Study On Integrated Stratigraphy,and Tectonic Differentiation Of Potassium Containing Depression Of The Majiagou Formation In Northern Shaanxi Ordovician Salt Basin

China is the lack of potash resources.The world’s largest potash deposits mainly occur in the ancient marine evaporite basins,such as Canadian Saskatchewan potash deposit of middle Devonian and Russian Niepa potash deposit of early Cambrian.China has a number of ancient marine evaporite basins,so the best way to solve the dilemma of potash is seeking a breakthrough in marine potash.Northern Shaanxi salt basin is the only Ordovician potash basin is the world and the preferred basin looking for ancient epicontinental large potash deposit.Based on the backbone of Northern Ordovician salt basin drilling,outcrop and seismic data.This article carry out the rock formations,paleontological and sequence stratigraphy comprehensive study.According to development of Salt strata,combined with seismic,to draw Paleostructure diagram of the 56 submember of Majiagou Formation.Using X-diffraction and Geochemistry to analysis clay samples from high potassium layers and study the potash paleoenvironmental of the 56 subperiod.Ordovician lithostratigraphic in Ordos Basin is significant different between the eastern and southwestern.Through comparative study of conodonts,we can know that southwestern strata of Ordovician in the Ordos Basin develop from Dapingian Age to Katian Age,and the middle and eastern have the stratas from Dapingian Age to Sandbian Age.Comprehensive stratigraphic framework of Ordovician in Ordos Basin is established based on the above.From first to fifth submembers of Majiagou Formation is included in the Middle Ordovician.and the sixth submember is classified as sandbian stage of the Upper Ordovician.From Sandaokan Formation to the lower part of Lashenzhong Formation is included in the Middle Ordovician and the upper of Lashenzhong Formation and Gongwusu Formation are classified to the Upper Ordovician.Northern Shaanxi salt basin of Majiagou peried has experienced three major transgressive-regressive cycles.The sixth period is smallest among the three transgressive cycles,for it only develop in the eastern edge of the basin.The second period is bigger and the forth is biggest.The third period is biggest among the three regressive cycles.The six members of Majiagou Formation in Northern Shaanxi salt basin can be divided into four desalination-salted Ⅳ grade salt rhythms.Sylvite usually is found in the 56 submember of Majiagou Formation,and its Roofs and floors are often in touch with mudstone.The mud content of Roofs and floors in the eastern basin is high,while the western is most consisted of dolomite,gypsum rock and limestone.The paleostructure diagram about the 56 subperiod in Northern Shaanxi salt basin is drew based on sixteen exploratory core data and seismic data.The diagram reveals a Palaeotectonics of two depression and one uplift,which can be devided five sags.The second eastern sag is best in conserving potassium.The 56 submember of Majiagou Formation of well SJ1 has three Halite layers,the upper layer is thickest.Mineral assemblage is rock salt,clay minerals,anhydrite,dolomite,quartz and minor feldspar.Clay mineral assemblage is illite and chlorite,and illite is than chlorite.The content of K+is 0.08%-1.24%,and the average of 0.23%,Br × 1000/Cl at 0.17-0.98,with an average of 0.5,which mean deposition stage of Sylvite-carnallite.The upper Halite layer was deposited accompied by semi-arid-climate,semi-deep lake and the higher reduction,and the middle was deposited accompied by arid-semi-arid climate,deep lake and high reduction.The upper Halite layer is better than the middle in conserving potassium.