The Cambrian System in the Bridger Range of south-central Montana is represented by the Sauk Sequence, transgressive-regressive package of fine-grained clastic and carbonate rocks. The sequence includes the transgressive Flathead Sandstone, followed by three shale-limestone couplets. The tectonic framework, which strongly influenced Cambrian and later Paleozoic sedimentation, was inherited from Precambrian time. The three couplets may be the products of sea level fluctuation and gentle tectonism along Precambrian structural elements.

In south-central Montana, the Park Shale is micaceous shale with siltstone at the base and limestone at the top. In the northern Bridger Range, however, the lower 30 m is interbedded arkosic sandstone and shale. The arkosic sandstone has two major facies, one rich in quartz and orthoclase and the other in arkose, glauconite, and grainstone intraclasts. It is interpreted to have been deposited in a shallow water, near shore, point source (island) environment. Variations in depositional energy and tectonic stability resulted in the two major facies.

The occurrence of Park sandstone beds containing grains of orthoclase and plagioclase and gneissic quartzo-feldspathic pebbles requires the presence of (1) a localized island of Precambrian crystalline rock, which was an erosional remnant that was exposed above the depositional interface through most of the Middle Cambrian, or (2) an island of Precambrian crystalline rock that was exposed by late Middle Cambrian tectonism.

The paucity of conspicuous feldspar grains and quartzo-feldspathic pebbles in the upper Wolsey Shale and Meagher Limestone, and the abundance of basement generated grains in the Park arkosic interval favor the second hypothesis. Stratigraphic and paleocurrent data do not define a source area proximal to the northern Bridger Range. Mineralogy of the arkosic sandstones suggests that the source terrain was located in the basement-cored Precambrian Dillon block.

The northern margin of the Dillon block is defined by the Willow Creek fault zone, which bisects the Bridger Range into northern and southern segments. Rocks north of the fault, which include the Park arkose, appear to be allochthonous. Thus, the original location of the source and depositional environment of the arkosic interval may be to the south and west of the present-day Bridger Range. This may help explain the lack of a readily apparent source area. If a source area for the Park arkose can be defined in the Dillon Block, these sandstones may prove useful in refining estimates of displacement along the Willow Creek fault zone.