Studies On The Jurassic Magnetostratigraphy And The Formation Conditions Of Salts In The Yanshiping Area,North Qiangtang Basin

With the breakup of the Gondwana,the opening of the Atlantic Ocean and the closure of the Palaeotethys Ocean,global paleoclimate underwent dramatic changes during the Middle-Late Jurassic.And an arid climatic zone was existed in the Tethys tectonic zone during the same interval,which led to large scale marine potash deposites were deposited in the Karakum and Amu Darya Basins of the Middle Tethys tectonic zone.The Qiangtang Basin,located in the East Tethys tectonic zone,is bounding the Karakum and Amu Darya Basins.The Qiangtang Basin is one of the most important prospective salt resource belts in China,in which there are a large number of the Jurassic marine strata.With the exploration projects of geological cartography,petroleum and salt lake resources going on,a lot of gypsiferous bed outcrops were founded in the Jurassic strata in the basin in the last decades.More importantly,salt springs with abnormal sodium?Na⁺?and potassium?K⁺?were recently founded in the Qiangtang Basin.Gypsum and salt springs are very important signs for investigating potash salt.However,there are few comprehensive studies on the forming ages and conditions?tectonism,provenance,paleoclimate and paleogeography?of evaporites in the Jurassic strata in the Qiangtang Basin besides some studies on the scope of gypsum and hydrochemical characteristics of the salt springs.In this paper the formation of halite and potash deposits in the Tethys tectonic zone as entry points and the forming conditions?tectonism,provenance,paleoclimate and paleogeography?of halite and potash deposits as main controlling factors,we subject the magnetostratigraphy and geochemical records of the Jurassic sedimentary sequences from the Yanshiping section,Qiangtang Basin,in order to study the forming conditions of evaporites.Firstly,combining with detailed paleomagnetic analyses and paleontological ages,magnetostratigraphic ages of the Jurassic sedimentary sequences in the Yanshiping section are suggested.Secondly combining with the analyses of sedimentary environments and stratigraphic sequences,a curve of sea level change in the Yanshiping region is constructed,which reveals that a regression event was happened in the upper member of the Xiali Formation.And then the correlation between the Yanshiping curve and the globle curve of Haq reveals that the change of global sea level is the first order factor controlling the sea level change in the Yanshiping region.Thirdly,combining with the analyses of the paleoclimate proxies?carbonate,sulfate ion,chloride ion,hematite,goethite and magnetic susceptibility?,the evolution sequences of paleoclimate in the Yanshiping section are constructed,which also reveal that an arid layer was happened in the upper member of the Xiali Formation.The above conclusions,a regression event,an arid layer and the obviously increased K⁺,Na⁺,Cl^-ions concentrations of the provenance proxies in the upper member of the Xiali Formation,reveal that the upper member of the Xiali Formation is the optimum layer to potentially deposit halite.On the other hand the conclusions of this dissertation will be provided the scientific suggestions for the delineation of target area of the formation of halite and potash deposits in the Qiangtang Basin in the future.Through the above study,the major conclusions are as the following:?1?Magnetostratigraphic study in the Yanshiping section in the Qiangtang Basin reveals 66normal and 65 reversed polarity zones.Constrained by fossil ages,the correlation with GPTS2012yields>171.2-165.5 Ma?an Aalenian-Callovian age in the Middle Jurassic?for the Quem Co Formation,165.5-163.9 Ma?a Callovian age in the Middle Jurassic?for the Buqu Formation,163.9-160.2 Ma?a Callovian-Oxfordian age in the Middle-Late Jurassic?for the Xiali Formation,160.2-157.6 Ma?an Oxfordian age in the Late Jurassic?for the Suowa Formation.?2?On the basis of the cognition of third order system tracts in the Yanshiping section,a curve of sea level change in the Yanshiping region is constructed,which reveals that two transgression events and two regression events were happened in the Qiangtang Basin during the Middle-Late Jurassic.Our curve coincides well with the Haq curve,which implies that the change of global sea level is the first order factor controlling the sea level change in the Yanshiping region.?3?Paleoclimate proxies of the Quem Co Formation featured by the lower carbonate,hematite,SO₄^2-,Cl^-content and the higher goethite,magnetic susceptibility content reveal that the climate in the formation is humid;Paleoclimate proxies of the Buqu Formation featured by the highest carbonate,magnetic susceptibility content,the medium higher SO₄^2-,Cl^-,hematite content and the medium lower goethite content reveal that the climate in the formation is semi-dry and semi-humid;Paleoclimate proxies of the lower member of the Xiali Formation featured by the lower carbonate,SO₄^2-,Cl^-content,the medium hematite content and the highest goethite content reveal that the climate in the sbu-formation is humid;Paleoclimate proxies of the upper member of the Xiali Formation featured by the highest SO₄^2-,Cl^-,hematite content,the medium higher carbonate content and the lowest goethite,magnetic susceptibility content besides the significant increased correlation coefficient of Ca²⁺-SO₄^2-ions from the lower?R=0.2706?to the upper?R=0.985?member of the Xiali Formation and the significant increased correlation coefficient of Na⁺-Cl^-ions from the lower?R=0.146?to the upper?R=0.8974?,reveal that the climate in the sbu-formation is dry;Paleoclimate proxies of the Suowa Formation featured by the medium carbonate,SO₄^2-,Cl^-,goethite,magnetic susceptibility content and the lowest hematite content reveal that the climate in the formation is humid.?4?The analyses of the conditions of tectonism?a depression basin in the passive continental margin?,paleoclimate?a dry climate in the upper member of the Xiali Formation?,sedimentary environment?a semi-closed tidal flat environment formed by the regression event?and provenance?the obviously increased K⁺and Na⁺ions concentrations in the upper member of the Xiali Formation?in the Yanshiping section reveal that the upper member of the Xiali Formation is the optimum layer to potentially deposit halite and potash.In addition,the correlation coefficient of Na⁺-Cl^-ions?R=0.8974?is obviously higher than the K⁺-Cl^-ions?R=0.7269?,which further reveals that there is more chance for the formation of halite in the upper member of the Xiali Formation than potash.?5?The correlations of tectonism,provenance,paleoclimate,sedimentary environment and saliferous strata reveal that the Qiangtang Basin has similar conditions for the formation of evaporites as the Amu Darya Basin during the Middle-Late Jurassic and the upper member of the Xiali Formation is a potential valuable layer to evaluate the possible formation of halite deposit.