The Period of Slope-Instability Observed in Tephra in a Coastal Area from Hachinohe City to Kuji City, Northeastern Japan

Abstract

On several coastal terraces along the northeastern margin of Kitakami Massif, it is observed that a certain tephra deposit was transported on slope surfaces and then buried by the younger tephra. The lowermost horizon of the covering younger tephras is stratigraphically settled at the upper part of Takadate Volcanic Ash and Haehinohe Pumice.
Such disturbed layer showing active movement of materials on the former slope surfaces is not observed in other horizons of the tephra sequence in the area.
From this fact, it is surmised as follows : there existed some conditions that caused slope-instability concurrent with Upper Takadate Ash fall and preceding Haehinohe Pumice fall. Slopes were disturbed in different periods in accordance with their topographic sites.
Upper Takadate Ash, covering the deposits of the coastal terrace of about 10m a.s.l, partly includes angular boulders splitted from the stacks on former abrasion platform and is partly varied in mxied deposits of clayey ash and breccia. There is organic clay directly overlain with Haehinohe Pumice of which C-14 age was dated to 12700±270 yrs. B.P. This clay is abundant in pollens of Picea. These data may suggest the age and environment in the period of slope-instability.