Abstract
Temporal variation of Holocene Osaka Bay environment was inferred from facies analysis and many highly dense 14C age determination of a core collected from the bay floor off-Kobe. The results indiate the following detailed variation of depositional environments and conditions of Osaka Bay in Holocene time.The depositional environments inferred from the core succession are as follows : The lower muds (8.5 m thick) are estuary and inner bay deposits and deposited rapidly (8 mm/yr) during rapid rising stage of sea-level ; the middle sand and mud (8 m thick) are tidal delta deposits showing rapid deposition (about 7 mm/yr) ; the upper sand and mud (9 m thick) are bay-floor deposits of slow deposition (0.9 to 1.8 mm/yr)during a marine flood and highstand in sea-level ; and the uppermost clay (4 m thick) is bottomset deposits of the modern Yodo River and shows increasing depositional rates (2.0 to 3.7 mm/yr).The following bay environments also are given from the core analysis : the sea-level at 11000 B.P., was -51 m below the modern sea-level in Osaka basin ; in 9700 B.P., -31 m in sea-level high, the Akashi Strait was formed ; in 8000 B.P., the sea-level of -12 m high, the Setouchi Inland Sea (Setonaikai) was formed when the seas were linked together at the Bisan-Seto ; during 5300 to 5000 B.P., the maximum highstand of sea level in the Osaka basin was +1 to 2 m in elevation ; during 8000 to 3000 B.P., coarser deposits were distributed in all Osaka Bay with slow deposition by strong tidal-currents in the Akashi Strait ; restarting of inflow of much flood clay from the Yodo River into Osaka Bay in about 1700 B.P. and spread the distribution until off-Kobe in 1000 B.P.
