Application Of Susceptibility On Cyclostratigraphic Analysis Of Carbonate Rock Successions

Susceptibility is a measurement of materials whether it is easy to be magnetized or is hard to be magnetized when it is placed in an applied field. Though susceptibility of carbonate rocks is affected by terristrial clastic supplies,relative sea level changes, varying carbonate accumulation rates and potential diagenesis, it is still able to encode stable Milankovitch cycles'signals.The Sinian Doushantuo Formation in south China deposited consecutively and well exposed. It is the first successions which deposited after the Neoproterozoic snowball earth events. Detailed cyclostratigraphic study on this succession has important significance on understanding astrodynamics of the Earth in Precambrian period,on setting up the astronomical time scale of the Doushantuo Formaton as well as on correctly evaluating the environmental significance of geologic events in the Doushantuo Formation.Principles and progresses of cyclostratigraphic analysis in marine and terrestrial deposits are detailly introduced. This thesis also systematicly introduced mathematical methods such as power spectrum estimation methods which are based on fourier transforms as well as other commonly used power spectrum estimation methods in cyclostratigraphic analysis and discussed the statistical significance of spectra peaks.A power spectrum estimation program which was writen by Matlab tools is also emphasized. The least record length for cyclostratigraphic analysis, filtering , infulences of accumulation rates changes, rectification, bioturbation on recognition of Milankovitch signals and the principal by which the fittest sampling interval is decided are introduced respectively.The platform Doushantuo Formation in Three Gorges area of Hubei Province can be lithologically divided into four members. Among these members,the third member is composed of 60-80-m-thick shaly or dolomitic limestones,and thick dolomite. The fourth member consists of~10-m-thick black shale containing abundant carbonate concretions. Lithological changes on crops all displayed obvious Milankovitch cycles character. This thesis carried out consecutive sampling and measuring of susceptibility on two sections of the third member and the fourth member of the Sinian Doushantuo Formation near Zigui county,Hubei province. Detailed cyclostratigraphic analysis on susceptibility data is conducted and potential cycles of long eccentricity,obliquity and precessions are recognized. Combined with the precise radiometric age of 551Ma on the top of the fourth part of the Doushantuo Formation and based on the long eccentricity signals, the floating astronomical time scale for the third part and the fourth part of the Doushantuo Formation was set up.The results reveal that the fourth part of the Doushantuo Formation lasts for 6.2 million years. The boundary age of the fourth part and the third part is 557.2 million years. The third part of the Doushantuo Formation lasts for 28 million years. Successions of thin- laminated limestones parts in the third part last for 14.1 millions years. The boundary age of the second part and the third part is 585.2 million years. The negativeδ13C anomalies which spans the the third part, the fourth part of the Doushantuo Formation and the bottom of the Dengying Formation lasts for 24.94 million years and it starts at 575.94 million years ago.Cyclostratigraphic study results provide a critical age support for precisely estimating significant changes on environments,biology and palaeoclimate in the third part and the fourth part of the Doushantuo Formation. This research also reveals that susceptibility can be used as a palaeoclimate proxy for Precambrian successions.