Chronostratigraphic Applications of Paleomagnetism and Rock Magnetic Cyclostratigraphy: Case Studies from the Ediacaran and Devonian Periods

This work extends the methods of paleomagnetism and rock magnetic cyclostratigraphy to aspects of North American paleogeography, Appalachian foreland basin history, stratigraphic correlation of Precambrian strata, and orbitally forced cyclicity of sedimentation in the Paleozoic and Neoproterozoic. Paleomagnetic study of the Marcellus Formation, sampled in the subsurface by drill core, demonstrates evidence of a primary Devonian magnetization. This result is critical given that most Appalachian basin sediments appear to have had their magnetizations overprinted in the Permian, leaving a gap in the history of the geomagnetic field, and the apparent polar wander path for Laurentia during the Devonian. Rock magnetic cyclostratigraphy of the Marcellus Formation, constrained by calibrated biostratigraphy, demonstrates encoding of precession scale orbital cycles in the rock magnetic properties of this organic rich, monolithologic basinal shale. We suggest the signal may arise from dilution of clastic input by variable production and burial of organic carbon. Paleomagnetic, rock magnetic, and carbon isotope stratigraphies of the Ediacaran aged Johnnie Formation (Death Valley, USA) and the Wonoka Formation (Flinders Ranges, Australia) provide a robust chronostratigraphic framework for the Shuram carbon isotope excursion (SE). Competing hypotheses suggest the SE is either a major global ocean oxidation event preceding the evolution of metazoan life, or the product of diagenetic alteration of globally distributed sediments. Magnetostratigraphy shows the nadir of the excursion to be synchronous between the Death Valley and Australia locations, and an astrochronology gives a duration of approximately 9 Myr for each expression of the excursion. The geochronologic consistency of the excursion between these two localities makes a diagenetic source unlikely. The duration of the 13C depleted strata can be used to obtain a more robust estimate of the necessary dissolved organic carbon pool and amount of oxidants in the Ediacaran ocean-atmosphere system necessary to explain the Shuram Excursion.