كليدواژه :
ﻧﺎﻧﻮﻓﺴﯿﻞﻫﺎي آﻫﮑﯽ , زﯾﺴﺖ ﭼﯿﻨﻪ ﻧﮕﺎري , زاﮔﺮس , ﻣﯿﺪان ﻧﻔﺘﯽ
چكيده فارسي :
ﺑﻪ ﻣﻨﻈﻮر ﻣﻄﺎﻟﻌﻪ زﯾﺴﺖ ﭼﯿﻨﻪ ﻧﮕﺎري ﺑﺮ اﺳﺎس ﻧﺎﻧﻮﻓﺴﯿﻞﻫﺎي آﻫﮑﯽ، ﺑﺨﺶ ﺑﺎﻻﯾﯽ ﺳﺎزﻧﺪ اﯾﻼم و ﺑﺨﺶ ﭘـﺎﯾﯿﻨﯽ ﺳـﺎزﻧﺪ ﮔـﻮرﭘﯽ در ﯾﮑـﯽ از ﭼـﺎه ﻫـﺎي ﻣﯿﺪان ﻧﻔﺘﯽ ﺑﻬﺮﮔﺎﻧﺴﺮ واﻗﻊ در ﺧﻠﯿﺞ ﻓﺎرس اﻧﺘﺨﺎب ﺷﺪ. ﺑﺮرﺳﯽ ﻧﺎﻧﻮﻓﺴﯿﻞﻫﺎي آﻫﮑﯽ در اﯾﻦ ﻗﺴﻤﺖ ﺑﻪ ﺷﻨﺎﺳﺎﯾﯽ 22 ﺟﻨﺲ و 35 ﮔﻮﻧﻪ ﻣﻨﺠﺮ ﮔﺮدﯾـﺪ ﻣﻄﺎﺑﻖ ﺑﺎ اوﻟﯿﻦ ﺣﻀﻮر ﮔﻮﻧﻪﻫﺎي ﺷﺎﺧﺺ و ﮔﺴﺘﺮش ﺗﺠﻤﻊﻫﺎي ﻓﺴﯿﻠﯽ ﻫﻤـﺮاه، ﭼﻬـﺎر زﯾﺴـﺖ زون Reinhardtites anthophorus zone (CC15/ late
Calculites obscurus zone (CC17/ Late Santonian- Early ،Lucianorhabdus cayeuxii zone (CC16/ Late Santonian) ،(Early Santonian Campanian( و Aspidolithus parcus zone (CC18/ Early Campanian) در ﺑﺨﺶ ﺑﺎﻻﯾﯽ ﺳﺎزﻧﺪ اﯾﻼم ﺷﻨﺎﺳﺎﯾﯽ ﺷﺪﻧﺪ ﮐﻪ ﺑـﺎ زﯾﺴـﺖ زون NC16 و ﻗﺎﻋﺪه زﯾﺴﺖزون NC18 از زﯾﺴﺖﭘﻬﻨﻪﺑﻨﺪي Roth )1978( و زﯾﺴﺖزون UC11 ﺗﺎ زﯾﺴﺖزون UC14 از زﯾﺴﺖﭘﻬﻨﻪﺑﻨـﺪي Burnett )1998( ﻣﻄﺎﺑﻘـﺖ دارﻧﺪ. ﻫﻤﭽﻨﯿﻦ زﯾﺴﺖزونﻫﺎي Calculites ovalis zone (CC19/ late Early Campanian) و Ceratolithoides aculeus zone (CC20/ late Early Campanian( از زﯾﺴﺖﭘﻬﻨـﻪ ﺑﻨـﺪي Sissingh 1977( ﮐـﻪ ﺑـﺎ ﺑﺨـﺶ ﺑـﺎﻻﯾﯽ زﯾﺴـﺖ زون NC18 و ﻗﺎﻋـﺪه زﯾﺴـﺖ زون NC19 از زﯾﺴـﺖ ﭘﻬﻨـﻪ ﺑﻨـﺪي Roth )1978( و ﺑﺨﺶ ﭘﺎﯾﯿﻨﯽ زﯾﺴﺖزون UC15 از زﯾﺴﺖﭘﻬﻨﻪﺑﻨﺪي Burnett )1998( ﻣﻄﺎﺑﻘﺖ دارﻧﺪ، ﻣﺘﻌﺎﻗﺒﺎً در ﺑﺨﺶ ﭘﺎﯾﯿﻨﯽ ﺳﺎزﻧﺪ ﮔﻮرﭘﯽ ﺷﻨﺎﺳﺎﯾﯽ ﺷﺪﻧﺪ. در ﻧﺘﯿﺠﻪ اﯾﻦ ﻣﻄﺎﻟﻌﻪ و ﺑﺮ اﺳﺎس زﯾﺴﺖزونﻫﺎي ﻣﺸﺨﺺ ﺷﺪه، ﺳﻦ ﺑﺨﺶ ﺑﺎﻻﯾﯽ ﺳﺎزﻧﺪ اﯾﻼم اواﺧﺮ ﺳﺎﻧﺘﻮﻧﯿﻦ ﭘﯿﺸـﯿﻦ ـ ﮐﺎﻣﭙـﺎﻧﯿﻦ ﭘﯿﺸـﯿﻦ و ﺳﻦ ﺑﺨﺶ ﭘﺎﯾﯿﻨﯽ ﺳﺎزﻧﺪ ﮔﻮرﭘﯽ اواﺧﺮ ﮐﺎﻣﭙﺎﻧﯿﻦ ﭘﯿﺸﯿﻦ ﺗﻌﯿﯿﻦ و ﺑﻨﺎﺑﺮاﯾﻦ ﻣﺮز ﺑﯿﻦ دو ﺳﺎزﻧﺪ ﭘﯿﻮﺳﺘﻪ در ﻧﻈﺮ ﮔﺮﻓﺘﻪ ﺷﺪ. ﻫﻤﭽﻨﯿﻦ ﮔﻮﻧـﻪ ﻫـﺎي ﺷـﺎﺧﺺ ﻧﺎﻧﻮﻓﺴﯿﻞﻫﺎي آﻫﮑﯽ ﻣﻮﺟﻮد، ﻣﺆﯾﺪ آب و ﻫﻮاي ﮔﺮم ﺣﻮﺿﻪ در زﻣﺎن ﺳﺎﻧﺘﻮﻧﯿﻦ ﭘﯿﺸﯿﻦ ﺗﺎ ﮐﺎﻣﭙﺎﻧﯿﻦ ﭘﯿﺸـﯿﻦ در ﻋـﺮض ﻫـﺎي ﺟﻐﺮاﻓﯿـﺎﯾﯽ ﭘـﺎﯾﯿﻦ ﺣﻮﺿـﻪ ﻧﺌﻮﺗﺘﯿﺲ اﺳﺖ.
چكيده لاتين :
Introduction
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.
Material and Methods
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.
Discussion
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. Conclusion
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.
