A POSSIBLE DISJUNCT OF PEORIA SILT (LOESS) IN NORTHERN LOWER MICHIGAN

Loess is rare in Michigan, and where present it is thin and usually incorporated into the sandy or clayey soil below. Most of the Peoria Silt, a widespread loess in the midwestern USA, was deposited before northern Michigan became ice-free (about 13 ka), and most meltwater streams in lower Michigan were short and terminated in one of the Great Lakes, minimizing any opportunity for eolian entrainment of silt. It has also long been thought that the Peoria Silt thinned so rapidly to the east of the Mississippi River that deposits in Michigan would be undetectable. Lastly, most of the glacial deposits in southern Michigan are sandy; the ice sheet was carrying less silt than in other areas of the midwest. Our research, however, has documented the existence of a thin, silt-rich, surficial deposit in northern lower Michigan, USA, that may be an notable outlier of Peoria Silt or a later correlative. The silt exists on flat uplands and in dry kettles on an otherwise sandy upland known locally as the Grayling Fingers. This landscape was an interlobate area between the Saginaw and Lake Michigan lobes of the Laurentide ice. Till in this interlobate area is extremely sandy, with 4-8% clay and almost no silt. Above this basal till is an ice-ablation deposit less, i.e., the possible Peoria Silt, than a meter thick that commonly has >60% silt. It is unlikely that the silt in this deposit originated from the ice proper, as it had almost no silt in its drift. Thus, we believe it to be an eolian deposit that was transported into this region and retained/concentrated on top of the ice sheet, as this region was probably a topographic low on the ice surface. Upon melting, much of the silt was retained, because this area was a subglacial high and many of the uplands within it were minimally influenced by meltwater. Work on the silt deposit is continuing.