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[HQR03-P08] Sedimentary environmental change of alluvium in the Kinugawa-Kokaigawa Lowland, central Japan
Keywords:sea level change, Jomon Transgression, Kinugawa-Kokaigawa delta, diatom fossil, flood plain deposit
The paleo-Kinu Bay region is a narrow and long alluvial lowland located in the northeastern part of the Kanto Plain. This is divided into two lowlands, the Kinugawa-Kokaigawa and the lower Tonegawa Lowlands at the junction of Tonegawa and Kokaigawa Rivers. Recently, Tanabe et al. (2022) reconstructed Holocene sedimentary environmental changes affected by sea level fluctuation in the lower Tonegawa Lowland. In the Kinugawa and Kokaigawa Lowland, there are few previous studies. We reconstructed sedimentary environmental changes and accumulation curve of a core material, GS-JIS-9 core, obtained in the central Kinugawa-Kokaigawa Lowland based on sedimentary facies, fossil diatom assemblages and radiocarbon ages.
The GS-JIS-9 core, 25 m long, were drilled at the distal of a crevasse spray of the Kinugawa River. Elevation of the coring sites is measured to be 16.02 m T.P. We described stratigraphic facies and conducted analyses on grain size composition, magnetic susceptibility, and diatomaceous fossils. In addition, 21 radiocarbon ages from the core were measured.
The alluvium deposits of the core is able to be divided into the following seven units.
Unit 1 (-6.73 to -8.98 m) consists of coarse to medium sands and gravels, and the lower part is mainly gravels. Depositional age of the upper units (Unit 2) suggest that this unit deposited at least before 9.0 ka. This unit is corresponding to the basal gravel bed of the paleo-Kinugawa-Kokaigawa River during the Last Glacial Maximum.
Unit 2 (-4.43 to -6.73 m) consists mainly of organic silt mixed with plant fragments, interbedded with thin very fine- to fine sand layers. Radiocarbon ages indicates that this unit deposited around 8.4 to 9.0 ka. Organic rich mud with abundant freshwater diatoms suggests that this unit is a floodplain deposit.
Unit 3 (-2.08 to -4.43 m) is massive silt to clay and high mud content. This unit is characterized by mixture of freshwater and brackish to marine water diatoms, and some inner bay species such as Cyclotella striata were found. This unit probably deposited around 7.6 to 8.4 ka. The elevation of the paleo sea level during this period was around -2 to -7 m (Tanabe 2020). Because the study area located at the innermost of narrow and long bay, the tidal range during this period was much larger than that of the present Tokyo Bay (about 2 m). This wide tidal range likely provided tidal flats or inner bay environment around the study area.
Unit 4 (2.87 to -2.08 m) consists of mainly silty to fine sandy and coarsening upward with abundant freshwater and fresh to brackish water diatom fossils. This unit probably to be a deltafront deposit. This unit deposited around 7.0 to 7.6 ka.
Unit 5 (11.00 to 2.87 m) consists mainly of organic silt with abundant freshwater diatoms. A sandy layer, medium to very coarse sand, was found in the muddy sediment at the range between 8.12 to 3.75 m T.P. This unit probably deposited between 4.0 and 7.0 ka. Organic rich mud and freshwater diatoms suggest that this unit is a floodplain deposits. The sandy layer may be channel deposit.
Unit 6 (14.25-11.00 m) is sandy mud layers and intercalated by very fine sandy layers. This unit is characterized by much smaller sedimentation rate, approximately 0.4 m/kyr, than the unit 5, in spite of slightly coarser sediments. This unit probably deposited around 0.7 to 4.0 ka.
Unit 7 (16.02-14.25 m) consist of mainly very fine to medium sand mixed with silt, and presumably deposited after 0.8 ka. This unit is a crevasse spray deposit inferred from the surface geomorphology.
Sedimentary environmental changes of the core suggest that the geomorphological developments divided into two stages. Stage 1 (7.6 to 4.0 ka) is characterized by deltafront progradation which provided accumulation of the floodplain deposits rapidly. On the other hand, Stage 2 (after ~4.0ka), the accumulation rate decreased significantly after 4.0 ka. This suggest that the infilling process have been changed response to the sea level fluctuation. Stable high sea level (2 to 3 m T.P.) during the Stage 1 induced deltafront progradation and rapid accumulation. During the Stage 2, sea level falling (-2 m T.P.) caused that depocenter of the rivers migrated seaward in the paleo-Kinu Bay, resulting in deltafront progradation in the lower Tonegawa Lowland (Tanabe et al. 2022). This depocenter migration associated with sea level fall likely caused slower accumulation in the inland area.
