Upper Cambrian feldspathic sandstones deposited across southeast Idaho, Montana, and Wyoming (USA) during the Sauk II-Sauk III regression boundary contain distinctive 500–490 Ma detrital zircon grains, derived from Late Cambrian plutons in the Lemhi arch of east-central Idaho. The Worm Creek Quartzite Member of the St. Charles Formation in the Paris plate of the southeast Idaho thrust belt contains as much as 320 m of feldspathic fine-grained sandstone within a thick section of carbonate rocks. The near-unimodal age of hundreds of detrital zircons from 8 samples of the Worm Creek is 497 Ma. This age and the initial εHf values from these detrital zircons (εHf of -8.0 ± 1.9 to 5.4 ± 1.2) overlap the age and isotopic composition of the Deep Creek and Beaverhead plutons intruded into the Lemhi arch (εHf of -6.3 ± 1.1 to 2.7 ± 1.4). This suggests rapid unroofing of the hypabyssal alkalic plutons, which were the primary source for the sandstones. In the plutons, intermediate initial εHf values are neither juvenile nor evolved, suggesting mixing with a Mesoproterozoic component. A 493–488 Ma detrital zircon age peak is also found in Upper Cambrian sandstones (from the Sauk II-III boundary) in the Wind River Canyon on the Wyoming craton, the Melrose area of the southwest Montana thrust belt, and the Leaton Gulch area of the central Idaho thrust belt. The detrital zircon signatures of these Upper Cambrian rocks is markedly different from that of the Lower Cambrian upper Brigham Group in southeast Idaho and the Middle Cambrian Flathead Sandstone at Teton Pass, Wyoming (1790 Ma age peak). The overlying Middle Ordovician Swan Peak and Kinnikinic Quartzites from Idaho south to Nevada contain a different detrital zircon age population, with almost all grains older than 1800 Ma and a peak at 1860 Ma.
We suggest that the Lemhi arch is a relatively unextended crustal block coincident with the northwest-trending Mesoproterozoic Lemhi subbasin of the Belt Supergroup and with ca. 1.37 Ga mafic magmatism. This magmatism strengthened the lower crust and predisposed the Lemhi arch to remain intact during extension and Neoproterozoic rifting of western Laurentia. Oblique normal faulting and subsidence along the dextral normal Snake River transfer fault produced the Late Cambrian Worm Creek basin and juxtaposed active Cambrian magmatism and exhumation with passive-margin sedimentation to the south.