Marine Transgression Equated with Oxygen Isotope Stage 7.3 in the Harima District, Hyogo Prefecture, Western Japan

摘要

Terrace sediments intercalating a marine clay layer were found in the Harima district along the northeastern coast of the Seto Inland Sea, Japan. We performed diatom and volcan- ic ash analyses to clarify the sedimentary envi- ronments and depositional age of the sedi- ments. The sediments are divided into four lithostratigraphical units, I, II, III and IV in ascending order. The results of the diatom analysis suggest that unit I was deposited in freshwater conditions. Units II and III are inferred to have been deposited in marine litto- ral and brackish-water conditions, and unit IV in terrestrial conditions. The Kakogawa vol- canic ash layer intercalated in unit IlI is correl- ative with the Koshienhama-I volcanic ash layer in the marine clay bed Mall (2) underly- ing the Osaka plain and the BT 51 volcanic ash layer (estimated eruption age 215ka) in the Takashima-oki 200m-core from Lake Biwa. Therefore, the marine facies of unitsII and III are stratigraphically equivalent to the Mall (2) bed, and the age of the transgression that deposited these marine units corresponds to oxygen isostope stage 7.3. From the succes- sion of diatom assemblages, the former shore- line indicating the relative sea-level position at 215 ka is recognized to be the Kakogawa vol- canic ash horizon at about 44 m above the pres- ent sea level. On the basis of this relative sea level and the paleo-sea level during stage 7 inferred from the coral-reef terraces in Sumba Island (Indonesia), average uplift rate during the last 215 ka is estimated to be 0. 2 to 0. 3 mm/ yr at the investigated site.