The studied section is located in the region of Izeh (Fig. 1), in the central part of the Zagros Basin, which is the northernmost part of the Arabian Platform constituting the southeastern margin of the Tethys palaeo-ocean. Development of the Zagros Basin began in the Late Cretaceous (Turonian to Maastrichtian) due to fore-deep subsidence preceding the continental closure along the Zagros Suture. This closure resulted in collision of the western Iranian and Afro-Arabian plates (Gealey, 1988). In the Zagros Basin, the Gurpi Formation constitutes the upper part of the Uppermost Turonian-Middle Maastrichtian progradational–retrogradational megasequence, as well as the basal strata of the overlying Uppermost Maastrichtian-Upper Eocene megasequence (Alavi, 2004, James and Wynd, 1965, Koop and Stoneley, 1982, Motiei, 2003, Murris, 1980 and Ziegler, 2001). The Gurpi Formation consists of dark gray marly shale (Turonian-Middle Maastrichtian in the lower part of the Formation), which is overlain by gray marls with interbedded clayey limestones of the Emam Hassan Member (Middle Maastrichtian), and the dark gray shaly marls (Upper Maastrichtian to Paleocene, in the upper part). These distinctive lithostratigraphical units represent depositional environments ranging from middle shelf to upper slope as indicated by the rich macro- and microfaunas (Hemmati-Nasab et al., 2008). In the studied area, the Gurpi Formation is considered to be a continuous stratigraphical succession through the Campanian–Paleogene interval. It overlies the Ilam Formation with minor disconformities and is covered conformably by the sandy to silty purple shales of the Lowermost Pabdeh Formation top. The K/Pg boundary lies within the upper part of the marly Gurpi Formation, at 275.82 m from its base. It is within the dark gray shaly marls unit composing the upper part of the formation, at about 68 m above the Emam Hassan Member. It is also at 61.2 m below the base of the Pabdeh Formation including three successive units: •

lower marly shales;

A total of 25 samples were collected, carefully for micropaleontological analyses from 2.8 m below to 1.5 m above the K/Pg boundary at the Izeh section. The section was sampled at 5, 10, 15 cm intervals close to the K/Pg boundary. Half of the collected samples were processed for foraminiferal study, following the standard method of Neumann (1967) and others. The samples have been soaked in water for several days and then washed through 200, 120, 63 μm and 36 μm sieve series with tap water and dried in an oven at 50 °C. One half of the each residue has been scanned for planktonic foraminifera species; the distribution interval of each recognized species is specified. For the semi-quantitative analyses, the planktonic foraminiferal assemblages have been counted in representative splits in the size fraction larger than 63 μm using a microsplitter in order to obtain, nearly 300 specimens from each sample (see Table 1). Additional foraminiferal specimens are selected for scanning electron microscopy (SEM, see Fig. 5, Fig. 6 and Fig. 7). The rest of the same samples were processed for palynostratigraphy following the standard palynological technique described in Wood et al. (1996). After HCl and HF treatment, the residues were sieved on 20 μm filters. Heavy liquid separation (using ZnCl2) was carried out on all samples, and five slides were prepared from each sample. At least three slides per sample were scanned until approximately 200 specimens (including debris) are identified and counted into larger than 20 μm sized fraction, as listed in Table 2.

Several biostratigraphic subdivisions (zonations) based on planktonic foraminifera for the Cretaceous–Paleogene transition interval, have been proposed (e.g., Berggren, 1972, Berggren and Miller, 1988, Berggren and Pearson, 2005, Berggren et al., 1985, Blow, 1969, Blow, 1979, Bolli, 1966, Canudo et al., 1991, Herm et al., 1981, Keller, 1988, Molina et al., 1996, Smit, 1982 and Toumarkine and Luterbacher, 1985).

The present study is based on the zonations of Zaghbib-Turki et al. (2000), Arenillas et al. (2004), Gallala et al. (2009), and Gallala and Zaghbib-Turki (2010), established for middle and lower latitudes.

