“SILTY-SAND” EOLIAN SEDIMENT IN THE LEE OF LARGE DUNES IN MICHIGAN’S UPPER PENINSULA

This research examines eolian deposits found in the lee of large dunes in Michigan’s eastern Upper Peninsula. Numerous small dunes occupy the sandy, but currently swampy lake bed of Glacial Lake Algonquin (GLA) – a product of widespread eolian activity that lasted well into the early Holocene. Upwind of our study area, near Germfask, MI, dunes get progressively larger downwind, to the southeast, eventually forming large dune complexes at the margins of low, bedrock uplands. We identified four of these large dune complexes in this study, and examined the thickness and texture of the thin eolian sediment in their lee, which we believe is loess. We fit the data from these samples to various statistical regression models, to examine the downwind spatial characteristics of the loess.

The loess in the lee of the dunes contains considerable amounts of very fine sand, but still retains many characteristics of more traditional loess deposits. On average, the loess contains ≈30% silt. Vandenberghe (2013) described this type of eolian sediment as “silty-sand”. In the field, sediment nearer to the dunes appeared to be richer in the finer sand fractions, but with considerably less silt than the sediment further downwind. Statistical analysis of textural data validated these observations; fine and very fine sand contents are greatest near the dunes, and decrease downwind. Conversely, silt contents increase with distance from the dunes, leading to sand/silt ratios that also decrease predictably downwind. These spatial patterns suggest that the dunes supplied sand to the loess deposits in their lee. When examined more broadly, total silt contents in the loess only increased slightly with distance from the bed of GLA, suggesting that silts derived from this source area must have been deposited more-or-less evenly across the landscapes to the southeast. Together, these data indicate that the loess on these uplands is a composite sediment: (1) fine sands were derived locally from dune “blow-over” and (2) silts were derived more distally from the bed of GLA. Spatial patterns and dune orientations suggest that the eolian sediments in this region were all transported on strong NW winds. This is the first study to report on Holocene loess deposits in the upper Midwest, and bridges an academic gap between the loess and eolian sand research communities.