UNUSUAL IRON CYCLING AND REDOX CONDITIONS IN THE LOWER CAMBRIAN HARKLESS FORMATION

The lower Cambrian Harkless Formation exposed near Gold Point, NV contains the last occurrence of archaeocyathan reefs in the Death Valley area. Prior to the reef bed, there are tens of meters of mostly unfossiliferous green shale with a few fossil packstones containing small shelly fossils. Geochemical conditions throughout this interval may provide insight into the redox state of shallow Cambrian seas and factors that controlled archaeocyathan reef growth. Here we present iron speciation, x-ray fluorescence (XRF), and x-ray diffraction (XRD) data which explore the redox state of the green shale underlying the reef.

Iron speciation results from the lower part of the section indicate that conditions were dominantly anoxic with some data falling into the range where iron speciation cannot distinguish between oxic and anoxic conditions. Upsection towards the strata that contain the reefs, iron speciation indicates that benthic conditions transitioned to oxic. Notably, relatively large quantities of iron (~50% of the highly reactive fraction) were obtained from all samples during the ammonium oxalate extraction, which is operationally designed to measure magnetite. However, XRD analyses indicate that there is no magnetite present and that this enrichment is instead due to the presence of Fe-rich clay minerals, such as glauconite and berthierine/chamosite, which were likely partially dissolved during the ammonium oxalate extraction. The occurrence of these clay minerals throughout the section suggests that there were fluctuating benthic redox conditions, even just prior to the onset of reef growth, which is supported by Fe/Al ratios and redox sensitive trace metal data. We conclude that a) the lower Cambrian Harkless Formation experienced variable redox conditions which facilitated the formation of Fe-rich clay minerals and may have affected biodiversity prior to the archaeocyathan reefs and b) caution should be used when interpreting iron speciation results when Fe-rich clay minerals are likely to be present.