The Permian in Northern Bulgaria. I. Formal lithostratigraphy related to the Lower Permian

Abstract

Two lithostratigraphic groups, corresponding to Lower Permian and Upper Permian, have been distinguished after stating the principles of division of the Permian sediments. In the first group, called North-Bulgaria Group, are distinguished: a variegated clayey-terrigenous formation, Nanevo and Komunari Formations (Lower Rotliegendes?), Severci Formation (Upper Rotliegendes?) and Bdin and Dolna Zlatica Formations (undivided Rotlegendes), as well as Kamen Briag, Bajacevo and Balgarevo Members in Nanevo Formation and Bononia, Delejnska and Rasovo Members in Bdin Formation. The officially distinguished lithostratigraphic units are documented by eight type sections and two reference sections, which are described according to drill-holes, taking into consideration the requirements of the Stratigraphic Code. The variegated clayey-terrigenous formati on comprises red and grey-green sandstones, silt stones and argillites. In the Nanevo Formation, from the bottom upwards, dominate various middle-acid volcanites and tuffs (Kamen Brjag Member), conglomerates, sandstones and a small amount of silt stones with tuffs and redeposited volcanic material (Bajačevo Member) and hypoargillitcs, hyposiltstones, a small amount of impure gravel-sandy rocks (Bălgarevo Member). Polygenetic brecc iaconglumcrates. conglomerates, red and black grey sandstones and siltstones alternate in the Komunari Formation (by a lateral disappearance of the psephilic rocks). The Belin Formation mainly contains conglomerates with carbonate fragments (with a small amount of admixtures of volcanic fragments, mainly in the Bononia Member), the composit ion of the Delcjnska Member being hypoa lcuropclitic. The Dolna Zlatica Formation is built of brown-reddish, mainly massive argillites and clayey siltstones with rare bedded interbeds consisting of siltstones and grained sandstones. Breccia conglomerates, sandstones and siltstones (a small amount of argillites), the psephites bearing fragments of sediments (carbone ones inclusive), low-metamorphic and volcanic rocks, are in an alternation in the Severci Formation. The Severci Formation overlies an eroded surface, while the other formations interfinger laterally with a correlation of the separate triads of members. The Komunari Formation is spread over the southern margin of the estern part of North Bulgaria, the Dolna Zlatica Formation - in the Tiirnovo Depression and the areas around, the Nanevo Formation - in the Bulgarian Coastal Dobrudza and around the North-Bulgaria Swell, the Severci Formation - in NE Bulgaria. Lower Permian, in the Bulgarian pari of the Moesian area, has a more detailed lithostratigraphic division than on the territory of Romania. The majority of the formations and members described may have analogues in the drill-holes north of the Danube.