The Formation Mechanisms Of Lacustrine Fine-grained Sedimentary Rocks In The Eocene Lower Es₃ Strata,the Jiyang Depression

The deposition of fine-grained sediments is poorly understood.Previous work on it was mainly conducted in marine environment,with little focus on lakes.Studies have shown that astrocycle is able to influence the deposition in lakes,but its effects on deepwater organic-rich fine-grained sedimentary rock(FGSR)are still unclear.This paper studies the depositional mechanisms of FGSR and its evolution constrained by astrocycle frame using data from the lacustrine organic-rich FGSR in the Eocene lower Es3 strata,Jiyang Depression of Bohai Bay Basin.Themain achievements of this paper are as follows.1.The petrology characteristics of the FGSR are made clear.The sum of calcite,clay and terrigenous debris accounts for more than 90% of the rocks.Calcite is mostly micrite that formed in lake water,whereas clay is mainly sourced from land.The total organic matter content(TOC)ranges from lower than 1% to ca.10%,with planktonic algae and bacteria acting as the main source.The sedimentary structures include horizontal laminae formed in still water,micro-wavy laminae and upward-fining laminae formed in weak turbulent water,massive structure and silty layer formed in relatively strong water current,lenticular calcite aggregates formed by re-suspension,and hummocky cross-stratification formed by windstorm.2.Totally 6 depositional mechanisms of the FGSR are distinguished.They are: 1)muddy water deposits,2)semi-muddy/semi-clear water deposits,3)clear water deposits,4)tubiditic deposits,5)microbial carbonates,6)re-depositional deposits.The muddy water depositswere formed in strong terrigenous input setting;terrigenous minerals and nutrition massively carried by rivers float to lake center with the nutrition promotes microbe blooms,thus therrigenous minerals and organic matter deposit together.The clear water deposits were formed in weak terrigenous input setting;the terrigenous minerals and nutrition are insufficient at this time,and so large amouts of calcite deposit in such environment.The semi-muddy water deposis were deposited in an intermediate condition between muddy and clear water deposits.Turbiditic deposits were deposited in the final stage of turbidity current,when the strength of the current became weak.Microbial carbonates were formed by binding or catchingof benthic microbes.The re-depositional depositsrefer to the re-suspension,transportation and deposition of previously deposited fine-grained sediments triggered by windstorm events.3.Three levels of astrocycle are proved to control FGSR deposition;they are sunspot with a cycle period tens to hundrends of years,precession with a cycle period of 23 ky,eccentricity with a cycle period of 405 ky.In a sunspot cycle,the cyclical changes of solar intensity influence the precipitation,then influence the strength of terrigenous input,and finally influences mineralogic composition of laminaes.In a precession cycle,the season of perihelion experiences a cyclical change from summar to winter and then back to summar;a humid climate and frequent flood occurs when the perihelion is in summer,whereas everything is opposite when the perihelion is in winter.Hence,the FGSR experienced cyclical changes from semi-muddy water deposis to clear water deposits and then back to semi-muddy water deposits,or muddy water deposits to semi-muddy water deposits and then back to muddy water deposits.The precession also influences the properties of wind,making the re-depositional deposits mostly sourced from muddy water deposits.The value of eccentricity controls the earth orbit around the sun,thereby controlled the climate,and finally controlled the deposition.It is mostly clear water deposits,semi-muddy water deposis,and microbial carbonates if the value of eccentricity is low,and mostly muddy water deposis,semi-muddy water deposis,and tubiditic deposits if the value of eccentricity is high.