Abstract

A Lower Triassic stage boundary (Dienerian-Smithian), defined by conodonts in two adjacent samples, occurs within the Mackenzie Dolomite Lentil of the Sulphur Mountain Formation 5 km south of Cadomin, Alberta. A sparse lower Dienerian (upper Induan) conodont fauna including Neogondolella carinata and Neospathodus kummeli occurs low in the exposed carbonate. An abundant, lower Smithian (lower Olenekian) fauna, 2.1 m higher in the section, consists primarily of Neospathodus pakistanensis and N. waageni. These two species are associated with a major episode of transgression that is recorded worldwide. In North America, both faunas are best represented in the Lower Triassic depocentre of southeast Idaho in the western United States. The apparent absence of upper Dienerian conodonts suggests the Mackenzie Lentil, a complex of offshore shell banks, may contain an interval of slow deposition or an episode of erosion. As a byproduct of this study, a modest revision of stratigraphic nomenclature is proposed. We recommend establishing the lower and upper boundaries of the Mackenzie Dolomite Lentil in its depositional area, respectively, as the upper boundary of the Phroso Siltstone Member and the lower boundary of the Vega Siltstone Member, thus establishing this carbonate lentil as a member of the Sulphur Mountain Formation.

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