DETRITAL ZIRCON CONSTRAINTS ON THE JEAN CONGLOMERATE AND SEVIER UNROOFING NEAR LAS VEGAS

A detrital zircon study was conducted on the Jean conglomerate, which sits on Permian sandstones and is overlain by the Tuff of Bridge Springs (15.12Ma) just east of the Birdsprings thrust, the easternmost Sevier related thrust fault southwest of Las Vegas. Two competing hypotheses were tested: 1) the Jean conglomerate was deposited in the Sevier foreland during the Mesozoic, or 2) the Jean conglomerate is Tertiary and linked to the Rainbow Gardens Conglomerate. Sampled strata included 1) the undated Jean conglomerate, 2) the Rainbow Gardens Conglomerate, a basal Tertiary conglomerate that unconformably overlies Triassic and Jurassic units east of Frenchman Mountain, and 3) two samples from the lower and upper portions of the demonstrably syntectonic Cretaceous Lavina Wash section near Goodsprings, which is over run by the Keystone thrust.

Results show that the Jean conglomerate has a diverse suite of detrital zircon ages that more closely, though not in detail, match the zircons in the Rainbow Gardens Conglomerate. More conclusively, two zircons from the Jean conglomerate (24.5 and 25.2Ma) restrict its depositional age to the Late Oligocene or Miocene, thus ruling out the Sevier foreland interpretation.

A surprising outcome of the analyses relates to the Lavina Wash samples. The presumed stratigraphically lower sample from volcaniclastic dominated units is dominated by zircons that statistically define a population whose mean age is 98Ma (i.e., early Late Cretaceous), which is in good agreement with a previously reported Ar/Ar tuff age of 99Ma from the section. Given the tight range of ages, the similarity of ages to the Ar/Ar age, and the euhedral aspect of most zircons, this age is interpreted to be a zircon tuff age. The presumed stratigraphically younger sample, collected from a carbonate clast facies portion of the section, also has euhedral zircons that are essentially one age population centered on 107Ma (i.e., late Early Cretaceous) and it too is interpreted as a zircon tuff age. Given these results, the two sections from which the samples were taken, which are not physically connected, appear to have been incorrectly correlated by previous workers and indicate a different unroofing that previously suggested.