Introduction to the geology of SW Bulgaria

Abstract

The geologic structure of Southwest Bulgaria is characterized by a complex Mid Cretaceous pile of thrust sheets, a complex system of horsts and grabens of Palaeogene age, and a neotectonic (Neogene - Quaternary) pattern dominated by the Strouma rift system and the Serbo-Macedonian neotectonic swell. Amphibolite-facies metamorphic rocks belong to the Ograzhdenian (Prerhodopian) and Rhodopian Supergroup and the Osogovo “Formation”. The last intensive metamorphic event is proven to be of Cadomian age, and later superimposed metamorphic and deformation events have had a local occurrence. The greenschist-facies Frolosh Formation (Vendian - Lower Cambrian) has a diabasephyllitoid composition and is typical for the Strouma tectonic superunit. The basement of the latter Ograzhdenian Supergroup, Osogovo “Formation”, Frolosh Formation) is covered with a major depositional unconformable contact by sedimentary complexes of Ordovician (only in the Bosilegrad District in Yugoslavia) and Permian and Triassic age. Palaeozoic formations of Ordovician to Devonian age are present only in the thrust sheets of the Morava superunit, and in parts of the southern edge of the Srednogorie zone. Permian formations (mostly continental red beds) have a restricted occurrence. The Triassic (only in the Strouma superunit and parts of the Srednogorie) consists of the Petrohan Terrigenous Group (continental red beds), the Iskur Carbonate Group (marine) and the Moesian Group (marine red beds). After folding, uplift and erosion, the transgressive Jurassic formations have been formed in several different environments, in latest Jurassic - earliest Cretaceous times represented by the carbonate platform to the North, and the Nish-Troyan flysch trough. The principal orogenesis occurred in Mid Cretaceous times, and Upper Cretaceous sedimentary formations are present only in parts of the Srednogorie zone. Late Cretaceous intrusive rocks are known from the Srednogorie (of mantle origin) and in Pirin (crustal granitoids). The Late Cretaceous orogenesis formed the Srednogorie superunit (to the North) and the Morava-Rhodope superunit (to the South). Thus, the Alpine structure consists of the following principal tectonic units: Late Cretaceous Srednogorie superunit (with fragments from the Mid-Cretaceous Lyubash, Golo-burdo, Melovete, Radomir and Verila units) and Morava-Rhodope superunit with the Mid-Cretaceous Morava superunit and Ograzhden unit (allochthonous), Strouma superunit (Louzhnitsa-Trun and Osogovo-Vlahina unit), Rhodope and Pirin-Pangaion superunit. The Palaeogene and Neogene formations have the character of a neoautochthone that is controlled by the Late Alpine and neotectonic block movements.