Bed thickness characteristics of inner‐shelf storm deposits associated with a transgressive to regressive Holocene wave‐dominated shelf, Sendai coastal plain, Japan

Abstract

AbstractHolocene inner‐shelf storm deposits preserved beneath the Sendai coastal plain facing the Pacific coast of north‐eastern Japan were formed during a transgressive–regressive cycle. The evolution of the Holocene wave‐dominated depositional system along the Sendai coast is reconstructed using 76 AMS (accelerator mass spectrometers) 14C ages and the origin of bed thickness variations in the inner‐shelf storm deposits is explored. The Holocene succession is <30 m thick and overlies latest Pleistocene to early Holocene non‐marine deposits above a transgressive ravinement surface. It comprises transgressive ravinement and inner‐shelf deposits, and regressive inner shelf, shoreface, and coastal plain deposits. The inner‐shelf deposits comprise alternating sand and mud layers interpreted as stacked storm beds. The average preservation interval of a single storm bed is shortest during the transgression (5·7–20·6 years), and then increases to a maximum during the early regression (83·3–250·0 years), decreasing to 7·7–31·3 years with shoreline progradation. Average accumulation rates decreased during the transgression and then increased during the regression, but the sand/mud ratio varies little, reflecting inefficient sediment segregation downdip on the inner shelf. The vertical pattern of sand‐layer thicknesses also shows no relationship to position within the cycle, although small‐scale intervals of upward thickening and thinning probably relate to lateral switching of river mouths and/or random storm processes. The average thickness of storm beds is the highest in the interval deposited during the period from maximum flooding to early regression. This is probably because of the low preservation potential of thin beds associated with frequent, low‐magnitude storms during this period of low accumulation rates and extensive reworking. This preservation bias and the nature of the Sendai inner shelf resulted in an absence of characteristic bed thickness trends in the preserved storm deposits.