ABSTRACT

The Bakken Formation comprises a grey sandstone bed between two distinctive, radioactive black shales. It straddles the Devonian-Mississippian boundary and is recognized as a premier stratigraphic marker and hydrocarbon source rock in the Williston Basin. Conodonts were observed scattered on bedding planes in both the upper and lower Bakken shales from 11 wells. Thirteen samples containing conodonts were collected in an effort to determine the age and thermal maturity of the formation. The conodonts are placed in 21 taxa, consisting mostly of platform elements of Siphonodella, Polygnathus, Branmehla, Bispathodus, Pseudopolygnathus, Palmatolepis and "Spathagnathodus". Conodont evidence indicates that the Bakken is as old as the Late Famennian Lower Palmatolepis gracilis expansa Biozone and as young as the Kinderhookian Lower Siphonodella crenulata Biozone. The Bakken sandstone lacks precise age-diagnostic conodonts, but paleobotanical evidence indicates that the Devonian-Mississippian boundary may coincide with the base of unit 3 of the Bakken sandstone in Saskatchewan. Conodont and lithologic evidence suggest that the two Bakken shales were deposited in a euxinic, reducing, offshore marine environment under conditions of very slow rates of sedimentation.