The BGS Lexicon of Named Rock Units — Result Details

Llanarmon Limestone Formation

Computer Code: LNML Preferred Map Code: notEntered
Status Code: Full
Age range: Holkerian Substage (CQ) — Asbian Substage (CR)
Lithological Description: Predominantly pale, thick-bedded, locally cross-stratified, shelly limestones (packstones and grainstones), subordinate thinner bedded dark grey limestones (packstones) and oolitic limestones. Upper parts (of early Asbian age), which interdigitate with the Leete Limestone Formation, include limestones rich in peloids, plates of dasycladacean algae, oncoids and reworked micrite intraclasts. On the Great Orme, the formation is extensively dolomitised. The Llanarmon Limestone Formation records the intoduction of carbonate depositional environments across North Wales during a major and sustained Arundian marine transgression. The initial distribution of facies suggests a ramp-like setting, with darker off-shore limestones deposited in the north and east (of the Clwydian Range) and shallower, inner ramp facies to the west (Vale of Clwyd). Subsequently, during the late Arundian, these high energy, inner ramp limestones prograded northwards and eastwards to establish a low-gradient carbonate platform. During the early Asbian, these shoal facies withdrew to the outer edge of the platform to form a protective barrier to the peritidal facies of the Leete Limestone Formation.
Definition of Lower Boundary: East of the Clwydian Range, the base is taken at lowest massive, thick-bedded packstones and grainstone above thinner bedded, heterolithic sequences of the Foel Formation. West of the Clwydian Range, the Llanarmon Limestone Formation rests conformably on red beds of the Ffernant Formation, or sits unconformably on older Silurian rocks.
Definition of Upper Boundary: Where overlain by the Leet Limestone Formation, the top is taken at the lowest porcellaneous limestone diagnostic of this formation; however, east of the Clwydian Range, the Leete Limestone Formation passes laterally northwards into the Llanarmon Limestone Formation so that north of Nannerch the top of the latter is taken at the entry of pseudobrecciated and mottled packstone of the Loggerheads Limestone Formation.
Thickness: Ranges from 75m in the Vale of Clwyd to 280m in the Dyserth-Caerwys area.
Geographical Limits: Crops out on the Great Orme at Llandudno [SH7683] and along the west side of the Vale of Clwyd [SJ1060]; in fault slices along the Vale of Clwyd Fault [SJ 1267]; in its type area east of the Clwydian Range between Dyserth [SJ0779 and Llandegla [SJ1852].
Parent Unit: Clwyd Limestone Group (CLWYD)
Previous Name(s): Gop Hill Limestone (-2910)
Llysfaen Limestone Formation [Obsolete Name And Code: Use LNML] (LLYL)
Llandudno Pier Dolomite Formation [Obsolete Name And Code: Use LNML] (LPD)
Ochr-Y-Foel Limestone Formation [Obsolete Name And Code: Use LNML] (OYFL)
Llandudno Pier Dolomite (-1665)
Moel Hraddug Limestone (-371)
Dyserth Quarry Limestone (-3537)
Lower Brown Limestone [Obsolete Name And Code: See FOF, LEEL, LNML, TNL] (LBRL)
Alternative Name(s): Llysfaen Limestone
Stratotypes:
Partial Type Section  Alyn Valley Borehole (SJ15NE/8) and Pistyll Gwyn Quarry, Llanarmon-yn-Ial. Davies et al., in press. Conformable contact with fine-grained argillaceous limestone of the underlying Foel Formation was met at 29.79m in the BH. Together with the quarry, this provides unbroken section through the lower 85m of the Formation. 
Reference(s):
Somerville, I D, Mitchell, M and Strank, A R E, 1996. An Arundian fauna from the Dyserth area, North Wales and its correlation within the British Isles. Proceedings of the Yorkshire Geological Society. Vol. 46, 57-77. 
Davies, J R, Wilson, D and Williamson, I T. 2004. Geology of the country around Flint. Memoir for 1:50 000 Geological Sheet 108. 
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. 
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. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E108 E096 E094 E095 E107 E121