The calcareous nannofossil record of the uppermost Maastrichtian-lower Palaeocene in the Kırıkkale Basin, in the Central Anatolian Region (Turkey)

The Late Cretaceous–Early Paleogene (K–Pg) was a critical period of transition in geological time. This period encompassed short-term climatic fluctuations on a global scale, changes in ocean circulation, and sudden and large extinctions of marine and terrestrial organisms. In the study area, located in the mid to low latitudes, the Late Cretaceous and Early Paleogene were very tectonically active due to the positioning of the site close to the collision zone of two large continents. The impacts of the global K–Pg crisis can be observed in the study area. In this study, the calcareous nannofossil contents of late Maastrichtian–Danian sediments were studied, and the nannofossil biostratigraphy determined, from samples from the Samanlık and Dizilitaşlar Formations, deposited in the Kırıkkale Basin. From three stratigraphic sections, 26 nannofossil genera and 36 nannofossil species were identified from the Late Maastrichtian UC20a TP and UC20b TP biozones and the NP1 and NP2 biozones of the Danian. Additionally, it was determined that the K–Pg boundary was not continuous in the study area. In the Kırıkkale Basin, relatively low abundances of Micula decussate Vekshina, 1959 signals a diagenetic effect and stressful environment in the Late Maastrichtian, whereas the relatively low abundances of Thoracosphaera operculata Bramlette & Martini, 1964 , Braarudosphaera bigelowii (Gran & Braarud, 1935) Def -landre, 1947 and Futyania petalosa (Ellis & Lohmann, 1973) Varol, 1989 in the Danian assem - blages indicate unstable environmental conditions and major environmental perturbations that reflect tectonic activity in the region. No nannofossils were encountered in those samples taken from turbiditic levels, which contained high proportions of sand.


INTRODUCTION
The study area is located close to the villages of Hacıbalı, Mahmutlar and Aşağısamanlık in Kırıkkale Province, in the Central Anatolian Region (Fig. 1). The depositional basins in the Central Anatolian Region formed above two continental units--the Sakarya Continent and Kırşehir Block-separated by a suture zone (GORUR et al., 1998) (Fig. 1). In the Late Cretaceous, the Sakarya Continent, Menderes-Tauride Platform and Kırşehir Block were separated by the İzmir-Ankara, Ankara-Erzincan and Inner Tauride Oceans. At the end of the Cretaceous, the Kırşehir Block and Sakarya Continent collided (TUYSUZ & DELLALOGLU, 1992;GORUR et al., 1998). The Kırşehir Block is surrounded by ophiolitic mélanges created as a result of the subduction of the İzmir-Ankara, Ankara-Erzincan and, later, the Inner Tauride Oceans (GORUR et al., 1998).
The Kırşehir Block, on which the study area is located, comprises mostly ophiolitic rock fragments and sedimentary cover rocks sitting above a metamorphic basement. The basement comprises gneiss, amphibolite, schists and marbles resulting from Cretaceous metamorphism (KETIN, 1961;ERKAN, 1975;ŞENGOR & YILMAZ, 1981;GONCUOĞLU, 1981;SEYMEN, 1982;ŞENGOR, 1985;GORUR et al., 1998). The cover rocks above this metamorphic basement extend from the Upper Cretaceous to the Holocene and mostly represent sedimentary sequences filling basins that varied in width and depth (GORUR et al., 1998).

The calcareous nannofossil record of the uppermost Maastrichtian-lower Palaeocene in the Kırıkkale Basin, in the Central Anatolian Region (Turkey)
The Kırıkkale Basin, which evolved between the Late Cretaceous and Late Palaeocene, has several features of a within-arc basin (GORUR et al., 1998), and is deep and narrow, forming a connection between the Çankırı Basin to the north and the Tuzgölü Basin to the south (NORMAN, 1972a, b;GOKCEN, 1977). The turbiditic sedimentation in the basin began before the Maastrichtian, with abundant volcanic and volcano-sedimentary contributions. GORUR et al. (1998) stated that the turbiditic sedimentation lasted until the end of the Lutetian and was coeval with shallow marine sedimentation in the east and southeast in the Late Thanetian-Ypresian.
Although several geological studies have been carried out in the region, biostratigraphic studies are limited. YILDIZ et al. (2000) investigated the biostratigraphy and palaeoecology of the Lower Maastrichtian-Paleocene sedimentary units, using trace fossils, planktonic foraminifera and calcareous nannofossils. They also determined the δ 18 O and δ 13 C isotope values from planktonic foraminifera tests and compared these with the abundance distributions of calcareous nannofossils purported to be sensitive to temperature, finding that the sea-surface water temperature and salinity were lower in the Early Maastrichtian than the Palaeocene, increasing from the beginning of the Danian. AKORALER (2018) and GORMUS & AKORALER (2019) investigated the benthic foraminiferal biostratigraphy and palaeoecology of the Late Cretaceous and Maastrichtian sedimentary deposits of the Kırıkkale and Kalecik regions. They posited that increased abundances of Orbitoides indicated tolerable temperature conditions and a shallow basin in this interval.
We investigated the nannofossil biostratigraphy in detail in the turbiditic sedimentary rocks in the Kırıkkale Basin in the İzmir-Ankara zone--one of the important tectonic belts in Turkey. In particular, we focused on determining changes in the Maastrichtian to Danian palaeoenvironment using the relative abundances of certain nannofossil species.

