松辽盆地白垩纪缺氧地质事件的地质地球化学特征

松辽盆地白垩纪出现多期缺氧地质事件,它与全球白垩纪的地质事件相对应。青山口组一段及嫩江组一段出现较明显的缺氧地质事件,表现为有机质丰度高的源岩沉积。可溶有机质具有较低的姥植比,生物标志物出现了28,30 双降藿烷及保存了相对较为完整的C34或C35藿烷、重排甾烷的含量很低、有伽马蜡烷的存在等特征。松辽盆地在白垩纪时存在缺氧事件和海侵事件,与此同时,缺氧事件造成了这些地层中富含并具有高度分散性区域特征的生物化石的组合,形成了层序地层学上的密集段;缺氧事件又反作用于底栖生物,导致了松辽盆地古生物演化史上的生物绝灭事件。松辽盆地以持续沉降为主,湖盆水域迅速扩大,水体加深,近岸湖水表层生物生产率提高,底层水循环不畅导致了湖水底层水体缺氧,引发了古湖泊缺氧事件。在松辽盆地的缺氧事件中形成了青山口组一段及嫩江组一段的黑色泥岩(部分地区发育页岩),成为松辽盆地的主力油源层,为大庆油田的形成奠定了良好的物质基础。

GEOLOGICAL AND GEOCHEMICAL EVIDENCES OF ANOXIC EVENT IN THE SONGLIAO BASIN,CHINA

Corresponding to the Cretaceous global events, there are several times of anoxic events in the Songliao Basin. The strata of Member 1 of the Qingshankou Formation (Albian-Cenomanian)and Member 1 of the Nenjiang Formation(Santonian-Campanian)have typical anoxic characteristics in geological and geochemical characteristics. Geologically, anoxic events cause richness in fossils and well preservation of local fossil assemblage and diversified fossil assemblages. The finely laminated sediments of anoxic event reflect a condensed strata in the geophysical data. Low sedimentation rates exist due to restricted detritus input. The anoxic events also lead to the dead benthic animal and another geological event--large number of animal annihilation event. Geochemically, the anoxic events are characterized by the widespread dark mudstone or shales which have high organic contents,lower Pr/Ph, appearance of 28,30 bisnorhopane and preservation of complete C35 hopanes series, lower diasteranes content in mudstone. Especially, 28,30 bisnorhopen originate from a typical anoxic bacteria and are identified only in the Songliao Basin. In addition, geological factors such as the rising water level, a warm climate,would promote water stratification and large scale palaeogeographic features and restrict free water mixing. The anoxic events may play an important role in the formation of main source rock in the Songliao Basin and settle an organic matter basis for large petroleum province--the Daqing oil field formation.