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Evidence for marine influence on a low-gradient coastal plain

Ichnology and invertebrate paleontology of the lower Tongue River Member (Fort Union Formation, middle Paleocene), western Williston Basin, U.S.A.

¹ Department of Geology, Amherst College, Amherst, MA 01002, U.S.A.
² Department of Environmental Science & Geology, Mary Washington College, 1301 College Avenue, Fredericksburg, VA 22401-5308, U.S.A.
³ Department of Geology, Smith College, Northampton, MA 01063, U.S.A.
⁴ Department of Geography and Earth Sciences, University of North Carolina, Charlotte, NC 28223, U.S.A.
⁵ Department of Geology and. Geological Engineering, and Energy & Environmental Research Center, University of North Dakota, Grand Forks, ND 58202, U.S.A.
⁶ Department of Biology and Geology, Bemidji State University, Bemidji, MN 56660, U.S.A
⁷ Department of Geosciences, University of Nebraska, Lincoln, NE 68588, U.S.A.

^* Correspondence should be addressed to: esbelt{at}amherst.edu

The Paleocene Tongue River Member of the Fort Union Formation contains trace-fossil associations indicative of marine influence in otherwise freshwater facies. The identified ichnogenera include: Arenicolites, Diplocraterion, Monocraterion, Ophiomorpha, Rhizocorallium, Skolithos linearis, Teichichnus, Thalassinoides, and one form of uncertain affinity. Two species of the marine diatom Coscinodiscus occur a few meters above the base of the member. The burrows occur in at least five discrete, thin, rippled, fine-grained sandstone beds within the lower 85 m of the member west of the Cedar Creek anticline (CCA) in the Signal Butte, Terry Badlands, and Pine Hills areas. Two discrete burrowed beds are found in the lower 10 m of the member east of the CCA in the little Missouri River area.

Abundant freshwater ostracodes include Bisulcocypridea arvadensis, Candona, and Cypridopsis. Freshwater bivalves include Plesielliptio and Pachydon mactriformis. We recognize four fossil assemblages that represent fluvio-lacustrine, proximal estuarine, central estuarine, and distal estuarine environments. Biostratal alternations between fresh- and brackish-water assemblages indicate that the Tongue River Member was deposited along a low-gradient coastal plain that was repeatedly inundated from the east by the Cannonball Sea.

The existence of marine-influenced beds in the Tongue River Member invalidates the basis for the Slope Formation.

Key Words: bivalves • diatoms • estuarine environment • Fort Union Formation • Tongue River Member • Ostracoda • trace fossils • Paleocene • Williston Basin

This article has been cited by other articles:

				D.J. Peppe, D.A.D. Evans, and A.V. Smirnov Magnetostratigraphy of the Ludlow Member of the Fort Union Formation (Lower Paleocene) in the Williston Basin, North Dakota Geological Society of America Bulletin, January 1, 2009; 121(1-2): 65 - 79. [Abstract] [Full Text] [PDF]

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