Date of Award
Master of Science (MS)
F.D. Holland, Jr
The Bakken Formation is a thin (maximum 145 ft., 44 m), predominantly elastic unit in the subsurface of the Williston Basin in the United States and Canada. The formation consists of two, mostly non-calcareous, grayish-black to brownish-black shales separated by light to dark gray, calcareous and dolomitic siltstone and fine-grained sandstone. The carbonaceous, black shales of the Bakken produce a distinctive geophysical marker and are a major source rock for hydrocarbons in the Williston Basin.
Conodonts were selectively sampled from cores of the Bakken in North Dakota in an effort to determine the age and thermal maturity of the formation. A diverse conodont collection of more than 700, mostly fragmentary, elements was obtained by disaggregating portions of the cores. Bakken conodonts are placed in 48 taxa and attributed to 17 form-genera. Specimens consist mostly of platform elements. Genera include Siphonodella, Pseudopolygnathus, Polygnathus, Bispathodus, "Spathognathodus", Palmatolepis, and Branmehla. Twenty-one biostratigraphically useful species of these genera were identified. Conodonts are unevenly distributed in the Bakken Formation. Most were obtained from thin (about 0.2 in., 0.5 cm), fossil-rich beds in the upper shale. Only rare, fragmentary conodonts were recovered from the middle member. Conodont evidence indicates that the Bakken Formation in North Dakota is Late Devonian and Early Mississippian in age. A small conodont fauna from the lower shale includes species of Palmatolepis and Polygnathus and is tentatively considered of the Upper Polygnathus styriacus Zone (Famennian). Species of Siphonodella, Pseudopolygnathus, and Polygnathus dominate the upper shale fauna. Conodonts from the uppermost beds of the upper shale of the Bakken are of the Lower Siphonodella crenulata Zone (Kinderhookian). Conodont evidence from the Bakken indicates that portions of the formation are correlative with the Exshaw Formation in Alberta, Sappington Member of the Three Forks Formation in Montana, Leatham Formation in Utah and Idaho, middle member (Leatham Member) of the Pilot Shale in Utah and Nevada, Cottonwood Canyon Member of the Lodgepole Limestone in Montana, and the Englewood Formation in South Dakota. Paleontologic evidence suggests that an unconformity may occur at the contact between the middle member and upper shale of the Bakken and that this contact may coincide with the Devonian-Mississippian boundary.
Conodont color alteration index (CAI) values from the upper shale of the Bakken range from 1.5 (about 7,500 ft., 2,290 m) to 2.5 (about 10,400 ft., 3,170 m) and indicate that the Bakken has reached formation temperatures capable of oil generation at 7,500 feet in depth but may have exceeded oil generation temperatures below 10,000 feet (3,050 m).
Lithologic and paleontologic evidence suggests that the carbonaceous shales of the two shale members of the Bakken were deposited in an anoxic, offshore, marine environment. Bedding features and fossils of the middle member indicate a current-influenced, mostly aerobic marine environment. Regional evidence indicates that the lower shale and upper shale of the Bakken were deposited during episodes of widespread marine transgression and that middle member deposition occurred during a time of regional marine regression between these transgressions.
Hayes, Michael D., "Conodonts of the Bakken Formation (Devonian and Mississippian), Williston Basin, North Dakota" (1984). Theses and Dissertations. 129.