DEFORMATION OF THE MIOCENE-PLIOCENE MUDDY CREEK FORMATION, SOUTHERN NEVADA: LAKE MEAD FAULT SYSTEM, SALT TECTONICS, OR BOTH?

Our research indicates that the uppermost Miocene-Pliocene Muddy Creek Formation (MCF) within the Overton sub-basin was syntectonic and related to the Lake Mead fault system (LMFS), salt tectonics, or both. Previous studies hypothesized that the MCF is post-tectonic infill of middle Miocene extensional basins, but our work invalidates this hypothesis. Our interpretations are based on field observations including measurement of 12 stratigraphic sections, as well as provenance, paleocurrent, and structural data collected near The Narrows of the Virgin River SE of Overton, Nevada. The results indicate that the MCF is comprised of sand-dominated fluvial redbeds with dominantly southward paleoflow, unconformably capped by pebble to boulder conglomerates. An ash layer near the top of the redbeds contains abundant sanidine and is being prepared for Ar/Ar analyses. Previous K-Ar analyses of an ash, in a similar stratigraphic position within the MCF further to the north, yielded an age of approximately 5Ma. The upper MCF redbeds are therefore presumed to be Pliocene in age. The MCF redbeds near The Narrows are folded with dips as steep as 45^o, but capping conglomerates fan from similarly steep dips to near horizontal within the span of a few tens of meters. MCF beds contain a syntectonic syncline/anticline pair south of a large (~2km diameter) circular depression near the Virgin River delta. Normal and strike-slip faults cut the MCF, as well as Pliocene-Quaternary(?) petrocalcic soil horizons developed atop the conglomerates. These faults, the aforementioned folds, and the close proximity of the Hen Spring fault of the LMFS, indicate a possible link between MCF deformation and movement along the LMFS since the Pliocene(?). South of the study area domal salt structures cropped out in areas now inundated by Lake Mead and thick subsurface Neogene salt has been documented. Therefore, salt may have extended under the study area, became diapiric, and evacuated during the Pliocene(?). The depression mentioned above might be the site of a collapsed/dissolved diapir. Subsidence adjacent to such a diapir could have produced the fanning of dips in the MCF. Therefore, salt tectonics may have played a key role in Neogene deformation of the area. MCF deformation appears to be related to LMFS deformation, salt tectonics, or a combination of both.