SEDIMENTOLOGY AND STRATIGRAPHY OF UPPER TRIASSIC CARBONATES FROM HOSSELKUS LIMESTONE AND LUNING FORMATION (WESTERN USA)

Although the significant research carried out during the last years, knowledge about Panthalassan shallow-water carbonates remains distinctly minor than their Tethyan counterparts. Considering the broad diffusion of these limestones, the comprehension of their depositional environment, ecologic conditions and geographic extent, represents a unique way to better assess life evolution and recovery after the main Permo-Triassic biological crisis. Hosselkus Limestone and Luning Formation, respectively located in Northern California and Western Nevada, represent two completely different scenarios of limestone deposition during Upper Triassic. The first, deposited on a volcanic arc far from the coast of the American craton, shows a rapid change from shallow water facies to deep marine deposits, offering a wide spectrum of calcareous and siliceous organisms. The second, deposited in a large embayment attached to the continent, represents a much wider and homogeneous environment, where calcareous organisms thrive for a long period. Paleontological studies, started at the beginning of the last century and exclusively counting on large-dimension fauna, already proved strong similarities with both Panthalassan and Tethyan carbonates. Now, for the first time, the two formations are described in terms of microfacies and microorganisms, allowing a more exhaustive picture of the depositional environments and a sharper comparison of biological contents. Ages have been revised too, thanks to numerous Conodonts specimens, enabling a high temporal resolution and leading to new insights about formations’ subdivision.