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Tectonic Setting and Evolution of the Sivas Basin, Central Anatolia, Turkey

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Tectonic Setting and Evolution of the Sivas Basin, Central Anatolia, Turkey1

A. POISSON, J. C. GUEZOU,

Laboratoire de Geophysique et Geodynamique Interne (Unite CNRS N° 1369), Universite de Paris-Sud, bat. 504, 91405 Orsay Cedex, France

A. OZTURK, S. INAN, H. TEMIZ, H. GURSOY, K. S. KAVAK, AND S. OZDEN

Department of Geology, Cumhuriyet University, 58140 Sivas, Turkey

Abstract The Sivas Basin is one of several Central Anatolian basins. It developed mainly after the closure of the northern branch of Neotethys. Its location between the Kirsehir Massif and the Taurides implies that it should not be confused with the Inner Tauride ocean located south of the Eastern Taurides. The basement of the Sivas Basin consists of ophiolitic nappes and melanges that were thrust toward the margins of the continental blocks present in this area—the Pontide belt to the north and the Anatolide-Tauride platform to the south. The basin was initiated by tectonic subsidence at the end of the Cretaceous, and it can be compared to a foreland basin during Paleocene and early to middle Eocene time. It was emergent during late Eocene and Oligocene time, although it continued to subside. A transgression in some parts of the basin occurred during the Oligocene and early Miocene (maximum flooding). During the Pliocene, it was affected by regional compression directed toward the NNW, which resulted from conver­ gence of the Arabian and Eurasian plates. This basin may have developed as an intracontinental basin within the Tauride platform and probably never had an oceanic basement. As a result of this work, the general paleogeographic organization of Central Anatolia and Northern Tethys during the Mesozoic should to be revised.

Introduction to Erzincan to the east and is considered as the suture of the oceanic realm and the line of SINCE LATE CRETACEOUS TIME, the Middle East collision between the southern margin of the has been involved in the formation of the Alpine Eurasian plate (Pontide belt) and the Apulo- belt, resulting from the convergence of the Anatolian block (Anatolide-Tauride platform, Eurasian, African, and Arabian plates. The cor­ Tauride belt) (Fig. 1). Thus, this lineament, responding N-S shortening led to the closure of called "the Central Anatolian thrust belt," rep­ the North Neotethyan ocean, and finally to the resents a fundamental structural boundary in collision between the Eurasian and Arabian Turkey (Tatar, 1982). This global model implies plates with the Anatolian block. The collision the existence of a Northern Neotethyan ocean was accompanied by obduction of the North located south of the Pontide belt. The organiza­ Neotethyan oceanic crust, toward and onto the tion of the southern margin of this oceanic neighboring continental margins. In Central basin also seems clear: the Kirsehir Massif and Anatolia, ophiolitic nappes and colored the Tauride belt, as well as the eastern part of melanges (the "Ankara melange" of Bailey and the Apulo-Anatolian microplate, all represent McCallien, 1950, 1953; Norman, 1984) parts of this southern margin. In the east­ resulted from these events and represent rem­ ernmost part of Anatolia, the Neotethys faces nants of the previous North Neotethyan ocean several continental blocks (Bitlis-Potiirge Mas­ basin. They crop out along a zone that runs E-W sif) and the Arabian plate (Fig. 1). Questions through Turkey: the Izmir-Ankara zone (Brink- regarding Central Anatolia concern: (1) the man, 1966, 1976). That zone can be extended relationship between the Kirsehir Massif and the Tauride belt; (2) the exact emplacement and ]Paper initially presented at the Second International Turk­ age of the Northern Neotethyan suture zone; ish Geology Symposium, Cumhuriyet University, Sivas, and (3) the mode of formation of the Central Turkey; Orhan Tatar, Convenor Editor.

0020-6814/96/214/838-16 $10.00 838 EVOLUTION OF THE SIVAS BASIN 839

FlC. 1. Location of the Sivas Basin in the geodynamic context of the Middle East.

Anatolian basins and their general evolution In fact, in many places on both sides of the during Cenozoic and Quaternary time. We shall basin, the oldest beds have been deposited on discuss these questions below, focusing atten­ top of ophiolitic sequences (mainly serpen- tion on the Sivas Basin. tinites and gabbros). Nevertheless, these ophio- lites are allochthonous, and their own basement may be found in the Tauride belt to the south Geologic Setting of the Sivas Basin and in the Kirsehir Massif to the north. All along the northern margin of the Sivas Basin, The Sivas Basin is one of the most important the Kirsehir Massif appears to be present of the Central Anatolian basins (Fig. 1). As a beneath the ophiolites. It crops out just north result of its location, its sedimentary fill of Sivas (Fig. 2) and to the northeast in tectonic (including the products of magmatic activity), windows through the ophiolites (beyond the and its structure, this basin offers a good oppor­ area depicted on the map). Thus the Sivas Basin tunity for study of the Paleogene to Recent appears to be a gutter between Kirsehir and the evolution of an active segment of the Alpine Tauride belt. It does not seem to lie within the belt in the Middle East and for discussion of the Ankara-Erzincan suture zone, as postulated by questions raised above. The western part of the Cater et al. (1991). In fact, this suture must be Sivas Basin is oriented NE-SW (SW of Sivas), located farther to the north of the Kirsehir but the eastern part has an E-W orientation. Massif. The northern limit of the basin (striking E-W) The nature and origin of the Kirsehir Massif corresponds to the Central Anatolian thrust must be reconsidered, as many questions con­ belt. The other margins also have been consid­ cerning this massif have arisen. It is not, how­ ered to be tectonic lineaments (e.g., prolonga­ ever, the purpose of this paper to discuss in tion of the Ecemis corridor). The basement of detail all the available data. We intend here only the basin is reputed to be the oceanic crust of to state precisely what we mean by "Kirsehir the "Inner Tauride suture" of Sengor (1984). Massif" and what other authors mean by "Kir- EVOLUTION OF THE SIVAS BASIN 841 suture zone. The Kirsehir block and the Tauride postulated. Complete revision of the general platform also were intruded by similar plutonic chronology of the basin (by means of bio- rocks. Thus we can say that Kirsehir and the chronology, magnetostratigraphy, isotope chro­ Taurides experienced a quite similar geo- nology, etc.), as well as other subordinate dynamic evolution at the end of the Cretaceous, studies in sedimentology, geochemistry, paleo- and we suggest, as a working hypothesis, that geography, etc., should recognize several struc­ Kirsehir may represent a northern extension of tural units, separated by numerous faults. The the Taurides. Kirsehir and the Taurides may majority of these faults are near-horizontal have been separated by a basin (the ancestor of thrust faults. Consequently, it appears to be the Sivas Basin), the nature and status of which essential to discuss first the general structural remain unknown. line and deformation style that prevailed during basin formation. Our studies have focused on the eastern tip of Geology the basin, on the central area near Sivas, and on Previous works the region along the basin margins. Figures 2 and 3 provide a general map of the central part The first general and detailed report (Yalcin- and two interpretive cross-sections. This map is lar, 1955) and the first important paper (Kurt- simplified from the 138 1:100,000 sheet cur­ man, 1973) concerning the general geology of rently being compiled. the basin have remained the state of knowledge until recently. Later, more specialized papers Previous data. Several unpublished reports were published on the pre-Neogene deposits (MTA, Ankara) preceded the publication of the (Gokcen, 1981; Gokten, 1983, 1986; Gokcen 1:500,000-scale Sivas sheet, which represents a and Kelling, 1985; Gokten and Floyd, 1987; synthesis of the work of Baykal and Erentoz Norman, 1990), the Neogene deposits (Temiz et (1966). Those authors pointed out the exis­ al., 1991), and the entire basin (Aktimur, 1988; tence of NE-SW-trending folds and also Cater et al., 1991; Yilmaz, 1994). The most reported an E-W, southward-verging thrust at important works concerning biostratigraphy the northern margin of the basin (north of are those of Stchepinsky (1939), Yalcinlar Hafik), supposedly of post-Oligocene to pre- (1955, general stratigraphy), Eriinal-Erentoz Miocene age. Arpat (1964) and Artan and Ses- (1956, molluscs), Dizer (1962, foraminifera), tini (1971) reported northward-verging reverse Siimengen et al. (1989, mammals), and De faults and thrusts along the southern margin of Bruijn et al. (1992, mammals). Geological maps the basin. Kurtman (1973) mapped several include the regular l:500,000-scale Sivas sheet important structures between Tecer Dag and (Baykal and Erentoz, 1966) and the new series Hafik, such as the N-verging Tecer thrust and at 1:100,000 scale: F23, F25, F26, and G23 the NW-SE strike-slip faults in the region of (General Directorate for Mineral Research and Karayiin. Nevertheless, he interpreted the Sivas Exploration [MTA], Geological Survey, thrust as a dip-slip fault. Aktimur (1988) inter­ Ankara). preted the same thrust as a left-lateral strike- slip fault, and Cater et al. (1991) interpreted it Our works have focused on the 138 sheet of as a S-verging "left-lateral transpressive strike- the regular l:100,000-scale map (equivalent to slip fault which probably has an earlier history F23 published by the MTA) and have concerned of down-to-south extensional displacement." the recognition of main structural units, whose These observations are correct in part, chronology (both biostratigraphic and radio­ especially concerning the N-verging thrusts, metric) is being revised completely. We present but they minimize the flat character of the here some initial interpretations resulting from this ongoing work. thrusts and the importance of the shortening. Gokten (1993) provided a good map of the Main structural lines and structural units tectonic structures around and south of the As has been noted earlier (Poisson et al., Tecer Dag. The corresponding models will be 1992; Gokten, 1993; Temiz et al., 1993), the discussed below. main structural lines of the Sivas Basin are General structures. The general type of defor­ younger and more important than previously mation is the classic tectonic fold-and-thrust 840 POISSON ET AL. sehir block." The question is important for the Central Anatolia, the ophiolites are allochtho- origin of the Sivas Basin and other Central nous, and as reported by Goriir et al. (1984), at Anatolian basins. The massif is not well defined the end of the Cretaceous, "major parts of the geographically. It is well known by local studies Kirsehir Massif were covered by an ophiolitic of its metamorphic sequence and by its Late nappe expelled from the northern branch of Cretaceous-Paleocene plutonic rocks. It is Neotethys." The same ophiolitic nappe also included in the Anatolides of Ketin (1966) and covered the northern margin of the Tauride in the Anatolide-Tauride platform of Sengor platform, probably located at that time just and Yilmaz (1981) and Sengor et al. (1982). south and southeast of the Kirsehir block. It This platform is bounded by the Erzincan therefore does not seem necessary to emplace suture to the north and by the Inner Tauride an oceanic trough between Kirsehir and the ocean to the south. The definition of the Inner Tauride platform to explain the presence of Tauride ocean is rather clear to the east, where ophiolites there, as all these ophiolites could it is located between the Bitlis-Potiirge Massif have been expelled from the northern suture. and the Eastern Taurides. The western pro­ Another argument has been presented on the longation of this basin remained poorly basis of paleomagnetic data. In their model of depicted in Sengor et al. (1982). On the basis of tectonic evolution of the Tuz Golii Basin com­ primarily unpublished data, Goriir et al. (1984) plex, Goriir et al. (1984) used the paleomag­ proposed a clear separation between a "Kirsehir netic data of Sanver and Ponat (1981), which block" and a "Menderes-Tauride block." Never­ suggested a counterclockwise rotation of the theless, no information is provided concerning Kirsehir block between the Late Cretaceous and the content of each block. More precisely, the the Lutetian. Such a rotation would provide a location of the Eastern Taurides is not clear: very good argument in support of a Kirsehir does it include the Kirsehir block, as in Sengor block independent from the Taurides. Neverthe­ et al. (1982)? Or does it end farther south? The less, the paleomagnetic data are not totally Sivas Basin, not named in these models, is reliable, as a result of the questionable strati- considered to be part of the large Oligo-Miocene graphic position of some of the samples used in Central Anatolian molasse basins. Neverthe­ the study. Our conclusion concerning the Kir­ less, in these models, the Sivas Basin cannot be sehir block, therefore, is that its geographic confused either with the Inner Tauride ocean, extension, paleogeographic affinities, and loca­ which is located south of the Tauride platform, tion with respect to the oceanic basins are or with the Ankara-Erzincan suture, located debatable. In our first purely geographic north of the Anatolides. Ricou et al. (1985) and approach, we attribute to the Kirsehir Massif Dercourt et al. (1985, 1986, 1993) interpreted all the metamorphic sequences located to the Kirsehir as an isolated block, mainly because of north and northwest of the Sivas Basin. Never­ its poorly known lithostratigraphic affinities theless, we are aware that metamorphism can­ and its presumed situation with respect to the not be the sole criterion for distinguishing the oceanic basins. Kirsehir Massif. Metamorphism also can affect the Tauride platform in Western Anatolia (the Two arguments concerning the paleogeo- Menderes Massif, for instance). These graphic emplacement of Kirsehir have been sequences disappear beneath the ophiolitic used in discussions of the models: (1) the nappes and melanges to the north. The exis­ position of the ophiolitic sutures, and (2) paleo- tence of an old basin to the south of Kirsehir magnetism. In all the above models, the ophio- (during Mesozoic time, for instance) beneath lites sensu stricto (interpreted as remnants of an the present-day Sivas Basin has been suggested, oceanic crust, generally highly deformed) play but: (1) it cannot be the Inner Tauride ocean, an important role. The existence of ophiolites which is located farther south; (2) it cannot be south of the Kirsehir block in the Ulukisla area the Erzincan suture, which is located farther (Oktay, 1982) and on the neighboring Tauride north; and (3) during Late Cretaceous time, the platform (Pozanti-Karsanti) represents an Kirsehir block probably was located close to the important argument for the emplacement of an Tauride platform and both were covered at the oceanic domain just to the north of the Tauride same time, probably by the same ophiolitic platform in the models of Goriir et al. (1984) nappe expelled from the Ankara-Erzincan and Ricou et al. (1985). Nevertheless, in EVOLUTION OF THE SIVAS BASIN 843

0 10

FIG. 3. Cross-sections. Legend: 1 = upper Miocene (detritus) and Pliocene (marls and lacustrine limestones); 2 = Oligocene-Miocene (marine marls and limestones); 3 - massive gypsum (Hafik Formation); 4 - Bahcecik Formation (Paleocene-Eocene in the north) and Karayiin and Selimiye formations (Oligocene); 5 = Bozbel Formation (Eocene); 6 = Tecer Dagi allochthon (Upper Cretaceous-Paleocene); 7 = Kirsehir Massif.

feature in the models. Along the western sec­ limit of the Pontide belt between Yozgat and tion, all the main thrusts are directed toward Erzincan. This limit is accepted as an the north. By contrast, along the eastern sec­ E-W-striking structural line—the "Central tion, we observe both N-verging and S-verging Anatolian ophiolite thrust" of Tatar (1982) — thrusts. In the eastern part of the basin (Kemah resulting from Late Cretaceous obduction. This area) (Temiz et al., 1993; Temiz, 1994), all the thrust was reactivated and duplicated later, dur­ main thrusts are S-vergent. The existence of ing the Neogene (Pliocene?) (Tatar, 1982), by two prominent vergence directions is one of the other thrusts located a short distance to the distinctive features of the Sivas Basin. Two south, along the well-exposed section between hypotheses are discussed below: (1) these Yildizeli and Camlibel Pass (road to Tokat). thrusts of opposite vergence are contempo­ Northeast of the area covered by our map, the raneous and some are back thrusts (Poisson et same type of thrust faults can be seen. In this al., 1992); and (2) these thrusts correspond to area, they have transported ophiolites from two distinct tectonic events. north to south onto the Bahcecik conglomer­ Two types of faults that have been somewhat ates, which crop out at the front of the nappes confused in previous work lie along the north­ and in tectonic windows in the back of the ern margin of the Sivas Basin (from Yildizeli in thrust front (to the north). Near Bahcecik, the the west to Imranli in the east). These two types lower Miocene section is involved in this south­ of faults do not have the same origin and age ward deformation. During Late Cretaceous (and thus the same significance along this mar­ time (Late Maastrichtian, according to Goriir et gin). In fact, it is most important to distinguish al., 1984), the Neotethyan domain was trans­ between them: (1) the thrust faults inherited ported southward onto the Kirsehir Massif, the from the Late Cretaceous obduction of the eastern part of which crops out north of Sivas. Neotethys oceanic domain that were reactivated Here, this initial thrust is masked by a more during Paleogene and early Neogene time as recent northward thrust. S-vergent thrusts; and (2) the thrust faults that The second type of structure consists of correspond to a more recent period of deforma­ N-vergent thrust faults. The Sivas fault system tion, affecting the Merakom lacustrine lime­ represents a good example of these faults; it stones (attributed to the Pliocene, but the consists of several parallel faults that have been Quaternary is also deformed) (see below). mapped along the northern margin of the basin Between Sivas and Hafik these thrusts, includ­ (Fig. 2). They can be high-angle thrust faults ing the Sivas thrust fault, are directed toward (e.g., near Sivas) (Fig. 4A) or completely hori­ the NNW (Poisson et al., 1992). zontal thrust faults (e.g., north of Hafik). Their The first type of structure corresponds, on NE-SW strike is oblique with respect to the many general maps of Turkey, to the northern general direction of the northern margin of the boundary of the Sivas Basin and the southern basin, which is E-W between Yildizeli and 842 POISSONET AL.

Fic. 2. Geological sketch map of the central part of the Sivas Basin. 1 = Quaternary (a = travertines, b = alluvium); 2 - upper Miocene (a) and Pliocene (b) continental facies; 3 - undifferentiated Miocene and Pliocene basalt flows; 4 = Oligocene-Miocene marine facies; 5 = Oligocene-Miocene (continental facies) (a = lacustrine limestones, b = sandstones and conglomerates); 6 - upper Oligocene marls, sandstones, conglomerates, lacustrine limestones, and bedded gypsum (Emirhan Formation); 7 " Hafik Formation (Oligocene massive gypsum); 8 - Karayiin and Selimiye formations (partially Oligocene red sandstones and conglomerates); 9 = Bozbel Formation (Eocene flysch); 10 = Paleocene and Eocene strata of the northern margin of the Sivas Basin (conglomerates, sandstones, and nummulitic limestones); 11 = Tauride belt (a = Paleozoic and Mesozoic, b = Upper Cretaceous-Paleocene of the Tecer Dagi allochthon); 12 - ophiolites; 13 = Kirsehir Massif and intrusives; 14 - Pontide belt (undifferentiated sedimentary and volcanic units); 15 = bedding in detritus; 16 = thrust faults. belt (Boyer and Elliott, 1982), with imbricate faults dominate, indicating that the basin fans and duplexes. A main decollement level is underwent major shortening that can be related located at the bottom of the massive gypsum to N-S convergence. Pliocene strata are sequence. affected by this deformation. We also see that The cross-sections in Figure 3 provide a gen­ the vergence of the thrusts can have either eral picture of the main structures. Flat thrust southward or northward orientation, a frequent EVOLUTION OF THE SIVAS BASIN 845

FlG. 5. The Hafik Formation and lateral equivalents in the western half of the Sivas Basin. In the Gemerek area, the sequence is continental and quite complete. In the Bahcecik area (north of Hafik), the marine sequence transgresses directly onto the northern margin of the basin where gypsum is absent. Localities on the sections are shown in Figure 2. Sections 8, 9, and 10 are outside of the map toward the west.

It could have remained active through Recent here on the revision of the main groups of time up to the present. formations in the central and western parts of In another interpretation, a tensional phase the basin. followed compression, giving rise to normal Basement. Along the southern margin, the faults and the present Kizilirmak Valley. We basement consists mainly of carbonates of the prefer the former alternative, which fits better Tauride belt, covered by ophiolitic nappes. The with the general model of a thrust-and-fold belt. northern margin consists of the metamorphic The kinematics of these faults (compression rocks of the Kirsehir Massif and the allochtho- toward the NNW) are in good agreement with nous Neotethyan sequences (ophiolites and the present dextral movement (striking N100°) melanges, defined above as remnants of oceanic along the eastern part of the North Anatolian crust). fault. These findings support a tectonic event Sequence of formations in the basin. Prior to (post-early Pliocene) independent of the the complete stratigraphic revision of the basin, southward translations, which could be a little we have used here the stratigraphic subdivisions older (post-early Miocene). previously proposed by Kurtman (1973) and others; these have been amended, completed, Lithostratigraphy and updated with recent chronological data. In The complete revision of the lithostrati- the Sivas area, the succession of formations graphic units (formations) in the basin was includes, from bottom to top : more difficult than expected, owing to several factors, which are summarized below. First, (1) Bahcecik conglomerates (Paleocene- because several structural units are separated lower Eocene) by faults, the lithostratigraphic correlations (2) Bozbel flysch (Eocene) from one area to the other are more complicated (3) Selimiye sandstone and conglomerate in the absence of key marker horizons. Second, (Oligocene) continental deposits (elastics and evaporites) (4) Karayiin sandstone and conglomerate remain difficult to date because of their (Oligocene) impoverished biostratigraphy. Third, rapid and (5) Hafik group important changes in facies occur in both hori­ A. Hafik gypsum (Oligocene) zontal and vertical directions. Therefore, com­ B. Emirhan marl, lacustrine lime­ plete revision will take time. We have focused stone (Oligocene) 844 POISSON ET AL.

FlG. 4. Tectonic style of deformation in the Sivas Basin, last tectonic event. Examples of decollement at the bottom of massive gypsum. A-A'. Sivas thrust fault (see Fig. 3 for location). Panoramic view from the Sivas-Ankara road. On the left: upper Miocene conglomerates (Incesu Formation) (I), overlain by lacustrine limestones (lower Pliocene Merakom Formation). Beds are overturned toward the NNW below the Sivas thrust. On the right is the gypsum sequence of the Hafik Formation (Ha) (Oligocene). B. Panoramic view toward the northeast from the old Sivas- Kayseri road. Superposed flat thrust faults and imbricate sheets of gypsum and detritus. C. Example of polyphased tectonics: an E-W-striking syncline transported toward the NNW during the final tectonic event. Me = Merakom Formation (lower Pliocene lacustrine limestones); I = Incesu Formation (upper Miocene sands and conglomerates); Ha - Hafik Formation (Oligocene massive gypsum); 01.-Mi. = undifferentiated Oligo-Miocene reddish detritus above massive gypsum. Abbreviations: r = Sivas-Ankara road; Ki - Kizilirmak.

Erzincan. In contrast, to the west of Sivas, the tectonic windows where they appear as very predominant NE-SW thrust-faults are parallel low-angle thrusts. The gypsum beds represent to the NW margin of the basin, although they the decollement level in the sequences. The are not coincident with it. This margin has been Sivas thrust system affects lacustrine lime­ generally interpreted as a strike-slip fault along stones (north of Sivas) that are considered to be the prolongation of the Ecemis corridor, which early Pliocene in age by comparison with an is considered to be left lateral. The Sivas fault identical formation of the Gemerek area also has been interpreted as a left-lateral fault (Siimengen et al., 1990) in the western part of (Aktimur, 1988; Cater et al., 1991). Neverthe­ the basin (Fig. 5). The underlying Incesu con­ less, the kinematic data collected along this glomerates are dated as upper Miocene. fault indicate compression toward the NNW, Near Sivas, Quaternary terraces have been which is incompatible with an important faulted (Gursoy et al., 1992), tilted, and folded wrench component. These kinematic data (unpubl. data), possibly because of a recent include meso-structures such as reverse folds, reactivation of the fault (normal faults could slickenslides on fault planes, and shear sense result in the formation of extensional basins on criteria in the gypsum above the contact. Far­ the back of the thrust). The Hafik area is the site ther south along the western section (Fig. 4B), of frequent low-intensity earthquakes. The age several other similar thrusts can be seen in of the Sivas fault is thus post-early Pliocene. EVOLUTION OF THE SIVAS BASIN 847

FlG. 6. Lithostratigraphic correlations of several sections near Sivas concerning the Karayun Formation and the overlying, mainly marine sequences. Sections are shown in Figure 2.

(5) The Hafik Group (Kurtman, 1973) is and fine-grained sands. In many places, the composed of (a) the massive gypsum sequence bedding is not apparent and the gypsum appears named the Hafik Formation sensu stricto, and (b) as massive bodies up to 100 or 200 m thick. In a lacustrine sequence that transitionally over­ such a case, the apparent great thickness and lies the massive gypsum and is known as the the disordered bedding have resulted from tec­ Emirhan Formation. tonic processes, and the massive bodies look like diapirs. The Hafik Formation sensu stricto (Kurtman, 1973) corresponds to the thick, massive gyp­ The age of the Hafik Formation has been sum deposits cropping out all along the north­ disputed, and many tectonic contacts have been ignored. Stchepinsky (1939) dated it at first as ern margin of the basin from Gemerek (western Oligocene. This age was confirmed by Lahn end) to Imranli in the east. According to Kurt­ (1950), who reconfirmed it later (1957), after a man (1973), the succession includes, from bot­ Miocene age was proposed by Nebert (1956). tom to top, red sandstones and conglomerates, More recently, it has been considered as Oligo- red clays interbedded with layered gypsum, and Miocene (Kurtman, 1961, 1973) or as massive gypsum. Because of the existence of a Oligocene again (Artan and Sestini, 1971). tectonic contact (decollement) below the gyp­ Recent work in the western part of the basin has sum, the transition with the underlying forma­ dated the top of the gypsum as Oligocene (verte­ tion cannot be observed, so we restricted the brate fauna) (Siimengen et al., 1990). In the Hafik Formation to the massive gypsum Sivas and Hafik areas, marine Oligocene and/or sequence. When only slightly to moderately lower Miocene layers stratigraphically cover the tectonized, the massive gypsum is thinly and gypsum in all localities (Figs. 5 and 6). Cater at very regularly bedded in layers that can reach a al. (1991) attributed a late Miocene age for the thickness of 10 m, intercalated with red clays Hafik Formation and correlated it with the 846 POISSON ET AL.

(6) Karacaoren marl, limestone (Oligo- of the formation is well established because of cene-lower Miocene) the existence of turbidites and planktonic (7) Benlikaya conglomerate, sandstone microfauna. The components of the sandstones (Miocene) are quartz, feldspar, volcanic fragments, sepen- (8) Incesu conglomerate, sandstone tinite and radiolarite, and resedimented plat­ (upper Miocene-Pliocene) form-type calcarenites with nummulites. The (9) Merakom lacustrine limestones and age of the formation is early to middle Eocene. marls (Pliocene) The detrital component originated from the (10) Quaternary fluvial terraces. south. The thickness of this formation is diffi­ cult to estimate, as a result of variation in (1) The Bahcecik conglomerates (Kurtman, facies, but it certainly exceeds 1000 m. 1973) (synonym: Ozderesi Formation of Gokten and Kelling, 1991), which represent the (3) The type locality for the Selimiye Forma­ first sequence that can be attributed to the tion (Kurtman, 1973) is in the south. It also has northern margin of the Sivas Basin, rest uncon- been observed in the Celalli oil exploration well formably on the basement. They consist of (MTA), where the formation reaches 2100 m in coarse-grained, thick-bedded polygenic con­ thickness. In his definition, Kurtman included glomerates. Cross-bedding and channel scours gypsum layers at the bottom of the formation. are common. Despite a lack of fossils, Kurtman We prefer to place the gypsum beds in the other attributed them to the lower Eocene. New dis­ gypsiferous formation, the Hafik Formation. coveries of fossils (Gokten and Kelling, 1991, Kurtman did not recognize the tectonic nature and our findings) suggest a late Paleocene age of many contacts at the bottom of the gypsum for the bottom (gastropods such as Batillaria layers, and consequently was hesitant in defin­ are well known in the Sparnacian south of ing the age of these layers as Oligocene and/or France [Plaziat, 1970]). The main part is proba­ Miocene. As we shall see below, Oligocene bly lower Eocene. The uppermost part appears seems to be the best determination for the Hafik to consist of nummulitic sands and marls Formation. The Selimiye Formation is (Lower Lutetian, to the south of Bahcecik), restricted to the red detritics cropping out in an which have been correlated to the Bozbel For­ elongated area in the south. It is partly marine mation (southern side of the Sivas Basin) (Kurt­ and is Oligocene in age (ostracod fauna) man, 1973). The thickness of the Bahcecik (Gokcen and Kelling, 1985), and a new find of conglomerates is difficult to estimate, as a planktonic foraminifera currently is under result of the numerous faults that cut the for­ study. mation and the Pliocene cover. The thickness is (4) In the type locality of the Karayiin For­ 1500 m according to Kurtman (1973), but only mation (Cater et al., 1991) near Karayiin, the 300 m according to Gokten and Kelling (1991). sequence includes three members—the Lower In this area, the sequence corresponds to the and Upper Karayiin sand bodies, separated by a first marine sediments deposited onto the fine-grained member, the Middle Karayiin sand obducted ophiolites. It is characterized by a body. The "sand bodies" consist of equal quan­ fluviomarine fan in an area of shallow-marine tities of sands and conglomerates, with subordi­ subsidence at the northern edge of the basin, nate shales and clays; red color dominates. whereas the deepest parts of the basin, located Erosional channels and cross-stratification are farther south and southwest, were the sites of common. Interaction between tectonics and more deep-marine deposits (Gokten, 1983, sedimentation seems probable, inasmuch as 1986). faults are observed along which gypsum has (2) We have not studied the Bozbel Forma­ been injected. The formation corresponds to tion (Kurtman, 1973), and therefore have no several deltaic sand bodies (Cater et al., 1991), new data. According to Artan and Sestini which have been mapped (Fig. 2). This forma­ (1971) and Kurtman (1973), it consists of sev­ tion is unconformably overlain by lower Mio­ eral members, all of them of flysch type, with cene marine deposits, and its age probably is not tuffites and volcanic intercalations. An olisto- Miocene (Cater et al., 1991), but Oligocene. strome is intercalated. These members grade It may be a lateral equivalent of Selimiye laterally into one another. The flysch character Formation. EVOLUTION OF THE SIVAS BASIN 849

the Kizilirmak valley during the Quaternary. At least four levels can be recognized (Poisson, in prep.). Several levels of travertine can be corre­ lated with levels of river terraces near Sivas.

Problems of chronology and sequence stratigraphy At the scale of the entire basin, lithostrati- graphic correlations can be attempted. The formations have to be considered first as litho- stratigraphic units, not as chronostratigraphic entities. The confusion between these two dif­ FIG. 7. Benthic indicator foraminifera at the Paleo- ferent notions has led to incorrect chro­ gene-Neogene boundary in the Westren Mediterranean. nostratigraphic attributions and erroneous paleogeographic reconstructions (e.g., Cater et al., 1991). From another point of view, the (8) The Incesu Formation (Derindere mem­ analysis of the basin sedimentary pile in terms ber) (Yilmaz, 1980) crops out essentially to the of sequence stratigraphy cannot be based on north and west of Sivas, and especially in the approximate chronologies. On the contrary, valley of the Incesu River (NW corner of Fig. this method requires rigorous chronologies at 2), where it has been dated by a vertebrate fauna the scale of the whole basin, which is not (Yalcinlar, 1955). It consists of poorly consoli­ currently the case for the Sivas Basin. dated, light-colored (grey) sands and conglom­ Figures 5 and 6 present correlations between erates, with intercalations of white marls and several sections through the western half of the lacustrine limestones. The bottom of the forma­ basin. In Figure 5, the massive gypsum (Hafik) tion cannot be seen in the Sivas area. The rich is depicted with its various covers. The gypsum vertebrate fauna, with Hipparion gracile, Sus beds remain only indirectly dated. Their cover erimanthius, Mastodon, Cervus, Gazella, and gives a minimum age of Oligocene in the Giraffa, was attributed to the Pontian. In the Gemerek area. The marine cover was dated first modern time scale, this fauna is Turolian (late as early Miocene, but in the Sivas area, Miocene) (S. Sen, 1995, pers. commun.). The Oligocene marine strata also exist above the fauna were discovered at the bottom of the gypsum (Sivas marls). The Selimiye Formation sequence in the valley, and the top therefore also is Oligocene in part. It appears that during may be Pliocene. the Oligocene at least three completely differ­ ent facies coexisted in the basin: planktonic (9) The Merakom Formation (Merakom marls, sandstones and conglomerates, and evap- member) (Yilmaz, 1980) constitutes the Mer­ orites. The possibility cannot be excluded that akom Plateau north of Sivas, and is composed of they are exactly, and completely, synchronous. green marls (at the bottom) and light-colored Figure 6 shows several sections of the lacustrine limestone. In the western part of the Karayiin Formation. This formation, with its Sivas Basin, a similar formation has been dated typical facies associations (primarily red sand­ as early Pliocene (Siimengen et al., 1990). stones and conglomerates), is well represented South of the Sivas Basin, at the northern mar­ near Sivas. It occupies a position similar to that gin of the Kangal Basin (Fig. 2), similar deposits of Hafik, below the marine sequences. Never­ are present, with basalts at the top of the theless, massive gypsum beds also are present at sequence. Similar deposits near Kangal also are the base of Karayiin, and a question arises: are Pliocene in age, whereas the bottom of the the Karayiin and Hafik laterally equivalent to sequence is dated as earliest Miocene (De each other (completely or in part)? These exam­ Bruijn et al., 1992). ples underline the complexity of paleogeo­ (10) Quaternary terraces and travertines cor­ graphic reconstructions of the basin and the respond to the successive stages of evolution of necessity of additional chronologic data. 848 POISSONETAL.

Messinian evaporites of the Eastern Mediterra­ crop out in a tectonic window through the nean, which is completely erroneous. The sec­ ophiolites 10 km farther north. This suggests a tion of Coragin Bayiri (near Eskibogazkesen deep erosion event prior to the Miocene. The southeast of Sivas) had been used as an example limestone member is the best known of the of intercalation of gypsum beds in a marine three members because of its wide extent, its sequence (Kurtman, 1961, 1973). Although typical shallow-marine lime muds, and its gypsum beds are present in this section, they richly fossiliferous calcarenites and reef lime­ correspond to resedimented gypsum (gypsum stones. Stchepinsky (1939), Yalcinlar (1955, pebbles and sands), and several minor tectonic general stratigraphy), Eriinal-Erentbz (1956, contacts had not been taken into account. In molluscs), and Dizer (1962, foraminifera) have other sites (north of Hafik), the gypsum tec- presented the main stratigraphic data. tonically overlies the marine sequence. We con­ The limestone member is dated as early Mio­ clude that the main gypsiferous sequence cene by foraminifera (benthic associations) (Hafik Formation) in the Sivas Basin is that characterize the Aquitanian (Miogyp- Oligocene in age at its top (the base could reach the upper Eocene, which also is known for its sinoides and Miogypsina) and the Burdigalian gypsiferous sequence in the area) (Lahn, 1950, (Miogypsina and Nephrolepidina) (Poignant and 1957; Oktay, 1982). The controversies concern­ Lorenz, 1985), but not the middle Miocene, as ing this age are essentially related to the non- proposed by Gokten and Kelling (1991), if we recognition of the tectonic nature of the bottom follow the fossil chart used in the Western of the massive gypsum in many sections. Lahn Mediterranean (Fig. 7). Around Sivas, the marls (1957) was the first to discuss this question. are Oligocene and Miocene, according to the planktonic associations. The Emirhan Formation (Fig. 5) is described The environment of deposition is a shallow- here as a lacustrine formation that surrounds water platform type, with small patch reefs or Hafik. It consists of marls and sandstones, larger reefs locally, to a shallow-basin type. Red which alternate with thin beds of gypsum at the algae and large foraminifera frequently are base, and limestones intercalated in marls and reworked and have accumulated in cross-bed­ sandstones near the top. The limestones con­ ded sand bodies around the reefs. tain charophytes, which indicate a late The upper detritic member corresponds to Oligocene age. the end of marine transgression. The arrival of (6) The Karacaoren Group (Kurtman, 1973) elastics (clays, sands, and conglomerates) is (synonym: Bahcecik Formation of Gokten and concomitant with a noticeable deepening of the Kelling, 1991) is a formation that will have to be basin (below the photic zone). Planktonic elevated to the status of "group" later, as it replace benthic foraminifera, but with only a includes quite different members. The area in the central-eastern part of the basin, where the limited number of species (mainly Globigeri- marine deposits are thicker, has been described. noides trilobus). The thickness of this member It resulted from new marine flooding of the may reach 100 m in the Sivas area and more basin from the east and southeast (Eriinal- than 200 m in the eastern part of the basin. Its Erentoz, 1956). The major part (but not all) of age is not precisely established. It may be mid­ the Sivas Basin was affected by this transgres­ dle Miocene, according to Yalcinlar (1955). sion, which unconformably overlies the Hafik (7) The Benlikaya Formation (Pisoni, 1965) and Karayun formations and overlaps the transitionally overlies the Karacaoren Forma­ ophiolites of the emergent northern land areas. tion. It consists of red elastics, mainly sand­ Facies and thickness vary considerably from stones, conglomerates, silts, and clays, with one place to another. The formation can be subordinate intercalations of thin beds of gyp­ subdivided into three members—lower detrit- sum and lignite. It corresponds to continental ics, limestones, and upper detritics. deposits and remains undated. As a result of its The lower detritics include conglomerates, position above lower (middle?) Miocene sedi­ red sands and clays (60 m thick near Giinyama) ments, it could be Miocene. However, it does not (Gokten and Kelling, 1991), and marls near look like the upper Miocene formations, which Sivas. The conglomerates include clasts of the generally are grey and poorly consolidated, as metamorphic rocks of the basement, which we shall describe below. 850 POISSON ET AL.

Discussion and Conclusions: Floyd 1987), and the authors are prudent in Origin and Nature of the Sivas Basin their interpretation, concluding that the data support a within-plate eruptive setting devel­ Several models have been proposed to oped on continental crust. It could be calc- explain the formation of the Sivas Basin. The alkaline arc volcanism, and, in that case, it general location of the basin between the Kir­ might be related to subduction of the Inner sehir Massif and the Tauride platform is well Tauride ocean below the Tauride platform. accepted, but the relationships between these Nevertheless, this ocean was necessarily two continental areas have not been discussed. located farther south, on the opposite side of We agree with Gokten (1986), Gokten and the Taurides, probably several hundred kilome­ Floyd (1987), and Yilmaz (1994) regarding the ters distant, and thus the relationship between non-oceanic nature of the basement. In fact, the volcanism and subduction is questionable. the general sedimentary fill of the Sivas Basin, From these data, the within-continental-plate as it has been described, does not appear to have setting seems important. affinities with sequences deposited above oceanic crust. A true oceanic series can be seen During Paleocene-early Eocene time, the north of the basin in the Pontide belt. There, basin was a foreland basin (i.e., a basin adjacent deep-marine limestones, radiolarites, volcanic- to a belt being tectonized and providing detri­ derived sands and breccias, and lava flows (pil­ tus—in this case the Tauride belt) (Artan and low lava and massive lava flows) characterize the Sestini, 1971; Gokten, 1986; Cater et al., 1991; Upper Cretaceous to Paleocene deposits. On Yilmaz, 1994). Its fill (flysch type) can be the opposite margin of the basin, to the south, related to the widespread tectonic events dis­ all along the northern margin of the Taurides, tributed throughout Anatolia during that the platform began subsiding en masse after the period (generally prior to the Lutetian). The Cenomanian, before the emplacement of the general immersion that characterizes Anatolia ophiolitic nappes. Large parts of the previously during late Eocene-Oligocene time is recorded neritic domains were invaded by pelagic sedi­ in the Sivas Basin by shallow-marine nummu- ments, such as pinkish planktonic-bearing litic limestones and gypsum in some places. limestones, during the Senonian and up to Nevertheless, the paleogeography was diversi­ Campanian or early Maastrichtian times. fied during the Oligocene: marine areas coex­ As a result, the ophiolitic nappes were isted with lagoonal and continental areas. emplaced onto pelagic sediments of variable Maximum marine flooding occurred during the water depths. After nappe emplacement, the early Miocene. Subsidence, which remained deposits were generally of shallow-marine type active from Paleocene up to late Oligocene and (corals and rudist limestones) in the Tauride late Miocene time, certainly was controlled by belt, and they have been dated as late Maastrich­ persistent tectonic activity (Gokten, 1993). tian. Thus a tectonic inversion occurred dur­ Thicknesses of sequences vary considerably ing, or just after, the nappe emplacement. from one place to another. The lower Miocene and the Pliocene strata are relatively thin. Fig­ The Sivas Basin started to develop at the same ure 6 shows how thick the red elastics can be time, with variable depth. Gokten (1983, 1986) (e.g., Karayiin, but Selimiye also is thick), i.e., described Upper Cretaceous pelagic limestones about 2000 m. These elastics were deposited (with globotruncanids), which implies a quite during a relatively short period, possibly during deep basin (more than 700 m, as estimated from a part of the Oligocene. At the same time, the volume of vesicles versus the total unit tectonic activity produced synsedimentary volume of lavas for the Paleocene). The subsi­ angular unconformities. These events have dence of the basin during Late Cretaceous- been related to normal faults (Cater et al., Paleocene time may have been controlled by 1991). faults located along the margins, which explains the differences in thickness and facies In summary, the Sivas Basin appears to have between platform and basin. Volcanic strata been a foreland basin rather than an oceanic (basaltic lava flows, tuffs, and breccias) are basin above oceanic crust. The following known to exist in the basin. The corresponding remarks and working hypotheses are proposed magmatism has been discussed (Gokten and in conclusion. EVOLUTION OF THE SIVAS BASIN 851

1. The basin appears to possess continental cooperation between CNRS (Centre National crust, and not oceanic crust, as basement. The de la Recherche Scientifique, France) and highest level of this basement consists of far- TUBITAK (Scientific and Technical Research travelled ophiolitic nappes, which were thrust Council, Turkey). The General Directorate for synchronously onto both the Kirsehir and Tau­ Mineral Research and Exploration (MTA) ride belts during Late Cretaceous time. This (Sivas bolge) of Turkey provided logistical sup­ pattern is well established in the Munzur Dag port during field work. Special thanks to Gen­ (Ozgiil and Tursucu, 1984), Central Tauride evieve Roche for her help in map preparation (Oktay, 1982; Ozgiil, 1984), and Kirsehir Mas­ and to Rachel Flecker for her revision of the sif (Goriir et al., 1984). English version. 2. The Sivas Basin does not coincide with the "Inner Tauride trough," according to the initial REFERENCES definition of this trough. Nevertheless, the exis­ tence of a basin in that location during Meso- Aktimur, H. T, 1988, Tiirkiye jeoloji haritalari serisi, zoic time cannot be excluded. 1/100,000 olcekli: Sivas F24 paftasi: Ankara, Gen­ eral Directorate for Mineral Research and Explora­ 3. The Kirsehir Massif is not necessarily an tion (MTA), Genel Miid. isolated block in the Northern Neotethys basin, Arpat, E., 1964, Giirlevik Dagi ve kuzeyinin genel jeolo- as it has been represented in several models jisi ve petrol imkanlari: 1/25,000 olcekli 139 dl-d4 (Goriir et al, 1984; Sengor, 1984; Dercourt et paftalari: Ankara, General Directorate for Mineral al., 1985, 1986, 1993). More simply, it may Research and Exploration (MTA), Report no. 4180. represent the northern extension of the eastern Artan, U., and Sestini, G., 1971, Geology of the Tauride belt. This hypothesis will have to be Beypinari-Karababa area (Sivas Province): Bull. confirmed by a detailed comparison between Min. Res. Explor. Inst. Turkey, v. 76, p. 72-88. corresponding lithostratigraphic successions, Bailey, E., and McCallien, W. J., 1950, The Ankara taking metamorphism into account. melange and the Anatolian thrust: Nature, v. 166, p. 4. The Sivas Basin has undergone a complex 938-940. tectonic evolution in the regional context of , 1953, Serpentine, lavas, the Ankara melange N-S collision between the Arabian, Anatolian, and the Anatolian thrust: Trans. Roy. Soc. Edin­ and Eurasian plates. It started as an extensional burgh, v. 62, p. 403-442. subsiding basin of the foreland-basin type, Baykal, E, and Erentbz, C., 1966, 1/500,000 olcekli which evolved into a lagoon and molassic basin Tiirkiye jeoloji haritasi: Sivas paftasi: Ankara, Gen­ (locally subsiding), connected with the other eral Directorate for Mineral Research and Explora­ Central Anatolian basins. The last episode that tion (MTA). was initiated during the Pliocene corresponds Baysal, 0., and Ataman, G., 1980, Sedimentology, min­ to a general compression that can be related to eralogy and geochemistry of a sulphate series (Sivas, the beginning of the strike-slip movement along Turkey): Sediment. Geol., v. 25, p. 67-81. the North and Southeast Anatolian faults. The Brinkmann, R., 1966, Geotektonische Gliederung von Westanatolien: Neues Jahrb. Geol. Palaontol., v. 10, effects of this compression are spectacular in p. 603-618. the Sivas Basin as a result of the gypsum beds, , 1976, Geology of Turkey: Stuttgart, Ferdi­ which are good decollement horizons. nand Enke Verlag, 158 p. Boyer, S. E., and Elliott, D., 1982, Thrust systems: Amer. Assoc. Petrol. Geol. Bull., v. 66, p. 1196-1230. Acknowledgments Cater, J. M. L., Hanna, S. S., Ries, A. C, and Turner, P., 1991, Tertiary evolution of the Sivas Basin, central We acknowledge the fruitful comments of Turkey: Tectonophysics, v. 195, p. 29-46. two anonymous reviewers. This work is part of a De Bruijn, H., Unay, E., Van den Hoek Ostende, L., and cooperative research and teaching program of Sarac, G., 1992, A new association of small mammals Cumhuriyet University (Sivas) and the Paris- from the lowermost Lower Miocene of Central Ana­ Sud University (Orsay). The academic tolia: Geobios, v. 25, p. 651. authorities of both universities are gratefully Dercourt, J., Zonenshain, L. P., Ricou, L. E., Kazmin, acknowledged, as are local authorities in Tur­ V. G., Le Pichon, X., Knipper, A. L., Grandjacquet, key. Financial support has been provided by C, Sbortshikov, I. M., Boulin, J., Sorokhtin, O., both universities, within the framework of Geyssant, J., Lepvrier, C, Biju-Duval, B., Sibuet, J. 852 POISSON ET AL.

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