DETRITAL SANIDINE AND COSMOGENIC BURIAL AGE CONSTRAINTS SUPPORT POST-2 MA INTEGRATION OF THE UPPER GREEN RIVER ACROSS THE UINTA MOUNTAINS

Since the time of John Wesley Powell’s historic explorations, geologists have puzzled over the anomalous course of the Green River across the Uinta Mountains uplift, and speculated on the timing of integration of the upper and lower Green River systems. A combination of new detrital sanidine (DS) and cosmogenic radionuclide (CRN) burial ages of Green River deposits indicate that the Green River established its present course across the mountains within the past 2 Ma.

In Browns Park, located immediately upstream of Lodore Canyon on the north flank of the eastern Uinta Mountains, 9 down-stepping Green River terraces rise up to 210 m above the modern river. Samples of the sand matrix from the oldest terrace (210 m) yielded 2 DS grains that are 1.96 and 2.37 Ma, and the next oldest terrace (130 m) produced 4 DS grains that are ~1.3 (Ma) (probably Mesa Falls Tuff). Upstream near Green River, Wyoming, 9 Green River terraces rise up to 175 m above the river. The oldest terrace (175 m) produced 5 DS grains that are 1.3 ± 0.006 Ma, and the next oldest terrace (150 m) produced 2 DS grains that are 1.0 ± 0.01 Ma as well as individual DS grains with ages of 1.3 and 2.1 Ma. Quartz-rich cobbles from this gravelly 150 m terrace also produced a CRN burial age of 1.38 ± 0.15 Ma. The youngest DS grain from the 120 m terrace is 0.85 ± 0.012 Ma; additional young DS grains from this terrace are 1.2 and 2.0 Ma. An existing CRN burial age from the 120 m terrace produced a date of 1.2 ± 0.3 Ma.

Because the age of the youngest DS grain in a given terrace deposit is progressively older in successively higher (and thus older) terraces, we hypothesize that the DS maximum depositional ages may approximate depositional ages. This suggestion is supported by generally similar DS and CRN dates from the same individual terrace deposit. In this hypothesis, episodic Yellowstone eruptions produced air fall that was reworked by the Green River as it slowly incised. We speculate that the post-2 Ma integration of the Green River across the Uinta Mountains substantially increased the river’s discharge, and contributed to rapid late Quaternary bedrock incision in downstream reaches of both the Green River (e.g., Desolation-Gray Canyon) and Colorado River (Glen Canyon). Thus, the major features of the broader Colorado-Green River system have probably formed within the past ~2 Ma.