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Revision as of 08:47, 18 May 2012 by Russell corbyn (talk | contribs)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)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 Terrane, Fenland Terrane, Wrekin Terrane and the Monian Composite Terrane. The Cymru Terrane is bounded to the northwest by the Menai Strait Fault System and to the southeast by the Ponteford Lineament. The geological terrane to the west is the Cymru Terrane and to the east is Charnwood Terrane. The majority of rocks in the area are associated with the outcrops that are evident at the faulted boundaries.
The Proterozoic rocks of the Cymru Terrane are typified in North Wales by the Arfon Group, Sarn Group and the Twt Hill Granite. The former is in the Bangor area (S & SW) with the latter two located on the Llyn 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).
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 604.7Ma+/-1.6Ma from the tuffs and 572.5Ma+/-1.2Ma from the Fachwen Formation. Information on the basement geology is somewhat sparse with no zircons noted to be older than 617 to 638Ma. However, there are outcrops of the Parwyd Gneiss which are retrogressed granitoid gneiss and garnet amphibolite that occur with the Llyn Shear Zone. 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 1350Ma which is in the same temporal region as the Sarn Granite
Intrusive Geology
In the southwest of Wales is the St David’s Granophyre the stratigraphical relationship of which was subject of heated debates towards the end of the 19th Century predominantly between Sir Archibald Geike and Henry Hicks. Geike (then director of the Geological Survey) insisted upon the Pebidian Supergroup and the high-level intrusion being placed in the Cambrian sequence with Hicks 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.
Sedimentary Geology
Palaeontology
References
- ^ P. J. Brenchley, P. F. Rawson The Geology of England and Wales, 2006, 2nd Ed
- Pharaoh, T.C., Gibbons, W., Precambrian Rocks in England and Wales south of the Menai Straight Fault System, 1987 A Revised Correlation of the Precambrian Rocks in the British Isles
- J. N. Carney, J. M. Horak, et al., Precambrian Rocks of England and Wales, Joint Nature Conservation Committee. Geological Conservation Review Series 20
- P.M. Allen, A.A. Jackson Bryn-Teg Borehole, North Wales., 1978, Bulletin of the Geological Survey of Great Britain, 61 1-52
- ^ 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.
- H. Hicks On the Precambrian (Dimentian and Pebidian) rocks of St David’s, 1877, Quarterly Journal of the Geological Society of London, 33, 199-241
- H. 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
External links
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