Abstract
The Pliocene to Middle Pleistocene Kazusa Group is widely distributed in subsurface of the Kanto Plain, central Japan. This study analyzed two drilling cores CRE-TAT-1 and CRE-TAT-2 obtained from the Fuchu city, Tokyo, where is located on the Tachikawa surface of Musashino uplands (terrace), the western part of Kanto Plain. The lithology and sedimentary structures of these cores were characterized in detail and grain-size distributions, water content, electrical conductivity and pH values in soil-stirred water were measured. Also, diatom assemblage analysis and X-ray fluorescence analysis were carried out. Both cores are divided into unit 1 to unit 7, in ascending order. Unit 1 is sandy channel and floodplain sediments. Unit 2, 4, and 6 are sandy gravels of river channels. Unit 3 consists of sand and silt with fresh-water diatom indicating floodplain environments. Unit 5 has sandy channel sediments at the bottom. The middle part of the unit 5 is bluish gray silt with abundant marine diatom. High sulfur content and electric conductivity suggest marine environment. It coarsens upward into the sandy channel and floodplain sediments showing upward shallowing trend.
Unit 1 to 5 might correspond to the Toneri formation of the Kazusa Group deposited during 1.219-0.78 Ma. Unit 6 is terrace gravel which forms the Tachikawa surface during 0.4-0.2 Ma, and unit 7 is the Tachikawa Loam.
