THE CHUAR GROUP-UINTA MOUNTAIN GROUP-PAHRUMP GROUP CONNECTION REVISITED: A “SNAPSHOT” OF THE INFRA-STURTIAN EARTH SYSTEM

The Chuar, Uinta Mountain, and middle Pahrump groups have long been correlated using lithostratigraphy, microfossils, and their stratigraphic positions. The ChUMP hypothesis (Dehler et al., 2001) furthers this correlation using detailed lithologic, paleontologic, C-isotope and physical stratigraphic characteristics.

Two of these successions now yield absolute ages. The Chuar Group has a 742 ± 6 Ma Ma tuff at the top of the 1.6 km section (U-Pb on zircon, Karlstrom et al, 2000). The Uinta Mountain Group now has a depositional age of 766.4 ± 4.8 Ma within 2 km of the base of the 7 km section (U-Pb detrital zircons; Fanning and Dehler, 2005). Both of these units sit unconformably on Proterozoic rocks. The Chuar Group is unconformably (?) overlain by the Sixtymile Formation, which contains a 15m-deep incised valley inferred to represent Sturtian-age base-level fall. The UMG, although unconformably overlain by Paleozoic strata, correlates with the Big Cottonwood Formation, which is overlain by the Sturtian Mineral Fork Formation. The only age control on the middle Pahrump Group (upper Crystal Spring Formation and the Beck Spring Dolomite; ~900 m thick) of Death Valley is that the lower contact cuts a 1.08 Ga dike and that the top is unconformably overlain by inferred Sturtian-age lower Kingston Peak Formation.

All three of these successions represent, in part, marine deposition. They show 8-15‰ variability in C-isotope values. They all have vase-shaped microfossils in the uppermost strata. Finally, the Chuar Group has marked lithostratigraphic and cyclostratigraphic similarities with parts of the other two successions.

The ChUMP model correlation of these successions indicates that the undated middle Pahrump Group is ~766 to 742 Ma, that there was a “Western Interior Seaway” at this time, and that the Sturtian started after ~742 Ma. These strata capture a “snapshot” of major perturbations in the Earth system prior to the Sturtian and offer insight into possible causes of ensuing low-latitude glaciation(s).