抄録
The K-Ar dating of volcanic rocks and study on diatom fossils from northern part of southwest Hokkaido lead to the following conclusions: The Otaru Formation, Fuso Agglomerate, Eboshidake Agglomerate, Kamui Agglomerate and Hoheikyo Agglomerate composed mainly of submarine volcaniclastic andesite and basalt, formed in Late Miocene age. These volcaniclastic rocks were erupted and deposited during the period from 10 Ma to 7.5 Ma. The volcaniclastic rocks unconformably overlay felsic volcanic rocks of Middle Miocene age. The unconformity formed probably between 11 Ma and 10 Ma. K-Ar ages of the andesite lavas, resting on these submarine volcaniclastic rocks are 7.5 Ma to 5 Ma. Therefore, the "flat lavas" were erupted during not only Pliocene or Pleistocene, but also Late Miocene age. The Late Miocene to Pliocene volcanic rocks are distributed in NW-SE trending.
