TECTONICALLY-DRIVEN DEEP VALLEY INCISION IN THE TERMINAL NEOPROTEROZOIC JOHNNIE FORMATION, EASTERN CALIFORNIA

Many terminal Neoproterozoic (Ediacaran) sedimentary successions contain stratigraphic evidence for glaciations (diamictite units and cap carbonates; e.g., Snowball Earth), as well as continental rifting (synsedimentary faulting and olistostrome shedding). Such events are represented in eastern California by the Kingston Peak Formation and Noonday Dolomite, which record evidence for glaciation, cap carbonates, and rifting. However, the overlying Johnnie Formation (ca. 580 Ma) contains pronounced incised valleys in the upper Rainstorm Member that have been attributed to either glacioeustatic drawdown or tectonic activity, implying that either eastern California is unique in recording three unequivocal glacial events or the tectonic evolution of the Cordilleran rifted margin is more complex than currently proposed. To distinguish between these models, we studied a spectacularly-exposed incised valley in the southern Nopah Range near Death Valley. We measured 11 stratigraphic sections and traced the lateral contacts of the incised valley and overlying Stirling Quartzite relative to the Johnnie oolite (a regional marker bed) to search for evidence of synsedimentary faulting or glaciogenic deposits. The maximum depth of incision was ~120 m, cutting below the underlying Johnnie oolite at its deepest point. Valley incision correlates strongly to the presence of synsedimentary normal faults, which offset underlying strata but are draped by coarse valley fill deposits. Maximum observed fault displacement was 18 m, but the regions of greatest valley downcutting are covered. Large olistostromes (up to 10 m in size) occur in valley fill near syndepositional faults. Although it is difficult to disprove contemporaneous glacioeustatic drawdown, the abundance of syndepositional normal faulting in association with significant valley downcutting and the occurrence of large olistostromes apparently shed from fault scarps imply a major tectonic event and indicate that rifting of the southern Cordilleran margin was more protracted than previously thought.