Middle to Late Pleistocene Shallow-Marine Sedimentary and Fannal Cycles Corresponding to the Orbital Precession or Obliquity in the Simosa Group, Boso Peninsula, central Japan

摘要

A total of nine depositional sequences corresponding to 20,000 or 40,000 year cycle, were identified in the non-marine to shallow-marine cyclic sequences of the Shimosa Group distributed in the Anesaki district, on the west coast of Boso Peninsula, east of Tokyo Bay. A typical cycle consists of freshwater mud and shallow marine sand (transgressive systems tract), and shallow marine sand (highstand systems tract). Analysis of the molluscan fossil assemblages in the sediments has shown that the depositional environment and the marine climate oscillated between cold-water, fresh- or brackish-water environment or upper sublittoral zone, and warm-water lower sublittoral zone. The water depth change of a scale of about 50-60 m and closely related climatic change strongly suggest a glacio-eustatic origin. Based on published data by fission track dating, the cyclothemic sequence span from the isotopic scale 5 to 11. From the Milankovitch theory, the timing of each cycle is inferred to corresponds to 20,000 or 40,000 yr. cycles caused by the cyclic changes in the orbital precession and obliquity.