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

Postdepositional greigite (Fe3S4; ferrimagnetic thiospinel) is of interest to sedimentary geochemists, because it reflects important reactions during diagenesis and to paleomagnetists because it can obscure a detrital paleomagnetic record. The presence, distribution, and origin of greigite are best understood through combined magnetic and geochemical studies. Such studies of greigite-bearing Upper Cretaceous siliciclastic beds from the Simpson Peninsula, North Slope, Alaska, reveal relations among sulfur species and magnetic properties, and they illustrate the use of geochemical analysis to constrain the age of secondary magnetization carried by greigite. Greigite is ubiquitous in marine mudstone of the Seabee Formation, and it dominates the magnetic properties of the Seabee (magnetic susceptibility [MS]: 5.9 x 10-4 volume SI; magnitude of natural remanent magnetization [NRM]: 6.6 x 10-2 amperes/meter [A/m]; averages of 22 specimens in which greigite is the only magnetic mineral). The Seabee rocks fill an ancient submarine canyon cut into marine, transitional, and nonmarine sandstone, siltstone, and mudstone beds of the undifferentiated Ninuluk and Seabee Formations. In these sandstone and siltstone beds, some of which contain biodegraded oil, 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). Samples that contain detrital iron-titanium oxides, principally titanohematite, as the only magnetic minerals have lower values of MS and NRM magnitude.

Different geochemical signatures in the Seabee Formation and undifferentiated Ninuluk and Seabee rocks indicate different origins of their greigite and associated iron disulfide minerals. In the Seabee, greigite and pyrite formed during early diagenesis via bacterial sulfate reduction utilizing indigenous sulfate and organic carbon. Evidence 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 presence of greigite and framboidal pyrite in detrital plant fragments. Ratios of total reduced mineral sulfur to organic carbon (S/C) indicate low contents of sulfur relative to those of normal marine sediments. In the undifferentiated Ninuluk and Seabee rocks, reactions that involved epigenetic sulfur produced greigite, pyrite, and rare marcasite that cement and surround early diagenetic pyrite. In many of these beds, S/C ratios are high relative to normal marine sediments. The epigenetic sulfur may have been derived from (1) sulfate-bearing Paleozoic units in deeper parts of the North Slope basin to the south, perhaps during much of the Tertiary to the present; or (2) the canyon-fill Seabee during 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 (formed from seawater sulfate) in any unit in the basin. Organic substances that supported such BSR in the undifferentiated Ninuluk and Seabee rocks may have been derived from hydrocarbons.