Cambrian, introduction, Wales
From: Howells, M F. 2007. British regional geology: Wales. Keyworth, Nottingham: British Geological Survey.
In earliest Lower Palaeozoic times, Wales, together with Belgium, England, southern Ireland and the Avalon area of eastern Newfoundland, formed part of a north-west-facing continent on the south-east side of the Iapetus Ocean at a latitude between 40° and 60° south (P916144). The intense volcanic, tectonic and metamorphic activity that occurred at the edge of this continent in late Precambrian times had dissipated by the time it was encroached upon by a major marine transgression with deposition of a carbonate-poor sequence in early Palaeozoic times. The elements of this continental plate (Avalonia) and its proximity to the Gondwana continent are still uncertain; it was separated from Baltica by the Tornquist Sea. On the north-west side of the Iapetus Ocean the Scottish Lower Palaeozoic carbonate-rich sequence accumulated at latitudes between 25° and 40° south on the edge of the North American continent of Laurentia. In Wales, the structural grain of the Precambrian basement influenced the development of a basin in which shallow and deep water sediments accumulated throughout early Palaeozoic times. The basin was defined by the Menai Straits Fault Zone at the edge of the Irish Sea Landmass in the north-west, and the Welsh Borderland Fracture Zone, including the Pontesford, Church Stretton and Twyi lineaments, at the edge of the Midland Platform in the south-east. Both fault zones continued to influence basin development and sedimentation throughout early Palaeozoic times (P916149).
The Cambrian System was defined in Wales, its historical type area, in the early years of the 19th century. However, in parts of the succession, particularly the lower parts, there is a dearth of faunas and the stratigraphy is difficult to establish. Consequently, the base of the Cambrian is now formally defined in south-east Newfoundland. The oldest rocks assigned to the Cambrian system in north Wales overlie an unconformity, but it is possible that some of the rocks below the unconformity may also be of Cambrian age; there is no direct contact between undisputed Cambrian rocks and the Monian Supergroup on Anglesey. There are no agreed international series subdivisions of the Cambrian System. The sequence has been divided into Lower, Middle and Upper parts (Table 2) and these have been casually regarded as series, although on the global scale they have been used inconsistently. Consequently, the British regional series names Comley, St David’s and Merioneth, respectively the Lower, Middle and Upper Cambrian as applied in Britain, have been used informally. Sea level rose progressively from early Cambrian times, with minor fluctuations, into late Cambrian times when three pulses are recognised, possibly caused by a fluctuating ice sheet in the southern hemisphere. Subsequently, there was a pronounced fall in sea level. The radioactive calibration of the time scale has been difficult because of the lack of suitable lithologies and the resetting of isotope systems in metamorphic events, but Welsh rocks have contributed important data nonetheless. On the geological timescale currently in use (2004) the Cambrian system spans the interval 542 to 488 Ma.
Cambrian strata crop out in the Harlech Dome, Arfon and Llŷn, in north Wales, and in the vicinity of St David’s in Pembrokeshire and Llangynog, south-west of Carmarthen, in south Wales. Local lithological correlation is good, although wider correlation is hindered by poor biostratigraphical control in the lowermost beds. The rocks contain some of the earliest remains of marine fossils. The communities are representative of the European Avalonian Province and are totally distinct from those of the North American Laurentian (Pacific) Province observed in equivalent strata in Scotland. The shallowest marine environments generally contain lingulate brachiopods whereas trilobites inhabited the more offshore settings. Trilobites only became firmly established in late Comley times. The richest faunas are found in the dark, blue-grey mudstone of the St David’s Series, which host a variety of sponges, hyoliths, echinoderms and bradoriid ostracods as well as several trilobite species. Most of these animals were benthic and relatively local in distribution, but some species of the large paradoxidid and tiny blind agnostid trilobites are widely distributed, being known from Scandinavia, Siberia, Spain and Newfoundland; they may have been bathypelagic, swimming widely in deep waters. These are the most useful macrofossils for dating the rocks. In the Merioneth Series, black mudstone contains the specialised olenid trilobites that evolved to occupy the most poorly oxygenated environments and characterise the ‘Olenid Biofacies’.
Bibliography
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
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 Llŷn 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.