ABSTRACT

Surface deposits of the Tuttle Canyon alluvial fan, eastern Sierra Nevada piedmont, California, mainly accumulated during a catastrophic outburst flood probably triggered by the failure of a moraine-dammed lake in the steep (16.4°), high-relief (2361 m), and high-elevation (to 4275 m) catchment. Moraines of the catchment and upper fan (Facies A) comprise unsorted sand to blocks derived from the granitic bedrock. The main flood facies (B) is texturally like the moraine, consisting of unsorted, matrix-supported, sandy pebbly cobbly blocky boulder gravel in a single unit 3 to 8+ m thick distributed across most of the 10.5 km long and 28.1 km² fan. Facies B was deposited during the initial outburst-flood phase as a high-volume, noncohesive sediment gravity flow (NCSGF) instigated by water rapidly descending over moraine. It differs from the moraine by a planar unit geometry and a slope-transverse long-axis alignment of boulders. NCSGF deposition was followed by a water-flood phase caused by continued lake drainage subsequent to removal of loose sediment in the flood path. This discharge carved channels 2-22 m deep and 5-60 m wide around and across fan NCSGF deposits, with channels departing from various points along Tuttle Creek. Horseshoe separation scours also formed around bedrock spurs, and laminated pebbly sand accumulated on the channel beds (Facies D). Flood discharge expanded on the distal fan into a sheetflood that first deposited a boulder unit (Facies E), and then 4+ m of cobbly sandy pebble gravel in planar beds 5-20 cm thick (Facies F). The final flood phase entailed incision of Tuttle Creek, wherein bouldery cobble gravel (Facies C) was concentrated. Post-flood modification of these deposits has been limited to secondary processes such as eolian deposition (Facies G) and bioturbation.

Outburst floods on fans of the Sierra Nevada piedmont ultimately are caused by the glacial transfer of colluvium from the upper catchment slopes into moraines on the valley floors that dam drainage to form lakes. Outburst floods differ from meteorological floods in the valley by the high water and sediment discharge achieved during a single event, by deposition of massive NCSGF beds, and by causing major changes to the landscape such as the carving of channels. These differences produce fans with distinctive facies and hazards, the former allowing their recognition in either modern or ancient settings.