SEDIMENTARY PETROLOGY OF THE TEDDY CREEK GROUP, BLACKTAIL MOUNTAINS, MT: IMPLICATIONS FOR PROVENANCE & LANDSCAPE EVOLUTION

A previously undocumented sequence of sedimentary rocks has been identified within SW Montana and informally termed the Teddy Creek Group (TCG). This project focuses on the field relations, mineral compositions, and textures of these rocks to determine their origin and relationship to the regionally extensive Renova Formation.

The TCG consists of a basal pebble-conglomerate with distinctive black chert clasts overlain by sandstones interlayered with white-purple volcanic ash. All lithologies have experience an episode of post-deposition silicification. Petrographic analysis reveals sandstones are compositionally mature with high abundances of quartz and minor amounts of feldspar and muscovite mica. Most grains and lithic clasts are subrounded-rounded, however monocrystalline quartz grains are subangular-angular. The TCG also preserve cross-bedded laminations indicating a fluvial depositional environment as well as fragments of petrified wood.

In contrast, the Renova Formation consists mainly of tuffaceous sandstones containing abundant muscovite and biotite micas. The presence of the micas is attributed to the unroofing of the Idaho batholith. However, the lack of micas, especially biotite, in the TCG suggests a different source than the Renova Formation.