Sedimentary environment, diagenesis and geochemistry of Parvadeh Formation (Middle Jurassic) at Kamar-Mahdi section (southwest of Tabas

  Parvadeh Formation is the first rock unit of second sedimentary cycle of Jurassic sedimentary deposits in Central Iran. At the type locality, Parvadeh Formation is overlain and underlain by sandstones of the Hojedk and the marls of Baghamshah formations (Seyed-Emami et al., 1991; Aghanabati, 1996; Valipoori Godarzi et al., 2014). Lithologically, this sequence sttarts with a siliciclastic at the base and continues with almost uniform gray limestones. This formation is completely different with the lower shale and sandstone deposits and also upper green gray marl that separates these two rock units. Based on biostratigraphic studies of this section, Middle to Late Bathonian age has been suggested for this sequence (Aghanabati, 1996). The studied section is located at Kamar-Mahdi area, 68 Km southwest of Tabas city, that contain sandstone and siltstone rock units and thin to medium bedded carbonate rocks that continues to sandstone-coal layers of Hojedk Formation (Valipoori Godarzi et al., 2014; Zarrin, 2014). For performance of laboratory studies, totally 86 rock samples were collected from Parvadeh Formation in Kamar-Mahdi section (eleven siliciclastic and seventy-five carbonate samples). All of the carbonate samples were stained using Dickson method (1965) for recognition of calcite from dolomite. Then, the stained microscopic samples were studied and photographed in detail at the laboratories of the department of Geology/University of Birjand. For the purpose of geochemical analysis of major and minor elements, totally 14 suitable micritic samples with low amount of insoluble residues (El-Hefnawi et al., 2010) were selected for atomic absorption spectrometery (AAS) analysis. This method is applied for determination of the amounts of Ca and Mg major elements and Sr, Fe, Mn and Na minor elements in carbonate rock samples. The AAS analyses were carried out using Shimadzu 670 device in analytical chemistry laboratory at the Ferdowsi University of Mashhad. Based on the obtained results from microscopic studies of thin sections, two siliciclastic lithofacies and eight carbonate microfacies were identified as follows: calclithite sandstone (C1), sandy bioclastic packstone (L1), siltstone (L2), extraclastic/peloidal packstone (L3), bioclastic/peloidal wackestone/packstone (L4), bioclastic/oncoidal packstone (L5), coral/sponge framestone (B1), oolithic/bioclastic grainstone (B2), bioclastic floatstone (M1) and bioclastic/oolithic packstone (M2). These lithofacies and microfacies were classified within four beach, lagoon, barrier and open marine facies belts. Based on the position of facies belts and compared with similar models, the suggested depositional environment for Parvadeh Formation in Kamar-Mahdi section is a rimmed shelf type carbonate platform. According to the petrography studies of thin section, the most significant diagenetic processes affecting on the carbonate samples of Parvadeh Formation in Kamar-Mahdi section are: micritization, dissolution and formation of intraparticle and interparticle porosities and incomplete bladed cement during marine diagenetic stage, physical and chemical compaction, creation of moldic and fracture porosities by dissolution, neomorphism, forming of equal mosaic and granular and syntaxial overgrowth cements, dolomitization and incomplete stylolitization in shallow burial diagenetic environment and formation of non-ferroan drusy mosaic and granular cements in phreatic meteoric diagenetic environment.
The analysis of major elements of carbonate samples reveals that the samples are composed of limestone, dolomite and dolomitized limestone. Also, based on the amounts of minor elements, primary mineralogical composition of the studied carbonate samples is aragonite. The Sr-Na crossplot diagram for the studied carbonate samples show a primary mineralogical composition similar to aragonite samples of Mozduran Formation. In addition, comparison of the obtained results with the previously published works (e.g. Brand & Veizer, 1981) indicates an open system geochemically for the studied carbonate samples. Furthermore, based on the amounts of minor elements, it has been concluded that the studied samples have been affected mostly within burial and meteoric diagenetic environments. Finally, based on Na/Sr ratio versus Mn values crossplot, paleoenvironmental conditions for this sequence have had similar to recent tropical environments. Based on the microscopic studies of thin sections and microfacies analysis, four different facies belts including coastal, lagoon, barrier and open marine belts have been identified and a rimmed shelf type carbonate platform depositional environment has been suggested for this sequence. Based on the study of thin sections and also results of geochemical analysis, diagenetic processes on the carbonate samples are mostly operated within burial and meteoric diagenetic environments. Geochemical analysis of the major elements revealed that the studied carbonate samples are composed of limestone, dolomite and dolomitized limestone. The results of geochemical analyses indicate an open geochemical system, aragonitic primary mineralogy and tropical paleoenvironmental conditions for the studied sequence