The BGS Lexicon of Named Rock Units — Result Details

Abercriban Oolite Subgroup

Computer Code: ABO Preferred Map Code: notEntered
Status Code: Full
Age range: Courceyan Substage (CF) — Chadian Substage (CI)
Lithological Description: Massive, pale grey, ooidal grainstone with some fine-grained micritic and dolomitic interbeds.
Definition of Lower Boundary: The base is placed at the sharp junction between a thin dolostone underlying the lowermost ooidal grainstone of the Abercriban Oolite Subgroup and the top of the grey mudstones forming the uppermost part of the Cwmynyscoy Mudstone Formation.
Definition of Upper Boundary: The top is placed at a palaeokarstic surface, below which the ooidal grainstone at the top of the Abercriban Oolite Subgroup becomes increasingly disordered upwards and irregular ooidal limestone masses become separated by grey-green mudstone films. The palaeokarstic surface marks a regionally developed unconformity, overlain by micritic to grainstone limestones of the Llanelly Formation.
Thickness: Up to 30m in the Chwar Pant-y-rhiw area [c.SO 200 157].
Geographical Limits: Northeast crop of the South Wales Coalfield, from Carn-yr-onen [SN 887 166] to Mynnyd Llangattock [SO 200 157].
Parent Unit: Pembroke Limestone Group (PEMB)
Previous Name(s): Abercriban Oolite (-629)
Abercriban Oolite Group (-3786)
Oolite Group (Part) (-3787)
Pendine Oolite [Obsolete Name And Code: Use ABO] (PDOL)
Abercriban Oolite Formation (-1942)
Alternative Name(s): Clydach Valley Subgroup
Stratotypes:
Reference Section  Baltic Quarry, Powys. 1km south of Aberdriban Quarry, provides a complete section of the Subgroup. Here, about 28m of beds are exposed in the main quarry face, with the overlying Llanelly Formation exposed on an upper step. Dickson and Wright, 1993. 
Type Area  Clydach Valley - Clydach River, Abergavenny, Wales. 
Type Section  Llammarch Dingle - Clydach Valley, Abergavenny, Wales. 
Partial Type Section  Abercriban Quarry, Powys, where 13.4m of pale grey ooidal grainstones with thin dolostone interbeds are exposed. Base and top of formation are not proved. Barclay et al., 1988. 
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. 
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.] 
Barclay, W J, Taylor, K and Thomas, L P, 1988. Geology of the South Wales Coalfield, Part V, the country around Merthyr Tydfil. Third Edition. Memoir of the British Geological Survey, Sheet 231 (England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E232 E231 E214