Lithostratigraphy, petrography, and geochemistry of sandstone in the middle part of the Upper Red Formation, Ghezeljeh area, NW Zanjan

Ghezeljeh area is located in the northeastern part of Mahneshan at 70 km northwest of Zanjan. The rock units exposed in the area are including the Lower Red, Qom, and Upper Red formations which the Upper Red Formation is the purpose of current research. According to field studies, the Upper Red Formation with 750 meters thickness includes 250 meters of evaporative units with intercalations of marls at the bottom and 500 meters of alternation of marls and sandstones intercalations at the top. According to microscopic studies, the grain size of the studied samples ranging from very fine sandstone to gravel. These sandstones show poorly orientation and well to poor sorted and are angular to sub-angular. The sorting and roundness parameters as well clay content show that these sandstones are immature and in some cases are sub mature. Based on a high percent of components and the Folk classification, the middle part of the Upper Red Formation displays lithic-arenite (sed-arenite) and feldspathic-lithic-arenite with an average composition of Q38F15Rf47. Geochemical studies show that these sandstones sourced from intermediate to felsic igneous rocks and deposited on the active continental margin. Furthermore, the weathering index shows an arid and semi-arid climatic condition during their deposition. Keywords: Stratigraphy, Geochemistry, Upper Red Formation, Ghezeljeh, Zanjan.   Introduction Sedimentary rocks are the main source of information about past conditions. According to the chemical composition of sediments and sedimentary rocks, the origin and other depositional processes such as weathering, transportation, and diagenesis can be evaluated (Mclennan et al. 1993). Meanwhile, the study of trace elements has more reliable results than the major oxide elements because these elements are immobile against processes such as transport, diagenesis, and metamorphism (Whitmore 2004; Von Eynatten 2004). So in the current study, in addition to petrographic studies, information obtained from rare earth elements was used to find out the origin of rocks, determine of tectonic setting, and palaeoclimate. The Upper Red Formation, which is the subject of this study, consists of three parts. In the current research, we investigated the middle part (M2) of Upper Red Formation, because it has potential as a cap-rock in gas fields and also a suitable host rock of ore deposits. Therefore, a detailed study of litho-stratigraphy, petrography, and geochemistry of different parts of this formation in Central Iran and Alborz sedimentary basins is very important. This study aims to investigate the tectonic setting, ancient climatic conditions, and the origin of these sandstones in the Ghezeljeh area, NW Zanjan.   Material & Methods In the current research, a suitable section was selected by using the Mahneshan geological map with a scale of 1:100000 (Lotfi 2001). In the second step, the stratigraphic column and profiles associated with the geological map (scale: 1:20000) of the study area were prepared. Besides, 30 thin-section fine- to medium-grained samples were prepared and carefully studied for sedimentary aspects and point counting. Furthermore, eight sandstone samples from the Upper Red Formation with minimum alteration and the least amount of carbonate cements, calcareous matrix, and bioclast were selected for whole-rock geochemical analysis. The whole rock geochemical analysis was done in the Zarazma Laboratory, Tehran. Several discriminatory plots of the trace elements were used to determine the provenance and tectonic setting of the studied sandstone of the Upper Red Formation.   Discussion of Results & Conclusions The study area is located in the central Iranian zone which is part of the Alpine‒Himalayan orogenic belt. The Upper Red Formation is the main outcropped rock unit in the study area which mainly consists of marls and sandstones. The investigated sandstones are mainly composed of sedimentary and metamorphic rock fragments. Additionally, there are some bioclasts in the studied sandstones that indicate that the carbonate and calcareous fragments in the Upper Red Formation may originate from older carbonate succession such as Qom Formation which has extensive outcrop in the study area. The studied sandstones are generally poor to well-sorted as seen in the thin sections. Petrographic studies revealed that sandstones of Upper Red Formation in the Ghezeljeh area are mainly lithic-arenite (sed-arenite) and feldspathic-lithic-arenite with an average composition of Q38F15Rf47. The grain size and particle geometry, as well as the degree of sorting of the studied sandstones, indicate that they are texturally immature to sub-mature in terms of textural maturity. The petrographic and geochemical studies of sandstones in the middle part of the Upper Red Formation indicate that the active tectonic setting and recycled orogeny for the investigated sediments which is consistent with the earlier studies. Moreover, the results of the modal analysis show that the climatic condition was dry to semi-humid during the deposition of this formation. According to geochemical analyzes, the investigated sandstones originated from acidic to intermediate igneous rocks. Furthermore, geochemical diagrams show that the studied sandstones formed in an active continental margin setting. These studies generally indicate that the investigated sediments were deposited probably in a foreland basin during the Arabia‒Eurasia collision.