HR: 1330h
AN: V32C-1040    [PDF]
TI: Basalt Chronostratigraphy of NE Iceland: Implications for the Geomagnetic Polarity Time Scale
AU: * Wijbrans, J R
EM: wijj@geo.vu.nl
AF: Department of Isotope geochemistry, Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085, Amsterdam, 1081HV Netherlands
AU: Langereis, C G
EM: langer@geo.uu.nl
AF: Paleomagnetic Laboratory 'Fort Hofddijk' Utrecht University, Budapestlaan 17, Utrecht, 3584 CD Netherlands
AB: The flood basalts exposed in the J\"{o}kuls\'{a}\'{a} dal valley in northeast Iceland show normal magnetic field directions consistent with emplacement during the Gauss Chron, and reversed magnetic field acquired during the Matuyama Chron. In the lower Matuyama basalts, an apparently continuous horizon containing normally magnetized flows has been recognized that was defined as belonging to the Olduvai event (Wensink 1964). A separate normal flow above the Olduvai was interpreted as the Gils\'{a} event. The age of this event was established at 1.61 $\pm$ 0.05 Ma by K-Ar dating of samples from the Hnjuks\'{a} valley, a small streambed draining into the main river from the basalt plateau to the west (McDougall and Wensink 1966). This age was revised to 1.62 $\pm$ 0.08 Ma in a subsequent study (Watkins et al. 1975. Both results have been recalculated to be consistent with current conventions concerning the decay constant and isotopic abundance of $^{40}$K). Based on the original paleomagnetic field work and the K-Ar results McDougall and Wensink (1966) suggested an average periodicity in the rate of basalt eruptions of once per 40 kyr, and implied regional continuity of individual flows. In a more recent paper, Udagawa et al. (1999) dated normally magnetized flows in the lower Matuyama in the main J\"{o}kuls\'{a} \'{a} dal river bed ca 10 km upstream from the original Hnjuks\'{a} section applying the conventional K-Ar dating technique. They found ages of 1.61 Ma for the normal flows, which they interpreted to be consistent with the Watkins et al. (1975) results from the Hnjuks\'{a} section, and thus belonging to the Gils\'{a} event. We independently resampled the Hnjuks\'{a} section and a second, parallel section along the Hnapp\'{a} streambed 3 km north of the Hnjuks\'{a} section. Paleomagnetic analysis indicates that both sections contain normal flows in the lower Matuyama at ca 500 - 550 m altitude, which could be interpreted as confirmation of the Olduvai subchron. There is no evidence for a separate normal flow above the Olduvai indicating a younger Gils\'{a} event. We applied high resolution $^{40}$Ar/$^{39}$Ar laser incremental heating to selected flows of both the Hnjuks\'{a} and the Hnapp\'{a} sections. We found that the normally magnetized flows of the Hnjuks\'{a} valley yielded ages of 1.82 $\pm$ 0.03 Ma and 1.71 $\pm$ 0.04 Ma. This age is consistent with emplacement of these basalts during the Olduvai Normal Subchron. As our results are based on well defined plateaus and isochrons, we conclude that in the original type section of the Gils\'{a} Subchron, our new $^{40}$Ar/$^{39}$Ar ages of the normal flows disproves the existence of a separate Gils\'{a} event. The ages of the normal flows in the adjacent Hnapp\'{a} section turned out to be even older: 2.01 $\pm$ 0.05 Ma and 2.26 $\pm$ 0.06 Ma, which would be at a 95 percent confidence level consistent with the R\'{e}union Subchron. From our analysis we draw two main conclusions: 1. In the type section, the normal flows previously attributed to the Gils\'{a} event, can be reconciled with emplacement during the Olduvai Subchron, and 2. Event at short distances the flood basalt flows in the J\"{o}kuldalur area are discontinuous, and cannot be correlated using magnetostratigraphy alone. This questions the conclusion of Udagawa et al. (1999) who argue that they identified flows belonging to the Gils\'{a} Subchron at much larger distances from the type section.
DE: 1035 Geochronology
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
DE: 9325 Atlantic Ocean
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting