PALEO-LAKE CHANGE DURING THE DEPOSITION OF NENJING FORMATION IN SONGLIAO BASIN: CONSTRAINS FROM THE REACTIVE IRON AND PYRITE SULFUR ISOTOPE EVIDENCES

Neijiang Formation deposited during the period for most obvious lake transgression in the Songliao Basin. As one of the main source rock of Daqing oilfield, the unit 1 and 2 of Nenjiang Formation were composed of deep lake facies of black shales and mudstones, intercalated with calcacious sediments. The possible marine incursion events during the deposition of unit 1 and 2 of Nenjiang Formation had been one of the focus for Songliao Basin study. The conduction of the SK-1 scientific drilling program has recovered the continuous upper Cretaceous strata including unit 1 and 2 of Nenjiang Formation which provide the best chance for this problem. This work conducted the reactive iron analysis on the mudstone of unit 1 of Nenjiang Formation which we got the content of reactive iron species including carbonate-associated iron (Fecarb), hematite-associated iron (Feox), magnetite-associated iron (Femag) and pyrite-associated iron (Fepy). The results shows that ratio between FeHR (the sum of the four reactive iron species) and the FeT (total iron) is higher than 0.38 but Fepy/FeHR is less than 0.80 for most samples. It suggests that ferrous bottom water predominated during the deposition of unit 1 of Nenjiang Formation. But at several interval the FeHR/FeT is higher than 0.38 and Fepy/FeHR is higher than 0.80 which suggests the bottom water condition may also have been sulfidic for some time. According to the pyrite sulfur isotope data, the δ³⁴S of unit 1 of Nenjiang Formation is relatively positive when compared to that of marine black shale with an average of 10.96‰ ranging from -37.69‰ to 37.46‰. The most negative pyrite sulfur isotope values mainly occur at lower part of the strata which shows dramatic fluctuation with very short interval and coincide with the proved occurrences of marine planktonic foraminifera. It is concluded that such dramatic fluctuation of pyrite sulfur isotope may be the consequences of the intermittent marine water incursion into the Songliao Basin.