11:00 AM - 1:00 PM
[HQR04-P04] Re-examination of Plio-Pleistocene tephrochronology around Oyabe City, Toyama Prefecture, Japan
Keywords:tephrochronology, Plio-Pleistocene, Hokuriku Group
Introduction: The Pliocene to Pleistocene strata of the Hokuriku Group are distributed in the Oyabe area, located in the western part of Toyama Prefecture, and include many tephra beds. There are two recent studies on the correlation and chronology of these tephra beds, but no correlation between them has been made in the Oyabe area. The chemical composition of the volcanic glass shards in the tephra beds is an important factor in these studies, but the compositional data cannot be statistically analyzed as the real data because of the constant-sum constraint (Aitchison, 1986). In this study, the Pliocene-Pleistocene tephra stratigraphy in the Oyabe area was re-examined by applying a method that solves the problem of constant-sum constraint.
Stratigraphy of the study area: The Pliocene-Pleistocene strata in the Oyabe area consist of the Otogawa, Omma and Hanyu Formations. In the study area, these strata are distributed in a NE-SW strike with a SW dip.
Method: Four routes were surveyed in the study area: Hachibuse, Hanyu, Yokodani and Tagawa, and tephra beds were collected at each route. Then, the mineral composition and volcanic glass shape classification of the tephra samples were measured. Also the chemical compositions of the major components of the volcanic glass shards were analyzed using EPMA, and log-ratio analysis (additive log-ratio transformation; Ohta and Arai, 2006) was applied to the analyzed values to free them from constant-sum constraints. The normalized components of the additive log-ratio transformation were selected based on the invariance test of Ohta et al. (2011) and element mobility, and the results of the subsequent analyses were compared. Then, a cluster analysis was performed using the mean value of the log-ratio data of samples as a multivariate to select the tephra layers that could be correlated.
Results: 13 tephra layers (Hb1 to Hb13) were identified in Hachibuse, 2 in Hanyu (Hny1 and Hny2), 1 in Yokodani (Yk-1), and 1 in Tagawa (Tg-1). Hb12, Hb13 and Hny1, Hny2 are successive units, respectively. These tephra beds have well-developed lamination and are partly embedded in siltstone to fine-grained sandstone containing shell fossils, suggesting that they were deposited at the bottom of the water. Of these, 15 layers are vitric ash except for 2 layers (Hb10 and Hb11) in the upper part of Hachibuse route, and the Hb10 and Hb11 are vitric crystal ash. The shapes of the volcanic glasses were diverse, and no trend was observed. K2O, SiO2, Al2O3, and TiO2 were used as normalized components. As a result of the cluster analysis, the data were roughly divided into 10-layered groups of Hb1 to Hb7, Hb9 to Hb11, and 7-layered groups of Hb8, Hb12 and Hb13, Hny1 and Hny2, Yk1, and Tg1 when normalized by any components. Only the latter was considered to be a tephra layer that could be correlated, and the distribution of log-ratio data was confirmed. As a result, the 7 tephra beds of FeO* were divided into 3 layers of Hb8, Yk1, and Tg1, and 4 layers of Hb12, Hb13, Hny1, and Hny2, regardless of the normalized composition. The compositional data also showed a tendency to divide into 2 groups, but it was not as clear as the log-ratio data.
Discussion and summary: Hb8-Yk1-Tg1 and Hb12-Hb13-Hny1-Hny2 can be considered to be correlated each other because of the similarities in the laminae and mineral compositions. Yk1 and Tg1 are corresponding to the "Dai-san pumice bed" (Makiyama, 1930; Sumi et al., 1989), which has been reported to be distributed in the lower part of the Omma Formation from Yokodani to Ueno in Oyabe City, and Hb12 and Hb13 are corresponding to the lower and upper parts of O2 reported by Tamura and Yamazaki (2004).