GEOLOGICAL SETTING
The study area was located in the İzmir-Ankara Zone. Rocks belonging to the İzmir-Ankara Zone were tectonically emplaced above the rocks of the Kırşehir Block, while rocks belonging to the Sakarya Zone were tectonically placed above these (DON-MEZ et al., 2008). The rocks of the İzmir-Ankara Zone have tectonostratigraphic relationships with each other, the oldest unit comprising the Late Cretaceous Artova Ophiolite Complex, and the youngest being Paleocene-Early Eocene in age (Figs. 1, 2). Rocks from the İzmir-Ankara Zone typically comprise sedi-ments from a closing basin (i.e. a basin with converging margins) that were deposited in the Late Cretaceous through to the Early Eocene. The relationships between sediments with turbiditic characteristics mainly occur in the form of underthrusts in a basin that was converging and closing (DONMEZ et al., 2008).
As explained above, the Artova Ophiolite Complex tectonically overlies rocks of the Kırşehir Block, while rocks of the Sakarya Zone have been emplaced as thrust units above this (DON-MEZ et al., 2008). The Kocatepe Formation occurs above the Artova Ophiolite and comprises sequences of fragmented, clayey pelagic limestone and radiolarite-mudstone with calciturbidite intercalations (Fig. 2). The unit has been interpreted as Cenomanian-Campanian in age based on benthic and planktonic fossils (AKYUREK et al., 1997).
The Samanlık Formation can be correlated to the Ilıcapınar Formation (AKYUREK et al., 1984), has a NE-SW orientation and is mainly turbiditic, comprising yellow, green, brown and grey conglomerate, sandstone and shale sequences (Figs. 1, 2). The unit comprises intermediate turbidites deposited in the lower part of a submarine fan at the same time as the Ilıcapınar Formation was being deposited (AKYUREK et al., 1984). The age of the Samanlık Formation is Maastrichtian based on fossils (AKY-UREK et al., 1984).
The Haymana Formation (RIGO DE RIGHI & CORTESINI, 1959) overlies the Samanlık Formation and comprises intermediate turbidites with conglomerate, sandstone and shale alternations deposited in the middle of the lower part of a submarine fan (AKYUREK et al., 1997) (Fig. 2). The unit is considered to be of Maastrichtian age, based on fossil evidence, and gradually grades into the overlying Dizilitaşlar Formation (DONMEZ et al., 2008) (Figs. 1, 2).
The Dizilitaşlar Formation (NORMAN, 1972a) comprises Palaeocene-Lower Eocene turbiditic sediments deposited in a basin that developed in front of the Artova Ophiolite Complex in the Late Cretaceous. The limestone blocks occurring in this formation are known as the Kuşkayatepe Limestone Olistolith Member (DONMEZ et al., 2008) (Figs. 1, 2). The Dizilitaşlar Formation transitions into the Maastrichtian Samanlık Formation in some places, whereas it sits on top of the ophiolite, above an angular disconformity, in others (BİRGİLİ et al., 1975). The Çayraz Formation unconformably overlies the Dizilitaşlar Formation (DONMEZ et al., 2008) (Fig. 1). Above this is the Sarıkaya Formation, comprising conglomerates, sandstones and agglomerates containing volcanic material and, dominantly, andesitic lava. In this region, this formation is probably related to arc volcanism linked to a subduction event in the İzmir-Ankara Zone in the Late Cretaceous-Palaeocene (DONMEZ et al., 2008) (Figs. 1, 2).

MATERIAL AND METHODS
The Samanlık and Dizilitaşlar Formations were the focus of this study, and their outcrops in the region were examined in three measured stratigraphic sections at different locations--the Kırıkkale Organised Industrial Zone, Aşağı Samanlık and the Organised Industrial Region (Fig. 1). The sample intervals varied based on the field conditions and the general structure of the geology, with samples being taken every 30 to 100 cm. A total of 99 samples were collected from the three measured sections--32 samples from the Kırıkkale Organised Industrial Zone (Samples KOS1-KOS32), 40 samples from Aşağı Samanlık (Samples AS1-  The calcareous nannofossils were analysed in smear slides (BOWN & YOUNG, 1998). Each slide was viewed at 1600x magnification with an oil-immersion lens under polarised light using a Leica DM 2500P microscope. Some of the nannofossils were photographed using a digital camera (Leica DFC 295). The relative abundances of the nannofossils were estimated using the method described in WEI (1988). Based on this method, one or more specimens of one species in each field of view (FOV) was classed as abundant (A), one in 2-10 FOVs as common (C), one in 11-50 FOVs as few (F), and one in 51-200 FOVs as rare (R).

RESULTS
The Upper Cretaceous (UC) standard nannofossil biozonation scheme developed by BURNETT et al. (1998) was used to determine the samples of Late Cretaceous age, whilst the NP biozonation scheme of MARTINI (1971) was used to date the Early Palaeocene samples. The species authors can be found in the book of Calcareous Nannofossil Biostratigraphy and website (Nannotax 3; https://www.mikrotax.org/Nannotax3/index.php?dir=ntax_ cenozoic).

Calcareous nannofossil implications of the deposits
Below, the distributions of the calcareous nannofossils and the biostratigraphy interpreted from them are discussed separately for each measured section.

Lithology
This section was measured along the road through the Kırıkkale Organised Industrial Zone (Fig. 1). The Samanlık Formation crops out at the base of the section, passing upwards into the Dizilitaşlar Formation with a tectonic contact. The total thickness of the section was 37 m, with 27 samples being taken (Fig. 3). Twenty-one nannofossil genera and 27 nannofossil species were identified ( Table 1).
The base of the section generally comprised brownish-grey, fine-to medium-bedded sandstones and greyish-dark green laminated shale alternations of the Samanlık Formation (Fig. 3). Finebedded, cream-coloured clayey limestone layers were observed between these. While the section passed upwards into the Dizilitaşlar Formation, with a faulted contact, this contact between the two formations was covered in the study area. At the base of the Dizilitaşlar Formation, the sandstones beds are fine, but they thicken (10-30 cm) towards the upper part of the formation. Fewer shales were observed in the upper part of the formation, where sandstone is more dominant. Faulting and deformation structures were frequently encountered throughout the section.

Calcareous nannofossils
The samples from the lowest 7 m of the section were determined to fall into the Nannofossil Biozone UC20a TP , the assemblages containing mostly very rare Lithraphidites quadratus Bramlette & Martini, 1964, the base of which defines the base of UC20 (Fig. 3, Table 1). Zone UC20b TP (defined from the base of Micula murus (Martini, 1961) Bukry, 1973 was identified in the same lithology, from 7 to 13 m (Fig. 3, Table 1). The species richness and abundance of Cretaceous nannofossil species in these samples was generally good, although the base of UC20b (Sample OSB7) exhibited low species abundance (Fig. 3, Table 1).
Low species abundances were noted in the alternating greenish-grey shale and dark brown sandstone beds of the Dizilitaşlar Formation (Fig. 3, Table 1). The samples from 13 to 28 m fall into Nannofossil Zone NP1--the interval from the top occurrence of Cretaceous nannofossils (and containing the bases of the earliest Danian Thoracosphaera operculata Bramlette & Martini, 1964 and Braarudosphaera bigelowii (Gran & Braarud, 1935)  Samples from the uppermost Dizilitaşlar Formation (above Sample OSB9) contained only scarce nannofossils (Table 1). Additionally, there were no nannofossils found in Samples OSB17-OSB19 (Table 1). Although Danian nannofossils were observed in Sample OSB20 and above, the species abundances were very low (Table 1). This data is compatible with nannofossil abundances decreasing in tandem with an increasing proportion of sand towards the top of the section.

