SMALL SHELLY-STYLE PRESERVATION OF FOSSILS IN LOWER TRIASSIC CARBONATES OF THE WESTERN US

Following Earth’s largest extinction, the end-Permian event, Lower Triassic carbonates are characterized by microbialites, carbonate crystal fans, and ecologically depauperate fossil communities. Most of our understanding of Early Triassic fossil communities comes from bulk counts on carbonates and siliciclastics, with just a handful of silicified occurrences known. Here, we report fossils preserved in the style of “small shelly fossils” in small size fractions (177 to 250 µm, and 250 µm to 420 µm) of insoluble residues of carbonates. This style of preservation is most commonly found in the Cambrian; however, these fossils represent an unusual occurrence of this taphonomic window much later in earth’s history. Twenty-six samples were collected from the Virgin Limestone Formation at the Muddy Mountains Ute and Muddy Mountains Overton localities. Of those initially dissolved, five were found to have abundant echinoderm spines replaced by apatite and glauconite and similar steinkerns of gastropods. Rarer forms include forams, crinoid ossicles, and ophiuroid fragments. The glauconitic and phosphatic mineralogical composition of samples was determined using SEM-EDS and XRD. The diversity of fossils is similar to what has been found in the macrofossil record: gastropods and echinoderms are the dominant fossils. Interestingly, bivalves, which are common components of silicified residues from these units, are not abundantly preserved by phosphate and glauconite in the samples. The nucleation of both glauconite and phosphate require that pore-water redox conditions oscillate between oxic and anoxic conditions, which implies that Lower Triassic carbonates periodically experienced oxygen depletion after deposition and during early diagenesis.