جستجوی مقالات مرتبط با کلیدواژه "جغرافیای دیرینه" در نشریات گروه "زمین شناسی"

The palynofloras of the Shemshak (upper part) and Dalichai (lower part) formations, Deh-Soufian, north of Semnan include 57 species of spores (34 genera), 25 species of pollen (13 genera), and 14 species of dinoflagellate cysts (8 genera). Based on the stratigraphic distribution of miospores, the Klukisporites variegatus-Striatella spp.-Contignisporites burgeri assemblage biozone (Middle Jurassic) is introduced. The co-occurrence in the palynofloras examined of Contignisporites burgeri and Striatella seebergensis allows for the Bajocian–Bathonian boundary to be located 75m below the boundary of Shemshak-Dalichai rock units. The presence in the strata below this chronostratigraphic horizon of Mendicodinium scabratum (Bajocian) supports this age assignment. The boundary of Shemshak and Dalichai (with basal sandstone) formations is discontinuous. While no significant change in the composition of miospores is discernible in palynofloras retrieved from the above rock units, the relative abundance of such terrestrial palynomorphs varies drastically. This might be attributed to side effects of the mid-Cimmerian Orogeny i.e., significant change in sedimentation rate resulting in change from the deltaic to marine depositional settings. It should be noted that 45 m below the boundary of the two rock units 24 miospore species exit whereas 11 different ones appear 20 m above that boundary. Notable correspondence of the Iranian Middle Jurassic palynofloras across the Alborz Ranges signifies to a rather significant coastal vegetation similarity of the time in the area. Whereas both Gondwanan and Eurasian miospore key species occur in the palynofloras examined those particularly endemic to the southern landmasses are missing from the palynofloras studied. Palynological evidence seemingly confirms the location of the Alborz Ranges on the northwestern margin of the Neotethys Ocean in the Middle Jurassic. The available paleontological data (ammonites and dinoflagellate cysts) apparently support this interpretation.

The Sibzar Formation (Middle Devonian) is located about 30 Km east of Neyshabur city and is composed of lower dolostone with intercalations of sandstone, middle basaltic sill, and upper dolostone to limy dolostone, with a thickness of 334 m. The sandstone intercalations of this formation have fine to sometimes medium-grained quartz, with low values of feldspar, and rock fragment. According to petrography studies, sandstones of the Sibzar Formation were mostly quartzarenite and subarkose, however some samples plotted on litharenite. From the perspective of geochemical studies, the most abundant major oxides in these sandstones are SiO2 and Al2O3 with an average concentration of 77.8 and 8.3 (wt%), respectively. According to the ratios of SiO2/Al2O3 and Na2O/K2O and other amounts of major oxides and trace elements, such as CaO, Fe2O3, TiO2, Zr, and V, these sandstones have medium to high values of quartz; therefore have a quartzose sedimentary provenance, and are located in an intracratonic basin on the passive continental margin. Source rocks of these sandstones are intermediate to felsic igneous rock and have been affected by moderate to extreme degrees of chemical weathering. According to the evidence, the source of siliciclastic sediments for Sibzar Formation sandstones may have been Arabian and African plates.

Lake basins have spatially and temporally index patterns and represent valuable maps of physical and chemical history. According to this point, that direct survey of climate changes has only been available as machinery in the past few decades, so paleo climatic changes indexes can give a better understanding of climate changes in the past era and its causes. Lake sediments have an extraordinary potantial to record climatic and environmental events with high sensitivity and resolution amounts. Therefore, lake sediments can be considered as regions for detecting paleo environments. Chegarman Lake formed in the northeast of Khuzestan province on the hillsides of folded Zagros and under the Izeh unit in a pit. This study has been tried to by Sedimentary geochemistry analyze, Elemental ratios, identification of sedimentary facies, and identification of sedimentary environments and changes of these environments set to the reformation the climatic and environmental conditions of Chegarman wetland in the Late Holocene.

