Preservation and Grain-Size Trends of Holocene Wave-Dominated Facies Successions in Eastern Japan: Implications for High-Resolution Sequence Stratigraphic Analysis

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-dom- inated successions in two contrasting coastal plains: the Kujukuri and Sendai, using high-resolution AMS 14 C dating, depositional facies, and grain size. Modal grain size is proposed as a good criterion for eval- uating 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 ravi- nement surface. In the Sendai area, the potential was much higher than in the Kujukuri area, because the Sendai ravinement surface had re- ceived greater sediment supply from the adjacent rivers during trans- gression. 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 consid- ered to be the result of mixing of a coarse transgressive lag with finer sediment derived from external sources. The finer mode forms a con- sistent 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 chang- es of depositional depth. Thus, the vertical trend of the modal grain size (and not mean or median) is best for evaluating changes of de- positional depth in ancient wave-dominated facies successions.