The BGS Lexicon of Named Rock Units — Result Details

Holyhead Formation

Computer Code: HHD Preferred Map Code: notEntered
Status Code: Full
Age range: Furongian Epoch (EF) — Tremadocian Age (OTR)
Lithological Description: Grey to white turbiditic metaquartzite and metasandstone (psammite), with subsidiary interbedded blue-grey silty metamudstone (phyllitic semipelite); pelite locally.
Definition of Lower Boundary: Drawn at base of first thick (up to 25 m) massive orthoquartzite conformably overlying the bedded turbiditic sandstones and mudstones of the South Stack Formation (NSS).
Definition of Upper Boundary: Conformably overlain by the Rhoscolyn Formation (NSR). The top of the formation is drawn at the top of the highest thick orthoquartzite, underlying the interbedded turbiditic sandstone and mudstones of the Rhoscolyn Formation.
Thickness: 150 - 200 m thick.
Geographical Limits: North Wales, Anglesey.
Parent Unit: Holy Island Group (NHI)
Previous Name(s): Holyhead Quartzite (NHQ)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Porth Saint, Anglesey. Phillips E, 1991. 
Partial Type Section  Bwa Ddu, Anglesey. Phillips E, 1991. 
Reference(s):
Treagus, J E, Treagus, S H, and Woodcock, N H. 2013. The significance of the boundary between the Rhoscolyn and New Harbour formations on Holy Island, North Wales, to the deformation history of Anglesey. Geological Magazine, 150(3), 519-535. 
Shackleton, R M. 1969. The Pre-Cambrian of North Wales. 1-22 in Wood, A (editor), The Pre-Cambrian and Lower Palaeozoic rocks of Wales. [Cardiff: University of Wales Press.] 
Phillips E. 1991. The lithostratigraphy, sedimentology and tectonic setting of the Monian Supergroup, western Anglesey, North Wales. Journal of the Geological Society, London, Vol. 148, 1079-1090. 
Greenly, E. 1919. The geology of Anglesey, Vol 1. Memoir of the Geological Survey of Great Britain (England and Wales). (London: H.M.S.O.). 
Shackleton, R M. 1975. Precambrian rocks of Wales. 76-83 in A correlation of Precambrian rocks in the British Isles. Harris, A L, and others (editors). Geological Society of London Special Report No. 6. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable