REVISITING THE AGE OF THE MOORHEAD 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 lake phase, and it has been interpreted to be coeval with the Younger Dryas cold period. Preliminary results from recent work dating moraines in northwestern Ontario would 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. Older reworked wood has commonly be reported from the Campbell beaches (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 sampling area for W-723 on the Grand Forks air base was revisited and a similar stratigraphy found in an excavation. New radiocarbon dates from wood and in-situ peat will be discussed. Based on existing radiocarbon dates from the Grand Forks air base region and elsewhere in the basin, 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.