Tsunami deposits in the estuary-fill:Holocene drill core in Minamisoma City, Fukushima Prefecture, Northeast Japan

Abstract

We studied the relationship between estuary-fill process and formation of tsunami deposits in the southern part of Minamisoma City, Fukushima Prefecture, Northeast Japan. We took a 26.5 m core of the Holocene estuary-fill sediments and reached underlying Pleistocene sandy siltstone. We described the lithology of the core and identified sedimentary facies, and carried out soft X-ray photograph, dry bulk density measurement, grain size analysis and dating of radiometric ¹⁴C ages. The Holocene estuary-fill is composed of the deposits of backshore, sandy to muddy tidal flat, subtidal estuary and salt marsh with intercalated event sand beds, and artificial fill and paddy soil in ascending order. The event sand beds yield many marine shell fragments. They indicate the sands were transported by landward flows. The beds contain rip-up clasts and gravels, and represent grading structures, basal reverse grading, multiple grading and periodic sedimentation. These characters suggest the possible tsunami deposits.