2:45 PM - 3:00 PM
[HQR04-10] Floodplain evolution of the Nobi Plain with special emphasis on organic-rich sediments
Keywords:peat, progradation, radiocarbon dating, Holocene
The organic mud layers are widely distributed in the western part of the plain, and its thickness tends to be larger to the west of the Ibi River. The elevation of the lower limit of the organic sediment layer, especially, is often lower than the mean sea level. In the Yoro Mountains, the lower limit of elevation was found to be -3.6 m. It may be explained by the subsidence of the plain caused by the activity of the Yoro fault. The age at which organic sediments begin to accumulate tends to be younger from landward to seaward, suggesting that it is related to the delta progradation. It can be explained without considering the influence of allogenic factors such as climate and sea-level changes. On the other hand, UK core sediments collected in the inner part of the plain show a transition from tidal flat to floodplain environments at 6000 cal BP, with active mud accumulation due to flood until approximately 4000 cal BP. However, slow deposition of organic mud and peats continued for approximately 2500 years since 4000 cal BP. The changes in facies and sedimentation rates may have been caused by allogenic factors such as weakened fluvial activity due to decreased precipitation and deceleration of eustatic sea-level rise.