DETRITAL ZIRCONS FROM THE NEOGENE MUDDY CREEK FORMATION IN SOUTHERN NEVADA INDICATE A MIXED COLORADO PLATEAU-BASIN AND RANGE PROVENANCE

The source of sediment within the Neogene Muddy Creek Formation (MCF) of southern Nevada is controversial. Early studies concluded the formation was deposited as post-tectonic infill with proximal sediment sources. Recent studies have varied in their interpretations of MCF provenance, arguing whether the formation was derived locally or from a distal source within the Colorado Plateau and transported via an ancestral Colorado River. The most recent of these studies (Pederson, 2008) concluded, based on petrographic and paleocurrent data, that the MCF north of Lake Mead was derived largely from local Miocene volcanic sequences that lie to the north of MCF exposures, with lesser contribution from the Colorado Plateau that was likely transported via an ancestral Virgin River.

Ten detrital zircon samples were collected as part of this study from the middle (sandstone) and upper (sandstone and conglomerate) MCF in the vicinity of the Overton Arm of Lake Mead and analyzed to evaluate sediment provenance since 6.62 +/- 0.03 Ma (new tephrochronology age; this study). Zircon analyses (total n = 803) resulted in population ages of 13 Ma, 19-20 Ma, 91-99 Ma, 176 Ma, 220-235 Ma, 378-621 Ma, 992-1248 Ma, 1334-1487 Ma, 1517-1599 Ma, 1624-2036 Ma, and 2710 Ma. One additional sample was collected from the lower (lacustrine) MCF; however it yielded only seven analyzable zircon grains that ranged in age from 347 to 2702 Ma. MCF zircon data were compared to data from the modern Virgin River, potential sources within the Colorado Plateau, and locally-exposed potential sources (some of which also occur, as correlative units, within the Colorado Plateau). These comparisons show that MCF zircon populations are consistent with Basin and Range and Colorado Plateau sources and require a mixed local and distal provenance for the formation since at least 6.62 Ma. Petrology and petrography data from the study area further indicate that the MCF, in the vicinity of the Overton Arm of Lake Mead, was derived largely from the Colorado Plateau, with lesser contribution from Basin and Range sources. Additionally, features within the study area show the MCF was, at least locally, deposited syn-tectonically.