Geology of the Central Sivas Basin (Turkey)

This paper presents a revised geological map at the 1/50,000 scale of the Central Sivas Basin together with a synthetic stratigraphic chart and cross-sections. The map covers an area of approximately 9840 km2 within the Eastern Anatolian orogen. The structure of the studied area is dominated by three major tectonic domains: (i) to the south, a north-verging thrust wedge involving Maastrichtian – Eocene sediments deposited onto an ophiolitic basement, (ii) in the center an Oligo-Miocene domain shaped by salt tectonics detached above the thrust wedge, along a late Eocene salt layer, and, (iii) to the north the Pliocene depocenter onlapping onto the Kırşehir basement. The central halokinetic domain exhibits two generations of minibasins (respectively, early Oligocene and late Oligocene to late Miocene), separated by an evaporite canopy. The map includes new stratigraphic correlations for the pre-salt stratigraphy and improve the comprehension of the southern fold-and-thrust-belt. ARTICLE HISTORY Received 5 March 2018 Accepted 18 August 2018

Based on the compilation and homogenization of existing maps (see Methodology), together with new data acquired during fieldwork, we updated the geological map of the central Sivas Basin (Sheet 1 (Main map)). The map covers the central part of the basin and encompasses stratigraphic levels ranging from the lower Mesozoic basement to the Quaternary deposits. It is complemented by additional data (Sheet 2 (Main map)) highlighting the basin architecture and stratigraphic relationships.
Based on this compilation, the team, including 4 PhD students (Kergaravat, 2016;Legeay, 2017;Pichat, 2017;Ribes, 2015) and their advisors, performed more than 20 months (cumulated) of field acquisition, involving several thousands of dip-data acquisitions, as well as about 33 km of sedimentological logs.
C. Ribes and C. Kergaravat studied the mini-basins core area and updated the stratigraphic chart of the post-salt deposits with a 1200 km 2 map at the 1/ 50,000 scales (e.g. Kergaravat et al., 2016;Ribes et al., 2015;Ribes et al., 2017). This later has been completed and extended over a surface of 9840 km 2 by E. Legeay and A. Pichat during their respective PhDs (Legeay, 2017;Pichat, 2017;Pichat et al., 2016;Pichat et al., 2018). This work was complemented by the updating of the pre-salt stratigraphic chart thanks to the compilation and homogenization of numerous studies dealing with the pre-salt deposits in different localities of the basin (e.g. Cater, Hanna, Ries, & Turner, 1991;Kurtman, 1973;Poisson et al., 1996).
Finally, the northern domain of the basin, mapped with less detail, follows the study of Özden et al. (1998).

Geological sheets organization
Two Geological Sheets are included in this paper, accompanied by a georeferenced map (geoTIFF format).
The Sheet 1 (A0 size) presents the geological map at the 1/50,000 scale with the associated legend. Below the main geological map displays (i) a basement map without sedimentary units, (ii) a structural map with the major's tectono-sedimentary units, and associated tectonic domains and (iii) a sketch map highlighting extension of different evaporite levels.
The Sheet 2 (A1 size) presents the architecture and sedimentary units of the basin. It displays a N-S and E-W correlations charts of pre-salt formations across the central part of the map. Furthermore, basin and regional-scale cross-sections modified from Legeay (2017) are presented to highlight the tectonic features of the Sivas basin.

Sivas Basin
On the basis of previous studies, Figures 2 and 3 summarize the configuration of the three main structural domains of the basin along a synthetic N-S cross-section Kergaravat et al., 2016;Temiz, 1996). Each domain is composed of specific tectono-sedimentary units (Unit 1 to 4), which will be further detailed. From south to north, the basin can be described as followed: 3.2.1. Fold-and-thrust-belt (pre-salt basin, Unit 1) On the southern edge of the basin, the fold-and-thrustbelt involves Maastrichtian -Paleocene carbonate platforms and associated slope deposits. They cover the ophiolite (Kurtman, 1973), and farther north, lower to middle Eocene marine flysch formations deposited in a foredeep, north of the growing compressional domain (Artan & Sestini, 1971;Cater et al., 1991;Kurtman, 1973). The fold-and-thrust-belt is covered by a regionally extensive marine salt layer deposited during the late Eocene (Kurtman, 1973;Özçelik & Altunsoy, 1996;Pichat et al., 2016;Pichat, 2017;Tekin, 2001). The salt acted as a passive-roof detachment that decoupled the halokinetic domain of the salt-andthrust-belt from the thrust wedge below (Figure 3).

Salt-and-thrust-belt (salt basin, Units 2 and 3)
In the central part of the basin, the salt-and-thrust-belt involves the late Eocene evaporite and the Oligocene to late Miocene continental to shallow-water marine deposits ( Figure 3). In this domain, salt walls and diapirs delineated a first generation of mini-basins (Unit 2) filled by early Oligocene continental clastics Pichat, 2017) and capped by an extended Oligocene evaporite canopy Ribes et al., 2015;Ribes et al., 2017). Secondary mini-basins (Unit 3) developed on this second evaporite layer and were filled with (i) upper Oligocene continental clastics Ribes et al., 2017) grading to (ii) early Miocene marine sediments, and, eventually (iii) middle to late Miocene continental clastics and reworked evaporites Kergaravat et al., 2017;Poisson et al., 2016;Ribes et al., 2017;Ribes et al., 2018). The late Eocene evaporites acted as an efficient decollement level that decoupled the mini-basins province from the compressional wedge at depth, which kept growing and propagating during Oligo-Miocene times . The first generation of mini-basins (lower Oligocene) and the earliest sediments of the secondary mini-basins (middle Oligocene) recorded a salt-controlled stage of mini-basin initiation and downbuilding. The second generation of mini-basins (late Oligocene to late Miocene) developed while being increasingly influenced by the compressive setting, with squeezing of the bordering diapirs and northward mini-basin tilting . The Northern border of the salt-and-thrust-belt is marked by an east-west trending, north verging thrust, namely the Sivas thrust Poisson et al., 1996;Temiz, 1996) rooted in the Miocene canopy front along the central domain, and in mother salt toward the east and west.

Stratigraphy
As previously introduced, the sediment succession within the Sivas Basin has been separated into five depositional groups (Figure 4). Sedimentary infill recorded basin migration from south to north (Figures 5 and Figure 6). On the basis of our own observations and published studies cited below, we shortly describe here the main lithostratigraphic units subdivided in five sedimentary groups. Groups I and II are pre-salt sediments and coeval to the structural unit 1. Groups III, IV and V are post-salt sediments and respectively coeval to the structural units 2, 3 and 4.

Group I: Maastrichtian -Paleocene
The Maastrichtian -Paleocene succession corresponds to a relative quiet tectonic phase of carbonate platform constructions to the south and turbiditic deposits further north. A general correlation chart of the presalt sedimentary formations is proposed in Figure 6.
The Tecer Formation (Bluementhal, 1938) (50-700 meters-thick) is dated as Maastrichtian to Thanetian (Inan & Inan, 1990;Yalçin & Inan, 1992). It constitutes the extended and highest relief of the southern edge of the Sivas Basin and directly rests on the ophiolitic basement or above local conglomerates reworking the ophiolite (Kurtman, 1973). The formation starts with tens of meters-thick rudist patch-reefs grading upward to marls and massive dark-grey and fossiliferous carbonates. The formation was interpreted as characterizing a shallow-water carbonate platform (Inan & Inan, 1990;Yalçin & Inan, 1992). Based on the microfauna content, the Paleocene Gurlevik formation described to the east in earlier studies (Kurtman, 1973) is considered as a lateral equivalent to the Tecer Formation (Inan & Inan, 1990).
The Yağmurluseki Formation (Meshur & Aziz, 1980) (50-300 meters-thick) was formerly attributed to the Maastrichtian to Paleocene, and considered as a lateral equivalent of the Tecer Formation (Kavak, Poisson, & Guezou, 1997), but without evidence for lateral facies variations. On the basis of microfauna content (especially Assilina sp.), we rather suggest a Paleoceneearly Eocene age. The formation crops out on the northern side of the Tecer Mountain. It comprises a lower basal reddish conglomerates, covered by red clastics which are capped by a ten meter thick carbonate bed (Kavak et al., 1997).
The Kaleköy Formation (Gökten, 1983) (1000-1500 meters-thick) was dated as late Cretaceous to early Paleocene and consists of coarsening upward sandstones rich in volcanic grains, ophiolitic clasts and interbedded with tuffaceous deposits (Gökten, 1983;Gökten, 1986). It outcrops to the western part of the Sivas Basin and at depth in the map area. The formation was interpreted as turbiditic deposits linked to a northward prograding fan system.
The Gazibey Formation (Gökten, 1983) (20-50 meters-thick), was dated as late Paleocene, and regionally covers the Konakyazi and Çerpaçindere formations. It is made of reddish shales containing radiolarian fauna and Globorotalia sp. and characterizing an open marine setting.

Group II: Eocene
The Eocene deposits consist of marine turbidites coeval with the initiation of contraction within the Sivas foreland basin. The open-marine basin was confined during the late Eocene, and was filled by evaporites.
The Bahçecik Formation (Kurtman, 1973) (50-500 meters-thick) was dated as early Eocene to middle Eocene (Kurtman, 1973)  . Along  Kurtman (1973) and Poisson et al. (1996). the northern edge of the basin, the formation rests unconformably on the IASEZ ophiolitic mélange. Along the southern edge, it is unconformable on the ITSZ peridotites and older sediments. The formation consists of thick debris flow and conglomerates that were fed by the dismantlement of (i) peridotites and carbonates of the Tecer Formation to the south and (ii) Kırşehir metamorphosed rocks and ophiolitic mélange to the north. North of the Gürlevik anticline (near the Aktaş village), the upper boundary of the formation exhibits fining up conglomerates and sandstones marking the transition with the overlying Kozluca Formation (Kurtman, 1973). Along the northern edge of the basin, the Bahçecik Formation ends with alternating sandstones and marls related to a deltaic environment .
We also include in the Bahçecik Formation the tens of meter-thick Eocene Sögütlü Formation which have only been locally described along the southern side of the Tecer and Gürlevik mountains (Aktimur et al., 1990;Aktimur & Tütüncü, 1988). This also includes debris flows and conglomerates made up of ophiolitic clasts and Eocene carbonate fossils (giving a distinctive cream to grey color) and related to alluvial deposits.
The Kozluca Formation (Kurtman, 1973) (700 meters-thick) was dated early Eocene (Kurtman, 1973). It is exposed in large detachment folds in the southern part of the basin. The formation consists of greenish conglomerates and sandstones made of volcanoclastics and ophiolitic material, alternating with marls, and displaying coarsening-up and thickeningup sequences. The formation was interpreted as turbiditic channel and lobe deposits that were fed from the eastern side of the basin (Artan & Sestini, 1971). The formation acts as a lateral and distal equivalent of the Bahçecik Formation.
The Tokuş Formation (150 meters-thick) was dated as middle to late Eocene, and only crops out along the northern edge of the basin where it locally developed around Eocene volcanoes Yılmaz et al., 1995). The formation includes sandstones interbedded with nummulitic limestone interpreted as shallow-water marine deposits (Yılmaz et al., 1995).
The Yapalı Formation (Yilmaz et al., 1989) (up to 150 meters-thick) was dated middle Lutetian. It mainly consists of calcarenites and thin-bedded calcareous mudstones and sandstones. Several competent pelagic to conglomeratic limestones beds characterize the top of the formation. These deposits were interpreted as calci-and siliciclastic turbidites.
The Bozbel Formation (up to 700 meters-thick) was dated Lutetian -Bartonian (Kurtman, 1973). It corresponds to regular alternation of thin-bedded sandstones and marls (Artan & Sestini, 1971;Kurtman, 1973). Olistostromes and slumped levels originating from the southern fold-and-thrust-belt are locally present (Figure 6), especially in the western part of the basin where olistoliths can be up to 50 meters-thick. The upper part of the formation become siliciclasticfree with azoic marls and mudstones. The formation was interpreted as characterizing proximal to distal turbidites becoming sediment-starved in the upper part.
mass of porphyroblastic gypsum. The gypsum beds were interpreted as gypsum turbidites and the capping crystalline gypsum as a caprock resulting from the dissolution of former halite deposits (Pichat, 2017). South of the Tecer Mountain, massive gypsum facies also outcrop directly over ophiolite and are interpreted as former shallow-water evaporites precipitated in piggyback basins. The deposition of the Tuzhisar Formation is interpreted as resulting from the tectonic isolation of the basin from the oceanic domain, in an arid to semi-arid climate (Cater et al., 1991;Gündogan et al., 2005;Kurtman, 1973).

Group IV: Middle Oligocene to Middle Miocene
The Group IV includes the Karayün, Karacaören and Benlikaya formations (e.g. Ribes et al., 2017;Ribes, 2015). Due to halokinesis, these successive formations are sometimes found incomplete . Moreover, their facies repartition in the minibasins is controlled by salt flow as well as by the increasing activity of south-verging thrusts Ribes, 2015;Ribes et al., 2018).
The Karayün Formation (< 2500 meters thick) was deposited between middle Oligocene and late Oligocene. Currently, only the top part of the Karayün formation is dated as late Oligocene by biostratigraphy . This formation is always cropping out above the Oligocene salt canopy, with the exception of the western end of the Sivas basin (Tatlicak area), where the Karayün basal contact is conformable on the Selimiye Formation (Yılmaz, Uysal, Ağan, & Göç, 1997). In the minibasins, the formation involves three successive members : (i) a lower member made of mudstone interbedded with sandstones and evaporite beds; (ii) a middle member made of amalgamated channelized sandstones and conglomerates ; and (iii) an upper member made of mudstone with few isolated sandstones, interbedded with lacustrine carbonate and evaporite beds. These deposits characterize a distributary fluvial system with each member, respectively corresponding to (i) playa lake and distal terminal splays deposits, (ii) fluvial braided deposits and (iii) saline lacustrine deposits .
The Karacaören Formation (< 2500 meters thick) was dated as Aquitanian to Burdigalian (Ribes, 2015;Sirel, Ozgen Erdem, & Kangal, 2013). Along salt ridges, it unconformably overlies the Karayün Formation, the evaporite canopy, and, farther east, the Selimiye Formation . Away from evaporite bodies, the contact is generally concordant over the Karayün Formation or discordant above the Selimiye Formation. North of Sivas thrust, the Karacaören Formation is deposited over deformed Eocene deposits Poisson et al., 1996). The Karacaören formation is made of bioclastic sandstones, grainstones, recifal patchs, marls and evaporites. It records a marine transgression with facies deposited in a mixed deltaic and carbonate ramp environment evolving to restricted coastal bays (Ribes, 2015;Ribes et al., 2018).
The Benilkaya Formation (< 1000 meters thick) is dated from late Burdigalian to Tortonian (Poisson et al., 2016;Ribes, 2015). It is mostly observed in the central part of the map, around the most uplifted and well-exposed secondary minibasins filled by the Karayün and Karaçaoren formations. The formation is conformable over the Karacaören Formation, or unconformable over older formations along the limit of the halokinetic domain. The lower part of the formation is composed of a fining-upward coarse conglomerates and sandstones related to a distributive fluvial system (Ribes et al., 2018). The upper part of the formation displays mudstones, carbonates beds and evaporites deposited in saline mudflat to saline lacustrine environment (Ribes et al., 2018).

Group V: Late Miocene -Pliocene
The upper part of the sedimentological column is made of upper Miocene and Pliocene continental deposits corresponding to the present-day foreland domain, and including the Incesu and the Merakom formations. These are well represented to the North of the Sivas Basin, forming the Kizilirmak basin, where the sedimentary thickness does not exceed a few hundred of meters . To the south, the time equivalent Kangal Formation also covers older formations, forming cuesta-like topography in the Kangal Basin with south directed, low dipping to tabular stratifications.
The lower boundary of the Incesu Formation is dated as upper Miocene . It consists of sandstones and conglomerates, with local intercalations of white marls and lacustrine limestones deposited in an alluvial environment.
The Merakom Formation is dated as lower Pliocene  and is unconformably deposited over the Incesu Formation. It is essentially made of lacustrine facies with limestone beds and green marl intercalations.

Conclusion
The geological map and the stratigraphic chart displayed in this paper act as a major regional synthesis of the Central Sivas Basin.
If the post-salt and halokinetic history of the basin had recently been updated, the pre-salt stratigraphic setting was up-to-now remaining quite confusing regarding the literature, with many formations that had been independently defined by different authors in different localities. The homogenization of the pre-salt stratigraphic units and their correlations enable to better understand 40 Ma of the basin history (corresponding to ∼5 km of sediments). It emphasis the lateral facies and thickness variations of the different formations and highlights a deep marine foredeep, affected by volcanic activity, and extensively filled by siliciclastic, carbonate-rich and volcanoclastic turbidites. These later were mainly fed from (i) the carbonate platforms and basement rocks dismantling in the growing southern foldand-thrust-belt initiated at Early Eocene, and (ii) north deltaic input along Kırşehir block and Pontides culminations.
The new geological map, together with the complementary documents improves thus the comprehension of the tectono-sedimentary evolution of the Sivas Basin, even if the stratigraphic chart remains unchanged for the post-salt formations. Our extended geological mapping of the central halokinetic domain enables to appreciate a large panel of new halokinetic structures (welds, diapirs, salt sheets and canopies) and salt-walled mini-basins. Their full description remains beyond the scope of this study but they should soon encourage further sedimentary and structural investigations of the Oligo-Miocene halokinesis in the Sivas Basin.

Software
Outcropping geological surfaces were mapped in the field with the support of orthophotographs provided by GoogleEarth©. The team used compass-clinometers or Fieldmove® (Middland Valley) on a tablet to collect dip data. The field data were geo-referenced and integrated into a digital environment (Quantum GIS) where the geological limits were extrapolated from satellite images. Final editing of the Geological Map was made using Adobe Illustrator and MAPublisher. The proposed cross-section were constructed and restored using 2DMove® (Middland Valley).