Jurassic stratigraphy and geodynamics in the eastern part of the Central Balkan Mountains (Gabrovo and Kazanluk Districts), Bulgaria

Abstract

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.