نشریه رخساره های رسوبی
سال ششم شماره 2 (پیاپی 11، زمستان 1392)

Late Oligocene deposits at the Tanbour section in the Central Iran are shallow water carbonates that can be correlated with the upper parts of the “C member” of the Qom Formation. In the study area, biostratigraphy of the larger benthic foraminifera indicate the age of Chattian stage for these deposits. Based on analysis of the larger benthic foraminiferal assemblages and microfacies, these sediments were deposited in three major depositional environments including open marine, barrier reef and lagoonal settings. Therefore, for the studied section, a carbonate platform in the homoclinal ramp has been suggested. In the inner ramp, the most abundant microfacies are wackestone-packstone with imperforated foraminifera. The middle ramp is represented by packstone-grainstone to floatstone with a diverse assemblage of larger benthic foraminifera with perforate wall, red algae, coral, and echinoids.

Clay minerals are important for interpretation and understanding of burial diegenesis in mudrocks and cementation history of sandstones during diagenesis. The clay mineral from 11 samples from the Middle Jurassic, Kashafrud Formation in east Kopet Dagh Basin, analyized by X-ray diffraction. The identified clay minerals in these samples include kaolinite, illite, vermiculite, motmorellonite, mixed layer montmorillonie/illite and mica/illite. Also 5 thin sections of sandstones were analyzed by microprobe to identify clay minerals in cements. Kaolinite, illite and chlorite are the most abundant clay minerals in sandstones. Additionaly, high abundance of stable clay minerals, such as illite and chlorite, and small amounts of kaolinite and mixed layer clay minerals in kashafrud deposits, particularly in mudrocks, interpreted that had formed as result of high temperature during deep burial diagenesis.

The Tirgan Formation (Barremian - Aptian) is well exposed in the Kopet Dagh in northeast Iran. Three stratigraphic sections in Esfidan, Dadanlu and Radkan were measured and sampled. Their thicknesses are 207.8, 498.2 and 238 metres, respectively. Based on study of 260 thin sections, 17 carbonate and 4 clastic lithofacies have been identified. Carbonate lithfacies are deposited in a carbonate ramp in open marine, shoal, lagoon and tidal flat environments. Sea level changes during Barremian - Aptian time led to formation of two large scale depositional sequences in all sections. Base and top boundaries of the Tirgan supersequence are type Two. We believe that this study can help in reconstruction of tectonic history of the area during Lower Cretaceous time.

The Khar-Rud river drainage basin (area=5500 Km2) is located in south of Qazvin Province. The maximum and minimum basin elevations are 1680 to 1200 m (above sea level) respectively; it occurs in the Central Iran structural zone. In this study, the style of river was determined based on morphological and sedimentological parameters. The quantitative morphological parameters include: sinuosity, width/depth ratio, entrenchment ratio of the bed, channel slope, the central angle of the meander and braiding ratio. In addition, some descriptive parameters such as the meander type were also discussed. In the western part of the study area (before entrance of Shurabeh tributary), this river is of F and B types (both single channel; sinuosity>1.5) mainly with a sandy bed (dominant facies: Sh, St and Sr). In the central part, the channel pattern changes to type D (Av. braiding index=3.73; gravelly bed; dominant facies: Gms and Gm). In the eastern part, this river is single channel (type C, dominant facies Gms, St and Sh). Textural parameters of single channel types (F, B and C) are similar, different from that of multi channel type (D).

In order to study the fossil contents of the Ilam Formation for biostratigraphical purposes in this study, a 172 meters thick section was measured and sampled at Shah Nakhjir Anticline. This sequence is mainly composed of marly limestone with interbed of shally marls. The lower contact of the formation with Surgah Formation and upper contact with Gurpi Formation are conformable. Thirty eight species belong to 16 genera were identified and 3 biozones were determined. These are: 1- Dicarinella asymetrica Total Range Zone. 2- Globotruncanita elevata Partial Range Zone. 3- Globotruncana ventricosa Interval Zone. Based on these assemblages, the age of this formation is Late Santonian – Middle Campanian. Biozones identified in this section compared with type section in Tang-e- Garab shows that the biozones and ages are similar in these two sections, and there are only differences in thickness of biozones.

The basal unit of the Pabdeh Formation at Darreh-Shahr section is composed of 153m and in type section 102m consists of shale and marl of purple to green and gray in color. These beds gradually change to shale and argillaceous limestone at the top of the sections. In order to study palaeobathymetry and palaeoecology of these beds, we used palynological (palynofacies) evidences. Three Palynofacies types (VI: Proximal suboxic – anoxic shelf, VIII: Distal dysoxic-anoxic shelf, IX: Distal suboxic-anoxic basin) in Darreh-Shahr and three Palynofacies types (VI: Proximal suboxic – anoxic shelf, II: Marginal dysoxic – anoxic basin, IX: Distal suboxic-anoxic basin) in type section of the Pabdeh Formation have been determined that propose a proximal - distal shelf and distal basin for Darreh-Shahr and proximal shelf and marginal to distal basin for type section as depositional setting. Correlation of palynofacies evidences between these two sections, high abundance of phytoclasts and very low abundance of marine palynomorphs in type section show that purple shale in type section have been deposited in a shallower and more oxygenated environment with more oxidation in type section in comparison with the Darreh-Shahr section.

Eastern Alborz zone contains paleosols and bauxites that demonstrate paleoclimatic changes in addition to widely geological events in this zone. The Khoshyeilagh region is located about 50 km southeast of Azadshahr. Stratigraphic sequence in this region contains calcrete paleosols in Devonian sandstones of the Padeha Formation and a karst bauxite horizon along the contact zone of Permian limestones and Triassic dolomitic limestones. Textural and mineralogical evidences indicate the formation of paleosols in arid to semi-arid climate and the formation of bauxites in warm and humid climate. Also, range of weathering indices point out increasing of chemical weathering from Devonian paleosols to Permo-Triassic bauxites. Geochemical data combined with the geological evidences suggest that the basaltic rocks of the Soltanmeidan Formation are the parent rock of the studied paleosols and bauxites. Combination of textural, mineralogical and geochemical data show that paleosol formed in low temperature, low rate of precipitation, and low chemical weathering. In contrast, high temperature and high precipitation rate led to formation of chemical weathering during bauxites formation. Devonian paleosols of the Padeha formation and Permo-Triassic bauxites of the study area are good indicators for recognition of paleoclimatic conditions in this area.

In current study, some surface sediment samples of the Oman Sea (Sirik area) were prepared and photographed for study of calcareous nannoplanktons. For the first time, 17 genera and 29 species introduced from this area. Some nannoplanktons are the dominant taxa and some species are the rare taxa such as Florisphaera profunda, Gephyrocapsa ericsonii and Braarudosphaera bigelowii. In this study, we also observed many reworked species, and mostly of them belong to Neogene age. Relationship between 4 elements Zn, Cu, Ba and Si, distribution of nannoplankton and variational changes are investigated in this area. The Zn, Cu and Ba are directly related to productivity of nannoplankton and the ratio of them were high in the relatively nearshore of Sirik area. The Si prevents growth and distribution of nannoplankton, therefore the ratio of this elementt in relatively nearshore of Sirik area has negative relationship to abundances of nannoplankton. Thus, we can concluded that the Oman Sea floor, especially in areas close to the nearshore is a suitable environmental for growth and distribution of nannoplankton and the Oman Sea basin is a marginal sea.