The BGS Lexicon of Named Rock Units — Result Details

Plastirion Member

Computer Code: PTN Preferred Map Code: Ptn
Status Code: Pending Upgrade
Age range: Longvillian Substage (OL) — Woolstonian Substage (OS)
Lithological Description: Fossiliferous grey calcareous mudstone and siltstone with fine- to medium- to coarse-grained sandstones.
Definition of Lower Boundary: Conformable and gradational with the underlying mudstones and sandstones of the Bodwrog Member. It is the presence of the latter that has allowed the Plastirion Member to be named thus. Where the Bodwrog Member is absent, the Plastirion Member overlies the Allt Fawr Rhyolitic Tuff Formation and forms the whole of the Dwyfach Formation. It is for this reason that it should be more aptly regarded as "Dwyfach Formation (undivided)".
Definition of Upper Boundary: Overlain by andesitic tuffs of the Penmaen Formation. The boundary is generally sharp but probably conformable, and is heralded by a marked coarsening at the very top of the Plastirion Member.
Thickness: 100 to 120m
Geographical Limits: Pwllheli and Nefyn areas, North Wales.
Parent Unit: Dwyfach Formation (DWF)
Previous Name(s): Llanbedrog Mudstone (-1777)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Plastirion, northeast of Pwllheli. Young, T P, Gibbons, W and McCarroll, D. 2002. 
Partial Type Section  Road cutting to the south of Llanbedrog. Young, T P, Gibbons, W and McCarroll, D. 2002. 
Reference(s):
Matley, C A, 1938. The geology of the country around Pwllheli, Llanbedrog and Madryn, southwest Carnarvonshire. Quarterly Journal of the Geological Society of London, Vol.94, 555-606. 
Young, T P, Gibbons, W and McCarroll, D. 2002. Geology of the country around Pwllheli. Memoir of the British Geological Survey, Sheet 134 (England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable