EVIDENCE FOR AN EARLIEST TRIASSIC MICROBIALITE FROM THE CONFUSION RANGE, UT

An understanding of global events during Permian Triassic boundary is limited by the paucity of complete boundary sections from Panthalassa and western Pangaea. High resolution ⁸⁷Sr/⁸⁶Sr and δ¹³C_carb isotope stratigraphy of the Gerster Limestone and Thaynes Formation in the Confusion Range, UT, indicates a relatively continuous PTB record. δ¹³C_carb decreases from +2 to -2‰, and within this interval are several distinctive lithologies including ~1.7m of microbially influenced carbonate. Within some Tethyan sections, stromatolitic, clotted fabrics, and digitate facies occur in microbial carbonates of the basal Triassic. Although earliest Triassic microbialites are unknown in Utah, their presence here was predicted by Kershaw et al. (2007) based on paleolatitude and ocean circulation models.

The Gerster Limestone includes several hundred meters of cherty, fossiliferous packstone of Permian age in the Great Basin region. However, the upper 10 m of the Gerster is unique in the region, including a 3.5 m thick bed of cross bedded red sandstone overlain by a ~6.5 m limestone unit containing large chert nodules in a micritic matrix. Overlying the chert unit, the basal Thaynes is made up of a ~0.2 m thick interval of laminated limestones with upturned margins described as tepee structures by Collinson et al. (1976). Above the tepees is a ~0.7 m thick fenestral limestone containing pellets, pisolites and interbedded stromatolitic fabric, characterized by mm-scale wavy bedding. Thin sections reveal a clotted texture, lobate structures, and ooids. The fenestral unit is capped by a ~0.7 m resistant limestone ledge with digitate columns (thrombolite texture) containing microgastropods. The digitate structures are abruptly truncated and overlain by a microgastropod packstone. The microgastropod packstone unit is overlain by brownish-gray, ammonite-bearing limestones and shales that are more typical of the Thaynes Formation throughout the Great Basin region.

These preliminary results show microbialite characteristics in the Gerster-Thaynes transition of the Confusion Range. However, future work is needed to determine if this microbial carbonate was deposited as part of a global microbialite event, or if these strata simply reflect the local depositional environment.