Proterozoic eon Geologic timescale of the Proterozoic (millions of years ago) (expand)
The Cymru terrane is one of five inferred fault bounded terranes that make up the basement rocks of the southern United Kingdom. The other notable geological terranes are the Charnwood, Fenland, Wrekin and the Monian Composite terranes.P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed In this article the definition of terrane is that implying rocks associated with the composition of the Precambrian basement. The Cymru terrane is bounded to the northwest by the Menai Strait Fault System and to the southeast by the Pontesford Lineament. The geological terrane to the west is the Monian Composite terrane and to the east is Wrekin terrane. The majority of rocks in the area are associated with the outcrops that are evident at the faulted boundaries.Pharaoh, T.C., Gibbons, W., Precambrian Rocks in England and Wales south of the Menai Strait Fault System, 1987 A Revised Correlation of the Precambrian Rocks in the British IslesJ. N. Carney, J. M. Horak, et al., Precambrian Rocks of England and Wales, Joint Nature Conservation Committee. Geological Conservation Review Series 20
The Proterozoic rocks of the Cymru terrane are typified in North Wales by the Arfon Group, Sarn Complex and the Twt Hill Granite.P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed The former is in the Bangor area (S & SW) with the latter two located on the Llŷn Peninsula. The St Davids Granophyre and the Pebidian Supergroup are located on the coastline of St Davids Peninsula. Inferred Proterozoic volcanic deposits are noted in the Bryn-Teg Borehole (Trawsfynydd, Gwynedd).P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd EdP.M. Allen, A.A. Jackson Bryn-Teg Borehole, North Wales., 1978, Bulletin of the Geological Survey of Great Britain, 61 1-52
The Padarn Tuff, which is thought to be contemporaneous with the Sarn Complex, is unconformable with the overlying Fachwen Formation and the unconformity has been constrained with dates noted of 604.7 Ma+/-1.6Ma from the tuffs and 572.5 Ma+/-1.2Ma from the Fachwen Formation.W. Compston, A. E. Wright, P. Toghill, Dating the Late Precambrian volcanicity of England and Wales. 2002, Journal of the Geological Society of London. 159 323-339. Information on the basement is somewhat sparse with no zircons noted to be older than 617 to 638 Ma.W. Compston, A. E. Wright, P. Toghill, Dating the Late Precambrian volcanicity of England and Wales. 2002, Journal of the Geological Society of London. 159 323-339. However, there are outcrops of the Parwyd Gneiss which are retrogressed granitoid gneiss and garnet amphibolite that occur with the Llyn Shear Zone.P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed This is conjectured to be one of two probabilities, either as an exotic sliver, or as a metamorphic slice from the Monian Composite terrane. The Granitoid Gneiss has Sm-Nd ages of 1350 Ma which is in the same temporal region as the Sarn Granite P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd EdW. Compston, A. E. Wright, P. Toghill, Dating the Late Precambrian volcanicity of England and Wales. 2002, Journal of the Geological Society of London. 159 323-339.
Intrusive geology
St David's Granophyre In the southwest of Wales is the St Davids Granophyre the stratigraphical relationship of which was subject of heated debates towards the end of the 19th Century predominantly between Sir Archibald Geikie and Henry Hicks. Geikie (then director of the Geological Survey) insisted upon the Pebidian Supergroup and the high-level intrusion being placed in the Cambrian sequence with Hicks H. Hicks On the Precambrian (Dimentian and Pebidian) rocks of St David's, 1877, Quarterly Journal of the Geological Society of London, 33, 199-241H. Hicks On the Precambrian rocks of Pembrokeshire with especial reference to the St David's district., 1878, Journal of the Geological Society of London. 40 507-560 taking the opposing view and suggesting that the Granophyre which cross cuts the Pebidian Supergroup should be placed in the Precambrian and by consequence placing the Pebidian Supergroup in the Precambrian. The issue was finally resolved by GreenJ. F. N. Green On the Geological Structure of the St David's area, Pembrokeshire., 1908, Journal of the Geological Society of London. 64 363-383 by revealing, in an excavation, an unconformity in which an excavation of a Cambrian basal conglomerate is shown to cut the granophyre.P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd EdJ. F. N. Green On the Geological Structure of the St David's area, Pembrokeshire., 1908, Journal of the Geological Society of London. 64 363-383
The St David's Granophyre has historically been incorrectly assigned to be alaskite or trondhjemite due to low concentrations of potassium feldspar but this has been shown to be an alteration product from its original calc-alkaline granite which is noted to exhibit a volcanic arc signature.T. W. Bloxham & M. H. J. Dirk The petrology and geochemistry of the St David's granophyre and Cwm Bach Rhyolite, Pembrokeshire, Dyfed., 1988, Mineralogical Magazine, 52 563-575 Correlation of the granophyric intrusion with the Arfon Group rather than the nearby Coomb Volcanic Formation has been suggestedR. E. Bevins, T. C. Pharaoh, J. C. W. Cope, T. S. Brewster, Geochemical Character of Neoproterozoic rocks in southwest Wales., 1995, Geological Magazine. 132 330-349 with a tentative U-Pb isotopic age of 625+/-25Ma being suggested.P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd EdP. J. Patchett & J. Jocelyn U-Pb zircon ages for late Precambrian igneous rocks in South Wales., 1979, Journal of the Geological Society of London. 136 13-19
Sarn Complex The largest plutonic body in the terrane has limited outcrop and is sheared by the Llyn Shear Zone in the west and covered by later (Arenig) sediments to the east.P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed Altered to Greenschist facies the pluton contains a bimodal suite of gabbro-diorite, monzogranite (Sarn Granite) and granodiorite.P. J. Patchett & J. Jocelyn U-Pb zircon ages for late Precambrian igneous rocks in South Wales., 1979, Journal of the Geological Society of London. 136 13-19
The Sarn Granite is leucocratic and covers an expanse of c.6 km^2 in contrast to the gabbro and diorite that exist as small and scattered exposures. The dioritic component has been confirmed as having a Neoproterozoic age of 614Ma+/-2Ma using U-Pb Zircon dating.P. J. Patchett & J. Jocelyn U-Pb zircon ages for late Precambrian igneous rocks in South Wales., 1979, Journal of the Geological Society of London. 136 13-19 So therefore, the shearing of the Llyn is also temporally constrained by the date.
