5:15 PM - 6:45 PM
[SGL17-P06] Tephrostratigraphy in the Kurehayama Hills, Toyama Prefecture: Reevaluation of the Znp-Ohta tephra
Keywords:Znp-Ohta tephra, tephrostratigraphy, U-Pb age, trace element composition, tephrochronology, Pliocene
The Kurehayama Hills, Toyama Prefecture, which consist to the Pliocene-Pleistocene sedimentary layers, are considered to have been uplifted relating to the Kurehayama fault. The Kurehayama Hills consist of the Nishitoyama Sandstone Member, the Anyobo Sandstone Mudstone Alternation Member, the Chokeiji Sand Member and the Kurehayama Gravel Bed in an ascending order. The Tsuribe 1 (TR1) and Znp-Ohta tephras were found in the Nishitoyama Sandstone Member, the Taniguchi (Tng) tephra was found in the Chokeiji Sand Member, and the Kamitakara tephra was found in the Kurehayama Gravel Bed (Tamura, 2005). The Znp-Ohta tephra is one of the wide-spread tephras distributed in central Japan (Tamura et al., 2008). The depositional age of the Znp-Ohta tephra was estimated by zircon fission-track dating and magnetostratigraphy (ca. 5.2-3.3 Ma). Recently, zircons in the pyroclastic flow deposit of the Znp-Ohta tephra yielded the U-Pb age of 3.94 ± 0.07 Ma (Ueki et al., 2019). Therefore, the Znp-Ohta tephra is an important Early Pliocene marker tephra. In this study, we reevaluated the tephrostratigraphy in the area near the Teramachi Keyakidai Park, Toyama City, which was the type locality of the Teramachi (Trm) tephra corresponded to the Znp-Ohta tephra in Kurehayama Hills. Although the Trm tephra with ca. 4-5 m thickness was only reported in this outcrop (Tamura and Yamazaki, 2004), we found the newly two tephra units and identified three tephra units including the Trm tephra.
Observations of the outcrop in the type locality of the Trm tephra revealed 1) > 50 cm tephra unit, 2) > 2 m tephra unit including pumice and 3) > 2.5 m tephra unit in an ascending order, and ca. 50 cm gravel bed sandwiched between the units 2) and 3) (Fig.). The upper part of the tephra unit 3) consists of a sand layer with well-developed lamination including pumice. Sample A, B and C were collected from the tephra units 1), 2) and 3), respectively, and we measured mineral composition and chemical composition and refractive index of the volcanic glass in these samples. Major and trace elements composition analyses were conducted with EPMA (JEOL, JXA-8530F) and LA-ICP-MS (LA: Photon Machines Inc., Analyte G2, ICP-MS: Thermo Fisher Scientific, iCAP TQ), respectively, at Tono Geoscience Center, Japan Atomic Energy Agency. The refractive index measurement with a refractometer by the thermal immersion method (MIOT) was commissioned to Furusawa Geological Survey.
All samples contained bubble-wall type glass, and bubble-junction type glass was especially included in the sample B. While the major element composition and refractive index of volcanic glass were similar among sample A, B and C, these samples were discriminated by trace element composition (e.g., Ba/La, La/Y): Ba/La = 38.2, La/Y = 1.65 for sample A; Ba/La = 38.2, La/Y = 0.663 for sample B; Ba/La = 20.0, La/Y = 1.80 for sample C. Based on these characteristics, it is most likely that the tephra units 1), 2) and 3) correlate with the TR1 (~4.2 Ma), Znp-Ohta, and Tng (~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 units 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 units2) and 3), and the age gap between these units 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 units to determinate more precise age. The identification of absolute ages will contribute to improve chronological importance of these tephra as tephra marker in tephrochronology.
Observations of the outcrop in the type locality of the Trm tephra revealed 1) > 50 cm tephra unit, 2) > 2 m tephra unit including pumice and 3) > 2.5 m tephra unit in an ascending order, and ca. 50 cm gravel bed sandwiched between the units 2) and 3) (Fig.). The upper part of the tephra unit 3) consists of a sand layer with well-developed lamination including pumice. Sample A, B and C were collected from the tephra units 1), 2) and 3), respectively, and we measured mineral composition and chemical composition and refractive index of the volcanic glass in these samples. Major and trace elements composition analyses were conducted with EPMA (JEOL, JXA-8530F) and LA-ICP-MS (LA: Photon Machines Inc., Analyte G2, ICP-MS: Thermo Fisher Scientific, iCAP TQ), respectively, at Tono Geoscience Center, Japan Atomic Energy Agency. The refractive index measurement with a refractometer by the thermal immersion method (MIOT) was commissioned to Furusawa Geological Survey.
All samples contained bubble-wall type glass, and bubble-junction type glass was especially included in the sample B. While the major element composition and refractive index of volcanic glass were similar among sample A, B and C, these samples were discriminated by trace element composition (e.g., Ba/La, La/Y): Ba/La = 38.2, La/Y = 1.65 for sample A; Ba/La = 38.2, La/Y = 0.663 for sample B; Ba/La = 20.0, La/Y = 1.80 for sample C. Based on these characteristics, it is most likely that the tephra units 1), 2) and 3) correlate with the TR1 (~4.2 Ma), Znp-Ohta, and Tng (~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 units 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 units2) and 3), and the age gap between these units 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 units to determinate more precise age. The identification of absolute ages will contribute to improve chronological importance of these tephra as tephra marker in tephrochronology.