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Mediterranean Geoscience Reviews

, Volume 1, Issue 1, pp 55–90 | Cite as

Integrated biostratigraphy of the middle to upper Eocene Kırkgeçit Formation (Baskil section, Elazığ, eastern Turkey): larger benthic foraminiferal perspective

  • Ercan ÖzcanEmail author
  • Gy. Less
  • L. Jovane
  • R. Catanzariti
  • F. Frontalini
  • R. Coccioni
  • M. Giorgioni
  • D. Rodelli
  • E. S. Rego
  • S. Kayğılı
  • M. Asgharian Rostami
Original Paper
  • 210 Downloads

Abstract

The Baskil section, located to the west of Elazığ in eastern Turkey, represents deep-marine facies of the Eocene Kırkgeçit Formation, deposited in a wide spectrum of environmental conditions ranging from shelf to basin. The 390-m-thick sequence was deposited at bathyal depths in the Tethys, at the edge of the subsiding Anatolide–Tauride plate and is highly promising for an integrated study of Bartonian to earliest Priabonian mid-latitude neritic and deep-marine biota. The section contains numerous allochthonous limestone beds, characterized either by turbidites or debris flows with resedimented larger benthic foraminifera (LBF). The section spans the planktonic foraminiferal Zones E10/11 to E14, the calcareous nannofossil zones NP15–NP18, shallow benthic zones (SBZ) SBZ16/17 to SBZ18A, including orthophragminid (OZ) zones OZ12–OZ14. The Bartonian–Priabonian boundary is placed at NP17/18 boundary by the lowest occurrence of Chiasmolithus oamaruensis, which lies within Subchron C17n.1n and Zone SBZ18A. The LBF, obtained as loose specimens from 17 turbiditic and debris flow beds, are characterized predominantly by 21 orthophragminid lineages and 13 nummulitid species as well as some other stratigraphically diagnostic genera. Most of the orthophragminid lineages straddle the Bartonian–Priabonian boundary, whereas Orbitoclypeus douvillei and Discocyclina pulcra appear to be the only orthophragminids confined to SBZ17 at its upper range. The Nummulites fabianii-lineage first appearing in zones E12 and NP16 in the Bartonian shows a well-documented evolution of the embryon, and is referred to Nummulites garganicus in the Bartonian, whereas its successor Nummulites hormoensis straddles the Bartonian/Priabonian boundary. The first appearances of N. hormoensis and Heterostegina armenica as well as the last occurrences of Nummulites ptukhiani and Assilina exponens in the section are almost coeval and are utilized to mark SBZ17–18 boundary. The first appearance of the H. armenica-lineage is recorded in zones E14 and upper part of NP17. Two important species, Chapmanina gassinensis and Silvestriella tetraedra, have been first recorded in SBZ18A just above the Bartonian–Priabonian boundary. Thus, the transition of N. garganicus to N. hormoensis, the first appearances of H. armenica, C. gassinensis and S. tetraedra as well as the last occurrences of N. ptukhiani, Assilina exponens, Discocyclina pulcra and Orbitoclypeus douvillei, all across the SBZ17–18 boundary or within the SBZ18A, appear to be the most useful bioevents in the transition from the Bartonian to the Priabonian in shallow-marine realms.

Keywords

Bartonian–Priabonian E Turkey Larger benthic foraminifera Calcareous nannoplankton Planktic foraminifera Magnetostratigraphy 

Introduction

The integration of shallow-marine and deep-marine biota is essential for high-resolution stratigraphic applications and correlations. It requires the study of suitable stratigraphic sections yielding fossil groups representing both neritic and deep-marine realms. During the Eocene, the integration of these fossil groups in the Bartonian–Priabonian interval has been notably hampered either by the rarity of such stratigraphic sections or the poor occurrence of larger benthic foraminifera (hereafter LBF) in deep-marine settings, such as in the Urtsadzor section (Armenia) (Cotton et al. 2017) and the Alano di Piave section (Italy), which is candidate for the global boundary stratotype section and point (GSSP) for the Priabonian stage (Agnini et al. 2011). The Alano di Piave section yielded only two horizons with limited LBF fauna, not yet described at species level, and the Urtsadzor section consists of seven levels with only Priabonian LBF. The Miralles–La Tossa composite section from the Eastern Ebro Basin (Spain) plays a significant role in the re-evaluation of the SBZ18 Zone with respect to the Bartonian–Priabonian boundary in shallow-marine environments (Costa et al. 2013). According to Costa et al. (2013), the Zone SBZ18, the lower boundary of which is traditionally accepted to mark the Bartonian–Priabonian boundary in the Tethys (Serra-Kiel et al. 1998), extends to the early Priabonian. This notion was recently followed in the updated scheme of Tethyan Eocene SBZ zones by Papazzoni et al. (2017). Some efforts to correlate the shallow benthic and calcareous plankton zones at the Bartonian–Priabonian boundary were also made at the Varignano section in northern Italy, consisting of several bioclastic levels with resedimented LBF (Luciani et al. 2016). In this section, an integrated correlation scheme was not presented, though the potential of the section for recording the LBF events was emphasized.

The Kırkgeçit Formation crops out in a wide area from Malatya to Lake Van in eastern Turkey, to the north of Bitlis–Zagros suture zone, a major tectonic element separating the Arabian and Anatolide–Tauride plates (Fig. 1). The unit consists of a wide range of lithofacies associations, and fossil assemblages indicating various depositional environments, from very shallow-marine to pelagic, and ages ranging from the middle Eocene to the Oligocene, showing the diachronic development of the unit in eastern Turkey (Türkmen et al. 2001; Herece and Acar 2016). In the Baskil region, the middle to upper Eocene Kırkgeçit Formation is about 2259 m thick (Özkul 1988). It consists of thick coarse clastic beds of inner-fan setting, followed by bathyal marls intercalated with calcarenites, turbidites, and debris flows with abundant LBF and massive shallow-marine limestone in the upper part (Fig. 2) (Özkul 1988; Özcan et al. 2006). The Baskil section corresponds only to a part of the Kırkgeçit Formation, encompassing the upper Lutetian–lower Priabonian interval.
Fig. 1

a Geological map of the area between Malatya and Elazığ in eastern Turkey and location of the Elazığ Basin in the plate tectonics context of the region (simplified from Akbaş et al. 2011). b Geological map of the Baskil region with the location of the Baskil section (Özkul 1988). EAF East Anatolian Fault, NAFZ North Anatolian Fault Zone, BZSZ Bitlis–Zagros Suture Zone

Fig. 2

a Generalized stratigraphy of the Baskil region and position of the Baskil section (Özkul 1988). b Cartoon showing the paleogeography of the Kırkgeçit Formation in the middle Eocene (Yazgan 1984; Cronin et al. 2000). c Paleogeographic map of the region during the middle-late Eocene (Aksoy et al. 2005)

Here, we present an integrated biostratigraphic scheme of calcareous nannoplankton, planktonic and benthic foraminifera, and LBF from the Baskil section, which corresponds to a mid-latitude paleo-setting in the Tethys. We emphasize the distribution and zonal significance of LBF, described from 17 levels in Bartonian-earliest Priabonian interval based solely on descriptions from the loose specimens. This study follows the works of Rodelli et al. (2018) and Rego et al. (2018) from the same section, having presented a magnetobiostratigarphic framework and environmental conditions, and Özcan et al. (2006), having presented the most comprehensive LBF data from the Kırkgeçit Formation in the same area with the Baskil section.

The Bartonian–Priabonian LBF record in the Tethys: a synthesis

The middle–upper Eocene shallow-marine deposits in the Tethys contain LBF mainly represented by nummulitids, orthophragminids, alveolinids and rotaliids (Schaub 1981; Racey 1995; Serra-Kiel et al. 1998, 2003, 2016; Özcan et al. 2010; Less and Özcan 2012; Hottinger 2014; Ali et al. 2018 and references therein). The dominant middle Eocene foraminiferal taxa belong to large (giant) Nummulites, various species of Assilina, Operculina, alveolinids and orthophragminids. The first appearance of Heterostegina, Spiroclypeus, Nummulites fabianii group, Pellatispira, Chapmanina, and Silvestriella, and disappearance of alveolinids and large Nummulites in the Bartonian and across the Bartonian/Priabonian boundary correspond to significant foraminiferal events in the Western Tethys (Serra-Kiel et al. 1998; Less and Özcan 2012). The superposition and the chronology of these events were used to better constrain the age of shallow-marine units in the Tethys. Recently detailed studies on several groups also enabled us to subdivide the shallow benthic zones SBZ18 and 19 into subzones in the late Bartonian–Priabonian interval (Fig. 3) (Less and Özcan 2012). The Operculina gomezi-group appears to be the most important LBF in the tentative delineation of Lutetian/Bartonian boundary since key foraminiferal events for this boundary have not been well defined yet. The Assilina schwagerialpina group also occurs abundantly in Bartonian–Priabonian Tethyan deposits, although the distinction between the two species, A. schwageri and A. alpina, presents difficulties due to the lack of morphometric data. The first appearance of Heterostegina is perhaps one of the most significant bioevents across the Bartonian/Priabonian boundary. The Heterostegina armenica and H. reticulata lineages display a very rapid evolution and, hence, are used as a valuable proxy to date LBF events and for biostratigraphic correlations (Less et al. 2008). The most important evolutionary trend in these lineages is the reduction in the number of operculinid chambers with time, which is used in the morphometric definition of their subspecies. Similarly, the N. fabianii-lineage displays a significant evolution, as reflected in the increase of the proloculus size, which helps the morphometric discrimination of the successive species (Özcan et al. 2009, 2010). The genus Spiroclypeus, a key Priabonian taxon, also displays the reduction in the operculinid chambers and is subdivided into different species morphometrically (Less et al. 2008). The first appearance of Silvestriella, Pellatispira and Chapmanina close to the Bartonian–Priabonian boundary represents a significant foraminiferal event and is very useful in the delineation of the middle/late Eocene boundary in the Tethys (Özcan et al. 2010; Luciani et al. 2016). Orthophragminids include various species of Discocyclina, Nemkovella, Orbitoclypeus and Asterocyclina (Less 1987; Özcan et al. 2006; Less et al. 2011). The extinction of taxa such as Discocyclina pulcra and Orbitoclypeus douvillei in the Bartonian, Discocyclina pratti, D. discus, Nemkovella strophiolata, Asterocyclina kecskemetii, and A. alticostata in the Priabonian before the onset of Priabonian/Rupelian boundary are key events. The Bartonian/Priabonian boundary, coincident with the SBZ18/19 boundary in Serra-Kiel et al. (1998), was lately lowered as to lie within the SBZ18 by Costa et al. (2013) and Papazzoni et al. (2017). In this study, we follow Less and Özcan (2012) in the assignment of subzones of SBZ18 to the studied section.
Fig. 3

View of the middle and upper part of the Baskil section and corresponding SBZ zones based on the platform-derived allochthonous LBF assemblages (M.G and R.S as a scale at the bottom of 6-sisters). Positions of some selected samples (numbers with stars) are shown

Geological setting and general stratigraphy

The Baskil section is situated in the western part of the Elazığ Basin in eastern Anatolia, where the basin fill is exposed in a SW–NE belt extending from east of Elazığ to Malatya region (Fig. 1a). The present-day plate boundary of the Arabian and Eurasian plates, determined by the Bitlis–Zagros Suture Zone, lies to the south of the study area (Fig. 1b). The Elazığ Basin, located to the north of this suture zone, is interpreted as a marine to non-marine Cenozoic basin that evolved from a Paleocene-early Miocene back arc setting to a collisional stage in the Miocene (Aktaş and Robertson 1984; Yazgan 1984; Bingöl 1984; Özkul 1988; Cronin et al. 2000; Aksoy et al. 2005) (Fig. 2). The generalized stratigraphy of the Baskil region is described as follows (Fig. 2).
  • Keban metamorphic complex and Yüksekova (Elazığ) magmatic complex: the basement of the upper Cretaceous–Paleogene basin in the Baskil region consists of the Permian–Triassic Keban metamorphics complex and the upper Cretaceous Yüksekova (or Elazığ) magmatic complex. The Keban metamorphic complex consists of marbles, calc-phyllites, calc-schists and metaconglomerates, which have undergone amphibolite-greenschist facies metamorphism (see Aksoy et al. 2005 and references therein). The upper Cretaceous magmatic complex consists of basalts, andesites, microdiorites and dolerites and plutonic rocks.

