DETRITAL ZIRCON EVIDENCE REQUIRES REVISION OF BELT STRATIGRAPHY IN SOUTHWESTERN MONTANA

In the Helena Embayment of southwest Montana, thin-bedded, fine-grained sandstone, siltstone and shale of the lower Belt Supergroup (Spokane and Empire Fms.) are overlain by the Middle Cambrian Flathead Sandstone, a prominent, cross-stratified medium to coarse grained quartz arenite that stands in bold relief above the recessive Belt rocks. The contact between these two packages is mapped throughout the region as a significant disconformity. However, recent mapping suggests the contact within the Helena Embayment is actually a conformable, coarsening upward gradational transition, suggesting that rocks mapped as Belt Supergroup may represent a previously unrecognized younger stratigraphic package.

Provenance analysis demonstrates that the two units have very different detrital zircon populations: the Spokane Shale (n=4) contains a distinct detrital zircon population of 1.45-1.60 Ga, which corresponds to the North American magmatic gap. This peak is absent in the overlying Flathead (n=5) which yields distinct peaks at 1.7-1.8 Ga, 2.6-2.7 Ga and 2.9-3.1 Ga, and suggests derivation from the Wyoming craton, or cannibalization of older Belt Supergroup (Greyson and LaHood) strata. It is important to note that neither package contains zircons younger than 1.4 Ga, which differs significantly from Neoproterozoic and Cambrian strata along the continental margin.

In the Big Belt Mountains, a thick succession of recessive red siltstone and shale typical of the Spokane Formation contains thin beds of coarse-grained, ripple marked, quartz arenite that is very similar to the overlying cross-stratified quartz arenite of the Flathead sandstone. These red strata yield a zircon population typical of the Spokane Formation, whereas both the intercalated and the overlying quartz arenite yield a typical Flathead detrital zircon signature. The interfingering of these two distinct zircon populations, combined with the coarsening upward nature of the succession and the apparently conformable, gradational contact, strongly suggests that some rocks mapped as the Spokane Formation are conformable with overlying Middle Cambrian strata, and are not part of the Middle Proterozoic Belt Supergroup. Preliminary ichnologic which includes the documentation of putative feeding structures not assignable to presently known ichnogenera suggests these strata may be Late Neoproterozoic.