Geological Survey Bulletin 1229 Geology of the Circle Cliffs Area, Garfield and Kane Counties, Utah

The Circle Cliffs area is a few tens of miles west of the Henry Mountains, on the west edge of the Colorado Plateau, and is bordered to the west by the High Plateaus of southeastern Utah. The climate is arid to semiarid, and the land is used mainly for cattle grazing. Rugged canyons, mesas, benches, and hogbacks, supported and walled by massive colorful sandstone beds, characterize the topography of the area; the maximum relief is about 4,800 feet.

The rocks in the central part of the area are gently to steeply folded along the northwest-trending axis of the Circle Cliffs anticline. Several thousand feet of stratified rocks were eroded from the anticlinal area during Mesozoic and late Tertiary time, and the oldest rocks now are exposed in the central Circle Cliffs area, along the axis of the anticline, and the youngest rocks on the perimeter of the area. The stratified sedimentary rocks that crop out are Paleozoic, Mesozoic, Tertiary, and Quaternary in age. Some of the Mesozoic sedimentary rocks contain concentrations of uranium minerals, and the study of these rocks and their included uranium deposits was one of the principal objectives of the investigation of the Circle Cliffs area.

Sandstone and mudstone are dominant lithologies in the rock sequence that is about 8,500 feet thick. Slightly more than half of the sequence was deposited in a continental or near-shore continental environment, and the rest was deposited in a marine or littoral marine environment. The Permian rocks are sandstone and dolomite and are chiefly of marine origin. The Triassic and Jurassic rocks are sandstone to mudstone and were deposited partly in continental and partly in marine environments. The Cretaceous rocks are mudstone and sandstone deposited mainly in a marine environment. The Tertiary and Quaternary rocks range from silt to gravel and were deposited entirely in a continental environment.

The sedimentary rocks are folded into an anticline, which is the major structural feature of the area. The anticline trends northwest and dips steeply to the northeast and gently to the southwest. It plunges gently northwest and southeast from the central part of the area. The vertical closure on the central part of the anticline is 1,200 feet, and total displacement of beds is about 9,000 feet. Few faults are present except on the southwest side of the anticline where a minor graben closely parallels the anticlinal axis. The dominant joint sets trend northeast and north to northwest. The anticline probably was formed in middle to late Paleocene time, but the joint systems may be as young as mid-Tertiary.

The uranium deposits are small, and opportunity for discovery of any large deposits seems poor. Most deposits are in the Moenkopi Formation on the edges of channels filled with sandstone of the Shinarump Member of the Chinle Formation. The criteria favoring minable deposits in the Shinarump are (1) the presence of a channel, (2) the presence of material in the channel sandstone capable of precipitating uranium, (3) a lenticular mudstone-rich sandstone in the channel that has considerable hydraulic continuity with the rest of the unit, and (4) possibly the presence of an overlying cap of relatively impermeable mudstone. Uranium deposits occur in the Salt Wash Sandstone Member of the Morrison Formation near an area where the Salt Wash changes from a massive thick-bedded sandstone to a thin- to thick-bedded lenticular sandstone, but the discovered uranium deposits in the area of change contain only low-grade ore.