CHRONOLOGY OF LAKE AGASSIZ'S SOUTH OUTLET

Two AMS radiocarbon-dated cores from lakes in the southern outlet of glacial Lake Agassiz indicate a minimum of two outlet occupations. A rotosonic core through the Browns valley alluvial fan, MN, penetrated and recovered 33m of alluvial fan, lake and flood sediment, and bedrock. The Browns valley fan is located in Lake Agassiz's south-outlet spillway where it separates Lake Traverse from Big Stone Lake, and is the mid-continent drainage divide between the Gulf of Mexico and Hudson Bay. Fan construction by the Little Minnesota River began after abandonment of the spillway. Two ¹⁴C ages on wood of 10,675±60 (AA37029) and 10,870±170 (AA 37030) are from within pebble gravel and silty-clay sediment overlying a 2m-boulder lag resting on top of Cretaceous shale. Unconformably above the silty clay is pebble gravel that fines up to sand 0.6m thick. Above the sand, the remainder of the core consists of 18.5m of silty clay and gyttja with one pebble-grading-to-sand unit. The sediment indicates a change in environment from spillway to lake at about 10.7 ka.

A 24m core from Big Stone Lake was recovered from a lake ice platform using piston corer, split spoon, and mud rotary methodologies. A 3.5m thick, fossiliferous, fine-pebble gravel unit unconformably overlies Cretaceous shale, and passes upward into sand-mud rhythmites. Populus (popular) wood fragments from this gravel date to 9640±70 (AA38303) and 9460±70 (AA38302) ¹⁴C BP. In the uppermost sand-mud rhythmites a popular wood fragment dates at 9630±70 (AA40113) ¹⁴C BP, while Scirpus (bulrush) seeds date at 9390±80 (AA40114) ¹⁴C BP. Above the rhythmites, 16m of mud with interbedded cross- and plane-laminated sand and fossils representative of aquatic, emergent, and shoreline plants record a lacustrine environment. The 9390 date obtained from bulrush seeds suggests that the final abandonment of the southern outlet of Lake Agassiz had occurred by this time.