MUDDY CREEK FORMATION: REEVALUATING MISCONCEPTIONS OF LATE MIOCENE TECTONICS AND SEDIMENTATION IN SOUTHERN NEVADA

Cenozoic Basin and Range sediments generally lack detailed study. Previous Basin and Range research has focused predominantly on Mesozoic and Paleozoic rocks and Cenozoic structural geology and volcanism to develop the geologic evolution of the region, with only minimal study of the Cenozoic sedimentary record. In the vicinity of Lake Mead, southern Nevada, Oligocene and younger sedimentary rocks record the tectonic evolution of the region. The Late Miocene Muddy Creek Formation (MCF) is part of this sedimentary sequence.

Limited previous studies of the MCF have loosely constrained the age, stratigraphy, and occurrence of the formation and concluded the formation was deposited as post-tectonic infill within local basins. Numerous workers cite the early MCF studies as partial evidence in support of tectonic reconstructions for the southern Basin and Range, for constraint on the timing of regional deformation events, or for tectonic studies of the Lake Mead fault system and Las Vegas Valley shear zone. However, poor constraint on the MCF limits the validity of these related studies and has led to misinterpretations. Detailed study of the MCF is in progress to constrain the timing, mode of deposition, and occurrence of the formation and validate or amend the conclusions of related studies.

Preliminary results from our in-progress study of the MCF are inconsistent with previous work and indicate that reevaluation of commonly-held conclusions is necessary. At present, the following preliminary conclusions are suggested, based upon exposures in the vicinity of the Overton Arm of Lake Mead: (1) Unambiguous syn- and post-depositional deformation is seen within MCF sediments. (2) Some units that were previously mapped as the MCF and are likely exposures of the Red Sandstone unit. (3) Some faults that cut the MCF during, and possibly after deposition of the formation, appear to be a southwest extension of the left-lateral Hen Spring Fault, which is part of the Lake Mead Fault system. (4) The Hen Spring Fault and Rogers Spring Fault are possibly kinematically linked through the Overton Arm area and were likely active during, and following, deposition of the MCF. (5) Numerous tuffs have been identified within the stratigraphy, which provide opportunities to improve the age resolution of the MCF, and possibly the Red Sandstone.