本州西部の下部中新統西市層：その化石海生貝類群・花粉群と年代論，関連する古地理的問題

抄録

The Nishiichi Formation in the Nishiichi district, which has been generally assigned to the Oligocene in age, yields in its Middle Member the marine molluscan fauna characterized by Cyclina hwabongriensis, Vicaryella notoensis, V. bacula etc. and the palyno-flora correlated to the Neogene palyno-floral zones, NP-1 and NP-2, in the northeastern Japan region. This, together with a comparative consideration of their occurrences in the Miocene strata in central Japan and southeastern Korea, leads to the conclusion that the member is the late Early Miocene in age (around 17 to 18.5 Ma). The Middle Member includes shallow marine deposits as well as fluvial and lacustrine ones, whereas the Lower and the Upper Members exclusively comprise fluvial braided-river deposits. Our paleocurrent measurement reveals that the stream system went down into some marine bay, a branch of the Pacific Ocean, to the east of the Nishiichi district. A westward transgression must have caused the shallow marine deposition of the Middle Member associated with the reported marine molluscan fauna. A northward paleocurrent measured in the Upper Member could have resulted from a change of the stream system in its depositional time accompanied by a significant change of paleogeographic condition from late Early Miocene to early Middle Miocene time.