Petrology of the Middle Cambrian Blacksmith Formation, Southeastern Idaho and Northernmost Utah

The Blacksmith Formation of Middle Cambrian (Albertan) age was studied in southeastern Idaho and northernmost Utah. Lithology and sedimentary structures were compared with modern environments and ancient rocks to determine environments of deposition, paleo-geography, diagenetic alterations and patterns of dolomitization. The Blacksmith Formation can be divided into five basic rock types. Rock type A has cryptalgalaminae, mudcracks, and lacks bioturbation. Rock type A is inferred to have been deposited in the upper intertidal to supratidal environment. Rock type B is extremely dolomitized and brecciated. It is inferred that rock type B was deposited in the upper intertidal to supratidal environment. Rock type C may have cryptalgal mats, stromatolites consisting of laterally linked hemispheroids, birdseye structures, vertical burrows and pseudomorphs after evaporite minerals. Rock type C is inferred to have been deposited in the lower intertidal environment. Rock type D is fine grained, often dark gray in color, has trails and often contains fossil fragments. Rock type D is inferred to have been deposited in subtidal-lagoon or open-platform environments. Rock type E is ooid-rich, and is often cross-stratified. Rock type E is inferred to have been deposited in an agitated-shoal or quiet-water shoal environment. All rocks of the Blacksmith Formation were deposited in supratidal to shallow subtidal environments. During Albertan time the study area was located in the tropics, and the adjacent area had little relief. Clay mineralogy of the insoluble residues suggests a relatively humid paleoclimate. Tidal amplitude was probably low, as suggested by small algal domes, LLH strornatolites, and cryptalgal mats. Water depth varied throughout the area. Less restricted fauna to the north suggest that water depth increased to the north. It is inferred that a transgression from the west, regression to the west, a second transgression, and possibly a second regression controlled the distribution of facies. Early diagenesis included minor compaction of intertidal or very shallow subtidal deposits, whereas deeper subtidal deposits may have undergone more compaction. Cementation occurred early in the inter-tidal or shallow subtidal environment. Dolomitization of the Blacksmith by a hypersaline brine is suggested by pseudomorphs after evaporites, authigenic quartz, desiccation features and cloudy dolomite rhombs. Chemical analyses for sodium also indicate a hypersaline fluid. The association of some of the dolomite with the oolite-shoal environment suggests that the dolomite distribution may be in part facies-controlled. The hypersaline brine likely developed on tidal flats south of the area, and percolated through the sediments via refluxion and through the permeable sediments via hydrostatic head. The amount of dolomite decreases to the north, farther from the source of the brine. As the dolomitizing brine moved downward, the Mg/Ca ratio was lowered so that a ferroan dolomite formed in the subsurface, under reducing conditions. Late diagenetic events include aggrading neomorphism by low-Mg calcite which may obliterate grains and cement or preserve grains and episodes of cementation. Late dolomitization, producing coarse dolomite rhombs exhibiting undulose extinction and curved crystal faces may have been epigenetic in origin