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Abstract

Chapter from:
AAPG Memoir 68:Regional and Petroleum Geology of the Black Sea and Surrounding Region, Edited by A.G. Robinson
AAPG Memoir 68: Regional and Petroleum Geology of the Black Sea and Surrounding Region. Chapter 6: Cimmerian and Alpine Stratigraphy and Structural Evolution of the Moesian Platform (Romania/Bulgaria), by Gabor Tari, Oprea Dicea, Joe Faulkerson, Georgi Georgiev, Svetlozar Popov, Mihai Stefanescu, and Gary Weir, Pages 63-90

Copyright © 1997 by The American Association of Petroleum Geologists. All arights reserved.

Chapter 6
Cimmerian and Alpine Stratigraphy and Structural Evolution of the Moesian Platform (Romania/Bulgaria)

Gabor Tari
Amoco Production
Houston, Texas, U.S.A.

Oprea Dicea
Prospectiuni S.A.
Bucharest, Romania

Joe Faulkerson
Amoco Production
Houston, Texas, U.S.A.

Georgi Georgiev
St. Kliment Ohridski University
Sofia, Bulgaria

Svetlozar Popov
Geology and Geophysics Corporation
Sofia, Bulgaria

Mihai Stefanescu
Amoco-Romania
Bucharest, Romania

Gary Weir
Amoco Production
Houston, Texas, U.S.A.


ABSTRACT

The vast thickness (>10 km) of relatively undeformed Cambrian to Recent sedimentary cover of the Moesian Platform in Romania and Bulgaria offers an exceptional record of Cimmerian and Alpine tectonics in the surrounding Carpathian and Balkan thrust-fold belts.

Above the Hercynian unconformity, Permian to Middle Triassic continental to shallow marine sediments deposited in a facies succession are quite typical for the European passive margin. Widespread Anisian-Carnian volcanism indicates an aborted rifting period also marked by locally very thick

 

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(>2000 m) evaporites. Based on systematic analysis of reflection seismic data and isopach maps, we interpret two large-scale subsurface features (Optasi-Peris uplift and North Bulgarian arch) as Middle Triassic rift flanks associated with two aborted, E-trending branches of the Paleotethys Basin. The striking appearance of the crystalline basement and the deeper levels of the Paleozoic succession (Cambrian-Silurian systems) in the subcrop of the overlying Jurassic can be understood in terms of rapid uplift and severe denudation of the rift shoulders.

The extensional period was replaced during the Norian-Rhaetian times by a compressional regime in the whole Moesian Platform. Although these Late Triassic Cimmerian folds were undoubtedly formed due to compression, detailed structural analysis permitted a more specific interpretation of the deformation in terms of fault-bend folding in a north-vergent, thin-skinned thrust-fold belt. Structural modeling of the anticlines suggests relatively minor shortening (9-18%). Whereas these structures are very characteristic for North Bulgaria, their structural significance is gradually diminishing to the north, in the Romanian part of the platform. In a wider paleotectonic scenario, the north-vergent anticlines beneath the Moesian Platform are interpreted as the frontal, foreland thrust-fold belt of the Mediterranean Cimmerides propagating into the foreland.

Above the Cimmerian unconformity, the sedimentary facies and thickness relations of Lower to Middle Jurassic carbonates clearly show the development of a south-facing passive margin. East-west-trending Jurassic troughs bounded by normal faults can be documented in the southern, Bulgarian side of the Moesian Platform. From the Early Cretaceous on, sedimentation was clearly influenced by successive compressional periods in the Balkans approaching the southern edge of the Moesian Platform. These consecutive Cretaceous through Eocene Alpine compressive periods, such as the Austrian, Mediterranean, Laramian, Illyrian, and Pyrenean, created the north-vergent thrust-fold belt of the Balkans separated from the platform sequence by a series of relatively narrow foredeeps. Whereas the final docking of the Balkans on the Moesian Platform margin occurred at the end of the Eocene, this did not happen on the northern edge until the late Miocene, when the Carpathians stopped moving onto the platform.

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