A NEW KEY SMITHIAN (EARLY TRIASSIC) QUANTITATIVE AMMONOID BIOCHRONOLOGY FROM THE WESTERN USA BASIN

Since the seminal work of Silberling and Tozer (1968), the western USA basin is known as including an excellent record of Early Triassic ammonoids. Smithian marine fossiliferous strata are widely distributed and cover southwestern Montana, southeastern Idaho, Utah, Wyoming and northeastern Nevada. The Smithian is a crucial time interval, recording the first global, major diversification-extinction cycle after the Permian-Triassic boundary mass extinction (PTBME). Early and middle Smithian corresponds to the main rediversification of ammonoids and conodonts, whereas the late Smithian witnessed the most severe intra-Triassic crisis for the nekton, about 1.4 myr after the PTBME. Our intensive sampling of the lower portion of the Thaynes Group within the Palomino Ridge area (northeastern Nevada) yielded abundant and well-preserved Smithian ammonoid faunas. Based on new data from Palomino Ridge and previous data from neighboring localities in Utah, we provide here the first quantitative Smithian ammonoid biochronological scheme for the western USA basin. This new zonation is based on the Unitary Associations (UA) method. The biochronological sequence comprises five unitary association zones that can be correlated with other localities from the Northern Indian Margin (Salt Range, Pakistan; Spiti northern India; and Tulong, South Tibet). Three unitary association zones (UAZ₁, UAZ₂ and UAZ₃) are defined for the early Smithian, one (UAZ₄) spans the entire middle Smithian and one (UAZ₅) comes into the first part of the late Smithian. Finally, a provisional UAZ₆ would represent the second part of the late Smithian.

This zonation stands in contrast to the fourteen UA zones previously established in the Northern Indian Margin (Brühwiler et al. 2010). The latter is shaped by much higher turnover rates, especially during the middle Smithian. This difference is discussed in terms of biogeography in the framework of the Early Triassic ammonoid biotic recovery.

Brühwiler, T. et al. (2010): High-resolution biochronology and diversity dynamics of the Early Triassic ammonoid recovery: the Smithian faunas of the Northern Indian Margin. – P. P. P. 297: 491–501.

Silberling, N.J. & Tozer, E.T. (1968): Biostratigraphic classification of the marine Triassic in North America. – Geol. Soc. Am. Spec. Pap. 110: 1–63.