HIGH RESOLUTION GRAIN-SIZE ANALYSES FROM PLEISTOCENE PROGLACIAL LAKE SEDIMENT IN WISCONSIN: LINKING SEDIMENTARY PROCESSES TO LANDSCAPE EVOLUTION

Grain-size is a basic lake sediment property that potentially records changes to the surrounding landscape. However, linking landscape evolution directly to grain-size data from lake sediment is difficult because of within-lake inputs such as diatom blooms, within-basin variability, changing basin geometry and water levels over the life of a lake, and climatic driven landscape instability. This is further compounded in deeply buried paleo-lake sediments where basin geometry is often unknown and samples are typically collected at few locations with coring. In addition, traditional settling-velocity laboratory methods required large sample sizes that often integrate strata. These problems can be addressed with laser-diffraction analyses, which requires much smaller sample sizes, allowing for high resolution sampling of discrete horizons as they are identified in the core.

This poster presents ~1,600 laser diffraction grain-size analyses from three settings in central Wisconsin that include proglacial lake strata interbedded with tills and outwash. Core samples were collected up to ~100 m depth from ice-marginal lakes associated with the Green Bay Lobe that represent distinct geomorphic settings: a proglacial lake that retreating ice advanced across multiple times, a proglacial lake dammed by ice only at the maximum position of the glacier, and an outwash-dammed lake removed from the maximum ice margin by several kilometers. Samples were collected at high resolution at a scale consistent with sedimentary processes that filled the basins, including subsampling of ~445 individual couplets in glacial Lake Oshkosh.

Lacustrine sediments are typically rhythmically bedded and tend to coarsen upward at the contact with tills and outwash, consistent with an advancing ice-margin or basin infilling. Basins associated with the last glacial maximum are punctuated with coarse bedding consistent with fluctuating glacial discharge. Modal grain sizes from basins formed during ice recession are finer (~6 microns) and less susceptible to eolian deflation than those formed at the ice maximum (~95 and 23 microns). These interpretations of lake history and surrounding landscape processes are aided by the high resolution nature of the grain-size data.