The palaeogeographic outlines of the Caucasus in the Jurassic : The Caucasian Sea and the Neotethys Ocean

The Caucasian Sea, fringing the northern margin of the Jurassic Neotethys Ocean, largely covered the Caucasus. Continental, shallow-marine and deep-marine palaeoenvironments delineate palaeogeographic outlines for three significant time slices: the Late Toarcian, the Early Bajocian and the Middle Oxfordian. These new palaeogeographic outlines of the Caucasus and adjacent territories match the Neotethys Ocean reconstructions. In the Late Toarcian, the Caucasian Sea embraced the Greater Caucasus Basin and the Black Sea – Caspian Sea Basin, which were divided by the Northern Transcaucasian Arc; it opened to the Neotethys Ocean which covered the Exterior Caucasian Basin. In the Early Bajocian, the Caucasian Sea only embraced the Greater Caucasus Basin; it opened the epicontinental seas of the Russian Platform, connecting them with the Neotethys Ocean by straits between islands of the Transcaucasian Arc. In the Middle Oxfordian, the Caucasian Sea which further embraced the Greater Caucasus Basin had its outer shelf fringed by carbonate build-ups. The connection between the Russian Platform shallow sea and the Neotethys Ocean was maintained. In the course of the Jurassic, a seaway developed along the northern margin of the Neotethys, of which the Caucasian Sea became a significant part.


Introduction
The Caucasus stretches over about 1000 km between the Black and Caspian seas (Fig. 1).In the Jurassic, it was located on the northern margin of the Neotethys Ocean, forming a "key" transition between western and central parts of the Northern Neotethys (STAMPFLI & BOREL, 2002;GOLONKA, 2004).Not only palaeogeogra-phically and palaeotectonically, but also palaeobiogeographically, the Caucasus was an important region.WES-TERMANN (2000) after UHLIG (1911) have defined the Mediterran-Caucasian Subrealm of the Mesozoic Tethyan Realm.
In spite of its importance, the Caucasusian Jurassic palaeogeography is still poorly known.Previous publications are often only available in Russian and/or lack the incorporation of modern palaeogeographic and palaeotectonic concepts.Outdated "formation" analysis or geosynclinal theory are the basis of many studies.To date, plate-tectoinc and terrane analysis of the Caucasus still remains sporadic and schematic.In many Russian reconstructions, the Caucasus was viewed as an isolated region and its border often delineated by the boundaries of the former USSR.To avoid misunderstanding, which is inevitable when dealing with a high amount of the sufficiently reliable sources, in this paper only a few Russian works have been considered.The first one is a book by JASAMANOV (1978), who presented general palaeogeographic information on the Caucasus for each of the Jurassic stages, while the second is a review by LORDKIPANIDZE et al. (1984), who presented the most acceptable palaeotectonic reconstructions, based on palaeomagnetic data.Tectonic models proposed by ERSHOV et al. (2003) were also employed.
Thus, in any somewhat more detailed palaeogeographic reconstruction of a larger portion of the Jurassic Northern Neotethys, the Caucasus remained a blank space.The general target of this article is to initiate a discussion on the highlighted topic.Simplified Jurassic palaeogeographic outlines of the Caucasus are proposed and discussed.It should be emphasized that this attempt is based on personal field investigations, as well as a revision of the available and trustwarthy data from recent studies of the entire Jurassic Neothethys (STAMPFLI & BOREL, 2002;GOLONKA, 2004).
The Jurassic deposits, widely distributed within the Caucasus, vary in distinct areas.Their stratigraphy has been reviewed by ROSTOVTSEV et al. (1992).The stratigraphic scale used in the Caucasus was revised by the author according to new developments in the Jurassic chronostratigraphy, using ammonoids, brachiopods (for detail see RUBAN, 2003), foraminifers and marker horizons (Fig. 2).Stratigraphic suggestions from both International Commission on Stratigraphy and the Groupe Français d'Étude du Jurassique (CARIOU & HANTZPERGUE, 1997) were taken account in doing this.A correspondence between the chronstratigraphic stages and substages (after GRADSTEIN et al., 2004) and stages in the regional sense (after ROSTOVTSEV et al., 1992) was established.The precise revision of the regional ammonoid-based zonation is a task for further special studies.It is also necessary to note that traditionally the Callovian stage in the Caucasus is attached to the Upper Jurassic (ROSTOVTSEV et al., 1992), in contrast to the present scale, recommended by the International Commission on Stratigraphy (GRADSTEIN et al., 2004).
Jurassic lithostratigraphy of the Caucasus has been reviewed in detail by ROSTOVTSEV et al. (1992).In general, two major sedimentary complexes are identified.The Sinemurian-Bathonian complex comprises argillaceous and clastic deposits with a total thickness upto 10000 m.The Callovian-Tithonian complex is represented chiefly by carbonates (thickness up to 3000 m) and also evaporites in the upper part.The accumulation of the Late Jurassic deposits was connected with the evolution of a large carbonate platform rimmed by carbonate buildups (KUZNETSOV, 1993;AKHMEDOV et al., 2003;RUBAN, 2005).In some areas (especially in the Lesser Caucasus), substantial amounts of volcanoclastic deposits are present.Two major regional hiatuses encompass the Hettangian-Early Sinemurian and the Bathonian.
In the Jurassic, the Caucasus was located in the central part of the northern margin of the Neotethys Ocean (Fig. 1) (STAMPFLI & BOREL, 2002;GOLONKA, 2004).Tectonic activity resulted from the dynamics between the terranes, which contacted with each other, and also with the larger Eurasian Plate.Several parallel subduction and spreading zones were located in this territory (LORDKI-PANIDZE et al., 1984;ERSHOV et al., 2003), although a precise interpretation of the Jurassic geodynamics in this region has not been made yet and many questions remain open.In the Early-Middle Jurassic, the Caucasus was located in a subtropical to moderate humid zone.The Toarcian palaeotemperatures are estimated as 15-20°C; in the Early Aalenian, they decreased to 5-15°C, but in the Late Aalenian, the temperatures increased again to 20-25°C, and apparently constant until the end of the Jurassic (JASAMANOV, 1978).After the beginning of the Callovian, the climate became subtropical to tropical and semi-humid.In the Late Kimmeridgian-Tithonian, evaporites were accumulated (JASAMA-NOV, 1978;ROSTOVTSEV et al., 1992), which indicated arid conditions.In the Early-Middle Jurassic, dysoxic to anoxic palaeoenvironments were typical for the Caucasian basins (RUBAN, 2004;EFENDIYEVA & RUBAN, 2005;RU-BAN & TYSZKA, 2005).The palaeobiogeographic position of the Caucasus is uncertain.While DOMMERGUES (1987) places it in the Euro-Boreal domain for the Early Jurassic, WESTERMANN (2000) includes it into the Tethyan Realm.An analysis of brachiopods suggests a rather transitional position (RUBAN, 2003).

Methods
Essentially, this study relies on palaeoenvironmental interpretation, realized in the same way as described by RUBAN (2006).The territory of the Caucasus is subdivided into several dozens of particular areas, which are traditionally called "zones".They are distinguished by the facies composition of the Jurassic succession.A total of 36 "zones" delineate the Hettangian-Bathonian interval (Fig. 3A), and 26 the Callovian-Tithonian interval (Fig. 3B) (ROSTOVTSEV et al., 1992).A palaeoenvironmental interpretation for all formations in each "zone" was made.The comprehensive information of ROSTOV-TSEV et al. (1992) and personal field observations in the Labino-Malkinskaya (see also EFENDIYE-VA & RUBAN, 2005;RUBAN & TYSZKA, 2005), Lago-Nakskaja and Labinskaja "zones" were used.Dashed lines mark uncertainty in the boundary definition.Regional ammonoid zonation does not correspond on this scale to the shown chronostratigraphy (it seems to be impossible to correlate them at present), but only to the stages in a regional sense.The Callovian macrocephalus and calloviense regional zones, and the Oxfordian vertebrale and cordatum regional zones are evidently not separated in the regional ammonoid succession.
Three main types of the palaeoenvironments were distinguished in general: continental, shallow-marine and deep-marine.Continental palaeoenvironments were usually documented by the hiatuses, while rarely by the subaeral deposits.Shallow-marine palaeoenvironments were interpreted by the presence of clastic or carbonate deposits, similar to those usually accumulated at a seashore or on a shelf.Deep-marine palaeoenvironments were traced mostly by the slope deposits (e.g., turbidites).In addition to lithology, also fossils, including plant remains, as well as sedimentological criteria, such as submarine slumps, concretions, etc., were used to determine the palaeoenvironments.
Special attention was paid to three time slices: the Late Toarcian, the Early Bajocian and the Middle Oxfordian, which all correspond to important phases in the evolution of the Caucasus.In the Late Toarcian, all the principal basins of the Caucasus were formed completely.The Early Bajocian and the Middle Oxfordian correspond to the time invervals after something like reorganizations of the Caucasian basins occurred, each following major regressions.
Maps showing the variety of the palaeoenvironments during these time slices were drawn for the Caucasus (Figs. 4A, 5A, 6A).They are attached to the present-day geography of the studied region.Therefore, the next step was to take into consideration the palaeotectonic reconstructions.In this paper, the reconstructions of LORDKIPANIDZE et al. (1984) were preferred, because they are based on reliable palaeomagnetic data.Additionally, the results of ERSHOV et al. (2003) were considered.Analyzing the composed maps of the palaeoenvironment distribution, attempt were made to recognize palaeogeographic elements (basins, arcs) highlighted by LORDKI-PANIDZE et al. (1984), and, when necessary, correct their location.
The final result, a set of the palaeogeographic sketches delineates what was the outline of the Caucasus at each of the studied time slices (Figs.4B, 5B, 6B).They embrace the whole territory of the Caucasus and adjacent regions, including the Pontides, Moesia, Iranian terranes and the southern periphery of the Eurasia continent.Although these sketch-maps remain at a relatively low resolution and the position of landmasses (i.e., continents and islands) is schematic, they may help to fill the gap in our knowledge of the Jurassic palaeogeography of the Caucasus.

The Late Toarcian (~177 Ma)
Marine palaeoenvironments prevailed over most of the Caucasus in the Late Toarcian (Fig. 4A).In its northern part, dominating deep-marine environments trace the elongated basin, which may evidently correspond to the Greater Caucasus Basin of LORDKIPANIDZE et al. (1984).Perhaps its western part was the widest and deepest.Sporadic shallow-water environments to the south support the idea of the presence of the Northern Transcaucasian Arc (LORDKIPANIDZE et al., 1984), related to the subduction zone.Moreover, there is no sound evidence for the presence of a large landmass there, as this is usually imagined (e.g., JASAMANOV, 1978).Presumably, only small islands might have been related to this arc.
Another deep basin is weakly delineated southwards, which may be related to the Black Sea -Caspian Sea Basin of LORDKIPANIDZE et al. (1984).Shallow-water environments to the south of it correspond potentially to the Southern Transcaucasian Arc, i.e. another subduction zone.This arc is considered as the eastern edge of the Pontide structure (LORDKIPANIDZE et al., 1984).In our palaeoenvironmental interpretation, there is no evidence to recognize the Lesser Caucasus Strait of the Tethys and the Nakhitchevan' Block, which were shown by LORDKI-PANIDZE et al. (1984).Another basin, with the proposed name "the Exterior Caucasian Basin", might have been located between the Southern Transcaucasian Arc and the main subduction zone of the Northern Neotethys.
In the Late Toarcian outline of the Caucasus (Fig. 4B), a large sea, for which the name Caucasian Sea is proposed, opens towards the Neotethys Ocean.Wide straits between the landmasses to the west and east of this region entered this sea.The Caucasian Sea embraced two sedimentary basins, divided by a submarine mountain range, united perhaps to the west.Possibly, two archipelagoes consisting of very small islands which formed the Northern and Southern Transcaucasian Arcs characterized this sea.The boundary between the Caucasian Sea and the Neotethys Ocean stretched along the Southern Transcaucasian Arc.Our sketch-map suggests that the Exterior Caucasian Basin was embraced by the Neotethys Ocean.

The Early Bajocian (~171 Ma)
The Early Bajocian times were characterized by laterally variable palaeoenvironments within the Caucasus (Fig. 5A).Deep-marine environments trace the Greater Caucasus Basin, while shallow-water and continental The palaeogeographic outline of the Caucasian in the Jurassic: The Caucasian Sea and the Neotethys Ocean Fig. 4. The Late Toarcian palaeoenvironments (A) and the palaeogeographic outline (B) of the Caucasus (explanation of "zones" -Fig.3A)."?" marks uncertainty in the interpretation of the continental palaeoenvironments, because of the doubtful establishment of hiatus.GCB, Greater Caucasus Basin, BCB, Black Sea -Caspian Sea Basin, ECB, Exterior Caucasian Basin, NTA, Northern Transcaucasian Arc, STA, Southern Caucasian Arc.The dotted line bounds the Caucasian Sea.
environments delineate an island arc to the south, including relatively large islands.Intriguing was the landmass in the western part of the studied territory, where several continental deposits suggest large islands, which appeared as the result of the collision between Northern and Southern Transcaucasian Arcs, which closed the Black Sea -Caspian Sea Basin, generating a single Transcaucasian Arc.Palaeomagnetic data that highlight the presence of the Black Sea -Caspian Sea Basin in the Middle Jurassic appear doubtful (LORDKIPANIDZE et al. 1984).The Exterior Caucasian Basin was located between the Transcaucasian Arc and the main subduction zone of the Northern Neotethys.Some islands can be locally evidenced from continental palaeoenvironments.
The Early Bajocian outline of the Caucasus is presented in Fig. 5B.The studied territory was occupied by the Caucasian Sea.It was isolated from the Neotethys Ocean by the island archipelago of the Transcaucasian Arc.Connection between the sea and ocean was realized by straits between these islands, as well as landmasses, located to the west.From the north, the Caucasian Sea was opened to the large, but shallow interior sea, occuping a waste area of the Russian Platform.Only one sedimentary basin was embraced by this sea.The transgression resulted in the appearance of a very large shelf to the north of this basin, and the structure of the sea in the Early Bajocian was characterized by a strong asymmetry.The boundary between the Caucasian Sea and the Neotethys Ocean stretched along the Transcaucasian Arc.Our sketch-map suggests that the Exterior Caucasian Basin was embraced by the Neotethys Ocean.The islands occurring there might have been of volcanic origin and, therefore, related to the wide belt of intense magmatism to the north of the main subduction zone of the Northern Neotethys.

The Middle Oxfordian (~158 Ma)
During the Middle Oxfordian, the Caucasus was dominated by shallow-marine palaeoenvironments (Fig. 6A).Marine environments trace the Greater Caucasus Basin.The composed map does not permit the idea of LORDKIPANIDZE et al. (1984) about the complete separation of the Western and Eastern Subbasins and the presence of island between them, to be supported.We observed deep-marine environments in the western, central and eastern parts of the Greater Caucasus Basin.Nevertheless, the existence of islands at the western and eastern edges of the latter, hypothesized by LORKIPA-NIDZE et al. (1984) and also by GOLONKA ( 2004), is confirmed by our results, because continental palaeoenvironments were interpreted for those areas.Another island (or a chain of islands), delineated by the continental environments to the south, may be related to the Transcaucasian Arc.In contrast to LORDKIPANIDZE et al. (1984), no evidence for the presence of the Northern and Southern Transcaucasian Arcs, separated by the Black Sea -Caspian Sea Basin, was found.Therefore, it is hypothesized that in the Middle Oxfordian, a unique arc existed, as it was already in the Early Bajocian.However, this arc migrated southwards in comparison with the earlier time slices.Shallow-marine environments in the south of the studied territory are attributed to the Exterior Caucasian Basin.
The Middle Oxfordian outline of the Caucasus is presented in Fig. 6B.The studied territory was oc-Fig.5.The Early Bajocian palaeoenvironments (A) and the palaeogeographic outline (B) of the Caucasus (explanation of "zones" -Fig.3A).GCB, Greater Caucasus Basin; ECB, Exterior Caucasian Basin; TA, Transcaucasian Arc.The dotted line bounds the Caucasian Sea.
cupied by the Caucasian Sea.It was only a little isolated from the Neotethys Ocean by the above mentioned island and submarine mountain range of the Transcaucasian Arc.Straits between landmasses to the west and east of this sea also connected it with the Neotethys Ocean.From the north, the Caucasian Sea opened into the interior sea, as in the Early Bajocian, but its area was diminished.Only one sedimentary basin was embraced by this sea.A large shelf existed to the north-east of this basin.Thus, the sea basin in the Middle Oxfordian was characterized by strong asymmetry in its eastern part, but it was quite symmetric in its western part.The boundary between the Caucasian Sea and the Neotethys Ocean stretched along the Transcaucasian Arc.Our sketch-map suggests that the Exterior Caucasian Basin was embraced by the Neotethys Ocean.
A distinctive feature of the Late Jurassic of the Caucasian basins was the wide distribution of carbonate buildups (JASAMANOV, 1978;KHAIN, 1962;LORDKIPA-NIDZE et al., 1984;KUZNETSOV, 1993;MARTIN-GARIN et al., 2002;ROSTOVTSEV et al., 1992;AKHMEDOV et al., 2003;CECCA et al., 2005;RUBAN, 2005).This coincided with the reef growth documented on the entire northern margin of the Neotethys Ocean (KIESSLING et al., 1999;LEINFELDER et al., 2002;MARTIN-GARIN et al., 2002;OLIVIER et al., 2004;CECCA et al., 2005).The term "carbonate buildups" is preferred to that of "reefs", as they are traditionally called in Russian literature (e.g., JASAMANOV, 1978;KHAIN, 1962;ROSTOVTSEV et al., 1992).SCHMID et al. (2001) mentioned the Caucasian buildups as mounds.The carbonate buildups are concen-trated around the deepest parts of the Greater Caucasus Basin (Fig. 7).It is suggested that to the north, they developed on the outer shelf periphery, connected to the stable landmass of the Russian Platform, while in the south, they occupied the narrow outer shelf of the Transcaucasian Arc.However, some buildups were also found crossing the basin, suggesting atolls, isolated or in groups, characterizing the Late Jurassic Caucasian Sea and Exterior Caucasian Basin.In general, the distribution of the carbonate buildups was tectonically controlled (KHAIN, 1962;AKHMEDOV et al., 2003).

Discussion
The presented palaeogeographic sketch maps suggest that during the Jurassic, the Caucasian Sea was located between the Eurasian landmass and large and little islands (Figs.4B, 5B, 6B).A string of large islands located west-and eastwards were the result of accretion of small terranes along the subducted margin of the northern Neotethys.Straits between these small landmasses made a connection with the Caucasian Sea possible.Together they were able to form an important seaway that stretched along the southern periphery of Eurasia.The tectonic origin of this Exterior Caucasian seaway is very different from thopse of the well-known Hispanic Corridor and the Viking Corridor, the results of break-up of continents (HALLAM, 1983;SMITH & TIPPER, 1986;RICCARDI, 1991;WESTERMANN, 1993;ABERHAN, 2001).It also differed from the other seaways, lsuch as the Cretaceous Western Interior Seaway The palaeogeographic outline of the Caucasian in the Jurassic: The Caucasian Sea and the Neotethys Ocean Fig. 6.The Middle Oxfordian palaeoenvironments (A) and the palaeogeographic outline (B) of the Caucasus (for an explanation of the "zones", see Fig. 3B).GCB, Greater Caucasus Basin; ECB, Exterior Caucasian Basin; TA, Transcaucasian Arc.Dotted line bounds the Caucasian Sea.
The western branches of this seaway included the oceanic basins of the Western Neotethys, such as the Meliata, Maliac, Pindos and Vardar, as well as the Alpine Tethys which opened during the Jurassic (STAMPFLI & BOREL, 2002;BROWN & ROBERTSON, 2004;GOLONKA, 2004).The central part of the seaway consisted of straits separating the blocks of Moesia, Rhodope and Western Pontides.Further east, the seaway communicated with the small Izmir-Ankara Ocean (STAMPFLI & BOREL, 2002).It was directly connected with the Caucasian Sea.The eastern branches of the mentioned seaway extended as straits between the Alborz, South Caspian, Aghdarband, Herat and other terranes of the central part of the northern Neotethyan margin (GOLONKA, 2004).STAMPFLI & BOREL (2002) additionally placed the so-called South Caspian Ocean eastwards of the Caucasus, which seems to be a fragment of the seaway.The latter ended in two branches, as is suggested from the palaeoreconstructions of GOLONKA (2004).Northwards, the seaway connected the basin between the Turan, Herat and Pamirs landmasses, while southwards it led directly to the Neotethys Ocean.

Conclusions
This study of the Jurassic palaeogeography of the Caucasus allows the formulation of some important conclusions: 1) the Caucasian Sea occupied most, although not all, of the studied area during the entire Jurassic; 2) in the Late Toarcian, the Caucasian Sea embraced most of the Caucasus, including the Greater Caucasus Basin and the Black Sea -Caspian Sea Basin, and was opened to the Neotethys Ocean, which covered the Exterior Caucasian Basin; 3) in the Early Bajocian, the Caucasian Sea comprised the Greater Caucasus Basin, it opened to the epicontinental seas of the Russian Platform, and it was connected with the Neotethys Ocean by the straits between islands of the Transcaucasian Arc; 4) in the Middle Oxfordian, the Caucasian Sea also covered the Greater Caucasus Basin and was open to both the epicontinental sea of the Russian Platform and the Neotethys Ocean; 5) during the Jurassic, the Caucasus was included in the long seaway, which stretched along the northern margin of the Neotethys.
Further studies are necessary to verify and detalize the very simple palaeogeographic reconstructions proposed in this paper.A significant task is the collection of data on the carbonate buildups, which has already been made for the Azerbaijanian part of the Caucasus (AKHMEDOV et al., 2003).These data will help to delineate the Late Jurassic carbonate platform.Special attention should also be paid to the high-resolution palaeotectonic interpretations.(1992); for the western part of the Caucasus, it was supported by personal field observations) (for an explanation of the "zones", see Fig. 3B)."Zones" where carbonate buildups were evidently documents are highlighted as gray, while "zones" where only coral founds are known are marked by x.The age of buildup-or coral-bearing deposits is indicated.
The palaeogeographic outline of the Caucasian in the Jurassic: The Caucasian Sea and the Neotethys Ocean

Fig. 1 .
Fig. 1.Geographic location of the studied region.GE -Georgia, AR -Armenia, AZ -Azerbaijan.The position of the Caucasus in the Jurassic is shown on the palaeogeographic map, strongly simplified after SCOTESE (2004).

Fig. 2 .
Fig. 2. Corrected stratigraphic scale of the Jurassic used in the Caucasus.Abbreviations: L -Lower, M -Middle, U -Upper.Unzoned intervals are shaded as gray.Dashed lines mark uncertainty in the boundary definition.Regional ammonoid zonation does not correspond on this scale to the shown chronostratigraphy (it seems to be impossible to correlate them at present), but only to the stages in a regional sense.The Callovian macrocephalus and calloviense regional zones, and the Oxfordian vertebrale and cordatum regional zones are evidently not separated in the regional ammonoid succession.

Fig. 7 .
Fig.7.The location of the Callovian-Tithonian carbonate buildups in the "zones" of the Caucasus (data was extracted fromROSTOVTSEV et al. (1992); for the western part of the Caucasus, it was supported by personal field observations) (for an explanation of the "zones", see Fig.3B)."Zones" where carbonate buildups were evidently documents are highlighted as gray, while "zones" where only coral founds are known are marked by x.The age of buildup-or coral-bearing deposits is indicated.