Основы литостратиграфии нижнемелового отдела в Болгарии

Pages: 
pp. 3-47
Софийски университет, 1000 София
Геологический институт, Болгарская акедемия наук, 1113 София
Геологический институт, Болгарская акедемия наук, 1113 София
Abstract: 

T. Nikolov, N. Ruskova, Kh. Krischev. Principles of the Lower Cretaceous lithostratigraphy in Bulgaria. The Lower Cretaceous deposits are widespread in North Bulgaria: in the Fore-Balkan, the Transitional Zone, and the Moesian Platform. The scheme of the formal lithostratigraphic units includes 37 Formations and has been elaborated for all this territory on the basis of outcrops and numerous boreholes. Four Groups (West-Balkan Carbonatic Group, Central-Balkan Flysch Group, Vraca Urgonian Group and Loveč Urgonian Group) consist of 19 Formations, and the other 18 Formations are independent. A total of 15 Members has been introduced within 9 of the Formations.
   The composition and interrelations of tbe lithostratigraphic units reflect the inhomogeneities and the different tendencies of the development of tile Early Cretaceous basin. The most expressive facies boundary in the west-east direction is the so-called Jablanica Line. West of it, carbonatic and marly deposits predominate in the Fore-Balkan. The carbonatic Formations belong to the West-Balkan Carbonatic Group and the Vraca Urgonian Group. The Salaš Formation is built up of clayey limestones and marls, and is widespread in the West Fore-Balkan, as well as in the western part of .the Moesian Platform.
   The asymmetry of the basin with a dry land as source region to the south of it, is clearly outlined to the east of the Jablanica Line, within: the Central and East Fore-Balkan and the adjacent Transitional Zone. This character of the basin controlled a facies differentiation with development of predominantly terrigeneous units in the south, and of carbonatic and marly units, to the north. This difference is expressed at its best at the lower levels of the Lower Cretaceous where units of the Central-Balkan Flysch Group are interfingering with units of the West-Balkan Carbonatic Group. The lateral transition takes place within the Transitional Zone.
   At higher stratigrapic levels (Hauterivian - Aptian) the asymmetry is expressed into a horizontal transition of terrigenous non-flysch deposits (Kamchija Formation and Roman Formation) with predominantly marly deposits (Gorna-Orjahovica and Trâmbeš Formations) to the north. A specific feature is the presence of Urgonian deposits in the Transitional Zone (Loveč Urgonian Group). At both sides of its development area, fans of the terrigeneous deposits of the Roman Formation are traced towards north. They are bounded to the Iskar (at the west) and Etar (to the east) depressions. Urgonian limestones occur also within the Moesian Platform in the area of Ruse (Ruse Formation).
   The highest (Albian) levels of the Lower Cretaceous section have been observed only in northwest Bulgaria. They are represented by the marls of the Sumer Formation, the glauconitic predominantly sandy deposits of Malo Peštene Formation, and the marls of Rabiša Formation.

VOLUME 21 (6)/December 1991

Софийски университет, 1000 София
|
Геологический институт, Болгарская акедемия наук, 1113 София
|
Геологический институт, Болгарская акедемия наук, 1113 София

Геологический институт, Болгарская акедемия наук, 1113 София
|
Предприятие геофизических исследований и геологического картирования, 1505 София
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Геологический институт, Болгарская акедемия наук, 1113 София