ROCK MAGNETIC CYCLOSTRATIGRAPHY: A NEW CHRONOSTRATIGRAPHIC TOOL FOR GEOLOGISTS

Rock magnetic cyclostratigraphy is a new technique that allows rock magnetic measurements to assign high-resolution time to a stratigraphic section. In its simplest application either ARM or IRM measures magnetic mineral concentration variations that record astronomically-driven global climate cycles. This allows time resolution at the precessional scale, better than magnetostratigraphy even at times of high reversal rate. Many samples can be processed rapidly because the samples do not need to be oriented. Remanence measurements are preferred over susceptibility, since they can be targeted at sub-populations of ferromagnetic grains while susceptibility is a more complicated signal making the climate encoding difficult to interpret.Three examples of rock magnetic cyclostratigraphy will be presented: The first example is the Cretaceous Cupido Formation, Mexico. ARM intensity variations in these platform carbonates show that magnetite concentrations record short eccentricity, obliquity and precession. These cycles become particularly prominent when sequence stratigraphy boundaries are tied to the long eccentricity cycle. Based on the magnetite’s grain size and from SEM observations it is sourced from eolian dust. The Eocene marine marls of the Arguis Formation from the Pyrenees provide the second example. In this study ARM variations record both short and long eccentricity as well as precession. The magnetic mineralogy is depositional magnetite plus secondary iron sulfides, showing that a rock magnetic cyclostratigraphy is possible even if the depositional magnetic minerals have suffered from reduction diagenesis. Coherency analysis shows that ARM maxima are in phase with precessional insolation during the rainy season. Ferromagnetics are encoding run off variations being driven by precession. Rock magnetic cyclostratigraphy has been pushed into the Precambrian by a study of the Neoproterozoic Johnnie Formation from Death Valley. Using a rock magnetic measure of the goethite:hematite ratio, a strong 5 m cycle is observed with a superimposed 0.6 m cycle. These cycles are interpreted to be short eccentricity and precession. This interpretation is supported by a magnetostratigraphy that shows 4 polarity zones in the 45 m thick section.