The BGS Lexicon of Named Rock Units — Result Details

Llwyn-y-frân Sandstone Member

Computer Code: LYFS Preferred Map Code: notEntered
Status Code: Full
Age range: Arundian Substage (CJ) — Arundian Substage (CJ)
Lithological Description: Calcareous sandstones and sandy limestones (grainstones), locally cross-bedded and dolomitised. Records high energy, near shore bar deposition during the early stages of the Arundian transgression of North Wales, at a time when siliciclastic detritus was still actively being supplied to the developing carbonate ramp margin.
Definition of Lower Boundary: Taken as corresponding to the entry of calcareous sandstone 13m above the base of the Llanarmon Limestone Formation in the Alyn Valley Borehole..
Definition of Upper Boundary: Entry of dark brown-grey, less sandy, crinoidal limestones (packstones) of the overlying part of the Llanarmon Limestone Formation.
Thickness: 20 to 30m
Geographical Limits: Llanarmon-yn-Ial area [SJ1958], east of the Clwydian Range, northeast Wales.
Parent Unit: Llanarmon Limestone Formation (LNML)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Alyn Valley Borehole [SJ15NE/8] and Pistyll Gwyn Quarry. Base of member proved at 16.72m depth in the borehole;top of member located c.14m above the base of the quarry section. Grid reference given is for Alyn Valley Borehole. Davies, et al., 1989. 
Reference(s):
Davies, J R, Wilson, D and Williamson, I T. 2004. Geology of the country around Flint. Memoir for 1:50 000 Geological Sheet 108. 
Davies, J R, Riley, N J and Wilson, D, 1989. Distribution of Chadian and earliest Arundian strata in North Wales. Geological Journal, Vol 24/1, 31-47. 
Somerville, I D and Strank, A R E, 1984. Discovery of Arundian and Holkerian faunas from a Dinantian platform succession in North Wales. Geological Journal, Vol.19, 85-104. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E108