Mass Extinction Déjà Vu: Seafloor Aragonite Fans near the Triassic-Jurassic Boundary

Decimeter-scale seafloor-precipitated calcium carbonate fans, exceedingly rare in post-Paleoproterozoic time, make a dramatic reappearance at several times in Earth's history: in the post-glacial cap carbonates associated with Neoproterozoic low latitude glaciation (snowball Earth), and during the Early Triassic, a time of environmental and biological upheaval following the end-Permian mass extinction. Here, we report primary aragonite seafloor fans near the Triassic-Jurassic boundary from the Lower Fernie Formation of the Western Canada sedimentary basin exposed at Williston Lake, British Columbia. The fans make up several layers of fibrous calcite ranging from about 1 to 10 cm in thickness and interbedded with very fine siliciclastics. Although their mineralogy is now calcite, blunt crystal terminations and pseudohexagonal crystal cross-sections suggest a primary aragonite composition. The fans were deposited in an outer shelf to slope setting, likely in oxygen-restricted conditions, and appear to have grown directly on the seafloor. Although the precise stratigraphic position of the Triassic-Jurassic boundary at Williston Lake is still debated, these fans are likely within 1-2 meters of the boundary. As such, they are the first Phanerozoic seafloor fans reported outside of the Early Triassic. Non-actualistic seafloor fan growth in the Early Triassic has been interpreted to be a symptom of the anoxic and euxinic oceanic conditions responsible for the end-Permian mass extinction. Although the fans at Williston Lake may prove an isolated regional occurrence, if fans are found associated with T-J boundaries in varied paleogeographic settings, this may indicate a similar kill mechanism for the end-Triassic mass extinction.