[HQR06-02] The emergence of coastal disasters during the mid-Holocene highstand: An example from the coast of Hidaka, Hokkaido
Keywords:Tsunami deposit, mid-Holocene highstand, Hokkaido, Diatom, Coastal disasters
The geological survey was conducted on the current sea cliffs and past peatlands behind the beach in the southern, central and northern Hidaka area. The upper limit of marine sediments revealed by diatom microfossils and chemical analysis was around 6000-4000 BP. This period corresponds to the mid-Holocene highstand period, and then changed the sedimentary environment to freshwater and dry condition. Two to nine sand layers were identified, and all of the sand layers became thinner in the inland direction. Therefore, the sand layers were determined to be event layers originating from the beach. As a result of radiocarbon dating, the sedimentation age of the event layers was found to be limited to around 2000 years from the upper limit of the marine sediments. This result suggests that the event layers were formed during the highstand period. The relative sea-level rise along the Hidaka coast is estimated to be 1-3 m (Okuno et al., 2014). We presumed that such sea-level changes affected the tsunami and storm run-up in coastal areas.