Migration and timing of an erosional surface of forced regression: A Holocene analogue from the Kanto Plain, Japan

Abstract

Erosional surfaces, including sequence boundaries within coastal sedimentary successions formed in response to sea-level fall, are known to propagate either seaward or landward. Verification of these processes requires millennial-scale reconstruction of entire coastal plains based on radiocarbon ages; however, no such systematic studies have been conducted to date. In this paper, we analyzed six sediment cores and 143 radiocarbon ages from the Nakagawa Lowland in the Kanto Plain, Japan, to reconstruct the migration process and timing of an erosional surface of forced regression (ESFR) formed since the mid-Holocene. In the Nakagawa Lowland, the ESFR is recognized between deltaic marine sediments and the overlying fluvial sediments. The ESFR formed between 6.2 and 1.2 cal kyr BP and truncates the underlying marine sediments in a diachronous manner. Excluding the influence of local topography, the ESFR developed on a tidal flat as a tidal ravinement generated in response to a sea-level fall of approximately 5 m around 4–3 ka. Because marsh-derived peaty silt directly overlies this surface without any time gap, erosion and subsequent deposition are inferred to have occurred nearly simultaneously. Furthermore, the downlapping isochrons of the fluvial sediments onto the ESFR indicate that the formation of the erosional surface did not propagate landward. The peaty silt overlying the surface, which is not eroded by upstream incision, therefore serves as a key marker for identifying the ESFR in the stratigraphic record.