The sedimentary succession in the Fachwen Formation, Arfon Group, North Wales.

The Arfon Group of North Wales (Reedman et al., 1984) has previously been interpreted to represent a Late Neoproterozoic to early Cambrian succession of extrusive igneous and sedimentary rocks formed in the Arfon Basin before and during uplift of the surrounding hinterland. It is overlain by the marine Llanberis Slates Formation (Brenchley et al., 2006). Recent mapping, combined with new dating of the sedimentary rocks, has indicated that the Arfon Group is entirely Neoproterozoic in age. The sedimentary sucession overlying the volcaniclastic rock appears to be derived entirely from the tuff and the clast assemblage suggests that the basin was essentially enclosed during deposition, with little evidence for supply of material from outside the Arfon Basin.