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Grand Junction, CO, United States

Dorsey R.J.,University of Oregon | Lazear G.,Grand Junction Geological Society
Geosphere | Year: 2013

Regional sediment budgets provide a useful method for quantifying erosion by large river systems over geologic time scales. The Colorado River (western United States) is well suited for such an analysis because the eroding source (Colorado Plateau) and sediment sinks in transtensional basins of the Salton Trough and northern Gulf of California are intact and well preserved. Using the distribution of Late Miocene basalt flows and new thermochronologic data, we calculate that ~3.4 ± 1.2 × 105 km3 of rock has been eroded from the Colorado Plateau since 10 Ma. Most of this erosion probably started ca. 5.5-6 Ma, when the river system became integrated and incision rates increased dramatically. We generate two estimates for the volume of Colorado River sediment that has accumulated in basinal sinks since ca. 5.3 Ma: (1) 2.8 ± 0.6 × 105 km3, assuming that crust between 5 and 10-12 km depth in the plate-boundary basins is young metasedimentary rock mixed with intrusions; and (2) 1.55 ± 0.35 × 105 km3, assuming that crust below 4-5 km is thinned pre-Cenozoic crystalline rock. The broad overlap of the fi rst estimate with the calculated volume of rock eroded from the plateau provides new support for a model of lithospheric rupture and rapid sedimentation in the Salton Trough. Assuming an average density of 2.3-2.5 g/cm3, and using the range of preferred volume estimates calculated here, the total mass transferred is 5.1-11.5 × 1014 t representing an average flux of 156 ± 60 Mt/yr since 5.3 Ma, the time when the Colorado River fi rst arrived in the Salton Trough, or 172 ± 66 Mt/yr if we assume that all sediment flux took place after 4.8 Ma. The calculated long-term flux is strikingly similar to historical pre-dam sediment discharge measured at Yuma (Arizona) in the early 1900s (172 ± 64 Mt/yr). The similarity of flux estimates suggests that rates of erosion and sediment discharge in this system have been consistent, on average, over modern to geologic time scales. We suggest that ongoing positive feedback between late Cenozoic erosion and flexural uplift on the Colorado Plateau provides a mechanism that could sustain steady rates of regional erosion and sediment production for millions of years after integration of the Colorado River ca. 5.5-6 Ma. © 2013 Geological Society of America.

Aslan A.,Colorado Mesa University | Hood W.C.,Grand Junction Geological Society | Karlstrom K.E.,University of New Mexico | Kirby E.,Oregon State University | And 6 more authors.
Geosphere | Year: 2014

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 (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 km3 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 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 shortterm events can trigger significant landscape changes, but the effects are more localized relative to regional climatically or tectonically driven events. © 2014 Geological Society of America.

Hood W.C.,Grand Junction Geological Society | Aslan A.,Colorado Mesa University | Betton C.,Grand Junction Geological Society
Geosphere | Year: 2014

Newly discovered lacustrine strata suggest that the most significant episode of stream capture in the upper Colorado River system (western USA), namely the abandonment of Unaweep Canyon, probably involved a combination of headward erosion and lake spillover. The abandonment of Unaweep Canyon occurred in two stages. The first stage was marked by the capture of the Colorado River, after which time the Gunnison River continued to incise. Continued incision by the Gunnison River created a wall of rock on the east side of the Gunnison River valley in Cactus Park and left the abandoned Colorado River bed well above the Gunnison River. The second stage involved two blockages, one created by the thick fill within Unaweep Canyon and one at the south end of Cactus Park, which led to the creation of a lake within Cactus Park. The lake level rose until it flooded the abandoned Colorado River bed and spilled over at the point where the Colorado River had been captured earlier, during the first stage of abandonment. Present-day East Creek was created by re-incision of the abandoned Colorado River course, which explains why the course of East Creek has a northeastward trend that is completely anomalous with respect to all other tributaries draining this area of the Uncom pahgre Plateau. The rapid incision created a large quantity of debris that deflected the Gunnison River eastward at the mouth of East Creek. The evidence suggests that the abandonment of Cactus Park and Unaweep Canyon by the Gunnison River and the creation and destruction of Cactus Park lake all likely occurred ca. 800 ka and shortly thereafter. The Unaweep Canyon classic example of stream piracy illustrates how piracy alone can dramatically influence landscape development even in the absence of significant tectonic and climatic influences. © 2014 Geological Society of America.

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