Reverse faults

Reverse fault zone, Ketobe Knob, Utah

Reverse fault zone, Ketobe Knob, Utah

Reverse faults shorten horizontal layers and are therefore contractional faults. This is an example of a reverse fault in fine-grained sandstones and siltstones of the Entrada Sandstone (lower reddish part) and Curtis Formation (upper part) – part of the Jurassic stratigraphy of the Colorado Plateau.

Reverse faults (and any other type of fault for that matter)  may look nice and clean on drawn-up cross-sections or seismic images, but in detail they tend to be more complicated.  In this case it is being composed of several strands and numerous small-scale deformation structures. Hence at this scale the structure is better termed a fault zone than a single fault.

This is an outstanding outcrop where the fault zone can be studied in three perpendicular vertical sections, in addition to the top section. The deformation is likely related to the Cretaceous Sevier orogeny, which is better expressed in western Utah and Nevada.

Detail of reverse fault. Note bending of hanging-wall layers (drag)

Detail of reverse fault. Note bending of hanging-wall layers (drag). If you magnify (click) and look hard you may even see a little back-thrust in the drag zone.

The locality is found along the western margin of Cedar Mountain in the northern part of the San Rafael swell east of Castle Dale, Utah. The place is known as the Ketobe Knob among structural geologists. Look out for rattlesnakes at this locality…

A quick interpretation of the locality

A quick interpretation of the locality

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About Haakon Fossen

Professor of structural geology, University of Bergen. Author of book Structural Geology, published at Cambridge University Press
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