Geochemical comparison of sandstone and shale in the study of Paleoclimate Indexes: An example from Shurijeh Formation in NE Iran

The relationship between the composition of siliciclastic sediments, tectonic setting, and the source rocks has been studied by many researchers. The siliciclastic deposits are always affected by the composition of source rock, chemical weathering, transport distance, and diagenetic changes after deposition. It is possible to determine the paleoclimatic conditions of siliciclastic rocks, particularly sandstones, using different methods such as petrography and geochemistry. Interpretation of paleoclimatic conditions has a significant impact on the reconstruction of paleogeography (Garzanti & Resentini, 2016). Due to their fine-grain and low permeability, shale can preserve the composition of source rock minerals and, therefore, are very important in interpreting the source rocks (Khanehbad et al., 2012). In this study, the distribution of the major elements in the sandstones and shales of the Shurijeh Formation is compared with the standard values, and the paleoclimatic conditions have been interpreted.

One of the best techniques to major elements values in the siliciclastic sediments is the analysis of rock by the XRF method. Petrographic studies on seven samples of sandstones were done using a polarizing microscope and classified based on Folk's (1980) classification scheme. Also, six shale samples with the lowest calcium carbonate content are selected. Fresh shale and sandstone samples were chosen for geochemical analysis and then were powdered to the size of fewer than 63 microns using Agate mortar. The presence of calcium carbonate on cement can cause an error in Ca contents, so the samples were washed with 10% hydrochloric acid to remove the CaCO3. The residual powders (IR) were analyzed using Philips Panalytical Model X-ray Fluorescence (XRF) to determine the major oxides and trace elements of siliciclastic particles in the Taban Gostar laboratory in Tehran.

The Suttner & Dutta (1986) binary diagram determines the paleoclimatic conditions and paleoweathering at the source area. In this diagram, the plotted values of the Shurijeh Formation sandstone samples show the semi-humid climatic condition. In contrast, the plotted values of shale samples in this diagram show different results. They indicate the semi-arid climatic conditions, which are entirely different from the results of the sandstone samples. Various statements can be presented for this difference. First, shale samples usually have higher K2O and Al2O3 than SiO2 due to the presence of higher amounts of clay minerals. However, in the sandstones (due to the presence of quartz and chert minerals), silica is higher than the shale samples (Gateneh, 2000). Therefore, these differences in the values of SiO2, Al2O3, and K2O cause the differences in the shale and sandstone samples in the binary diagram (Suttner and Dutta, 1986). By comparing this chart with the "A-CN-K" ternary chart, better results can be obtained. This indicates that it is better to select the sandstone samples which are least affected by diagenetic processes based on detailed petrography studied.Calculation of the CIA index values in the studied sandstone and shale samples shows high weathering conditions in the source rock area within the semi-humid climatic conditions. The use of binary diagrams for sandstone samples confirms this subject, and the results are the same as the triangular diagram "A-CN-K" for the sandstones. However, in the case of shale samples containing clay minerals, the binary diagram shows completely different results, indicating cold and dry climatic conditions. One of the main reasons for this difference is the increase of the SiO2/Al2O3 ratio in shale samples. Therefore, binary diagram is not recommended for the shale samples, including clay minerals. Instead, it is better to use only the triangular diagram "A-CN-K", which shows the wet conditions and semi-humid climate. Small changes in the amount of Al2O3 due to diagenesis, on the one hand, and large changes in the amounts of Na2O, CaO, K2O, and SiO2 in the diagenetic condition, on the other hand, can even exacerbate the error of using the triangular diagram A-CN-K. Because the values of these elements, based on the geochemical analysis data, are less than the original values; therefore, it causes analytical errors in the interpretation of the data. For this reason, it is recommended to select the least affected sandstone samples by diagenetic processes after a detailed petrographic study.