Sedimentary Environmental And Its Evolution Of Mid-yixian Formation In Early Yixian Formation,Western Liaoning

The volcanic sedimentary basin was well developed in the lower Cretaceous in western Liaoning Province,such as the Yixian Basin and the Jin-Yang Basin.The basin was filled with thick volcanic rocks and many sedimentary layers appear in the basin.Abundant "Jehol Biota"fossils have been found in the sedimentary layer of Yixian Formation.Dinosaurs,birds and Archaefructus have attracted much attention from the world.Many scholars have put into the study of "Jehol Biota" fossils,paleoecology,paleogeography and paleoclimate,and gain a lot of achievements.Based on previous data and comprehensive field sedimentology work,this paper makes a further study on the Zhuanchengzi layer and the Dakangpu layer of Mid-term of Yixian FormationAccording to detailed description of standard section of Zhuanchengzi layer and Dakangpu layer,the lithofacies types are classified.Then the sedimentary facies can be identified based on to the lithofacies combination.On the basis of establishing vertical sequence,the evolution of sedimentary environments are analyzed,and the main controlling factors of sedimentary process are discussed.A profound study on the varve and the volcaniclastic marl rock in the sedimentary layer,with geochemical assistance,the influence of provenance,paleoclimate and volcanic activity on sedimentary layer.Seventeen lithofacies types have been recognized in Zhuanchengzi layer.There are including matrix supported conglomerate facies(Gm),massive tuffaceous sandstone(Sm),planar-stratified sandstone facies(Sp)et al.The characteristics of the vertical sequence indicate that this period is fan-delta and lacustrine sediment include:debris flow facies,shore-shallow lake subfacies and deep and semi-deep lake facies.The evolution of the lake environmental during early stage of sedimentary controlled by tectonic activity obviously,and during middle to late stage is mainly controlled by paleoclimate.Eleven lithofacies types were recognized in Dakangpu layer include:matrix supported conglomerate facies(Gm)、planar cross-stratified tuffaceous medium-coarse grained sandstone facies(Spi)and planar cross-stratified medium-fine grained sandstone facies(Sp2)et al.The characteristics of the vertical sequence indicate that this period is lacustrine sediment include:shore shallow lake facies,deep and semi-deep lake facies and deep lake turbidity facies.The evolution of the lake environmental during sedimentary period is mainly controlled by paleoclimate change.Sedimentology shows that the varve is very high(about 60～70%)in Dakangpu layer,and periodic(seasonal)climatic changes of the deposition period is obvious.The changes of the varves thickness indicates the fluctuation of precipitation and the changes of temperature(high temperature season)during the deposition period.The precipitation of the Dakangpu layer gradually decreased and the deposition rate gradually decreased.The flood occurred frequently in the early and the climate was relatively warm.Geochemical analysis of the varves show that the chemical index of alteration(CIA)is between 65 and 67 with in a small range;w(Rb)/w(Sr)is low and increases gradually from bottom to top.Thus the climate in Dakangpu depositional period is warm and moist with the trend from dry-cold to warm-humid.The index of component variability(ICV)changes from 1.3 to 11.6,more than 1,representing the existence of initial depositional product and active tectonic setting during the depositional period.According to the standardized REE pattern distribution diagram,A-CN-K and Co/Th-La/Sc source rock attribute discrimination diagrams,the source of Dakangpu layer is dominated by partial acid volcanic rocks that was the primary volcanic rocks of Yixian Formation.The differential degree of REE indepositional period w(La)n/w(Yb)n is greater than 1,and the retention time of REE is longer in the lake with a low sedimentation rate;w(La)n/w(Yb)n decreases gradually from bottom to top,and the sedimentation rate and the lake salinity are both gradually increased.Comparing with modern lacustrine varve,Dakangpu layer lacustrine varve was formed in the middle-and-high latitude crater lake.The Dakangpu layer varve were controlled by sediments that precipitated in different ways at different stages(fluvial input,chemical precipitation,atmospheric dust,etc.).The lake water is stratified,with a closed hydrological condition.The vegetation around the lake is better,and the proportion of coniferous forests is larger,which is slightly colder than that of modern climate.The eastern plateau may exist in western Liaoning.The volcanic activity has great influence on the sedimentary layer,which is characterized by the control and transformation of sediments.The geochemical analysis of the volcaniclastic marl rock(volcanic events strata),laminated marl interlayers in Zhuanchengzi layer,and laminated marl in Dakangpu layer show that SiO2,K2O,Li,Sc,As,Sr and Ag are enriched in volcanic events strata.The chemical index of weathering(CIW)range from 82 to 91 with gradually increasing trend in samples,indicating strong weathering.There are many special deposits in the sedimentary layer,including the purplish red-reddish brown fine grained sandstone,it is a typical continental redbeds indicate an arid-hot climatic conditions of early Yixian Formation.The lamellar silicified dolomite in Zhuanchengzi layer indicate an hot climatic conditions.Lacustrine carbonate and calcareous deposits are founded in each sedimentary interlayer,indicating warm and humid climate conditions,existing arid and semi-arid climate conditions.The CIA,CIW,w(Rb)/w(Sr)and the oxygen isotope are analyzed in each sedimentary layer.There is a good consistence between element ratios and paleotemperature(Calculation by oxygen isotopes)in Laogonggou layer,Zhuanchengzi layer and Dakangpu layer.The CIA value of Jingangshan layer is relatively low,however paleotemperature is very high.Carbonate deposits in sedimentary layer are related to volcanic activities during sedimentary period.Base on the changes in the characteristics of the sedimentary layer during the volcanic inactive period,the evolution of Yixian Basin was divide into 5 stages:initial rifting stage,continuous rifting stage,the strongest stage of rift,the end of the rifting and the ended.The evolution of Yixian Basin is strongly influenced by tectonic movement and volcanic activity.