A post–6 Ma sediment budget for the Colorado River

Abstract

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 fl ows and new thermochronologic data, we calculate that ~3.4 ± 1.2 ◊ 10 5 km 3 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 ◊ 10 5 km 3 , 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 ◊ 10 5 km 3 , 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/cm 3