Abstract An ~90-m-thick interval of mixed siliciclastic-carbonate strata, including a cap dolostone unit, overlies diamictite of the upper Scout Mountain Member of the Pocatello Formation in the Fort Hall Mine area south of Portneuf Narrows, southeastern Idaho, and is ≤ ca. 665 Ma. Six facies comprise this interval: silty sandstone (reworked diamictite matrix), laminated dolomite, dolomite and sandstone, sandstone, dolomite-chip breccia, and argillite and limestone. Sedimentary structures and bedding geometries of facies indicate paleoenvironments ranging from below storm wave base to upper shoreface. The edgewise, mounded, and parallel-bedded dolomite-chip breccia indicates slope failure and reworking of the lower shoreface during large storms. Facies relationships allow generalized division of these strata into three units. The lowermost unit, Unit A, comprises intimately interbedded laminated dolomite (“cap dolostone”), dolomite-chip breccia, and sandstone facies and is 17 m thick. Unit A apparently grades upward into Unit B, a 45-m-thick interval of the sandstone facies. Unit C, 28 m thick, rests sharply on Unit B, and comprises a basal laminated dolomite facies and the limestone and argillite facies. Units A and B may indicate a regressive wave-dominated coast that was influenced by large storms (highstand systems tract). Unit C indicates near storm wave base deposition and an overall deepening, as shown by dark argillite beds of the overlying upper member of the Pocatello Formation (transgressive systems tract). δ13C and δ18O values from dolomite and limestone samples of Units A and C are similar to values from local, regional, and transglobal cap carbonate intervals. δ13C values range from −1.9 to −5.6‰ and δ18O values range from −10.2 to −17.4‰, with no systematic correlation between C- and O-isotope values. δ13C values are consistent with previously reported values from the Pocatello Formation and are similar to values from the alleged Marinoan Noonday Formation in Death Valley, California, and the Marinoan Maieberg Formation in Namibia. Collective data from the cap dolostone and associated strata of the Pocatello Formation suggest protracted mixed siliciclastic-carbonate deposition on a stormdominated shelf at ca. 665 Ma.

Abstract This one-day field trip examines two Neoproterozoic sections in the Portneuf Narrows area, SE of Pocatello, Idaho. Rocks to be examined on the first traverse belong to the Scout Mountain Member, Pocatello Formation, and include <710 Ma glacial diamictites, dolomite cap, 667 Ma tuff, and upper caplike carbonate. The second traverse, in Blackrock Canyon, exposes the cyclic Blackrock Canyon Limestone, with upward-shallowing siliciclastic to carbonate cycles and microbial mounds.