| Research Abstract |
1)Surface geologlcal investigation was conducted on the Paleogene Tertiary formations in northwestern Kyushu and westernmost part of Honshu in terms of (a)sedimentary facies analysis, (b)sampling of fine-grained sediments for dinoflagellate cyst analysis, and (c)sampling of molluscan fossils. Logged sections include the Shimoshima Group in the Amakusa area, the Takashima and Iojima Groups in the Takashima area, the Matsushima and Nishisonogi Groups in the Sakito-Matsushima area, the Ainoura and Sasebo Groups in the Sasebo area, the Ouchi and Kishima Groups in the Karatsu area, the Ashiya Group in the Chikuho area, the Hioki Group in the Yuyawan area, and the Manda Group in the Miike area. The overall sampled interval ranges from the Middle Eocene to latest Oligocene in age, based on previous studies.
2)Successive occurrences of fossil dinoflagellate cysts were confirmed from most marine intervals of the logged section. The assemblages, being generally diverse, include genera such as Gla
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phyrocysta, Spiniferites, Homotryblium, Cordosphaeridium, Hystrichokolpoma, Heteraulacacysta, Deflandrea, Dracodinium and Wetzeliella. They showed apparent stratigraphic changes, which is positive for feasible blostratigraphic potential in Southwest Japan. A preliminary biostratigraphic zonation was presented. In addition, the constituents of the assemblages are mostly common to those from the coeval sediments in the East China Sea, although at the same time they are very different from those from the Hokkaido-Sakhalin region. This supports the paleobiogeographic differentiation along the northwestern coast of the Pacific that became apparent probably during the Terminal Eocene, which requires independent biostratigraphic schemes for higher latitudes and lower latitudes.
3)Our investigation of molluscan fossils indicated that the assemblages from the Sasebo Group should be attributed to the Ashiya Fauna, lacking the elements typical to the Kadonosawa Fauna. This implies that the age range of the Saseboan Stage should not extend up to the Miocene. Less
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