The BGS Lexicon of Named Rock Units — Result Details

Coed Duon Volcanic Formation

Computer Code: CDVO Preferred Map Code: CD
Status Code: Full
Age range: Abereiddian Stage (O2) — Abereiddian Stage (O2)
Lithological Description: The Coed Duon Volcanic Formation comprises around 60m of green, fine-grained felsic tuff with abundant mafic crystal and rhyolitic lithic clasts, locally overlain by around 15m of white-weathering, fine-grained, pink rhyolite.
Definition of Lower Boundary: In the type area, the Formation rests conformably upon pale grey, thin- to thick-bedded arkosic sandstone of the Ffairfach Grit Formation. The lower boundary is not exposed but is taken at the incoming of green, tuffaceous beds, constrained near the summit of Coed Duon.
Definition of Upper Boundary: The Formation is conformably overlain by buff-weathering, thinly interbedded sandstone, siltstone and limestone of the Llandeilo Flags Formation. The upper boundary is not exposed but is taken at the top of the last outcrop of pale rhyolite or tuff.
Thickness: Around 75m
Geographical Limits: The Coed Duon Volcanic Formation is restricted to the summit and southeastern slopes of Coed Duon, located around 8km eastnortheast of the town of Llandeilo in Carmarthenshire.
Parent Unit: Not Applicable (-)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Characteristic exposures of the Coed Duon Volcanic Formation crop out on the hill top of the same name, located around 8km eastnortheast of the town of Llandeilo, Carmarthenshire. 
Reference(s):
Williams, A. 1952. The geology of the Llandeilo district, Carmarthenshire. Quarterly Journal of the Geological Society, London, Vol.145, 915-923. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable