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

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  • Cosmogenic-burial and U-series dating, identification of fluvial terraces and lacustrine deposits, and river-profile reconstructions show that capture of the Gunnison River by the Colorado River and abandonment of Unaweep Canyon occurred between 1.4-0.8 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/Ma. The Gunnison River removed 400-500 km3 of erodible Mancos Shale and incised up to 360 m deep in 0.17-0.76 My (incision rates of ~470-2250 m/Ma). Prior to canyon abandonment, long-term (~11-1 Ma) Gunnison River incision averaged ~100 m/Ma. 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 (up to ~500 m/Ma) but localized river incision. As regional river incision progressed, the juxtaposition of resistant Precambrian bedrock and erodible Mancos Shale within watersheds favored the development of significant 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 fluvial incision are punctuated by short-term geomorphic events such as stream piracy. These short-term events can trigger significant landscape changes, but the effects are more localized relative to regional climatically- or tectonically-driven events. Powered by
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  • Aslan, A., Hood, W. C., Karlstrom, K. E., Kirby, E., Granger, D. E., Kelley, S., ... & Asmerom, Y. (2014). Abandonment of Unaweep Canyon (1.4–0.8 Ma), western Colorado: Effects of stream capture and anomalously rapid Pleistocene river incision. Geosphere, 10(3), 428-446. doi:10.1130/GES00986.1
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  • 10
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  • 3
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  • NSF grant EAR-1119635 (Aslan) supported this research. We wish to also thank the Grand Junction Geological Society (GJGS) for its financial support of the Cactus Park drillholes, and numerous society members for help in the field during the drilling. Without this financial support and the help of society members during the drilling, this study would not have been possible.
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