal classification of the Wenlock Shales of the Welsh Borderland. Quarterly Journal of the Geological Society of London, Vol. 56, 370–414.

FORTEY, R A, and COCKS, L R M. 1986. Marginal faunal belts and their structural implications with examples from the Lower Palaeozoic. Journal of the Geological Society of London, Vol. 143, 151–160.

HOLLAND, C H, and LAWSON, J D. 1963. Ludlovian facies patterns in Wales and the Welsh Borderland. Liverpool and Manchester Geological Journal, Vol. 3, 269–288.

MERRIMAN, R J. 2006. Clay mineral assemblages in British Lower Palaeozoic mudrocks. Clay Minerals, Vol. 41, 473–512.

WARREN, P T, PRICE, D, NUTT, M J C, and SMITH, E G. 1984. Geology of the country around Rhyl and Denbigh. Memoir of the British Geological Survey, Sheet 107 and parts of sheets 94 and 106 (England and Wales).

WOOD, A, and SMITH, A J. 1959. The sedimentation and sedimentary history of the Aberystwyth Grits (Upper Llandoverian). Quarterly Journal of the Geological Society of London, Vol. 114 (for 1958), 163–195.