نشریه رخساره های رسوبی
سال هشتم شماره 1 (پیاپی 14، تابستان 1394)

Four stratigraphic sections of the Middle Cambrian Fasham Formation sediments exposed in the Central Alborz are described and interpreted on the basis of field observations and facies analysis in order to reconstruct their depositional environments and sequence stratigraphic framework. Relying on the facies characteristics and stratal geometries, the size of the bedforms, the sedimentary structures and the complex arrangement, the erosional base, the predominantly sandy grain size, the significant height of the sandbodies together with the tidal action evidences such as extensive cross-stratified sandstone, tidal bundles, reactivation surfaces and mud drapes, the siliciclastic successions of the Fasham Formation are deposited in a tide-dominated open-mouthed estuarine environment. During the initial inundation of the present-day Alborz region in the early Middle Cambrian, siliciclastic deposits formed in open-mouthed estuarine. Basal sequence of Middle Cambrian sediments is composed of a transgressive systems tract (TST) comprising the basal siliciclastic-dominated succession of the Fasham Formation and a carbonate-dominated highstand systems tract (HST) with in the lower part of the Deh-Sufiyan Formation. It records the drowning of the terrigeneous sources and subsequent establishment of the carbonate system. Based on sequential infilling model for open estuaries during transgression and initial flooding, tidal flats were developed along the margins and sandy dunes formed along the central part of the estuary and their lower surface was interpreted as a subaerial unconformity formed due to lowstand erosion. The last stage of infilling involves the development of sand bars that gradually migrated towards the open sea until they wholly occupy the mouth of the open estuary.

Chehel-Kaman Formation is one of the Paleogene formations of Kopet-Dagh basin in east of Iran that conformably overlies and underlies the siliciclastic Pesteligh and Khangiran Formations, respectively. The lithology of it mainly consists of limestone and dolomite with interbeds of sand, shale and evaporate sediments. Two Paleosol in Unit 2 and 6 were identified and studied. Based on biostratigraphic studies, 33 genere and 32 species of Larger benthic foraminifera have been identified that according to recognition of Laffitteina, Miscellanea and Rotalid the age of Chehel-Kaman formation is Selandian to Ypresian (?). The Paleocene/Eocene boundary has been probably recorded as a thin red paleosol horizon (~10-15 cm). Identified Trace fossils are belonging to Skolithos and Cruziana ichnofacies that indicates this settings have been deposited in a carbonate Ramp system such as semi consolidated substrate to sublittoral carbonate zone.

In this research biostratigraphy and paleoecology of the Asmari Formation in Kooh-Siah anticline (north of Dehdasht) has been studied. The thickness of the Asmari Formation in this section is 214.1m and consists of thick, medium and thin bedded limestones. This study led to identification 23 genera and 20 species of benthic foraminifera. Based on the distribution of foraminifera 3 assemblage zones with ages upper Chattian, Aquitanian and Burdigalian were recognized. Faunal associations indicate that the Asmri Formation in the study area was deposited under tropical and oligotrophic conditions that it is accordance with photozoanassociation. The zooaxanthellate corals were not able to build three-dimensional wave-resistant structures up to sea level, therefore, heterozoan association were developed. Faunal association shows that the Asmari Formation was deposited in water with variety of salinity (normal to hyper-saline conditions).

The Pabdeh Formation is one of the major rock units in the Zagros Basin is the source rock. Biostratigraphic study Pabdeh formation led to the identification of 62 species and 23 genera of calcareous nannofossils. According to the first occurrence and Last Occurance of index species and fossil assemblages, Fourteen biozones of Martini (1971) (Tribrachiatus contortus Zone (NP10), Discoaster binodosus Zone (NP11), Tribrachiatus orthostylus Zone (NP12), Discoaster lodoensis Zone (NP13), Discoaster sublodoensis Zone (NP14), Nannotetrina fulgens Zone (NP15), Discoaster tani nodifer Zone (NP16), Discoaster saipanensis Zone (NP17), Chiasmolithus oamaruensis Zone (NP18), Isthmolithus recurvus Zone (NP19), Sphenolithus pseudoradians Zone (NP20), Ericsonia subdisticha Zone (NP21), Helicosphaera reticulata Zone (NP22), Sphenolithus predistentus Zone (NP23)) equivalent to biozones CP9a- CP17 of Okada and Bukry (1980) have been recognized. As a result of this study and based on the identified zones, age of the pabdeh formation in the North of Dezful Embayment is Ypresian to Rupelian.

