PRELIMINARY 40AR/39AR LASER DATING OF DETRITAL BIOTITE FROM CENOZOIC SANDSTONES, RUBY VALLEY, MONTANA

Single detrital biotite crystals from two distinctive sandstone units, the Teddy Creek Group (TCG) and the Renova Formation, were analyzed using ⁴⁰Ar/³⁹Ar laser dating. The goals were to determine the absolute age of the micas and provide information on; 1) the depositional age of the sandstones; 2) the provenance/unroofing history of their source region; and 3) speculate on the tectonic evolution of this region following the Laramide Orogeny. The question of whether high standing topography, with Precambrian basement exposed, existed during the time of deposition or whether the region was characterized by a broad basin surrounded by low lying topography will be evaluated in light of these new dates.

Although the TCG and Renova Formation have been mapped as a single unit, sandstone petrology indicates that the Teddy Creek and Renova units are distinct mineralogically, texturally, and stratigraphically. The TCG consists of a basal pebble-conglomerate with distinctive black chert fragments overlain by interlayered sandstone and white-purple volcanic ash. Petrographic analysis reveals sandstones are compositionally mature with high abundances of quartz and minor amounts of feldspar, muscovite, and biotite mica. Most grains and lithic clasts are subrounded; however, monocrystalline quartz grains are commonly subangular. The TCG has experienced an episode of post-deposition silicification. In contrast, the Renova Formation consists mainly of poorly cemented tuffaceous sandstones containing abundant muscovite and biotite micas. The presence of the micas has been attributed to the unroofing of the Idaho batholiths located approximately 100 km to the west. However, the general low abundance of detrital micas in the TCG sandstones suggests a different source than the Renova Formation.

⁴⁰Ar/³⁹Ar results from the TCG yielded Cretaceous to Eocene dates. The youngest dates suggest deposition during Late Eocene time. ⁴⁰Ar/³⁹Ar dates from Renova sandstones revealed a larger age distribution, ranging from ~30 Ma to 1550 Ma. The lack of ages correlative with the Idaho Batholith suggests it did not provide a significant component of detrital material. Rather, the data support a multi-source model in which crystalline basement was exposed and contributed detrital material, at least during Renova deposition.