MARINE INCURSION EVENTS DURING THE DEPOSITION OF UNIT 1 OF QINGSHANKOU FORMATION IN CRETACEOUS SONGLIAO BASIN OF CHINA: EVIDENCES FROM PYRITE MORPHOLOGY AND PYRITE SULFUR ISOTOPE

The deposition of Unit 1 of Qingshankou Formation, the main source rocks of Daqing oilfield, consisting of black mudstone and blackshales, was correspondent to the first transgression of Cretaceous Songliao lake. The possible marine incursion events during this time had been focus of studies for Chinese geologists for tens of years. The implementation of SK-1 drilling program has recovered the whole upper Cretaceous strata, including Unit 1 of Qingshankou Formation, of Songliao basin therefore provided a unique chance to do so. Reflected light microscope and scanning electronic microscope had been used to study the pyrite morphology in Unit 1 of Qingshankou Formation. According to the diametric distribution of pyrite framboids in Unit 1 of Qingshankou Formation, it was discovered that the bottom water had been in oxic-dysoxic condition during the deposition of sediments from 1782 to 1770m, but changed to anoxic-euxinic status from 1770 to 1752m. The redox condition of bottom water again turned to oxic-dysoxic from 1752m above to 1702m within the Unit 1 of Qingshankou Formation. The pyrite sulfur isotope of Unit 1 of Qingshankou Formation varied from 14.4 `24.06‰ with an average of 18.48‰ which was quite close with that of sulfate in the coeval marine water (~19‰). It was concluded that the incursion of marine water may have provided the sulfate to be reduced forming the pyrite. However the marine incursion events were not strong and the sulfate concentration in the lake water is not high which was almost totally reduced.