Spectacular crinkled crust—A detachment fold train in the Carmel Formation, western San Rafael Swell, Utah

Abstract

Imagine slipping on a small rug overlying a hardwood floor. In the process of sliding along the floor the rug produces a series of small folds and the rug moves forward from its original position. The same could be said for the “crinkled crust,” or folded layers of rocks in a detachment fold train. A spectacular detachment fold train, consisting of over 100 small, regularly spaced convex-upward folds called anticlines in gypsum-rich rock layers of the Middle Jurassic (about 168 million years ago [Ma]) Carmel Formation, is exposed immediately north of Interstate 70 (I-70) in the San Rafael Swell of east-central Utah (figures 1 and 2). The San
Rafael Swell, a large anticlinal uplift, is an icon for everything that makes the Colorado Plateau dramatically scenic and geologically classic. However, the fold train is located in drab-colored, relatively featureless rock layers of the Carmel Formation in an area called Reed Wash along the gently dipping west flank of the Swell. After passing magnificent canyons, buttes, and mesas both to the east and west along I-70, the fold train typically goes unnoticed by not only the average tourist but geologists as well. Once the fold train is pointed out, the geologic observer is immediately struck with awe at this large, well-exposed, complex structural feature.

Literally hundreds of classic geologic sites are well displayed in the San Rafael Swell; many are easily accessed overlooks and viewpoints. The detachment fold train, by contrast, is chosen as a geosite for its geologic uniqueness, educational instruction, and research opportunities in structural geology.