Abstract

Vertical grain-size trends provide significant information for the sequence stratigraphic analysis of siliciclastic wave-dominated facies successions. This paper discusses the relationships between the preservation timing and the grain-size trends of Holocene wave-dominated successions in two contrasting coastal plains: the Kujukuri and Sendai, using high-resolution AMS 14C dating, depositional facies, and grain size. Modal grain size is proposed as a good criterion for evaluating qualitative changes of depositional depth.

The Kujukuri and Sendai successions are both fining-upward to coarsening-upward wave-dominated successions that have developed upon ravinement surfaces. The fining-upward part of the Kujukuri succession formed in regressive deposits during a highstand period, whereas that of the Sendai formed during transgression. A difference in the timing of the generation of upward fining is due to the varying preservation potential of transgressive shoreface deposits on the ravinement surface. In the Sendai area, the potential was much higher than in the Kujukuri area, because the Sendai ravinement surface had received greater sediment supply from the adjacent rivers during transgression. A differing rate of tectonic uplift between the areas was not critical to the preservation potential because of the short time scale.

The successions show bimodal grain-size distributions in the basal parts and unimodal distributions elsewhere. The bimodality is considered to be the result of mixing of a coarse transgressive lag with finer sediment derived from external sources. The finer mode forms a consistent trend with unimodal grain sizes in overlying deposits. The trend is of a simple upward coarsening in the Kujukuri and an upward fining to coarsening in the Sendai, showing distinctive correlation with changes of depositional depth. Thus, the vertical trend of the modal grain size (and not mean or median) is best for evaluating changes of depositional depth in ancient wave-dominated facies successions.

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