PALEOLIMNOLOGICAL ENVIRONMENTS AND ORGANIC ACCUMULATION OF THE NENJIANG FORMATION IN THE SOUTHEASTERN SONGLIAO BASIN, CHINA

Abstract

Thick layers of dark lacustrine mudstone in the Nenjiang Forma­tion record the evolution of local depositional environments in the Songliao Basin. This evolution in lake water is accurately reflected in variations in trace element compositions in sedimentary rocks. In this study, element geo­chemistry and clay mineralogy in successive cores were investigated to have a closer insight into the paleolimnological environment and organic accumula­tion during the Nenjiang epoch. Analysis of the contents of Mn, Ca and Ti, as well as Rb/Sr and Sr/Cu revealed that paleoclimate cycled between warm and humid to semi-arid and hot. The study of Sr/Ba ratios, clay minerals and stable isotopes indicated that both high and low salinity existed in two stages, and that high salinity in Member 1 of the Nenjiang Formation is likely correlated with transgressive events. Analysis of the ratios of V/V + Ni, Ni/V and Th/U suggested that the paleolimnological environment was reduc­ing. The investigation of paleotemperature demonstrated that the Nenjiang Formation was deposited in a warm water environment, analysis of carbon and oxygen isotopes revealed its deposition in open paleolake. High paleoproductivity and salinity as well as redox potentials represent the most favorable environment for oil shale enrichment.