CARBON ISOTOPIC CHEMOSTRATIGRAPHY OF THE MICROBIALITE-BEARING CAMBRO-ORDOVICIAN NOTCH PEAK FORMATION, LAKESIDE MOUNTAINS, UTAH

The uppermost Notch Peak Formation in the southern Lakeside Mountains (Clark et al., 2017), Utah, consists primarily of dolomitic microbialites with few metazoan fossils. The lack of fossils has led to some uncertainty in the age of the formation, but the rocks are thought to be late Cambrian to early Ordovician in age (e.g., Garcia, 2015; Clark et al., 2017). Here we report novel carbon isotopic data that helps to better constrain the age of the unit.

Samples were collected at a one meter resolution from seventy meters of measured section. Thin section billets were drilled after petrographic analysis to determine the least altered phases. Powders were analyzed for carbon isotopic composition via cavity ring down spectrometry at Bryn Mawr College.

Despite extensive dolomitization, the analysis yielded useful results. The basal ten meters of the measured section exhibited isotopic values around -0.5 permil V-PDB, increasing to a maximum of about 1.5 permil approximately fifty meters upsection. The remaining fifteen meters of the section exhibit a decrease to 0.5 permil. Thus, the isotopic profile of the section is best described as a 2 permil positive excursion. This positive excursion correlates best in both magnitude and absolute values with the basal Stairsian positive excursion (as described in Saltzman et al. 2015) of the Tremadocian stage of the Lower Ordovician. However, the excursion is also similar to the positive excursion at the base of the Skullrockian stage of the uppermost Cambrian. Ongoing work will help to better refine our understanding of the age of this unit.