[S-GL44_2PM1] The Lower-Middle Pleistocene boundary of the Kazusa Group

[SGL44-01] Distribution of stratigraphic units of Middle Pleistocene Izumiyatsu Formation and their arsenic concentrations

*Takeshi YOSHIDA¹, Takashi KUSUDA², Hisashi NIREI³ (1.Research Institute of Enviornmental Geology, Chiba, 2.Foremer Research Institute of Enviornmental Geology, Chiba, 3.Institute of medical geology)

Keywords:Member unit, Groundwater flow, Arsenic

We identified the strata units of the Pleistocene Izumiyatsu Formation, which extends from the central part of Chiba Prefecture to the northeast, and the distribution of arsenic in these strata. In our summary of the geology, we refer to the Shimofusa Group.It is possible to divide the facies of the Izumiyatsu Formation, a type locality, into five beds ? a muddy sand layer (facies 1: an estuarine sediment), an interbed of fine sand and mud (facies 2: a tidal flat sediment), a sand layer (facies 3: a tidal channel sediment), a silt layer (facies 4: a freshwater?seawater marsh sediment), and a medium sandy mud layer (facies 5: a inner bay marine sediment). The Izumiyatsu Formation, with changing facies, exhibits the following distribution pattern: facies 1, 2, 3, 4, and 5 in the southwest area, facies 5 only in the central area, and facies 4 and 5 in the northeast area. Only facies 5 is continuously distributed throughout the research areas.Silt layer(Facies 4), the freshwater?seawater marsh sediment, has lower arsenic concentrations in sediment and in leachate than the other facies. Facies 5, the inner bay sediment, has higher arsenic concentrations in sediment and in than the other facies.