Since the time of John Wesley Powell, integration of the upper Green River (western United States) across the eastern Uinta Mountains has been a subject of great interest and significance for understanding Colorado Plateau evolution. We address the question of the timing of integration by making novel use of detrital sanidine (DS) 40Ar/39Ar geochronology, which, due to the precision of the method, is emerging as an excellent way to constrain terrace ages that are >0.5 Ma in the western United States. The DS dates, in combination with cosmogenic burial and luminescence dates, produced a long-term (∼2 m.y.) record of the upper Green River in southwestern Wyoming (USA; Peru Bench) and along the Colorado-Utah, USA, border (Browns Park).
DS dating of 3461 sanidine grains from 10 terrace deposits (five from Peru Bench and five from Browns Park) produced maximum depositional ages (MDAs), seven of which show a linear relationship of increasing terrace height and age. The DS MDAs at Peru Bench are: 1.304 ± 0.011 Ma (175 m terrace), 1.020 ± 0.046 Ma (150 m terrace), 0.847 ± 0.009 Ma (125 m terrace), and 0.574 ± 0.045 Ma (75 m terrace). The DS MDAs in Browns Park are: 1.980 ± 0.025 Ma (210 m terrace), 1.283 ± 0.018 Ma (130 m terrace), and 0.670 ± 0.004 Ma (45−60 m terrace). Two cosmogenic burial dates for Peru Bench terraces are: 1.24 ± 0.44 (150 m terrace) and 1.40 ± 0.96 Ma (125 m terrace). Two luminescence dates for Peru Bench terraces are: 59.0 ± 10.4 ka (20−30 m terrace) and 35.4 ± 4.2 ka (5−10 m terrace).
Regressions of terrace heights and DS ages from both locations show that long-term bedrock incision rates measured over the past ∼2 m.y. have remained remarkably constant through >40 glacial-interglacial cycles, with no apparent influence of the Mid-Pleistocene Transition. This constancy is interpreted to record long-term regional epeirogenic uplift and erosional isostasy. Minimum bedrock incision rates are ∼169 m/m.y. at Peru Bench (upstream) and ∼114 m/m.y. in Browns Park (downstream). The slower incision rate in Browns Park is interpreted to reflect dampening of incision due to Quaternary subsidence within the Browns Park graben.
Stratigraphic relations demonstrate that integration of the upper Green River across the eastern Uinta Mountains occurred after the end of Browns Park Formation sedimentation ca. 8 Ma. Published Colorado River detrital zircon analyses from the Salton Trough suggest that upper Green River integration could have occurred as early as ca. 5 Ma. New DS dates show that upper Green River integration was likely complete by ca. 2 Ma. It is also probable that integration of the upper Green River into the Colorado River system contributed to late Cenozoic exhumation of the Colorado Plateau.
