SMALL SHELLY FOSSIL PRESERVATION AND THE ROLE OF EARLY DIAGENETIC REDOX IN THE EARLY TRIASSIC

Abstract

Minute fossils from a variety of different metazoan clades, collectively referred to as small shelly fossils, represent a distinctive taphonomic mode that is most commonly reported from the Cambrian Period. Lower Triassic successions of the western United States, deposited in the aftermath of the end-Permian mass extinction, provide an example of small shelly style preservation that significantly post-dates Cambrian occurrences. Glauconitized and phosphatized echinoderms and gastropods are preserved in the insoluble residues of carbonates from the Virgin Limestone Member of the Moenkopi Formation. Echinoderm plates, spines and other skeletal elements are preserved as stereomic molds; gastropods are preserved as steinkerns. All small shelly style fossils are preserved in the small size fractions of the residues (177 to 420 μm), which is consistent with the size selection of small shelly fossils in the Cambrian. Energy-dispersive X-ray spectra of individual fossils coupled with X-ray diffraction of residues confirm that the fossils are dominantly preserved by apatite and glauconite, and sometimes a combination of the two minerals. The nucleation of both of these minerals requires that pore water redox oscillated between oxic and anoxic conditions, which, in turn, implies that Lower Triassic carbonates periodically experienced oxygen depletion after deposition and during early diagenesis. Long-term oxygen depletion persisted through the Early Triassic, creating diagenetic conditions that were instrumental in the preservation of small shelly fossils in Triassic and, likely, Paleozoic examples.