The BGS Lexicon of Named Rock Units — Result Details

Llyn Shear Zone Complex

Computer Code: LSZ Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Ediacaran Period (AD) — Comley Series (EC)
Lithological Description: Fine-grained, siliceous schists and phyllitic rocks containing porphyroclasts and larger bodies of sheared limestone, red phyllite and metamorphosed basaltic lavas.
Definition of Lower Boundary: Not applicable. The shear zone involves Neoproterozoic rocks.
Definition of Upper Boundary: Not applicable. Mostly involves pre-Arenig rocks but associated brittle faults penetrate the adjacent outcrops of Ordovician rocks.
Thickness: Not applicable.
Geographical Limits: Lleyn Peninsula, but may connect offshore with the Berw Shear Zone Complex of Anglesey. Boundaries are tectonic and either gradational with, or locally faulted against rocks of the Gwna Group and the Sarn Complex.
Parent Unit: Not Available (!)
Previous Name(s): LLYN SHEAR ZONE ROCKS (UNDIFFERENTIATED) (LSZ)
PENMYNYDD ZONE OF METAMORPHISM
Alternative Name(s): LLEYN SHEAR ZONE
Stratotypes:
Type Section  Foreshore, immediately south of Penrhyn Nefyn. 
Reference Section  Cliff section on, and to west of Trwyn Bychestyn. 
Reference(s):
Gibbons, W and McCarroll, D. 1993. Geology of the country around Aberdaron, including Bardsey Island. Memoir for 1:50 000 Geological Sheet 133. 
Matley C A. 1928. The Pre-Cambrian complex and assocciated rocks of south-western Lleyn (Carnarvonshire). Quarterly Journal of the Geological Society of London, Vol. 84, 440-501. 
Gibbons, W. 1981. Glaucophanic amphibole in a Monian shear zone on the mainland of N Wales. Journal of the Geological Society, London, Vol.138, 139-143. 
Gibbons, W. 1983. The Monian 'Penmynydd Zone of Metamorphism' in Llyn, North Wales. Geological Journal, Vol.18, 21-41. 
Gibbons, W. 1987. Menai Strait Fault System: An early Caledonian terrane boundary in North Wales. Geology, Vol.15, 744-747. 
Gibbons, W. 1989. Basement-cover relationships around Aberdaron, Wales, U.K. : the fault reactivated northwestern margin of the Welsh Basin. Geological Magazine, Vol.126, 363-372. 
Gibbons, W, Tietzch-Tyler, D, Horak, J M and Murphy, F C. 1994. Precambrian rocks in Anglesey, southwest Llyn and southeast Ireland. 75-84. In Gibbons, W and Harris, A L (editors), A revised correlation of Precambrian rocks in the British Isles. Geological Society of London Special Report No 22. 
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.). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E133 E118 E134