The BGS Lexicon of Named Rock Units — Result Details

Gilwern Clay Member

Computer Code: GWCL Preferred Map Code: notEntered
Status Code: Full
Age range: Arundian Substage (CJ) — Arundian Substage (CJ)
Lithological Description: Green and red-brown and purple mottled clay with limestone ("calcrete") nodules and, locally, a grey rootlet bed (seatearth) and a thin coal at the top.
Definition of Lower Boundary: The base is placed at the karstified top of the underlying Penllwyn Oolite Member, where the basal green clay of the Gilwern Clay Member drapes the irregular surface of the Penllwyn Oolite Member.
Definition of Upper Boundary: The top is placed at the sharp, disconformable junction between the topmost clay (a seatearth at Llanelly Quarry) and the overlying sandy limestone forming the base of the Dowlais Limestone Formation. Where the Garn Caws Sandstone is present, the top is placed at the irregular scoured surface where the topmost clays of the member are erosively overlain by the Garn Caws Sandstone Formation [note that Wright (1981) included this sandstone in the Gilwern Clay, but it was subsequently given separate formation status by Barclay (1989)].
Thickness: 5m (Cwar yr Hendre) to 7.4m (Llanelly Quarry).
Geographical Limits: Northeast crop of the South Wales Coalfield, from Cwar yr Ystrad [SO 081 142] to the Gilwern Hill area [SO 247 120].
Parent Unit: Llanelly Formation (LLY)
Previous Name(s): Gilwern Clay Formation (-299)
Alternative Name(s): none recorded or not applicable
Type Section  Llanelly Quarry provides the type locality, where the Member is completely exposed at its thickest development, although largely inaccessible. It comprises 7.4m of mottled clay containing platey calcrete at its base, and calcrete nodules for 5.3m above, where pyrite and carbonized rootlets appear, culminating in a grey seatearth capped by a thin coal. Wright, 1981. 
Wright, V P. 1981. Llanelly Formation. 36-72 in Wright, V P, Raven, M and Burchette, T P (editors), A field guide to the Carboniferous Limestone around Abergavenny. [Cardiff: Department of Geology, University College of Wales.] 
George, T N. 1954. Pre-Seminulan Main Limestone of the Avonian Series in Breconshire. Quarterly Journal of the Geological Society of London, Vol.110, 282-322. 
Dickson, J A D and Wright, V P. 1993. Carboniferous Limestone of the north crop of the South Wales Coalfield. 331-349 in Woodcock, N H and Bassett, M G (editors), Geological excursions in Powys, central Wales. [Cardiff: University of Wales Press, National Museum of Wales.] 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable