Transition from wave- to tide-dominated estuary: An example from the Eocene Urahoro Group, eastern Hokkaido, northern Japan

Estuarine morphologies are commonly classified into two end-member categories based on dominant sediment transport processes: wave-dominated and tide-dominated estuaries. Although estuaries are generally presumed to retain their fundamental morphotypes throughout their evolutionary history, this study presents the first documented ancient example of a morphological transition from a wave-dominated to a tide-dominated estuary, identified in the Eocene Urahoro Group in eastern Hokkaido, Japan. A three-dimensional outcrop model was utilized to generate continuous stratigraphic columns extending into the upper parts of the outcrop for detailed facies analysis. Seven facies were identified and grouped into three stratigraphically successive facies associations: (1) an alluvial fan association characterized by braided river channels and floodplain deposits; (2) a wave-dominated estuary association comprising the bayhead delta, central basin, and flood-tidal delta deposits; and (3) a tide-dominated estuary association consisting of tidal sand bar and tidal flat deposits. This study proposes a hypothesis to explain this unusual evolutionary transition: an accelerated relative sea-level rise likely enhanced tidal influences, causing barrier disintegration within the initially wave-dominated estuarine system. These findings emphasize the potential for more dramatic morphological changes in estuarine systems than previously recognized, providing critical insights into predicting estuarine responses to future environmental changes.