Abstract
This paper discusses the period of slope instability and the formative processes of block deposits on the Kobugahara Plateau, northern part of the Ashio Mountains. The plateau is mostly underlain by granodiorite and characterized by many core stones and tors derived from the deep weathering profile of the bedrock. Some block deposition features, similar to so-called block streams, also occur on the plateau. Many water streams flow through the block deposition features. It suggest that transportation of fine weathered materials by running water and accumulation of core stones as lag deposits were important formation processes of the features. The topographic characteristics of the block deposition features, however, indicate that slow mass movement also played an important role in their formation. Three types of finer slope deposits, gully-fill colluvium, upper fine deposits, and lower fine deposits, also occur as surficial deposits on the plateau. The upper fine deposits are composed of nonbedded humic soil and silt layers. The lower fine deposits include granodiorite blocks, fragmented weathered rocks, Ag-KP pumice (ca. 45-50 ka), and Nt-I scoria (ca. 14-15 ka). The blocks, rock fragments, and Ag-KP grains are randomly scattered in a brown silty matrix, while Nt-I grains occur only in the upper part of the deposits. The gully-fill colluvium, composed of silt and scattered granodiorite blocks, has infilled gullies entrenched in the lower fine deposits. The broad occurrence of the lower fine deposits and the intercalated tephras point to the period of slope instability between ca. 50 ka and 15 ka. The facies and distribution of the lower fine deposits as well as their age suggest that they were formed by slow periglacial mass movement. Deposits forming the block deposition features are unconformably underlain by the lower fine deposits, suggesting that the block deposition features formed after the period of widespread slope instability. A relatively wet and cold climate around the Pleistocene-Holocene transition may have facilitated the formation of the block deposition features due to both slow mass movement and the fluvial transportation of fine materials.
