PROVENANCE AND TIMING OF SEVIER FORELAND BASIN SEDIMENTS IN THE VALLEY OF FIRE, SOUTHERN NEVADA, FROM U-PB GEOCHRONOLOGY

Baseline Sandstone and Overton Conglomerate, exposed in Valley of Fire State Park, were deposited along the eastern edge of the Sevier thrust belt in southern Nevada, and are interpreted to be synorogenic foreland basin deposits. U-Pb dating of detrital zircons from the Baseline Sandstone places constraints on both the source of foreland basin sediment and the timing of thrusting.

SHRIMP-RG U-Pb zircon ages of 101.6 +/- 1 Ma to 99.9 +/- 2 Ma from three ash beds in the immediately underlying Willow Tank Formation provide a precise age for the timing of thrust initiation and beginning of foreland basin sedimentation to late Early Cretaceous time. ICP-MS U-Pb detrital zircon data from seven samples of Baseline Sandstone show little change in age populations with stratigraphic height. Dominant Baseline Sandstone zircon populations, from about 350 to 470 Ma, 530 to 650 Ma, and 930 to 1250 Ma, are very similar to zircon ages in the Jurassic Navajo Sandstone, preserved in the local footwall. Populations of basement-aged zircons (1400 to 1450 Ma and 1600 to 1800 Ma) in the Baseline Sandstone are not high enough to suggest derivation from exhumed basement rock or local lower Paleozoic sedimentary rocks; rather they are likely sourced from the Navajo Sandstone as well.

The Baseline Sandstone is comparable in age to the lower McCoy Mountains Formation to the south, but differs in source material, probably due to differences in structural style between the Sevier and Maria thrust belts. The provenance of Baseline Sandstone implies gently dipping thin-skinned thrust faults, versus deep basement-exhuming thrust or reverse faults as in the Maria thrust belt.