Conodonts of the Bakken Formation (Devonian and Mississippian), Williston Basin, North Dakota

Abstract

The Bakken 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 Formation are major source rocks for hydrocarbons and a distinctive fog marker in the Williston Basin. Samples were selectively collected from Bakken cores from 17 wells in northwestern North Dakota in an effort to determine the age and thermal maturity of the formation using conodonts. A diverse conodont collection of more than 650, 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 of Siphonodella, Pseudopolygnathus, Polygnathus, Bispathodus, "Spathognathodus", Palmatolepis, and Branmehla. Conodonts are unevenly distributed in the Bakken Formation. Most were obtained from thin (about 0.2 in; 5mm), 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 Siphonodoella, 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 as reported by other workers. Paleontologic evidence suggests that an unconformity occurs 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 7500 ft; 2290 m) to 2.5 (about 10,400 ft; 3170 m) and indicate that the Bakken has reached formation temperatures capable of oil generation at 7500 ft in depth but may have exceeded oil generation temperatures below 10,000 ft (3050 m). Lithologic and paleontologic evidence suggests that 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. Conodonts from the Bakken Formation, together with regional evidence, indicate 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.