The BGS Lexicon of Named Rock Units — Result Details

Aberkenfig Formation

Computer Code: ABKF Preferred Map Code: notEntered
Status Code: Full
Age range: Pendleian Substage (CE) — Arnsbergian Substage (CG)
Lithological Description: Dark grey fissile mudstones with thin bedded radiolarian cherts and siliceous mudstones.
Definition of Lower Boundary: The base is a gradational boundary with white-weathering, black, fine-grained limestones and interbedded black mudstones of the underlying Oystermouth Formation passing upwards into the thin-bedded radiolarian cherts and mudstones of the Aberkenfig Formation. In the type section of the Kenfig Borehole, the base is at 48.46m depth.
Definition of Upper Boundary: The top is taken at the top of highest chert or siliceous mudstone of the Formation, where it is overlain by the lowest quartzite of the Twrch Sandstone Formation. In the partial type section of the Kenfig Borehole, the top is not seen, the Formation being unconformably overlain by marl and red clay of the Mercia Mudstone Group at 12.8m depth.
Thickness: To c.35m
Geographical Limits: Parts of the northern and southern peripheries of the South Wales Coldfield:- on the north crop in the Ammanford district, from Gam [c.SN 540 139] east, to east of Llandybie [c.SN 645 173]; on the south crop, from west Gower [c.SS 430 945] (not exposed) east to Swansea Bay, and east of Swansea Bay to the Sarn Park area [c.SS 920 830].
Parent Unit: Marros Group (MARR)
Previous Name(s): Plastic Clay Beds (-3)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Kenfig No.2B Borehole (SS 88SW/81), near North Cornelly, Porthcawl, south Wales; proves the Formation from 13.46 to 48.46m depth, but the top is not preserved beneath unconformably overlying Triassic rocks. Wilson et al, 1990. 
Reference Section  Sarn Park M4 Motorway cutting, where the formation comprises about 15m thickness of black fissile mudstones and cherts. The lowermost beds contain pebbly lags indicating a non-sequence at the base, as confirmed by a mid- to late Arnsbergian fauna in the basal lag, overlying argillaceous limestones and mudstones of the Brigantian Oystermouth Formation. Wilson et al. 1990. 
Reference(s):
Strahan, A, Cantrill, T C, Dixon, E E L and Thomas, H H. 1907. Geology of the South Wales Coalfield, Part VII, the country around Ammanford. Memoir of the Geological Survey of England and Wales, Sheet 230 (England and Wales). (London: HMSO.) 
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). 
Waters, C N, Barclay, W J, Davies, J R and Waters, R A. In press. Stratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/05/06. 
Ware, W D. 1939. The Millstone Grit of Carmarthenshire. Proceedings of the Geologists' Association, Vol.50, 168-204. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable