Named as a member of Plateau Rhyolite for exposures on Obsidian Creek (its type area) northwest Yellowstone National Park, WY in Yellowstone province. Is comprised of a series of isolated rhyolitic lava flows and domes wholly outside the Yellowstone caldera. Is therefore difficult to define in traditional stratigraphic terms. Plagioclase is a common phenocryst type. The following are included in the member: mixed lavas of Gardner River and Grizzly Lake; the Willow Park, Apollinaria Spring, Landmark, Gibbon Hill, Geyser Creek, and Pointpot eruptive domes. Considered younger than Mallard Lake Member (new) of Plateau. No radiometric determinations have been made. Thought to be younger than Lava Creek Tuff (new) of Yellowstone Group (rank raised). Is part of the third volcanic cycle of the Yellowstone Plateau.

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