The BGS Lexicon of Named Rock Units — Result Details

Hunts Bay Oolite Subgroup

Computer Code: HBO Preferred Map Code: notEntered
Status Code: Full
Age range: Arundian Substage (CJ) — Holkerian Substage (CQ)
Lithological Description: Mainly ooidal limestones, with subordinate skeletal, peloidal and oncolitic limestones and calcite mudstones. Mainly dolomitised on the southeast and east crop of the South Wales Coalfield.
Definition of Lower Boundary: The base of the Subgroup is gradational and taken at the incoming of a predominantly oolitic grainstone sequence, locally with units of calcite mudstone, above the skeletal packstones or skeletal/peloidal/oolitic packstone/grainstones of the underlying High Tor Limestone Formation.
Definition of Upper Boundary: In the Vale of Glamorgan the top is taken at the sharp contact, defined by a palaeokarstic surface, between the heterolithic lithologies of the Subgroup and the overlying partially dolomitised sandy skeletal packstones and the calcareous sandstones of the Pant Mawr Sandstone Member [Oxwich Head Limestone Formation]. In the Gower and Pembrokeshire it is taken at the palaeokarstic surface separating the heterolithic lithologies of the Subgroup from the mottled and pseudobrecciated skeletal packstones of the overlying Oxwich Head Limestone Formation.
Thickness: 245m in central Vale of Glamorgan thinning northwards to a feather edge in the Forest of Dean.
Geographical Limits: Pembroke [SM 97 03], Tenby [SN 13 01], Gower [SS 50 90], Vale of Glamorgan [ST 00 74], east crop of the South Wales Coalfield [ST 17 85] and Monmouth and Chepstow [ST 53 93] areas. Passes eastwards into the Clifton Down Limestone Formation of south Pembrokeshire [SM 99 01] and northwards in South Wales into the Dowlais Limestone Formation.
Parent Unit: Pembroke Limestone Group (PEMB)
Previous Name(s): Drybrook Limestone (-874)
Hunts Bay Group (-3459)
Hunts Bay Oolite Formation (-3460)
Clifton Down Group [Obsolete Name And Code: See PEMB] (CDGP)
Drybrook Limestone Formation [Obsolete Name And Code: Use HBO] (DL)
Hunts Bay Oolite Group (-4087)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Cliffs and foreshore reefs between Bacon Hole and Pwll ddu Head, Southgate, Gower, South Wales. Entire unit present apart from a few minor exposure gaps and comprises a lower oolitic dominated unit [Cornelly Oolite Formation] and an upper heterolithic unit of peritidal limestones (Stormy Limestone Formation). Contacts with the underlying High Tor Limestone and overlying Oxwich Head Limestone seen. 
Reference(s):
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. 
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, 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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E263 E262 E247 E246