The BGS Lexicon of Named Rock Units — Result Details

Graig Fawr Limestone Member

Computer Code: GFLS Preferred Map Code: notEntered
Status Code: Full
Age range: Asbian Substage (CR) — Asbian Substage (CR)
Lithological Description: Knoll reef limestones comprising highly fossiliferous, massive, pale, fine-grained, recrystallised and locally dolomitised limestones with abundant calcite-lined former cavities with internal geopetal sediment (stromatactis). Contains a rich brachiopod and goniatite fauna. During the time of late Asbian carbonate platform development in North Wales a series of knoll reefs formed along, and serve to define, the platform margin. The knoll reef facies interdigitate with platformal facies on one side, and resedimented basinal facies of the Craven Group on the other.
Definition of Lower Boundary: Entry of massive "reef" limestones above well bedded Loggerheads Limestone; also passes laterally southwards into the Loggerheads Limestone, and passes laterally northwards into the Prestatyn Limestone Formation (Craven Group).
Definition of Upper Boundary: Massive "reef" limestones overlain by dark grey mudstones and thin argillaceous limestones of the Teilia Formation.
Thickness: c.100m
Geographical Limits: Dyserth area (SJ 06 80), North Wales.
Parent Unit: Loggerheads Limestone Formation (LGHL)
Previous Name(s): Reef Limestone (-927)
Knoll Reef (-3429)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Disused railway quarry (Station Quarry) at Meliden, near Dyserth exposing 14.4m of fossiliferous "reef" limestone near to the base of the member. Warren et al., 1984. 
Reference(s):
Warren, P T, Price, D, Nutt, M J C and Smith, E G. 1984. Geology of the country around Rhyl and Denbigh. Memoir for 1:50 000 geological sheets 95 and 107 and parts of sheets 94 and 106. 
Waters, C N, Barclay, W J, Davies, J R and Waters, R A. In press. Stratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/05/06. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable