Implication of Polygonal Features Discovered in a Middle Miocene Deep-Sea Sediment Sequence (Kawabata Fm., Hokkaido, Japan)

Numerous landslides appeared in the region of Atsuma (Hokkaido, Japan) as a result of the 2018 Hokkaido Eastern Iburi Earthquake. Despite their devastating consequences, the emergence of mass movements may provide a unique opportunity to gain insight into specific chapters of Earth’s history by exposing new natural outcrops and revealing rock strata. In the region, geological formations, including the studied Middle Miocene Kawabata Formation, composed primarily of sedimentary rocks, reflect various deep-sea environments, especially the continental slope and basin plain of a foreland basin. For such a reason, the exposed bedding plane consisting of polygonal features in an outcrop formed at the scarp of a landslide aroused interest. Mud cracks or desiccation marks often form in the tidal zone due to exposure to the subaerial environment, but not in the aforementioned deep-sea facies. An additional series of evidence of wet and dry cycles in a near-shore (coastal) environment was revealed, such as limonite grain coatings and hypocoatings, and biotite exfoliation. Based on that evidence, a new Middle Miocene facies, a newly discovered stratigraphic member, was proposed in the Kawabata Formation, indicating a tidal palaeoenvironment.