Chronology and Process of Fluvial Terrace Formation in the Eastern and Western Parts of the Ohmi Basin

Abstract

 The process of terrace formation in Japan is discussed in the context of advances in tephrochronology. In particular, the ages of fluvial and marine terraces correlate with climate and glacio-eustatic sea-level changes. However, previous studies do not distinguish between the influences of climate changes and base-level changes (glacio-eustatic sea-level changes) for terrace formations, and mainly target northeast Japan due to advantages related to tephrochronology and terrace development. Therefore, this study focuses on the Ohmi Basin, Shiga Prefecture, southwest Japan, to reveal the process of terrace formation under conditions with no base-level changes, and compares them to those in northeast Japan. To achieve this aim, a drilling survey at terrace surfaces and a cryptotephra analysis are conducted. Widespread tephras, Kikai-Akahoya (K-Ah) tephra, Aira-Tn (AT) tephra, and Kikai-Tozurahara (K-Tz) tephra, are used to correlate fluvial terraces and establish the chronology of fluvial terraces in the eastern and western parts (Koto and Takashima regions, respectively) of the Ohmi Basin. Terrace correlation shows that terraces formed during Marine Isotope Stage (MIS) 2 are distributed in both regions under different tectonic settings. This indicates that climate change is the main factor of terrace formation in the Ohmi Basin. Therefore, river conditions during MIS 1, 2, and 5 are compared, and influences of climate changes and crustal movements for terrace formation are estimated. As a result, terrace formation in the Takashima region is explained by climate changes and fault movements. On the other hand, terrace formation in the Koto region is explained by climate changes and tectonic tilting. Consequently, these results suggest that the fluvial terraces in the Ohmi Basin are climatic terraces and that older to younger terrace steps could be the result of a combination of climate changes and crustal movements without base-level changes.