Holocene coastal evolution and paleogeography of the Izumo Plain and Lake Shinji, Western Japan: Incised-valley fills in a wave-dominated estuary environment

The sedimentary infill of the Izumo Plain was studied with emphasis on the influence of coastal evolution and paleogeography. The Izumo Plain and Lake Shinji belong to Paleo-Shinji Bay, an east-west incised valley formed during the Last Glacial Maximum and drowned by the early Holocene sea-level rise. Using micro-XRF, CNS elemental analysis, grain size, diatom analysis, and robust radiocarbon dates for three newly collected cores from the plain, complemented by three previously studied cores from the plain and Lake Shinji, five depositional phases have been identified. The phases include estuarine to coastal plain (up to 9.8 cal. kyBP), transgressive central basin (9.8–7.7 cal. kyBP), regressive central basin (7.7–3.7 cal. kyBP), regressive inner central basin (3.7–0.7 cal. kyBP), and delta front (<0.7 cal. kyBP). Beginning at ∼10.2 cal. kyBP, marine inundation caused by early Holocene sea-level rise shifted deposition from fluvial gravel to muddy sediment in the transgressive central basin. The high sediment accumulation during the transgression suggests that coastal erosion transported sediment into the bay. The regressive central basin, driven by decelerated sea-level rise, is characterized by thinly laminated muddy sediments deposited at a lower accumulation rate due to the shallowness of the bay mouth and the topography of the depression, resulting in anoxic conditions. Northward progradation of a bay-side delta began at ∼6.5 cal. kyBP, eventually dividing the regressive central basin into western and eastern basins by ∼3.7 cal. kyBP, with the eastern inner basin evolving into a more enclosed environment. By 0.7 cal. kyBP, intensified bay-side delta expansion, caused by a change in the main river channel, partially filled the eastern inner basin and formed the Izumo Plain. However, the unfilled estuary, now known as Lake Shinji, remained in a coastal lake. The infill history of Paleo-Shinji Bay was characterized by rapid marine sediment accumulation during the early Holocene sea-level rise, followed by bay-side delta progradation during the later Holocene under relatively stable to falling sea levels, providing insights into the evolution of estuarine infill in wave-dominated systems.