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Home My WebLink AboutGMC Data Report No. 069Magnetic Properties of Greigite-bearing Cretaceous Strata, North Slope, Alaska (abstract only) 2 September 1987 total of I page in report Geologic Materials Center Data Report 69 l~agnetic Properties of Greigite-bearing Cretaceous Strata, North Slope, Alaska R-L. REynOLDS, J.A. KARACHEWSKI, N.S. FISHHAN, and J. G. ROSENBAUH (USGS, Box 25046, Denver, CO 80225) Hagnetic properties of Upper Cretaceous sandstone, siltstone, and mudstone at the Simpson oil field (Alaskan North Slope) reflect contributions from authigenic greigite and from detrital re-Ti oxide minerals. Hagnetic minerals, separated from 103 samples taken from 9 borehole cdres (10-300 m depths), were identified by thermomagnetic,'petrographic, and X- ray diffraction analyses. Greigite (Fe3S4; spinel structure) is the major magnetic mineral in most samples (G-group) from each rock type. Detrital Fe-Ti oxide minerals, principally ferrimagnetic titano- hemitite and titaniferous magnetite, occur with greigite in some samples but are dominant remanence carriers in only 20-30I of the samples (FT-group). Inten~ities of ~RH of the G-group samples range from 3x10- to 2x10-' A/m, whereas t~se of .the FT-group samples range from 6x10~~ to 10-3 A/m. Low-field susceptibilities (4xl0- (X<3xl0- SI [volume] units) are similar for both groups. Clear distinctions between the groups can be made from plots of ARH vs. X, and from the ranges in values of the coercivity of - - remanence (B__) and of the median destructive induc- tance (HI)I) ~om the AF demagnetization of NRH.. Slopes of ARH vs. X are steep for G-group samples; slopes appear to decrease systematically w/th higher relative content of detrital Fe-Ti oxides. Values of Bcr (55-71 mT) and HD! of ~ (38-48 mT) in the G-group samples contrast with those values (39-48 mT and 13-22 respectively) in the FT-group samples. The different magnetic characteristics of the groups probably reflect mainly the differences in size between greigite (<0.7 pm, based on SEH examination) and the detrital oxides (typically 5-50 pm), but compo~itional influences cannot be discounted. The value of these characteristics as discriminators a2ong fine (<l magnetic spinels of different compositions and origins is uncertain. GMC Data Report 69