Abstract
The Middle Pleistocene to Holocene stratigraphy under the interior of the Kanto Plain was constructed by analyzing two drilling cores (GS-FK-1 and SA-GD-1). We performed sedimentary facies analysis, determined lithological compositions of gravels, and measured grain size, electronic conductivity (EC), total sulfate contents (TS), and magnetic susceptibility. Both cores were divided into five depositional units, FK-U1 to FK-U5 and GD-U1 to GD-U5, in descending order, respectively. All depositional units except for GD-U1 consist of gravel layers overlain by fine-grained layers. These units were influenced considerably by transgressiveregressive cycles, which are associated with glacio-eustatic sea-level changes. The gravel layers were deposited as channel sediments mainly during sea-level falls and/or lowstands, while the fine-grained layers were presumably accumulated as floodplain and/or marine sediments during sea-level rises and the following sea-level highstands. The units can be correlated with marine isotope stage (MIS) based on 14C ages, tephras, and palynological records.
Paleogeography and sedimentary facies distribution were discussed based on longitudinal and transverse stratigraphic cross-sections drawn by using the both cores and existing drilling columns. The sections indicate that the sea spread to the interior of the plain in MIS 11 and MIS 9. After the maximum transgressions of each interglacial stage, beachs were probably developed at the GS-FK-1 site. In contrast, delta progradation occurred at the SA-GD-1 site. The difference may reflect the water depth in each core. The water depth at the GS-FK-1 site would be smaller than the SA-GD-1 site, and the GS-FK-1 site would be located at the margin of the seacoast.
The grain size and the thickness of gravel layers of GS-FK-1 are larger than those of SA-GD-1. This is probably because the GS-FK-1 site is much closer to the Kanto Mountains than the SA-GD-1 site.
