Lithology and petrology of the Ustrem Formation (Lower Triassic) in the Sakar Mountains (SE Bulgaria)

The Ustrem Formation (Lower Triassic) belongs to the Sakar (metamorphic) Triassic. This Triassic type is probably in allochtonous position. It includes the most strongly metamorphosed Triassic rocks in Bulgaria. The Ustrem Formation belongs to the Spathian. A great number of bivalvia prints have been established in marbles, metaquartzites and amphibolites. It is built up of rather varied rocks. They reach up to 2250 min thickness. Main rocks are quartz-mica schists with porphyroblasts of biotite, garnet ±staurolite, marbles and quartz-mica calcitic schists with porphyroblasts of biotite. Metasandsones and amphybolites are secondary rocks. Manganese-ferrous and lead-zink mineralizations are specific formations. The presence of metamorphic quartz veins is characteristic. The Ustrem Formation sediments have undergone regional dynamothermal metamorphism of medium degree (amphibolite facies). The study of the original conditions of sedimentation (protogenesis) allow to be distinguished two main environments – a shallow-water marine environment strongly affected by the dry land and a shallow-water marine environment void of terrigenous influence. Terrigenous-carbonate facies are formed in the first one while the Mramor carbonate buildup grows up in the second one. Both environments are determined by specific paleogeomor phologic conditions.