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HPS-05 Recent developments in the Geologic Timescale
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The magnetostratigraphy of early Cretaceous Liupanshan group of Liupanshan basin, central China
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Shuang Dai, Key Laboratory of Western China's Environmental Systems, Ministry of Education of China, Lanzhou University (China)
Qiang Zhu, Key Laboratory of Western China's Environmental Systems, Ministry of Education of China, Lanzhou University (China)
Xiaomin Fang, Institute of Tibetan Plateau Research, CAS (China)
Hongfei Hu, Key Laboratory of Western China's Environmental Systems, Ministry of Education of China, Lanzhou University (China)
Yuhu Tang, Key Laboratory of Western China's Environmental Systems, Ministry of Education of China, Lanzhou University (China)
Maodu Yan, Key Laboratory of Western China's Environmental Systems, Ministry of Education of China, Lanzhou University (China)
Yongbo Huang, Key Laboratory of Western China's Environmental Systems, Ministry of Education of China, Lanzhou University (China)
Junwei Liu, Key Laboratory of Western China's Environmental Systems, Ministry of Education of China, Lanzhou University (China)
Li Kong, Key Laboratory of Western China's Environmental Systems, Ministry of Education of China, Lanzhou University (China)
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The Early Cretaceous nonmarine sediments, named the Liupanshan Group in the Liupanshan basin in the central of China, consists of the four characteristic color units which composed by the alluvial redpupple conglomerates and fluvial redbrown sandstones (red unit), lacustrine redbrown mudstones and graygreen, greenyellow calcareous mudstones, marls and limestones (1st red-green unit ), lacustrine green mudstones, calcareous mudstones and marls (green unit) and lacustrine redbrown and graygreen calcareous mudstones interbedded with the sandstones and the gypsum bed (2nd red-green unit) from the bottom to the top. The paleomagnetic sampling using portable gas drill was conducted at 330 stratigraphic levels (total 1100 specimen) from the upper of red units thorough 2nd redgreen unit (total 730 meters in thick) along Huoshizhai section with 1380 meters in thick in the north of the basin. The samples were thermally demagnetized in 15 to 24 steps at 10°C-50°C intervals between 50°C and 690°C, and the remanent magnetizations and directions were measured on a 2G cryogenic magnetometer in a magnetically shielded room. Most samples have strong remanent magnetizations with clear isolation of characteristic remanent magnetization (ChRM) directions between 400 - 580(690)°C except for the greenyellow calcareous marls between 300-450°C. The measurement of the isothermal remanent magnetization acquisition and back field, thermal demagnetization of three axis IRM components suggests that the ChRM was carried by the magnetite in the mudstones, calcareous mudstones in the samples of the upper red units thorough the 2nd red-green unit, the hematite in the lower red units, and maghemite in a few greenyellow calcareous mudstones and marls in the green unit. The ChRM directions of the samples were determined by using principal component analysis.
The interpretable ChRM directions could be obtained from 312 stratigraphic levels and the ChRMs of specimens at a level were averaged for determination of the magnetic directions at each level. Virtual geomagnetic polarity latitudes were calculated from these ChRM directions and used for polarity determination. The obtained magnetostratigraphic record indicates 11 long normal polarity zones N1- N11 and 11 short reversed zones R0 - R10. Except for R0, R7, R9, and R10, the R3, R4, R5, R6 zones are questionably defined by only one sample which represents stratigraphic intervals as 1-3 meters, and R1 and R2 are the mixed polarity zones. The obtained polarity zones could be correlated with the M3 through the M-2r of GPTS of Gradstein et al. (1994), which suggests that the Liupanshan Group was deposited between ca. 127 - 100 Ma. This is collaborated by a rich fossils record of Ostracod, Estheria, plant, spollen and fish which has yielded age correlation of Barremian-Albian to Liupanshan Group.
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