Thrust Fault in the San Raphael Swell
May 31, 2018
This outcrop is certainly one of the most photogenic examples of a thrust fault on the planet. Thrust faults and reverse faults, unlike normal faults, are formed by compressional motion. Converging forces, confounding the principle of superposition, will cause the fault system’s hanging wall to slide above the foot wall. This fault system contains several obvious fault planes and exhibits a vertical offset of approximately 10 m (bottom photo – I’m standing on the lower fault plane for scale). Besides the dramatic vertical off-set, several drag folds are easily recognizable. Additionally, careful examination of the talus around the outcrop may reveal rocks with slickenside features. (inset photo).
Geologically this feature is located at the foot of Cedar Mountain near the northern extent of Utah’s San Raphael Swell. The fault, which may be attributed to the Sevier orogeny, is clearly younger than the Jurassic sedimentary layers that it cuts across, including the red Entrada Sandstone and the lighter colored member of the Curtis Formation.
Geographically, the exposure, known to some structural geologists and geocaching enthusiasts as Ketobe Knob, is in the San Raphael Swell Recreation Area, located approximately 16 kilometers ESE as the drone flies from Lawrence, Utah. Photo taken on April 2, 2018.
Photo Details: Top - Camera: NIKON D5100; Exposure Time: 0.0080s (1/125); Aperture: ƒ/11.0; ISO equivalent: 100; Focal Length (35mm): 27. Bottom - Same except: ISO equivalent: 110; Focal Length (35mm): 54. Inset - Same as top photo.