Tephrostratigraphy in the Kurehayama Hills, Toyama Prefecture; Reevaluation of the Znp-Ohta tephra

Kagami, Saya   ; Yasue, Kenichi*; Nagata, Mitsuhiro   ; Yokoyama, Tatsunori   ; Tamura, Itoko*

The Kurehayama Hills consist to the Pliocene-Pleistocene sedimentary layers, where the Znp-Ohta tephra was found. The Znp-Ohta tephra is one of the wide-spread tephras and an important Early Pliocene marker tephra. In this study, we reevaluated the tephrostratigraphy in the type locality of the Teramachi (Trm) tephra corresponded to the Znp-Ohta tephra in Kurehayama Hills. Observations of the outcrop in the type locality of the Trm tephra revealed three tephra layer 1), 2), and 3) in an ascending order, and gravel bed sandwiched between the layers 2) and 3). Based on tephra characteristics (major and trace element composition and refractive index of volcanic glass), it is most likely that the tephra units 1), 2) and 3) correlate with the Tsuribe1 (4.2 Ma), Znp-Ohta (3.9 Ma), and Taniguchi (2.2 Ma) tephras, respectively. Previously, the Znp-Ohta tephra had been only identified at the type locality of the Trm tephra. This study clarified that the tephra deposits from 4.2 to 2.2 Ma were overlying in this outcrop of several meters. In addition, no other outcrops have been found where three tephra layers could be continuously identified. The results in this study are important to understand the formation history of the Kurehayama Hills. Furthermore, the gravel bed was found between the tephra layer 2) and 3), and the age gap between these layers was 1 Myr. This unconformity suggests that the Znp-Ohta tephra in this area was thicker than the currently deposited layer thickness. In the future, we plan to conduct zircon U-Pb dating for three tephra layers to determinate more precise age. The identification of absolute ages will contribute to improve chronological importance of these tephra as tephra marker in tephrochronology.