RODINIAN DISASSEMBLY: PERSPECTIVES FROM SOUTHWEST LAURENTIA

The Cambrian explosion and its Neoproterozoic precursors occurred in a context of extraordinary environmental change. Tectonic events are generally suspected to be the primary cause of both climatic and polar wander events. Significant progress has been made over the last decade in constraining the absolute age and global correlation of both climatic and biological events, primarily through U/Pb dating of rare ash horizons in key sequences. However, our understanding of the disassembly of Rodinia, as constrained by field study of the structures that accommodated it, is relatively primitive. The Neoproterozoic succession in the Panamint Range, eastern California, is the single most extensive exposure of Neoproterozoic rift structure and related sedimentation along the southwest flank of Laurentia. Here, sedimentation records two major glacial intervals and their cap carbonate sequences that are strong correlatives with the Sturtian and Marinoan events in Australia (Surprise and Wildrose diamictites, Kingston Peak Formation), and a later incision event that we tentatively correlate with the Gaskiers event (informal Conglomeratic member, upper Johnnie Formation). These correlations yield a quasi-linear subsidence profile through the Ediacaran of ~50 m/Myr (not decompacted). We have identified at least four north-trending normal fault zones that control the stratigraphic architecture. An older period of east-side-down faulting generated a regional angular unconformity, tilting Sturtian strata westward, with growth relations during Sturtian time. A distinctly younger period of both east-side- and west-side-down faulting, which tectonically defines the breakup unconformity of the Cordilleran miogeocline, was active during the Marinoan glaciation, persisted well into its cap carbonate sequence (Sentinal and Radcliff Members of the Noonday Formation), and ended stratigraphically ~1000 m below the presumed Gaskiers-correlative incisions in the upper Johnnie. If we have read Neoproterozoic time correctly, the preserved faults record strong rifting (without local breakup) at 700 to 750 Ma, another strong event between 630 and 600 Ma, and the onset of thermal subsidence in the miogeocline (linearly interpolating between Gaskiers and Marinoan correlation points) at 600 to 610 Ma.