A high-resolution sequence stratigraphic framework for the eastern Ellis Bay Formation, Canada: A record of Hirnantian sea-level change

Anticosti Island (Québec, Canada) contains one of the thickest Ordovician/Silurian boundary sections in the world, providing a fossiliferous and well-preserved record of the Late Ordovician mass extinction; however, the absence of a comprehensive correlation framework for the island prevents the full incorporation of data from across Anticosti into a global understanding of the extinction event. Here, we combine sedimentological, stratigraphic, and chemostratigraphic data to develop a sequence stratigraphic framework for the Ellis Bay and lowermost Becscie formations along the northeastern shore of Anticosti Island. These formations record six fourth-order (∼100–400 k.y.) depositional sequences bounded by regionally traceable unconformities. Evidence of subaerial weathering and exposure at many of these unconformities is subtle and has long gone unrecognized, which highlights the complexity of correlation in this and other stratigraphically thin icehouse records. Quartz pebble lags that mantle these surfaces punctuate the otherwise fine-grained, mixed carbonate–siliciclastic strata of the Ellis Bay Formation and provide the means to trace unconformities across localities; these surfaces are also marked by depleted carbon isotopic values, meteoric cement, microkarst, and incised valleys. This sequence stratigraphic framework provides the first correlation framework for exposures from the eastern end of this classic Ordovician/Silurian boundary section, and thus enables the integration of paleobiological, stratigraphic, and geochemical data into a basin-wide perspective of the Late Ordovician mass extinction. Critically, comparison to sequence stratigraphic frameworks from coeval sections suggests that other regions may be incomplete at the level of the fourthorder cycles recorded on Anticosti Island and may contain similarly cryptic unconformities.