  • Upper Cretaceous–Paleogene cover units: the Harami Formation is stratigraphically the lowest unit covering the basement rocks consisting of a basal conglomerate and overlying shallow-marine limestone (Fig. 2a). The age of the unit with larger benthic foraminifera such as Orbitoides, Lepidorbitoides, Omphalocyclus, Siderolites and rudists, is commonly accepted as (late) Maastrichtian (Yazgan 1984; Bingöl 1984; Turan 1984; Türkmen et al. 2001). The upper age range of the unit has been lately extended into middle Paleocene by Herece and Acar (2016) who recorded a Selandian foraminiferal fauna including Sistanites, Cuvillierina, and Miscellanea further to east of the study area. The lower Paleocene conglomerates of the Kuşcular Formation rest unconformably on the Harami Formation, and are unconformably overlain by a prominent carbonate unit, the Seske Formation. The latter consists of shallow-marine limestone containing late Paleocene-early Eocene foraminiferal assemblages of nummulitids, miscellaneids, and orthophragminids (Türkmen et al. 2001). The upper age range of this unit has been recently extended into the middle Eocene by Herece and Acar (2016). The Kırkgeçit Formation, a thick siliciclastic-carbonate unit indicating a significant subsidence in the basin, is a widespread unit in eastern Turkey, with the type-locality situated near the city of Van, ca. 400 km to the east of Baskil. This unit has variable facies in the Elazığ Basin, ranging from shallow-marine limestone and clastic rocks to bathyal bioclastic limestone and marls, and deeper marine siliciclastic rocks (Özkul 1988; Türkmen et al. 1999, 2001; Özcan et al. 2006; Herece and Acar 2016) (Fig. 2c). The most expanded outcrops of the Kırkgeçit Formation are more than 2 km thick, and occur to the west of the Baskil town, where four distinct members are identified (Fig. 2a).
    • Marik Member, 795-m-thick lower conglomeratic unit (inner-fan conglomerates of Özkul 1988 and slope basin basal apron fill deposits of Cronin et al. 2000):

    • Seher Dağı Member, ca. 1420-m-thick (including the thickness of its lateral equivalent Keklik Tepe Member) unit composed of deep-marine mudstone, marl, sandstone and conglomerate intercalations laterally passing into the bioclastic limestone beds of the Keklik Tepe Member with abundant LBF. According to Özkul (1988) and Cronin et al. (2000), the conglomerates represent the deposition in mid-fan channels (slope-channels).

    • Keklik Tepe Member, a bathyal bioclastic limestone with abundant allochthonous LBF, intercalated with marls containing pelagic fauna and flora. The Seher Tepe and Keklik Tepe members are laterally interfingering.

    • Gökbelen Member, a thick-bedded to massive shallow-marine limestone with limited outcrops in the study area.

In this context, according to previous studies, the Baskil section corresponds to part of the Seher Dağı and Keklik Tepe members, suggesting a deposition at bathyal depths for most of the section (Fig. 2c). The age of the Kırkgeçit Formation has been variously reported and its upper age is uncertain. The various ages ranging from middle Eocene (Lutetian) to Priabonian (Bingöl 1984; Avşar 1991, 1996) or from middle Eocene to Oligocene (Özkul 1988; Cronin et al. 2000) were assigned to the unit mostly based on LBF. Although the presence of Oligocene beds is uncertain in the Baskil region, elsewhere in the basin they occur in shallow to deep-marine facies (Hüsing et al. 2009; Herece and Acar 2016). In the study area, the Kırkgeçit Formation is unconformably overlain by the lower Miocene Alibonca Formation, which is the youngest marine unit in the Baskil area.

Baskil section: lithology and paleontological context

The Baskil section is located about 19.8 km NW of the town of Baskil in Elazığ province, eastern Turkey (Fig. 1a). The section was sampled near the main road from Baskil to Aydınlar (beginning of the section: 38°36′21.35″N, 38°36′1.38″E; end of the section: 38°36′31.56″N, 38°36′31.56″E) to the west of the Resulkahya village, where the Kırkgeçit Formation is exposed along the cliffs of the Seher Mountain (Figs. 1b, 3). The section encompasses 387 m of the Kırkgeçit Formation and corresponds to the Keklik Tepe Member of Özkul (1988) (Figs. 2a, 3). The Baskil section is composed of hemipelagic and turbidite deposits and debris flows with abundant LBF, studied at 17 levels throughout the section (Fig. 4). The key datum planes based on calcareous nannofossils, planktonic foraminifera, and LBF are presented in Table 1.
Fig. 4

Distribution of LBF and corresponding SBZ zones in Baskil section (after Serra-Kiel et al. 1998; Less and Özcan 2012), their correlation to calcareous nannofossils (NP, CP and MNP after, Martini 1971; Okada and Bukry 1980; Fornaciari et al. 2010), planktonic foraminifera (E after Wade et al. 2011) and magnetostratigraphy (after Rodelli et al. 2018). Chronostratigraphy is after Gradstein et al. (2012)

Table 1

Bioevents, and Chron boundaries, with their numerical ages for the Baskil section

Biostratigraphic event and Chron

m

NP

CP

MNP

CNE

P

E

SBZ

Gradstein et al. (2012) age (Ma)

Baskil (Ma)

AE C. erbae

348.0

  

MNP18A/MNP18B

CNE17/CNE18

   

37.40

LO Ch. oamaruensis

342.0

NP17/NP18

      

37.80

HO Ch. grandis

336.0

 

CP14b/CP15a

     

37.20

37.20

AB C. erbae

332.0

  

MNP17B/MNP18A

CNE16/CNE17

   

37.60

37.90

LO Heterostegina

321.0

      

SBZ17/SBZ18A

38.40

38.20

HO M. crassatus

321.0

     

E13/E14

 

38.10

37.90

C17n.1r/C17n.1n

320.6

       

37.79

C17n.2n/C17n.1r

313.7

       

37.91

C17n.2r/C17n.2n

305.8

       

38.14

C17n.3n/C17n.2r

301.3

       

38.20

C17n.3r/C17n.3n

285.1

       

38.38

HO S. obtusus

282.0

  

MNP17A/MNP17B

CNE15/CNE16

   

39.60

39.50

HO Ch. solitus

269.0

NP16/NP17

      

40.40

38.50

C18n.1n/C17n.3r

268.3

       

38.67

LO S. obtusus

200.5

  

MNP16Bc/MNP17A

    

39.60

39.50

C18n.1r/C18n.1n

199.8

       

39.69

C18n.2n/C18n.1r

193.0

       

39.76

HO O. beckmanni

191.0

    

P13/P14

E12/E13

 

40.00

39.70

HO S. spiniger

190.0

  

MNP16Bb/MNP16Bc

    

40.10

39.70

LO D. bisectus

154.0

  

MNP16Ba/MNP16Bb

CNE14/CNE15

   

40.60

40.10

C18n.2n/C18n.2r

153.4

       

40.20

LO O. beckmanni

145.0

    

P12/P13

E11/E12

 

40.50

40.20

HCO S. furcatolithoides

141.0

  

MNP16A/MNP16Ba

    

40.70

40.20

HO? B. gladius

61.0

NP15/NP16

      

43.30

41.10

C19n.1n/C18n.2n

58.1

       

41.20

LCO C. reticulatum

50.0

   

CNE13/CNE14

   

41.20

C19n.1r/C19n.1n

43.3

       

41.44

HO D. bifax

42.0

 

CP14a/CP14b

     

40.40

41.40

Calcareous nannofossil Zones and Subzones are after Martini (1971) (codified as NP), Okada and Bukry (1980) (codified as CP), Fornaciari et al. (2010) (codified as MNP) and Agnini et al. (2014) (codified as CNE). Planktonic foraminiferal Zones are after Wade et al. (2011) (codified as E and P). Larger benthic foraminiferal Zones and Subzones are after Serra-Kiel et al. (1998) and Less and Özcan, 2012). Chronostratigraphy is after Gradstein et al. (2012). The ages in the section were calculated using linear interpolation between magnetostratigraphy tie points of Rodelli et al. (2018)

AB, acme beginning (defined on abundance > 10%); AE, acme ending (defined on abundance < 10%); LO, occurrence; HO, highest occurrence; LCO, lowest common occurrence; HCO, highest common occurrence

The lower part of the Baskil section (0–182 m) contains massive marls and turbiditic siltstone and sandstone beds (Fig. 5a). This part of the section is devoid of coarse turbidites and debris flow deposits and also LBF, except for sample 78, the stratigraphically lowest sample. The lowest occurrence (LO) of Orbulinoides beckmanni is marked at 145 m in the upper part of the section.
Fig. 5

Field aspects of the Kırkgeçit Formation in the studied section and position of some selected samples (numbers with stars) showing important paleontological datum planes and boundaries. S1–6: 6-sisters

A 25-m-thick turbiditic section (named six sisters, S1-6 herein) between 182 and 207 m consists of intercalation of hemipelagic marls and turbidites/debris flows with abundant LBF (Fig. 5b). The highest occurrence (HO) of O. beckmanni at 191 m is within this turbiditic part, which contains LBF in 6 distinct levels (S1–S6). The boundary between SBZ16/17 and 17 is tentatively placed at 182 m after recording the first occurrence of taxa such as Nummulites garganicus Tellini, 1980 and Operculina ex gr. gomezi Colom & Bauzà within the beds just above this level.

This is followed by a succession of marls, turbidites, and debris flows (Fig. 5c–e). The HOs of Chiasmolithus solitus at 269 m and of Sphenolithus obtusus at 282 m are recognized in this part. The SBZ17/18 boundary is tentatively placed at a level above 300 m and below 314 m based on the LOs of Nummulites hormoensis Nuttall & Brighton, 1931 at 314 m and H. armenica (Grigoryan, 1986) at 323 m. The SBZ17/18 boundary stays below the Bartonian/Priabonian boundary, which is established by the LO datum of Chiasmolithus oamaruensis at 342 m.

The upper part of the Baskil section consists of massive marls with thick debris flow deposits and turbidites (Fig. 5f) with characteristic Priabonian LBF. This interval is characterized by the occurrence of N. hormoensis Nuttall & Brighton, 1931, H. armenica (Grigoryan, 1986), Chapmanina gassinensis (Silvestri, 1905) and Silvestriella tetraedra (Gümbel, 1868).

Results

Paleomagnetism

Throughout alternated field demagnetization, stable paleomagnetic behavior permitted to recognize reliable characteristic remanent magnetization, which defined the geomagnetic polarity. The magnetic polarity record recognized by Rodelli et al. (2018) subdivided the studied section into 12 magnetozones, of which six are reverse polarity and six normal polarity intervals. The position of the chrons boundary is precisely pinpointed along the section as reported in Table 1. Due to the continuous, undisturbed, and undeformed sedimentary record, using the ages of the geomagnetic reversals and fossil assemblages, Rodelli et al. (2018) established a detailed age model, which spans from the upper part of Subchron C19n.1r to the base of Subchron C17n.1n (42.4–37.8 Ma).

Calcareous nannofossils

About 360 m of section were sampled for calcareous nannofossils study. The samples were prepared from unprocessed material as smear slides following standard procedure and observed under a light microscope at 1250X magnification using principle transmitted light techniques. The relative number of key calcareous nannofossils was evaluated through counting methods. The percentages of Reticulofenestra umbilicus, Cribrocentrum reticulatum, Dictyococcites bisectus and Cribrocentrum erbae were obtained counting 300 specimens of the total assemblage; the percentages of Sphenolithus furcatolithoides, S. spiniger and S. obtusus were evaluated counting 100 taxonomically related forms; the number of the less frequent Discoaster bifax, Nannotetrina cristata, Blackites gladius, Chiasmolithus solitus, C. grandis and C. oamaruensis was counted on 300 fields of view (roughly corresponding to 6 mm2). The distribution of calcareous nannofossil index species is shown in Fig. 6, and microphotographs are shown in Fig. 7. The Baskil section spans the Bartonian to lowermost Priabonian, for this interval of time the biostratigraphic MNP scheme of Fornaciari et al. (2010) and CNE scheme of Agnini et al. (2014) were also adopted for improving the time resolution of the standard NP scheme of Martini (1971) and the CP scheme of Okada and Bukry (1980).
Fig. 6

Range chart of calcareous nannofossil index species from the Baskil section. LO (lowest occurrence), HO (highest occurrence). Numbers express the frequency of index species both as percentage (R. umbilicus, C. reticulatum, D. bisectus, C. erbae, S. furcatolithoides, S. spiniger, S. obtusus) and number in an area of 6 mm2 (D. bifax, N. cristata, B. gladius, C. solitus, C. grandis, C. oamaruensis)

Fig. 7

Microphotographs of calcareous nannofossil index species from the Baskil section. 1Nannotetrina cristata (Martini, 1958) Perch-Nielsen 1971, parallel nicols, sample 25. 2Discoaster bifax Bukry, 1971, parallel nicols, sample 2. 3Cribrocentrum reticulatum (Gartner & Smith 1967) Perch-Nielsen 1971, crossed nicols, sample 269. 4Blackites gladius (Locker, 1967) Varol 1989, crossed nicols, sample 61. 5Sphenolithus furcatolithoides Locker 1967, crossed nicols, sample 109. 6Dictyococcites bisectus (Hay et al. 1966) Bukry and Percival 1971, crossed nicols, sample 269. 7Sphenolithus spiniger Bukry 1971, 0°, crossed nicols, sample 140. 8Sphenolithus spiniger Bukry 1971, 45°, crossed nicols, sample 140. 9Sphenolithus obtusus Bukry 1971, 0°, crossed nicols, Sample 269. 10Sphenolithus obtusus Bukry 1971, 22°, crossed nicols, sample 269. 11Sphenolithus obtusus Bukry 1971, 45°, crossed nicols, sample 269. 12Chiasmolithus solitus (Bramlette and Sullivan 1971) Locker 1968, crossed nicols, sample 37. 13Reticulofenestra umbilicus (Levin 1965) Martini and Ritzkowski 1968, crossed nicols, sample 345. 14Cribrocentrum erbae Fornaciari et al. in Fornaciari et al. 2010, crossed nicols, sample 345. 15Chiasmolithus grandis (Bramlette & Riedel 1954) Radomski 1968, crossed nicols, sample 345. 16Chiasmolithus oamaruensis (Deflandre, 1954) Hay et al. 1966, crossed nicols, sample 345. Scale bar 10 µm

The following bioevents and Zones–Subzones were identified as shown in Fig. 6. The standard bioevents are the LO of C. oamaruensis that was detected at 342 m and easily allows the identification of the base of Zone NP18, and the HO of C. solitus, that was detected at 269 m and defines the base of Zone NP17 and the base of Subzone CP14b. The HO of C. grandis was detected at 345 m, after additional counting, and identifies the CP14b–CP15a boundary, whereas the HO of B. gladius seems to be unreliable as the species was observed only at 61 m, so that the position of the base of Zone NP16 is uncertain. Considering the more recent biozonations, the LO of common C. reticulatum was observed at 50 m and defines the base of Zone CNE14, the HO of common S. furcatolithoides was recognized at 142 m and sets the base of Subzone MNP16Ba, the LO of D. bisectus was detected at 154 m and defines the base of Subzone MNP16Bb and Zone CNE15, the HO of S. spiniger was observed at 190 m and identifies the MNP16Bb–MNP16Bc boundary. The total range of S. obtusus, recorded between 200.5 and 282 m, allows the recognition of Zone MNP17A, and its HO indicates the CNE15–CNE16 boundary. The LO and HO of the high abundance interval of C. erbae, recorded between 332 and 348 m, identify Zone MNP18A and Zone CNE17, the sharp decrease of abundance of C. erbae is also used to define the base of Zone MNP18B and Zone CNE18.

Foraminifera

Samples for planktonic foraminiferal biostratigraphy and benthic foraminiferal quantitative analyses were treated following the cold acetolysis technique of Lirer (2000) by sieving through a 63 μm mesh and drying at 50 °C. The so-obtained residues were studied with a binocular microscope to identify planktonic foraminiferal biostratigraphic marker species and to characterize the benthic foraminiferal assemblages. The planktonic foraminiferal standard zonation scheme of Wade et al. (2011) was followed. A total of 116 and 31 samples were processed for planktonic and benthic foraminifera, respectively. For benthic foraminifera, at least 300 specimens were picked from the fraction greater than 125 µm.

Planktonic foraminifera

Planktonic foraminifera are generally abundant and well diversified with moderate to good state of preservation. They are represented by typical middle-late Eocene low-latitude taxa, with different species of the genera Acarinina, Catapsydrax, Dentoglobigerina, Globigerina, Globigerinatheka, Globorotaloides, Globoturborotalia, Guembelitrioides, Morozovelloides, Orbulinoides, Paragloborotalia, Parasubbotina, Planorotalites, Subbotina and Turborotalia. The main biostratigraphical events are (from bottom to top): (1) the LO of O. beckmanni at 145 m; (2) the HO of O. beckmanni at 191 m; and (3) HO of Morozovelloides crassatus at 321 m (Rodelli et al. 2018) (Fig. 8). On the basis of these bioevents, the Baskil section spans from the E10/E11 to E14 planktonic foraminiferal Zones of Wade et al. (2011) (Fig. 4 and Table 1).
Fig. 8

Microphotographs of index planktonic foraminifera from the Baskil section. 13Orbulinoides beckmanni (Saito, 1962), sample 177. 4acMorozovelloides crassatus (Cushman, 1925), sample 275. 5acMorozovelloides crassatus (Cushman, 1925), sample 270. 6acMorozovelloides crassatus (Cushman, 1925), sample 275

Smaller benthic foraminifera

Smaller benthic foraminifera are abundant and highly diversified throughout the study interval of the Baskil section. Although the state of preservation is poor in the 160–180 m interval corresponding to the Middle Eocene Climatic Optimum, elsewhere in the section the preservation is good. A total of 224 species belonging to 111 genera were identified. The assemblages are dominated by calcareous taxa whereas agglutinated and porcelaneous forms represent only a minor part. Anomalinoides, Bathysiphon, Bulimina, Cibicidoides, Lagena, Lenticulina, Melonis, Nodosaria, Nuttallides, Oridorsalis, Praebulimina, Pullenia, Stilostomella, and Uvigerina are the dominant genera. The most abundant calcareous taxa are Lenticulina sp. (5.9%), Nodosaria sp. (5.5%), Lagena sp. (5.2%), Nuttallides truempyi (3.1%), Laevidentalina sp. (2.4%), Gyroidinoides beisseli (2%), Cibicidoides eocaenus (1.8%), Cibicidoides sp. (1.6%), Nuttallides sp. (1.5%), Gyroidinoides sp. (1.4%), Cibicides sp. (1.3%), Stilostomella sp. (1.3%), Hanzawaia ammophila (1.1%), Bulimina alazanensis (1.1%), Anomalinoides sp. (1%), Uvigerina sp. (1%), and Gyroidinoides globosus (1%), Melonis sp. (0.9%) and Oridorsalis umbonatus (0.9%). The agglutinated species are mainly represented by Bathysiphon sp. (8.5%), Dorothia sp. (1.4%), Rhabdammina sp. (1.2%), and Hormosina sp. (1%). Porcelaneous species are rare and mainly include Quinqueloculina sp. (0.1%) and Sigmoilopsis sp. (0.1%). Several diagnostic taxa with an upper depth limit in the upper bathyal are recognized, namely Anomalinoides alazanensis, Anomalinoides capitatus, B. alazanensis, B. jacksonensis, B. tuxpamensis, Cibicidoides bradyi, Cibicidoides eocaenus, C. mexicanus, C. micrus, H. ammophila, Osangularia plummerae, Planulina ambigua, Spiroplectammina spectabilis, Uvigerina rippensis, Uvigerina spinulosa, N. truempyi and Globocassidulina subglobosa (Fig. 9). On the basis of the dominance of calcareous tests, the state of preservation, and the upper depth limit of diagnostic taxa, the section was likely deposited at an upper bathyal (~ 300–600 m) environment (Rodelli et al. 2018).
Fig. 9

Distribution of smaller benthic foraminifera in Baskil section

Larger benthic foraminifera

In Baskil section, LBF are dominated by orthophragminids and nummulitids with sparse rotaliids. Taxa listed in the Appendix ‘List of larger benthic foraminifera from the Baskil section’ were identified. Their assemblages suggest SBZ16/17 (OZ12), 17 (OZ13) and 18A (OZ14) throughout the section (Fig. 4).

