LOWER TRIASSIC AMMONOID BEDS IN THE CONFUSION RANGE OF WESTERN UTAH

The Thaynes Formation was deposited in the Sonoma Foreland Basin during the Early Triassic (Olenekian). Outcrops of these epicontinental marine rocks can be seen now in the Confusion Range of western Utah. Most of the Thaynes Formation in this area consists of yellow-gray fissile shale and gray-brown limestone that reflect deposition in relatively calm conditions equivalent to basin and outer shelf facies. Occasional, short-term regressions produced a few thin layers of inner shelf siltstone, sandstone, and limestone.

The Thaynes Formation is well known for its densely concentrated ammonoid beds. Indeed, the best locality in the Confusion Range seems to be inexhaustible, despite the sporadic but dedicated efforts of collectors over at least the past several decades. We recognize three distinct Smithian ammonoid assemblages here, and we informally refer to these occurrences as the Meekoceras Beds, the Inyoites Beds, and the Anasibirites Beds. The occurrences are mostly restricted to thin, isolated beds, which can be confidently identified in outcrops within the study area (~100 km2) and even beyond (e.g., in southeastern Utah). The lower and middle assemblages appear to lie within the Romunderi Zone, whereas the upper assemblage appears to lie within the Tardus Zone; these ammonoid biozones subdivide the Smithian Substage of the Olenekian Stage (~250 Ma).

There are numerous possible explanations for dense concentrations of ammonoids, including reproductive mass mortality (i.e., semelparity), taphonomic biases (e.g., condensed intervals and post-mortem hydrodynamic transportation), and mass mortality generated by environmental catastrophe. There is some evidence that the ammonoid beds of our study may have resulted from multiple different causes. However, we believe the primary cause of these fossil concentrations is related to the unusual ocean conditions documented for the Early Triassic by many other workers. Various lines of evidence, such as sedimentology, paleontology, and geochemistry, indicate that anoxia and hypercapnia are likely culprits for large-scale death horizons during the Early Triassic in general. The fact that our ammonoid beds can be identified across hundreds of kilometers strongly suggests that they were produced by basin-wide catastrophic environmental events.