Detrital Zircon Evidence Requires Revision of Belt Stratigraphy in Southwest Montana.

Abstract

The Belt Supergroup was originally named for widespread exposures of thin-bedded red clastic strata in the Big Belt Mountains in southwestern Montana. Subsequent studies extended the geographic extent and thickness of the Belt Supergroup throughout Montana, Idaho and British Columbia. The nonfossiliferous character of these strata necessitated purely lithostratigraphic correlations. The advent of detrital zircon analyses has provided a method for more rigorous evaluation of proposed correlations. In southwest Montana, the Belt Supergroup consists primarily of thin-bedded, fine-grained sandstone, siltstone and shale of the Spokane, Empire and Greyson Formations. These rocks are overlain by the Middle Cambrian Flathead Sandstone, which is a prominent, cross-stratified medium to coarse grained quartz arenite that stands in bold relief to the underlying recessive Belt rocks. The contact between these two packages is mapped throughout southwest Montana as a profound unconformity, but recent mapping suggests the contact is actually a conformable, coarsening upward gradational transition.