The stratigraphically lowest level with LBF (sample 78) contains the species/subspecies of the following orthophragminid and nummulitid taxa; Discocyclina pratti pratti, D. trabayensis trabayensis, D. augustae olianae, D. dispansa hungarica, D. discus, N. evae cf. reinechensis, Orbitoclypeus douvillei pannonicus, O. haynesi, O. varians scalaris, Asterocyclina stellata stellata, A. stella stella, A. sireli, A. alticostata cuvillieri, Nummulites perforatus-biedai, N. maximus, N. biarritzensis, N. anomalus, A. schwageri, and A. exponens. This sample is just above the Lutetian and Bartonian boundary and the assemblage characterizes the lower Bartonian (Fig. 4). Based on the presence of Orbitoclypeus douvilleipannonicus, O. varians scalaris and Asterocyclina alticostata cuvillieri the assemblage of the above taxa is characteristic for the SBZ16/17, and in terms of the orthophragminid zonation by Less (1998a) for the OZ12 Zone. This interval is tied to C18n.2r, and stays within NP16, MNP16A, and E10/E11 zones.

The interval between 182 and 207 m (herein informally referred to as ‘six sisters’) yielded Discocyclina pratti pratti, D. augustae olianae, D. radians cf. labatlanensis, D. dispansa dispansa, D. pulcra balatonica, D. discus, D. euaensis, Orbitoclypeus douvillei malatyaensis, O. varians varians, O. zitteli, Orbitoclypeus n. sp. A, Asterocyclina stellata stellata, A. stella stella, A. kecskemetii, Asterocyclina alticostata alticostata, N. garganicus, Nummulites perforatus-biedai, N. maximus, Nummulites ptukhiani, N. striatus, N. biarritzensis, N. anomalus, Operculina ex gr. gomezi, A. schwageri, A. exponens and Linderina sp. Based on the presence of Orbitoclypeus douvilleimalatyaensis, O. varians varians, A. alticostata alticostata and N. garganicus, the assemblages of the above taxa are characteristic for SBZ17, and in terms of the orthophragminid zonation for OZ13 Zone. This interval is correlated with the O. beckmanni Zone (E12) and lower part of E13, implying a Bartonian age (Fig. 4).

The section between 207 and 250 m is dominated by marls and turbidites with sporadic LBF. Sample 242 in the upper part yielded only nummulitids such as Nummulites perforatus-biedai, N. ptukhiani, N. striatus, N. biarritzensis, and A. exponens. Marls and turbidites between 265 and 290 m yielded a diverse LBF represented by Discocyclina pratti pratti, D. trabayensis elazigensis, D. augustae, D. radians cf. labatlanensis, D. dispansa sella, D. pulcra cf. balatonica, Nemkovella strophiolata n. ssp. Padragkút, Orbitoclypeus douvillei malatyaensis, O. varians varians, O. zitteli, Asterocyclina stellata stellata, A. stella stella, A. sireli, A. kecskemetii, A. alticostata alticostata, N. garganicus, Nummulites perforatusbiedai, N. maximus, N. gizehensis-lyelli, N. ptukhiani, N. striatus, N. biarritzensis, N. anomalus, Operculina ex. gr. gomezi, A. schwageri and Linderina sp. The assemblages of above taxa are basically identical with those from the 6-sisters and also characteristic for the SBZ17 (and in terms of the orthophragminid zonation for the OZ13 Zone). This part of the section overlaps with E13, NP16 and lower part of NP17 zones and implies a Bartonian age.

The upper part of the section between 290 and 387 m yields Discocyclina pratti, D. augustae, D. radians, D. dispansa, D. discus, Nemkovella strophiolata n. ssp. Padragkút, O. varians varians, O. zitteli, Asterocyclina stellata, A. stella, A. sireli, A. kecskemetii, A. alticostata alticostata, N. hormoensis, Nummulites perforatus-biedai, N. maximus, N. striatus, N. biarritzensis, N. anomalus, Heterostegina armenica armenica, Operculina ex gr. gomezi, A. schwageri, C. gassinensis and S. tetraedra. Based on the disappearance of N. garganicus, N. ptukhiani, Assilina exponens, Discocyclina pulcra and Orbitoclypeus douvillei and also on the appearance of N. hormoensis, H. armenica armenica, C. gassinensis and S. tetraedra, this part belongs to the SBZ18A Zone (and to the OZ14 Zone in terms of the orthophragminid zonation), and spans NP17, lower part of NP18, upper part of E13 and E14, implying a transition from Bartonian to Priabonian (Fig. 4).

Orthophragminids

Orthophragminids in Baskil section belong to 21 lineages (Figs. 4, 11). General test features in Tethyan orthophragminid genera and parameters used in species/subspecies identification (based on Less 1998a and updated by Zakrevskaya et al. 2011; Özcan et al. 2016) are shown in Fig. 10. Brief information for each group is given below.
Fig. 10

General test features in Tethyan orthophragminid genera (a) (after Less 1987; Ferràndez-Cañadell 1997; Özcan et al. 2016), qualitative parameters (b): a—types of embryon configurations, b—types of the adauxiliary chamberlets, c—different growth patterns of the equatorial annuli, d—types of granules and lateral chamberlets on the test surface, and parameter D (diameter of deuteroconch) used in the morphometric description of orthophragminids as illustrated in D. pseudodispansa (c)

Discocyclina trabayensis Neumann, 1955

Discocyclina trabayensis is an unribbed species having a very small, iso- to nephrolepidine embryon, very low and relatively wide “varians”-type adauxiliary chamberlets, and narrow equatorial chamberlets with “trabayensis”-type growth pattern. This species includes tree subspecies in the Tethys: D. trabayensis trabayensis Neumann, 1955 (Dmean < 125 μm); D. trabayensis elazigensis Özcan & Less, 2006 (Dmean = 125–170 μm); D. trabayensis vicensensis Less, 1987 (Dmean > 170 μm).

This species is sparse and is characterized by D. trabayensis trabayensis (Fig. 12a–c) and D. trabayensis elazigensis in SBZ16/17 and 17, respectively (Fig. 11). Discocyclina trabayensis does not occur in the upper part of the section, though its stratigraphic range extends until the end of Priabonian (Less et al. 2011).
Fig. 11

Distribution of some key orthophragminid species/subspecies in the Baskil section. Dashed line shows probable occurrence. The extinction of two orthophragminid lineages (Discocyclina pulcra and Orbitoclypeus douvillei) in SBZ17 is marked

Discocyclina augustae van der Weijden, 1940

Discocyclina augustae is an unribbed species with a small, semi-iso- to nephrolepidine embryon, narrow and low “archiaci”-type adauxiliary chamberlets and also narrow and relatively low equatorial chamberlets mostly with “strophiolata”-type growth pattern. This species includes four subspecies in Tethys: D. augustae sourbetensis Less, 1987 (Dmean < 145 μm); D. augustae atlantica Less, 1987 (Dmean = 145–180 μm); D. augustae olianae Almela & Rios, 1942 (Dmean = 180–225 μm); D. augustae augustae van der Weijden, 1940 (Dmean > 225 μm).

This species occurs throughout the section and is represented by D. augustae olianae Almela and Rios in the lower part of the section in SBZ16/17 (Fig. 12d, e). It is rare in the upper part and no subspecific designation was made (Fig. 12f–i). Discocyclina augustae became extinct at SBZ20/21 boundary in the Tethys (Less and Özcan 2012).
Fig. 12

Equatorial sections of Discocyclina pratti, D. radians, D. dispansa and D. pulcra from the Baskil section. acD. pratti (Michelin), a 78–30, b 182b-43, c 276b-10, dD. radians (d’Archiac), 276b-26, eD. dispansa (Sowerby) sella (d’Archiac), 276b-13, fD. dispansa (Sowerby), 385b-8, gD. pulcra (Checchia-Rispoli), 182b-33

Discocyclina dispansa (Sowerby, 1840)

Discocyclina dispansa is an unribbed species with a small- to medium-sized, semi-nephro- to trybliolepidine embryon, moderately wide and high, “archiaci”-type adauxiliary chamberlets and equatorial chamberlets. This species includes six subspecies in the Tethys: D. dispansa broennimanni Less, 1987 (Dmean < 160 μm); D. dispansa taurica Less, 1987 (Dmean = 160–230 μm); D. dispansa hungarica Kecskeméti, 1959 (Dmean = 230–290 μm); D. dispansa sella (d’Archiac, 1850 (Dmean = 290–400 μm); D. dispansa dispansa (Sowerby, 1840) (Dmean = 400–520 μm); and D. dispansa umbilicata (Deprat, 1905) (Dmean > 520 μm).

At its type-locality in Kutch (W India), this species is invariably represented by flat forms (Özcan et al. 2018). In the Western Tethys (peri-Mediterranean region and Europe), both flat and saddle-shaped forms with similar embryonic configurations but showing a wide range in embryon diameter were previously assigned to this species (Less 1987; Özcan et al. 2006, 2010). Therefore, it seems necessary to re-evaluate D. dispansa in the Western Tethys. This species is represented by D. dispansa hungarica Kecskeméti in the lower part of the section (sample 78), by D. dispansa dispansa (Sowerby) in the lower part of 6-sisters (sample 182), and by D. dispansa sella (d’Archiac) in sample 276 (Fig. 13e). Few specimens in the upper part (samples 385) were identified at species level (Fig. 13f). In the Western Tethys, D. dispansa became extinct at SBZ20/21 boundary (Less and Özcan, 2012).
Fig. 13

Equatorial sections of Discocyclina trabayensis and D. augustae from the Baskil section. acD. trabayensis trabayensis Neumann, a 78–61, b 78–59, c 78–33. d, e, g, iD. augustae van der Weijden olianae Almela & Rios, d 78–63, e 78–64, g 182b-53, h 78-66, i 182b-55. fD. augustae van der Weijden, 375b-9

Discocyclina pratti (Michelin, 1846)

Discocyclina pratti is an unribbed species having a medium-sized to large, tryblio- to excentrilepidine embryon, numerous moderately wide and high, “pratti”-type adauxiliary chamberlets and narrow but high equatorial chamberlets with “pulcra”-type growth pattern. This species is subdivided into three subspecies in the Tethys: D. pratti montfortensis Less, 1987 (Dmean < 510 μm); D. pratti pratti (Michelin, 1846) (Dmean = 510–700 μm); D. pratti minor Meffert, 1931 (Dmean > 700 μm).

Discocyclina pratti is represented by D. pratti pratti (Michelin) in the lower and upper part of the section (Figs. 11, 13a–c). Few specimens from the uppermost part were identified at species level. In the Western Tethys, D. pratti became extinct across the SBZ19A/B boundary before the Eocene–Oligocene boundary (Less and Özcan, 2012).

