JOURNAL OF STRATIGRAPHY AND SEDIMENTOLOGY RESEARCHES
Year:2019 | Volume:35 | Issue:2 (75) |Papers Count:9

In this study, petrography and geochemistry of the sandstones and shales of Padeha Formation in Khoshyeilagh section in Eastern Alborz have been used to obtain more information on their provenance. The Padeha Formation in the studied section has a thickness of 341 meters and overlies the Late Ordovician-Silurian Soltan-Meydan basalts and disconformably underlies the Middle to Upper Devonian Khoshyeilagh Formation. After field studies, 20 medium-grained sandstone samples were selected for point-counting and eight sandstone and two shale samples were selected for geochemical analysis. Based on the calculated percentages of point counting data, the composition of these sandstones has been detected as quartzarenites, subarkose and sub litharenites. Based on Q/Q+F in sandstones and geochemical properties of sandstones and shales, such as Al2O3/TiO2 ratio and TiO2 against Zr, the dominant parent rock of these deposits can be considered as felsic igneous rocks. The QmFLt and QtFL diagrams show the tectonic setting of the Padeha sandstones as a stable craton. The geochemistry has also largely confirmed the tectonic setting of passive continental and rifted margins. Considering paleogeography of the region, tectonic uplifts and erosion of the Cadomian granitic basement and also the entrance of clastic materials from granitic rocks of Arabian craton can be considered as other source rocks of the sediments of Padeha Formation. Introduction Petrography and the composition of siliciclastic sediment and sedimentary rock components provide important information on parent rock, which is one of the main factors in the reconstruction of tectonic evolution and sedimentation. In addition, by using the chemical composition (major, trace and rare earth elements) of siliciclastic sediments and sedimentary rocks, one can also estimate the provenance and other sedimentary processes such as weathering, transport and diagenesis (McLennan et al. 1993; Armstrong-Altrin et al. 2012; Armstrong-Altrin 2015). In this study, petrography and geochemistry of the sandstones and shales of the Padeha Formation in Khoshyeilagh section in Eastern Alborz have been used to obtain more information on their provenance. Material & Methods The Padeha Formation in the studied section has a thickness of 341 meters and overlies the Late Ordovician-Silurian Soltan-Meydan basalts and disconformably underlies the Middle to Upper Devonian Khoshyeilagh Formation. In this study, 30 samples of sandstones of the Padeha Formation in Khoshyeilagh section were selected after field sampling and petrographic studies. Twenty medium-to fine-grained sandstone samples were selected for modal analysis, and about 300 points were counted on every thin section using the Gazzi-Dickinson method (Ingersoll et al. 1984). According to the point counting, the main and accessory components of this sandstones are identified and quartz, feldspar and rock fragments modes are utilized for naming the sandstones according to the Folk classification (Folk 1980) as well as different diagrams of the provenance (Dickinson 1985). Based on the calculated percentages of point counting data, the composition of these sandstones has been detected as quartz arenites, sub-arkose and sub litharenites. Eight samples of medium-grained sandstone samples with the lowest amount of carbonate cement and two samples of shales in the middle member of the Padeha Formation were selected, crushed and powdered with a mill and sent to the Zarazma Laboratory, Tehran for geochemical analysis (ICP-MS method) to determine the major, trace and rare earth elements. Discussion of Results & Conclusions According to the ratio of quartz to total feldspar and quartz in the sandstones of the Padeha Formation, which is 0. 95 and geochemical properties of sandstones and shales, such as Al2O3/TiO2 ratio and TiO2 against Zr, the dominant parent rock of these deposits can be considered as felsic igneous rocks. The QmFLt and QtFL diagrams show the tectonic setting of the Padeha sandstones as a stable craton. Passive continental and rifted margins for Padeha Formation has also largely confirmed by geochemistry of sandstone and shale samples. Considering paleogeography of the region, tectonic uplifts and erosion of the Cadomian granitic basement and also the entrance of clastic materials from granitic rocks of Arabian craton can be considered as the other source rocks of the sediments of Padeha Formation.

In this study, for the first time depositional conditions, sequence stratigraphy and elemental geochemistry of the Taleh Zang Formation with Paleocene to Lower Eocene age in the Kermanshah province (Kaboutar Bala section) have been evaluated. In this section, the Taleh Zang Formation conformably overlies the Amiran Formation and is overlain by the Kashkan Formation with a disconformity surface. Field and microscopic studies led to recognition of six microfacies within the succession. Gradual microfacies change, the absence of calciturbidites and lack of extensive barrier reefs with considerable thickness, confirms a carbonate ramp. Sequence stratigraphic studies led to the identification of one-third order depositional sequence which includes TST and HST facies association. The lower boundary of this sequence, specified as type II and the upper boundary with evidences of subaerial exposure is type I. The maximum flooding surface (MFS) is determined with bioclastic coral corallinacea floatstone/rudstone microfacies of proximal middle ramp. The results of geochemical analysis of major elements (Ca, Mg) and minor elements (Sr, Na, Mn and Fe) indicate that the original carbonate mineralogy is aragonite and diagenetic system is semi-close to open system. Trend of elements changes along this stratigraphic section shows that in the HST sediments, the amount of Sr is decreased, while Fe and Mn are increased due to more meteoric diagenetic effect.

