Mathematics – Logic
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja....11595w&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #11595
Mathematics
Logic
Scientific paper
The basalt stratigraphy of northeast Iceland exposed in the Jokuldalur valley has been the focus of continued paleomagnetic and geochronological research for almost 40 years since in the mid 1960's McDougall and Wensink (1966) proposed the existence of a short-lived normal event in the reversed Matuyama chron at 1.61 ± 0.05 Ma. This subchron was named the Gilsa, and it demonstrated the possibility of short normal polarity excursions of the magnetic field from the dominant -- in the case of Matuyama Chron, reversed -- direction of the earth magnetic field. The Gilsa subchron proved elusive, however: stratigraphic continuity was questioned, and lower in the stratigraphy a second normal subchron, considered as the Olduvai subchron, could not conclusively be distinguished from the Gilsa. Recently, Udagawa et al. (1999) proposed that the Gilsa is a short-lived subchron younger than the Olduvai subchron. We independently resampled two sections (Hnjuksa and Hnappa) of the type locality of the Gilsa subchron. Paleomagnetic analysis of the basalt stratigraphy yielded two virtually identical patterns: normal polarity basalt flows could be correlated with the Gauss Chron, and overlying reversed polarity basalt flows with the lower Matuyama Chron. Both sections contain a short normal level ca. 150 m above the base of the Matuyama Chron. In an attempt to conclusively resolve the magnetostratigraphy of the Gilsa type sections, we have dated these samples using newly developed protocols for 40Ar/39Ar laser incremental heating of ground mass basalt samples. Surprisingly, the two parallel sections gave drastically different results. While the Gauss-Matuyama boundary seems well established, in the Hnjuksa section the reversed basalt immediately underlying the normal subchron is 2.271 ± 0.074 Ma, while the first normal flow, yielded an age of 1.836 ± 0.041 Ma: an age commonly accepted for the Olduvai subchron. In the Hnappa section, so far, we obtained a poorly defined age of 2.26 ± 0.33 Ma for the highest reversed flow immediately below the normal subchron; an age indistinguishable from that of the overlying normal flow, 2.255 ± 0.056 Ma. The normal subchron ends at 2.006 ± 0.050 Ma. This is an age range that cannot be attributed to the Olduvai, but only to the Reunion subchron. Reversed basalts immediately overlying the normal basalts of the Reunion subchron yielded ages of 1.507 ± 0.021 Ma and 1.441 ± 0.019 Ma indicating a hiatus in the stratigraphy. We have found no evidence for the Gilsa subchron at 1.61 Ma. Instead, we demonstrate that both sections are incomplete: in the Hnjuksa section a hiatus from 2.271 Ma until 1.836 Ma has caused the absence of the Reunion subchron, whereas, surprisingly, in the Hnappa section the Reunion subchron is present, but a hiatus immediately above the Reunion subchron caused the absence of the Olduvai subchron. While having resolved the controversy surrounding the Gilsa subchron, we have in fact demonstrated that because of the chaotic nature of volcanic flow, basalt stratigraphies can only be regarded as ideal records of earth magnetism on the condition that accurate and precise dating has been done. -- McDougall I., Wensink H. (1966) Earth and Planet. Sci. Lett. 1. 232-236. -- Udagawa S., Kitagawa H., Gudmundsson A., Hiroi O., Koyaguchi T., Tanaka H., Kristjansson L., Kono M. (1999) Phys. Earth and Planet Int. 115. 147-171. -- Watkins N.D., Kristjansson L., McDougall I. (1975) Earth and Planet Sci. Lett. 27. 436-444.
Langereis C.
Wijbrans Jan
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