The BGS Lexicon of Named Rock Units — Result Details

Cwmyniscoy Mudstone Formation

Computer Code: CCM Preferred Map Code: notEntered
Status Code: Full
Age range: Courceyan Substage (CF) — Courceyan Substage (CF)
Lithological Description: Interbedded grey mudstones and thin- to medium-bedded skeletal packstones deposited in a mid-shelf environment.
Definition of Lower Boundary: Where back barrier mudstones, calcite mudstones and calcisiltites occupy the uppermost part of the underlying Castell Coch Limestone Formation, the base of the Cwmyniscoy Mudstone Formation is taken at the sharp incoming of marine, interbedded grey mudstones and packstones. The contact is commonly erosional and overlain by a packstone or grainstone bed rich in intraclasts of the underlying back barrier mudstones. Where the uppermost part of the Castell Coch Limestone Formation comprises barrier grainstones, the base of the Cwmyniscoy Mudstone Formation is taken at the sharp incoming of interbedded grey mudstones and packstones.
Definition of Upper Boundary: The contact of the Cwmyniscoy Mudstone Formation with the overlying Barry Harbour Limestone Formation is gradational. In the uppermost part of the Cwmyniscoy Mudstone Formation, the proportion of packstone beds increases to exceed that of mudstone. The top of the Cwmyniscoy Mudstone Formation is taken at the top of the last significant mudstone bed above which the succession becomes dominated by stacked fine and coarse skeletal packstones with scattered thin beds and partings of calcareous mudstone. In the north crop of the South Wakes Coalfield, the Cwmyniscoy Mudstone Formation is overlain by the Sychnant Dolomite Formation or the Pwll-y-cwm Oolite Formation. The junction with the Sychnant Dolomite Formation is gradational. Thin beds of calcite mudstone and dolomitic limestone appear in the uppermost part of the Cwmyniscoy Mudstone Formation mudstone/packstone sequence. The top of the formation is taken at the incoming of abundant, thin-bedded, fine-grained, dolomitised skeletal packstones of the Sychnant Dolomite Formation. The contact with the Pwll-y-cwm Oolite Formation is sharp and taken at the incoming of thick-bedded pale grey oolitic grainstone above the interbedded mudstones and packstones of the Cwmyniscoy Mudstone Formation.
Thickness: 60m in the south of the Vale of Glamorgan, thinning northwards to 10m on the north crop of the South Wales Coalfield [Abergavenny].
Geographical Limits: Vale of Glamorgan [ST 00 74] and South Wales Coalfield [ST 06 81], east of Swansea. Present but unmapped in the east crop of the South Wales Coalfield [ST 25 90], Pembrokeshire [SM 99 01] and Monmouth and Chepstow [ST 53 93] areas.
Parent Unit: Avon Group (AVO)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Quarry, 300m south of Cwmyniscoy, Pontypool. Complete section [29m] through the unit, comprising interbedded mudstones and limestones. Contacts with the underlying Castell Coch Limestone Formation and the overlying, currently undivided, Clydach Valley Subgroup seen. 
Reference(s):
Wilson, D, Davies, J R, Fletcher, C J N, and Smith, M. 1990. The Geology of the South Wales Coalfield, part VI the Country around Bridgend. Memoir of the British Geological Survey, 1:50 000 Geological Sheets 261 and 262 (England and Wales). 
Squirrell, H C and Downing, R A. 1969. Geology of the South Wales Coalfield, Part 1, the country around Newport (Monmouthshire). Memoir of the Geological Survey of Great Britain, Sheet 249 (England and Wales). 
Waters, R A and Lawrence, D J D. 1987. Geology of the South Wales Coalfield - Part III: the country around Cardiff. Memoir of the British Geological Survey, Sheet 263. (England and Wales). 
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:
E263 E262 E232 E214