COLORADO RIVER TERRACE GRAVELS AT THE BIG HOLE RINCON IN WESTWATER CANYON, UTAH, AND CONSTRAINTS ON THE TIMING OF WESTWATER CANYON INCISION

Big Hole is a spectacular entrenched meander of the Colorado River in Westwater Canyon. Comparisons between compositions of Big Hole and modern river gravels as well as regional age estimates, constrain the age of these deposits and the timing of incision of Westwater Canyon.

Colorado River gravels are present at two levels. The highest gravel is 190-192 m above the modern river and the other deposit is 100-105 m above the river. The higher deposit consists of rounded pebbles mixed with angular sandstone, which suggests the gravels represent reworking of higher and perhaps older river gravels. Comparisons between Big Hole and modern Colorado River gravel compositions suggest that both gravel deposits represent the combined Colorado-Gunnison Rivers. The 190-m gravels at Big Hole have abundant (30%) intermediate volcanic clasts of the Gunnison River, which drains Tertiary volcanics of the San Juan and West Elk Mountains. Similar intermediate volcanic clasts are also found in the 100-m-terrace gravels.

Based on regional incision rates (140-150 m/Ma), an estimated age for the 190-m river gravels is 1.27-1.36 Ma. Because the Big Hole terrace gravels contain volcanic clasts derived from the Gunnison River, the Big Hole river gravels must post-date abandonment of Unaweep Canyon by the ancestral Gunnison River. Cosmogenic ages indicate that the maximum age for Unaweep Canyon abandonment is 1.46 +/- 0.33 Ma (Balco et al., 2012). This age estimate also represents a maximum age for the 190-m Big Hole gravels. Modeled U-series ages for 140- to 160-m Colorado River terraces near Grand Junction are slightly <1 Ma, which supports the interpretation of a ca. 1.4 Ma age for the 190-m river gravels at Big Hole. This maximum age estimate (~1.4 Ma) and the height of the 190-m gravels, produces a long-term incision rate of 136 m/Ma.

In summary, Big Hole gravel compositions and age constraints on the abandonment of Unaweep Canyon indicate that Westwater Canyon was carved in <1.4 My. Incision rates calculated using a maximum age of 1.4 Ma for the Big Hole gravels are remarkably similar to longer-term regional incision rates measured over the past 10 Ma. This similarity suggests that the effects of glacial-interglacial cycles have been secondary to the effects of Neogene uplift on Colorado River incision in Westwater Canyon.