Reconstruction of the lower Miocene depositional environment in Honshu, Japan

在本学年(2000-2001)，我们从层序地层学的角度对本州大陆下中新世地层进行了研究，并在这些地层中采集了化石。我们还利用采自这些下中新统的软体贝壳进行了锶同位素地层学研究。研究区域包括Echizen沿海地区和福井市地区。福井县。霍塔-明科卡地区。博索半岛。长野县和群马县的Saku-Uchiyama地区，Gumma县的Shimonita地区，崎玉县的千户盆地，Joban煤田，福岛县和宫城县的Marumori盆地。此外，我们还对冲绳县Iriomote-Jima岛的下中新统进行了研究，以了解这些植物群的变化，从而建立了Joban煤田下中新统的标准剖面。它覆盖了22~15 Ma，包括两次变暖和变冷事件。总的来说，利用软体动物贝壳进行锶同位素地层学研究取得了成功的成果。此外，我们还利用碎屑铬尖晶石等重矿物从沉积学的角度研究了下中新世，如博索半岛的下中新世，以了解日本岛屿早中新世的构造环境。基于这些结果，我们希望在未来继续这一项目的研究，扩大到太平洋门户的视野，并包括邻近地区如台湾、韩国、萨哈林、堪察斯卡等的锶等位地层学，以阐明更广泛的全球范围内的下中新世古环境。

During this academic year (2000-2001), we studied lower Miocene stratigraphy in Honshu main land form a veiw point of sequence stratigraphy and collected fossils from these strata. We also studied the Sr isotope stratigraphy using molluscan shells collected from these lower Miocene strata. The studied areas includes Echizen costal area and the Fukui City area. Fukui Prefecture. Hota-Mincoka area. Boso Peninsula. Saku-Uchiyama area of Nagano and Gumma Prefectures, Shimonita area of Gumma Prefecture, Chichibu Basin of Saitama Prefecture, Joban coal-field, Fukushima Prefecture and marumori Basin of Miyagi Prefecture. In addition, we Studied lower Miocene of the Iriomote-jima Island, Okinawa Prefecture to understand these floral changes.As a result, we have established the lower Miocene standard section in the Joban coal-field. It covered from 22 Ma to 15 Ma and included two-time warming and cooling events. In general, we obtained successful results of the Sr isotope stratigraphy using mollusean shells. In addition, we also studied lower Miocene from sedimentological view as exemplified by the lower Miocene of the Boso Peninsula using heavry-minerals such as detrital chromian spinel to understand tectonic setting during the early Miocene of the Japanese Islands.Based on these result, we would like to continue the study concerned this project in future by expanding to view from the Pacific gateways and also include Sr isopote stratigraphy of the adjacent area such as Taiwan, Korea, Sakhalin, Kamchatska and so on to clarify lower Miocene paleoenvironments for more wider global scope.