SEDIMENTARY FACIES
Year:2010 | Volume:3 | Issue:1|Papers Count:8

South west of Qayen area is one of the platform basins in the Thethys, which contains microfossils such as orbitolinids. Six species belonging to the genera Praeorbitolina, Palorbitolina, Palorbitolinoides, Mesorbitolina, and Conicorbitolina were identified in Qomenjan area. Based on orbitolinids species, two biozones including Mesorbitolina texana and Palorbitolina lenticularis are suggested. The age of sediments is attributed to Late Baremian -Early Cenomanian and Early Aptian–Early Cenomanian. Based on occurrence and stratigraphic distributions of orbitolinids in the Qomenjan section and comparison with Cherchi and Schroeder's evolutionary model, Praeorbitolina cormyi–Conicorbitolina cuvilleri and Palorbitolina lenticularis- Conicorbitolina cuvilleri phylogenetic lineage is suggested for the Qomenjan area herein. The most important changes in the phylogenetic lineage, represented in our research, are as follows: the change in the position of embryonic apparatus, the shape of the protoconch and subembryonic zone, increase in the size of embryonic apparatus and protoconch, increase in the number of subembryonic and deuteroconch partition.

Petrographic, major and trace element composition of sandstones from Ordovician Rahdar Mountain have been investigated to determine their provenance, tectonic setting and paleo-weathering condition in the source area. Petrographically and geochemically, these sandstones are classified as quartzarenite and sublitharenite. All sandstone samples are enriched in quartz but poor in feldspar and rock fragments. These sandstones are commonly well cemented by silica (overgrowth or syntaxial cements) and rarely by carbonate, hematite and clay materials. The petrographic data suggest a recycled orogen and craton source terrain. Geochemical data suggest that these sediments were derived from quartzose sedimentary provenance (older sandstones). The mean CIW values (chemical index of weathering) for these sandstones are about 91 that suggest intense-weathering condition in source area. Humid climate prevailed during deposition of these sandstones.

Barremian-Early Aptian Tirgan Formation has an excellent northwest- southeast trending exposure in eastern part of the Kopet-Dagh Basin. This formation consists mainly of oolitic and Orbitolina limestone and shale interbeds. It is conformably overlain and underlain by the Shurijeh and Sarcheshmeh Formations respectively. In order to recognize the sedimentary facies and study sequence stratigraphy, two stratigraphic sections were measured and sampled in northeastern and southwestern flanks of Khor syncline. The results of this study revealed that these sediments are deposited in shallow carbonate ramp within 4 facies belt including open marine, bar, lagoon and tidal flat. Sequence stratigraphic analysis led to identification of 2 third-order depositional sequences, bounded by type 2 sequence boundaries. Interpreted sea level curve in the Khor synclinal area can be relatively correlated with global curves during Barremian-Aptian time.

The Abtalkh Formation is one of the sedimentary units of the Kopet-Dagh Basin that has been studied based on palinomorphs, nannoplankton fossils and foraminifers several times up to now. The studied section with geographical coordinates N59o 46' 54" and E36o 55' 42", is located in 135 Kilometers along the Mashhad-Kalat road, near the Bafarji Village. It is mainly composed of blue-gray shale with layers of marls. Its lower boundary with Abderaz Formation and upper boundary with Neyzar Formation is conformable and it is about 980m thick. In order to study the Abtalkh Formation Ostracods, 46 samples have been taken. For separating Ostracoda from other sediment, samples were washed and then were studied by SEM. 54 species of 25 genera of Ostracods are identified and most of them are from Platycopidae and Podocopidae family. According to them, age of Abtalkh Formation is Santonian-Masterishtian. Also, it is interpreted that this formation was deposited in a oxic shallow marine environment with warm climatic conditions.