The steps of this research include book studies, field works, laboratory works, statistical data processing and then interpretation and conclusion. To investigate the paleoclimate and conditions of the sedimentary environment, one intact sediment cores with a maximum depth of 7.2 m collected with using Auger corer. 21sediment samples were selected for elemental analysis with ICP-OES and XRF (X-Ray fluorescence). Statistical parameters, histogram diagrams of element frequency and cluster analysis were performed to determine the relationships between the elements in SPSS software. In this study, elemental ratios of V/Cr, Mn/Al, K/Al, Rb/Al, Si/Fe, Ti/K were used to reconstruction of the paleo climatic and environmental conditions. Which led to the study of the amount of detrital material, the severity of weathering and erosion in the catchment and fluctuations in the water level of the lake.

In the studied core, 5 major sedimentary facies identified. Facies often contains clay, silt, gravel, and interstitial sediments along with vegetation and shell fossils which in most cases frequency and interference in the mentioned sediments are often seen. Changes were in particle size between clay to gravel due to process type changes, environment's energy and river input's amounts and sediment color variations concerning sedimentation conditions, presence of organic matter, pH amount, salinity and temperature variations, and drought occurrence, throughout the cores. Two wetland sedimentary and alluvial environments identified. According to the Chronology, the average sedimentation rate in the Chegarman wetland is 1.4 mm / year, which this amount is different for various depths (Darvishi et al., 2022). Due to the type of sediments, the high level water period of the Chegarman wetland has been established between 1250 and 150 years ago. The study of the facies sediments of the wetland bed shows that climate change occurred gradually (not suddenly) about 1250 years ago and this trend has taken at least 300 years. The relatively dry and cold period lasted about 1850 years from 3100 years to 1250 years ago and is confirmed with the reported dry periods in the Maharlou Lake about 1800 years ago, in the Mir Abad Lake about 1500 years ago and in the Urmia Lake between 2500 and 1500 years ago. From 4000 years ago to about 3100 years ago, stable and slow wetland conditions was prevailed. The conditions for creating a peatland environment have been provided in the wetland and were associated with increasing the amount of organic matter, vegetation and shell fossils and consequently increasing sedimentation rate. This period which lasted for about 900 years, has been accompanied by extremely humid conditions, high groundwater level and activity of springs in the region. The existence of light brown mud which has the age ranges from 4900 to 4000 years ago, indicates a dry period (900 years). This dry period shows suitable approximate conformity with a dry event of 4200 years ago in the northern hemisphere. This period in the studied region appeared with at least 100 years of temporal delay. After this dry period, we see again a gray sequence with plant and shell fossils that continue to a depth of 7 meters (about 5000 years ago). At depths of 7 to 7.2 m, the existence of brown-muddy sediments along with coarse-grained sediment in size of gravel, pebble and rubble, strengthen the possibility of very strong flood in the region that sent the coarse-grained sediments of around formation to bed of the wetland or maybe has hit to the area of wetland sediments bed in the region.

According to the results of surveys in the Chegarman wetland, at least 2 high water periods with humid climate and at least 2 dry periods during the Late Holocene were identified. Dry periods appear more suddenly and with more intensity and shorter duration than humid periods in the region and gradually lead to from very dry periods to semi-arid, semi-humid, and eventually humid periods.

Paleocene-Miocene sediments in the central part of the Gorgan Plain were studied to evaluate lithostratigraphy, biostratigraphy and reconstruction of the paleogeography of the South Caspian Basin (SCB). Based on nannofossils distribution in the studied interval, the following points were identified: Foram layers equivalent sediments have Thanetian –Eocene age and Upper Maykop- Diatom equivalent sediments have Middle-Late Miocene age. Evaluation of the nannofossils distribution in the studied interval has clarified a major unconformity between lower Paleocene-Eocene sediments and the upper Middle-late Miocene sediments. This unconformity is coincident with collisional event of the Afro-Arabia and Eurasia Plates and rising of the surrounding mountain ranges around the Caspian Basin including Alborz and Kopet-Dagh Mountains. Nannofossils distribution of the studied succession displays that the SCB was connected to the Black Sea and Mediterranean Basin in the lower Middle Miocene and was isolated in the Middle-Late Miocene. The interpreted east-west running seismic lines displays a clear increase in the sediment thickness and accommodation space from eastern to western part of the Gorgan Plain. The higher available accommodation space from east to west of the Gorgan Plain led to a delay in the subaerial exposure in the western part of the Gorgan Plain due to rising of the Kopet-Dagh and Alborz Ranges. This interpretation is consistent with the lack of the Paleocene-Eocene sediments and presence of the Cretaceous sediments below the major unconformity in the eastern part of the Gorgan Plain and presence of the Paleocene-Eocene sediment below the unconformity in the studied area.   

