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 carbonate rocks from the Paris Basin, France, consist mainly of oolitic and bioclastic limestones that are hydrocarbon reservoirs in the subsurface. Despite a preliminary positive study, these deposits have been considered to be largely remagnetized (Rochette, private communication), and hence not amenable to palaeomagnetic dating. To establish their magnetic mineralogy and test this remagnetization hypothesis, we have used an integrated investigation combining petrographic, geochemical, rock-magnetic and palaeomagnetic measurements on samples extracted from five cores from the Paris Basin and from outcrops in Burgundy. Magnetic minerals in the Bathonian-Oxfordian carbonates include: (1) primary biogenic single-domain magnetite and detrital multi-domain Ti-magnetite and their oxidized form, maghemite; (2) authigenic spheres of magnetite probably related to hydrocarbons; and (3) goethite, either restricted to ferruginous ooid layers or resulting from surficial alteration, notably replacement of pyrite framboids. Rock-magnetic experiments carried 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 clearly distinct from diagnostic values for a chemical remagnetization. Pyrrhotite was not identified within these sediments. Moreover, the average 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 direct implication for the preservation of the primary magnetization; consequently, these deposits are selectively amenable for magnetostratigraphic dating and possible regional correlations.