Skip to Main Content

The Upper Proterozoic Pocatello Formation, southeastern Idaho, contains heterogeneous clastic and carbonate strata (Scout Mountain Member), volcanic and volcaniclastic rocks (Bannock Volcanic Member), and laminated argillite (upper member). These rocks are interpreted to have been deposited during a period of regional glaciation and local volcanism during initial formation of the Cordilleran miogeocline. They are presently exposed in two bands of outcrop, as allochthons, in the northern Bannock Range south and east of Pocatello and the southern Bannock Range north and west of Preston.

Strata of the Scout Mountain Member are divided into lithofacies interpreted to have been deposited in a variety of shallow to deep marine sedimentary environments. Massive diamictite containing rare striated clasts is interpreted as subaqueous lodgement tillite or flow tillite. Interbedded graded sandstones and diamictites are interpreted as turbidites and deep water mass flow deposits. Thick-bedded cobble conglomerate is interpreted as a shallow to deep marine channel-fill deposit. Clast assemblages in these coarse strata indicate that both extrabasinal quartzitic and granitic terranes and intrabasinal volcanic and siltstone terranes contributed sediment. Sandy and silty facies, locally with thin carbonate interbeds, are interpreted as intertidal to subtidal shallow marine deposits.

Syn-sedimentary tectonism is suggested by local abrupt facies changes, the presence of interbedded volcanics, and the abundance of clasts in diamictite interpreted to have been derived from exposed and uplifted portions of the Pocatello Formation. Strata exposed in the southern Bannock Range are interpreted to have been deposited in a northward-deepening sub-basin of the early Cordilleran miogeocline. A separate, northward-deepening sub-basin, containing glaciated volcanic islands at the north end, is inferred for deposition of strata now exposed in the northern Bannock Range. An alternate explanation for the disparity in facies between the northern and southern Bannock Range involves differential tectonic transport along previously undetected tear faults between the two areas during the Mesozoic Sevier Orogeny.

You do not currently have access to this chapter.

Figures & Tables





Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal