Jökull - 01.12.1974, Page 46

Fig. 2. Tillite of the 4th
glacial horizon exposecl
in Deildargil. A rhyolite
sheet of the 3rcl acid
phase is seen intruding
between the tillite and
the fluvial conglomerate
overlying it.
Mynd 2. Jökulberg í
Deildargili; hrœrigrautur
úr leirsteini og kantnúnu
grjótrusli. ÆÖ úr liparíti
hefur skotist inn á milli
jökulbergsins og lagskipts
völubergs, sem ofan á þvi
liggur.
described in this paper were erupted from the
central volcano alone, except the rocks above
the volcano. From the 4th glacial horizon on-
wards a hitherto simple layer upon layer strati-
graphy became very much disturbed due to
thick rhyolite flows of small lateral extent and
piles of hyaloclastite erupted under subglacial
conditions.
2.2.3 The second acid phase.
Tltis phase is represented by a number of
rliyolite flows, tuffs of minor importance and
a huge ignimbrite sheet. Also associated with
it are some minor basaltic intrusions in the
form of dykes and inclined sheets. The occurr-
ence of pumice and rhyolite pebbles in the
fluvioglacial material of the glacial horizons 3
ancl 4 was described in the last section. It
comes as no surprise therefore to find the strati-
grapliically lowest rhyolites occurring beneath
the hyaloclastite piles of Tunga and Bæjarfell
which as described in the previous section were
erupted during the 4th glacial event. These
rhyolites occur in the south of Tunga and at
the mouth of Selgil. The visible thickness
amounts to 40 m in Selgil and 80 m in Tunga.
Flow surfaces have been preserved locally in
Tunga where overlain by basalt flows, but else-
where the rhyolites show evidence of erosion
prior to their burial by the hyaloclastites.
44 JÖKULL 24. ÁR
The bulk of the rhyolite belonging to the
second acid phase occurs in Tunga and in
Fljótstunguháls overlying the hyaloclastites of
the 4th glacial event. Altogether two plugs and
nine flows which are of limited lateral extent
but up to 100 m in thickness, make up tliis,
the largest rhyolite outcrop of the area. The
flows characteristically have a dense pitclistone
base and a flow laminated interior which be-
comes contorted upwards and which grades in-
to spherulitic rock and finally a pitchstone
breccia towarcls the top and margins. The inter-
layering of a widespread ignimbrite sheet (to
be discussed below) and of an horizon with
olivine tholeiites and glacial products among
the rhyolite flows make it possible to pin down
their position in the stratigraphic column.
Three flows in Fljótstunguháls, one west of
Merkjagil and two flows in the east of Tunga
and on the col between Tunga and Strútur
preceded the ignimbrite. These flows are the
most extensive among tlie second acid pliase
rhyolite flows and they all occur to the north-
east of the preexisting hyaloclastite hills of
Tunga.
The second acid phase culminated in a great
ignimbrite eruption. The ignimbrite was traced
for 20 km along the strike from the upper
reaches of Thorvaldsdalur in the NE where its
thickness is still about 30 m to Giljahnúkar in