Preliminary Hypothesis About the Polygonal Features Discovered in a Middle Miocene Deep-Sea Sediment Sequence (Kawabata Fm., Hokkaido, Japan)

Numerous landslides occurred 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 has provided a unique opportunity to gain insight into specific chapters of Earth’s history by exposing new natural outcrops and revealing rock strata, including the Middle Miocene Kawabata Formation. The formation primarily comprises sedimentary rocks that reflect various deep-sea environments, especially the continental slope and basin plain of a foreland basin. For this reason, an exposed bedding plane consisting of polygonal features, recently (September 2022) discovered in an outcrop formed at the scarp of a landslide, has 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 subaerial exposure, with potential wet and dry cycles in a near-shore/coastal environment, was revealed, such as limonite grain coatings and hypocoatings, as well as biotite exfoliation. Based on this evidence, a putative new Middle Miocene facies and a potential new stratigraphic member was proposed in the Kawabata Formation.