The Cambrian System in the Bridger Range of south-central Montana is part of a 450 to 500-m (1,475 to 1,640-ft) thick transgressive-regressive sequence of fine-grained clastic and carbonate rocks. Above the Flathead Sandstone at the base, this sequence is composed of three shale-limestone couplets, possible products of the complex interaction of sea level fluctuations and gentle tectonism along reactivated Precambrian structural elements. In ascending order, these couplets are the Wolsey Shale and Meagher Limestone, the Park Shale and Pilgrim Limestone, and the Dry Creek Shale and Sage Pebble members of the Snowy Range Formation.
In south-central Montana, the Park Shale is 50 m (165 ft) of 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 (100 ft) is a prominent interval of interbedded arkosic sandstone and micaceous shale. Here, thin sandstone beds are characterized by sharp, commonly graded bases, weakly developed cross-stratification, load structures, and a distinctive suite of glauconite, quartz, orthoclase, and plagioclase grains. Quartzofeldspathic gneiss pebbles and biomicrite intraclast pebbles and cobbles occur in these beds, in striking contrast to the fine to medium sand that composes most of the sandstone beds of the interval. These arkosic sandstone beds are localized in the northern Bridger Range and are unknown in the southern Bridgers and in Cambrian outcrops of surrounding areas.
The occurrence of Park sandstone beds that contain orthoclase and plagioclase grains and pebbles of quartzofeldspathic gneiss requires (1) the presence of a localized island of Precambrian crystalline rock, an erosional remnant that must have risen at least 200 m (650 ft) above the surrounding Cambrian/Precambrian erosion surface and 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 reactivation of zones of Precambrian structural weakness. Probably the strongest argument favoring the second alternative is the paucity of conspicuous feldspar grains and gneiss clasts in the middle and upper Wolsey Shale and Meagher Limestone of the Bridger Range. This evidence, coupled with the abundance of these basement-generated grains in the basal part of the overlying Park Shale, strongly suggests that the arkosic interval of the Park is the product of weathering and erosion of a nearby island. This island was tectonically activated during the late Middle Cambrian and was available as a source of coarse clastic sediment principally during that time.
The location, size, and shape of this island are unknown: no abbreviated Cambrian section is known in the area, and the arkose beds show no strong paleocurrent or paleosource directions. However, because the arkose beds are restricted to the northern Bridger Range and because a relatively local source is required for this type of sediment, the sediment-producing island must be close by. The most spatially and lithologically feasible tectonic feature along which late Middle Cambrian movement might have produced an island or series of islands is the Willow Creek-Jefferson Canyon fault zone, along which significant movement occurred during deposition of the LaHood Formation (Precambrian Y); the fault zone structurally divides the northern and southern parts of the Bridger Range, and later Paleozoic movement has been documented along this zone.