Date of Award


Document Type


Degree Name

Master of Science (MS)



First Advisor

A.M. Cvancara


Nine molluscan species are known to occur in Tongue River Sentinel Butte sediments of parts of Billings, Golden Valley, and Slope Counties in southwestern North Dakota. Tongue River sediments contain all nine of the species which include: two unionids, Plesielliptio priscus (Meek and Hayden) and Rhabdotophorus senec tus (White); one corbulid, Bicorbula mactriformis (Meek and Hayden); and six viviparids, Viviparus retusus (Meek and Hayden), Viviparus trochiformis (Meek and Hayden), Campeloma nebrascensis (Meek and Hayden), Lioplacodes limnaefonnis (Meek and Hayden), Lioplacodes nebrascensis (Meek and Hayden), and Lioplacodes tenuicarinata (Meek and Hayden). Only five of these species (P. priscus, Q. nebrascensis, V. trochiformis, L. limnaeformis, and L. tenuicarinata) occur within the Sentinel Butte Formation of North Dakota.

Four molluscan species (R. senectus, B. mactriformis, V. retusus, and L. nebrascensis) may be of value as stratigraphic indicators for the Tongue River Formation since preliminary work indicates that they do not occur in Sentinel Butte sediments.

The fresh-water molluscan fauna concludes a fresh-water origin for the sediments, but the presence of a corbulid may indicate that some Tongue River deposits may have been brackish-water in origin.

The fossil assemblage suggests low-energy environments such as shallow water lakes with sluggish streams. This conclusion contradicts the work of others who favor primarily a fluvial origin for the sediments.

The Tongue River Formation is considerably more fossiliferous than the Sentinel Butte Formation. This discrepancy of fossil abundance may be accounted for by characteristics of the sediments themselves, such as the coarser grain size, greater bentonite, and lower calcium content of the Sentinel Butte Formation.

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