Structural-Tectonic Elements as a Factor in Cave Development

Milena Zlokolica-Mandic

Geotectonic structures of Serbia - klick to enlarge, size:  25 K

Geotectonic structures of Serbia

1 - Serbian Massif,
2 - Carpatho-Balkan Arc,
3 - Vardar Zone,
3a - Jadar Block,
4 - Dinarides,
4a - Ophiolitic Belt;
A - Borders of structural units;
B - Considerably karstified limestones;
C - Caves.

Geotectonic Structures of Serbia

    There are many geotectonic divisions and regionalizations of the territory of Serbia. They are based on differences in composition, distribution, and mutual relations of separate geological formations and differ in acceptance of one of the universal interpretations of development of the earth’s crust (plate tectonics, pro and con). These divisions refer to the area of Western and to some extent Central Serbia, where carbonate formations (limestones and dolomites suitable for development of larger caves) are lacking, occur in a lesser extent, or as the youngest members retain an upper superpositional relationship (for example, Triassic limestones over porphyrite–chert series).
    The territory of Serbia in the geotectonic sense makes up  part of the Alpine fold region or Alpine Orogenic Belt of Southern Europe. Precisely in this part, the Alpine Orogenic Belt exhibits typical development according to the principle of bilateral divergent composition: the northern Alpine branch is represented in the Carpatho-Balkan Arc of Eastern Serbia; Western Serbia fits into the Dinarides and encompasses an area with the most characteristic representatives of inner zones of the orogenic belt’s southern branch; and between these branches lies an “intermountain region”, referred to recently as the Serbo-Macedonian or Serbian Massif (Belt). Finally, parts of all these units are covered by relatively younger basins with Tertiary and Quaternary formations; among the basins, the Pannonian Basin and corresponding Paratethys inlets stand out in the northern part of the country (Ciric B., 1996., p. 16). Such a conception, which places the eastern boundary of the Dinarides alongside the Serbian Massif, differs significantly from the one that distinguishes the Vardar Zone between them. The general disagreement of views is also contributed to by the terrane concept, which is increasingly present on the Serbian geological scene.
    The Serbian Massif occupies the central part of the territory, from the Vrsac Hills in the north to the boundary with Macedonia. Deep reverse faults constitute the boundary with other geotectonic units. No significant masses of limestones or dolomites are developed in it.
    The Carpatho-Balkan Arc of Eastern Serbia encompasses the eastern parts of the territory, to the national boundaries with Romania, Bulgaria, and Macedonia. Rocks of Proterozoic to Quaternary age take part in the composition of this region. Widely disseminated are limestones and dolomites (in a lesser extent) of Jurassic (J2, J3) and Lower Cretaceous (K1+2, K3+4) age whose thickness can be more than 1,000 m. Triassic limestones are represented to a lesser extent and occur in two isolated belts: along the nappe of Permian sandstones in the west and in the zone of the Stara Planina Mountains in the east. A small number of caves are developed in them.
    The Carpatho-Balkanides are characterized by a tectonic model of zonal composition with a north-south orientation of structures, which rotates into north-northeast in the northern part and south-southeast in the southern part, thereby forming an arc. Limestones mainly occupy upper superpositional and hypsometric parts and only sporadically are covered with older sediments (along the nappe of red sandstones in the west) and Neogene sediments in basins. Folds with gentle or monoclinal slopes are formed. Through transversal and longitudinal faults along which downthrows or upthrows of terrains occurred, they are divided into massifs separated from each other by Neogene basins, older formations, and Timok eruptives. Topographic characteristics (peaks rarely exceed an elevation of 1,000 m a.s.l.), high levels of noncarbonate base formations (in places as high as 1,000 m a.s.l.), and deeply downthrown bottoms of Neogene basins (hundreds of meters deep) with limestones in the base provide a specific framework of macrostructural conditions for development of karst. The ongitudinal faults that separate zones are eastwardly oriented and occur repeatedly at distances measurable in kilometers. Reverse and dextral transcurrent (horizontal) movements took place along them, as well as the youngest gravitational movements (on small areas). Transversal faults are characterized by sinistral transcurrent movements and gravitational movements (older and younger). If they are formed as tension faults together with north-south transcurrent faults, only gravitational movements are present in them (Marovic M., Djokovic I., 1993).
    The Vardar Zone borders on the Serbian Massif and the Dinarides. It is characterized by complex structural composition and high extent of schists with a medium degree of metamorphism, whose age has not yet been finally determined. Also present among the schists are recrystallized limestones and marbles in which caves are developed (caves 12, 14, and 37). According to the classical formulation, the Vardar Zone also includes the Jadar region (Jadar Sub-block) with significant areas of Triassic limestones south (the Lelicki karst) and north (in the floor of the Tamnava Neogene deposits) of the city of Valjevo. This part of the Vardar Zone is also characterized by structures of “Dinaric direction”, which significantly distinguishes it from other parts of the geostructural unit within which it is separated. Until recently the deepest pit in Serbia, Dragov Ponor (cave 42) is located precisely in this region.
    The Dinarides, in the guise of their inner belt, are located in Western Serbia and extend continuously into neighboring territories in the southeast and northwest. In relation to the Carpatho-Balkan Arc, the main differences in development of limestones are as follows: significant dissemination of Triassic limestones; development of the Jurassic in the form of a diabase-chert formation (ophiolitic belt) with subordinate representation of limestones in upper superpositional parts; and development of the Cretaceous predominately in a flysch formation.
    Together with the Jabuka limestones and dolomites, Triassic limestones and dolomites with significant dissemination on the terrain surface can be distinguished along the Uzice - Nova Varos - Tutin axis (ophiolitic belt). They are deposited over older formations (primarily sandstones, schists, and a porphyrite-chert series) and are partially covered by elements of a diabase-chert formation, which is most widespread precisely in this part of Serbia. A relatively small number of caves have been investigated in this region. Among the best known are the caves Tubica Pecina and the Usacki Cave System (15 and 23), as well as the cave Pecina nad Vrazjim Firovima, the longest cave in Yugoslavia (with a length of 10,450 m), whose entrance is in Montenegro and which extends to the Pestersko Polje. Cretaceous limestones occur subordinately along the eastern border of the region as the final superpositional members of flysch sedimentation.
    The central position of limestones in relation to noncarbonate rocks (between noncarbonate sediments below and adiabase-chert formation above) and the extremely closed nature of the system constitute a particular characteristic of this region vis-a-vis other parts of the Dinarides.
    Together with complex tectonic conditions, this has dictated predominantly siphonal circulation of underground waters.
    The Dinarides are distinguished by zonal composition with a characteristic northwest-southeast orientation of structures. Significant reverse movements toward the southeast (tens of kilometers long) occur along longitudinal faults, and this has also to some extent given rise to differences of opinion about the superpositional interrelations and genesis of limestone masses within the ophiolitic belt (olistoliths being individual shells that overlie a diabase-chert formation or limestone massifs in its base which protrude in places). A significant contribution to the resolution of these problems has been made by hydrogeological investigations (Kresic N., 1991), which established continuous occurrence of Triassic limestones over significant areas.