Geology of Romania

Abstract

Romania consists of four major areas of Mesozoic and older rocks: the southern Carpathians, eastern Carpathians, Apuseni Mountains, and Dobrogea. Late Tertiary sedimentary rocks occupy areas among these four regions and overlie older rocks in the Pannonian basin, Transylvanian basin, and Skythian (Russian) and Moesian platforms. Although the Carpathian erogenic belt appears to form a continuous easterly arc through Romania, this belt formed through a series of events that began in Triassic time and continue to the present. The southern Carpathians consist of three structural and paleogeographic units. From east to west they are the Danubian terrane, Severin nappe, and Getic terrane. The Danubian and Getic terranes consist of shallow-water, non-marine Mesozoic rocks that overlie Paleozoic and Precambrian sedimentary and crystalline rocks. Rocks of the Getic terrane have been thrust relatively eastward over rocks of the Danubian terrane, along the Getic thrust fault. A slice of Late Jurassic to Late Cretaceous flysch-type rocks occurs between the Getic and Danubian rocks. The Severin nappe may represent remnants of oceanic deposits that probably extended eastward into the flysch terrane of the eastern Carpathians. Emplacement of the Getic and Severin nappes is not well dated but most likely occurred during latest Cretaceous to earliest Tertiary time. The eastern Carpathians consist of two main paleogeographic and structural units, which are, from west to east, the inner crystalline zone and the outer flysch zone. Mesozoic rocks of the inner crystalline zone are mainly shallow marine and nonmarine and overlie crystalline rocks of Paleozoic and Precambrian age. They are similar to rocks of the southern Carpathians, but continuation is not observable because Tertiary rocks of the Brasov depression conceal the relation between the two units. Structurally, the inner crystalline zone consists of a sequence of relatively east-directed thrust slices; all but the uppermost slice contains pre-Mesozoic crystalline rocks. From the base to the top, the main tectonic units are the Bertila unit, sub-Bucovinian nappe, Bucovinian nappe, and Transylvanian nappes. The Transylvanian nappes consist of isolated masses of Mesozoic sedimentary rocks, some incorporated, in wildflysch, that were probably emplaced by gravity during Early Cretaceous time. Thrusting of the lower three units occurred in Late Albian-Early Vraconian time; the crystalline nappes were thrust relatively eastward over the westernmost nappes of the flysch zone. The flysch zone contains sedimentary rocks of Late Jurassic to Late Sarmatian age, and the older rocks (Jurassic and Cretaceous) are present mainly in the western part of the zone. The flysch zone consists of seven nappes, from west to east: Black flysch unit, Ceahleau nappe, Curbicortical nappe, Audia nappe, Tarcau nappe, marginal folds nappe, and sub-Carpathian nappe. Stratigraphic units in the flysch nappes change markedly from one nappe to the next; this change can best be explained in terms of the migration of flysch deposition outward in time. Migration of the axis of the flysch basin resulted from westward underthrusting of the flysch zone beneath the inner crystalline zone. Deformation of the flysch zone extended over a long period of time, probably from Albian to Holocene time. Pleistocene rocks are folded at the south end of the flysch zone; these folds occur within a region of deep (to 200 km) earthquakes, suggesting that underthrusting is still active. The Apuseni Mountains are situated in central Romania, north of the southern Carpathians and west of the Transylvanian basin. They consist of two different structural elements, the northern and southern Apuseni Mountains. The northern Apuseni Mountains consist of Mesozoic shallow marine and nonmarine sedimentary rocks that overlie Paleozoic and Precambrian sedimentary and metamorphic rocks. North-directed thrust faults, nearly all involving pre-Mesozoic basement rocks, cut the southern part of the northern Apuseni Mountains. The nine thrust plates can be divided into two groups: the Codru nappes to the north, containing mainly Mesozoic rocks, and the Crystalline or Biharia nappes of pre-Mesozoic rocks to the south. The nappes were emplaced during Late Cretaceous time, but only the northernmost nappes are clearly dated as Late Turanian-Early Coniacian. The southern Apuseni Mountains consist of several major bodies of mafic and rare ultramafic rocks, probably representing a Middle Jurassic ophiolite sequence, and thick sections of flysch, wildflysch, and molasse of Late Jurassic to Late Cretaceous age. The stratigraphy and structure of the sedimentary rocks are poorly known but appear to result from several different sedimentary basins that were brought together during a series of Cretaceous structural events. Structural events are recorded as Aptian, Albian, Turonian, Santonian(?), and Maestrichtian; the earlier deformations were south-vergent, and the later events were north-vergent, giving the southern Apuseni Mountains a crude fan structure. The Dobrogea area comprises a group of low mountains in southeastern Romania, east of the main Carpathian chain. Dobrogea can be divided into four northwest-trending structural zones, each bounded by faults of probable large displacement. The pre-Dobrogea depression is buried beneath Holocene sediments of the Danube delta, north of Dobrogea proper. Its north flank forms the Skythian platform, and its south flank is marked by a south-dipping thrust(?). The depression, or basin, forms an asymmetric syncline with a steep south flank and is filled with a thick sequence of Jurassic rocks. North Dobrogea contains deformed and metamorphosed basement rocks of Hercynian age, overlain by a thick sequence of alpine-type Mesozoic rocks. These rocks comprise northeast-directed folds and thrusts, probably formed either at the end of Jurassic or in late Early Cretaceous time. Central Dobrogea contains Pre-cambrian metamorphic rocks that are overlain by weakly folded Jurassic rocks. South Dobrogea contains rocks that are similar to those of central Dobrogea; they dip gently southward, forming the east margin of the Moesian platform. The structures in Dobrogea can be traced in the subsurface toward the Carpathians, but here they are buried by late Tertiary rocks of the Carpathian foredeep. Only the pre-Dobrogea depression can be traced east of the Carpathians into Poland; its linear trend is unaffected by the arc of the Carpathians. The Pannonian and Transylvanian basins are superposed on the structural elements of the Carpathians. They clearly represent post-tectonic basins, created partly by extension in middle and late Tertiary time. Unfortunately, these Tertiary basins conceal relations among the structural elements of the Carpathian orogen.