Biostratigraphy, Facies and sedimentary environment of Tirgan Formation in eastern parts of Kopeh- Dagh Basin, based on palynomorphs and foraminifera

Introdaction The Kopet-Dagh sedimentary basin was formed in northeast Iran, southwestern Turkmenistan and north Afghanistan after closure of the Paleotethys Ocean following the Middle Triassic orogeny (Ruttner, 1993). Tirgan Formation, in Kopeh-Dagh basin consists of thick-bedded orbitolinid limestones, limy shale – marl with thin intercalation of limestone.
Materials and Methods 20 rock samples were selected for palynology and 60 for foraminiferal studies. Palynological preparation procedures followed those of Traverse (1988). Removal of carbonates and silicates were done with HCL and HF, then remaining samples sieved with 200µm and 20µm mesh (nylon sieve), using zinc chloride (ZnCl 2) (separation of organic material). The prepared slides were scanned and their palynological contents were identified and counted. The identified specimens were grouped in dinoflagellate cysts and sporomorphs. Moreover, 60 thin sections of Chalky limestone and limy marl have been prepared and studied. Thin sections were stained using Alizarin red and studied using standard petrographic microscopic techniques. Carbonate rocks were classified according to Dunham’s carbonate classification (1962).
Discussion This study led to identification of 16 species of dinocysts and six benthic foraminifera (Orbitolinids). Of the 20 rock samples prepared for palynology eight sample were from Baghak and 10 sample from Gallehvar sections. Some samples have good preservation and diversity of dinocysts. Based on the obtained biostratigraphy data, a late Barremian-early Aptian age is suggested for these deposits.
According to Palynological studies of Oosting et al. (2006), Skupien & Vašíček (2002) and Torricelli (2000), Barremian- Aptian boundary have been identified by presence of Pseudoceratium pelliferum, Cribroperidinium tenuiceras. Palynological contents of the slides were used for environmental interpretations. These include of three main organic groups: Marine Palynomorphs (MP), Amorphous Organic Matter (AOM) and palynomacerals or Phytoclasts (PH). Palynofacies are related to the composition and state of preservation of organic content of a rock unit or loose sediments (Batten, 1996). Based on Tyson diagram (1993), four palynofacies have identified for both sections: II: Marginal dysoxic-anoxic basin. IV: Shelf to basin transition. VI: Proximal suboxic-anoxic shelf. IX: Distal suboxic-anoxic basin. According to the quantitative palynological studies of the Tirgan Formation, depositional environment of the formation starts with a shallow, near shore environment and eventually leds to deeper environments (in the terminal shales).
Microfacies characteristics allow interpretations of lateral and vertical shifts in facies zones that reflect major and minor variations in environmental patterns, such as climate, water circulation, and siliciclastic input or sea-level fluctuations (Bachmann et al., 2006). Based on petrographic and washed samples, four lithofacies (A, B, C and D) have been identified in the Tirgan Formation. Petrographic studies were carried out to describe the microfacies and interpret the depositional environments. Therefore, 4 facies associations were identified.
Facies association A: This association is composed of 3 microfacies A1, A2, A3 (Mudstone and Sandy Mudstone). Abundant carbonate mud with quartz grains in this association represent shallowing parts of the basin and indications of the supratidal conditions (Shine, 1983). This facies association has been observed in both sections (Baghak and Gallehvar) and most abundant in the lower parts of the sections.
Facies association B: This facies association includes 3 microfacies B1, B2 and B3 (Peloidal Wackstone, Bioclastic Wackstone and Bioclastic Oolithic Wackstone). Abundant of Peloids in this association show deposition in low energy condition such as lagoon (Adachi et al., 2004). The evidences represent formation of this facies association in a lagoon environment. This association has been observed in both sections but its frequency is in Gallehvar section.
Facies association C: This facies association contains 3 microfacies C1, C2 and C3 (Oolitic grainstone, Biocalstic Oolitic garinstone and Orbitolina Oolitic grainstone). Presence of abundant tangential ooids in this association represent formation in the shallow, warm, agitated and calcium carbonate supersaturated conditions. Generally, ooids are index for sedimentation in shallow turbulent waters (Milliman, 1974). Presence of cross-stratification represents high-energy conditions during the formation of this facies (Bachmann & Hirsch, 2006). Aboundant discoidal and elongated orbitolina in the oolitic grainstone show formation of this facies in the seaward margin of the shoal. Given this evidence, it can be said that high-energy conditions of shoal prevailed in both studied sections.
Facies association D: This facies association is mainly composed of Orbitolina Oolithic Packstone. Due to existance of stenohaline organisms such as Echinoderms, Brachiopods, Briozoers and discoidal Orbitolina, this association have been deposited in an open marine environment. Presence of aboundant mud lime in this association represents low energy conditions (Adachi et al., 2004) and deposition under the normal waves.
The facies model of Tirgan Formation in the studied area can be interpreted as a shallow carbonate platform of ramp type where carbonate rocks have been deposited in four facies belt: tidal flat, lagoon, shoal and open marine. One the reasons that prevented the reef formation is, supply of siliciclastic particles from the land that are seen in some of the facies in both studied sections. The results of the palynofacies study can also support the interpretation of sedimentary facies in these sections.