Ynys Llanddwyn late Neoproterozoic – Cambrian Mélange

Ynys Llanddwyn featured in one of the first geological investigations and mapping exercises in the world (Henslow, 1822), and the term ‘mélange’ was first coined by Greenly (1919) for the mixture of rocks found on the western tip of the island. Rock exposures on and around Ynys Llanddwyn showcase some of the UK’s most important geology. Late Neoproterozoic-Cambrian rocks of both continental and oceanic settings belong to the Gwna Group (now the Bodorgan Formation (Schofield et al., 2020) and include pillow and massive basalt, hyaloclastite, carbonate and basalt-carbonate, chert, sandstone, mudstone and mélange-bearing units. These rocks are superbly exposed in coastal sections and outcrops that extend northeast from the southern tip of Ynys Llanddwyn across a tidal causeway into Newborough Forest. The assemblage is one of several mélanges preserved on Anglesey and northwest Llŷn, which are of controversial origin and of considerable ongoing research value. Debate continues as to whether these mélanges were formed entirely by tectonic emplacement, as part of an accretionary wedge or prism in a subduction zone, or if gravity slide or mass flow, mobilised by a catastrophic event such as continental shelf collapse, was a contributory process (Maruyama et al., 2010; Groome, 2022).