The Balkan Thrust Wedge and Foreland Basin of Eastern Bulgaria: Structural and Stratigraphic Development
H.D. Sinclair, S.G. Juranov, G. Georgiev, P. Byrne, N.P. Mountney, 1997. "The Balkan Thrust Wedge and Foreland Basin of Eastern Bulgaria: Structural and Stratigraphic Development", Regional and Petroleum Geology of the Black Sea and Surrounding Region, A. G. Robinson
Download citation file:
The Balkan Mountains of Bulgaria run east-west and outcrop along the north-south-running Black Sea coastline. Immediately north of the Balkan thrust front is the Kamchia Depression, which has been interpreted to repre-sent the North Balkan foreland basin. To the south is the Srednogorie Zone, comprising Cretaceous calc-alkaline volcanics representing a remnant vol-canic arc. A north-south structural cross section can be generated by the inte-gration of coastal exposures, with deeper level constraint from onshore and offshore seismic data. In this section, the Balkans comprise two large syn- clines bounded by major faults. This folding and thrusting detached at a horizon within the Jurassic succession at ~5 km depth. Section restoration across the Balkans from the remnants of the volcanic arc in the south to the Balkan thrust front in the north gives a minimum of 18 km of shortening.
Seismic stratigraphy indicates two periods of shortening across the Balkans. Initially, deep-seated normal faults that offset the Triassic were reactivated as reverse faults at the end of the Cretaceous. The sea-floor topography generated during this compression was subsequently draped by Paleocene and lower and middle Eocene strata. At end-middle Eocene times, thin-skinned thrusts propagated into the basin, initiating the main Balkan structures. The termination of shortening is recorded by the blanketing of thrust-related topography at end-Oligocene times. Therefore, the>18 km of shortening took place from early Paleocene to end-Oligocene times; this indicates a time-averaged rate of shortening of ~0.5 km/m.y.
The sedimentary fill of the Kamchia Depression is intimately linked to the growth of the Balkan Mountain belt. At end Cretaceous/early Tertiary times, it was characterized by emergence in the north linked to the deep-level fault reactivation, and deepening in the south to bathyal depths, where calci-turbidites accumulated. During the early Eocene times, siliciclastics were deposited in the south and center of the basin, and the northern margin experienced a marine transgression; this is thought to be related to the load-induced subsidence of the southern margin of the Moesian Platform by the Balkan Mountains. By the middle Eocene, immediately prior to thrust encroachment into the basin, the northern margin underwent mass wasting in the form of debris flows, slumps, and gravity glide sheets. At a similar time, micropaleontological indicators suggest reduced oceanic circulation, possibly linked to physical isolation of the Black Sea region by growing mountain belts to the south.
Subsidence analysis offshore indicates accelerated tectonic subsidence during middle and upper Eocene times, thus strengthening the proposed link between early thrusting and deepening of the basin. The structural, sed-imentological, and subsidence history of this basin strongly supports its interpretation as the north Balkan retro-arc foreland basin. Rates of tectonic subsidence (>0.05 km/m.y.) and crustal shortening (<0.5 km/m.y.) are slow compared to other thrust wedge/foreland basin settings.
Figures & Tables
Regional and Petroleum Geology of the Black Sea and Surrounding Region
In 1967 and 1969, two oceanographic cruises were made in the Black Sea under the guidance of the Woods Hole Oceanographic Institute: The cruises included scientists from many countries and disciplines. Their aims were to determine the recent geological and geochemical evolution of the Black Sea, to map the shallow structure of the basin, and to study the interaction between the oxidized surface waters and the anoxic waters beneath them. The results were published 23 years ago, as AAPG Memoir 20 (Ross and Degens, 1974). During the 1969 cruise, the vessel Atlantis II collected 40 piston cores, which formed the basis of most of the subsequent geological studies that were restricted to very recent sedimentation. Speculations concerning the origin of the basin and the relationship of the geology offshore to that exposed around the margins of the Black Sea were rooted in pre-plate tectonic concepts of basin formation and were in any case hampered by a lack of relevant data (Brinkmann, 1974).
In 1976, the Glomar Challenger visited the Black Sea on Leg 42B of the Deep Sea Drilling Project and drilled and cored three deep-water sites (379, 380, and 381). Well 381 north of the Bosporus encountered sediments as old as Miocene, including some apparently deposited in shallow water (Ross, 1978).
The next major volume in Western literature to deal with the Black Sea was published a decade later, collecting papers presented two years earlier at a conference in Yalta. In this volume, a number of seismic reflection lines