GRANULOMETRY OF GLACIALLY-GENERATED SEDIMENT IN THE MATANUSKA RIVER, ALASKA: EVALUATING THE ROLE OF FLUVIAL SYSTEMS IN THE FORMATION OF LOESS DEPOSITS

Loess deposits are regarded as a high-resolution archive of continental climate change, yet the transport and sorting of glacially-generated sediment prior to deflation and deposition as loess is not well understood. To address this, we present preliminary granulometry (grain size and shape) data that detail the physical characteristics of windblown and fluvially-transported fines in a proximal modern glacial system—the Matanuska River, Alaska. Assessment of fine-grained sediment shape in source-to-sink studies is generally lacking. Fluvial sediment was sampled at a high resolution from bars along a transect from the Matanuska Glacier terminus to the confluence with Cook Inlet macrotidal estuaries (~100 km down-stream), with a focus on bar-top fines and slackwater deposits in zones of low shear stress behind bars. Four stationary dust traps were also constructed at strategic locations on land within ~3 km of the river to capture wind-blown sediment along the transect. All samples were processed to remove organics prior to analysis with the SYNC particle analyzer, which allows for synchronous measurement of grain size and shape via simultaneous laser diffraction and image analysis measurements on a single sample. Splits of mixed (bedload-suspended load) fluvial bar samples were sieved (No. 200 mesh) to isolate the silt-sized fraction for comparison with eolian silt samples. We compare fine (silt) fraction shapes (e.g., sphericity, symmetry, aspect ratio) both along the fluvial transect and between fluvial and eolian samples. This robust dataset is critical for understanding analogous processes in ancient glacial systems, including the role of fluvial systems in the formation of loess deposits. Future work includes microtextural analysis of individual silt-sized quartz grains to assess polygenetic textures associated with glacial and fluvial influences on transport, and XRF geochemistry to assess along-transect mineralogical variation.