Wj. Fritz et Cj. Stillman, A SUBAQUEOUS WELDED TUFF FROM THE ORDOVICIAN OF COUNTY WATERFORD, IRELAND, Journal of volcanology and geothermal research, 70(1-2), 1996, pp. 91-106
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.