OPTICAL AGES FOR DEEP LAST-GLACIAL LAKE MISSOULA, MONTANA

Glaciolacustrine sediments in the Clark Fork River valley at Garden Gulch, near Drummond, MT allow for documenting highstand positions in glacial Lake Missoula and periodic subaerial exposure of the lake bottom. Ten cycles of lake deepening, subaerial exposure, and periglacial modification are recorded in a 9 m-thick section. The deposits, at 1170 m elevation, are ~100 m below the highest shorelines in the lake basin. The section thus records lake stands of >65% of maximum capacity. Fourteen samples of alluvial sand or sand in periglacial wedges were dated using 180-250 µm quartz and standard optically stimulated luminescence (OSL) dating of single aliquots. All ages were determined using the central-age model. Infrared stimulated luminescence (IRSL) signals from K-rich feldspar were used to test the accuracy of quartz ages. A comparison between the quartz OSL and K-feldspar IRSL ages implies that quartz fractions were likely well bleached.

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.