Ground-penetrating radar profiles of Holocene raised-beach deposits in the Kujukuri strand plain, Pacific coast of eastern Japan

Abstract

Ground-penetrating radar (GPR) profiles of Holocene beach deposits were documented from the Kujukuri strand plain, Pacific coast of Japan, to examine a marker of past sea level recognized in these profiles. The Kujukuri strand plain has been uplifted over the last 6000 years, with a total relative sea-level fall of about 5 m suggested by seaward lowering of the lithologic boundary between the foreshore and upper shoreface facies. A GPR survey was performed on a modern aeolian dune and beach ridges of the strand plain using a PulseEkko100 system with 100-MHz antennae. The profiles obtained contain five vertically stacked radar units (R1–R5) with characteristic reflections. Ground truthing using sediment cores and the morphology of the modern beach suggest that these radar units correspond to aeolian dune (R1), small-scale swale (R2), backshore and foreshore (R3), upper shoreface (R4), and lower shoreface (R5) deposits, in descending order. The backshore and foreshore radar unit shows a sequence of continuous reflections dipping seaward at an angle consistent with the modern beach, whereas the underlying upper shoreface radar unit represents complicated and less-continuous reflections. The boundary between these two units is a downlap surface of the foreshore reflections, and is thought to have formed at about 1 m below the mean sea level at the time of deposition. The lateral tracing of the boundary in radar profiles across the strand plain indicates a total fall in relative sea level of 5 m, which is consistent with the drill core results. Radar profiles of the foreshore and upper shoreface deposits in the Kujukuri strand plain thus provide a past sea-level indicator that is available without drilling sediment cores.