Redefinition of the Precambrian-Cambrian Contact in Southwestern Montana

The Cambrian Sauk transgression migrated across a deeply eroded cratonal surface, and is recognized by a profound stratigraphic break between Archean or Proterozoic rocks and overlying Cambrian strata. In southwest Montana, this boundary is traditionally defined as the contact between the Cambrian Flathead Sandstone and Middle Proterozoic Belt Supergroup.

The Belt Supergroup in southwest Montana consists primarily of thin-bedded, fine-grained sandstone, siltstone and shale of the Spokane, Empire and Greyson Formations. The Cambrian Flathead sandstone is a distinct medium to coarse grained, cross stratified quartz arenite that forms prominent exposures above the underlying recessive units. Mapping and stratigraphic analysis suggests a conformable contact between rocks mapped as Spokane Shale and Flathead Sandstone. The discovery of trace fossils within the Spokane Shale is curious, as coeval burrows have not been reported outside SW Montana. Do they represent an early phase of multicellular evolution? Or do they imply the underlying shale is not Middle Proterozoic, but instead a later transgression immediately preceding the Sauk transgression?

The two units have very different detrital zircon populations: the Spokane Shale contains a major detrital zircon population of 1.45-1.60 Ga, which corresponds to the North American magmatic gap and suggests derivation from a western cratonal source. The overlying Flathead has distinct peaks at 1.7-1.8 Ga, 2.6-2.7 Ga and 2.9-3.1 Ga, suggesting derivation from the Wyoming craton, or cannibalization of older Belt Supergroup strata.

The recognition of a conformable, coarsening upward gradational contact between rocks mapped as Belt Supergroup and Flathead sandstone is problematic. If this contact represents a continuous depositional sequence, 1) underlying rocks have been mis-mapped and are a much younger package of rocks, (Late Proterozoic-Cambrian?), 2) rocks mapped as Flathead Sandstone are actually Middle Proterozoic, 3) the contact is a disconformity, suggesting SW Montana was tectonically inactive for >900 million years (1440-550 Ma).