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) a braid plain association characterized by braided river channels and floodplain deposits; (2) a wave-dominated estuary association comprising 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. The transition from wave- to tide-dominated estuary is recorded within a relatively thin (c. 150 m) stratigraphic succession and represents a short-lived depositional shift without significant temporal hiatus. This indicates that the transition occurred at a parasequence to parasequence-set scale, rather than reflecting long-term, basin-wide reorganization. This study proposes a hypothesis to explain this unusual evolutionary transition: an accelerated relative sea-level rise, likely driven by tectonic subsidence, enhanced tidal influences associated with increased tidal prism, causing barrier disintegration within the initially wave-dominated estuarine system. These findings emphasize the potential for more drastic morphological changes in estuarine systems than previously recognized, providing critical insights into predicting estuarine responses to future environmental changes.