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Water Science and Applications

 

Keywords

  • Paleohydrology
  • Floods

Index Terms

  • 1821 Hydrology: Floods
  • 1035 Geochemistry: Geochronology
  • 1824 Hydrology: Geomorphology
  • 1749 History of Geophysics: Volcanology, geochemistry, and petrology

Article

WATER SCIENCE AND APPLICATION, VOL. 5, PP. 191-215, 2002

Cosmogenic 3He ages and geochemical discrimination of lava-dam outburst-flood deposits in western Grand Canyon, Arizona

C. R. Fenton, T. E. Cerling, B. P. Nash, R. H. Webb, and R. J. Poreda

Quaternary terrace gravels almost entirely composed of basalt clasts are preserved on basalt ledges in western Grand Canyon between river miles 185 and 222. Sedimentological evidence preserved in these gravels suggests that they were emplaced by the catastrophic failure of lava dams, leading to deposition at elevations that decreased abruptly with increasing distance from potential damsites near the Uinkaret volcanic field (between river miles 179 and 188). At least two lava dams on the Colorado River may have failed in the last 160 ka, and deposits that correlate on the basis of exposure age and chemistry are preserved up to 26 river miles (43 km) downstream from the damsites in western Grand Canyon. These possible outburst-flood deposits range in height from 53–200 m above current river level. The deposits generally consist of a poorly–sorted basal hyaloclastite ash overlain by poorly-sorted, sub–angular to well-rounded basalt boulders, cobbles, and pebbles. Clast size and deposit thickness decrease with increasing distance from the damsites. Cosmogenic 3He exposure ages and chemistry of whole rock basalt, basalt glass, and olivine phenocrysts in the deposits document at least two distinct flood deposits. The best preserved deposit has an average surface exposure age of 159±2 ka at river miles 189.5 and 202; the second flood deposit, at river mile 204.4, has a problematic exposure age of 66±6 ka and is likely older than 160 ka. A terrace cut into a pre-existing gravel deposit at river mile 204.6 has an exposure age of 100±6 ka. In the Uinkaret volcanic field, the Whitmore Cascade (177±9 ka), Younger Cascades (108±11), and Bar Ten (88±6 ka) lava flows were investigated as potential lava dams correlative with the outburst-flood deposits. Channel morphology and clast size can be used for future analysis of the flood wave that propagated downstream after the dam failures.

Citation: Fenton, C. R., T. E. Cerling, B. P. Nash, R. H. Webb, and R. J. Poreda (2002), Cosmogenic 3He ages and geochemical discrimination of lava-dam outburst-flood deposits in western Grand Canyon, Arizona, in Ancient Floods, Modern Hazards: Principles and Applications of Paleoflood Hydrology, Water Sci. Appl., vol. 5, edited by P. K. House et al., pp. 191–215, AGU, Washington, D. C., doi:10.1029/WS005p0191.

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