Slope Deposits in the Northern Margin of Kitakami Massif, Northeast Japan

Abstract

Several types of slope deposits were observed in undulating lands near the northern margin of Kitakami Massif. Stratigraphic relations of the deposits were investigated tephrochronologically. The radiocarbon ages of several tephra layers have already been known. The processes by which the slope deposits were formed were studied from their facies, their relations to landforms and lithology, and paleoenvironmental data including pollen analysis. As the result, the following four periods were discerned as 'periods of slope instability' when mass-movements on slope surfaces were more active than at the other periods.
The period I: It is earlier than the late Pleistocene and might be warmer. Piedmont gentle slopes composed of removed materials of granodiorite were built at the foot of the mountains of the same rock.
The period II: It is the cooler late Pleistocene when gentle slopes composed of screes were formed, smoothly continuing to or covering terrace surfaces.
The period III: It is the latest Pleistocene and is subdivided into the subperiod III-1 about 30, 000 years B. P. and the subperiod III-2 about 15, 000 or 20, 000 years B. P.. Palynological data suggest colder climate of those days which is concordant to common knowledge. The same landforms as those of the period II were formed during the subperiod III-1, and derasional valleys were configurated through the subperiod III-1 and III-2. Those shallow concave valleys were terraced and flat-floored valleys were excavated in the Holocene. The morphological evidence suggests the intensification of fluvial action at about the time of the Pleistocene-Holocene boundary. Rocks were deeply weathered on the summits of the residual mountains and tali were formed at the foot of steep slopes during the period III.
The period IV is in the Holocene and was the least unstable among the four periods of slope instability.