Jökull - 01.12.1979, Side 15

towards the northwest. The sediments consist of
fossiliferous sandstone of various grades, con-
glomerate and lignite, deposited in a nearshore en-
vironment within a basin open towards the north-
west. The sedimentary Tjörnes sequence may have
formed in the early stage of opening of a rift axis
that occupied a new position in the area west of
Tjörnes. A major transcurrent fault system, known
as the Húsavík faults, terminated this axis and also
the sedimentary basin towards the south.
Studies of dips and as yet somewhat fragmentary
K/Ar ages of the lavas indicate that segments of the
rift zone in western Iceland jumped to more south-
erly and easterly positions during build up of the
Tertiary series. The Tertiary lava pile defines a
number of shallow synclines and low anticlines,
perhaps more appropriately referred to as flexures.
Most pronounced is the structural syncline within
which lies the present day axial rift zone. Other
synclines may have formed in a similar way from
crustal extension and downsagging, thus indicating
the position of extinct rift zones. There are two such
in western Iceland one trending SW-NE in the
Snaefellsnes area and the other trending N-S in the
area west of Skagi (Fig 6). Extinction of volcanism
in these syncline areas occured about 6—7 m.y.
ago. At the same time new rift zones were initiated,
out of which grew the present day Reykjanes-
Langjökull axial rift zone and the northern part of
the axial rift zone in northeastern Iceland.
In northeastern Iceland an age and structural
discontinuity is present within the Tertiary basalts
on both sides of the axial rift zone. The older
sequence of basalts is downwarped and commonly
dips 20—30° below the edge of the younger unit.
The latter goes back in age more or less unbroken to
över 5 m.y. Approaching the Vatnajökull region no
such break is present which implies that the axial
rift zone in that part of eastern Iceland remained
unaffected.
In western Iceland the shift of the Snaefellsnes
rift zone led to an anticlinal structure of the Ter-
tiary lavas referred to as the Borgarnes anticline
(Fig. 1). The ages of its flanks are widely contrast-
ing. The western flank was formed prior to 7 m.y.
ago within the now extinct Snaefellsnes rift zone
and theeastern flank formed from about 7 m.y. ago
up to Recent within the Reykjanes-Langjökull rift
zone.
Plio-Pleistocene
The Plio-Pleistocene areas cover about 25.000
km2, occupying broad zones intermediate between
the Tertiary areas and the neovolcanic zones. The
boundary between the Tertiary and Plio-
Pleistocene series is somewhat arbitrarily fixed at
the base of the Mammoth event 3.1 m.y. ago.
About this time the first tillites appear inter-
stratified with the lavas in southwestern Iceland
(Esja-Húsafell), and in northeastern Iceland (Jök-
uldalur-Vopnafjördur). Also at this time a marked
climatic cooling occured, which is well documented
both in the fossiliferous Tjörnes strata and in lignite
bearing interbasaltic horizons elsewhere (see
chapter 6).
Volcanism proceeded along the same pattern
during the Plio-Pleistocene as during the Tertiary
with elongate volcanic systems. Six central volca-
noes have been defined and partly mapped in
southwestern Iceland and another 4 may exist in
southeastern Iceland. No central volcanoes of this
age are known so far in northern Iceland. Extrusion
rates in southwestern Iceland were on the order of
1000 m/m.y. outside the central volcanoes, but
elsewhere they are poorly defined except in Tjörnes
and northeastern Iceland where they are only a few
100 m/m.y.
The main part of the Plio-Pleistocene series is
conformable with the Tertiary series, there being
no stratigraphic or structural breaks between the
two. Continuous sections representing the lower
2/3 of the Plio-Pleistocene are common where
erosion has dissected the sequences. Exposures of
the upper 1/3 are more sporadic and seldom reveal
more than short segments because they are still to a
large extent buried beneath younger deposits on
the margin of the neovolcanic zone unaffected by
deep erosion.
The rocks of the Plio-Pleistocene series are more
varied than in the Tertiary. Among the inter-lava
sediment appear grey unsorted tillite beds; the
proportion of detrital beds increases considerably,
conglomerates of fluvial or fluvioglacial origin be-
come very frequent whereas the red soil or dust
beds become less conspicuous. Instead of primarily
subaerial flows of the Tertiary, from now on sub-
glacial volcanic material with pillow lavas and
various types of breccia and hyaloclastite are com-
monly interstratified with the lavas (Fig. 7). The
Plio-Pleistocene was thus clearly characterized by
alternating stages of warm climate and cold
climate when glaciers formed or advanced to a
degree that much of Iceland was covered by ice.
The most complete single section discovered so
JÖKULL 29. ÁR 13