Provenance and depositional environment of the Neogene alluvial deposits of the Birjand Plain (South Khorasan)

Abstract

AbstractThe study area includes the Neogene alluvial terrace units of Birjand city, which are composed of conglomerate, sandstone and mudstone deposits. Field and petrographicstudies of eight alluvial profile sediments were carried out to investigate the provenance and sedimentary environment. Petrography of the conglomerate gravels shows the presence of all types of ultramafic and intermediate igneous clasts (intrusive and extrusive) along with sedimentary rock fragments in these samples. According to the modal analysis results, the sandstones are divided into two categories of sublitharenite and feldspathic litharenite and belong to the recycled orogen and mixed magmatic arc tectonic settings. These sediments were probably derived from the exposed and weathered sedimentary, igneous (intrusive and extrusive), ophiolitic and, to a lesser extent, metamorphic rocks. The interpretation of the depositional conditions of the Neogene alluvial terrace is based on the analysis of sedimentary facies and the vertical and lateral facies changes. The facies changes and the various architectural elements show that the above sediments were deposited by a shallow proximal gravel-bed braided river.Keywords: Neogene Alluvial deposits, Petrography, Sedimentary environment, Tectonic setting, Birjand plain. IntroductionThe provenance study is an effective method for identifying the types of sedimentary parent rocks, reconstructing depositional conditions, and determining the palaeogeographical condition of the source area. The detrital sedimentary framework, particularly the sandstones, and their mineralogical composition are sensitive to the natural processes and tectonic environment of the source area. Therefore, the method of useing the composition of detrital deposits to determine the tectonic nature of the source area is widely used in basin analysis (Yuan and Wang 2016; Hauhnar et al. 2018; Chima et al. 2018; Baiyegunhi et al. 2020). Thus, petrographical analysis can be used to understand and reconstruct the depositional history of ancient sedimentary rocks. Identifying the vertical and lateral facies changes and architectural elements of alluvial deposits play an important role in the interpretation of ancient sedimentary succession and help to reconstruct the pattern of ancient channels (Benito et al. 2023). The objectives of this study are (1) to analyze the petrographic data in order to identify the components that make up the detrital sediments, classify and infer the tectonic setting and provenance, and (2) to identify and analyze the sedimentary facies and present the depositional model. Materials & MethodsThe field study of eight Neogene alluvial terrace profiles is conducted in terms of textural characteristics and sedimentary structures (based on the field criteria of Stow 2005), and the lithofacies types are separated by textural characteristics and structural, geometric shape and boundary surfaces are separated according to the classification of Miall (2006, 2000). Gravel clasts of conglomerate facies and sandstone samples are considered for laboratory investigation. To study the petrography of the gravel clasts of the conglomerate, 200 clasts were sampled and their microscopic thin sections were prepared according to the method of Stow (2005). Modal composition and textural analysis of sandstone samples were also performed on ten prepared thin sections. The percent abundance of major sandstone constituents such such as quartz, feldspar and rock fragments, as well as accessory minerals such as mica and heavy minerals, and cement, is determined by the Gazi-Dickinson method (Gazzi 1966; Dickinson 1970; Ingersoll et al. 1984; Zoffa 1985). Folk (1980) compositional classification were used to name sandstone samples. Existing standard comparator charts are used to check the roundness, sphericity and sorting of grains (Powers 1953; Tucker 2001) and the size of the grains is also measured on the Udden-Wentworth classification. Discussion of Results & ConclusionsIn this study, the provenance and sedimentary environment of the Birjand Neogene alluvial deposits were investigated based on petrographic and field data. Microscopic study of the gravel samples of conglomerate facies shows the presence of ultramafic, mafic and intermediate igneous rocks (intrusive and extrusive) along with sedimentary rock fragments. Petrographic studies and modal composition of sandstones show that the main detrital grains include quartz (mostly single-crystal quartz), feldspar (mostly plagioclase), and rock fragments (mostly sedimentary and volcanic rock fragments). Based on petrographic data, igneous (intrusive and extrusive) and sedimentary rocks along with minor amounts of metamorphic rocks are the parent rocks of these sediments. Furthermore,, modal analysis of the sandstone composition shows that the tectonic setting of the Neogene sandstones is recycled orogen and mixed magmatic arc, and these sediments are the result of weathering and erosion of sedimentary, igneous rocks and to a lesser extent metamorphic rocks. The facies analysis of each profile based on grain size, textural characteristics and sedimentary structures shows that these facies are placed in three facies associations: conglomerate, sandstone and mudstone lithofacies. Several field evidence such as the abundance of coarse-grained gravel facies, the low thickness of fine-grained flood plain sediments, the large number of finning-upward cycles, the erosional base of each cycle, the vertical and lateral changes of the facies, the presence of sediment gravity flow deposits and the presence architectural elements are confirming the deposition of these sediments by the proximal shallow gravel-bed braided river.

Keywords

• neogene alluvial deposits
• petrography
• sedimentary environment
• tectonic setting
• birjand plain