Exhumed river channel

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An exhumed river channel is a ridge of sandstone that remains when the softer flood plain mudstone is eroded away. The process begins with the deposition of sand within a river channel (typically a meandering river) and mud on the adjacent floodplain. Eventually the channel is abandoned and over time becomes buried by flood deposits from other channels. Because the sand is porous (grain-to-grain contact leaves spaces between), groundwater flows more easily through the sand than through the mud of the floodplain deposits.

An exhumed rived channel (red arrow) in the Cedar Mountain Formation near Green River, Utah.

Exhumed river channels were once the site of a river system. Sand was deposited in the channel and mud on the adjacent floodplain as seen with this meandering river in western South Dakota.

Minerals (typically calcium carbonate) can cement the grains together converting the loose sand into sandstone. Meanwhile, pressure from overlying sediments compresses the floodplain mud converting it to mudstone. Millions of years later, erosion can remove the softer, less cemented mudstone and leave the more resistant sandstone as a linear ridge. Thus, the local landscape where these occur is an inverted topography: what was previously low is now high, and vice versa. Exhumed channels are important indicators for ancient stream flow direction.

Some of the best examples of exhumed river channels occur in the Ruby Ranch Member of the Cedar Mountain Formation southwest of Green River, Utah (Harris 1980). These channels are so large that they are visible from low orbit satellites. Exhumed tributaries can be seen with some of these channels.

[edit] References

Harris, D. R. 1980. Exhumed paleochannels in the Lower Cretaceous Cedar Mountain Formation near Green River, Utah. Brigham Young University Geology Studies 27:51–66.