فصلنامه پژوهش های چینه نگاری و رسوب شناسی
سال سی و یکم شماره 4 (پیاپی 61، زمستان 1394)

Silisiclastic deposits of Member 5 of the Mila Group in Central Alborz that call Lashkarak Formation in this research, were studied in Shahmirzad, Tueh-Darvar, and Deh-Molla sections in order to reconstruct their depositional environments and sequence stratigraphic framework. The integration of sedimentological and ichnological characteristics of estuary and open marine deposits of the Lashkarak Formation has led to more reliable determinations of the relative degree of influence imposed by river influx, waves, and storms on the coastal regime. Relying on the facies characteristics and stratal geometries, the siliciclastic successions of the Lashkarak Formation are divided into two facies associationswave-dominated shoreface-offshore complex andwave-influenced estuary. High resolution sequence stratigraphic data have been used to subdivide the strata into two third-order sequence sets contains five fourth-order parasequence sets. Lowstand fluvial deposits of the first stage of the valley-system fill occur in sequence 1. These fluvial deposits are overlain by coastal-plain and wave-dominated estuarine strata during TST and HST. A drastic change to open-marine conditions is marked by a marine flooding surface during HST. Open-marine strata are dominatly lower-shoreface and offshore-transition deposits. Although the interpreted depositional systems and stratigraphic architecture of sequence 1 of the Lashkarak Formation resemble transgressive incised-valley-fill successions, the greater thickness and larger size of the Lashkarak valleys suggest valley development during a relative sea-level fall. The sequences 2 of the lashkarek Formation is composed of transgressive and highstand systems tract couplets interpreted as reflecting eustatic sea-level changes and the continuous creation of accommodation during eustatic sea-level changes. These sequences indicate the transgressive and regressive cycles coeval with eustatic sea-level fluctuations during Ordovician.

The Lower to Middle Triassic aged Kangan Formation is one of the most significant carbonate gas reservoirs in Iranian territory. In this study, thin sections data were used to recognize microfacies, sedimentary environments and the interaction between diagenetic processes and facies stacking pattern in a sequence stratigraphic framework. Petrographic studies leaded to recognition of eight microfacies related to three facies belts including tidal flat, lagoon and shoal. Moreover, the observed microfacies patterns indicate a ramp carbonate platform as depositional environment for this carbonate succession. The main diagenetic processes of Kangan Formation include micritization, isopachous and fibrous cements (primary marine diagenesis), dissolution and moldic porosity (meteoric diagenesis), compaction and stylolitization (secondary diagenesis). Based on facies changes, two third-order sequences were specified, each of which could be divided into two systems tracts including transgressive systems tract (TST) and highstand systems tract (HST). In addition, sequence boundaries were identified with bedded, massive and nodular anhydrite. These facies, that are indicative of maximum sea level fall, were deposited in hypersaline lagoons. There is a close association between diagenetic processes and relative sea level changes of Kangan Formation, so that diagenetic processes of studied succession have been controlled by sediments stacking patterns during transgression and regression of sea level. During the transgression, the main diagenetic processes in shoal facies are marine cementation and dolomitization in lagoon and tidal flat facies. However, during the sea level fall, these processes include dissolution in shoal facies and dolomitization, anhydrite nodule formation and cementation in lagoon and tidal flat settings.

Miocene deposits in the Middle East such as Iran are famous and significance because of good oil reservoirs and a lot of studies has been carried out for precise exploring. Iran in this region, especially in Central Iran zone, in comparison with neighbor countries is a lack of exact and comprehensive information and need more investigation. In this way, study of foraminiferal assemblages for ascertain paleoenvironment characteristics and age determination is one of the researchers aims. Complicate and various characteristics of the Qom Formation in Central Iran such as depositional environments, faunal assemblage, age and lithology has caused that the information from its investigation be utilized and noticed for understanding connection of Indian Ocean and Mediterranean sea as a part of Tethyan Sea way in Oligo- Early Miocene. Agglutinated smaller foraminifera are one of the faunal groups which present in the Qom Formation in Dobaradar section. However, comparison with other forams with hyaline and porcelaneous tests, have less diversity and abundance. In this research, for the first time the early Miocene uncoiled agglutinated foraminifera from the Qom Formation in type area has been studied. Investigation of foraminifera in southeast of Qom, led to recognition a new species and report of 4 genera and 6 species for the first time from Iran. Among these Agglutinated forams, 2 species belong to Aquitanian and 2 present in Burdigalin, and the rest of them belong to Aquitanian- Burdigalian.

Zaladou section is located in Ozbak-Kuh mountains in the nourthen part of Tabas block and consist of shale, limy sandstone, limestone and dolomite. Continous relationship of Upper Carboniferous and Lower Permian deposits, is quite evident in Zaladou section. The lower boundry of this section is located on the Absheni formation of Sardar Group with disconformity surface, and upper boundry’s is covered by disconformity surface and bauxite horizon of Bagh-e-Vang formation. According to the lithological Characters and microscopic studies, tidal flat, lagoon, bar, tidal inlet and open marine sub-environment are identified for Zaladou section. Results of this study show that Zaladou section was deposited in silisiclastic-carbonate mix homoclinal ramp in late Carboniferous and in carbonate homoclinal ramp in early Permian. Field study, microfacies and analysis through the sequence led to recognition of main sequence surface, such as: sequence boundry, maximum flooding surface, marine flooding surface, system tracts and two depositional sequences.

Jurassic deposits are well exposed in the Calshaneh area, north western of the Tabas city, Central-East Iran. It contains plant macrofossils belonging to seventeen species allocated to eleven genera of various orders such as of Equisetales, Filicales, Bennettitales, Cycadales, Caytoniales, Ginkgoales and Pinales. One biozone was recognized in the Hojedk Formation at Calshaneh section. According to occurrence of Klukia exilis and Coniopteris hymenophylloides a Middle Jurassic age is suggested for this biozones. One Assemblage zone, Nilssnonia sp. cf. N. bozorga-Klukia exilis Assemblage zone, and three subzones were recognized in this section. Subzones are from base to up 1- Ptilophyllum harrisianum Taxon range zone 2- Elatides thomasii-Sphenobaiera longifolia concurrent range zone and 3- Ginkgoites sp. cf. G. parasingularis - Coniopteris hymenophylloides concurrent range zone. This Assemblage biozone is comparable to other biozones of the same age in Iran. Furthermore, relative abundances of taxa was studied and relative abundances of order Nilssoniales, Ginkgoales, Filicales, and Equisetales were 44.44%, 20.64%, 15.87, and 7.94% respectively. It is noteworthy that variety and relative abundance of the species of Genus Nilssonia (six species and 42.27%) and species of Order Ginkgoales (two taxa and 17.46%), as a macrophyllous cycadophyte, was high within the Calshaneh area. Therefore, a humid subtropical to temperate climate is considered for this period of time in this area.