Abstract

G. Orton writes: In a recent paper outlining the eruption and emplacement of the mid-Ordovician Lower Rhyolitic Tuff Formation (LRTF), Howells et al. (1986) invoke water depths of up to 500 m for the initial eruptions and a shallow marine environment for the second (main) phase of caldera development. Whilst not doubting that the second phase occurred in a shallow marine environment, I question their evidence for the initial phase submarine eruptions in considerable depths of water. The water depths associated with these initial eruptions have important implications regarding the formation of subaqueous welded ash flow tuffs.

The principal arguments of Howells et al. are the presence of marine sedimentary rocks and subaqueously emplaced basaltic lavas immediately subjacent to the LRTF, as well as the absence of significant Plinian fallout tuffs beneath ash flow deposits which they suggested may be a consequence of the suppression of the eruption column in substantial depths of water.

With regard to the depositional environments of the subjacent strata, the LRTF largely rests directly on the Pitts Head Tuff (PHT) to the SW of Snowdon. The absence of any intercalated sediments from Moel Hebog to Cwm Llan (Fig. l), a distance of 10 km, suggests that this area was subaerial. The lack of significant erosion channels in the top of the PHT, such as one observes above subaerial ash flow tuffs on Tenerife would be a function of both the pervasive welding of the PHT and the absence of any significant intra-basinal topography; most pyroclastic volcanic fields are

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