Rock Magnetism Of The Black Shales Of The Member One, Qingshankou Formation In Songliao Basin

The Cretaceous is one of the most important periods in the geological history, during which many significant events affect the surface of the earth,resulting in great global change and bringing profound influence to the environment.Songliao Basin,in northeastern China,is one of the biggest cretaceous lakes in the world.It is the key area to investigate the terrestrial stratigraphic records of major geological events and global climate changes in the Cretaceous.This thesis focuses on the high resolution(the resolution is 5 cm) rock magnetic and environmental magnetic research of the black shale from the first member of the Qingshankou Formation recovered by CCSD-SK-â… s borehole(China Cretaceous Continental Scientific Drilling-Songkeâ… -the south borehole).The character of the magnetic assembly has been distinguished,including the magnetic composition, content and granularity.The significance of different magnetic parameters has been analyzed to find the magnetic proxies of the environmental changes.This work may provide a base for the further research of the past environment and paleoclimate changes during the late Cretaceous in Songliao Basin.1990 specimens were made for environmental magnetic measurements.Detailed rock magnetic measurements were carried out,including magnetic susceptibility(χ), anhysteretic remanent magnetization(ARM),the medium destructive field (MDF_-ARM),saturation isothermal remanent magnetization(SIRM),isothermal remanent magnetization at reverse field of 100 mT and 300mT(IRM_-100 and IRM_-300), temperature-dependence of magnetic susceptibility(χ-T curve),isothermal remanent magnetization acquision curve and its demagnetic curve,the thermal demagnetization of ARM and SIRM and the Lowrie experiment et.In addition to rock magnetic measurements,mineralogical analyses were made,including the thin observation and X-ray diffraction spectrum.The results are as below:(1) Magnetite is the main magnetic remanence cartier in sediments from the first member of the Qingshankou Formation recovered by CCSD-SK-â… s borehole.(2) Susceptibility of the sediments is strongly influenced by the content of paramagnetic minerals and pyrite is identified in some specimens.The values of SIRM may mainly be controlled by the content of magnetite grains,while ARM supplies more information about fine magnetite grains.The ratio ARM/SIRM could show the granularity characteristic of the magnetite.(3) For the black shale profile:Susceptibility values of the sediments from the 1700～1745 m section show weak fluctuation,while those values of the sediments from the 1745～1782 m section fluctuate greatly.The values ofχ_ARM and SIRM of the sediments from the 1700～1745 m section are slightly higher than those of the 1745～1782 m section,which indicates that the sediments from the upper section possess more(fine) magnetite.The S-₁₀₀ ratios(-IRM_-100/SIRM) of the sediments from the 1700～1760 m section are higher than those of the sediments from the 1760～1782m,suggesting that,among all magnetite grains,the soft magnetite(its coercivity is less than 100 mT) is some more in the section of 1700～1760 m.