SEDIMENTARY FACIES
Year:2009 | Volume:2 | Issue:1|Papers Count:8

Field Study and petrography of Shirgesht Formation (Ordovician), in Kuh-e Asheghan and Kuh-e Rahdar sections, located in Kalmard block in Cent ral Iranian Zone, led to identification of 4 carbonate microfacies associations and 7 siliciclastic petrofacies. These facies associations in conjunction with trace fossils, such as cruziana and skholithos ichnofacies, as well as the results of calcimetry, XRD analysis and SEM images from fine grained samples (shales), show that these strata have been deposited in barrier island-lagoon depositional system. The trends of interpretated sea level curve, from bottom to the top of the two studied sections, represent relative falling stage in sea level. Moreover, this study shows the effect of regional and local mechanisms in deposition, such as movement of Kalmard basement fault that effected the spatial and temporal variations in sediment supply.

The colonial coral faunas of Tarbur Formation in the Zagros area (SW Iran) were studied in terms of paleontological criteria and faunal relationships with adjacent areas. In total 9 genera in 8 species of Scleractinian corals have been identified through the specimens collected from the rudistid carbonate unites of Semirom section. Common stratigraphic ranges of determinated coral taxa beside the presence of coexistence index fossils, emphasized on the Late Cretaceous age (Maastrichtian) for these layers. The identified types of corals are containing endemic species with regional distribution and pandemic species which have a worldwide expansion. Comparing the present endemic taxa with those are reported from the other parts of eastern Tethyan realm (Arabian platform) such as Turkey, Saudi Arabia, UAE and Oman pointed out on clear faunal interaction between these areas during the Late Cretaceous.

In this survey, in order to sedimentological studies and determine the Caspian Sea level fluctuations by variations in magnetic susceptibilities of the Caspian bottom sediments, 5 cores were taken from southeastern basin of the Caspian Sea. These samples have been analyzed for grain size, total organic carbon content, carbonate content, mineral composition and magnetic susceptibility. The results show a close relation between particle size distribution and MS magnitude due to variation in detrital influx, caused by sea-level fluctuations in different time. This process is reflected in increasing particle size and magnitude of MS simultaneously with sealevel rise and decreasing particle size and magnitude of MS simultaneously with sea-level fall. However such a relationship between magnitude of MS and carbonate and total organic carbon was not observed.

Detrital sedimentary rocks of Agh-darband area with Paleozoic to late Triassic age, which is the only outcrop of Kopet-Dagh basement, consist of three zones. 1- Paleozoic Metamorphic rocks that processed in deep marine of continental slope by turbidity flow. 2- Qara-gheitan formation formed maximum outcrop in Agh-darband area. According to sedimentary structures, this formation formed in debris flow and sheet flood by baraided stream in proximal environment with flowing from North to South. Qara-gheitan’s conglomerates have volcanic sources. Magmatic debris fragments are calk-alkaline granodiorite to tonalite and volcanic debris fragments are high potassic calk-alkaline rhyolite to dacite. Petrological studies show that these rocks are "I- type". These plutonic and volcanic fragments in Qara-gheitan’s conglomerates is due to existence of a magmatic arc in north of Agh-darband which now buried under the young sediments. 3-Sina formation is sequence of shale and sandstone rocks that formed in continental slope basin by classic turbidites. All sandstone rocks of Qara-gheitan and Sina formation are lithic arkose to feldspatic litharenite that originate from dissected to transitional arc. Detrital sedimentary rocks that created in Paleozoic to late Triassic in basement of Kopet-Dagh basin produce in foreland basin that formed in front of magmatic arc.

Sediments of the Qom Formation in the Jazeh area, south of Kashan, are Ropelian- Chatian in age and have a thickness of 299 meters. It consists of medium to thick bedded and massive limestone, shale and marl. They overlie on the Eocene volcanic rocks with unconformity and its upper boundary is covered by alluvium. 230 samples have been collected and thin section was prepared. Based on petrographical studies of depositional textures, and fauna, 10 microfacies have been identified that were deposited in open marine and lagoon subenvironments. Based on microfacies analysis and field observation, Qom Formation in the study area is deposited on an open shelf.

Ferruginous shallow marine limestones of Late Devonian age -mainly Famennian- are widely distributed across central and eastern Iran. The iron, almost hematite, is concentrated in oolitic and skeletal grainstones, skeletal packstones and rarely in lime-mudstones. Hematite occurred as clay coatings on various allochems (ooids, bioclasts and intraclasts), and also as finely dispersed clays within micritic beds. Most grains are concentrically laminated with alternations on ferruginous clays and micro-laminae suggesting the clays were syndepositionally accreted to grain surfaces. The ferruginous beds are commonly associated with ooides and oncoides. The planar to wavy and sometimes discontinuous laminae with individual thickness of 2 to 5 mm resemble oncolitic microbialites. The microbial tubules from laminae show several concentric layer of stepwise growth around a core. Micro-morphological and mineralogical characteristics suggest biogenic factors are essential for the formation of ferruginous grains.

The Shemshak Formation is wellexposed in northern Ghoznavi– Farsian area which is located along Shahrud– Azadshahr road. The thickness of this formation in the studied area is about 1200 meters and consists of sandstones, siltstones and shales with a few coal seams interbeds. The Shemshak Formation disconforably rests on the Elika Formation and it is overlain by the Farsian Formation. In this area, most parts of this formation contain plant fossil including leaves stems and reproductive organs. The identified mega fossils consist of 25 species (12genera) such as Baiera gracilis, Baiera muensteroana, Cladophlebis cf. nebbensis, Cladophlebis denticulate, Cladophlebis raciborski, Clathropteris meniscoides, Ctenozamites iranica, Cycadatites sp., Dictyophyllum sp., Nilssonia cf. acuminate, Nilssonia feriziensis, Nilssonia tenuinervus, Nilssoniopteris vittata, Otozamites eichwaldi, Peterophyllum bavieri, Podozamaites lanceolatus, Pterophyllum braunianum, Pterophyllum tietzei and Scyhtophyllum persicum. The above mentioned plant mega fossils belongs to various plant orders such as Bennetitales, Coniferales, Cycadales, Filicales, Ginkgoales, incertae sides ferns and Peltaspermales. Based on the abovementioned plant fossil taxa, a Late Triassic (Rhetiain) – Middle Jurassic age is suggested for the Shemshak Formation. Based on identified thermophitic plants, such as Dictyophyllum, Otozomites, Pterophyllum and Zamites, it is interpreted that a humidsubtropical climate has been prevailed during deposition of Shemshak Formation in the studied area.

Sanganeh Formation was introduced as one of the Early Cretaceous Formations in the Kopet-Dagh basin. By using polarized microscope, 15 genera and 36 species were identified at Mozduran locality. According to stratigraphic distribution of calcareous nannoplanktons of the studied sections, the first occurrence of index species and fossil assemblages, 4 biozones of Sissingh biozonation (1977) are suggested which are coincide with biozones of CC8- CC11. Based on the mentioned biozones, the Albian/ Early- Middle Turonian age is proposed for the studied section.