| Qiangtang Basin is the most developed regions for the middle-Late Jurassic marine strata,and also is the regions where hopes to achieve a major breakthrough for the oil and gas exploration and potassium prospecting in China.Based on the chronological study of Jurassic Qiangtang Basin is nearly blank,we systematically studied the Jurassic in Yanshiping area,Qiangtang Basin at the rock magnetism and magnetic stratigraphy to provide important geochronology data for the reaserch of the Tethys evolution,the regional tectonic,and the sedimentary environments evolution and so on.In this study,we also have developed the system rock magnetism research for the sedimentary rock samples of Jurassic Yanshiping Group which collected from Yanshiping area,Northern Qiangtang Basin.The results show that:The main magnetic minerals of Quemocuo Formation(lower member,Middle member and upper member)performed as coexistence of magnetite and hematite.And it is expressed as mainly to the small particles magnetite,and containing iron sulfides(pyrite gum,pyrrhotite)and goethite which are the mineral of high coercivity in Buqu Formation.In Xiali Formation,the magnetic minerals showed mainly to magnetite and a certain amount of maghemite.In Suowa Formation,the magnetic minerals showed as magnetite-based.In addition,the natural remanence(NRM)mathematical statistics show:The average value of NRM in Buqu Formation(2.2×10-4 A/m)was significantly higher than it in Quemocuo Formation(2.2×10-5A/m),Xiali Formation(2.4×10-5A/m)and Suowa Formation(4.4×10-5A/m).Therefore,we believe that the small particles magnetite is the mainly factor which caused the NRM enhanced.Contrast the changes of the magnetic mineral assemblages and the ancient depth and sedimentary faces for every formation in Yanshiping Profile,we have found that there are good correspondence between the two of them,and indicating that the magnetic mineral changes in Yanshiping Group is mainly controlled by the sedimentary environment changes.In short,different types of magnetic mineral assemblages reveal different sedimentary environments.Magnetic mineral assemblages,which are the low coercivity magnetite(gray strata)and high coercivity hematite(red strata),represent the periodic tidal effects,such as in the middle section of Quemocuo Formation and the Xali Formatiom.Magnetic mineral assemblages with high coercivity hematite dominant represent the deltaic sedimentary environment,which have characteristics of rapid supply of terrigenous and long-term in an oxidizing environment,such as in the lower section and the upper section of Quemocuo Formation.And magnetic mineral assemblages which are the low coercivity magnetite and iron sulfide,represents a steady reduction of sedimentary environments,such as in Buqu Formation and Suowa Formation.By the measurement of the high-density magnetic stratigraphy and combined with a large number of paleontological research for the Jurassic stara in Yanshiping era,Qiangtang Basin,we have established the magnetic stratigraphic sequence:The precise ages of the Quemocuo Formation,Buqu Formation and Xiali Formation,Which are belong to the stratum of Middle Jurassic,are at 172.4Ma?166.4Ma,166.4Ma?162.6Ma,162.6Ma?159.2Ma.And the precise ages of the Suowa Formation are at 159.2Ma?156.4Ma,Which are belong to the stratum of Late Jurassic.Using the ages,we have calculated the deposition rate of every Formation in Yanshiping Profile.And the results show that:The deposition rate of Quemocuo Formation can be divided into three segments obviously.From the fist segment to the third segment of Quemocuo Formation,the deposition rates respectively are 300 m/Ma,223.43 m/Ma and 206.16 m/Ma. |
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