جستجوی مقالات مرتبط با کلیدواژه "palaeoclimate" در نشریات گروه "زمین شناسی"

The Sarvak Formation, as one of Iran's most important hydrocarbon reservoirs, has been deposited and evolved under active tectonic conditions in various regions of the Zagros, including the Persian Gulf. The present study aims to investigate the effects of tectonic activities on the facies characteristics and diagenetic history of the Sarvak Formation in the eastern and western regions of the Persian Gulf. Facies analysis results indicate the deposition of the Sarvak Formation on a carbonate platform of ramp type. In the eastern part of the Persian Gulf, sedimentary facies of the Mishrif Member (Upper Sarvak) are predominantly deposited in inner platform areas (lagoon, shoal, reef deposits), while facies of the Khatiyah Member (Lower Sarvak) are mainly found in deep and outer platform areas (outer ramp). In the western part of the Persian Gulf, the Sarvak Formation encompasses a spectrum of shallow to deep-water facies, with open marine facies (middle and outer ramp) being notably more abundant. Two lower-order sequences (fourth-order) and one higher-order sequence (third-order) have been identified in this formation, indicating a high correlation with other areas of the Zagros and the Arabian Plate. Diagenetic effects related to a palaeoexposure surface have been observed in the uppermost part of the Sarvak Formation in both the eastern and western regions of the Persian Gulf. Dissolution porosity, brecciation, and iron oxide staining are among the most important diagenetic processes associated with these exposure surfaces. The thickness of the Sarvak Formation shows considerable variations in different parts of the Persian Gulf. Additionally, sedimentary facies of this formation exhibit significant lateral variations. The changes in thickness and facies variability of the Sarvak Formation were directly related to basement structures, indicating their activity during the deposition of this formation in the Persian Gulf. These tectonic events have had a significant impact on the burial history of the Sarvak Formation. Ultimately, the culmination of these facies and diagenetic characteristics has influenced the reservoir quality of the Sarvak Formation in the study areas.

Sporomorph EcoGroups data and the relevant plant communities are considered as a possible routine used to draw palaeoecological inferences for their host strata. Occurrence of high abundant and diverse miospores in the Qadir Member (Nayband Formation), from the exploratory well no. 954 (Exploratory Area of Parvadeh 4), south of Tabas, central Iran allows for this method to be used to obtain certain palaeoecological implications. By classifying spores and pollens (sporomorphs) in Sporomorph EcoGroups (SEGs), all six plant ecogroups (including plants adapted to Upland, Lowland, River, Pioneer, Coastal, and Tidally-influenced) were identified in examined material. The high ratio of warmer/cooler sporomorphs and the low ratio of drier/wetter sporomorphs suggests deposition under a moist, ±warm climate during the Late Triassic in south of Tabas. From the investigation of miospore parent plants, it was approved that, in descending order, pteridophyta (44%), cycadophyta (34%), coniferophyta (9%), lycophyta (8%), pteridospermophytes (2%), ginkgophytes (2%) and bryophyta (1%) were surrounded the environment of the studied formation. Notable abundance of fern spores (Kyrtomisporis and Foveogleicheniidites) and cycads pollen (Ovalipollis and Ricciisporites) in the palynofloras studied indicate the predominance of warm to semi-warm climate with high humidity. Paleogeographic position of Iran during the Late Triassic in the southern active margin of Eurasia (Turan Plate) tends to support this palaeoclimate generalization.

The Sarvak Formation, a crucial reservoir rock in the Abadan Plain, is extensively studied due to its sedimentary attributes and diagenetic evolution, heavily influenced by tectonic activities and palaeoclimatic conditions. This research focuses on analyzing palaeoclimatic indicators within the Sarvak Formation in a selected oil field in the Abadan Plain. Through a comprehensive approach integrating core data, thin section analyses, and electron microscopy, the study characterizes various facies, diagenetic processes, and sequence stratigraphy of the formation. Five distinct microfacies representing different depositional environments, such as lagoon, shoal, reef, reef-talus, and open marine belts, are identified, suggesting a ramp-type depositional setting. The investigation also reveals a paragenetic sequence of diagenetic processes, including transitions from marine to meteoric diagenesis and from shallow to deep burial environments. Notably, two palaeoexposure surfaces are identified, characterized by meteoric dissolution, brecciation, iron oxide staining, and silicification. Scanning electron microscopy analysis indicates prevailing kaolinite and montmorillonite clay mineral assemblages, indicative of warm and humid palaeoclimatic conditions. These findings provide insights into the palaeoclimatic conditions and paleaogeographical positioning of the Arabian Plate during the Cenomanian–Turonian period, suggesting a close proximity to low latitudes near the Equator.

