LIKELY TECTONISM DURING REED DOLOMITE DEPOSITION, WHITE-INYO MOUNTAINS, CALIFORNIA AND NEVADA

Much of the Lower and Upper Reed Dolomite of the White-Inyo succession, eastern California/western Nevada, represents typical shallow water carbonate platform sedimentation along southwestern North America in Neoproterozoic time. In western exposures, however, it has long been known that the Hines Tongue, a coarse feldspathic sandstone unit, separates the lower member from the Upper Member. Regional mapping now demonstrates that ~100 meters of carbonate clast megabreccia crops out discontinuously across the region and separates the Lower Member from the Hines Tongue. The megabreccia displays large, angular, and predominantly intra-formational clasts derived from the underlying Lower Member. The megabreccia is associated with syn-depositional faults that are subsequently buried by the overlying Hines Tongue in at least one locality. The age of the megabreccia is unclear, but is certainly older than ~548 Ma based on the presence of Cloudina in the Upper Member. The presence of simple trace fossils in the underlying Wyman Formation and the Hines Tongue itself constrain the age further and suggest megabreccia development occurred between ~555 Ma and 548 Ma.

Volcanism, syn-depositional faulting, and abrupt facies changes noted in the Death Valley succession to the east have been used to suggest rifting initiated in the southern Cordillera ca. 750-700 Ma. Other workers interpret the evidence to represent a two-stage or multi-stage rift history that extended to ca. 635 Ma, in association with glacial deposits that are currently unknown in the White-Inyo succession. We cannot rule out a partial glacio-eustatic origin for the megabreccia in the Reed Dolomite, but the locally-derived clasts and association with syn-depositional faults suggest a tectonic component as late as 555 Ma to 548 Ma along southwestern North America. The megabreccia development in association with syndepositional faulting in the Reed Dolomite may suggest that passive margin evolution was more complicated than previously thought.