Jökull - 01.12.1970, Blaðsíða 74

Fig. 5. Columnar jointing in the northernmost
hill of the Hólahnúkar. The length of the col-
umns is about 20 m. Photo: I. B. Fridleifsson.
Mynd 5. Stuðlaberg i Hólahnúkurn.
precipitous cliffs along the western side. The
two-tiered columnar jointing will be discussed
in detail below. Within the olivine basalts this
jointing habit is conspicuously absent. They
usually show columnar jointing also but it is
poorly developed except in the Akbraut area
just south of Thjórsá.
Foreset breccias were found in a few locali-
ties, usually forming the basal facies of the
young lavas. They attain a thickness of more
than 10 m in the southern slopes of Gíslholts-
fjall, where the base of the young lavas merges
downwards into pillows, lobes, and southwest-
erly dipping steeply inclined sheets. An out-
crop of a non stratified hyaloclastite with small
lava lumps sealed with black glass can be seen
above the Gíslholt farm. On top of it a small
outcrop of westward dipping, stratified, fine-
grained hyaloclastite occurs. North of Gamla
Akbraut, just above Thjórsá river about 3 m
of pillow breccia occur at the base of the young
lavas overlying a 2—5 m thick sandstone and
conglomerate. Examples of hyaloclastite forma-
tion north of Thjórsá were seen in the south-
eastern part of Skardsfjall (Midhúsafjall) and
in the western slope of Hlídarfjall where 15—
20 m thick hyaloclastites occur below the young
lavas. As in the former examples the hyalo-
clastites represent foreset breccias genetically
related to the overlying lavas as a result of
flow into lakes.
Pillow lavas. In the slopes of Galtafell en-
72 JÖKULL 20. ÁR
tablatures can sometimes be seen to grade into
pillow lavas and related breccias. Because o£
the loose consistency of these rocks the other-
wise precipitous cliffs tend to be scree covered
where this occurs. In the example reproduced
in Fig. 4 an entablature is seen to continue
for 100—150 m distance greatly reduced in
thickness and partly disintegrated into a numb-
er of sheets. The attenuation is compensated
by rounded pillows and irregular lobes of low
porosity and a hackly jointed interior forming
the lower part of the flow. The boundary be-
tween the pillow lavas and the attenuated
entablature is fairly sharp, but pillow like
downward projections are commonly developed.
Glassy material between individual pillows is
present although very insignificant at this local-
ity. On the northeastern slope of Galtafell an
entablature of another young lava flow (no. 3
of Fig. 4) grades into breccias with abundant
glass containing pillows and lava lumps sealed
with glass. The formation of the pillow lavas
may have taken place where small water ponds
developed, probably near the margins of an
advancing flow. After they were dried up the
flow could attain its usual characteristics.
Fluviatile deposits between the lava flows.
In the western slope of Galtafell a sedimentary
layer up to 3 m in thickness is developed be-
tween flows 1 and 2 and flows 4 and 5 of
Fig. 4. These sediments are mainly composed
of poorly rounded, coarse glass fragments and
angular lithic fragments up to 30 cm in size.
Well rounded boulders occur occasionally. The
coarse lithic debris has been derived almost
exclusively from the young lavas. It occurs in
distinct layers separated by a more finegrained
material. Cross bedding with southerly dips is
characteristic of the sediments. A sort of tor-
rential bedding, thus indicated, is in accorcl-
ance with an origin related to catastrophic
flooding immediately after the emplacement of
the flows as referred to below.
INTERPRETATION
OF THE TWO-TIERED FLOWS
The remarkable twofold division of flows
such as observed in the Hreppar district has
been known for many years, in various places
throughout Iceland mostly in Quaternary rocks.