Discocyclina discus (Rütimeyer, 1850)

Discocyclina discus is a large, saddle-shaped and unribbed form with a giant, mostly umbilicolepidine (rarely also tryblio- or excentrilepidine) embryon, numerous, wide and high, ‘archiaci’ or transitional ‘archiaci-pratti’-type adauxiliary chamberlets and wide and high equatorial chamberlets with ‘archiaci’ or transitional ‘archiaci-pulcra’-type growth pattern. This species includes two subspecies in the Tethys: D. discus discus (Rütimeyer, 1850) (Dmean < 1350 μm) and D. discus sowerbyi Nuttall, 1926 (Dmean > 1350 μm).

This species occurs sporadically in three levels (samples 78, 182 and 375) (Figs. 11, 14a, b) not permitting subspecific identification. In the Western Tethys, D. discus became extinct in the Priabonian across the SBZ18/19 boundary (Less and Özcan, 2012).
Fig. 14

Equatorial sections of Discocyclina discus, Nemkovella evae, N. strophiolata and equatorial and vertical sections of Orbitoclypeus douvillei from the Baskil section. a, bD. discus (Rütimeyer), a 78–75, b 375b-8, cN. evae cf. reinechensis Özcan, İsmail-Lattrache & Boukhalfa, 78-72, dN. strophiolata n. ssp. Padragkút, 314b-18, e, gO. douvillei pannonicus Less, e 78–45, g 78–41, fO. douvillei malatyaensis Özcan & Less, 182b-19. E axial, others equatorial sections

Discocyclina pulcra (Checchia-Rispoli, 1909)

Discocyclina pulcra is an unribbed species having a large to giant centrilepidine embryon, numerous wide and high to very high, “pratti”-type adauxiliary chamberlets and relatively narrow but high to very high equatorial chamberlets with “pulcra”-type growth pattern. It is subdivided into five subspecies in the Tethys: D. pulcra landesica Less, 1987 (Dmean < 800 μm); D. pulcra pulcra (Checchia-Rispoli, 1909) (Dmean = 800–1000 μm); D. pulcra n. ssp. Angoumé Less, 1998 (Dmean = 1000–1250 μm); D. pulcra balatonica Less, 1987 (Dmean = 1250–1600 μm); D. pulcra baconica Less, 1978 (Dmean > 1600 μm).

This species occurs in three levels (samples 182, 198 and 279), assigned to D. pulcra balatonica in sample 182 (Fig. 13g) and D. pulcra cf. balatonica in samples 198 and 279. Discocyclina pulcra became extinct within SBZ17 in the Western Tethys (Less and Özcan 2012).

Discocyclina euaensis Whipple, 1932

Discocyclina euaensis is an unribbed species with a small- to medium-sized semi-nephro- to trybliolepidine embryon, wide and moderately high, “pratti”-type adauxiliary chamberlets and narrow and high equatorial chamberlets with “pulcra”-type growth pattern.

The species has not yet been subdivided into subspecies. Discocyclina euaensis occurs only in 6-sisters and has been previously illustrated from the coeval beds of the Kırkgeçit Formation in the same area by Özcan et al. (2006; pl. 3, Figs. 20–21).

Discocyclina radians (d’Archiac, 1850)

Discocyclina radians is a ribbed species with a small- to medium-sized semi-nephro- to trybliolepidine embryon, wide and moderately high, “pratti”-type adauxiliary chamberlets and narrow and high equatorial chamberlets with “pulcra”-type growth pattern. It includes four subspecies in the Tethys: D. radians n. ssp. Caupenne (Dmean < 240 μm); D. radians noussensis Less, 1987 (Dmean = 240–300 μm); D. radians radians (d’Archiac, 1850) (Dmean = 300–375 μm); D. radians labatlanensis Less, 1987 (Dmean > 375 μm).

This species occurs sporadically in samples 182, 276 (Fig. 13d), 314 and 375, not permitting subspecific identification.

Nemkovella evae Less, 1987

Nemkovella evae is an unribbed species with a relatively small to moderate semi-iso- nephrolepidine embryon, low and relatively wide, very diagnostic, arcuate, “varians”-type adauxiliary chamberlets, and moderately narrow and low equatorial chamberlets with mostly “varians” and “archiaci” types of the growth pattern. It includes tree subspecies in the Tethys: N. evae evae Less, 1987 (Dmean < 260 μm); N. evae karitensis Özcan & Less, 2007 (Dmean = 260–550 μm); N. evae reinechensis Özcan, Ismail-Lattrache & Boukhalfa, 2014 (Dmean > 550 μm).

This species occurs only in 6-sisters, and is represented by N. evae cf. reinechensis (Fig. 14c). The upper age limit of this taxon, although not surely known, seems to be within the Bartonian (SBZ17). At its type-locality in Tunisia, this subspecies is of Bartonian age (SBZ17) (Ben Ismail-Lattrache et al. 2014).

Nemkovella strophiolata (Gümbel, 1870)

Nemkovella strophiolata is an unribbed species with a small semi-iso to nephrolepidine embryon, low and relatively wide, very diagnostic, arcuate, “varians”-type adauxiliary chamberlets, and moderately narrow and low equatorial chamberlets with “strophiolata”-type growth pattern. It includes four subspecies in the Tethys: N. strophiolata fermonti Less, 1987 (Dmean < 150 μm); N. strophiolata strophiolata (Gümbel, 1870) (Dmean = 150–185 μm); N. strophiolata n. ssp. Padragkút (Dmean = 185–230 μm); N. strophiolata tenella (Gümbel, 1870) (Dmean > 230 μm).

This species occurs in samples 276 and 314 (Fig. 14d) and is represented by N. strophiolata n. ssp. Padragkút. Less and Özcan (2012) reported its upper stratigraphic range in the Tethys as SBZ19A-B boundary within the Priabonian.

Nemkovella daguini (Neumann, 1958)

Nemkovella daguini is a small-sized, unribbed species with a very small iso- to nephrolepidine-type embryon, with few and “daguini”-type adauxiliary chamberlets, the equatorial chamberlets around the embryon and adauxiliary chamberlets isolated and arcuate in shape, cyclic growth attained in neanic stage, the annular chamberlets typically hexagonal, annuli displays wavy pattern.

This species is not yet discriminated into subspecies. Nemkovella daguini occurring in 182 m and upwards in the Baskil section was previously illustrated from the coeval beds of the Kırkgeçit Formation in the same area by Özcan et al. (2006; pl. 2, Figs. 1–4). Common in the peri-Mediterranean region, this species was recently reported from the lower Priabonian Tahwah Formation in the Arabian Peninsula and from the transitional middle-upper Eocene Prang Formation in Meghalaya (India) suggesting a wide geographic distribution in the Tethys (Özcan et al. 2019). The upper stratigraphic limit of the species was proposed to extend into SBZ19 by Less and Özcan (2012), although recently Yücel et al. (2019) discovered it from SBZ20 in the Thrace Basin in Turkey.

Orbitoclypeus varians (Kaufmann, 1867)

Orbitoclypeus varians is an unribbed species with “marthae”-type rosette, small- to medium-sized excentri- to eulepidine embryon, adauxiliary chamberlets of “varians” type with moderate size and shape and also moderately wide and high equatorial chamberlets arranged into undulated annuli with “varians”-type growth pattern. This species includes six subspecies in the Tethys: O. varians portnayae Less, 1987 (Dmean < 165 μm); O. varians n. ssp. Caupenne (Dmean = 165–205 μm); O. varians angoumensis Less, 1987 (Dmean = 205–255 μm); O. varians roberti (Douvillé, 1922) (Dmean = 255–320 μm); O. varians scalaris (Schlumberger, 1903) (Dmean = 320–400 μm); O. varians varians (Kaufmann, 1867) (Dmean > 400 μm).

Orbitoclypeus varians is the commonest species in the Baskil section occurring in eight levels (Fig. 11). The species is represented by O. varians scalaris in the stratigraphically lowest sample (78) and by O. varians varians in the upper samples (182, 198, 269, 279, 314, 375, 385) (Fig. 15e–g). According to Less and Özcan (2012), this species became extinct in the Tethys at Eocene–Oligocene boundary.
Fig. 15

Equatorial sections of Orbitoclypeus haynesi, O. varians, and O. zitteli from the Baskil section. adO. haynesi (Samanta & Lahiri), a 78–17, b 78–47, c 78–50, d 78–46, egO. varians varians (Kaufmann), e 314b-21, f 385b-6, g 375b-7, hO. zitteli (Checchia-Rispoli), 385b-7

Orbitoclypeus zitteli (Checchia-Rispoli, 1908)

Orbitoclypeus zitteli is an average-sized, slightly flattened unribbed species with ‘marthae’-type rosettes. The rather large embryon is excentrilepidine (very rarely eulepidine). The adauxiliary chamberlets are of ‘varians’ type, moderately wide and high. The annuli are almost circular or very slightly undulated; their growth pattern is of the ‘varians’ type.

This species is very similar to O. varians externally and internally. The annuli in O. varians, however, display strong undulation and this is used as the primary criterion for their distinction. In Özcan et al. (2006), these forms were described as O. aff. varians, and since then the species name O. zitteli was adopted (Özcan et al. 2010). The species was identified in three levels: samples 182, 276 and 385 (Fig. 15h).

Orbitoclypeus douvillei (Schlumberger, 1903)

Orbitoclypeus douvillei is an unribbed species having “chudeaui”-type rosette, a small to relatively large, eu-, tryblio and excentrilepidine embryon, wide and moderately high, “varians”-type adauxiliary chamberlets and also wide and moderately high equatorial chamberlets arranged into circular annuli with “varians”-type growth pattern. This species includes six subspecies in the Tethys: O. douvillei douvillei (Schlumberger, 1903) (Dmean < 200 μm); O. douvillei yesilyurtensis Özcan, 2002 (Dmean = 200–260 μm); O. douvillei n. ssp. Gibret (Dmean = 260–340 μm); O. douvillei chudeaui Less, 1987 (Dmean = 340–425 μm); O. douvillei pannonicus Less, 1987 (Dmean = 425–580 μm); O. douvillei malatyaensis Özcan & Less, 2006 (Dmean > 580 μm).

This species occurs in four samples, and belongs to O. douvillei pannonicus in stratigraphically lowest level (sample 78; Fig. 14e, g) and is represented by O. douvillei malatyaensis in samples 182 (Fig. 15f), 202 and 276. The highest occurrence of the species in Baskil section is within SBZ17, consistent with the available data from the Western Tethys.

Orbitoclypeus haynesi (Samanta & Lahiri, 1985)

Orbitoclypeus haynesi is an unribbed species with ‘marthae’-type rosette, small eulepidine (rarely excentrilepidine) embryon, adauxiliary chamberlets of ‘varians’-type of average size and shape, moderately wide and high equatorial chamberlets arranged into strongly undulated annuli with ‘varians’-type growth pattern.

This species, not yet discriminated into subspecies in the Tethys, occurs only in sample 78 (early Bartonian) (Fig. 15a–d). At its type-locality in Kutch (India), it is abundant in the early Bartonian (Özcan et al. 2018). Recorded only in Turkey and Tunisia in the peri-Mediterranean region, O. haynesi occurs as the only orbitoclypeid species in the Bartonian of the Indian subcontinent.

