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[SGL38-02] Litho-stratigraphy and sedimentary environment of upper part of Kokumoto Formation with the L-M Pleistocene boundary
Keywords:GSSP, The Lower-Middle Pleistocene boundary, Kokumoto Formation, Kazusa Group, Tabuchi section
Kokumoto formation, about 350 meter thick, is composed of lowermost part, lower part, upper part and uppermost part in ascending order. Lowermost part, about 60 meter thick, consists of thick siltstone with thin sandstone bed and marker tephras, Ku6 and ku5. Lower part, about 120 meter thick, consists of sandy alternation of sandstone and siltstone with Ku3 tephra. Upper part, about 80 meter thick, consists of thick siltstone without slump bed and with thin sandstone and marker tephra (Byakubi zone (Byk-E, Byk-D, Byk-C, Byk-B, Byk-A), Tap-B, Tap-A, Tas-C, Tas-B, Tas-A, Ku2). The Matuyama?Brunhes boundary is in Byk zone. Uppermost part, about 90 meter thick, consists of sandy alternation of sandstone and siltstone with Ku0.1 tephra.
The upper part, thick siltstone, is interbedded with thin, 1-3cm thick, sandstone every 0.3-3 m thick and thin, 1-5 cm thick, sandysiltstone every 0.1-0.25 m thick without slump bed and thick mudflow bed. The siltstone have bathyal and sublittoral benthic foraminifera and many trace fossils. Grain size distribution in the siltstone have bimodal grain group. Main grain group is composed of fine silt and sub group consists of very fine sand. These characteristics show hemipelagic sedimentary environment in deep sea and very fine sand flow often into, namely deep sea slope.