Late Devonian Magnetostratigraphy from a Condensed Limestone, Canning Basin, Western Australia: ABSTRACT

Abstract

Detailed thermal demagnetization experiments have been carried out on 31 samples (93 specimens) of fore-reef limestones from the northern Canning basin, Western Australia. Samples are from a condensed sequence composed of deep-water (over 100 m) stromatolites in the marginal-slope facies of the Virgin Hills Formation. These stromatolites are made up of shrublike cyanobacterial filaments of Frutexites, an organism that probably precipitated iron hydroxide from seawater. Under the oxidizing conditions created by extremely slow sedimentation rates, it is likely that iron hydroxides were converted to hematite at or near the sediment-water interface. When plotted within the microstratigraphic framework for the study area, the observed characteristic directions from the sampled interval (14.5 cm thick) lie in five discrete normal and reversed polarity zones. The measured northeast-southwest declinations and shallow inclinations in these polarity zones probably record Late Devonian magnetostratigraphy on a centimeter scale. Using sedimentation rates from detailed conodont studies, a 56,000-year periodicity is calculated for Late Devonian polarity reversals. This value assumes no breaks in sedimentation, and is comparable to minimum lengths of late Mesozoic and Cenozoic polarity reversals. However, this study shows that iridium is present over the span of five magnetic reversals. Evidence therefore favors a biologic, rather thanmore » extraterrestrial, origin for anomalous iridium concentrations.« less