Determining the age and depositional model of the Doig Phosphate Zone in northeastern British Columbia using conodont biostratigraphy

Abstract Conodont biostratigraphy of the upper Montney and Doig formations in the subsurface of northeastern British Columbia provides the first age constraints on the boundary between these two formations. Previously presumed to be broadly but uniformly equivalent to the Spathian-Anisian boundary, it is shown herein to be highly diachronous, ranging from Spathian to Middle Anisian in age. It is oldest in the Altares area (at 56.201389 N; 121.906667 W) and youngest in the Dawson area (at 55.846389 N; 120.203333 W). The fact that the Montney-Doig boundary is oldest in the centre of the study area and younger in all directions suggests that the basal Doig Formation does not represent simple west-east transgression as previously thought. Rather, the Doig Formation was apparently deposited in the Altares region first and transgression must have proceeded away from this point. The lowest part of the Doig Formation, the Doig Phosphate Zone, has long been recognised as a condensed horizon. However, it is not condensed equally throughout northeastern British Columbia. It is most condensed in the Swan and eastern Groundbirch areas, and most expanded in the Altares and western Groundbirch areas. Together, these observations support the presence of palaeo-highs within and to the west of the Western Canada Sedimentary Basin during the Middle Triassic, a hypothesis that has been proposed previously on the basis of sedimentary thickness variation and provenance studies.