Rocky Mountain Section - 64th Annual Meeting (9–11 May 2012)

Paper No. 2
Presentation Time: 1:45 PM


MAHON, Robert C., Geology and Geophysics, University of Wyoming, Dept. 3006, 1000 E. University Ave, Laramie, WY 82071-2000, DEHLER, Carol M., Department of Geology, Utah State University, 4505 Old Main Hill, Logan, UT 84322-4505, KARLSTROM, Karl E., Earth and Planetary Science, University of New Mexico, Albuquerque, NM 87131 and LINK, Paul K., Department of Geosciences, Idaho State University, Pocatello, ID 83209,

U-Pb zircon analyses were conducted at the Arizona LaserChron Center on a full suite of detrital samples from the Pahrump Group in the southeastern Death Valley region. The samples analyzed represent a complete section from the base of the Crystal Spring Formation (>1080 Ma) through the glaciogenic Kingston Peak Formation (age unconstrained, but inferred 600-700 Ma). The lower and middle Crystal Spring Fm. each exhibit strongly unimodal populations at ca. 1680 Ma and 1760 Ma, respectively. The upper Crystal Spring Fm. data (n=6; to be redefined as the Horse Thief Springs Fm.) show a pronounced shift in provenance, with age peaks at 1880, 1790, 1450, 1220 Ma and a few young grains in several samples between 850-780 Ma. Samples analyzed from the other Pahrump Group units, including the Beck Spring Dolomite (n=2) and Kingston Peak Fm. (n=3) exhibit age populations similar to the underlying Horse Thief Springs Fm.

The majority of these age populations are likely derived from proximal sources (<1000km) in the Mojave, Yavapai and Mazatzal crustal provinces with minor contributions from farther travelled Laurentian grains. Archean grains are present in most samples and are interpreted to be recycled from earlier Paleoproterozoic and Mesoproterozoic strata. Possible known sources for the 850-780 Ma grains include south Australia and south China.

The youngest age grouping of grains from the basal Horse Thief Springs Formation provides a maximum depositional age for Rodinia rift-related sediments in the Death Valley region of <774 Ma, consistent with observations from the Chuar and Uinta Mountain Groups of western Laurentia. This maximum age requires a 300 m.y. hiatus between the Crystal Spring Formation, which is intruded by 1.08 Ga diabase (correlative with the Unkar Group of the Grand Canyon, cut by 1.1 Ga diabase), and the <774 Ma Horse Thief Springs Fm. (correlative with the Chuar Group, ca. 770-742 Ma). The ca. 300 m.y. hiatus between the Crystal Spring and Horse Thief Springs Formations suggests the term Pahrump Group should also be elevated to Pahrump Supergroup, analogous to the Grand Canyon Supergroup.