Biostratigraphy of the Gurpi formation with emphasis on Calcareous nannofossil in the Qutbabad section, Northeast Jahrum

Introduction :

One of the most extensive Cretaceous deposits is the Gurpi Formation in the Zagros Basin, which was studied based on calcareous nannofossilstudies. Type section of the Gurpi Formation is located in the southwestern Tang-e Pabdeh, North of Lali oilfield (Jams & Wynd, 1965). One of the most important achievements obtained in the Gurpi Formation is the evaluationof biozones. In the present study, calcareous nannofossils have been investigated in the Qutbabadsection located in Northeastof Jahrum. The thickness of the studied section is about 458 m. The study in the Qutbabad section show that sediments of Gurpi Formation have a drop in the depth of sedimentary basin at the late Maastrichtian and suddenly lead to limestones of Tarbur Formation.

In this study 78 samples from the Gurpi Formation have been studied. Samples were prepared following standard smear slide method (Bown and Young 1998). All slides were studied under polarized light microscope at × 1000 magnification. The nomenclature of calcareous nannofossils follow the taxonomic schemes of Perch-Nielsen (1985).

In order to study biostratigraphy based on calcareous nannofossils, the Gurpi Formation in the Qutbabad section was selected. In this section, Gurpi Formation mainly consists of gray argillaceous limestones. For introducing calcareous nannofossil assemblages, slides have been studied, which led to the recognition of 22 genera and 37 species of calcareous nannofossils. According to the first and last occurrence of index species and recording bio-events, eight zones including Aspidolithus parcus ZonetoArkhangelsiella cymbiformis Zonehave been recognized. According to the first and last occurrence of index species the following bio-zones are identified:Aspidolithus parcus parcus Zone
The first zone in the Gurpi Formation is recorded from the FO of A. parcus parcusto LO of M.  furcatus. The age of this zone is early Campanian. The thickness of this zone is 53 m.

 Calculites ovalis Zone:

This zone spans the interval from the last occurrence of M. furcatusto the FO of C. aculeus. The age of this zone is late Early Campanian. The thickness of this zone is 40 m.

Ceratolithoides aculeus Zone:

This zone spans the interval from the FO ofC.aculeusto the FO ofU. sissinghii. The age of this zone is late Early Campanian. The thickness of this zone was measured about 54 m.

Quadrum sissinghiiZone :

This zone spans the interval from the FO of U.  sissinghiito the FO ofU.  trifidum. The age of this zone is early Late Campanian. The thickness of this zone was measured about 78 m.

Quadrum trifidum Zone:

This zone spans the interval from the FO of U.trifidumto the LO ofR. anthophorus. The age of this zone is late Campanian. The thickness of this zone was measured about 20 m.

Tranolithus phacelosusZone:

This zone spans the interval from the LO of R. anthophorusto the LO ofT.  phacelosus. The age of this zone is latest Campanian–early Maastrichtian. The thickness of this zone was measured about 50 m.

Reinhardtites levis Zone:

This zone spans the interval from the LO of Tranolithus phacelosusto the LO ofReinhardtites levis. The age of this zone is early Maastrichtian. The thickness of this zone was measured about 133 m.

Arkhangelsiella cymbiformisZone:

The last bio zone recorded is Arkhangelsiella cymbiformis Zone. This zone spans the interval from the LO of Reinhardtites levisto the FO ofNephrolithus frequens. The age of this zone is late Maastrichtian. The thickness of this zone was measured about 27 m.

As a result of this study, 22 genera and 37 species of calcareous nannofossil have been recognized. Based on distribution of index species calcareous nannofossils biozones of the zonation of Sissingh (1977) have been recognized, including CC18, CC19, CC20, CC21, CC22, CC23, CC24 and CC25, that correspond to UC14aTP till UC20bTP zones of Burnett (1998). According to the identified biozones, the age of the Gurpi Formation in this section is early Campanian to late Maastrichtian.