Petrography and Clay Mineral Variations Across the Cretaceous/Paleogene (K/Pg) Boundary, Sulaimani, Kurdistan Region, Iraq

Abstract


Introduction
The Cretaceous-Paleogene (K/Pg) extinction event is the most recently documented mass extinction and the only one, which has been linked to a major asteroid impact (Alvarez et al. 1980).The exact cause of this mass extinction is intensely debated.In the last decades, numerous theories have been proposed to explain the mass extinction including an asteroid impact (Alvarez et al., 1980;Hildebrand et al., 1991;Alvarez et al., 1995), and widespread volcanism eruption (McLean, 1985;Courtillot et al., 1986;Vandamme et al., 1991).Timing and which one was most effective from these two catastrophic event remain debated (Schulte et al., 2010;Keller, 2014).According to Renne et al. (2013), the age of the boundary transition was about 66 Ma.
The K/Pg boundary in High Zagros Folded Thrust Belt (HZFTB) around Sulaimani city located between Tanjero and Kolosh formations (Al-Qayim et al., 2020) (Fig. 1).Research works on the bounadry between mentioned formations was mostly focused on biostratigraphy without detailed record of other parameters in the geological record of the boundary.These researches was mostly discussed about is sedimentation was continue or there is a break in sedimentation, especially Danian sediments.Most of the recent studies use planktonic foraminifera and nannofossil emphasizes that lower Danian sediments exist between Tanjero and Kolosh formations, and mark the boundary as continue (i.e.without break in sedimentation) (Al-Qayim and Al-Shaibani, 1989;Sharbazheri et al., 2009;Kharajiany et al., 2019;Al Nuaimy et al., 2020;Al-Qayim et al., 2020;Kharajiany et al., 2020).While there are many studies that marked the boundary as unconformity (Kassab, 1972;Jassim and Goff, 2006;Sissakian and Al-Jiburi, 2014).One of the most oldest work on the contact between the formations was conducted by van Bellen et al. (1959).They discussed this contact in detail and also marked the contact as a widespread unconformity, however; the study cocluded that the boundary could be continuing in some areas without determination of location, in this case, paleontological pull-out will be helpful for determining the boundary.Research on other proxies besides paleontology in Tanjero and Kolosh formations boundary are really scarce, except that work using stable isotope to study the boundary by Al-Qayim et al. (2021).Using various proxies will be helpful to reconstruct the optimum possible model for what was the exact situation between Tanjero and Kolosh formations.This study aims to define petrographic charactersitics and clay mineralogy of different lithologies across the boundary, and using these charactersitcs to decipher tectonic setting, provenance, paleoclimate, and depositional environments across K/Pg boundary.

Geologic Setting
Collision between the Arabian plate and the Iranian plate made a foreland basin in front of obducted Iranian plate.The basin striking (NW-SE) parallel to Zagros Orogenic Belt (Al-Qayim et al., 2012).The selected section (Dartw section) which was studied passes across boundary between Tanjero and Kolosh formations.Geographically; this section located NE of Iraq about 18.5 Km from N60W of center Sulaimani city and about 7 km from N30E of Tasluja town at (35 38' 55.12" N and 45 16' 18.15"E).Geologically; it is at SE plunge of Yakhyan anticline (part of Piramagrun mountain) (Fig. 1).Yakhyan Anticline is one high folded structures in HZFTB zone according to tectonic subdivision of Iraq (Lawa et al., 2013;Fouad, 2015).The Zagros Folded Thrust Belt has 25-50 km wide parallel to main trend of Zagros belt, the variation was caused by transversal faults.This zone is characterized by strong deformations in different lithostratigraphic units as it appears in many valleys and road cut outcrops, the anticlines have high amplitude with narrow synclines between them, units of the Cretaceous deposits comprise most of anticlines.They have whale back anticlines which are verge to S and SW, all of these characteristics were a consequence of convergent of the Arabian and the Eurasian (Iranian) plate starting from late Cretaceous (Jassim and Goff, 2006;Aqrawi et al., 2010;Fouad, 2015).K/Pg boundary at HZFTB in Sulaimani area is between Tanjero and Kolosh formations.Tanjero Formation is a turbidite facies deposit belonging to late Campanian-late Maastrichtian early foreland basin, that was deposited in a submarine fan parallel to the main trends of Zagros belt (Jaza, 1992;Karim and Surdashy, 2005a;Ahmed et al., 2015), It was first described by Dunnington (1952), its type locality in Sirwan valley, near Halabja, north of Iraq.Lithology of this formation is mainly composed of clastics but it also contains beds of limestone and argillaceous limestone (van Bellen et al., 1959).In the studied area, it has a well alternation of olive green variable grain size sandstones with bluish and olive green marlstone and argillaceous limestone.The lower part of the section has lithology more likely to be missinterpreted as Shiranish Formation but it has many very thin beds of sandstone (Fig. 2).This is the same as reported by Jaza (1992) and studied by Ismael et al. (2011) in Sulaimani area where they found that Tanjero Formation contain intervals similar to Shiranish Formation in lithology, this is because these two formations are deposited in single basin at initial stage of Zagros Foreland Basin (Karim, 2004).
The Paleogene Kolosh Formation was starting to be deposited in nearly same depositional basin as of Tanjero Formation, it represents complex low density turbidite deposited in submarine fan (Jassim and Goff, 2006;Al-Qayim et al., 2008).This formation also first described by Dunnington (1952), its type locality located from north of Koya town at Kolosh Village.It comprises bluish shale, green sandstone (with variable grain composition), calcareous shale, and limestone.The formation mainly shows mainly rhythmic alternation between clastic sediment and pelagic carbonate as a reflection of cycles of turbidite current (Van Bellen et al., 1959;Al-Qayim and Salman, 1986).In the studied area, this formation shows no abrupt change with underlying Tanjero Formation, except variation, which can be noted with changing color from bluish calcareous shale and green sandstone to darker color (dark green to dark grey), and sandstones of Kolosh Formation in the section are less tough compared with sandstones of Tanjero Formation (Fig. 2).Moreover; the amount of silt in calcareous shale totally declined upward in lower part of Kolosh Formation.

Materials and Methods
Seventy-nine samples from both Tanjero and Kolosh formations were collected across K/Pg boundary in Dartw section outcrop.The samples are representative to lithology variation of turbidite cycles.Sample collection was at four stages, in order to closeness to the boundary interval.Intervals between individual samples are based on proximity to the boundary, at boundary transition sample intervals are much smaller compared to distant samples.Thin sections from all samples for petrographic study prepared at workshop of the Department of Geology, University of Sulaimani.
The X-Rey diffraction (XRD) analyses were carried out on fifteen samples to investigate clay minerals type in beds of different lithology.This analysis was done to samples with close distance to the K/Pg boundary.Three samples of the sandstones selected to define their clay mineral content, Samples are DT-46, DT-59, DT-69 from below, boundary transition, and above the boundary respectively.The other 12 samples composed of marlstone and argillaceous limestone, which is divided in half between the two formations (six samples for each formation).Clay fraction of samples are separated form samples according to Moore and Reynolds (1997) by digesting sample powder with 1N HCl to remove carbonate fraction, then adding drops of H2O2 in order to remove organic matter (washing was applied in each step), after that drain the upper part of the solution into graduated cylinder (500 ml).In order to disperse clay particles, 1 gm of Na-Hexametaphosphate was added to the solution.By following Stoke's law clay size fraction separated and concentrated, by adding drops of concentrated part to glass slides and leaving it to dry at room temperature to make the slide ready for analysis.
Identification of clay mineral types was based on Moore andReynolds (1997), Hillier (2003), and Środoń (2006) and to calibrate the result of identified main phases present in the analyzed samples, the results were compared to Morrison and Parry (1986), Shau et al. (1990), Beaufort et al. (1997Beaufort et al. ( , 2015)), Leoni et al. (2010), andDrits et al. (2011).In order to study the microstructure of corrensite, the SEM was used.A small freshly surface chip of sample (DT-69) was gold-coated using Emitech K550X sputter coater at Kurdistan Institute for Strategic Studies and Scientific Research (KISSR).This observation was performed using FEI Quanta 400 at University of Sulaimani/Department of Geology.

Petrography
Petrographic study of collected samples subdivided based on lithology, carbonates and sandstones:

Carbonates
From all collected samples, 57 samples are from marlstone and argillaceous limestone beds.The samples show no variation across the boundary on the petrographic base.Fine grain samples are classified based on Wright (1992).Most of the samples are belonging to wackestone and mudstone (Fig. 3), one of the samples is packstone (DT-14) at 8 meters below the boundary which contains a large number of fossils packed together.In Dartw section Tanjero and Kolosh formations overlap, so the K/Pg boundary was marked by means of planktonic foraminifera in wackstone and mudstone samples.
The boundary was determined by appearance of lowermost Paleogene indexes of foraminifera (Parvularugoglobigerina eugubina) with (Guembelitria cretacea) (Fig. 3) in sample DT-61.Below this sample, there is a barren zone of 20cm in thickness which is composed of very fine sand and silt (DT-58, DT-59 and DT-60).All other planktonic foramineferas in samples below DT-58 belong to late Cretaceous.Nannofossil assemblages of Dartw section was studied by Al-Qayim et al. ( 2020), according to their study the section has continues record of nannofossil biostratigraphy.

Sandstone
Petrographic study of 22 sandstone samples manifest that sandstone are fine to coarse grained, poorly sorted and rounded.Sandstones are classified as lithic wacke (Fig. 4).sandstone beds across K/Pg boundary gradually varied in detrital composition at long distant samples from Tanjero to Kolosh formations (Fig. 5).Three major components (i.e.Argillaceous RF, Quartz, and Carbonate RF) make up about 40% of all compositions.Argillaceous rock fragment is the major constituent with highest fraction below the K/Pg boundary which reaches 31%, it does not show great variation except anomalous increase near the boundary.Total quartz content as the second major constituent shows a gradual increase across the boundary (Fig. 5).The majority of quartz grains are monocrystalline quartz (Fig. 3), amount of polycrystalline quartz in the Tanjero Formation (below the boundary) is really scarce, only some grains can be seen, but in the studied Kolosh Formation (above the boundary) polycrystalline quartz increases in its amount, sometime reaches 3% in some samples.The third major constituent is carbonate rock fragments (Fig. 3).Carbonate and chert rock fragments variation across the boundary shows a slight decrease (Fig. 5).
The most dramatic decline was recorded in radiolarian chert content of sandstone, which tends to decrease from 20% in lowermost sample to 1% and is scarce to be found at top of the section (Table 1).Radiolarian chert are well round grain with small spikes and filled with chalcedony (Fig. 3).Igneous and metamorphic rock fragments can be found in the boundary, below the boundary only a few grains of both components can be found, but above the boundary igneous detritus tend to increase up to 6%, while metamorphic detritus does not show variation only trace amount exists from both above and below the boundary.The composition of grains is varied, most igneous detritus were mafic minerals (Fig. 3), while metamorphic detritus is represented by serpentine, chlorite, schist, and some traces of micas.Feldspar detritus similar to igneous detritus show a small increase after the boundary, it tends to be increased to about 5% near top of the section, but below the boundary, feldspar has a greater occurrence, unlike igneous detritus (Table 1).
Amounts of matrix and cement are presented in Table 1.Matrix material between framework grains is mostly mud material but appears to be affected by chloritization (diagenesis) by its light green color and replaces the grain and filling as fibrous material between grains (Fig. 3).Other components were represented by iron oxide (as patches and fossil replacement), reworked fossils, heavy minerals (zircon, rutile, garnet...….etc.), and plant remains in some samples.Matrix content across the section increased to its maximum content at the K/Pg boundary after about 20 m above the boundary decreased to its average level.Cementing material contains calcite as a major form of binding material in all samples.Iron oxides and siliceous cement appear in some samples as trace content.Matrix material as binding material is hard to be distinguished with an optical microscope (Fig. 3).As mentioned before, sandstones contain a substantial matrix with a high amount of lithic fragments.They are classified as lithic wacke expect five samples which contain less amount matrix classified as lithic arenite (Table 1).In the population of samples in the ternary plot, the two formation samples variation in component appears with quartz and feldspar increase and some samples do overlap (Fig. 4).

