Jökull - 01.12.1974, Síða 45

and at between 1.57—1.58 for the brownish
variety.
The ignimbrite is covered by an exceptionally
thin andesite lava of only 4—10 m wliich ex-
tends for 10 km along the strike. This andesite
flow terminates the first acid phase. Rhyolites
are not represented in this phase, but small
fragments occur as xenoliths in the ignimbrite.
2.2.2 The succession between the first and
second acid phases.
Above the first acid phase is found a varied
succession of basalts, hyaloclastites and clastic
beds having a total thickness of about 200 m.
The lowest 150 m of this group are tholeiites
but olivine tholeiites and hyaloclastites are
common in the upper part. The succession be-
gins with numerous thin tholeiite flows totall-
ing 70 m in thickness, without sedimentary inter-
beds. Above them is found a glacial horizon
(no. 3 of Plate 1), ~ 10—20 m thick, which be-
gins with a tillite but which grades upwards
into reworked hyaloclastites, sandstone and
conglomerate. Occasional thin lava flows are
found interbedded with the fluvial facies. A
layer of reworked acid pumice also occurs inter-
bedded with it, and fragments of the pumice
are dispersed tliroughout the upper part of the
fluvial material.
Above tliis glacial horizon the sequence con-
tinues witli tholeiite flows which have no or
only minor red interbeds between them. In
Valagil the flows are distinctly thicker than in
Deildargil nearer to the volcanic center. The
tholeiites may represent a shield building stage
of the volcano. This is supported by a westerly
residual dip at the mouth of Deildargil. The
tholeiites give way towards the top of this
succession to olivine tholeiites which are thick-
est in the slopes south of Reykjadalsá and west
of Thorvaldsdalur, whereas sporadic flows only
are found in Deildargil nearer to the core area
of the volcano. Perhaps therefore the vents to
those flows were located on the lower flanks of
the volcano or even öutside it.
Another glacial horizon (no. 4 of Plate 1)
occurs where the rock type changes from tliolei-
ite flows to hyaloclastite and compound olivine
tholeiite lavas. Two glacial advances can be re-
cognized within this horizon each represented
by its own characteristic succession. The two
subhorizons are separated by only two lava
flows. North of Hvítá the subhorizon indicat-
ing the first glacial advance consists of a tillite
at the base but well bedded sandstone and
conglomerate of mainly hyaloclastite origin in
the upper part. The layer interfingers with a
hyaloclastite pile of great thickness which crops
out in Merkjagil. The rock is of the olivine
tholeiite variety and grades through pillow
lavas, breccias and bedded tuffs all thoroughly
zeolitized. South of Hvítá this horizon is re-
presented in Deildargil by a 4—6 m thick fluvial
conglomerate containing some pumice and
rhyolite pebbles indicating that the second
phase rhyolitic volcanism had already started.
The subhorizon indicating the second glacial
advance is found both to the north and to the
south of Hvítá varying considerably in thick-
ness and constitution. Nearest to Merkjagil
it is composed of a tillite at the base overlain
by coarse liyaloclastite debris evidently derived
from the hyaloclastite pile itself. In Deildargil
the base of the layer is also a tillite (Fig. 2)
overlying a striated basement, whereas the
upper part consists of varved clay and fluvial
conglomerates.
To the east, in Tunga and in the lower part
of Bæjarfell, this glacial horizon is represented
by hyaloclastites which may reach up to 150 m
in thickness. In Selgil and at the western tip
of Tunga a tillite is exposed at the base of this
pile. The rock of the hyaloclastite is a tholeiite
with breccias and tuffs prevailing but pillows
occurring locally as well. Strong hydrothermal
alteration has affected this unit imparting to it
dominantly greenish tints; this may be the
reason why is was mistaken for acid breccias
(Tr. Einarsson, 1962). The intimate association
of these hyaloclastite piles with tillites hardly
leaves a doubt about their subglacial origin.
The thick and extensive hyaloclastites of this
glacial event accentuated the relief of the pre-
ceding shield building stage. From there until
the end of its development the Húsafell central
volcano remained topographically so high that
lavas from the surroundings only managed to
onlap the outskirts. The last possible repre-
sentatives of those are the olivine tholeiite
flows which occur above the 4th glacial horizon
at tlie mouth of Thorvaldsdalur and on the
slopes south of Reykjadalsá. Later rock series
JÖKULL 24. ÁR 43