Twt Hill Granite
This is a small fine-grained, leucocratic intrusive body that cuts the Padarn Tuff at the western end of the Bangor-Caernarfon Ridge.P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed
Volcano-sedimentary geology
The thick sequences of volcano-sedimentary facies are present in North Wales (as the Arfon Group) and South Wales (as the Pebidian Supergroup) and are generally considered to be coeval but are geochemically distinct, showing acidic and basic qualities respectively.R. E. Bevins, T. C. Pharaoh, J. C. W. Cope, T. S. Brewster, Geochemical Character of Neoproterozoic rocks in southwest Wales., 1995, Geological Magazine. 132 330-349P. J. Patchett & J. Jocelyn U-Pb zircon ages for late Precambrian igneous rocks in South Wales., 1979, Journal of the Geological Society of London. 136 13-19 Both are cut by minor granitic intrusions.
The Pebidian Supergroup is interbedded basic lavas and acid tuffs, is cut by the St David's Granophyre.P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed There is evidence presented that subaerial and subaqueous deposition has occurred in a Welsh Basin that is likely to have been undergoing varying rates of subsidence thus promoting aerial emergence. Greenschist alteration is in keeping with the regional metamorphism that is characteristic of Welsh Neoproterozoic rocks.R. E. Bevins, T. C. Pharaoh, J. C. W. Cope, T. S. Brewster, Geochemical Character of Neoproterozoic rocks in southwest Wales., 1995, Geological Magazine. 132 330-349
In the Pebidian Supergroup the basal unit is largely basaltic and exhibits columnar jointing, autobrecciated lavas, scoriaceous and fine grained tuffs. The volcano-sediments have an increased acid component until the uppermost beds of the unit exhibit the basaltic influence once again. Such information is preserved in the Rhosson Group as autobrecciated lava, tuffs and scoria deposits.P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed
The Arfon Group This volcano-sedimentary group incorporates over 4000m of deposits and were previously considered to be of Cambrian age. U-Pb data suggests that the whole succession is indeed Precambrian Neoproterozoic age.A. J. Reedman, B. E. Leveridge, R. B. Evans, The Arfon Group (Arvonian) of North Wales., 1984, Proceedings of the Geologists Association, 95, 313-321 Exposure of the lower unit (Padarn Tuff) is exhibited on a ridge between Bangor and Caernarfon and also on a ridge near Llyn PadarnP. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed This is noted to be a thick sequence of acid ash flow tuffs and exhibits welding and are thought to be rapid deposition of thin air-fall tuffs and rhyolite flows placed in a bounded half-graben (or graben) .A. J. Reedman, B. E. Leveridge, R. B. Evans, The Arfon Group (Arvonian) of North Wales., 1984, Proceedings of the Geologists Association, 95, 313-321 Isotope data shows a U-Pb zircon from the lower part of the succession as 614+/-2MaR. D. Tucker & T. C. Pharaoh, U-Pb Zircon ages for late-Precambrian rocks in southern Britain., 1991, Journal of the Geological Society of London, 148, 435-433 and 604.7+/-1.6MaW. Compston, A. E. Wright, P. Toghill, Dating the Late Precambrian volcanicity of England and Wales. 2002, Journal of the Geological Society of London. 159 323-339. confirms a Neoproterozoic age.P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed
Minfordd and Bangor Formations
These strata have an angular and discordant relationship with both formations having variable thickness controlled by bounding fault lines that are likely to have controlled the depositional regime of the rocks.P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd EdA. J. Reedman, B. E. Leveridge, R. B. Evans, The Arfon Group (Arvonian) of North Wales., 1984, Proceedings of the Geologists Association, 95, 313-321 The formations exist to the west of the Aber Dinlle Fault. The Minfordd Formation is a sandstone dominated epiclastic and tuffaceous lithology which has a broad upward fining sequence with the presence of thick welded and non-welded ash-flow tuffs that elucidates an amount of volcanism.P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd EdA. J. Reedman, B. E. Leveridge, R. B. Evans, The Arfon Group (Arvonian) of North Wales., 1984, Proceedings of the Geologists Association, 95, 313-321 The basal rocks of the Minfordd Formation contain clasts of granite, vein quartz, quartzitic sandstone, quartzose schists and were thought to be Cambrian basal conglomerate. The Bangor Formation overlies the Minfordd Formation and it underlies the Llanberis Slates and is a similar lithology to the Minfordd Formation excepting the basal conglomerate.P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd EdA. J. Reedman, B. E. Leveridge, R. B. Evans, The Arfon Group (Arvonian) of North Wales., 1984, Proceedings of the Geologists Association, 95, 313-321
Fachwen Formation