Abstract:
The middle to late Pleistocene Shimosa Group distributed in the Kanto district, is characterized by sedimentary cyclothems originated from glacio-eustatic sea-level fluctuation. On the basis of detailed analysis of sedimentary facies and molluscan fossil content, the Jizodo Formation which represents the lowest formation of the Group, is subdivided into three depositional sequences. Each depositional sequence comprises two systems tracts: (HST). The TST is characterized by estuarine mud, transgressive lag and offshore sand. The HST of regressive facies is characterized by shoreface and beach sands. Abundant molluscan fossils in the Jizodo Formation are the excellent indicator of both water depth and marine climate. The ratio of lower sublittoral warm-water molluscan fossils increase toward the top of the TST, forming a condensed section. While in the HST, upper sublittoral cold-water molluscan fossils become abundant in upward sequence and no lower sublittoral warm-water molluscan fossils are found. The HST is, therefore, interpreted as formed during an initial stage of sea-level fall under a condition of climatic cooling. Judging from the fission track dating of a tephra and comparison of the inferred relative sea level curve with the oxygen isotope curve derived from V 28-239 and DSDP site 502, we consider that the three depositional sequences in the Jizodo Formation correspond to the three cycles of the isotope fluctuation from stage 12 to 10. It follows that the each depositional sequence of the Jizodo Formation was formed under the influence of glacio-eustacy with a period of the c. 20,000 or 40,000-years. Our preliminary survey shows that the overlying Yabu Formation is, similarly, subdivided into two depositional sequences. Also, the other formations of the Shimosa Group (Yabu, Kamiizumi, Kiyokawa, and Yokota formations) are known to have formed under the influence of glacio-eustacy with a period of the c. 20,000 or 40,000-years. It is, therefore, suggested that all the depositional sequences of the Shimosa Group were originated from glacio-eustacy with the period of orbital precession or obliquity by the Milankovitch theory.