Dr Graham Leslie, BGS Edinburgh
Edward Greenly viewed the older schists and quartzites of Anglesey with ‘great trepidation’ – perhaps with great foresight. Some 100 years on, it is now apparent that Late Neoproterozoic accretion at the outboard margin of East Avalonia is recorded on Anglesey in ca. 650 Ma metamorphism in the Coedana Complex, the ca. 615 Ma supra-subduction zone Coedana Granite, and ca. 560 Ma exhumation of the Penmynydd Zone blueschists. Thrusting complexity upon complexity however, Anglesey’s present architecture is largely a product of repeated cycles of accretionary tectonics against peri-Gondwana that commenced in the Early Ordovician when coaxial to intensely non-coaxial SE-vergent deformation assembled the Late Neoproterozoic rocks with the Middle Cambrian (to earliest? Ordovician) Monian Supergroup. This cycle is consistent with Penobscottian accretion in the northern Appalachians. Those Monian rocks were at surface (and deeply weathered?) before sub-aerial eruption of the (early Arenig?) ca. 300 m thick, acid Church Bay Tuff Formation. The tuffs are overlain unconformably by a Middle Ordovician to early Silurian marine foreland basin succession now arranged, with its basement, in a SE-vergent (Salinic?) thrust stack. All of that orogenesis pre-dates Acadian deformation recorded in Devonian strata on Anglesey.