Application of elemental data in reconstruction of paleoredox conditions of the Qom Formation at Khani Abad section, southeast of Kashan

The abundances of minor-elements in carbonate rocks depend on the noncarbonate materials, including detrital minerals, accessory authigenic precipitates, noncarbonate skeletal material, organic matter, phases formed during diagenesis and the elements adsorbed upon all of these materials (Graf, 1960; Lian et al. 2008). Mo, V, Cu and Ni abundancey in carbonate rocks depend on CaCO3, organic carbon (Le-Riche, 1959; Snow et al., 2005; Lian et al., 2008). The carbonate materials rich in organic matter should be enriched in Ag, As, Mo, V, Ni, Pb, Cu, Ge, Br and I whereas carbonate rocks and sediments containing manganese oxides have high amounts of Co, Mo and Ba trace elements (Krauskopf, 1955; Madhavaraju et al., 2015).
Due to the economic importance of the Qom Formation as a hydrocarbon reservoir and presence of celectite and chalk deposits, this formation studied mostly on petrology, stratigraphy and paleoenvironment view (e.g. Seyrafian et al. 2007; Mohammadi et al. 2009; Reuter et al. 2009; Safari et al. 2014; Daneshian et al. 2017; Nasiri Ghareh Shiran 2017). However, minor geochemical studies were done on the Qom Formation deposits.
In this research, the Qom Formation was studied by geochemical data in Khani Abad stratigraphic section from southeast of Kashan. The aim was to investigate the trace and major elements to reconstruction of paleoredox conditions and find their relationship with sedimentary environment.
Material and To determine sedimentary characteristic of the Qom Formation, 70 thin sections have been studied. Thin sections were stained with potassium ferricyanide and alizarin-red S solution (Dickson, 1965). The petrographic classification for carbonates is based on the Dunham limestone classification (1962) and Embry and Klovan (1971). Wilson (1975) and Flügel (2010) facies belts and sedimentary models were also used.
After petrographic studies, twenty powdered micrite samples were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) by 4-Acid Digestion method for trace, major and rare earth element contents at the Met-Solve Analytical Services Inc., BC, Canada.
Discussion of Results & The Qom Formation has 70 m thickness in Khani Abad stratigraphic section and consists of thin to thick bedded and massive limestone, argillaceous limestone and marl. This formation stay among non-marine rocks of lower and upper Red Formations. The petrographic study shows that the main grains in Qom Formation are foraminifers, red algae, echinoderm, bryozoan, pelecypods, gastropods, corals and peloids that have precipitated in lagoon and shoal facies belts.
Geochemical data indicates that amounts of Al, Ti, Th and Zr are low in studied samples and might show the low terrigenous input during the precipitation of the Qom Formation carbonates. The amount As in selected samples are lower than 5 ppm. In two samples with argillaceous limestone and marl, the As content are increased in compare to others. There are a positive correlation between Sr contents with bivalve frequency and Na contents with foraminifera frequency as skeletal allochems in stratigraphic section.
Sr/Ca versus Man and Mg show that the carbonate deposits of the Qom Formation are situated in mostly semi-closed to open diagenetic system. Also, variations in amounts of Mn, V/Cr, V/(V) and Mo/Zr ratios in studied carbonate rocks are indicative of mainly dysoxic conditions of precipitation in Qom Formation carbonates. Correlation of V/Cr ratio and amounts of Mn, Na and Ba versus skeletal allochems and facies belts shows that the frequency of these elements are different in depth of lagoon toward the land and shoal and in mid parts of lagoon. In middle parts of the lagoon with low level of energy, amounts of Ba, Mn and V/Cr are higher than other parts which indicates more dysoxic condition in comparison to other parts. In lagoonal parts toward the shoal, the V/Cr, Mn and Ba amounts are lower comparing to other parts that indicates oxic to dysoxic conditions.