Geochemical Characteristics Of Source Rocks And Plant Fossil Cuticles From The Middle Jurassic In Yaojie Basin,Gansu And Their Significance

The Jurassic was one of coal-forming periods in the history. The sedimentary environment and climatic change during the period are important study areas for geologists. The Jurassic formations of the northern China are rich in good source rocks, and Yaojie Basin, one of the typical areas, containing very abundant plant fossils, is a key site for organic geochemical and paleoenvironment research. biomarkers and carbon isotope, two traditional methods on studying organic matter, are widely used in paleoenvironmental reconstruction.According to the GC-MS analysis in Middle Jurassic carbonaceous mudstone, oil shale, mudstone and plant fossil cuticles from the Yaojie Basin, we analyzed the biomarkes of n-alkanes and arene. In addition, we discussed the paleoclimate in Middle Jurassic, based on the comparison of the fossil and modern leaves.The findings showed that the parameters of fossil cuticles and source rocks are identical, including the characterization of microorganisms, sedimentary environment and maturity, indicating that all samples in this study deposited in a weak oxidizing to reducing environment and developed from a shore-shallow lake or semi-deep lacustrine with fresh to brackish water. Additionally, the organic matter had been degraded by microorganisms, and reached to a phase ranging from the un-matured to low-matured. Moreover, the Pr/Ph and carbon isotope indicated that water was deepened from J2y2 to J2y5 of Yaojie Basin in the Middle Jurassic.We detected many biomarker characteristics that are related to terrigenous high plants, like the series of tetracyclic, lupane, oleanane, cadalene, retene, fluoranthene, pyrene and perylene etc, indicating that the fossil cuticles of gymnosperms are significant matter of source rocks. The biomarkers, the type of parameter of horpance, C27-C28-C29???20R sterane and carbon isotope indicated that the sources of organic matter are mainly from terrigenous high plants, as well as from abundant aquatic organisms, The type of organic matter is ?1-?, which had a lower potential in generating hydrocarbon.Based on the empirical formula, atmospheric carbon isotope composition (?13Ca) in the Middle Jurassic was calculated from carbon isotope composition (?13Cp) of Solenites cf. murrayana, and was compared to ?13Ca from carbon isotopic and zoo-plankton fossil records, suggesting that the empirical formula is not suitable for ?13Ca estimates. The results showed that the carbon isotopic discrimination (?13C) of fossil Solenties is mainly related to environmental factors which interacted with each other., The concentration of carbon dioxide was a crucial factor resulting in the variation of atmospheric carbon isotope composition (?13Ca) between the fossil and the modern leaves, Water played a second role,and the influence of light intensity and temperature on the variation was weak. According to the model of carbon isotope fractionation and the comparison of carbon isotopic discrimination (?13C) between fossil cuticles and the extant relative species, we concluded that the Jurassic had a warmer and more humid climate environment and higher concentrations of atmospheric carbon dioxide, but lower light intensities.