RATES OF RIVER INCISION AND SCARP RETREAT IN EASTERN GRAND CANYON'S HERMIT-BOUCHER AREA DETERMINED USING U-SERIES DATING OF TRAVERTINE

Nine outcrops of interbedded travertine and gravel exist in Grand Canyon between Hermit and Boucher creeks. We present U-series ages for them and use these to determine the rate of Colorado River incision, of scarp retreat for the Redwall/Muav cliff, and to date tributary aggradation and incision episodes.

Two possible interpretations exist for the age of the four oldest outcrops. Nine of 22 samples yield ages near 500 ka. We conclude this is the depositional age of all four outcrops. The other 13 samples have endured diagenesis but display ²³⁴U /²³⁸U disequilibrium, meaning they are younger than 1.2 Ma. An alternative interpretation is that the 500 ka ages record a period of travertine remobilization, with depositional ages somewhere between 500 ka and 1.2 Ma.

Our data reveal:

1) Travertine Creek aggraded between 53-11 ka and has since been incising at 1910 +/- 10 m/m.y.

2) The Redwall/Muav escarpment has been retreating at 319-373 m/m.y., assuming the conservative 1.2 Ma outcrop age. That rate is 724-946 m/m.y using the preferred 500 ka age.

3) The Inner Gorge was cut to within 59 m of the river’s current elevation by 394 +/- 32 ka. The maximum Colorado River incision rate since that time is 164-247 m/m.y.

4) The four oldest outcrops, which are perched on the Tonto Platform 250 m above the modern river, were deposited by the Colorado River when it flowed across the Tonto Platform. Using the oldest possible outcrop age of 1.2 Ma yields a

conservative river incision rate of 207-232 m/m.y. Using the preferred outcrop age of 500 ka, the river incision rate has been 460-721 m/m.y. and requires a 500-400 ka period of rapid Colorado River incision followed by slowing after

400 ka.

5) Two basaltic dikes at river mile 159 predate carving of the 430-meter-deep Muav Gorge. They are dated at 405-780 ka (K-Ar), requiring a minimum incision rate of 547 m/m.y. with similar slowing after 400 ka.

6) Passage of a knickpoint explains the progression from very high incision rates before 400 ka to slower rates thereafter. That knick passed Hermit-Boucher near 500 ka. The comparatively low, steady incision rate documented for the entire

eastern Grand Canyon since about 400 ka is the background rate after knickpoint passage.