Magnetostratigraphy, Late Devonian iridium anomaly, and impact hypotheses

Details Magnetostratigraphy, Late Devonian iridium anomaly, and impact hypotheses Magnetostratigraphy, Late Devonian iridium anomaly, and impact hypotheses

Apr 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990geo....18..291h&link_type=abstract

Geology, vol. 18, Issue 4, p.291

Mathematics

Logic

7

Scientific paper

Paleomagnetism, sedimentology, and fine-scale stratigraphy have been integrated to explain the origin of an iridium anomaly in the Late Devonian of Western Australia. Thermal demagnetization experiments were carried out on 93 specimens of marginal-slope limestone from the northern Canning Basin. Samples are from a condensed sequence of deep-water (>100 m) Frutexites microstromatolites. Frutexitesis a shrublike cyanobacterial organism that probably precipitated hematite, or a metastable precursor, from sea water. When plotted within the microstratigraphic framework for the study area, the observed characteristic directions from the sampled interval (14.5 cm thick) are in five discrete, layer-parallel, normal-and reversed-polarity zones. The measured northeast-southwest declinations and shallow inclinations probably record Late Devonian magnetostratigraphy on a centimetre scale. The Frutexitesbed studied here occurs close to the Frasnian/Famennian (Late Devonian) boundary, a time of mass extinction of a wide variety of marine organisms throughout the world. Anomalously high iridium concentrations observed in the Frutexitesbed have suggested to some authors that the mass extinction was caused by meteorite impact. This study concludes that iridium, which is present over the span of five layer-parallel magnetic reversals, was concentrated over a long period of time by biologic processes. Thus, the Canning Basin iridium anomaly may be unrelated to meteorite impact.

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