Orbitoclypeusn. sp. A

This species is characterized by very large piles (pillars) in the central part of the test, which are fused forming a thick central pile, covering almost 2/3 of the test surface (Fig. 16c). Internally, the size of protoconch ranges between 110 and 165 μm and that of deuteroconch between 230 and 260 μm. Thus, among the similar taxa, the deuteroconch is much smaller than that of O. varians and larger than that of O. haynesi. The equatorial chambers are wavy as in O. haynesi and O. varians (Fig. 16a, b). This species, occurring in samples 182 and 202 in the Bartonian, will be formally described elsewhere.
Fig. 16

Equatorial and axial sections of Orbitoclypeus n. sp. A and equatorial sections of Asterocyclina alticostata, A. kecskemetii, A. stellata, and A. sireli from the Baskil section. acOrbitoclypeus n. sp. A, a 182b-29, b 182b-27, c 182b-66, dA. alticostata cf. alticostata (Nuttall), 279b-6, e, fA. kecskemetii Less, e 314b-20, f 314b-27, g, hA. sireli Özcan & Less, g 78–24, h 78–83, iA. stella stella (Gümbel), 314b-25, jA. stellata stellata (d’Archiac), 276b-33

Asterocyclina sireli Özcan & Less, 2006

Asterocyclina sireli is a medium to large-sized species with four radial ribs and ‘marthae’-type rosette. The embryon is small, iso- to nephrolepidine. The deuteroconchal wall corresponding to the position of the successive stage of the developing rib is mostly depressed. The adauxiliary chamberlets are few (2–4) in number, low and moderately wide. The equatorial annuli are arranged usually in four rays.

This species, not yet discriminated into subspecies in Tethys, occurs in three levels: samples 78 (Fig. 16g, h) and 314. Recorded in SBZ16 at its type-locality in Sivas, Turkey (Özcan et al. 2016), its upper range extends into SBZ18. This species, not found in Europe, occurs abundantly in Turkey (Özcan et al. 2006), Tunisia (Ben Ismail-Lattrache et al. 2014) and the Indian subcontinent (Ali et al. 2018; Özcan et al. 2018), all records coming from the upper Lutetian and Bartonian deposits.

Asterocyclina stellata (d’Archiac, 1846)

Asterocyclina stellata is a star-shaped species usually with five rays and “marthae”-type rosette. It has a small semi-iso- to nephrolepidine embryon, few, wide and low, “stellata”-type adauxiliary chamberlets and narrow and low equatorial chamberlets arranged into asteroidal annuli with “strophiolata”-type growth pattern. This species includes four subspecies in the Tethys: A. stellata adourensis Less, 1987 (Dmean < 150 μm); A. stellata stellata (d’Archiac, 1846) (Dmean = 150–190 μm); A. stellata stellaris (Rütimeyer, 1850) (Dmean = 190–240 μm); A. stellata buekkensis Less, 1987 (Dmean > 240 μm).

Asterocyclina stellata in Baskil section belongs to A. stellata stellata in samples 78, 182 and 276 (Fig. 16j). It was identified at species level in sample 385.

Asterocyclina stella (Gümbel, 1861)

Asterocyclina stella is a star-shaped species commonly with five rays and “marthae”-type rosette. It has a small semi-iso- to nephrolepidine embryon, few, wide and low, “varians”-type adauxiliary chamberlets and narrow and low equatorial chamberlets arranged into asteroidal annuli with “strophiolata”-type growth pattern. This species includes two subspecies in the Tethys: A. stella praestella Less & Kovács, 2009 (Dmean < 150 μm); A. stella stella (Gümbel, 1861) (Dmean > 150 μm).

Asterocyclina stella in Baskil section belongs to A. stella stella in samples 78, 182, 276, 314 (Fig. 16i), 375 and 385.

Asterocyclina alticostata (Nuttall, 1926)

Asterocyclina alticostata is a star-shaped species with five to ten ribs and “chudeaui”-type rosette. It has a medium-sized to relatively large isolepidine embryon, very few, very wide and moderately low, “alticostata”-type adauxiliary chamberlets and also wide and moderately high equatorial chamberlets arranged into asteroidal annuli with “strophiolata” or “varians”-type growth pattern. This species includes four subspecies in the Tethys; A. alticostata gallica Less, 1987 (Dmean < 275 μm); A. alticostata cuvillieri (Neumann, 1958) (Dmean = 275–350 μm); A. alticostata alticostata (Nuttall, 1926) (Dmean = 350–450 μm); A. alticostata danubica Less, 1987 (Dmean > 450 μm).

This species occurs in four levels and was assigned to A. alticostata cuvillieri in sample 78 and to A. alticostata cf. alticostata in samples 182, 279 (Fig. 16d) and 314. According to Less and Özcan (2012), this species become extinct at SBZ19A-B boundary within the Priabonian. Our recent data from Thrace Basin in Turkey, however, show that this species occurs in SBZ20 suggesting that it survived in the latest Priabonian (Yücel et al. 2019).

Asterocyclina kecskemetii Less, 1987

Asterocyclina kecskemetii is a star-shaped form with five to six rays and “chudeaui”-type rosette. It has a medium-sized nephro- to semi-isolepidine embryon, with relatively wide and high, “varians”-type adauxiliary chamberlets. The two principal auxiliary chamberlets are characteristically well developed. The equatorial chamberlets are of average width and height, arranged into sharp asteroidal annuli while their growth pattern is of “strophiolata” type.

This species, not yet subdivided into subspecies in the Tethys, occurs in three levels; samples 182, 276 and 314 (Fig. 16e, f). According to Less and Özcan (2012), this species became extinct at the SBZ19–20 boundary.

Nummulitids

For the generic classification of the family, we apply Hottinger (1977) principles and subdivision with the addition by Romero et al. (1999) on the distinction of Assilina and Operculina. Four genera are recorded, three of them (Nummulites, Assilina and Operculina) with unsubdivided chambers, and Heterostegina with subdivided chambers.

According to Hottinger (1977) and Schaub (1981), the taxonomical concept of nummulitids with no secondary chamberlets is typological and we mostly followed this practice in the determinations. Simultaneously, we apply the morphometric method, used for the orthophragmines, to Heterostegina (Less et al. 2008) and to the Nummulites fabianii-lineage (Özcan et al. 2009, 2010). Biometric calibration of some other nummulitid lineages is also in progress, and preliminary results (Less 1998b and unpublished data) are used particularly for the N. perforatus- and A. schwageri-alpina-lineages.

Brief information on the identified taxa is given below, “P” indicates the inner cross-diameter of the proloculus of megalospheric forms.

Genus Nummulites Lamarck, 1801

The determination of Nummulites is based on both the surface characteristics and the features of the equatorial section. Based on their surface characteristics, the representatives of the genus Nummulites in the studied area can be classified into three categories as follows: N. garganicus, N. hormoensis and N. ptukhiani belong to the reticulate, N. perforatusbiedai and N. gizehensislyelli to the granulate, while N. maximus, N. striatus, N. biarritzensis and N. anomalus to the radiate forms. Among these taxa, the diameter of microspheric forms of N. perforatusbiedai, N. gizehensislyelli and N. maximus exceeds at least 1.5 cm, they are called giant Nummulites. Their proloculus size is also large, around 1 mm.

Among the reticulate forms N. ptukhiani with relatively large proloculus considerably differs (Cotton et al. 2015) from N. garganicus and N. hormoensis, which belong to the N. fabianii-lineage with much smaller proloculus. Numerous populations of this lineage from the Western Tethys, spanning from the early Bartonian to the early Chattian, are elaborated and the lineage is revised according to the measurement and parameter system introduced by Drooger et al. (1971) and Less (1999). According to Özcan et al. (2009, 2010), the lineage is subdivided into six species, among which N. garganicus is characterized by Pmean = 100–140 µm, whereas this parameter is between 140 and 200 µm for N. hormoensis.

Among giant Nummulites, the size increase of the proloculus by time is well documented by Schaub (1981), Serra-Kiel (1984) and Less (1998b). A biometric calibration for the Middle Eocene representatives of this group is in progress, the preliminary results are used in this work.

According to Schaub (1981), radiate Nummulites listed above belong to different lineages distinguishable using Schaub’s typological criteria. The developmental stage within these lineages was also determined by the Schaub (1981) and by the measurements listed in Less (1998b) too.

Nummulites garganicusTellini, 1890 (Fig.  17 a–e)
Fig. 17

Equatorial sections of Nummulites garganicus and N. hormoensis from the Baskil section. aeN. garganicus Tellini, a 182b-2, b 182b-9, c 182b-1, d 202b-1, e 202b-2, fhN. hormoensis Nuttall & Brighton, f 314b-1, g 385b-1, h 314b-3

This small reticulate taxon (represented exclusively by A-forms) belongs to the Nummulites fabianii-lineage, preceded by N. bullatus and followed by N. hormoensis, from which it is bounded by the arbitrary morphometric limits of Pmean = 100 and 140 µm, respectively. More information on the lineage and its segmentation can be found in Özcan et al. (2009, 2010) and in Less et al. (2011, 2018). N. garganicus can be commonly found in the lower part of the Baskil section starting from sample 182 up to sample 279; however, higher up it is substituted by N. hormoensis (see there). Pmean varies between 97 and 120 µm and also fluctuates between samples 182 and 276 (Fig. 18). In strict sense, the population from sample 186.5 is transitional between N. bullatus and N. garganicus. This population is, however, arranged also here because both the under- and overlying populations surely belong to N. garganicus. This taxon was also mentioned from the corresponding sample KEÇ 1 of the nearby Keçili section by Özcan et al. (2006, Table 3 and Pl. 1, Fig. 4, and erroneously under the name of N. n. sp. aff. ptukhiani on text—Fig. 8). N. garganicus characterizes the SBZ17 Zone in most cases, the only exception where it could be found in SBZ18B is recorded by Less et al. (2011).
Fig. 18

Change in the inner cross-diameter of the proloculus (p) of reticulate Nummulites in the Baskil section

Nummulites hormoensisNuttall & Brighton, 1931 (Fig.  17 f–h)

This taxon represents the next evolutionary step of the Nummulites fabianii-lineage after N. garganicus. Pmean = 140 µm is the morphometrical limit, which arbitrarily bounds them, while Pmean = 200 µm separates it from the successive N. fabianii. N. hormoensis (only megalospheric specimens) is common in the upper part of the Baskil section starting from sample 314 up to sample 385. As seen in Fig. 18, Pmean fluctuates around 150–160 µm in this interval, which, however, differs considerably from the Pmean values of N. garganicus from the stratigraphically lower samples. N. hormoensis was also mentioned from the corresponding sample KEÇ 3 (and also from the stratigraphically higher samples KEÇ 11 and 15) of the nearby Keçili section by Özcan et al. (2006, Table 3 and Pl. 1, Figs. 9 and 17, and erroneously under the name of N. ptukhiani on text—Fig. 8). This taxon characterizes the SBZ18 Zone, and in this work we report for the first time the almost synchronous first appearance of N. hormoensis with that of the genera Heterostegina, Chapmanina and Silvestriella, above the last occurrence of N. ptukhiani, Assilina exponens, Discocyclina pulcra and Orbitoclypeus douvillei.

