Processes influencing organic matter accumulation in hypersaline lake sediments: Geochemical and petrologic assessment of Eocene Qianjiang shales, Jianghan Basin, China

Intra-salt shales from the middle Eocene Qianjiang Formation, Jianghan Basin, China, epitomize shales formed in hypersaline lacustrine settings, although understanding how environmental factors govern their deposition remains enigmatic. Combining geochemical and petrologic evidence helps constrain how the accumulation of organic matter (OM) in this lacustrine system reflects sequential changes in hydroclimate. The initial stage of the depositional succession forms sediments containing enhanced contributions of OM from halophilic archaea that reflect high evaporation rates favoring formation of OM-lean mudstones under oxic conditions. They transition into strata accumulated during episodes of moderate salinity where water-column stratification coupled with anoxic bottom waters, or an anoxic hypolimnion, form OM-rich mudstones dominated by algal biomarkers. These intervals, in turn, alternate with strata recording intermittent mixing of the water column that promoted bottom-water oxygenation, a scenario that enhanced recycling of oxidized OM when coupled with greater inputs of clastic material and higher nutrient influx. This sequence reflects alternating wet/dry climate cycles that govern precipitation, affect salinity and lake levels, and control biological sources of OM and its preservation. However, continued repetition of this succession was interrupted by clastic dilution events that decreased sedimentary OM concentrations and reset the stratification cycle. This evidence of depositional scenarios derived from combining geochemical profiles and petrologic data demonstrate the utility of this approach in establishing new paradigms for interpretation of environmental changes during deposition of multiple series of rhythmites in a hypersaline lake setting.