The Sub‐Balkan graben system in central Bulgaria forms the present northern boundary of the Aegean extensional region. This east‐trending graben system lies along the southern flank of the Stara Planina range and consists mainly of half‐grabens. The sedimentary fill in the grabens ranges in age from late Miocene to Recent and records the initiation and evolution of the graben system. The sedimentary fill in the grabens is oldest in the central graben and becomes progressively younger to the west and east, indicating a diachronous development of the grabens. Grabens are formed in the hangingwalls of south‐dipping low‐angle normal faults which have been displaced by younger higher angle normal faults along the foot of the Stara Planina. Hangingwall rocks have been complexly faulted and rotated such that some graben fill has been rotated down‐to‐the‐north. The Sredna Gora range south of the grabens is part of a complexly faulted and rotated hangingwall block bounded on the south by south‐dipping normal faults forming the northern boundary of the Thracian Basin. The Stara Planina range has been formed by uplift and rotation due to footwall unloading along the low‐angle normal faults and forms the northern margin of the graben system. Most of the topography of Bulgaria south of the Sub‐Balkan graben system is the result of late Miocene to Recent extensional processes linked to the Aegean region that have been superposed on convergent features and earlier extensional features that extend back to late Eocene time.

The Quaternary in the Ogosta River valley system (the Danubian plain) is represented by various genetic types of continental Quaternary sediments (eolian, alluvial, deluvial, proluvial). They lie on a diverse pre-Quaternary base. It is uneven and with various denudation. The accepted correlation schemes are based on local data in which the tectonic factor and duration of deformation have not been accounted for. The present work considers the complex results obtained during the investigation of the Quaternary sediments and formations. The filling consists of clayey-sandy or calcareous-sandy materials, coloured in rusty nuances by iron hydroxides. Their age has been determined on the basis of the found fauna: Elephas meridionalis Nesti,Anancus arvernensis Cr. et Gob (Bakalov, Nikolov, 1962).

The Holocene sediments in the western part of the Ajdemir Lowland are covered by two formal lithostratigraphic units: Todoranka clayey Formation (new formation) for the swampy sediments and Târlica pebble-sandy-clayey Formation (new formation) for the alluvial sediments. They are deposited over small block of different dynamics and form superimposed, interbedded and contiguous discordences. The Todoranka Clayey Formation sediments are formed in swampy environment under conditions of constant sinking, while those of the Târlica Pebble-Sandy-Clayey Formation are formed in lacustrine environment under changing lithodynamic conditions – from slow tectonic activity and dynamic balance to continuous sinking resulting from negative tectonic movements. The lower boundary represents a distinguished erosional washout over various lithological basement of different age.

UDC: 551.44(497.2) Dora Angelova: Karst types in Bulgaria The karst in Bulgaria occupies an area of 26 170 km 2 or 22.7 % of the territory of the country. The karst water resources are estimated to be 2.3 billion m 3 or 11.6 % of the total water resources of the country. The interest in karst in Bulgaria has become higher during the last years because there are a number of practical problems that have to be solved. Karst in Bulgaria is characterized by great diversity due to the complex combination of factors (geological, tectonic, geomorphologic, hydrological and hydrogeological, climatic, etc.) and to the geodynamic development of this part of Europe. This work presents a new zoning of karst in Bulgaria. The following types have been distinguished: plain karst (the Danubian Plain); marine and transformed marine karst into plain and plain-marine karst (Black Sea subaqual and subareal plain); plateau-like karst (the Fore Balkan) and mountainous karst. The karst wetlands and karst phenomena provoked by paleoearthquakes are separately outlined and sample models for the different karst types, genesis, dynamics, lithostructural control, relations, etc. are presented.