Very few palynological studies have been conducted on the Nayband Formation, as a part of the Upper Triassic coal deposits of central Iran. Mousavi, 2002 (seen in Aghanabati 2004, pp. 209-210) considered the abundance and diversity of marine palynomorphs compared to terrestrial types indicate the shallow sea environment for Nayband Formation. Cirilli et al., 2005 attributed the Nayband Formation palynofloras to tropical plants such as Bennettitales, Matoniaceae, and Marattiaceae accompanied by characteristic forms of the Siberian Province. Sabbaghiyan et al., 2015 examined the palynology of the Nayband Formation, Tabas Block. They believed the associated marine palynomorphs (dinocysts), accompanied by spores indicate a nearshore depositional environment for the Late Triassic deposits. Sajjadi et al., 2015 studied the palynology of the Nayband Formation, Kamar Macheh Kuh, southeastern Tabas. They concluded that the abundance of ferns and Coniferophytes in parent flora implies that the host strata accumulated under a moist warm climate with progressively decreasing temperatures during Late Triassic. Sabbaghiyan et al., 2020 investigated the palynology of the Upper Triassic Bidestan and Howz-e-Sheikh members in Tabas Block. They believed that co-occurrence of marine elements (dinoflagellate cysts, bivalves, corals, and gastropods) with terrestrial elements (spores and pollen) shows a shallow marine depositional environment for the Nayband Formation.
A total of 62 samples were collected from the Nayband Formation (Qadir Member), at exploratory well no. 948b. All samples were prepared following standard palynological processing procedures (Phipps & Playford, 1984), including HCl (10-50%) and HF (40%) utilized for the dissolution of carbonates and silicates, respectively. Then the residues were saturated with ZnCl2 solution (specific gravity 1.9 g/ml) for density separation. All the residues were sieved with a 20 μm mesh sieve previous to making strew slides. Three slides for each preparation were examined by a transmitted light microscope.
To reconstruct the paleoclimate of the Nayband Formation (Qadir Member) in the south of Tabas, 62 samples were collected from exploration well no. 948b for palynological investigations. The assemblage of diverse palynomorphs includes spores and pollen of land plants, dinoflagellate cysts, foraminiferal test linings, acritarchs, and algal spores with moderate to good preservation. The parent plants of existing miospores show that the diversity and abundance of ferns is 64%, cycadophytes 16%, lycophyta 9%, conifers 4%, gynophytes 4%, pteridospermophyta 2% and Bryophyta 1%. The maximum diversity and relative abundance belong to miospores attributed to ferns (64%), which indicates the predominance of warm to semi-warm climate with high humidity at that time of deposition. Based on the model of Sporomorph EcoGroups, miospores typifying all six plant communities are present in the studied strata, but the highest frequency is related to the lowlands SEGs. To reconstruct the paleoclimate variation, the large quantity percentage of four main plant groups (Hydrophilic, xerophilic, Thermophilic, Psychrophilic) was calculated and the paleoclimate study was determined by the pattern of relative abundance of drier/wetter and warmer/cooler elements. The consequences of these calculations approve the warm to semi-warm climate with high humidity conditions. Also, the paleogeographic position of Iran during the Late Triassic in the southern active margin of Eurasia (Turan Plate) is another confirmation of this type of climate.
Diverse palynomorphs with high abundance including spores, pollen, dinoflagellate cysts, foraminiferal test linings, acritarchs, and algal spores with moderate to good preservation are present in the studied material. Inferred natural relationships of the Nayband sporae dispersae imply derivation from a diverse parental flora such as ferns, Conifers, Lycophyta, Cycadophytes, Gynophytes, Pteridospermophyta, and Bryophyte. Ferns have relative abundance and maximum diversity in the composition of the vegetation around the sedimentary environment. It implies that the host strata accumulated under moist warm to semi-warm climates. In addition to, the high ratio of warmer/cooler, wetter/drier palynomorphs and the palaeogeographic position of Iran during the Late Triassic in the southern active margin of Eurasia (Turan Plate) and the location of Iran at a latitude of about 35°N, all confirm the above results that hot and humid climate prevails.

