Tran-sborder ( south-east Serbia / west Bulgaria ) correlations of the Jurassic sediments : Infra-Getic Unit

The Infra-Getic Unit is a palaeogeographic unit, predestined by palaeotectonics. From the point of view of geological heritage, it represents a geosites framework. For the purpose of the correlation, the Serbian sections of Lukanja, Bogorodica Monastery, Rosoma~ and Senokos, as well as the Bulgarian sections of Komshtitsa, Gintsi, and Stanyantsi were used. The Jurassic sediments of the Infra-Getic Unit crop out on the southern slops of the Stara Planina Mountain in east Serbia and west Bulgaria. The Lower Jurassic started with continental and continental-marine sediments (clays and sandstones) (Lukanja clastics and Lukanja coal beds in Serbia and the Tuden Formation in Bulgaria) and continue with Lukanja quartz sandstones (Serbia) and the Kostina Formation (Bulgaria). These sediments are covered by Lukanja brachiopod beds and Lukanja limestones (Serbia) and the Romanov Dol, Ravna and Dolni Loukovit Members of the Ozirovo Formation (Bulgaria) predominantly consist of bioclastic limestones. The sedimentations follow with Lukanja belemnites-gryphaea beds (marls and clayey limestones), which in Bulgaria correspond to the Bukorovtsi Member (also marls and clayey limestones) of the Ozirovo Formation. The Middle Jurassic sedimentation started with black shales with Bossitra alpine. These sediments are individualized in Serbia as Senokos aleurolites and clays and in Bulgaria they are known as the Etropole Formation. In Serbia the section continues with sandstones called Vodeni~ki sandstones of Bajocian age, known in Bulgaria as the Dobrogled Member of the Polaten Formation. However, in Bulgaria, the age is Upper Bajocian–Lower Bathonian, and it cover the marls of the lower member (Gornobelotintsi Member) of the Bov Formation and is covered by the upper member – alternation of marls and clayey limestones – the Verenitsa Member of the Bov Formation. The Vodeni~ki sandstones–Dobrogled Member which ended their distribution in the section of Komshtitsa, to the east (in the Gintsi section), they are not represented – build a body of sandstones, a prodelta coming from the west to the east. The Bov Formation corresponds to the Senokos ammonite beds in east Serbia. The upper boundary of the Senokos ammonite beds and of the Bov Formation is sharp. It is covered by grey limestones of the Yavorec Formation in Bulgaria and by the Kamenica limestones in eastern Serbia. They are covered by grey or red nodular/lithoclastic limestones (“ammonitico rosso” type) of the Gintsi Formation in Bulgaria and the Pokrovenik ammonitic (acanthicum) limestones in Serbia. The Jurassic section in the Infra-Getic ended with grey micritic and lithoclastic limestones, which belong to the Rosoma~ and Rsovci limestones in east Serbia and to the Glozhene Formation in Bulgaria.


Introduction
In this paper we expose our essay to make correlations accross the Serbian/Bulgarian state border of the existing in the published literature Jurassic formal lithostratigraphic units in the framework of the Infra-Getic paleotectonic and paleogeographic unit (Fig. 1).This unit is known in the Serbian literature presumably as the Staroplaninska-Pore~ka units (ANDJELKOVI] et al., 1996) and as the Izdremets Jurassic paleograben (SAPU-NOV et al., 1986, etc.).

