[S-GL38] 上総層群における下部ー中部更新統境界GSSP

[SGL38-03] 上総層群国本層の生痕化石と生痕相：堆積環境と底生生態系への示唆

*泉 賢太郎¹、西田 尚央²、風岡 修³、菅沼 悠介⁴、岡田 誠⁵、吉田 剛³、荻津 達³、中里 裕臣⁶、亀山 瞬³、香川 淳³、森崎 正昭³、兵頭 政幸⁷、楡井 久⁸ (1.国立環境研究所　生物・生態系環境研究センター、2.産業技術総合研究所　地質情報研究部門、3.千葉県環境研究センター地質環境研究室、4.国立極地研究所、5.茨城大学　理学部理学科、6.農業・食品産業技術総合研究機構農村工学研究所、7.神戸大学　自然科学系先端融合研究環　内海域環境教育研究センター、8.国際地質科学連合環境管理研究委員会)

In the Tabuchi section, along the Yoro river in the Boso Peninsula, the Kokumoto Formation (Kazusa Group) represents an expanded well-exposed, continuous marine succession across the Lower-Middle Pleistocene boundary. Since the Tabuchi section contains the Matuyama-Brunhes boundary whose age was highly accurately constrained, it is considered as a candidate for the Global Boundary Stratotype Section and Point (GSSP). In addition, due to the high sedimentation rate and continuous deposition, the Kokumoto Formation is suitable for high-resolution paleoenvironmental studies. However, paleoecological studies of the formation are very few, responses of marine organisms to paleoenvironmental changes remain unclear. Therefore, this study systematically described the trace fossils and ichnofabrics of the Kokumoto Formation of the Tabuchi section. Twelve ichnogenera and another indeterminate U-shaped burrow were recognized from the silty beds of the formation, which are typical components of ichnofacies characterizing the bathyal zone (i.e. Zoophycos ichnofacies). In addition, no graphoglyptid trace fossils, which commonly occur in abyssal plain environments, were observed. Based on the trace-fossil assemblage combined with other sedimentological features, it is most likely that the silty beds of the Kokumoto Formation were deposited in a continental slope setting. Furthermore, two types of ichnofabrics were identified, which are Phycosiphon-dominated ichnofabric (Phy ichnofabric) and Chondrites-Planolites-Thalassinoides ichnofabric (Ch-Pl-Th ichnofabric). Observation of the thin-sliced slabs revealed the distinctive stratigraphic changes of these two ichnofabrics; namely, both the Phy-ichnofabric and Ch-Pl-Th ichnofabric occur in the lower and upper part of the Tabuchi section, whereas the Ch-Pl-Th ichnofabric is exclusively recognized in the middle part. The vertical change in ichnofabrics is not correlated with changes in sedimentation rate. Instead, the ichnofabrics are well correlated with changes in benthic-food contents, which are estimated by the results of high-resolution XRF analysis. In particular, the lower and upper parts of the Tabuchi section, which are characterized with the presence of Phy-ichnofabric, are synchronized with food-poor intervals. Since the Phycosiphon-producer is regarded as a grain-selective deposit feeder, which may have effectively ingested organic matter, it is reasonable that the Phy-ichnofabric occurs only in food-poor intervals.