PHOSPHATIC AND GLAUCONITIC FOSSIL PRESERVATION LINKED TO REDOX OSCILLATIONS IN CAMBRIAN OCEANS

Phosphatic and glauconitic fossils are well known features of Cambrian successions worldwide. In particular, abundant small shelly fossils (SSFs), commonly preserved in phosphatic and glauconitic deposits, are thought to indicate unusual depositional environments during Cambrian time. What is lacking is a mechanistic understanding of the preservation of fossils by apatite and glauconite and what these minerals indicate about redox conditions. Here, we report new occurrences of phosphatized and glauconitic fossils from southwestern Mongolia and western US. Phosphatized archaeocythans are preserved in a series of horizons in the lower Cambrian carbonate-dominated Salaagol Formation and the overlying siliciclastic-dominated Khairkhan Formation on the Zavkhan terrane in southwestern Mongolia. Eight fossiliferous samples were dissolved in 10% buffered acetic acid to remove carbonate matrix. Fossils of archaeoacyathans and SSFs were picked from insoluble residues and imaged under SEM. Complementary petrographic analysis revealed that phosphate occurs as grains, cement, replaced SSFs and internal fill of archaeocyaths. Although phosphatized internal molds of archaeocyathans and replaced SSFs were the most common modes of preservation, one glauconitic internal mold of an archaeocyathan was found in residue.

In the middle Cambrian Carrara Formation exposed at Eagle Mountain, CA, similar phosphatized and glauconitic fossil debris has been recovered from two insoluble residues of calcareous siltstone facies. Although the fossil debris is broken and abraded, thin sections reveal an abundance of trilobite hash. We suggest that in each of these settings, phosphatic and glauconitic fossils formed when organic matter decayed in environments that experienced oscillating episodes of anoxic and oxic conditions. These oscillations fostered the nucleation and rapid growth of apatite and glauconite. The relative abundance of glauconite and phosphate minerals may relate to the availability of phosphorus and organic matter in these environments. We hypothesize that fluctuating redox amplified the accumulation and retention of both sedimentary phosphorus and glauconite during Cambrian redox instability and fostered the unusual fossil preservation that is widely observed in Cambrian facies globally.