FOSSILS PRESERVED BY IRON-BEARING MINERALS IN THE LOWER-MIDDLE CAMBRIAN CARRARA FORMATION

The lower to middle Cambrian Carrara Formation is exposed at Eagle Mountain, near Death Valley, California, where the basal portion sits in contact with the Zabriskie Quartzite and is overlain by the Bonanza King Formation. In the basal ~60 m of the Carrara Formation at Eagle Mountain, lenticular carbonates are preserved in a dominantly siliciclastic part of the section. An insoluble residue analysis of eleven thin limestone beds reveals that these samples preserve abundant fossil material. The samples were dissolved in 10% acetic acid and then separated into 3 size fractions (>841µm, 841 to 400 µm, and 400 to 250 µm). Trilobites, echinoderms, and various cone-shaped fossils and fragments are preserved in all three size fractions but are most abundant in the >400 µm size fraction. Trilobite fragments include genal spines and other parts of the cephalon. Echinoderm plates and fragments are identified by their porous stereom, and Salterella and unidentified cones are also preserved. Energy dispersive x-ray analysis of fossils (SEM-EDS), x-ray diffraction data (XRD) on the insoluble residues, and petrographic analysis confirm that glauconite, apatite, and iron oxides are fossil-preserving minerals. Echinoderms were replaced mostly by iron oxides whereas trilobites were often replaced by a variety of minerals, including quartz, apatite, and glauconite, and often with many minerals replacing a single fossil grain. The taphonomic history of this assemblage indicates that these organisms were living where oxygen was available but deposited in an area where pore waters likely experienced periodic low oxygen conditions in the basal Carrara Formation at Eagle Mountain.