بازسازی شرایط دیرینه بوم شناسی سازند قم در نواحی کهک، نوبران و اندآباد (حوضه پشت کمان قم)

به منظور بازسازی شرایط دیرینه بوم شناسی رسوبات الیگوسن میوسن حوضه دریایی قم، سه برش چینه شناسی به ترتیب در نواحی کهک (جنوب قم)، نوبران (شمال غرب ساوه) و اند آباد (شمال غرب زنجان) انتخاب گردید. در مجموع از نواحی کهک 146 نمونه، اند آباد 137 نمونه و نوبران 118 نمونه برداشت شد. سازند قم در هر سه ناحیه شامل تناوبی از شیل و سنگ آهک است. این سازند در نواحی اند آباد و نوبران بر روی سازند قرمز پایینی و در ناحیه کهک بر روی سنگ ‌های آتش‌فشانی ائوسن قرار دارد. مرز بالایی سازند قم نیز در نواحی کهک و اند آباد با سازند قرمز بالایی و در ناحیه نوبران به صورت ناپیوستگی با آبرفت ‌های عهد حاضر پوشیده شده است. بر اساس مطالعات زیست چینه نگاری، سن رسوبات سازند قم در ناحیه کهک روپلین شاتین، در ناحیه نوبران آکیتانین و در ناحیه اندآباد بوردیگالین تعیین شد. از دیدگاه دیرینه بوم شناختی، طی زمان ‌های روپلین شاتین (ناحیه کهک)، آکیتانین (ناحیه نوبران) و بوردیگالین (اند آباد) دریایی با شوری غالباً متوسط تا بالا حاکم بوده است. در طی زمان ‌های یاد شده شرایط نوری یوفوتیک و شرایط غذایی الیگوتروفیک مزوتروفیک حکمفرما بوده است. عمق دیرینه آب دریا در نواحی کهک (روپلین شاتین) و اند آباد (بوردیگالین) کمتر از 10 متر تا کمتر از 30 متر و در نوبران (آکیتانین) عمق کمتر از 10 متر تا کمتر از 40 متر در نوسان بوده که نشان دهنده عمق بیشتر دریا در زمان آکیتانین می‌باشد. در نواحی مورد مطالعه حضور فراوان مرجان‌ها، جلبک‌های قرمز و روزن‌داران کف‌زی بزرگ گویای شرایط آب و هوایی گرمسیری نیمه‌گرمسیری (18 تا 25 درجه سانتی‌گراد) و اجتماعات کربناته از نوع هتروزوئن بوده و شامل دو اجتماع دانه‌ ای (فورامل و فورآلگال) می‌باشد.

Reconstruction of paleoecological conditions of the Qom Formation in the Kahak, Nowbaran, and Andabad areas (back-arc Qom basin)

IntroductionThe Tethyan seaway was located between the Gondwana and Eurasia supercontinents (Reuter et al., 2009). The Qom basin developed at the northern margin of the Tethyan seaway (Reuter et al., 2009). In addition, the Qom Formation was deposited in this basin during the OligoceneMiocene ages (Reuter et al., 2009). The researchers indicated that the Qom basin was divided into three subbasins by the UrumiehDokhtar magmatic arc, and the subbasins are inclusive of the forearc subbasin (EsfahanSirjan basin), the backarc subbasin (Qom basin), and the Intraarc subbasin (Reuter et al., 2009; Mohammadi et al., 2013). The study areas (Andabad, Nowbaran, and Kahak areas) lie in the backarc subbasin (Qom basin). In this research, the paleoenvironmental conditions (such as light, nutrient supply, salinity, substrate nature, depth of seawater, and temperature) of the OligoMiocene deposits (Qom Formation) were studied. Materials and MethodsThe Andabad area, with the coordinates of N 36˚ 48’ 12.6& and E 47˚ 59’ 28&, is located 19 km away from northwestern Mah Neshan city. The Nowbaran area (coordinates of 35˚ 05’ 22.5& and E 49˚ 41’ 00&) which is situated at 53 km from northwestern Saveh city, as well as the Kahak area (coordinates of N 34˚ 23’ 01.1& and E 50˚ 50’ 20.2&), lies 30 km from south Qom city. The Qom Formation in the Andabad (with 220 m thickness), Nowbaran (with 458 m thickness), and Kahak (with 645 m thickness) areas mainly contain shale and limestone rocks. The lower boundary between the Qom and Lower Red formations is unconformable with the Andabad and Nowbaran areas. In the Kahak area, Eocene volcanic rocks are covered by the Qom Formation sediments. The Upper Red Formation unconformably overlies the Qom Formation in the Andabad and Kahak areas. The upper boundary between the Qom Formation and Recent Alluvial deposits is unconformable. A total of 401 rock samples from the study areas were collected. Microfacies analysis and paleoecological studies were performed by resources such as Dunham (1962), Embry and Klovan (1971), Pomar (2001), Mutti Hallock (2003), Mossadegh et al. (2009), MateuVicens et al. (2009), Brandano et al. (2009), and Pomar et al. (2014). Discussion and ResultsThe deposits of the Qom Formation were formed in the Kahak, Andabad, and Nowbaran areas during the RupelianChattian, Aquitanian, and Burdigalian ages, respectively. The paleoecological studies show the high marine salinity to hypersaline conditions in the Kahak (RupelianChattian), Nowbaran (Aquitanian), Andabad (Burdigalian) areas. The euphotic condition can be considered for the deposition environment of the Qom Formation in the Kahak (RupelianChattian), Nowbaran (Aquitanian), Andabad (Burdigalian). In the study areas, the oligotrophic to mesotrophic conditions can be observed. The paleo depth of seawater in the Kahak and Andabad areas was variable from less than 10 m to more than 30 m. The seawater depth in the Nowbaran area fluctuated from less than 10 m to more than 40 m. Therefore, the sea in the Aquitanian stage was deeper than the RupelianChattian and Burdigalian ages. The abundance of coral, red algae, and large benthic foraminifera in the study areas indicate tropical and subtropical environments (18 to 25 °C). The grain associations such as foramol and foralgal (belong to heterozoan association) were identified in the study areas. ReferencesBrandano, M., Frezza, V., Tomassetti, L., Pedley, M., 2009. 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