In order to identify microfacies of the Sarvak Formation in Firozabad area, 137 samples, considering microfacial changes and lithological properties, were collected and studied. 13 microfacies and 3 depositional sequences, bounded by SB2 boundaries, in these sequences were identified. Microfacies analysis and field observations revealed that the upper and lower parts of Sarvak Formation in study area have been deposited in 2 relatively different environments; sedimentation in the lower part has taken palace in an open shelf, while upper part deposited in a carbonated ramp setting.  Sequence 1 (Late Albian age) includes a set of deepening TST parasequences and a set of shallow HST parasequences that are concurrent with K110 sequence in Arabic plate. Sequence 2 (Late Albian-Early Cenomanian age) includes a set of deepening TST parasequences and a set of shallow HST parasequences that are concurrent with K120 sequence in Arabic plate. Finally, Sequence 3 in (middle-late Senomanian age) includes a set of deepening TST parasequences and a set of shallow HST parasequences that are concurrent with K130 sequence in Arabic plate.

Sedimentrary succession of Cenomanian-Turonian boundary at Shurab section is located in east Kopet-Dagh and consists of upper most layers of Aitamir Formation and lower most layers of Abderaz Formation.The lithology of this succession is mainly composed of green shale and glauconitic sandstone and alternating of grey to white shale and marl. Based on biostratigraphic studies of Foraminifera in this section, four biozones have been recognized:1- Rotalipora reicheli Total Range Zone (Late Cenomanian)2- Rotalipora cushmani Total Range Zone (Late Cenomanian)3- Whiteinella  archaeocretacea Partial Range Zone (Cenomanian-Turonian)4- Marginotruncana  schneegansi Interval Zone (Middle  Turonian).In this study, the boundary between Aitamir Formation with Abderaz Formation considered to be within Whiteinella archaeocretacea zone. In upper part of this biozone, an intense decrease in fauna can be seen. This decreasing can be considered to be as result of biotic and paleoecologic changes that can be related to global events during this time. Therefore, most probable part of B can be considered as the base of Abderaz Formation and C/T boundary is located in this part.

Three assemblage zones (Rupelian-Chattian age) have been identified by distribution of large benthic foraminifera in the study area. The Firozabad section was compared with Chahangir (Rupelian) and Dil (Chattian-Burdigalian) sections. Early Oligocene carbonate (Firozabad and Chahangir sections) was deposited at the edge of Zagros foreland basin whereas towards the center of the basin, the Pabdeh Formation continued to accumulate. The Miocene part of the Asmari Formation, as seen in the Dil section, is absent in Firozabad and Chahangir sections. This suggests that Oligocene sea level fall restricted marine deposition and Asmari Formation replaced laterally by the Gachsaran Formation. The biotic associations and palaeolatitudinal reconstructions suggest that carbonate sedimentation took place in tropical waters under oligotrophic conditions in the study area, a favorable environment for photozoan assemblages. The dominance of heterozoan assemblages seems to be related to the palaeoecology and evolution of zooaxanthellate corals, which were not able to construct reefs against the destructive waves; actually they lived in the meso-oligotrophic zone.

A detailed study of Rhizocorallium assemblages from the Late Santonian chalky limestone in the Kopet Dagh basin led to identification of Glossifungites horizon in uppermost part of the Abderaz succession. Rhizocorallium jenense shows straight, short and slightly curved U shape structure with almost protrusive spreiten. Scratches are very abundant, which are less continuous and non parallel on the marginal tubes and shorter scratches are visible on the spreiten. Rhizocorallium irregulare is scarce. It exhibits the form of straight to slightly sinus U shapes with scratch marks on the surface. The higher dominance of R.jenense than R.irregulare reflects high energy condition of the sea water. The presence of abundant Rhizocorallium traces in firm ground consistency reveal Glossifungites ichnofacies. The occurrence of dominant suspension feeding R. jenense and scarce deposit feeding R. irregulare reflects a high concentration of nutrients in the water column. Scratch mark casts suggest crustaceans as trace makers with some appendages in continuous contact with the substrate.