SUBSIDENCE HISTORY OF THE EDIACARAN JOHNNIE FORMATION AND RELATED STRATA OF SOUTHWEST LAURENTIA: IMPLICATIONS FOR THE AGE AND DURATION OF THE SHURAM ISOTOPIC EXCURSION AND ANIMAL EVOLUTION

The Johnnie Formation and associated Ediacaran strata in southwest Laurentia are some 3000 m thick, beginning with a Marinoan cap carbonate sequence, and ending with a transition from Ediacaran to Cambrian fauna. About halfway through the sequence, the Shuram negative carbon isotopic excursion occurs within the Rainstorm Member at the top of the Johnnie Formation, followed by a remarkable valley incision event. At its type locality in the northwest Spring Mountains, Nevada, the Johnnie lithostratigraphy consists of three distinctive sand-rich intervals alternating with four siltstone/carbonate-rich intervals, which appear correlative with other regional Johnnie outcrops. Carbon isotope ratios in the sub-Rainstorm Johnnie are uniformly positive for at least 400 m below the Shuram excursion, and compare well with sub-Shuram excursion profiles from the Khufai Formation in Oman. There is historical consensus that the Johnnie and overlying formations were deposited on a thermally subsiding passive margin. Following previous authors, we used Paleozoic horizons of known biostratigraphic age to define a time-dependent exponential subsidence model, and extrapolated the model back in time in order to estimate the ages of the Shuram excursion and other prominent Ediacaran horizons. The model suggests that the Shuram excursion occurred from 585 - 579 Ma, and that incision of the Rainstorm shelf occurred during the 579 Ma Gaskiers glaciation. It further suggests that the base of the Johnnie is about 630 Ma, consistent with the underlying Noonday Formation representing a Marinoan cap carbonate sequence. These results contrast with suggestions that the Shuram excursion followed the Gaskiers event by some 20 - 30 Myr. They suggest instead that the Shuram and Gaskiers events are coeval with the transition from the acanthomorphic to leiospherid acritarchs, and ushered in the first appearance of widespread macroscopic animal life, 38 Myr prior to the Ediacaran-Cambrian boundary.