Fig. 2 compares some of the more frequently used biozonations for the Maastrichtian–Danian transition. Four main planktonic foraminiferal zones are recognized in the studied section: •

Abathomphalus mayaroensis Zone (including the Plummerita hantkeninoides Subzone);

The planktonic foraminiferal assemblages in the Izeh section are very similar to those of the well-known Tunisian section at El Kef (i.e., the GSSP for the K/Pg boundary).

On the other hand, age assignments of the dinoflagellate assemblages reported here are based on comparison with worldwide ranges of individual taxon with emphasis on low to mid latitude records (Williams et al., 2004). It was done in combination with a detailed planktonic foraminiferal stratigraphy from the Izeh section. Several local and worldwide biostratigraphical biozonations based on dinoflagellate cysts have been proposed (Brinkhuis and Zachariasse, 1988, Brinkhuis et al., 1998, Costa and Manum, 1988, Lentin and Williams, 1980, Mao and Mohr, 1992, Powell, 1992, Williams and Bujak, 1985 and Williams et al., 1990). Especially a widely accepted worldwide standard zonation has been proposed by Brinkhuis and Powell, 2004 in Gradstein et al., 2004 (see also Geologic Time Scale, therein). The microfossils from the Izeh section that we have recognized include 61 planktonic foraminiferal species and 57 dinocyst taxa. The important planktonic foraminiferal and dinocyst taxa are illustrated on Fig. 4, Fig. 5, Fig. 6 and Fig. 7.

In the Zagros Basin most of the Gurpi Formation represents continuous deposition in an open marine environment. The upper part of the formation indicates sedimentation in an outer shelf environment (Fig. 1E). In most of the outcrops of the Gurpi Formation in southwestern Iran, a hiatus spanning the Cretaceous/Paleocene boundary interval is observed (Ghasemi-Nejad et al., 2006, James and Wynd, 1965, Motiei, 2003, Sampo, 1969, Setudehnia, 1972, Setudehnia, 1978 and Wynd, 1965). However, the Izeh section exhibits a continuous and well-preserved transition across the boundary.

The K/Pg boundary is exposed at 275.82 m from the base of the Gurpi Formation. It is marked and overlain by a few millimeters thick rust-colored ferruginous layer. This layer composed of reddish hematitic and goethitic laminae containing very low percentage of CaC0₃. It is marked by an abnormal high values of Ir (an abrupt enrichment from 0 to 4.3 ppb: unpublished data) and Ni (from 0 to 0.24 ppm, unpublished data).

Therefore, all the main criteria defining the K/Pg boundary are recorded at the Izeh section, including the mass extinction of planktonic foraminifera and spike in iridium (see Fig. 3 and Fig. 4). The mass extinctions affected particular species of planktonic foraminifera, and especially the keeled globotruncanids and large heterohelicids like at the other sections showing a complete and continuous Cretaceous–Paleogene transition interval. However, the Cretaceous species crossing the K/Pg boundary are commonly unkeeled globotruncanids, hedbergellids, globigerinelloids and small heterohelicids. Among these latter, we note that: •

the species may be identical or different from section to other. To ensure that such species is reworked or it is a real survivor, it needs eventually to realize stable isotopic analysis on each individual specimen of these species and compare their values below and above the K/Pg boundary;

Besides, in the Izeh section no Cretaceous species was extinct below the K/Pg boundary and both Archaeoglobigerina blowi and A. cretacea became extinct at the K/Pg boundary. In contrast, in the El Kef section (Gallala, 2010) and the Agost and the Zumaya sections these species became extinct at few decimeters below the K/Pg boundary (Gallala, 2010 and Molina et al., 1998). In the Bidart section only A. cretacea became extinct below the K/Pg boundary (Gallala and Zaghbib-Turki, 2010 and Gallala et al., 2009).

