A Geochemistry study of the Sargelu Formation (Middle Jurassic) in terms of original mineralogy and palaeotemperature in the Lorestan area

The carbonate Sargelu Formation (Middle Jurassic) in the Lorestan area mainly consists of black calcareous shales and thin- to thick-bedded limestones with remarkable organic material. Integrated binary diagrams of trace elements and stable isotopes such as Na-Mn, Sr-Na, Sr/Na-Mn and petrography, SEM and CL suggest an original low Mg calcite  (LMC) composition of the carbonate Sargelu Formation. Deep-sea sedimentation of the Sargelu Formation and its outer ramp to basin depositional settings are consistent with the formation and preservation of the LMC carbonate sediments. Binary diagrams of 1000Sr/Ca-Mn and δ18O-Mn as well as the presence of the framboidal and cubic pyrites, organic material and pseudocement infilling the bivalves display a predominant anoxic to the sub-oxic condition of the sedimentary substrate of the Sargelu Basin and marine and burial diagenetic effects in a closed to the semi-closed diagenetic system. The high content of the Mn of the studied sediments supports the anoxic condition in the sedimentary substrate as this condition facilitates incorporating Mn within calcite composition. Greenhouse condition in the Middle Jurassic period, which led to the stratification of the seawater of the Sargelu Basin and as a function of low water circulation, poor ventilation and formation of the anoxic condition, caused this high-temperature degree.