THE RIVER RAISIN FLOODWAY AS A SOURCE OF SAND TO THE LAKE WARREN SHORELINE

On the ancestral Lake Erie (ALE) lake plain of northwestern Ohio and southeastern Michigan, the shorelines of glacial Lakes Whittlesey and Warren are prominent landforms. In most areas, the Whittlesey shoreline is narrow but sharply demarcated. In contrast, the Warren shoreline typically is broad, sand-rich, and has the geomorphic character of having developed through several depositional episodes, possibly related to lake-level fluctuations of several meters. West of Toledo, Ohio, the Warren shoreline comprises the Oak Openings Ridge (OOR), which supports a unique dry-prairie ecosystem. Ground-penetrating radar transects across the OOR show large-scale sigmoidal beds of a prograding barrier-island system. The main body of the barrier can be 3 to 4.5 km wide with sand as thick as 8-9 m. Recent age-dating constrains deposition of the Warren barrier to time scales of 500 to 1000 years. In that context, a long-standing question has been, What was the source of that much sand?

Recent field investigations of another, unusual, feature on the ALE lake plain in SW Monroe County, MI – the Ottawa Lake basin – led to a regional evaluation of the sources and relative timing of the supply of large volumes of sediment to the ALE system. As a working hypothesis that will guide upcoming field studies, the River Raisin Floodway appears to have channeled large volumes of sediment and water, presumably as glacial outwash derived primarily from the southern flank of the Saginaw Lobe. Upstream to the west, and at the margin where higher-elevation shorelines rework glacial deposits, the main stem of the Raisin and its largest tributary, Black Creek, show evidence of several episodes of braided-river and fan or fan-delta deposition. Downstream near Dundee, MI, the River Raisin valley shows evidence of the catastrophic breaching of the Warren shoreline as a geomorphic dam, and possible smaller, lateral flood drainage which appears to have interacted with the lagoon behind the Warren shoreline. The River Raisin Floodway lies north of a subtle topographic lineation that may separate a smaller glacial lobe extending from the Huron Lobe from the main body of the Erie Lobe in the modern valley of the Maumee River. In this model, sand delivered to the Lake Warren shoreface was carried by littoral transport to the S-SW to build the Oak Openings Ridge.