八ケ岳東麓野辺山原周辺の第四系 : とくに広瀬層群の層序と古地磁気について

概要

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A thick lacustrine deposits with large amount of various volcanic products of the middle Pleistocene age are widely developed around Nobeyamahara, eastern foot of the Yatsukatake Volcanic Chain. These deposits have been called the Hirose Group (GOHARA, Y. et. al., 1978). The Hirose Group is divided into five stratigraphic units on the basis of the presence of disconformity: the Lowest, Lower, Middle, Upper and the Uppermost Unit. A close relation between volcanic activity and growth of the palaeo-lake basin is clearly recognized. Disconformities in the Hirose Group might have resulted from the fluctuations of the palaeolake basin. The results of the analysis of palaeo-lake water level changes in this region suggest that some regional tectonic movement occurred after the deposition of the Upper part of the Hirose Group. The palaeomagnetic study on the lacustrine sediments of the Hirose Group revealeded the following results. The remanent magnetization of the present samples demagnetized at 150oe or 200oe in 400Hz alternating field show the best grouping which estimated to be primary magnetizations. The palaeomagnetic polarity inferred from most of the Hirose Group was NORMAL, the results obtained from the three horizons belonging to the Lower and Upper part suggest the presence of INTERMEDIATE polarity. This INTERMEDIATE polarity suggests the presence of new geomagnetic excursion in the Brunhes normal epoch.
地学団体研究会の論文
1980-07-25