Early Holocene coastal environment change inferred from deposits at Okinoshima archeological site, Boso Peninsula, central Japan

Abstract A series of deposits recording the early Holocene drastic changes in coastal environment was investigated in terms of sedimentology and paleontology at the Okinoshima archeological site on the southwest coast of Boso Peninsula, central Japan. The deposits, ca. 3 m thick exposed in trenches, consist of structureless mud and overlying stratified sand, divided into three units, I, II, and III in ascending order. Unit I is poorly sorted sandy mud with abundant opal phytoliths. Unit II, gradually overlying unit I, is poorly sorted sandy mud containing more organic material and pieces of wood, and archeological remains. Unit III overlies unit II with a sharp erosional contact and is composed of well-sorted medium-grained sand intercalated by mud layers with abundant plant fossils. Unit I suggests a dry surface on a coastal plain near uplands. Unit II was transported from the upland onto the coastal plain by a debris flow. Unit III deposits resulted from high-energy river and storm surge flows onto the coastal plain. The succession records that the sedimentary environments of the study site changed successively from a slope near the uplands, to a fluvial plain, to a coastal plain in response to a sea-level rise in the early Holocene. AMS ages of the units are approximately 10,000 cal BP, and deposition occurred within 1000 years.