Substratum
The substratum of the Jurassic sediments in the studied area of the Infra (Sub)-Getic consists of Triassic rocks.In the Serbian Bogorodica, Rosoma~ and Senokos sections, the substratum consists of redish aleurolites, marls to argillites with concretions of sphaero-siderites, inter-bedded by sandstones, from 5 up to 100 m thick.They are called the Senokos red series (ANDJELKOVI], 1996, p. 78) (Pl. 1, Fig. 2).These sediments cross the state border near the village Komshtitsa and continue to the east up to the Gintsi village.They are the Bulgaria Komshtitsa Formation in the Bulgaria (TRONKOV, 1969).The problem of the age is controversial because of the lack of characteristic fossils: in Serbia two opinions exist: (1) that of ANDJELKOVI] et al. (1996, p. 78, etc.), after which the Senokos red series is with the Late Triassic age; (2) this of URO [EVI] & RADULOVI] (1990), after which they are Rhaeto-Liassic.ANDJELKOVI] (1996, p. 78) considered the Senokos Formation as Upper Rhaetian because they were formed under a dry and hot climate, while Jurassic sediments were formed under hu-mid conditions.In Bulgaria, the Komshtitsa Formation, after TRONKOV (1993, p. 170) is connected by a progressive lithologic passage with the Carnian Russinovdol Formation and for that reason it is considered as Carnian-Norian.
In the out crops near the villages Stanyantsi, Berende Izvor, Tuden, etc. in Bulgaria, the Jurassic substratum is built up of grey Middle Triassic limestones of the Iskar Carbonate Group.
The Lukanja coal beds (8-150 m thick) started with fine grained quartz sandstones are gradually intercalated by clay and clayey sandstones with coal beds (Pl. 1, Fig. 3).
The Tuden Formation (about 30 m thick) consists predominantly of clays, intercalated with sandstones.The sections started with grey to black clays and coal shales; the higher sections are intercalated with fine grained sandstones, often laminated (Pl.3, Fig. 1).
Marine sandstones (Fig. 3) also lie on continental Lukanja coal beds and the Tuden Formation, as well as directly on Triassic sediments.In East Serbia, they are known as Lukanja quartzy sandstones (ANDJELKOVI] et al., 1996, pp. 86-87) and in Bulgaria as the Kostina Formation (SAPUNOV in SAPUNOV et al., 1967) (Pl. 3, Fig. 2) The Lukanja quartzy sandstones (Pl. 1, Fig. 5) (2.8 m thick in Rosoma~, 8 m in Senokos and up to 120 m in the Mala Lukanja River) are built of coarse to middle   grained quartz with silica cement and a transition to quartzite of Early Liassic age.
The Kostina Formation (Pl.3, Fig. 3) lies directly on the Komshtitsa Formation in the vicinities of the villages Komshtitsa and Gintsi and on the Tuden Formation near the village of Stanyantsi.It consists of coarse to middle grained sandstones, from 3.5 m (near Stanyantsi village) up to 12 m thick (near Komshtitsa village).Its age is Middle Hettangian.
The Lower Jurassic sections continue with calcareous sedimentation.In east Serbia, the Lukanja brachiopod beds developed, which, in western Bulgaria, correspond to the Dolniloukovit Member of the Ozirovo Formation (Pl. 3,Figs. 4,5).
The Lukanja brachiopod beds (ANDJELKOVI], 1958, p. 15) near Senokos village started with dark grey to black crinoidal limestones (1.5 m thick), which contain many well-rounded quartz pebbles (Pl. 1, Fig. 6), as well as many brachiopods and bivalves.The same limestones with quartz pebbles and fossils also crop out in the Bulgarian section of Komshtitsa.In Senokos above them crop out marls (4-5 m) with rare interbeds of clayey limestones (Pl. 1, Fig. 7) (of Hettangian-Sinemurian age).Analogous sediments are individualized in Bulgaria as the Ravna Member of the Ozirovo Formation.The largest part of the Lukanja brachiopod beds is structured by sandy and bioclastic dark grey bituminous limestones, with many brachiopods, bivalves and belemnites (Pl. 1, Fig. 8).Near Senokos village, they were separated by ANDJELKOV] & MITROVI]-PETROVI] (1992) as the Senokos beds.The thickness of the Lukanja brachiopod beds is 40-45 m.
The Lower Jurassic sedimentation finished with rocks, individualized in east Serbia as the Lukanja marlstones and Lukanja belemnitic-gryphaean beds and in Bulgaria as the Bukorovtsi Member of the Ozirovo Formation.
The Bulgarian Bukorovtsi Member is represented by grey silty marls, interbedded by thin (10-15 cm thick) beds of clayey limestones with many sideritic and phosphoritic concretions (Pl.3, Fig. 6).They contain many belemnites and large bivalves (aequipectens and gryphaeas).They are not subdivided into different parts and encompass the Domerian and the Toarcian.
Middle Jurassic (Fig. 3) The Middle Jurassic sediments, in south-east Serbia, are subdivided into the following lithostratigraphic units: Senokos siltstones and shales (Aalenian), Vodenica sandstones (Middle Bajocian) and Senokos ammonitic beds (Upper Bajocian, Bathonian and Lower Callovian), and in western Bulgaria into: the Etropole Formation (Aalenian-Bajocian) and the Bov Formation (Upper Bajocian-Upper Bathonian), intercalated by the Dobrogled Member of the Polaten Formation (Bathonian, lower part).The Senokos ammonitic beds and the Etropole Formation are similar to the "black shales with Bossitra alpine" from the Alpes.
The Senokos silstones and shales (ANDJELKOVI], 1958) are structured by dark grey to black aleurolitic argillites and marly sandstones with phosphoritic, sideritic and calcareous concretions (Pl.2, Fig. 1).Near Senokos and Rosoma~ villages, they are 50-70 m thick.The upper boundary with the Senokos ammonitic beds represents a transition.In Bulgaria, the Etropole Formation is analogous (SAPUNOV in SAPUNOV et al., 1967).It is built up of dark grey to black shales, generally aleurolitic with sideritic and rare phosphoritic concretions (Pl. 3,Figs. 7,8).Near Komshtitsa village it is about 30 m thick and encompasses the Aalenian up to the lower part of the Upper Bajocian.
The Dobrogled Member of the Polaten Formation (SAPUNOV et al., 1993) is about 4 m thick in the section of Komshtitsa (Bulgaria).This lithostratigraphic unit is represented by yellow to brown thick bedded limy sandstones in alternation with thin bedded calcareous limestones (Pl.4, Fig. 1).Its age is Lower Bathonian.
Between the Senokos siltstones and shales and the Vodenica sandstones, as well as between the Etropole and the Bov Formation, the boundary is connected with a progressive transition.In the section of Komshtitsa (Bulgaria), between them grey-greenish silty marls with rare sideritic concretions, greenish marls developed, which are the horizontal prolongation of the lower member -Gornobelotintsi Member (Pl.3, Fig. 8) of the Bov Formation.Such a lithostratigraphic unit is not individualized in the sections of south-east Serbia.
In Bulgaria, this unit corresponds to the Verenitsa Member of the Bov Formation (TCHOUMATCHENCO, 1978), represented by an alternation between clayeysilty limestones and thin beds of silty marls, about 8 m thick containing Zoophycos sp.indet of Late Bathonian age (Pl.4, Fig. 2).From the uppermost part were collected Rhopaloteuthis gillieroni and Homoeoplanulites homoemorphus, which prove the middle part of the Upper Bathonian (Oppelia (Oxycerites) aspidoides Zone).
To west of Rosoma~a, in the valley of Jelovica River, Middle-Upper Callovian sandy limestones and sandstones crop out.

Middle Callovian-Tithonian (Fig. 4)
In the base, near the villages of Senokos and Roso-ma~, crop out the Kamenica limestones (ANDJELKOVI] et al., 1996, p. 133) represented in the base by brecciated limestones, which are covered by micritic, well bedded limestones, on the beds surfaces of which there are lumachelles of ammonites.These limestones are Lower and Upper (p.p.) Oxfordian.
To west of Rosoma~ village crop out sub reef sediments known as the R`ana limestones.They are represented by grey, well bedded limestones, 10 m thick, which contain many bivalves, gastropods, bryozoans, brachiopods, sponges, etc.
The hickness is about 20 m.
The Gintsi Formation in Bulgaria is analogous to the Pokrovenik limestones in Serbia.They are lithoclastic and/or nodular, red or grey limestones, with marly cement, of Middle Kimmeridgian (upper part)-Middle Tithonian (upper part) age, in the Komshtitsa section 29.15 m thick and in Gintsi section about 40 m.
The Jurassic sediments in east Serbia are covered by the Berriasian-Lower Barremian R`ana beds, consisting of slaty, grey biomicritic limestones with intercalations of chert nodules, situated in the Berriasian parts in 5 levels.Their analogous in Bulgaria are the clayey limestones of the Salash Formation (NIKOLOV & TZANKOV, 1971) (Pl.4, Fig. 8).

Fig. 4 .
Fig. 4. Simplified columnar sections of the Middle Callovian-Upper Jurassic sediments of the Infra-Getic domain in the Pirot-Godech area.Legend as for Fig. 3.