Neil E. Tibert¹, H. Allen Curran¹, Joseph H. Hartman², Edward S. Belt³, John A. Diemer⁴

(1) Smith College, Northampton, MA
(2) University of North Dakota, Grand Forks, ND
(3) Amherst College, Amherst, MA
(4) University of North Carolina, Charlotte, NC

ABSTRACT: Marine Flooding Intervals Within the Freshwater Tongue River Member (Fort Union Formation - Paleocene), Western Williston Basin, U.S.A

The Tongue River Member is laterally extensive across eastern Montana and western North Dakota. Throughout this region, sediment accumulated primarily on an alluvial plain with a mosaic of episodic crevasse splay and interdistributary bay environments. Until recently, most invertebrate fossils reported were freshwater unionid bivalves and gastropods, and the Tongue River facies were interpreted as nonmarine.

Trace fossils of likely marine origin were reported at discrete horizons within the Tongue River from three districts in Montana (Miles City, Locate, and Terry) and one in North Dakota (Little Missouri River). On the basis of more recent paleontological investigations, we report burrowed sandstone beds that occur on average every 10 to 16 m within the 85 m of lower Tongue River strata. Trace fossils include in order of decreasing abundance: Skolithos linearis, Teichichnus, Thalassinoides, Diplocraterion, Arenicolites, Monocraterion, and Rhizocorallium. In addition to trace fossils, agglutinated foraminifera occur at Signal Butte (Montana) 40 cm below a burrow bed and include Trochammina and Eggerella indicative of estuarine influence. Furthermore, the brackish-water bivalve Corbula mactriformis forms coquina beds in the Little Missouri River section. Finally, nonmarine ostracodes include Bisulcocypridea, Candona, Eucypris?, and Darwinula. Bisulcocypridea dominates the ostracode assemblages associated with the C. mactriformis coquina beds.

The Cannonball Sea that lay to the east in central North Dakota was the source for marine deposition in the Tongue River Member where the youngest brackish beds are Tiffanian-2 (North American Land Mammal Age). Controls on the stratal cyclicity include a complex interplay of processes such as delta-lobe switching and tectono-eustasy. Given the lenticularity of the fluviodeltaic lithofacies, units that contain marine biota associated with thin coal seams, typical of the lower Tongue River Member, are the most favorable for regional correlation of marine influence.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado