Transition from incised valley to barrier island systems during MIS 5e in the northern Chiba area, Kanto Plain, central Japan

Abstract

Facies analysis, together with palynostratigraphic studies of the Kioroshi Formation, which lies beneath the northern Chiba area of the Kanto Plain of central Japan, reveal the sedimentary processes in the fluvial to coastal settings during marine isotope stage (MIS) 5e in this region. The depositional sequence of the Kioroshi Formation comprises a lower incised valley system and an upper barrier island system. The incised valley system consists of fluvial, estuary, and central basin facies with sandbar facies in the baymouth area. In the study area, the overlying barrier island system includes flood tidal delta, spit, and lagoon facies. These facies form part of the barrier island system known as Paleo-Tokyo Bay, which covered the present-day Kanto Plain. On the basis of palynostratigraphic correlation with the oxygen isotope curve in an offshore core (MD01-2421), the fluvial and estuary facies, together with the lower half of the central basin facies of the incised valley system are interpreted to have accumulated during a sea level rise in the early stage of MIS 5e. The baymouth sandbar also developed and retrograded with the rising sea level and was abruptly covered by the central basin facies during the middle stage of MIS 5e, probably due to the migration of the sandbar to the outer-bay area. The abrupt change corresponds with the initial stage of development of the barrier island system. The landward (westward) retreating bay ravinement erosion resulted in the transition to the barrier island system during middle MIS 5e. Maximum transgression also occurred during middle MIS 5e. The back-barrier lagoon was infilled during the late stage of MIS 5e. The sedimentary processes of the Kioroshi Formation are almost in accordance with the marine isotope curve.