The Cambrian System in the Bridger Range, south-central Montana, is represented by the Sauk sequence, a transgressive-regressive package of fine-grained clastic sand carbonates. In ascending order, the sequence consists of the Flathead Sandstone, Wolsey Shale, Meagher Limestone, Park Shale, Pilgrim Limestone, and Snowy Range Formation.

Regionally, the Park is a green, micaceous shale with interbedded siltstone at the base and intercalated limestone at the top. However, in the northern Bridger Range, the lower 30 m (98 ft) is a prominent interval of interbedded arkosic sandstone and shale. A quartz- and/or orthoclase-rich facies and a biomicritic, arkosic, glauconite-rich facies comprise this interval. Individual sandstone beds, 5-17 cm (2-7 in.) thick, are characterized by sharp contacts, scoured surfaces, load structures, and weakly developed cross-stratification. Gneissic, quartzofeldspathic pebbles and biomicritic cobbles occur in sharp contrast to adjacent shales. The arkosic sandstones were deposited in a nearshore island environment adjacent to an areally restricted source of clastic detritus. Variations in en ironmental energy regime and tectonic stability resulted in the two facies.

The abundance of basement-generated grains in the basal Park Shale, their absence in the upper Wolsey Shale and Meagher Limestone, and the localized occurrence of arkose indicate late Middle Cambrian tectonism and exposure of Precambrian crystalline basement. Subsequent weathering resulted in an easily erodable source of coarse-grained clastics.

Due to its unique mineralogy and stratigraphic setting, the Park arkosic interval may provide a key stratigraphic marker for refining estimates of displacement along Laramide structures within the study area.

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