Deep valley incision in the terminal Neoproterozoic (Ediacaran) Johnnie Formation, eastern California, USA: Tectonically or glacially driven?

Neoproterozoic sedimentary successions worldwide display evidence for multiple low-latitude glacial events and continental rifting episodes related to the fragmentation of the supercontinent Rodinia. In Death Valley, eastern California, these events are represented by two intervals of glacial diamictite and two episodes of synsedimentary extensional faulting in the Kingston Peak Formation. Deep incised valleys of uncertain origin in the upper Johnnie Formation, approximately 1 km stratigraphically above the Kingston Peak Formation, may provide evidence either for a third glacial episode or for an additional tectonic event. Eleven measured sections indicate that one of these incised valleys is 120 m deep and approximately 2.5 km wide. Detailed mapping of the paleovalley has revealed the presence of numerous synsedimentary normal faults that offset underlying units but are capped by incised valley breccia. Abrupt incision occurs across buried normal fault scarps. Several large olistoliths (up to 15 m in diameter) are located near the fault scarps and the average clast size of the breccia decreases in general away from mapped normal faults. Incised valley clasts are derived exclusively from underlying Neoproterozoic formations, nearly entirely from the Johnnie Formation itself. There is no evidence for direct glacial influence as no striated clasts or dropstones were observed. The absence of glacial indicators coupled with the strong correlation between incision and synsedimentary normal faulting imply a primary tectonic control on valley formation. The valley represents a significant tectonic reactivation of the Neoproterozoic passive margin, similar to early Paleozoic extensional events in the northern Cordillera of western North America. © 2005 Elsevier B.V. All rights reserved.