The BGS Lexicon of Named Rock Units — Result Details

Friars Point Limestone Formation

Computer Code: FPL Preferred Map Code: FPL
Status Code: Full
Age range: Courceyan Substage (CF) — Chadian Substage (CI)
Lithological Description: Thin- to thick-bedded, dark grey to black, foetid, fine- to coarse-grained skeletal [mainly crinoid] packstones with subordinate thin beds of shaly argillaceous skeletal packstones and mudstone. Widespread burrowing and a lack of tractional structures characterises all but the lowermost part of the formation. The upper part of the formation is dolomitised in the south, while along the southern and eastern crop of the South Wales Coalfield the entire Formation is dolomitised. Formation deposited in a mid ramp enviroment.
Definition of Lower Boundary: In the Barry area, the base of the Formation is taken at the sharp contact between the crinoidal grainstones of the Yorke Rock Bed that caps the Barry Harbour Limestone Formation, and the overlying thin- to medium-bedded, fine- to coarse-grained, skeletal packstones with scattered thin beds and partings of mudstone of the Friars Point Limestone Formation. North of Barry, the base of the Formation is taken at the sharp junction of the oolitic grainstones of the Brofiscin Oolite Formation and the overlying thin- to medium-bedded skeletal packstones of the Friars Point Limestone Formation. Along the southeast and east crop of the South Wales Coalfield the base is taken at the sharp junction between the dolomitized oolite of the Brofiscin Oolite Formation and the very fine- to fine-grained dolomitised packstones of the Friars Point Limestone Formation.
Definition of Upper Boundary: The top of the formation is taken at the sharp contact between the dolomitised packstones of the Formation and the overlying oolitic grainstones or dolomitised oolite of the Gully Oolite Formation. In the south the junction appears conformable but in the north a thin red clay overlying a palaeokarstic surface is locally present; in such cases the top is taken at the top of the red clay.
Thickness: 410m+ in the south of the Vale of Glamorgan, thinning northwards to 76m on the south crop of the South Wales Coalfield.
Geographical Limits: Vale of Glamorgan [ST 00 74]. Present in Gower [SS 50 90], Pembrokeshire [SM 99 01], east crop of the South Wales Coalfield [ST 25 90], Monmouth/Chepstow [ST 53 93] and Bristol/Mendip [ST 50 65] areas but not mapped. Passes northwards in Wales into the Clydach Valley Subgroup.
Parent Unit: Black Rock Limestone Subgroup (BRL)
Previous Name(s): Tears Point Limestone [Obsolete Name And Code: See FPL] (TPLM)
Rudry Formation [Obsolete Name And Code: See FPL] (RDY)
Laminosa Dolomite [Obsolete Name And Code: See FPL] (LAD)
Penmaen Burrows Limestone Formation [Obsolete Name And Code: See FPL And BHL] (PBL)
Black Rock Limestone (-3850)
Lower Dolomite Formation [Obsolete Name And Code: See FPL And BHL] (LD)
Penmaen Burrows Limestone (-3213)
Black Rock Dolomite (-913)
Black Rock Dolomite Formation [Obsolete Name And Code: See BRL] (BRD)
Lower Dolomite (-1985)
Langland Dolomite [Obsolete Name And Code: See FPL] (LGD)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  1:50,000 Sheet 263 (Cardiff). 
Partial Type Section  Coast section at Friars Point, Barry Island, Vale of Glamorgan. Lower three quarters of the Formation [300m] exposed, which consists of thin to thick bedded skeletal packstones with shaly partings. Lower contact with the Barry Harbour Limestone Formation and capping Yorke Rock Bed exposed. About 100m of unexposed skeletal limestones passing up into dolomites is estimated to be present above the top of the exposed section. Upper contact with the overlying Gully Oolite Formation not seen. 
Reference(s):
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. 
Kellaway, G A and Welch, F B A, 1955. The Upper Old Red Sandstone and Lower Carboniferous rocks of the Bristol and the Mendips compared with those of Chepstow and the Forest of Dean. Bulletin of the Geological Survey of Great Britain, No.9, p.1-21. 
Institute of Geological Sciences. 1973. Swansea, England and Wales sheet 247. Solid Geology, 1:63,360. [Southampton: Ordnance Survey]. 
Vaughan, A, 1905. The palaeontological sequence in the Carboniferous Limestone of the Bristol area. Quarterly Journal of the Geological Society of London, Vol.61, 181-307. 
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). 
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). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E263 E262