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Major Paleozoic-Mesozoic Unconformities in the Greater Caucasus and Their Tectonic Re-Interpretation: a Synthesis

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Major Paleozoic-Mesozoic Unconformities in the Greater Caucasus and Their Tectonic Re-Interpretation: a Synthesis File: L:/3b2/RIVISTE/GeoActa/7055_2007/091-102_Ruban.3d Ð Pagina: 91 GeoActa, vol. 6, 2007, pp. 91-102, Bologna Major Paleozoic-Mesozoic unconformities in the Greater Caucasus and their tectonic re-interpretation: a synthesis DMITRY A. RUBAN Department of Geology, University of Pretoria, Pretoria 0002, South Africa. Contact address: P.O. Box 7333, Rostov-na-Donu, 344056, Russian Federation. E-mail: [email protected], ruban- [email protected] Abstract Four major unconformities are present within the Paleozoic-Mesozoic strata of the Greater Caucasus, a region that was derived from the Afro-Arabian margin of Gondwana during the Ludlow, then docked at the Laurussian margin near the European Alpine terranes, and moved eastwards during the Late Triassic-earliest Jurassic to occupy its present position. The Ordovician unconformity, which includes part of the Middle Or- dovician, formed due to uncertain tectonic activity on the Gondwanan margin. The mid-Permian unconformity, which encompasses the Guadalupian to early Lopingian, might have resulted from the Saalian Orogeny ac- companied with strike-slip activity. The Triassic/Jurassic unconformity, which encompasses the mid-Rhaetian to early Sinemurian, developed after the long-distance movement of the Greater Caucasus Terrane along a shear zone or, alternatively, after the global sea-level fall. These three mentioned major unconformities are correlated with similar unconformities in some European, north-central African, and Arabian basins. The mid-Jurassic unconformity, which encompasses the mid-Bathonian to the Early Callovian, might have been caused by arc-arc collision in the Caucasian sector of the northern Neotethys or by the major sinistral transtension from Europe to the Himalayas. Keywords: Unconformity, Palaeotectonics, Paleozoic, Mesozoic, Greater Caucasus. Introduction The Paleozoic-Mesozoic sedimentary succession of the Greater Caucasus is characterized by the The Greater Caucasus is a large region, which presence of several major unconformities (Ruban, stretches between the BlackSea and the Caspian 2006a). However, the stratigraphical data on these Sea, and it embraces some areas of southwestern unconformities are ``dispersed'' in numerous re- Russia, northern Georgia, and northern Azer- ferences, published mostly in Russian. baijan (Fig. 1). The whole Caucasus is a large This article presents a brief synthesis of Alpine-type region, which is presentely located at knowledge on the major Paleozoic-Mesozoic un- the junction of the Eurasian, Anatolian, Arabian conformities of the Greater Caucasus, including plates and Iranian terranes (Bird, 2003). It is new data collected by the author and new data composed of two principal domains (terranes), from recent studies by Kotlyar et al. (1999, 2004), which are the Greater Caucasus and the Lesser Davydov and Leven (2003), and Gaetani et al. Caucasus. They are divided by the Transcaucasian (2005). In addition, this article presents a new depressions, which are named the Rioni Depres- tectonic interpretation of the major un- sion and the Kura Depression (Fig. 1). The geo- conformities, according to recent models of the logical record from this region provides many new regional evolution of the Greater Caucasus (Ta- and intriguing information to recognize and ex- wadros et al., 2006; Ruban, in press), and it also plain the tectono-sedimentary events common for demonstrates that they may be traced in some the entire Tethys. Unfortunately, the Greater other regions. Caucasus is less known than the adjacent Eur- opean, Asian, and African regions. The use of traditional, but outdated and false concepts like Geological setting ``geosyncline paradigm'' and ``formational analy- sis'' by the Russian geologists additionally mini- The geological history of the Greater Caucasus mizes the significance of information from this has been briefly reviewed by Tawadros et al. region for the international scientific community. (2006), who presented a new model of the tectonic 91 File: L:/3b2/RIVISTE/GeoActa/7055_2007/091-102_Ruban.3d Ð Pagina: 92 Dmitry A. Ruban nian, the Greater Caucasus and the other Eur- opean Hunic terranes were shifted to the Laur- ussian margin and docked somewhere near the Carnic Alps and Bohemia), (3) Proto-Alpine Phase (the Greater Caucasus was located at above- mentioned place until the Late Triassic), (4) Left- Shear Phase (the clockwise rotation of Africa provoked the left-shear displacements along the Northern Palaeotethyan Shear Zone and, during the earliest Jurassic, the Greater Caucasus reached its present position), and (5) Arc Phase (until the end of the Mesozoic, the Greater Cau- Fig. 1 - Geographic location of the Greater Caucasus. casus was dominated by the parallel arcs and elongated sea basins on the northern margin of the Neotethys Ocean). The sedimentary history of the Greater Cau- evolution of this region. To develop this new casus has been also reviewed by Tawadros et al. model, various palaeontological, stratigraphical, (2006), whereas Ruban (2006a) has presented a and palaeomagnetic data and global plate re- composite lithologic section (Fig. 3). Although constructions (Dercourt et al., 2000; Cocks and numerous unconformities are known within the Torsvik, 2002; Stampfli and Borel, 2002; Vai, Paleozoic-Mesozoic strata of the Greater Cauca- 1991, 2003; von Raumer et al., 2002, 2003; Vai, sus, only four of them may be called major (Ru- 2003; Golonka, 2004; Scotese, 2004; Torsvik and ban, 2006a). As described below, the major un- Cocks, 2004) were taken into account. A key point conformities are present in strata of Ordovician, in this model is the activity of planetary-scale mid-Permian, end-Triassic-early Jurassic, and mid- strike-slip movements on the northern periphery Jurassic age. The chronostratigraphic framework of the Palaeotethys Ocean (Arthaud and Matte, and numerical ages used in this paper are all taken 1977; Swanson, 1982; Vai, 1991, 2003; Rapalini from Gradstein et al. (2004). The term ``un- and VizaÂn, 1993; Stampfli and Borel, 2002; Ruban conformity'' is used following Bates and Jackson and Yoshioka, 2005). To develop this model, it (1987) and Catuneanu (2006). was argued first, that the Greater Caucasus was located somewhere close to the Carnic Alps and Bohemia in the Middle-Late Paleozoic. This was Major unconformities concluded from the similarities of their sedimen- tary successions and palaeontological records. The next step was to explain how the studied Ordovician unconformity terrane might have reached its present position. The first Paleozoic unconformity described in This involved the idea of long-distance displace- this article is present in strata of Ordovician age ments along the major shear zone. As for the pre- (Ruban, 2006a). The Ordovician unconformity, Ludlow time, the position of the Greater Cauca- which is present between the Upper Formation of sus was evaluated approximately with the global the pre-Middle Paleozoic metasedimentary com- reconstructions of Stampfli and Borel (2002), plex and the siliciclastic strata of the ?Ordovician- which permit to understand the palaeogeody- Silurian Urleshskaja Formation (Fig. 3), has been namics of the Hunic terranes in the whole. Ac- observed in outcrops in the valley of the Khasaut cording to this new model of Tawadros et al. River (Robinson, 1965). It seems that the Cam- (2006), the following five tectono-depositional brian complexes were altered by metamorphism phases are recognized within the Paleozoic-Me- (Robinson, 1965), and this unconformity is es- sozoic history of this region (Tawadros et al., sentially a nonconformity (sensu Bates and Jack- 2006) (Fig. 2): (1) Gondwanan Phase (before the son (1987) and Catuneanu (2006)). However, the Ludlow, the Greater Caucasus was a part of the structural frameworkof the Lower Paleozoic Afro-Arabian margin of Gondwana open to the complexes of the Greater Caucasus remains a Prototethys and then Rheic oceans), (2) Hunic subject for further discussions. Phase (from the Ludlow and until the end-Devo- Below the unconformity, the upper part of the 92 File: L:/3b2/RIVISTE/GeoActa/7055_2007/091-102_Ruban.3d Ð Pagina: 93 Major Paleozoic-Mesozoic unconformities in the Greater Caucasus and their tectonic re-interpretation: a synthesis Fig. 2 - The palaeotectonic position of the Greater Caucasus Terrane (marked by asterisk). The palaeotectonic maps are simplified from Stampfli and Borel (2002). Abbreviations: A - Avalonia, S - Serindia, L - Laurentia, Prt - Prototethys, Pt - Palaeotethys, Hun - Hun Superterrane (European+Asiatic Hunic terranes), Ci - Cimmerian Superterrane, Nt - Neotethys, At - Alpine Tethys. Upper Formation of the pre-Middle Paleozoic Ordovician and the Silurian deposits corresponds metasedimentary complex is composed of phyl- with the appearance of fine-grained siliciclastic lites with rare interbeds of sandstones up to 900 m rocks (Obut et al., 1988). thick. This metasedimentary complex is thought Thus, the age of the lower part of the Urleshs- to be of Ordovician age (Paffengol'ts, 1959, 1965), kaja Formation is unclear, and we cannot con- and it overlies the ?middle Cambrian carbonates clude, how long was the hiatus, established since with Archaeocyathus sp. Following these assump- the Early Ordovician. If possibly sedimentation tions, the uppermost horizons of the Upper For- restarted already in the Middle Ordovician or mation may be upper Cambrian - lowermost Or- even early as suggested from microfossils, this dovician. means hiatus was only 10-15 Ma long. In contrast, Above
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