HOW DO WE KNOW IT'S A SABKHA?

One of the more difficult problems we face when analyzing environments of deposition is knowing which factors are the key indicators of a particular environment. The sabkha environment, because it is primarily a subaerial one, may lose many of its prime indicators through erosion and dissolution. As a consequence, we must use more subtle indicators.

I will use the Middle Cambrian Blacksmith Formation of northern Utah and southern Idaho as an example. Wide spread dolomitization that characterizes the Blacksmith Formation in northern Utah, but the degree of dolomitization decreases to the north into Idaho. Several different processes of dolomitization, including burial dolomitization, fault-controlled dolomitization, mixing zone dolomitization, and seepage reflux dolomitization, have been proposed as the origin for this dolomite. Chemical analysis of the dolomite and coexisting calcite shows that the dolomite contains two to three times the amount of sodium than the calcite, suggesting that seepage reflux is probably a major contributor to the dolomitization. That result also implies that there was an area in which significant evaporation could occur.

A tidal flat/supratidal facies was identified in the Blacksmith Formation. Is it simply a tidal flat/supratidal facies, or is it a sabkha? The facies is characterized by an abundance of mudcracked, crytpalgalaminated mudstones that contain no macrofossils or bioturbation. This facies also has the largest amounts of insoluble residue in all 16 sections that were measured for this study. Insoluble residue is dominated by quartz, and microcline, both which are commonly authigenic, and illite, which is identified by x-ray diffraction and is probably detrital. In addition, there are halite crystal casts, replaced gypsum crystals, and solution breccias both within and below the facies. All of these suggest that the facies is a sabkha.