Geochronological Examination and Geomorphic Evolution of River Terraces Distributed along the Hiji-kawa River in Ehime Prefecture, Southwest Japan: Based on Topographic Continuity and Weathered Degree of Gravels

Terrace profiles along rivers based on accurate correlations of terraces and geochronological examination are needed to reveal regional crustal deformation from the distribution of river terrace surfaces. Tephra beds, which are intercalated or cover terrace sediments, are used as beneficial information to correlate terrace surfaces accurately. On the other hand, in cases lacking tephric sediments, the correlation of terrace surfaces is mainly based on geomorphic characteristics, such as degree of dissection, relative elevation, and geologic properties, such as facies of sediments and weathered degree of terrace gravels. A terrace surface correlation based on qualitative indicators may cause uncertainty or variability depending on the criteria defined in each study. In this research, weathered degree of gravels in terrace sediments is introduced as another quantitative indicator to validate or compensate for the terrace surface correlation. The examination process is as follows. The research target is the Hiji-kawa River in western Shikoku, along which many terrace surfaces (P, HH, H, M, and L) are distributed. Few tephric sediments exist in terrace sediments. The 0.6 Ma tephra is intercalated at the upper part of P Surface deposits. 0.3 Ma tephra is intercalated at the upper part of Hf1 Surface deposits. An analysis of volcanic ash grains suggests that M surfaces are covered with Aso-4 tephra (85 to 90 ka). First, the ages of other non-aged-terrace surfaces are estimated based on height differences from aged-terraces. Second, real density values are measured for several gravels collected from 15 stages of terrace sediments. Finally, measured real density values are normalized by measured values of current riverbed gravels. Relative density values (RDV) show a negative cor* 株式会社阪神コンサルタンツ ** 株式会社四国総合研究所 *** 四国電力株式会社 **** 株式会社ナイバ + 現所属:ケイジオ * Hanshin Consultants CO., LTD., Tokyo, 101-0037, Japan ** Shikoku Research Institute INC., Takamatsu, 761-0192, Japan *** Shikoku Electric Power Co., INC., Takamatsu, 760-8573, Japan **** Naiba CO., LTD., Takamatsu, 763-0062, Japan + Present address: K-Geo, Matsuyama, 791-1101, Japan 地学雑誌 Journal of Geography(Chigaku Zasshi) 131(5)521­544 2022 doi:10.5026/jgeography.131.521