PRELIMINARY GEOLOGIC AND GEOMORPHIC MAP OF LATE NEOGENE AND YOUNGER SEDIMENTS IN THE EASTERN PORTION OF THE OVERTON, NE 7.5 MINUTE QUADRANGLE, NEVADA

The goal of this EDMAP-sponsored project was to produce a 1:24000 map of Late Neogene and younger sediments contained in the eastern half of the Overton NE 7.5 minute quadrangle of Nevada. The field area is located approximately 60 miles north of Las Vegas, NV near Mesquite. Miocene-Recent sedimentary deposits and soils are well exposed adjacent to the Virgin River, a tributary of the Colorado River. We used depositional characteristics, lithology, soil development, and surface features to differentiate and define individual geologic and geomorphic units. We mapped four fluvial terraces and related deposits in Halfway Wash, a small tributary to the Virgin River located in the northwest section of the field area. Other mapped units include sediments of the Virgin River modern floodplain, older elevated river gravels and sands capping fluvial terraces that are well exposed on the western side of the river, and river gravels found between 500-600 meter elevations that represent the highest river levels in the mapping area. Fine-grained silts and sands interpreted as low-energy Virgin River deposits were difficult to differentiate from the Miocene Muddy Creek Fm. We mapped these deposits as part of the older Virgin River alluvium. Other Quaternary units include alluvial fan deposits that cover the Virgin Mountain piedmont on the eastern side of the river and alluvium on the western side of the river deposited on slopes off of the elevated Mormon Mesa landform. Silts and sands of the Muddy Creek Fm. capped by more resistant material (i.e. Mormon Mesa petrocalcic soil horizons, or Virgin Mountain derived alluvium) form cliff-exposures throughout the mapping area. Although this map is preliminary, it provides additional information on the depositional history of the Virgin River. Further mapping is needed to refine and further describe and differentiate units in this area. Specifically, more work needs to be done on the older Virgin River fine-grained deposits to distinguish them from Muddy Creek Fm. sediments.