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The reconstruction of geological history depends on reliable geochronologic constraints, principally radio-isotopic ages intercalibrated with bio-, chemo-, magneto- and cyclostratigraphy.
Recent radio-isotopic ages from volcanoclastics interbedded within marine fossil bearing sediments demonstrate that the Late Triassic is exceptionally long, in excess of 30 Ma (1), and that the end-Triassic extinction appears to be coincident with a major episode of continental scale volcanism, the Central Atlantic Magmatic Province (2-4). Although some successions are well-calibrated locally (e.g., (5)), the timing of events in the Late Triassic is poorly understood because of a lack of absolute age control with robust radio-isotopic data. The correlation between events in marine and terrestrial environments remains elusive even though the understanding of the timing of environmental change leading up to the End Triassic extinction is critically urgent. This lack of correlation also hinders our understanding of the global tempo and mode of major biotic events, including the origin and early diversification of dinosaurs (5), the potential effects from the Manicouagan impact at c. 215.5 Ma (6), and faunal turnover at or near the Triassic-Jurassic boundary.
We present initial TIMS U-Pb single zircon ages from a tuffaceous sandstone at the base of the Blue Mesa Member, directly overlying the Mesa Redondo/Bluewater Creek Member (lower portion of the Upper Triassic Chinle Formation) in the Six Mile Canyon area of western New Mexico. A coherent cluster of zircon analyses yields a preliminary weighted mean 206Pb/238U age of 219.2 ± 0.7 Ma, which is a maximum age because the layer is not a primary ashfall. This age suggests that previous global biostratigraphic correlations of the Chinle Formation (e.g., (7)) are untenable, and that many traditional "late Carnian" terrestrial assemblages may be mostly or wholly within the Norian. Ages of similar quality from both terrestrial and marine successions are necessary to establish a firm chronostratigraphy and ultimately result in coherent reconstruction of events in the Late Triassic.
1. S. Furin et al., Geology 34, 1009 (2006).
2. U. Schaltegger, J. Guex, A. Bartolini, B. Schoene, M. Ovtcharova, Earth and Planetary Science Letters 267, 266 (2008).
3. B. Schoene, J. L. Crowley, D. J. Condon, M. D. Schmitz, S. A. Bowring, Geochimica et Cosmochimica Acta 70, 426 (2006).
4. A. Marzoli et al., Geology 32, 973 (2004).
5. R. B. Irmis et al., Science 317, 358 (2007).
6. J. Ramezani, S. A. Bowring, M. S. Pringle, F. D. Winslow, E. T. Rasbury, Geochimica et Cosmochimica Acta 69, A321 (2005).
7. S. G. Lucas, Palaeogeography Palaeoclimatology Palaeoecology 143, 345 (1998).
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