MIOCENE RIVERS OF WESTERN COLORADO: IMPLICATIONS FOR THE TIMING OF RIVER INTEGRATION IN THE ROCKY MOUNTAINS

New detrital sanidine ⁴⁰Ar/³⁹Ar age constraints on fluvial deposits suggest that Middle to Late Miocene (ca. 17-11 Ma) paleorivers of western Colorado record the onset of river integration in the upper Colorado River system. Prior to river integration, internal drainage, characterized by fine-grained basin-fill sedimentation, predominated. At Columbine Pass (elevation 2700 m), paleoriver deposits consist of ~5 m of volcanic-clast-dominated gravel derived from Oligocene volcanics of the San Juan Mountains. Fluvial detrital sanidine dates produced a weighted-mean maximum depositional age (MDA) of 16.59 ± 0.02 Ma (n=2 grains of 199). Grand Mesa (elevation 2900-3100 m) consists of Late Miocene (ca. 10.8-9.5 Ma) basalt flows that locally overlie river gravel. On western Grand Mesa, river gravel is heterolithic (quartzite, granite, sandstone, volcanics). To the east, river gravel is dominated by volcanic clasts of the West Elk Mountains. Detrital sanidine dating of the youngest fluvial sand produced a weighted-mean MDA of 10.80 ± 0.02 Ma (n=3 grains of 239). Grand Mesa basalt also locally overlies tens of meters of unconsolidated mud, sand and gravelly sand as well as minor beds of micritic carbonate. This mud-rich sequence is informally referred to as the Goodenough unit, and detrital sanidine dating produced a weighted-mean MDA of 13.29 ± 0.06 Ma (n=4 grains of 140). Cimarron Ridge (elevation 3000 m) is the northern tip of the San Juan Mountains where ancient fluvial deposits are represented by ~5 m of boulder-cobble gravel. The gravel is dominated by volcanic clasts and granite; likely source areas for the clasts include the Gunnison Uplift (east) and San Juan Mountains (south). Cimarron Ridge river gravel overlie ~15 m of unconsolidated mud and sand similar to Grand Mesa.

Collectively, mud-rich deposits at Grand Mesa and Cimarron Ridge are interpreted to represent low-energy, internally drained landscapes created by a combination of Laramide uplift/subsidence and Oligocene volcaniclastic sedimentation. By comparison, overlying fluvial gravels are similar in caliber to the bed load of modern rivers of western Colorado. We interpret the appearance of gravelly fluvial deposits as a record of Late Miocene river integration and accelerated uplift and erosion of the Rocky Mountains.