Lithology
This section was sampled at the entrance to the Organized Industrial Zone immediately outside Kırıkkale, on the road, almost 1 km south of Hacıbalı village (Fig. 1). The Dizilitaşlar Formation in this section is almost 135 m thick (Fig. 4) and comprises yellow and brown, medium-bedded sandstones (Fig. 5), greyishbrown, poorly-sorted conglomerate and greyish-brown, fine-bedded, occasionally laminated shale alternations. The same lithology continued from the base to the top, and a total of 32 samples were collected (Fig. 4).
The sandstones are generally medium to coarse grained, with normal grading. In some places in the middle part of the section, there are coarse-grained, unconsolidated sandstone layers reaching up to 1 m in thickness. The thickness of the turbiditic sandstones in the section reach almost 1 m. The shales beds are 30-40 cm thick. Small-scale faulting and sliding are observed, causing disruption to the stratigraphy in a large part of the section.

Calcareous nannofossils
Generally, the samples contain low nannofossil species abundances, with 17 nannofossil genera and 19 species being identi-     (Table 2). Two biozones were identified (Fig. 4). The lowest 6 m of the section contained both Cretaceous and Danian nannofossils (Fig. 4). The alternating brown sandstone and greyish-dark green shales were determined as representing Zone NP1, from the last Cretaceous nannofossils to the base of Cruciplacolithus primus Perch-Nielsen, 1977 (Fig. 4, Table 2). The relative abundance of Cretaceous nannofossil species decreased up-section. Above Sample KOS9, the taxa present were mostly Danian (Table  2), although their species abundances were generally very low.
The first occurrence of Cruciplacolithus intermedius van Heck & Prins, 1987 was found in the same lithology, at 5 m, and was used to indicate the base of the NP2 zone (Fig. 4, Table 2). This marker species has been found to be more appropriate for dating marginal sea sediments  where the definitive marker, Cruciplacolithus tenuis (Stradner, 1961) Hay & Mohler 1967, is absent, as was the case here, possibly due to the unstable environmental conditions in the region. Some samples were barren, while other samples only contained very low numbers of species. Although few in number, Thoracosphaera operculata Bramlette & Martini, 1964 and Braarudosphaera bigelowii (Gran & Braarud, 1935) Deflandre, 1947 dominated in this section (Table 2) as in the previous section (Table 1).

Lithology
The Aşağı Samanlık section was sampled on the road to Aşağı Samanlık village (at the junction with the Kırıkkale-Çankırı road) (Fig. 1). The section is 50 m thick and comprises dark green and grey shales and dark brown sandstone alternations belonging to the Dizilitaşlar Formation throughout (Fig. 6). Forty samples were taken, which yielded 22 nannofossil genera and 25 nannofossil species (Table 3).
The Aşağı Samanlık section exhibits typical turbiditic features, with laminated shales and fine-to medium-bedded sandstones (Fig. 7). Small faults and slides were observed in the section. The shale beds at the base of the section are generally 70-80 cm thick, while the sandstone beds vary from 2 to 5 cm thick. Towards the top of the section, the sandstone beds occasionally reach 10 cm thick. The number of occasional sandstone beds increases towards the top of the section, where they reach 10-15 cm thick, while the shale beds are relatively decreased in number.
Rarely, trace fossils were observed in the sandstone layers. Within the sequence, there are unconsolidated, loose-textured conglomerate levels, 70-80 cm thick.

Calcareous nannofossils
The samples from the lower part of the section contained Danian and Cretaceous nannofossils. There were relatively higher species abundances in the shale samples (Table 3). Cruciplacolithus intermedius van Heck & Prins, 1987 has been found in the region in the upper part of NP1, close to the base of NP2, according to THIBAULT et al. (2018) (Figs. 6, 8, Table 3). Zone NP2 was interpreted up to a height of 50 m in the section (Fig. 6    level. The Cruciplacolithus lineage was not observed in NP2 here possibly because of the unstable regional conditions. Nannofossil species abundances decreased towards the top of the section, particularly from Sample AS30, with only a few reworked Cretaceous specimens among a small but greater proportion of Danian species being identified in the uppermost beds (Table 3). Samples with a high proportions of sand appeared to have reduced species richness.

