A SUBAQUEOUS WELDED TUFF FROM THE ORDOVICIAN OF COUNTY WATERFORD, IRELAND

The Metal Man Tuff (MMT) from the Ordovician of County Waterford, Irel and was emplaced and welded in water depths greater than the thickness of the pyroclastic flow. The MMT is the basal member of the Middle Tr amore Volcanic Formation (MTVF) of the 5-km-thick Tramore Group. The M MT consists of a 10-m-thick basal graded zone that represents a pyrocl astic flow consisting of angular clasts of black mudstone, pumice, gra y flow banded rhyolite, and pink massive rhyolite set in a matrix of n on-deformed ash shards and pumice, Maximum grain size grades from larg e cobbles and small boulders to pebbles. The basal 10-30 cm is deplete d with respect to the largest boulders resulting in an inversely grade d basal layer. The basal graded zone passes upward into a transition z one with a strong eutaxitic foliation defined by elongated fiamme of m udstone and flattened pumice, Overlying this is an upper welded zone w ith a pronounced eutaxitic foliation, columnar jointing, flattened ash shards and shards deformed around phenocrysts and spheroids. The pres ence of these features indicate that the deposit is welded, was hot, a nd was in motion as the shards deformed. The MMT represents a pyroclas tic flow that was a hot primary product of an eruption rather than re- mobilized cold pyroclastic debris. The MMT is bounded by suspension de posited fine-grained tuff, tuffaceous mudstone and terrigenous mudston e deposited below storm wave base. Many of the mudstone horizons conta in brachipod faunas from shelf-depth water. Nowhere in the 5-km-thick Tramore Group is there terrigenous sandstone, evidence of unidirection al flowing water, nor any indication of shoreline, alluvial environmen ts, or subaerial exposure. It thus seems reasonable to conclude that t he MMT was emplaced and welded subaqueously. The geochemistry of the M MT is typical of other high-silica (70-78 wt.% SiO2) rhyolite from the Ordovician of Ireland. The chemistry of the MMT is consistent from to p to bottom allowing it to be distinguished from associated syndeposit ional intrusive rock and fine-grained tuffs.