زیست چینه نگاری سازند گورپی در برش تاقدیس آغار واقع در جنوب غرب شیراز بر مبنای نانوفسیل‌های آهکی

نانوفسیل ‌های آهکی از گروه ‌های فسیلی شاخص و مفید در تعیین سن نسبی ردیف رسوبات کرتاسه بالایی هستند. در این مطالعه 143 متر از سازند گورپی در یال جنوب غربی تاقدیس آغار در جنوب غرب شیراز مورد بررسی قرار گرفت و 21 جنس و 40 گونه نانوفسیل‌ آهکی در توالی سازند گورپی شناسایی شد. بر اساس پراکندگی شاخص‌های نانوفسیلی ده زیست‌زون تشخیص داده شد که شامل زیست‌زون ‌های lucianorhabdus cayeuxii zone (cc16) به سن سانتونین پسین تا arkhangelskiella cymbiformis zone (cc25b) به سن مایستریشتین میانی از زیست‌پهنه‌بندی (sissingh (1977 شد. این زیست‌زون‌ها با زیست‌زون‌های nc17 تا nc22 از زیست‌پهنه‌بندی roth (1978) و زیست‌زون‌های‌ uc13? تا uc19 از زیست‌پهنه‌بندی burnett (1998) مطابقت دارند. مطالعه سازند گورپی در برش تاقدیس آغار نشان می‌ دهد رسوب‌گذاری در زمان سانتونین پسین آغاز شده و تا مایستریشین میانی ادامه داشته و سپس با کاهش ناگهانی عمق در اواخر مایستریشین میانی، حوضه رسوبی از آب خارج شده است. نوسانات عمق حوضه رسوبی در این بخش از ایران می تواند متاثر از تغییرات جهانی سطح آب دریا باشد.

Calcareous nannofossil biostratigraphy in the Gurpi Formation Aghar anticline section located in SW Shiraz

IntroductionOne of the most extensive Cretaceous strata is the marine strata of the Gurpi Formation in Zagros, which was studied based on paleontology studies. The Gurpi Formation type section is located in Tange Pabdeh, North of Masjed Soleyman city (James Wynd, 1965). One of the most important achievements obtained from the Gurpi Formation is the evaluation of stage boundary and biostratigraphy of it. In this section, the Gurpi Formation consists of 143 m thick which mainly consists of marl and marly limestone. Materials and MethodsIn this study, 73 samples from the Gurpi Formation interval have been studied. Samples were prepared following the standard smear slide method (Bown Young 1998). All the slides were examined under the polarized light microscope at ×1000 magnification. The nomenclature of calcareous nannofossil follows the taxonomic schemes of PerchNielsen (1985) and Burnett (1998). Discussion and ResultsTo study biostratigraphy based on calcareous nannofossils, the Gurpi Formation succession in the southwest of the Aghar anticline, SW Shiraz, was selected. In this section, the Gurpi Formation with 143 m thickness mainly consists of marly limestones and marl. For introducing index species, calcareous nannofossil assemblages, and biozones, slides have been studied at the Gurpi Formation, which led to the determination of 40 species and 21 genera of calcareous nannofossils. In this study, ten biozones were recognized in the Gurpi Formation. Biozones including Lucianorhabdus cayeuxii Zone (CC16), Calculites obscurus Zone (CC17), Aspidolithus parcus Zone (CC18), Calculites ovalis Zone (CC19), Ceratolithoides aculeus Zone (CC20), Quadrum sissinghii Zone (CC21), Quadrum trifidum Zone (CC22), Tranolithus phacelosus Zone (CC23), Reinhardtites levis Zone (CC24) and Arkhangelsiella cymbiformis Zone (CC25) with Late Santonian middle Maastrichtian in age. According to the first and last occurrence of index species the following biozones are identified:1 Lucianorhabdus cayeuxii Zone This biozone is recorded from the FO Lucianorhabdus cayeuxii to the FO of Calculites obscurus. The age of this zone is Late Santonian. The thickness of this zone has been measured approximately 4 m.2 Calculites obscurus Zone The second zone is recorded from the FO of Calculites obscurus to the FO of Aspidolithus parcus. The age of this zone is Late Santonian Early Campanian. The thickness of this zone is 4 m.3 Aspidolithus parcus Zone This zone spans the interval from the FO of Aspidolithus parcus to the last occurrence of Marthasterites furcatus. The age of this zone is Early Campanian.4 Calculites ovalis Zone The next nannofossil unit recorded in this study is the CC19. This zone spans the interval from the LO of Marthasterites furcatus to the FO of Ceratolithoides aculeus. The age of this zone is late Early Campanian.5 Ceratolithoides aculeus Zone The next biozone recorded is CC20. This zone spans the interval from the FO of Ceratolithoides aculeus to the FO of Quadrum  sissinghii. The age of this zone is late Early Campanian. The thickness of this zone was measured at about 15/6 m.6 Quadrum sissinghii Zone This zone spans the interval from the FO of Quadrum sissinghii to the FO of Quadrum trifidum. The age of this zone is early Late Campanian. The thickness of this zone was measured at about 4/4 m.7 Quadrum trifidum Zone This zone spans the interval from the FO of Quadrum trifidum to the LO of Reinhardtites anthophorus. The age of this zone is late Late Campanian. The thickness of this zone was measured at about 57 m.8 Tranolithus phacelosus Zone This zone spans the interval from the LO of Reinhardtites anthophorus to the LO of Tranolithus phacelosus. The age of this zone is late Late Campanian Early Maastrichtian. The thickness of this zone was measured at about 38 m.9 Reinhardtites levis Zone             This zone spans the interval from the LO of Tranolithus phacelosus to the LO of Reinhardtites levis. The age of this zone is Early Maastrichtian. The thickness of this zone was measured at about 4 m.10 Arkhangelsiella cymbiformis Zone The last nannofossil unit recorded in this study is the CC25. This zone spans the interval from the LO of Reinhardtites levis to the FO of Nephrolithus frequens. The age of this zone is middle Maastrichtian.So, as a result of this study and according to the identified biozones, the age of the Gurpi Formation is Late Santonian middle Maastrichtian. ReferencesBown, P.R., Young, J.R., 1998. Techniques. In: Bown, P.R., (ed.), Calcareous Nannofossil Biostratigraphy. Chapman and Hall, London, 1628.Burnett, J.A., 1998. Upper Cretaceous. In: Bown, P.R. (ed.), Calcareous Nannofossil Biostratigraphy, British Micropalaeontological Society Publication Series. Chapman and Hall Ltd. Kluwer Academic Publisher, London, 132165.James, G.A., Wynd, J.G., 1965. Stratigraphic Nomenclature of Iranian Oil Consortium Agreement Area. American Association of Petroleum Geologists Bulletin (AAPG), 49: 2182–2245.        PerchNielsen, K., 1985. Cenozoic Calcareous Nannofossils. In: Bolli, H.M., Saunders, JB., PerchNielsen, K., (eds.), Plankton Stratigraphy. Cambridge University Press, 427554.