INTERPRETATION OF “EXTREME” SEDIMENTATION IN THE CONTEXT OF LATE GLACIAL, HOLOCENE AND ANTHROPOCENE ENVIRONMENTS OF QUESNEL LAKE, BRITISH COLUMBIA, CANADA

At 512 m Quesnel Lake is the third deepest in North America and at 100 km long its drainage basin spans from the arid interior plateau to the high mountains of the eastern Cordillera in British Columbia, Canada. From the late Pleistocene to present the lake has experienced sediment inputs and accumulation rates that range from “extreme” during deglaciation (but normal in the context of ice sheet recession), to minimal – as is the case for most of the Holocene – and in the range of “catastrophic” most recently as a result of human disturbance. Ice sheet ablation and active glacier front recession pumped in as much 100 m of sediment in less than a few thousand years via low energy turbidity and overflow currents. In sharp contrast, over at least 9000 years of the Holocene average annual accumulation varied from 0.22 to 0.72 mm a^-1 delivered by limited inflow, and significant hypolimnic circulation which probably flushed water and suspended sediment from the water column. On August 4, 2014 the failure of a mine tailings dam near the western arm of the lake introduced approximately 18.6 million m³ of tailings and displaced coarse sediment to the lakebed. Local deposit thickness was in the order of 10 m following the short 12 hour event. This “catastrophic” event created subaqueous sediment density flows generated from the slurry and eroded two channels in a lake margin fan-delta (Hazeltine Creek). A model for subaerial debris flow erosion overestimates measured submerged delta-front erosion rates in the channels because of evolution of the subaqueous debris flows and incomplete liquefaction of the eroding bed. The subaqueous debris flows deposited a localized debrite on the older lakebed sediments and is characterized by a hummocky, acoustically opaque surface composed of fine tailings and coarse displaced fan-delta and fluvial sediment. We argue that concept of “extreme” when inferring processes from accumulation rates and deposit geometry in these lake types – and in any subsequently lithified geologic units - needs to be carefully considered in terms of both spatial and temporal variations of the environmental setting.