References
Tanabe, S. (2020) Quaternary Science Reviews, 248, 106575.
Tanabe, S. et al. (2022) Marine Geology, 447, 106795.
The GS-JIS-9 core, 25 m long, were drilled at the distal of a crevasse spray of the Kinugawa River. Elevation of the coring sites is measured to be 16.02 m T.P. We described stratigraphic facies and conducted analyses on grain size composition, magnetic susceptibility, and diatomaceous fossils. In addition, 21 radiocarbon ages from the core were measured.
The alluvium deposits of the core is able to be divided into the following seven units.
Unit 1 (-6.73 to -8.98 m) consists of coarse to medium sands and gravels, and the lower part is mainly gravels. Depositional age of the upper units (Unit 2) suggest that this unit deposited at least before 9.0 ka. This unit is corresponding to the basal gravel bed of the paleo-Kinugawa-Kokaigawa River during the Last Glacial Maximum.
Unit 2 (-4.43 to -6.73 m) consists mainly of organic silt mixed with plant fragments, interbedded with thin very fine- to fine sand layers. Radiocarbon ages indicates that this unit deposited around 8.4 to 9.0 ka. Organic rich mud with abundant freshwater diatoms suggests that this unit is a floodplain deposit.
Unit 3 (-2.08 to -4.43 m) is massive silt to clay and high mud content. This unit is characterized by mixture of freshwater and brackish to marine water diatoms, and some inner bay species such as Cyclotella striata were found. This unit probably deposited around 7.6 to 8.4 ka. The elevation of the paleo sea level during this period was around -2 to -7 m (Tanabe 2020). Because the study area located at the innermost of narrow and long bay, the tidal range during this period was much larger than that of the present Tokyo Bay (about 2 m). This wide tidal range likely provided tidal flats or inner bay environment around the study area.
Unit 4 (2.87 to -2.08 m) consists of mainly silty to fine sandy and coarsening upward with abundant freshwater and fresh to brackish water diatom fossils. This unit probably to be a deltafront deposit. This unit deposited around 7.0 to 7.6 ka.
Unit 5 (11.00 to 2.87 m) consists mainly of organic silt with abundant freshwater diatoms. A sandy layer, medium to very coarse sand, was found in the muddy sediment at the range between 8.12 to 3.75 m T.P. This unit probably deposited between 4.0 and 7.0 ka. Organic rich mud and freshwater diatoms suggest that this unit is a floodplain deposits. The sandy layer may be channel deposit.
Unit 6 (14.25-11.00 m) is sandy mud layers and intercalated by very fine sandy layers. This unit is characterized by much smaller sedimentation rate, approximately 0.4 m/kyr, than the unit 5, in spite of slightly coarser sediments. This unit probably deposited around 0.7 to 4.0 ka.
Unit 7 (16.02-14.25 m) consist of mainly very fine to medium sand mixed with silt, and presumably deposited after 0.8 ka. This unit is a crevasse spray deposit inferred from the surface geomorphology.
Sedimentary environmental changes of the core suggest that the geomorphological developments divided into two stages. Stage 1 (7.6 to 4.0 ka) is characterized by deltafront progradation which provided accumulation of the floodplain deposits rapidly. On the other hand, Stage 2 (after ~4.0ka), the accumulation rate decreased significantly after 4.0 ka. This suggest that the infilling process have been changed response to the sea level fluctuation. Stable high sea level (2 to 3 m T.P.) during the Stage 1 induced deltafront progradation and rapid accumulation. During the Stage 2, sea level falling (-2 m T.P.) caused that depocenter of the rivers migrated seaward in the paleo-Kinu Bay, resulting in deltafront progradation in the lower Tonegawa Lowland (Tanabe et al. 2022). This depocenter migration associated with sea level fall likely caused slower accumulation in the inland area.
References
Tanabe, S. (2020) Quaternary Science Reviews, 248, 106575.
Tanabe, S. et al. (2022) Marine Geology, 447, 106795.