Abstract

The northern coast of the Oga Peninsula is a type locality for Quaternary sedimentary exposures on the Sea of Japan side of northeast Japan. Middle to late Pleistocene shallow marine to nonmarine sediments are continuously exposed on sea cliffs along the Anden Coast. These sediments have been mapped and classified into nine sedimentary facies, based on grain size, sedimentary structures, intensity of bioturbation, and associated fossils. The depositional environments and paleo-water depths of each facies have been estimated, based on comparison with the present sub-bottom sedimentary features observed in coastal systems around the Oga Peninsula. On the basis of sedimentary facies analysis, variations in the depositional depth through time have been reconstructed and correlated with the SPECMAP δ18O curve, using four marker tephras as time controls. The two curves generally agree between approximately 80 and 450 ka, suggesting that the sedimentary cycles at the Anden Coast faithfully record 5th- and 6th-order glacio-eustatic sea-level changes. Peak-to-peak comparison between the reconstructed depositional depth curve and the δ18O curve reveals that (1) unconformities are formed during the late eustatic sea-level falls and/or the following lowstands, (2) transgressive lag deposits are mainly formed below wave base, (3) deepening-upward successions, which sometimes occur below the ravinement surfaces, are formed in the inner part of estuary or barrier-island environments where wave action was minor, (4) shoaling-upward successions formed during eustatic sea-level falls are deposited under the influence of intense wave action in a strand plain-coastal plain system, and (5) the characteristic combination of thinner deepening-upward successions and thicker shoaling-upward successions reflects the asymmetric pattern of glacio-eustatic sea-level changes and/or lower sediment supply during transgression.

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