Abstract
The Nishikubiki area in the northwestern part of the Shin'etsu back-arc sedimentary basin, central Japan faces the Japan Sea. Turbidites and sandy siltstones of the Middle Miocene to Pleistocene are distributed in this area. The records of relative sea-level changes in the paleo-Japan Sea are preserved in the Middle Miocene to Pleistocene strata distributed in the sedimentary basin and it is known that the fourth-order depositional sequences in the sedimentary basin are controlled by eustasy. The purpose of this study is to investigate the driving force that formed the third-order depositional sequence in the Nishikubiki area.Facies analysis of the Pliocene strata in the Nishikubiki area has revealed that they were deposited in a series of environments ranging from basin plain to shelf. A third-order depositional sequence which was set up on the basis of stacking patterns of sedimentary facies is composed of a coarse gravelly channel-levee system on slope (lowstand systems tract), condensed section on outer shelf (transgressive systems tract) and shallowing upward facies succession on shelf (highstand systems tract).Maceral composition ratio and trace fossil assemblages are related to relative sea-level of the third-order depositional sequence. Vitrinite and sporinite of terrigenous origin have minimum content in the maximum flooding surface of the third-order depositional sequence. In contrast, they increase upward within the highstand systems tract of the sequence. Size and diversity of the trace fossils decrease upward in the transgressive systems tract of the third-order depositional sequence. On the other hand, they increase upward within the highstand systems tract of the sequence.The sea-level positions presumed from third-order depositional sequence, the assemblages of trace fossils, and maceral composition, coincide with the oxygen isotope curve. These results suggest that the third-order depositional sequence in this area was formed in response to global eustasy.
