西南北海道における新第三紀後期から第四紀初頭における古環境変遷 : 層位学的・微古生物学的解明

Neogene and Quatemary deposits are extensively di stributed in the Oshima Peninsula, southwestern Hokkaido. The sedimentary basin of these strata were split into several smaller basins during late Pliocene, and disappeared by Middle Pleistocene. The Kuromatsunai and Setana Formations, the Pliocene and Pleistocene in this region, have recorded paleoenvironmental changes during this period. In the present study, the author investigated lithostratigraphy of the Kuromatsunai and Setana Formations, their ages assignments based on a K-Ar radiometric analysis and microbiostratigraphic analyses, and the paleoenvironmental changes particularly during the deposition of the upper part of the Setana Formation based on foraminifera, and concluded as follows; 1) The Kuromatsunai Formation was subdivided into three members, the Sumiyoshi Siltstone, Sakkurubetsu Pyroclastic and Okusawa Sandstone, in ascending order in the Imakane-Kamiyakumo area. The Okusawa Sandstone Member, which have been regarded as a member of the Setana Formation, is unconformably overlain by this formation and separated from it. 2) The Kuromatsunai Formation contacts clinounconformably with the Setana Formation in all the areas studied. This fact indicates that tectonic movements and time-gap between deposition of both formations. 3) The Kuromatsunai Formation was deposited in early Pliocene to early Pleistocene (ca.5.0 to 1.4Ma), and its paleodepth was middle bathyal zone (over 700m) in maximum. 4) The Setana Stage, the biggest transgressive time in Quaternary of Hokkaido, was subdivided into two substages; the earlier substage began at about 1.2Ma, and thi s area was under inner sublittoral depth (O to 45m). This substage is characterized by cold-water molluscs and sirenian mammals lived in such water temperatures as the recent Okhotsk and Bering Seas. 5) The later substage began at about 1.0Ma, and the sea areas were expanded and deepened rapidly. Paleodepth sometimes attained to over 150m. This sea was then shallowed and dried up about 0.6 to 0.4Ma. 6) A detailed faunal analysis was performed on foraminifera and molluscs from the sequence of the upper part of the Setana Formation deposited in 0.95 to 0.85Ma. The sequence consisting of sandstone and four shell concentrations are divided into seven units. In the sequence the author distinguishes the three types of shell concentration, event-concentratlon, condensed-concentration, and lag-concentration. The author also showed that there are two condensations related to rapid deepening with transgressions. 7) The paleodepths curves based on benthic foraminifera indicate transgressive-regressive cycles between 50 and 100m or more influenced by glacial eustatic sea-level changes. The similarity of paleodepths curves and calcareous nannofossil biostratigraphy suggest that the age of this sequence is assigned to the Oxygen Isotopic Stage 21. 8) The estimated paleodepths on molluscs are usually shallower than those on benthic foraminifera. A detailed stratigraphic study indicates that such a discrepancy is brought from the effect of time-averagings during rapid deepenings of the sea, and a higher resolution by foraminiferal analyses