Geochemical investigations of the Sangestan Formation, west of Yazd: using multivariate analysis to identify sediment source, sedimentary environment, and palaeoclimate conditions

Abstract The Early Cretaceous succession in the southern part of Yazd Block begins with terrestrial sediments of the Sangestan Formation. Little attention has been paid to this formation and most studies have focused on its lithostratigraphy. In this study, some geochemical studies have been performed on this formation. Twenty seven siliciclastic samples were taken from the Sangestan Formation in two sections of Bidakhvid and Alavi Mountain in order to identify the type of siliciclastic sediments, the nature of the sediments, and palaeoclimate conditions. Mineralogical studies and modal analysis showed that the siliciclastic sediments have been classified as sandstone (arkose) and siltstone. The results of factor and cluster analysis depicted that the chemical composition of siliciclastic sediments is different in the study areas. Based on multivariate analysis, siltstones that have high positive values of Zr, V, U, Sc, Sr, REE, MgO, Fe2O3, TiO2, and CaO elements are related to cement, while sandstones showed high positive values for K2O, Na2O, SiO2, Al2O3 elements. The results showed that siltstone sediments have more calcite and iron cement than the sandstone samples. By studying the variations of these elements versus the profile of the Sangestan Formation, it was observed that the elements related to rock cement are concentrated in siltstones due to the increasing depth of the sedimentary basin or more extension related to rift event, and the entrance of new elements into the basin. Also, the results of weathering indices depicted that both rocks have been formed in the same climate conditions which is consistent with the palaeogeography of Yazd Block during the Early Cretaceous.Keywords: Sangestan Formation, Yazd Block, Geochemistry, multivariate analysis, sedimentary environment, weathering index  IntroductionThe Sangestan and Taft formations dating back to the Early Cretaceous were deposited in many locations of Yazd Block which is located in the western part of the Central–East Iranian Microcontinent.In general, most studies have focused on the Taft Formation because it hosts lead and zinc deposits such as Mahdiabad, Farahabad, Mansourabad, etc. (Mojtahedzadeh 2002; Maghfouri and Hosseinzadeh 2018; Maghfouri et al. 2019; Maghfouri et al. 2020). On the other hand, a few studies have been performed on the Sangestan Formation focusing on its stratigraphy and paleontology. Until now, the geochemistry of Sangestan sediments and their sedimentary environment have not been paid attention to.The use of major, trace, and rare earth elements of siliciclastic sediments is very common in sedimentary geochemistry studies used by researchers to identify various purposes, including the tectonic setting, palaeoweathering conditions of the source area, and the nature of the source rock (Shadan and Hosseini-Barzi 2013; Wang et al. 2013; Nowrouzi et al. 2014; Salehi et al. 2014; Zaid et al. 2015; Moallemi et al. 2017; Okon et al. 2017; Periasamy and Venkateshwarlu 2017; Somasekhar et al. 2018; Abubakar et al. 2019; Etesampour et al. 2019; Sci et al. 2019; Xu et al. 2019).Multivariate statistical methods have been utilized to determine the relationship between several elements and the nature of sediments; something impossible by univariate statistical analysis. Despite the advantage of multivariate statistical methods, they have not been paid considered in siliciclastic geochemistry and they have been used only in a few studies.The main purpose of this study is to evaluate the composition and petrography of siliciclastic rocks of the Sangestan Formation, the relationship between the composition of the sediments and the deepening of the sedimentary basin, the relation between the nature of the sediments and the major and trace elements by multivariate analysis, and the climate conditions in Yazd Block during the Early Cretaceous. Material & MethodsAfter field observation, sampling was performed from the Sangestan Formation at the Bidakhavid and Alavi Mountain sections. Seventeen samples (nine siltstone and eight sandstone samples) from Alavi Mountain and 10 samples (three siltstone and seven sandstone samples) were collected from Bidakhavid.Sixteen thin sections from siliciclastic samples were studied petrographically by the Olympus model polarizing microscope.  The point counting of more than 500 points per thin section was performed on twelve sandstone samples by the Gazzi-Dickinson method.Twenty-seven samples of siliciclastic rocks of the Sangestan Formation were selected for geochemical objectives. The preparation steps of the samples which include the cutting of the samples in order to obtain an un-weathered surface, crushing, and grinding to achieve the particle sizes under 200 mesh were performed at Yazd University. Then, the representative samples were analyzed by Zarazma Mineral Studies Company (Iranian company). To determine the major oxide elements, the samples were fused by lithium metaborate and dissolved in dilute nitric acid and, finally, determined by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES). An ICP-OES 735 instrument in Australia was used to analyze the samples. Moreover, the rare earth elements (REE) and other trace elements were determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The instrument used was an ICP-MS Agilent series HP 4500 in the United States.SPSS and GCDkit software were used to carry out multivariate statistical studies and to plot ternary diagrams, respectively. In addition, Rockwork 14 software was used to draw the stratigraphic column. Discussion of Results & ConclusionsThe preliminary study of thin sections showed that the siliciclastic samples of the Sangestan Formation include sandstone and siltstone in the two studied sections. Sandstone samples are often medium-grained, with a small percentage of coarse-grained, moderately rounded and immature. Modal analysis was performed on sandstone samples for more detailed studies. The main constituents are quartz grains (56% and rock fragments ( less than 1%), as well as feldspar (25%).Siltstone samples are similarly composed of quartz, feldspar, and rock fragments. They have about 52% quartz, most of which is monocrystalline, and only less than 1% is polycrystalline quartz. The amount of feldspar is less than sandstone samples and they constitute about 16% of the rock volume. Similar to the sandstone samples, a few rock fragments were observed in siltstone samples. Furthermore, on the basis of the classification of Folk (1980), sandstone samples of the Sangestan Formation showed arkosic composition.  This result was validated by geochemical data confirmed by the geochemical classification of Herron (1988).In factor analysis, the positive and large factor loadings demonstrate a significant correlation between the variable and corresponding factor in the matrix. The results of factor analysis identified two first factors which discriminated the elements associated with the main constituents of siliciclastic sediments and those  related to the cement of siliciclastic sediments.For example, factor 1 showed positive factor loadings with Zr, V, U, Sc, Sr, REE MgO, Fe2O3, TiO2, CaO and negative factor loading with Cr, Na2O, SiO2, and Al2O3.  This factor depicted that the cement of rocks was formed from carbonate, iron oxide, and so on whereas the main constituents of siliciclastic samples were formed from elements such as Na2O, SiO2, and Al2O3. Furthermore, the results of the hierarchical cluster analysis shown in the dendrogram confirmed those of factor analysis.In order to evaluate the changes of these groups of elements, they were plotted along the sedimentary log. The results showed that the elements which are associated with the framework constituents of siliciclastic samples are more in sandstone rocks than siltstone rocks. Conversely, the elements related to the cement of siliciclastic sediments are more in siltstones than sandstone rocks.  Thus, it can be interpreted that the time of siltstone deposition was probably accompanied by an increase in the depth of the basin and the presence of more cement in the basin. Finally, the results of (A-CN-K) Al2O3-(CaO*+Na2O)-K2O ternary diagram and diagram which was proposed by Suttner and Dutta (1986) depicted that these sediments have been deposited in arid to semi-arid climatic conditions.