| Computer Code: |
RBVO |
Preferred Map Code: |
RBV |
| Status Code: |
Full |
| Age range: |
Llanvirn Series (OE)
— Llanvirn Series (OE) |
| Lithological Description: |
Felsic ash-flow tuffs, fine-grained dust tuffs and tuffites with interbedded silty mudstones and, locally, conglomerates, as well as intercalated high-level intrusions and extrusions of both dioritic and rhyolitic composition. |
| Definition of Lower Boundary: |
Conformably overlies or is intruded into undivided mudstone of the Nant Ffrancon Subgroup immediately above the top of the Allt Lwyd Formation, and passes laterally into them. |
| Definition of Upper Boundary: |
Overlain conformably by undivided mudstones of the Nant Ffrancon Subgroup. |
| Thickness: |
To 640m |
| Geographical Limits: |
Southern Snowdonia and northeast of the Harlech Dome, North Wales. |
| Parent Unit: |
Aran Volcanic Group (AVG)
|
| Previous Name(s): |
Rhiw Bach Formation [Obsolete Name and Code: Use RBVO]
(-1786)
|
| Alternative Name(s): |
Serw Formation [Obsolete: use BGL, RBVO]
|
| Stratotypes: |
| Partial Type Section |
Rhiw Bach Quarry, ezast of Blaenau Ffestiniog, Snowdonia, North Wales. Howells, M F and Smith, M. 1997. |
| Reference(s): |
| Zalasiewicz, J A. 1992. Graptolite dating of Ordovician vulcanicity in the Arenig area, North Wales. Geological Journal, Vol.27, 379-389. |
| Rushton, A W A and Howells, M F. 1998. Stratigraphical framework for the Ordovician of Snowdonia and the Lleyn Peninsula. British Geological Survey Research Report, RR/99/08. |
| Howells, M F and Smith, M. 1997. Geology of the country around Snowdon. Memoir of the British Geological Survey, Sheet 119 (England and Wales). (London: The Stationery Office). 104pp. |
| Lynas, B D T, 1973. The Cambrian and Ordovician rocks of the Migneint area, North Wales. Journal of the Geological Society, London, Vol.129, 481-503. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
|
E119
|
