A widely distributed upper Aptian shallow-marine tsunami deposit in the Miyako Group, northeast Japan

We report a newly discovered characteristic event deposit of late Aptian age in the Tanohata Formation of the Lower Cretaceous Miyako Group in northeast Japan. The event deposit is 1.5–5.5 m thick and contains internal erosional surfaces that divide the event deposit into multiple sub-layers. Sigmoidal cross-stratification, which is often formed under hydraulic conditions transitional between upper and lower flow regimes, was observed at the top of the deposit. The event deposit lies on an erosional surface cut approximately 4 m into the underlying strata and contains sand rip-up clasts of underlying strata. The presence of sand rip-up clasts reflects the powerful effect of this event on the shallow seafloor, which caused erosion of the cemented substrate and generated boulder-size clasts. On the basis of these sedimentological features, we interpret the event deposit as a tsunami deposit. A previous study reported a tsunami deposit about 20 km away from this study site at exactly the same lithostratigraphic horizon as the tsunami deposit reported in this study. The fact that tsunami deposits were found at the same stratigraphic horizon in two distant areas indicate that sediment transport and deposition by the tsunami was substantial and extensive. We hypothesize that other deposits formed by the same tsunami may be present in other upper Aptian strata across the western Pacific region.