NATURE AND ORIGIN OF MAGNETIC MINERALS WITHIN THE MIDDLE JURASSIC SHALLOW-WATER CARBONATE ROCKS OF THE PARIS BASIN, FRANCE - IMPLICATIONS FOR MAGNETOSTRATIGRAPHIC DATING

The Middle and Upper Jurassic Bathonian-Oxfordian shallow-water carbon ate rocks from the Paris Basin, France, consist mainly of oolitic and bioclastic limestones that are hydrocarbon reservoirs in the subsurfac e. Despite a preliminary positive study, these deposits have been cons idered to be largely remagnetized (Rochette, private communication), a nd hence not amenable to palaeomagnetic dating. To establish their mag netic mineralogy and test this remagnetization hypothesis, we have use d an integrated investigation combining petrographic, geochemical, roc k-magnetic and palaeomagnetic measurements on samples extracted from f ive cores from the Paris Basin and from outcrops in Burgundy. Magnetic minerals in the Bathonian-Oxfordian carbonates include: (1) primary b iogenic single-domain magnetite and detrital multidomain Ti-magnetite and their oxidized form, maghemite; (2) authigenic spheres of magnetit e probably related to hydrocarbons; and (3) goethite, either restricte d to ferruginous ooid layers or resulting from surficial alteration, n otably replacement of pyrite framboids. Rock-magnetic experiments carr ied out on 68 samples reveal H-cr/H-c and M-rs/M-s ratios ranging from 1.88 to 5.58 and 0.017 to 0.314, respectively. These values are clear ly distinct from diagnostic values for a chemical remagnetization. Pyr rhotite was not identified within these sediments. Moreover, the avera ge H-cr/H-c ratio of 3.14 is significantly different from the value of 1.333 for natural pyrrhotite (Dekkers 1988). These results have a dir ect implication for the preservation of the primary magnetization; con sequently, these deposits are selectively amenable for magnetostratigr aphic dating and possible regional correlations.