فصلنامه پژوهش های چینه نگاری و رسوب شناسی
سال بیست و ششم شماره 3 (پیاپی 40، پاییز 1389)

The Abtalkh Formation in the Kopeh-Dagh basin in northeastern Iran with an age of Late Cretaceous (Turonian – Santonian) is selected to evaluate resolution of magnetic susceptibility for assessing sea level fluctuations. Thirty-nine shale samples were collected and studied for magnetic susceptibility and palynology. Assuming the average MS magnitude as the proxy for mean sea level, positive Magnetic Susceptibility (MS) values were considered to present sea level falls and negative values for sea level rises. Based on these data a curve showing water level fluctuations throughout the stratigraphic column was prepared which shows a general trend of regression from base to the top of the formation. To test accuracy of this method, magnetic susceptibility results were compared with those gained from palynology and palynofacies studies and with those from geochemical analyses. Based on the changes in palynological factors (e.g. the terrestrial/marine particles ratio, chorate/proximate, proximochorate, and cavate dinocysts ratio, differences in dinoflagellate assemblages, rarity and abundance of dinoflagellate groups), palynofacies and fossil contents, three Third-order sequences were differentiated, confirming the results gained from magnetic susceptibility method. In addition, the results gained from Rock-Eval pyrolysis indicate that the Abtalkh Formation at the type section was deposited under an oxic condition and confirm the results gained from palynology and magnetic susceptibility.

The Asmari Formation of the Zagros Basin is a thick sequence of shallow water carbonate. In the study area, it is subdivided into 11 microfacies that are distinguished on the basis of their depositional textures, petrographic analysis and fauna. In addition, four major depositional environments were identified for the Asmari Formation. These include peritidal, inner ramp, middle ramp and outer ramp environmental settings which are interpreted as a ramp carbonate platform. In the Asmari carbonate succession four third-order depositional sequences are recognized from deepening and shallowing trends in the depositional facies and sequence boundary features.

The Khoshyeilagh Formation in the eastern Alborz Mountains (north-east Iran) is Middle to Late Devonian post-rift succession. Facies analysis led to the recognition of five facies associations that are deposited in terrestrial (fluvial) and marine (carbonate ramp platform) environments. Its marine sediments are formed in inner ramp, mid-ramp, outer ramp, and basin sub-environments. Sequence stratigraphy of the Khoshyeilagh Formation is composed of five complete (and one incomplete) third-order depositional sequences with tectono-eustatic origins. Tectonic setting of the Khoshyeilagh Formation is an extensional post-rift or passive margin basin across the Paleo-Tethys Basin (northern rim of the Gondwana). This basin underwent various changes from Middle to Late Devonian and its ramp platform passed evolutionary stages from homoclinal type to distally steepened type. Depositional system of this basin is similar to mixed siliciclastic-carbonate system.

Based on the stratigraphical distribution of the larger benthic foraminifera in the subsurface sedimentary successions of the Asmari Formation (Oligocene – Miocene) from the Rag-e-Safid oil field four assemblage zones have been recognized. Assemblage zone I suggests a Rupelian age and includes Nummulites fichteli, Nummulites vascus, Eulepidina, Nephrolepidina, Austrotrillina paucialveolata, Heterostegina assilinoides, Praerhapydionina delicate, Pseudolituonella reicheli; Assemblage zone II represents a Chattian age and is introduced by association of Archaias, Austrotrillina, Amphistegina, Miogypsinoides complanatus; Assemblage zone III suggests an Aquitanian age and consists of Miogypsinoides deharti, Miogypsina sp. and peneropelidae; Assemblage zone IV refers to Burdigalian age and is recognized by the first appearance of Boreli s melo and Borelis curdica. The paleoecology and biofacies of the larger benthic foraminifera suggest four depositional settings included open marine; ooids shoals; lagoon and tidal flat. These sedimentary settings correspond to the inner and middle parts of a carbonate ramp.

The Surmeh Formation with the age of Jurassic composed of shallow-water limestones and dolomites. The study area is located in the southwest of Iran in the Chaharmahal and Bakhtiari Province. Petrographic observation of the Surmeh Formation reveals the presence of several different of cement types related to various diagenetic environments. This study attempts to determine the temperature of cement precipitation, the salinity of the fluid from which the cement precipitated and the diagenetic environments. Bivariate plot of Th (temperature of homogenization) and Tmice (temperature of final ice melting) show distinct areas for each cement. In addition, the petrographic observation and fluid inclusion data (Th and Tmice) revealed that timing of precipitation of 5 different types of coarsely-crystalline cements in this successtion, began with meteoric diagenesis and then affected by deep burial diagenesis. All of fluid inclusions exhibit two-phases (liquid-vapour phase), which the liquid ratios is more than vapour ratios. In this research, petrographic evidence, elemental, oxygen and carbon isotope values indicate that aragonite was the original carbonate mineralogy in this formation. Bivariate plots of Sr/Ca versus Mn values, also illustrate that Surmeh carbonates were affected by non-marine diagenesis in a closed to semi-closed system.

Two coral assemblages are described in the Ozbak-Kuh (Zaladou section) and Kalmard (Gachal section) areas. They consist of Tabulata, solitary, fasciculate and massive rugosa corals They clearly belong to the Carboniferous system. The first assemblage consist of Syringopora distans Fischer and Kueichouphyllum cf. yabei Minato and Kueichouphyllum cf. laosense Fontaine that accompanied with the foraminiferas such as Eostaffella pseudostruvei Rauser&Belaiev and Archaeodiscus cf.krestovnikovi Rauser indicate Late Visean to Early Serpukhovian age for the member D of Gachal formation.The second assemblage consist of Minatoa cf bulla Flugel, M. alternata Flugel and Fomichevella smithi Fontaine that suggest Middle Carboniferous(Bashkirian age) for lower part of Sardar Formation.

Platform carbonate sediments of Oligocene-Miocene age (Asmari Formation) in the Zagros Basin (SW Iran) have been investigated in order to determine their biostratigraphy and palaeoecolgy. The Asmari carbonate system was dominated by large benthic foraminifera. Five assemblage biozones have been recognized by distribution of the larger benthic foraminifera from the Oligocene—Miocene Asmari Formation of the Zagros Basin in southwest of Iran. Assemblage I is Rupelian in age. Assemblage II suggests Rupelian-Chattian. Assemblage III is Chattian in age. Assemblages IV and V show Aquitanian and Burdigalian age, respectively. The distribution of the Oligocene-Miocene larger benthic foraminifera indicate that shallow marine carbonate sediments of the Asmari Formation at the study area have been deposited in the photic zone of tropical to subtropical ocean.

One stratigraphic section of the Sanganeh Formation (Lower Cretaceous), north-west of Kopeh Dagh, was measured and sampled. This formation consists of shale, calcareous shale and marls. The lower boundary with Sarcheshmeh Formation is conformable and upper boundary with Kalat Formation is unconformable. Biostratigraphic studies led to identification of 21 genera and 48 species of benthic foraminifera. Based on foraminifera fauna, a Late Aptian age is suggested for Sanganeh Formation at this locality. Using identified foraminifera, Gavelinella flandrini assemblage zone is proposed. This biozone coincides with global biozones. Palaeoecological studies led to recognition of three calcareous benthic and two agglutinated benthic morphogroups.