In this paper, we deal with the Miocene Ukan Formation, sporadically distributed in Ukan-cho, Okayama prefecture, from a viewpoint of analysis of lithology and molluscan assemblages.
The Ukan Formation is lithostratigraphically divided into the Suzuoka Conglomerate, Yamagata Muddy Sandstone, Shimoyokomi Sandstone and Conglomerate, Azechi Sandstone and Conglomerate, Tsuneyama Siltstone Members in ascending order.
The following 6 molluscan assemblages are recognized.
The lower four members and the last member is correlative to the Lower Sandstone and Upper Shale Formation of the Bihoku Group, respectively judging from lithology and fauna.
The temporal chages of the above mentioned molluscan assemblages represents that gradual deeping of sea during the depositional period from the Suzuoka Conglomerate and Yamagata Muddy Sandstone Members to the Azechi Sandstone and Conglomerate Member via the Shimoyokomi Sandstone and Conglomerate Member, and abrupt deeping of sea seems to have happened between the Azechi Sandstone and Conglomerate Member and the Tsuneyame Siltstone Member.