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
Humus-rich organic mud and peat are commonly recognized at the near-coastal floodplains. Such organic sediments have been formed on land after the shallow sea, which was appeared by the Holocene transgression, was buried mainly by fluvial sediments since the middle Holocene. Previous studies have discussed the relationship between allogenic factors such as climate changes and sea-level fluctuations and the floodplain evolution based on the distribution depth and radiocarbon dates of these sediments. However, the formation of organic sediments may have been influenced not only by allogenic factors but also by autogenic factors. This study analyzed the spatial distribution of organic sediments and their formation ages in the Nobi Plain west of the Kiso River, and discussed the formation of organic sediments and the evolution of floodplains.
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.
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.