The Pitts Head Tuff Formation: a subaerial to submarine welded ash-flow tuff of Ordovician age, North Wales

Abstract

The Pitts Head Tuff Formation, of Ordovician (Caradoc) age, was emplaced as a thick (c. 700 m) intracaldera sequence and two outflow units comprising welded acidic ash-flow tuff. The Pitts Head pyroclastic flows were erupted subaerially but the lower and most extensive of the outflows crossed a shoreface, and continued for several kilometres offshore. The flow entered the sea without disruption and, following deflation and tuff emplacement, displaced the shoreface several kilometres to the east and northeast. Post-eruption subsidence in the northeast resulted here in the rapid establishment of environments deeper than had previously existed.The lower outflow tuff is parataxitically to eutaxitically welded in both the subaerial and marine environments. The extremely regular plane-parallel welding foliation of the subaerial tuffs, however, contrasts with the locally highly deformed foliation of the tuff deposited beyond the shoreface. The deformed foliation, associated with irregular zones of intense siliceous nodule development, is ascribed to the upward streaming of water vapour generated at the tuff/sediment boundary. Elsewhere rheomorphism within the tuff was caused by instability resulting from emplacement on slopes related to faulting. Continued movement initiated extensive brecciation, detachment, and local gravity sliding of large rafts of tuff.

Middle Jurassic rhyolite volcanism of eastern Graham Land, Antarctic Peninsula: age correlations and stratigraphic relationships

“…However, in general terms the Mapple Formation is dominated by ignimbrite units and crystal tuffs (about 80 %). The volcanic rocks are almost entirely subaerial, although rare chert horizons and development of siliceous nodules within some horizons may imply localized subaqueous emplacement (Reedman et al 1987). Eruptive centres have proved very difficult to identify, although close proximity to source can be locally inferred from mass flow deposits and co-ignimbrite breccias.…”

Large volume silicic volcanism along the proto-Pacific margin of Gondwana: lithological and stratigraphical investigations from the Antarctic Peninsula

“…The volcanic rocks are almost entirely subaerial, although rare chert horizons and development of siliceous nodules within some horizons may imply localized subaqueous emplacement (cf. Reedman et al 1987). Eruptive centres have not been identified, although close proximity to source can be locally inferred from mass flow deposits and co-ignimbrite breccias.…”

Sedimentary response to ignimbrite emplacement across a fluvial–shallow‐marine transition: Ordovician Mweelrea Formation, South Mayo Trough, Western Ireland

“…In several locations, Ignimbrite 1 is characterized by nodules up to ca . 9 cm across; a similar texture, although with larger (10–40 cm) nodules, was interpreted in the Pitts Head Tuff, UK, as due to water vapour streaming from the tuff/sediment interface (Reedman et al ., ). In some cases, metamorphism and alteration has destroyed vitroclastic textures in pyroclastic deposits, and in all cases, original glass fragments are devitrified to quartz and feldspar microphenocrysts.…”