4:30 PM - 4:45 PM
[HCG23-10] Sedimentary features of event deposits interbedded with the annually laminated sediments in Lake Ichinome-gata, Oga Peninsula.
Keywords:Ichinome-gata, Turbidite, Event deposit
Lacustrine sediments are formed by seasonal changes of depositional processes. The annually laminated sediments provide us high-time resolution climatic and environmental archives with at least annual and sometimes even seasonal resolution. In addition, turbidites interbedded with the annually laminated sediments enable us to reconstruct the history of natural disasters such as earthquakes and floods. According to previous research(Okuno et al., 2011), the annually laminated sediment of Lake Ichinome-gata, NE Japan, covers at least 30ka years-long record. To identify various event deposits with precise ages, we took lake sediment cores from Lake Ichinome-gata.
In this study, we analyzed a 32.5 cm long sediment core taken from the central part of Lake Ichinome-gata. We also took surface sediment in the lake bottom slope and sediment from the inflow stream area to reveal correlations of sediment core between them. Based on counting the annually laminated sediments and visual observation, we identified four event layers (E1, E2, E3, E4) interbedded with the annually laminated sediments. We counted the number of varves (including varve thickness) and accurately detected the age of event layers. The result showed that the event layers matched paleo-disasters. E1 and E3 matched the 1983 Sea of Japan earthquake and the 1964 off Oga Peninsula earthquake, respectively. E2 and E4 matched the heavy rainfall disaster in 1979CE and 1955CE, respectively. Grain size distribution of these event deposits was different from that of annually laminated sediments. The grain size distribution of event layers caused by earthquake (E1, E3) is similar to that of surface sediment in the lake bottom slope. The grain size distribution of event layers caused by heavy rain (E2, E4) was similar to that of sediment taken from inflow stream area. These results suggest that event layers by earthquake (E1, E3) consist of reworked surface sediments along the slope and event layers caused by heavy rain (E2, E4) consist of the river-sourced deposits. Our study suggests that the trigger and the timing of previous natural disasters can be distinguished from the sediment core of Lake Ichinome-gata.
In this study, we analyzed a 32.5 cm long sediment core taken from the central part of Lake Ichinome-gata. We also took surface sediment in the lake bottom slope and sediment from the inflow stream area to reveal correlations of sediment core between them. Based on counting the annually laminated sediments and visual observation, we identified four event layers (E1, E2, E3, E4) interbedded with the annually laminated sediments. We counted the number of varves (including varve thickness) and accurately detected the age of event layers. The result showed that the event layers matched paleo-disasters. E1 and E3 matched the 1983 Sea of Japan earthquake and the 1964 off Oga Peninsula earthquake, respectively. E2 and E4 matched the heavy rainfall disaster in 1979CE and 1955CE, respectively. Grain size distribution of these event deposits was different from that of annually laminated sediments. The grain size distribution of event layers caused by earthquake (E1, E3) is similar to that of surface sediment in the lake bottom slope. The grain size distribution of event layers caused by heavy rain (E2, E4) was similar to that of sediment taken from inflow stream area. These results suggest that event layers by earthquake (E1, E3) consist of reworked surface sediments along the slope and event layers caused by heavy rain (E2, E4) consist of the river-sourced deposits. Our study suggests that the trigger and the timing of previous natural disasters can be distinguished from the sediment core of Lake Ichinome-gata.