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The AAPG/Datapages Combined Publications Database

Williston Basin Symposium

Abstract

SKGS-AAPG

Fifth International Williston Basin Symposium, June 14, 1987 (SP9)

Pages 53 - 67

MACROFOSSILS AND STRATIGRAPHIC SUBDIVISIONS OF THE BAKKEN FORMATION (DEVONIAN-MISSISSIPPIAN), WILLISTON BASIN, NORTH DAKOTA

LARRY C. THRASHER, U.S. Bureau of Land Management, P.O. Box 1229, Dickinson, ND 58602

ABSTRACT

Well over 500 macrofossils representing more than 50 taxa were collected from 39 cores of the Bakken Formation in North Dakota. Brachiopods, the most common fossils, represent 17 genera, 11 of which have not been previously reported from the Bakken. Nonbrachiopod fossils, mostly not previously reported from the Bakken, include a syringoporid coral, several genera of gastropods and pelecypods, straight and coiled cephalopods, a trilobite, the conchostracan Cyzicus (Lioestheria) sp., a shrimp-like organism, trace fossils characterized by Scalarituba missouriensis, and Foerstia sp. and other plants.

Macrofossils of the Bakken were found mostly within five stratigraphic intervals; each interval contains a different fauna and thus the same vertical sequence of fossils was found from core to core. 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 per cent of the lower shale member in one core, appears to mark a widespread time-stratigraphic interval of Late Devonian age. Cyzicus (Lioestheria) sp. is prolific in the upper few inches of the lower shale near the center of the basin; this species is concentrated at a similar interval in correlatives of the Bakken that crop out in the western interior region. 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 fauna 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 1 appears to be the only regressive unit in an otherwise transgressive formation. 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 locally mark the base of the type Mississippian section in Illinois. This correlation, the correlation of the brachiopods in units 1 and 3, and the transgressive nature of unit 2 suggest an early Mississippian age for this unit. The contact between units 1 and 2, although poorly defined, thus appears to mark the systemic boundary in North Dakota. Unit 3, comprising about the top 5 feet of the middle member, contains brachiopods that seem to correlate with those of the McCraney Limestone in the type Mississippian section and the Spirifer marionensis fauna of the Exshaw Formation in Alberta, both of early, but not earliest, Mississippian age; this may be the first correlation made between the McCraney and Exshaw. The structural attitudes of unit 3 and the upper shale member suggest that these beds were deposited after a reorientation of the Bakken sea. 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.

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