Characteristics And Palaeoenvironmental Significance Of Lacustrine Carbonate Rocks From The Early Cretaceous Yixian Formation,Western Liaoning,China

Since the 1990s,a mass of well-preserved biota fossils has been discovered in Western Liaoning,providing an ideal site for paleontology research.However,the study on sedimentology and paleoenvironment in the study area is relatively weak.With the deepening of the study,the work on sedimentology and paleoenvironment becomes more and more urgent.Lacustrine carbonate is a kind of unique sediments,which cannot be studied using the principle of terrigenous clastic sediments,nor just mechanically applying the research ideas from marine carbonate.As the most widely distributed continental carbonate,lacustrine carbonate is very sensitive to paleoclimate,paleo-hydrodynamics,paleo-aqueous chemical conditions,terrestrial materials and so on.However,it is at the low level of research.The research on the lacustrine carbonate still rests on sedimentary characteristics of individual basins in different geological periods.The Yixian basin is one of the Mesozoic basins where the Yixian Formation develops,with large scale of lacustrine carbonate rocks distributed in numerous beds.This paper identifies the position and spatial distribution of 40 layers of lacustrine carbonate in eight sections of the Yixian Formation in the basin.According to the type of particulate matters,matrix types,sedimentary structures and biological fossils,eight standard kinds of microfacies were identified in the Yixian Formation carbonate.According to types of particles from the carbonate rocks observed under the microscope,five kinds of biological structures have been identified,including micro-oncoids,microbial-caddisfly bioherms and a large number of single larval cases,biospheres and boreholes,and Petroxestes borings,most of which is the first time to be discovered in the Yixian Formation.The morphological and structural characteristics of these five biological structures are described in this paper.The genetic types,formation mechanisms,growth patterns and possible builders are also analyzed.This research also discusses the indication of these biological structures on the paleoclimate,paleo-hydrodynamic and other sedimentary environments.There are various sedimentary structures in the carbonate rocks of the Yixian Formation in Western Liaoning,including mud crystal structure,laminated structure,halite pseudocrystal,nodular chert,collapse structures,marlstone breccia with laminated structures and so on.This paper analyzes the paleo-sedimentary environment revealed by each sedimentary structure.In particular,the collapse structures,which were produced in the carbonate layer at the top of the northern Yingwoshan section,is described and discussed its indication of the paleo-sedimentary environment.According to the biological structure,sedimentary structure and field characteristics of the carbonate rocks in the Yixian Formation,the carbonate formation environment of the Yixian Basin in Western Liaoning is diverse,and we divide them into six groups.1.The hydrodynamic conditions are weak,the sedimentary rate is slow,and the water bodies are stratified in an anoxic environment.In such a situation,carbonate rocks mainly belonging to microfacies of LMF1 and LMF2 were formed.2.Carbonate rocks formed in an extremely arid climate,with a large number of structures of halite pseudocrystal and nodular chert.3.Clastic carbonate rocks formed in a strong hydrodynamic condition and rapid sedimentary environment,which are characterized by the presence of large amounts of rock fragments,feldspar and quartz grains.4.Particles in carbonate rock mainly consist of micro-oncoids,which reflect the turbulent,oxygen-rich water and relatively slow deposition rate.And the environment was suitable for large-scale reproduction of microorganisms.5.Ostracod limestone.Carbonate rocks contain a large number of ostracod shells indicate a suitable environment occurred in a short time.6.Algal limestone deposition in SFT-1,indicate medium to hydrodynamic conditions,oxygen enrichment,rich nutrition,low content of water in clastic,slow sedimentary rate conditions.Based on the sedimentary structures and biological structures of the Yixian Formation carbonate rock with wide distribution and low reproducibility in a single section,the stratification relationship of each section in the basin is discussed.In a word,the position of the Yenangou section is relatively lower than other sections.The top part of the Yenangou section(YNG-1)and the lower part of the North Yingwoshan(YWS-B-1)were formed in the same period.The middle part of the North Yingwoshan section(YWS-B-3,YWS-B-5-5),the bottom of the South Yingwoshan section(YWS-N-1),the middle part of the Wangjiagou section(WJG-0,WJG-1 and WJG-2),the upper part of the Sifangtai section(SFT-2,SFT-3)and the lower part of the Panjiagou section(PJG-1)were formed at the same time.There is a corresponding relationship between the middle part of the Zaocishan section(ZCS-1,ZCS-3)and the middle part of the North Yingwoshan section(YWS-B-4-4,YWS-B-5-3).The corresponding relationship between the Laogonggou section formed at the same time with LMF1 and LMF2 carbonate rocks in the other sections.According to the sedimentary characteristics of carbonate and clastic sedimentary rocks in the Yixian Formation of Yixian Basin,the evolution of Yixian Lake has experienced 5 main stage:1.the early stage when the basin was occurred and developed;2.the middle stage characteristed by the flourish of flora and fauna;3.the nest stage during which hot and dry paleoenvironment caused the lake to atropharid;4.the late stage containing a brief revival of the lake and the end;5.the last stage with A brief period of biological prosperity.Along with the evolution of the lake basin,the sedimentary center of the lakes moves eastward and terminates near the Zaocishan section.