SUBDIVISIONS OF THE MESOPROTEROZOIC YELLOWJACKET FORMATION AND HOODOO QUARTZITE, SALMON RIVER MOUNTAINS, CENTRAL IDAHO

The thickest exposed unbroken section of Mesoproterozoic Yellowjacket Formation in the type area, east of the Bighorn Crags, Salmon River Mountains, is over 3000 m thick. Six thick units can be identified. The lowest contains evenly laminated siltite with calc-silicate pods, intruded at the base by Eocene granite. It is abruptly overlain by a coarse-grained sandstone bed of unit 2 which passes upward to evenly laminated quartzite with sparse calc-silicate-bearing zones. The 3rd unit is thin-bedded poddy calc-silicate mudrock, and the 4th contains couplets of siltite capped by biotite, with scapolite-chlorite pods. The 5th unit of rippled and mudcracked couplets, and the 6th, of reddish trough cross-bedded and laminated fine sandstone with uncracked silt to clay couplets, correlate with shallow-water, mud-cracked strata of the uppermost type Yellowjacket exposed north of Middle Fork Ranger Station. Overlying unit 6 is fine-to medium-grained cross-bedded feldspathic sandstone of the Hoodoo Quartzite.

About 30 km to the west, near Taylor Ranch on Big Creek, an east-flowing tributary to the Middle Fork Salmon River, three units are distinguished in what had been previously mapped as undivided Yellowjacket Formation and Hoodoo quartzite. The lowest strata (equivalent to the upper part of the type Yellowjacket Formation) are hundreds of meters thick and contain olive green thin-to medium bedded siltite and argillite, and calc-silicate schist. The overlying Hoodoo Quartzite is a cliff-forming heavy-mineral laminated feldspathic arenite. Above the Hoodoo is at least 150 m of medium to coarse arenite and siltite with hummocky couplets, plus gray microlaminated wavy siltite, with local calc-silicate pods at the base. It is equivalent to the argillaceous quartzite of Shovel Creek, and lowest part of the "Cobalt Yellowjacket" of the Panther Creek area.

We correlate the "type" Yellowjacket Formation with the lower Ravalli Group. The Hoodoo Quartzite represents the southern extension of the Revett Formation, and the strata above the Hoodoo are the lowest parts of the "Cobalt Yellowjacket" (Apple Creek Formation) near the Blackbird Mine and Apple Creek Formation. They also correlate with the St. Regis or lowest Wallace formations.