ORIGINAL PAPER

Evaluation of Cretaceous–Paleogene boundary basedon calcareous nannofossils in section of Pol Dokhtar,Lorestan, southwestern Iran

Saeedeh Senemari & Masoomeh Sohrabi Molla Usefi

Received: 31 January 2012 /Accepted: 25 June 2012 /Published online: 8 July 2012# Saudi Society for Geosciences 2012

Abstract The Pol Dokhtar section of southern Lorestan,faulted Zagros range of southwestern Iran, contains oneof the most complete Early Campanian to Daniansequences. The lack of a good fundamental paleontolog-ical study is a strong motivation for investigating cal-careous nannofossils in southwestern Iran. The majorityof the section is made of shale, marl, and partly ofmarly limestone and clay limestone, respectively. As aresult of this study, 24 genera and 45 species of nanno-fossils have been identified and presented for the firsttime. This confirms the existence of biozone CC18 ofzonation scheme of Sissingh (Geologie en Minjbouw 56:37–65, 1977) to NP1 of zonation of Martini, which suggeststhe age of Early Campanian to Danian. All Early Cam-panian to Danian calcareous nannofossil biozones fromCC18 (equivalent to the Aspidolithus parcus zone) toNP1 (equivalent to the Markalius inversus zone) arediscussed. Also, the zonal subdivision of this sectionbased on calcareous nannofossils has shown continuityin Cretaceous/Paleocene boundary in south part of Lore-stan Province. We can also learn about the predominantconditions of the studied sedimentary basin that was in

fact part of the Neotethys basin with the existence ofindexed species calcareous nannofossils that indicatewarm climate and high water depths of the basin inlow latitudes.

Keywords Calcareous nannofossils .Cretaceous–Paleoceneboundary . Paleoecology . Lorestan

Introduction

The Zagros basin extends in northwest southeast trendand consists of a gently folded rock succession. Thebasin is composed of more than 10,000 m of Mesozoic–Cenozoic strata. Tectonically, the area is part of a fore-land basin deposited dominantly with a thick sedimen-tary sequence of carbonate and clastic composition,which was formed in the Late Triassic. Also, the Zagrosbasin is one of the largest oilfields in the world, thearea known as Agreement Area since 1954. The GurpiFormation is exposed throughout most of the AgreementArea along the south Lorestan of southwestern Iran.This formation in Zagros basin has changed laterally.All micropalaeontologists believed that the Gurpi Formationhas various ages based on Foraminifera studies in variouslocalities in the Zagros basin. (Ghasemi-Nejad et al. 2006;Khosrowtehrani 2008). The Pol Dokhtar section is located inKuh-E-Sultan, about 15 km northeast of Pol DokhtarCity, south of Lorestan Province (Fig. 1). At this local-ity, the Gurpi Formation conformably overlies the lime-stone of the Ilam Formation. This section consists of360-m-thick succession and covers the area between longi-tudes 47°45′–48°00′ E and latitudes 33°00′–33°15′ N. The

S. Senemari (*)Department of Mining, Faculty of Engineering,Imam Khomeini International University,Qazvin, Irane-mail: senemari2004@yahoo.com

M. Sohrabi Molla UsefiDepartment of Geology, Faculty of Science,Islamic Azad University,Islam Shahr, Irane-mail: sohrabi@iiau.ac.ir

Arab J Geosci (2013) 6:3615–3621DOI 10.1007/s12517-012-0620-8

majority of the section consisted of shale, medium beddedbluish gray marl, and partly of marly limestone and

argillaceous creamy limestone and is itself disconformityoverlain by the shale Pabdeh Formation. The Gurpi

33º00

48º00

Lorestan

Fig. 1 General geologic map of studied area. Ilam limestone, Gurpi shale, marl, and marly limestone, Pabdeh shale (scale 1:250,000, afterLlewellyn 1974)

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Formation includes the Cretaceous–Tertiary transition. Thisformation at the type section (E49°13′47″, N32°26′50″)is composed of 320 m gray to blue marl and shale bedsand occasionally thin beds of argillaceous limestones(James and Wynd 1965; Setudehnia 1978; Darvishzadeh1992). Few studies on Upper Cretaceous calcareous nanno-fossils have been carried out in Iran and are focused onthe Kopet Dagh Range (Hadavi 2004). Most of palae-ontological studies on the Cretaceous of the foldedZagros have been performed using foraminiferalassemblages (Jalali 1971; Zahiri 1982; Ghasemi-Nejadet al. 2006; Darvishzadeh et al. 2007). These micro-palaeontologists defined the Santonian–Maastrichtianage of the Gurpi Formation based on various localitiesin the faulted Zagros.

In recent years, the Upper Cretaceous in the Zagros basinwas studied based on the calcareous nannofossils (Hadavi etal. 2007; Senemari 2007). The samples for the present studywere taken almost every 2–3 m from the base to the top ofthe Pol Dokhtar section. The main scope of this study was toestablish a biostratigraphic zonation or biofacies of UpperCretaceous and to determine the nature of Cretaceous–Paleocene boundary. This study is based on the recognizedcalcareous nannofossil assemblages and the defined age of theformation.

