Abstract

Both linear and tabular sand bodies occur in the nonmarine Tongue River Formation (Paleocene) of western North Dakota. The tabular sand bodies have sharply scoured bases, become finer grained upward, and the vertical sequence of sedimentary structures shows a decrease in the flow regime upward. These tabular bodies are overlain by gray lignitic silt and clay, and are interpreted as high-sinuosity stream deposits. Linear sand bodies are usually straight and have deeply channeled bases and flat tops. Vertical changes in grain size show no consistent pattern, but the upper parts of many bodies become finer grained upward. Sedimentary structures are about 60 percent planar (omikron) cross-stratification, 25 percent horizontal or low-angle planar laminae (plane bed) with parting lineation, and 15 percent small-scale ripple cross-stratification. Paleocurrent indicators are parallel to the axes of the linear bodies, which are interpreted as being low-sinuosity stream deposits.

Elongate concretions as much as 5 m wide and a few hundred meters long occur in the linear bodies. The concretions consist of cal-cite-cemented sandstone, and usually occur at or near the tops of the bodies. The long axes of the concretions are parallel to both paleocurrent indicators and the long axes of the sand bodies, and so the concretions can be used to map paleochannel patterns.

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