The BGS Lexicon of Named Rock Units — Result Details

Oystermouth Formation

Computer Code: OB Preferred Map Code: notEntered
Status Code: Full
Age range: Brigantian Substage (CX) — Brigantian Substage (CX)
Lithological Description: Thin- to medium-bedded dark grey argillaceous limestones and mudstones. Chert nodules and beds, and silicified limestones common. Interbedded argillaceous limestones and mudstones commonly deeply weathered to decalcified rottenstone and pale grey, white and yellow clay. In western Gower and south Pembrokeshire, units of medium- to thick-bedded skeletal packstones and oolitic limestone with cherts occur in the lower part of the Formation.
Definition of Lower Boundary: Taken at the palaeokarstic surface seperating the thick-bedded mottled and pseudobrecciated skeletal packstones of the Oxwich Head Limestone Formation and the overlying thin- to medium-bedded argillaceous limestones, cherts and mudstones of the Formation.
Definition of Upper Boundary: Taken at the top of the last limestone in the gradational junction between the interbedded limestones and mudstones of the Formation and the overlying thin-bedded banded cherts and mudstones and silaceous mudstones of the Aberkenfig Formation.
Thickness: 61m on the Gower, thinning northwards to a feather edge on the northwestern crop of the South Wales Coalfield.
Geographical Limits: South Pembrokeshire [SM 99 01], Gower [SS 50 90], northwestern crop of the South Wales Coalfield [SN 63 17], and western Vale of Glamorgan [SS 80 80].
Parent Unit: Pembroke Limestone Group (PEMB)
Previous Name(s): Bullslaughter Limestone Formation [Obsolete Name And Code: Use OB] (BULL)
Rottonstone Beds (-431)
Oystermouth Beds (-1736)
Black Lias (-4820)
Upper Limestone Shales [Obsolete Name And Code: Use OB] (ULSH)
Upper Limestone Shales (*320)
Alternative Name(s): Upper Limestone Shales
Stratotypes:
Partial Type Section  Oystermouth Quarry, immediately east of Oystermouth Castle, Oystermouth, Swansea, Glamorganshire. 25m of thin bedded limestones and mudstones from the middle part of the Formation. Lower contact with the underlying Oxwich Head Limestone Formation and upper contact with the Aberkenfig Formation not seen. 
Reference(s):
Dixon, E E L and Vaughan, A. 1912. The Carboniferous succession in Gower [Glamorganshire]. Quarterly Journal of the Geological Society of London, Vol.67, 477-571. 
Geological Survey of Great Britain. 1901. Summary of Progress for 1901, p.41. 
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. 
Strahan, A. 1907. The geology of the South Wales Coalfield, Part VIII. The country around Swansea. Memoir of the Geological Survey. England and Wales. 
Waters, C N, Waters, R A, Barclay, W J, and Davies, J R. 2009. Lithostratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/09/01. 184pp. 
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. 
Institute of Geological Sciences. 1973. Swansea, England and Wales sheet 247. Solid Geology, 1:63,360. [Southampton: Ordnance Survey]. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E262 E246 E247