Jökull - 01.12.1957, Blaðsíða 4

Einarsson’s views received further support a
few years ago when thin layers of lignite were
found in the grey section (personal communica-
tion by Kjartansson).
Recent paleomagnetic studies by Einarsson
and Sigurgeirsson appear to throw further light
upon these conditions. Einarsson (3) found in
middle western and northern Iceland that bas-
alts resting on the peneplaine could be group-
ed into two distinctive sections according to
the magnetization. The upper section is magne-
tized in tlie direction of the present field
whereas the lower one has an opposite polarity.
The lower section appears to extend down to
the beginning of the Quaternary period, which
is in accordance with the results of Roche (4)
in France and Akimoto (5) in Japan.
The peneplaination appears to have occurred
during the upper Pliocene and the grey section
should consequently be of a Tertiary age. The
section has a reverse magnetization but it is
not possible to correlate this reverse section to
the succession of the magnetic events as there
is a gap at the end of the Tertiary.
The erosion during the peneplaination has
been considerable. In places sections up to a
few thousand metres have been eroded away.
At the end of the Pliocene or in the early
Quaternary the peneplaine was lifted to its
present position, that is, to an elevation of
some 1,000 metres, resulting in the erosion of
the fiords and valleys.
Furthermore, Einarsson finds 30 magnetic
sections in a column of 6,000 metres of lower
or middle Tertiary age in Eastern Iceland. He
concludes that the Tertiary period may in-
clude not less than 50 to 100 reversals of the
magnetic field and that the average duration
of each interval between reversals may be as
short as y2 million years.
Thus, there appears to be strong evidence
for a Tertiary age of the grey section and that
the peneplaination took place in the latter half
of the Pliocene.
On the other hand, the paleomagnetic dating
is still only semi-quantitative. The length of
the two Quaternary intervals of constant geo-
magnetic direction can as yet not be computed
or even estimated directly. The only clue is
that Einarsson’s estimate of y2 million years for
each interval leads to a length of the Quater-
nary period of al.'out one million years which
is of the same order as the current estimates
based on geological and astronomical evidences.
The uplift of the basalt plateau is an event
of major importance for the creation of the
present topography of Iceland. According to
the data available the uplift can have taken
place in one or more steps sometime during
the last few million years. The main uplift may
be as recent as a few hundred thousancl years
olcl but it may also be up to ten times older.
The present data do not allow a discrimina-
tion between these possibilities.
It is at this juncture appropriate to inquire
whether there are other methods of dating the
succession of geologic events. In fact, there
appear to be such possibilities and one method
will be discussed below, that is, the thermal
method.
The application of the thermal method to
the present case is based on the fact that a
rapid erosion may lead to positive transient
temperature anomalies in the upper few kilo-
metres. These anomalies may be detectable and
a quantitative evaluation may furnish data on
the progress of the geologic events. This will be
discussed below.
The writer has elsewhere (Bodvarsson, 6)
discussed the terrestrial heat balance in Iceland.
It was concluded that multiple intrusion of
magma into the upper crust could lead to sub-
stantial heat flow anomalies. This process could
very well be the cause of the abnormal tempera-
ture conditions in the zones of recent or late
Pleistocene volcanism in Iceland. However,
the anomalies that arise l'rom this process are
of a transient nature and decrease relatively
rapidly when the intrusive processes cease.
Consequently, it appears difficult to apply the
process as an explanation in the case of the
heat flow anomalies observed in zones where
volcanism has not been active since the late
Tertiary period.
THE OBSERVED TEMPERATURE
ANOMALIES
There are two indications of abnormal sub-
surface temperature in Iceland. Firstly, the great
number of hot springs, and secondly, the ob-
servation of abnormal temperatures in shallow
wells.
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