Evaluation of Ilam-Gurpi Formations boundary based on study of Calcareous nannofossils at the oil field Bahr-e Gansar, Persian Gulf

One of the most extensive Cretaceous deposits in Zagros, the marine sediments of the Ilam and Gurpi Formations, the first in the Zagros based on stratigraphy and paleontology was identified (James & Wynd, 1965). The type section of the Gurpi Formation is studied in the Lali oil field, located in the NE of Masjed Suleiman. The type section of the Ilam Formation is situated in the Kabirkuh area, located in Lurestan. The most important things that can be done given of these formations in Zagros, determining the exact boundary of by calcareous nanofossil. At the oil field Bahr-e-Gansar, the upper part of the Ilam Formation and the lower part of the Gurpi Formation consists of 17 m thick was selected which mainly consists of marl, marly limestone, shale, and limestone.

In this study, twenty-seven samples from Ilam and Gurpi Formations have been studied. Samples were prepared following a standard smear slide method (Bown & and Young 1998). Calcareous nannofossil nomenclature follows the taxonomic schemes of Perch-Nielsen (1985). As a result of this study, twenty-two genera and thirty-five species of calcareous nannofossils have been identified.

Calcareous nannofossils recorded in the Cretaceous strata are appropriate and index taxa for biostratigraphic studies. Abbreviations used in this study are the CC (Cretaceous Coccoliths), UC (Upper Cretaceous), and the NC (Nannofossil Cretaceous). The nannofossil zonation used in the present study is based on the Nannoplankton Cretaceous zonation of Sissingh (1977), Burnett (1998), and Roth (1978). According to our biostratigraphic data, CC15 to CC20 biozones are introduced for the Ilam-Gurpi formations boundary. Four biozones of the zonation of Sissingh (1977) are recognized at the upper part of Ilam Formation as follows:Reinhardtites anthophorus zone (equivalent to UC11/ NC16): The first nannofossil unit recorded in this study is the CC15 zone. This biozone is recorded from the FO Reinhardtites anthophorus to the FO of Lucianorhabdus cayeuxii. The age of this zone is late Early Santonian.Lucianorhabdus cayeuxii zone (equivalent to UC12/ NC16): The second nannofossil unit recorded at the upper part of the Ilam Formation is the CC16 zone. This biozone is recorded from the FO Lucianorhabdus cayeuxii to the FO of Calculites obscurus. The age of this zone is early Late Santonian.Calculites obscurus zone (equivalent to UC13 / NC17): This zone spans the interval from the FO of Calculites obscures to the FO of Aspidolithus parcus parcus. The age of this zone is Late Santonian-Early Campanian.Aspidolithus parcus zone (equivalent to UC14/ NC18): The last nannofossil unit recorded in the uppermost part of the Ilam Formation is CC18 zone. This zone spans the interval from the FO of Aspidolithus parcus parcus to the LO of Marthasterites furcatus. The age of this zone is Early Campanian.Two biozones of Sissingh’s zonation (1977), recognized at the lower part of Gurpi Formation are as follows:Calculites ovalis zone (equivalent to UCl5aTP / NC18): The next nannofossil unit recorded in this study is the CC19 zone. This biozone is recorded from the LO of Marthasterites furcatus to the FO of Ceratolithoides aculeus. The age of this zone is late Early Campanian. Ceratolithoides aculeus zone (equivalent to UCl5bTP/ NC19): The last nannofossil biozone recorded at the lower part of the Gurpi Formation is the CC20 zone. This biozone is recorded from the FO Ceratolithoides aculeus to the FO of Quadrum sissinghii. The age of this zone is Late Early Campanian.

The detailed study based on calcareous nannofossils enables the subdivision of the studied deposits (upper part of the Ilam and lower part of Gurpi Formations) into six biozones and thus the boundary between the two formations appears to be transitional suggesting an age of the late Early Santonian to Late Early Campanian.