GEOCHEMICAL DISCRIMINATION OF FIVE PLEISTOCENE LAVA-DAM OUTBURST-FLOOD DEPOSITS, WESTERN GRAND CANYON, AZ

Cosmogenic 3He dates and geochemical signatures of basalts distinguish Pleistocene lava flows, lava dams, and lava-dam outburst-flood deposits. REE content and major element concentrations of whole-rock basalts and hyaloclastites are used to suggest correlations between lava-dam remnants and flood deposits of similar age. At least 5 lava dams failed catastrophically between 525 and 100 ka ago producing outburst-flood deposits preserved up to 53 km downstream of dam sites in western Grand Canyon of the Colorado River. Basalts in deposits classified as units Qfd1, Qfd2, Qfd3, and Qfd5 exhibit distinct alkali olivine basalt signatures and unit Qfd4 is tholeiitic. The two youngest deposits (Qfd5 and Qfd4) yielded average cosmogenic 3He (3Hec) ages of 104 ± 12 and 165 ± 18 ka. On the basis of overlapping ages and similar REE patterns, the Younger Cascade (3Hec = 108 ± 11 ka) and the informally-named Hyaloclastite Dam may have constructed lava dams that failed catastrophically to produce the Qfd4 and Qfd5 outburst-flood deposits. Likewise, the Upper Prospect lava flow (3Hec = 395 ± 35 ka) may have been part of a lava-dam that failed and produced the Qfd1 outburst-flood deposit. Toroweap Dam (Flow A; K-Ar = 1.2 Ma; McKee et al., 1968) may be as young as 410 ka based on 44.5 m of displacement in the basalt along the Toroweap fault and an average displacement rate of 110 m/Ma (Fenton et al., 2001). Qfd3 deposits may preserve 2 separate outburst floods that could have resulted from the failure of the Toroweap Dam and Whitmore Cascades (3Hec = 180 ± 6 ka). Stage III and IV soil-carbonates exist in limestone talus covering the deposits (Lucchitta et al., 2000). REE content, major element concentrations, and accurate age constraints greatly improve mapping and interpretation of how Quaternary basalt flows, lava dam remnants, and lava-dam outburst-flood deposits affected the Colorado River in western Grand Canyon.