近畿・東海地方の鮮新・更新統古地磁気層序

抄録

Since 1951, many studies have been published on the magnetostratigraphy of the Plio-Pleistocene strata of the Kinki district. In the 1970's, especially, various sections of the Osaka and Kobiwako Groups, including bottom sediments of Lake Biwa, as well as their correlatives in the Tokai district, have been studied repeatedly.
The sediments are mostly lacustrine and fluvial, and are frequently intercalated with volcanic ash. The Osaka Group also contains interbedded marine clay layers. The sediments, therefore, are suitable for establishing a combined litho-, tephro-, magneto- and biostratigraphy and radiometric chronology. Such a chronology can be used for testing the reliability of primary remanent magnetization of the sediments by the measurements of multiple samples from various localities and correlated by means of volcanic ash intercalations. Based on the latest studies, including unpublished results of studies by the present writer, the Plio-Pleistocene magnetostratigraphy of the Kinki district has been compiled (Fig. 2). Important points include:
1. The Gauss/Matuyama boundary is drawn between the Kosaji and Hazama Volcanic Ash in the Kobiwako Group, and between the T2 and Shimakumayama Volcanic Ash (2.3-2.4m.y.), in the Osaka Group.
2. The Matuyama/Brunhes boundary is about 10m above the Biotite Volcanic Ash (0.7m.y.) in the Kobiwako Group, and under the Ma4 clay layer in the Osaka Group.
3. Five normal polarity events have been recognized in the Matuyama Reversed Epoch. They are, in ascending order, as follows:
a-. Shiba event (about 2.3m.y.)
b-. Sushirodani event (about 2.2m.y.)
c-. Naka-Nango event (1.7-1.8m.y.)
d-. Pink-Komyoike event (1.1-1.2m.y.)
e-. Kamikatsura event
Some of the five events remain uncertain as to correlation with those reported previously in the Matuyama Epoch.
4. Five reversed polarity events or excursions were detected in the Brunhes Normal Epoch. The older three were named Biwa I, II and III events, and the younger two may be correlatives with the Lashamp and Blake events. The age of the Biwa III Event has been estimated to be about 0.33m.y..