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[MIS10-P02] Environmental changes of peat bogs formed in shallow depressions on large landslide body since Late Holocene, Happo-sawa Basin, Northern Japanese Alps
Keywords:Northern Japanese Alps, Holocene, Environmental changes, Landslide
In the southern part of the Northern Japanese Alps, topographic development, paleoenvironments, and biological natural history of alpine-subalpine bogs on closed shallow depressions have been clarified. In this study, we focused on the northern part of the Northern Japanese Alps, where a wetter climate of heavy rain and snow is dominant than the southern part, and conducted an excavation survey in shallow depressions with bogs. Here we present preliminary results on paleoenvironments based on lithofacies changes in drilling cores.
The study sites are located a landslide body developed in the uppermost area of the Happo-sawa basin on the south face of Mt. Happo (1974 m ASL). The geology around the study sites is composed mainly of ultramafic rocks of unknown age and granitic rocks of Cretaceous to Paleogene period. Although late Pleistocene till are distributed, a few have been studied in detail.
There are five major landslide bodies in this basin. The largest landslide body A (A) has a 980 m long cliff just below the main ridge. The body A subdivided into east and west (Ae, Aw) sub-bodies at the upper domain of the whole landslide feature, but they merge at the lower domain. The area of body A is ca. 7.64x 105 m2, which corresponds to a volumetric large scale landslide feature if the average thickness of the body is 10 m. Secondary minor landslides, pressure ridges, small bulges, and closed depressions are dominant on the body A. A handy auger drilling was conducted at three bogs (e.g., HDA=1820 m ASL, HDB=1708 m ASL, and HDC=1733 m ASL) in shallow depressions on the body A. Lengths of recovered cores were as follows: HDA=239 cm (core ID: HPO-2021), HDB=183 cm (HPO-2022A), and HDC=127 cm (HPO-2022B). Among these cores, we already reported initial description of HPO-2021 in JpGU2022 (Sato et al., 2022; M-IS15 P05).
Based on visual observations of the core sections, HPO-2022A was divided into 7 lithofacies zones (A1: 179 to 183 cm, A2: 78 to 179 cm, A3: 65 to 78 cm, A4: 54 to 65 cm, A5: 35 to 54 cm, A6: 9 to 35 cm, A7: 0 to 9 cm), HPO-2022B was also divided into four zones (B1: 72-127 cm, B2: 61-72 cm, B3: 18-61 cm, and B4: 0-18 cm from the bottom).
Based on the lithofacies classification and 14C ages, including HPO-2021, environmental changes were generally common to the three sites. At every site, sand and gravel layers were continuously deposited from deeper part to 50-70 cm depth from the ground surface (equivalent age: 3.1-1.6 cal ka BP).
From these horizons to the ground surface, lithofacies changed to humic silt layers or decomposed peat layers. These facts indicate that the slopes around each site was almost unstable and less vegetation cover continued until ca. 1.6 cal ka BP at most. Recent deposition of humus-rich soil layers show slope stabilization and vegetation encroachment around the sites. However, a humic silt layer found at the deepest part of HPO-2022A was dated ca. 7.8 cal ka BP, and this suggests that vegetation encroachment into the site HDB occurred in the early Holocene.
Deposition of sand and gravel layers until ca. 3.1 to 1.6 cal ka BP would be related to enlargement of snowpatch rubble slopes and/or minor landslides around the bogs due to cooler climate in the late Holocene. Studies on pollen analysis and periglacial geomorphology done in and around the northern Japanese Alps as well as the snowy mountains in Northern Japan (the Tohoku District) clarified a temporary climatic deterioration in the late Holocene after 5 cal ka BP. Similar climatic cooling may took place around the Happo-sawa basin. In addition, paleoearthquake activities generated by the Kamishiro fault, located 6 km east of the study sites, should be taken into consideration.
The study sites are located a landslide body developed in the uppermost area of the Happo-sawa basin on the south face of Mt. Happo (1974 m ASL). The geology around the study sites is composed mainly of ultramafic rocks of unknown age and granitic rocks of Cretaceous to Paleogene period. Although late Pleistocene till are distributed, a few have been studied in detail.
There are five major landslide bodies in this basin. The largest landslide body A (A) has a 980 m long cliff just below the main ridge. The body A subdivided into east and west (Ae, Aw) sub-bodies at the upper domain of the whole landslide feature, but they merge at the lower domain. The area of body A is ca. 7.64x 105 m2, which corresponds to a volumetric large scale landslide feature if the average thickness of the body is 10 m. Secondary minor landslides, pressure ridges, small bulges, and closed depressions are dominant on the body A. A handy auger drilling was conducted at three bogs (e.g., HDA=1820 m ASL, HDB=1708 m ASL, and HDC=1733 m ASL) in shallow depressions on the body A. Lengths of recovered cores were as follows: HDA=239 cm (core ID: HPO-2021), HDB=183 cm (HPO-2022A), and HDC=127 cm (HPO-2022B). Among these cores, we already reported initial description of HPO-2021 in JpGU2022 (Sato et al., 2022; M-IS15 P05).
Based on visual observations of the core sections, HPO-2022A was divided into 7 lithofacies zones (A1: 179 to 183 cm, A2: 78 to 179 cm, A3: 65 to 78 cm, A4: 54 to 65 cm, A5: 35 to 54 cm, A6: 9 to 35 cm, A7: 0 to 9 cm), HPO-2022B was also divided into four zones (B1: 72-127 cm, B2: 61-72 cm, B3: 18-61 cm, and B4: 0-18 cm from the bottom).
Based on the lithofacies classification and 14C ages, including HPO-2021, environmental changes were generally common to the three sites. At every site, sand and gravel layers were continuously deposited from deeper part to 50-70 cm depth from the ground surface (equivalent age: 3.1-1.6 cal ka BP).
From these horizons to the ground surface, lithofacies changed to humic silt layers or decomposed peat layers. These facts indicate that the slopes around each site was almost unstable and less vegetation cover continued until ca. 1.6 cal ka BP at most. Recent deposition of humus-rich soil layers show slope stabilization and vegetation encroachment around the sites. However, a humic silt layer found at the deepest part of HPO-2022A was dated ca. 7.8 cal ka BP, and this suggests that vegetation encroachment into the site HDB occurred in the early Holocene.
Deposition of sand and gravel layers until ca. 3.1 to 1.6 cal ka BP would be related to enlargement of snowpatch rubble slopes and/or minor landslides around the bogs due to cooler climate in the late Holocene. Studies on pollen analysis and periglacial geomorphology done in and around the northern Japanese Alps as well as the snowy mountains in Northern Japan (the Tohoku District) clarified a temporary climatic deterioration in the late Holocene after 5 cal ka BP. Similar climatic cooling may took place around the Happo-sawa basin. In addition, paleoearthquake activities generated by the Kamishiro fault, located 6 km east of the study sites, should be taken into consideration.