Quantifying Sediment Transport Dynamics on Alluvial Fans From Spatial and Temporal Changes in Grain Size, Death Valley, California

Abstract

How information about sediment transport processes is transmitted to the sedimentary record remains a complex problem for the interpretation of fluvial stratigraphy. Alluvial fan deposits represent the condensed archive of sediment transport, which is at least partly controlled by tectonics and climate. For three coupled catchment‐fan systems in northern Death Valley, California, we measure grain size across 12 well‐preserved Holocene and late‐Pleistocene surfaces, mapped in detail from field observations and remote sensing. Our results show that fan surfaces correlated to the late Pleistocene are, on average, 30–50% coarser than active or Holocene fan surfaces. We adopt a self‐similar form of grain size distribution based on the observed stability of the ratio between mean grain size and standard deviation downstream. Using statistical analysis, we show that fan surface grain size distributions are self‐similar. We derive a relative mobility function using our self‐similar grain size distributions, which describes the relative probability of a given grain size being transported. We show that the largest mobile grain sizes are between 20 and 35 mm, a value that varies over time and is clearly lower in the Holocene than in the Pleistocene; a change we suggest is due to a drier climate in the Holocene. These results support recent findings that alluvial fan sedimentology can record past environmental change and that these landscapes are potentially sensitive to climatic change over a glacial‐interglacial cycle. We demonstrate that the self‐similarity methodology offers a means to explore changes in relative mobility of grain sizes from preserved fluvial deposits.