Temporal and Spatial Variations of Accommodation and Sediment Accumulation during Transgressive to Highstand Stages as Reconstructed from a Latest Pleistocene to Holocene Sequence in the Intra-Arc Osaka Basin, Japan

Abstract

Temporal and spatial variations in accommodation (i.e., paleo-water depth) and sediment accumulation (amount of deposition) in the intra-arc Osaka Basin, Japan, were reconstructed from the post-glacial transgression through the sea-level highstand, a total of 9000 years. At the beginning of the marine transgressive stage (about 11,000 cal y BP), paleo-water depths were shallow and the sediment accumulation was large. The area occupied by Osaka Bay gradually extended and sediment deposition decreased from 11,000 to 6000 cal y BP. During the period of maximum transgression (6000 - 5000 cal y BP), an inner bay, Kawachi Bay with a water depth of 5 - 10 m, was expanded in the inland eastern Osaka area, and paleo-water depths reached a maximum and depositional rates reached a minimum. During the subsequent highstand and small regression (about 5000 cal y BP to the present), however, deposition increased rapidly as a result of river delta and shoreline progradations. Regional differences were observed in accommodation and accumulation between the outer bay area and the inner bay area. During both the transgressive and regressive stages, deposition decreased in the inner bay area. In contrast, in the outer bay area and in the basin overall, deposition was high during the first part of the transgressive stage but it decreased during the maximum transgression, before reaching a maximum during the subsequent highstand and regression. During the regressive stage, fluvial delta progradation led to the formation of a thick sequence of delta body sediments. Sediment accumulation was 30% - 40% higher during the regressive stage than that during the transgressive stage.