ASSESSING LPSA PARTICLE SIZE DISTRIBUTIONS IN KNOWN SAMPLES TO CLARIFY SORTING AND DISAGGREGATION STATES

Key words: Loess, Silt, Peru, Glacial, Deposition

Grain size analysis provides information essential to assessing conditions of transport, sorting, and deposition of sediment. Laser particle size analysis (LPSA) is widely used and produces a particle size distribution (PSD) in the form of a frequency distribution of sizes by volume or number percent. Here, we apply LPSA analyses to a suite of modern samples collected in the mountains to coastal regions of western Peru, including loess, eolian dune/interdune deposits, fluvial bar and overbank, colluvium, and glacial moraine, to build a library of PSDs of these environments for this region.

Samples were sieved to isolate the <2000μm fraction, then subjected to standard H₂O₂ and acetic acid treatments to remove organic matter and carbonate, respectively. Dispersant was added prior to LPSA analysis.

Not surprisingly, both similarities and differences in PSDs occur among the different environments. Samples from the eolian dune exhibit the coarsest mode (235μm) and are strongly unimodal with a narrow range in PSD. Glacial moraine and river bar environments also exhibit unimodal PSDs (118μm, and 203μm, respectively) and both are left skewed, but the moraine sample is strongly left skewed whereas the river bar sample PSD is slightly left skewed. Colluvium sample PSDs exhibit the largest range and are strongly multimodal with three discrete modes. The loess samples exhibit the most fine-grained modes (17.7, and 27.4 μm), and the narrowest range in PSDs. Some loess samples exhibit multimodal PSDs, perhaps attributable to storm events during loess accumulation. Other loess samples exhibit minimal partitioning, perhaps indicative of a more constant wind velocity during deposition. These results can provide the basis for further studies that tie characteristic trends in particle size distributions to specific transport processes.