Gwna Group
Gwna Group | |
---|---|
Stratigraphic range: Ediacaran / cambrian | |
![]() Pillow lavas at Gwddw Llanddwyn, Anglesey | |
Type | Group |
Unit of | Monian Supergroup |
Underlies | Fydlyn Group |
Overlies | Church Bay Tuffs |
Thickness | several hundred metres |
Lithology | |
Primary | mélange |
Other | schist, granite, limestone, sandstone, mudstone |
Location | |
Region | north Wales |
Country | Wales |
Type section | |
Named for | Afon Gwna |
The Gwna Group is a late Precambrian (Ediacaran) / Cambrian lithostratigraphic group (a sequence of rock strata) in northwest Wales. The name is derived from the Afon Gwna, a river near Bodorgan on Anglesey where the strata are exposed.[1] This rock sequence is also commonly referred to as the Gwna Mélange.
Outcrops
[edit]These rocks are exposed across various parts of Anglesey and along the northern coast of Llŷn and at Bardsey Island.[2][3] The extensive coastal cliffs at each of these locations affords excellent exposure.
Lithology and stratigraphy
[edit]The Group is considered to represent the result of an olistostrome, a giant underwater gravity slide, which occurred probably as a result of tectonic activity at some time after 614 million years ago. It includes clasts, at all sizes from millimetres up to a kilometre or more, of a diverse range of both sedimentary and igneous rocks. Since deposition the group as a whole has been subject to low grade metamorphism.[4] Very large limestone clasts within the mélange which have been dated to between 860 and 800 Ma contain stromatolites which are the oldest known fossils in Wales.[5]
References
[edit]- ^ http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=NGW (BGS on-line lexicon of rock units)
- ^ British Geological Survey 1:50,000 scale geological map (England & Wales) sheet 133, 134 & Special sheet Anglesey
- ^ "Geology of Britain viewer | British Geological Survey (BGS)".
- ^ Howells, M.F. 2007 British Regional Geology: Wales (Keyworth, Notts, British Geological Survey)
- ^ Campbell, Stewart; Wood, Margaret; Windley, Brian (2014). Footsteps through Time. The Rocks and Landscape of Anglesey Explained. Isle of Anglesey County Council. p. 30.