The BGS Lexicon of Named Rock Units — Result Details
Clydach Valley Subgroup
| Computer Code: | CLD | Preferred Map Code: | notEntered |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Courceyan Substage (CF) — Chadian Substage (CI) | ||
| Lithological Description: | Massive ooidal grainstones with interbedded fine-grained, thinly bedded dolostones. | ||
| Definition of Lower Boundary: | The base is placed at the base of the lowest bed of fine-grained dolostone (of the Sychnant Dolomite Formation), where there is a conformable, interdigitating gradation from the mudstones of the underlying Cwmynyscoy Mudstone Formation. | ||
| Definition of Upper Boundary: | The top is a major, regionally developed palaeokarstic surface that truncates the massive, pale grey ooidal grainstone of the Gilwern Oolite Formation. It is unconformably overlain by grey, micritic limestones and green clays (Clydach Halt Member) at the base of the Llanelly Formation, which comprises mainly well-bedded, cyclical micrite to grainstone limestones with green clay interbeds. | ||
| Thickness: | 20m in the type area of the Clydach valley [SO 225 125] to 60m near Pontypool [SO 289 015]. | ||
| Geographical Limits: | In a narrow outcrop around the northeast and east crops of theSouth Wales Coalfield, extending from about 2km north of the Clydach valley [SO 219 142] south to the Pontypool area. [c.ST 280 985]. | ||
| Parent Unit: | Pembroke Limestone Group (PEMB) | ||
| Previous Name(s): | Clydach Beds [Obsolete Name and Code: Use DDLM, CLD]
(-1440)
Oolite Group (OGP) Oolite Group (OGP) |
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| Alternative Name(s): | Abercriban Oolite Subgroup |
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| Stratotypes: | |||
| Type Area | Clydach Valley, near Brynmawr, Monmouthshire, South Wales, in which the Subgroup is exposed in numerous quarries on both sides of the valley and in the river gorge. Barclay, 1989 for details. | ||
| Reference Section | Roadside exposures at Blackrock on the north side of the Clydach valley provides an easily accessible, well exposed section through the lower part of the Subgroup. The Pwll-y-Cwm Oolite, Pantydarren and Blaen Onnen Oolite formations are exposed. Barclay, 1989. | ||
| Reference(s): | |||
| George, T N. 1954. Pre-Seminulan Main Limestone of the Avonian Series in Breconshire. Quarterly Journal of the Geological Society of London, Vol.110, 282-322. | |||
| 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 | |||