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[HQR05-08] Geomorphological development and estimate of crustal movement since MIS6 along the Omoi and Watarase Rive
Keywords:sea-level change, crustal movement, Quaternary, Kanto Plain
1.Introduction
Long-term crustal movement can be estimated by geomorphological and geological methods. There are two major methods. First, by comparing the present-day elevation of the marine strata with the relative sea-level elevation at the time of deposition; second, by comparing the elevation of terraces or buried valleys formed under similar climatic and sea-level conditions. The former method can provide an almost accurate amount of vertical crustal movement, while the latter method should not be equivalent to crustal movement because the stream channel elevation varies with other factors. However, there are few other methods to estimate crustal deformation in inland areas, and its accuracy has not been examined enough. This study compared above two methods along the Omoi and Watarase River, where marine and fluvial strata are deposited alternately, and both methods are available.
2.Method
To three all core borings: GC-OY-2, GC-OY-1, and GC-NG-1 cores drilled on the Takaragi Terrace [1], observation of facies, particle size analysis by laser diffraction device, elemental analysis by WD-XRF, magnetic susceptibility measurement, and identification of gravel lithology were performed. Third, using the borehole database (Geo-station), the buried valleys formed in MIS2 and MIS6 were mapped.
3.Result
3.1 Facies analysis
The three cores contain fluvial strata in MIS6, marine strata in MIS5.5, and fluvial strata in MIS4 as Miyamoto et al. [2] had reported. The gravel lithology in MIS6 is mainly sedimentary and rhyolite (95-97%), while those in MIS4 also contain basalt and andesite (28-30%).
3.2 Mapping of buried valleys
The basal gravel of the buried valley incised in MIS2 and MIS6 were confirmed widely in the study area.
4.Discussion
4.1 Crustal deformation rates estimated from marine strata.
The top elevation of the marine strata deposited in MIS 5.5 (125 ka) is 3.76 m in the OY-2 core, 4.75 m in the OY-1 core, and -3.43 m in the NG-1 core. The average uplift rates since 125 ka are estimated as -0.04 ~ -0.01, -0.03 ~ 0.00, and -0.10 ~ -0.07 m/kyr.
4.2 Geomorphological development after MIS6 and crustal deformation rate estimated from river strata.
In MIS6, both the Omoi and Watarase River formed incised valleys. After that, aggradation occurred to MIS4 along the Omoi River, however, did not occur along the Watarase River probably because of the large amount of basaltic and andesitic sediment supply to the Omoi River from the Nikko Volcanos through the Daiya River. After that, the Daiya River no longer flows into the Omoi River, and degradation occurred along both the Omoi and Watarase River to MIS2. In the postglacial period, aggradation occurred along both rivers. The difference in elevation between the top of the gravel beds in MIS2 and 6 is about 20 m along the Omoi River and about 12 m along the Watarase River, and thus the average uplift rates for about 120,000 years for MIS6~2 are estimated at -0.16 and -0.10 m/kyr.
4.3 Differences between the two methods and their factors
The crustal deformation rates estimated by the two methods were inconsistent along the Omoi River. It can be that the degradation that occurred in MIS2 was insufficient. It is possible that aggradation to MIS4 caused the river to take a longer time to incise, and that the Daiya River did not flow into the Omoi River in MIS2, unlike MIS6, which reduced the discharge and slowed down the incision rate. Another factor is that the period when the sea level dropped significantly was shorter in MIS2 than in MIS6[3].
Acknowledgments: This study was conducted under a contract with the Agency of Natural Resources and Energy, part of the Ministry of Economy, Trade and Industry of Japan as part of its R&D program supporting development of technology for geological disposal of high-level radioactive waste (JPJ007597).
Reference:[1]Kaizuka et al., The Univ. of Tokyo Press, 2000. [2]Miyamoto et al. ,JpGU-AGU Joint Meeting, 2020. [3]Waelbroek et al., Quat. Sci. Rev., 21, 2002.
Long-term crustal movement can be estimated by geomorphological and geological methods. There are two major methods. First, by comparing the present-day elevation of the marine strata with the relative sea-level elevation at the time of deposition; second, by comparing the elevation of terraces or buried valleys formed under similar climatic and sea-level conditions. The former method can provide an almost accurate amount of vertical crustal movement, while the latter method should not be equivalent to crustal movement because the stream channel elevation varies with other factors. However, there are few other methods to estimate crustal deformation in inland areas, and its accuracy has not been examined enough. This study compared above two methods along the Omoi and Watarase River, where marine and fluvial strata are deposited alternately, and both methods are available.
2.Method
To three all core borings: GC-OY-2, GC-OY-1, and GC-NG-1 cores drilled on the Takaragi Terrace [1], observation of facies, particle size analysis by laser diffraction device, elemental analysis by WD-XRF, magnetic susceptibility measurement, and identification of gravel lithology were performed. Third, using the borehole database (Geo-station), the buried valleys formed in MIS2 and MIS6 were mapped.
3.Result
3.1 Facies analysis
The three cores contain fluvial strata in MIS6, marine strata in MIS5.5, and fluvial strata in MIS4 as Miyamoto et al. [2] had reported. The gravel lithology in MIS6 is mainly sedimentary and rhyolite (95-97%), while those in MIS4 also contain basalt and andesite (28-30%).
3.2 Mapping of buried valleys
The basal gravel of the buried valley incised in MIS2 and MIS6 were confirmed widely in the study area.
4.Discussion
4.1 Crustal deformation rates estimated from marine strata.
The top elevation of the marine strata deposited in MIS 5.5 (125 ka) is 3.76 m in the OY-2 core, 4.75 m in the OY-1 core, and -3.43 m in the NG-1 core. The average uplift rates since 125 ka are estimated as -0.04 ~ -0.01, -0.03 ~ 0.00, and -0.10 ~ -0.07 m/kyr.
4.2 Geomorphological development after MIS6 and crustal deformation rate estimated from river strata.
In MIS6, both the Omoi and Watarase River formed incised valleys. After that, aggradation occurred to MIS4 along the Omoi River, however, did not occur along the Watarase River probably because of the large amount of basaltic and andesitic sediment supply to the Omoi River from the Nikko Volcanos through the Daiya River. After that, the Daiya River no longer flows into the Omoi River, and degradation occurred along both the Omoi and Watarase River to MIS2. In the postglacial period, aggradation occurred along both rivers. The difference in elevation between the top of the gravel beds in MIS2 and 6 is about 20 m along the Omoi River and about 12 m along the Watarase River, and thus the average uplift rates for about 120,000 years for MIS6~2 are estimated at -0.16 and -0.10 m/kyr.
4.3 Differences between the two methods and their factors
The crustal deformation rates estimated by the two methods were inconsistent along the Omoi River. It can be that the degradation that occurred in MIS2 was insufficient. It is possible that aggradation to MIS4 caused the river to take a longer time to incise, and that the Daiya River did not flow into the Omoi River in MIS2, unlike MIS6, which reduced the discharge and slowed down the incision rate. Another factor is that the period when the sea level dropped significantly was shorter in MIS2 than in MIS6[3].
Acknowledgments: This study was conducted under a contract with the Agency of Natural Resources and Energy, part of the Ministry of Economy, Trade and Industry of Japan as part of its R&D program supporting development of technology for geological disposal of high-level radioactive waste (JPJ007597).
Reference:[1]Kaizuka et al., The Univ. of Tokyo Press, 2000. [2]Miyamoto et al. ,JpGU-AGU Joint Meeting, 2020. [3]Waelbroek et al., Quat. Sci. Rev., 21, 2002.