Stratigraphy of Neogene volcanic rocks in Hakodake area, northern Hokkaido, Japan

Abstract

A large amount of andesitic volcanic rocks are widely distributed in northern Hokkaido. These volcanic rocks have formed a high topography with flat surface, and therefore have been called "flat lavas". The "flat lavas" distributed around the Hakodake area to the east of Bifuka town, northern Hokkaido are divided into three units in ascending order ; the Sakkuru andesites, Otoifuji andesites and Hakodake lavas. Each andesitic unit is composed of lava flows and debris flow deposits and, their maximum thickness is about 400 m for the Sakkuru andesites, more than 40 m for Otoifuji andesites and about 500 m for the Hakodake lavas. Field observations revealed that three volcanic units are interfingered with the middle Miocene Onnenai Formation composed of fluvial and shallow sea deposits. This suggests that the lavas and debris flow deposits were formed on the flat fluvial plains and subsequently buried in a relatively shallow depth due to subsidence of the basin. The present stratigraphical results are good consistent with the previously obtained radiometric ages of 10∼14 Ma for the volcanic rocks in this area. After eruption of the Hakodake Lava, the volcanic and sedimentary rocks in the area may have been uplifted by tectonic movement accompanied by NS trending faulting and folding. A very short period of erosion on the subaerial ground seems to have resulted in preservation of flat plains consisting mainly of lava flows in the Hakodake area.