In order to study the biostratigraphy, paleoecology and paleogeography of Qom formation ostracods, the Gonjan stratigraphical section was sampled and studied. 36 species and 23 genera of ostracods from Gonjan stratigraphical section have been studied and identified, which belong to two podocopid and platycopid orders. Based on the diversity of the identified foraminifera in this section and abundance of lepidocyclinids, the suggested age of  Qom Formation deposits in Gonjan section are Rupelian?-Chattian. The presence and abundance of ostracoda with larger benthic foraminifera, Coralline red algae and Corals indicates the oligotrophic to mesotrophic conditions in the tropical to subtropical environments, with normal salinity and oxygen richness during the deposition of the Qom Formation.The Qom Formation consisting of limestones, marine marls, gypsum and siliciclastics was deposited at the last sea transgression in the Oligo-miocene age, with northwest-southeast trending in three basins of Sanandaj-Sirjan fore arc, Urumieh-Dokhtar intra arc and Central Iran back arc.

In order to conduct this research, based on lithology of layers, 45 marly and calcareous samples were collected.

Qom Formation in the study area consists of five limestone and marl informal members. These informal members are lower limestone, lower marl, middle limestone, upper marl, and upper limestone. 36 species and 23 genera of ostracoda from Gonjan stratigraphical section have been studied and identified:Cytherella sp., Cytherella dissimilis, Cytherella strangulat, Cytherelloidea sp., Paracypris sp.A, Paracypris sp.B, Krithe sp.A, Krithe sp.B,  Trachyleberis sp., Ambostracon sp., Xestoleberis sp., Dentokrithe comma, Krithe trinidadensis, Loxocorniculum sp., Acanthocythereis sp., Buntonia sp., Loxoconcha ? sp., Cyprideis sp.A, Cyprideis sp.B, Actinocythereis sp., Actinocythereis iragensis, Legitimocythere sp., Parakrithe sp.A,  Parakrithe sp.B, Cytheridea josephina, Cytheridea eggenburgensis, Ruggieria sp., Bairdia sp.A, Bairdia sp.B, Bairdia rafidainensis, Loxoconcha psedohamrinensis, Loxoconcha hamrinensis, Aurila sp., Costa tricostata, Mutilus sp., Ambocytherea sp.

The presence and abundance of ostracoda with larger benthic foraminifera, Coralline red algae and Corals indicates the oligotrophic to mesotrophic conditions in the tropical to subtropical environments with normal salinity and oxygen richness during the deposition of the Qom Formation. Studies on the Qom Formation ostracoda in Gonjan section, indicate the similarity of these samples with other areas of Iran, India, north and north-east of Iraq, Turkey, eastern Mediterranean and North Africa. Thus it represents a seaway connection between these areas during Oligo-Miocene, which is considered as the Tethyan Seaway. The Qom Formation had a communicative role between Eastern Tethys and the Western Tethys region in the Iranian Plate.

Lake basins have spatially and temporally index patterns and represent valuable maps of physical and chemical history. According to this point, that direct survey of climate changes has only been available as machinery in the past few decades, so past’s climate changes indexes can give a better understanding of climate changes in the past era and its causes. Of these, lake sediments have an extraordinary ability to record climatic and environmental events with high sensitivity and resolution and very high- resolution amounts. Therefore, lake sediments can be considered as regions for detecting paleontological environments. Abzalou Lake formed in the northeast of Khuzestan province on the hillsides of folded Zagros and under the Izeh unit in a pit. This study has been tried to by sedimentary examination, identification of sedimentary facies, identification of sedimentary environments and changes of these environments set to the reformation the climatic and environmental conditions of Abzalou wetland in the end Holocene.

