40AR/39AR ISOTOPIC DATES FOR THE FOSSILIFEROUS WILLOW TANK FORMATION (CRETACEOUS) IN VALLEY OF FIRE STATE PARK, NEVADA

The Willow Tank Formation of Southern Nevada in Valley of Fire State Park contains fossils of a diverse Cretaceous biota. This biota includes fish, turtles, crocodylomorphs, and a number of different dinosaur taxa. Previous age control placed the formation in the late Early Cretaceous (Albian), these ages were based upon biostratigraphy of the tree fern Tempskya along with limited zircon U/Pb dates. We report here dates that help further constrain the timing of deposition of the Willow Tank Formation and the biota recorded within this unit. Samples of volcaniclastic sediment were taken from both the top and near the base of the Willow Tank Formation, which are separated stratigraphically by roughly 30 m. Sanidine crystals from each location were dated by the 40Ar/39Ar method. The preferred ages of the samples were 98.56 Ma for the top of the formation and and 98.68 Ma for the base, which places the Willow Tank Formation in the early Late Cretaceous (Cenomanian). The new dates for the formation have some interesting implications for the paleontological research that is being conducted in the area. The new dates do not indicate an early occurrence of Tyrannosauroidea and Hadrosauriodea (advanced Iguanodontia), as suggested by the published dates of previous studies. These dates will also help provide a rough sedimentation rate for this portion of the foredeep of the Sevier retroarc foreland basin.