The BGS Lexicon of Named Rock Units — Result Details

Oxwich Head Limestone Formation
Computer Code: OHL Preferred Map Code: notEntered
Status Code: Full
Age range: Asbian Age (CR) — Brigantian Age (CX)
Lithological Description: Thick bedded fine- to coarse-grained, recrystalised, bioturbated skeletal packstones with distinctive pale to dark grey mottling and pseudobrecciation and ooidal limestones. Units of dark grey, irregularly bedded skeletal packstones with shaly partings are developed at intervals. Thin unit of calcareous sandstone and sandy skeletal packstone [Pant Mawr Sandstone Member and Honeycombed Sandstone Member] at base in the Vale of Glamorgan and northwest crop of the South Wales Coalfield respectively. Unit of ooidal limestone in lower part [Penderyn Oolite Member] on northwest crop of the South Wales Coalfield. Palaeokarstic surfaces, overlain by red and grey clay palaeosols, punctuate the Formation; thin coals developed in palaeosols in Gower.
Definition of Lower Boundary: In the Vale of Glamorgan the base is taken at the sharp contact, defined by a palaeokarstic surface, between the heterolithic lithologies of the underlying Stormy Limestone Formation and the overlying partially dolomitised sandy skeletal packstones and calcareous sandstones of the Pant Mawr Sandstone Member. On the northwestern crop of the South Wales Coalfield, it is taken at the sharp junction of the calcareous sandstone of the basal Honeycombed Sandstone Member and the underlying dark bituminous skeletal/peloidal grainstone/packstone/wackestone of the Dowlais Limestone Formation. In Gower and Pembrokeshire it is taken at the palaeokarstic surface separating the heterolithic lithologies of the underlying Hunts Bay Oolite Subgroup from the mottled and pseudobrecciated skeletal packstones of the Formation. In the Bristol/Mendip area the Formation overlies the Clifton Down Limestone Formation. The base here needs further work but is generally taken at the gradational change from calcite mudstones and ooidal limestones of the Clifton Down Limestone Formation to ooidal and skeletal limestones of the Formation. In the north of the Bristol area the Formation overlies the middle tongue of the Cromhall Sandstone Formation, the base being taken at the incomming of the ooidal and skeletal limestones above the sandstones of the Cromhall Sandstone Formation.
Definition of Upper Boundary: Taken at the palaeokarstic surface separating the thick-bedded mottled and pseudobrecciated skeletal packstones of the Formation and the overlying thin- to medium-bedded argillaceous limestones, cherts and mudstones of the Oystermouth Formation.
Thickness: 125m in the central Vale of Glamorgan thickening southwards to 183m in the Mendip Hills.
Geographical Limits: Western and central Vale of Glamorgan [ST 00 74], Gower [SS 50 90], northwest crop of the South Wales Coalfield [SN 90 13] and Bristol/Mendip Hills area [ST 50 65]. Present but unmapped in south Pembrokeshire [SM 99 01]. Interfingers with and passes northwards into the Cromhall Sandstone Formation in the Bristol area [ST 67 03].
Parent Unit: Pembroke Limestone Group (PEMB)
Previous Name(s): Penwyllt Limestone [Obsolete Name And Code: Use OHL] (PNWL)
Crickmail Limestone [Obsolete Name And Code: Use OHL] (CKL)
Llandyfan Limestone Formation [Obsolete Name And Code: Use OHL] (LLB)
Crickmail Limestone (-434)
Llandyfan Limestone Formation (-2351)
Penwyllt Limestone (-4234)
Hotwells Limestone Formation (-4824)
Mynydd-Y-Garreg Limestone (-4825)
Hotwells Limestone Formation [Obsolete Name And Code: Use OHL] (HL)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Cliff and foreshore section on eastern and southern side of Oxwich Head, Gower. Most of the Formation is present apart from the uppermost part. It mainlycomprises thick bedded pseudobrecciated skeletal packstones with prominent scattered palaeosols. The Pant Mawr Sandstone Member is not developed here. The contct with the underlying Hunts bay Oolite Subgroup is exposed but the contact with the overlying Oystermouth Formation is not seem. 
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. 
Institute of Geological Sciences. 1973. Swansea, England and Wales sheet 247. Solid Geology, 1:63,360. [Southampton: Ordnance Survey]. 
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. 1977. Carmarthen, England and Wales sheet 230. Solid Geology, 1:50 000. [Southampton: Ordnance Survey]. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E262 E247 E246 E230
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