LACUSTRINE FACIES ARCHITECTURE AND DEFORMATION OF THE MUDDY CREEK FORMATION (MIOCENE), NELLIS BASIN, CLARK COUNTY, NEVADA

The Nellis Dunes Recreation Area (NDRA) lies 6 km northeast of Las Vegas, NV in the Nellis Basin, north of Frenchman Mountain and south of the Dry Lake Range. Ongoing field mapping identified four facies that are assigned to the Muddy Creek Formation. These facies document filling of a lacustrine basin and subsequent integration with regional fluvial systems.

The facies from oldest to youngest are 1) white limestone and claystone, 2) brown claystone and siltstone, 3) yellow sandstone, and 4) red sandstone. The white limestone and claystone (~25 m thick) is interbedded laminated limestone with massive, white, calcareous silty claystone. This facies is observed in the lowest elevations of the study area and is interpreted as offshore lacustrine deposits. The brown claystone and siltstone (8-10 m thick) is a succession of gray-brown laminated claystone and siltstone with 3-5 cm thick white claystone interbeds and nodules. Gypsum layers, 1-2 cm thick, are also present. The brown claystone and siltstone facies represent a shallower mudflat environment. The yellow sandstone facies (10-15 m thick) consists of yellow-white, very fine to fine-grained sandstone with interbeds of laminated siltstone, claystone, and gypsum. Root casts are present within the interbedded sandy siltstone units. This facies was deposited as a basin margin sandflat that onlaps Paleozoic bedrock. Within the yellow sandstone facies are zones of cemented and brecciated sandstone with plant remains that may represent groundwater seepage from springs. The stratigraphically youngest facies is pink-red cross bedded and rippled sandstone and siltstone. This facies is similar to the “middle Muddy Creek Formation” described in the Mesquite basin further east and reflects a shift from lacustrine to fluvial-dominated deposition during interconnection of regional drainage systems.

The Muddy Creek Formation within the NDRA has been faulted and folded, particularly in the southern parts of the study area. The zones of the most extensive deformation appear to be associated with west-east trending faults that are the eastward extension of the Las Vegas Valley Shear Zone, though the dominant sense of motion observed is dip-slip. The lack of datable horizons within the succession currently limits determination of the timing of this deformation.