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

• Main Authors: REYNOLDS, RL, TUTTLE, ML, RICE, CA, FISHMAN, NS, KARACHEWSKI, JA, Sherman, David M
• Format: Article in Journal/Newspaper
• Language: English
• Published: 1994
• Subjects: CALIFORNIA CONTINENTAL BORDERLAND; ANOXIC MARINE-SEDIMENTS; IRON SULFIDE MINERALS; CEMENT OIL-FIELD; SOUTH TEXAS; SULFATE REDUCTION; HOLOCENE SEDIMENTS; WESTERN INTERIOR; PYRITE FORMATION; SULFUR-ISOTOPE; Simpson Peninsula; north slope; Alaska
• Online Access: https://hdl.handle.net/1983/2d888250-e9a7-4716-b3c5-ba689f2e9df5; https://research-information.bris.ac.uk/en/publications/2d888250-e9a7-4716-b3c5-ba689f2e9df5

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 ...