Development of the Kiso River Delta during the Last 10,000 Years Based on Analyses of Sedimentary Cores and C-14 Datings

The depositional process of the Latest Pleistocene to Holocene shallow-marine and fluvial successions is investigated using five C-14-dated borehole cores from the Kiso River delta, central Japan. The delta succession provides high-quality records of the transgression and regression of an enclosed delta system and reveals the development of fluvial-dominated coastal lowlands. Based on a facies analysis, the cored sediments are classified into five sedimentary units: A) basal gravel, B) fluvial and intertidal sand and silt, C) transgressive lag deposit, prodelta mud, and sandy silt, D) delta-front-slope sandy silt and sand, and delta-front-platform sand, and, E) delta-plain and fluvial sand and silt. Detailed age-depth curves of these cores are reconstructed based on 108 AMS C-14 ages including 83 new C-14 ages. These curves show patterns similar to those of temporal variations in accumulation rates, and are divided into four sections (I to IV) from the base to the top: I mainly consists of unit B with rapid accumulation (7.3-21.4 mm/yr); II mainly consists of unit C with moderate accumulation (2.4-2.8 mm/yr); III mainly consists of unit D with rapid accumulation (6.7-17.8 mm/yr); and, IV consists of unit D and E with moderate accumulation (1.3-3.9 mm/yr). The section boundaries of I/II and II/III, which coincide with the unit boundaries of B/C and C/D, reflect the inundation of a flood plain by seawater, developing a bay environment, and the subsequent progradation of the delta-front slope, respectively. The ages of these boundaries constrain the estimation of the expanding rate of the bay and progradation rates of the delta. The expanding rate of the bay is estimated at 10 m/yr during the period 10200-7900 cal yrs BP, and the progradation rates of the delta are 3-4 m/yr (6500-4100 cal yrs BP), 5 m/yr (4100-1300 cal yrs BP), and 10 m/yr (1300 cal yrs BP to the present). These age data indicate that the transition from transgression to regression occurred between 7800 and 7300 cal yrs BP. The geological cross-section along the Kiso River along with the isochrones constructed by over a hundred C-14 dates indicates the following depositional history of the Kiso River delta:(1) from 10000 to 7280 cal yrs BP(K-Ah horizon)-During the expansion of the bay, onlap of unit C on unit B is visible, and unit C overlapped all the core sites just before the K-Ah fall, and (2) from 7280 cal yrs BP to present-Isochrones younger than 6500 cal yrs BP are similar to each other and cross the unit boundaries of C/D and D/E. This demonstrates that units C. E are contemporaneous heterotopic facies formed in response to delta progradation after the K-Ah fall.