5:15 PM - 7:15 PM
[MIS12-P04] Geo-environmental changes around Hakuundake, central Daisetsuzan Mountains
Keywords:peaty soil layer, tephrochronology, radiocarbon dating, Daisetsu-zan Mountains
Multiple ridges composed of sediments are recognized in the valley head of the Yanbetappu River, on the southeastern slope of Mount Hakuundake, located in the central part of the Daisetsuzan Mountains. These landforms are considered to be moraines formed by Holocene glaciation (Hasegawa et al., 2012; Takahashi, 2024). The authors obtained chronological data from the soil layers on these depositional landforms, as well as from fluvial deposits and periglacial gravel deposits distributed in the head of the Chubetsu River. This study describes these slope-forming materials, determines their formation or deposition ages, and examines the geomorphological development of the southeastern slope of Mount Hakuundake, as well as the environmental changes behind it.
The soil layers formed on the ridges at the head of the Yanbetappu River contain the Tarumae-a tephra (AD 1739) and the Komagatake-c2 tephra (AD 1694), and in some cases, the Baitoushan-Tomakomai tephra (AD 946) is also observed. Therefore, these ridges are likely to be divided into those that were formed before the fall of the Baitoushan-Tomakomai tephra and those that were formed afterward. Additionally, on the outer slopes of the head of the Yanbetappu River, the presence of a scoria layer that fell 5,000 cal BP at the base of the soil suggests that the formation of the soil layers around Mount Hakuundake generally began 5,000 cal BP. A radiocarbon date of 12064–11760 cal BP was obtained from the muck like soil layer in the fluvial slope deposits of the Chubetsu River head on the southeast slope of Mount Hakuundake. According to Takahashi (2024), glaciation considered to be occurred in the head of the Chubetsu River during the Holocene. Therefore, sand and gravel transported by meltwater may have been deposited in the glacial lake. In the snow patch hollow on the southeast slope of Mount Hakuundake, a radiocarbon age of 2304–2147 cal BP was obtained from the base of the soil layers. Since the 5000 cal BP scoria layer commonly seen in the soil layers around Mount Hakuundake is not found in the soil layers overlying the snow patch hollow, it is believed that the snow patch hollow on the southeast slope of Mount Hakuundake was formed after 5000 cal BP, and further geomorphic formation likely occurred in most parts of the snow patch hollow thereafter.
The soil layers formed on the ridges at the head of the Yanbetappu River contain the Tarumae-a tephra (AD 1739) and the Komagatake-c2 tephra (AD 1694), and in some cases, the Baitoushan-Tomakomai tephra (AD 946) is also observed. Therefore, these ridges are likely to be divided into those that were formed before the fall of the Baitoushan-Tomakomai tephra and those that were formed afterward. Additionally, on the outer slopes of the head of the Yanbetappu River, the presence of a scoria layer that fell 5,000 cal BP at the base of the soil suggests that the formation of the soil layers around Mount Hakuundake generally began 5,000 cal BP. A radiocarbon date of 12064–11760 cal BP was obtained from the muck like soil layer in the fluvial slope deposits of the Chubetsu River head on the southeast slope of Mount Hakuundake. According to Takahashi (2024), glaciation considered to be occurred in the head of the Chubetsu River during the Holocene. Therefore, sand and gravel transported by meltwater may have been deposited in the glacial lake. In the snow patch hollow on the southeast slope of Mount Hakuundake, a radiocarbon age of 2304–2147 cal BP was obtained from the base of the soil layers. Since the 5000 cal BP scoria layer commonly seen in the soil layers around Mount Hakuundake is not found in the soil layers overlying the snow patch hollow, it is believed that the snow patch hollow on the southeast slope of Mount Hakuundake was formed after 5000 cal BP, and further geomorphic formation likely occurred in most parts of the snow patch hollow thereafter.