Jökull - 01.12.1982, Blaðsíða 100

Fig. 4. Sampling locations of
early published laboratory
studies og remanence in Ice-
landic rocks. M: Mercanton.
Mynd 4. Sýnat'ókustaðir vegna
sumra Jyrstu rruelinga á segulmögn-
un íslensks bergs í rannsóknastofum.
a tillite (in turn overlain by a short event in N2) at 3.
1M. y.
McDougall and Wensink hence suggested that
their lower event was the same as the Mammoth
event, and that glacialclimateset in 3. 0 M. y. agoin
Iceland. This result has been found subsequently to
be consistent with climatic evidence from elsewhere
in Iceland, but it should be noted that two events
are now assumed to have taken place in the lower
Gauss epoch.
McDougall and Wensink’s (1966) suggestion of the
existence of a geomagnetic event named Gilsá
around 1.6 M. y. ago became the focal point of a
controversy that was to range across most of the
Earth for several years. Scientists working on the
geomagnetic polarity time scale as derived from
volcanics, sediments, or ocean-floor anomalies had
quite divergent views on the number, timing, and
duration of events in the lower Matuyama epoch.
As no absolutely reliable way of constructíng this
time scale has yet been discovered, it may be said
that the controversy has not been finally settled.
However, it is currently considered that the major
event in the lower Matuyama must have been that
first discovered at Olduvai, but it may have been
preceded by one or two short (Reunion) events, and
succeeded by one shortevent (McDougall 1979).
Due to relatively discontinuous exposures and
the effects ofglacial climates, stratigraphic work in
the Jökuldalur area is difíicult, so a comprehensive
and detailed stratigraphy in the area is not yet
available. Watkins et al. (1975) sampled all flows in
Wensink's Hnjúksá section as well as three shorter
sections for paleomagnetic measurements and dat-
ing. Polarities and ages found by Wensink (1965; his
Figs. 7-XII and XIII should be interchanged) and
McDougall and Wensink (1966) were in general con-
firmed, as the lavas in this area are relatively un-
affected by secondary alteration. However, it
appeared that Wensink had assigned an “R?”
polarity to a weakly but normally magnetized flow,
and hence that the normal event in R1 was not split.
Wensink‘s polarity sequence in Tregagilsá across
the valley (1965, Fig. 2) was also supposed to in-
clude a normal event which Watkins et al. did not
Iocate; because of a westerly tilt, that section only
contains seven lava flows.
In 1979, H. Ó. Bragason (1981) sampled 60 lava
flows in Hnjúksá and Arnórsstaðahnjúkur,
Hnappá, Gilsá, Yzta-Rjúkandi, and Garðá at
Hjarðarhagi for laboratory measurements. He
confirmed the presence of a short normal geo-
magnetic polarity zone in Hnappá and a reverse one
in Yzta-Rjúkandi, butagain no evidence was found
for a split in the normal zone occurring in Hnappá,
Hnjúksá and Arnórsstaðahnjúkur.
It must therefore be concluded that the existence
ofaLower Matuyama event distinct from theOldu-
vai has not been proved in Jökuldalur. Should such
an event be found elsewhere, the name Gilsá would
not be appropriate for it.
Whiie Wensink was working in the Jökuldalur
96 JÖKULL 32. ÁR