The BGS Lexicon of Named Rock Units — Result Details

Llanelly Formation

Computer Code: LLY Preferred Map Code: notEntered
Status Code: Full
Age range: Arundian Substage (CJ) — Arundian Substage (CJ)
Lithological Description: Well bedded peritidal, cyclic micritic to grainstone limestones with green clay interbeds at base, overlain by ooidal grainstone and capped by green and red-mottled clay palaeosol.
Definition of Lower Boundary: The base is placed at a major palaeokarstic surface truncating the ooidal grainstones of the underlying Gilwern Oolite Formation and overlain by green clay and micritic limestones (Clydach Halt Member) of the Llanelly Formation.
Definition of Upper Boundary: The top is a sharp unconformity at which the topmost green-red mottled clay (the Gilwern Clay) of the Llanelly Formation is overlain by dark grey limestones of the Dowlais Limestone Formation.
Thickness: 20m at type locality.
Geographical Limits: Northeast crop of the South Wales Coalfield, from the Hepste Valley [SN 971 129] east to the Clydach Gorge [SO 214 125] and southwards down the east crop on the west side of the Afon Lwyd Valley to about 1km south of Blaenavon [SO 261 077].
Parent Unit: Pembroke Limestone Group (PEMB)
Previous Name(s): Whitehead Limestone [Obsolete Name And Code: Use LLY] (WTHL)
Whitehead Limestone (*628)
Calcite Mudstone Group (-1033)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Llanelly Quarry, Clydach Gorge, near Brynmawr, South Wales. The Formation is 20m thick, rests disconformably of the Gilwern Oolite Formation and is overlain unconformably by the Dowlais Limestone Formation. Barclay, 1989. 
Reference(s):
Barclay, W J, Jackson, D I, Mitchell, M, Owen, B, Riley, N J, White, D E, Strong, G E and Monkhouse, R A. 1989. Geology of the South Wales Coalfield, Part II, the country around Abergavenny. Memoir of the British Geological Survey. 
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:
E232 E231