Material and methods

A total of 171 samples representing the strata of the360-m succession were collected up to the contact withthe Pabdeh Formation. The most detailed sampling wasperformed in the intervals at 0.5 m below and aboveboundaries of the Gurpi Formation in Pol Dokhtar sec-tion. From this set of samples, the Early Campanian toDanian biozones were identified and the species rangeswere determined. Samples examined for calcareous nan-nofossils content were prepared using the smear slideand or to centrifuged techniques. For this work, allouter surfaces of the samples were trimmed with a razorblade to obtain a new fresh material. Then, a smallamount of sediment was scraped onto a glass slideand diluted with distilled water to make a thick sedi-ment suspension using a flat-sided toothpick and thendried on a hot plate. Between preparations, the razorused in the preparation was washed in distilled water.The counter and the hot plate used in making the smearslides were wiped with 10 % hydrochloric acid betweensample preparations. Because of the extremely smallsize of coccoliths, every care has to be taken to avoidcontamination during sampling or in the laboratory. Allslides were examined under an Olympus BH-2 lightmicroscope at ×1,000 magnification. The marker and

the most common species are illustrated in plate 1(Fig. 2).

Calcareous nannofossils

Calcareous nannofossil abundances are high within thestudy area. The species Watznaueria barnesae, Watznaueriabiporta, Ceratolithoides aculeus, Eiffellithus turriseiffelii,and Microrhabdulus decoratus are the major componentsand abundant of the assemblages. Arkhangelskiella cymbifor-mis, Arkhangelskiella maastrichtiana, Quadrum trifidum,Quadrum sissinghii, Quadrum gothicum, Lithraphidites car-niolensis, Micula decussate, Lucianorhabdus maleformis,Prediscospheara cretacea, Thoracosphaera operculata, andLucianorhabdus cayeuxii are relatively numerous. Tranoli-thus phacelosus, Ceratolithoides arcuatus, Marthasteritesfurcatus, Micula swastika, L. cayeuxii, Micula praemurus,Micula murus, and Micula prinsii are rare. Most Cretaceousnannofossil taxa became extinct below the first violetmarly intercalation, a bioevent that is synchronous withthe Cretaceous/Paleocene (K/Pg) boundary event inlow–mid-latitude areas (Perch-Nielsen 1981; Perch-Nielsen1985; Lees 2002).

The presence of the species mentioned above in thestudied samples could indicate a very deep basin andtropical climate conditions. The paleoecology and depthof the sedimentary basin can be explained using theindex species of calcareous nannofossils. From therecords on abundance and diversity of the low-latitudespecies, which are known to be very useful indexes forthe Late Cretaceous, we concluded that the sedimentarybasin was located in low latitude, tropical environment,and a very deep basin (James and Wynd 1965; Sissingh1977).

Biostratigraphy

Calcareous nannofossils recorded in the Cretaceousstrata are believed to be appropriate means for bio-stratigraphic studies (Cepek and Hay 1970; Crux1982; Bown and Young 1998). The importance ofthese calcareous nannofossils for correlation has beendiscussed at length by Sissingh (1977), Perch-Nielsen(1985), and Burnett (1998). The examination of calcar-eous nannofossils of the Gurpi Formation at Zagrosbasin (south of Lorestan) enabled us to recognize mostof the standard biozones defined in Mediterraneanregions, especially Tethysian domain (Thierstein 1976;Loeblich and Tappan 1987; Watkins 1996). In Zagrosbasin, few studies of cretaceous calcareous nannofossilshave been carried out on Gurpi Formation and K/Pg

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1 2 3 4

5 6 7 8

12

13 14

9 10

15 16

17 18 19 20

11

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boundary (Hadavi et al. 2007; Hadavi and Ezadi 2007;Senemari 2007). Calcareous nannofossil abundances arehigh within the study area. In the Early Campanian toDanian, the biozones CC18 to CC26 (zones of Sis-singh) and zone NP1 (zone of Martini) were identifiedusing the zonal scale that subdivides the upper Creta-ceous and Paleocene to biozones (Sissingh 1977;Perch-Nielsen 1985). Ten zones (CC18–NP1) are intro-duced as follows:

1. Aspidolithus parcus zone (CC18)—19/90 mAge: Early Campanian2. Calculites ovalis zone (CC19)—54/90 mAge: Late Early Campanian3. C. aculeus zone (CC20)—40/20 mAge: Late Early Campanian4. Q. sissinghii zone (CC21)—49/70 mAge: Early Late Campanian5. Q. trifidum zone (CC22)—16/30 mAge: Late Late Campanian6. T. phacelosus zone (CC23)—49/80 mAge: Latest Campanian–Early Maastrichtian7. Reinhardtites levis zone (CC24)—24/20 mAge: Early Maastrichtian8. A. cymbiformis zone (CC25)—71/40 mAge: Late Maastrichtian9. Nephrolithus frequens zone (CC26)—30 mAge: Late Late Maastrichtian10. Markalius inversus zone (NP1)—3/60 mAge: Early Paleocene (Early Danian)

These zonal schemes are shown in Fig. 3 and arecompared with the other commonly used zonations. The

taxa discussed in this section are illustrated in plate 1(Fig. 2).

