Jökull - 01.12.1975, Page 29

shape, while the smaller valleys of the system
were not so affected, and either still retain
their V-shape cross section or their smooth floor
at 250—300 m elevation. Thus, most of the
erosional work was done before the beginning
of the Pleistocene Ice Ages. After a further
general uplift of the Tertiary country bv about
200 m, the large valleys were graded anew to
a lower base level (3rd generation). At the same
time a widespread strandflat was eroded. When
this was practically finished, it was covered by
lavas of reverse polarity, the Iv/Ar-date of which
is close to 1 My.
Using this — probably too low — age for the
late strandflat stage as a basis, and comparing
the quantity of material eroded and carried
away during the older erosional stages, the
present author came many years ago to the
minimum figure of 15—20 My for the whole
erosional time. But instead of a comparable
figure as to order of magnitude, K/Ar-datings
in Iceland leave even less than 1 My for all
this erosional work. We think that every scient-
ist, acquainted with geomorphology, will agree
that these K/Ar-datings are too low by more
than an order of magnitude. This includes the
paleomagnetic Gilsá event.
In this connection we shall return to the
question of reconstructing groundwater history.
Near-horizontal zeolite zones in the Tertiary
basalts of the Eastern Fjords were first described
by Walker (1960) in a long section co-direc-
tional with the dip (6—8°) of the basalts. Corre-
sponding zones have later been studied in other
Tertiary areas in the country, and can probably
be considered as general.
The near-horizontality of the zones implies,
as can be demonstrated by onyxes, that the
process of tilting was nearly finished and had
stopped during the development of the zones,
for the onyxes and the zones dip 1—2°, to in-
crease the earlier tilting.
The formation of the zeolite zones suggests a
relatively flat horizontal land surface at the
time, emphasized further by the indications of
near-stagnancy of the groundwater (Walker, 1. c.,
found that the material of zeolites is derived
from the local rock), ancl the zones may thus
be linked with horizontal isotherms under the
peneplained, previously tilted blocks. The zeo-
lites imply such temperatures, and such solution
of feldspars by groundwater, as to make it
highly questionable whether argon could have
been retained in feldspars — disregarding glass
— to be still found in the exposed rocks.
But after the blockfaulting and uplift of the
peneplained land during the Second Tectonic
Phase, as defined by the author in earlier
papers, ancl especially after the dissection of
the raised land by the lst, and especially the
2nd valley generation, the rocks which are now
exposed, and were above or a little below sea-
level, would have been permeated and cooled
by a relatively cold downward flowing ground-
water because of the then existing hydrostatic
head. From now on, the feldspars in such rocks
might better retain the produced argon, if they
were deep enough under the surface. The
highest K/Ar-ages (12—16 My) found in rocks
rapidly uncovered during the Pleistocene, might
then give a lower age limit for this dissection
stage of the basalts. This interpretation of the
measured ages is in a remarkably good harmony
with the author’s earlier estimate, on morpho-
logical grounds, of the age of the earliest val-
lays.
On the basis of palynological work (Pflug,
1959), an Eocene age was inferred for the base
of the basalts of Eastern Iceland (12 My by
K/Ar-dating), and by the lack of any signific-
ant erosional or depositional intervals, the
higher lava groups are not much younger than
the lowest ones. Also the Brjánslækur flora in
western Iceland is considered Eocene by Pflug
(1959). A more general study of that flora
(Friedrich, 1966) led to this result: "nothing
speaks against a Lower Tertiary age but, on
the other hand, nothing against a Miocene age”.
We suggest that the safest way of dating
Tertiary rocks in Iceland, and the tectono-
morphological stages of these rocks, may in the
end be the coupling of climatological, tectonic,
magnetic polarity, and marine-paleontological
data, as indicated in a separate paper soon to
be printed.
Our interpretation of the maximum ages in
Iceland leads to the question why higher ages
(50—60 My) are found in the basalts of NW-
Britain, because these are all altered, to even
a higher degree than the Icelandic ones. A
temperature of up to 200°C or even higher,
under an original cover of the present expo-
JÖKULL 25. ÁR 27