Evolution of fan delta systems in temperate-humid and tectonically active region

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This research obtained sediment cores (TR1 and TR2) from the fan delta developed at the lower reaches of the Tenryu River, central Japan to clarify stratigraphy and sedimentary environments of the fan delta. Two sediment cores are similar to each other, and are divided into three depositional units, units 1 to 3. Unit 1 consists of alternation of pebble to pebbly sand, which is interpreted as fluvial sediments. Unit 2 is characterized by organic-rich, sand-mud alternation. Electric conductivity suggests that the unit was formed under marine influence. Unit 3 is composed of clast-supported gravels underlain by mud. Radiocarbon dates show that each unit was deposited almost simultaneously between the both sites. Large accumulation rates, approximately 12 m/kyr, occurred between about 11000 and 8000 cal BP. In contrast, accumulation rates decreased considerably after about 8000 cal BP. The change is probably related to the rate of post-glacial sea-level rise. Moreover, upward-coarsening successions observed at both cores around 8000 cal BP suggest the beginning of regression.