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[MIS11-P04] Constraining depositional age of tsunami deposit using rip-up clast
Keywords:tsunami deposits, rip-up clasts, OxCal
In case of dating using materials within the tsunami deposit, shells and plant fragments are commonly selected (e.g., Bondevik et al., 1997; Clark et al., 2011). However, 14C dating results of individual materials often have large statistical errors due to wiggles in the calibration curve. Instead, rip-up clast, which is considered to have been eroded from the ground sediment by the tsunami and incorporated into the tsunami deposit, has an advantage because sequential 14C dating can be performed. In this study, we tested sequential measurement of 14C dating of rip-up clast to better constrain the depositional age of tsunami deposit.
Study area is the Rikuzentakata City, Iwate prefecture. In this area, we observed a tsunami deposit with abundant rip-up clasts in few centimeters in size. Presence of rip-up clasts indicates strong basement erosion during the tsunami flow. We performed 14C dating using samples taken from above and below the tsunami deposit and rip-up clasts within the tsunami deposit. Results of sequential measurement of 14C dating were constrained by the stratigraphic order and depth information using OxCal ver.4.2.4 (Bronk Ramsey, 2008; Bronk Ramsey and Lee, 2013). The depositional age of the tsunami deposit is AD 681–1184 if we only use dating results from sediments above and below the tsunami deposit. On the other hand, the age of the youngest part of the rip-up clast was AD 776–887. The age obtained from the rip-up clast can be used to constrain the limiting maximum age of the tsunami deposit. In this case, depositional age of the tsunami deposit is AD 776–1184.
As stated above, rip-up clast can further constrain the depositional age of the tsunami deposit. This results in turn suggest that the depositional age of the tsunami deposit sometimes be older than few tens to hundreds of years due to the basement erosion by the tsunami flow if we only use dating results from sediments above and below the tsunami deposit.