Nummulites ptukhianiZ.D. Kacharava,1969(Fig.  19 g, h)
Fig. 19

Some important Nummulites species from the Baskil section. a, bNummulites ex gr. gizehensis (Forskål)–lyelli d’Archiac & Haime juvenile specimen, 186-5b, E 2019.1.1. ceNummulites perforatus (Montfort), c 276b, E 2019.2.1; d, e 186-5b, E 2019.3.1. f, lNummulites maximus d’Archiac, f 78b, E 2019.4.1; l 276b, E 2019.5.1. g, hNummulites ptukhiani Z. Kacharava, g 276b, E 2019.6.1; h 242b, E 2019.7.1. iNummulites anomalus de la Harpe, 78b, E 2019.8.1. j, kNummulites biarritzensis d’Archiac, j 266b, E 2019.9.1; k 276b, E 2019.10.1. mpNummulites striatus (Bruguière), m, n 186-5b, E 2019.11.1; o 314b, E 2019.12.1; p 266b, E 2019.13.1. a, b, m, n B-forms, rest: A-forms. b, d, m External views, rest: equatorial sections. a, b, d, m, n × 5, rest: × 10

Megalospheric specimens of this reticulate taxon with large (450–1200 µm) proloculus can commonly be found in the middle part of the Baskil section between samples 199 and 279 (with no apparent trend in change of the proloculus size); however, it is absent from the stratigraphically higher samples. Özcan et al. (2006) reported this form from the corresponding sample KEÇ 1 of the nearby Keçili section under the name of N. hottingeri. This can be explained that N. ptukhiani was formerly used (e.g., by Schaub, 1981 and Serra-Kiel et al. 1998) to identify the precursor form of N. fabianii (what is called in this work as N. hormoensis). However, the original N. ptukhiani introduced by Z.D. Kacharava (1969) does not belong to the N. fabianii-lineage (for details see Papazzoni, 1998 and Cotton et al. 2015) but corresponds clearly to Schaub’s (1981) N. hottingeri introduced for reticulate forms with much larger proloculus from the Bartonian. Thus, N. ptukhiani is a senior synonym of N. hottingeri and should be used instead of it. Cotton et al. (2015) reported the Lutetian precursors of this taxon from Tanzania. However, it became widespread in the Tethys only in the early Bartonian SBZ17, since it is reported from Spain through Turkey and Armenia to Tanzania and Kutch (Saraswati et al. 2017, who—in our opinion—described the same forms also as N. aff. hormoensis). N. ptukhiani became extinct most probably at the SBZ17/18 boundary since no record of the taxon is known from SBZ18.

Nummulites perforatus(de Montfort, 1808)–biedaiSchaub, 1962 (Fig.  19 c–e)

Advanced representatives of the Nummulites perforatus-group are widespread throughout the Baskil section. The sexual dimorphism of this group is quite remarkable. The diameter of inflate, densely granulated B-forms is 1.5–2 cm, whereas that of the A-forms with similar external characteristics is commonly 3–5 mm. Schaub (1981) typologically distinguished more than 20 middle Eocene species of this group, and arranged them into 6 lineages. We hypothesize that several characteristics used for the separation of these taxa are ecophenotypical features because this meticulous taxonomical segregation has not yet been followed in the subsequent works.

The most important numerical feature given by Schaub (1981) for the N. perforatus-group is “P”, increasing from about 500 µm in the early Lutetian to more than 1000 µm at the top Bartonian. Based on the measurements of all megalospheric specimens of the Schaub-collection in Basel, Less (1998b) confirmed this trend. In collaboration with Dr. Botond Kertész (Budapest, Hungary), we also morphometrically investigated the A-forms of the N. perforatus-group from several middle Eocene localities throughout the Western Tethys. Based on our preliminary results (partly published in Özcan et al. 2006, 2010 and Less et al. 2011), upper Lutetian to uppermost Bartonian (and also basal Priabonian according to the most recent interpretation of the Bartonian/Priabonian boundary) representatives of the N. perforatus-group are characterized by an average “P” above 700 µm, and (considering also “L”, the average length of chambers in the third whorl, introduced by Less, can be arranged into four species. These are: N. perforatus with Pmean = 700–950 µm and Lmean < 420 µm (SBZ16–17), N. aturicus with Pmean = 700–950 µm and Lmean = 420–600 µm (SBZ15–18A), N. meneghinii with Pmean = 700–950 µm and Lmean > 600 µm (SBZ16–17) and N. biedai with Pmean > 950 µm (SBZ18).

No detailed morphometrical studies were performed in the Baskil section, only “P” was measured and “L” was estimated on some specimens. Based on these, “P” is around 950 µm throughout the section with no significant trend from the base to the top, whereas chambers in the third whorl are relatively narrow, and “L” is commonly smaller than 400 µm. Therefore, we determined the representatives of the N. perforatus-group throughout the Baskil section as intermediate between N. perforatus and N. biedai. It is worth mentioning that formerly a detailed morphometrical study of the group was performed in sample KEÇ 1 from the closest vicinity (Özcan et al. 2006), corresponding to the interval between samples 199b and 276b of the Baskil section, and that population was determined as N. perforatus.

Nummulites maximusd’Archiac, 1850 (Fig.  19 f, l)

This other giant Nummulites in the Baskil section belongs to the N. millecaput-group, and is less common than the representatives of the N. perforatus-group. They occur in large quantity only in samples 78 and 276, whereas in the other samples they can only be subordinately found. Giant (up to 5 cm in diameter), flat B-forms occur sporadically. A-forms can be recognized based on the lack of surface granulation and on the characteristic, rather densely spaced, strongly arched septa. “P” is ca. 1000 µm throughout the whole section, and based on this they are considered as Nummulites maximus, the most developed member of the N. millecaput-group. In the nearby Keçili section, these forms were also found (Özcan et al. 2006) but only in the lowest samples (KEÇ 1 and 3), corresponding to the middle and upper part of the Baskil section. The nomenclatural problems of this group are detailed in Less et al. (2011). Similar forms in slightly higher stratigraphical position (certainly within the SBZ18C Zone, whereas the very rare occurrence of A-forms only in the SBZ19A Zone) were found in the Urtsadzor section of Armenia by Cotton et al. (2017) from where they are reported as N. millecaput-group.

Nummulites gizehensisForskål, 1775–lyellid’Archiac & Haime, 1853 (Fig.  19 a, b)

One single juvenile microspheric specimen of this giant Nummulites was found in sample 202, unequivocally recognized based on its characteristic surface and almost straight septa; however, determination on the species level was not possible. The characteristic spiral multiplication is well visible in Fig. 19a (for details see Ferràndez-Cañadell 2012). In the nearby Keçili section, Özcan et al. (2006) reported N. gizehensis from sample KEÇ 3 corresponding to the highest part of the Baskil section, and also N. lyelli from the even higher sample KEÇ 11. This latter species was also found in both the southern (Özcan et al. 2010) and northern (Less et al. 2011) part of the Thrace Basin in basically the same stratigraphic level as in Keçili.

Nummulites striatus(Bruguière, 1792) (Fig.  19 m–p)

Both A- and B-forms of this taxon commonly occur throughout the Baskil section except in sample 78, the lowest one containing LBF. Based on the characteristic surface, on the tight spire and also on the densely spaced, moderately arched chambers, it can confidently be identified with Nummulites striatus, although Özcan et al. (2006) determined them as N. cyrenaicus and N. aff. cyrenaicus from the nearby Keçili section. No marked trend in parameter “P” (varying between 180 and 360 µm) along the section can be observed. According to Schaub (1981) and Serra-Kiel et al. (1998), this taxon is characteristic for the SBZ18 and also for the SBZ19A Zones. However, in the Baskil section, it starts to occur already from sample 186.5, thus it can also be found in the SBZ17 Zone. Meanwhile, we have never found it above the SBZ18 Zone, and, therefore, we think that the occurrences of N. striatus (and also of N. boulangeri) from the SBZ19A Zone (e.g., Faudon, Arguis, Villa Marbella) listed by Schaub (1981) should be revised.

Nummulites biarritzensisd’Archiac in Bellardi, 1852 (Fig.  19 j, k)

This radiate taxon occurs quite commonly in the Baskil section and it is quite frequent in samples 266 and 276. It bears a relatively small embryon, evenly coiled spiral and slightly arched, more or less isometric chambers. Parameter “P” varies mostly between 100 and 200 µm, with no real trend throughout the section. Özcan et al. (2006) described similar forms from the nearby Keçili section under the name of Nummulites incrassatus. Those forms are mostly from slightly younger levels (samples KEÇ 3–16) and possess a slightly larger proloculus (between 140 and 250 µm). Less et al. (2011) also described similar forms from both the SBZ18 and 19 Zones of N Thrace (Turkey) as N. incrassatus. Parameter “P” of forms from the SBZ19 Zone is only slightly larger than that from the SBZ18 Zone. Thus, the separation of N. biarritzensis and N. incrassatus is problematic and not yet solved. However, the former name has priority (de la Harpe, 1883). If further studies demonstrate a real difference between the two species, they can be arranged into a lineage starting with N. biarritzensis (?SBZ17–18), followed by N. incrassatus (?SBZ19–20) and finishing with N. vascus in the Oligocene. The connection between the last two species (although their separation is not yet morphometrically defined, too) was already recognized by Boussac (1911) and followed by several authors as summarized in Nemkov (1967). This is not mentioned by Schaub (1981), although he supposed a probable link of N. biarritzensis with N. vascus.

Nummulites anomalusde la Harpe, 1877 (Fig.  19 i)

This radiate taxon is easily recognized based on its quite regular, relatively tight spire and especially on the sparsely spaced, extremely arched septa. A-forms occur in large numbers only in sample 78 representing SBZ16/17, however, they can also be found sporadically in the higher levels of the Baskil section (samples 279 and 358, representing SBZ17 and 18A, respectively). Microspheric forms were not found. Parameter “P” varies between 140 and 210 µm. Similar forms were also found in the nearby Keçili section (Özcan et al. 2006) in samples KEÇ 3, 11, 15 and 16 (SBZ18A for the first and SBZ18B for the rest) as well as in sample V1.25 of the Urtsadzor section in Armenia (Cotton et al. 2017), corresponding to the SBZ18C Zone. A similar form (but with much smaller proloculus), Nummulites stellatus is believed to be characteristic for the SBZ19–20 Zones; however, recently it was also reported from the SBZ18B Zone of northern Thrace Basin (Less et al. 2011). Otherwise, N. anomalus is very rarely reported, particularly in the last 50 years, and a modern taxonomic revision is required. Therefore, our taxonomic identification is somewhat tentative.

Genus Assilina d’Orbigny, 1839

Representatives of this genus belong to two groups. The first of them (Assilina in traditional sense) is formed by the large-sized, semi-involute forms of the early and middle Eocene that are arranged by Schaub (1981) into the main A. spira- and A. exponens-lineages and into the side-lineage of A. reicheli. Based on Romero et al. (1999), the small evolute forms ranging through the whole Eocene, formerly generally referred to as Operculina alpina, O. schwageri, etc., also belong to the genus Assilina.

