PROXIMAL-TO-DISTAL GRANULOMETRY IN A MODERN GLACIAL SYSTEM (ALASKA): PERSPECTIVES INTO THE GENERATION AND FORMATION OF CONTINENTAL LOESS DEPOSITS

Abundant hypotheses exist about the complexities of loess generation and formation, yet the transport, recycling, and deposition of glacially-generated sediment from source to sink have not been characterized in a modern setting. Detailing the mechanisms of silt transport and sorting (prior to deflation and ultimate deposition as loess) in modern environments is important for understanding analogous processes in ancient systems with implications for past climates.

Three sediment samples were collected from a bar (~30 sq. m) between two active channels in the Matanuska River, Alaska, within ~15 km of the Matanuska glacier terminus. These proximal proglacial bars consist of abundant gravel and sand deposited as bedload in addition to the finer fractions that were sampled and analyzed. Samples were processed in H 2 O 2 and 1N HCl to remove organics and carbonate (respectively) prior to grain size and shape analysis with the CAMSIZERX2. We present granulometry (grain size and shape) data, sandstone petrography, and microtextural analysis of individual quartz grains from each sample to comprehensively document the physical properties of fluvially transported fines proximal to the glacial origin. Preliminary analysis of these samples yields silt-sized median (D-50) grain sizes of 55, 62, and 64 µm. These samples were collected from a single bar and thus may not be representative of the whole system, but preliminary results exhibit low variability in the overall distributions and D- 50 values, suggesting a well-sorted and silt-rich suspended load (even within the proximal fluvial system), which is consistent with glacially-generated sediments that may eventually become deflated and redeposited as continental loess. Future work to build on these pilot data begins with a reconnaissance field campaign to densely sample fluvial systems draining the Matanuska and Knik glaciers (Alaska) along a transect through to more distal, tidally-influenced bars near the confluence with a macrotidal estuary (Cook Inlet). Spatial connections between silt deposition and geomorphology will be explored with the use of digital imagery and mapping. Evaluating the role of fluvial systems in the formation of loess deposits is critical for a better understanding of loess as an archive and as an agent of climate change.