The Lacustrine Carbonate And Their Sedimentary Environment Of Early Cretaceous Yixian Formation In Western Liaoning

The lacustrine carbonate distributed from Paleozoic to Cenozoic on a global scale, but it’s the most prosperous period of development of continental basin in Mesozoic and Cenozoic in China, a number of continental sedimentary basins formed on the continental crust, and also deposited a large number of lacustrine carbonates in the continental sedimentary basins. Early Cretaceous Yixian Basin in western Liaoning Province is one of the many continental basin that formed in Mesozoic, in this sedimentary basin, a large number of Jehol bita fossils buried in the volcanic-sedimentary strata, and well preserved. The Yixian Formation becomes world well-known by this fancy Jehol bita fossils. With further study the Jehol bita, people gradully began to focus on its environmental characteristics of these ancient lives, but as a result of deuteric tectonic destroy and weathering effect, the Jehol bita’s paleogeograpic and basin sedimentary characteristics are hard to make detailed field works and nethermore research. In the course of field work found that there were several lacustrine carbonate intervals in the Yixian Formation and relatively well preserved, and the lake sediments very sensitive to environmental changes, therefore, we could get some information about the sedimentary environment of the Jehol beta in western Liaoning from lacustrine carbonates.Based on the rocks characteristics, mineral composition, lithofacies, carbon and oxygen stable isotope compostion of the lacustrine carbonates in Yixian Formation, systematically studied the palaeogeography, palaeoclimate and palaeoenvironment of the lacustrine carbonates, and compared the sediment environment of Yixian Formation with the lacustrine carbonate of Qihai Lake.The lacustrine carbonate of Yixian Formation, as thin layer or lenticular occurred in the volcanic-sedimentary strata, and had a complex shape. The complex spatial distribution of lacustrine carbonates could reflect the characteristics of sedimentary environment. We find that the thin layer carbonate developed in shallow lake, the lenticular carbonates developed in short-lived isolated pounds on the floodplain and lake margin.The lacustrine carbonate of Yixian Formation is divided into nine kinds of carbonate rock types according to the rock samples and microscopic identification, include paleocaliche, mudstone, wackestone, packstone, grainstone, rudstone, floatstone, cherty packstone and grainstone, laminated dolostone and so on. Six main carbonate lithofacies have been differentiated in lacustrine carbonate based on the distribution of different rock types, include lenticular paleocaliche facies(facies A), packstone and grainstone facies(facies B), peloidal packstone and grainstone facies(facies C), micritic limestone facies(facies D), rudstone and floatstone facies(facies E), laminated dolostone facies(facies F) and so on. Facies A and some facies B developed in short-lived isolated shallow pounds and lake margin; facies C and some facies B deposited mainly in offshore lake; facies E and some facies F deposited mainly in shallow lake region; lamina flat facies F and facies D deposited mainly in shallow to semi-deep lake.Analysised the mineral composition by microscopic identification, X-ray diffraction and scanning electron microscopy, find that the mineral composition include calcite dolomite, small amount of clay mineral composition and siliceous components, combining the carbon and oxygen stable istope and the relationship between the carbonate and other sedimentary beds, we find that the lacustrine carbonate sedimentary environment in Yixian basin represents short-lived isolated ponds developed in very shallow depressions on the floodplain as well as brackish water in offshore lakes. The climate is relatively dry and hot, the average water temperature of carbonate deposition around20℃, the average water temperature of Zaocishan-Jingangshan Member is higher than other, around30℃, this may be related to volcanic activity. The lake hydrology tend to be closed, water supply is less than evaporation. Alkaline lake water, and contains more magnesium components. The clay mineral composition is the product of tuffaceous alteration. Volcanic activity may influence the hydrology and water temperature of the Yixian basin, and may be also the main source of silica composition.Comparing the sedimentary environment of the carbonate in Yixian Formation with the sedimentary environment of Qinghai lake, we find that the sedimentary environment of these two places has some similarities, could more accurately recovery the sedimentary environment of lacustrine carbonate in Yixian Formation.