Trans-border (north-east Serbia/north-west Bulgaria) correlations of the Jurassic lithostratigraphic units

Herein, correlations of the Jurassic sediments from NE Serbia with those of NW Bulgaria are made. The following Jurassic palaeogeographic units: the Eastern Getic, the Infra-Getic and the Moesian Platform are included in the study. The East Getic was studied in the outcrops near Rgotina, where the sedimentation started in the Hettangian and continued during the Callovian–Late Jurassic and is represented by platform carbonates. The Infra-Getic is documented by the sections of Dobra (Pesača) and the allochtonous sediments near the Štubik. Very important for the Infra-Getic are the Late Jurassic volcano–sedimentary deposits of the Vratarnica Series, which crop out near Vratarnica Village. The Jurassic Moesian platform was studied in the sections near D. Milanovac and Novo Korito (Serbia) and in their prolongation in NW Bulgaria into the Gornobelotintsi palaeograben. Very important are the correlation in the region of Vrška Čuka (Serbia) and Vrashka Chuka (Bulgaria) – Rabisha Village (Magura Cave). A revision of the Jurassic sediments on the Vidin palaeohorst, which were studied in the Belogradchik palaeohorst, Gorno-Belotintsi palaeograben, Belimel palaeohorst and the Mihaylovgrad palaeograben, is made. The sedimentation on the Vidin palaeohorst started during different parts of the Middle Jurassic, and in the Mihaylovgrad palaeograben during the Hettangian (Lower Jurassic) where the sediments were deposited in relatively deeper water conditions. To south, the relatively shallow water sediments deposited on the Jurassic Vratsa palaeohorst on the southern board of the Mihaylovgrad palaeograben are described.


Introduction
It is well known that the rock strata -sedimentary, igneous or metamorphic are organized in lithostratigraphic units based on macroscopically discernable lithologic properties or combination of lithologic properties and their stratigraphic relations.Often these units cross the state boundaries.However, it is the general practice that on geological maps, the lithostratigraphic units from the two sides of a state border are different and have different names.This is also the case with the Jurassic lithostratigraphic units on the two sides of the Serbian/Bulgarian state boundary.Our aim is to make correlations of the Jurassic lithostratigraphic units from the two sides of this state border.For the beginning, in the literature existing trans-border units (TCHOUMA- TCHENCO et al. 2006aTCHENCO et al. , 2008) ) are correlated.With the present paper, this stage of the correlation in the northeast Serbia/north-west Bulgaria is finished.The second step, must be, after an analysis of all data, to replace the later names of the lithostratigraphic units by the earlier ones, other considerations being equal, following, in general, the principle of reasonable priority.In this way, the exigencies of the International Stratigraphical Guide (SALVADOR 1994; "Relation of Names to Political Boundaries") will be satisfied.
The Jurassic sediments within the framework of the study area of eastern Serbia and western Bulgaria enter the following large palaeogeographic units (TCHOUMA- TCHENCO et al. 2006aTCHENCO et al. , 2008) ) (from west to east): the Getic (Dragoman Jurassic Pale horst in Bulgaria and Karpatikum in Serbia), divided into two sub-units -Eastern and Western Getic (TCHOUMATCHENCO et al. 2008) based on the presence (in Eastern Getic) or absence (in Western Getic) of Liassic sediments; the Infra-Getic Unit (Izdremets Jurassic Palaeograben in Bulgaria and Stara Planina-Poreč Unit in Serbia); the Moesian Platform.The purpose of this paper is the correlation of the Jurassic sediments across the state border in north-east Serbia and north-west Bulgaria.
The Serbian lithostratigraphic units have been described by D. Rabrenović, V. Radulović, N. Malešević and B. Radulović with the participation of P. Tchoumatchenco and of the Bulgarian, by P. Tchoumatchenco.

Jurassic Lithostratigraphic units
Nota Bene.The number in the description of the lithostratigraphic units corresponds to the number of the lithostratigraphic units in Figures 1, 2, 3; N -indicates that the beds are not nominated.

Getic Palaeogeographic Unit
The Getic is developed on the back-ground of the Serbo-Macedonian -Thracian Massif.During the Ear-ly Jurassic the whole territory represents dry land.In eastern Serbia in this unit enters large parts of the Carpatho-Balkan region of ANDJELKOVIĆ et al. (1996).In study area enters only the East Getic Unit, where the sedimentation in the vicinities of Rgotina started in the Early Jurassic.

Infra-Getic Palaeogeographic Unit
During the Jurassic, the Supra-Getic and the Getic units developed on the framework of the Thracian Massif, which had been separated from the other large palaeogeographic unit, the Moesian Platform, by the Infra Getic Unit with a relatively deep water sedimentation since the Hettangian.In the study area, the sediments of the Infra-Getic crop out in the region of the Dobra Village (Pesača River, etc.), the Štubik Village and the Vratarnica Village.

Moesian Platform Palaeogeographic Unit
The Moesian Platform is a crustal block, located beyond the south-western margin of the European craton.It was divided during the Jurassic (according PATRULIUS et al. 1972;SAPUNOV et al. 1988) into three parts: the West and East Carbonate Moesian Platforms separated by the Central Moesian Basin.Herein, only the West Moesian Platform, the sediments of which crop out from the two sides of the Bulgarian/Serbian state border, is studied.To west, it is limited by the Infra-Getic Unit.
The West Moesian Platform is structured by the Vidin Horst and the Vratsa Horst separated by the Mihaylovgrad Graben (with its Gornobelotintsi-Novo Korito Branch -Basin (Graben)).
The Vidin Horst (SAPUNOV et al. 1988) is the north-western part of the West Moesian Platform and  is built up (from north to south) by the Gomotartsi Step, the Belogradchik Step, the Belotintsi Step, the Prevala Step and the Belimel Step.
The Belogradchik Step (SAPUNOV et al. 1988).During the Early Jurassic and the Aalenian, in the Belogradchik Step of the Vidin Horst entered the Miroč-Vrška Čuka Zone from NE Serbia and the Rabisha-Vrahska Chuka Zone from NW Bulgaria, which represented a dry land with a continental environment -a terrigenous, coal bearing formation.During the Bajocian, a large part was covered by sea water, in which existed a shallow, sublittoral environment with a sandy-pebbly bottom and agitated water with the sedimentation of oligomictic sandstones and conglomerates, covered by the aleuritic marls.To the north of Belogradchik Town, the sediments of the Bov Formation were subsequently eroded (under sub-marine condition) and the Callovian part of the Javorets Formation (micritic limestones) lied directly on the Lower Bathonian parts of the Polaten Formation.
Gornobelotintsi-Novokorito Graben (Basin) (Belotintsi Step) (SAPUNOV et al. 1988).During the Early Jurassic, this was also an area of continental sedimentation.At the beginning of the Bajocian started the formation of a new graben with sandy sedimentation under conditions of a shallow sublittoral environment with a sandy bottom and agitated water during the beginning of the Bajocian -the Gornobelotintsi-Novo Korito Graben.At the end of the Bajocian-Bathonian-Early Callovian, in it existed the conditions of a deep sublittoral environment with a muddy bottom and slightly agitated water with the sedimentation of marls, interbedded by clayey limestones.During the Middle Callovian-Late Jurassic started a stage of bathymetric differentiation and pelagic micritic and nodular limestones were sedimented.In east Serbia, will be described the sediments near Milanovac-Novo Korito and in Bulgaria, the Belogradchik TV Tower, Yanovets, Dolni Lom and Gornobelotintsi sections in the Gornobelotintsi Graben are described herein.
The sedimentation in the two areas was analogous during the Late Jurassic and the Callovian and differed only during the Middle Jurassic.Here was deposited the following sediments: (10) -Novokorito limestones: clayey biomicrites and dolomitic biomicrites with ammonites, Saccocoma and calpionelidsin the upper parts (Kimmeridgian-Tithonian) (analogous to the Glozhene Formation); (11) -Greben ammonitic limestones (ANDJELKOVIĆ et al. 1996) analogous to the Kichera Formation).In the Novo Korito Section, during the Bajocian and the Bathonian, sedimented (18a) -grey-greenish aleuritic marls (locally with many Zoophycos) (horizontal analogue to the Bulgarian Gornobelotintsi Member of the Bov Formation).
Mihaylovgrad Graben (SAPUNOV et al. 1988) (Figs. 1, 2, 3).This is a SW-NE negative palaeostructure, in which the Jurassic sediments show an integrated section from the Hettangian up to the end of the Early Cretaceous.To the east, it is connected transitionally with the Central Moesian Basin and to west, with the N-S oriented Infra-Getic Palaeogeographic Unit and in both of them existed analogous palaeogeographic conditions.The transgression started with the deposition of oligomictic sandstones, followed during the Pliensbachian by sandy bioclastic limestones deposited also in a shallow sublittoral environment, but with a calcareous bottom (Gresten facies -grosso modo).During the Aalenian and the Bajocian, in the Mihaylovgrad Graben existed a deep sublithoral environment when argilites of the facies "black shales with Bositra-Possidonia alpina" were deposited.During the Middle Callovian and the Late Jurassic, micritic limestones (Middle Callovian-Middle Oxfordian) were deposited, followed by nodular and lithoclastic limestones -"ammonitico rosso facies" (Late Oxfordian-Early Tithonian), lithoclastic and micritic limestones (Middle-Late Tithonian) and platform limestones (Late Tithonian-Berriassian).
The section Gaganitsa Village is the section where the complete development of the Jurassic sedimentation deposited in the central parts of the Mihaylovgrad palaeograben crop out.The Gaganitsa Lake section shows a lack of the upper part of the Gintsi Formation and the Glozhene Formation.The section Vinishte demonstrates the lateral changes connected with the board of the palaeograben and the section Nikolovothe effect of a big slumping -the Shugavitsa Slump.
Gaganitsa Village Section (Figs. 1: Bg-8, 2, 3).In the vicinities of Gaganitsa Village (SAPUNOV & TCHOU-MATCHENCO 1995i), in the central parts of the Mihaylovgrad Palaeograben, were deposited: (39) -the Brestnitsa Formation (NIKOLOV & KHRISCHEV 1965) -Slivnitsa Formation (after SAPUNOV & TCHOUMATCHENCO 1995i): light-grey to whitish, thick bedded limestones, often containing corals, bivalves, gastropods (Nerinea), Gaganitsa Lake Section (Figs. 1: Bg-9, 2, 3).This section is situated 3 km south-east from the Gaganitsa Village Section.Its sediments are the same as in the Gaganitsa Village Section, but differs from the latter by the fact that the thick bedded limestones with debris of corals, gastropods, etc. of the Brestnitsa Formation lie directly on the reddish clayey lithoclastic limestones of the middle packet of the Gintsi Formation; the uppermost part of the Gintsi Formation and the limestones of the Glozhene Formation are missing.The situation is the same in the east direction, on the Peak Ludeno and east wards.This is, after P.T., the effect of the Shugovitsa Slump; the missing parts of the section were slumped to north and now crop out as big folds in the Shugovitsa River Valley near the village of Nikolovo, Montana District, at a distance of 16 km.
Nikolovo Section (Figs. 1: Bg-10, 3) (SAPUNOV & TCHOUMATCHENCO, 1995k).The Lower and partly the Middle Jurassic sediments were studied in the drill cores of many bore holes.On the surface crop out the sediments since the upper parts of the Etropole Formation.In this section, the Jurassic sediments are very similar to the Jurassic sediments developed in the Gaganitsa Village Section.The difference between them lies in the fact that the sediments of the Glozhene Formations here are folded in a few horizontal to reversed folds (40b) (Fig. 5F) to reverse faults (TCHOUMATCHENCO & SAPUNOV 1998), which slid on the surface, formed by the reddish lithoclastic limestones of the Gintsi Formation -the Shugovitsa Slump is probably the result of the slumping of these sediments from the region of the Gaganitsa Lake Section -the result of seismic shock.