Clay Mineralogy
Diffractogram patterns from XRD data after (AD) (EG) and (H) run reveal that clay fractions for sandstones, marlstones and argillaceous limestones were predominantly dominated by corrensite (regularly interstratified chlorite smectite mixed layer).Corrensite is the second most abundant mixed layer in the natural environment after illite/smectite mixed layer.it is composed of regular interstratifications R1 (50:50) of chlorite layers with smectite layers (low-charge corrensite) or chlorite layers with vermiculite layers (high-charge corrensite) (Moore and Reynolds 1997).On XRD diffractograms of oriented samples, corrensite is characterized by a series of harmonic basal reflections from very low angles of 2 toward higher angles based on Mering's principle between chlorite and smectite basal reflections (Fig. 6).
On AD state patterns show a series of basal reflections related to corrensite (Table 2).Upon ethylene glycol salvation treatment all harmonic reflections of corrensite shifted toward lower angles of 2 except (003) reflection which is extinct to almost the background record, it is an indication of existence of expandable layers and approaching to result of low charge corrensite, as vermiculite does not expand (Fig. 6A).Major peaks d-spacing of EG (Table 2) compared to corrensite of published works are in perfect match, appearance a peak at exact 7.8 Å (11.3 2) for 004 basal reflection is a diagnostic characteristic for identification corrensite.After heating peaks shifted toward higher angles due to dehydration of smectite layers (Fig. 6B).Basal reflections of samples after heating (Table 2) also coincide with corrensite in published works.The superstructure of corrensite ( 001) which has d-spacing 29.2 Å from a combination of chlorite and smectite d-spacing (14.2+15Å) is difficult to record because of some instrumental artifacts, but the high intensity of very low angles of all samples is also a good indicator of corrensite.Another indication for assuring the existence of this mineral can be found on peaks of dehydrated state of samples, with this treatment smectite structure is dehydrated and its structure collapse to about 9.7 Å, this marks corrensite superstructure collapse to have about 24 Å d-spacing (Fig. 6B).Finally obtained patterns from all different condition of samples confirms that corrensite is the major clay component of our samples without changes in it is dominance across K/Pg boundary in Dartw section.Chlorite and smectite from XRD data were also recorded as the discrete face.Because of their closeness to each other, (001) basal reflections of both discrete face chlorite (14.2 Å) and smectite (15 Å) was masked by high and broad peak of corrensite (002) in both AD and EG states.While upon dehydration they are separated to (14.1, 12.00, 10 Å) for chlorite, corrensite, and smectite, respectively.Chlorite (002) reflection at 7.1 Å in most of the samples obviously appear on both AD and EG state but heating to 350 C for 1 hr may cause this peak to reduce its intensity in sample DT-57 (Fig. 5).Heating the same sample to 550 C for 1 hr caused chlorite (002) to disappear, this was also documented by Hillier and Velde (1992), If this reflection was absent at 350 C must be regarded as kaolinite but it was chlorite.Heating all other samples to 550 C for 1 hr caused to increase in the intensity of chlorite (001) and some decreased intensity of all other peaks of corrensite (Fig. 6).Traces of illite were also recorded in some samples when a small peak at 10 Å remain in its position at all treatments.
SEM micrograph images showed in Fig. 7. Corrensite observed as in photos as it has flake with webby morphology making a boxwork texture (Fig. 7A).Corrensite webby morphology filling pores as dense flakey mass is visible (Fig 7B), which fills spaces between sand grains to block porosity and spread out among the rock.
Petrographic observation of sandstones reveals that corrensite crystals grow between grains and replaces some grains (Fig. 8).From XRD analysis, three indications can be used to interpret corrensite as a diagenetic product.First, existing discrete smectite and chlorite, they are end members of intermediate corrensite (R1, Chlorite smectite mixed layer) is a good indication for formation of corrensite from smectite.Second, is the progress of chloritization between corrensite and chlorite through a mixed layer of corrensite/chlorite. this can be traced between corrensite (004) to chlorite (002) and corrensite (006) to chlorite (003) to corrensite (007) on EG state (Beaufort et al. 1997;Leoni et al. 2010) (Fig. 6).Third, it cannot be regarded as weathering product in the outcrop, because during telogenesis retrograde of chloritization chlorite/vermiculite mixed layer formed instead of chlorite/smectite mixed layer (Moore and Reynolds, 1997).

