| In the Borah Peak horst, central Lost River Range, east-central Idaho, >1.2 km of Neoproterozoic-Cambrian strata crop out to the west of the Lemhi Arch. The Wilbert Formation, comprising the lowermost strata, is divided into four units: (Lower Wilbert: unit one shallow marine cross bedded quartzose sandstone, unit two shallow marine siltstone, unit three purple fluvial sandstone with rip up clasts), and unit four Upper Wilbert, Skolithos-bearing quartz arenite. At South Creek, southern Lemhi Range, the Upper Wilbert is exposed. The Wilbert and overlying strata are litho- and chronostratigraphically correlative with the Brigham Group and overlying strata in SE Idaho. Grenville-age (1200-950 Ma) detrital zircons are present in the Lower Wilbert. Wilbert unit two contains a major ~663 Ma detrital zircon population interpreted to come from proximal 665-650 Ma Big Creek plutons. Grenville-age detrital zircons disappear between Wilbert four and the overlying shaly Tyler Peak Formation, which contains a sharp ~1780 Ma detrital zircon peak. A sandstone in the overlying lower Summerhouse Formation contains a ~500 Ma detrital zircon population interpreted to be derived from proximal Beaverhead plutons. The youngest detrital zircon grains in the Summerhouse are about 475 Ma, suggesting that Beaverhead belt plutonism continued into the Early Ordovician. Key Words: Neoproterozoic, Cambrian, Lemhi arch, Borah Peak horst, Wilbert Formation |