PETROGRAPHIC INVESTIGATION OF INVERTEBRATE FOSSIL DIVERSITY IN THE CAMBRO-ORDOVICIAN NOTCH PEAK FORMATION, LAKESIDE MOUNTAINS, UTAH

During the Cambro-Ordovician transition, the area now known as the Lakeside Mountains, Utah, was a shallow marine carbonate-dominated environment in which microbialites were abundant. In the Notch Peak Formation at the Lakeside Corral locality, microbialite reefs are interrupted stratigraphically by high-energy packstone beds and bioturbated beds (ichnofabric indices of 3-5 (Garcia, 2015)), all of which have been secondarily dolomitized. A low-diversity fauna consisting primarily of polyplacophorans (Matthevia) and gastropods is evident from outcrop observations (Garcia, 2015). Although the Notch Peak Formation in the Lakeside Mountains has been included in regional mapping projects (e.g., Clark et al., 2017), the paleontology and paleoecology of these exposures has been relatively little-studied. Recent paleoecological work has emphasized the trace fossils, particularly Thalassinoides, that are present in the Notch Peak Formation in the Lakeside Mountains (Garcia, 2015), but no detailed petrographic investigations of fossil diversity have been undertaken.

The goals of our study were to assess the diversity of fossil material in the Lakeside Corral locality of the Notch Peak Formation in the Lakeside Mountains using thin section petrography and to compare our results with the outcrop-scale diversity observed at this locality. Samples were collected at 1-meter intervals throughout the ~70 meters of Notch Peak Formation exposures at the Lakeside Corral locality. Fossils identified in thin section from these samples include fragmentary gastropods, polyplacophorans, trilobites, putative bivalves, putative ostracods, echinoderms, and sponges (spicules). Some of the bioclasts observed in thin section could not be identified because the dolomitization obscured fossil boundaries, resulting in ghost preservation. Overall, the total faunal diversity documented from our thin sections exceeds what has been observed in the Lakeside Mountains outcrops, indicating that the effects of secondary dolomitization at this locality do not pose an insurmountable obstacle to fossil identification.