2:15 PM - 2:30 PM
[HDS11-03] Formation age of liner depression from tephrochronology on the Sekita Mountains, central Japan.
Keywords:tephrochronology, deep-seated gravitational slope deformation, Sekita mountains
Introduction
Deep-seated gravitational slope deformation (DGSD) features are considered phenomenon preceding deep-seated landslides (Chigira et al., 2013). On the other hand, Kojima et al. (2015) and Kojima (2018) pointed out the existence of long-term existing DGSD features in the Kii peninsula that cannot be regarded as this phenomenon. To assess DGSD features as this phenomenon, it is important to understand when and how DGSD features were formed and how they will change in the future. Therefore, it is necessary to accumulate more data of the formation ages of long-term existing DGSD features.
In our study site, the Sekita Mountains, linear depressions, double ridges, and uphill-facing scarps with lengths of ca. 20 km are intermittently developed along the ridge. Around Nonomi, Sekita and Hiramaru passes, large scale linear depressions, wetlands and ponds are formed, which are considered DGSD features (e.g., Yagi and Inokuchi, 2017). In addition, Sekiguchi et al. (2003) reported a widespread tephra DKP (ca. 60 ka) in the Nonomi Moor sediment. This implies that this moor was formed before 60 ka. In the 50 km west of the Sekita Mountains, there is the Myoko Volcano Group. Its tephrostratigraphy has been established by Hayatsu (2008) and Volcanic Ash Research Group for Nojiri-ko Excavation (1993). Tephrochronological studies in this area can provide the formation age of DGSD features which is beyond the applicability of radiocarbon dating. In this study, we aimed to reveal the formation age of linear depressions on the ridge of the Sekita Mountains based on tephrochronology.
Method
We conducted geomorphic mapping using aerial photographs, coring survey in Nonomi Moor and Chayaike Moor using a Portable engine-driven Percussion Coring System (PPCS; Kaneda et al., 2018) and a hand auger (HA), and tephra analyses such as counting mineral composition, measuring refractive index of volcanic glass shards and heavy minerals, and determining major element composition of volcanic glass shards.
Result and Discussion
Around the Nonomi pass, there are several ridge-parallel uphill-facing scarps with WNW-ESE-trending. On the ridge, liner depressions, Nonomi Moor and Nonomi Pond, were formed by uphill-facing scarps. The NNM-P1 core obtained from the Nonomi Moor was 7 m in length. However, NNM-P1 core did not reach the base of filling sediment. This core is composed of fine grained organic-rich silty sediments. Three tephra layers, AT (ca. 30 ka), DKP, and Iz-Kt (MIS6), were identified at 58 cm depth, 276–279 cm depth, and at 610–620 cm depth, respectively.
Around the Sekita pass, there are several ridge-parallel uphill-facing scarps with NE-SW-trending. Double ridge and linear depressions are formed by DGSD (Yagi and Iguchi, 2017). The Chayaike Moor is located in a linear depression in the center of the gentle slope. The CYI-P1 core obtained from the Chayaike Moor was 6 m in length. However, CYI-P1 core did not reach the base of filling sediment. The upper 2 m of the CYI-P1 core was peat, the depth of 200–280 cm was inorganic gray to blue-gray silt, and below that was composed silt with small gravel. Five tephra layers, Yk-KGc (ca. 1 ka), My-Ot (5 ka) or My-A (6 ka) K-Ah glass shards (ca. 7.3 ka), As-K (17 ka), and AT were identified at 25–27 cm depth, 133 cm depth, 170–180 cm depth, 200 cm depth, at 250 cm depth, respectively.
Although the NNM-P1 and CYI-P1 cores did not reach the base of filling sediments, the formation ages of these linear depressions are estimated to be before 30 ka in the Chayaike Moor and 130 ka in the Nonomi Moor based on tephrochronology. Because there are many fresh collapsed landforms in the Sekita Mountains, it is necessary to study the relationship between the long-term existing DGSD feature and deep-seated landslides in the future.
Deep-seated gravitational slope deformation (DGSD) features are considered phenomenon preceding deep-seated landslides (Chigira et al., 2013). On the other hand, Kojima et al. (2015) and Kojima (2018) pointed out the existence of long-term existing DGSD features in the Kii peninsula that cannot be regarded as this phenomenon. To assess DGSD features as this phenomenon, it is important to understand when and how DGSD features were formed and how they will change in the future. Therefore, it is necessary to accumulate more data of the formation ages of long-term existing DGSD features.
In our study site, the Sekita Mountains, linear depressions, double ridges, and uphill-facing scarps with lengths of ca. 20 km are intermittently developed along the ridge. Around Nonomi, Sekita and Hiramaru passes, large scale linear depressions, wetlands and ponds are formed, which are considered DGSD features (e.g., Yagi and Inokuchi, 2017). In addition, Sekiguchi et al. (2003) reported a widespread tephra DKP (ca. 60 ka) in the Nonomi Moor sediment. This implies that this moor was formed before 60 ka. In the 50 km west of the Sekita Mountains, there is the Myoko Volcano Group. Its tephrostratigraphy has been established by Hayatsu (2008) and Volcanic Ash Research Group for Nojiri-ko Excavation (1993). Tephrochronological studies in this area can provide the formation age of DGSD features which is beyond the applicability of radiocarbon dating. In this study, we aimed to reveal the formation age of linear depressions on the ridge of the Sekita Mountains based on tephrochronology.
Method
We conducted geomorphic mapping using aerial photographs, coring survey in Nonomi Moor and Chayaike Moor using a Portable engine-driven Percussion Coring System (PPCS; Kaneda et al., 2018) and a hand auger (HA), and tephra analyses such as counting mineral composition, measuring refractive index of volcanic glass shards and heavy minerals, and determining major element composition of volcanic glass shards.
Result and Discussion
Around the Nonomi pass, there are several ridge-parallel uphill-facing scarps with WNW-ESE-trending. On the ridge, liner depressions, Nonomi Moor and Nonomi Pond, were formed by uphill-facing scarps. The NNM-P1 core obtained from the Nonomi Moor was 7 m in length. However, NNM-P1 core did not reach the base of filling sediment. This core is composed of fine grained organic-rich silty sediments. Three tephra layers, AT (ca. 30 ka), DKP, and Iz-Kt (MIS6), were identified at 58 cm depth, 276–279 cm depth, and at 610–620 cm depth, respectively.
Around the Sekita pass, there are several ridge-parallel uphill-facing scarps with NE-SW-trending. Double ridge and linear depressions are formed by DGSD (Yagi and Iguchi, 2017). The Chayaike Moor is located in a linear depression in the center of the gentle slope. The CYI-P1 core obtained from the Chayaike Moor was 6 m in length. However, CYI-P1 core did not reach the base of filling sediment. The upper 2 m of the CYI-P1 core was peat, the depth of 200–280 cm was inorganic gray to blue-gray silt, and below that was composed silt with small gravel. Five tephra layers, Yk-KGc (ca. 1 ka), My-Ot (5 ka) or My-A (6 ka) K-Ah glass shards (ca. 7.3 ka), As-K (17 ka), and AT were identified at 25–27 cm depth, 133 cm depth, 170–180 cm depth, 200 cm depth, at 250 cm depth, respectively.
Although the NNM-P1 and CYI-P1 cores did not reach the base of filling sediments, the formation ages of these linear depressions are estimated to be before 30 ka in the Chayaike Moor and 130 ka in the Nonomi Moor based on tephrochronology. Because there are many fresh collapsed landforms in the Sekita Mountains, it is necessary to study the relationship between the long-term existing DGSD feature and deep-seated landslides in the future.