Mid-Cretaceous Largest Lake Transgression And Its Biological And Environmental Responses In Songliao Basin

In the early Nenjiang stage, a largest lake transgression occurred in the Songliao basin, depositing a group of black shale with intercalation of oil shale in the major areas of basin. Based on the detailed sampling from the Yaojiachezhan section in the Dehui region and Houjingou section in the Nong'an region, a further study of micropaleontology and geochemistry has been carried. The Yaojiachezhan section has been divided as upper Member 3 of the Yaojia Formation and lower Member 1 of the Nenjiang Formation, and the Houjingou section been divided as upper Member 1 and lower Member 2 of the Nenjiang Formation. The boundary between Yaojia and Nenjiang formations has been located. The first appearance of Halysestheria yui (Chang) was considered as the datum of bottom of Nenjiang stage, but it has been found to occur 40 cm above the boundary by the present work.Based on the ecology of ostracodes, sedimentary and geochemical analysis, the changes of the lake level and paleohydrology have been recovered. The lake level rised guickly at the sedmentary period of lower Member 1 of the Nenjiang Formation and lowemost Member 2 of the Nenjiang Formation respectively. The latter was larger. Along with the lake level rising, the ostracodes underwent a progress of species disappearance, recovery and radiation. In the upper Yaojia Formation, TOC is lower, organic matter was mainly origined from terrestrial plant; in the lower Member 1 of the Nenjiang Formation, TOC is relative high, organic matter was mainly origined from alga, as well as a few plant. Upward, the TOC decrease dramatically, and the origin of plant increased. In the lowermost Member 2 of the Nenjiang Formation, TOC is very high, and organic matter was mainly originated from the mixture of alga and plant. According to the analysis of Sr/Ba ratio, in conjunction with study of palaeontology, it is suggested that although the Songliao basin was not affected by large marine transgression, there were at least two short periods of seawater incursions, resulted in the deposition of thick oil shale. Based on the present study, the author refers a model to explain the oil shale formation, i.e. salinity stratification cased by the seawater intrusion.Oxygen and carbon isotope study of Cretaceous ostracodes has been carried. Oxygen isotopes show a ~10℃decrease in temperature from the sedimentary period of lower Member 2 of the Qingshankou Formation to upper Member 1 of the Nenjiang Formation. Carbon isotoperecords imply a biologically productive lake in the sedimentary period of lower Member 2 of the Qingshankou and less productive in the period of upper Member 1 of the Nenjiang Formation.