High-Precision Re-Os 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 Re-Os 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 Re-Os ORS age dates, at least for the Toarcian, has been linked to subtle chemostratigraphic variations in Os-187/(188)Osi values. Here, we report the first high-precision Re-Os ages of organicrich 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 Re-Os age results bracket sediment deposition between the late Sinemurian-early Pliensbachian (192.11 +/- 0.77 Ma [2s] and 193.83 +/- 1.34 Ma [2s]) and late Toarcian (180.10 +/- 2.68 Ma [2s]). When coupled with chemo- (Sr-87/Sr-86 and Os-187/Os-188 isotopes) and bio-stratigraphy (ammonite and coccolith), our Re-Os 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.