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[MIS15-P05] Stratigraphy and ages of a drilled core recovered from a shallow depression on a landslide body in Happosawa Basin, Happo One-ridge, Northern Japanese Alps, Japan
Keywords:Landslide, Happo-One, 14C age
In recent years, there has been an attempt to elucidate the interrelationships among geomorphic development, vegetation history, and biogeography of ponds and wetlands in the alpine and subalpine zones of the Northern Japanese Alps. These studies have been conducted mainly in the Kamikochi area in the southern part of the Northern Japanese Alps. In this study, we focused on the northern part of the North Japanese Alps, in which humid climate with heavy rain and snow is dominant than in the southern part of the North Japanese Alps, and conducted a geological and geomorphological survey from the same perspectives as previous studies.
A large landslide site exists in Happosawa Basin, Happo One-ridge. We obtained a columnar core (HPO-2021) 239 cm long by a hand auger in a shallow depression (HDA) formed on the large landslide body. Based on visual observation of the core, HPO-2021 was divided into four sedimentary facies zones: H1 (136 cm to 239 cm), H2 (42 cm to 136 cm), H3 (22 cm to 42 cm), and H4 (0 cm to 22 cm). The H1 to H3 zones consist of gravel layers with little or no humus. In the H3 zone, thin layers of brownish humic silt are sandwiched between the sand layers. In the H4 zone, humic silt is dominant. Also, six samples were collected for dating. Depositional ages of the four sedimentary facies zones are estimates as follows: H1 zone is: before ca. 3930 cal BP (up to about 4270 cal BP); H2 zone is: 3100 to 3930 cal BP; H3 zone is: 1330 to 3100 cal BP and H4 zone is: 0 to 1330 cal BP. Environmental changes around the bottom slope of HDA are considered as follows: The HDA already existed at least ca. 3930 cal BP, and sand and gravel were deposited from about 3100 cal BP (H1 to H2 zones). During the period of H1 and H2 zones, unstable slopes were thought to have existed around the HDA. This may be related to global climate change during the middle Holocene and paleo seismicity generated by nearby active faults. In the H3 zones, thin layers of brown humic silt were sandwiched between sand layers, suggesting that the slope has gradually been stabilized. In the H4 zone, humic silt layers were deposited continuously, and no major slope change is expected to have occurred during this period.
A large landslide site exists in Happosawa Basin, Happo One-ridge. We obtained a columnar core (HPO-2021) 239 cm long by a hand auger in a shallow depression (HDA) formed on the large landslide body. Based on visual observation of the core, HPO-2021 was divided into four sedimentary facies zones: H1 (136 cm to 239 cm), H2 (42 cm to 136 cm), H3 (22 cm to 42 cm), and H4 (0 cm to 22 cm). The H1 to H3 zones consist of gravel layers with little or no humus. In the H3 zone, thin layers of brownish humic silt are sandwiched between the sand layers. In the H4 zone, humic silt is dominant. Also, six samples were collected for dating. Depositional ages of the four sedimentary facies zones are estimates as follows: H1 zone is: before ca. 3930 cal BP (up to about 4270 cal BP); H2 zone is: 3100 to 3930 cal BP; H3 zone is: 1330 to 3100 cal BP and H4 zone is: 0 to 1330 cal BP. Environmental changes around the bottom slope of HDA are considered as follows: The HDA already existed at least ca. 3930 cal BP, and sand and gravel were deposited from about 3100 cal BP (H1 to H2 zones). During the period of H1 and H2 zones, unstable slopes were thought to have existed around the HDA. This may be related to global climate change during the middle Holocene and paleo seismicity generated by nearby active faults. In the H3 zones, thin layers of brown humic silt were sandwiched between sand layers, suggesting that the slope has gradually been stabilized. In the H4 zone, humic silt layers were deposited continuously, and no major slope change is expected to have occurred during this period.