CARVING GRAND CANYON'S INNER GORGE: A TEST OF STEADY INCISION VERSUS PASSAGE OF AN INCISION TRANSIENT

A recent study posits that the 250-m-deep inner gorge of Grand Canyon was carved between 500 and 400 ka due to passage of a transient knickpoint (Abbott et al., 2015) at rates of ~1600 m/Ma; this was based on dating of a ca. 500 ka travertine deposit perched on the rim of the inner gorge near Hermit Rapid and a ca. 400 ka travertine drape that extends to within 60 m of river level nearby. However, this model is challenged by our new U/Th age of 517 ± 13 ka (n=5) from the base of the same drape; the drape top is newly dated at 246 ± 13 ka. The presence of 517 ka travertine, just 95 m above river level, requires that most of the inner gorge was already carved before 500 ka. The resulting maximum bedrock incision rate of >180 m/Ma is consistent with independent results from both up and downstream and with models for semi-steady Quaternary bedrock incision.

New field observations show that the discontinuous travertine deposits near Hermit Rapid were deposited by springs that emanated from the Redwall-Muav aquifer, mantled the Tonto Platform, and locally built downwards into the inner gorge. The range of U/Th ages from ca. 10 to 600 ka indicates these were long-lived spring systems. No far-traveled Colorado River gravel was observed by us; instead, the travertine cements colluvium and sub-rounded local gravel, which are often steeply dipping and interpreted to be deposited by lower-order side streams on the Tonto Platform. Sub- to well-rounded gravel of unknown lithology was documented by Abbott et al. (2015) at one of the deposits and used to suggest the Colorado River base level was at the rim of the inner gorge at 500 ka. If so, the rounded gravels could be explained if: 1) they were deposited at about 2 Ma, which is when steady incision rate models suggest the inner gorge began to incise. This is compatible with the available U/Th geochronology on infillings, which give a minimum age on the gravel, and is testable by additional U/Th and U/Pb dating on infillings and detrital travertine clasts; 2)a ca. 400-600 ka lava dam at Toroweap, 140 km downstream, backed water up to the rim of the inner gorge in the Hermit area for a short time, however lacustrine deposits have not been found; 3) regional climate-driven aggradation of the entire inner gorge took place at 500 ka, but aggradation of this scale or at this time has not been recognized elsewhere in the Colorado River system.