Jökull - 01.12.1982, Page 99

Fig. 3. Rock-generator magne-
tometer of A. Brynjólfison and T.
Sigurgeirsson, constructed at
Copenhagen University in 1954.
Samples were fixed inside the
inner ring and the outer ring was
then rotated by means of an ex-
temal motor. An induced voltage
signal was then fed to a mirror
galvanometer through a commu-
tator. On the right, one-half of a
mu-metal screen surrounding the
pickup coil. Base of instrument is
33 cm across. Photo L. Kr. 1981.
Mynd3. Bergsegulmœlir Ara Bynj-
ólfssonar og Porbjöms Sigurgeirsson-
ar.
•n lavas was also carried out, chieíly by Brynjolfs-
son. Thus, inconsistencies in NRM directions
within lavas were shown to be due to V. R. M.
acquired in the earth’s field over a long period of
time. The effect of VRM in these rocks could be
eliminated by demagnetizing in a peak a-c field of
100 Oe or so; a negative correlation between the
VRM intensity and the stability of primary re-
manence was also demonstrated. This was the first
successful application of alternating field de-
magnetization to actual geological strata.
Brynjoljsson (1957) measured the remanence
directions in samples of 25 dated Recent lavas from
unspecified locations in Iceland, and plotted a
secular variation path from these.
UTRECHT GROUP
M. G. Rutten (see above) continued his lield
mapping in Iceland into the late fifties, and some of
his later papers include paleomagnetic polarity re-
sults or discussions of these (Rutten 1960, Rutten and
IVensink 1959, 1960a,b). Rutten and Wensink (1960a)
pointed out that cold climate deposits occur in the
N2 zone of NE-Iceland and hence that glacial
climates may have set in earlier here than in
Europe. (A similar conclusion was being considered
on different grounds by Einarsson (1957a,1959)).
They suggested that the R1/N2 boundary should be
used as a global definition of the base of the Quater-
nary.
Rutten's mapping in Iceland cannot be said to be of
lasting value. His stratigraphic studies in Iceland
were based on concepts like the “Graue Stufe“,
which are now discounted (see e.g. Piper (1973b));
he also assumed that N2 covered most or all of the
Pliocene, R2 and N3 the Miocene, and R3, N4most
of the Oligocene.
Along with a party ofstudents, H. Wensink (1964,
1965) carried out field mapping of polarity in
basalts during stratigraphic studies in Jökuldalur,
Fljótsdalur, Tjörnes and Vopnafjörður. 'I’he most
extensive effort was in Jökuldalur, and included the
sampling of 16 flows for laboratory magnetic mea-
surements from asequence injökulsá and Hnjúksá.
Not all these samples (6 per fiow) were demagnetiz-
ed.
YVensink noted the presence of short “extra”
polarity zones within the R1 and N2 in Jökuldalur
and elsewhere. Such short zones were just then
being reported from localities in Africa and the U.
S.,and were named “events“, as distinct from the
major geomagnetic epochs. The four youngest
epochs and some of these events were delineatcd
and named in 1964-65 (see review by Watkins,
1972).
In the very first paper to report radiometric (K-
Ar) dates from Iceland, McDougall and Wensink
(1966) confirmed that R1 and N2 in Jökuldalur-
corresponded to the lower Matuyama and the
Gauss epoch respectively. They also dated the
possibly split normal event in R1 at 1.6 M. y., the
R1/N2 boundary at 2. 5 M. y. and a lavaovcrlain by
JÖKULL 32. ÁR 95