The major crisis affecting the planktonic foraminifera was caused by the sudden environmental deterioration induced by the meteorite impact, which generated climate change and trophic starvation in the ocean superficial waters (Hallam, 1984, Hildebrand et al., 1991, Huber et al., 2002, MacLeod et al., 1997, McLean, 1991 and Schulte et al., 2010, among others). At least two thirds of the Maastrichtian planktonic foraminifera species became extinct within the first few centimeters above the K/Pg boundary (higher they are poorly preserved and probably reworked). The Upper Maastrichtian planktonic foraminiferal assemblages of the Izeh section are characterized by the highest diversity (43 species) observed in the entire studied interval (Table 1 and Fig. 3). They are dominated by cosmopolitan species dwelling in near surface and intermediate photic water depths (e.g., Heterohelix globulosa, Rugoglobigerina pennyi; see Abramovich et al., 2003). Small biserial species (heterohelicids and pseudoguembelinids) are frequent (e.g., Heterohelix globosa and H. navarroensis represent respectively 19% and 8%; see Fig. 3). Planispiral (globigerinelloids) and simple trochospiral species (hedbergellids and globotruncanellids) are also common, particularly Hedbergella monmouthensis and Hedbergella holmdelensis. These taxa are all relatively small and morphologically simple. The triserial heterohelicids species (Guembelitria spp.) are common just below the K/Pg boundary (approximately 28%); this may be due to burrowing organisms which reworked the Lowermost Danian material. Large, complex and tropical-subtropical taxa are less abundant than the cosmopolitan ones, but they are very diverse. For hedbergellids (i.e., Hedbergella monmouthensis and H. holmdelensis), which have been reported to cross the K/Pg boundary in several middle to lower latitude sections (Keller et al., 1995), their relative abundance decreases progressively towards bipolar and eventually austral habitats just prior to their extinction. H. monmouthensis, H. holmdelensis and Heterohelix globulosa, which dominated the Uppermost Maastrichtian planktonic foraminiferal assemblages, are as abundant in the Lowermost Danian as in the Maastrichtian. H. globulosa exhibits a similar pattern of decrease toward higher latitudes just before they disappeared from the Danian assemblages, while others (e.g., Pseudoguembelina palpebra, P. costulata, Rugoglobigerina hexacamerata, R. macrocephala, Pseudotextularia elegans) became extinct at the K/Pg boundary in middle and low latitudes, as in Tethyan region (Obaidalla, 2005). Simple and double-keeled trochospiral of the genera Globotruncana, Globotruncanita, Gansserina, Contusotruncana and Abathomphalus are generally rare in the Uppermost Maastrichtian, except Globotruncana arca and Globotruncana aegyptiaca, which are common. The unkeeled and rugose trochospiral species, e.g., Rugoglobigerina rugosa, R. macrocephala and Plummerita hantkeninoides, are more common than the keeled taxa. The large biserial helerohelicids belonging to Pseudotextularia, e.g., Pt. intermedia and Pt. elegans, are also common. However, the multiserial heterohelicids (e.g., Planoglobulina spp., Racemiguembelina spp. and Gublerina spp.), as well as high trochospiral keeled species, i.e., Contusotruncana spp. are rare. The index species of the Abathomphalus mayaroensis Biozone is also rare, and in the Uppermost Maastrichtian becomes increasingly rare. The index species of the Plummerita hantkeninoides Subzone is slightly more frequent within the Uppermost Maastrichtian sediments (Fig. 3). Thus, this subzonal index species allows further characterization of the terminal Maastrichtian biozonation. Moreover, the high diversity of the generic and specific composition of the Uppermost Maastrichtian assemblages in the Izeh section suggests optimal mesotrophic and oxygenated tropical to subtropical marine conditions. The Lower Danian Guembelitria cretacea PRZ, Parvularugoglobigerina eugubina TRZ and Parasubbotina pseudobulloides PRZ at the Izeh section are less expanded than at the El Kef and Ellès sections (see Fig. 8) and they are similarly developed as other continuous and complete sections known in the Tethyan region and elsewhere (e.g., El Melah, Aïn Settara, Zumaya, Bidart). Through these biozones, the planktonic foraminiferal species content is very similar to that of El Kef and Ellès sections in Tunisia. As in the other sections with a complete K-Pg transition, the assemblages of the Gt. cretacea Zone are poorly diversified and the specimens are all tiny, and species richness appears to be similar to that in the low and middle latitude Tethyan sections (Coccioni and Luciani, 2006, Gallala and Zaghbib-Turki, 2010, Gallala et al., 2009, Molina et al., 1998 and Zaghbib-Turki et al., 2000). The basal Danian planktonic foraminiferal assemblages are dominated by guembelitriids (e.g., Gt. cretacea and Gt. trifolia), which are the most frequent “Cretaceous-survivor” of the K/Pg boundary event. As compared to the other sections (Fig. 8), the G. cretacea Zone interval is expanded but without exceeding 25 cm in thickness of dark clayey. It covers the K/Pg boundary indicated by a few millimeters thick rust-colored ferruginous layer composed of reddish hematitic and goethitic laminae and lacking fossils. Within the lower part (over 13 cm) of the G. cretacea Zone especially the “Cretaceous-survivors” co-occurring with reworked species are present. The foraminifera species restricted to the Danian, which are of the genera Eoglobigerina, Woodringina and Parvularugoglobigerina, appear few centimeters above the K/Pg boundary. Most of the globotruncanids and large heterohelicids represented by altered tests (i.e., broken, dissolved and/or not filled by iron oxide) are probably reworked. However the small heterohelicids associated with tiny hedbergellids are better preserved. They are the main survivors crossing the K/Pg boundary event.

