Paper No. 33-2
Presentation Time: 1:50 PM
CONCURRENT LOESS AND SALTATING SAND DEPOSITION ON THE BUCKLEY FLATS OUTWASH PLAIN OF NORTHWESTERN LOWER MICHIGAN
Loess was first studied in Michigan in 2008, on the Buckley Flats, an outwash plain in the NW Lower Peninsula. Here, sandy outwash parent materials are overlain by ≈70 cm of loamy sediment, which was then interpreted as silt-rich loess that had been mixed with the outwash sands below, via pedoturbation. This work centered on re-examining the loess on the Buckley Flats by (1) auger sampling at a larger number of sites, across a wider area, and (2) manipulating or “filtering” the textural data in Microsoft Excel to estimate the original silt contents. We also sampled and examined key sites from deep soil pits. Data indicate that the mantle on the Flats is siltiest in the center, where the landscape is flattest. Mapping the soil mantle’s silt content, after statistically filtering the data to remove mixed-in sand, did not suggest that the silt was derived from any one nearby source, such as the nearby Manistee River valley or the Mancelona-Thompsonville outwash plain, both of which carried meltwater. Thus, we concluded that silts were derived from surrounding deglacial landscapes, and that the silt contents were highest in the center of the Flats because here, slope gradients are low, enabling the paleolandscape to retain silt accumulating via suspension. Pedoturbation of sand into an (originally) silty upper profile, from the outwash below, did not seem to be a dominant process here, because the contact between the mantle and the underlying outwash is commonly abrupt. This information, in conjunction with evidence of relict patterned ground, indicative of permafrost, suggested a different origin for the sands. Sand contents in the “unfiltered” data showed high values near large gullies that surround the western, southern, and eastern margins of the Flats. This finding led us to develop a model whereby sand in the loamy mantle was derived via saltation, up and onto the Flats from the peripheral gullies as they were being incised, exposing sand-rich outwash to potentially strong katabatic winds. Permafrost would have exacerbated runoff, facilitated gully formation. Saltating sands from the gullies were eventually transported onto the upland surface. The result is a sandy loam mantle on the Flats that has distinct textural bimodaility – sands (mainly medium and coarse) that saltated in, and silts that arrived via suspension.