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.

The only boundary which can be used to differentiate the Bulgarian Black Sea shelf into stratigraphic zones is the spatial distribution of mollusc fauna. This boundary is bathymetrically determined and divides the central morphological shelf zone into two uneven parts given inner and outer shelf. Another boundary is drawn which follows roughly the parallel of Cape Kaliakra and divides it into two uneven parts. The sediments of Low Pleistocene series were not deposited in the inner shelf zone and here a stratigraphic gap is established there which coincides with Chaudian Age. A similar gap at the time of Early Chaudinian Subage is established in the outer zone of the shelf where the Dreissena rostriformis tschaudae local mollusc Range-zone is indicated. In both shelf zones the sedimentation continues by formation of the Middle Pleistocene Old Euxinian and Uzunlarian Regional Stage in whose sediments the corresponding are Didacna crassa pontocaspia – Dreissena euxinica and Didacna crassa pontocaspia – Abra ovatа local mollusc Parallel Range-zones. The Upper Pleistocene (Karangatian) mollusc zones are established only at the inner parts of the Bourgas Bay and there three: Corbula gibba – Rissoa parva, Paphia senescens – Cardium tuberculatum and Cardium edule – Bittium reticulatum local mollusc Parallel Range-zone are found out which correspond to the three Substages of the Karangatian Regional Stage. The succession of the mollusc zones from the inner shelf is interrupted by a barren zone which corresponds to the continental deposits sedimented at the time of the Early New Euxinian Substage. During the greater part of the Late Pleistocene Subepoch in the zone of the outer shelf a stratigraphic gap is established. In the composition of the zone community of the next in superposition Dreissena polymorpha – Dreissena rostriformis distincta local mollusc Parallel Range-zone two associations are established – one in the inner and another in the outer shelf. The climatic changes which took place after the Würm glaciation restored the connection of the Black Sea with the World Ocean. In the Black Sea basin this Holocene transgression causes uplift of the sea level and increasing of the salinity to its recent values. The latter determines the successional changes of the mollusc communities which from the beginning of the Holocene became more halophilic. The community of Hydrobia ventrosa – Monodacna caspia caspia local mollusc Parallel Range-zone and Mytilus galloprovincialis local mollusc Range-zone comprise the sediments of Old Chernomorian Substage. The two New Chernomorian Spisula subtruncata triangula and Modiolus phaseolinus local mollusc Range-zone are fingering spatially in the zone of the outer shelf together with increase of the bottom at practically one and same composition of the zonal community.

Seven instructive Jurassic sections in the Gabrovo and the Kazanluk Districts are described. It was established that in the Gabrovo District the lower part of the Ozirovo Formation belongs to the Lower Sinemurian, and the underlying Kostina Formation to the Hettangian. In the Kazanluk District (section in the locality Ikischa) the lower part of the Ozirovo Formation belongs to the Lower Pliensbachian, and the Kostina Formation to the Upper Sinemurian. The sections in the Gabrovo District are situated within the limits of the Kostina Step (second order block) of the Izdremets Complex Graben of the Moesian Early- Middle Jurassic Platform. The sections in the Kazanluk District are situated within the Neshkovtsi Step of the same graben. The marine ingression on the Moesian Early-Middle Jurassic Platform began during the Hettangian with the most descended Kostina Step. The sea advanced from the Toundzha Fault-Angle Valley. During the Sinemurian and the Early Pliensbachian the ingression expanded and covered all the rest, more uplifted steps of the Izdremets Graben. The sea gradually overflowed the Sevlievo Graben and the western part of the Veliko Turnovo Graben of the Moesian Early-Middle Jurassic Platform. At the initial stage of the marine ingression the supplying dryland was active, therefore quite shallow water terrigenousclastic sediments and very little amount of more or less sandy calcareous silts with bioclasts were deposited. Later on (at the end of the Sinemurian and in the beginning of the Pliensbachian) the calcareous silts began to prevail over the terrigenous sediments, the shallow-water environments having remained. More or less significant Middle Jurassic and Early Jurassic stratigraphic breaks were established in all of the described sections. They realized in marine conditions. Strong submarine currents led to a marine abrasion. As a result, condensed Middle-Upper Callovian limestones lie at present over Lower Bajocian or Lower Sinemurian sediments, or directly overlay Upper Triassic rocks. In the section near the Sokolski Monastery, Gabrovo District, Middle Kimmeridgian limy shales to marls and a quite limited amount of thin layered limy sandstone intercalations directly overlie Lower Sinemurian, sandy bioclastic limestones.

The results of the stratigraphical and pollen-analylical investigation of shelf and deep sea sediments from the Bulgarian sector of the Black Sea are presented. The sediments referred to the complex pollen zone I belong to the New Euxinian Regional Stage (upper substage). They were deposited during a regression of the sea under the conditions of dry and cold climate typical of the Black Sea region during Lateglacial time. The sediments referred to the complex pollen zone II belong to the Holocene or Chernomorian Regional Stage. The age of the sediments was confirmed by several radiocarbon dates which fit the biostratigraphic evidence.

Detailed biostratigraphical study (on brachiopods, bivalves and ammonites) on seven instructive sections established that only a part of them are complete, while the others are more or less incomplete. The reason for this phenomenon is the gradual marine Early Jurassic transgression which has occurred during the Hettangian in some areas (i.e. the section near the Gaganica Village), and later – during the Early Pliensbachian (the section at the Ledenika Cave). Locally the marine transgression had been preceded by continental lacustrine and (or) paludal accumulation (the sections near the villages of Belimel, Kamenna Riksa and Gorno Ozirovo). The area studied belonged to three major block palaeostructures of the Mocsian Early-Middle Jurassic Platform - the Eastern margin of the Belimel Step of the Vidin Early-Middle Jurassic Complex Horst, the Selanovci Step of the Mihajlovgrad Early-Middle Jurassic Complex Graben and the Ozirovo Step of the Vraca Early-Middle Jurassic Complex Horst. Tile Ozirovo Step possessed a complex block structure with step-like arrangement of lowest range blocks in descending order. On their basis the gradual transition to the Izdremec Early-Middle Jurassic Graben or to the Selanovci Step of the Mihajlovgrad Graben has been achieved. The lower Jurassic sediments studied in the sections have been deposited in the shallow shelf while the water dynamics intensity has increased approaching the adjacent shore.