Tectonic Setting and Provenance
Different kinds of rock fragments in sandstone can hold a record about its source rock, weathering condition, transportation distance to the basin, and the depositional environment (Boggs Jr and Boggs, 2009).Rock fragments can be also used to study any changes that occurred after deposition by diagenesis.Petrographical study of sandstone from K/Pg boundary at Dartw section reveals that all sandstones from below to above boundary are texturally immature as evidenced by their high matrix content, grain size variability, sharp angular edges, high variability of grain composition, and high content of unstable grains (carbonate rock fragments and argillaceous rock fragments).These all properties are characteristics of mixed provenance area and proximity of the provenance to the depositional environment (Dickinson, 1985).From QmFL ternary diagram plot (Fig. 9) (Dickinson, 1985), it is inferred that the area where particles derived from belonging to recycled orogen, with showing variation of samples population for the units from above and below the boundary.Most Cretaceous (Tanjero Formation) sandstones belong to lithic recycled orogen, while Paleogene (Kolosh Formation) sandstones represent transitional recycled orogen.Paleocurrent analysis conducted by Karim and Surdashy (2005b) and Çelik and Salih (2021) of the Tanjero Formation indicate that the source area is located NE of the basin.The area where an uplifted older sedimentary rocks (Fig. 10A), exposes to tectonic force of Arabian Plate movement toward NE (Al-Qayim et al., 2012).As a consequence of Zagros Foreland Basin formed by uplifting older sedimentary rocks, it is represented by flysch deposits of the Tanjero and Kolosh formations.Petrographic variations revealed the evolution of Zagros Foreland Basin provenance from lithic recycled to transitional recycled orogeny, by increasing the amount of quartz content (Fig. 9).This evolution of the foreland basin can be traced from other component variations, like a-increasing both igneous rock fragments and feldspar, b-decreasing amount of carbonate rock fragments, argillaceous rock fragments and bioclast fragments (radiolarians) (Fig. 3).Variation in detrital composition of grains just below the boundary is most probably caused by global high-stress environment at the end of Maastrichtian caused by Deccan trap volcanism before bolide impact (Keller, 2014;Font et al., 2018), from field observation, this zone is characterized by an abundance of plant remain content in sandstone beds.Changes in detrital composition between late Cretaceous and early Paleocene is the consequence of different rock type uplift in the adjacent hinterland (obduction of ophiolites) (Fig 10B).The gradual increase in values of igneous rock fragments is a good indication of tectonic evolution between Arabian and Iranian Plates.Ophiolites step by step become a source for flysch deposits in its front foreland basin, it is indicated by mineral types of igneous rock fragments which is mainly belong to ultramafic and mafic minerals (Fig. 5).There is also a record of high ultramafic and mafic content in Kolosh formation (Al-Mashaikie and Mohammed, 2018).Thus Qulqula group, carbonates of Bisotun block (Avroman Formation), and Zagros ophiolites were potential source rock lithostratigraphic units for sediments across K/Pg boundary in Dartw section.These units are mainly composed of radiolarian chert, limestones, mudstones, shale, peridotites, pyroxenites, and basalt (Mohammad, 2008;Baziany, 2014;Mirza et al., 2021).

