Lacustrine Shale Stratigraphy And Eocene Climate Recorded In The Jiyang Depression In East China

As paleo-climate has significant influence on the depositional process of lacustrine mudstone and organic matter accumulation,paleo-climate reconstruction in lake basin area is of great importance to hydrocarbon exploration.Taking the lower third Member(Es3L)of the Eocene Shahejie Formation in the Zhanhua Depression,Bohai Bay Basin,eastern China as an example,detailed core descriptions,microscopic observations,and high-resolution mineralogical and geochemical analyses are made to identify two major lithofacies including massive calcareous mudstone and laminated calcareous mudstone in Luo 69 drilling core.The sedimentologic observations and changes of geochemical proxies,including Ln(Al2O3/Na2O),B/Ga,and V/(V+Ni),suggest that the massive calcareous mudstone was deposited in a small,shallow,salt lake that was dysoxic-anoxic,with cool and arid paleoclimate in the lake basin.While,laminated calcareous mudstone was deposited in a large and deep stratified lake with anoxic,high salinity bottom layer water and oxic,low salinity upper layer water,forming in a more humid and warm climate.The dominant lithofacies changes from laminated calcareous mudstone to massive calcareous mudstone in the studied core,indicate that paleoclimate became cooler and drier through time.Meanwhile,the lake became shallower and smaller.Such a climate trend may be a response to global cooling during the middle Eocene.Even though factors such as lake water chemistry and amount of terrigenous input,oxidation and reduction conditions,and lake level changes in Luo 69,average total organic content(TOC)in both lacustrine highstand and lowstand are comparable.It is inferred that the accumulation of organic matter within the lacustrine highstand was controlled by the combination of primary productivity,carbonate production,and preservation in anoxic bottom water,while only by primary productivity accumulation in lacustrine lowstand.Except for paleoclimate influence on shale deposition,tectonic activities control shale depositional environment and deposition process.Here we study the upper fourth Member(Es4U)of the Eocene Shahejie Formation,the major source rock of the Dongying Depression,eastern China,in order to understand stratigraphic heterogeneity and tectonic and climate controls on depositional processes in lacustrine mudstone.Integrated with core descriptions,microscopic observations,and studies of mineralogy,TOC and chemical weathering proxies of CIA* and Ln(Al2O3/Na2O),and connected wells profile in Niu 1 and Fan 1 well,four major lithofacies were identified in upper fourth Member,including interbedded evaporite and mudstone deposited in a shallow salt lake;silt-bearing clay-rich mudstone deposited in a pro delta within a shallow open lake;laminated calcareous mudstone deposited in a deep,stratified meromictic lake with frequent earthquake activities;and massive calcareous mudstone deposited in a shallow and oxygenated lake.Lithofacies changes of the Es4 U Member from the interbedded evaporite and mudstone-dominated lithofacies association to silt-bearing clay-rich mudstone-dominated lithofacies association,and finally to laminated calcareous mudstone-dominated lithofacies association from below,suggest that basin-wide varies in depositional processes,lake level,lake water chemistry,and paleohydrology,which reflect ratio changes between sediment and water supply and potential accommodation of the lake basin governed by paleoclimate in the lake drainage and extensional tectonics.We infer that during the deposition of the Es4 U Member,the lake basin evolved from an underfilled salty lake basin to shallow balanced-fill lake basin,then to deep balanced-fill basin.During the process,paleoclimate altered from acid to humid.Besides,basin changed from underfilled shallow salty lake to deep balanced-fill basin which is mainly controlled by subsidence caused by tectonic activity.Therefore,it is demonstrated that climate and tectonics play main roles in paleolimnological changes and depositional processes of lacustrine mudstone.On the basis of the above researches,the main features and evolution regularity of east Asia monsoon in the middle-late Eocene in eastern China was carried on thorough analysis of typical drilling core datas of isotope geochemistry in Zhanhua sag and Dongying sag.The results show that carbonates in the Jiyang Depression have not experienced significant diagenesis,and ?18O values still carry geochemical signature of original surface temperature and surface water during deposition.Compared with ?18O values and modern climate data,it's considered that Category1 with more negative ?18O values largely reflect variation of rainfall amount,further inferring East Asian monsoon changes as stronger East Asian monsoon brings in heavier summer rains resulting in minor ?18O value and vice versa.Based on the distribution pattern of Eocene China climate zone,it's thought that Jiyang Depression was located in the junction of temperate zone and arid zone(subtropical high)in the middle-late Eocene.Although subtropical high coverage is usually hot and dry,it would ease as the east Asian monsoon enhances.And due to the Eocene global climate gradually cools,subtropical highs shift southward.In the middle-late Eocene,the east Asian monsoon in the eastern part of China consisted of five evolutionary stages,and summarized as two phase of cycle from strong to weak,with one mainly in upper fourth Member(Es4U),and the other in the lower third Member(Es3L).