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. This rock sequence is also commonly referred to as the Gwna Mélange.

Outcrops

These rocks are exposed across various parts of Anglesey and along the northern coast of Llŷn and at Bardsey Island. The extensive coastal cliffs at each of these locations affords excellent exposure.

Lithology and stratigraphy

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. 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.