High-accuracy synchronism for seismic reflectors and 14C ages: Holocene prodelta succession of the Kiso River, central Japan

Horizons at 500-yr intervals for the past 6000 years can be inferred in two cores drilled along a high-resolution seismic line in the prodelta of the Kiso River, Ise Bay, central Japan. These horizon ages clearly indicate that a given seismic reflection line in the profile is isochronous. Synchronism between ages and reflectors in the succession is especially distinct for the interval from 5000 to 2500 yr BP, despite the reflectors having inclined planes and having the same ages at different depths. These data indicate that the Kiso River delta prograded to form a downlap surface before the maximum flooding stage of the post-glacial relative sea-level rise at about 6000 yr BP. Delta progradation beginning late in the transgressive stage was caused by the flow of large amounts of sediment into this steep river that is part of the active Japanese volcanic arc.