The Distribution of Silty Soils in the Grayling Fingers Region of Michigan: Evidence for Loess Deposition onto Frozen Ground
Randall J. Schaetzl
This paper presents textural,
geochemical, mineralogical, soils, and geomorphic data
on the sediments of the Grayling Fingers region of northern Lower Michigan. The
Fingers are mainly comprised of glaciofluvial sediment, capped by sandy till.
The focus of this research is a thin silty cap that overlies the till and
outwash; data presented here suggest that it is local-source loess, derived
from the Port Huron outwash plain and its down-river extension, the Mainstee River valley. The silt is geochemically and
texturally unlike the glacial sediments that underlie it and is located only on
the flattest parts of the Finger uplands and in the bottoms of upland, dry
kettles. On sloping sites, the silty cap is absent. The silt was probably
deposited on the Fingers during the Port Huron meltwater event; a loess deposit
roughly 90 km down the Manistee River valley has a comparable origin. Data
suggest that the loess was only able to persist on upland surfaces that were
either closed depressions (currently, dry kettles) or flat because of erosion
during and after loess deposition. Deep, low-order tributary gullies (almost
ubiquitous on Finger sideslopes) could only have
formed by runoff, and soil data from them confirm that the end of gully
formation (and hence, the end of runoff) was contemporaneous with the
stabilization of the outwash surfaces in the lowlands. Therefore, runoff from
the Finger uplands during the loess depositional event is the likely reason for
the absence of loess at sites in the Fingers. Because of the sandy nature and
high permeability of the Fingers' sediments, runoff on this scale could only
have occurred under frozen ground conditions. Frozen ground and windy
conditions in the Fingers at the time of the Port Huron advance is likely
because the area would have been surrounded by ice on roughly three sides. This
research (1) shows that outwash plains and meltwater streams of only medium size
can be significant loess sources and.