11:30 AM - 11:45 AM
[HQR05-10] Alluvial fan aggradation during MIS 6 in the downstream of Tenryu River
Keywords:Luminescence dating, Alluvial fan, Pleistocene
In the lower reaches of the Tenryu River, alluvial fan terraces have developed. Previous studies have suggested that these alluvial fan terraces were formed in response to sea-level rise. However, no chronological framework based on the depositional ages of sediments has been established thus far. In this study, we focused on the Iwatahara Plateau in the lower Tenryu River basin, determining the depositional ages of alluvial fan sediments using luminescence dating and examining their formation process.
We conducted comparative analyses of post-IR IRSL50/150 and post-IR IRSL50/225 dating using multiple grains, as well as single-grain post-IR IRSL50/225 dating. The g-values for the post-IR IRSL50/150 signal ranged from 2.0% to 2.7%/decade, whereas those for the post-IR IRSL50/225 signal were lower, at 1.1% to 1.5%/decade. The low g-values of the post-IR IRSL50/225 signal indicate that the impact of age underestimation due to fading is negligible. The uncorrected ages of the uppermost layer of the terrace deposits were approximately 125 ka for post-IR IRSL50/150 and 150–135 ka for post-IR IRSL50/225. Since the uncorrected post-IR IRSL50/150 age, which slightly underestimates the true age due to fading, corresponds to MIS 5e, the formation of the terrace can be correlated with MIS 6.
The alluvial fan gravel layers that constitute the Iwatahara Plateau have a total thickness of approximately 60 m. Since an age of approximately 200 ka was obtained from the lowermost part of the deposits, it is inferred that the aggradation initiated during the sea-level highstand of MIS 7. A significant aggradation of fan deposits ~40 m thick occurred around 150–135 ka, accounting for two-thirds of the total thickness of alluvial fan deposits accumulated since MIS 7. This significant aggradation is presumed to have been induced by the weakening of the East Asian summer monsoon. The results of this study suggest that the formation of coastal alluvial fans is strongly influenced not only by sea-level changes but also by climate changes.
We conducted comparative analyses of post-IR IRSL50/150 and post-IR IRSL50/225 dating using multiple grains, as well as single-grain post-IR IRSL50/225 dating. The g-values for the post-IR IRSL50/150 signal ranged from 2.0% to 2.7%/decade, whereas those for the post-IR IRSL50/225 signal were lower, at 1.1% to 1.5%/decade. The low g-values of the post-IR IRSL50/225 signal indicate that the impact of age underestimation due to fading is negligible. The uncorrected ages of the uppermost layer of the terrace deposits were approximately 125 ka for post-IR IRSL50/150 and 150–135 ka for post-IR IRSL50/225. Since the uncorrected post-IR IRSL50/150 age, which slightly underestimates the true age due to fading, corresponds to MIS 5e, the formation of the terrace can be correlated with MIS 6.
The alluvial fan gravel layers that constitute the Iwatahara Plateau have a total thickness of approximately 60 m. Since an age of approximately 200 ka was obtained from the lowermost part of the deposits, it is inferred that the aggradation initiated during the sea-level highstand of MIS 7. A significant aggradation of fan deposits ~40 m thick occurred around 150–135 ka, accounting for two-thirds of the total thickness of alluvial fan deposits accumulated since MIS 7. This significant aggradation is presumed to have been induced by the weakening of the East Asian summer monsoon. The results of this study suggest that the formation of coastal alluvial fans is strongly influenced not only by sea-level changes but also by climate changes.