Name Blacktail Deer Creek basalts applied to several basalt flows associated with the Blacktail Deer Creek formation of early Miocene age. The basalts are mapped at northeast edge of map (geologic map) in Beaverhead and Madison Cos, MT in the Montana folded belt province, northeast of Blacktail Deer Creek. They have a dark, locally vesicular groundmass with phenocrysts of feldspar. Small crystals of augite seen in thin section. Similar basalts occur elsewhere in mapped area as small remnants attesting to a greater extent in the Blacktail, Ruby, and Snowcrest Ranges. Assigned a Miocene age; younger than Medicine Lodge volcanics and older than Edie School rhyolites.

Source: GNU records (USGS DDS-6; Denver GNULEX).