Paleoclimate and Depositional Environment
XRD and SEM results of clay fraction, show that most of clay mineral assemblage across the K/Pg boundary was composed of corrensite, with chlorite and smectite as a discrete face.clay mineral assemblage across the boundary does not show any significant variation.Corrensite can be formed in a variety of geological environments.High Corrensite content at the boundary between the Tanjero and Kolosh formations in Dartw section interpreted as an intermediate stage of smectite chloritization.
Corrensite as a major neoformed component assemblage clay mineral from precursor smectite must be reached temperature realm range, its formation temperature based on vitrinite reflectance was estimated by Chang et al. (1986).Sandstones permit more fluid transfer, so corrensite can form as low as 60 C, while in fine grained sedimentary rock, it formed at 70 C as lower limit.Upper limit temperature for corrensite is at about 120 C (Worden et al., 2020).Chloritization of smectite through corrensite reaction mechanism two models were proposed (Beaufort et al., 2015): 1-Solid-state transformation by increasing amount of chlorite layers continuously.2-dissolution-crystallization, this mechanism is stepwise transition from smectite to corrensite and from corrensite to chlorite.The results of clay mineral study support that the interlayering of mudstone, marlstone, and sandstones across K/Pg boundary was dominated by detrital smectite at the time of deposition, however, it had been subjected to diagenesis after burial.These high abundances of smectite-like clay in sandstone further support immature sediment, because smectite is the transient mineral in weathering progress and it is more likely to be the first mineral forming during weathering of source rock (McKinley et al., 2003), Thus before clay mineral maturation, smectite deposited in the basin across the boundary.The present of smectite suggests arid and seasonal variation weathering paleoclimate of mixed source area and implies lowintensity chemical weathering (Chamley, 1989;Hillier, 1995;McKinley et al., 2003;Worden et al., 2020).Furthermore, the presence of thick beds of mudstone also support semi-arid to arid climate as its ideal condition for clay accumulation with episodic flood (Ketzer et al., 2003).The abundance of smectite in the Cretaceous mainly formed as the consequence of a large amount of pedogenic layer produced by hot and seasonal changes in humidity (Chamley, 1997).Results of petrography also support smectite-dominated paleoclimate.Sandstones with high content of unstable grains (argillaceous and carbonate rock fragments) reflect low intensity of weathering and arid to semi-arid climate, with a slight difference in decreasing unstable detritus and increasing quartz detritus between sandstones from the Tanjero to Kolosh formations (Fig. 11).The Tanjero and Kolosh formations were previously mentioned as marine deposits in Zagros foreland basin.Boundary transition between these two formations hosted smectite rich mudstone because hinterland produces smectite and this is most probably the result of sandstone also being smectite rich (McKinley et al., 2003).Detrital smectite is the last clay mineral to flocculate from water, so it is deposited furthest from the shore (Worden and Morad, 2003).It has a smaller size and is easily transported by wave action to a deeper part of the ocean, so smectite increase basinward (Hillier, 1995;Šimkevičius et al., 2003).This two factors of flocculation sequence and grain size of clay minerals illustrates that clay mineral of Dartw section was deposited in deep marine environment.High mudstone and marlstone beds to sand ratio also support distal turbidite facies in deep marine (Shaw and Conybeare, 2003), while sandstone cycles may reflect changes in sea level.

Conclusions
The petrographic and clay mineral study across K/Pg boundary between Tanjero and Kolosh formations lead to the following conclusions: • All sandstones are immature evidenced by the high amount of unstable content and angularity of grains with mixed many kinds of grain.• Clay mineral assemblage dominated by corrensite with discrete faces of chlorite, smectite, and corrensite/chlorite mixed layers.Corrensite was formed by burial diagenesis from precursor smectite.It is a middle stage of smectite chloritization.• Sandstones have varied gradually across the boundary by increasing the amount of quartz, plagioclase, and igneous rock fragments with decreasing argillaceous and carbonate rock fragments.• Changes in grain components and sandstone immaturity reflect evolution of tectonism in the Zagros Foreland Basin.Uplifting of Zagros ophiolite zone was contribute to sandstone provenance with formerly uplifted sedimentary units (i.e.Qulqula Group and Avroman Formation).• The precursor detrital smectite, similar to sandstones are immature clay mineral, therefore; they are belonging to low intensity of weathering and arid to semi-arid climate in the source area.• High smectite enrichment in clay minerals in both Tanjero and Kolosh formations is good proxy for a deep depositional environment, because of its late flocculation and its small size.Thin sandstones compared to marlstones and argillaceous limestones indicate distal turbidite facies at deeper part of the basin in Dartw section.

Fig. 2 .
Fig. 2. Stratigraphic column of the studied section across the K/Pg boundary near Sulaimani city, Kurdistan Region, Iraq.

Fig. 5 .
Fig. 5. Petrographic record across studied section.A) Main individual framework grain percentage vertical variation; B) All component (porosity was not detectable in the samples).

Fig. 8 .
Fig. 8. Photomicrographs of corrensite.a&b) PPL and XP view of corrensite (cr) replacing a grain; c&d) PPL and XP view of corrensite spread out between grains and three radiolarians filled by chalcedony (Cha).

Fig. 11 .
Fig. 11.Bivariant log/log plot of the ratio of polycrystalline quartz to feldspar plus rock fragments against the ratio of total quartz to feldspar plus rock fragments (after Suttner and Dutta, 1986).