Vratsa Horst (SAPUNOV et al. 1988).The dry land in a continental environment under conditions of erosion and denudation on the Vratsa Horst progressively diminished (destroyed during the Callovian) and was encountered by a shallow and moderately deep sublittoral environment with the sedimentation of sandy, bioclastic limestones (type of Gresten faciesgrosso modo) during the Early and part of the Middle Jurassic (at the beginning of the Aalenian).During the end of the Aalenian and the Early, Middle and the early part of the Late Bajocian the conditions of a deep sublittoral environment dominated with the sedimentation of silty argillites (facies of black shale with Bositra-Possidonya alpina).At the end of the Late Bajocian and the Bathonian, sandy bioclastic limestone was deposited.During the Middle Callovian-Middle Oxfordian, micritic limestones were deposited, followed upwards by lithoclastic during the Middle Oxfordian-Tithonian.During the Latest Late Tithonian and the Berriasian, the conditions in the Vidin and Vratsa Horsts, and in the Mihaylovgrad (and in its branch, the Gornobelotintsi Graben) became more or less uniform and thick bedded, sometimes bioclastic limestones were deposited. Ledenika

Notes on the Jurassic lithostratigraphy in NW Bulgaria
In this paper, one of us (P.Tchoumatchenco) express some opinions on the lithostratigraphy of the Jurassic which slightly differ from the "official" points of view of the Bulgarian lithostratigraghic interpretation, expressed, e.g., in SAPUNOV & METO-DIEV (2009) -" Jurassic Geology", Chapter 5.3 of the "Mesozoic Geology of Bulgaria" and in many others publications, of which P. TCHOUMATCHENCO is also a co-author.They will be studied in stratigraphical order.
Kostina Formation (SAPUNOV in SAPUNOV et al. 1967).It is used as in the original paper -quartz sandstones (Hettangian, some time up to the Lower Pliensbachian).
Ozirovo Formation (SAPUNOV in SAPUNOV et al. 1967).Herein it is used as in the original paper; later it was named "Homogenous Ozirovo Formation", without subdivision into members.Hence, it is the Ozirovo Formation in the holostratotype (NACHEV et al. 1963).In this meaning, the Ozirovo Formation has a spotted distribution and is a very important palaeogeographic marker; it was deposited in shallow water conditions in a sublittoral area, often with the formation of iron-bearing sediments in the limestones (upper part of the Hettangian-Toarcian).It enters into the Malaplanina carbonate group.
Dolnilukovit Formation (SAPUNOV 1983).It was introduced as a member of the Ozirovo Formation.Here, it is used as an independent lithostratigraphic formation, built of bioclastic dark grey limestones.In some localities, it contains a few lithostratigraphic members: the Ravna, Romanovdol and Teteven members.Maximal range: Sinemurian-Toarcian.This Formation was sedimented in more quiet and deep localities in the Early Jurassic Basin.It enters into the Mala Planina carbonate group.
Bukorovtsi Formation (SAPUNOV in SAPUNOV et al. 1967).Here, is restituted its original meaning as an independent lithostratigraphic formation, as was created by SAPUNOV (1967).The Formation is built of clayey limestones and marls (Upper Pliensbachian-Toarcian or the lower part of the Aalenian).The Bukorovtsi Formation was sedimented in quiet and relatively deep environments.It enters into the Mala Planina carbonate group.
Etropole Formation (SAPUNOV in SAPUNOV et al. 1967).Black shales and argillites (similar to the "Black Shales with Possidonia" in the Southern Alps).In the study area, it is divided into three members: the Shipkovo, Nefela and Stefanets Members.They enter into the Chernivit terrigenous group.
Polaten Formation (STEPHANOV 1966).In northwestern Bulgaria, it is divided into three members: the Yanovets, Vratnitsa and Desivitsa members.They enter into the Mala Planina carbonate group.
Vratnitsa Member (STEPHANOV 1966).It is built of calcareous sandstones to sandy limestones.The lectostratotype is described by TCHOUMATCHENCO (1978) (upper part of the Lower Bajocian-lower part of the Bathonian).
Desivitsa Member (STEPHANOV 1966).TCHOUMA-TCHENCO (1978) gave to these sediments the range of Formation which SAPUNOV & TCHOUMATCHENCO (1986) accepted for them, the range of one marker -the Desivitsa Oolite Marker.Herein, the original meaning of the beds of the Polaten Formation, i.e., above the Vratnitsa Member and below the Bov Formation, in which exists a local Oolite marker in the lower parts of the Desivitsa Member, is returned.It is not logic to have in the Polaten Formation in NW Bulgaria, in the base, a Vratnitsa Member, an Oolite Marker and the Polaten Formation.In the Desivitsa Valley, where these sediments are better developed, in the base exist a bed with oolites and above it, red micritic limestones and marls (Fig. 5E), which are the most important part of this Member.This is the reason for the return to the wider meaning, which was introduced by TCHOUMA-TCHENCO (1978) (middle part of the Lower Bathonian-lower part of the Upper Bathonian).These sediments were deposited in a shallow part of the basin, with relatively strong water movement.The Desivitsa Member of the Polaten Formation is the probable equivalent of the Klaus Schichten in the Southern Carpathians.
Bov Formation (SAPUNOV 1969).It is characterised by clayey limestones and marls (maximal range Upper Bajocian-Middle Callovian).Often it is divided in two members: the Gornobelotintsi and Verenitsa Members.
Javorets Formation (NIKOLOV & SAPUNOV 1970).The Formation is based on micritic limestones with chert concretions (Middle Callovian-Oxfordian). Elsewhere in the basal part of this Formation, a horizon exists with zoogenous breccia-conglomerates, herein individualized as an independent lithostratigraphic unit, with the range of a lithostratigraphic marker, the Sokolov Venets Zoogenous Marker.It enters into the West-Balkan Carbonate Group.
Sokolov Venets Zoogenous Marker (Соколов Венец зоогени репер -new unit) (0.42 m thick in the holostratotype) (named after the peak Sokolov Venets, situated 3.5 km to the NNE of Belogradchik Town).The type section is situated along the road Belogradchik-Oreshets Railway Station (Fig. 4E), described by STEPHANOV (1961), rediscribed by SA-PUNOV & TCHOUMATCHENCO (1995e) and by BELIVA- NOVA & SAPUNOV (2003).Here the description is also after STEPHANOV (1961)."Bed 7. Thickness 0.30 m; brown-red limestones with ferrous hydroxide ooids: Macrocephalites macrocephalus (SCHLOTHEIM) (abundant), Hecticoceras hecticum (REINECKE) (rare), Choffatia spirorbilis (BONCHEV & POPOV).Cover: lithoclastic, pinkish limestones (the Gintsi Formation).Bed 6. Thickness 0.12 m; yellowish-red clayey limestones with scattered ooliths and with large flat-spherical lenticular ferrous hydroxide nodules up to 25 cm in diameter around Bathonian calcareous pieces or Callovian ammonites: Macrocephalites macrocephalus (SCHLOTHEIM) (frequent) and others ammonites.In the two beds, there are many Perisphinctidae, Phylloceratidae, Litoceratidae, etc."The fossils were probably resedimented in the horizontal direction from the Verenitsa Member of the Bov Formation.In this sediment, the Callovian Stage was proven for the first time in Bulgaria by BONCHEV & POPOV (1935), substratum (Lower Bathonian Zigzag Zone): sub-marine hard ground, erosional surface over the Desivitsa Member: sandy, biodetritical limestones with ferrous ooids.Cover: sharp boundary with red nodular limestones containing Hecticoceras (Middle Callovian).Previous uses: "Macrocephalites beds" (BONCHEV & POPOV 1935) and "Red oolitic Callovian limestones" (ATANASOV & ALEXIEV 1956; ATANASOV 1957).Regional aspect: in the section near the Television Tower of Belogradchik, the substratum of the Sokolov Venets Zoogenous Marker is represented by alternation of marls and micritic limestones -the Verenitsa Member of the Bov Formation; cover: beige micritic lithoclastic limestones -the Gintsi Formation.The Sokolov Venets Marker is composed of a few centimetres of dark grey marly limestones.This marker is distributed in the Western, Central and East Stara Planina, the Pre-Balkan and in West Bulgaria.In some localities, it represent an erosional surface between the Polaten Formation and the cover of the Gintsi Formation (Staro selo, region of Pernik, western Bulgaria (TCHOUMATCHENCO et al. 2010a(TCHOUMATCHENCO et al. , 2010b)), between the Polaten Formation and the Belediehan Formation (TCHOUMATCHENCO et al. 2010a), between the Polaten Formation and the turbidite Cerniosam Formation (in Konyava Planina Mt., demonstrated by I. Zagorchev), or between the Polaten and the Lobosh Formation (TCHOUMATCHENCO et al. 2010a).More complicated is the situation in the area of Godech, to north-west of Sofia, near the Villages Gintsi and Komshtitsa.BELIVANOVA & SAPUNOV (2003) wrote that "The section of the Gintsi Cliff, District of Sofia is an uninterrupted Bathonian-Callovian section".This is demonstrated in their figs. 1 and 2 and proven by the ammonitic data, the results of a microfacies study (samples 1, 2) and the analysis of the faunal spectra.This situation is true for the north Gintsi Venets (Cliff) and partly, for the south Gintsi Venets (Cliff), where the beds are concordantly.Interestingly, BELIVANOVA & SAPUNOV (2003) do not comment on the paper of TCHOUMATCHENCO & SAPUNOV (1998), in which the folded upper beds of the Polaten (or Bov?) Formation is demonstrated, and they comment only on this part of the section where it is represented by two horizontal beds, lower (called in the Komshtitsa Section, the Polaten Formation by TCHOUMATCHENCO et al. (2001) and the Bov Formation by BELIVANOVA & SAPUNOV (2003)) and the upper "Niveau condensé à ammonites" with indications of the "Z.à Gracilis (s/Z.à Michalskii)".BELIVANOVA & SAPUNOV (2003) indicated that in the "Bov Formation" is collected Clydoniceras cf.discus (J.SOWERBY 1813) (Upper Bathonian, upper part) the C. discus Zone and the upper bed, the Yavorets Formation -dark grey micritic limestones, in which, from the very base of the packet, is found Macrocephalites cannizaroi (GEM-MELLARO 1868) (Lower Callovian), Grossouvria sp.(Callovian) and Macrocephalites spp.(Lower Callovian).The determination of these ammonites is out of doubt.However, in the south Gintsi Venets (Cliff), these two beds in horizontal direction change over a short distance (in 2-3 meters) (Fig. 5B).The lower bed became folded (TCHOUMATCHENCO & SAPUNOV 1998;refigured in TCHOUMATCHENCO et al. 2010a, Fig. 4D), and the upper bed (with the Callovian Trans-border (north-east Serbia/north-west Bulgaria) correlations of the Jurassic lithostratigraphic units ammonites) became thick, up to 1.80 m.This bed fully filled the negative parts of the folded lower bed (Fig. 5D).What is demonstrated in Figure 5d?At the end of the Bathonian, the beds were folded (the cause is unknown, probably slumping of the Bathonian sediments), then the negative folds were fully filled by current accumulations from the Lower Callovian sediments, which were the result of erosion and redeposition of sediments containing ammonites and other fossils (especially from the Lower Callovian part of the Bov Formation).In the vicinities of Komshtitsa Village (at the piedmont of the Elenine Vrah), the Sokolov Venets Zoogenous Marker was structured by a zoogenous breccia or locally, by an intraformational conglomerate (TCHOUMATCHENCO et al. 2010b, Fig. 4F).In the Nechinska Bara Valley, near Gorno Belotintsi Village, Montana District, the Sokolov Venets Marker is expressed only by an angular discordance (SAPU-NOV & TCHOUMATCHENCO, 1998; SAPUNOV & TCHOU-MATCHENCO, 1995d).In addition, a local discordance between the Verenitsa Member and the Gintsi Formation is expressed in the Dolni Lom Village section (SAPUNOV & TCHOUMATCHENCO 1995g).The substratum of the Sokolov Venets Zoogenous Marker is with different ages: Lower Bathonian in the type section, in western Bulgaria, etc.; Upper Bathonian in the Gintsi Village sections, in the Komshtitsa section, etc.; Lower Callovian in the Belogradchik TV Tower section, in the Nechinska Bara Valley section, in the Dolni Lom section, etc. and depends on the energy of the sub-marine erosion (after STEPHANOV 1961 -emersion and transgression).The Sokolov Venets Marker is analogous to the Middle Callovian Sultanci Formation in the bore holes in the area of Provadiya Town, eastern Bulgaria.
The Sokolov Venets Zoogenous Marker can be correlated with the Lower Callovian sediments with many ammonites in the area of Rosomač-Senokos (Stara Planina Mts.), near D. Milanovac (23a in the present paper) (ANDJELKOVIĆ 1975), etc.
What is the cause of the formation of the Sokolov Venets Zoogenous Marker?We agree with the notion of NACHEV (2010) that at the end of the Bathonian in the "Balkanids", there were many hesitant movements with a tendency of general swallowing.One of us (P.T.) thinks that the post-Bathonian erosion was the most developed during the early Callovian and at this time, the Bathonian-Lower Callovian sediments of the Bov Formation (Verenitsa Member), which are well preserved in the section of the Belogradchik-T.V. Tower, were completely destroyed to the north-west (in the holostratotype of the Sokolov Venets Zoogenous Marker).Probably during the early Callovian, the north part of a syn-Early Callovian fault, crossing the seabed near the present day Belogradchik, was elevated and the sediments destroyed, and their ammonites resedimented in the sediment of the Sokolov Venets Zoogenous Marker.The aspect of this marker is different in different localities: in some localities, it is represented by an erosional surface, where there was only elevation of the area, and in other, where the erosion was weaker, after the erosion followed stage of deposition of sediments of Sokolov Venets Zoogenous Marker.It is interesting that the new sedimentation started in many localities during the Middle Callovian and in others later, during the Early or the Late Kimmeridgian.
Magura Formation (NIKOLOV & TZANKOV 1996) -it is based on whitish to grey-beige thick bedded, shallow water limestones, in many localities containing corals, gastropods and bivalves.They also contain some calpionellids (Upper Tithonian-Berrisaian).It enters into the West-Balkan Carbonate Group.
Slivnitsa Formation (ZLATARSKI 1885) -it is based on bright thick-bedded limestones, in some locality with many corals and gastropods.It contain some calpionellids in its upper parts (Upper Tithonian-Berriasian; after some colleagues up to the Hauterivian).It enters into the West-Balkan Carbonate Group.

N. B.
These three lithostratigraphic units are with the same or very similar lithological characteristics and with almost the same stratigraphical position.The problems for the delimitation of their area of distribution are, to a high percent, subjective.Their outcrops are separated one from others by younger sediments and it is question of personal opinion whether one outcrop is connected with another or not.The discussion was opened by the paper of TCHOUMATCHENCO & SA-PUNOV (1986).At this time, we did not know that in East Serbia, the Vratarnica Series of ANDJELKOVIĆ (1975) existed, and by analysis of the geological liter-ature (known to this moment in Bulgaria), P.T. came to the decision that the Slivnitsa Formation is connected in the region of Vrashka Chuka with the Brestnitsa Formation.This idea was adopted by his colleague, IVO SAPUNOV, and it was published by TCHOUMA-TCHENCO & SAPUNOV (1986).In many of the sheets of the geological map of Bulgaria on the scale 1:100 000, concerning NW Bulgaria, this idea was adopted and on them is indicated the presence of the Slivnitsa Formation, but in the neighbouring map, for the same rocks, the Brestnitsa Formation was adopted.Later, NIKOLOV & TZANKOV (1996) created for analogic rocks, a new lithostratigraphic formation -the Magura Formation.Finally, his (of P.T.) present day opinion is that the Slivnitsa Formation is developed only in western Bulgaria and is separated from the same rocks in NW Bulgaria and NE Serbia by the volcano-sedimentary rocks of the Vratarnica Formation ("Series") (Figs. 1, 2 and 3) of ANDJELKOVIĆ (1975), ANDJELKOVIĆ et al. (1996), ANDJELKOVIĆ & MITRO-VIĆ-PETROVIĆ (1992).His last opinion is that the Magura Formation is connected with the Brestnitsa Formation, but in text I, the idea of RUSKOVA & NI-KOLOV ( 2009) is followed because it is a predominantly a Lower Cretaceous problem, i.e., that they are separate lithostratigraphic bodies.

Syn-sedimentary discordances
In the Jurassic sediments of NW Bulgaria exist two regional syn-sedimentary discordances: Sokolov Venets Discordance -in the subsided parts of the basin, it is expressed only by an angular discordance (without erosion of the substratum and new sedimentation -Nechinska Bara, Dolni Lom).In the moderately elevated region, the discordance is fossilized by few centimetre deposits -Belogradchik TV Tower, Teteven, etc.In the more elevated area was effectuated an erosion and subsequent accumulation of calcareous breccia-conglomerate, Belogradchik-Oreshets, Gintsi-Komshtitsa; the substratum is spotted; the erosion goes up to the Lower Bathonian beds, Belogradchik-Oreshets, Kremikovtsi, etc.In western Bulgaria, the result is a lack of sedimentation going up to the Early Kimmeridgian (Kremikovtsi -Sofia District), Staro Selo (Pernik District) or to the Late Kimmeridgian (Konyava Planina Mt.; a phenomenon demonstrated to me by I. Zagorchev).
Granitovo Discordance -as result of a seismic shock during the Late Kimmeridgian, in the sediments of the upper part of the Gintsi Formation, in the area of Belogradchik, a fold formed (Fig. 4D), which passed into an overthrust with NW vergency (Figs.4E, F).In the Mihaylovgrad Paleograben, the result of this seismic shock was the formation of the Shugovitsa Slump (Fig. 5F).It commences in the region of the Gaganitsa Lake area and goes to the north of the Shugovitsa Valley, near Nikolovo Village.It is interesting that here the vergency of the folds and overthrusts are opposite to the fold-over thrust in the area of Belogradchik with north vergency and transport of material from south to north.
Fig. 4. A, Vratarnica Series, the "matrix" of the formation, near the road south of Vratarnica Village, Zaječar District; B, Vratarnica Series, olistolite of volcanogenous rock in the matrix, near the road south of Vratarnica Village, Zaječar District; C, Vratarnica Series, olistolite of coarse grained whitish limestone (analogous to the Crni Vrh Limestones of east Serbia or the Bulgarian Slivnitsa Formation) in the matrix, near the road south of Vratarnica Village, Zaječar District; D, Granitovo, Gradishteto Hill, syn-sedimentary fold in the upper part of the Gintsi Formation; E, Reverse fault in the upper part of the Gintsi Formation near Granitovo Village, Sokolov Venets Hill -in the upper part of the Gintsi Formation; F, reverse fault in the upper part of the Gintsi Formation, near the road Belogradchik-Railway Station Oreshets.

Fig. 5 .
Fig. 5. A, Holotype section of the Sokolov Venets Zoogenous Marker, along the road Belogradchik-Railway Station Oreshets, general view and upper bed No 7; B, Holotype section of the Sokolov Venets Zoogenous Marker, along the road Belogradchik-Railway Station Oreshets, specimen from the lower bed No 6 -zoogenous breccia; C, Sokolov Venets Zoogenous Marker, view of the outcrop on the cliff by the road near Mitrovtsi Village, Montana District; D, Sokolov Venets Zoogenous Marker, view of the outcrop on the southern hill of the Gintski Venets (Cliff), near Gintsi Village, Sofia District, folded uppermost bed of the Bov (Polaten?)Formation; the sediments of the Sokolov Venets Zoogenous Marker fully fill the negative part of the ancient relief; in the right part of the photograph is seen the bed of the Polaten (Bov?) Formation overlaid by zoogenous breccia of the Sokolov Venets Zoogenous Marker; E, Desivitsa Member of the Polaten Formation in the Desivitsa Valley, near Prevala Village, Montana District; F, view of the Shugovitsa Slump; in the Shugovitsa River Valley, near Nikolovo Village, Montana District, the folds are in the Gintsi and the Glozhene Formations.