The BGS Lexicon of Named Rock Units — Result Details

High Tor Limestone Formation

Computer Code: HTL Preferred Map Code: notEntered
Status Code: Full
Age range: Arundian Substage (CJ) — Arundian Substage (CJ)
Lithological Description: Mid grey, predominantly thick-bedded, fine- to coarse-grained, burrowed, skeletal packstones, with thin beds and partings of shaly dolomite mudstone and siltstone. Some thin-bedded dark grey bituminous skeletal packstones interbedded with thin argallaceous wackestones and mudstones. Coarse, locally cross bedded, peloidal/skeletal/oolitic packstones/grainstones developed in lower and upper parts in northern sections. On the southeast crop of the South Wales Coalfield the Formation is predominantly dolomitized.
Definition of Lower Boundary: The base is taken at the sharp erosional contact between the thin-bedded calcite mudstones and the mudstones of the underlying Caswell Bay Mudstone Formation and the overlying coarse peloidal/skeletal/oolitic packstone/grainstones forming the lower part of the Formation in South Wales and the Weston-super-Mare areas. A basaltic lava is present in the lower part of the sequence in the Weston-super-Mare area. In the southern parts of the Welsh outcrop, the Formation overlies the Gully Oolite Formation. Here the base is taken at the sharp erosional contact between the oolitic grainstones of the Gully Oolite Formation and the overlying coarse skeletal packstone/grainstones of the Formation.
Definition of Upper Boundary: The top of the formation is gradational and taken at the incoming of a predominantly oolitic grainstone sequence [Hunts Bay Oolite Subgroup] above the skeletal packstones or skeletal/pelioidal/oolitic packstone/grainstones of the Formation.
Thickness: 55m on the south crop of the South Wales Coalfield, thickening southwards to 133m in west/central Vale of Glamorgan.
Geographical Limits: Gower [SS 50 90], Vale of Glamorgan [ST 00 74] and the Weston-super-Mare [ST 32 60] areas. Present but not mapped in the Tenby [SN 13 01] and Pembroke [SM 97 03] areas. Passes northwards into the Llanelly Formation in Wales and into the Clifton Down Mudstone in the Bristol/Broadfield Down [ST 50 650 area. Passes eastwards into the Burrington Oolite Subgroup in the Mendip Hills [ST 50 54].
Parent Unit: Pembroke Limestone Group (PEMB)
Previous Name(s): Birnbeck Limestone Formation [Obsolete Name And Code: Use HTL] (BBL)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Sea cliffs and foreshore reefs below High Tor, Southgate, Gower. Unit is completely exposed apart from a few minor gaps and comprises mainly skeletal limestones. Contacts with the underlying Casewell Bay Mudstone Formation and the overlying currently undivided Hunts Bay Oolite Subgroup are seen. 
Reference(s):
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. 
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). 
Institute of Geological Sciences. 1973. Swansea, England and Wales sheet 247. Solid Geology, 1:63,360. [Southampton: Ordnance Survey]. 
Ramsey, A T S, 1987. Depositional enviroments of Dinantian limestones in Gower, South Wales. 265-308 in European Dinantian Environments. Millar, J, Adams, A E and Wright, V P (editors). [Chichester: John Wiley and Sons.] 
Whittaker A and Green G W. 1983. Geology of the country around Weston-super-Mare. Memoir of the Geological Survey, England and Wales, Sheet 279 with parts of sheets 263 and 295. 
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). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E263 E262 E247 E246