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 Vauréal 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 Vauréal occur in the rest of the formation. Moreover, the eastern Schmitt Creek and Mill Bay members of the Vauréal Formation are older than the uppermost Vauréal 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 Dob's Linn in Scotland.