Characterization of depositional conditions for lacustrine oil shales in the Eocene Jijuntun Formation, Fushun Basin, NE China

Depositional conditions in the paleo-Jijuntun Lakes were synoptically reconstructed based on sedimentary features, trace element and organic geochemical proxies. Oxic to suboxic, fresh-water conditions were prevailing in the paleo-Jijuntun Lakes during deposition of the LOSU, and then evolved into oxygen-deficient, fresh- to mildly brackish-water conditions during deposition of the ROSU, as revealed by the concentrations of redox- and salinity-sensitive trace elements (e.g. V/Ni, V/Cr, Ni/Co and Sr/Ba ratios, and boron content) in the oil shales. Warm to cool and humid to arid climate (represented by e.g. Sr/Cu and Rb/Sr ratios) could result in the deepening of water column (from ca. 20-30 m to ca. 100 m) and saline stratification superposed on pre-existing thermal stratification in the paleo-Jijuntun Lakes. A systematical decrease (approximately one order of magnitude) in the concentrations of most trace elements (except for boron and Co) in the coaly oil shale in the lowermost LOSU, relative to those in the other oil shales in the Jijuntun Formation indicates the significant change of organic matter (OM) input from higher plant (especially coaly materials) to aquatic plankton. This is partly consistent with the variations of OM input from terrestrial higher plant and aquatic organisms (type II kerogen in the LOSU) to lacustrine algae (type I kerogen in the ROSU) as revealed by biomarkers. The slight to significant enrichment or depletion of few trace elements (i.e. Mn and Mo) in these immature to low-mature oil shale deposits may be associated with low to moderate productivities of aquatic plankton and/or incompletely oxygen-free conditions in the paleo-Jijuntun Lakes.