The BGS Lexicon of Named Rock Units — Result Details

Avon Group

Computer Code: AVO Preferred Map Code: notEntered
Status Code: Full
Age range: Courceyan Substage (CF) — Courceyan Substage (CF)
Lithological Description: Interbedded grey mudstones and thin- to medium-bedded skeletal packstones with one to several thick units of ooidal and skeletal grainstones. Thin units of calcite mudstone and mudstone locally present. Sparse thin ironstones. Represents mid to inner shelf/ramp deposits with coeval barrier, back barrier and coastal plain sediments.
Definition of Lower Boundary: Conformable in southern settings, where it is taken at the sharp to gradational incoming of grey calcite mudstones, mudstones and limestones of the Group, above the red sandstones, siltstones and mudstones of the Upper Old Red Sandstone [Quartz Conglomerate Group, Tintern Sandstone Formation and Portishead Formation]. Boundary sharp in the Vale of Glamorgan and southeast crop of the South Wales Coalfield, where it is taken at the base of the Tongwynlais Formation. Elsewhere, boundary reported to be gradational at base of Shirehampton Formation or Avon Group undivided. Such boundaries are in urgent need of revision, especially that in South Pembrokeshire, where the uppermost part of the Skrinkle Sandstones is of similar facies to the Shirehampton Formation. In the Newport area of South Wales and the Bristol district the lower part of the Avon Group this has mistakenly been included in the Upper Old Red Sandstone formations on BGS maps. Disconformable along the north crop of the South Wales Coalfield, and Monmouth and Chepstow area where it is taken at the sharp erosive base of the transgressive skeletal and ooidal grainstones [Castell Coch Limestone Formation] of the group above the sandstones of the underlying Quartz Conglomerate Group, Tintern Sandstone Formation, Plateau Beds Formation, Grey Grits Formation and Brownstones Formation.
Definition of Upper Boundary: Gradational in southern settings, and taken at the incoming of stacked dark grey fine to coarse skeletal packstones with scattered mudstone partings [Barry Harbour Limestone Formation] of the Black Rock Limestone Subgroup above the interbedded mudstones and skeletal packstones [Cwmyniscoy Mudstone Formation] of the Avon Group. Boundary taken at the top of the last significant bed of grey mudstone. Along the eastern part of the north crop of the South Wales Coalfield, taken at the gradational incoming of the stacked dolomitised skeletal packstones [Sychnant Dolomite Formation] of the Clydach Valley Subgroup above the interbedded mudstones and skeletal packstones of the Avon Group. Along the remainder of the crop, the Group is overlain unconformably by the Dowlais Limestone Formation.
Thickness: 156m in the south Mendip Hills, thinning northwards to 96m in the Bristol area, 54m in the Monmouth area. 140m in the southern Vale of Glamorgan, thinning northwards to 10m on the northwest crop [Ammanford area] of the South Wales Coalfield.
Geographical Limits: South Wales [SS 90 95], Monmouth and Chepstow [ST 53 94], Bristol [ST 60 74], the Mendip Hills [ST 50 53] and Clee Hill [Shropshire] [SO 60 74] areas. Passes southwards into the Pilton Shales Formation of North Somerset [ST 00 20].
Parent Unit: Carboniferous Limestone Supergroup (CL)
Previous Name(s): Lower Limestone Shale Group [Obsolete Name And Code: Use AVO] (LSH)
Cefn Bryn Shales [Obsolete Name And Code: Use AVO] (CBSH)
Cefn Bryn Shales (*674)
Maesbury Mudstone Formation [Obsolete Name And Code: Use AVO] (MBYM)
Lower Limestone Shale (-1320)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Avon gorge, Bristol; various sections along the river and Portway Road section. Reported by Kellaway and Welch [1993] to have deteriorated over the years and to comprise only the upper part of the Shirehampton Formation, including the Bryozoa Bed. No upper or lower contacts seen. 
Reference(s):
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]. 
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. 
Barton, C M, Strange, P J, Royse, K R and Farrant, A R. 2002. Geology of the Bristol District - a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 Sheet 264 Bristol [England and Wales]. 
Buckland, W and Conybeare, W D, 1822. Observations on the south-western coal district of England. Transactions of the Geological Society, Vol.1, p.222. 
George, T N. 1939. The Cefn Bryn Shales of Gower. Geological Magazine, Vol.76, 1-6. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable