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Studies of tephrochronology in Japan have revealed a detailed time and space diagram of widespread marker tephra layers for the last 1Ma (for example, Machida, 2002), and these tephra layers have greatly contributed to the reconstruction of recent paleoenvionments in the Japan Islands and adjacent Seas. Since the middle 1980s, many tephra correlation data have been accumulated for older (early Pleistocene and Pliocene) strata in central Japan.
In central Japan, these Plio-Pleistocene strata are distributed in several sedimentary basins, i.e. the Osaka Group and the Kobiwako Group in Kinki area (for example Yoshilawa,1975), the Tokai Group and the Kakegawa Group in Tokai area ( for example Mizuno et al, 1987), the Kazusa Group in Kanto area (for example Satoguchi, 1995), the Sarumaru and Omine Formations and Hokuriku Group in Chubu area ( for example Tamura and Yamazaki,2004), the Uonuma Group in the Niigata area (or example Kishi and Miyawaki,1997). Thereby, some widespread tephra layers have been correlated precisely between the Kinki, Tokai, Kanto, Chubu and Niigata areas using stratigraphical data, i.e. magnetic stratigraphy, fission-track and K-Ar methods and petrographic data, i.e. megascopic features, mineral assemblages and the volcanic glass analysis of major elemental composition by electron microprobe (EPMA) and refractive indices of volcanic glass and phenocryst minerals. By this means, the Omine tephra (1.65Ma: Nagahashi, 1998), Ebisutoge-Fukuda tephra (1.75Ma: Yoshikawa et al., 1994), UN-MD2 tephra (2.65Ma: Kurokawa and Tomita, 2000), Habutaki1 tephra (2.7Ma: Tomita and Kurokawa, 1999) and Siuri-Itayama tephra (3.3-3.5Ma: Kurokawa et al., 1998) and Znp-Ohta tephra (3.7Ma: Kurokawa and Tomita, 1998) are recognized as widespread tephras covering much of central Japan. However, compared with the late Pleistocene deposits, there are relatively few widespread tephra deposits and localities where these are exposed are sparse. For these reasons, future broader-based research into the more substantial tephra layers deposits a Plio-Pleistocene age is expected.
This research was undertaken to establish tephra correlations based on the major and trace element composition of volcanic glass determined by EPMA and inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis. This has enabled many widespread tephra layers of Pliocene and early Pleistocene to be identified and correlated extensively in central Japan. Here, a catalog of 15 widespread tephra layers is presented which provides the foundations for the establishment of a regional Pliocene-Pleistocene chronostratigraphy, and helps to explain the formation of unconformities and volcanic activity in central Japan during the Pliocene and early Pleistocene.
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