How can ancient river channels end up standing above the surrounding land?

Details

This observational geological study describes two types of sand bodies—linear and tabular—in the nonmarine Tongue River Formation of Paleocene age in western North Dakota. It examines their shapes, grain-size patterns, sedimentary structures, paleocurrent indicators and concretions, and describes how the concretions can be used in aerial-photo and field mapping.

The tabular sand bodies have sharply scoured bases and become finer upward. Their vertical sequence of sedimentary structures indicates a decrease in flow regime upward. They are overlain by gray lignitic silts and clays and are interpreted as deposits of high-sinuosity streams.

Linear sand bodies are the most common type in the formation. They are usually straight, with deeply channeled bases and flat tops. Vertical grain-size changes have no consistent pattern, although the upper parts become finer upward in many places. Their sedimentary structures comprise about 60% planar (omikron) cross-stratification, 25% horizontal or low-angle planar laminae (plane bed) with parting lineation, and 15% small-scale ripple cross-stratification. Paleocurrent indicators run parallel to the axes of these bodies, which are interpreted as deposits of low-sinuosity streams.

Elongate concretions, up to about 15 ft wide and hundreds of feet long, occur only in the linear bodies. They consist of calcite-cemented sandstone and usually occur at or near the tops of the bodies. They help protect the sand bodies from erosion, so that in places the bodies survive as exhumed paleochannel systems forming an inverted topography. The axes of the concretions parallel both the paleocurrent indicators and the axes of the sand bodies. The concretions are clearly visible on aerial photographs and can readily be used to map ancient channel patterns, either on those photographs or in the field.