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

本研究从日本中部天龙江下游发育的扇三角洲中获取了沉积岩芯(TR1和TR2)，以澄清扇三角洲的地层和沉积环境。两个沉积岩心相似，分为三个沉积单元，单元1至单元3。单元1由卵石到卵石沙的交替作用组成，被解释为河流沉积。单元2具有富含有机质、砂泥交替的特点。电导率表明，该单元是在海洋影响下形成的。第三单元由碎屑支撑的砾石组成，砾石被泥浆覆盖。放射性碳测年显示，每个单元几乎同时在两个地点之间沉积。巨大的堆积速率，大约12米/年，发生在大约11000到8000卡BP之间。相反，在大约8000卡BP之后，累积速率显著下降。这种变化可能与冰川后海平面上升的速度有关。此外，在两个岩心8000卡BP附近观察到的向上粗化的序列表明了退缩的开始。

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.