Bed-thickness frequency distributions and recurrence intervals of sediment-gravity-flow deposits intercalated in lacustrine varved diatomite: examples from the middle Pleistocene Hiruzenbara Formation, southwest Japan

ABSTRACT Bed-thickness frequency distributions of sediment-gravity-flow deposits, especially turbidites, are one of the major interests of sedimentology. Lognormal, power-law, exponential, and truncated normal distributions have all been proposed for their frequency distributions. Although these frequency distributions have been obtained from many field observations and estimated from statistical models, problems associated with the complexity of sedimentary processes have remained. In this study, bed-thickness frequency distributions and the recurrence intervals of sediment-gravity-flow deposits intercalated in the Pleistocene lacustrine varved diatomite in southwest Japan were analyzed. The results reveal that the bed thicknesses of sediment-gravity-flow deposits induced by different mechanisms show different types of frequency distribution. For example, flood-induced sediment-gravity-flow deposits show power-law-like distributions, whereas such deposits caused by lake-slope-failure show lognormal distributions. The suggestion is that flood-induced types are deposited from floods having a power-law scale. However, the bed thicknesses of the deposits induced by lake-slope-failure do not purely reflect the event scale, because the lateral variation of their thicknesses reflects the depositional processes. The recurrence intervals of both types of event show Poisson-like distributions except for lake-slope-failure deposits at the slope-base section. Despite observed ranges, the distributions of lake-slope-failure deposits at the slope-base section have a high amount of zero value and the recurrence interval show exponentially decreasing. These results suggest that both events were generated randomly, despite different origins. In addition, the incompleteness of the records was also suggested from sediment bypass of a lake-slope-failure event at the slope-base section.