فهرست مطالب مریم نادریان

The Dalichai Formation in Parikhan section in the west of Shahrood with a thickness of 250 meters consisting of an alternation of bluish-gray was studied. This formation overlies the Shemshak formation disconformity, and it has gradually been covered by the thick-bedded limestones of the Lar formation. The biogenic components consist of brachiopods, gastropods, bivalves, ammonites, echinoderms, belemnites, sponges and sponge spicules, radiolarians, ostracods, benthic foraminifers. The aim of the present study is to discuss the age of the formations and paleobiogeography relationship based on foraminifera and dinoflagellate cyst assemblage.

The Parikhan section is located on the central part of Alborz Mountains, approximately 14 km west of Shahrood (coordinates with E 54° 49΄11 ˝ and N 36° 24΄20˝). 27 samples were collected systematically and processed for palynostratigraphy following the standard palynological technique described in Traverse (2007). Cold hydrochloric (30 %) and hydrofluoric acids (30 %) were used to dissolve carbonate sand silicates, respectively. The residue was neutralized and centrifuged in ZnCl2 with specific gravity (1.9 g/cm3). The materials were then sieved using a 15-μm nylon mesh and mounted on microscope slides using liquid Canada balsam. The present study shows that the Dalichai Formation is rich in terrestrial (spores and pollen grains) and marine palynomorphs (dinoflagellate cysts, acritarchs, and scolecodonts). In the laboratory, thin sections were prepared from all marly limestones and hard resistant shaly marl samples to achieve further microfossils evidence and aid sedimentological descriptions. All samples were processed for foraminiferal analysis following the standard method of Lirer (2000). Thus, 21 species belonging to 17 genera of planktonic and benthonic foraminifera were identified.

In the present study, 30 species of dinoflagellate cysts belong to 22 genera and 10 species of spore, which belong to 8 genera, and 7 species of pollen, which belong to 4 genera, have been identified. Based on the presence and stratigraphic distribution of the most dominant miospore, Klukisporites variegatus and Callialasporites dampieri of pollen are identified in the Dalichai Formation (late Bajocian - Callovian). Based on recorded dinocyst assemblages, especially marker species, three palynozone, including Cribroperidinium crispum Total Range Zone (late Bajocian), Dichadogonyaulax sellwoodii Interval Zone (Bathonian – early Callovian), and Ctenidodinium continuum Interval Zone (late Callovian) were identified.

The Dalichai Formation in Darjazin section in NE Semnan with a thickness of 631 meters consisting of an alternation of bulish-gray was studied. This formation overlies the Shemshak formation disconformity with conglomerate bed and it has gradually been covered by the thick-beded limestones of the Lar formation. In order to study paleoenviroment of the Dalichai Formation the authors examined many different factors including Microfacies and palynological data (abundance and diversity of Dinocyst, proximate to chorat Dinocyst, palynofacies analysis and "selective preservation of organic matter"). A high percentage of brown wood and sporomorphs and the dominance of cavate and proximate dinocysts such as Nannoceratopsis gracilis, and pareodinia ceratophora and fluorescent amorphous organic matter, as well as Miliolid benthic foraminifera are evidence of suboxic to anoxic condition, which favored accumulation and preservation of organic matter in open marine lagoon, in the lower part of Dalichai Formation. A decrease in the brown wood and abundance and species richness of marine palynomorph (mainly chorate dinocysts) reflects more offshore settings in the upper part of Dalichai Formation. On the other hand, Appearance and increase of planktonic foraminifera and Radiolar and posidonia indicate development of strong marine transgression and deposition in an open marine environment.