Decapod crustaceans from the lower Miocene Qom Formation of the Isfahan area, Central Iran

From the lower Miocene (Burdigalian) of the Qom Formation, exposed in three sections (Kuh-e-Donbeh, Bagher-Abad, and Vartun) in Central Iran, a deca-pod crustacean assemblage is described. The specimens exhibit two modes of preservation: carapaces (either isolated or with attached appendages) and isolated cheliped elements. All studied specimens are fractured and/or eroded. Based on this moderately preserved material, three brachyuran crab taxa are identified, including Mursia cf. lienharti (Bachmayer, 1962), Palaeocarpilius rugifer Stoliczka, 1871, and Necronectes sp. The occurrence of P. rugifer represents the youngest confirmed occurrence of the species, whereas other two taxa represent the first confirmed Iranian occurrences of respec-tive genera. This report enriches our knowledge on Miocene decapod assemblages of Iran, and thus helping to better understand the decapod migratory patterns along the Tethyan Sea-way.

Structural style of the Kashan-Ardestan syn-tectonic sedimentary basin in Central Iran, Arabian-Eurasian collision zone

<p>The Kashan-Ardestan sedimentary basin in Central Iran was initially formed by back-arc extension due to the subduction of Neo-Tethys oceanic lithosphere beneath the Iranian Plate during Eocene time. Following rifting and the onset of the Arabian-Central Iranian continental collision in the Oligocene, the basin was infilled by a sequence of continental clastic and evaporitic sediments referred to as the Lower Red Formation. Post-rift cooling and thermal subsidence led to the development of a shallow marine environment for the accumulation of Qom Formation carbonates and shales in the late Oligocene–early Miocene. The Qom Formation is the most significant hydrocarbon target in Central Iran, containing both source and reservoir rocks. The continental collision triggered the reactivation of pre-existing normal and strike-slip fault systems. The basin was subjected to compressional tectonism during the deposition of the Miocene Upper Red Formation and overlying Plio-Quaternary sediments. This long-lasting and multi-episodic tectono-sedimentary evolution of the Kashan-Ardestan Basin has led to the formation of a complex structural style, which must be resolved before petroleum system modeling and drilling of prospects can take place.</p><p>In this study, several transverse and longitudinal 2D seismic lines were converted to depth and interpreted to define the deep-seated geometry of structures in the basin. The seismic lines were tied to the data from three exploration wells, reaching depths of ~ 4 km. In addition, ~ 15000 gravity and magnetic stations, covering the entire Kashan-Ardestan Basin, were integrated into our model.</p><p>The results of our study indicate that two major strike-slip fault systems, including the Qom-Zefreh and Ardestan faults in the south and the Gazu fault zone in the north, control the geometry and evolution of the Kashan-Ardestan Basin. In this basin, the rheological profiles of the sedimentary sequences control the folding style and deformation mechanisms. Both basement-involved and thin-skinned faults developed in the basin and formed different types of fault-related anticlines. The reactivation of pre-existing strike-slip faults has produced positive flower structures during compression. There is some evidence that the Navab Anticline in the SW developed as a forced fold, with basement involvement. In addition, several thin-skinned detachment folds are observed above the evaporites of the Lower Red Formation at the base of the sedimentary cover. The Lower Red Formation thins and pinches out toward the eastern limit of the basin, where the Qom carbonates directly overly the Eocene volcanic basement. Meanwhile, the Upper Red Formation thins toward the north and northeastern limits of the basin, and towards the crests of anticlines. These syntectonic thickness variations allow us to define the geometric evolution of the Kashan-Ardestan Basin through geologic times, allowing for the burial history of the source rock and timing of trap formation at the reservoir level to be described.</p>

Amount and location of tectonic uplift in the Urmia region of northwest Iran from the Permian to the Neogene

