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Graptolites recovered from the organic-rich intervals, previously named the Boas River Formation in the Upper Ordovician succession on Southampton, Akpatok, and southern Baffin islands provide a reliable age assessment for the Upper Ordovician petroleum source rocks in the northern Hudson Bay, Hudson Strait, and Foxe basins. They are characterised by Anticostia lata and Anticostia hudsoni in the lower Red Head Rapids Formation on Southampton Island; Anticostia decipiens and Rectograptus socialis in the lower Foster Bay Formation on Akpatok Island; and Diplacanthograptus spiniferus and Amplexograptus praetypicalis in the lower Amadjuak Formation on southern Baffin Island. These data suggest that the organic-rich intervals in the northern Hudson Bay and Hudson Strait basins can be correlated to the Dicellograptus anceps and Paraorthograptus pacificus zones of the upper Katian, and the horizon in the Foxe Basin to the Diplacanthograptus spiniferus Zone of the lower Katian. The Boas River Formation is not deemed appropriate to use as it occurs as an organic-rich interbed in different stratigraphic units in different basins; therefore, it is suggested to abandon it as a stratigraphic term.

New high-resolution chemostratigraphy, in combination with updated biostratigraphy, refines the chronostratigraphic resolution of the nearly 2 km thick Ordovician section below Anticosti Island. A total of 1414 horizons, spaced at 1.0–1.5 m intervals, were sampled from the New Associated Consolidated Paper (NACP) drill core and analyzed for major and trace elemental composition by portable X-ray fluorescence analyzer (pXRF). Select micrite and calcareous shale powders were then analyzed for δ13Ccarb (number of samples, N = 364) and 87Sr/86Sr (N = 25). Our results indicate a Floian to early Darriwilian (F3–Dw1) age for the Romaine Formation, a middle Darriwilian to Sandbian (Dw2–Sa1) age for the Mingan Formation, a lower to mid-Katian (Ka1–Ka2) age for the Macasty Formation, an upper Katian (Ka3) age for the lower Vauréal Formation, an uppermost Katian (Ka4) age for the upper Vauréal, and a Hirnantian (H1) age for the Ellis Bay Formation. This integrated chemostratigraphic and biostratigraphic synthesis establishes the position of numerous unconformities, the duration of the intervening depositional sequences, and rates of sedimentation.

The Ellis Bay Formation cropping out in the central and western parts of the Island of Anticosti, is one of the thickest uppermost Ordovician carbonate shelf successions. Three new species of Hercochitina : Hercochitina florentini , Hercochitina changi and Hercochitina longi , are described from the lower part of the formation. These species are associated with Clathrochitina concinna , Belonechitina concava and Ancyrochitina longispina , and define the new H. florentini – C. concinna Biozone. This zone is present at the Anse aux Fraises section in the western part of the island and in the lower Lousy Cove and probably the Prinsta members of the Ellis Bay Formation in a section on the northeastern coast. The H. florentini – C. concinna Biozone follows the Hercochitina crickmayi Biozone of the Vaureal Formation and is overlain by the B. gamachiana Biozone, which ranges through most of the Ellis Bay Formation in western Anticosti. The B. gamachiana Biozone is succeeded by the Spinachitina taugourdeaui Biozone just below the reefal Laframboise Member. In eastern Anticosti the B. gamachiana Zone has only been recognized in the upper Lousy Cove Member of the Ellis Bay Formation. The presence of a Hirnantian shelly fauna, including the typical genus Hirnantia , the comparison of the Anticosti chitinozoan assemblages with those from other sections dated by graptolites and the available stable isotope geochemistry information suggest that the Ellis Bay Formation is of Hirnantian age.

Thickness and facies variations in the Upper Ordovician succession of Anticosti Island have made correlations between the western and eastern ends of the island difficult. This chitinozoan study reveals that most strata of the Ellis Bay Formation at the eastern end of the island correlate with the Vaureal Formation at the western end. In the eastern end, chitinozoans typical of the Ellis Bay Formation occur only in the upper part of the Lousy Cove Member, whereas chitinozoans diagnostic of the mid-upper Vaureal occur in the rest of the formation. Moreover, the eastern Schmitt Creek and Mill Bay members of the Vaureal Formation are older than the uppermost Vaureal eponym members in the western end. New east-west correlations, supported by geochemical, sedimentological, and sequence stratigraphic data, are proposed. Comparison with coeval Upper Ordovician successions suggests a Hirnantian age for the gamachiana Zone that occurs in the lower members of the Ellis Bay Formation. It also indicates that the Hirnantian Isotopic Carbon Excursion (HICE) on Anticosti begins in the D. anceps Zone, just below the base of the Hirnantian Stage, reaches a first peak in the lower N. extraordinarius Zone, a second modest peak high in the N. extraordinarius Zone, and a third, more important one, in the lower part of the N. persculptus Zone. These results are in agreement with the biostratigraphic and chemostratigraphic data at the Global Boundary Stratotype Section and Points (GSSPs) for the base of the Hirnantian and Silurian in south-central China and at Dob9s Linn in Scotland.

