Jökull - 01.12.1979, Síða 13

same period palaeomagnetic mapping of lava
groups was first applied on a regional scale by T.
Einarsson in western Iceland, a decade before the
polarity time scale was established.
Systematic mapping on a regional scale has been
carried out in.most parts of the Tertiary areas, but
is far from complete. The longest continuous sec-
tion studied so far is in eastern Iceland, and in-
volves an 8.5 km thick lava succession composed of
some 700 individual lava flows. This is not a true
vertical thickness because of successive offlapping
of younger flows as the growing lava pile was
transported sideways (Fig. 2). The section spans a
time interval of over 10 m.y. from about 13.4 m.y.
up to the Plio-Pleistocene. The build up rates taken
at face value uncorrected for down dip thickening
are variable by a factor of seven. The lowest 4 km
were extruded at an average rate of 720 m/m.y.
The next 3.0 km at 2600 m/m.y., and the upper-
most 1.5 km at rate of only 360 m/m.y. The lowest
build up rate was found for the period from about
6.5 m.y. up to the top of the section. On strike to the
north rocks representing this time interval are
partly lacking and a hiatus is present.
In northern Iceland a 5 km section with an age
range of about 3 m.y. (beginning at about 12 m.y.
and ending at 9 m.y.) yields a build up rate of about
1000 m/m.y. for the lowest 2.5 km, but an excessive
rate of 4000 m/m.y. for the upper 2.5 km of the
section. A 3.5 km section in western Iceland rang-
ing in age from about 6.5 m.y. up to 2 m.y. was
formed at a more or less constant growth rate of
about 780 m/m.y.
The down dip thickening of the lava groups
suggests that generally higher eruption rates would
be obtained within the hidden part of the lava pile.
A ~ 2 km continuous core obtained in 1978 from a
borehole in eastern Iceland may soon give infor-
mation about this.
The sections on which these values were ob-
tained avoided central volcanoes because of associ-
ated structural and stratigraphic complexities.
During their active periods they are known to pro-
duce an excessive thickness of flows so that they
stand apart topographically, later to become gra-
dually buried by onlapping lavas from younger
volcanic systems.
Sometimes a period of erosion and considerable
sediment accumulation seems to have followed the
extinction of the volcanic systems. Prominent sedi-
ment horizons that occur in western Iceland and
are traceable over tens of kilometers along strike
Fig. 5. Northwest Peninsula of Iceland. The trace
of major sedimentary horizons mapped to date is
shown. Most of these horizons include several
sedimentary layers.
(Fig. 5) appear to have formed during such tran-
sition periods. The sediments are of terrestrial
origin, fluvial or lacustrine, with occasional lignite
and plant impressions in the more fine grained
strata. The palaeobotanical record is more or less
continuous for the last 16 m.y. providing a unique
opportunity to study climatic trends of the North-
Atlantic region over this period (see chapter 6).
The most common type of interbeds in the Ter-
tiary series are thin layers of red or redbrown clayey
or tuffaceous material consisting of an iron stained
opaque groundmass and glass in various stages of
alteration. The origin of these interbeds is thought
to be aeolian, i.e. wind blown ash that has suffered
chemical weathering towards laterite. Deep
weathering of the underlying substratum is rarely
seen; however, this is likely to have occurred at the
base of the lowest thick lignite/sediment horizon of
the Northwest Peninsula (Botn-horizon of Fig. 5).
There the underlying porphyritic basalt has been
altered to clay (montmorillonite and kaolinite).
In southeastern Iceland ice sheets developed
locally in the Pliocene under conditions perhaps
similar to the present Vatnajökull area, i.e. high
ground and high precipitation. Deposits of glacial
origin are found there interspersed within the lava
pile back to about 5 m.y. A thick sequence of pre-
dominantly marine Pliocene sediment is exposed
along the west coast of the Tjörnes Peninsula in
northern Iceland amounting to some 500 m in
aggregate thickness (Fig. 8). The underlying basalt
sequence is at least 9—10 m.y. old, dipping steeply
JÖKULL 29. ÁR 11