珪藻化石層序によるサハリン島南部のMaruyama層・Kurasi層の年代と北海道天北地域の新第三系との対比 Diatom Biostratigraphy and Geologic Age of the Maruyama and Kurasi Formations, Southern Sakhalin, and Their Correlation to the Neogene of the Tenpoku Area, Hokkaido
A diatom biostratigraphic study of the Maruyama Formation in the Makarov and Dolinsk areas of the southern part of Sakhalin Island indicates that its geologic time span ranges from ca. 14 Ma to 3.5 Ma or possibly younger, which is much longer than previous estimates (<I>e.g.</I> 9.5-5.3 Ma by Ingle, 1992). A middle Miocene age of the Kurasi Formation in the Makarov area is also given on the basis of diatoms. These results are presented with correlation to the Neogene sections in the Tenpoku area, northern Hokkaido.<BR>The diatom zones of Akiba (1986) and Yanagisawa and Akiba (1998) recognized in the Makarov area include the <I>Denticulopsis simonsenii</I> Subzone of the <I>D. hyalina</I> Zone (NPD4Bb; middle middle Miocene, 14.6/14.5-13.1 Ma) in the Kurasi Formation and the 1st Member of the lower part of the Maruyama Formation, and the <I>D. praedimorpha</I> Zone (NPD5B; upper middle Miocene, 12.9-11.5 Ma) in the 2nd Member of the lower part of the Maruyama Formation. In the middle part of the Maruyama Formation in the Dolinsk area, the type area for the formation, numerous specimens of reworked diatoms were observed. The youngest zone shown by those assemblages is the Thalassiosira oestrupii Subzone of the <I>Neodenticula kamtschatica</I> Zone (NPD7Bb; lower Pliocene, 5.5-3.9/3.5 Ma), thus the middle part of the Maruyama Formation in the Dolinsk area should be of early Pliocene or younger ages. As similar assemblages are known from the Yuchi Formation in the Tenpoku area of northern Hokkaido, which is of Pleistocene age at least in part, the middle part of the Maruyama Formation in the Dolinsk area may be Pleistocene in age, which should be tested by further studies. In addition, our re-evaluation of Sheshukova-Poretzkaya's (1967) data from the Aniva area, south of Yuzhno-Sakhalinsk, suggests that the formation also contains an interval correlative to the <I>Rouxia californica</I> Zone (NPD7A; upper upper Miocene, 7.6-6.4 Ma).<BR>The present study geochronologically constrains other geological data such as paleontology, paleomagnetism, and paleoenvironments from the studied sections and their correlative strata. The considerably long geochronologic interval represented by the Maruyama Formation suggests stratigraphic gaps within the formation, thus a further study of the formation may elucidate the history of relative sea-level changes which have localor global implications.
地學雜誌 109(2), 203-217, 2000-04-25
Tokyo Geographical Society