Jökull - 01.12.1979, Page 18

LEGEND
1 ; ]|] Basaltlava (4flows) |ooo | Tilllte
Cong I omer ate/Sand stone J Eault
100.000—120.000 years. For the upper part of the
Plio-Pleistocene, information about the frequency
of glaciations is available only from the Tjörnes
sequence on Iceland’s north coast. There 6 glaci-
ations are reported for the period from the Gilsá
event up to the end of the Matuyama epoch
(~ 1.8—0.7 m.y. ago).
During the Plio-Pleistocene the topographic en-
vironment of Iceland must have changed mark-
edly. Instead of the flat lava plains of the Tertiary,
from now on subglacial volcanism created an un-
even topography with elongate ridges or conical
hills of hyaloclastite and pillow lava. Glacial
erosion began to dissect the Tertiary basalts and
the fjord landscape gradually developed.
Volcanism during the Plio-Pleistocene was not
only confined to the axial rift zones. Two outliers
occur, one on the Snaefellsnes Peninsula in western
Iceland and the other in the Skagi area in northern
Iceland (Fig. 1). In both areas the volcanic products
overlie unconformably deeply eroded Tertiary
basement. In Snaefellsnes the lavas are transitional
between tholeiite and alkalic rocks; in Skagi they
are tholeiites. The young volcanism of Snaefellsnes
was more or less continuous throughout the Plio-
Pleistocene until Holocene. Two central volcanoes
(Setberg) developed in the middle part of Snae-
fellsnes in the lower Plio-Pleistocene. They were
subsequently eroded and are capped by a younger
sequence. A glacial marine deposit forms a base to
this young sequence which totals about 800 m in
thickness and is crowned by the active Snaefells-
jökull strato-volcano (1446 m) on the tip of the
peninsula. Detailed sections through this thick
sequence remain to be worked out.
Volcanism in the Skagafjördur area resumed in
the Matuyama epoch and reached into the Upper
Pleistocene. The ~ 250 m thick sequence of lava
flows, conglomerates and tillites is floored by a
marine deposit (Fig. 10). Feeder dykes, plugs and
vent fillings exposed among the young volcanics
Fig. 10. East-West cross
section through the Plio-
Pleistocene sequence of
Skagi the topmost flow (4)
is of Upper Pleistocene age
(Brunhes epoch age). From
Everts, 1972 (modified).
indicate that the volcanism accompanied late N-S
normal faulting of which there is evidence both east
and west of the fjord.
The island of Grímsey north of Iceland is com-
posed of Plio-Pleistocene basalt lavas and interba-
saltic detrital rocks of possibly fluvioglacial origin.
Preliminary datings on the basalts yield an age of 1
m.y. and less. The rocks dip to the SW towards a
N-S graben feature, the Eyjafjördur Deep, which
probably marks a former spreading axis in line with
the Kolbeinsey Ridge. This spreading axis existed
at least until late Plio-Pleistocene time when it was
replaced by a set of en echelon fissure swarms con-
necting the Kolbeinsey Ridge with the neovolcanic
zone east of the Tjörnes Peninsula (Fig. 6).
In Plio-Pleistocene time the opening of a rift zone
began to the southwest of Vatnajökull parallel with
the rift zone south of Langjökull. Double rifting has
persisted on this segment of the ridge axis since.
The young age of the eastern branch is apparent
from the age and attitude of the flanking piles of
volcanic rocks which range in age back to probably
no more than 2 m.y. and are untilted. A marine
ingression is known to have occured in the extreme
south as is evident from Plio-Pleistocene fossilife-
rous sedimentary rocks brought up as xenoliths
during volcanic eruptions in Mýrdalur and Vest-
mannaeyjar Islands. An unconformity is nowhere
seen between an old basement and rocks produced
on the young axis. However, west of it the lava pile
becomes progressively younger away from this axis
or towards NE parallel with it; the younging is
towards the parallel Reykjanes-Langjökull and the
Central Iceland volcanic zones which show an un-
diminished rate of extrusion, at least during the
lower Plio-Pleistocene.
Uþper Pleistocene
The Upper Pleistocene series comprises rocks
formed during the Brunhes magnetic epoch which
began 0.7 m.y. ago, excluding the Postglacial. It
16 JÖKULL 29. ÁR