Stratigraphic variations in lacustrine sediment gravity-flow deposits intercalated in varved diatomite: An example from the Hiruzenbara Formation, Okayama Prefecture, southwest Japan

Abstract

The Hiruzenbara Formation, which is distributed within the Hiruzenbara Basin of southwest Japan, includes a section of varved diatomite, representing ca. 8000 y of deposition in a dammed lake formed during the Middle Pleistocene. The varve, on average, ca. 1.5 mm thick, consists of light- and dark-green laminae sets, and is predominately composed of planktonic diatoms. Several facies types of sediment gravity-flow deposits are intercalated in the varved diatomite. In this study, we examined the sedimentary facies, lateral changes, and stratigraphic variations of these sediment gravity-flow deposits. In addition, we also analyzed a time series of varve thickness fluctuations.Based on their sedimentary facies, composition, and diatom species, sediment gravity-flow deposits in the varved diatomite can be classified as flood-generated or slope-failure-generated. Sedimentary facies analysis indicates that the slope-failure deposits were deposited by slope failure within the lake. Since no significant segregation of rip-up clasts is observed in the deposits, a proximal depositional setting at the base of the lake slope is suggested. The flood-generated deposits have eroded lower contacts, which indicate deposition by hyperpycnal flows. In contrast, deposits with no basal erosion indicate deposition by homopycnal or hypopycnal flows. The thickness time series of the varve shows a thickening-upward trend indicating an increase in diatom production or in diatom shells sizes. Running mean thicknesses show non-stationary periodicities of 200–500 y. The frequency of the deposit varies, but is approximately 10 events per 400 y. Flood deposits in the middle part of the section are dominated by volcaniclastic silt. This suggests that vegetation around the lake was reduced by ash fall, resulting in frequent flood events during this interval. The frequency and thickness of slope failure deposits increases in the lower part of the succession, together with the occurrence of slump scars. The frequency of the deposits in the upper part of the section is approximately 100–500 y, similar to the postulated interval at activity of the nearby Yamasaki Fault System. The results of this study improve our knowledge of paleoenvironmental changes in the study site, and also provide new criteria for classifying sediment gravity-flow deposits in lacustrine sedimentary cores.