,(2012) Stratigraphic and sedimentologic analysis of the latest Pleistocene to Holocene sediment core GS-HIS-1 recovered from the Ishikari coastal plain, Oyafune, Ishikari, Hokkaido, Japan. Bull. Geol. Surv. Japan, vol. 63(5/6), 14 figs, 5 tables. Abstract:The stratigraphy and sedimentary facies of the latest Pleistocene to Holocene incised-valley fills were analyzed at Oyafune, Ishikari, in the Ishikari coastal plain, Hokkaido, northern Japan. A core sample, named GS-HIS-1, was obtained at a site in the present strand plain, where the axis of the paleoIshikari River valley was located in the Last Glacial Age. The core sediments can be divided into seven stratigraphic units: flood-plain deposits (Unit 1), gravelly river deposits (Unit 2), meandering river and tidal river / salt marsh deposits (Unit 3), inner-bay deposits (Unit 4), lower-shoreface deposits (Unit 6), lagoonal deposits (Unit 7), and upper-shoreface to foreshore / backshore deposits (Unit 8), in ascending order.The lower shoreface sands (Unit 6) erosively overlie the inner-bay deposits (Unit 4), and the erosive boundary is considered to be a wave ravinement surface. Delta-front / delta-plain deposits (Unit 5 in Kawakami et al., 2012a; on inner-bay deposits in inland area are absent at the drilling site. Units 6-8 constitute a prograding sand body, which was formed after the mid-Holocene sea-level highstand. The muddy deposits (Unit 7) intercalated with this sand body yield abundant freshwater diatom species, and they were probably deposited in a lagoonal fluvial channel running parallel to the shoreline. The channel would have been rapidly buried as a result of shore progradation.The sediment accumulation curve provides information on the timing of periods of environmental change at the studied site. As the sea level rose, the sedimentary system changed from a gravelly river to a meandering sandy river at about 13,000 cal BP. The river was influenced by marine water intrusion after 10,600 cal BP, and the inner-bay environment was generated around 8,900 cal BP. The shallow marine sands prograded onto the wave ravinement surface after 5,300 cal BP.

The physical properties of the deposits correspond to their sedimentary facies and stratigraphic positions. The pH and electric conductivity values of the muddy sediments are in good agreement with the interpreted depositional environments. The accumulation curve obtained from the cores provides information on the timing of periods of environmental change at the studied site during a Holocene transgression: ( ) the onset of deposition in a meandering river system, succeeding a gravelly river system, occurred at about cal kyr BP; ( ) the inner-bay environment rapidly expanded to the study site during -cal kyr BP; and ( ) progradation of the bay-head delta on the bay floor occurred at around cal kyr BP.