Depositional age and triggering event of turbidites in the western Kumano Trough, central Japan during the last ca. 100 years

For correlating a turbidite record with a detailed land record of natural disasters during the last ca. 100 years, the depositional age of the surface hemipelagic sediments of the Kumano Trough located in central Japan were estimated. We attempted to extract mass accumulation rate of “hemipelagic fallout” from hemipelagic intervals on the basis of 137Cs geochronology, X-radiograph, dry bulk density, and grain-size distribution. Based on assumptions of the biogenic mixing effect on the 137Cs appearance horizon and hemipelagite erosion by turbidity current, the reconstructed turbidite record from hemipelagic fallout mass accumulation rate is well correlated with the record of major natural disasters in the Kumano area. Based on the correlation, most of turbidite layers are flood/storm origin except for a well-sorted seismo-turbidite induced by the Tonankai Earthquake that occurred the mid 20th century. Although the wide distribution of a flood-induced turbidite layer due to the 19th century Tostukawa flood, which was a huge disaster in the area caused by heavy rain and dams breaching, it does not show a hyperpycnite succession. The flood by dams breaching may cause surge-like flow. Angular to subangular sand-sized lithic fragments in the turbidite show that sand grains were transported from the Kumano River directly. Plant debris and microfossils are common in both seismo-turbidite and flood/storm induced turbidites.