LANDFORM AND SEDIMENTARY RELATIONSHIPS OF GLACIAL LAKE AGASSIZ SPILLWAYS

During deglaciation, numerous proglacial lakes developed within drainages blocked by retreating ice sheets. The largest proglacial lake that developed in this way is glacial Lake Agassiz which was located over central North America. The spatial extent of Lake Agassiz was controlled by an overall retreating, but fluctuating, northern ice margin, episodic switching of outlets, and isostatic rebound. The flat lake basin is ringed by isostatically-warped strandlines and deltas. The ice-contact lake margin is poorly constrained in time, but large moraines composed of sand and gravel record significant inputs of meltwater into the lake during deglaciation. Locally, kames and subaqueous fans record point sources of meltwater. The geomorphology of the northwestern and southern spillways commonly consists of an upper scoured zone (USZ) above an inner channel of the spillway. The USZ is usually identifiable by streamlined residuals that resemble drumlins in form and scale, and represent initial sheetflows kilometers wide before incision of the inner channel. Rarely are there terraces or alluvium associated with the inner channel. Instead, isolated boulders, boulder clusters, and boulder sheets (lags?) are found within the spillways. At the distal end of the northwestern spillway, large boulder-gravel bedforms are found where the spillway ends at a delta built into glacial Lake McConnell. Scour lakes within proximal ends of spillways are on, or adjacent to, sub-continental drainage divides, and contain important paleogeographic data. Such data can include the elevation of strandlines graded to particular outlets, and basal radiocarbon dates from lacustrine sediment provide minimum estimates of outlet abandonment. Moreover, the transition (i.e., contacts) between flood and lacustrine sediment within the overall lithostratigraphy, can record the style of outlet abandonment, and thus deglaciation in other areas of the proglacial lake basin.