Abstract

Middle Miocene to early Pliocene beds of the Muddy Creek Formation near Mesquite in southeastern Nevada are at least 200 meters thick. The formation consists of discontinuous beds of sandstone, siltstone, mudstone, and claystone deposited as intrabasinal fill. The sediments were derived locally from adjacent block-faulted mountains. The beds have a general strike of about N 50 E and a dip of 3–5 to the northwest. The beds are not continuous, but pinch and swell laterally, due to the many stream channels that are superimposed upon each other. Mesa-capping limestones, thought perhaps to be lake deposits in the Muddy Creek Formation, are younger and unconformably sit on top of the Muddy Creek Formation. The limestones contain well developed pisolites and appear to be caliche deposits.

Sedimentary structures in the formation include clay rip-up clasts, armored mud-balls, trough cross-beds, mudcracks, and ripple marks. The ripple marks range in wavelength from 2 cm to 40 cm and in amplitude from 1 cm to 5 cm. They occur in a number of ripple types.

Many of the sandstone beds are relatively unconsolidated. In places, thin mudstone and claystone beds are present within channels, along with coarse sandstone and occasionally larger mud rip-up clasts indicating a highly variable flow regime. Thin overbank deposits of green mudstone are laterally continuous with channel sandstones in many places. Thicker deposits (1–2 m) of light green mudstone and siltstone are laterally continuous with some of the larger channels and may represent oxbow lake deposits in abandoned meanders.

Fossils and trace fossils in the Muddy Creek Formation are locally abundant, such as “camel” tracks and bones, bird and lizard tracks, many types of burrows, and impressions of grass or small sticks in what may have been tufa deposits.

Sedimentary structures and fossils seem to indicate an environment of deposition not too dissimilar to the climate of today with some periods of higher rainfall and more moderate climate producing a savannah-like environment. Both braided and meandering stream deposits occur in the formation, along with associated pond, lake, and flood plain deposits.