Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 31

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time before the 1 My old Mosfellsheiði dolerite appeared. This
means that the block lifting is pre-Pleistocene. Furthermore,
Gagnheiði is directly connected with the lower to middle-height
layers of the Súlur mountain, which can be shown to be older
than deep and time consuming valley formation in the Tertiary
basalts at the head of Hvalfjörður (18). Drilling, such as indi-
cated for Laugarvatnsfjall, would be of no less interest at Ár-
mannsfell, and possible also at Hrafnabjörg.
These facts have already given us some indication of a con-
siderable age of the untilted Zonal plateau rocks. Their zonal
origin is indicated by a slight dip towards the present Zone, of
the next older group of extensive plateau basalts, termed locally
N2 in the original paleomagnetic grouping in the surrounding
area by Þorbjörn Sigurgeirsson and the present author (18
and 19). This denotation will now have to be revised, as not
only are the Mosfellsheiði and Lyngdalsheiði (of normal pol-
arity) older than reverse rock at Sandfell (15), the untilted
plateau rocks (of normal polarity) are also superposed directly
by reverse rocks (in northern Iceland). A provisory new re-
gional denotation might be: Lowground shieldvolcanoes (N2n),
the n in the index indicating the new system; the superposed
reverse rock at Sandfell would then be (Rln); the untilted
plateau (N3n); and their covering rocks in Tjörnes and in Kálf-
borg (to be considered later) might then become (R2n). The
lower part of what would be (N4n) consists of plane-parallel
lavas which dip slightly (2-3°) towards the Median Zone on
both sides, and can be traced almost continuously across the
zone, just south of Langjökull.
It seems to us highly desirable to keep Iceland out of the
so-called international Geopolarity Time Scale, which is based
on K/Ar-dating of rocks from the most various parts of the
earth, thus violating intentionally the fundamental principle
of all sound stratigraphy. Furthermore, there are serious doubts
as to the meaning of the datings (20). Within Iceland we can
in many cases proceed stratigraphically from one polarity
group to the other or distinguish the individual lava groups
by erosional and/or tilting unconformities, together with their