CHRONOLOGY OF MONTANA’S GLACIAL LAKE MISSOULA: CURRENT STATUS (Invited Presentation)

Glacial Lake Missoula (GLM) has been recognized for >105 years, but the geochronology of deposits and features related to catastrophic discharge of water from the lake are poorly known due to the lack of recognized fossils, charcoal, or tephra in the sediments. Quartzose feldspathic sand occurs below (alluvial basal sands) and within cyclic lacustrine deposits and at subaerial exposure surfaces within the deposits. Optical ages of quartz and feldspar, and tenuous event correlations to deposits in the Channeled Scabland form the basis of the current knowledge of chronology.

Here we present 17 new optical ages of basal sands at 7 sites; these range from 17.6±2.2ka near a late-glacial ice dam (630 m altitude) to 32.3±3.4ka (n=2) at 840 m. The average age of the basal sand found at the highest altitude (1170 m) is 20.4±1.4ka (n=4). A lack of interlayering between the basal sands and overlying glaciolacustrine sediments at most locations suggests that there may be hiatuses at the top of the basal sands, i.e. they do not necessarily record the time of lake transgression but are maximum ages for the lake at different locations. Glacier Peak G tephra (13.4-13.6 ¹⁴C yr BP), which is found in alluvial deposits inset into GLM deposits, sets the minimum age for cessation of lacustrine sedimentation.

Ages of a basal sand which is interlayered with GLM deposits at 1170 m (n=4), and of subaerial surfaces within that section (n=7), show that the lake fluctuated around >65% of maximum capacity between averages of 20.4±1.4ka and 19.3±1.1ka (n=2). These may have been the deepest stand(s) of the late-glacial lake, early in its history. Lake-level lowering may have been rapid, producing large floods after 20.4±1.4ka, but was possibly not associated with complete draining of the lake, which allowed for preservation of sediment along canyon floors at altitudes of <950 m. Catastrophic flooding produced giant gravel bars in the canyons. However, preservation of unconsolidated sandy sediment in the bottom of canyons previously occupied by GLM suggests that the gravel bars must be older than 32.3±3.4ka. Basal sand ages of 17.6±2.2ka and 18.2±0.4ka (n=5) document alluvial deposition prior to lake transgression to altitudes between 630-~910m, representing <15% of maximum lake capacity. The lake likely did not drain catastrophically from this last stand.