THE USE OF TRAVERTINE TO MEASURE RATES OF RIVER INCISION AND SCARP RETREAT IN THE EASTERN GRAND CANYON, ARIZONA

Lava flows help constrain the incision history and scarp retreat rate for the western Grand Canyon (e.g. Hamblin 2003). Direct comparison of these rates with those in the eastern Grand Canyon is desirable for several reasons. Lava flows are absent but travertine is abundant in the eastern canyon's late Cenozoic record. Travertine can be dated using U-series techniques, making it a useful datum.

Two travertine populations, both issuing from paleo-springs on the Tonto Platform, occur along the river corridor between Hermit and Travertine canyons. Type 1 outcrops are interbedded with colluvium and flow into the Inner Gorge, thus providing a lower bound on Inner Gorge incision age. Type 2 outcrops display nearly horizontal beds, do not extend below the Tonto Platform, and are interbedded with alluvial gravels derived from a broad area. These characteristics argue for Type 2 deposition when the Colorado River occupied the Tonto Platform, thus providing an upper bound on Inner Gorge incision age.

We obtained a U-series age of 394 +/- 32 ka for the Type 1 travertine that flows the farthest into the Inner Gorge and similar ages for other Type 1 outcrops. The Type 2 outcrops are too old to be directly dated using the U/Th technique. However, high 234U/238U ratios (some in excess of 1.1) for several samples indicate that Type 2 outcrops are younger than 1.2 Ma. These ages, combined with sample elevations, imply cutting of the Inner Gorge entirely within the last 1.2 m.y. Rapid early incision slowed by a factor of three or more during the last 400 kyr.

Travertine-cemented, boulder breccia present on the uphill sides of several outcrops implies their accumulation at the base of the Redwall Limestone cliff, which has since retreated. U-series dates for five outcrops yield retreat rates between 0.35-0.75 m/kyr. This range agrees well with arid region rates obtained in the American Southwest and elsewhere, but is less than the 1.6 m/kyr cumulative rate Hamblin (2003) obtained for western Grand Canyon using the lava flows.