GEOCHEMICAL AND PETROGRAPHIC ASSESSMENT OF THE EOCENE QIANJIANG SHALE AS AN EXAMPLE OF ORGANIC MATTER ENRICHMENT IN HYPERSALINE LAKES

The peculiar characteristics of intra-salt shales in the Qianjiang formation have been of interest for decades as representative examples of source rocks deposited in a hypersaline lacustrine setting. However, the precise combination of environmental factors that govern and favor shale deposition remains enigmatic. This study explores how organic geochemical (e.g., biomarker ratios such as gammacerane index, hopanes & steranes distributions), elemental (e.g., Ti, Al, Sr, Ba), and petrologic & petrographic data help elucidate how organic matter (OM) accumulation reflects depositional conditions and determine the stratigraphic occurrence of shales. It focuses on five distinct mudstone lithofacies characterized by powder-XRD that attest to variations in redox conditions and salinity: laminated strata that include (i) calcareous, (ii) dolomitic, (iii) glauberitic, and (iv) anhydrite-bearing intervals, and (v) massive shales. These different lithofacies are attributed to fluctuations in water levels and oxygenation levels within the lake. Lower water levels during episodes of decreased precipitation and run-off with intensified evaporation led to salinity stratification forming the laminated glauberite and anhydrite mudstone, whereas a dysoxic freshwater environment led to deposition of massive mudstones. Episodes of reducing conditions and deeper water stratification are characterized by laminated calcareous and dolomitic mudstones, also associated with increased terrigenous input, formed during intervals of high primary productivity with moderate OM preservation. By contrast, enhanced inputs of clastic material coupled with water column mixing during intermittent oxygenation formed laminated glauberitic and dolomitic mudstones. The overall aim of this research is to assess the nature of depositional conditions that favor OM preservation in the Qianjiang shale to better constrain the factors influencing shale deposition in hypersaline settings.