Biostratigraphy
The Samanlık and Dizilitaşlar Formations contain turbidites with high sand contents. Upper Maastrichtian nannofossil species were only identified in the Kırıkkale Organized Industrial Zone section (Fig. 3). In this section, Upper Maastrichtian species abundances were generally very low, apart from Micula decussate Vekshina, 1959, Microrhabdulus decorates Deflandre, 1959and Watznaueria barnesiae (Black, 1959 Perch-Nielsen, 1968 (Table 1), with the species richness and relative abundances of the nannofossils showing a significant decrease at the K-Pg boundary (Table 1). In the study area, the K-Pg boundary was not continuous, but was identified based on the increased abundances (albeit still only a few specimens) of Thoracosphaera operculata Bramlette & Martini, 1964 and Braarudosphaera bigelowii (Gran & Braarud, 1935) Deflandre, 1947 (Fig. 3, Table 1) in the Danian in the Organised Industrial Zone section. Nannofossils were still rare and generally poorly preserved in the Danian samples, while samples that contained high proportions of sand tended to be barren of nannofossils (Tables 1, 3).
The base of NP2 is defined by the base of Cruciplacolithus tenuis (Stradner, 1961 The Kırıkkale Basin was a deep-sea basin that prevailed during unstable environmental conditions due to tectonic activity in the Maastrichtian through the Early Danian. In the study area, Cruciplacolithus tenuis (Stradner, 1961) Hay & Mohler 1967, the actual marker for the base of NP2, was not found in this unstable environment except for in one section, and so the base of NP2 had to be correlated using the base of Cruciplacolithus intermedius van Heck & Prins, 1987 (Fig. 8).
In Fig. 8, the nannofossil zones identified in this study are compared with the stratigraphic distributions of selected nannofossils from other Tethyan localities with similar assemblages and similar ages, including the Kırıkkale Basin.

