OPTICAL AGES FOR DEEP LAST-GLACIAL LAKE MISSOULA, MONTANA
Three preliminary ages on alluvial sediments below the glaciolacustrine sequence show initial lake transgression at 21.0 ± 1 to 19.7 ± 1.5 ka. Eight preliminary ages from the tops of six cycles range from 21.4 ± 1.6 ka to 17.2 ± 1.1 ka for sand captured in downward-tapering wedges interpreted to represent sedimentation in seasonal frost cracks. A bioturbated sandy unit with three ages ranging from 11.7 ± 0.7 to 10.5 ± 0.6 ka caps the upper, undated, four cycles.
Sandy alluvial sediment below the glaciolacustrine section locally interfingers with the lowermost lake-bottom sediments, showing concurrent deposition of the alluvium with the transgressing lake. Ages for the basal sediment suggests that the glaciolacustrine sediments at Garden Gulch are ~2 to 5 ka older than similar sediments 200 m lower in elevation in the Missoula and Ninemile valleys. The uppermost ages at Garden Gulch are apparently younger than the last lake drainage.
These data show that glacial Lake Missoula reached >1170 m by 17-22 ka and either partially or completely drained ten times from this position. We interpret that the lower part of the Garden Gulch section was deposited during earlier lake stands than those near Missoula. One or more rapid drainage events, due to ice dam failure, would have eroded glaciolacustrine silt and clay at lower elevation locations, explaining the younger ages at sections near Missoula.