The BGS Lexicon of Named Rock Units — Result Details

Holyhead Formation

Computer Code: HHD Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Ediacaran Period (AD) — Ediacaran Period (AD)
Lithological Description: Massive, coarse orthoquartzites, locally pebbly, with a few thin beds of schist. Following work by Phillips (1991), the Holyhead Quartzite Formation has been re-defined to include the South Stack Moor Beds of Greenly (1919). The latter are a sequence of orthoquartzites, lithologically similar to the Holyhead Quartzite, but included by Greenly in his South Stack Series. The term Holyhead Quartzite is now an informal name reserved for a 50m thick quartzite unit within the core of the Rhoscolyn Anticline, and the name 'Holyhead Quartzite Formation' is obsolete. The preferred designation for the division is Holyhead Formation (Phillips 1991), which accords with Shackleton's (1969) definition.
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.
Definition of Upper Boundary: Conformably overlain by the Rhoscolyn Formation. The top of formation is drawn at top of 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  Bwa Ddu, Anglesey. Phillips E, 1991. 
Partial Type Section  Porth Saint, Anglesey. Phillips E, 1991. 
Reference(s):
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. 
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. 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.] 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable