Abstract

Dr R. V. Beavon writes questioning submarine welding of ash-flow tuffs as suggested by Francis & Howells (1973). The irregular basal contacts, and associated soft-sediment structures below mildly transgressive ash-flow deposits, were probably caused by pre-eruption uplift, tilting and superficial slumping of wet sediment during and immediately preceeding ash-flow eruption, emplacement and consolidation.

Beavon (1963) has described the emplacement of a grossly transgressive (unconformable) ash-flow deposit, preceeded by a volcano-tectonic uplift that induced significant gravity gliding of an older tuff sheet and its weak basement of watery sediment. The resultant landslide (or lahar) may have been partly contemporaneous with one of the most voluminous ash-flows in Snowdonia. Smaller scale slump structures, analogous in age and origin to those described by Francis & Howells (1973) have been described in thin welded tuffs that were subjected to penecontemporaneous erosion (Beavon 1963, p. 486). If we imagine eruptive mechanisms consisting of (1) volcano-tectonic uplift, (2) eruption and emplacement of ash-flow and (3) welding of ash-flow during volcano-tectonic subsidence (cf. Smith & Bailey 1968), operating, to a degree commensurate with the volume of effusions, in an extensive near-shore tidal flat, then the Welsh Ordovician characteristic of subaerial welded tuffs interbedded with marine sediments should be regarded as the norm.

If the interpretation offered herein is correct, then it may be possible to recognize important eruptions where the volume of welded tuff was reduced by penecontemporaneous erosion. Perhaps the authors will comment on the possibility of stratigraphic correlation on the basis of the degree of

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