The BGS Lexicon of Named Rock Units — Result Details

Black Rock Limestone Subgroup

Computer Code: BRL Preferred Map Code: notEntered
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 packstone and mudstone. Widespread burrowing. Tractional structures and silicified limestones characterise the lowermost part of the Subgroup. Unit of ooidal limestone [Brofiscin Oolite Formation] developed in lower part in Gower, Vale of Glamorgan and east crop of the South Wales Coalfield. The upper part of the Formation is dolomitized in the south, while along the southern and eastern crop of the South Wales Coalfield and in the Monmouth and Chepstow area, the entire Formation is dolomitized. Tuffs developed in the lower part of the Subgroup in the Weston super Mare areas [Middle Hope Volcanic Member]. Subgroup represents deposition in an inner to mid ramp setting with local ooid shoal development.
Definition of Lower Boundary: Conformable and taken at the gradational incoming of dark grey fine- to coarse-grained skeletal packstones with scattered mudstone partings of the Subgroup above the interbedded mudstones and skeletal packstones [Cwymyniscoy Mudstone Formation] of the Avon Group.
Definition of Upper Boundary: The top of the Subgroup is taken at the sharp contact between the dolomitised packstones of the Subgroup 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. In the eastern Mendip Hills it is taken at the incoming of pale grey coarse crinoidal limestones of the Vallis Limestone Formation, above the dolomitised packstones of the underlying Black Rock Limestone Subgroup. Little is known about this contact, which needs further investigation. In the Bristol area the top is taken at the junction of the sparsely crinoidal dolomites of the Black Rock Subgroup and the crinoidal limestones of the overlying Sub-Oolite Bed of the Gully Oolite Formation. Little is known about this contact, which needs further investigation.
Thickness: 500m in the southern Vale of Glamorgan, thinning northwards to 70m in the Monmouth area. 365m in the eastern Mendip Hills, thinning northwards to 158m at Bristol and 104m to the north of Bristol at Cromhall.
Geographical Limits: South Pembrokshire [SM 19 90], Gower [SS 50 90], southeast crop of the South Wales Coalfield [ST 17 85], Vale of Glamorgan [ST 00 74], Monmouth and Chepstow [ST 53 93], and Bristol [ST 60 74], areas and the Mendip Hills [ST 50 53]. In Wales, passes northwards into the Clydach Valley Subgroup.
Parent Unit: Pembroke Limestone Group (PEMB)
Alternative Name(s): none recorded or not applicable
Partial Type Section  Black Rocks Quarry and crags, immediately west of Durham Down, on the north side of the Avon Gorge, Bristol. 
Type Section  Burrington Coombe (east and west Twin Valleys), Somerset. 
Waters, C N, Brown, M A E, Dean, M T and Powell, J H. In press. Lithostratigraphical framework for Carboniferous successions of Great Britain (Onshore). British Geological Survey Research Report, RR/05/06. 
Kellaway, G A and Welch, F B A. 1993. Geology of the Bristol district. Memoir of the British Geological Survey Special Sheet [England and Wales]. 
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. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E263 E262 E280 E281 E264 E279 E265 E247 E250 E251