Tash and Astaneh bauxite deposits are located about 40 km north of Shahroud and 30 km northwest of Damghan, respectively. The Tash deposit has been developed as a stratiform horizon along the contact zone of Ruteh limestones and Elika dolomitic limestones, whereas the Astaneh deposit located along the contact zone of Triassic dolomitic limestones and Jurassic shales and sandstones. Pelitomorphic and microgranular textures are the most abundant matrix textural elements and ooids, pisoids and pellets are the main separated textural elements in these deposits. Textural analyses indicate both allochthonous and autochtonous origins for both deposits. Based on petrographic and facies evidences the Tash and Astaneh deposits can be divided into five and three distinct units, respectively. The Tash deposit is mainly composed of diaspore, anatase, kaolinite, hematite, goethite and berthierine, whereas the quartz, hematite, goethite kaolinite and anatas are the most abundant minerals in Astaneh. Combination of stratigraphic, petrographic and facies data show that Tash and Astaneh deposits formed in three and two main stages, respectively. First, bauxite materials and clay minerals were developed as authigenic bauxitization processes of the parent rock. During the second stage, these materials were transported to karst depressions, where they accumulated as a relatively thick bauxite horizon. Finally, during the third stage which has occurred only in Tash deposit, the ore upgraded by in situ leaching and desilicification. Based on geological, mineralogical and textural evidences both deposits can be grouped as karst bauxite type.

The Maestrichtian rudist-bearing Tarbur Formation with abundant large foraminifera was deposited in Zagros basin. For studying of the Tarbur Formation in the Fars province, the deposits of Tarbur Formation in 5 Km to south west of Semirom was sampled and studied. The Semirom area is located in the east of Zagros Mountain; in the interior Fars of the High Zagros. In the studied area the Tarbur Formation with 462 m thickness, composed of carbonate and terrigenous rocks. Tarbur Formation has a gradual and conformable contact with red shale of red detritus unit (S2) and the upper boundary with Kashkan Formation is marked by an erosional surface.
On the basis of fossil assemblages from the Tarbur Formatin, the identified Biozone is Omphalocyclus-Loftusia-Siderolites calcitrapoides assemblage Zone. The biostratigrphic data based on benthic and planktonic foraminifera and rudists represents the late Maestrichtian age for the study section. Among distinguished foraminifers and rudists, Omphalocyclus cideensis, Omphalocyclus anatoliensis, Pseudomphalocyclus blumenthali, Elphidiella multiscissurata, Sirtina orbitoidiformis, Rotalia skourensis, Loftusia anatolica, Loftusia persica, Pseudedomia sp., Laffitina sp. and Hippurites cornucopiae reported for the first time at the deposits of Tarbur Formation in Semirom section of Isfahan.

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.

Ab- Deraz Formation is from the Lithostratigraphic units of the Kopet Dagh Basin in upper Cretaceous located in the North Eastern of Iran. According to the palynological studies by the use of Dinoflagellates, the age of Ab- Deraz in the section of West Sanganeh village Touronian to the middle Santonian was determined. The aim of this study is separation sequence units and determining water sea level changes by using palynological factors which is used for the first time on Ab- Deraz Formation. After reading the systematic of 41 samples of this Formation with the use of the palynological factors and agents three palynological facies identified and with the use of the triple a chart of Tyson was separated that includes three environment II: Marginal dysoxic-anoxic basin, V: Mud-dominated and oxic shelf, IV: Shelf to basin transition In order to separate sequence units based on three main changes of organic materials (Phytoclasts, palynomorphs and amorphs organic materials) and integration of these data with identified palynofacies, three sedimentary sequence kind of rank three with four sequence boundary ( three boundary of second kind and a type of first border) identified and determined.