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

Abstract

A detailed correlation of the tufts of the Capel Curig Volcanic Formation is presented. The depositional environment of the sediments associated with the tufts varied from subaerial to submarine. Variation in the ash-flow tufts are related to this changing environment. Subaerially emplaced tufts are characterised by even bases, distinctive recrystallisation textures, the absence of reworked tops and sparse siliceous nodules. Tufts emplaced in a subaqueous environment show irregular undersurfaces with welding to the lower contact, reworked tops and common siliceous nodules.

Distribution and significance of crystalline, perlitic and vesicular textures in the Ordovician Garth Tuff (Wales)

“…Care must be taken where changes in the emplacement envi-ronment contribute to the development of textures, as in the case of the Garth Tuff. However, the distribution of the highest-grade facies in the Garth Tuff corresponds to the general northerly position of the vent inferred by separate studies (Howells et al 1979;Kokelaar 1988;Orton 1991).…”

“…Conglomeratic alluvial fan sediments were deposited in proximal areas adjacent to the inferred eruptive centre (Howells et al 1979;Orton 1991). In medial to distal settings, the Garth Tuff was emplaced across a wet sandy to muddy substrate and displays an irregular basal contact (Howells et al 1979). Depositional environments have here been interpreted as: (a) fluvial-lacustrine to shallow subtidal (Howells and Leveridge 1980;Howells et al 1985); (b) fluvial-deltaic (Orton 1991); and (c) tidally Fig.…”

“…McArthur, unpublished data). The eruptive centre is not exposed, although a northerly location is favoured (Howells et al 1979;Kokelaar 1988;Orton 1991). The absence of widespread precursory Plinian fall deposits and dense welding suggest a "boiling-over" eruptive style .…”

“…Four calc-alkaline, dacitic to rhyolitic volcanic members are defined, locally separated by volcaniclastic sedimentary rocks ( Fig. 1; Howells et al 1979). The Garth Tuff is the first and most voluminous ignimbrite of this eruptive phase.…”

“…Conglomeratic alluvial fan sediments were deposited in proximal areas adjacent to the inferred eruptive centre (Howells et al 1979;Orton 1991). In medial to distal settings, the Garth Tuff was emplaced across a wet sandy to muddy substrate and displays an irregular basal contact (Howells et al 1979).…”