Stratigraphic Re-examination of the Upper Pleistocene in the Oga Peninsula, Akita Prefecture, Northeast Japan.

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  • Shiraishi Tateo
    Faculty of Education, Akita University Present address; Faculty of Engineering and Resource Science, Akita University
  • Takeuti Sadako
    Saito Ho-on Kai Museum of Natural History

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  • 秋田県男鹿半島における上部更新統層序の再検討
  • アキタケン オガ ハントウ ニ オケル ジョウブ コウシントウソウ ジョ ノ サイケントウ

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Abstract

Middle and Upper Pleistocene deposits successively crop out at a sea cliff near Anden at the northern coast of the Oga Peninsula, Akita Prefecture, Northeast Japan. The deposits have been historically classified into the Shibikawa, Anden, and Katanishi formations in ascending order. The type localities of the Shibikawa and Anden formations are located in this sea cliff, and that of the Katanishi formation is placed in the Katanishi area, in the easternmost part of the Peninsula. The aim of this report is to stratigraphically re-examine these formations.<br>The Katanishi formation is subdivided into the following four members: the Tarusawa Sand, Kakumazaki Mud, Matsukizawa Sand and Gravel, and Honnai Sand. The Kakumazaki Mud member conformably overlies the Tarusawa Sand member, the lowermost of the formation, and interfingers with the Matsukizawa Sand and Gravel member. The Honnai Sand member lies upon the Matsukizawa Sand and Gravel member with conformity.<br>Recently, volcanic ash intercalated both in the Anden formation and in the Kakumazaki Mud member was identified as Toya ash, which erupted from the Toya Caldera, Hokkaido, and which is one of the prominent marker tephras in Japan. Consequently the Anden formation is regarded as a part of the Katanishi type formation tephrostratigraphically. Furthermore, faunal, floral, and lithological evidence shows that the Anden formation consists of offshore deposits correlatable with the Kakumazaki Mud and Matsukizawa Sand and Gravel members of the Katanishi type formation. Thus, the Anden formation is here called the Anden Sand member of the Katanishi formation.<br>The lower boundary of the Katanishi formation coincides with the basal unconformity of the Anden facies at Anden, where submarine sedimentation continues over the emergence horizon in the type area of the formation. The Tarusawa Sand member yields pollen and cold water molluscan fossils. The pollen assemblage of the member is characterized by dominance of coniferous tree pollen, which resembles that of the uppermost Shibikawa formation. These facts indicate that the definition of the upper and lower boundaries of the Katanishi formation is still questionable.

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