LAKE AGASSIZ IS A CONUNDRUM - SHOULD THE CHRONOLOGY BE LIKEWISE?

The history of glacial Lake Agassiz has been debated for over a century and assigned ages for various events within the basin have evolved. Relative events within the former lake include initial inception, an expansion phase, a low phase, continued expansion, and its demise. Although geomorphic features formed during each of these events, surprisingly few events have been assigned unambiguous ages. Here we synthesize the chronology emerging from recent research to suggest a revised working history.

To assign ages to geomorphic features during events we employ a statistical approach that combines all chronologic data related to a specific event, and summarizes it as probability distribution. Note that a variety of dating techniques have been used (14C, OSL, 10Be). Thus depending on the stratigraphic context and dating method, the probability distributions can form brackets (taken at the 95% level) or highest probability age assignments for those events that, to date, have yielded information. In practice, these only provide snapshots of events occurring in that portion of the glacial Lake Agassiz basin located ~south of 50° N latitude.

As a working model we suggest that: 1) the lake could not have formed until after ~15.3 ka as that is the minimum age for the formation of the Big Stone moraine; 2) expansion of the lake occur after 13.1 ka; 3) major shorelines formed at least at 14.2, 13.5, and 10.5 ka; 4) the beginning of the low phase was 13.1 ka or 12.7 ka depending upon location, and rising water terminated the low phase at ~11.5 ka; 5) any lake expansion during or after the formation of the 10.5 ka (Campbell) shoreline, and the lakes eventual demise remain to be placed into a solid chronology. This working model, if validated, implies that most events within or adjacent to the lake Agassiz basin still remain poorly constrained in time as the evolving lake followed the receding ice margin during the late Glacial and early Holocene time.