HR: 15:25h
AN: GP23B-08    [Abstracts]
TI: Normal and Inverse Magnetic Fabrics in Mesozoic Black Shales, Northern Siberia, Russia: Siderite Controlled?
AU: * Chadima, M
EM: chadima@sci.muni.cz
AF: Institute of Geology, Academy of Sciences, Rozvojova 135, Prague, 16502 Czech Republic
AU: Pruner, P
AF: Institute of Geology, Academy of Sciences, Rozvojova 135, Prague, 16502 Czech Republic
AU: Grygar, T
AF: Institute of Inorganic Chemistry, Academy of Sciences, Rez, Prague, 25068 Czech Republic
AU: Hirt, A M
AF: Institute of Geophysics, ETH-Hoenggerberg, Zuerich, 8093 Switzerland
AB: A 28-meter-long profile situated on the coast of the Arctic Ocean, Russia (74N, 113E) was extensively sampled primarily for the purpose of magnetostratigraphic investigations across the Jurassic/Cretaceous boundary. The profile consists predominantly of marine black shales with abundant concretions and several distinct diagenetically cemented layers. Newly formed siderite can be macroscopically observed in cemented layers. Its presence was further verified using X-ray powder diffraction. Magnetic volume susceptibility of the shales is between 8 x 10-5 and 2 x 10-3 SI with concretions and cemented layers possessing susceptibilities with an order of magnitude higher compared to the neighboring rock. The intensity of NRM varies between 1 x 10-5 and 6 x 10-3 A/m. Several samples with anomalous paleomagnetic directions and extremely high values of NRM were found. Unlike susceptibility, the relatively high NRM values show no apparent correlation to the profile lithology. The anisotropy of magnetic susceptibility of the majority of samples is consistent with a sedimentary fabric (both magnetic foliation and lineation are sub-parallel to the bedding). The anisotropy degree ranges from 1.01 to 1.15, the shape of the susceptibility ellipsoid is distinctly oblate. Despite the well-preserved sedimentary fabric, in some samples the K1 and K3 principal susceptibility directions are interchanged and have distinctly prolate ellipsoid shapes. Although the majority of such inverse samples are bound to the cemented layers, some can be found within "uncemented" sections of profile. Magnetic fabric using anisotropy of anhysteretic remanent magnetization is bedding-parallel with oblate ellipsoid shapes. This suggests that the inverse magnetic fabric is due to the paramagnetic mineral siderite, which is confirmed by high-field anisotropy measurements. We conclude that the presence of normal or inverse magnetic fabric is controlled by the relative siderite content in studied rocks. The samples with high NRM values may contain oxidation products of siderite (possibly hematite) which could acquire the stable chemical remanent magnetization with anomalous directions.
DE: 1714 Geomagnetism and paleomagnetism
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting