Sedimentary History of the Late Jurassic-Paleogene of Northeast Bulgaria and the Bulgarian Black Sea
Neil Harbury, Martin Cohen, 1997. "Sedimentary History of the Late Jurassic-Paleogene of Northeast Bulgaria and the Bulgarian Black Sea", Regional and Petroleum Geology of the Black Sea and Surrounding Region, A. G. Robinson
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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 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 continen-tal 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/car-bonates, 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 sedi-mentation over a broad platform.
In late Paleocene times, shallow-marine coralline algal buildups developed in the Ravna Gora region and are correctable with similar algal-rich deposits recorded in outcrop in northeast Bulgaria. Early Eocene facies include sandstones and packstones, often containing abundant nummulite foraminifera.
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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