Jökull - 01.12.1989, Page 87

crystallized parts of the rock has led to subsequent
weathering along the joint planes with a splitting of
the rock into thin plates. In the cliffs at the southem
side of Skúmhattardalsbrík and Flatafjall the feeder
dyke for the dacite is exposed. At higher levels apo-
physes from this dyke intruded the country rock
parallel to the red partings which are sandwiched
between the basaltic lava flows.
SILICIC ROCKS OUTSIDE THE
KÆKJUSKÖRÐ RHYOLITIC VOLCANO
kollutungur rhyolite
The oldest rhyolite in the mapped area is exposed
in its northem part. (Fig. 4) in two gullies. It has a
thickness of at least 40 m. The colour of the rhyolite
ranges from white to red and it resembles in some
parts a strongly welded ignimbrite. Bands of large
lithophysae occur in this Iava flow . They occasion-
ally reach a thickness of 0.1 m. Propylitisation
processes have altered the rock, because numerous
cavities can be seen filled with calcite and quartz,
and a slight green discoloration of the rock in some
places is due to the formation of secondary chlorite.
The exposures of the flow are insufficient to deter-
mine its origin.
HERFELLIGNIMBRITE
The Herfell ignimbrite appears at altitudes of
about 700 m above sea level, where it is exposed
between basaltic lava flows. Its stratigraphic posi-
hon (Fig. 9) indicates that it must have been depo-
sited when volcanic activity of the Kækjuskörð vol-
cano had come to an end and the volcano was about
t0 be buried by basaltic lava flows. The ignimbrite
covers an area of more than 75 km2 between Her-
tell, Miðfell, Karlfell, Miðfjall, Bungufjall and
Skælingur (Fig. 12). It often forms prominent cliffs
and is especially preserved at places where it is pro-
tected from erosion by the overlying basalts. Its
thickness averages 60 m with a maximum of 120 m.
This thickness maximum (100-120 m) of the ignim-
brite occurs on the mountains between Herfell, Mið-
fell, Karlfell and Miðdalshnjúkur. The sheet thins
°ut to the north and east and cannot be traced
beyond the above mentioned mountains, nor is it
Fig. 12. Isopach map of the Herfell ignimbrite with
20 m isopachites. Contours are with a 200 m inter-
val. Outcrops of the Herfell ignimbrite appear in
black.
Mynd 12. Dreifing flikrubergsins í Loðmundarfirði.
Jafnþykktarlínur með 20 metra millibili.
exposed in the mountains on the south side of Loð-
mundarfjörður. Relatively good exposure has
allowed the construction of an isopach map of the
Herfell ignimbrite (Fig. 12), which makes it possible
to determine the eruption site of the ignimbrite at a
location named ’Askja’ between Herfell and Norð-
dalshnjúkur. To avoid confusion with the famous
Askja in Dyngjufjöll central volcano in the eastem
rift zone, the name of this presumed volcano has
been taken from the next mountain to the south,
Herfell.
The partly welded, pink to grey coloured rock
with a fine grained matrix contains lithic fragments
of up to 5 cm in diameter, both basaltic rock frag-
ments and spherulithic rhyolite. Feldspar crystals are
also quite common. At Herfell, Miðfjall and Karlfell
the ignimbrite is capped by ash fall tuffs of up to one
metre in thickness. Due to their low resistance to
weathering and erosion they have probably been
removed at other locations. Close to the proposed
eruption centre between Herfell and Norðdalshnjúk-
ur the ignimbrite is covered by rhyolites.
The relatively wide distribution of the ignimbrite
JÖKULL, No. 39, 1989 85