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 9: Sedimentary History of the Late Jurassic-Paleogene of Northeast Bulgaria and the Bulgarian Black Sea, by Neil Harbury and Martin Cohen, Pages 129-168

Neil Harbury
Geological & Geophysical Research School, Birkbeck College
London, United Kingdom

Martin Cohen
Enterprise Oil
London, United Kingdom

ABSTRACT

Callovian to Paleocene strata outcrop in the Varna region of northeast Bulgaria, and have also been recorded from several wells in both the Black Sea and onshore locations to the northeast and the south of Varna in the Kamchia Basin. Sedimentological data collected from the abundant core material recovered from these wells allow detailed facies schemes to be developed and regional paleofacies maps to be constructed.

During the Callovian to Valanginian, eastern Bulgaria and the Bulgarian Black Sea were divided into areas dominated by shallow-marine carbonate sedimentation and deeper water environments. The boundary between these facies belts is a broadly east-west line south of Shumen and Varna, and becomes more northeast-southwest in the Black Sea. Seismic data and well studies suggest the platform had a ramp morphology with a gradation of facies from shallow-marine through deeper ramp to basinal facies from north to south. Limited reworking of shelfal material into the basin is recognized, with some intraformational calcirudites and slumping, which suggest that the ramp may, in places, have been distally steepened. Local faulting of the ramp allowed carbonate breccias derived from a shallow-marine area to be reworked into the basin in places. Hauterivian and Barremian marls were deposited, and turbiditic sandstones are recognized in the more basinal areas.

The ramp morphology of the platform was lost by Aptian times. Aptian facies include sandstones, mixed carbonate/siliciclastic facies, and marls. These shallow-marine sedimentary rocks indicate that the Kamchia Basin had become filled by the Aptian. Sedimentary rocks of Albian age are not recorded in the subsurface nor in outcrops of the eastern Moesian Platform; possibly, the late Aptian was a period of erosion and/or nondeposition in

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eastern Bulgaria. The "mid" Cretaceous elevation of the region above sea level may be interpreted as rift-flank uplift, which was a local response to a Western Black Sea rift event. Basement uplift may also explain the continental basement clasts that provided an important siliciclastic source to the Aptian shallow-marine deposits in an otherwise carbonate-dominated basin.

Upper Cretaceous-Paleocene rocks are commonly exposed in the Varna region. Facies include a variety of carbonates and mixed siliciclastics/carbonates, in which pelagic, shallow-marine, and siliciclastic components were mixed by shallow-marine hydrodynamic processes and bioturbation. Deposition of laterally extensive lime mudstones during Campanian times suggests widespread flooding of the northern platform region. Intensely bioturbated intervals, abundant glauconite, and increased proportions of siliciclastic detritus in certain intervals imply periods of condensed sedimentation over a broad platform.

In late Paleocene times, shallow-marine coralline algal buildups developed in the Ravna Gora region and are correlatable with similar algal-rich deposits recorded in outcrop in northeast Bulgaria. Early Eocene facies include sandstones and packstones, often containing abundant nummulite foraminifera.