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40Ar/39Ar Dating of Detrital Sanidine in the Goodenough Unit, Grand Mesa, Colorado


Goodenough is the informal name given to sedimentary deposits that underlie Late Miocene basalt flows of Grand Mesa in western Colorado, and which overlie the Eocene Green River Formation. These poorly consolidated deposits are extremely heterolithic and include pebbly sandstone, sandstone, sandy siltstone, variegated mudrocks and minor carbonate lenses. The unit can be subdivided into the sand-rich Eggleston facies, which overlies the mud-rich Vela facies. The wide range of clast lithologies (granite, diorite, volcanics, sedimentary) suggest that the sediments were derived from the Sawatch Range located to the east, and were transported westward where coarse- and fine-grained sediments accumulated up to ~100 m in thickness. Carbonate lenses are micritic and bioturbated (rooted), which suggests that periods of paludal conditions existed episodically during overall aggradation of Goodenough deposits.

The age of the Goodenough unit is poorly constrained. The sediments underlie the ca. 9.5-11 Ma Grand Mesa basalt flows and river deposits, locally. Volcanic breccia clasts that were derived from the ca. 30 Ma West Elk Mountains are found in Goodenough sediments locally, and suggest that the deposits are <30 Ma. 40Ar/39Ar dating of detrital sanidine was used to provide radiometric ages for the Goodenough interval. Four samples were acquired from silty and sandy units within both the Eggleston and Vela facies from locations near South Mesa Lakes. Sample preparation included sieving, magnetic separations, and mineral separation using lithium metatungstate. The final processing step involved hand picking 125-250 micron sanidine grains from processed samples. Only two of the four samples had sufficient quantities of sanidine to proceed with the radiometric dating. The two sanidine-bearing samples were irradiated and ~140-160 sanidine grains from each sample were dated at the New Mexico Tech Geochronology Lab.

One sample of the Vela facies contained no grains younger than Eocene; the youngest grain is 48.89 ± 0.20 Ma, and most of the grains are Cretaceous in age. The other sample (Eggleston facies) yielded numerous Miocene- and Oligocene-age grains. The youngest single grain is 13.11 ± 0.10 Ma and the youngest four grains yielded a weighted-mean age of 13.29 ± 0.06 Ma. This Late Miocene date is only slightly older than the ca. 11 Ma maximum ages for the overlying Grand Mesa basalt flows and locally present river sediments.

The dating results suggest that the Goodenough interval represents a Late Cenozoic phase of localized aggradation in the western Piceance Basin, which continued until the Late Miocene. Aggradation was probably associated with filling of an Early Miocene-Late Oligocene(?) paleovalley carved into Laramide basin-fill units (e.g., Green River and Uinta formations), rather than tectonic subsidence. Valley filling involved reworking of the Oligocene volcanic rocks that mantled much of west-central Colorado in the Miocene (e.g., San Juan and West Elk volcanic fields), and aggradation at this time was probably aided by volcaniclastic inputs from areas such as the Basin and Range, and perhaps climatic drivers such as Late Miocene aridity.