MAGNETIZATION AND GEOCHEMISTRY OF GREIGITE-BEARING CRETACEOUS STRATA,NORTH-SLOPE BASIN, ALASKA

Postdepositional greigite (Fe3S4; ferrimagnetic thiospinel) is of inte rest to sedimentary geochemists, because it reflects important reactio ns during diagenesis and to paleomagnetists because it can obscure a d etrital paleomagnetic record. The presence, distribution, and origin o f greigite are best understood through combined magnetic and geochemic al studies. Such studies of greigite-bearing Upper Cretaceous silicicl astic beds from the Simpson Peninsula, North Slope, Alaska, reveal rel ations among sulfur species and magnetic properties, and they illustra te the use of geochemical analysis to constrain the age of secondary m agnetization carried by greigite. Greigite is ubiquitous in marine mud stone of the Seabee Formation, and it dominates the magnetic propertie s of the Seabee (magnetic susceptibility [MS]: 5.9 x 10-4 volume SI; m agnitude of natural remanent magnetization [NRM]: 6.6 x 10-2 amperes/m eter [A/m]; averages of 22 specimens in which greigite is the only mag netic mineral). The Seabee rocks fill an ancient submarine canyon cut into marine, transitional, and nonmarine sandstone, siltstone, and mud stone beds of the undifferentiated Ninuluk and Seabee Formations. In t hese sandstone and siltstone beds, some of which contain biodegraded o il, greigite occurs sporadically but is locally concentrated to yield high values of MS (5 x 10(-3) vol. SI) and NRM magnitude (0.5 A/m). Sa mples that contain detrital iron-titanium oxides, principally titanohe matite, as the only magnetic minerals have lower values of MS and NRM magnitude. Different geochemical signatures in the Seabee Formation an d undifferentiated Ninuluk and Seabee rocks indicate different origins of their greigite and associated iron disulfide minerals. In the Seab ee, greigite and pyrite formed during early diagenesis via bacterial s ulfate reduction utilizing indigenous sulfate and organic carbon. Evid ence for early diagenetic iron sulfide includes (1) negative deltaS-34 values (typically between -22 and -30 permil) of acid-volatile sulfur (sulfur in greigite) and disulfide sulfur; and (2) the common presenc e of greigite and framboidal pyrite in detrital plant fragments. Ratio s of total reduced mineral sulfur to organic carbon (S/C) indicate low contents of sulfur relative to those of normal marine sediments. In t he undifferentiated Ninuluk and Seabee rocks, reactions that involved epigenetic sulfur produced greigite, pyrite, and rare marcasite that c ement and surround early diagenetic pyrite. In many of these beds, S/C ratios are high relative to normal marine sediments. The epigenetic s ulfur may have been derived from (1) sulfate-bearing Paleozoic units i n deeper parts of the North Slope basin to the south, perhaps during m uch of the Tertiary to the present; or (2) the canyon-fill Seabee duri ng compaction of the marine mud. Bacterial sulfate reduction (BSR) in a sulfate-limited environment is indicated from many deltaS-34 values (> +22 permil) that exceed the expected values for sulfate minerals (f ormed from seawater sulfate) in any unit in the basin. Organic substan ces that supported such BSR in the undifferentiated Ninuluk and Seabee rocks may have been derived from hydrocarbons.