Assilina exponens (Sowerby, 1840)

Megalospheric forms of this taxon occur constantly in the lower and middle part of the Baskil section (up to sample 276), and they were also reported from the corresponding sample KEÇ 1 of the nearby Keçili section (Özcan et al. 2006). Unfortunately, we failed to obtain good equatorial sections; however, we could measure parameter “P” (350–550 µm). The specific determination of large Assilina (Schaub, 1981) is purely based on typological features. In the case of the Baskil material, we used also the fact that A. exponens was first described from the Fulra Limestone of Lakhpat (Kutch, W India) having according to Özcan et al. (2018) and Saraswati et al. (2018) the same age (SBZ17) as the Baskil samples containing Assilina. Since large-sized semi-involute Assilina specimens have never been reported from stratigraphically higher levels, their highest occurrence marks reliably the SBZ17/18 zone boundary together with the highest or lowest occurrence of some other forms listed at the end of the description of N. hormoensis.

Assilina schwageri(Silvestri, 1928) (Fig.  20 f, g)
Fig. 20

Stratigraphically important nummulitids from the Baskil section. ac Heterostegina armenica armenica (Grigoryan), a 385b-16, b 375b-4, c 375b-3, d, e Operculina ex gr. gomezi Colom & Bauzá, d 375b-1, e 314b-16, f, g Assilina schwageri (Silvestri), f 385b-19, g 314b-17, h Chapmanina gassinensis (Silvestri), 375b-14, i Asterigerina rotula (Kaufmann), 197, j Silvestriella tetraedra (Gümbel), 375b-15, k Gyroidinella sp. 353, l Calcarina sp., 297, m Linderina sp. 197, n Sphaerogypsina globulus (Reuss), 201, o Fabiania cassis (Oppenheim), 353, pr rotaliid foraminifer, 186.5

Based on Romero et al. (1999) the evolute forms, formerly referred to as Operculina alpina, O. schwageri, etc., belong to the genus Assilina, since their septa are not folded and not intersected by stolons characteristic for Operculina. According to Hottinger (1977), this group forms a single evolutionary lineage. It needs, however, a serious revision since biometric limits between chronotaxa are not yet established. However, basing on the data of Papazzoni (1998), Less (1999), Özcan et al. (2010) and Less et al. (2011), it can be recognized that for forms from the SBZ17–18 Zones “P” below 120 µm is characteristic whereas for those from the SBZ19–20 Zones this value is commonly above 120 µm.

In the entire Baskil section, megalospheric specimens of these evolute Assilina occur constantly as accessorial elements of the LBF fauna as well as in the nearby Keçili section (Özcan et al. 2006 where they were determined as Assilina ex. gr. schwagerialpina), and “P” is mostly between 60 and 135 µm with no stratigraphically increasing trend. Since traditionally the forms from older levels are called A. schwageri whereas the name of A. alpina is used for those from the younger strata, we tentatively apply the name of A. schwageri for specimens from both the Baskil and Keçili sections.

Genus Operculina d’Orbigny, 1826

This genus with folded septa intersected by stolons is represented in the Eocene by the involute Operculina gomezi-group with very dense and high chambers.

Operculinaex gr.gomeziColom & Bauzá, 1950 (Fig.  20 d, e)

The first appearance of the Operculina gomezi-group nearly coincides with the Lutetian/Bartonian boundary (see Özcan et al. 2006; Less and Özcan 2012). In the Baskil section, A-forms of this group were first recognized in sample 186.5, and then, starting from sample 276b they constantly occur up to the top of the profile, as well as in the neighboring Keçili section (Özcan et al. 2006, under the name O. ex gr. roselligomezi). Hottinger (1977) arranged the representatives of the O. gomezi-group into a single evolutionary lineage starting with O. bericensis, followed by O. roselli and ending with O. gomezi; however, their biometric limits have not been given. Therefore, here we apply the name of O. ex gr. gomezi. In our material, the inner cross-diameter of the proloculus does not show a clear increasing trend and remains in the range between 80 and 130 µm.

Genus Heterostegina d’Orbigny, 1826

Based on a rather wide Mediterranean material the Eocene representatives of this genus have been revised by Less et al. (2008), who arranged them into three lineages considered as species. These are H. armenica, H. reticulata and H. gracilis; among which only the first of them occurs in the Baskil section.

Heterostegina armenica armenica(Grigoryan, 1986) (Fig.  20 a–c)

The representatives of the H. armenica-lineage have an involute, flat biconvex test, a central pile and slightly sigmoid septal sutures passing into an irregular sutural network in the edges. The proloculus is relatively large (as compared with the representatives of the H. reticulata lineage with similar X values; see later), the chamberlets (often with incomplete secondary septa) are rather irregularly arranged and characteristically polygonal. The number of undivided chambers is subjected to nepionic acceleration. Based on this, the species has been subdivided by Less et al. (2008, 2011) into three chronosubspecies as follows (parameter X marks the number of post-embryonic, pre-heterosteginid chambers): H. armenica armenica (Xmean > 8), H. armenica tigrisensis (Xmean = 5–8) and H. armenica hacimasliensis (Xmean < 5). According to Less et al. (2008), H. armenica armenica is characteristic for the SBZ18A, while the other two, more advanced subspecies for the SBZ18B Subzone.

In the Baskil section, megalospheric forms of the lineage are absent in the lower and middle part, they first appear only in sample 323b, and upsection they occur also in samples 375b and 385b. In the Keçili section, they are also missing from the lowest sample KEÇ 1 but are present upsection in samples KEÇ 3 (corresponding to the upper part of the Baskil section), and further in samples KEÇ 11, 15 and 16. Based on the above diagnosis, Less et al. (2008) identified the population from sample KEÇ 3 as H. armenica armenica, and those from the other samples as H. armenica tigrisensis. Since the developmental degree of specimens from the upper part of the Baskil section is very similar to that from sample KEÇ 3, these populations are also identified as H. armenica armenica.

Chapmanina, Silvestriella, Linderina and Calcarina

The genera Linderina (Fig. 20m) and Calcarina (Fig. 20l) occur rarely in the Bartonian part of the Baskil section (Fig. 4). Chapmanina, represented by C. gassinensis (Silvestri) (Fig. 20h), and Silvestriella, represented by S. tetraedra (Gümbel) (Fig. 20j) appear first in the lower part of SBZ18A, close to the Bartonian–Priabonian boundary. Thus, the first appearance datum level of these taxa is significant to the tentative placement of the Bartonian–Priabonian boundary. We have also identified the following additional LBFs in the section; Asterigerina rotula (Kaufmann) (Fig. 20i), Gyroidinella sp. (Figure 20k), Sphaerogypsina globulus (Reuss) (Fig. 20n), Fabiania cassis (Oppenheim) (Fig. 20o), and some rotaliid foraminifera (Fig. 20p–r).

Conclusions

The Baskil section is an expanded Bartonian-early Priabonian deep-marine sequence consisting of diverse assemblages of platform-derived LBF. The magnetic polarity record previously recognized by Rodelli et al. (2018) subdivides the studied section into 12 magnetozones, with 6 reverse polarity and 6 normal polarity intervals spanning from the upper part of Subchron C19n.1r to the base of Subchron C17n.1n (42.4–37.8 Ma). The section spans the planktonic foraminiferal Zones E10/11 to E14, the calcareous nannofossil zones NP15–NP18, LBF zones SBZ16/17 to SBZ18A, including OZ zones OZ12–OZ14. The Bartonian–Priabonian boundary is placed at NP17/18 boundary by the lowest occurrence of Chiasmolithus oamaruensis, which lies within Subchron C17n.1n and Zone SBZ18A. This section yielded the richest LBF assemblages in Bartonian and across the Bartonian–Priabonian boundary in the Tethys, which are correlated to deep-marine fauna and biota. We have identified 21 orthophragminid lineages and 13 nummulitid species as well as some stratigraphically diagnostic other genera, such as Chapmanina, Silvestriella, Linderina and Calcarina, listed in the Appendix. Most orthophragminid species straddle the Bartonian–Priabonian boundary, while two species, O. douvillei and D. pulcra appear to be confined to SBZ18 at their upper stratigraphic ranges. The representatives of Nummulites fabianii-lineage show a clear increase in embryon diameters and are represented by N. garganicus and N. hormoensis, respectively. Their transition, together with the first appearance of the genera Heterostegina, Chapmanina and Silvestriella and also with the last occurrence of N. ptukhiani, Assilina exponens, Discocyclina pulcra and Orbitoclypeus douvillei in the upper part of the section suggest that these are the best LBF markers to tentatively infer to Bartonian–Priabonian boundary in the Tethys.

Notes

Acknowledgements

The Baskil project was developed within the framework of the CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) “Ciências do Mar II” and Marie Curie Actions (FP7-PEOPLEIEF-2008, proposal 236311). L. Jovane, D. Rodelli, E.S. Rego and M. Giorgioni were supported by grants from the Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP) 2011/22018-3, 2012/21212-3, 2015/16501-4 and 2012/15995-5, respectively. Thanks are due to Johannes Pignatti (Rome) and to Pratul Kumar Saraswati (Mumbai) for constructive reviews. We thank editor Attila Çiner for his comments and suggestions.

Funding

The Baskil project was developed within the framework of the CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) “Ciências do Mar II” and Marie Curie Actions (FP7-PEOPLEIEF-2008, proposal 236311).

Compliance with ethical standards

Conflict of interest

No potential conflict of interest.

Informed consent

Informed consent was obtained from all individual participants included in the study.

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  • Ercan Özcan
    • 1
    Email author
  • Gy. Less
    • 2
  • L. Jovane
    • 3
  • R. Catanzariti
    • 4
  • F. Frontalini
    • 5
  • R. Coccioni
    • 5
  • M. Giorgioni
    • 3
    • 6
  • D. Rodelli
    • 3
  • E. S. Rego
    • 3
    • 7
  • S. Kayğılı
    • 8
  • M. Asgharian Rostami
    • 9
  1. 1.Department of Geological Engineering, Faculty of Minesİstanbul Technical University (İTÜ)IstanbulTurkey
  2. 2.Institute of Mineralogy and GeologyUniversity of MiskolcMiskolc-EgyetemvárosHungary
  3. 3.Instituto OceanográficoUniversidade de São PauloSão PauloBrazil
  4. 4.Istituto di Geoscienze e Georisorse CNRPisaItaly
  5. 5.Dipartimento di Scienze Pure e Applicate (DiSPeA)Università degli Studi di Urbino “Carlo Bo”UrbinoItaly
  6. 6.Instituto de GeociênciasUniversidade de BrasíliaBrasíliaBrazil
  7. 7.Instituto de GeociênciasUniversidade de São PauloSão PauloBrazil
  8. 8.Department of Geological EngineeringFırat UniversityElazığTurkey
  9. 9.EECB and Biology DepartmentUniversity of Nevada, RenoRenoUSA

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