WHEN WAS THE MOORHEAD LOW-WATER PHASE OF GLACIAL LAKE AGASSIZ?

For over 15 years drainage events from glacial Lake Agassiz have been implicated as triggers for abrupt climate change. However, the chronology of Lake Agassiz events is poorly known with only a few moraines, beaches, and outlets accurately dated. Lake phases corresponding to high and low water are stratigraphically well understood in the main basin, but are poorly constrained in time. More radiocarbon dates are associated with the Moorhead low-water phase than any other phase, and it has been interpreted to be coeval with the Younger Dryas cold period. Preliminary results from recent work dating moraines west of Thunder Bay, Ontario indicate that an eastern outlet was not open at the beginning of the Younger Dryas. Consequently, the apparent coincidence in time of a low lake level and eastern outlet flow may not be correct, and the timing of the Moorhead phase should be revisited. Cores from the southern outlet spillway contain evidence that the lake did not drop below the southern outlet sill until sometime after 10,675±60, well after the beginning of the Younger Dryas. One piece of wood about two thousand years too old has been reported from a Campbell beach (e.g., Teller et al., 2000), thus it is also feasible that the few old wood dates from the Moorhead-age Poplar Creek Formation (e.g., W-723 10,960±300) are similarly reworked. Only two of 36 dates associated with the Moorhead phase are >10,500 14C BP. The end of the Moorhead phase may also be younger. Dates of 9530±70 and 9490±70 14C BP on wood from within an estuarine-fill below Campbell Beach gravels at the Snake Curve section indicate the rise in lake level from the Moorhead to Emerson Phase (Campbell Beach) was still underway at 9500 14C BP. Thus, we would propose that the Moorhead phase of Lake Agassiz is younger than previously thought. The reason for the Moorhead low however, remains a perplexing problem.