Trilobites from thrombolitic reefs in the Ore Hill Member of the Upper Cambrian Gatesburg Formation are assigned to (in ascending order) the Pseudosaratogia magna. Cliffia lataegenae, and Irvingella major Subzones of the Elvinia Zone. These reef faunas contrast markedly with those from coeval off-reef grainstones (tempestites), representing separate biofacies rather than biostratinomic variants. Associated lithologies suggest that the fauna of the P. Magna Subzone at the base of the member comprises a biofacies associated with shallower microbial reef environments than those occupied by faunas in the overlying C. lataegenae Subzone. Reefs in the C. lataegenae Subzone yield a fauna dominated by Buttsia drabensis, extending the documented range of catillicephalid abundance in microbial reef biofacies downward into the highest strata of the Steptoean Stage. This reef fauna also includes several new taxa, including Cheilocephalus quadratus new species (described herein), an atypical species of Cheilocephalus with catillicephalid-like cranidial features. The fauna in off-reef tempestites low in the C. lataegenae Subzone contains the genera that characterize the Xenocheilus-Kindbladia Biofacies, expanding the known environmental and geographic range of that biofacies. Sulcocephalus, typically a minor component of faunas in the I. major Subzone, dominates the fauna in reefs within this subzone such that a distinct Sulcocephalus Biofacies can be recognized immediately above the extinction horizon that defines the base of the Sunwaptan Stage.