The BGS Lexicon of Named Rock Units — Result Details
Blaen Onnen Oolite Formation
| Computer Code: | BLO | Preferred Map Code: | notEntered |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Courceyan Substage (CF) — Courceyan Substage (CF) | ||
| Lithological Description: | Massive medium-grained, ooidal grainstone and crinoidal dolostone. | ||
| Definition of Lower Boundary: | In the Clydach Valley, the base is a sharp, palaeokarstic surface cut in dolostones of the Pantydarren Formation, overlain by basal ooidal grainstones of the Blaen Onnen Oolite Formation. At the type locality, the base is placed at the base of massive ooidal grainstones that overlie a prominent 0.3 to 0.5m-thick rubbly bed of radiating fibrous calcite nodules. | ||
| Definition of Upper Boundary: | In the Clydach Valley the top is a sharp junction, where fine-grained dolostones of the Coed Ffyddlwn Formation overlie the topmost ooidal grainstones of the Blaen Onnen Oolite Formation. At the type locality, the top is an irregular palaeokarstic surface unconformably overlain by green clay forming the base of the Llanelly Formation. | ||
| Thickness: | To 14m | ||
| Geographical Limits: | Northeast crop of the South Wales Coalfield; its correlation with parts of the Abercriban Oolite Group in the Merthyr Tydfil district is unclear, and thus its known limits extend from Cwar yr Hendre [SO 090 146] east to Craig-y-Gaer [SO 2290 1263]. To the south of the Clydach Valley it becomes dolomitized, but it is present at Craig Quarry [SO 2723 0738] and may be crinoidal dolostone at Gallowsgreen Quarry, Varteg Hill [SO 2662 0677]. | ||
| Parent Unit: | Clydach Valley Subgroup (CLD) | ||
| Previous Name(s): | Middle Oolite [Obsolete Name and Code: Use BLO]
(-3227)
Bed 4 Oolite Group [Obsolete Name and Code: Use BLO] (-3858) Blaen Onnen Oolite [Obsolete Code: Use BLO] (BLOO) |
||
| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Type Section | Blaen Onneu Quarry. 2km south of Llangyndr, Powys, where the Formation is a maximum of 14m thick; it rests sharply on a 0.3 to 0.5m-thick bed of calcite nodules and has a spectacular pinnacled palaeokarstic top blanketed by a 5m-thick rubbly palaeosol. Barclay 1989. Dickson and Wright, 1993. See Barclay, 1989, for explanation of different spellings (Blaen Onneu and Blaen Onnen). | ||
| Reference(s): | |||
| George, T N, Johnson, G A L, Mitchell, M, Prentice, J E, Ramsbottam, W H C, Sevastopulo, G D and Wilson, R B. 1976. A correlation of the Dinantian rocks of the British Isles. Special Report of the Geological Society of London, No 7. | |||
| Dickson, J A D and Wright, V P. 1993. Carboniferous Limestone of the north crop of the South Wales Coalfield. 331-349 in Woodcock, N H and Bassett, M G (editors), Geological excursions in Powys, central Wales. [Cardiff: University of Wales Press, National Museum of Wales.] | |||
| Owen, T R, Rhodes, F T, Jones, D G and Kelling, G. 1965. Summer (1964) field meeting in south Wales. Proceedings of the Geologists' Association, Vol.76, 463-495. | |||
| Barclay, W J, Jackson, D I, Mitchell, M, Owen, B, Riley, N J, White, D E, Strong, G E, and Monkhouse, R A. 1989. Geology of the South Wales Coalfield, Part II, the country around Abergavenny. Memoir of the British Geological Survey. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E232 | |||