Fossils preserved by iron-bearing minerals in the Lower-Middle Cambrian Carrara formation

The early Cambrian is often characterized by an unusual fossil record reflecting the complex environmental conditions of the Cambrian. To study this enigmatic time period in depth, we chose the Carrara Formation as the focus unit. The lower to middle Cambrian Carrara Formation is located in the Basin and Range Province in southeastern California and southern Nevada. The Carrara Formation consists of alternating shaly and silty members with limestone units. In the Eagle Mountain Shale Member and Thimble Limestone of the Carrara Formation, there are a series of thin resistant silty limestone beds that preserve abundant macrofossils. Analysis of the insoluble residues of these limestone beds revealed abundant fossils in 250, 400 and 841µm sieve sizes preserved by apatite and iron-bearing minerals. Petrographic and FTIR examination of thin sections of these samples confirmed that apatite and iron-bearing minerals replaced, infilled, and grew on the surfaces of fossil fragments. These fossils are similar to other small shelly-style fossils except that here, echinoderms and trilobites comprise the majority of the fossils. We show that the presence of fossils preserved by apatite, chamosite, vivianite and other iron oxides suggest the occurrences of oxidation-reduction reactions in the pore water, suggesting a complex redox history of the lower Carrara Formation during fossilization of these organisms in late early Cambrian time