The BGS Lexicon of Named Rock Units — Result Details

Derfel Limestone Member

Computer Code: DEL Preferred Map Code: DEL
Status Code: Pending Upgrade
Age range: Caradoc Series (O4) — Caradoc Series (O4)
Lithological Description: Dark, blue-black pyritous, slightly calcareous, shelly mudstone, sporadically but richly fossiliferous.
Definition of Lower Boundary: Not seen, but interpreted as a fault against falsic ash flow tuffs of the Aran Fawddwy Formation (formerly the Llyn Conwy Formation) at its stratotype by BGS mapping in 1993. (Corwen provisional 1:50 000 Sheet 120). Elsewhere considered to be conformable on the Aran Fawddwy Formation.
Definition of Upper Boundary: Not seen, but appears to be conformable with the mudstones of the Ceiswyn Formation.
Thickness: 1m to 2m.
Geographical Limits: Corwen and Bala areas.
Parent Unit: Ceiswyn Formation (CSW)
Previous Name(s): Orthis Limestone (-1496)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  West wall of the gorge of Nant Aber Derfel at Pont Aber Derfel. Bassett, D A, Whittington, H B and Williams, A. 1966. 
Type Area  Corwen, Sheet 120. 
Reference(s):
Bassett, D A, Whittington, H B and Williams, A. 1966. The stratigraphy of the Bala district, Merionethshire. Quarterly Journal of the Geological Society of London, Vol.122, 219-271. 
Elles, G L. 1922. The Bala country: its structure and rock succession. Quarterly Journal of the Geological Society of London, Vol.78, 132-175. 
Whittington, H B and Williams, A. 1955. The fauna of the Derfel Limestone of the Arenig district, North Wales. Philosophical Transactions of the Royal Society, London. Series B, Vol.238, 397-430. 
Fearnsides, W G. 1905. On the geology of Arenig and Moel Llyfnant. Quarterly Journal of the Geological Society, Vol.61, 608-640. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E120