Date of Award
1985
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Geology
First Advisor
F.D. Holland, Jr
Abstract
The Bakken Formation, up to 145 feet thick in North Dakota, is a subsurface formation in the Williston Basin that typically consists of two black shale members separated by a middle member of predominantly gray siltstone or silty limestone up to 85 feet thick.
Well over 500 macrofossils representing more than 50 taxa were collected from cores of 40 wells. Brachiopods, the most common fossil, represent 17 genera, 1 of which have not been previously reported from the Bakken. Nonbrachiopod fossils of this study, mostly not previously reported from the Bakken, are a hylothyrid, a conulariid, a syringoporid coral, several genera of gastropods and pelecypods, straight and coiled cephalopods, a trilobite, a conchostracan, a shrimp-like organism, pelmatozoan columns, fish fragments, trace fossils, and Foerstia sp. and other plants. Fossils not treated herein are foraminiferids, ostracods conodonts, and palynomorphs.
The macrofossils of the Bakken occur mostly within five stratigraphic intervals; each interval contains a different assemblage and appears to represent a geographic extension of a previously known fauna. The basal few feet of the lower shale member locally contains a benthic rhynchonellid fauna that appears similar to fossils from near the base of Devonian black shales in the eastern United States. Foerstia sp., found in the basal 20 to 30 percent of the lower shale member in one core, appears to mark a widespread time-stratigraphic interval of Late Devonian age. The conchostracan Cyzicus (Lioestheria sp. is prolific in the upper few inches of the lower shale near the center of the basin; it is concentrated at a similar interval in correlatives of the Bakken that crop out in the Western Interior. The other fossiliferous intervals of the Bakken occur in the middle member and are found in association with lithologies that are used here to divide the middle member into three stratigraphic units: units 1, 2, and 3. The lowest of these, unit 1, up to about 30 feet thick, contains a Syringothyris brachiopod assemblage similar to that of the Louisiana Limestone of latest Devonian age in the upper Mississippi Valley and in siltstones of Bakken correlatives in the western United States, also of latest Devonian age. Unit 2 is up to about 35 feet thick and is poorly fossiliferous. Blade-like "leaves" are abundant in the basal few feet of unit 2 near the center of the basin, and correlate with similar “leaves” that are locally abundant at the base of the Mississippian type section in Illinois. This correlation, together with the correlation of the brachiopods in units 1 and 3, suggest an early Mississippian age for unit 2. The base of unit 2, a poorly defined contact, thus appears to mark the base of the Mississippian in North Dakota. Unit 3 of the Bakken, about 5 feet thick, contains brachiopods that correlate with those of the McCraney Limestone in the type Mississippian section and the Spirifer marionensis assemblage of the Exshaw Formation in Alberta, both of early, but not earliest, Kinderhookian age; this may be the first correlation made between the McCraney and Exshaw. Macrofossils in the upper shale member are generally small, thin-shelled, and rare; no fossiliferous intervals or age-diagnostic fossils were found in this member.
The lower Bakken shale appears to have been deposited in a deepened sea during a period of worldwide rise in sea level. Unit 1 of the middle member was deposited during a major regression and units 2 and 3 are transgressive, shallow-water deposits. The upper Bakken shale was deposited during a major, worldwide rise in sea level.
Recommended Citation
Thrasher, Lawrence C., "Macrofossils and biostratigraphy of the Bakken Formation (Devonian and Mississippian) in western North Dakota" (1985). Theses and Dissertations. 297.
https://commons.und.edu/theses/297