西南北海道北部の新生界とその特徴  [in Japanese] Cenozoic system and its geotectonic features of the northern part of southwest Hokkaido, Japan  [in Japanese]

Geotectonic features of the Cenozoic terrane in the northern part of southwest Hokkaido are described with some comments on the stratigraphic obscurity of the Neogene type sections in this area. The igneous activity commenced at 30 Ma in the latest Paleogene. Terrestrial volcanism took place in the western part (Okushiri Island) and the plutonic rocks intruded in small scale in the eastern part. In early Miocene time (Fukuyama Stage and Yoshioka Stage; 25-16 Ma), basaltic to rhyolitic volcanic rocks were erupted on the land area in all around southwest Hokkaido. In the Yoshioka stage (18-16 Ma), the first marine transgression proceeded in the restricted area extending from Matsumae in the south to Mt. Yuhrapu in the north. The regional marine transgression with intense subaqueous volcanism occurred in early Middle Miocene time (Kunnui Stage). Enormous amounts of acidic lavas and pyroclastic flows were accumulated in the northern part of this region (Akaigawa-Johzankei). In the southern part, the Kunnui sedimentary basin where the marine Kunnui Formation was deposited were expanded to the northwest in this stage. In Middle to Late Miocene time, the regional subsidence proceeded with the marine transgression at 12-11 Ma. The sedimentary basins were NW-SE in trend. The upper Middle Miocene sediments (Yakumo Stage) overlay the lower Middle Miocene and Cretaceous formations with unconformity in the marginal areas of the sedimentary basin. This evidence suggests that the hinterlands of the basins were uplifted with a NW-SE trending axis prior to the regional transgression. The subaqueous volcanism was more active in the subsiding zone in the sedimentary basin in Pliocene time than in the Miocene. Uplifting of the hinterland was intensified, and it made the sedimentary basins smaller. The Kuromatsunai lowland area appeared in Late Pliocene to Pleistocene time, which divided the Southwest Hokkaido into the two parts, the eastern and the western. The terrestrial volcanism in the eastern part started in the Late Pliocene time and continued to the Middle Pleistocene.