The end-Ordovician chitinozoan zones of Anticosti Island, Québec: Definition and stratigraphic position

Abstract

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 Vauréal 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.