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Jökull - 01.12.1982, Qupperneq 100

Jökull - 01.12.1982, Qupperneq 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
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