Abstract
Four basic lithologic types are present in the Tongue River Formation in western North Dakota. These are arranged in a basic cyclic unit consisting, from bottom to top, of (1) gray clay and silt, (2) lignite, (3) yellow silt and sand that may be clayey, and (4) sand. This cyclic unit probably originated during the formation and filling of flood basins on an alluvial plain.
The units of gray clay and silt show irregular stratification, and are commonly lignitic and fossiliferous. These units are interpreted as flood-basin deposits. As the flood basins filled, swamps became established and lignite beds formed. Crevassing and expansion of natural levees across the flood basins would have destroyed the swamps and deposited yellow silt and sand over the lignite. The yellow silt and sand commonly show climbing ripples and remnants of vertical tree trunks, indicating high rates of deposition characteristic of natural levees and crevasse splays.
Two types of sand bodies are present. Tabular sand bodies generally are about 10 ft thick and thousands of feet wide; they become finer grained upward and have a vertical sequence of sedimentary structures that shows a decrease in the flow regime upward through the deposit. These sand bodies probably are point-bar deposits formed by lateral accretion in high-sinuosity streams. Linear sand bodies are straight and show deeply eroded bases. Paleocurrent indicators trend parallel with the axes of these bodies, which are interpreted as low-sinuosity channel deposits. Elongate concretions, visible on air photographs also trend parallel with the axes of the sand bodies and can be used to map paleochannel patterns.
