Stratigraphy and correlation of tephras in the Lower Pleistocene Kiwada Formation and its correlative beds, Kanto, Central Japan

Abstract

We describe the characteristics of tephras preserved in Lower Pleistocene marine sediment in the middle part of the Kiwada Formation (Kazusa Group) and its correlative beds in the Tama hills, Tokyo (from drillcore), the Boso Peninsula, and the Choshi area, Kanto, central Japan. The tephras were correlated based on their mineral contents, refractive indices of minerals, and the shapes, refractive indices, and chemical compositions (major and rare earth elements) of volcanic glass shards. We re-examined 22 tephras (in descending order: HM-Kd16, GS, NK, NB-Kd17, SG-Kd18, MT, Kd19, NG-YR, YU, PK, YM, KK, TN, Sr-Kmd, Ob4e-Kd23A, Ob4d-Kd23B, Ob4c-Kd24, Ob4b-6 to 4, Ob4b-1, and Omn-SK110), revising their definitions, correlations, and distributions. The biostratigraphy of the Choshi area, based on calcareous nannofossil datum 9—11, indicates the tephras range in age from 1.343 to 1.633 Ma. The accumulation rate of sediments during this period, in all four study areas, was 1.4—225 cm/kyr, depending on the sedimentary environment.