ریزدیرینه‌شناسی و محیط رسوبی سازند قم در شمال غرب زنجان

به منظور انجام مطالعات ریزدیرینه‌شناسی و بازسازی محیط رسوبی بر روی توالی‌های سازند قم، یک برش چینه‌شناسی در محدوده شمال غربی زنجان نمونه‌برداری گردید. سازند قم در این ناحیه متشکل از 168 متر سنگ‌های کربناته متوسط تا ستبرلایه با میان لایه ‌های سنگ آهک رسی می‌ باشد که با مرز فرسایشی بر روی توالی ‌های کنگلومرا، ماسه ‌سنگ و مارن سازند قرمز زیرین و در زیر ردیف رسوبات تبخیری، مارنی و ماسه ‌سنگی سازند قرمز بالایی قرار گرفته‌اند. مطالعات فسیل ‌شناسی بر روی 52 مقطع نازک میکروسکپی به شناسایی 23 گونه از روزن‌داران کف‌ زی و شناور منجر شد که شامل گونه‌هایی همچون amphistegina sp.، borelis melo curdica، dendritina rangi، globigerinoides sp.، meandropsina anahensis، meandropsina iranica و sphaerogypsina globulus می‌باشند. در تمامی زیست‌زون‌های غیررسمی ارائه شده برای سازند قم، ظهور و گستره گونه شاخص borelis melo curdica شاخصی برای شروع آشکوب بوردیگالن در نظر گرفته شده است. بر این اساس، با توجه به حضور این گونه از ابتدا تا انتهای توالی‌های مورد مطالعه، سن سازند قم در این منطقه بوردیگالین (میوسن پیشین) پیشنهاد می‌گردد. مطالعات رخساره‌ای مقاطع نازک میکروسکپی به تفکیک 6 ریزرخساره منجر گردید که بخش‌های سکوی داخلی تا انتهای شیب قاره از یک محیط رسوبی شلف کربناته لبه‌ دار را شامل می‌شوند. وجود دو بخش ستبرلایه کربناته حاوی کلنی‌های بزرگ مرجانی و رخساره‌ های ریزشی و توربیدایتی در جلوی آنها، سکوی کربناته نوع شلف را ‌تایید می‌کند. نمودار تغییرات ریزرخساره‌ها در طول ستون چینه نگاری یک روند افزایش نسبی سطح آب دریا از ابتدای برش تا متراژ 54 متری و یک روند کاهش نسبی سطح آب دریا از این افق تا انتهای برش را نشان می‌دهد. بدین ترتیب یک سکانس رسوبی رده 3 را می‌توان برای این توالی‌ها در نظر گرفت.

Micropaleontology and sedimentary environment of the Qom Formation in the northwest of Zanjan

IntroductionQom Formation carbonate succession unconformably deposited over the red clastic layers of Lower Red Formation after the Eocene tectonic movements in Iran (Berberian King, 1981; Rahimzadeh, 1994). The type area of the Qom Formation is located in the South East of Qom city. Qom Formation has been divided into six members in the type area (Furrer Soder, 1955). Based on biostratigraphic studies, Eocene to Middle Miocene age has been proposed for the Qom Formation (Bozorgnia, 1966; Dozy, 1944; Furrer Soder, 1955; Jaafari, 1963; Zhu et al., 2007). Later studies by (Daneshian Aftabi, 2010; Daneshian et al., 2008; Ruter et al., 2009) show Oligocene – Miocene age for this Formation. Sedimentological studies in different sections show carbonate shelves and ramps in different locations. So, to biostratigraphy and sedimentary environment reconstruction of Qom Formation, one stratigraphic section has been sampled in the North West of Zanjan. Geological setting The studied section is located in the North West of Zanjan near AndAbad village. To access the area, we can follow the main road of Zanjan to the Mahneshan. Geological coordination of the studied section is: E 47ᴼ 59՜ 10.77ؘ ؘ    N 36ᴼ 48՜ 08.54ؘ ؘ. In the studied section, we can see the F member of the Qom formation that created the heights of the area. The AndAbad village is located on the Upper Red Formation in this area. Qom Formation in this area overlies the Lower Red Formation (with the angular unconformity boundary) and underlies the Upper Red formation (with the sharp stratigraphic boundary).Material and methods Fiftythree thin sections have been prepared from 168 meters of carbonate layer related to the Qom Formation. We used Adams Bourgeois (1967) and Leoblich Tappan (1988) for our biostratigraphic studies. The microfacies studies and reconstruction of the sedimentary environment have been done based on Flugel (2010), and sequence stratigraphy studies carried out according to Van Wagoner et al. (1988). 23 species of foraminifera and 6 microfacies groups have been identified using a transmitted light microscope.DiscussionBiostratigraphic studies lead to the identification of 23 foraminifera species: Borelis melo curdica, Textularia sp., Amphistegina sp., Schelumbergerina sp., Quinquloculina sp., Sphaaerogypsina globulus, Pyrgo sp., Dendritina rangi, Ammonia beccarii, peneroplis thomasi, Meandropsina anahensis, Meandropsina irnica, Triloculina trigonula, Orbulina sp., Bolivina sp., Operculina complanata , Globigerinoides sp., Globigerina praebulloides, Triloculina tricarinata, Elphidium sp., Rotalia viennotti, Asterigerina rotula, Archaias kirkukensis. The total range zone of the Borelis melo curdica index species shows Burdigalian age in many studies such as (Adams Bourgeois, 1967; Ehrenberg et al., 2007; Laursen et al., 2009). In this study, the first occurrence datum (FOD) of this species can be seen at the base of the section from the lowermost part of the Qom Formation layers and the last occurrence datum of this species is located at the top of the section. So we can propose the Burdigalian age for all studies layers of the Qom Formation in this section. Microfacies analysis leads to the identification of 6 different microfacies as follow: 1 Grainestonepackstone with small benthic foraminifera, 2 Floatstonerudestone with benthic foraminifera and red algae, 3 Wackstonepackstone with peloid, small benthic foraminifera, and sand particles, 4 Boundstone with coral, 5 Wackstone with planktonic foraminifer, 6 Packstonegrainstone with benthic foraminifera. These facies are related to the platform interior, platform margin, and slope and toe of slope parts of the carbonate shelf. Sequence stratigraphic studies is led to identifying one depositional sequence (3rd order) with one sequence boundary type I and one sequence boundary type II. Relative sealevel changes curve and the stratigraphic log show one transgressiveregressive trend during all sections.ConclusionTo biostratigraphy, microfacies, sedimentary environment, and sequence stratigraphy analysis, one stratigraphic section related to the Qom Formation has been sampled in the North West of the Zanjan. All 53 thin sections have been investigated based on microfacies and fossil contents. 23 foraminifera species have been identified. Based on the total range zone of the Borelis melo curdica index species, Bourdigalian age has been proposed for the studied layers. 6 different microfacies have been identified that show interior platform to lower slope areas of a carbonate platform as a sedimentary environment of the studied samples. On depositional sequence has been determined based on microfacies analysis that shows one transgressiveregressive trend during all sections.ReferencesAdams, T.D., Bourgeois, F., 1967. Asmari biostratigraphy. Iranian Oil Operating Companies. Geological and Exploration Division, Report 1074: 137. (Unpublished)Berberian, M., King, G., 1981. Towards a paleogeography and tectonic evolution of Iran. Canadian Journal of Earth Sciences, 18: 210265.Bozorgnia, F., 1966. Qom formation stratigraphy of the Central Basin of Iran and its intercontinental position. Bulletin of the Iran Petroleum Institute, 24: 6976.Daneshian, J., Aftabi, A., 2010. Foraminiferal biostratigraphy of the Qom Formation on the basis of new investigation at Navab anticline, in southeast Kashan. Journal of Science University of Tehran, 35: 137154.Daneshian, J., Mosaddegh, H., Khalaj, H., Ghasemi, A., 2008. Sequence stratigraphy of the Qom Formation at type area (KuheBichareh section), in southeast Qom, north of Central Iran. Research Journal of University of Isfahan (Science), 34: 1954.Dozy, J., 1944. Comments on geological report no. 1 by Thiebaud (on the QumSaveh area). Geological report, 308.Ehrenberg, S.N., Pickard, N.A.H., Laursen, G.V., Monibi, S., Mossadegh, Z.K., Svana, T.A., Aqrawi, A.A. M., McArthur, J.M. Thirlwall, M.F., 2007. Strontium isotope stratigraphy of the Asmari Formation (Oligocene–Lower Miocene), SW Iran. Journal of Petroleum Geology, 30: 107–128.Furrer, M., Soder, P., 1955. The Oligo–Miocene marine formation in the Qom region (Central Iran): Proceedings of the 4th World Petroleum Congress, Rome, Section I/A/5: 267–277.Flugel, E., 2010. Microfacies of carbonate rocks: analysis, interpretation and application. Springer Science Business Media, 1006 p.Jaafari, A., 1963. History and development of the Alborz and Sarajeh fields of Central Iran, 6th World Petroleum Congress, Frankfurt am Main, Germany.Laursen, G.V., Monibi, S., Allan, T.L., Pickard, N.A.H., Hosseiney, A., Vincent, B., Hamon, Y., Van Buchem, F.S.P., Moallemi, A., Druillion, G., 2009. The Asmari Formation revisited: changed stratigraphic allocation and new biozonation. First International Petroleum Conference and Exhibition Shiraz, Iran.Leoblich, A.R., Tappan, J.H., 1988. Foraminiferal Genera and their Classification. Van Nostrand Reinhold Company, 2: 869.Rahimzadeh, F., 1994. Treatise on the Geology of Iran: Oligocene, Miocene, Pliocene. Geological Survey of Iran, 12: 1311.Reuter, M., Piller, W. E., Harzhauser, M., Mandic, O., Berning, B., Rogl, F., Kroh, A., Aubry, M. P., WielandtSchuster, U., Hamedani, A., 2009. The Oligo/Miocene Qom Formation (Iran): evidence for an early Burdigalian restriction of the Tethyan Seaway and closure of its Iranian gateways. International Journal of Earth Sciences, 98: 627650.