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 (e.g. 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.
The diatom zones of Akiba (1986) and Yanagisawa and Akiba (1998) recognized in the Makarov area include the Denticulopsis simonsenii Subzone of the D. hyalina 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 D. praedimorpha 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 Neodenticula kamtschatica 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 Rouxia californica Zone (NPD7A; upper upper Miocene, 7.6-6.4 Ma).
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.
