Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Belt, E. S. Articles by Curran, H. A. Search for Related Content GeoRef GeoRef Citation

Unconformities and age relationships, Tongue River and older members of the Fort Union Formation (Paleocene), western Williston Basin, U.S.A.

¹ Department of Geology, Amherst College, Amherst, MA 01002, U.S.A.
² Department of Geology and Geological Engineering, and Energy & Environmental Research Center, University of North Dakota, Grand Forks, ND 58202, U.S.A.
³ Department of Geography and Earth Sciences, University of North Carolina, Charlotte, NC 28223, U.S.A.
⁴ Department of Biology and Geology, Bemidji State University, Bemidji, MN 56660, U.S.A.
⁵ Department of Geology, Smith College, Northampton, MA 01063, U.S.A.

^* Author to whom correspondence should be addressed esbelt{at}amherst.edu

An unconformable relationship is observed within the Paleocene Fort Union Formation in the western Williston Basin at the contact between the Tongue River Member and the underlying Lebo and Ludlow Members. Isotopic dates and pollen biozone data reported here are integrated with previously published data. A new correlation of these facies results in a revised history of localized depositional and tectonic events. One unconformity occurs at this lithological contact in the Pine Hills (PH), Terry Badlands (TB), and Ekalaka (E) areas west of the Cedar Creek anticline (CCA), and another unconformity occurs at the same lithological contact in the Little Missouri River (LMR) area east of the CCA. The two unconformities differ in age by about two million years. The older is the U₂ and the younger is the U₃, which initially were recognized in the Ekalaka area of southeastern Montana (Belt et al., 2002). The U₂ crops out in the TB, PH, and E areas, where at least 85 m of Tongue River strata bearing palynomorphs characteristic of biozone P-3 are found above the unconformity. Radiometric dates from strata (bearing palynomorphs characteristic of biozone P-2) below the U₂ range in age from 64.0 to 64.73 Ma. The U₂ unconformity west of the CCA thus occurs in strata near the base of the lower P-3 biozone.

The U₃ crops out in the LMR area (east of the CCA), where only 13 m of strata characterized by the P-3 pollen biozone occur above it. Radiometric dates from an ash <1 m above the U₃ in that area range in age from 61.03 to 61.23 Ma, and the P-3/P-4 pollen biozone boundary is located 13 m above the ashes. The U₃ thus occurs in strata characterized by upper parts of the P-3 pollen biozone east of the CCA. The U₃ is also identifiable in the middle of the ca. 200 m-thick Tongue River Member west of the CCA, where mammal sites 40 to 80 m above it are Tiffanian-3 in age. The strata below this unconformity are tilted gently to the northwest; strata above the unconformity are flat lying. This mid Tongue River unconformity probably correlates with the unconformity at the base of the Tongue River Member in the LMR area east of the CCA, where a Ti-2 mammal site (the "X–X" locality) occurs <10 m above it.

Depositional and tectonic events can be summarized using North American Mammal Age nomenclature as a relative time scale. From latest Cretaceous through Puercan time, paleodrainage was toward the east or southeast, in the direction of the Cannonball Sea. The Black Hills did not serve as an obstruction at that time. During early Torrejonian time, the Miles City arch (MCA) and Black Hills were uplifted and partially eroded, leading to the U₂ unconformity. When deposition resumed, paleodrainages shifted to a northeasterly course. During middle and late Torrejonian time, facies of the lower Tongue River ("Dominy") sequence and the Ekalaka Member of the Fort Union Formation were deposited in the middle of a subbasin between the MCA and the CCA. Simultaneously, smectite-rich components of the Ludlow Member were being deposited east of the CCA. During latest Torrejonian time, uplift of the Black Hills tilted the "Dominy" sequence toward the northwest and local erosion led to the U₃ unconformity. Following this tilting, during Tiffanian time, deposition of the upper Tongue River ("Knobloch") sequence shows continuity from western North Dakota across eastern Montana and into the northern Powder River Basin.

Key Words: mammal ages • Paleocene • pollen biozones • radioisotopic ages • stratigraphy • tectonics • unconformities • 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]

				R. Secord, P. D. Gingerich, M. E. Smith, W. C. Clyde, P. Wilf, and B. S. Singer Geochronology and Mammalian Biostratigraphy of Middle and Upper Paleocene Continental Strata, Bighorn Basin, Wyoming Am J Sci, April 1, 2006; 306(4): 211 - 245. [Abstract] [Full Text] [PDF]

				E. S. Belt, N. E. Tibert, H. A. Curran, J. A. Diemer, J. H. Hartman, T. J. Kroeger, and D. M. Harwood 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. Rocky Mountain Geology, July 1, 2005; 40(1): 1 - 24. [Abstract] [Full Text] [PDF]