Transgressive welded ash-flow tuffs among the Ordovician sediments of NE Snowdonia, N. Wales

Abstract

In the type area, and southwest of it, the Capel Curig Volcanic Formation comprises up to three ash-flow tuffs interbedded with marine sediments. The tuffs are generally massive and welded, but pass laterally and upwards into current bedded, ripple marked tuffs. They are believed to have been deposited in a subaqueous environment, after originating from subaerial eruptions farther north where the Formation is represented by an uninterrupted sequence of intensely welded ash-flows. The lower contacts of the submarine ash-flows contrast with subaerial ash-flows in being welded and by locally transgressing the underlying sediments at angles up to 90°, with minor apophyses resembling magmatic intrusions. Eutaxitic foliation, however, remains generally parallel to the regional dip. The sediments adjacent to the transgressive undersurfaces are disturbed and reconstituted. The irregular contacts, together with adjacent discrete pipe-like tuff bodies are interpreted as down-sags analogous to load casts but of unprecedentedly large scale, up to 100 m, formed by the liquefaction and yielding of the sediments during and after the emplacement of the tuffs.

The caradocian rocks of the north and west berwyn hills, north wales

“…Brenchley 1969). The ash-flows of the Swch Gorge Tuff were assumed by Brenchley to be sub-aerial, but the work of Francis and Howells (1973) in Snowdonia has thrown doubt on such assumptions about ignimbrites. However the associated volcaniclastic sediments show evidence of a high energy, probably channel environment and the underlying and overlying sediments are shallow marine.…”

The Capel Curig Volcanic Formation, Snowdonia, North Wales; variations in ash-flow tuffs related to emplacement environment

“…The outcrop at Capel Curig was remapped by Williams (1922) who interpreted most of the volcanics of the formation as rhyolite lavas emplaced in a marine environment. Francis & Howells (1973) reinterpreted these rocks as welded tufts.…”

Volcanogenic mineralization related to the Snowdon Volcanic Group (Ordovician), Gwynedd, North Wales

“…The uppermost formation, the Cape1 Curig Volcanic Formation consists of welded acid ash-flow tuffs and the two most extensive flows were erupted from subaerial centres in the north and transgressed southwards into a marine environment (Francis & Howells 1973;Howells et al 1979;Howells & Leveridge 1980;Howells et al 1985).…”

“…To the NE, outside the caldera, a single complete non-welded ash-flow tuff, the lowermost member of the Lower Crafnant Volcanic Formation (Howells et al 1973) was emplaced and preserved within the marine sedimentary sequence. It travelled for at least 20 km from source.…”