REEXAMINING THE DEPOSITIONAL SETTING OF THE KANOSH FORMATION (MIDDLE ORDOVICIAN, UTAH): INTEGRATING GEOCHEMISTRY INTO A SEDIMENTARY AND PALEOECOLOGICAL STUDY

The Great Ordovician Biodiversification Event (G.O.B.E.) was the most significant radiation of multicellular life in the Paleozoic. Within approximately 25 million years, marine animal life diversified threefold at the family level, leading to the emergence of the Paleozoic Evolutionary Fauna (Sepkoski, 1979). By the end of the G.O.B.E., family-level diversity had reached a plateau that persisted, with minor fluctuations, until the End-Permian mass extinction (Droser and Finnegan, 2003).

The Middle Ordovician (Whiterockian) Kanosh Formation was deposited during the G.O.B.E. and is well-exposed near Ibex, western Utah. The Kanosh Formation consists of interbedded carbonates and shales with minor sandstones and was deposited in a semi-restricted basin within a tropical carbonate shelf (Ross et al., 1989). The depositional setting of the Kanosh Formation has been interpreted as dysoxic to anoxic based on the presence of well-preserved graptolites and absence of bioturbation in the thick shale-dominated intervals (Ross et al., 1989). Ross and colleagues (1989) interpreted the thinner carbonate units as storm-transported material derived from outside the intra-shelf basin. Subsequently, Wilson et al. (1992) demonstrated that a “black shale” interpretation for the Kanosh Formation is precluded by the presence of an in-situ benthic community preserved on hardground surfaces. In this study, we combined stratigraphic, sedimentological, geochemical, and paleoecological approaches to examine the depositional setting of the Kanosh Formation at Fossil Mountain, western Utah, within the context of the G.O.B.E. Our results support the conclusion of Wilson et al. (1992) that the Kanosh Formation was deposited under oxic conditions. Geochemical analyses performed on carbonate units in the Kanosh Formation revealed low weight percentages of organic carbon and total sulfur, which do not support anoxic deposition, and rare black-colored shales were observed during our field investigation. We propose, in addition, that deposition occurred in shallower water than has been suggested. Imbricated fossils and cross-bedding are found throughout the section but not always in association with storm deposition, indicating that moderate- to high-energy conditions may have persisted beyond isolated storm events.