Evidence for an Eolian Origin for the Silt-Enriched Soil Mantles on the Glaciated Uplands of Eastern Upper Michigan, USA
Randall J. Schaetzl and Walter Loope
We provide textural,
geochemical, and mineralogical data on a thin, silty deposit that unconformably mantles glaciated uplands in the eastern Upper
Peninsula of Michigan. Previous research on this deposit, which we hypothesize
to be loess, is nonexistent. The uplands were islands or narrow peninsulas
within one or more glacial lakes. We compare the distribution, likely source
and nature of the 2060 cm thick silty mantle by using the loess formation
model of Mason et al. [Mason, J.A., Nater, E.A., Zanner, C.W., Bell, J.C., 1999. A new
model of topographic effects on the distribution of loess. Geomorphology 28, 223236], which focuses on the generation of
eolian silt by saltating sand across upwind, barren surfaces.
Parabolic dunes, with arms open to the NW, are common on former lake floors
upwind of the silt-mantled uplands, attesting to the strength and direction of paleowinds. The abrupt termination of the dunes at the footslopes of the uplands, associated with silt deposition
on upland soil surfaces in downwind locations, are both consistent with the
model of Mason et al. [Mason, J.A., Nater, E.A., Zanner, C.W., Bell, J.C., 1999. A new model of topographic
effects on the distribution of loess. Geomorphology 28,
223−236]. Sediments on former lake floors contain abundant strata
of fine/medium sand and silt, and thus are likely sources for the silt and dune
sand. The cap, dune and lake sediments are similar along many different
geochemical axes, whereas the substrate sediment, i.e., the drift below the
cap, is unique. Cap sediments, normally containing roughly 30% silt, are
enriched in quartz and depleted in Ti and Zr,
relative to dune sediment. The dune sediment, a more residual eolian deposit,
is enriched in Ti and Zr, relative to the cap,
probably due to its greater abundance of heavy minerals. Therefore, we conclude
that the silty cap is loess that was deflated from abandoned lake floors after
nearby glacial lakes drained, probably contemporaneously with dune migration across
the former lake floors.