DEGLACIAL RIBBON LAKES, THOMPSON VALLEY, BRITISH COLUMBIA: IMPLICATIONS FOR THE STYLE OF CORDILLERAN ICE SHEET DECAY

During the decay of the Cordilleran Ice Sheet (CIS), ~9 to 12 ka BP, numerous deglacial ribbon lakes developed within the moderately deep valleys of the Interior Plateau of British Columbia. The sediments and landforms of these lakes provide a detailed record of deglaciation. Within the Thompson Valley, this record reflects the severe adjustments of a landscape undergoing deglaciation and hints at a new model of CIS decay, at least locally.

The sediment record is one of high sedimentation rates and energetic inflows. Tributaries supplied abundant meltwater and sediment and encouraged the formation of large deltas and subaqueous fans, and thick (> 100 m) accumulations of lake bottom sediments. Thick beds of diffusely graded sand in subaqueous fans, cross-bedded sand in delta foresets and numerous and thick turbidites in lake bottom sediments record energetic, sediment laden river-generated (possibly jökulhlaup) underflows. Such conditions suggest that ribbon lakes likely received their meltwater and sediment from ice remnant on the plateau. This inference is counter to the accepted model of CIS decay that describes plateaus becoming ice-free prior to valleys. We attribute early valley deglaciation to the role and routing of meltwater within the CIS.