The Spetrum Anylysis And Its Significance Of The Middle-Late Jurassic Sediments In Yanshiping Area,Qiangtang Basin

Qiangtang Basin is the most developed regions for middle-Late Jurassic marine strata in China,which is the most possible place to explore ore and sylvite.The sediments show that it is very complete mid-up Fm.,which also have the features of clear sequences,layered marker significantly and rich fossils,thus its ideal searching vector for Jurassic in Yanshiping section.There are four group in Yanshiping section,which contain Quemocuo Fm.,Buqu Fm.,Xiali Fm.and Suowa Fm.and their lithology changes shows the characteristics of good cycle according to the sequence of "clastic rocks-limestone-rocks-limestone".Based on the characteristics of lithology's good cycle,through the Redfit spectrum analysis for calcium carbonate content and NRM target which they are high resolution,we find the calcium carbonate content and NRM target spectrum analysis result have good corresponding,we consider that the earth's orbital cycles is an important factor for developing sedimentary cycles in Yanshiping area.Further more,we make use of Gaussian filter to anylysis the main frequency cycles of calcium carbonate content and NRM target,combing with the characteristics of the lithology,fossil and layers of gypsum outcrops,as a result,we gain those results:1)The sediments recorded the stratum have Milankovitch cycles in Yangshiping Area,Qiangtang Basin(172.4-156.4Ma),we think that the Earth's orbital cycles is the main factor for the area sedimentary cycle occurring and developing.2)During the period of 172.4-156.4Ma,eccentricity cycle and precession cycle are in nonlinear relationship in Yanshiping section.Because eccentricity cycle and precession cycle have the feature of "as one is strong,the other is weak"on climate controlling,it lead to climate change with the cycle of "wet(early Quemocuo Fm.)-more drier(middle-late Quemocuo Fm.)-wetter(Buqu Fm.)-wet(early Xiali Fm.)-dry(middle-late Xiali Fm.)-wetter(Suowa Fm.)",and the lithology also shows fine cycle as "scrap(Quemocuo Fm.)-carbonate rock(Buqu Fm.)-scrap(Xiali Fm.)-carbonate rock(Suowa Fm.)".3)During the period of 172.4-166.4Ma(Quemocuo Fm.)and 162.6-159.2Ma(Xiali Fm.),the paleoclimate is mainly controled by the precession cycle.As the power of the precession cycle increasing and high contribution rate of precession cycle for climate in this two section(76.9%),the amount of sun radiation added and the climate become drier.Combined with the layers of gypsum outcrops in the field and the calcium carbonate content,we find that Xiali Fm.is the most arid and also is the most possible layers to find potassium salt.4)During the period of 166.4-162.6Ma(Buqu Fm.)and 159.2-156.4 Ma(Suowa Fm.),the power of precession cycle become smaller obviously,which make the sun radiation reduction(the contribution of sun radiation is-73.8%).The eccentricity cycle plays a major role in controlling solar radiation,as power value of the precession cycle increasing,and the contribution rate of eccentricity cycle to climate is upright(positive 26.2%),all of which cause the amount of solar radiation reduced in total amount.Otherwise,the increasing eccentricity cycle and the reducing of the precession cycle make the climate of Buqu Fm.and Suowa Fm.become moist.