High-precision ReOs dating of Lower Jurassic shale packages from the Western Canadian Sedimentary Basin

The Lower Jurassic rock record contains abundant organic-rich mudrocks suitable for refining Early Jurassic time using ReOs organic-rich shale (ORS) geochronometry, such as those associated with the Sinemurian–Pliensbachian boundary event or the Toarcian Ocean Anoxic Event (T-OAE). Despite this, mos...

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Bibliographic Details
Published in:	Palaeogeography, palaeoclimatology, palaeoecology Vol. 560; p. 110010
Main Authors:	Toma, Jonathan, Creaser, Robert A., Pană, Dinu I.
Format:	Journal Article
Language:	English
Published:	Elsevier B.V 15-12-2020
Subjects:	Gordondale Member Organic-rich shales Osmium isotopes Pliensbachian–Toarcian boundary Sinemurian–Pliensbachian boundary Strontium isotopes Organic-rich shales Osmium isotopes Pliensbachian–Toarcian boundary Gordondale Member Sinemurian–Pliensbachian boundary Strontium isotopes
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Summary:	The Lower Jurassic rock record contains abundant organic-rich mudrocks suitable for refining Early Jurassic time using ReOs organic-rich shale (ORS) geochronometry, such as those associated with the Sinemurian–Pliensbachian boundary event or the Toarcian Ocean Anoxic Event (T-OAE). Despite this, most studies to date have failed to obtain high precision, geologically meaningful, ages for such Lower Jurassic rocks. The cause of imprecision in ReOs ORS age dates, at least for the Toarcian, has been linked to subtle chemostratigraphic variations in 187Os/188Osi values. Here, we report the first high-precision ReOs ages of organic-rich shale packages of the Gordondale and basal Poker Chip Members of the Lower Jurassic Fernie Formation of Northwestern Alberta, Canada from the Western Canadian Sedimentary Basin (WCSB). Our ReOs age results bracket sediment deposition between the late Sinemurian-early Pliensbachian (192.11 ± 0.77 Ma [2σ] and 193.83 ± 1.34 Ma [2σ]) and late Toarcian (180.10 ± 2.68 Ma [2σ]). When coupled with chemo- (87Sr/86Sr and 187Os/188Os isotopes) and bio-stratigraphy (ammonite and coccolith), our ReOs ages correlate with regional and global stratigraphic tie points corresponding to pivotal points of Early Jurassic time, such as the Sinemurian–Pliensbachian boundary event, T-OAE, and late Toarcian, that are otherwise weakly constrained by isotopic methods. This integrative approach helps to place these events in a more robust temporal framework. Based on seawater Os isotope data, we also speculate that a previously unidentified positive Os isotope excursion exists at the Sinemurian–Pliensbachian boundary - perhaps triggered by late-stage Central Atlantic Magmatic Province (CAMP) volcanism. •Sinemurian–Pliensbachian Gordondale shale (Canada) yields ReOs age of 192.11 ± 0.77 Ma.•Pliensbachian–Toarcian Gordondale shale (Canada) yields ReOs age of 183.86 ± 1.54 Ma.•Upper Toarcian Gordondale shale (Canada) yields ReOs age of 180.10 ± 2.68 Ma.•~0.5 seawater 187Os/188Osi signature confirmed in the late Sinemurian-early Pliensbachian.
ISSN:	0031-0182 1872-616X
DOI:	10.1016/j.palaeo.2020.110010