Abstract
Abstract The taphonomy of a newly discovered petrified forest in the Two Medicine Formation, south of Choteau, Montana, provides new constraints on the pre-thrusting position of the Boulder batholith and Elkhorn Mountains Volcanics. The petrified forest consists of more than 200 charcoalified, subaligned trees encased in an ash flow tuff and an immediately overlying bentonite. The ash flow tuff has been dated at 80.002± 0.114Ma (39Ar/40Ar on plagioclase). The ash flow and bentonite are interpreted to record a single catastrophic eruptive event that toppled and entombed the forest. The mean azimuth of the prostrate trunks, which are interpreted as the direction of travel of the ash flow, is 037° (2σ = 35°). Based upon age, mineralogy, and the absence of other contemporaneous volcanic centers, it is concluded that the Late Cretaceous Elkhorn Mountains Volcanics were the most likely source of the tuff and bentonite that encase the trees. The mean azimuth of the tree trunks is consistent with palinspastic res...
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