Late Holocene Environmental Changes at Kuninaka Plain, Sado Island, Central Japan, Deduced from Sediment Facies and Diatom Assemblages

Abstract

This paper describes the Holocene stratigraphy and depositional environment at Kuninaka Plain on Sado Island, off Niigata, on the Japan Sea side of central Japan, using stratigraphy, ¹⁴C dates, and diatom analysis of cores, and it discusses the upper limit of Holocene marine deposits and paleogeographical changes during the Holocene. Kuninaka Plain is the largest plain on Sado Island, and is located between two active tilted blocks, Osado to the northwest and Kosado to the southeast. The northeast-trending Kuninaka-minami fault is an approximate boundary between the plain and the Kosado mountains. Kuninaka Plain is divided into two parts by the last interglacial marine terrace (T3): the northeastern part, facing Ryotsu Bay, includes Kamo Lake, filled with brackish water. The southwestern part is a large alluvial plain facing Mano Bay. Diatom analysis and facies observation of Holocene deposits at the southwestern part of the Kuninaka Plain indicate that the upper Holocene marine limit is 1 to 2m above sea level and that the emergence took place ca. 4, 000-5, 000yrs BP. Since that time, no transgression has occurred. In contrast, deposits of nearly the same age occur below the present sea level in the northwestern Ryotsu Bay area, suggesting the relative subsidence of that area, where minor transgressions and regressions have recurred since that time. This may not be caused eustatically, but rather may have originated from the opening and closing of the barrier. The marine limit is rather high, ca. 3-4m on the Holocene terrace along Ryotsu Bay. This is probably a result of the uplifting of the upthrown side of Kuninaka-minami fault. Holocene paleo-geographical changes are reconstructed for three stages.