Structure and Stratigraphy of Southern Part of Wah Wah Mountains, Southwest Utah

ABSTRACT The area investigated is in the southern part of the Wah Mountains of southwest Utah. With regard to regional structural relations, the area lies a short distance west of the frontal belt of Laramide thrusts developed at the eastern border of the Great Basin. Two large thrusts are present in the Wah Wah Mountains; these thrusts divide the area into three structurally and stratigraphically distinct units: an “autochthon,” the Blue Mountain thrust plate, and the Wah Wah thrust plate. The lower or Blue Mountain thrust brings a sequence of rocks ranging in age from Middle Cambrian through Early Pennsylvanian over the “autochthon,” which consists of clastic Mesozoic strata ranging in age from Early Triassic to Late Jurassic. The “autochthon” is essentially a plateau-type continental section. Some of the Triassic units are partly metamorphosed to hornfels by a postulated intrusive body in the subsurface. The “autochthon” is exposed at Blue Mountain in the eastern part of the area and in several windows in the south-central part of the range. The Blue Mountain thrust can be traced for 12 miles along the strike and has a minimum horizontal displacement of 9 miles. It is at least as young as latest Jurassic and probably formed during the Laramide orogeny. The rocks of the Blue Mountain thrust plate are an easterly facies of the Paleozoic sequence of the eastern Great Basin. This easterly facies is characterized chiefly by thinning and omission of units found in the more westerly facies toward the interior of the geosyncline. A sequence of Upper Devonian-Mississippian rocks disconformably overlies the Middle Devonian Simonson Dolomite; this break presumably is the same as the regional unconformity that is found farther northeast in the Great Basin. The Mississippian sequence within the Blue Mountain thrust plate displays a distinct facies in contrast to that of the Mississippian of eastern Nevada and westernmost Utah. Several imbricate thrusts are present within the Blue Mountain thrust plate, and overfolding indicates an east-southeasterly direction of yielding. The upper or Wah Wah thrust has brought a sequence of rocks ranging in age from latest Precambrian to Late Cambrian over the Paleozoic rocks of the Blue Mountain thrust plate. The main part of the Wah Wah Mountains is composed of the Cambrian sequence of the upper plate. The Cambrian sequence continues northward into the House Range. The Paleozoic sequence of the Wah Wah thrust plate continues unbroken northwestward into the Confusion Range. Thus, the Wah Wah, House, and Confusion Ranges are assigned to the same major thrust plate. Farther northeast the Wah Wah thrust is extended by the Frisco thrust mapped by E. H. East (1956) and possibly also by the Canyon Range thrust mapped by F. W. Christiansen (1952). The Wah Wah thrust is interpreted as being genetically related to a frontal breakthrough of the Snake Range decollement (Hazzard et al., 1953; Misch, 1960). A large part of the area is covered by Tertiary volcanic rocks which have been moderately faulted, tilted, and in part weakly folded.