Chemostratigraphy of the Qom Formation by considering its influence on palaeotemprature determination in Dobaradar section, south east of Qom

Chemostratigraphy is a part of stratigraphy which study chemical changes of sedimentary units in a sequence. Nowadays, the changes of elements in carbonate sediments use as a criteria for the detection of their old sediments equivalents (Adabi, 2004). Marine carbonate rocks often contain low and high magnesium calcite and aragonite which their frequency are not constant in the environment, and their changes depend on environmental factors such as temperature, salinity, carbon dioxide, pressure and the ratio of magnesium to calcium (Rao, 1996).  Kasaei Najafi (1993) investigated the elements of Sr, Na, Mn, Fe, Ca from the Qom deposits in south to southeast of Qom city and determined a concentration of elements in carbonate rocks depend on the microfacies features.  Okhravi (1999) considered a concentration of rare elements and their relation to the facies by studying of  "f " Member in the same areas. The study area and the stratigraphic section are available through the Tehran-Qom and the cross road of Qom-Jamkaran. This area belong to Central Iran Zone which is formed by collision of Arabian-African plate to Iranian plate. Central Iran connected eastern and western Tethys (Schuster & Wielandt, 1999; Reuter et al., 2009; Mohammadi et al., 2011) and the Qom Formation was deposited during the final marine transgression in Central Iran Basin during the Oligo-Miocene (Daneshian & Ramezani Dana, 2007; Reuter et al., 2009; Seddighi et al., 2012; Mohammadi et al., 2011, 2013, 2015; Daneshian et al., 2017). Dozy (1944) was the first  geologist that  used the term of  Qom Formation for marl and limestone of shallow water with high variations in facies and was not permitted  to introduce a type section for this formation in southern Qom plain by having good outcrops and considerable thickness (Bozorgnia, 1966; Stocklin & Setudehnia, 1971; Hadavi et al., 2010).

The sedimentary sequence of the studied section consists of a, b, c-1, c-2, c-3, c-4, d, e, f and g members with 1328 meters thickness. 676 samples, systematically, collected with approximate intervals of 0.5 to 3 meters. Among them, 30 samples were selected based on lithological characteristics and stratigraphic boundaries of the members for the purpose of investigation of elements and their changes. The specimens were powdered up to 60 microns and these specimens were analyzed by the ICP AES method for understanding the variations of Ca, Mg, Fe, Na, Zn, Mn, Sr elements, and Mg / Ca ratio.  Then, the variations of the examined elements compared to the benthic and planktic foraminifera diversity. Among the diversity of benthic species, the type of test (agglutinated, hyaline and porcelaneous) were determined separately for recognition of the paleotemperature changes.

The chemical analysis of elements including Ca, Mg, Fe, Mn, Na, Zn, Sr, and changes in species diversity of benthic foraminifera indicate sharp variations of the elements in the lower part of the succession between  "a" to "e" members, while in the upper parts of the section, "f" and "g" members, the variations are not very sharp. Also, the highest rate of variation for elements is observed in the base of the studied sequence, in the boundary between "b" and "c-1" members especially for Manganese, and in the boundary between "d" and "e" members for Strontium. All elements other than Manganese significantly reduced on the boundary between "b" to "c-1" members, but between "d" and "e" members, all the elements show increasing trend, except Sodium, Manganese and Strontium which indicate deduction and the greatest decreasing  is belong to the Strontium. Besides, species diversity changes of the foraminifera, particularly benthic forms show a definite increase between the boundary of "d" and "e" members.In general, the high variation of the species diversity of benthic foraminifera suggests a relative increase in temperature of the paleoenvironment. Reducing the species diversity at the boundary of "d' Member depends on changes in the depth of sedimentary environment and existing the evaporate environment. In the "e" Member, the diversity of the benthic and planktonic foraminifera show an increasing trend that indicates a high temperature in the environment. In "g" Member, the diversity of foraminifera is low at the base, and is almost constant.  It seems the temperature with a slight increase has reached to a relative stability in this part. But due to the gradual decrease in the depth of the environment at the end of the stratigraphic section and the sea level fall, appear faunal changes to be affected by changes in environment depth.

Study of the elements and their variations along the examined section show the high variety at the bottom (Aquitanian), and a low variety at top with Burdigalian age. This state of variations is observed in foraminiferal diversity changes and corresponds to palaeoenvironmental conditions. Fluctuations of the elements, especially Magnesium and Mg/Ca ratio with foraminifer’s diversity changes are environmental indicators and show paleotemperature changes. Increasing concentration of Mg and the proportion of Mg / Ca ratio in the most samples coordinate with increment of foraminifer’s diversity. Both are indicators of increasing of paleotemperature in the Qom basin. These evidences show that the Qom sedimentary basin during Burdigalian were warmer than Aquitanian in Dobaradar section. In fact, the temperature fluctuations at the Aquitanian and Burdigalian are relatively high, and in the middle part of the Burdigalian, and maximum at the beginning of the Late Burdigalian, there is a relative stabile in temperature.