FRASNIAN STROMATOPOROID-CORAL REEF ABOVE ALAMO IMPACT STORM-RELATED BACKWASH CHANNEL, NEEDLE RANGE, WESTERN UTAH
The >200-m-wide Needle Range reef comprises three beds in ascending order: (1) bulbous stromatoporoids, 1m; (2) massive core of predominantly disoriented laminar stromatoporoids and solitary rugose corals in an ~50% muddy carbonate matrix, with in situ columnar stromatoporoids in the upper part, 1.8 m; and (3) twig-like Amphipora stromatoporoids,1 m. The reef core has an irregular top laterally but culminates in a 3.8-m-high central dome. The reef thins laterally to less than 2 m. Its top is separated by ~45 m of peritidal carbonate beds from the widespread Orecopia gastropod horizon. We interpret these paleoenvironments from the faunal succession within the reef. The bulbous stromatoporoids of Bed 1 indicate a shallow-subtidal environment close to wave base. The disoriented laminar stromatoporoids and solitary rugose corals of Bed 2 indicate a deepening event characterized by less turbulent and more turbid conditions. The closely packed Amphipora of Bed 3 represent a return to a very shallow subtidal setting.