Abstract
Recent studies using well density distributed borehole logs have revealed undulating topography at the bases of incised valleys formed during the Last Glacial Maximum (LGM). In this study, from analysis of 4702 borehole logs, undulating topography forming a series of pits 1–2 km long, < 1 km wide, 5–10 m deep, and spaced at 1–2-km intervals was discovered at the bases of LGM incised valleys beneath the Tama River Lowland on the west coast of Tokyo Bay. This undulating topography can be attributed to scouring at braided river channel confluences. In the study area, single borehole logs are available within each 187 m × 187 m grid cell, and the logs sample both the bottom and marginal portions of the scouring, which suggests that this undulating topography is not an artifact of erroneous values arising from mathematical interpolation. The morphologies and incision depths of two incised valleys in the study area show a cover effect of the gravel bed at the base of the post-LGM incised-valley fills. The basal age of this basal gravel bed (BG) is confirmed at < 30 ka because the LGM incised valleys dissect the MIS 3 Tachikawa buried terrace overlain by the AT tephra dated 30.0 ka. This means that the BG, which represents braided-river sediments, is interpreted as resulting from the LGM sea-level lowstand after 30 ka.
Highlights
An incised valley is a topographic feature formed by river incision related to allogenic forcing such as sealevel lowering (Dalrymple et al 1994, 2006)
The BG is correlated to lowstand braided river sediments, the lower clay bed (LC), middle sand bed (MS), and the lower portion of the upper clay bed (UC) are correlated to transgressive estuary sediments, and the upper portion of UC, the upper sand bed (US), and the uppermost alluvium bed (UA) are correlated to regressive delta sediments (Fig. 5)
6 Conclusions In this study, we revealed the detailed topography of the Last Glacial Maximum (LGM) incised valley of the Tama River Lowland, central Japan, by analyses of 4702 borehole logs
Summary
An incised valley is a topographic feature formed by river incision related to allogenic forcing such as sealevel lowering (Dalrymple et al 1994, 2006). Incised valleys were formed in the Last Glacial Maximum (LGM) in many places in the world because of the global lowering of sea level. LGM incised valleys are filled by riverine sediments deposited in association with the subsequent deglacial sea-level rise and stillstand; these sediments, which can comprise fluvial, estuarine, and deltaic sediments from the bottom to the top of the sequence, are termed post-LGM incised-valley fills (e.g., Zaitlin et al.1994; Tanabe et al 2015). Modern riverine coastal plains are situated on these post-LGM incised-valley fills. The post-LGM incised-valley fills have a highwater content and form unconsolidated soft strata, contributing to various geological hazards such as subsidence caused by groundwater pumping and amplification of earthquake ground motion (e.g., Kaizuka and Matsuda 1982; Endo et al 2001). The study of the morphology of LGM incised valleys and of the depositional architecture of the post-LGM incised-valley fills are important for mitigation of geological hazards as well
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