The Sarvak Formation, the second important oil reservoir in the Zagros Basin, has a complicated diagenetic history. This study focuses on the diagenetic processes of this formation in the Abadan Plain. To achieve this goal, petrographic investigations of core samples, thin sections, X-ray diffraction, and scanning electron microscopy are integrated with the results of elemental geochemical data. Intensive meteoric dissolution (karstification), paleosol formation, dissolution-collapsed brecciation, meteoric cementation, and silicification are major meteoric diagenetic processes. Such intensive meteoric diagenesis along with the dominance of kaolinite and montmorillonite, as predominant clay types within the paleosol, all indicate a warm and humid paleoclimatic condition at the time of exposure. These diagenetic alterations provided special trends of variations in the elemental contents of altered carbonates. They include a clear increase in Na, Mn, Fe, and Rb along with the decrease in Sr contents recorded below the Cenomanian–Turonian and mid-Turonian disconformities. The variations in Mg contents depend on the original mineralogy of carbonates that can result in variable trends in response to the meteoric diagenesis. Below the mid-Turonian disconformity, the development of mud-dominated facies hampered the free fluid circulation and, consequently, diagenetic alterations and their related geochemical trends are limited within the Turonian sequence.

The Baghamshah Formation (Middle Jurassic) at the Senj stratigraphic section, northeastern Tabas, central Iran, consists of diverse and relatively well-preserved palynomorphs, including spore and pollen of land plants, microscopic algae, acritarchs, Tasmanites, fungal spores, foraminiferal test linings and dinoflagellate cysts. The miospores are attributed to the Pterophyta (11%), Lycophyta (1%), Coniferophyta (80%), Cycadophyta (3%) and Pteridospermophyta (5%). Among the miospores, the maximum frequency is related to Classopollis from the Chirolpidiaceae family of conifers. The abundance of Classopollis in the studied samples may represent the abundance of Chirolpidiaceae in the composition of vegetation nearby the sedimentary environment, the abundance of produced pollen, and/or the vicinity of the habitat of the parent plants to the depositional environment. Previous investigations about the environment of the Chirolpidiaceae family indicate the predominance of hot and dry climates. The abundance of Botryococcus algae separately or attached to Classopollis also supports this type of weather. The dominant size of the miospores in the stratigraphic section (25–35 µm) indicates the transport by a wind from highland to lowland. Based on the model of Sporomorph EcoGroups (SEGs), miospores typifying all the six plant communities are present in the studied strata. The palaeoclimate study was determined by the pattern of relative abundance of drier/wetter and warmer/cooler elements. The consequences of these calculations approve the hot and dry climatic conditions during the accumulation of the strata examined.

Early and Middle Jurassic, continental sediments of considerable thickness are widespread in the northern, central, and central-eastern Iran (Kilpper, 1964, 1968, 1971, Barnard and Miller, 1976, Sadovnikov, 1976, 1984, 1991, Corsin and Stampfli, 1977, Fakhr, 1977, Schweitzer and Kirchner, 1995, 1996, 1998, 2003, Schweitzer et al., 1997, 2000, 2009, Vaez-Javadi and Pour-Latifi, 2004, Vaez-Javadi and Mirzaei-Ataabadi, 2006, Vaez-Javadi, 2008, 2011, 2012, 2014, 2015a, b, 2018, Saadatnejad et al., 2010, Vaez-Javadi and Abbasi, 2012, 2018, Vaez-Javadi and Allameh, 2015). For a more detailed study, plant macrofossils from Chah-Rekhneh were considered here.
Material, metho

The plant fossils have been driven from a borehole in Chah-Rekhneh, 62km of SW Tabas city, 33˚17´56ʺ North latitude, 56˚24´01ʺ East longitude. A total of 84 specimens have been collected from a 240 m stratigraphic-core section. Some specimens yielded more than one fossil. The flora from this locality here is introduced for the first time. Material cited in this work (prefixed MJHCh; acronym for Mehdizadeh, Javadi, Hojedk, and Chah-Rekhneh) is held in the collection of the Palaeobotany Laboratory of the Department of Geology at the University of Ferdowsi, Mashhad. Biostratigraphy of the Chah-Rekhneh The Hojedk Formation consists of shale, siltstone, and fine-sandy silt alternating with thick medium-grained sandstone at the base of column. Several coal seams are present at the middle and upper part of the core column. One assemblage biozone was established in this section with lower and upper boundaries identified by FOO (First Observed Occurrence) and LOO (Last Observed Occurrence) of Coniopteris hymenophylloides and Klukia exilis, respectively. Furthermore, three informal subzones were recognized which are upward: I- Equisetites beanii- Ptilophyllum harrisianum Interval zone; II- Ptilophyllum harrisianum- Elatides thomasii Interval zone, III- Elatides thomasii - Nilssonia macrophylla Interval zone.Palaeoclimate Analysis In order to recognize palaeoclimate of the Chah-Rekhneh area during the Middle Jurassic, plant macrofossils relative abundance analysis and floral gradient method were used. Ziegler et al. (1996) assigned all Jurassic leaf genera to ten coarser morphological categories (or 'morphocats'). Rees et al. (2000) explained "Floral gradient" analysis. They show how this analysis can be used to interpret phytogeographic patterns based on the axis 1 scores of individual leaf genera and corresponding plant localities, due to their relative degrees of association. Then the climatical distribution in terms of the basic morphological characteristics of individual leaf genera and the palaeogeographic distribution of plant localities can be understood. By calculating the average of the scaled (0 to 100) axis 1 scores of the 32 genera common to all three intervals, a Jurassic 'floral gradient' has been derived. Floral gradient score of this flora in Chah-Rekhneh is 41.81. It shows that the flora assigned to the middle part of Floral Gradient table and suggests a humid and sub-tropical climate during early Middle Jurassic.

