[B-PT04] Biotic History

[BPT04-P04] Miocene radiolarian and diatom stratigraphy near the Biratori Fault, Hae River, Hidaka Town, Hokkaido, Japan.

*Isao Motoyama¹, Erika Shoji², Fumio Akiba³ (1.Faculty of Science, Yamagata University, 2.Graduate School of Science and Engineering, Yamagata University, 3.Diatom MiniLab Akiba Ltd.)

Keywords:Radiolaria, diatom, Takinoue Fauna

According to previous studies, the Middle Miocene Abetsu and Nibutani Formations and the Upper Miocene Nina Formation are in contact with each other across the Biratori Fault in the vicinity of the Hae River, Hidaka Town, central Hokkaido. However, no exposure of the fault has been observed so far in this area, and it is difficult to classify strata on the basis of lithofacies alone, since the strata in this area are mainly composed of fairly monotonous mudstone although accompanied by minor sandstone and conglomerate. Therefore, we determined the age of the strata and fault locations, taking radiolarian and diatom stratigraphy into consideration. As a result, the radiolarian Lithocampe subligata, Dendrospyris sakaii, lower Eucyrtidium inflatum and Lychnocanoma magnacornuta Zones and the diatom NPD3B, NPD4A, NPD4B and NPD5C Zones were recognized, with the lack of the subzone B of the E. inflatum Zone and the NPD5A and NPD5B Zones approximating the location of the fault. By digging the gravel of the riverbed at the presumed location and observing the bedrock, we were able to confirm the fault outcrop where the Abetsu and Nina Formations are in contact with each other across the fault crush zone. The estimated age of the Sakae Formation indicates that the Takinoue Fauna is correlated with the Burdigalian stage.