References: Aitchison, J., ed., Chapman & Hall, 416p, 1986; Makiyama, Chikyu, 14, 161-174, 1930; Ohta and Arai, The Jour. Geo. Soc. Japan, 112, 173-187, 2006; Ohta et al, Math. Geosci, 421-434, 2011; Sumi et al., 1:50,000 Geological Map of the Isurugi area, Geo. Sur. Japan, 1989; Tamura and Yamazaki, The Jour. Geo. Soc. Japan, 110, 417-436, 2004.
Stratigraphy of the study area: The Pliocene-Pleistocene strata in the Oyabe area consist of the Otogawa, Omma and Hanyu Formations. In the study area, these strata are distributed in a NE-SW strike with a SW dip.
Method: Four routes were surveyed in the study area: Hachibuse, Hanyu, Yokodani and Tagawa, and tephra beds were collected at each route. Then, the mineral composition and volcanic glass shape classification of the tephra samples were measured. Also the chemical compositions of the major components of the volcanic glass shards were analyzed using EPMA, and log-ratio analysis (additive log-ratio transformation; Ohta and Arai, 2006) was applied to the analyzed values to free them from constant-sum constraints. The normalized components of the additive log-ratio transformation were selected based on the invariance test of Ohta et al. (2011) and element mobility, and the results of the subsequent analyses were compared. Then, a cluster analysis was performed using the mean value of the log-ratio data of samples as a multivariate to select the tephra layers that could be correlated.
Results: 13 tephra layers (Hb1 to Hb13) were identified in Hachibuse, 2 in Hanyu (Hny1 and Hny2), 1 in Yokodani (Yk-1), and 1 in Tagawa (Tg-1). Hb12, Hb13 and Hny1, Hny2 are successive units, respectively. These tephra beds have well-developed lamination and are partly embedded in siltstone to fine-grained sandstone containing shell fossils, suggesting that they were deposited at the bottom of the water. Of these, 15 layers are vitric ash except for 2 layers (Hb10 and Hb11) in the upper part of Hachibuse route, and the Hb10 and Hb11 are vitric crystal ash. The shapes of the volcanic glasses were diverse, and no trend was observed. K2O, SiO2, Al2O3, and TiO2 were used as normalized components. As a result of the cluster analysis, the data were roughly divided into 10-layered groups of Hb1 to Hb7, Hb9 to Hb11, and 7-layered groups of Hb8, Hb12 and Hb13, Hny1 and Hny2, Yk1, and Tg1 when normalized by any components. Only the latter was considered to be a tephra layer that could be correlated, and the distribution of log-ratio data was confirmed. As a result, the 7 tephra beds of FeO* were divided into 3 layers of Hb8, Yk1, and Tg1, and 4 layers of Hb12, Hb13, Hny1, and Hny2, regardless of the normalized composition. The compositional data also showed a tendency to divide into 2 groups, but it was not as clear as the log-ratio data.
Discussion and summary: Hb8-Yk1-Tg1 and Hb12-Hb13-Hny1-Hny2 can be considered to be correlated each other because of the similarities in the laminae and mineral compositions. Yk1 and Tg1 are corresponding to the "Dai-san pumice bed" (Makiyama, 1930; Sumi et al., 1989), which has been reported to be distributed in the lower part of the Omma Formation from Yokodani to Ueno in Oyabe City, and Hb12 and Hb13 are corresponding to the lower and upper parts of O2 reported by Tamura and Yamazaki (2004).
References: Aitchison, J., ed., Chapman & Hall, 416p, 1986; Makiyama, Chikyu, 14, 161-174, 1930; Ohta and Arai, The Jour. Geo. Soc. Japan, 112, 173-187, 2006; Ohta et al, Math. Geosci, 421-434, 2011; Sumi et al., 1:50,000 Geological Map of the Isurugi area, Geo. Sur. Japan, 1989; Tamura and Yamazaki, The Jour. Geo. Soc. Japan, 110, 417-436, 2004.