The Hojedk Formation in the Chah-Rekhneh area, south west Tabas contains 33 plant macrofossil species (21 genera) of various orders. On the basis of FOO and LOO of index species, one assemblage zone- Coniopteris hymenophylloides-Klukia exilis Assemblage zone and three informal subzones were recognized. These subzones are upward: I- Equisetites beanii- Ptilophyllum harrisianum Interval zone, II- Ptilophyllum harrisianum- Elatides thomasii Interval zone, and III- Elatides thomasii - Nilssonia macrophylla Interval zone. The occurrence of index fossils such as Klukia exilix and Elatides thomasii, Aalenian-Bajocian are assigned for this assemblage. Statistical analysis studies reveal that Order Filicales, Cycadales, Pinales and Equisetales with relative abundance of 46.31%, 23.50%, 12% and 9.26%, and Nilssonia, Klukia, Coniopteris and Equisetites were the most abundant genera with 22.81%, 13.38%, 8.40% and 8.40%, respectively in the Chah-Rekhneh area. Therefore, based on relative abundance of Filicales and Cycadales, the occurrence of Equisetites beanii as index species of climatic and ecologic, and average floral gradient score (41.81), a humid sub-tropical palaeoclimate was dominant in this area during the Aalenian-Bajocian interval.

Aeolian deposits are important palaeoclimate archives, partly because they are direct records of past atmospheric circulation (An et al. 2012). Distinguishing aeolian signature from lacustrine sediments is important for understanding the frequency and timing of palaeostorms. Efforts have been invested on deciphering aeolian records from lacustrine sediments in the United States (Dean 1997; Parris et al. 2010), Greenland (Mayewski et al. 2004), New Zealand )McGowan et al. 1996( and Japan )Xiao et al. 1997) providing a continuous record of Holocene aeolian activity.
Sistan Basin is a remarkable environment to study Aeolian activity changes in eastern Iran – southwestern Afghanistan. The area is one of the driest regions in the world whose sparse water resources and fragile ecosystems are very sensitive to climate change. In moist periods, fluviolacustrine and palustrine conditions are dominant while in dry periods, aeolian activities prevail. Hence, variations of aeolian deposits in sedimentary successions could be a useful tool to evaluate palaeoenvironmental conditions over the Sistan Basin during the Holocene.
Material & The closed depression of Sistan, lying on the Iran-Afghanistan border, contains four shallow basins (mean water surface at about 471m asl) that receive the discharge of Hirmand River. Sistan is located whitin the Asian part of the desert belt of the temperate subtropical zone of northern hemisphere, with a semi-desert climate (Whitney 2007). In late spring, throughout summer and early autumn the Sistan basin is dominated by the northerly “wind of 120 days”. The winds are related to the north–south pressure gradient between a persistent cold high-pressure system over the high mountains of the Hindu Kush in northern Afghanistan and a summertime thermal low-pressure system common over the desert lands of eastern Iran and western Afghanistan as a result of sustained surface warming (Alizadeh-Choobari et al 2014).
Two cores of H1 and H2 (6.2m and 6.8m long) were retrieved using a Cobra vibra-corer in west of Kuh-i Khawjeh in the dry lake bed Hamoun-e Hirmand in the Sistan Basin. Magnetic susceptibility (MS) determination (using a MS2C Bartington magnetic susceptibility meter), analyzing features of grain-size frequency curves (using a FRITSCH laser particle sizer) and petrography were conducted on sediment samples.
A bulk sample was selected from the lower half of the core H1 for radiocarbon dating. The calibrated date suggests late-glacial age for lower parts of the core H1 (13.5 ka).
Discussion of Results & The core lithology generally consists of clayey silt, silt and sandy silt layers. Based on basic sedimentological data, MS and sediment color we have recognized three main facies, A (lateglacial - early Holocene), B (early- mid Holocene) and C (late Holocene).
Since the late-glacial, the climatic regime of interior west Asia has been under the influence of various atmospheric circulation patterns and intensities (Fleitmann et al. 2007). High pressure cells of the Siberian Anti-cyclone (Siberian High: SH) from the north, low pressure cells belonging to the Indian summer monsoon (ISM) from the south, and mid latitude westerlies (MLW) governed the regional climate during thid period.