Palaeoecological interpretation
The asteroid impact at the K-Pg boundary caused dramatic biotic and biogeochemical changes in the oceans, and planktonic foraminifera and calcareous nannofossils were severely affected by the ensuing environmental crisis in the marine pelagic ecosystem (SMIT, 1982;POSPICHAL & WISE, 1990;BERGGREN & NORRIS, 1997;MOLINA et al., 1998;HUBER et al., 2002;BOWN, 2005;FUQUA et al., 2008;SCHULTE et al., 2010;GUERRA et al., 2021). Some nannofossil species survived the impact event, adapting to the changed environmental conditions (BOWN, 2005). Here, the K-Pg boundary was determined based on a significant decrease in the abundance of nannofossil species, an increase in survivor Cretaceous species, such as Thoracosphaera operculata Bramlette & Martini, 1964, and observations of the new Palaeocene taxa (LAMOLDA et al., 2005;BER-NAOLA & MONECHI, 2007).
Few to rare abundances of Micula decussate Vekshina, 1959and Watznaueria barnesiae (Black, 1959) Perch-Nielsen, 1968 occurred in the Upper Maastrichtian samples in the study area (Table 1). Micula decussate Vekshina, 1959 has been interpreted, in some studies, as a species that thrives in low-productivity, relatively cool and stressed environments (WATKINS & SELF-TRAIL, 2005;KELLER et al., 2007;THIBAULT & GARDIN, 2007;MAHANIPOUR et al., 2022). Watznaueria barnesiae (Black, 1959) Perch-Nielsen, 1968 is considered to be a warm water, cosmopolitan, eutrophic and opportunistic species in lowlatitude regions by MUTTERLOSE (1996), POSPICHAL (1996 and LEES (2002), and as being more efficiently able to adapt to different environmental and stress conditions by, for example, AGUADO et al. (2016). LEES et al. (2005) explained that Watznaueria barnesiae (Black, 1959) Perch-Nielsen, 1968 is likely to have been ecologically r-selected for rapid reproduction in heightened nutrient. Both Watznaueria barnesiae (Black, 1959) Perch-Nielsen, 1968and Micula decussate Vekshina, 1959 are considered to be highly resistant to dissolution and have been used to test the preservation of nannofossil assemblages (HILL, 1975;THIERSTEIN, 1980). In the Kırıkkale Basin, however, rare occurrences of Micula decussate Vekshina, 1959and Watznaueria barnesiae (Black, 1959) Perch-Nielsen, 1968, in tandem with low abundances of other taxa, are interpreted as being possibly due to the unstable and stressful environment of the Late Maastrichtian, which is in agreement with previous foraminifera data determining a shallower basin and less saline environment with decreased temperature.
Braarudosphaera bigelowii (Gran & Braarud, 1935) Deflandre, 1947 and Thoracosphaera operculata Bramlette & Martini, 1964 were found to be rare in the Maastrichtian, increasing to the 'few' abundance category, except in some samples, in the Danian (Tables 1, 3). Braarudosphaera spp. became important components of the nannofossil assemblages in the Danian after the K-Pg extinction event. Braarudosphaera bigelowii (Gran & Braarud, 1935) Deflandre, 1947 evolved in the Cretaceous and is common and/or persistent in the Cretaceous of some Tethyan regions (BERNAOLA & MONECHI, 2007). JONES et al. (2019) stated that this species survived after the K-Pg event because it was able to adapt to the ensuing unstable environment, as also explained by POSPICHAL & BRALOWER (1992), peaking in abundance just above the K-Pg boundary. Braarudosphaera bigelowii (Gran & Braarud, 1935) Deflandre, 1947 is also very resistant to diagenetic alteration (JONES et al., 2019). High abundances of Braarudosphaera bigelowii (Gran & Braarud, 1935) Deflandre, 1947 have mostly been associated with low-salinity coastal waters (PELEO-ALAMPAY et al., 1999;BARTOL et al., 2008), the influx of terrestrial material into oceanic waters (ŠVÁBENICKÁ, 1999) and eutrophication (CUNHA & SHIMA-BUKURO, 1997). It is believed to have evolved in unusual palaeoceanographic conditions and has been viewed as an opportunist that responds to reduced competition (THIERSTEIN et al., 2004). Several researchers have reported episodes of Braarudosphaera bigelowii (Gran & Braarud, 1935  . Selected calcareous nannofossil images identified in the sections. All photomicrographs were taken using polarised light. A-Braarudosphaera bigelowii (Gran & Braarud, 1935 PELEO-ALAMPAY et al., 1999;KELLY et al., 2003;EISEN-ACH & KELLY, 2004;GAMBOA & SHIMABUKURO, 2006). The cause of unstable environmental conditions coincides with the tectonic and volcanic activity in the region that induced dramatic change in nannoplankton assemblage linked to the subduction event in the İzmir-Ankara Zone.
Thoracosphaera operculata Bramlette & Martini, 1964 is another species that evolved in the Cretaceous and survived into the Danian, being observed at the K-Pg boundary in most studies on mid-and low-latitude regions (TANTAWY, 2003 (Arkhangelsky, 1912) Gartner 1968, Sample AS1; C-Retecapsa crenulata (Bramlette & Martini, 1964) Grün & Allemann 1975, Sample AS1; D-Chiastozygus amphipons (Bramlette & Martini, 1964 of this species across the K-Pg suggests it is an opportunistic species, and it has been interpreted as indicating a high-stress environment (TANTAWY, 2003;AGUADO et al., 2005;KAYA-OZER, 2014). Acmes of Thoracosphaera operculata Bramlette & Martini, 1964 may correlate with episodes of increased environmental stress, higher atmospheric CO 2 contents, fluctuations in marine salinity and pH, and/or significant warming over a short period of time, such as the conditions that followed the K-Pg boundary event (LAMOLDA et al., 2005).
In the Danian, the few relative abundances of Thoracosphaera operculata Bramlette & Martini, 1964 and Braarudosphaera bigelowii (Gran & Braarud, 1935) Deflandre, 1947 in assemblages also containing rare Futyania are interpreted as indicating stressful conditions and significant environmental perturbations in the Kırıkkale Basin that resulted from the tectonic activity that occurred in the Danian in the study area. The lack of species in some samples, especially those from sandstonedominated intervals, was notable.
Some of the species found in this study are illustrated in Figs. 9 and 10.

CONCLUSIONS
A detailed nannofossil biostratigraphic analysis was performed on the Samanlık and Dizilitaşlar Formations, deposited in the Kırıkkale Basin. Four nannofossil biozones-UC20a TP and UC20b TP and NP1 and NP2-were identified, indicating the Late Maastrichtian to Danian periods, in three stratigraphic sections.
The nannofossil species abundances were generally low in both the Upper Maastrichtian and the Danian. The low abundances of Micula decussate Vekshina, 1959 and the lack of species richness in the Upper Maastrichtian suggest stressful environmental conditions with lower temperature and salinity. The K-Pg boundary was not continuous but was identified by the relatively increased abundances of Braarudosphaera bigelowii (Gran & Braarud, 1935) Deflandre, 1947 and Thoracosphaera operculata Bramlette & Martini, 1964. In the Danian, the low species richness and the presence of opportunistic species, such as Braarudosphaera bigelowii (Gran & Braarud, 1935) Deflandre, 1947 and Thoracosphaera operculata Bramlette & Martini, 1964, were interpreted as indicating stressed environmental conditions in the study area. The finding of reworked fossils and the low species abundances accord with the tectonically active environment of the Maastrichtian through the Palaeocene here.