Tephrochronological study of the Hokuriku Group—The age of the Hokuriku Group by tephra stratigraphy and correlation to the wide-spread tephra layers—

Abstract

The Plio-Pleistocene Hokuriku Group includes the Otogawa and the Omma Formations distributed along the Japan Sea in Ishikawa and Toyama Prefectures, central Japan. In order to characterize the tephra layers within the Hokuriku Group, authors investigated the stratigraphy, thickness, color, shape of volcanic glass, mineral composition, refractive indices of volcanic glass and orthopyroxene, and the major and trace element composition of volcanic glass. Thirty tephra layers were detected from the Hokuriku Group in the study area. Some tephra layers are traceable over several kilometers, and are important as key beds. They were named as the Tsuribe 1, Sunakodanil, Sunakodani2, Teramachi, MT2・Chk, O1, O2, O3 tephra layers in an ascending order. These tephra layers in the Hokuriku Group are correlated with the tephra layers of the other Plio-Pleistocene Groups in central Japan by lithologic and petrographic properties. Ten wide-spread tephra layers : Ya-4, Ya-5 and Sakai, Kosugaya :4 Ma, Ohta-Znp : 3.7Ma, T2, Tng: 2.2-2.3 Ma, Bando2 : 2.1 Ma, Eb-Fukuda : 1.75 Ma, Om-SK110 : 1.65 Ma, KMT : 0.6 Ma were found from the Hokuriku Group. Therefore, the age of the Takakubo Formation is estimated to be 3 Ma at the period of the Gauss chron, and the age of lower part of the Omma Formation is about 2.3 Ma. The unconformity between the Takakubo and the Omma Formations, in which it was previously thought that a long-term period of non-sedimentation existed, does not indicate a long-term interval of time. The age of the Kurehayama gravel bed at the eastern edge of Toyama Plain is correlated with the lowest member of the Omma Formation which interbeds Taniguchi tephra (2.2-2.3Ma). Therefore, it is clear that the Hida Mountain began to be upheaved before 2.2-2.3 Ma.