NUMERICAL PREDICTION OF SEDIMENT GRAIN SIZE DISTRIBUTION SUPPLIED FROM A SIMPLE DRAINAGE: DEATH VALLEY QUATERNARY FAN CASE STUDY

Application of source to sink methods to predict grain sizes in a sedimentary system is limited by our ability to capture the various geologic processes that act on the sediments as they are weathered from bedrock and conveyed toward final deposition. A key source of uncertainty that has traditionally received less attention than hydraulic sorting or erosion in source to sink modeling is the soil grain size distribution derived from weathering of bedrock in the source region. The grain size distribution of hillslope soil is the result of multiple geologic processes interacting on the landscape. Through mining a large soil dataset, empirical functions have been constrained that describe the grain size distribution resulting from in situ weathering of different lithology groups. These functions are implemented in a 2-dimensional sediment transport model that simulates erosion in the source region and the hydraulic sorting during transport within the sink region.

Here we present the results of a numerical modeling case study conducted for two Quaternary alluvial fans in Death Valley, USA. The two fans show a clear difference in primary mode of transport (water-flow vs. debris-flow) and in mean grain size within the considered stratigraphic interval. Despite the variation between fans and the uncertainty in classifying the parent rocks within the upstream drainage basin, our predicted grain size distribution closely fits the observed distribution for both fans. Results demonstrate that this model, with a more accurate link between parent rock lithology and the hillslope soil grain size distribution, makes it possible to accurately predict the grain size distribution of sediment exiting a small and tectonically simple drainage basin over the timescales considered. Also, our results are consistent with the interpretation that variations in both primary transport mode and parent rock lithology in the drainage basin caused the stark contrast in mean grain size between the fans.