Geology of the Wilson Cliffs-Potosi Mountain Area, Southern Nevada

Abstract

Detailed mapping of seven lithologic subunits in the Bonanza King Formation in the Wilson Cliffs-Potosi Mountain area, eastern Spring Mountains, Nevada, demonstrates the existence of two thrust plates between the NW-trending Cottonwood and La Madre faults. The structurally lower Wilson Cliffs thrust plate is thrust eastward along a spectacularly exposed contact juxtaposing nearly black Cambrian Bonanza King dolomite above white to pale red Jurassic Aztec Sandstone. This thrust has been called the Keystone thrust by previous workers, but the Keystone thrust can be traced from its type area in the Goodsprings District northward, where it forms the base of the Keystone thrust plate lying structurally above the Wilson Cliffs plate. The Wilson Cliffs plate is a remnant of the Contact thrust plate to the south and the Red Spring thrust plate to the north. Emplacement of the Contact-Wilson Cliffs-Red Spring thrust plate preceded the emplacement of the Keystone thrust and the two events are separated by movement on the La Madre fault and a period of erosion that locally removed the Wilson Cliffs plate. The Cottonwood fault displaces the Wilson Cliffs plate, but ends to the northwest by warping of the Keystone plate. The deformation along the Cottonwood fault can be explained by post-Keystone southside-down displacement of 3500 to 3800 feet (1060 to 1150 m), and suggests that Cenozoic deformation may be more important within the Spring Mountains than currently recognized. Presently the Contact-Wilson Cliffs-Red Spring thrust plate lies below and east of the Keystone thrust plate, a relationship that has been used to demonstrate that this part of the Cordilleran thrust belt did not become progressively younger eastward. However, at a low structural level, the ramp for the Keystone thrust fault lies east of the ramp for the Contact-Wilson Cliffs-Red Spring thrust fault and thus is in sequence. At a higher structural level, the Keystone thrust fault propagated across the Contact-Wilson Cliffs-Red Spring thrust plate, placing the leading edge of this plate east and below the surface trace of the Keystone thrust. Thus, the present map pattern gives the erroneous impression of an out-of-sequence relationship.