At the Izeh section, the Pv. eugubina Biozone is recognizable by the LO of Pv. eugubina at the base and the HO of Pv. eugubina and/or the LO of Ps. pseudobulloides at the top. Throughout this zone the assemblage still contains Cretaceous-survivors.

The assemblages of the Ps. pseudobuloides Zone are dominated by biserial species belonging to Chiloguembelina (15%) and Woodringina (30%). Guembelitria cretacea is frequent (18%) in the lower part of the zone. Thus, in the Early Danian the planktonic foraminiferal assemblages are very similar to those of Tunisian sections, e.g., at El Kef, Ellès and Aïn Settara and Spanish sections, e.g., Zumaya, San Sebastian, Osinaga, Músquiz and the French section at Bidart (Arenillas et al., 1998, Arenillas et al., 2004, Gallala and Zaghbib-Turki, 2010, Keller, 1988, Zaghbib-Turki et al., 2000 and Zaghbib-Turki et al., 2001). Although, the Ps. pseudobulloides Zone is well developed in the Gurpi Formation, only its lower 85 cm have been investigated in the present work. Many species appearing in the Guembelitria cretacea and Parvularugoglobigerina eugubina biozones have short stratigraphical ranges. As elsewhere, increasing size, diversification of wall structure, and increasing species diversity are the main trends in the behavior of the planktonic foraminiferal species and assemblages within the Ps. pseudobulloides Biozone as recorded in the GSSP for the K/Pg boundary and the auxiliary sections.

The palynomorphs were less affected by mass extinctions and most species crossed the K/Pg boundary indicating that there was not a major crisis in the dinocysts. Nevertheless, there were major changes in dinocyst assemblage structure beyond the K/Pg boundary. The LO of Manumiella seelandica, Phelodinium magnificum, Hystrichokolpoma bulbosum, and Cordosphaeridium fibrospinosum are the main dinocyst events within The Upper Maastrichtian Abathomphalus mayaroensis Zone. The K/Pg boundary coincides with the boundary between the Manumiella seelandica PRZ and Alisocysta reticulata PRZ. The important LO events of D. californicum, Carpatella cornuta, Cordosphaeridium gracile, and the HO of Dinogymnium acuminatum characterize the Damassadinium californicum/Carpatella cornuta CRZ and Cordosphaeridium gracile PRZ occurring at the Danian.

In short, the recognized dinocyst assemblages from the Izeh section are similar to that from the El Kef section containing several palynomorphs which indicate cold surface water (e.g., Manumiella seelandica) as described by Brinkhuis and Zachariasse (1988). Our organic-walled dinoflagellate semi-quantitative results are based on three slides per sample; it may that very rare dinocyst morphotypes are missed. In all the cases, in the Izeh section, the Danian samples do not contain reworked Cretaceous dinocysts. Mohamed et al. (2012) working on organic-walled dinocyst from the Cretaceous–Paleogene transition in Austria noted that the Lower Danian samples contain many Cretaceous dinoflagellates that they consider reworked. This was confirmed also by the enrichment of the Danian deposits in detrital lithology elements, lacking in the Izeh section.