Data revealed that during the lateglacial - early Holocene, dominance of low pressure system of ISM on Sistan Basin and the western Hindu Kush provided a moist environment with less wind action. As shown by primary modal peak of grain-size distribution curves (20-30 µm), massive clayey silts with low MS (4.2 - 5.2 ×10-4 SI) and abundant plant remains, charophytes and carbonate shells in facies A are deposited in a highly productive lake with no signs of wind action. This period is also characterized by weakening MLW as well as elevated sea surface temperature in Atlantic Ocean and Arabian Sea. In addition, this humid period in the Sistan Basin is concurrent with the early Holocene long-term weakening of SH, inferred from increases in concentration of Kþ in GISP2 ice core (Mayewski et al. 2004) causing dramatic decrease in north-south pressure gradient between the Hindu Kush Mountains in northern Afghanistan and the desert lands of the Sistan Basin. High solar irradiance and weakening and northward migration of territory influenced by the SH also pushed the ISM domain northward (Mayewski et al. 2004) affecting Sistan catchment basin.
During the early- mid Holocene gradual weakening ISM (Sirocko et al. 1993) in addition to strengthening SH and MLW (Bradbury et al. 1993) caused dominance of severe wind storms in Sistan, as shown by modal peak more than 100 µm in facies B. During the mid-late Holocene, episodic high aeolian inputs in the basin by high energy dust storms comparable with the present day “winds of 120 days” are evident. Traces of palaeostorms during this time is evidenced by high oscillations of MS and presence of some sand-bearing lacustrine sequences in both cores. During this period, southward migration of ITCZ led to weakening of the ISM (Fleitmann et al. 2007), and consequently drought periods in Sistan Basin and semi-arid conditions over NW Himalaya (Dortch et al. 2013). Establishment of a high pressure gradient between the Sistan depression and the high Hindu Kush Mountains caused the occurrence of severe and frequent dust storms over the area.
Our results suggest that the late Holocene in the Sistan Basin (facies C3) was characterized by frequent changes in MLW and SH activity. Palaeoclimatic records show since the mid Holocene to the present time, the climate of Sistan and its catchment area more or less oscillated around a steady state comparable with modern situations (Hamzeh et al. 2016). During this time, the hydroclimatic regime and Aeolian activity of the Sistan Basin and NW Himalaya have been mostly governed by MLW-associated precipitation. Periods of prolonged droughts are indicated in proxy records of NW Iran such Lake Neor (Sharifi et al. 2015), presumably consistent with high MS values in our record. It is possible that weakening of ISM, along with distal influences of the MLW during the late Holocene exposed the Lake Hamoun basin to frequent droughts. Frequent lake level fluctuations show unstable climate of the Sistan Basin during mid to late Holocene with frequent wind storms.

Jurassic deposits are well exposed in the Calshaneh area, north western of the Tabas city, Central-East Iran. It contains plant macrofossils belonging to seventeen species allocated to eleven genera of various orders such as of Equisetales, Filicales, Bennettitales, Cycadales, Caytoniales, Ginkgoales and Pinales. One biozone was recognized in the Hojedk Formation at Calshaneh section. According to occurrence of Klukia exilis and Coniopteris hymenophylloides a Middle Jurassic age is suggested for this biozones. One Assemblage zone, Nilssnonia sp. cf. N. bozorga-Klukia exilis Assemblage zone, and three subzones were recognized in this section. Subzones are from base to up 1- Ptilophyllum harrisianum Taxon range zone 2- Elatides thomasii-Sphenobaiera longifolia concurrent range zone and 3- Ginkgoites sp. cf. G. parasingularis - Coniopteris hymenophylloides concurrent range zone. This Assemblage biozone is comparable to other biozones of the same age in Iran. Furthermore, relative abundances of taxa was studied and relative abundances of order Nilssoniales, Ginkgoales, Filicales, and Equisetales were 44.44%, 20.64%, 15.87, and 7.94% respectively. It is noteworthy that variety and relative abundance of the species of Genus Nilssonia (six species and 42.27%) and species of Order Ginkgoales (two taxa and 17.46%), as a macrophyllous cycadophyte, was high within the Calshaneh area. Therefore, a humid subtropical to temperate climate is considered for this period of time in this area.