2025 Volume 131 Issue 1 Pages 45-58
The coastal lowlands of Japan are mostly covered by unconsolidated sediments deposited since the last glacial maximum (LGM). The sedimentary facies and physical properties of post-LGM deposits filling small incised valleys with catchments in upland areas are distinct from those of the major alluvial lowlands, due mainly to a low rate of sediment supply. We studied two borehole cores (GS-SUM-1 and GS-SMS-1) to investigate the depositional environments, facies, and physical properties of the post-LGM fills in a small incised valley in the Shibakawa Lowland, one of the largest valleys in the Ōmiya Upland in Saitama Prefecture, central Japan. Both cores consist of Pleistocene deposits, post-LGM deposits, and artificial fill in an ascending order. Post-LGM deposits in the two cores consist of fluvial, inner-bay to brackish–fresh water, and fresh-water swamp deposits. The inner-bay deposits are mainly mud with little to no sand and yield very low S-wave velocities (~100 m/s). We reconstructed the post-LGM depositional environments in the Shibakawa Lowland using the sedimentary facies and radiocarbon ages. After 9 ka, seawater entered the incised valley. The inner bay extended furthest upstream at ~6 ka, forming a ravinement surface. The valley floor transformed into a freshwater swamp by 4 ka. In addition to the low rate of sediment supply due to the small catchment area, the progradation of deltaic to fluvial deposits in the Arakawa Lowland downstream blocked the valley outlets at the time of the highstand, resulting in the deposition of very muddy soft sediments in the Shibakawa Lowland. On the margins of the uplands, similar soft post-LGM deposits filling the incised valleys are likely to be present beneath the alluvial plain; consequently, attention should be paid to the ground strength in such areas.