The Cretaceous–Paleogene transition was a period of extremely stressful environmental conditions for many living organisms due to rapid and extreme climate fluctuations (Abramovich et al., 1998, Coccioni and Luciani, 2006, Hallam, 1984, Keller, 2003, Koeberl and MacLeod, 2002, Li and Keller, 1998a, Li and Keller, 1998b, Ryder et al., 1996 and Sharpton and Ward, 1990). The climate change and trophic starvation conditions caused by the asteroid impact added to widespread volcanism (Arenillas et al., 2006, Gertsch et al., 2011 and Keller et al., 2007 and references therein) generated high stress conditions and triggered the mass extinctions of tropical and subtropical species at the K/Pg boundary. This critical time of high stress environments is widely characterized by blooms of the opportunist Guembelitria species which have fast reproduction and very small size and may arise at any time of stressful environment [Albian, Cenomanian, Late Maastrichtian and Early Cenozoic (see Keller and Pardo, 2004, Leckie, 1987, Leckie et al., 1991 and Leckie et al., 1998)].

The Lowermost Danian Guembelitria bloom is associated with low productivity and rapid cooling induced by the asteroid collision (Abramovich and Keller, 2002 and Keller, 2003). The post-K/Pg bloom of guembelitriids in the world ocean occurred both in shallow and deep-water environments, near shore, and in the open ocean, and at high and low latitudes (Abramovich et al., 2002, Abramovich et al., 2003, Hull et al., 2011 and Liu and Olsson, 1992).

The relative abundance of Spiniferites shows a general increasing trend in relative abundance upward in the section (Firth, 1993 and Schiøler et al., 1997). This also shows a complex signal. In the other hand, high abundances of the Spiniferites group are in most cases associated with high sea level and/or low energy (Firth, 1993 and Schiøler et al., 1997). Harland (1983) considered the morphology of Spiniferites ramosusto be closely related to taxa observed in inner to outer neritic environments.

Our high-resolution biostratigraphical investigation confirms that the Izeh section from SW Iran, in the Zagros Basin, is complete and continuous across its Cretaceous–Paleocene transition interval and does not contain any stratigraphic hiatus. The planktonic foraminiferal biozones and subzones and dinocyst events characterizing the Uppermost Maastrichtian–Lower Danian interval are recorded and easily recognized by their well-preserved index species. The K/Pg transition at the Izeh section is characterized by the following features: •

the uppermost part of the Abathomphalus mayaroensis Zone including the Plummerita hantkeninoides Subzone is well developed in Gurpi Formation. This interval of deposition is rich in diverse planktonic foraminifera with cosmopolitan and tropical-subtropical species (e.g., globotruncanids, large heterohelicids, e.g., Pseudotextularia elegans, see Fig. 6);

The similarity of Izeh section with the other complete sections (in Tunisia, Spain, France, and other countries) in the model of planktonic foraminiferal species extinctions (mass extinctions and sudden loss of at least 43 of 61 species), coinciding with an Ir anomaly, confirms its catastrophic pattern compatible by an asteroid impact.

Minor differences in the assemblages may be due to their different specific composition background. The stressful conditions caused by the asteroid impact favor a bloom of the opportunist Guembelitria species and a few probable Cretaceous “survivors” (e.g., Hedbegella holmdelensis, H. monmouthensis, Heterohelix globulosa).

The palynomorphs were less affected by the catastrophic bioevent defined by sudden mass extinctions of species from diverse biological groups. Therefore and most dinocysts crossed the K/Pg boundary. Nevertheless, they showed changes in their assemblages’ structure beyond the K/Pg boundary. Of particular biostratigraphical importance, Manumiella seelandica and M. druggii typical species of Antarctic Maastrichtian dinocysts assemblages occur in coeval deposits at the Izeh section and they persist through the lower Danian. Like in Tunisia (e.g., El Kef section, Ellès section; see Brinkhuis and Leereveld, 1988, Brinkhuis and Zachariasse, 1988, Brinkhuis and Powell, 2004 and Galeotti et al., 2004), these boreal palynomorphs show an obvious increase. Besides, Alisocysta reticulata and Senoniasphaera inornata, the Earliest Danian markers first occur respectively at the K/Pg boundary and at 25 cm above.