Re-investigation for the delta succession of the Plio-Pleistocene Kobiwako Group in the Yasu River, Shiga Prefecture, Japan

Abstract

A re-investigation was conducted for a continuous outcrop of the Plio-Pleistocene Kobiwako Group located on the riverbed at the Yasu River in Shiga Prefecture, Japan. The outcrop is famous for producing fossil footprints of elephants, deers and others of about 2.6 million years ago. As a result of applying sedimentary facies analysis to the strata, the following new facts were found out. The strata are constructed by the progradation of shallow lacustrine delta system. The delta topset shows horizontal alternation of floodplain silt and sand of distributary channels and/or mouth bars with wave ripples, cross stratification, mud-drape, flood sand, plant debris, tree stumps and footprints. The delta foreset is gently inclined down-lap alternation of sand and mud with subaqueous gravity-flow sand and slump beds of delta slope or delta lobes. The bottomset is massive clay of prodelta deposits. The delta deposits record the lake-level changes caused by weather change and tectonic movement. Paleocurrent directions obtained from the delta deposits, such as forward direction of fluvial bed-form indicated by cross-lamination and prograding direction of delta by foreset stratification, are approximately east to west that is significantly different from previously reported direction. These paleocurrents indicate that the paleogeography at the time needs to be reexamined.