A PROPOSAL FOR NEW CHRONOSTRATIGRAPHIC STAGE SUBDIVISIONS OF THE UPPER TRIASSIC SERIES

Hounslow, Mark W and Lucas, Spencer G. (2023) A PROPOSAL FOR NEW CHRONOSTRATIGRAPHIC STAGE SUBDIVISIONS OF THE UPPER TRIASSIC SERIES. Albertiana, 48. ISSN 0169-4324

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Abstract

Chronostratigraphic stages that have a short duration are desirable because they identify brief units that can be used to make more detailed and precise age assignments and correlations. Finer-scale substage and zonal subdivisions naturally remain for correlation, but these are outside the formal decision processes of the Subcomission on Triassic Stratigraphy (STS). The longest Phanerozoic stage is the Norian (approximately 18 Myr duration), which is both mismatched in terms of duration with other Phanerozoic stages, and the Lower and Middle Triassic Series, which together are shorter than the Norian Stage. If the Norian was formally divided it would also enhance our understanding of this interval as well as improve chronostratigraphic resolution. We propose the Norian should be raised in rank, and its substages used as new stages. However, such a modification would also require a re-evaluation of the hierarchical position of the Carnian and Rhaetian stages of the Upper Triassic. Given that the Carnian (approximately 10 Myr duration) is one of the longer stages in the Mesozoic it would also benefit with a raised ranking and subdivision to maintain its historical hierarchy with the Norian. We examine three alternative scenarios that raise the rank of the Carnian and Norian (and the Rhaetian Stage) in the chronostratigraphic hierarchy to either series or subseries rank. The existing Upper Triassic substages are also proposed to be raised in rank to stages, allowing formal definition by GSSP boundary stratotypes. The positive and negative issues with any changes in stage and substage hierarchy are examined.