The first nannofossil unit recorded from shale of theGurpi Formation is the zone CC18 defined as the inter-val from the first occurrence (FO) of Aspidolithus ex.gr. parcus to the last occurrence (LO) of M. furcatus.The next biounit CC19 is distinguished as the intervalfrom the LO of M. furcatus to the FO of C. aculeus.The FO of C. aculeus is close to the Lower/UpperCampanian boundary and the age zone (CC20) is lateEarly Campanian (Perch-Nielsen 1985). The next bio-zone (CC21) recorded from the shale and marls of theGurpi Formation is the FO of Q. sissinghii. The follow-ing event in the studied succession is the FO of Q.trifidum (CC22) also referred to the uppermost Campa-nian stage (Perch-Nielsen 1981). The successive LO ofnannofossils Q. trifidum, T. phacelosus, R. levis, Rein-hardtites anthophorus, and Eiffellithus eximius (CC23 toCC24) are recorded in the marls of the Early Maastrichtianage (Burnett 1998; Perch-Nielsen 1985). The next bio-events, recorded towards the top of the formation, arethe successive FO of Lithraphidites quadratus, M.murus, and M. prinsii (CC25 to CC26), dated as LateMaastrichtian (Thierstein 1976; Sissingh 1977; Perch-Nielsen 1985; Ehet and Moshkovitz 1995; Watkins 1996;Lees 2002). The last unit recorded in the shale andmarl of the Gurpi Formation is the zone NP1 definedas the interval from the LO of Cretaceous coccolithsor FO of acme of T. operculata (a calcareous dino-flagellate), Cruciplacolithus primus, and Biantholithussparsus to the FO of Cruciplacolithus tenuis. A bio-event that is synchronous with the K/Pg boundaryevent in low–mid-latitude areas. This interval followsthe typical succession observed at the Pol Dokhtarsection and includes the LO of M. prinsii at the topof zone CC26. Above this extinction are two succes-sive blooms, one of the dinoflagellate cyst genusThoracosphaera and the other of the nannofossilBraarudosphaera bigelowii. Indeed, dinoflagellatespecies of Cretaceous origin flourished in the earlyDanian, at a time when the calcareous-shelled biotawas still impoverished by extinction. These bloomshave been recorded in many mid-latitude areas,slightly above the K/Pg boundary event, in the low-ermost Paleocene (Thierstein 1976; Perch-Nielsen1981; Burnett 1998).

Conclusions

As a result of this study, 24 genera and 45 species ofnannofossils have been identified for the first time in southof Lorestan (Kuh-E-Sultan), southwestern Iran. Based on

�Fig. 2 Plate: All figures in XPL except 18 in PPL, light micrographs.1 Micula decussata Vekshina (1959); 2 W. barnesae (Black in Black &Barnes, 1959) Perch-Nielsen (1968); 3 M. prinsii Perch-Nielsen(1979a); 4 Calculites obscurus (Deflandre, 1959) Prins & Sissingh inSissingh (1977); 5 L. cayeuxii Deflandre (1959); 6 Rhagodiscus angus-tus (Stradner, 1963) Reinhardt (1971); 7 Q. gothicum (Deflandre,1959) Prins & Perch-Nielsen in Manivit et al. (1977); 8 Quadrumgartneri Prins & Perch-Nielsen in Manivit et al. (1977); 9 M. praemu-rus (Bukry, 1973) Stradner & Steinmetz (1984); 10 Q. sissinghii Perch-Nielsen (1984b); 11 R. anthophorus (Deflandre, 1959) Perch-Nielsen(1968); 12 A. parcus constrictus (Hattner et al. 1980) Perch-Nielsen(1984a); 13 A. parcus parcus (Stradner, 1963) Noel (1969); 14 Q.trifidum (Stradner in Stradner & Papp, 1961) Prins & Perch-Nielsenin Manivit et al. (1977); 15 Lithastrinus grillii Stradner (1962); 16. M.decoratus Deflandre (1959); 17 E. turriseiffelii (Deflandre in Deflandre& Fert, 1954) Reinhardt (1965); 18 M. furcatus (Deflandre in Deflan-dre & Fert, 1954), Deflandre (1959); 19 E. eximius (Stover, 1966)Perch-Nielsen (1968); 20 T. operculata Bramlette & Martini (1964)

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the obtained nannofossils, the age of the studied section isdefined as Early Campanian to Danian, which correspondsto zones CC18–CC26 and NP1 zonation. The GurpiFormation in section Pol Dokhtar consists of ten

biozones including: (1) A. parcus, (2) C. ovalis, (3) C. acu-leus, (4)Q. sissinghii, (5)Q. trifidum, (6) T. phacelosus, (7) R.levis, (8) A. cymbiformis, (9)N. frequens, and (10)M. inversuswere determined.

Fig. 3 Nanno-stratigraphic chart of Gurpi Formation, Pol Dokhtar section, Lorestan

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Acknowledgments This work has been supported by research creditof Imam Khomeini International University under contract number of751541-91.

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