Recognition Of Milankovitch Cycles In The Eocene Terrestrial Formation And Environmental Responses In Dongying Sag

The Eocene is an enigmatic and important period in the geological history because it is a transitional period from the greenhouse climate(bipolar ice-free)to a current icehouse climate(bipolar ice-sheet).The Es3 l and Es4 u formations of the Eocene are the main source rocks in eastern China,Bohai Bay Basin,which are characterized by strong continuity and thick deposition.The widely developed and high-quality source rocks record the climate and environmental changes in lacustrine basin which are good carriers to recognize the Milankovitch cycles and to explore climate-driven mechanism.Taking the systematic coring intervals of Well Fanye-1 in the Jiyang Depression as an object of the study,this paper obtained the accurate depositional ages using the magnetostratigrphic method.The effects of the astronomical forcing on lacustrine fine-grained sedimentary rocks are explored combined with core observations,thin section identifications,geochemical tests,magnetic susceptibility testing and cyclostratigraphy analysis.Finally,the research achievements provide new ideas and tools for the exploration of the continental shale oil in China.The related achievements are as follows:1.The recognition of Milankovitch cycles from the Es3 l and Es4 u formations in the Dongying Sag.Taking the magnetic susceptibility,Fe/Mn,Al/Ti,and Sr/Ca as the climate proxies,we eliminated the non-orbital signals though date pre-proccessing.In addition,the evolutive harmonic analysis(EHA),multi-taper method(MTM),and average spectral misfit(ASM)techniques are applied in depth domain.Finally,the Well Fanye-1 can be divided into four sections according to the change of sedimentary accumulation rates and the optimal sedimentary accumulation rates are 6.997-13.14 cm/kyr.The filtering results of magenetic susceptibility,Fe/Mn,Al/Ti,and Sr/Ca show that the Milankovitch signals are well preserved in Well Fanye-1.In addition,the eccentricity and precession signals were more dominant in Es3 l while the obliquity signals were more dominant in Es4 cu.2.The high-resolution astronomical time scale in the Dongying sag has been established and the high-frequency sequence divisions of the fine-grained sedimentary rocks have been carried out.The oriented core samples of Well Fanye-1 were demagnetized by 13-step alternating field demagnetization(AFD)method to obtain their stable remanent magnetization.Four normal polarity magnetozones(N1-N4),three reverse polarity magnetozones(R1-R3),and one chaotic polarity magnetozone(M)were obtained.The 100-kyr filter curves of magnetic susceptibilty were tuned to the La2010 d solution and adopted the C18n1n/C18n1 r as the accurate anchor point to establish the high-resolution ATS of the Dongying sag.Furthermore,the boundary ages of Es3 l and Es4 u have been recalibrated by the ATS.Finally,Taking the long eccentricity,short eccentricity and tilt curves as the references for mid-term,short-term and super short-term base level cycles,11 fourth-ordered cycles,44 fifth-ordered cycles,111 sixth-ordered cycles of Well Fanye-1 were identified.3.The climate-driven mechanism of the astronomical parameters are revealed combining with the paleo-climate evolution analyses.According to the geochemical characteristics,the evolutions of the Es3 l and Es4 cu have been divided into four dry-wet cycles,the durations of which are 0.829 Myr,0.954 Myr,1.25 Myr and 1.231 Myr,respectively.The amplitude modulation analysis of magnetic susceptibility,Fe/Mn and climate index “C” in time domain all showed significant ?1.2 Myr and ?0.84 Myr super long-term cycles.The ?1.2 Myr cycles were related to the amplitude modulation of the obliquity.The short eccentricity has been divided into warm-wet climate semi-circle and cold-dry climate semi-circle based on the analyses of the geological characteristics of eccentricity and precession.The lacustrine fine-grained sedimentary rocks depositional model controlled by astronomical parameters has been established.Finally,the warm-wet climate semi-cycle maybe more conducive for shale oil exploration combined with oil-bearing properties and organic geochemical data.