A number of hummocky hills created by the Zenkoji debris avalanche (Zd) are distributed around the southern foot of Usu volcano and extend out into Uchiura Bay in the form of rocky reefs. Previous studies have estimated that Zd occurred 9-6ka ago BP and was caused by a collapse of the Usu somma, composed of basaltic lava flows.
In this study we investigated the thick peaty deposits in the lowlands (2-4m above sea level) surrounded by hummocky hills of Zd origin near the coast. Two boring core samples showed continuous peat and organic clay sediments from altitudes of +2m to －6m, which through diatom analysis were traced to a freshwater environment. The basal parts of the cores yielded AMS 14C ages of ～20calkaBP. Pollen analysis indicated dominant subarctic coniferous forest elements in the lower half of the deposits (20-10ka). Tephra layers, such as Nigorikawa tephra (Ng) of 15calkaBP, were intercalated into the middle of the deposits. After 10calkaBP broadleaf deciduous forest became dominant, intercalated with Ko-g tephra (6.6calkaBP) and B-Tm. The bottom of the deposits contained deposits rich in pumice similar to the Toya pyroclastic flow deposits of ～110ka.
These findings suggest that a 6-7m thick peaty layer of freshwater lake and marsh sediments was deposited in a small, closed basin formed among several Zd hummocky hills, and that the debris avalanche occurred during the Last Glacial Maximum (LGM).
This revised chronological position of Zd also necessitates amendments to the placement of the basaltic lava flows during the somma stage of Usu volcano, since the activities of the Usu somma must have begun during or shortly prior to the LGM and ceased during the LGM.