جستجوی مقالات مرتبط با کلیدواژه "دونین میانی" در نشریات گروه "زمین شناسی"

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.

In this research, a section of carbonate rocks of Bahram Formation in east of Ozbak-kuh village, located in north of Tabas city (Southern Khorasan Province), has been measured and studied. This sequence generally has been made of gray to bluish gray colored, thin to thick bedded limestones. Based on the filed and petrographic studies, totally 11 different carbonate microfacies have been identified. These microfacies have been deposited in five facies belts, including tidal flat, lagoon, tidal channel, shoal and open marine belts within a ramp-type carbonate platform. The most significant diagenetic processes which have been observed in Bahram Formation samples are micritization, compaction, neomorphism, cementation, dolomitization, dissolution and cavity fillings. Geochemical studies of elemental analysis of the samples show that the formation time condition of these deposits has been similar to the present temperate regions. Furthermore, these carbonate rocks have been situated in an open-system geochemically. In addition, petrographic evidences combined to the geochemical data revealed that the studied carbonate rocks have been influenced by marine, meteoric and burial diagenesis and also the effect of phreatic-meteoric diagenesis on these deposits has been more significant than the others.

Devonian corals of Bahram Formation in Cheshme Shir area (South of Osbak-kuh) were studied. In the study section, more than 18 species belonging to 10 genera of Rugose corals were identified. The following genera are: Sinodisphyllum, Spinophyllum, Glossophyllum, Acantophyllum, Temnophyllum, Cystihexagonaria, Marisastrum, Hexagonaria, Ceratophyllum and Pseudozaphrentis, which suggest an Upper Givetian-Frasnian age.According to coral morphology, 2 assemblages were distinguished. The first assemblage occurs in the reef areas and is composed of colonial corals forming patch reefs. The second assemblage is composed of dissepimented corals of median size that occurs in shallow water of an open shelf.