Initiation of delta formation and subsequent fluvial sedimentation in the upper delta plain: A case study from the Kiso River delta, central Japan

This study investigates the Holocene evolution of the upper Kiso River delta in central Japan, focusing on sedimentary processes and accumulation patterns in the upper delta plain near the upstream limit, an area directly influenced by transgressive and regressive phases driven primarily by eustatic sea-level change and less explored than the lower delta plain. The analysis is based primarily on a 23-m sediment core (UK) retrieved from a floodbasin approximately 40 km inland from the present mouth of the Nagara River. The core was analyzed for sedimentary facies, grain size distribution, colour, electrical conductivity, loss on ignition and AMS radiocarbon dating. Six depositional units were identified, ranging from basal river gravels to marine-influenced sands and overlying floodbasin muds and peats. An upward-coarsening succession from tidal river to distributary channel deposits along with the presence of gravels marks the onset of regression at approximately 8 ka. Radiocarbon data indicate that sediment accumulation initially almost kept pace with eustatic sea-level rise but slowed to approximately 1.6 m/kyr during the highstand phase, exceeding the contemporaneous sea-level rise. Stratigraphic correlation with nearby boreholes along the Kiso and Nagara rivers reveals consistent depositional patterns and a lack of prodelta mud in upstream areas. A turning point in N -values within the sand layer may represent the maximum flooding surface. Floodplain aggradation has persisted since approximately 6 ka, likely accompanied by vertical accretion within the channel belt. Despite the slowdown in sea-level rise and regional crustal uplift associated with glacio-hydro isostatic adjustment, tectonic subsidence appears to have maintained sufficient accommodation. Peat accumulation, which became increasingly prominent after 4 ka, reflects reduced clastic input and associated hydrological changes. Comparative stratigraphic and geomorphological analyses suggest that floodbasin deposits have been less frequently replaced by channel migration and avulsion along the Ibi and Nagara rivers than along the Kiso River.