PALEOREDOX GEOCHEMISTRY AND BIOTURBATION LEVELS OF THE EARLY CAMBRIAN INDIAN SPRINGS LAGERSTÄTTE, POLETA FORMATION, NEVADA

The early Cambrian Indian Springs Lagerstätte is preserved in exposures of the Poleta Formation in the Montezuma Range of western Nevada and exhibits Burgess Shale-type (BST) preservation of a diverse array of animal phyla, including the earliest definitive echinoderms. It therefore provides an important 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 BST deposit in order to characterize the paleoenvironmental conditions in which these animals lived and their fossils were preserved. A total of 28 rock samples were collected from outcrops at the three previously reported intervals of exceptional preservation at the Indian Springs locality (English and Babcock, 2010), as well as at one interval not known to exhibit such preservation. Additional random samples (20 total) were collected from the talus using a square meter quadrat. In the laboratory, the samples were geochemically analyzed for trace metal paleoredox indices (V/Cr and V/(V+Ni) ratios) and bioturbation levels were assessed through X-radiography and petrographic thin sections using the ichnofabric index (ii) method. Additional samples from coeval strata of the Poleta Formation at Westgard Pass, CA were also geochemically analyzed with the same methodology. Preliminary results indicate that oxic bottom water conditions dominated during deposition of these strata, despite consistently low bioturbation levels. This pattern holds for intervals with BST preservation and those without. These results suggest that oxic bottom waters were dominant during the deposition of these rocks. 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 BST preservation to occur even in this setting dominated by oxic bottom waters. Paleoecological reconstructions and taphonomic hypotheses relating to the Indian Springs biota should account for the paleoredox conditions revealed in this study.