FRAMBOIDAL AND DISSEMINATED PYRITE IN THE UPPER CRETACEOUS SEDIMENTARY ROCKS AT SONGLIAO BASIN: EVIDENCE FOR THE REDOX FLUCTUATION FROM THE UNIT 1 OF NENJIANG FORMATION

Marine incursion events in the Songliao Basin have been discussed in the previous study, yet lack primary mineral evidence, which are vital to the reconstructing and confirming the anoxic event. Sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in sedimentary environment over geologic time. The mechanisms of pyrite formation in the fluctuation redox environments indicated that framboid pyrite size distribution in sedimentary rock can reconstruct the ancient environment of the oxidation reduction and it is effective to the state.

The present study shows that framboids and disseminated pyrite (inter layer with pseudomorphic framboids) are the dominant pyrite forms in the mudstone samples of the unit 1 of Nenjiang Formation (K₂n¹) in Songliao Basin, NE China. The framboids pyrite size distribution indicates that the deposition of K₂n¹, redox conditions in the bottom water changed from oxic-dysoxic to fully anoxic to oxic-dysoxic. During the oxic-dysoxic environment the water were fluctuation and the oxygen were sufficient than the fully anoxic condition. In the depth from 1025.63 to 1098.62m the hydrogen sulfide is excess. May be the whole basin was covered with sea water. With disseminated and pseudomorph framboid pyrite occurred in the rock layer, it is revealed that the deposition rate is slower that other depth of K₂n¹ and has strong reduction and weakly oxidizing conditions. May be in this sedimentary period, with the enhanced activity of the bacteria the environment is relatively warm and important to the formation of hydrocarbon source rocks in the Songliao Basin. The presence of fully anoxic bottom water reveal that anoxic event developed in the Songliao Basin ancient lake, and continued for about 528Ka. The anoxic bottom environment and the whole group richer in sulfate were the most important factor in hydrocarbon source rock formation in Songliao Basin. And it is necessary to pay attention to the disseminated and pseudomorphic ptrite study.