Pyrite Morphology And The Evolution Of Ancient Lake In Unit 1 Of Qingshankou Formation Of Late Cretaceous, Songliao Basin

Songliao Basin, located in northeastern China, is a large-scale Cretaceous sedimentary Basin in China, and one of the most big Cretaceous lake in Asia. The Qingshankou period in Cretaceous is the time when the first large-scale lacustrine transgressive events happened in the Songliao Basin. For a long time, transgressive events and the properties of the ancient lake during deposition of the Qingshankou Formation Unit 1(K2qn1) have been the research focuses in the Songliao Basin."SLCORE I"has taken continuous high-resolution sedimentary records. So it has provided ideal materials for the high-resolution paleoenvironment and paleoclimate reconstruction. In the organic matter-rich sedimentary pyrite are a characteristic mineral and an important indicator for recovering depositional environment. Numerous studies show that study of sedimentary pyrite morphology may provide useful information on depositional environments and early diagenic processes.In this study reflecting microscope and Scanning Electron Microscopic (SEM observations indicate that euhedral crystals and framboids are the dominant textural forms of pyrite in the K2qn1 mudstone samples from the south well of"SLCORE I". Pyrite framboidal size distribution indicates that redox condition in the bottom water had been changed from oxic-dysoxic to euxinic-anoxic to oxic-dysoxic during K2qn1 deposition. Moreover, organic-to-pyrite sulfur ratio indicates that paleosalinity change in the lake had a certain stage during K2qn1 deposition. Transgression may be the most direct reason caused for the water salinization.Mainly based on results of the analysis of the bottom water redox conditions and paleosalinity change, the evolution of lake water environment has been divided into five stages during K2qn1 deposition, i.e., in the first stage (I) the lake is a freshwater environment; In the second stage (II) transgression occurred and the water beginning salinization, but salinity stratification was weak, and the bottom water environment was in an oxic-dysoxic state. In the third stage (III) transgression continued, strong salinity stratification, and the bottom water environment was in a euxinic-anoxic state. In the fourth stage (IV) transgression ended, and the bottom water environment was in an oxygen state. In the fifth stage (V) the bottom water environment was in an oxygen-dysoxic state. In addition, Energy Dispersive X-ray Spectrometer (EDS) analysis results indicate S:Fe atomic ratio may be a indicator for the stage of pyrite formation. Using X-ray diffraction (XRD) analysis results, we found that there may be a positive correlation between framboid diameter and minicrystal size of pyrite. However, in this regard remains to be further studied.