Pleistocene lava-dam outburst floods, western Grand Canyon, Arizona

Five lava dams failed catastrophically between 100 and 525 ka ago producing basalt-rich outburst-flood deposits preserved between river miles 185 and 222 downstream of dam sites in western Grand Canyon of the Colorado River. 3 He exposure ages and chemistry of whole rock basalt, basalt glass, and olivine phenocrysts distinguish flood deposits, lava flows, and lava dams. REE concentrations indicate 10 of 11 basalt flows sampled in the Uinkaret volcanic field are alkalic. The two youngest deposits (Qfd4 and Qfd5) yielded average 3He ages of 104 ± 12 and 165 ± 18 ka and resulted from the dam failures of the Hyaloclastite Dam and Younger Cascades Dam (108 ± 11 ka), respectively. The Upper Prospect lava flow (395 ± 35 ka) may have constructed a lava dam that failed catastrophically and produced the Qfd1 outburst-flood deposits. Qfd3 deposits may preserve two flood events that resulted from the failure of the Toroweap Dam (∼410 ka) and Whitmore Cascades (180 ± 6 ka).; The sedimentology and elevations of the deposits indicate large-scale floods with a voluminous but unsteady discharge; flood-deposit elevations indicate an exponential-decay surface that could be reproduced only by unsteady flow. The deposits generally consist of a poorly-sorted basal hyaloclastite ash overlain by poorly-sorted, subangular to well-rounded basalt boulders, cobbles, and pebbles. Clast size and deposit thickness decrease with increasing distance from the damsites.; The Toroweap and Hurricane faults, both normal faults, are downthrown to the west, and the Colorado River crosses them as it flows west in the Grand Canyon. 3He_c dates on volcanic landforms are used to calculate middle and late Quaternary vertical displacement rates of the Toroweap and Hurricane faults that are 70–180 and 70–170 m/m.y., respectively.; Fault displacement rates are used to reconstruct canyon geometry during the 165 ka Qfd4 flood. FLDWAV produced a flood with a peak discharge estimate for the Qfd4 outburst flood of 4.4 · 105 m3/s based on Saint-Venant equations for unsteady flow. The flood dissipated to 4.3 · 105 m3/s 23 km downstream of the dam failure and it approached steady, uniform flow conditions; however, a 1.4 · 105 m3/s flood more accurately reproduces the water-surface profile preserved in flood deposit elevations at this location.