NUMERICAL AGES OF HOLOCENE TRIBUTARY DEBRIS FANS INFERRED FROM DISSOLUTION PITTING ON CARBONATE BOULDERS IN THE GRAND-CANYON OF ARIZONA

Carbonate boulders transported down steep tributary channels by debris flow came to rest on Holocene debris fans beside the Colorado River i n Grand Canyon National Park. Weakly acidic rainfall and the metabolic activity of blue-green algae have produced roughly hemispheric dissol ution pits as much as 2-cm deep on the initially smooth surfaces of th e boulders, The average depth of dissolution pits increases with relat ive age of fan surfaces. The deepening rate averages 2.4 mm/1000 yr (s tandard error = 0.2 mm/1000 yr), as calculated from several radiometri cally dated surfaces and an archeological structure. This linear rate, which appears constant over at least the past 3000 yr, is consistent with field relations limiting the maximum age of the fans and with the physical chemistry of limestone dissolution. Dissolution-pit measurem ents (n = 6973) were made on 617 boulders on 71 fan surfaces at the 26 largest debris fans in Grand Canyon, Among these fan surfaces, the av erage pit depth ranges from 1.2 to 17.4 mm, and the resulting pit diss olution ages range from 500 to 7300 cal yr B.P. Most (75%) surfaces ar e younger than 3000 yr, probably because of removal of older debris fa ns by the Colorado River, Many of the ages are dose to 800, 1600, 2300 , 3100, or 4300 cal yr B.P. If not the result of differential preserva tion of fan surfaces, this clustering implies periods of heightened de bris-flow activity and increased precipitation. (C) 1998 University of Washington.