11:00 AM - 1:00 PM
[HDS11-P01] Yakedake Nakao tephra: an attempt to use the local tephra as a Late Holocene age marker for the southern part of the Northern Japan Alps
Tephrostratigraphy is one of the methods to reveal sedimentary age of Quaternary period layers by using tephras with a known eruptive age. Machida and Arai (2003) compiled the characteristics of the tephras in and around Japan and McLean et al. (2018) investigated Quaternary wide-spread tephras in east Asia in the core of Lake Suigetsu in Fukui Prefecture. Owing to many previous studies including the above studies, the method of sedimentary age determination by using Quaternary wide-spread tephra in east Asia was adequately established. However, in areas with many volcanoes such as Japan, it is difficult to detect the output peaks of wide-spread tephras due to the strong influence of nearby volcanic ejecta. However tephras with characteristic geochemical signatures by minor eruptions from nearby volcanos has potential utility as an important age marker.
Area around Kamikouchi in the southern part of the Northern Japan Alps is strongly affected by the eruption from Mt. Yakeake. We found three distinctive tephras, and prepared 1) one sample of glassy local tephra between 141 cm and 143 cm in depth in a hand-auger boring core drilled at near the Kinugasano-ike pond, located about 4.7 km northeast of Mt. Yakedake (Kariya and Takaoka, 2019) and 2) two samples of glassy tephras embedded the Nakao pyroclastic flow deposit, which is considered to be the ejecta from the last magma eruption about 2,300 years ago, distributed about 1.5 km north-northwest of Mt. Yakedake. We investigated particle composition of ultrafine-grained sand in these tephras, chemical composition and refractive index of volcanic glasses.
More than 200 particles per sample, we classified volcanic glasses (microlite-bearing, blocky, fluted and vesicular type glasses) and crystal minerals (quartz, plagioclase, hornblende, biotite and pyroxene) according to McLean et al (2018) by using a polarizing microscope. As a result, approximately 70% of constituent particles were volcanic glasses and most of them were microlite-bearing and blocky type glasses. We measured chemical composition of mainly blocky type glasses using Electron Probe Micro Analyzer (EPMA) at Tono Geoscience Center, Japan Atomic Energy Agency. As a result, these volcanic glasses were characterized by high SiO2, K2O, and Na2O and low Al2O and FeO. The chemical composition was distinguishable from those of volcanic glasses in the wide-spread tephras in east Asia shown by McLean et al (2018). We measured refractive index of 30 thin volcanic glassed of microlite and blocky types for each tephra using a refractometer (RIMS) by the thermal immersion method at Chuo University. As a result, the refractive index of volcanic glasses showed 1.4958-1.5011 with the peak top in the range of 1.4970 to 1.4990.
In terms of the particle composition of ultrafine-grained sand, chemical composition of volcanic glasses and refractive index, the tephras found at the Kinugasano-ike pond core was identified with the tephra embedded in the Nakao pyroclastic flow deposits from eruption of Mt. Yakedake about 2,300 years ago. Therefore, the tephra with the above characteristics can be used as a useful index for the southern part of the Northern Alps about 2,300 years ago.
Area around Kamikouchi in the southern part of the Northern Japan Alps is strongly affected by the eruption from Mt. Yakeake. We found three distinctive tephras, and prepared 1) one sample of glassy local tephra between 141 cm and 143 cm in depth in a hand-auger boring core drilled at near the Kinugasano-ike pond, located about 4.7 km northeast of Mt. Yakedake (Kariya and Takaoka, 2019) and 2) two samples of glassy tephras embedded the Nakao pyroclastic flow deposit, which is considered to be the ejecta from the last magma eruption about 2,300 years ago, distributed about 1.5 km north-northwest of Mt. Yakedake. We investigated particle composition of ultrafine-grained sand in these tephras, chemical composition and refractive index of volcanic glasses.
More than 200 particles per sample, we classified volcanic glasses (microlite-bearing, blocky, fluted and vesicular type glasses) and crystal minerals (quartz, plagioclase, hornblende, biotite and pyroxene) according to McLean et al (2018) by using a polarizing microscope. As a result, approximately 70% of constituent particles were volcanic glasses and most of them were microlite-bearing and blocky type glasses. We measured chemical composition of mainly blocky type glasses using Electron Probe Micro Analyzer (EPMA) at Tono Geoscience Center, Japan Atomic Energy Agency. As a result, these volcanic glasses were characterized by high SiO2, K2O, and Na2O and low Al2O and FeO. The chemical composition was distinguishable from those of volcanic glasses in the wide-spread tephras in east Asia shown by McLean et al (2018). We measured refractive index of 30 thin volcanic glassed of microlite and blocky types for each tephra using a refractometer (RIMS) by the thermal immersion method at Chuo University. As a result, the refractive index of volcanic glasses showed 1.4958-1.5011 with the peak top in the range of 1.4970 to 1.4990.
In terms of the particle composition of ultrafine-grained sand, chemical composition of volcanic glasses and refractive index, the tephras found at the Kinugasano-ike pond core was identified with the tephra embedded in the Nakao pyroclastic flow deposits from eruption of Mt. Yakedake about 2,300 years ago. Therefore, the tephra with the above characteristics can be used as a useful index for the southern part of the Northern Alps about 2,300 years ago.