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AAPG Bulletin

Abstract

AAPG Bulletin, V. 95, No. 4 (April 2011), P. 649673.

Copyright copy2011. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/08181008061

Structural and depositional evolution of the East Balkan thrust belt, Bulgaria

Charles J. Stuart,1 Michal Nemcok,2 Dian Vangelov,3 Eric R. Higgins,4 Chelsea Welker,5 David P. Meaux6

1Energy and Geoscience Institute, 423 Wakara Way Suite 300, Salt Lake City, Utah 84108; [email protected]
2Energy and Geoscience Institute Laboratory at Geological Institute of Slovak Academy of Sciences, Dubravska cest 9, 840 05 Bratislava, Slovakia; [email protected]
3Department of Geology, Sofia University, bul. Tzaz. Osvoboditel 15, Sofia 1000, Bulgaria; [email protected]
4Vintage Petroleum, 110 West Seventh Street, Tulsa, Oklahoma 74119; present address: Chesapeake Energy Corporation, 6100 N. Western Avenue, Oklahoma City, Oklahoma 73118; [email protected]
5Energy and Geoscience Institute, 423 Wakara Way Suite 300, Salt Lake City, Utah 84108; [email protected]
6AOA Geophysics, 1300 West Sam Houston Parkway S, Suite 225, Houston, Texas 77042; present address: BP America, 501 Westlake Park Blvd, Houston, Texas 77079; [email protected]

ABSTRACT

Analysis of structural and sedimentologic data from onshore outcrops, offshore wells, and offshore seismic profiles indicates that the thrust belt geometry in eastern Bulgaria from the Paleocene to the Holocene is characterized by a southeastward plunge toward the western Black Sea Basin. This plunge was caused by (1) a combination of eastward-thinning continental crust in the west and oceanic crust in the east; (2) a postrift thermal subsidence of the continental crust; (3) buttressing and no buttressing of the Moesian platform against the thrust belt in its western and eastern parts, respectively; and (4) northeastward thrust belt advance. These factors controlled the overall eastward-diminishing uplift of the thrust belt and associated eastward sediment funneling into the Black Sea.

Evidence for the eastward-fading uplift and buttressing includes the (1) eastward decreasing amount of shortening along constructed cross sections, yielding 30, 10.5, 11, and 4 km (18.6, 6.5, 6.8, and 2.5 mi, respectively) from west to east, respectively; (2) eastward trend of more complete stratigraphic sections and shallower erosional levels; and (3) eastward increase in decollement depths, being 3.7, 3.8, 9.5 to 13.5, and 12.3 to 14.1 km (2.3, 2.4, 5.9–8.4, and 7.6–8.8 mi). The age of the last thrusting is progressively younger toward the east from the middle Eocene through the late Eocene to the Oligocene from west to east, respectively. Onshore parts of the thrust belt, which were significantly affected by buttressing against the Moesian platform, exhibit thrusting followed by late Eocene gravitational collapse, Oligocene quiescence, and Neogene extension. The thrust belt part farther east exhibits thrusting followed by Oligocene–Neogene extension. A Paleocene–middle Eocene piggyback basin formed in the onshore part of the thrust belt, centered in the East Balkan zone, with a southeastward-plunging axis, which migrated northeastward with basin shortening and filling.

The development of the East Balkan thrust belt and its later extensional modification had a dominant control over sediment transport, lithofacies, and depositional patterns. Developing thrust belt fold structures, together with the orogenic hinterland and highs in the foreland, formed a northeastward and eastward expanding system of sediment input. Southeastward-plunging axes of the foreland basin and the Paleocene–middle Eocene piggyback basin were the principal sediment transport pathways, together with subordinate internal synclinal axes. These depressions funneled sediments toward and into the western Black Sea Basin. As orogenesis advanced to the northeast, former depositional areas were uplifted and eroded, providing local sources of sediment.

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