HISTORY AND CHRONOLOGY OF LAKE MAUMEE’S IMLAY CHANNEL, MICHIGAN

The Imlay Channel (IC), located in Lapeer County, Michigan, is one of two outlets that drained glacial Lake Maumee in the Lake Erie basin. The other outlet head is at Ft. Wayne, Indiana. Occupancy history and chronology of the IC is poorly understood. Earlier work in the 1970’s, including a MS thesis by W.A. Burgis, resulted in a calibrated age of 16.1–17.3 cal ka BP from macrofossils in a neighboring valley with an uncertain relationship to the IC. A similar age (16.6–17.0 cal ka BP) from wood fragments was collected from a fossiliferous diamicton (colluvium?) within a scour channel incised into the proximal side of a large kame separating the IC from Burgis’ sample site, which confirms this minimum age for deglaciation. The controlling sill elevation for the IC was considered the modern drainage divide within the channel. To elucidate the history and chronology of the IC, a variety of field techniques that included geophysics, hand auguring, outcrop descriptions and sediment coring, resulted in a surficial geology map and cross-sections within the IC. Surficial map units, which mostly overlie the Mississippian Marshall Formation, have an average thickness of ~34 m and include three types of diamicton (till), as well as glaciolacustrine, outwash, undifferentiated glaciofluvial-glaciolacustrine deposits, and Holocene and Pleistocene-aged peat and alluvium. A channel cross-section to bedrock near the IC drainage divide reveals a minimum 12 m of post-glacial alluvium channel fill consisting of sandy gravel with minor amounts of mud and usually overlain by peat. Radiocarbon ages from macrofossils within the sandy gravel agree with OSL ages from sand—both indicating that channel sedimentation had begun before 14.3–15.5 ka and the transition to a shallow wetland (peat) occurred by 12.8–13.0 cal ka BP. Bedrock elevations from well logs and other data indicate a sill elevation for drainage was 235 m (uncorrected for rebound), 15 m below Burgis’ estimate, and ~25 m below the closest and highest Lake Maumee strandline. After adjusting for glacial isostatic rebound the sill of the IC is ~11 m lower than the Fort Wayne outlet sill. Presumably then, when uncovered by ice, both outlets drained glacial Lake Maumee.