Abstract
Sedimentary facies analysis for the deposited around the Pink Volcanic Ash Layer in the lower part of the Pliocene to Pleistocene Osaka Group distributed in the Southern Osaka Prefecture revealed the initiation of tectonic movement that generated folds and dome structure. Ten sedimentary facies are recognized, indicating a series of environments ranging from fan, river channel, fan delta, flood plain, estuary, lagoon, foreshore, tidal flat, Shoreface to bay mud floor. Relative sea-level curve at each study point suggests that discriminative basin subsidence occurred when the Pink Volcanic Ash Layer was deposited; The areas with a low subsidence rate evolved to present anticlines, whereas the areas with a high subsidence rate correspond to present synclines. This implies that tectonic movement that formed present fold structures has already begun in the Early Pleistocene when the Pink Volcanic Ash Layer was deposited. Reconstructed paleogeography at that time shows complicated shorelines with an arm of sea deeply projecting into the land and an elongated inlet extending north or northeast.
