Abstract: The Upper Triassic chronostratigraphic scale consists of one Series, the Upper Triassic, divided into three stages (in ascending order)—Carnian, Norian and Rhaetian. Only the base of the Carnian currently has an agreed on GSSP (global boundary stratotype section and point), though agreement on GSSPs for the bases of the Norian and Rhaetian is imminent. Substages of the Carnian and Norian provide more detailed subdivisions of Late Triassic time than do the relatively long Carnian and Norian stages. These substages need boundary definitions and greater use in Late Triassic correlations. Numerical chronology of the Late Triassic is based on very few radioisotopic ages from volcanic ash beds directly related to marine biostratigraphy. The numerical calibration of the Late Triassic favored here is Carnian ~220–237 Ma, Norian ~205–220 Ma and Rhaetian ~201–205 Ma. Late Triassic magnetostratigraphy is fraught with problems because the most complete record from the Newark Supergroup of eastern North America cannot be correlated based on pattern matching to any co-eval magnetostratigraphy from a marine section. The long Norian (beginning at ~228 Ma) was created by magnetostratigraphic correlations that abandoned biostratigraphic constraints and has produced extensive miscorrelation, particularly of nonmarine Carnian strata. A reliable Late Triassic magnetostratigraphy is a succession of multichrons that identifies the Carnian-early Norian and late Norian-Rhaetian as dominantly of normal polarity. Late Triassic cyclostratigraphy of the Newark Supergroup has been advanced as a floating astrochronology of the Late Triassic, but is problematic given evident hiatuses in the Newark record and the presence of non-cyclical lithofacies. Isotope stratigraphy of the Late Triassic, for example the late Rhaetian carbon-isotope excursion, has great potential for use in Late Triassic correlations. The Late Triassic timescale is still very much a work in progress that needs more precise chronostratigraphic definitions, additional numerical ages directly related to marine biostratigraphy, a wholesale rethinking of magnetostratigraphic correlations and additional cyclostratigraphic and isotopic data to achieve greater precision and stability.
Publication Year: 2017
Publication Date: 2017-11-16
Language: en
Type: book-chapter
Indexed In: ['crossref']
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Cited By Count: 14
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