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The Pliensbachian/Toarcian boundary in Canada: ammonite succession and correlations

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Macchioni F. , Palfy J. , Smith P.

tom Vol. 4, no. 1

182-182

The Pliensbachian-Toarcian transition in North America is best typified by the Queen Charlotte Island ammonite succession where 3 assemblages are recognized, all of which include representatives of Lioceratoides, Protogrammoceras and Tiltoniceras. The lower assemblage also includes Fanninoceras, Amaltheus and Arieticeras, and is placed in the Pliensbachian (Carlottense Zone). The upper assemblage also includes the first occurrences of Dactylioceras and is placed in the Toarcian (Kanense Zone). The correlation of the intermediate assemblage is uncertain and it was initially placed in the Pliensbachian because it occurred beneath Dactylioceras and above Amaltheus. However, Pleuroceras that characterizes the northwest European uppermost Pliensbachian does not occur in British Columbia and it is also questionable whether the incoming of the genus Dactylioceras can be used as a means of correlation with the basal Toarcian (Tenuicostatum Zone) of northwest Europe. Consequently, it has been difficult to confidently correlate the Pliensbachian-Toarcian transition as defined in North America with the boundary as defined in northwest Europe. Light is being shed on this problem by recent work on faunas from the Laberge Group in the Yukon Territory and the Spatsizi Formation (Hazelton Group) in northcentral British Columbia. As has often been noted, Canadian sequences show stronger affinities with Mediterranean successions rather than the Boreal successions of northwest Europe. In addition to representatives of Fontanelliceras, Neolioceratoides, Canavaria, Bouleiceras and Tauromeniceras, there are several new occurrences of Dactylioceras. The Tethyan Dactylioceras cf. simplex occurs low in the Kanense Zone and suggests a correlation with the D. simplex horizon of the basal Polymorphum Zone in the Mediterranean area, which predates the basal Tenuicostatum Zone of NW Europe.