The end Ordovician was a critical period in Earth history. It was a time of global climatic change with large-scale continental glaciations. A wealth of recent work has documented the stratigraphical records from the near-field areas, i.e., from various Gondwanan continental relics where glaciation was more severely felt. There is an urgent need to re-examine the stratigraphic record from the far-field areas at greater levels of resolution than it has been hitherto possible in order to understand the timing, duration, and character at the glaciation centred around the Ordovician/Silurian (O/S) boundary. One of the best exposed and most complete stratigraphic records from paleotropical areas near the boundary is on Anticosti Island in eastern Canada. The east–west trending, ∼ 200 km long Ellis Bay (Hirnantian) outcrop belt, with superb coastal exposures at both ends of Anticosti Island, is slightly oblique to the paleoshoreline with near-shore siliciclastic-dominated facies restricted to the eastern part of the island and more offshore carbonate-dominated facies present along the central and western parts. These mixed carbonate–siliciclastic facies accumulated along a storm-dominated beach-to-offshore profile. They are organized into a stacked set of shallowing-upward offshore to shoreline facies successions (typically coarsening upward) separated by deepening upward facies successions (typically fining upward) formed during intervening transgressions. High-frequency glacio-eustatic fluctuations were the dominant control on the development of TR sequences observed in the Ellis Bay succession of Anticosti Island, although tectonic subsidence had a substantial secondary effect on thickness. Five TR sequences are recognized allowing for precise correlation along the Ellis Bay outcrop belt. In the eastern sections, relatively thin TR sequences are locally bounded by unconformities. A typical sequence begins with thin transgressive limestone overlain by a maximum flooding, calcareous shale grading up into regressive, storm-influenced, proximal to transitional zone fine sandstone/grainstone and shale, which are sharply overlain by shoreface to foreshore sandstone/grainstone. In the central and western sections, the thicker TR sequences have more symmetrical transgressive–regressive components reflecting greater subsidence compensated by a higher carbonate sediment supply. Two discrete glacial periods are recognized during the deposition of the Hirnantian Ellis Bay succession with the younger one recording large sea level changes (> 50 m in magnitude) caused by the development of very large ice sheets over western Gondwana. Abundant ice conditions on the globe during the end Ordovician, and the resulting high-amplitude eustasy, appear to have left distinctive stratigraphical imprints on mixed, paleotropical, siliciclastic–carbonate ramps.

Mélanges are common in the Humber and Dunnage zones of the Quebec Appalachians. Humber Zone mélanges are spatially associated with Taconian nappes and believed to be of tectonic origin. Dunnage Zone mélanges are interpreted as relicts of an Ordovician accretionary prism. The Ruisseau Isabelle Mélange (RIM) in the Gaspé Appalachians was first interpreted as a Dunnage Zone mélange, because of its tectonic position along the Baie Verte – Brompton Line and lithological components, such as dark mudstone, blocks of ultramafic rocks, and greenish chromite-bearing sandstone. The RIM comprises sedimentary rock assemblages (Composite Shale, Black Shale, and Chromite-Bearing Sandstone assemblages), an Exotic Block, a Pebbly Mudstone Assemblage and slivers of Serpentinized Peridotite, and Metamorphic Tectonite. Most mélanges of the Dunnage Zone in the Quebec Appalachians contain partly similar rocks, but their age is not well constrained. Recent work on the RIM and Cap-Chat Mélange of the Humber Zone and new fossil finds in the RIM rock assemblages allow us to conclude that previous distinction between mélanges of the Dunnage and Humber zones on the basis of rock-type associations is not valid. According to the age of the RIM rock assemblages, mélanges in the northern Gaspé Appalachians continued forming after the mid-Caradocian, later than the Dunnage Zone mélanges of southern Gaspé Appalachians. The juxtaposition and structural features of the RIM rock assemblages result from repeated faulting along the Shickshock Sud fault from Late Ordovician to Middle Devonian.