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

Results of a paleomagnetic, rock magnetic, geochemical, and petrographic st udy on Jurassic and Cretaceous carbonates in the Vocontian trough support a hypothesized connection between burial diagenetic alteration of smectite a nd the widespread occurrence of a chemical remanent magnetization (CRM) car ried by magnetite. Where smectite has altered to other clay minerals, limes tones are characterized by a prefolding, secondary, normal polarity magneti zation throughout the basin. The magnetization is interpreted to be a CRM b ased 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 th e amount of smectite and burial. Isothermal, anhysteretic, and natural rema nent magnetization intensities increase where smectite has altered, both st ratigraphically and geographically. This is interpreted to indicate magneti te authigenesis associated with clay diagenesis. Superparamagnetic magnetit e is more dominant in highly altered units based on the results of low-temp erature experiments. All sections away from the Alps have Sr-87/Sr-86 value s that are similar to coeval seawater, and stable isotopes of carbon and ox ygen show no sign of alteration. Orogenic-type fluids therefore are not a l ikely agent of remagnetization. Near the Alps the rocks are characterized b y 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 de velopment of the widespread CRM in the Vocontian trough and that this mecha nism might explain widespread chemical remagnetization elsewhere.