Jökull - 01.12.1971, Síða 58

Gilsá river, and there called Rj by Wensink.
The still lower normal group at the mouth of
the Gilsá river, Wensink calls N,. Our normal
group, which for convenience we shall simply
call NG, can however not be called Ni, neither
in the absolute sense nor regionally, for it is
again covered by a new reverse group, as shown
in my map. The contact of these two groups
is in rather flat terrain and mostly covered by
debris. In the Hnappá section the contact is
mO'St probably the Fljótsdalsheidi peneplain.
NG, together with the lower groups, falls gently
towards W, and it is quite clearly a part of the
dipping plateau basalts, which means that it is
older than the Fljótsdalsheidi peneplanation.
It is worth stressing that NG is in no way
peculiar among polarity groups, and it shows
its individuality and time span by intercalated
sediments. The following notes on sediments
within NG I find in my record of the four
sections, from base to top.
Hnappá: a) Light-grey coarse conglomerate.
The lower part is fluvioglacial, the upper part
is clay and brown sandstone. b) Tliin sand-
stone. c) Greyish conglomerate, 10 m thick. At
the bottom is a coarse breccia, possibly a lava
crumbled by weathering. Tlien fluvioglacial
layers and, finally, brown sandstone.
Hnjúksá: a) The lowest normal lava rests on
a brown conglomerate. b) Sandstone.
Kringilsá: a) Same as for Hnjúksá. b) Thin
sandstone. c) 2—3 m conglomerate, river gravel.
d) 3 m conglomerate, scattered rhyolite pebbles.
Búdará: a) The lowest normal lava rests on
a sediment. b) 1—2 m river gravel. c) Very
coarse secondary (i. e. non-volcanic) breccia,
covered by a thick conglomerate.
Wensink and his group studied one of these
four sections, the Hnjúksá (Wensi.nk, 1964).
They crossed NG between about 530 and 620 m,
found here two normal lavas ancl a reverse
one, with a question mark. They also reached
the bottom of the overlying reverse group.
However, for a reason which is not made
understandable in that paper, these normal
lavas are thrown into a special category which
Wensink calls lavas “inverted in relation to the
series in which they are intercalated”. In short,
he simply seems to define that some polarity
groups should be accepted as such, while some
groups are just called invertecl to the underly-
56 JÖKULL 21. ÁR
ing and overlying groups, although they prove
their individuality, and sorne non-trivial length
of time, by a number of lavas and intercalated
sediments.
Later, for his paper of 1966, Wensink studi-
ed the above mentioned sections and found
NG. But his attitude towards it is the same
as before: this group falls into the category
“inverted” to the polarity below ancl above it.
However, here one seems to understand his
reason, for he says, p. 368: “The presence of
more reversals than has been generally assumed
(italics mine), involves that a paleomagnetic
stratigraphy must be built up with the utmost
care.” His trouble obviously is that he is fac-
ing “generally assumed” ideas, and is trving to
make observations agree with them.
Now, in the early years of paleomagnetic
mapping, it was tentatively assumecl that a
polarity period would be of a duration around
1 My, ancl that the Pleistocene contained two
such periods, the Upper Pleistocene normal one
and the Lower Pleistocene reverse one. The
“events” had not been introduced. Further-
more, the cold climate vestiges in the upper-
rnost part of the tilted and peneplained plateau
basalts were sometimes mistakenly assumed to
correspond to the known scale of major Plei-
stocene glaciations in the Alps, ancl generally in
Europe and North America. We shall try to
clarify this point.
First, this uppermost part of the pile of
plateau basalts is not generally separated from
the basalts below by an unconformity. This is
so in the Borgarfjördur region in West Iceland
and in the present East Iceland area according
to my studies, and verified here by Wensink
(1966). It is the aim of may 1962-paper to
separate Pleistocene and late Tertiary “Young
Rocks” from the main Tertiary plateau basalts,
and this lack of an unconformity, or a clear
indication of a long time interval between the
main plateau basalts and their uppermost part,
containing cold climate vestiges, was observed
and pointed out. Secondly, there are so many
cold climate sediments at short intervals be-
tween these upper plateau basalts that a general
glaciation in each case of glacial vestiges is
out of the question. Small local glaciers, or the
sediments of a glacier river coming from a
distant source, is the most likely explanation.