The Devonian/Carboniferous (DC) transition is characterized by several transgressive/regressive cycles which led to a widespread ocean anoxia known as the Hangenberg Black Shale Event (HBS), close to the D/C boundary a major sea-level fall (Hangenberg Sandstone, HSS), can be recognized in many sections around the world. Both events known as the Hangenberg Crises. In order to examine the D/C transition, the Mighan section, in Eastern Alborz was selected and studied. The outcrop section is located 25 Kms NE Shahrood city. Frothy five conodont samples (4-5 kg) were systematically taken from 91 ms of the D/C transition interval. Although the conodonts show low frequency but high diversity exhibit important zonal index taxa of the widely applied conodont standard zonation. Twenty-three conodont species belong to five genera were identified and let to discrimination of six following zonal boundaries: Bispathodus aculeatus aculeatus zone, Bispathodus costatus zone, Bispathodus ultimus zone, praesulcata zone, CKl interregnum, sulcate zone. At the D/C transition in Mighan section a black shale and sandstone units are observable that corresponds to the Hangenberg Crisis, that highly affected trilobite, ammonoid, brachiopod and conodont faunas. Kockeli Zone at the latest Famennian is missing at the studied profile due to the major sea level regression.

The Ruteh Formation is the second sedimentary cycle in Alborz basin. The petrography and laboratory studies of these deposits led to the identification of 15 microfacies which were deposited in seven sub-environments including supratidal, intertidal, lagoon, shoal, open marine, middle ramp, and outer ramp. Since there were no turbidity deposits, the lack of marginal reef belt and the gradual changes of facies, it turned out that these sediments were deposited in a homoclinal carbonate ramp. The study of relative sea level changes was done according to vertical distribution of facies and their stacking patterns. These studies lead to identification of four depositional sequences. The petrography data and geochemical analyzes were used in order to identify the diagenetic processes and geochemical changes. The recognized diagenesis processes include micritization, cementation, compaction, stylolitization, silicification, ferrugenation and neomorphysm. These diagenetic processes took place in three diagenetic environments: marine-phreatic, meteoric-phreatic and burial. Geochemical analyzes confirms that the carbonate sediments of Ruteh Formation were originally composed of aragonite, which was eventually recrystallized to calcite during diagenesis. Plotting Sr/Ca versus Mn diagram approved that sediments was subjected to a meteoric diagenesis in a semi-closed to open diagenetic system. The results of this research can be used to provide a sedimentary model for these deposits which completes the palaeogeographical data for the Tethys Ocean.

The Mishrif Formation (Cenomanian– Turonian) is one of the most important hydrocarbon reservoirs in the Persian Gulf. In this study, 151 microscopic thin sections and the porosity and permeability data on wells SIE-5 and SIE-6 in Esfand Oilfield were studied to identify the microfacies, depositional environment and the diagenetic processes of the Mishrif reservoir, to evaluate the effect of these processes on the reservoir quality. Lucia's petrophysical classification and the reservoir quality descriptive classification were also used in the optimal analysis of the reservoir characteristics. Petrographic analyses revealed 11 microfacies that belonged to the peritaidal zone, lagoon, shoal, middle and outer ramp sub-environments on a homoclinal carbonate ramp. Most of the microfacies of these two wells are belong to the shoal and lagoon facies belts. The succession in both wells show shallowing upward sequences. The most important diagenetic characteristics of the Mishrif Formation include dissolution, cementation, compaction, dolomitization and stylolitization. Porosity type consists of intercrystalline, vuggy, moldic and fracture porosities. Almost all microfacies in both wells are also in classes one and two of Lucia's petrophysical classification. Furthermore, the facies in the peritidal zone and shoal sub-environments in SIE-5 are in a better reservoir condition than SIE-6. On the other hand, the open marine facies in SIE-6 have a higher quality than the facies in SIE-5. In summary, it could be stated that diagenetic processes have had a larger role in the increase or decrease in the reservoir quality than the depositional environment. Thus, the Mishrif reservoir could be considered a diagenetic reservoir

In order to investigate the concentrations of heavy metals in stream sediments of the Baycheh-Bagh area, northwest of Zanjan province, 131 samples were collected from intended stations. These samples after dissolution by four acid method, were analyzed by ICP-OES. To determine the type of spatial distribution of data, two important spatial statistics, i. e. global Moran’ s I and Anselin local Moran’ s I were employed. The average concentrations of As, Bi, Cd, Ni, Pb, Sb and Cu in stream sediment samples were 21. 83, 0. 32, 0. 35, 35. 57, 68. 96, 37. 67, 1. 96 and 82. 71 ppm, respectively which are higher than the average of crust whereas the average concentration of Co is 20. 30 ppm. The obtained results indicate that the heavy metals in the study area has a spatial structure and distributed in cluster form. According to the Local Moran’ s I results, the most contaminated area in the study area was detected in western part of the Baycheh-Bagh abounded mine and located on the present polymetallic vein-veinlet mineralizations and their related alterations. Analysis of obtained data and investigation of their relationship with the geological rock units of the study area show that concentration of the studied elements have geogenic source and is related to the mineralizations and alterations.