Abstract

The northwest part of the Three Forks sheet, mapped by Peale (1896), has been remapped. The Pebbly limestones (Cambrian) of Peale's section could not be identified. The contact of the Dry Creek shales with the overlying Jefferson limestone represents a hiatus involving the Ordovician, Silurian, and Lower Devonian. The Jefferson is correlated with the Devils Gate of Nevada; upper Middle Devonian age is indicated in the lower beds by Spirifer cf. S. engelmanni (Spirifer argentarius zone, Devils Gate), and the base of the Upper Devonian is marked by the occurrence of Phillipsastraea, 175 feet above the base of the formation. Sixty feet of yellow sandstone overlying the Cyrtospirifer zone of the Three Forks shale (Upper Devonian), and previously included in that formation, contains a Lower Carboniferous Syringothyris fauna (Kinderhook) and is lithologically a recognizable unit. The name Sappington sandstone is here proposed. The Belt is not present in the southern part of the area but is represented by 4000 feet of conglomerate, arkosic sandstone, and micaceous shale in the northern part. A relatively steep shore line is indicated, and the section probably is the near-shore facies of one or more of the formations distinguishable farther north. The Belt shore line is marked by an east-west zone of Laramide high-angle thrusting, including the Jefferson Canyon fault, with an apparent throw of approximately 12,000 feet. The faulting was probably controlled by the declivity in the basement surface, which in turn was localized by a zone of weakness within the basement. North of the Jefferson Canyon fault, the structures conform to those of the Northern Rockies in their orientation and plainsward thrusting. South of it, the trends are west and northwest, and thrusting and overturning are to the southwest. This conforms to the general structural pattern of southern Montana and is not explainable by local resolution of east-northeastward Laramide stresses.

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