Foraminifera of shallow and very shallow facies from the upper Eocene–lower Oligocene Kazandere Member, Soğucak Formation, Thrace Basin, northwest Turkey

 E. Sirel, T. Ayyıldız, A. Deveciler, 2020 CC BY-SA


INTRODUCTION
The main objective of this study is to define the reference section (holostratotype) of the new Kazandere Member of the Soğucak Formation and its parastratotype, in the northeastern Thrace ( Fig. 1A-C). These sections were investigated as to their microfacies, taxonomy and biostratigraphy of the shallow/very shallow-water Priabonian foraminifera.
The Thrace region has been geologically investigated since the mid 20th Century. Daci (1951) described middle−late Eocene foraminifera, especially from genus Nummulites lamarck in the Küçükçekmece and Çatalca regions, East of Thrace. Akartuna (1953) performed a geological study based on this genus in the same region (Çatalca-Karacaköy). The chronostratigraphic record of foraminifera from this region was improved by Sirel and Gündüz (1976), who were the first to report the existence of marine Oligocene in the Thrace region based on Nummulites.
New insights in the geology of the Thrace Basin were provided by Şengör and Yılmaz (1981) who studied the tectonic evolution of the Tethys in Turkey. Later, Turgut et al. (1991) studied the evolution of the Thrace Basin and its hydrocarbon potential, and Görür and Okay (1996) suggested a fore-arch origin for this basin.

Foraminifera of shallow and very shallow facies from the upper Eocene-lower Oligocene Kazandere Member, Soğucak Formation, Thrace Basin, northwest Turkey
As regards the stratigraphy and sedimentology of the Thrace Basin, Varol et al. (2009) studied the carbonates in Bozcaada and Kıyıköy. Okay et al. (2010) identified the basement rocks of the southern Thrace Basin, and studied the lower Eocene series and the upper Eocene olistostromes. Less et al. (2011) studied the larger foraminifera and shallow-marine rocks of the middle-Eocene to lower− Oligocene in the North and East of Thrace Basin. Okay et al. (2017) described in Çatalca the connection between the west Black Sea and the Thrace Basin during the late Eocene to the early Oligocene. Recently Okay et al. (2020), Özcan et al. (2020) and Yücel et al. (2020) have investigated the biostratigraphy of foraminifera in the Thrace Basin.
According to the current stratigraphical and paleontological knowledge, very shallow/shallow-water marine rocks containing rich assemblages of larger benthic foraminifera of Bartonian-Priabonian age are widespread in Turkey (Deveciler, 2010;Deveciler, 2014;Less et al., 2011;Özcan et al., 2019;Özcan et al., 2020;Sirel and Gündüz, 1976;Sirel, 2003;Sirel, 2015;Yücel et al., 2020). Nonmarine deposits are abundant after the Bartonian while marine Priabonian and lower Oligocene rocks yielding foraminifera of very shallow-water facies were confined to small areas (Sirel and Acar, 1982;Sirel, 1996;Sirel et al., 2013). This process of progressive marine regression may have been connected with orogenic uplift of the Anatolian region. A similar situation is found all over the Mediterranean region, where shallow and especially very-shallow marine environments with porcellaneous foraminifera such as alveolinids, soritids, peneroplids and miliolids predominate in the Priabonian to lower Oligocene depositional sequences.  Okay et al., 2017); C) Geological map of the Kıyıköy region and location of the studied section (simplified from Çağlayan and Yutsever, 1998).
Very-shallow-water foraminifera from the same time span are known in other Mediterranean regions. For instance, the Bartonian?−Priabonian species Neoalveolina vonderschimitti was first described and figured from samples from Colli Berici, North Italy, by Schweighauser (1951). Hottinger (1963) reported early Oligocene veryshallow-water foraminifera in southern Spain. Borelis vonderschimitti (schweighauser) was reported by Drobne and Pavlovec (1979) and Drobne et al. (1985) as an important marker for this environment in the Priabonian of the west side of the Pannonian Basin, North Slovenia.

MATERIALS AND METHODS
Foraminifera were described with the help of 80 oriented thin sections from limestone samples 16-TA and KZ. Microfacies and fossils were photographed using a Leitz microscope. The suprageneric classification followed the systematics of Loeblich and Tappan (1987). All the studied samples are stored in the Museum of the Department of Geological Engineering, University of Ankara, Turkey.

Lithostratigraphy
Numerous informal lithostratigraphic units have been proposed in former geological studies of the Thrace Region. Some of them have been certified as formal units by the Stratigraphy Committee of Turkey (see Siyako, 2006). The relevant lithostratigraphic unit in the present study is the Soğucak Formation (Siyako, 2006), to which the new Kazandere Member belongs. The type section of this formation is situated in the Soğucak village (map reference E 19-d2). The lithological features and the foraminiferal content of the Soğucak Formation have been described by Daci (1951), Akartuna (1963), Sirel and Gündüz (1976), Less et al. (2011), Okay et al. (2017, 2020, Özcan et al. (2020) and Yücel et al. (2020) in different areas of the Thrace region. According to Siyako (2006) the Soğucak Formation is mainly composed of shallow-water limestone alternating with marl and sandstone with benthic foraminifera including nummulitids, orthophragminids and other groups. The formation is late Bartonian to early Oligocene in age.

Kazandere Member of the Soğucak Formation
The Kazandere succession ( Fig. 2A-C) is defined here as a new member of the Sogucak Formation, and the Kazandere section ( Fig. 2A) is chosen as its reference section (holostratotype) because of its completeness, accessibility, and foraminiferal content. Other sequences (Fig. 2B Lower and upper boundaries. The Kazandere Member overlies with conformity the Koyunbaba Formation at the holostratotype ( Fig. 2A). However, at the parastratotype the Kazandere member overlies the basement metamorphic rocks of the Paleozoic (Fig. 2B, C). Its upper boundary is unknown in the studied area.

E X P L A N A T I O N S
GNEISS V e r y s h a l l o w l e s t o n e s v s l a x s p r a n . s p . n a g a n e n s s n a e l o n g a t a n . s p . Foraminiferal content and age. From sample 16 TA-19 to sample 16 TA-25A the limestone is Priabonian in age and contains very shallow-water foraminifera. This contitutes the first report of such assemblages in both the Thrace region and the whole Anatolia. The lithology and the foraminiferal content of samples in holostratotype (  Environmental interpretation: In a micropaleontological study performed in the Gulf of Aqaba, the depth distribution of Borelis spp. was reported to be between 5 and 60m, mainly 20-40m (Reiss and Hottinger, 1984, p. 267, fig. G 42). According to Hottinger (1974, fig. 5), Orbitolites lamarck species occur mainly in restricted shelves with normal salinity. The chrysalidinids seem to be restricted to extremely shallow-water deposits (Hottinger and Drobne, 1980, p. 13, 35). The data available support a very shallowwater (inner ramp) environment for samples 16-TA-19 to 25A from the reference section ( Figs. 2A; 3) and for samples KZ-2 to 7 from the parastratotype (Figs. 2B-C; 4).
The presence of N. fichteli, N. vascus (Fig. 5E), O. complanata (Fig. 13D) and corals (Fig. 5D, F), in the samples 16-TA-17, 25 B and 26 suggested a shallow-water marine environment. A similar picture with N. fichteli has been introduced from the fore-reef shoal limestone of the lower-middle Oligocene Kirkuk well, North Iraq by Henson (1950b, p. 219, Fig. 6), thus supporting the idea of a shallowwater environment for the top of the newly defined member.    Serra-Kiel et al. (1998) and Cahuzac and Poignant (1997) established the shallow-water benthic biozones of the Tethyan Paleocene-Eocene (SBZ 1-20) and of the European Oligo-Miocene (SBZ 21-26). The following SBZs identified in the Kazandere Member are based on those studies.

SBZ 21 (early Oligocene)
The first occurrence of N. fichteli and N. vascus, define the lower boundary of the SBZ 21 in the Kazandere Member ( Figs. 2A; 3). O. complanata (Fig. 13D) occurs rarely in this interval. According to Cahuzac and Poignant (1997, p. 155), the first appearance of N. fichteli and N. vascus define the lower boundary of SBZ 21.

Eocene-Oligocene boundary
The boundary between the Eocene and the Oligocene is defined by the last occurrence of the Priabonian foraminifera B. voderschimitti, C. gassinensis, C. elongata n. sp., P. globulus n. sp. and O. minimus or by the first appearance of the Oligocene species N. fichteli, N. vascus and O. complanata ( Figs. 2A; 3).
Remarks. This genera occurs from the Priabonian to the recent. (schweighauser, 1951) (Fig. 7A-G Description. The species has a small, almost spherical test (Fig. 7A-G). The equatorial diameter ranges from 0.63 to 0.83mm and the axial diameter is 0,63mm−0.83mm. The small proloculus is followed by one to two rows of streptospiral undivided early chambers (Fig. 7F-G). The following whorls are coiled tightly and divided by numerous narrow chambers (Fig. 7E). The small chamberlets and septula are arranged continuously from one chamber to the next. The chamberlets have an upright elongated oval shape ( Fig. 7A-B). The preseptal passage is narrow (Fig. 7E).

Origin of name.
It refers to the characteristic loosely coiled whorls.
Material. 40 specimens from oriented axial, equatorial and tangential sections from the type locality.

Repository. Museum of the Department of Geological
Engineering, University of Ankara, Turkey. Type level. Very shallow-water limestone of Priabonian age , that occurs below shallow-water limestone with the early Oligocene species N. fichteli and N. vascus (SBZ 21).

Diagnosis.
Spherical, rarely nautiloid test (Fig. 8D, G, L) with characteristically loosely coiled whorls displaying robust chamberlets (Fig. 8). The equatorial and axial diameters of the subspheric form range from 0,83mm to 1,08mm and the index of elongation varies between 0,85 to 0,96. A very small proloculus (0.05mm in diameter) is followed by one or two rows of undivided streptospiral chambers (Figs. 8B, F-G; 6A). Later, the loosely divided whorl becomes planispiral and involute. In cross section, chamberlets are an upright oval and arranged tightly. The comparatively large chamberlets and thin septula are lined up from one chamber to the next (Figs. 6B; 7D; 8J-K). In equatorial section there are two streptospiral and 6-7 divided whorls, and the equatorial diameter measures 1 mm (Fig. 6A).
The new species differs from B. vonderschmitti in its larger test and loosely coiled whorls with larger chamberlets. There are 5 whorls in an equatorial section of B. vonderschmitti, which measured 0.65mm in diameter (Schweighauser, 1951, fig. 2), while with the same number of whorls, an equatorial section of the new species reached up to 0.92mm in diameter (Fig. 8F). Furthermore, the new species has larger chambers than B vonderschmitti, a preseptal passage and chamberlets.

Distribution. As for Boretis vonderschmitti.
Family: Rotaliidae ehrenberg, 1839 Subfamily: Chapmaninidae thalmann, 1938 GENUS Chapmanina silvestri, 1931 Type species: Chapmania gassinensis, silvestri, 1905 Description. The test is conical with a hyaline calcareous wall. A bilocular-trilocular embryonic cone is followed by numerous uniserial discoidal chambers that enlarge rapidly in diameter as they are added. The marginal zone of the cone is divided by short vertical partitions that are arranged closely in a circumference and form almost elongated ovoid chamberlets between two successive rectilinear chambers (Fig. 6D). In the central part of the cone, the septa are folded and generate small chamberlets (Fig. 6C). The numerous pillars are distributed irregularly in the central part of the cone (Fig. 6C-D).

Description.
The type species has a low conical test with hyaline calcareous wall. The apical angle is nearly 90º. The diameter of the cone base is 1.2-2.13mm and the cone is 0.53-1.48mm in height. The ratio of the cone height to the diameter of the cone base is 0.51−0.71. The cross section of the marginal chamberlets is almost elongated ovoid in shape (Figs. 6C; 9A, E). There are 14 septa per 1mm in the vertical length (Figs. 6C; 9A). The secondary chamberlets are generally semi-lunar in shape (Fig. 6C).  l o g i c a A c t a , 1 8 . 1 4 , 1 -2 1 ( 2 0 2 0   Type level. Very shallow-water limestone of Priabonian age . Diagnose. The megalospheric generation has a characteristic globular-subglobular small test with agglutinated, canaliculate external wall (Fig. 11A-D). The diameter of the vertical section (parallel to the coiling axis) ranges from 1.0 to 1.2mm. The early globular megalosphere (0.150−0.175mm in diameter) is followed by three to four trochospiral early chambers, positioned at the apex of the globular test (Fig. 11A). The two neighbouring trochospiral early chambers are connected by intercameral foramina (Fig. 11A). The adult stage consists of large wedge-like chambers arranged in a uniserial pattern. The thick septa are characteristically incomplete and spur-like ( Fig. 11A-D). There are large apertural foramina and pillars in the central part of the test (Figs. 11J; 6F).
Large size and early chambers suggest that both megalospheric and microspheric generations occurred (Fig. 11F, H-I). The largest vertical diameter ranges from 1.4 to 2mm. The vertical section shows that the small microsphere is followed by four to five early trochospiral chambers (Fig. 11H). The adult stage is divided by spur-like shaped septa resulting in numerous wedge-like uniserial chambers as in the megalospheric form ( Fig. 11F-I).
Remarks. Very shallow-water Pfendericonus was first described and figured as a new sub-genus of Chrysalidina d'orbigny (type species Lituonella (Pfendericonus) macarskae van soest (Hottinger and Drobne, 1980, p. 13, 224). Later, Pfendericonus was elevated to generic status by Loeblich and Tappan (1987, p. 187). Due to the chamber arrangement in the early and adult stages, most authors share the view of Loeblich and Tappan (1987, p. 187).

Description.
The megalospheric generation has a small discoidal test with fineness characteristic of the internal structure as in Henson (1950a, p. 58) (Fig. 12A-C). The diameter of the test ranges from 3.3 to 3.65mm. A very large elongated ovoid megalosphere (0.75-0.8mm in diameter) is followed by a second chamber (Fig. 12E) and numerous cyclical chambers with chamberlets.

Remarks.
Because of the presence of the small discoidal test with fine internal structure and very large megalosphere, the studied specimens ( Fig. 12A−G) have been described and figured as O. minimus, although Priabonian species O. complanatus var. minima had been described and figured from inadequate equatorial section by Henson (1950a, p. 58, pl. 3, fig . 1).

Distribution. As given for B. vonderschmitti.
Family: Peneroplidae schultze, 1854 GENUS Coscinospira ehrenberg, 1839 Type species: Coscinospira hembrichii ehrenberg, 1839 Coscinospira sp. (Fig. 12H) Description. Elongated crosier-shaped test with imperforate, calcareous porcellaneous wall as in Coscinospira elongata sirel and özgen-erdem (Sirel et al., 2013, p.93). The diameter of the planispiral early stage is 0.275mm and the length is 1.53mm. The small globular megalosphere (0.075mm in diameter) is followed by 10 Description. The megalospheric form has a small and inflated lenticular test with rounded-pointed peripheral margin ( Fig. 13A-B). The diameter of the test ranges from 3.1 to 3.6mm and the thickness 1.25-1.5mm. The rectangular or subrectangular septal filaments form a meshwork on the test surface. Weakly developed granules totally merge to form a regular reticulation. The equatorial sections clearly show that the spire interval increases gradually from the first whorl to the last one (Fig. 13C). The globular megalosphere is followed by subrectangular chambers that line up in 5-6 whorls. The width of the subrectangular chambers is always greater than the height (Fig. 13A). There are 6-7 whorls in an equatorial section measuring 3.4mm in diameter (Fig. 13C).
Distribution. N. fichteli occurs in the lower Oligocene shallow-water limestone, located at the top of the Kazandere Member with N. vascus, O. complanata and an amphisteginid. It also occurs in the early Oligocene (SBZ 21) of the Dolhandere, East of Kırklareli, central Thrace with N. vascus and Operculina (Sirel and Gündüz, 1976). Furthermore, it occurs in the Rupelian-lower Chattian of eastern and southern Turkey (Sirel, 2003, figs. 2-12).
The megalospheric form has a small inflated lenticular test with a slightly rounded periphery (Fig. 13E-G). The diameter of the test ranges from 2.1 to 3.2mm and the thickness is 1.0-1.5mm. The surface of the test is ornamented with a small central knob and slightly curved thick septal filaments (Fig. 13D). The globular megalosphere (0.200-0.250mm in diameter) is followed by small subrectangular chambers that are lined up in 4 whorls (Fig. 13F). The spire interval increases gradually from the megalosphere to the last whorl. Thin septa are curved backward.

CONCLUSIONS
Throughout the Mediterranean region, very shallowwater (inner ramp) marine deposits with porcellaneous foraminifera (particularly alveolinids) are virtually absent in the Priabonian. An exception to this situation are the very shallow-water limestone facies with mainly Borelis species, such as that of the Kazandere Member from the Thrace region (Turkey), newly defined herein, which contains well preserved and abundant B. vonderschimiti, B. laxispira n. sp., C. gassinensis, C. elongata n. sp., P. globulus n.sp, O. minimus and Coscinospira sp. This Priabonian sequence with B. vonderschimitti and B. laxispira n. sp. occurs just below the Oligocene limestone with N. fichteli, which allows the precise definition of the Eocene-Oligocene boundary in the study area.
In addition, B.vonderschimiti, B. laxispira n.sp. are significant to understand the evolution of the Alveolinidae ehrenberg in the region. At the end of Bartonian, all over the Mediterranean region, large tectonic movements (especially orogenic) resulted in major regresions, which lead to the disappearance of very shallow species of Alveolina d' orbigny. However, the alveolinid genus Borelis and particularly B. vonderschimitti and B. laxispira n.sp. remained, thus the alveolinids carried on in the Tethyan realm.