THE FLUVIAL MUDDY CREEK FORMATION NEAR OVERTON, NEVADA: PROVENANCE, STRATIGRAPHY, AND MIOCENE-RECENT EVOLUTION OF THE MORMON BASIN

The Muddy Creek Formation (MCF) near the Overton arm of Lake Mead, NV was mapped in order to 1) constrain models pertaining to the establishment of the modern Colorado River drainage; 2) constrain the age of the Mormon Mesa geomorphic surface which is developed on the MCF and is presumed to be one of the oldest geomorphic surfaces in the world; and 3) refine the history of Miocene-recent fluvial and erosional processes in the Mormon Basin.

Our mapping of the Overton SE 7.5-minute quadrangle produced data that help constrain the Miocene-Pliocene (~10-4 Ma) provenance and paleodrainage systems of the southern Virgin River. Paleocurrent analyses of MCF conglomerates indicate southerly flow, similar to the flow of the modern Virgin River. Provenance data indicate a dominantly Colorado Plateau source with lesser input from local volcanic (Caliente Caldera complex) and metamorphic (Black Ridge amphibolite) sources. We also refine the Quaternary history of the study area by documenting at least four post-MCF depositional base-level changes. A partially preserved alluvial fan facies within the MCF reveals that the Virgin River’s channel has shifted approximately 2.5 km east to its current location since the middle Pleistocene. Future mapping in the area aims to gain age control for the upper MCF, map structures that may have been responsible for the river channel’s movement, and measure stratigraphic sections to further constrain the provenance of, and lateral variation within the MCF.