猫魔火山西麓の火山麓扇状地
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The geomorphological history of two volcanic-fans (Kumakura-fan and Shiokawa-fan) on the western foot of Mt. Nekoma in the Fukushima prefecture, Northeast Japan, were discussed in this paper.The development of the volcanic-fans has to be described from the viewpoint of interaction of deposition and erosion.Three key tephras, Numazawa-Numazawako Tephra (Nm-NM: ca. 5ka.), Numazawa-Kanayama Tephra (Nm-RN: ca. 50-55ka.), HG2 (ca. 60ka.) and brown loam are observable on the volcanic-fan surfaces. They are very useful for identifying and making chronology of the volcanic-fan surfaces.The both fan surfaces were subdivided into 5 geomorphic surfaces, terrace 1 to terrace 5 from the highest to the lowest relatively, based on the photographic interpretation and observation of the facies of their formation. The ages of them are estimated from the three key tephras, the mean depositional rate (av. 0.02mm/y) of aeolian deposits, and 14C dating.The terrace 1, which is constituted of Kanagawayama debris avalanches or Oohara pyroclastic flow deposits, was formed before ca. 160ka. B. P.. Terrace 2 consists of traction deposits dating back to ca. 110ka. B. P., as depositional surface.Terrace 3 is locally strath terrace slightly cutting the terrace 1 and 2, and locally fill-top terrace. It was formed ca. 50-55ka. B. P.. Not only debris flow facies at the upstream but also traction ones at the downstream are observed in the formation of terrace 3. Terrace 4 was dated back to ca. 20ka. B. P., and terrace 5 was formed after ca. 5ka. B. P., as depositional surface. They also consist of traction deposits.It is considered that the four terraces from terrace 2 to terrace 5 are climatic terraces, because they were formed on their own base levels that have relatively changed up and down comparing to the previous age's level, they can be correlated to the other climatic terrace from Nagai I to Nagai V developing along the Aga river in the Aizu basin judging from their ages, and their ages are near the periods of the mass movement and the development of the slope formation in the Central Kitakami Mountains.These facts may indicate that the volcanic-fan surfaces are also formed under a certain control of climatic and base level changes so much as river terrace building.