Grain Size Characteristics And Sedimentary Significance Of Cretaceous Clastic Rocks In MZK-3 Borehole Of Simao Basin

The characteristics of grain size and quartz grains surface are important property of clastic rocks,which are controlled by provenance,transport mode,transport medium and sedimentary environment.Therefore,sedimentary environment and dynamic condition of rocks can be judged by researching the distribution of grain size and surface morphology of quartz grains.Simao Basin is the only ancient solid-sylvite producing area in Yunnan Province,China.At present,the Cretaceous clastic rocks is the main strata of sedimentary deposits,but there are relatively few studies concerning on the sedimentary environment and dynamic conditions..In this paper,petrological characteristics,grain size characteristics and surface morphology of quartz grains of Cretaceous clastic rocks in MZK-3 borehole are analyzed by means of thin sections,grain size analysis and scanning electron microscopy.The hydrodynamic conditions and sedimentary environment of Cretaceous strata are comprehensively discussed in order to provide new information for the Cretaceous Paleoenvironment.The results show that:(1)The sediments of Nanxin Formation is mainly fine and very fine sand,mainly sub-angular type and high textural maturity,and suspension transport.There are less mechanical wear and collision between grains,and less mechanical action characteristics on the surface of quartz.However,chemical precipitation is the main depositional mode,which indicates that the hydrodynamic force is weak during deposition.It is presumed that the sedimentary environment is shallow lake deposit.(2)The sediments of Hutousi Formation is dominated by medium-fine sand,subcircular type and followed by angular type.During the sedimentary period,the hydrodynamic force and the mechanical wear and collision between the grains was increasing,contributing to the surface of the quartz with high-energy feature.,In addition,the grains were screened by the wave,causing the sediment were well sorted.It is inferred that the depositional environment of Hutousi Formation is shore-shallow lake environment.(3)The lithology of the lower part of Mengyejing Formation is purple-red quartz fine sandstone.The clastic components are mainly quartz without matrix.The grains are mainly subcircular-circular,and were very well sorted.The probability cumulative curve is mainly a straight line with a slope of about 60 degrees.The skewness is mainly symmetrical and the kurtosis is moderate.A large number of dish shape crater appear on the surface of quartz,and more siliceous spheres and corrosion pits are deposited.These characteristics reflect that the clastic sediments of Mengyejing Formation had been transported and reformed over a long distance,and mainly transported by leaps and bounds.Dish shape crater are usually formed in strong storms,which are the unique signs of wind-induced environment.The special environment and climate of desert resulting in strong dissolution and precipitation.Therefore,it is presumed that the lower depositional environment of Mengyejing Formation is wind-desert environment.