U-Pb Geochronology of non-marine Upper Triassic strata of the Colorado Plateau (western North America): implications for stratigraphic correlation and paleoenvironmental reconstruction
Abstract
The Triassic is a critical period in Earth history that witnessed the origin of modern ecosystems and frequent climate fluctuations, as well as major environmental events such as flood basalt volcanism and bolide impacts. The Chinle Formation contains a primary non-marine archive for past ecosystems in North America due to its fossil richness and well-studied sedimentology. Moreover, within these highly fossiliferous strata, a biotic turnover has been reported that has been hypothesized to coincide with one or more of the aforementioned environmental events. Unfortunately, few radioisotopic ages have been published for the Late Triassic, limiting our ability for lithological and paleoenvironmental correlations. In addition, the superposition of the Chinle Formation remains illusive due to frequent lateral facies changes and discontinuous outcrops across the Colorado Plateau. The 520 m long core 1A of the Colorado Plateau Coring Project from Petrified Forest National Park (PFNP) (Arizona) provides, for the first time, a continuous section of these early Mesozoic sedimentary strata. Many of the sand- and siltstones from this continuous succession throughout most of the Upper Triassic Chinle Formation contain euhedral zircons suitable for U-Pb analyses. We analyzed >300 crystals each from 10 samples using LA-ICPMS; these results indicated abundant Late Triassic crystals that appear to be closely associated with the depositional age of the host rock. We then selected the youngest grains from these samples to obtain precise CA-TIMS U-Pb single zircon ages in order to constrain the maximum depositional ages (using quantitative methods) of these formations. We are able to revise the proposed time scale (based on outcrop samples) for Upper Triassic strata at PFNP and evaluate whether the biotic turnover observed within the Sonsela Member of these strata coincides with the Manicouagan bolide impact event. This revised chronostratigraphic framework allows intercalibration with the Newark astrochronostratigraphic polarity time scale and the correlation with other key non-marine and marine Late Triassic sections globally (e.g., Ischigualasto-Villa Uníon Basin, Argentina; Tethyan region, Europe) improving our understanding of paleoenvironmental and evolutionary changes during the Triassic.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFMPP44A..04R
- Keywords:
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- 1115 Radioisotope geochronology;
- GEOCHRONOLOGY;
- 1527 Paleomagnetism applied to geologic processes;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1535 Reversals: process;
- timescale;
- magnetostratigraphy;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 4994 Instruments and techniques;
- PALEOCEANOGRAPHY