High resolution stratigraphy across the early-middle Pleistocene boundary from a core of the Kokumoto Formation at Tabuchi, Chiba Prefecture, Japan

We present a detailed stratigraphy of the lithology, sedimentary facies, magnetic susceptibility, and chemical composition of a 54-m thick core sequence drilled adjacent to the Chiba section, a candidate for the Global Boundary Stratotype Section and Point of the early-middle Pleistocene boundary. Siltstones rich in trace fossils are dominant throughout the core, except for the lowermost sand beds. The Byakubi tephra is divided into three units, of which the uppermost (39.54-39.49 m core depth) is identified as a primary bed showing evidence of fallout. The preliminary Matuyama-Brunhes magnetic polarity boundary lies 1 m above the top of the fallout unit. Maximum and minimum susceptibility axes for anisotropy of magnetic susceptibility have shallow (<11°) and steep (>73°) inclinations, respectively, throughout the core. The results suggest that sedimentary structures formed in calm environments remain undisturbed in the sediment. Clustered maximum axis directions indicate the influence of a persistent NW-SE contour current. The homogeneous depositional environment for the siltstone interval is supported by relatively smooth variations in magnetic susceptibility and chemical composition. Stratigraphic correlation with published oxygen isotope data indicates that the depositional age of the core may extend from the end of marine oxygen isotope stage (MIS) 20 to just after mid-MIS 19, from which an average sedimentation rate of 2-2.5 m/ky is estimated. These results suggest that the Chiba section has a high potential to provide an excellent stratotype for the early-middle Pleistocene boundary.