Fig. 23

Lithology bar at boreholes sites OB-1 and OB-2 (offshore Kobe) and sites Suminoe-A and B (Osaka Basin), modified after Masuda et al. (2000), Masuda and Miyahara (2000), Nanayama et al. (2000, 2001), and Kitada et al. (2001). Ages (yr) are based on these studies (red: tephra, black: radiocarbon dating, blue: pollen assemblage). The Holocene alluvium consist of a lower sequence of clay, silt, and sand interpreted to be marine clay deposited in an estuary and inner bay environment during marine transgression via the Kitan Strait, overlain by middle and upper sequences of coarser-grained, silt, sand, and clay in marine and tidal delta environment, following the opening of the Akashi Strait (~ 9.7 ka) (Masuda and Miyahara 2000; Nanayama et al. 2001). Green, blue, yellow, and pink lines mark the base of seismic units defined in this study. Bases of Units A1 and A1’ in the northern area in this study are correlated with the bases of the middle and upper sequences (tidal delta facies), and the base of Unit A2 with the that of lower sequence (inner bay and estuary facies). Unit A1 comprises the contemporary Okinose sand bank, which have likely initiated during the sea-level highstand at ~ 5 ka (also see Fig. 24). Unit B1 is correlated to the finer-grained fluvial facies of the early Holocene alluvium, and Unit B2 to the coarser-grained fluvial facies of the Late Pleistocene colluvium. Note that in the central to southern area, Unit A1 is absent, and Unit A1’ is deposited to the surface (~ 9.7 ka to present)