Major and trace elements in mid-Eocene lacustrine oil shales of the Fushun Basin, NE China: Concentration features and paleolimnological implications

Abstract

Mid-Eocene tectonic subsidence of the pull-apart Fushun Basin, NE China accommodated an up to 300-m-thick oil shale succession within the lacustrine Jijuntun Formation (Lutetian). Based on sedimentary records and bulk element geochemistry, accumulation of this oil shale succession is modeled to evaluate the principal controls on enrichment patterns of organic matter (OM) and trace metals in lacustrine organic facies. The Jijuntun Formation, consisting primarily of massive oil shale and interbedded siltstone and sandstone that grade rapidly upward into well-laminated oil shale, records an overall evolution from lacustrine–deltaic facies associations to profundal facies associations. Sedimentological characteristics of the oil shale succession display a gradual change of moderately overfilled to balanced-fill lake fill mode, consistent with the up-section drop in siliciclastic inputs reflected by detrital proxies like decreased Si/Al and K/Na ratios. Variations in the sedimentary fill states are accompanied by increasing hydrographic restriction of the Jijuntun paleo-lake, in agreement with low Mo/TOC (consistently <0.54 × 10−4), Mo depletions (average ~1 ppm), and slight U deficiencies (average <2 ppm). Slightly elevated (V + Cr)/Al values in the oil shale suggest reduced oxygen contents in the sediment column relative to oxic bottom waters. The up-section decrease in both FeT/Al and Mn/Al ratios further reveals a shift in bottom-water redox state toward less oxidizing conditions throughout deposition of this oil shale succession. In response to such a change in redox, reactive Fe–Mn shuttling across the chemocline could become intense and facilitate P recycling in water masses, resulting in the coupled Fe–Mn–P enrichments. Euphotic zones of this paleo-lake likely had moderate to high primary and export productivity, supported by the magnitude of enrichments of total organic carbon (TOC) and P nutrient as well as increasing TOC to P ratios. Aerobic respiration of sinking organic particles should occur at the sediment–water interface to reduce benthic O2 levels in the paleo-lake. However, greater aerobic remineralization of labile OM was required to consume relatively more dissolved oxygen during deposition of the lower relative to upper oil shale succession. Hence, different degrees of aerobic OM remineralization are responsible for the contrast in TOC contents between the lower (avg. = 4.7 wt%) and upper (avg. = 12.0 wt%) Jijuntun Formation.