Jökull - 01.12.1971, Síða 56

1968). He has defined, as lst tectonic phase,
the tilting of the basalts. Then followecl a
period of very widespread, if not quite general,
peneplanation of Iceland. During the second
main tectonic phase, the country was cut into
large blocks (along shear faults in a stress field
which can now be considered as known by the
cutting off and shear movement of dykes)
which were uplifted to different elevations. In
the Borgarfjördur area the peneplain is very
clear at a level of 400 m, and here covered by
flatlying younger rocks, of reverse and normal
polarity (Einarsson, 1960, Fig. 3a, 1962).
The time span represented by the period of
peneplanation is difficult to estimate. The
author has discussed this (Einarsson, 1968, p.
7567) and pointed out that there is a discrep-
ancy between the time which geology seems to
demand for a peneplanation (hardly less tlian
5—10 My) and K/Ar-datings of the topmost
dipping groups which are certainly older than
the peneplanation, and are found to be 2—
3 My. “If this age is accepted, planation and
over 90% of the valley erosion would have
taken place during 2 My, and during the re-
maining 1 My relatively insignificant erosion
would have taken place. This seems unlikely,
and it would appear that radioactive dating of
the basalts must still be used with caution,
and that the age of the second tectonic phase
is uncertain.” I still consider it an unsolved
geological problem how a peneplanation of the
hard basalts of Iceland could have been per-
formed in about 1 My, to leave another 1 My
for the valley erosion, in three generations
with threefold grading of the main valleys to
the respective base levels. Is K/Ar-dating of
basalts so certain that such geological enigmas
can be thrust upon us?
The uppermost magnetic groups of the pene-
plained plateau basalts contain vestiges of cold
climate. This has mostly been taken to mean
Pleistocene or Late Pliocene age, always by in-
vestigators who have not known the later pene-
planation, nor realized the three-generation
development of the present valley landscape of
the basalt areas. But taking that erosional work
into consideration, one should realize that there
is really no reason why Iceland could not have
had a spell of cold climate long before Plei-
stocene time. Cold climate flora would natur-
ally accompany this cold climate, and by the
content of the flora the age could not be deter-
mined within the Upper Tertiary. Yet, it should
be mentioned that the cold climate flora of the
upper plateau groups has been considered to
be of late Pliocene age (Jux, 1960). This would
naturally be in keeping with the K/Ar-datings,
but leave us with our erosional enigma.
In the Hvalfjördur area, six polarity groups
lie above the deepest horizon of cold climate
(and local glacial) vestiges, but by extending the
survey into middle southern Iceland, I con-
cluded (Einarsson, 1962) that about 10 polarity
groups do lie above this horizon. In the present
East Iceland area, the number of polarity
groups above the lowest cold climate vestiges is
commensurate or a little larger.
The Gilsá event was i. a. suggested by an ob-
servation by one member, Hupjé, of a students
training group led by Wensink (1964). A single
normally polarized lava was found “ in tlie Gilsá
river” within what was taken to be an otherwise
uninterruptecl reversely polarized group of bas-
alts. The elevation of this normal flow is given
as 560—570 m above sea-level. The locality is not
indicated closer, but “in the Gilsá river” at this
altitude, indicates a locality where the river
flows in flat terrain at about 65° 20.5' N, 15°
17' W, or outside the map showing the localities
of samples, Fig. 1 in Wensink’s paper. Nor is
the locality indicated in Wensink (1966), in the
polarity map of Fig. 1. The Gilsá event was,
however, basecl on the dating of other lavas, as
indicated later.
On the south side of the lower Gilsá river
gorge, there is the high valley side of Jökul-
dalur, reaching 636 m in the north (Arnórs-
stadahnjúkur), high above the Gilsá gorge.
Conditions for the study of polarity groups are
in various respects much better here than up
along the Gilsá river.
Four sections of the northerly part of this
Jökuldalur valley slope — besides further stu-
dies farther south — were studied by me, jointly
with Th. Sigurgeirsson and G. P. L. Walker.
These sections were along the brooks Hnappá,
Flnjúksá, Kringilsá, and Búdará. In all sections
we founcl an about 60 m thick normal lava
group overlying, with a clear contact, a sedi-
ment that separates it from the underlying re-
verse group which is found along the lower
54 JÖKULL 21. ÁR