The BGS Lexicon of Named Rock Units — Result Details
Offrwm Volcanic Formation
| Computer Code: | OF | Preferred Map Code: | notEntered |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Llanvirn Series (OE) — Llanvirn Series (OE) | ||
| Lithological Description: | Thick- to thin-bedded, locally welded, felsic ash-flow tuffs, subordinate rhyolitic lavas and dark grey mudstones. | ||
| Definition of Lower Boundary: | Conformably overlies the siltstones and sandstones of the Allt Lwyd Formation or the conglomerates of its highest subdivision, the Aran Boulder Bed Member in the Harlech area and western part of the Cadair Idris district, but is disconformable on the Dol-cyn-afon, Dolgellau and Ffestiniog Flags formations in the south and east. | ||
| Definition of Upper Boundary: | Conformably overlain by the Cregennen Formation (locally the Bryn Brith Member) over much of the Cadair Idris district, but unconformably overlain by the Ty'r Gawen Mudstone Formation (Fron Newydd Member) in the southwest. In the Harlech district it is variably overlain by the Cregennen, Melau and Benglog formations, with possible unconformity/disconformity. | ||
| Thickness: | 0 to 290 m. | ||
| Geographical Limits: | Cadair Idris and Harlech districts, North Wales. | ||
| Parent Unit: | Aran Volcanic Group (AVG) | ||
| Previous Name(s): | Lower Acid Volcanic Group
(LAV)
Beacon Hill Acid Group [Obsolete Name and Code: Use OF] (-3593) Lower Acid Volcanic Group (LAV) Mynydd-Y-Gader Group [Obsolete Name and Code: Use MGV, OF] (-1064) Lower Acid Group [Obsolete Name and Code: Use OF] (-432) Upper Volcanic Series [Obsolete Name and Code: Use OF] (-1101) Rhyolitic Series and Ashy Series [Obsolete Name and Code: Use OF] (-4823) Mynydd Gader Volcanic Series [Obsolete Name and Code: Use OF] (-4822) |
||
| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Type Section | Crags on the slopes around Foel Offrwm, North Wales. Allen and Jackson, 1985. | ||
| Reference(s): | |||
| Cox, A H, and Wells, A K. 1916. The Ordovician sequence in the Cader Idris district (Merioneth). British Association for the Advancement of Science Report for 1915 (Manchester), p.424. | |||
| Cox, A H, and Wells, A K. 1921. The Lower Palaeozoic rocks of the Arthog-Dolgellau district (Merionethshire). Quarterly Journal of the Geological Society, London, Vol.76, p. 254-324. | |||
| Pratt, W T, Woodhall, D G and Howells, M F. 1995. Geology of the country around Cadair Idris. Memoir of the British Geological Survey, Sheet 149. (England and Wales). (London: HMSO), 111pp | |||
| Allen, P M, and Jackson, A A. 1985. Geology of the country around Harlech. Memoir of the British Geological Survey, Sheet 135 with part of Sheet 149 (England and Wales). (London: HMSO.) 112 pp. | |||
| Jones, B, 1933. The geology of the Fairbourne-Llwyngwril district, Merioneth. Quarterly Journal of the Geological Society, Vol.89, 145-171. | |||
| Lake, P and Reynolds, S H. 1896. The Lingula Flags and igneous rocks of the neighbourhood of Dolgelly. Quarterly Journal of the Geological Society of London, Vol.52, 511-527. | |||
| Waters, C N, Smith, K, Hopson, P M, Wilson, D, Bridge, D M, Carney, J N, Cooper, A H, Crofts, R G, Ellison, R A, Mathers, S J, Moorlock, B S P, Scrivener, R C, McMillan, A A, Ambrose, K, Barclay, W J, and Barron, A J M. 2007. Stratigraphical Chart of the United Kingdom: Southern Britain. British Geological Survey, 1 poster. | |||
| Cox, A H. 1925. The geology of the Cadair Idris Range, (Merioneth). Quarterly Journal of the Geological Society of London, Vol.81, p.539-592. | |||
| Cox, A H, and Wells, A K. 1927. The geology of the Dolgelley district, Merionethshire. Proceedings of the Geologists' Association, Vol.38, 265-318. | |||
| Davies, R G, 1959. The Cader Idris Granophyre and its associated rocks. Quarterly Journal of the Geological Society, London. Vol.115, 189-216. | |||
| Lake, P and Reynolds, S H, 1912. The geology of Mynydd-y-Gader, Dolgelly. Quarterly Journal of the Geological Society of London, Vol.68, 345-363. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E149 E135 | |||