LACUSTRINE CARBONATE DEPOSITION AND FACIES DISTRIBUTION WITHIN THE MUDDY CREEK FORMATION (MIOCENE-PLIOCENE), NELLIS BASIN, SOUTHERN NEVADA

The Nellis basin lies north and east of the Sunrise-Frenchman Mountain area in southern Clark County, Nevada. Here, alluvial gravels cover an area of critical transition (~ 1 km) from basin margin lacustrine carbonates to basin-centered gypsum and gypsiferous marls, within the Muddy Creek Formation (Miocene-Pliocene). These units were deposited under fluctuating climatic conditions within an arid, continental, closed lacustrine basin that formed during Basin and Range extension along strike-slip faults. Field mapping has established interfingering relationships within this suite of sedimentary facies. The facies from the basin center to the margins, are: (1) gypsum with minor silt, (2) interbedded gypsiferous marls and silt, (3) interbedded white limestone, marls, and gypsiferous silt, (4) interbedded white limestone and marls. Sedimentary structures are largely absent in most lithofacies, with occasional horizontal laminations, indicating a low energy depositional system.

The facies represent shifting depositional environments in response to climatic fluctuations. This evaporitic lake system contains nearshore, interbedded white limestone (1-7 m) and marls (1-15 cm). In addition, the marls contain zones of magadi-type chert further supporting the interpretation of an alkaline lake system. To the east, we see an increase in red, green, and gypsiferous silts (1 cm-2 m) and thinning interbeds of limestone (1-15 cm) and marls (1-2 m). This is interpreted as dry-saline mud flats, fringing the evaporitic basin center. The basin center represents a more abrupt transition to massive gypsum (1-5 m). The gypsum is approximately 150 m thick.

A well-defined geochronology of the lake system allows determination of the timing of lake-basin infilling. Furthermore, age control helps to constrain the establishment of the modern Colorado River system, which represents integration of internally-draining basins into the regional drainage network. Tephrochronological analyses of several tuff layers found within the silty marls will help to provide age control on this facies succession.