Late Cretaceous Lake Environmental Change Revealed By Biomarkers From Sk-1, Songliao Basin

In the climate period corresponding with the Qingshankou Formation and theNenjiang formation, Songliao Basin, various characteristics of the stratum have verysimilar performance with the oceanic anoxic events reflected in the strata duringCenomanian~Turonian period. Therefore, it has become a very meaningful topic, torestore the ecological structure and the trend of water environment change of theancient basin, to pointed out that the interaction between the anoxia of lake waterand productivity of lake during late Cretaceous in Songliao Basin, and the influencemechanism how this two affect the deposition and preservation of organic matter,and to inquire into the relationship between the lake water environment and the oilgas generation.Via the analysis of elemental composition, δ13Corg, biomarkers with indicatingsignificance, and the correlation of each index of the organic matter in the141samples from the upper Quantou Formation to the Mingshui Formation, SK-1, thetrend of the water environment changes and the correlation among them during thetime period of91.88Ma~65.49Ma has been obtained as below:(1) During the period when the upper Quantou Formation to the MingshuiFormation deposit, the lake water environment has experienced several largechanges such as the starts and ends of the marine transgression, the rises and falls insalinity, the stratification and mixing of the water column, and the hypoxia andrecovery in the bottom water. All these changes occurred in the period when the firstmember of Qingshankou Formation and the first member of Nenjiang Formationand their adjacent strata deposited, and the most severe changes occurred in theperiod the first member of Nenjiang Formation and its adjacent strata deposited.There also were several changes during the period when the second and thirdmember of Qingshankou Formation and the second member of Nenjiang Formationdeposited, but in general, very weak.(2) In the period when the first member of Qingshankou Formation and the first member of Nenjiang Formation deposited, the various changes of the lake waterenvironment showed obvious stages.(3) As one of the important factors affecting water salinity, the marinetransgressions plays a crucial role in the rises of water salinity in the Songliao basin;similarly, as an important cause of the bottom water hypoxia, the water columnstratification could be very favorable for the preservation of depositional organicmatter. In addition, although the water salinity increases caused by the marinetransgressions did not occupy the major position in the causes of water columnstratification, but also to a certain extent, promoted the formation of stratification,and then provided a favorable condition for the deposition and preservation oforganic matter.