| The Mesozoic fluvial and lacustrine sediments are well developed in the northwest China,which recorded significant tectonic evolution information,and which contains abundant resources such as coal,oil and gas.The Jurassic is an important transitional period of tectonic evolution in the northwest China,with a large number of sedimentary basins initiated or expand.Due to the lack of accurate geologic time data,there are still great controversies over the understanding of the tectonic and sedimentary evolution as well as resource accumulation mechanism of these basins.In this paper,two basins in this region,the Hexi Corridor Basin and Ordos Basin were addressed to study the magnetostratigraphy and astronomical time scale for the Middle-Upper Jurassic strata.The obtained geologic time scales of great significance to the study of the tectonic and sedimentary evolution process in the northwest China over the Middle and Later Jurassic,and of formation mechanisms of multiple energy resources in the same basin.The following results have been achieved.(1)Based on lithologic and sedimentary features of the ZB26-32 borehole in Fengxiang county,southwestern Ordos Basin,and the ZK1901 borehole and the measured profile in Kuantaishan(Chijin town),west Hexi Corridor,the Middle Jurassic Zhiluo Formation in Fengxiang composed of braided river and meandering river facies,and the Anding Formation is lacustrine facies.The Upper Jurassic Chijinbao Formation in the kuantaishan area consisted of the piedmont alluvial fan facies,fan delta facies and the lacustrine facies.(2)Demagnetization and rock-magnetism experiment indicate that the characteristic remanents magnetizations(ChRMs)of gray and greenish samples are mainly carried by magnetite,with small amount of maghemite and goethite.The ChRMs of brown and reddish,and containting red breccia samples are main carried by hematite,containing small amounts of goethite and magnetite and somewhat pyrrhotite.In some gray brownish,brown and gray-black samples the ChRMs are carried both by hematite and magnetite,and with small amount of maghemite.(3)Magnetostratigraphy study show that the Upper Jurassic strata of the ZK1901 borehole in the Kuantaishan recorded 20 normal and 20 reverse magnetozones,which can be correlated to the M28A-Dr.2r to M34 Bn.1n of GPTS2012,yielding an age of 162.39 Ma-158.30 Ma,equivalent to the mid-Oxfordian stage.This determine the existence of the Upper Jurassic strata in the Kuantaishan ares.Middle Jurassic Zhiluo Formation of ZB26-32 core in southwestern Ordos,recorded 17 normal and 16 reverse magnetozones,which can be correlated to the M39 n.4n to M42 n.7n of GPTS2012.This correlation yields the age of 168.63 Ma-166.27 Ma(the late Bajocian and early Callovian stage),The age of the lower Anding Formation designed between 166.27 Ma and 165.65 Ma,which corresponds to the middle period of Callovian.(4)Spectral analysis and continuous wavelet transform analysis for natural gamma ray data demonstrate that well-preserved Milankovitch cycle are identified in the sedimentary strata.The Chijinbao Formation in ZK1901 borehole recorded following 400 ka long eccentricity cycle,95 ka short eccentricity cycle,44 ka oblique and 18 ka precession cycle,which corresponds to the stratigraphic thickness is 30.66 m,6.33 m,3.17 m and 1.25 m.The Zhiluo-Anding Formation ZB26-32 core only contain following 125 ka short eccentricity cycle and 44 ka obliquity cycle,which corresponds to the stratigraphic thickness is 6.54 m and 1.98 m.After filtering for the eccentricity,obliquity and precession cycle of the GR logs and tuning them to the Laskar astronomical model,the middle Chijinbao Formation on ZK1901 recorded 10.5 long eccentricity cycles of 400 ka and 44 short eccentricity cycles of 95 ka.This yield a floating astronomical time scale of ca.4.2 Myr.The Zhiluo Formation and lower part of Anding Formation on ZB26-32 corerecorded 21 short eccentricity cycles of 125 ka,yielding a floating astronomical time scale of ca.2.63 Myr. |
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