Oligocene larger benthic foraminifera and sedimentation of the Burdur Basin, SW Anatolia, Turkey

ABSTRACT This study focuses on the Oligocene successions that contain predominantly siliciclastic, limestones with larger benthic foraminifera in the Burdur Basin of southwest Anatolia, Turkey. The Burdur-1 and Burdur-2 sections in the Oligocene deposits are described sedimentologically and were sampled for lithology, biostratigraphy, microfacies analysis and fossil content. Larger benthic foraminifera were mainly recovered from limestones in the upper part of the Burdur-2 section. Thin-section analysis of the larger benthic foraminiferal assemblage reveals that the upper part of the Burdur-2 section includes Nummulites fichteli Michelotti, 1841, Operculina complanata (Defrance, 1822), Nephrolepidina praemarginata (Douvillé, 1908), Eulepidina sp., Planorbulina sp., Amphistegina sp., and Asterigerina sp. This foraminiferal association represents the Shallow Benthic Foraminifera Zone 22 (SBZ 22) of Rupelian-early Chattian age. Sedimentological analysis in the Burdur-1 and Burdur-2 sections has revealed an association of massive-unorganized conglomerate, parallel-bedded sandstone, massive mudstone, graded-massive conglomerate, graded-massive sandstone, well-sorted organized conglomerate, well-sorted parallel-bedded sandstone and microfossil-bearing parallel-bedded limestone facies. Alluvial fan /river, fan-delta, beach and shallow shelf carbonate facies communities were identified by lateral and vertical correlation of facies. It is concluded that the large Burdur Fan developed in a graben basin that opened and was flooded by a branch of the Tethyan Ocean that invaded southwest Anatolia during the Oligocene.


INTRODUCTION
Turkey is part of the Orogenic Belt of the Alpine-Himalayan Mountain Range (Fig. 1). The geological evolution of southwestern Turkey is unique in the Mediterranean region since several well-preserved tectonic units exemplify the whole tectonic history of the Alpine orogen (Şengör & Yılmaz 1981;Şenel 1997;Collins & Robertson 1998, 2003. The rock succession outcropping in the western Taurids (SW Anatolia), was placed into the following four tectonic units by Collins & Robertson (1998, 2003: Menderes Massif, Bey Dağları platform, Lycian thrust sheets and melanges, and supra-allochton sediments. As shown by Poisson (1977), Özcan et al. (2009), and several workers, the Oligocene-Miocene development of the region is characterized by both the emplacement of Lycian Allochthon over the Bey Dağları platform in the foreland and the development of extensional basins in the hinterland. Although a very tentative foraminiferal composition of the Oligo-Miocene units of both autochthonous and supra-allochthonous units was documented by several workers (Dizer 1962;Poisson 1977;Göktaş et al. 1989;Örçen 1991;Şenel 1997), updated palaeontological studies with zonal precision are lacking. Gedik & Karadenizli (2007) and Karadenizli et al. (2009) presented unpublished detailed palaeontological and biostratigraphical data for the Oligocene deposits in the Burdur region. The most recent studies of larger foraminifera from the Burdur area include Özcan et al. (2009) and Matsumaru et al. (2010). Özcan et al. (2009) studied the Korkuteli (Antalya) and Tavas-Burdur basins, and presented a detailed foraminiferal inventory of both Oligo-Miocene units from the Bey Dağları platform and coeval supra-allochthonous units (piggy-back basin units of Sözbilir 2002). The latter units are deposited in an extensional tectonic regime. Matsumaru et al. (2010) focused their study on the Karabayır Formation in the Korkuteli Basin, which was deposited in a shallow-marine regime well-marked by lower Miocene transgressive sediments, which in turn are overlain by basinal units of the Karakuştepe Formation that developed prior to the emplacement of the Lycian Nappes (Poisson & Poignant 1974;Poisson 1977;Şenel 1997;Robertson 2000). These authors have hypothesized that tectonics were the most influential control on the depositional history of the Bey Dağları foreland basins during the Oligocene-Miocene.
The marine Oligocene units of southwestern Anatolia, deposited under different tectonic regimes, are important in interpreting the regional palaeogeography of the Neotethys (Şenel et al. 1989;Karadenizli et al. 2009Karadenizli et al. , 2017. During the Oligocene, thick siliciclastic sediments with limestones formed on the basement belonging to the Lycian Nappes in the Burdur Basin (Şenel et al. 1989;Karadenizli et al. 2009Karadenizli et al. , 2017. These limestones contain diagnostic larger foraminifera such as nummulitids and lepidocyclinids, which are of great value for biostratigraphic dating and regional correlation (Karadenizli et al. 2009(Karadenizli et al. , 2017. Oligocene sediments outcrop in the Burdur area along the Burdur Lake (Fig. 1A) where are situated the sections Burdur-1 and Burdur-2. Their geographic coordinates are 37°48'22"N, 30°06'31"E and 37°47'38"N, 30°07'33"E, respectively. In this study, the stratigraphy of the region was re-examined and correlated with the Acıgöl region. The Oligocene units consist of the Çardak and Hayrettin formations (Göktaş et al. 1989). Their sedimentological characteristics are interpreted to represent the development of Oligocene sedimentary environments in the Burdur Basin. Micropalaeontologic studies and facies analysis were carried out in the Burdur-1 (840 m)
and Burdur-2 (140 m) sections to interpret the sedimentary environments (Figs 1A; 2). Large foraminifera were analysed from the shallow shelf carbonates of the Hayrettin Formation, in the upper 20 m of the Burdur-2 section.

MATERIAL AND METHODS
Two stratigraphic sections, in total about 1000 m, were logged. The foraminiferal assemblages were mainly recovered from limestones in the upper part of the Burdur-2 section. Taxonomic identification of the benthic foraminifera was performed on randomly oriented sections. Integrated sedimentology and micropalaeontology have enabled lateral and vertical interpretation of the facies, allowing recognition of four facies associations and their sedimentary environments. For systematic study of foraminifera, the classification of Loeblich & Tappan (1987) is followed. All the non-oriented and oriented thin sections are deposited in the collection of the General Directorate of Mineral Research and Exploration (MTA), Ankara, Turkey, under the numbers shown in Figures 4 and 5.

GEOLOGICAL SETTING AND STRATIGRAPHY
The southwestern Anatolian region was initially under the influence of a compressive regime until the late Eocene (Şenel 1997). It was later influenced by an extensional regime (Seyitoğlu & Scoot 1991, 1992Karadenizli et al. 2009Karadenizli et al. , 2017. The basement Lycian Nappes are settled over the continental crust as a result of the collision of the oceanic crust to the continental crust during the Cretaceous (Şenel 1997). The region was later influenced by the extension regime that is explained by various tectonic models such as back-arc expansion, tectonic escape, orogenic collapse and episodic two-stage graben formation.
The back-arc expansion model involves subduction of the African Plate under the Anatolian Plate along the Aegean-Cyprus zone, and this caused crustal expansion behind the subduction zone in the Aegean and western Anatolian regions (McKenzie 1978;LePichon & Angelier 1979, 1981Jackson & McKenzie 1988). The tectonic escape model suggests collision of the Arabian Plate and the Eurasian Continent, which resulted in the formation of the Anatolian wedge during the late Serravallian (Middle Miocene), bordered by the North Anatolian and East Anatolian strike-slip faults ( Fig. 1), with westward tectonic escape of the Anatolian Plate and the beginning of the expansion regime in Western Anatolia (Şengör 1979, 1982Dewey & Şengör 1979).
The orogenic collapse model considers that the collision between the northern Sakarya Continent and southern Anatolite-Tauride Platform resulted in the closure of the Neotethys Ocean, and caused a crustal thickening along the Izmir-Ankara-Erzincan suture zone during the late Paleogene. The later thinning of the crust along this suture zone by orogenic collapse leds to the extensional regime in Western Anatolia during the late Oligocene-early Miocene (Dewey 1988;Seyitoğlu & Scoot 1991, 1992. The episodic two-stage graben model claims that none of the above models are satisfactory to explain the origin and age of the crustal expansion, and suggests that the expansion regime evolved in two different structural styles (Koçyiğit et  . 1999;Bozkurt & Rojay 2005). According to this model, the primary graben system developed, depending on the formation of the core complex, in an extensional tectonic regime during the early-middle Miocene, and this tectonic regime ended during the late Serravallian-early Tortonian interval with the start of a north-south compressive regime. The model then stated that the second graben formation occurred during the Pliocene-Quaternary time interval. Burdur Basin opened by normal faulting that developed mainly by extension tectonics. It supports the orogenic collapse model with the available tectonic data.

Stratigraphy of Burdur BaSin
The Oligocene-Miocene marine units in the southwestern Turkey were defined with a variety of different names. We here adopt the lithostratigraphic nomenclature proposed by Şenel 1997) who also provided a geological map of the region. The oldest units in the study area are the Lycian Nappes that consist of ophiolitic melange, olistostrome, harzburgite, serpentinite, dunite, limestone, calciturbidite, volcanite, radiolarite, chert and shale (Fig. 1B). This basement unit is unconformably covered by the Varsakyayla Formation (Şenel 1997), which consists of Eocene sandstones, claystones and limestones. The Varsakyayla Formation is overlain disconformably by the Çardak and Hayrettin Formations that are the subject of the present study (Fig. 1).
The Çardak Formation was first defined by Göktaş et al. (1989) in an unpublished report, who divided it into four members. In this study, we observed that the Maymundağı Member of Göktaş et al. (1989), which consists of thick conglomerate-sandstone facies, constitutes the predominant lithofacies of the Çardak Formation while the other members reflect its lateral and vertical facies changes. All the members defined by Göktaş et al. (1989) are, consequently, grouped in a single unit in the present study, the Çardak Formation. In the Burdur area Yalçınkaya et al. (1986) distinguished this unit as the Ardıçlı Formation, and Şenel 1997) and Şenel et al. (1989) included it in the Acıgöl Group. The Hayrettin Formation was also first defined by Göktaş et al. (1989). It conformably overlies the Çardak Formation, and it is covered unconformably by Miocene sediments. The main lithologies of the Hayrettin Formation are sandstone and claystone, although locally conglomerates and fossiliferous limestones are also observed. Göktaş et al. (1989) dated this formation as "middle-late Oligocene" based on its invertebrate fossil association.

SEDIMENTOLOGY OF OLIGOCENE DEPOSITS
Facies analysis was carried out in the Burdur-1 (840 m) and Burdur-2 (140 m) sections that comprise almost entirely the Çardak and Hayrettin formations. Ten facies were identified using their lateral and vertical relations (Fig. 2). These are here grouped in four facies associations: alluvial fan-fluvial, fan-delta, beach and shallow water carbonate sediments.   (13) facieS Massive-unorganized conglomerate This facies consists of red 90-140 cm thick layers, with lateral continuation up to 250 m. The base does not show evidence for erosion, and the sorting is poor. Reverse grading is rare. Clasts are generally aligned along a north-south direction. The granular components are usually limestone, serpentine, radiolarite, derived from the underlying Lycian and ophiolite nappes. Although the average grain size is 5-8 cm, clasts may have dimensions up to 75-80 cm. The presence of thick and massive layers, locally reverse grading, poor grain sorting and large blocks is consistent with mass flow deposition facies (Bull 1977).

Parallel-bedded sandstones
These sandstones reveal parallel layers, and their thickness varies between 20-55 cm and they extend laterally for several hundred meters. Medium to coarse-grained sandstones are mostly lithic-wackes, and the cement is composed of clay. Despite their relatively thin beds, their lateral continuity suggests that they were formed at the end of the flooding period by the lateral accumulation of bed load in the form of sand flats (Rust 1978a, b).
These sandstones composed of middle-coarse sand grains are lithic and binding rocks. Although the thickness of beds is low, their lateral continuation is a sign of the accumulation of the bed burden at the end of the flood (Rust 1978a, b).

Massive mudstone
The red mudstones display lateral continuity of hundreds of meters and contain grass, wood and coal fragments. The clayey part of the mudstones is formed from suspension while their silt component was deposited during the flooding stage from the mixture of suspended sediments and bed-load that settled as floodplain deposits (Miall 1987).

Reverse grading, massive conglomerates
This facies, which is most commonly found in the Burdur-1 section, varies in thickness from 70 to 175 cm and lateral continuity reaches several tens of meters. The matrix ratio increases upwards in each layer of conglomerates, displaying reverse grading. Water escape, load and flame structures are observed in some places. The average grain size is between 6-12 cm, while the maximum grain size reaches 60 cm, and preferred grain orientation is observed. The lateral orientation of grains, the increase of matrix rate upwards, load and flame structures indicate that this facies settled as an underwater debris flow (Nemec & Steel 1988).

Reverse graded, massive sandstone
The green and grey sandstones are composed of fine to medium parallel layers with eroded upper surfaces. Medium to coarse-grained sandstones display reverse grading, water escape, load, deformation and obstrusion structures. The load, deformation and obstrusion structures observed in sandstones and conglomerates indicate that they settled in subaqueous conditions (Nemec & Steel 1988).

Well-sorted, organized conglomerates
Well-sorted and grain-supported conglomerates form 60-80 cm thick parallel layers, with coarse aggregates composed of disc or blade-shaped grains. The top of the sheets are convex upward. These conglomerates pass laterally into marine facies. This facies represents beach platform and berm environments (Postma & Nemec 1990;Bluck 1999).

Well-sorted, parallel-bedded sandstones
Well-sorted and parallel-bedded sandstone includes scattered medium to coarse-grained gravels and the layer thickness is between 15 to 55 cm. Fine gravels form pockets extending parallel to the stratification. These sandstones show lateral transition towards the terrestrial facies. This facies fits with Bluck's (1999) beach "outer-frame" or represents shore side/ shore front transition in a coastal environment.

Well-sorted, cross-bedded sandstones
These green-grey sandstones have cross-bedding, and their thickness is between 12-20 cm, pre-set thickness 3.9 mm and cross-bedding dip between 10 o -14 o . The top surface of layers is eroded. Medium to fine-grained sandstones are well sorted and do not contain a clay matrix. This facies displays obstrusion and load structures. The presence of cross-bedding and bed-load transport in the lateral direction indicate that this facies is deposited by water flows (Rust 1978b). In addition, these well-graded sandstones are formed by wave-reworking of fan-delta deposits, and they may have formed beach and near-shore bars (Crowell & Link 1982).

Laminated claystones
Although these grey-green coloured claystones are usually laminated, they can also appear massive owing to amalgamation. The claystones contain benthic foraminiferal fragments. These claystones were formed by suspension sediments in a coastal marine environment. They are locally mixed with shore-derived silt and the presence of thin coal levels indicates periodic deposition in restricted marshlands.

Fossiliferous, parallel-bedded limestones
This facies is located in the uppermost parts of Burdur-2 section (Fig. 3). The yellow coloured limestones, which vary in thickness between 15 and 70 cm, locally alternate with claystones. Petrographic analysis indicates grainstone, packstone and wackestone fabrics with abundant benthic foraminifera. These limestones were formed in regions of minimal siliciclastic input in lagoons within a carbonate platform (Heckel 1974).

facieS aSSociationS:
Based on lateral and vertical relations of the ten facies described above, four facies associations can be distinguished: alluvial fan-fluvial deposits, fan-delta deposits, beach deposits, and shallow shelf carbonate deposits (Fig. 3).

Alluvial fan and fluvial deposits
This facies association, including massive-unorganized conglomerates, parallel-bedded sandstones and massive mudstones, is observed at the Burdur-1 section (Fig. 1). It is included in the Çardak Formation (Fig. 1), and it indicates alluvial fan and fluvial depositional environments (Miall 1991(Miall , 1996.

Fan-delta deposits
This facies association consists of reverse grading, massive conglomerates and sandstone and is observed in the Burdur-1 section and in the basal part of the Burdur-2 section (Fig. 3). A large part of the Çardak Formation consists of this facies association. The term fan-delta was first used by Holmes (1965), followed by McPherson et al. (1988) and later defined by Nemec & Steel (1988).

Beach deposits
This facies association is composed of well-sorted conglomerates, bedded sandstones, cross-bedded sandstones and laminated claystones (Fig. 3). It constitutes most of the Hayrettin Formation and is especially widespread in the Burdur-2 section and at three intermediate levels in the Burdur-1 section (Fig. 2). These sediments, within beach and longshore bars, are considered to be the result of reworking of fan-delta deposits by wave action (Crowell & Link 1982). The process is considered to be associated with a sea level rise. Coal seams are considered to have accumulated in protected parts.

Shallow shelf carbonate deposits
The shallow shelf marine carbonates are composed of fossiliferous, parallel-bedded limestones and laminated claystones. They are located at the highest levels of the Hayrettin Formation (Fig. 2). Wright & Burchette (1996) found that similar facies developed in the ramp-type carbonate platform. Aigner (1985) also noted such deposits in carbonate benches or shallow marine environments. The mud banks are considered to have accumulated in local depositional lows (Basan 1973;Turnel & Swensen 1976). Increased carbonate production produced skeletal benches, that later developed into shell islands and beaches.

Oligocene Basin development
According to the facies and facies assemblages obtained from the sequence of Oligocene in the Burdur region, the Çardak Formation is formed in alluvial fan /fluvial, fandelta environments and the Hayrettin Formation occurs in beach and shallow shelf carbonate environments (Fig. 3). The basin fill is controlled by the Burdur Normal Fault which formed the southern edge of the basin. The fan controlled by this fault spread to the south (Fig. 3). The clastics material were then deposited along the beaches, bringing with the effect of waves. In the absence of clastic yield, carbonate precipitation has occurred and many organisms have found shelter. remarkS This reticulate species belongs to the N. fabianii lineage. This form has inflated lenticular shell with rounded periphery. The diameter of test ranges from 2 to 6.4 mm and the thickness from 0.5 to 1.8 mm. The test surface is covered with a reticulation that has rectangular or subrectangular mesh ( Fig. 4A, C-E; Fig. 5C). The large microsphere (diameter 0.3 to 0.4 mm) is followed by the regular coiled whorls.

remarkS
The test is thin, bilateral and symmetric. Its diameter ranges from 2.5 to 2.9 mm, and the thickness from 0.4 to 1.8 mm.
The test is planispiral and evolute, with numerous narrow chambers in many rapidly expanding whorls, so that the height of the chambers may be five times the width of the chambers (Fig. 5A, B). The proloculus is spherical and small (90 µ diameter).
Family lepidocyclinidae Scheffen, 1932Genus Nephrolepidina Douvillé, 1911 Nephrolepidina praemarginata (Douvillé, 1908 remarkS The identification is based only upon axial sections. The small test is inflated, lenticular with a central umbo, so that the shell is thicker towards the center. The diameter of the test ranges from 1.5 to 2.1 mm and the thickness from 0.6 to 0.9 mm. The large, central umbo consists of numerous small pustules (Fig. 5E, G). Lateral chambers are numerous at the surface of test and rosette shaped, and this feature forms a comb-like surface appearance (Fig. 5E). The embryo consists of subspherical small protoconch (diameter 0.25 mm) and reniform deuteroconch (diameter 0.22 mm). Secondary chambers are very small and their sizes are almost equal to each other. The equatorial chambers are subrectangular or rhombic in outline. There are 6-7 orders of lateral chambers in the center of the test.

Genus Eulepidina Douvillé, 1911
Eulepidina sp. (Fig. 5H, I) remarkS The identification is based only upon axial sections. The test is lenticular, its diameter ranges from 8.1 to 11.2 mm and its thickness from 1.1 to 1.7 mm. The equatorial chambers are polygonal in shape.

DISCUSSION ON THE LARGER FORAMINIFERAL RECORD
The study of Oligocene hyaline larger foraminifera, based on the biometric study of the nummulitids and lepidocyclinids, from western Taurids, gives insights on their palaeobiogeography in the Tethys. These groups show that they are closely related with the coeval assemblages known from the European basins.  is used as key criteria in Oligo-Miocene biostratigraphy.
Although Douvillé (1908Douvillé ( , 1925 reported N. praemarginata from the early Oligocene of Italy and from the Oligocene-Miocene transition in France, Cahuzac & Poignant (1997) demonstrated that this form is limited to the biozone SB 22. This taxon is poorly documented in Turkey. Some specimens of this species have been described in eastern Turkey as Nephrolepidina partita based on the presence of coarse granules in the umbonal part of the test (Sirel 2003;Gedik 2014Gedik , 2015. According to Sirel (2003), N. praemarginata is associated in the Sarıbuğday section (Elazıǧ region, eastern Turkey) with Nummulites fichteli, N. vascus, N. cf. germanicus, Borelis pygmaea, B. inflata, B. merici, Operculina complanata, Neorotalia lithothamnica, N. pinarensis, N. cf. tectoria, N. viennoti Karadenizli et al. 2009Karadenizli et al. , 2017, with thick sequences deposited in graben basins opened by normal faults. An example of this system is seen in the Burdur region where a 840 m thick fan sequence developed in a graben basin (Fig. 3). A simple stratigraphic succession developed in the Burdur Fan during the Oligocene, which consists of the Çardak and Hayrettin formations. The ten lithofacies have been grouped into four facies associations (Fig. 2) based on their lateral and vertical relations (Fig. 3). Sedimentary environments of these facies associations show that the Burdur region was invaded by a shallow sea from the west, and that the sea penetrated into Anatolia.
The basin fill is controlled by the Burdur Normal Fault which formed the southern edge of the basin. Along this margin, alluvial-fluvial fan and fan-delta facies associations developed, while in the northern part sedimentation was limited to fan-delta successions (Fig. 3). These facies mainly consist of coarse-grained sediments derived from the Taurus tectonic unit. There is no evidence for sediment derivation from the metamorphics of the Menderes Massif. A beach facies is present towards the center of the basin (Fig. 3). Ramp-type carbonate platforms also developed depending on the shelf topography in the intervals when clastic input was minimal (Fig. 3).
A moderately diverse and well-preserved assemblage of hyaline and porcellaneous larger foraminifera was found in the late Rupelian-early Chattian shallow marine deposits of the Hayrettin Formation in the Burdur Basin, SW Anatolia. The foraminiferal association includes 7 genera. Larger foraminifera from the Hayrettin Formation, classified under the genera Nummulites, Operculina, Nephrolepidina, Eulepidina, Planorbulina, Amphistegina and Asterigerina show close similarity to the coeval assemblages already known from the European and circum-Mediterranean marine sedimentary sequences. Their western Tethyan affinity allowed us to apply the standard biozonation scheme (SB zonation) for our study area in SW Turkey. The late Rupelian-early Chattian age, i.e., SB 22 Zone of Cahuzac & Poignant (1997) for the Burdur-2 section is documented by the presence of Nephrolepidina praemarginata, together with Nummulites fichteli. This evidence dates the Hayrettin Formation as late Rupelian-early Chattian (SB 22).