Abandonment of Unaweep Canyon (1.4–0.8 Ma), western Colorado: Effects of stream capture and anomalously rapid Pleistocene river incision

Abstract

Cosmogenic-burial and U-series dating, identifi cation of fl uvial terraces and lacustrine deposits, and river profi le reconstructions show that capture of the GunnisonRiver by the Colorado River and abandonment of Unaweep Canyon (Colorado, USA) occurred between 1.4 and 0.8 Ma. This event led to a rapid pulse of incision unlike any documented in the Rocky Mountains. Following abandonment of Unaweep Canyon by the ancestral Gunnison River, a wave of incision propagated upvalley rapidly through Mancos Shale at rates of ~90-440 km/m.y. The Gunnison River removed 400-500 km 3 of erodible Mancos Shale and incised as deep as 360 m in 0.17-0.76 m.y. (incision rates of ~470-2250 m/m.y.). Prior to canyon abandonment, long-term (ca. 11-1 Ma) Gunnison River incision averaged ~100 m/m.y.The wave of incision also caused the subsequent capture of the Bostwick-Shinn Park River by the ancestral Uncompahgre River ca. 0.87-0.64 Ma, at a location ~70 km upvalley from Unaweep Canyon. This event led to similarly rapid (to ~500 m/m.y.) but localized river incision. As regional river incision progressed, the juxtaposition of resistant Precambrian bedrock and erodible Mancos Shale within watersheds favored the devel-opment of signifi cant relief between adjacent stream segments, which led to stream piracy. The response of rivers to the abandonment of Unaweep Canyon illustrates how the mode and tempo of long-term fl uvial incision are punctuated by short-term geomorphic events such as stream piracy. These shortterm events can trigger signifi cant landscape changes, but the effects are more localized relative to regional climatically or tectonically driven events.

Aftermath of a stream capture: Cactus Park lake spillover and the origin of East Creek, Uncompahgre Plateau, western Colorado

“…This is a 0.5 m.y. older than the burial age of the Cactus Park gravels (800 ± 240 ka, discussed herein; also see Aslan et al, 2014). Based on the evidence that the Gunnison River did not contribute to Cactus Park lake, whereas it sourced the western Unaweep lake, the improbability that the western Unaweep lake level could have been high enough to reach the top of the Cactus Park lake beds, and the considerable difference in apparent ages of the two lakes, we conclude that the two lakes were distinct and that the western Unaweep lake is older than the Cactus Park lake.…”

“…Our clast counts indicate that the gravel in the pit matches these values reasonably well, with 65% intermediate volcanic clasts, 14% metamorphic, 14% felsic on June 9, 2014 geosphere.gsapubs.org Downloaded from volcanic and granites, and 7% other rock types. The age distribution of detrital zircons is also similar to the modern Gunnison River, adding more evidence that these gravels are ancestral Gunnison River gravels (Aslan et al, 2014). Caskey (2008) also examined several outcrops of Cactus Park gravel, comparing the composition of Cactus Park gravel to that of the Colorado, Uncompahgre, and Gunnison Rivers, and also concluded that the Cactus Park gravel originated in the Gunnison River.…”

“…After the capture the ancestral Gunnison alone would have continued through Unaweep Canyon. Between the Colorado River and Gunnison River captures, the Colorado River would have continued to deepen its valley, giving perhaps as much as 300 m difference in elevation between it and the ancestral Gunnison River before the Gunnison was captured (Aslan et al, 2014).…”

“…A second sample, of the same material, was submitted later and returned an age of 620 ± 390 ka. The average of these two ages is 800 ± 240 ka (Aslan et al, 2014). Although the cosmogenic age indicates when the gravel was buried and not necessarily the age of abandonment by the river, there is some evidence that suggests that the lake beds closely post-date abandonment.…”

Geology of Unaweep Canyon and its role in the drainage evolution of the northern Colorado Plateau

“…The prevailing fl uvial hypothesis posits original canyon formation by the ancestral Gunnison River (Peale, 1877;Cater, 1966Cater, , 1970Sinnock, 1981), the ancestral Colorado River, or a combined Colorado-Gunnison (Gannett, 1882;Stokes, 1948;Shoemaker, 1954;Cater, 1955a;Hunt, 1956;Lohman, 1961Lohman, , 1981Steven , 2002;Aslan et al, 2008Aslan et al, , 2010Aslan et al, , 2014Hood, 2011;Hood et al, 2014). Some suggested that the Dolores River fl owed northeastward to carve Unaweep Canyon (Peale, 1877;Hunt, 1956).…”