Keywords:Byk zone, Lower-Middle Pleistocene Boundary, Kokumoto Formation upper part, Yoro River, Chiba Section
Kazusa group distributed in the Ichihara City southern part, Chiba Prefecture, is composed of Umegase formation, Kokumoto formation, Kakinokidai formation. Kokumoto formation in the geological structure of N67-69E 7- 9N, and the tilt direction falling direction of Yoro River is substantially coincident. Kokumoto formation is 4 divided by facies, massive mud layer, sand layer rich sand layer and mud layer alternated layers (sand layer:mud layer=10:1-6:4), equivalent sand and mud alternated layers(mud layer:sand layer =4:6-6:4), mud layer rich sand layer and mud layer alternated layers(mud layer:sand layer=10:1-6:4), massive sand layer. And Kokumoto formation is divided 4 parts (uppermost part: sand layer rich sand layer and mud layer alternated layers, upper part: massive mud layer,middle part: sand layer rich sand layer and mud layer alternated layers, lower part: massive mud layer). The Brunhes / Matuyama chron boundary (B / M boundary) is confirmed in Byk zone, at upper part base of Kokumoto formation. This location is Chiba section. Byk zone is divided 5 tephras (Byk-A: off-white silt grain volcanic ash and fine sand grain scoria, Byk-B and Byk-C and Byk-D: medium sand grain scoria, Byk-E: white silt grain volcanic ash). Byk-E is identified as a conventional TNTT. Byk zone (Byk-A - E) has also been confirmed in Koshikiya River east of Yoro River, and Byk zone is distance Byk-A and Byk-E with a deposition rate change of the side. Distance of Byk-A and Byk-E is 3.5m at Yoro River location(Tabuchi Section of Chiba Section), and distance of Byk-A and Byk-E is 3.0m at Koshikiya River location(Koshikiyagawa section of Chiba section). From the measured value, Tabuchi section is faster deposition rate than Koshikiyagawa section in Chiba section.