| Abstract: | The Yr Arddu Tuffs (Ordovician) are a sequence of predominantly welded acid ash-flow tuffs in the outlier centred on Yr Arddu (North Wales). The tuffs accumulated above a NNE-trending fracture, the Yr Arddu Fracture, which had previously influenced sedimentation. The heterogeneity of the tuffs and their restricted development suggest that they represent proximal accumulations from small or suppressed (boiling-over) eruption columns. The eruptions and emplacement developed, at least in the initial stages, in a submarine environment. Lithological, geochemical and palaeontological evidence indicates that the tuffs represent the earliest eruptive phase of the Lower Rhyolitic Tuff Formation. They were later intruded by comagmatic rhyolite domes, whose alignment reflects the continued influence of the early fracture. |
