FRASNIAN STROMATOPOROID-CORAL REEF ABOVE ALAMO IMPACT STORM-RELATED BACKWASH CHANNEL, NEEDLE RANGE, WESTERN UTAH

A unique reef development is fortuitously displayed in cross section, approximately parallel to the original shoreline, above a channel-fill deposit on the east side of the northern Needle (Mountain Home) Range. This deposit is one of several early Late Devonian (Frasnian) Alamo Impact (punctata conodont Zone, 382 Ma) uprush-backwash deposits mapped by Sandberg and Jared Morrow in 2007. This reef is in a similar position to the only known reef developed directly atop the Alamo Breccia at Mt. Irish in southern Nevada. However, it is older than the Early hassi Zone stromatoporoid reef tract located along strike to the northeast in the Burbank Hills and Confusion Range but absent in the Needle Range. In the Burbank Hills that 60-m-high reef is separated by 66 m of peritidal carbonate beds from the Alamo-related deposit.

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.