Marine emplacement of welded ignimbrite: the Ordovician Pitts Head Tuff, North Wales

Abstract

The Pitts Head welded ignimbrite records a subaerial, large‐volume eruption and associated pyroclastic current that flowed into the sea. The current 15 km from source was steadily sustained with high mass flux and high particle concentration towards its base, and it entered the sea without substantial mixing with water and thus without large‐scale hydroexplosivity and without general cooling. It continued to aggrade ignimbrite at >580°C for at least 3–4 km from the original shoreline, in water initially ≥50 m deep. Entirely subaqueous, hot‐state, progressive aggradation and welding of the ignimbrite occurred where the water could not be wholly displaced by the current, although eventually the deposit displaced the shore >4 km seawards. Wet sea‐floor sediments buried by the ignimbrite were heated and locally fluidized by steam, and several square kilometres of hot ignimbrite with variable thicknesses of sedimentary substrate detached and slid downslope. Directions of sliding and the order of piling‐up of slide sheets are shown by hot‐state (rheomorphic) deformation fabrics and the geometric relations of detachment surfaces. Extensional disruption of the ignimbrite is marked by breaks in the sheet via which fluidized sediments were mobilized, locally to form rootless vents. Both the initial incursion of the pyroclastic current into the sea and the subsequent submarine sliding of the ignimbrite are likely to have caused tsunamis. Similar occurrences at modern coastlines presently susceptible to incursion of high mass flux pyroclastic currents (e.g. Taupo Volcanic Zone, New Zealand; Neapolitan region, Italy; vicinity of Manila (Taal), Philippines) would, according to tsunami propagation behaviour, cause significant near‐and far‐field coastal hazard.