The Oonogawa river basin is covered mainly by Tertiary and Quaternary volcanic rocks and ashes . See the following documents:

Geology of Southern Hokkaido (PDF)
Geological Map of Hokkaido (Written in Japanese)（548KB）

Recent Major Volcanic Eruptions in Western Hokkaido (PDF)
Volcanic Ash in Hokkaido (Written in Japanese)（244KB）

Apart from lithic fragments(including volcanic glass), the dominant minerals are all of volcanic origin: plagioclase, orthopyroxene, clinopyroxene, and magnetite.


Heavy minerals
Apart from magnetite, the dominant heavy minerals are orthopyroxene, clinopyroxene, cummingtonite (Fe-Mg amphibole), and hornblende (Ca-bearing amphibole). Almost no other minerals were observed.

The iron-magnesium ratio (Mg number) in orthopyroxene from the Oonogawa River peaked at 62, but there are wide variations in composition. It appears that the river has transported a variety of volcanic ash and rock. For reference, we also show the Mg number of cummingtonite. It too shows scattering around the general range of 58. By comparing samples with the chemical compositions of cummingtonite in volcanic ashes and rocks from the upstream area, it could be possible to determine their sources.