Abstract

In and near the Bridger Range area north and east of Bozeman, Montana, in Gallatin County, about 27,000 feet of sedimentary rocks, from Beltian to Recent, is exposed. Strata of known Ordovician, Silurian, Permian, and Triassic age are not present. Middle Cambrian Flathead sandstone is underlain by coarse Beltian arkoses in the northern part of the area and by Archean (?) metamorphics in the southern part. The depositional edge of the Belt arkoses has been fault-controlled. The Sappington formation (Berry, 1943) is separated from the underlying Three Forks formation by a minor disconformity. The Big Snowy group is recognized in the Bridger Range for the first time. The problem of the Livingston formation is reviewed, and it is apparent that the andesitic phases of these Montanan, Lancian, and Paleocene sediments can be used to interpret the tectonic history of the Crazy Mountain basin and adjacent mountain areas.

Analysis of the structural features of the Bridger Range and adjacent areas indicates two phases of folding and faulting. One developed generally north-trending structures in mid-Paleocene time, and the other developed north-northwest trending structures probably in latest Paleocene or early Eocene.

In Oligocene time isostatic arching produced normal faults on the west side of the Bridger Range. The range was lifted relative to the Gallatin Valley, and the east-flowing drainage was impeded so that sediments accumulated in the basins. These movements and deposition have continued sporadically to the present.

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