Литология триaсовых пород в Северной Болгарии и связанные с ними микрофации (по данным бурения)

A. Vaptzarova. Lithology of the Triassic rocks in North Bulgaria and related microfacies (according to borehole data). The present study is a review of the knowledge on the lithology and composition of the Triassic sediments in North Bulgaria. The rocks aro referred to three large lithostratigraphic units – Groups – Petrohan terrigenous, lskar carbonate and Moesian Group.
   Petrohan terrigenous Group includes three lithostratigraphic units ranging in age from the early to the lowermost parts of the Upper Spatian Substage. The rocks of the Group, terrigenous with high mineralogical and textural maturity, and silty-clayey wen: formed in continental conditions. Andesite-basalt(?) lava flows were extruded in subaeral environment.
   lskar carbonate Group comprises 8 Formations and 2 Members. The age range of the group varies from Middle-Upper Spatian to the Carnian. Initially formed are limestones and dolomites of diverse texture, related to the stable stage in the development of the basin, followed by clay-terrigenous, dolomitic and evaporitic deposits which mark the beginning of the regressive stage in the evolution of the Triassic Sea in this area. A number of microfacies have been proved in the carbonate and clay sediments but only those of them which are widely used in lithological correlations are described in the paper. The activization of the Triassic volcanic activity in the Northwest Fore-Balkan is related to the initial and middle stages of carbonate deposition (Anisian-Ladinian). Its termination coincided with the beginning of the general intensive uplift of the territory.
   The Moesian Group is subdivided into 9 Formations and 2 Members. The age range of the Group is conventionally assumed as Norian (?) - Rhaetian Substage. A complex of multicoloured terrigenous, clayey, carbonate and sulphate sediments of normal, unsaturated and mixed composition were formed in the regression phase of the development of the Triassic basin. Their diverse quantitative combinations, the rapid vertical and horizontal changes and the abrupt variation of the thickness reflect the regressive tendencies in the deposition and the more intensive tectonic activity of the territory studied in the course of the final stage of the Triassic Period.