The BGS Lexicon of Named Rock Units — Result Details

Gilwern Oolite Formation

Computer Code: GWO Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Chadian Substage (CI) — Chadian Substage (CI)
Lithological Description: Massive ooidal grainstone, shelly, crinoidal bed at base; becomes dolomitized on the east crop of the coalfield.
Definition of Lower Boundary: The base is placed at an erosion surface. In the type area, this is cut in micritic limestones of the underlying Coed Ffyddlwn Formation (Darren Ddu Member), which is overlain by coarse shelly, crinoidal grainstone (Craig-y-Gaer Coral Bed) at the base of the Gilwern Oolite Formation.
Definition of Upper Boundary: The top is a major palaeokarstic surface, with extensive dissolution and palaeosol formation of the ooidal grainstone at the top of the Formation. This is overlain by micritic limestones and green clay (Clydach Halt Member) forming the basal part of the overlying Llanelly Formation.
Thickness: 10m in the Clydach Gorge to 20m on the east crop.
Geographical Limits: Northeast crop of the South Wales Coalfield, from between Blean Onnen Quarry and Chwar Pant-y-rhiw [c.SO 185 165] east to the Clydach Gorge and south down the east crop of the coalfield, where it is dolomitized, but perhaps correlatable with 14.4m of massive, cross-bedded dolomites at Gallowsgreen Quarry [SO 2662 0677], Varteg Hill.
Parent Unit: Clydach Valley Subgroup (CLD)
Previous Name(s): Gilwern Oolite (-2224)
Alternative Name(s): none recorded or not applicable
Type Area  Names from Gilwern Hill, near the Clydach Gorge, Brynmawr, South Wales. Extensively exposed in disused quarries (for example at Craig-y-Gaer in the Clydach Gorge) [e.g. SO 2238 1330] 2km east of Brynmawr, South Wales. Barclay, 1989. 
Reference Section  Llanelly Quarry, Clydach Gorge, where the Formation is up to 10m thick and well exposed at the base of the quarry face. The basal beds are not exposed at the base of the face, but can be examined nearby at the entrance to the quarry. Barclay, 1989. 
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. 
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: