PRELIMINARY MAP OF LATE NEOGENE AND YOUNGER SEDIMENTS IN THE WESTERN PORTION OF THE OVERTON, NE 7.5 MINUTE QUADRANGE NEVADA

The goal of this EDMAP-sponsored project is to produce a 1:24000 map of Late Neogene and younger sediments contained in the western half of the Overton NE 7.5 minute quadrangle of Nevada. The mapping area contains Miocene-Recent sedimentary units and soils that are well exposed by erosion from the Virgin River (a tributary to the Colorado River). Regionally, these Miocene-Recent deposits occur within extensional basins that developed during the formation of the Basin and Range province. Locally, the uppermost portion of this sequence is capped by a set of mature calcium carbonate soil horizons (petrocalcic horizons). This unique soil is considered to represent a soil-geomorphic surface that is one of the oldest in the world. Miocene Muddy Creek Fm. sediments are identified throughout the mapping area as red, horizontally bedded, closed basin-fill sandstones and mudstones. We use elevation above modern floodplain, soil development, surface characteristics such as desert pavement development, and gravel lithology to define individual geomorphic units. Preliminary mapping has identified and characterized five surfaces (Quaternary-Recent) formed by fluvial downcutting of the Virgin River and its tributaries. Three of these surfaces appear equivalent to the Overton, Underton, and Lost City surfaces described by earlier workers. The older (Tertiary) Mormon Mesa soil-geomorphic surface occupies the western half of the mapping area and has evidence of minor faulting and folding. In the northern section of the mapping area, a conglomerate unit of predominantly carbonate lithologies with rare intrusive rocks sits atop the Muddy Creek Fm with stage VI soil development. These sediments are interpreted to be derived from the Mormon Mountains to the north and may represent a pulse of alluvial deposition in response to an increase in aridity or fluvial deposition associated with the initial opening of the basin. The exposure in the proposed mapping area is world-class and mapping should result in a better understanding of the development of the modern day Colorado River.