The response of alluvial plains or basins located upstream from the limit of marine deposition to allogenic forcing, such as sea level and climate change, has received little attention compared with adjacent coastal plains. This study investigates the stratigraphy and evolution of an alluvial basin in Nara, Japan, which has formed along the middle reaches of the Yamato River and its tributaries since around the Last Glacial Maximum (LGM), based on the analysis of newly collected radiocarbon-dated sediment cores and by collating existing borehole logs, radiocarbon ages, and burial depths of dated archaeological remains. Major fluvial incision did not occur in the basin during the sea-level lowstand around the LGM. In addition, the latest Pleistocene to Holocene strata (<5 m thick) are extremely thin compared with the southern part of the Osaka Plain, which is located in the lower reaches of the river. These observations indicate that the base level fluctuations caused by eustatic sea level change, and tectonic subsidence related to active faults in the surrounding mountains, have had little influence on deposition in the basin since around the LGM and that the basin has been a zone of sediment transfer or transport. The burial depths of dated archaeological remains suggest that the thickness of sediment that has accumulated over the last 2000 years is around 1–2 m across large areas of the basin. The expansion of secondary forest caused by human disturbance in the historical period, and repeated landslide-induced river-bed uplift downstream from the outlet of the basin, may have promoted flood deposition in the basin.
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