Sedimentary facies and sedimentary processes of the 2011 Tohoku-oki tsunami deposit with diversified grain size distribution

Most of the reported modern tsunami deposits are composed of sand, but gravelly tsunami deposits are rarely reported so far. While, gravely deposits that are considered as the possible paleotsunami deposits have often been identified in the geologic stratum. To expand our knowledge about diversity of tsunami deposits and provide criteria to identify tsunami deposits in the strata, we described a deposit with diversified grain size distribution of the 2011 Tohoku-oki tsunami. As well as describing the deposit, we also reconstructed its sedimentary processes. The height of the 2011 Tohoku-oki tsunami reached up to 28.1 m above sea level and inundated up to 1.8 km inland at the study site in Miyako city, Iwate prefecture, Japan. Tsunami deposit with diversified grain size distribution (silt-cobble) covered the lowland that had been used to paddy fields. The layered tsunami deposit is composed of pebble and cobble near the shore, whereas it is composed of sand and silt near the inundation limit, showing general fining landward trend. The thickness of the deposit is 1 m or more near the shore and generally thins inland to be a few centimeters. Separate from the layered deposits, many boulders that are composed of broken pieces of breakwaters, wave-dissipating blocks and volcanic rocks were scattered on the lowland. The transportation and deposition of the boulders and layered deposits at the study site is one of the rare cases in the sense that the wide range of sedimentary grains was deposited concurrently by a tsunami. Most boulders were deposited until 750 m from the shoreline, and the current velocities estimated from the boulder sizes show rapid declination at the point. The seaward stretched local scours around the larger boulders represent that they were not moved by the return flow. The gravelly tsunami deposit shows a sharp decline in thickness and grain size where many of the boulders were stopped. These indicate that deceleration in run-up flow caused rapid deposition of the sand and gravel. Furthermore, boulders were deposited on the sand and gravel layer, but not buried in it. This reflects that sand and gravel were transported in the bore front of the run-up flow and deposited first, and boulders followed them.