Associations between burial diagenesis of smectite, chemical remagnetization, and magnetite authigenesis in the Vocontian trough, SE France

• Published in: Journal of Geophysical Research: Solid Earth 2000-01, Vol.105 (B1), p.851-868
• Main Authors: Katz, Bodo, Elmore, R. Douglas, Cogoini, Monika, Engel, Michael H., Ferry, Serge
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Geophysics: general, magnetic, electric and thermic methods and properties; Internal geophysics; Isotope geochemistry; Isotope geochemistry. Geochronology; Stratigraphy
• ISSN: 0148-0227; 2156-2202
• DOI: 10.1029/1999JB900309

Results of a paleomagnetic, rock magnetic, geochemical, and petrographic study on Jurassic and Cretaceous carbonates in the Vocontian trough support a hypothesized connection between burial diagenetic alteration of smectite and the widespread occurrence of a chemical remanent magnetization (CRM) carried by magnetite. Where smectite has altered to other clay minerals, limestones are characterized by a prefolding, secondary, normal polarity magnetization throughout the basin. The magnetization is interpreted to be a CRM based on low burial depths which cannot cause thermoviscous resetting. Where significant smectite is still present, the CRM is absent/weakly developed, and where the clays show no evidence for burial alteration, the units are characterized by a primary magnetization. CRM intensity also varies with the amount of smectite and burial. Isothermal, anhysteretic, and natural remanent magnetization intensities increase where smectite has altered, both stratigraphically and geographically. This is interpreted to indicate magnetite authigenesis associated with clay diagenesis. Superparamagnetic magnetite is more dominant in highly altered units based on the results of low‐temperature experiments. All sections away from the Alps have 87Sr/86Sr values that are similar to coeval seawater, and stable isotopes of carbon and oxygen show no sign of alteration. Orogenic‐type fluids therefore are not a likely agent of remagnetization. Near the Alps the rocks are characterized by an additional reversed polarity component which is interpreted to reflect acquisition of the CRM through a reversal. A postfolding magnetization is also present there and strontium isotopic ratios are higher than elsewhere in the basin and might indicate some alteration by orogenic‐type fluids. We conclude that burial diagenesis of smectite is the likely cause for the development of the widespread CRM in the Vocontian trough and that this mechanism might explain widespread chemical remagnetization elsewhere.