This biostratigraphic study of the conodont faunas from the Devonian-Carboniferous boundary beds in southeastern Alberta and southwestern Saskatchewan suggests that there is a significant hiatus between the Big Valley and Bakken formations. The hiatus may span the Upper trachytera to Upper postera zones. It divides the Kaskaskia sequence of the Williston and Western Canada Sedimentary basins into lower and upper parts. The Devonian Big Valley Formation is composed of interbedded calcareous claystones and shallow marine carbonates. The uppermost limestone bed is overlain with an erosional contact by a rubble deposit that grades upward into a silty and fossiliferous claystone. This limestone bed contains conodonts of the Uppermost marginifera to Lower trachytera zones. The claystone bed is sharply overlain, by means of a pyritized lag deposit, by the euxinic, ＇＇offshore＇＇ shale facies of the lower shale members of the Bakken and Exshaw formations, which contain conodonts of the Famennian lower to Upper expansa zones. The middle (Coleville Sandstone) member of the Bakken Formation and the siltstone unit of the Exshaw Formation include fossils and sedimentary structures typical of elastic, shallow marine deposits. These units, composed of sandstone, siltstone and claystone, did not yield diagnostic, short-ranging conodonts but probably include the Devonian-Carboniferous boundary. The sandy and silty beds are overlain by another black, euxinic ＇＇offshore＇＇ shale facies: the upper shale member of the Bakken Formation. This black shale unit contains an early Tournaisian conodont fauna and changes upward and westward into a greenish-grey marine shale facies that forms the basal part of the Carboniferous Banff and Lodgepole formations. The greenish-grey shale facies is locally calcareous and yielded early Tournaisian conodonts of the sulcata to Lower crenulata zones.