Four Middle Cambrian lagerstättten occur in the Great Basin; each contains exceptionally preserved soft-bodied fossils in strata deposited under anoxic conditions that precluded a benthic fauna. A microstratigraphic approach reveals that the four units that house the soft-bodied faunas (Spence Shale, "deep" Wheeler Shale, "shallow" Wheeler Shale, Marjum Formation (Robison 1991)) additionally contain mudstones deposited under more oxic bottom waters. During transitions from typical soft-bodied biofacies, which are characterized by allochthonous fossils, an absolute lack of bioturbation and abundant authigenic pyrite, to well-oxygenated biofacies characterized by moderate levels of bioturbation indicating an in-situ benthic fauna, intermediate communities dominated by trilobites occur; the best example is the Elrathia biofacies of the "deep" Wheeler Shale. The strata which contain these communities show clear evidence of oxygen-deficient bottom waters: very low levels of bioturbation relative to overlying strata and common authigenic pyrite. Evidence that these communities inhabited oxygen-poor environments and are not allochthonous is provided by size distribution and orientation data which demonstrate no influence of sorting or current alignment. Levels of bioturbation within this biofacies range from ichnofabric index (ii) 1 to ii 2. Communities contained within ii 1 strata commonly contain unusually abundant, monospecific trilobite faunas, often known only from a single locality. Communities contained within ii 2 strata exhibit reduced abundances, and commonly consist of multiple trilobite taxa. All trilobite-bearing strata share a common taphonomic mode whereby carapaces are filled with fine dolomite crystals and preserved in three dimensions, demonstrating that the observed pattern is not taphonomically generated. Thus, all available evidence constrains these monospecific "blooms" to dysaerobic settings. These factors suggest the presence of a Middle Cambrian oxygen-deficient biofacies, in which low-diversity trilobite communities lived near the anaerobic-dysaerobic boundary, perhaps utilizing microbial communities that are well known to thrive at this interface.