The Ordovician-Silurian boundary (late Katian-Hirnantian) of western Anticosti Island: revised stratigraphy and benthic megafaunal correlations

摘要

The type sections of the Ellis Bay Formation are revised to incorporate recent stratigraphic correlations east to west on Anticosti Island. This is one of the most complete tropical carbonate sequences spanning one of the major Phanerozoic mass extinction episodes. A richly fossiliferous benthic fauna faithfully records the change-over from the Ordovician (Richmondian, late Katian), the mass extinction around the boundary, and the recovery within the earliest Silurian (Rhuddanian, Llandovery). Critical in this revision is the Hirnantian shelly fauna that begins at or close to the base of the Ellis Bay Formation at the west end (as revised herein), and ends at the top of the reefal Laframboise Member, the O/S boundary as used herein. Equivalent strata at the east end contain Hirnantia sp. at the base of the Prinsta Member, as the basal Ellis Bay Formation, and Hirnantia sagittifera in reef-capping beds at the top of the Laframboise Member in the central area of the island. The Ellis Bay Formation is rich in typical Hirnantian brachiopods such as Eospirigerina and Hindella. The earliest Silurian recovery brachiopod fauna is usually small-shelled, composed of both Ordovician hold-over taxa (ca 30%) and new arrivals such as the pentameride Viridita, athyridide Koigia, and atrypide Zygospiraella. In this study, three new members are proposed for the 80-90m thick Ellis Bay Formation at the west end of Anticosti Island, beginning with the basal shales and limestones of the Fraise Member (overlying a recessive shaly unit of the Katian Vaureal Formation), followed by the Juncliff Member resistant limestones, and the overlying limestones and shales of the Parastro Member. The uppermost two members are correlated directly with the Lousy Cove and Laframboise members at the east end, where the same sequence is thinner and contains several discontinuities. The top of the reef-capping grainstorie beds above the reefal Laframboise Member marks the major end-Ordovician extinction of Hirnantian brachiopods, stromatoporoids, corals, crinoids and nautiloids, signaled by the return to background δ~(18)O and δ~(13)C values. The overlying Rhuddanian Becscie Formation comprises a lower Fox Point Member of thin, evenly bedded limestones, with a low-diversity but high-abundance Rhuddanian brachiopod fauna, and an upper Chabot Member of irregularly bedded coralline, non-reefal limestones, marking the appearance of typical Silurian benthic faunas.