Abstract: The Middle Member of the Poleta Formation (Cambrian, Stage 3) Esmeralda County, Nevada, was deposited in shallow marine, siliciclastic-dominated settings during a transgressive to highstand cycle in western Laurentia. Carbon isotopic stratigraphy suggests correlation with the Cambrian Arthropod Radiation isotopic Excursion (CARE). The Middle Member of the Poleta Formation includes at least three less than 1 m-thick and discontinuous horizons of exceptional fossil preservation termed the Indian Springs Lagerstatte. Most fossils were collected from the horizons of exceptional preservation and mostly consist of biomineralizing animals such as trilobites, helicoplacoids, hyolithids, and inarticulate brachiopods. Examples of preserved nonbiomineralized animals and anatomy include mantle setae in brachiopods, demosponges, hyolithid gut tracts, trilobite appendages, and body fragments of nonbiomineralizing arthropods. Exceptional fossil preservation was facilitated through preference of durophageous carnivores, obrution deposition, and favorable geochemical conditions during fossil diagenesis after burial.

The early Cambrian Indian Springs biota, western Nevada, USA exhibits Burgess Shale-type preservation of a diverse array of animal phyla, including the earliest definitive echinoderms. It therefore provides a window on animal life during the Cambrian radiation. The objective of this study is to analyze the trace metal paleoredox geochemistry and bioturbation levels of this deposit in order to characterize the paleoenvironmental conditions in which these animals lived and their fossils were preserved. 29 rock samples were collected from outcrops. An additional 20 random samples were collected from the talus for comparison. The samples were geochemically analyzed for trace metal paleoredox indices and bioturbation levels. Additional samples from coeval strata of the Poleta Formation in the White Inyo Mountains, CA that lack good fossil preservation were also geochemically analyzed with the same methodology. Preliminary results indicate that oxic bottom water conditions dominated during deposition of these strata, despite low bioturbation levels. This pattern holds for intervals with Burgess Shale-type preservation and those without. Although ephemeral incursions of low-oxygen waters may have taken place, there is no evidence for persistent oxygen restriction in these paleoenvironments. The low levels of bioturbation indicate limited mixed layer development and a redox boundary near the sediment-water interface, likely allowing post-burial preservation to occur even in this setting dominated by oxic bottom waters. Paleoecological reconstructions and taphonomic hypotheses relating to the Indian Springs Lagerstätte should account for the paleoredox conditions revealed in this study. Several models propose the roles of clay minerals, the presence of hypersaline brines, and the actions of iron reducing bacteria as mechanisms for exceptional preservation.--adapted from abstract.

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