Abstract

We acquired three geo-slicer cores along a longitudinal profile on the subaqueous portion of the Hii River delta built since the 1630s in Lake Shinji, western Japan. Grading patterns of 26 individual sand beds in the cores vary spatially. We interpret these sand beds as subaqueous flood deposits from their sedimentary structures, the geomorphology of the delta lobe, and the absence of hydrodynamic influences other than river floods entering the lake. Most sand beds at shallow depths (< 4 m below the water surface) display triple stacks of inverse-to-normal grading. Single inverse-to-normal grading appears between 4 and 5 m depth, and normal grading dominates in deeper beds. Inverse and normal grading could reflect the waxing and waning of the parent flow, respectively. We explain the occurrence of triple stacks of inverse-to-normal grading in shallower horizons and single inverse-to-normal grading in deeper horizons as consequences of lateral movement of the plunge point of flood plumes during the course of flood events. Spatially decelerating sediment-laden river plumes steeply increase their velocity after they plunge beneath the water surface. In depth-limited proximal areas of a subaqueous delta, back-and-forth translation of the plunge point over a fixed point due to the waxing and waning of river discharge leads to three cycles of waxing and waning of flow velocity, and the resulting deposits at the fixed point would have triple stacks of inverse-to-normal grading. In the distal parts of the delta, velocity of fully plunged hyperpycnal flow increases and then decreases, reflecting directly the waxing and waning of river discharge, and the resulting deposits would be composed of single inversely-to-normally graded beds. In the most distal parts, the slower initial part of the hyperpycnal flow may be overtaken by the succeeding faster part to yield a monotonically waning flow at a fixed point. This process may have resulted in sand beds with uniformly normal grading in lower (distal) horizons. Assuming that flood hydrographs for the Hii River have not changed since the 17th century, these findings mean that the momentum of river flow during floods propagates to the outflow differently at different distances from the river mouth. Our results show that in parts of the delta-front slope, the outflow of river floods accounts for the inverse-to-normal grading considered to be an important basis for identifying hyperpycnal flow turbidites. However, in other parts of this environment, grading patterns of flood deposits do not necessary reflect the flood hydrograph. Overdependence on inverse-to-normal grading in identifying hyperpycnal flow deposits may lead to inaccurate estimations of their frequency in the sedimentary record. On the other hand, special care is needed for the reconstruction of paleo-flood hydrographs using sedimentary records.