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Taphonomy of a Petrified Forest in the Two Medicine Formation (Campanian), Northwest Montana: Implications for Palinspastic Restoration of the Boulder Batholith and Elkhorn Mountains Volcanics

¹ Department of Geology, University of Montana, Missoula, MT 59812

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° (2 = 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 restorations of the northern Rocky Mountain fold-thrust belt that place the Elkhorn Mountains Volcanics and their plutonic equivalent, the Boulder batholith, 65–110 km northwest of their present position during Campanian time. This restored position is 150–200 km from the petrified forest, making the ash flow one of the farthest-traveled documented flows with enough power to topple mature trees.

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