The steps of this research include book studies, field works, laboratory works, data processing and then interpretation and conclusion. To investigate the paleontological climate and conditions of the sedimentary environment, 8 intact sediment cores with a maximum depth of 7.5 m and total of 35 m collected with using Auger corer. In this regard, tried to the position of the core be in a way that makes it possible to collect the maximum thickness of wetland bed’s sediments, give maximum coverage to wetland’s sedimentary environments and at least collect one sample in each homogenization work unit as much as possible. Based on field observations, core’s description and lab results, the cores separated into sedimentary units and their stratigraphy columns drew and determined under each sedimentary unit. Also, 3 sediment samples sent to ETH Zürich Institute of geology to age/ date measurement analyzing by the radiocarbon method of organic matters.

in the studied cores, 8 major sedimentary facies identified. The facies often contains clay, silt, sand, gravel, and interstitial sediments along with vegetation and shell fossils which in most cases frequency and interference in the mentioned sediments are often seen. Changes were in particle size between clay to gravel due to process type changes, environmentchr('39')s energy and river inputchr('39')s amounts and sediment color variations concerning sedimentation conditions, presence of organic matter, pH amount, salinity and temperature variations, and drought occurrence, throughout the cores. Three wetland sedimentary, river and alluvial environments identified. According to the age measurement, the average sedimentation rate in the Abzalou wetland is 1.22 mm / year, which this amount is different for various depths. Due to the type of sediments, the Abzalou wetland has undergone human changes for about 600 - 500 years ago. The high water period of the Abzalou wetland has been established between 1800 and 500 years ago. The study of the facies bed sediments of the wetland shows that climate change occurred gradually (not suddenly) about 1800 years ago and this trend has taken at least 200 years. The relatively dry period lasted about 700 years from 2500 years to 1800 years ago and is confirmed with the reported dry periods in the Maharlou lake about 1800 years ago, in the Mir Abad lake about 1500 years ago and in the Urmia lake between 2500 and 1500 years ago. From 4400 years ago to about 2500 years ago, stable and slow wetland conditions prevailed. The conditions for creating a peatland environment have been provided in the wetland and were associated with increasing the amount of organic matter, vegetation and shell fossils and consequently increasing sedimentation rate. This period which lasted for about 1900 years, has been accompanied by extremely humid conditions, high groundwater level and activity of springs feeder in the region. The existence of light brown mud which has the age ranges from 4400 to 4700 years ago, indicates a dry period (300 years). This dry period shows suitable approximate conformity with a dry event of 4200 years ago in the northern hemisphere. This period in the studied region appeared with at least 200 years of temporal delay. After this dry period, we see again a gray sequence with plant and shell fossils that continue to a depth of 7 meters (about 5300 years ago). At depths of 7 to 7.5 m, the existence of brown-muddy sediments along with coarse-grained sediment in size of gravel, pebble and rubble, strengthen the possibility of very strong flood in the region that sent the coarse-grained sediments of around formation to bed of the wetland or maybe has hit to the area of wetland sediments bed in the region.

According to the results of surveys in the Abzalou wetland, at least 3 high water periods with humid climate and at least 2 dry periods during the Late Holocene were identified.  Dry periods appear more suddenly and with more intensity and shorter duration than humid periods in the region and gradually lead to from very dry periods to semi-arid, semi-humid, and eventually humid periods.

To assess biogeographic affinity of the Late Cambrian (Furongian) and the Early Ordovician (Late Tremadocian) faunas of Eastern Alborz، a multivariate cluster analysis using the pair-group Raup-Crick similarity index has been applied. The Late Cambrian fauna of Alborz shows clear Gondwanan signatures with the closest affinities to the contemporaneous faunas of temperate latitude peri-Gondwana (Central Iran، Oaxaquia terrane in southern Mexico) and in slightly lesser degree with low latitude faunas of the Karatau Naryn microplate (southern Kazakhstan) and the Australasian segment of Gondwana. Thus، outcome of the analysis suggests that by the second half of the Furongian، which was the time of proliferation of the early euconodonts، the Alborz terrane approached and probably entered temperate latitudes. In the Late Tremadocian links of Alborz conodont faunas with temperate latitudes faunas of Gondwana and Baltica became more evident. Assessment of the Cambrian (Furongian) and Early Ordovician (Late Tremadocian) conodont faunas of the Alborz are in favor of its isolated position at some distance the western Gondwanan coast.