Rootless vents in welded ash-flow tuffs from northern Snowdonia, North Wales, indicating deposition in a shallow water environment

Abstract

In northern Snowdonia welded ash-flow tuffs, correlated with the Pitt's Head Tuff Formation occur intercalated with Caradoc shallow marine shelf sediments. Within the tuffs there are vent-like structures which have a diapiric form where fiamme are drawn in towards a central core. The cores may have a constrictional linear fabric of streaked out fiamme, or may consist of vitrous tuff surrounded by a zone of buckled planar fiamme. There are associated zones of quartz nodules which are thought to represent quartz filled vesicles. The diapiric structures are believed to be rootless vents. Steam generated by the heat of the flow in contact with shallow water is thought to have caused explosive disruption of the tuffs as consolidation and welding occurred. These structures are thus themselves indicators of deposition of ash-flow tuffs in a shallow water environment.