MESOPROTEROZOIC STRATA SOUTHWEST OF SALMON IDAHO

We present a progress report on several years of reconnaissance stratigraphic examinations of Mesoproterozoic strata in the Yellowjacket and Cobalt areas southwest of Salmon, Idaho. We suggest that the Yellowjacket Creek to Blackbird Mountain area contains strata correlative with the Ravalli Group and Piegan Group (middle Belt carbonate) of the Belt Supergroup. Significant questions remain as to degree of structural repetition, nature of specific contacts, and assignment of various outcrop areas to stratigraphic units. In its type area, the Yellowjacket Formation consists of thin to medium beds of flat-laminated and climbing ripple cross-laminated sandstone interstratified with locally desiccation-cracked and rippled mudstone beds containing scattered mudchips (Burke correlative). These rocks probably record episodic floods across periodically dried muddy sandflats, that were distal to alluvial fans. Tabular flat-laminated, planar and trough crossbedded fine- to coarse-grained feldspathic quartz sandstone of the overlying Hoodoo Formation records a large prograding and retreating alluvial fan complex (Revett correlative). The upward transition to small antidunes grading to current ripples and desiccation-cracked muddy layers above the Hoodoo (argillaceous quartzite of Ekren, 1988) probably records the return of sandflat environments, somewhat muddier than those of the type Yellowjacket. This repetition of facies has contributed to the problem of separating the type Yellowjacket below the Hoodoo with the miscorrelated "Cobalt Yellowjacket" above the Hoodoo. Along Iron Creek, south of Salmon, strata mapped as Apple Creek Formation (Ekren, 1988) contain flat-laminated sandstone correlative with the Hoodoo Quartzite overlain by hummocky siltite correlative with the Cobalt Yellowjacket Formation. To the west of the Middle Fork of the Salmon River Belt rocks are discordantly overlain by Windermere Supergroup in roof pendants of the Idaho batholith (Lund, 1999).