Precambrian and ?Cambrian, introduction, Wales

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

Terranes of the Caledonian and Variscan orogens (adapted from Cope et al., 1992). P916145.

Precambrian (Proterozoic) rocks in Wales crop out most extensively on Anglesey and Llŷn but also occur in inliers in Arfon, between Bangor and Caernarfon, and smaller inliers in Pembrokeshire, Carmarthenshire and Radnorshire. Throughout the 20th century, the relationship between the rocks at these localities has been a subject of intense discussion and it is only in recent years that the terrane model provided the most satisfactory explanation of the disparate relationships seen at outcrop. The model proposes that all the outcrops are of late Precambrian age, and that they formed as a series of smaller terranes of markedly different geological characteristics, which were accreted together between 700 and 540 Ma.

Two major terranes have been defined: the Monian Composite Terrane, which includes Anglesey and the main part of the Llŷn outcrops, and the Avalon Terrane, which includes small localities on Llŷn, Arfon, Pembrokeshire and the Welsh borders (P916145). These terranes are juxtaposed by the Menai Straits Fault System, a complex of steeply inclined shear zones and brittle faults, but there are no exposures displaying the stratigraphical relationships between rocks on either side. The Monian Composite Terrane is equivalent to the Rosslare Terrane in south-east Ireland and both are bound to the north-west by the Leinster–Lake District Terrane.

Bibliography[edit]

Main reference[edit]

GIBBONS, W, and HARRIS, A L (editors). 1994. A revised correlation of Precambrian rocks in the British Isles. Geological Society of London Special Report, No. 22.

In addition[edit]

BEVINS, R E, PHARAOH, T C, COPE, J C W, and BREWER, T S. 1995. Geochemical characters of Neoproterozoic volcanic rocks in southwest Wales. Geological Magazine, Vol. 132, 339–349.

BOUMA, A H. 1962. Sedimentology of some flysch deposits. (Amsterdam: Elsevier.)

COLLINS, A S, and BUCHAN, C. 2004. Provenance and age constraints of the South Stack Group, Anglesey, UK: U-PB SIMS detrital zircon data. Journal of the Geological Society of London, Vol. 161, 743–746.

COPE, J C W, INGHAM, J K, and RAWSON, P F. 1992. Atlas of palaeogeography and litho-facies. (London: The Geological Society.)

CRIMES, T P. 1970. A facies analysis of the Cambrian of Wales. Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 7, 113–170.

GIBBONS, W, and HORAK, J M, 1990. Contrasting metamorphic terranes in north-west Wales. 315–328 in The Cadomian Orogeny. D’LEMOS, R S, STRACHAN, R A, and TOPLEY, C G (editors). Geological Society of London Special Publication, No.  51.

GIBBONS, W, and MCCARROLL, D. 1993. Geology of the country around Aberdaron, including Bardsey Island. Memoir of the British Geological Survey, Sheet 133 (England and Wales).

GIBBONS, W, TIETZSCH-TYLER, D, HORAK, J M, and MURPHY, F C. 1994. Chapter 9 Precambrian rocks in Anglesey, south-west Llyn and south-east Ireland. 75–84 in A revised correlation of Precambrian rocks in the British Isles. GIBBONS, W, and HARRIS, A L (editors). Geological Society of London Special Report, No. 22.

HICKS, H. 1877. On the Precambrian (Dimentian and Pebidian) rocks of St David’s. Quarterly Journal of the Geological Society of London, Vol. 33, 299–241.

HICKS, H. 1884. On the Precambrian rocks of Pembrokeshire with special reference to the St David’s district. Quarterly Journal of the Geological Society of London, Vol. 40, 507–547.

KUENEN, PH H. 1953. Graded bedding with observations on Lower Palaeozoic rocks of Britain. Verh. K. Nederland, Akad Wet Afd Nat (B) 20 (3).

KOPSTEIN, F P H. 1954. Graded bedding of the Harlech Dome. Diss Groningen Rijksuniv, 1–97.

PHILLIPS, E. 1991. The progressive deformation of the South Stack and New Harbour groups, Holy Island, western Anglesey, north Wales. Journal of the Geological Society of London, Vol. 148, 1091–1100.

PHILLIPS, E. 1991. The lithostratigraphy, sedimentology and tectonic setting of the Monian Supergroup, western Anglesey, north Wales. Journal of the Geological Society of London, Vol. 148, 1079–1090.

RUSHTON, A W A. 1974. The Cambrian of Wales and England. 43–120 in Cambrian of the British Isles, Norden and Spitzbergen. HOLLAND, C H (editor). (London: John Wiley.)

RUSHTON, A W A, and HOWELLS, M F. 1999. Stratigraphical framework for the Ordovician of Snowdonia and the Lleyn peninsula. British Geological Survey Research Report, RR/99/08.

TIETZSCH-TYLER, D. 1996. Precambrian and early Caledonian Orogeny in south-east Ireland. Irish Journal of Earth Sciences, Vol. 15, 19–39.