[H-CG21] Earth surface processes related to deposition, erosion and sediment transport

[HCG21-08] Transition from Wave- to Tide-Dominated Estuaries: An example from the Eocene Urahoro Group, eastern Hokkaido, Northern Japan

*Ryusei Sato¹, Hajime Naruse¹ (1.Kyoto University)

Keywords:coastal geomorphology, barrier disintegration, morphodynamics

Estuaries are coastal depositional systems typically developing in drowned incised valleys during transgressive periods. Estuarine morphologies are commonly classified into two end members based on dominant sediment transport processes: wave-dominated and tide-dominated estuaries. While estuaries are generally expected to maintain their fundamental morphotypes throughout their evolutionary history, some studies have reported that tide-dominated estuaries can temporarily change their geomorphological features due to environmental changes or the topographical characteristics of the incised valley. However, the evolutionary models describing morphotype transitions from wave- to tide-dominated estuaries remain scarce. This study analyzes the depositional history of the Eocene Urahoro Group in eastern Hokkaido, Japan, as an exceptional ancient example of a wave- to tide-dominated estuarine transition. The Urahoro Group in the Konsen-coastal region was selected for its well-exposed coastal cliffs. 3D outcrop models were leveraged to generate columnar sections continuous to the upper part of the outcrop, supplementing the observation by the naked eye. Seven facies were identified based on the bed geometry, grain size, sedimentary structures, and fossils. These facies comprise three facies associations in ascending stratigraphic order: (1) Alluvial fan facies association, consisting of braided river channels and floodplain deposits; (2) Wave-dominated estuary facies association, including bayhead deltas, central basin, and flood-tidal deltas; (3) Tide-dominated estuary facies association, comprising tidal flats and tidal sand bars. Although further research is needed to confirm the mechanisms driving the morphotype transition, this study explores two potential hypotheses. One suggests that barrier disintegration in the wave-dominated estuary resulted from an accelerated rate of relative sea-level rise. The other posits that increasing tidal influence, driven by autogenic processes, facilitated the development of a tide-dominated estuary as the system evolved. This study highlights the possibility that estuarine morphotypes can change in response to allogenic or autogenic factors, offering valuable insights for predicting future estuarine evolution.