The Urmia Lake Basin is located between the West and East Azerbaijan provinces in the northwest of Iran. Lake Urmia is the twentieth largest lake and second largest hypersaline lake in the world. Stratigraphic columns have been constructed, using published information, to compare the sedimentary units deposited from the Permian to the Neogene on the east and west sides of the lake, and to use these to quantity subsidence and uplift. East of the lake, the sedimentary section is more complete and has been the subject of detailed stratigraphic studies, including the compilation of measured sections for some units. West of the lake, the section is incomplete and less work has been done; three columns illustrate variations in the preserved stratigraphy for the time interval. In all cases, the columns are capped by the Oligocene–Miocene Qom Formation, which was deposited during a post-orogenic marine transgression and unconformably overlies units ranging from Precambrian to Cretaceous. Permian to Cretaceous stratigraphy is used to measure subsidence in the Lake Urmia basin up to the end of the Cretaceous, and then, the subsequent orogenic uplift, which was followed by further subsidence recorded by the deposition of the Qom Formation in the Oligocene–Miocene.

Reconstruction of the depositional sedimentary environment of Oligocene deposits (Qom Formation) in the Qom Basin (northern Tethyan seaway), Iran

During the Rupelian–Chattian, the Qom Basin (northern seaway basin) was located between the Paratethys in the north and the southern Tethyan seaway in the south. The Oligocene deposits (Qom Formation) in the Qom Basin have been interpreted for a reconstruction of environmental conditions during deposition, as well as of the influence of local fault activities and global sea level changes expressed within the basin. We have also investigated connections between the Qom Basin and adjacent basins. Seven microfacies types have been distinguished in the former. These microfacies formed within three major depositional environments, i.e., restricted lagoon, open lagoon and open marine. Strata of the Qom Formation are suggested to have been formed in an open-shelf system. In addition, the deepening and shallowing patterns noted within the microfacies suggest the presence of three third-order sequences in the Bijegan area and two third-order depositional sequences and an incomplete depositional sequence in the Naragh area. Our analysis suggests that, during the Rupelian and Chattian stages, the depositional sequences of the Qom Basin were influenced primarily by local tectonics, while global sea level changes had a greater impact on the southern Tethyan seaway and Paratethys basins. The depositional basins of the Tethyan seaway (southern Tethyan seaway, Paratethys Basin and Qom Basin) were probably related during the Burdigalian to Langhian and early Serravallian.

FACIES ANALYSIS AND STRATIGRAPHIC SEQUENCE ARCHITECTURE IN A BACK-ARC BASIN IN CENTRAL IRAN: A CASE STUDY FROM THE EARLY MIOCENE OF THE QOM FORMATION

Marine deposits of the Qom Formation, which is an important gas reservoir in Central Iran with the age of Early Oligocene to Early Miocene, is studied to determine facies, sedimentary paleoenvironment, and depositional sequences. The primary lithology is limestone, which is accompanied by a conglomerate, sandy marl, marl, and sandy limestone. Based on siliciclastic content, textural analysis, and the biotic constituents, ten facies have been identified. These facies belong to five depositional settings, including delta, a tidal inlet, lagoon, shoal, and open marine. According to the absence of continuous and large barrier reefs, gradual vertical variation in facies from the transitional environment (delta) to shallow open marine, the absence of oncoid, pisoid and aggregate grains that are mostly present in rimmed carbonate shelf environments, the absence of calciturbidites and slump and slide structures, the Qom Formation has been deposited in a homoclinal ramp setting (inner, middle and outer ramp). Field studies and vertical facies variation architecture in the framework of depositional system tracts led to the recognition of two 3rd order depositional sequences in the Early Miocene (Aquitanian) time. Sedimentary facies in the Qom Formation that mainly occurred in the middle ramp setting reveal a mostly aggredational stacking pattern in depositional sequences. The Early Miocene sequences stratigraphic architecture of the Qom Formation based on correlation charts are similar to the regional sequences of the Arabian plate and Zagros basin.