A. Simeonova¹ and D. Vangelov²

¹RS Group Ltd., Sofia Bulgaria
²Sofia University "St. Kliment Ohridski", Bulgaria

ABSTRACT: New Insights on Tectono-Sedimentary Evolution of Eastern Balkans Thrust Belt and Sub-thrust Zone, East Bulgaria

The Eastern Balkans Thrust Belt (EBTB) is a part of the northern branch of the Alpo-Himalayan chain. It is represented by intensively folded, Late Albian to Middle Eocene turbiditic sequences (over 6 km thick) deposited in a back-arc setting along the southern margin of the Moesian Platform. The present-day sedimentary pattern allows recognition of a variety of depositional environments and makes possible correlation of the deposits on the Moesian Platform (foot wall) and EBTB (hanging wall).

The aims of the study are to document structural deformations and to examine vertical and lateral facies distribution, that to enable balanced cross-sections construction in order to achieve better understanding on the tectono-sedimentary evolution of the thrust and sub-thrust zone.

The Upper Cretaceous-Paleogene sedimentary section in the EBTB comprises various sandprone basin floor and slope fan turbidite systems with complicate interactions. The systems' architecture and facies distribution indicate tectonic influence on deposition. The same stratigraphic interval on the Moesian Platform is characterized by mixed siliciclastic-carbonate deposits with restricted thickness (200-400 m), many erosional surfaces, hiatuses and stratigraphic gaps. The sediments from both areas show depositional features in a polyphase basin which underwent all stages of evolution.

The onset of basin formation is initiated by dextral transtanssion in Late Albian-Cenomanian. Cenomanian-Campanian time is characterized by development of a pull-apart basin with volcanosedimentary deposition in the central part and low-density turbidites deposited on rotated extensional blocks along its northern board. The synchronous sediments on the Moesian Platform indicate shallow-water deposition in a basin, separated by the pull-apart, due to the effect of "foot-wall cut-off" uplifting and stable platform roll-under folding.

Tectonic activity had ceased during late Campanian-Mastrichtian and uniform, low-density carbonate/siliciclastic turbidite sediments were deposited in the basin plain, while amalgamated carbonate turbidites were developed along the northern flank. In the area of the Moesian Platform carbonates were the main lithologies, indicating that the deposition was realized in a common basin with well-expressed facies belts development, from shore-face to basin plain. Thus the Campanian-Mastrichtian level appears an excellent interval for correlation of the footwall and hanging-wall sequences.

The fieldwork observations and further interpretations show, that synistral transpression was the major deformation mechanism during Paleocene - Middle Eocene. Two stages of shortening across EBTB could be distinguished: syn-rift normal faults reverse stage, resulting in basin morphology and depositional style changes (Paleocene), followed by thin-skinned thrusting, prograding NE-wards, leading to basin closure and foredeep development with coarse-grained syn-tectonic deposition.

The beginning of the Paleocene is marked with compressional deformations which affected mainly the basin's flanks and associated with inversion of pre-existing normal faults and reverse faulting. That resulted in disintegration of the "shelf " (MP) and deep-water (EBTB) areas of existing basin, and led to significant changes in depositional style. The EBTB is characterized with syntectonic, coarse-grained, high density turbidite deposition, whilst in the area of the Moesian Platform shallow-water carbonates were deposited in a narrow basin.

The ongoing compressional regime during Early Eocene, gradually lead to formation flysch type "piggy-back" basin and a foredeep. Within the latter, coarse-grained, gravely-dominated turbidite systems were developed along its steep southern margin. It propagated north-northeastwards with the thrust plain, starving northern slant flank, where predominantly mudstone and shallow water sandstone sediments were deposited.

The final episodes of thrusting at the end of Lutetian are well marked by ceasing of deposition, deformation of syn-tectonic sediments and the end of thrust progradation. A large anticline, developed in front of the thrust belt, can be explained by development of subthrusts and duplex zones or fault propagation fold with complicated core structures within the Upper Jurassic- Lower Cretaceous basement.

The final post-compresional stage of the basin's evolution is characterized by molasse deposition in front and partly over of the thrust belt and associated with development of relatively thick shallow water siliciclastic and deltaic sequences.

This study points out that that Upper Cretaceous - Paleogene sequences from the Moesian Platform and the East Balkans were deposited in different parts of a common polyphase basin. The re-interpreted structural and stratigraphic frameworks of the region indicates that the East Balkans Thrust Belt should not be treated as a simple thin-skinned thrust belt. Pre-existing structures in the region had influenced the later deformations and played a substantial role in the tectonic evolution of the area. Thus the developed structural and stratigraphic patterns could lead to significant re-evaluation of the petroleum prospectivity of the East Balkans Thrust Belts and subtrust zone.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado