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The pre-Callovian stratigraphic gap in the Central Balkanids is distributed in the Etropole, Teteven, Troyan, Kazanluk, Gabrovo and Turgovishte Districts. To the west (village of Gintsi, Sofia District) and to the east (the section of well R-3 Yunak, District of Yarn a) of the gap uninterrupted Bathonian-Callovian rock successions exist. In its western and eastern parts the pre-Callovian stratigraphic gap is a diastem. In these localities the gap embraces minimum duration. In the western diastem the duration of the gap is from the Late Bathonian to the Early Callovian. In the eastern diastem the duration of the gap is probably in the framework of the Late Bathonian. In many localities where the gap is diastem the boundary between the basement and the cover is marked by hardgrounds. In its central part (Troyan, Kazanluk and Gabrovo Districts) the pre-Callovian stratigraphic gap is with much longer duration. This is caused by the intensive submarine washout of the earlier deposited Middle and Lower Jurassic sediments. In limited localities the Callovian from the cover lies directly even above Upper Triassic rocks. Almost everywhere the cover of the gap is represented by condensed Middle-Upper Callovian micritic limestones. They are rich of ammonites. The main microfacies type is mudstone/wackestone with filaments from thin-shelled bivalves. It is correlated with the Standard Microfacies Type 3 (SMF 3) - "pelagic mudstone and wackestone". It is formed in the lower part of the deep sublittoral, which is in consent with the ba thymetric interpretations based on the characteristics of the respective faunal spectra. The Callovian limestones in conditions of uninterrupted sedimentation show similar microfacies characteristics. In the localities where the pre-Callovian gap is a diastem, the conclusions for the microfacies characteristics and for the faunal spectra of the Bathonian sandy bioclastic limestones are similar. The microfacies type is bioclastic-lithoclastic packstonelrudstone, which can be attributed to some extent to the SMF 4 - "bioclastic-lithoclastic packstone". It may be supposed that the features of this texture direct to a more shallow parts of the deep sublittoral. Almost the same characteristics have the Bathonian limestones and marls in conditions of uninterrupted sedimentation. In general, the pre-Callovian gap is realized in submarine conditions, which are connected with the deep sublittoral. There are four stages in the transition between normal sedimentation and submarine gap: normal sedimentation (in which biostratigraphic superposition is traceable) → slow sedimentation (condensations) → submarine stratigraphic gaps (as a result of non-deposition - diastems) → submarine stratigraphic gaps (as a result of submarine washout).

As a result of the biostratigraphic study of brachiopods, bivalves and ammonites made in eight instructive sections it was established that most of the Lower Jurassic rock sequences were more or less incomplete. The sections in the valleys of the rivers Kostina and Cherna Reka, Teteven Region, are most complete (from Hettangian to Toarcian). They are situated in the central, the most deeply sinking block one – called Kostina Step of the Izdremets Complex Graben of the Moesian Early-Middle Jurassic Platform. There, the most deep-sea Early Jurassic distal facies were existing. The section near the village of Gorno Shipkovo, Troyan Region, has more shallow-water character and is situated in the Shipkovo Step, the most deeply sinking step of the southern edge of the Izdremets Graben. There, the Lower Jurassic section begins with the Lower Sinemurian. The Lower Jurassic rock sequences in the Central Stara Planina Mts. between the hamlet of Neshkovtsi and the locality Brousovete, Troyan Region, are connected with the Neshkovtsi Step (the sections near the hamlet of Neshkovtsi) and the Ambaritsa Step (the section near the mountain hut Ambaritsa and in the locality Brousovete). There, the Lower Jurassic sequences are most incomplete and they begin with the Upper Sinemurian (near the hamlet Neshkovtsi) or with the Pliensbachian (in the rest sections). The lithofacies and the thanatocoenoses possess a more coastal character and are referred to the proximal part of the basin. During the Early Jurassic the studied south-eastern part of the Izdremets Graben subsided gradually, so that in the end of the Early Jurassic, when the maximum deepening took place, the depths corresponded to the deep sublittoral (150-200m). At that time the marine transgression extended considerably to the south.

In south-east Bulgaria the presence of Hettangian (by indirect data) and Sinemurian, Pliensbachian, Toarcian. Aalenian, Bajocian and Bathonian (on the basis of direct biostratigraphic proofs) has been established. The Kostina Formation is represented by quartzites, and almost everywhere is related to the Hettangian. However, in a restricted area, in the vicinity of Stara Zagora Town, the Kostina Formation probably belongs to the Upper Sinemurian. The Ozirovo Formation (represented by shallow water, sandy, bioclastic, partly ferrugineous limestones) and the Bliznak Formation (mainly consisting of fine-grained quartz sandstones) are related to the Sinemurian. The Ozirovo Formation and the Bliznak Formation comprise also the Pliensbachian, the Toarcian, the Aalenian and the lowermost part of the Lower Bajocian (the lower part of Hyperlioceras discites ammonite Zone). The Lower Bajocian (without the lower part of Hyperlioceras discites ammonite Zone) belongs to the Zvezdets Formation, represented by shales, rich in organic matter, with siderite and phosphorite concretions, in some places being interbedded with sandstones, or the Boyanovo Formation (new formation), represented by an alternation of terrigenous-clastic and pelitic rocks and bioclastic limestones. In the south-eastern part of the Strandzha Mountains, sandstones are more abundant in the Zvezdets Formation, and in addition some olistostromes, consisting mainly of limestone breccias, are found – Kazanski Member of the Zvezdets Formation. Only the Zvezdets Formation and its Kazanski Member belong to the Upper Bajocian. The Brushlyan Formation (represented by shallow water bioclastic limestones in alternation with phyllites) and the Hranova Formation (shallow-water calcareous phyllites and a lesser amount of limestones with bioclasts) belong to the Bathonian. All autochthonous Lower and Middle Jurassic rocks in southeast Bulgaria occurred within the limits of the Tundzha Early-Middle Jurassic Fault-Angle Valley which gradually subsided during Early Jurassic and the beginning of Middle Jurassic times. As a result of these phenomena, the sea rapidly ingraded from south-east, from the Vardar Ocean side. In the valley, 7 blocks (steps) were formed and separated bathymetrically from each other: Yambol, Indzhe Voyvoda, Botevo, Kraynovo, Stara Zagora, Elhovo and Kondolovo Steps. Along the valley, a sea sound was formed and carried out the connection between the Vardar Ocean and the epicontinental basin of the Moesian Early-Middle Jurassic Platform since the Hettangian up to the Bathonian time. The valley passed two stages in its development: (a) transgressive one (Hettangian-Bajocian) and (b) regressive one (Bathonian). During the transgressive stage the sea gradually overflowed the Yambol Step (during the Hettangian), the Indzhe Voyvoda, the Botevo and the Kraynovo Steps (during the Early Sinemurian), the Stara Zagora Step (during the Late Sinemurian), the Elhovo and the Kondolovo Steps (during the Early Bajocian). During Hettangian, Sinemurian and Pliensbachian, everywhere in the sea sound, a shallow sublittoral biotop with calcareous or sandy bottom and agitated waters existed. The depths did not exceed 50 m. During Toarcian, Aalenian and Early Bajocian, some bathymetric differentiation took place. The shallow sublittoral biotop remained near the sound coasts, while in the parts distant from the coasts, bottom subsidence took place. An environment of moderate deep sublittoral biotop mainly with calcareous and partly sandy bottom appeared there. The depths were of the order of 50-100 m. During the Early Bajocian, in the south-eastern part of the sea sound the most intensive deepening took place. There, environments of a deep sublittoral biotop with a silty and partly sandy bottom and relatively calm water have set in. The depths were 150-200 m. In the central parts of the sound, the sea overflowed new considerable parts of the Tundzha Valley. There, a shallow sublittoral biotop existed, and the sedimentation regime often changed (shallow-water limy silts, pelites and terrigenous-clastic sediments were deposited). In the north-western part of the sound, probably deeper-water pelitic sediments were deposited, but in the interval Callovian-Early Cretaceous, under conditions of emergence, they were eroded. During the Bathonian, a rapid general shallowing arose there, and under conditions of a shallow sublittoral biotop with calcareous and muddy bottom, limy silts with bioclasts and pelites were deposited. The depths did not cxceed 50 m. In considerably large parts of the Tundzha Fault-Angle Valley, a shallow sublittoral biotop with strong bottom currents existed and caused a sedimentation gap. At the end of the Bathonian, the Tundzha Fault-Angle Valley was uplifted over the sea level and joined the dry land of the Thracian Massif. As a result of this, the sea-strait between the Vardar Ocean and the Moesian Early-Middle Jurassic Platform stopped existing.

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