EARLY CENOZOIC VOLCANISM, SEDIMENTATION, AND FAULTING, SOUTHEAST BLACKTAIL MOUNTAINS, MONTANA

A faulted and highly dissected sequence of maroon gneiss-clast rhyolitic flow breccia, crystal-lithic tuffs, and red pebbly mudstone unconformably overlies Archean metamorphic basement rocks in the Price and Teddy Creek drainages of the southeasternmost Blacktail Mountains, MT. We informally designate this previously undifferentiated sequence the Price Creek Volcanics (PCv). The PCv are overlain by pebbly sandstone and conglomerate containing clasts of quartzite, black chert, and granitic gneiss. These basal lithologies are in turn overlain by interbedded grey to purple rhyolitic tuffs, tuffaceous mudstones, and silicified wood-bearing sandstones (informally designated the Teddy Creek Fm-TCfm).

Preliminary mapping of the pebble to boulder sized flow breccia at the base of the PCv suggests it was emplaced on a paleosurface of substantial relief in which Archean metamorphic rocks were exposed. We have yet to identify an unequivocal vent for the PCv. Several dike-like exposures at the front of the Blacktail range near the mouth of the Price Creek valley suggest the volcanics may have erupted near the intersection of the NW-striking, moderately NE-dipping Jake Canyon fault zone and the steeply-dipping, NE-striking, Teddy Creek fault zone.

The age of the PCv is unknown, but the fact that they are cut by NE-striking faults suggests, from regional tectonic arguments, that the PCv predate mid-Miocene Basin and Range extension. We interpret the TCfm as younger than the PCv, but it is sedimentologically distinct from typical Eocene-Miocene Renova Fm. We base this on the abundance of conglomerate with multicolored quartzite clasts. We infer that the quartzite clasts are recycled Proterozoic Belt Supergroup derived from the Cretaceous Beaverhead Formation located ~10 km to the southwest. The TCfm is overlain by the Miocene-Pliocene Six Mile Creek Fm to the east in the upper Blacktail Valley area. We interpret the TCfm as accumulating in an isolated basin that recorded episodic volcanism from a more distal source than the PCv.