A new key Smithian (Early Triassic) quantitative ammonoid biochronology from the western USA basin R. Jattiot, H. Bucher, A. Brayard, M. Brosse, J. Jenks & K. G. Bylund GSA Meeting 25-28 September 2016 Palaeontological Institute and Museum University of Zürich

Introduction - The Smithian, a crucial time interval end-Smithian event Modified after Brayard & Bucher, 2015

Introduction - The Smithian, a crucial time interval Modified after Brayard & Bucher, 2015 end-Smithian event

- The Smithian, a crucial time interval Introduction - The Smithian, a crucial time interval China, Brayard & Bucher 2008 South Primorye, Shigeta et al. 2009 Pakistan/Spiti, Brühwiler et al. 2012

- Pioneering contributions from the western USA basin Introduction - Pioneering contributions from the western USA basin Smith, 1932 Hyatt & Smith, 1905 Mathews, 1929 Smith, 1904

- The western USA basin (Sonoma Foreland Basin) Introduction - The western USA basin (Sonoma Foreland Basin) Early Triassic 252-247 Ma Palomino Ridge

Introduction - Palomino Ridge Intensive Smithian ammonoid sampling at Palomino Ridge section. Jattiot et al., submitted.

Introduction - Palomino Ridge First comprehensive taxonomical and biochronological study of the Smithian ammonoid faunas from this locality. Jattiot et al., submitted.

Taxonomy Jattiot et al., submitted. Lanceolites compactus, Parussuria compressa Hemiprionites typus

Biochronology - Unitary Associations method allowing intrabasinal correlation between Nevada and Utah. Before this study, only a few correlations were established. The UA method produces biozonations based on maximal association zones, instead of interval zones. It also helps solving biostratigraphical contradictions. < Raw data matrix

Biochronology - Results 5 UA zones, one (UAZ4) spanning the entire middle Smithian. A hypothetical UAZ6 would represent the second part of the late Smithian, but undisputed specimens of Pseudosageceras augustum need to be found in Utah. Jattiot et al., submitted.

Discussion Peculiar middle Smithian diversity pattern for the western USA basin: - occurrences of mostly cosmopolitan, rather long-ranging genera, diversity continually increases with additional taxa through time.  the maximum set of species for the middle Smithian is observed in the late middle Smithian, and includes all previous middle Smithian sets of species.  this leads to the creation of a single UAZ for the entire middle Smithian. Modified after Jattiot et al., submitted.

Discussion Brühwiler et al. 2010, PPP. Accretionary pattern of diversity in western USA versus high turnover rates in the Northern Indian Margin (NIM).  UAZ 4 from the western USA basin corresponds to 5 UAZs from the NIM! < Jattiot et al., submitted.

Based on the taxonomical data from the western USA basin and the NIM: Discussion Based on the taxonomical data from the western USA basin and the NIM: It seems that nearly all genera from western USA are also found in the NIM. On the contrary, no good markers of the middle Smithian from the NIM are found in western USA. Brayardites Escarguelites Nammalites Pseudoceltites Nyalamites

 One-sided oceanic currents during the middle Smithian?? Discussion  One-sided oceanic currents during the middle Smithian?? Oceanic currents

Conclusion 1) First comprehensive taxonomical and biochronological study of the Smithian ammonoid faunas from this section. 2) This study allowed intrabasinal correlation between Nevada and Utah.. 3) .. and led to the creation of 5 UA zones for the western USA basin. 4) The patterns of diversity during the middle Smithian are highly contrasted between the western USA basin (1 UAZ) and the Northern Indian Margin (5 UAZs). 5) The absence in the western USA basin of many good markers of the middle Smithian from the NIM might suggest one-sided oceanic currents at that time.

THANK YOU