SILICICLASTIC-CARBONATE CYCLES OF THE NEOPROTEROZOIC BLACKROCK CANYON LIMESTONE, SOUTHEASTERN IDAHO

The Neoproterozoic Blackrock Canyon Limestone, southeastern Idaho, is positioned above the siliciclastic-dominated, fining-upward Pocatello Formation and below the siliciclastic-dominated, coarsening-upward Brigham Group. In the type area, the Blackrock Canyon “Limestone” is 174 m thick. Siltstones and sandstones dominate the formation while cyclic carbonates comprise only ~25 percent. About 20 km south along strike, on the south slope of Scout Mountain, the formation contains only a few meters of thin dark limestone.

The Blackrock Canyon type exposures contain siliciclastics and carbonates, arranged in five cycles; each cycle is composed of a siliciclastic lower half and a carbonate upper half (these could be considered “grand cycles” but most likely represent shallowing upward parasequences in a larger sequence stratigraphic framework). The siltstones of the siliciclastic half cycles are identical to those in the overlying Papoose Creek Formation. They contain unusual shrinkage cracks in thinly interbedded, parallel-laminated, siltstone and fine-grained sandstone.

The carbonate units are predominantly laminated mudstones to packstones in the lower three cycles and cross-bedded oolitic grainstones in the upper two cycles. The lower carbonate units also contain poorly defined stromatolitic and/or thrombolitic bioherms on the order of a few meters in width and height. Dolomitized, irregular exposure surfaces are present in the uppermost carbonate units. Each carbonate half-cycle represents shallower deposition than in the preceding cycle. Therefore, the carbonate units record a forestepping pattern and are interpreted to represent the initiation of highstand deposition.

The localization of these thick parasequences suggests a tectonically controlled south-facing hinge, where the Proterozoic basin shallowed to the north on the flanks of the Bannock Volcanic eruptive center.