Temporal development of prograding beach–shoreface deposits: the Holocene of Kujukuri coastal plain, eastern Japan

Abstract

Prograding beach–foreshore deposits with a basal ravinement bed are recognized from detailed AMS 14C dating and grain size analyses of Holocene deposits of the Kujukuri coastal plain facing the Pacific coast of Japan. The Holocene deposits are about 20 m thick and consist of an upward-shoaling beach–foreshore succession that has a basal unconformity overlain by a thin layer of relatively coarse sand. The unconformity is a downlap surface, which originated from a ravinement surface. The overall succession was formed by a prograding beach–foreshore system during the highstand in sea level of the last 6000 years. Poorly preserved fossil shells, enclosed in the thin layer of coarse sand at the base of the succession, date from the transgressive stage in sea level before 6000 cal yr BP. Grain size distribution patterns are polymodal in the basal sand layer and unimodal in the remainder of the succession. Modal classes of the basal part are fine (3.3–2.6φ), medium (2.3–1.8φ), and coarse (1.2–1.0φ). The fine mode and the unimodal grain sizes define an overall upward-coarsening trend, which correlates with the upward-shoaling succession. In contrast, the medium and coarse modes are restricted to the basal sand, and are interpreted as related to the ravinement process. The thin basal sand layer, an admixture of these modal classes, is interpreted as a type of ravinement deposit preserved during sea-level highstand. No transgressive deposit was preserved on the ravinement surface because sediment supply was so small that the sediment on this surface experienced thorough reworking by storm waves.