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

Abstract 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. Keywords: Zagros, Sargelu Formation, Jurassic, Geochemistry, Palaeotemperature Introduction The palaeoecology condition of the sedimentary basin and characterizations of the water column such as temperature, salinity, water circulation, redox condition could be identified by some variable sedimentary and geochemical parameters as type, abundance and diversity of the components, texture, and the content of the major and trace elements and stable isotopes (e.g., 18O, 13C) (e.g., Adabi & Rao 1996; Adabi et al. 2010; LaGeange et al. 2019; Grabowski et al. 2019; Briard et al. 2020). Such studies are important to the reconstruction of the environmental condition of the sedimentary basin, especially about the carbonate sediments as they are sensitive to salinity, temperature, pCO2, bathymetry, etc. (e.g., Hay 2008; Kennedy et al. 2014; Rudko et al. 2014; Kalanat & Vaziri-Moghaddam 2019). In the studied area, the Sargelu Formation (Middle Jurassic) includes bituminous, black, thin-bedded limestone, dolomitic limestone, and black fissile shale with high organic materials, known as source rock in the Zagros Basin. An integrated approach including petrography, cathodoluminescence, SEM, trace elements and stable isotopes was used to evaluate the primary mineralogy and palaeotemperature of the marine water of the Sargelu Formation during the Middle Jurassic in the Lurestan area. Material & Methods Seven sections of the Sargelu Formation including, Bizel, Kezi, Dowdan, Homajgah, Hawandeh, Mahpareh and Tenge-mastan in the Lorestan area of the Zagros fold-thrust belt were studied in this research. One hundred and eighty thin sections were examined to identify the composition, components, and the texture of the studied succession. Thirty-five samples were analyzed by SEM in the Razi Metallurgical Research Center (Tehran) to recognize the organic material, framboidal and cubic pyrites, as well as the degree of preservation of the fossils. Twelve samples were studied by CL in the RIPI to evaluate diagenetic processes in the selected samples. Thirty-one samples were analyzed by ICP-MS in the Actalabs (Canada) for the trace elements and stable isotopes. Finally, fifty-one samples were analyzed by XRF in the Zarazma Company to assess the content of the major and trace elements in the selected samples. Discussion of Results & Conclusions Petrography, CL and SEM analyses of the studied samples of the Sargelu Formation display that the selected samples have good quality for the geochemical analysis. The samples do not show heavy diagenetic imprints such as calcite cementation, as proved by the non-luminescence character of these samples. The SEM analysis of the studied samples displayed a nearly compact fabric for the micritic matrix of the studied samples. The bivalves, as the main components of the studied sediments, also show a well preservation mode. In this context, the major diagenetic process in the studied samples is framboidal and cubic pyrites, which were formed in the sedimentary substrate and burial stage, respectively. The binary cross-plots including Na-Sr, Na-Mn and Mn-Sr/Na reflect an original LMC mineralogy for the Sargelu sediments in the studied area, as the selected samples were located within Permian subpolar cold-water carbonate and recent temperate bulk carbonate sediment fields. This interpretation is consistent with the bathymetry of the Sargelu sediments, which were deposited in the outer ramp to basin settings of a distally steepened ramp carbonate platform. The binary cross-plots of 1000Sr/Ca-Mn and δ18O-Mn indicate a closed to semi-closed diagenetic system for the Sargelu sediments as supported by the presence of the framboidal and cubic pyrites. The palaeotemperature of the Sargelu Basin was calculated as 27° C by using Anderson & Arthur (1983) equation. The calculated temperature is consistent with the climate conditions of the Middle Jurassic, which is known as a greenhouse period. In this relation, an increase in the CO2 concertation in the atmosphere and within marine water resulted in dominant low water circulation and stratification of the marine and oceanic waters around the world. This special condition normally led to intensive deposition of the high organic sediments in the Sargelu environment in the Lurestan area of Zagros Basin.

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