NEOPROTEROZOIC TO CAMBRIAN PASSIVE MARGIN? EVIDENCE FOR ACTIVE TECTONISM DURING THE SAUK TRANSGRESSION IN CENTRAL IDAHO AND MONTANA
North of the Helena Salient of the Montana fold and thrust belt, near Lewis and Clark Pass, the Flathead Sandstone unconformably overlies both the Ravalli (lower Belt) and Missoula (upper Belt) groups. This relationship requires substantial pre-middle Cambrian uplift (>5 km) and erosion of the Belt Supergroup. South of the Helena Salient, near Ermont, MT, boulder fanglomerate within the Flathead Sandstone where it unconformably overlies probable pre-Belt “Argenta” strata suggests structurally-controlled sedimentation. In central Idaho, where Cambrian strata are absent, rapid uplift and erosion of Neoproterozoic plutons yields 500 Ma zircon within the ca. 500 Ma Worm Creek quartzite of SE Idaho.
In contrast, within the Helena Salient area, the mapped contact between coarse-grained Flathead Sandstone and underlying fine-grained strata of the Ravalli Group appears to be a conformable transition suggesting the putative Spokane Formation is in fact Neoproterozoic to Middle Cambrian strata conformably below the Flathead Sandstone. This relationship is supported by the intercalation of two distinct zircon populations.
The highly variable nature of the basal Cambrian contact (locally an angular unconformity, a disconformity about crystalline basement, and an apparent conformable contract), the coarse grained nature of basal Cambrian strata, and evidence for uplift of both Belt Supergroup strata (>5 km) and Neoproterozoic plutonic rocks suggests the continental margin was tectonically active in Neoproterozoic to Cambrian time, in markedly contrast to the traditional interpretation of a passive margin setting.