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[HGM03-02] Temporal variation of filling processes of valley-head hollows in the Ohmatsuzawa Hills, Sendai, northeastern Japan
Keywords:head hollow, subhollow, mass-wasting, Holocene, climate change
The Ohmatsuzawa Hills extend between the Naruse and Yoshida River lowlands, about 25 km north of Sendai. The hilltops align along the skyline below the level of 140 m a.s.l. and represent erosional surfaces with gravel beds (Akojima, 1971). The study area of the Showa Man-yo Forest Park is located in western part of the Ohmatsuzawa Hills. The highest and lowest elevations occur on the crest slope at 70 m a.s.l. and in the stream floodplain at 20 m a.s.l., respectively. The local basement is the Pliocene Miyatoko Tuff. Fluvial boulders of volcanic origin are found on some of the hilltops and less often on middle to lower parts of slopes. Eight valley-head areas form in the study area and micro-landforms of channel, subhollow, head hollow, upper sideslope and crest slope are identified and aligned in this order upstream.
Profiles of regolith in test pits and by the cone penetration test show layered structure. At channel heads, for instance, a gravel layer of 40-50 cm thick overlies the weathered (in-situ) Miyatoko Tuff at c.a. 80 cm deep and is occasionally covered by a buried humus layer of 10-20 cm thick. The gravels must have been transported (colluvium) given that patches of buried humus are found within the gravel layer. Spatial extent of the colluvial gravel layer is narrower than the area of head hollow but wider than subhollow. 14C dating of a patch of buried humus within the colluvial gravel layer indicates an age of the middle of the Holocene and ages of a buried humus layer overlying the colluvial gravel layer are younger than the patch of buried humus.
It has been reported that shallow landslides can occur in (relatively steep) upper sideslopes once intensive rain and/or strong ground shake (earthquake) affect the slope stability and therefore evolution in valley-head areas is driven not only by continuous diffusional processes such as soil creep but also occasional mass-wasting (Tamura et al. 2002, 2011). In the preset study area, the colluvial gravels are sourced to the hilltop gravel deposits and were likely transported through shallow landslides occurred across the crest slope and (relatively gentle) upper sideslope. The time of the mass-wasting falls in the warmer (and probably wetter) period in the Holocene, suggesting forest fire and/or intensive rainfall may have caused the mass-wasting although earthquakes are known to induce landslides on convex slopes. The period of the mass-wasting was followed by a static period indicated by development of the buried humus.
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