I CAN’T QUIT YOU, DAVE: NEW TAPHONOMIC INSIGHTS FROM THE LOWER TRIASSIC VIRGIN LIMESTONE FORMATION, WESTERN UNITED STATES

The Lower Triassic Virgin Limestone Formation of the western United States has received decades of close attention, in part because it encompasses the critical interval after the end-Permian mass extinction just before sustained recovery begins in the Middle Triassic. Study of this unit has revealed many critical features of Lower Triassic seafloors including the preservation of wrinkle structures, microbial reefs and other microbialites, and post-extinction metazoan bioherms. More recently, taphonomic aspects of Lower Triassic carbonates of the Virgin Limestone have been explored. Limestone samples from this unit in the Muddy Mountains region produced insoluble residues dominated by silicified fossils that are not necessarily quantitatively abundant in thin section. This suggests that preferential silicification imparts a strong taphonomic filter on these assemblages. In the small size fractions of some of these same residues, an abundance of glauconitic and phosphatic fossils (177 to 250 µm, and 250 µm to 420 µm) illustrates that small shelly-style fossilization occurred in Lower Triassic carbonates. The preservation of these fossils likely represents periodic low oxygen conditions in pore waters during early diagenesis, as has been suggested in Cambrian deposits. Glauconitic and phosphatic fossils are steinkerns and replaced fossils of echinoderms, gastropods, and a few foraminifera. The diversity of these fossils is not surprising and mirrors what is found in many Early Triassic assemblages; however, the small size of the fossils and unusual taphonomy of Early Triassic seafloors likely fostered their preservation. Much work remains in untangling the taphonomic implications of mass extinction, and the Virgin Limestone Formation continues to serve as an important recorder of this time.